The present invention relates to photocurable silicone compositions comprising an olefin polysiloxane and a compound containing a plurality of thiol groups (such as a mercapto polysiloxane), as well as methods of photocuring such silicone compositions and methods of additive manufacturing using the compositions.

BACKGROUND

Ultraviolet-curable silicone compositions have been known in the art and used in many applications including conformal coatings, optical fiber coatings, electrical encapsulation, adhesive compositions and others. The unique properties from silicones, such as excellent thermal and oxidative stability, optical transparency, the ease of fabrication, weather resistance, and high gas permeability, make them ideal for such applications.

The curing chemistry of silicone compositions that can be cured by exposure to UV radiation is mostly free-radical in nature. The conventional radical reaction, however, is subject to the inhibition by atmospheric oxygen, which becomes more troublesome in a highly oxygen-permeable silicone system. Recently, the application of the thiol-ene reaction in silicone cross-linking chemistry has found its advantages over the conventional radical reaction where it can tolerate to an extent the inhibition of oxygen in surrounding environment whilst attaining the characteristic of fast curing.

Generally, the thiol-ene click reaction was found relatively insensitive to the presence of oxygen, which was proposed that the peroxy radicals formed by the reaction between the carbon-centered propagating radicals and molecular oxygen in air still be able to abstract hydrogens from the thiol groups to produce new thiyl radicals avoiding radical termination. However, previous work indicates that the thiol-vinyl silicone system still suffered from the inhibition of oxygen on the surface of the silicone substrate with some unreacted oily residue after the UV irradiation. That could be explained by the high concentration of oxygen at this layer. For example, in Nguyen et al., 2016, Polym. Chem., 7:5281-5293 describes a fast diffusion-controlled thiol-ene based crosslinking of silicone elastomers with tailored mechanical properties for biomedical applications. However, the systems described in that paper suffer from incomplete reactions, exhibiting as an oily residue after curing. The curing reaction also does not take place with sufficient speed for some applications. Improvements on such photo-curable silicone compositions are therefore required, in particular ones that provide a more complete reaction in combination with a fast reaction speed. To date, such photocurable silicone systems have not been possible.

In the present disclosure, the inventors describe a novel ultra-fast, highly oxygen-tolerant photoinitiated silicone cross-linking system comprising an olefin polysiloxane and a mercapto polysiloxane, which can be activated by various initiating systems and added with different types of fillers. The reaction proceeds quickly to completion, without leaving an oily residue. SUMMARY OF THE INVENTION

In a first aspect of the invention, there is provided a photocurable silicone composition comprising an olefin polysiloxane comprising one or more side chains comprising a strained cyclic functional group and/or one or more terminations comprising a strained cyclic functional group, wherein the strained cyclic functional group of the one or more side chains and/or the one or more terminations comprises a carbon-carbon double bond; and a mercapto polysiloxane having a plurality of thiol functional groups.

In a second aspect of the invention, there is provided a method for curing silicone, comprising contacting: an olefin polysiloxane comprising one or more side chains comprising a strained cyclic functional group and/or one or more terminations comprising a strained cyclic functional group, wherein the strained cyclic functional group of the one or more side chains and/or the one or more terminations comprises a carbon-carbon double bond; with a mercapto polysiloxane having a plurality of thiol functional groups; to provide a mixture, and then irradiating the mixture with light to cure the silicone composition.

In a third aspect of the invention, there is provided a cured, cross-linked silicone composition comprising an olefin polysiloxane comprising one or more side chains comprising a cyclic functional group and/or one or more terminations comprising a cyclic functional group; and a mercapto polysiloxane having a plurality of thiol functional groups, wherein the olefin polysiloxane and the mercapto polysiloxane are crosslinked to each other via sulphide bonds. There is also provided a cured, cross-linked silicone composition obtainable according to any method of the invention.

In a fourth aspect of the invention, there is provided methods of additive manufacturing. For example, there is provided a method of additive manufacturing to provide a 3D product, comprising: a) providing a print cartridge comprising a mixture of an olefin polysiloxane comprising one or more side chains comprising a strained cyclic functional group and/or one or more terminations comprising a strained cyclic functional group, wherein the strained cyclic functional group of the one or more side chains and/or the one or more terminations comprises a carbon-carbon double bond; and a mercapto polysiloxane having a plurality of thiol functional groups; b) depositing a portion of the mixture on to a build platform by printing using a 3D printing apparatus to provide a layer of the 3D product; c) irradiating the deposited portion of the mixture with light; and d) repeating steps (b) and (c) to provide the 3D printed product.

In a further aspect of the invention, there is provided a 3D printer cartridge comprising a chamber, the chamber comprising a mixture of an olefin polysiloxane comprising one or more side chains comprising a strained cyclic functional group and/or one or more terminations comprising a strained cyclic functional group, wherein the strained cyclic functional group of the one or more side chains and/or the one or more terminations comprises a carbon-carbon double bond; and a mercapto polysiloxane having a plurality of thiol functional groups. In a further aspect of the invention, there is provided a product incorporating a photocured silicone composition of the invention.

In a still further aspect of the invention, there is provided a kit, comprising an olefin polysiloxane comprising one or more side chains comprising a strained cyclic functional group and/or one or more terminations comprising a strained cyclic functional group, wherein the strained cyclic functional group of the one or more side chains and/or the one or more terminations comprises a carbon-carbon double bond; and a mercapto polysiloxane having a plurality of thiol functional groups.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 is a representation of the photoinitiated crosslinking of silicone via thiol-ene chemistry.

Figure 2 shows the Impact of UV irradiance on the kinetics and the mechanical properties of cross-linked PDMS network via thiol-norbornene reaction A. Time sweep (oscillating amplitude of 1% strain at 25 Hz) shows the evolution of storage modulus (G') at different UV irradiance (see legend). The UV curable resin was prepared with norbornene:thiol ratio of 1:2, and DMPA as photoinitiator (thiokphotoinitiator molar ratio of 1:0.05). B. Comparison of gelation times between the present study (norbornene) and previous work (vinyl, as described in Nguyen et al.). C. Frequency sweeps (oscillation amplitude of 1% strain) were carried out for the characterisation of cross-linked PDMS. D. Comparison of storage modulus G' (at frequency of 1 Hz obtained from frequency sweeps) of the uncured resin and the cross-linked silicones at different UV intensities.

Figure 3 : A. Comparison of photorheological profiles of norbornene-based (in this study), and vinyl-based (described in previous work, i.e. Nguyen et al.) when irradiating with low UV intensity (5mW/cm ² ). B. Plot of normalised inhibition time vs inverse light intensity. C. Disappearance of the oily layer (unreacted residue) on the surface of the norbornene-based cross-linking system as a result of greater oxygen tolerenace to the inhibition of air.

Figure 4 shows the impact of norborne to thiol ratio on the kinetics and the mechanical properties of cross-linked PDMS network via thiol-ene chemistry. A. Time sweep (oscillating amplitude of 1% strain at 25 Hz ) shows the evolution of storage modulus (G') during UV irradiation (UV irradiated at 94 mW/cm2). The UV curable resins were prepared from different norbornene-to-thiol molar ratios (see legends), with DMPA as photoinitiator (thiokphotoinitiator molar ratio of 1:0.05). B. Gelation times were determined from the rheological profiles by in-situ rheology. C. Frequency sweeps (oscillation amplitude of 1% strain) were carried out for the characterisation of cross-linked PDMS. D. Comparison of storage modulus G' (at frequency of 1Hz obtained from frequency sweeps) of the resins before UV irradiation and the cross-linked silicones.

Figure 5 shows the photo-crosslinking of silicone elastomer via thiol-norbornene reaction using different initating systems (Type 1 and 2 photoinitaitors). A. Time sweep (oscillating amplitude of 1% strain at 25 Hz). The UV curable resins were prepared with norbornene:thiol ratio of 1:2, and thiokphotoinitiator molar ratio of 1:0.05. B. Comparision of gelation time and storage modulus at 1Hz of different photoinitiating systems. Figure 6 shows visible-light-induced cross-linking of silicone elastomer via thiol-norbornene chemistry. A. Time sweep (oscillating amplitude of 1% strain at 25 Hz ) shows the evolution of storage modulus (G') at different UV irradiance (see legend). The UV curable resin was prepared with norbornene:thiol ratio of 1:2, and BOPA as photoinitiator (thiokphotoinitiator molar ratio of 1:0.05). B. Gelation times were determined from the rheological profiles by in-situ rheology.

Figure 7 shows dual-cure mechanism of the blending material via different routes. A. Moisture cure following by UV irradiation at 94mW/cm ² . B. UV cure at 94mW/cm ² following by an exposure to air for further curing. The blend was mixed at the ratio of 50:50 between UV curable and moisture curable precursors.

Figure 8 shows the impact of blending ratios of dual-cure compounds on each of the cure chemistry. A. Time sweep (oscillating amplitude of 1% strain at 25 Hz ) shows the evolution of storage modulus (G') at different blending ratios between Moisture-curable and UV-curable parts (see legend). B. Comparision of gelation time and storage modulus when cross-linking the dual-cure materials via moisture-cure. C. Comparision of gelation time and storage modulus when crosslinking the dual-cure materials using UV irradiation at 94mW/cm ² . D. Further toughening of the silicone network after the UV irradiation via moisture-cure mechanism.

Figure 9 shows the impact of silica loading on the curing kinetics and network's properties. Rheological properties and curing kinetics of silica filled silicone composites showing great ability for 3D printing.

Figure 10 : A. 3D printing of silicone composites via thiol-norbornene chemistry. The UV curable composites was prepared with norbornene:thiol ratio of 1:2, DMPA as photoinitiator (thiokphotoinitiator molar ratio of 1:0.05) and loaded with 5wt% of fumed silica. B. SEM image showing the stacking of 3D printing layers of silicone and the possibility of printing the overhanging part.

Figure 11 shows ultra-fast curing of PDMS-based graphene composites via thiol-ene chemistry. A. Time sweep (oscillating amplitude of 1% strain at 25 Hz ) shows the evolution of storage modulus (G') at different loading levels of Graphene oxide (see legend). B. Gelation time as a function of graphene oxide content. C. Electrical conductivity as a function of GO loading, for composites annealed at 300°C for 3h00 D. Mechanical properties obtained for sprayed GO/norbornene-PDMS composites.

Figure 12 is a comparison between the systems of Nguyen et al. 2016 (vinyl) and the compositions of the present invention (labelled "TDF" in the figure). A. Using different irradiation intensities. The UV curable resins were prepared with norbornene:thiol ratio of 1:2, and DMPA as photoinitiator (thiokphotoinitiator molar ratio of 1:0.05). B. Using different ene:thiol ratios. The UV curable resins were prepared from different norbornene-to-thiol molar ratios (see legends), and DMPA as photoinitiator (thiokphotoinitiator molar ratio of 1:0.05). The resins were irradiated at 94 mW/cm ² of UV light.

Figure 13 is a comparison between the systems of Muller et al. (1996) and the compositions of the present invention. Figure 14 shows cure kinetics (A) and mechanical properties (B-C) of silicone elastomers prepared from different side-chain PDMS-NB. The side-chain norbornene PDMS were synthesised from PDMS-OH having molecular weight ranging between 550 and 18,000 g/mol (named accordingly as NB550, NB1100 NB2500, NB18000). The photocurable resins were prepared with ene:thiol (Thiol 4-6) ratio of 1:2 and 1:4, and DMPA as photoinitiator (thiol :photoinitiator molar ratio of 1:0.1).

Figure 15 shows the impact of thiol-containing PDMS design on the cure kinetics and mechanical properties of thiolnorbornene formulations. (A, B). PDMS-thiol having different thiol-content. Thiol 4-6 and Thiol 15 are [(mercaptopropyl)methylsiloxane-dimethylsiloxane] copolymers (numbers indicating the % of mercapto residues) whilst Thiol 100 is a Poly((mercaptopropyl) methylsiloxane) homopolymer. (C,D). Blending Thiol 4-6 and Thiol 100 at different ratios. (E). Addition of Pentaerythritol tetrakis(3-mercaptopropionate) (Tetra-thiol).

Figure 16 shows the impact of blending of side-chain norbornene PDMS in the thiol-norbornene silicone systems. The silicone reins were prepared with Thiol-100 (norbornene:thiol of 1:2) and DMPA as photoinitiator(thiol:photoinitiator molar ratio of 1:0.1).

Figure 17 shows the impact of use of copolymer (synthesised with different chain length between norbornene groups). The silicone resins were prepared with Thiol-100 (norbornene:thiol of 1:2) and DMPA as photoinitiator (thiol :photoinitiator molar ratio of 1:0.1).

Figure 18 shows the properties of visible light-cured thiol-norbornene formulations based on side-chain norbornene PDMS. (A) Photorheological profiles of silicones using different photoinitiators and a commercial Photocentric resin. (B) Gelation times determined from photorheology. Storage modulus of Photocentric's resin already exceeded its loss modulus before irradiation, so there was no cross-over of modulus (indication of gelation) observed. The silicone resins were prepared with Thiol-100 (norbornene:thiol of 1:2) and thiokphotoinitiator molar ratio of 1:0.1. 4- methoxyphenol(MEHQ) was added at lOOOppm to stabilise the resins and the visible light irradiance intensity was used at 23mW/cm2

Figure 19 shows the 3D printing trial on SLA printer using silicone resin based on side-chain norbornene PDMS with : (A) Microfluidic chip designed by the inventors' lab. (B) Tensile dumbbell design (Left: 3D printed part using silicone developed in this study, Right: 3D printed part using commercial resin).

Figure 20 shows compositions of various silicone formulation based on side-chain norbornene PDMS. Thiol XXX corresponds to poly[(mercaptopropyl) methylsiloxane-dimethylsiloxane] copolymers with the XXX numbers indicating the % of mercapto residues.

Figure 21 shows the mechanical properties of some of the formulations of Figure 20. Thiol XXX corresponds to poly[(mercaptopropyl) methylsiloxane-dimethylsiloxane] copolymers with the XXX numbers indicating the % of mercapto residues. DETAILED DESCRIPTION OF THE INVENTION

The present invention provides photocurable silicone compositions (elastomers) comprising an olefin polysiloxane comprising one or more side chains comprising a strained cyclic functional group and/or one or more terminations comprising a strained cyclic functional group, wherein the strained cyclic functional group of the one or more side chains and/or the one or more terminations comprises a carbon-carbon double bond; and a mercapto polysiloxane having a plurality of thiol functional groups. The compositions of the invention provide a number of advantages compared to curable silicones of the art, including extremely fast and highly efficient reaction even in the presence of oxygen of air (a gelation time of less than 1 second for the silicone), the ability to fully cure the formulations in air (no oily residue left uncured at the surface in contact with air), they are versatile and effective over a wide range of photoinitiating systems and light activation (very fast even with low light absorption in the visible range), the retain a high cure speed even when incorporated with light scattering (fumed silica) or light-absorbing (graphene oxide) filler (less or around 1 second at 94mW/cm ² ), and they have the ability to perform a dual-cure mechanism without any impact on its fast curing features.

Olefin polysiloxane component

The photocurable silicone composition comprises an olefin polysiloxane. In some embodiments, the olefin polysiloxane is polydimethylsiloxane (PDMS). The structure of PDMS is shown below, although in the invention the PDMS further comprises one or more side chains comprising a strained cyclic functional group and/or one or more terminations comprising a strained cyclic functional group.

The olefin polysiloxane (for example PDMS) is functionalised to provide an olefin polysiloxane having one or more side chains comprising a strained cyclic functional group and/or one or more terminations comprising a strained cyclic functional group, wherein the strained cyclic functional group of the one or more side chains and/or the one or more terminations comprises a carbon-carbon double bond. For example, in some embodiments, the olefin polysiloxane may be functionalised with norbornene, a norbornene derivative, nadimide or a nadimide derivative. In some embodiments, the olefin polysiloxane is functionalised with norbornene, and may be di-norbornene polydimethylsiloxane (structure shown below, in which each end of the PDMS molecule is functionalised with a norbornene), or the PDMS may be functionalised with one or more norbornene functional groups as more or more side chains.

In embodiments in which the olefin polysiloxane is functionalised with one or more side chains comprising a strained cyclic functional group, the number of side chains and the distance between them may be varied. The distance between the side chains can be controlled by using different starting weights of olefin polysiloxane when manufacturing the functionalized olefin polysiloxane. The distance between side chain strained cyclic functional groups can be measured according to the length of the chain (i.e. the siloxane chain aka the polysiloxane chain) between adjacent side chains, for example as defined by g/mol. In some embodiments, the molecular weight of the chain between strained cyclic functional groups is at least about 500 g/mol, although shorter chains may be employed. The maximum length of the chain is not particularly limited, and could be up to, for example, about 20,000 g/mol or higher. A mixture of different chain lengths may also be employed. Shorter distances may be used provide a higher Young's modulus whilst longer distances may provide superior stretchability (elongation) of the resulting polymer. In some embodiments, the distance between the side chains (i.e. the molecular weight of the olefin polysiloxane chain between side groups) is from about 500 g/mol to about 20,000 g/mol. In some embodiments, the distance between the side chains (i.e. the molecular weight of the olefin polysiloxane chain between side groups) is from about 500 g/mol to about 4,500 g/mol, since such formulations may display a sufficiently high Young's modulus, whilst retaining sufficient stretchability.

Olefin polysiloxanes having different molecular weights and different viscosities can be used in the invention, and these may influence the properties of the final cured silicone-containing composition.

For example, the olefin polysiloxane (such as PDMS) may have a molecular weight of at least about 5kDa. The weights and viscosities of the olefin polysiloxanes may differ according to the type of functionalisation (for example terminator groups and/or side chains). Accordingly, in embodiments in which the olefin polysiloxane is functionalised with one or more side chains comprising a strained cyclic functional group, the olefin polysiloxane (such as PDMS) may have a molecular weight of at least about 5kDa. In embodiments in which the olefin polysiloxane is functionalised with one or more terminations comprising a strained cyclic functional group (such as di-norbornene polydimethylsiloxane), the olefin polysiloxane may have a higher molecular weight. For example, the olefin polysiloxane may have a molecular weight of at least about 15 kDa when the olefin polysiloxane is functionalised with one or more terminations comprising a strained cyclic functional group

The olefin polysiloxane may have particular viscosities. For example, the olefin polysiloxane (such as PDMS) may have a viscosity of at least about 0.001 m ² /s.

Olefin polysiloxanes having certain combinations of molecular weights and viscosities may be used, at this may be influenced by the functionalisation of the olefin polysiloxane. For example, in embodiments in which the olefin polysiloxane is functionalised with one or more terminations comprising a strained cyclic functional group (such as di-norbornene polydimethylsiloxane), the olefin polysiloxane may have a molecular weight of at least about 15 kDa and/or a kinematic viscosity of at least about 0.001 m ² /s. For example the olefin polysiloxane may have a molecular weight of at from about 15 kDa to about 25kDa and/or a kinematic viscosity of from about 0.001 m ² /s to about 0.0025 m ² /s. Long olefin polysiloxanes will provide molecular weights and viscosities in this range.

Shorter olefin polysiloxanes having a lower molecular weight and a lower viscosity may be used, although the longer olefin polysiloxanes have been found to perform surprisingly better than the shorter olefin polysiloxanes.

In embodiments in which the olefin polysiloxane is functionalised with one or more side chains comprising a strained cyclic functional group, the olefin polysiloxane may have a molecular weight of at least about 5 kDa and/or a kinematic viscosity of at least about 0.001 m2/s. For example the olefin polysiloxane may have a molecular weight of at from about 5 kDa to about 25kDa and/or a kinematic viscosity of from about 0.001 m2/s to about 0.0025 m2/s.

Viscosities of any of the components may be measured according to any suitable method known to the skilled person. For example, viscosities may be measured at 40°C. Viscosity may be measured according to ISO 3448:1992 (2 ^nd edition, published September 1992).

In embodiments in which the olefin polysiloxane is functionalised with one or more side chains comprising a strained cyclic functional group, the olefin polysiloxane may be considered a copolymer. In embodiments in which the olefin polysiloxane is functionalised with one or more side chains comprising a strained cyclic functional group, the olefin polysiloxane may be copolymer with chain length between strained cyclic functional group of at least about 500g/mol or a combination of homopolymers and/or copolymers with different chain lengths between strained cyclic functional groups. In some embodiments, the olefin polysiloxane is a copolymer functionalised with one or more side chains comprising a strained cyclic functional group, with a chain length between strained cyclic functional group of from about 500 g/mol to about 20,000 g/mol. In some embodiments, the olefin polysiloxane is a copolymer functionalised with one or more side chains comprising a strained cyclic functional group, with a chain length between strained cyclic functional group of from about 500 g/mol to about 4,500 g/mol. A range of chain lengths between adjacent side chains may be achieved using a blend of (co)polymers. For example, a range of chain lengths between adjacent side chains may be achieved using a blend of 2 (co)polymers, the first having a chain length between strained cyclic functional group of about 550 g/mol and the second having a chain length between strained cyclic functional group of about 18,000 g/mol. Alternatively, a range of chain lengths between adjacent side chains may be achieved using a blend of 2 (co)polymers, the first having a chain length between strained cyclic functional group of about 550 g/mol and the second having a chain length between strained cyclic functional group of about 4,200 g/mol. Alternatively, a range of chain lengths between adjacent side chains may be achieved using a blend of 2 (co)polymers, the first having a chain length between strained cyclic functional group of about 550 g/mol and the second having a chain length between strained cyclic functional group of about 2,500 g/mol. Alternatively, a range of chain lengths between adjacent side chains may be achieved using a blend of 2 (co)polymers, the first having a chain length between strained cyclic functional group of about 2,500 g/mol and the second having a chain length between strained cyclic functional group of about 18,000 g/mol. In embodiments in which the olefin polysiloxane is functionalised with one or more side chains comprising a strained cyclic functional group, the olefin polysiloxane may be (random) block copolymer containing different chain lengths between strained cyclic functional groups. A block copolymer may be employed to achieve a range or combination of different polysiloxane chain lengths between adjacent side chains, as discussed above for the blends of copolymers (the same ranges and combinations of chain lengths are explicitly contemplated herein).

For example the olefin polysiloxane may be a block copolymer having chain lengths between strained cyclic functional groups of more than about 500 g/mol. In some embodiments, the olefin polysiloxane may be a block copolymer having chain lengths between strained cyclic functional groups of from about 500 g/mol to about 20,000 g/mol. In some embodiments, the olefin polysiloxane may be a block copolymer having chain lengths between strained cyclic functional groups of from about 500 g/mol to about 4,500 g/mol.

In some embodiments the olefin polysiloxane may be a block copolymer comprising a first set of blocks with chain length between strained cyclic functional groups of about 550 g/mol and a second set of blocks of chain length of about 18,000 g/mol. Alternatively, the olefin polysiloxane may comprise a first set of blocks with chain length between strained cyclic functional groups of about 550 g/mol and a second set of blocks of chain length of about 4,200 g/mol. Alternatively, the olefin polysiloxane may comprise a first set of blocks with chain length between strained cyclic functional groups of about 550 g/mol and a second set of blocks of chain length of about 2,500 g/mol.

The ratios between number of blocks comprising different chain lengths between strained cyclic functional groups is not fixed and may be varied to achieve stronger mechanical properties or higher stretchability of cross-linked silicone materials. For example, the ratio of the different blocks may be about 1:1 (i.e. about 50% of each block type).

As noted, the olefin polysiloxane component may be a homopolymer, a mixture of different homopolymers or it may be a copolymer or a mixture of different copolymers, or using a block copolymer. Blending different homopolymers or copolymers, or using a copolymer, or using a block copolymer, may improve the mechanical properties of the resulting cured compositions, since the length of polysiloxane chain between adjacent side chains can be varied. When blending different homopolymers or copolymers, the amount of each polymer may vary, although a ratio of about 1:1 for the different polymers may be preferred. In some embodiments, the blend is a blend of 2 homopolymers or copolymers, although other blends are possible. Similarly, when using a block copolymer, the ratio of the amount of different blocks of the copolymer may vary, although a ratio of about 1:1 for the different blocks may be preferred. In some embodiments, the block copolymer is a block copolymer of 2 different blocks, although other combinations are possible. Blends and block copolymers can be used to provide compositions having specific combinations of siloxane chain lengths in embodiments with side chain functionalised olefin polysiloxanes.

For example, a blend of olefin polysiloxanes having a plurality of side chains comprising the strained cyclic functional group may be used, wherein the blend comprises a first olefin polysiloxane having a plurality of side chains comprising the strained cyclic functional group in which the molecular weight of the chain length between adjacent side chains is from about 500 to about 600 g/mol, and a second olefin polysiloxane having a plurality of side chains comprising the strained cyclic functional group in which the molecular weight of the chain length between adjacent side chains is from about 2,500 to about 20,000 g/mol. The ene:thiol molar ratio of the composition may be from about 1:1 to about 1:5, for example about 1:2. The blend may be a 1:1 blend of the different polymers.

A blend of olefin polysiloxanes having a plurality of side chains comprising the strained cyclic functional group may be used, wherein the blend comprises a first olefin polysiloxane having a plurality of side chains comprising the strained cyclic functional group in which the molecular weight of the chain length between adjacent side chains is from about 500 to about 600 g/mol (for example about 550 g/mol), and a second olefin polysiloxane having a plurality of side chains comprising the strained cyclic functional group in which the molecular weight of the chain length between adjacent side chains is from about 2,500 to about 4,500 g/mol. The ene:thiol molar ratio of the composition may be from about 1:1 to about 1:5, for example about 1:2. The blend may be a 1:1 blend of the different polymers.

A blend of olefin polysiloxanes having a plurality of side chains comprising the strained cyclic functional group may be used, wherein the blend comprises a first olefin polysiloxane having a plurality of side chains comprising the strained cyclic functional group in which the molecular weight of the chain length between adjacent side chains is from about 500 to about 600 g/mol (for example about 550 g/mol), and a second olefin polysiloxane having a plurality of side chains comprising the strained cyclic functional group in which the molecular weight of the chain length between adjacent side chains is about 2,500 g/mol. The ene:thiol molar ratio of the composition may be from about 1:1 to about 1:5, for example about 1:2. The blend may be a 1:1 blend of the different polymers.

A blend of olefin polysiloxanes having a plurality of side chains comprising the strained cyclic functional group may be used, wherein the blend comprises a first olefin polysiloxane having a plurality of side chains comprising the strained cyclic functional group in which the molecular weight of the chain length between adjacent side chains is from about 500 to about 600 g/mol (for example about 550 g/mol), and a second olefin polysiloxane having a plurality of side chains comprising the strained cyclic functional group in which the molecular weight of the chain length between adjacent side chains is about 4,200 g/mol. The ene:thiol molar ratio of the composition may be from about 1:1 to about 1:5, for example about 1:2. The blend may be a 1:1 blend of the different polymers.

A blend of olefin polysiloxanes having a plurality of side chains comprising the strained cyclic functional group may be used, wherein the blend comprises a first olefin polysiloxane having a plurality of side chains comprising the strained cyclic functional group in which the molecular weight of the chain length between adjacent side chains is from about 500 to about 600 g/mol (for example about 550 g/mol), and a second olefin polysiloxane having a plurality of side chains comprising the strained cyclic functional group in which the molecular weight of the chain length between adjacent side chains is about 18,000 g/mol. The ene:thiol molar ratio of the composition may be from about 1:1 to about 1:5, for example about 1:2. The blend may be a 1:1 blend of the different polymers.

A blend of olefin polysiloxanes having a plurality of side chains comprising the strained cyclic functional group may be used, wherein the blend comprises a first olefin polysiloxane having a plurality of side chains comprising the strained cyclic functional group in which the molecular weight of the chain length between adjacent side chains is about 2,500 g/mol, and a second olefin polysiloxane having a plurality of side chains comprising the strained cyclic functional group in which the molecular weight of the chain length between adjacent side chains is about 18,000 g/mol. The ene:thiol molar ratio of the composition may be from about 1:1 to about 1:5, for example about 1:2. The blend may be a 1:1 blend of the different polymers.

An olefin polysiloxane block copolymer may be used, in which the molecular weight of the chain length between adjacent side chains is from about 500 to about 600 g/mol (for example about 550 g/mol) in about 50% of the blocks, and the molecular weight of the chain length between adjacent side chains is from about 2,500 to about 20,000 g/mol in about 50% of the blocks. The ene:thiol molar ratio of the composition may be from about 1:1 to about 1:5, for example about 1:2.

An olefin polysiloxane block copolymer may be used, in which the molecular weight of the chain length between adjacent side chains is from about 500 to about 600 g/mol (for example about 550 g/mol) in about 50% of the blocks, and the molecular weight of the chain length between adjacent side chains is from about 2,500 to about 4500 g/mol in about 50% of the blocks. The ene:thiol molar ratio of the composition may be from about 1:1 to about 1:5, for example about 1:2.

An olefin polysiloxane block copolymer may be used, in which the molecular weight of the chain length between adjacent side chains is from about 500 to about 600 g/mol (for example about 550 g/mol) in about 50% of the blocks, and the molecular weight of the chain length between adjacent side chains is about 2,500 in about 50% of the blocks. The ene:thiol molar ratio of the composition may be from about 1:1 to about 1:5, for example about 1:2.

An olefin polysiloxane block copolymer may be used, in which the molecular weight of the chain length between adjacent side chains is from about 500 to about 600 g/mol (for example about 550 g/mol) in about 50% of the blocks, and the molecular weight of the chain length between adjacent side chains is about 4,200 in about 50% of the blocks. The ene:thiol molar ratio of the composition may be from about 1:1 to about 1:5, for example about 1:2.

An olefin polysiloxane block copolymer may be used, in which the molecular weight of the chain length between adjacent side chains is about 2,500 g/mol in about 50% of the blocks, and the molecular weight of the chain length between adjacent side chains is about 18,000 g/mol in about 50% of the blocks. The ene:thiol molar ratio of the composition may be from about 1:1 to about 1:5, for example about 1:2.

An olefin polysiloxane block copolymer may be used, in which the molecular weight of the chain length between adjacent side chains is from about 500 to about 600 g/mol (for example about 550 g/mol) in about 50% of the blocks, and the molecular weight of the chain length between adjacent side chains is about 18,000 g/mol in about 50% of the blocks. The ene:thiol molar ratio of the composition may be from about 1:1 to about 1:5, for example about 1:2.

Other combinations are possible, including at least those disclosed in Figures 20 and 21. Strained cyclic functional group component

A strained cyclic functional group is a cyclic functional group in which one or more bond angles in the ring is under strain. For example, the cyclic functional group comprises one or more bond angles smaller than 109.5°. Cyclic functional groups under strain are more readily reacted in a cross-linking reaction due to their higher level of reactivity. In the invention, the strained cyclic functional groups are able to cross-link with the thiol groups of the mercapto polysiloxane. The strain on the strained cyclic functional group must be sufficient for such a cross-linking reaction to take place when exposed to light.

In some embodiments, the strained cyclic functional group is a bicyclic functional group. Such functional groups naturally have a higher strain that monocyclic molecules.

In the invention, the strained cyclic functional group comprises at least one carbon-carbon double bond. The presence of the carbon-carbon double bond allows the functional group to cross link the olefin polysiloxane component with the mercapto polysiloxane component of the silicone composition. Most likely, the strained cyclic functional group comprises only one carbon-carbon double bond.

In preferred embodiments of the invention, the olefin polysiloxane component of the photocurable silicone composition comprises one or more side chains comprising a bicyclic alkene and/or one or more terminations comprising a bicyclic alkene. Bicyclic alkenes are an example strained cyclic group that may be used in the present invention.

The strained cyclic functional group may be bonded to the siloxane backbone via a linker. For example, as already noted, the photocurable silicone composition comprises one or more side chains comprising a strained cyclic functional group and/or one or more terminations comprising a strained cyclic functional group. In some embodiments, the one or more side chains are the strained cyclic functional group, or the one or more terminations are the strained cyclic functional group. In such embodiments, the strained cyclic functional group is bonded directly to the siloxane backbone. However, the strained cyclic functional group can be bonded to the siloxane backbone via a linker. The linker may be any suitable linker known to the skilled person, for example an alkane, a cycloalkane, a cyclohexane, an ester, an ether, a thioether, an amine, an imine, an oxime or an amide. Simple alkane linkers may be used. For example, a Ci-Ci, linker may be used to link the strained cyclic functional group (such as norbornene) to the siloxane backbone of the polysiloxane polymer.

The strained cyclic functional group may be a heterocyclic functional group or a homocyclic functional group. Heterocyclic functional groups may comprise one or more heteroatoms. Possible heteroatoms includes nitrogen, oxygen and sulphur. In some embodiments, the heteroatom may be nitrogen or oxygen.

The one or more side chains or terminations comprising the strained cyclic functional group may be optionally substituted. Possible optional substitutions include an alkyl, a carboxylic acid, an ester, an amide, an amine or an anhydride, a heteroalkyl, an aryl, a heteroaryl, an arylalkyl, or a heteroarylalkyl. In some embodiments, the optional substitution is selected from the group consisting of Ci-Co alkyl, a carboxylic acid, ester or amide.

The strained cyclic functional group may be present as a number of side chains extending from the siloxane backbone of the polysiloxane monomer units. Alternatively, the strained cyclic functional group may be present as terminator groups at one or each end of the olefin polysiloxane polymer. In some embodiments, the strained cyclic functional group may be present as a combination of one or more side chains extending from the siloxane backbone of the polysiloxane monomer units and as terminator groups at one or each end of the olefin polysiloxane polymer.

In some embodiments, the olefin polysiloxane polymer comprises a strained cyclic functional group as a terminator group according to the following structure:

Formula I wherein y is absent or is a linker, and R is a strained cyclic functional group comprising a carbon-carbon double bond. For example, R may be a bicyclic alkene. The linker (if present) may be an alkane, a cycloalkane, a cyclohexane, an ester, an ether, a thioether, an amine, an imine, an oxime or an amide. In some embodiments, the linker (if present) may be a Ci-Ce alkyl. The number of monomer units in the above polymer can vary, for example according to the desired molecular weight and/or viscosity of the polymer. For example, n can be from about 60.

In some embodiments, R is: wherein z is H, Ci-Ce alkyl, carboxylic acid, ester or amide and X is CH2 or O.

In some embodiments, R is wherein x is CHz or oxygen.

In some embodiments, the olefin polysiloxane polymer comprises a strained cyclic functional group as one or more side chains according to the following structure:

Formula II wherein y is absent or is a linker, and R is a strained cyclic functional group comprising a carbon-carbon double bond. For example, R may be a bicyclic alkene. The linker (if present) may be an alkane, a cycloalkane, a cyclohexane, an ester, an ether, a thioether, an amine, an imine, an oxime or an amide. In some embodiments, the linker (if present) may be a Ci-Ce alkyl. The number of units in the above polymer can vary for example according to the desired molecular weight and/or viscosity of the polymer. For example, n can be from about 3, and each m can independently be from about 2 (for example from about 5 to about 25). As noted elsewhere, the length of the chain between adjacent side chains can be tuned to by modifying n.

In some embodiments, R is: wherein z is H, Ci-Ce alkyl, a carboxylic acid, ester or amide and X is CHz or O.

In some embodiments, R is wherein x is CHz or oxygen. In the most preferred embodiments, R is norbornene.

Embodiments of the invention include olefin polysiloxanes having combinations of side chain and terminator strained cyclic functional groups. For example, the olefin polysiloxane polymer may have a structure according to Formula II above, but may additionally comprise a strained cyclic functional group at one or both ends. In such embodiments, one or both of the terminator hydroxyl groups are optionally replaced with the strained cyclic functional group, for example a bicyclic alkene. In some embodiments, the strained cyclic functional group has the structure according to the following formula: wherein z is H, Ci-Ce alkyl, a carboxylic acid, ester or amide and x is CH2 or O, or strained cyclic functional group has the structure according to the following formula: wherein x is CH2 or oxygen. The terminator strained cyclic functional groups may be attached to the polymer via a linker. The linker (if present) may be an alkane, a cycloalkane, a cyclohexane, an ester, an ether, a thioether, an amine, an imine, an oxime or an amide. In some embodiments, the linker (if present) may be a Ci-Ce alkyl.

Combinations of different strained cyclic functional groups may be used in the name olefin polysiloxane, although it is more likely that each strained cyclic functional group in the functionalised olefin polysiloxane is the same (i.e. has the same structure). Similarly, linkers (if used) may vary within the same polymer, or the linkers (if used) may all have the same structure.

More specific example structures of olefin polysiloxanes used in the invention are those comprising norbornene or norbornene derived functional groups. For examples, olefin polysiloxanes comprising one or more strained cyclic functional groups that may be used in the invention include: where X can be a H or a Ci-Cs alkyl, carboxylic acid ester or amide , and Y is absent or is a linker. For example, the linker (if present) can be an alkane, a cycloalkane, a cyclohexane, an ester, an ether, a thioether, an amine, an imine, an oxime or an amide linker. In some embodiments, the linker (if present) may be a Ci-Ce alkyl.

As can be seen from the above, norbornene derivatives used in the invention include norbornene derivatives having the structure wherein z is H, Ci-Ce alkyl, a carboxylic acid, ester or amide and x is CH2 or O.

Further examples that may be used in the invention include those based on nadimide or nadimide derivatives, for example: where X is CH2 or oxygen, and Y is absent or is a linker. For example, the linker (if present) can be an alkane, a cycloalkane, a cyclohexane, an ester, an ether, a thioether, an amine, an imine, an oxime or an amide linker. In some embodiments, the linker (if present) may be a Ci-Ce alkyl.

A further example based on nadimide or nadimide derivatives that may be used in the invention includes: where X is CH2 or oxygen, and Y is absent or is a linker. For example, the linker (if present) can be an alkane, a cycloalkane, a cyclohexane, an ester, an ether, a thioether, an amine, an imine, an oxime or an amide linker. In some embodiments, the linker (if present) may be a Ci-Ce alkyl.

As can be seen from the above, nadimide derivatives used in the invention include nadimide derivatives having the structure: wherein X is CH2 (as in nadimide) or is oxygen.

An example of a specific strained cyclic functional group-terminated olefin polysiloxane polymer is di-norbornene terminated PDMS, which has the following formula:

Here the strained cyclic functional is norbornene and is attached to the terminal monomer units via -C2H4- linkers. However, other linkers could be used, for example an alkane, a cycloalkane, a cyclohexane, an ester, an ether, a thioether, an amine, an imine, an oxime or an amide linker. In some embodiments, the linker may be a Ci-Cs alkyl linker.

An example of an olefin polysiloxane polymer having a strained cyclic functional group as one or more side chains is side-chain norbornene PDMS (poly norbornene), which has the following formula:

Again, the strained cyclic functional is norbornene and is attached to the siloxane backbone via a -C2H4- linker. However, other linkers could be used, for example an alkane, a cycloalkane, a cyclohexane, an ester, an ether, a thioether, an amine, an imine, an oxime or an amide linker. In some embodiments, the linker may be a Ci-Ce alkyl linker.

Although the invention is not limited to specific arrangements, the use of olefin polysiloxane polymers having one or more strained cyclic functional groups present as side chains may offer a number of advantages. For example, providing the strained cyclic functional groups as side chains may provide the ability to alter the spacing between the side-chain norbornene groups, and as a result, its mechanical properties (e.g. elongation, strength, etc.), as discussed above. Therefore, the provision of the present invention which employs side chain functionalised polysiloxane polymers allow for tuning of the mechanical properties by changing the distance between side chains, using combinations of polymers with different distances between side chains, and using combinations of side chain and end chain functionalisation. They may also provide a significant cost reduction as compared to the use of, for example, commercially available terminal norbornene PDMS. They may also provide a wider range of molecular weights and viscosities for various applications. Furthermore, it may be easier to ensure the absence of undesired strained cyclic functional group molecules (such as mono-norbornene molecules) in the starting olefin polysiloxane component, which is known to be a problem with commercially available olefin polysiloxanes that comprise the strained cyclic functional groups as terminator groups.

The number of strained cyclic functional groups may be controlled by the skilled person, as the number and location of the strained cyclic functional groups may influence the physical chemical properties of the resulting cured silicone composition.

The provision of olefin polysiloxanes comprising one or more strained cyclic functional groups (as side chains and/or as terminator groups) assists in speeding up the reaction time significantly and also helps the reaction to completion, without leaving any residue formed from unreacted components. Although the use of a more highly reactive functional group such as a strained cyclic functional group (compared to, for example, a vinyl group as used in the prior art) might be expected to give a faster reaction time, this could not have been predicted for a reaction system comprising heavy polymer components. The inventors have also surprisingly found the reaction is particularly fast for a photocuring silicone reaction. For example, the photocurable silicone compositions of the invention have a reaction time of less than 1 second. Moreover, despite the speed of the reaction, the inventors surprisingly found the reaction is more complete that with photocurable silicone compositions of the prior art. The reaction is able to complete at the surface of the composition, despite the presence of atmospheric oxygen. As such, no oily residue forms, meaning the compositions of the invention are feasible for use in additive manufacturing (3D printing). For 3D printing, the unreacted oily layer of silicone on the surface of each printing layer is a major issue as it can affect the control of layer thickness and the printing of the subsequent layers (and integrity of the resulting printed material as the different layers would then separate). The removal of the oily layer is also not feasible for small and complex soft devices like microfluidic devices. For coatings, it is obviously essential as by definition the coating is only a surface layer and therefore absence of curing at the surface would lead to complete failure of the coating proposed.

The combination of a very fast reaction and one that reacts to completion is particularly advantageous.

Example specific olefin polysiloxane comprising one or more strained cyclic functional groups that may be used in the invention include :

(bicycloheptenyl)ethyl terminated Poly[(ethyl bicycloheptenyl)

Polydimethylsiloxane dimethylsiloxane]

For the side-chain olefin polysiloxane, the distance between adjacent side chains may be varied to the resulting mechanical properties. A mixture of distances may be employed, for example using blends of homopolymers or copolymers or using block-copolymers.

Mercapto polysiloxane component

The mercapto polysiloxane component may be a homopolymer or a copolymer. Although either can be used, the inventors have surprisingly found the use of mercapto polysiloxane homopolymers in the photocurable silicone systems results in a faster reaction as compared to reaction times when using copolymers. The reaction time is significantly faster in addition to the reaction having better tolerance to oxygen inhibition.

The mercapto polysiloxane may be selected from the group consisting of (mercaptopropyl)methylsiloxane homopolymer and (mercaptopropyl)methylsiloxane dimethylsiloxane copolymer. In some embodiments, the (mercaptopropyl)methylsiloxane dimethylsiloxane copolymer is (4-6% mercaptopropyljmethylsiloxane dimethylsiloxane copolymer. The structures are as follows:

Homopolymer PDMS-thiol (ThiollOO) Copolymer PDMS-thiol 4-6% (Thiel 4-6)

The number of thiol groups (as provided by the mercaptopropyl functional group of this component of the composition) can influence the reaction with the olefin polysiloxane components, since the thiol groups react with the carbon-carbon double bond in the olefin polysiloxane as the composition is cured. The skilled person may therefore desire to control the number of thiol groups in the mercapto polysiloxane component. The number of thiol groups as provided by the mercaptopropyl functional group may differ according to how the olefin polysiloxane component has been functionalised. For example, in some embodiments, at least 50% of the silicone atoms in the mercapto polysiloxane polymer comprise a mercaptopropyl functional group, for example when the olefin polysiloxane component has been functionalised with one or more terminator groups comprising the strained cyclic functional group having the carbon-carbon double bond (e.g. the bicyclic alkene). Alternatively, a lower thiol content, for example from about 2% (for example form about 2% to about 10%), may be used when the olefin polysiloxane component has been functionalised with one or more side chains comprising the strained cyclic functional group having the carbon-carbon double bond (e.g. the bicyclic alkene).

The molar ratio of ene groups (carbon-carbon double bonds in the strained cyclic functional group) provided by the olefin polysiloxane component and the thiol groups provided by the mercapto polysiloxane component (i.e. the stoichiometric ratio) can be adjusted by the skilled person to alter the properties of the final product as well as the reaction times. The present invention works for a range of different molar ratios. In some embodiments, the components are provided in such a way to have a greater amount of -thiol groups from the mercapto polysiloxane component than -ene groups from the olefin polysiloxane component. Accordingly in some embodiments there is an excess of thiol groups from the mercapto polysiloxane compared to carbon-carbon double bonds from the olefin polysiloxane. In some embodiments, the -ene:thiol molar ratio is from about 1:1 to about 1:10, or from about 1:1 to about 1:5.

Although the thiol group may be provided using a mercapto polysiloxane component, it is possible to use nonsilicone components and non-vinyl. For example, in some embodiments, non-silicone thio-containing polymer may be used instead of a mercapto polysiloxane. For example, pentaerythritol tetrakis(3-mercaptopropionate) (tetra thiol):

Pentaerythritol tetrakis(3-mercaptopropionate) (tetra thiol)

Accordingly, in some embodiments of the invention, there is provided a photocurable silicone composition comprising an olefin polysiloxane comprising one or more side chains comprising a strained cyclic functional group and/or one or more terminations comprising a strained cyclic functional group, wherein the strained cyclic functional group of the one or more side chains and/or the one or more terminations comprises a carbon-carbon double bond; and a compound having a plurality of thiol functional groups. The compound having a plurality of thiol functional groups may be, for example, the mercapto polysiloxane component discussed above, or other compounds may be used, for example pentaerythritol tetrakis(3-mercaptopropionate). Accordingly, in some embodiments, the compound having a plurality of thiol functional groups may be selected from the group consisting of (mercaptopropyl)methylsiloxane homopolymer, (mercaptopropyl)methylsiloxane dimethylsiloxane copolymer and pentaerythritol tetrakis(3-mercaptopropionate). Embodiments employing pentaerythritol tetrakis(3- mercaptopropionate) may have any of the more specific features as described herein for the mercapto polysiloxane containing embodiments.

Various combinations of features

In some embodiments, the photocurable silicone composition comprises a) an olefin polysiloxane comprising one or more optionally substituted bicyclic alkene side chains and/or one or more optionally substituted bicyclic alkene terminations, wherein the bicyclic alkene is optionally substituted with a Ci-Ce alkyl, a carboxylic acid, an ester or an amide, and where the bicyclic alkene groups are optionally bonded to the siloxane backbone of the polysiloxane via a linker; and b) a compound having a plurality of thiol functional groups, for example a mercapto polysiloxane having a one or more thiol functional groups.

In some embodiments, the photocurable silicone composition comprises a) an olefin polysiloxane comprising one or more optionally substituted norbornene side chains and/or one or more optionally substituted norbornene terminations, wherein the norbornene is optionally substituted with a Ci-Cs alkyl, a carboxylic acid, ester or amide, and where the norbornene groups are optionally bonded to the siloxane backbone of the polysiloxane via a linker; and b) a compound having a plurality of thiol functional groups, for example a mercapto polysiloxane having a one or more thiol functional groups.

In some embodiments, the photocurable silicone composition comprises a) an olefin polysiloxane comprising one or more norbornene side chains and/or one or more norbornene terminations, wherein the norbornene groups are optionally bonded to the siloxane backbone of the polysiloxane via a linker; and b) a compound having a plurality of thiol functional groups, for example a mercapto polysiloxane having a one or more thiol functional groups.

In some embodiments, the photocurable silicone composition comprises a) polydimethylsiloxane comprising one or more norbornene side chains and/or one or more norbornene terminations, wherein the norbornene groups are optionally bonded to the siloxane backbone of the polydimethylsiloxane via a linker; and b) (mercaptopropyl)methylsiloxane homopolymer.

In some embodiments, the photocurable silicone composition comprises a) polydimethylsiloxane comprising one or more norbornene side chains and/or one or more norbornene terminations, wherein the norbornene groups are optionally bonded to the siloxane backbone of the polydimethylsiloxane via a linker, and wherein the polydimethylsiloxane has a molecular weight of at least 5 kDa and/or a kinematic viscosity of at least 0.001 m ² /s; and b) (mercaptopropyl)methylsiloxane homopolymer.

In some embodiments, the photocurable silicone composition comprises a) polydimethylsiloxane comprising one or more norbornene side chains and/or one or more norbornene terminations, wherein the norbornene groups are optionally bonded to the siloxane backbone of the polydimethylsiloxane via a linker, and wherein the polydimethylsiloxane has a molecular weight of at least 5 kDa and/or a kinematic viscosity of at least 0.001 m ² /s; and b) (mercaptopropyl)methylsiloxane homopolymer; wherein the ratio of carbon-carbon double bonds provided by one or more norbornene side chains and/or one or more norbornene terminations to thiol groups provided by the mercaptopropyl)methylsiloxane homopolymer is from about 1:1 to about 1:10.

In some embodiments, the photocurable silicone composition comprises a) polydimethylsiloxane comprising a norbornene termination at each end, wherein the norbornene groups are optionally bonded to the polydimethylsiloxane via a linker, and wherein the polydimethylsiloxane has a molecular weight of at least 15 kDa and/or a kinematic viscosity of at least 0.001 m ² /s; and b) (mercaptopropyl)methylsiloxane homopolymer; wherein the ratio of carbon-carbon double bonds provided by one or more norbornene side chains and/or one or more norbornene terminations to thiol groups provided by the mercaptopropyljmethylsiloxane homopolymer is from about 1:1 to about 1:10.

In some embodiments, the photocurable silicone composition comprises a) polydimethylsiloxane comprising one or more norbornene side chains, wherein the norbornene groups are optionally bonded to the polydimethylsiloxane via a linker, and wherein the polydimethylsiloxane has a molecular weight of at least 5 kDa and/or a kinematic viscosity of at least 0.001 m ² /s; and b) (mercaptopropyl)methylsiloxane homopolymer; wherein the ratio of carbon-carbon double bonds provided by one or more norbornene side chains and/or one or more norbornene terminations to thiol groups provided by the mercaptopropyljmethylsiloxane homopolymer is from about 1:1 to about 1:10.

In some embodiments, the photocurable silicone composition comprises a) polydimethylsiloxane comprising a plurality of norbornene side chains, wherein the molecular weight of the chain between adjacent norbornene side chains in the polydimethylsiloxane is from about 500 g/mol to about 20,000 g/mol, wherein the norbornene groups are optionally bonded to the polydimethylsiloxane via a linker, and wherein the polydimethylsiloxane has a molecular weight of at least 5 kDa and/or a kinematic viscosity of at least 0.001 m ² /s; and b) mercapto polysiloxane; wherein the ratio of carbon-carbon double bonds provided by one or more norbornene side chains and/or one or more norbornene terminations to thiol groups provided by the (mercaptopropyl)methylsiloxane homopolymer is from about 1:1 to about 1:3.

In some embodiments, the photocurable silicone composition comprises a) polydimethylsiloxane comprising a plurality of norbornene side chains, wherein the molecular weight of the chain between adjacent norbornene side chains in the polydimethylsiloxane is from about 500 g/mol to about 4,500 g/mol, wherein the norbornene groups are optionally bonded to the polydimethylsiloxane via a linker, and wherein the polydimethylsiloxane has a molecular weight of at least 5 kDa and/or a kinematic viscosity of at least 0.001 m ² /s; and b) mercapto polysiloxane; wherein the ratio of carbon-carbon double bonds provided by one or more norbornene side chains and/or one or more norbornene terminations to thiol groups provided by the mercaptopropyljmethylsiloxane homopolymer is from about 1:1 to about 1:3.

Of course, the present invention extends at least to methods to make the above compositions. In addition, the above embodiments may be combined with further features of the invention as described elsewhere herein.

Other components of the composition

The compositions may further comprise one or more photoinitiators. Photoinitiators are compounds that undergo a photoreaction on absorption of light, producing reactive species. These are capable of initiating or catalysing chemical reactions that result in significant changes in the solubility and physical properties of suitable formulations. Example photoinitiators that can be used in the invention include Type I photoinitiators (such as a Type I photoinitiator selected from the group consisting of a phosphine oxide, an a-hydroxyketone, an a-aminoketone, a benzyl ketal and a benzoin, or a combination thereof) and Type II photoinitiators (such as a Type II photoinitiator selected from the group consisting of a benzophenone and a xanthone, or a combination therefore). In some embodiments the photoinitiator is selected from the group consisting of 2,2-Dimethoxy-2-phenylacetophenone (DMPA), bis-acylphosphine oxide (BAPO), Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) and 4,4'-bis(N,N- diethylamino) benzophenone (DEABP), or combinations thereof.

There is no restriction on the precise about of photoinitiator the composition may include. In some embodiments, the photoinitiators are present in an amount of up to about 10 wt% of the total weight of the composition, or up to about 5 wt% of the total weight of the composition. In some embodiments, the photoinitiators are present in an amount of from about 0.01 wt% of the total weight of the composition. In some embodiments, the photoinitiators are present in an amount of from about 0.01 to about 5 wt% of the total weight of the composition. The compositions may comprise a combination of photoinitiators, for example 2 or more different photoinitiators (such as the combination of DMPA and BAPO, although other combinations are feasible). The amount of the photoinitiator may also be expressed as a molar ratio of thiokPI. In some embodiments, the molar ratio of thiokPI may be from about 1:0.05 to about 1:0.2, for example about 1:0.1. The photocurable silicone composition may also comprise one or more solvents. The solvents allow for more efficient mixing of the components, and hence may increase the efficiency of the curing reaction. Example solvents that may be used in the invention include dichloromethane, ethylbenzene, chloroform, tetrahydrofuran, cyclohexane and hexane.

The photocurable silicone composition may also comprise one or more fillers. Fillers may be used to increase the mass of the composition or to alter the physical characteristics of the composition. Fillers include silicon dioxide (silica, including fumed silica), graphene oxide, calcium carbonate, kaolin, magnesium hydroxide (talc), wollastonite (CaSiOa) and glass. Other fillers include metal particles (such as silver particles), magnetic particles (such as iron oxides (Fe3O4), ferrites (e.g. SrFel2O19 or BaFel2O19), alnico, samarium cobalt (SmCo)) or metal-coated particles (such as nickel-coated particles), carbon black or nano clay. Fumed silica (also known as pyrogenic silica), is a colloidal form of silicon dioxide. In preferred embodiments of the invention, the filler used may be silica (for example fumed silica) or graphene oxide. The filler may be present in an amount of up to 20 wt% of the total weight of the composition.

The presence of graphene oxide (also referred to as graphitic oxide or graphitic acid) may provide compositions and cured products that are electrically conductive and anti-corrosive. In some embodiments, when graphene oxide is included in the composition, the graphene oxide may be functionalised to assist mixing with the other components. GO may be functionalised with Poly[dimethylsiloxane-co-(3-aminopropyl)methylsiloxane] before being mixed into the silicone resin to help with the dispersion. GO is hydrophilic whilst silicone is hydrophobic, so the abovementioned polymer may act as a surfactant in such embodiments. This polymer will be coupled to the GO via electrostatic interaction whilst PDMS segment would help functionalised-GO be dispersed nicely in the silicone matrix.

Methods involving the use of graphene oxide my also comprise a step of heat treating the cured elastomer, for example at a temperature of at least 100°C for at least one hour to increase the conductivity of the final product.

Physical and chemical characteristics of the curable silicone composition

The photocurable silicone composition of the invention is curable in the visible range of light and/or in ultraviolet (UV) light.

As used herein, "visible range of light" refers to the range of light visible to the naked human eye. Generally, the visible range of light is electromagnetic radiation with wavelength greater than or equal to about 380nm, 390nm, 400nm, 410nm, 420nm, 430nm, 440nm or 450nm, or up to about 650nm, 660nm, 670nm, 680nm, 690nm, 700nm, 710nm, 720nm. For example, the visible range of light may be from about 390nm to about 700 nm.

As used herein "UV light" refers to the range of light having electromagnetic radiation with wavelength from about 10 to about 400nm or from about 10 to about 390nm. Methods of the invention may use UV light in the range of about 200 nm to about 390 nm. Accordingly, the photocurable silicone composition of the invention is curable in light having an electromagnetic radiation wavelength of from about 200 to about 700 nm.

The photocurable silicone composition of the invention is advantageously curable in air. Air comprises about 78% nitrogen, about 21% oxygen, less than 1% argon and about 0.04% carbon dioxide. Other gases make up the remainder. The provision of photocurable silicone compositions that are able to be cured in air is advantageous as it does not require the composition to be cured in a protection atmosphere or with reduced oxygen, since the curing reaction is oxygen tolerant, reacting quickly to completion without leaving an oily residue. The photocurable silicone composition of the invention is also advantageously curable at room temperature (from about 15°C to about 25°C). This is also advantageous since it does not required the photocurable silicone composition to be heated to speed up or to complete the reaction between the components.

The photocurable silicone composition is photocurable within 1 second. This means gelation (cross-linking) of the components occurs and complete within 1 second from irradiation with light. The gelation point is when both the storage modulus (G') and the loss modulus (G") display the same power law variation with respect to the oscillation frequency.

Once cured the compositions of the invention advantageously have a storage modulus of at least about lOkPa. In some embodiments, the cured compositions may have a storage modulus from about lOkPa to about lOMPa. The storage shear modulus, G' characterises the ability of a material to store energy via an elastic mechanism (as opposed to a dissipative, viscous mechanism). It is defined as the real component of the dynamic modulus G, according to the formula G=G'+/G", where / is a complex number and G" is the loss shear modulus. G is defined as a time function of the stress (o) and strain (e) according to G (t)= o (t) / £.

Once cured the compositions of the invention advantageously have a Young's modulus of at least about 30 kPa. In some embodiments, the cured compositions may have a Young's modulus from about 30 kPa to about 30 MPa. In some embodiments, the cured compositions may have a Young's modulus from about 30 kPa to about 9 MPa. The Young's modulus, E, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied lengthwise. It quantifies the relationship between tensile/compressive stress o (force per unit area) and axial strain s (proportional deformation) in the linear elastic region of a material and is determined using the formula: E = (o / e).

The formulations have superior stretchability. For example, stretchability, or elongation, can be measured as the % elongation of the composition at break. For example, in some embodiments, the compositions have an elongation at break of at least about 50%. However, higher values may be obtained.

Some specific embodiments are discussed below, although the invention is not limited to such compositions.

Method of curing The curing of the photocurable silicone compositions of the prior art takes place via a thiol-ene click reaction. The method comprises contacting: an olefin polysiloxane comprising one or more side chains comprising a strained cyclic functional group and/or one or more terminations comprising a strained cyclic functional group, wherein the strained cyclic functional group of the one or more side chains and/or the one or more terminations comprises a carbon-carbon double bond; with a mercapto polysiloxane having a plurality of thiol functional groups; to provide a mixture, and then irradiating the mixture with light to cure the silicone composition.

The step of contacting the components may comprise adding one component to the other (e.g. adding the olefin polysiloxane component to the mercapto polysiloxane component, or adding the mercapto polysiloxane component to the olefin polysiloxane component). Alternatively, the two components may be added together successively, for example using an additive manufacture method.

The relative amount of the two main components (the olefin polysiloxane component and the mercapto polysiloxane component) are such that there is at least an equal molar amount of thiol groups (provided by the mercapto polysiloxane) to -ene groups (carbon-carbon double bonds, provided by the functionalised olefin polysiloxane), or the components are provided to provide an excess of thiol groups compared to -ene groups. In some embodiments, the -ene:thiol molar ratio is from about 1:1 to about 1:10. In some embodiments, the -ene:thiol molar ratio is from about 1:1 to about 1:5.

In some embodiments, the method may comprise the use of a solvent. The solvent may be added to one or both of the additive manufacturing, or the solvent may be added after the olefin polysiloxane and mercapto polysiloxane components have been contacted with one another to form the mixture. Any suitable solvent may be used to assist the mixing of the components. Example solvents include, for example, dichloromethane, ethylbenzene, chloroform, tetrahydrofuran, cyclohexane and hexane.

In some embodiments, the method may comprise the use of a photoinitiator. The photoinitiator may be added to one or both of the olefin polysiloxane and mercapto polysiloxane components, or the solvent may be added after the olefin polysiloxane and mercapto polysiloxane components have been contacted with one another to form the mixture. Any suitable photoinitiator may be used to assist the mixing of the components. Example photoinitiators include, for example 2,2-Dimethoxy-2-phenylacetophenone (DMPA), bis-acylphosphine oxide (BAPO) and 4,4'- bis(N,N-diethylamino) benzophenone (DEABP), Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO).

The method may comprise a mixing step, in which the components are mixed together. A mixing step may be present after each successive addition of each component. Alternatively, mixing may occur after all of the components have been added. Mixing can be achieved by any suitable means, for example by stirring or shaking. The mixing step may comprising mixing the components for a time sufficient to produce a substantially homogenous mixture. In some embodiments the mixing step may comprise mixing the components for at least about 1 minute or for at least about 5 minutes.

The method may comprise a step of removing solvent from the mixture after the mixing step, if solvent has been used. The solvent can be removed by any suitable means, for example by rotary evaporation (using a rotary evaporator) and/or using a vacuum.

As the composition is photocurable, the above steps may occur without exposing the mixture to light sufficient to cure it. For example, the mixture might only be exposed to up to about 2 mW/cm ² of light, in particular to up to an intensity of about 2 mW/cm ² of light having an electromagnetic wavelength of from about 200 to about 700 nm. In some embodiments, the mixture might not be exposed to any light at all prior to curing.

When the reaction is ready to be cured, however, it will of course be exposed to light sufficient to cause the olefin polysiloxane component and the mercapto polysiloxane component to crosslink.

In some embodiments, the methods comprise a step of irradiating the mixture with light having an intensity of from about 1 mW/cm ² to about 1000 mW/cm ² , wherein the light has an electromagnetic wavelength of from about 200 to about 700 nm.

Advantageously, the reaction takes place very quickly. Depending on the intensity of the light and the molar ratio of thiokene, the cross-linking (i.e. curing) reaction between the olefin polysiloxane and mercapto polysiloxane components may reach completion in less than 20 seconds. In some embodiments, the reaction may reach completion in less than 10 seconds. In some embodiments, the reaction may reach completion in less than 1 second. To achieve faster curing times, the step of irradiation may comprise irradiating the mixture with light having an intensity of at least about 50 mW/cm ² , wherein the light has an electromagnetic wavelength of from about 200 to about 700 nm. Other steps that may be alternatively or additionally be taken is to increase the thiokene ratio, and/or to increase the photoinitiator content. Generally, the should be at least a ratio of thiokene of 1:1, although ratios providing an excess of thiol increase the reaction time.

The step of irradiation may expose the mixture to the light source for a period longer than is required for completion of the reaction. In some embodiments, for example those using additive manufacturing apparatus, the step of irradiation may occur after a portion of the reaction mixture is exposed to light.

The method can take place at room temperature (for example from about 15°C to about 25°C), meaning the reaction does not need to be heated. In addition, the reaction can take place in air. This means the reaction does not need to take place in an inert atmosphere.

In some embodiments, the method may be carried out by additive manufacturing, also known as 3D printing. The additive manufacturing method employed can be any suitable additive manufacturing method. For example, the method may employ any of binder jetting, directed energy deposition, material extrusion, material jetting, powder bed fusion, sheet lamination, and stereolithography (SLA). In the present invention, additive manufacturing methods comprising stereolithography (SLA), material extrusion, or material jetting may be preferred. Methods of additive manufacturing are generally known to the skilled person, and the present invention is advantageously able to be used in existing additive manufacturing methods and apparatuses due to the fast curing times achieved.

Additive manufacturing may use a computer. For example, an additive manufacturing printing apparatus may be under the control of a computer. A user can input instructions or a design into the computer to determine the size and shape of the product to be made. The computer then instructs the 3D printer accordingly to provide a product having the desired specifications.

Methods of additive manufacturing of the invention may comprise providing the olefin polysiloxane and mercapto polysiloxane components, and any other components that may be being used (for example a photoinitiator and/or a filler), in a chamber to provide a reaction mixture in said chamber. The method may further comprise selectively irradiating the mixture to produce a layer of photocured elastomer fixed to a build platform. The build platform is progressed and the layer of photocured elastomer fixed to the build platform is coated with reaction mixture. A further step of selective radiation takes place to provide the second layer of photocured elastomer, before the build platform is progressed again. Selective irradiation may occur by use of a light beam (such as a LED, lamp or laser that focusses on an area where curing should take place, to successively build the photocured product according to a pre-determined design. This is one example of a possible additive manufacturing methods that may be used (stereolithography), although other methods may be employed according to requirements.

Other methods of manufacturing of the invention may comprise providing a print cartridge comprising a mixture of the olefin polysiloxane and mercapto polysiloxane components, and printing a 3D product using an additive manufacturing apparatus. The additive manufacturing apparatus provides a first layer of the 3D product by depositing a portion of the mixture on a build platform, before moving on to the second and subsequent layers of the 3D product. Given the fast curing times of the methods and compositions of the present invention, successive layers of the product can be built quickly, with minimal or no stopping required between layers to allowing for curing of the silicone elastomer. In some embodiments, there may be a step of irradiating the deposit silicone mixture between each layer deposition. This may be achieved by irradiating the building platform and the product that is being successively built consistently during the printing process, or the irradiation may occur periodically between the printing of each layer.

As with the other aspects of the invention, methods of additive manufacturing may employ the use of one or more photoinitiators and/or one or more fillers. For example, the print cartridge may comprise one or more photoinitiators and/or one or more fillers that are printed with the silicone components. Alternatively, in stereolithographic methods, the chamber may comprise the one or more photoinitiators and/or one or more fillers.

The present invention also provides a 3D printer cartridge comprising a chamber, the chamber comprising a mixture of an olefin polysiloxane comprising one or more side chains comprising a strained cyclic functional group and/or one or more terminations comprising a strained cyclic functional group, wherein the strained cyclic functional group of the one or more side chains and/or the one or more terminations comprises a carbon-carbon double bond; and a mercapto polysiloxane having a plurality of thiol functional groups. The mixture is therefore provided as 3D printing ink.

The cartridge may comprise several other features. For example, the cartridge may further comprise an indicator providing information on the amount of mixture contained within the chamber.

In some embodiments, the cartridge further comprises an interface unit. The interface unit may interface between the cartridge and a 3D printing apparatus when the cartridge is installed in the 3D printing apparatus. The interface may be operable to cause the release of at least a portion of the mixture from the chamber. For example, the interface is operable to cause the release of at least a portion of the mixture from the chamber via a release mechanism.

In some embodiments, the chamber of the cartridge further comprises a valve, wherein the valve provides for the deposition of the mixture when the cartridge is used in a 3D printing apparatus.

The printer chamber may be a flexible chamber, such as a bag. Alternatively, the chamber may be rigid.

The chamber may prevent light from reaching the reaction mixture prior to printing, to prevent accidental curing of the silicone mixture. For example, the chamber maybe opaque, and the chamber may be of sufficient opacity to be impervious to light. The chamber is generally sealed prior to use.

The cartridge may comprise a fill port to allow the chamber to be filled with the silicone mixture. The mixture may preferably be not exposed to light prior to printing.

The mixture in the chamber can comprise other components, as discussed elsewhere. For example, in some embodiments, the chamber further comprises a filler and/or one or more photoinitiators. As above, the features and relative amounts of each of the components of the mixture may be as provided for the other aspects of the invention, for example the curable compositions of the invention. When using the mixture is an ink, the mixture may preferably further comprise a filler, and the filler may preferably be a thixotropic additive, such as fumed silica.

As noted above, the mixture in the chamber is a 3D or additive manufacturing ink. Accordingly, in some embodiments there is provided a 3D or additive manufacturing ink comprising a mixture of an olefin polysiloxane comprising one or more side chains comprising a strained cyclic functional group and/or one or more terminations comprising a strained cyclic functional group, wherein the strained cyclic functional group of the one or more side chains and/or the one or more terminations comprises a carbon-carbon double bond; and a mercapto polysiloxane having a plurality of thiol functional groups. The additional features of the ink are as provided elsewhere for the additional features of the photocurable mixture. The ink may be provided or disposed in an opaque container (for example a printing cartridge) to prevent exposure of the ink to light. Kits

The method also provides kits comprising the various components of the invention. In embodiment there is provided a kit, comprising: an olefin polysiloxane comprising one or more side chains comprising a strained cyclic functional group and/or one or more terminations comprising a strained cyclic functional group, wherein the strained cyclic functional group of the one or more side chains and/or the one or more terminations comprises a carbon-carbon double bond; and a mercapto polysiloxane having a plurality of thiol functional groups. As with the other embodiments of the invention, the silicone components may be provided having particular features or in particular amounts, and/or with additional components. For example, the kit may further comprise one or more photoinitiators and/or a filler. The kit may also comprise instructions for use.

The components of the kit may be disposed separately. Some of the components may be disposed together. If the olefin polysiloxane and mercapto polysiloxane components are disposed together, they may be provided in an opaque container. The container may be impervious to light to prevent accidental or premature curing of the silicone components prior to use.

In one embodiment, there is provided a photocurable silicone composition comprising: a) polydimethylsiloxane comprising one or more norbornene side chains and/or one or more norbornene terminations, wherein the norbornene groups are optionally bonded to the siloxane backbone of the polydimethylsiloxane via a linker, and wherein the polydimethylsiloxane has a molecular weight of at least 5 kDa and/or a kinematic viscosity of at least 0.001 m ² /s; b) (mercaptopropyl)methylsiloxane homopolymer; and c) one or more photoinitiators; wherein the ratio of carbon-carbon double bonds provided by one or more norbornene side chains and/or one or more norbornene terminations to thiol groups provided by the mercaptopropyl)methylsiloxane homopolymer is from about 1:1 to about 1:10.

Preferred features for the second and subsequent aspects of the invention are as provided for the first aspect of the invention mutatis mutandis.

The invention is further described by reference to the following non-limited examples.

EXAMPLES

The in-situ photorheology was carried out to monitor the evolving dynamic rheological properties during the photoinitiated cross-linking of the thiol-norbornene silicone systems. As detailed in our previous work and literature ¹ ' ⁴ , the gelation point is a crucial transition reflecting the transformation of the materials from viscous state to solid state, or simply, the weight average molecular weight diverges to infinity. It must be mentioned that a true definition of the gelation point is when both the storage modulus (G') and the loss modulus (G") display the same power law variation with respect to the oscillation frequency ^1,5 . However, the dynamic moduli at several frequencies during the course of the crosslinking process cannot be monitored simultaneously as a result of the extremely fast reaction presently studied. Therefore, the gelation point can be determined by an alternative approach by using the crossover point of storage modulus (G') and loss modulus (G") obtained from the fast-sampling time sweep rheological measurements.

Ultrafast and highly oxygen-tolerant cross-linking of silicone via thiol-norbornene reaction

The thiol-norbornene system was first evaluated by looking at the reaction kinetics when varying the intensity of UV irradiation. The silicone resins were formulated with the ratio of norbornene to thiol of 1:2 and 2,2-Dimethoxy-2- phenylacetophenone (DMPA) as photoinitiator (thiol : photoinitiator = 1 : 0.05 molar ratio, c.a. 0.1 wt%). The rheological profiles in Figure 2A show a strong dependence of the cross-linking rates on the intensity of UV light while the storage moduli of cross-linked networks reached a similar plateau level once the reaction finished, which was also strongly confirmed in the frequency sweep after cure (Figure 2C, 2D). This result indicates that the UV irradiation only has impact on the kinetics of the reaction rather than the silicone network properties. A noticeable result observed in Figure 2B is that the gelation times of thiol-norbornene system were significantly reduced in comparison with those prepared from the vinyl-terminated PDMS in the earlier study ² . The vinyl-based system was prepared with the same PDMS-thiol and photointiatior (DMPA), but at higher concentration of photointiator (thiol :photoinitiator = 1:0.1, c.a. 0.2wt%). It has been reported ^{6 8} that the type of functionality in the end groups of the thiol-ene chemistry exerts an influence on the reaction kinetics, physical and mechanical properties of the materials. Among different ene- functional groups, bicyclic olefins such as norbornene- were found to have a higher affinity to the addition of thiyl radicals and less susceptibility to undergo the reverse reaction. The exceptionally high reactivity toward thiol-ene coupling of the norbornene functional groups can be explained by the release of a significant amount of ring strain energy when reacting with thiyl radicals, and the rapid rate of abstraction of thiol hydrogen atom by the carbon-centred radicals ^{9 u} . In particular, it was reported ¹² that norbornene functional group has low propagation free energy of AG=6.9kcal/mol among other olefins. Cramer et al. ⁶ also recorded the ratio of propagation to chain transfer (k _P /kct) near unity and a fast polymerisation rate for the thiol-ene system comprising of dinorbornene and tetra-thiol functional groups.

In addition, the improved cure rate of the thiol-norbornene system as compared to its vinyl analogue is more distinct at the lower end of UV intensity in Figure 2B, for examples, at 5mW/cm ² (6.4 ± 1 s with norbornene-based resin as compared to 21.5 ± 0.5 s with vinyl-based resin). This could be due to the better tolerance of the thiol-norbornene reaction to the inhibition of oxygen in the atmosphere. The evolution of the storage modulus with irradiation time, shown in Figure 2A, revealed a lagging period after exposed to UV light, which increased with decreasing light intensities. This lagging period was also observed in the earlier study of thiol-vinyl system ² and might be owing to a combination of the increase of mixture viscosity and the retardance from oxygen that highly dissolved in silicone materials. Figure 3A illustrates an example of comparing the lagging time between thiol-norbornene and thiol-vinyl when irradiated at 5mW/cm ² . The storage modulus of norbornene-based material started to increase almost after the UV exposure (about 5s) while the thiol-vinyl reaction was retarded for more than 15s, highlighting the greater tolerance to oxygen and higher efficiency of the former system. It is well known that the thiol-ene chemistry is generally not inhibited by the presence of oxygen thanks to its fast transfer to thiyl radicals and the regeneration of thiyl radicals via hydrogen-abstraction from peroxy radicals ^{11 3} . However, the presence of oxygen still affects the kinetics of the reaction as the thiol-ene coupling only starts in the region where the concentration of oxygen is sufficiently low. Qualitatively, the influence of the oxygen can be expressed as an inhibition period in our study. Since there was no development of the network (G' and G") observed throughout this lagging time, it can be hypothesised that the inhibition period reflects only the scavenging of radicals generated by the initiation of photoinitiator, which could be expressed simply as shown in Eql. where inhibition is the inhibition period, [O ] is the oxygen concentration in the system and Ri is the initiation rate.

On the other hand, the rate of photochemical initiation Ri is defined by the quantum yield of the initiation cP, the extinction coefficient of the initiator (per unit of length of the sample) s, the concentration of photoinitiator [I], and the intensity of incident UV light Io as ¹⁴

R _t = 2 <t>I _o e [I] (Eq2)

In the case of using the same photoinitiator and a fixed gap between the two plates of the rheometer, the inhibition time must be directly proportional to the inverse intensity of UV incident light (l/l ₀ ). As the photoinitiator concentrations used in this study were different with previous work, the inhibition time obtained from both systems was normalised with the starting concentrations of photoinitiator (DMPA for both studies) and plotted in Figure 3B. A linear relationship between the normalised inhibition time and the inverse light intensity l/l ₀ has confirmed our proposed hypothesis. The difference of slopes between two systems is likely attributed to the efficiency of adding thiyl radicals across the double bonds ¹⁵ , where norbornene- terminated silicone was found to be approximately 4.8 times more effective than the vinyl- derivative in the present thiol-ene silicone chemistry. This result highly agreed with the Raman spectroscopy studies of the photocross-linking between a thiol siloxane copolymer and different ot,<u-ene PDMS from Muller and Kunze ¹⁵ which indicated that the relative addition efficiency varied by a factor of 5 from norbornenyl to the vinyl olefins. To confirm such high reactivity and extremely oxygen-tolerant features from the thiol-norbornene reaction, the UV curable resins prepared from vinyl- and norbornene- silicones were cast on a mould and irradiated with UV light in an ambient atmosphere (Figure 3C). A strip of silicon wafer was placed in contact with the surfaces of cross-linked silicones showing no residue of the unreacted materials when using norbornene-terminated PDMS. This is particularly useful for many applications that oxygen-reduced atmosphere is not convenient or practical including spraying and coating of silicone.

Impact of thiol-norbornene functional ratios on kinetics and mechanical properties

To further investigate the thiol-norbornene cross-linking, a wide range of molar ratios (from 1:1 up to 1:3) between thiol- and norbornene- functionalities in the UV curable resins were characterised using a fixed UV irradiation (94 mW/cm ² ) and concentration of photoinitiator (2,2-Dimethoxy-2-phenylacetophenone (DMPA), thiol photoinitiator = 1 : 0.05 molar ratio, c.a. 0.1 wt%). Figure 4A shows the rheological profiles obtained from the in-situ photorheology and gelation times were summarised in Figure 4B. As expected from our discussion above, the reaction between thiol- and norbornene siloxanes occurred rapidly straight after activating the UV light with the gelation point staying well below Is of reaction time, ranging from 632 + 3ms (for ene : thiol of 1:1) to 352 + 1ms (for ene : thiol of 1:3). The fast sampling mode used in our in-situ rheological measurements allowed the rheometer to follow such extremely fast reaction, and as a result, captured precisely the crossover between storage and loss moduli, which can be used as an estimation of the gelation point. The gelation times were shown to decrease with increasing the thiol content in the system (Figure 4B). A correlation between the gelation time, representing the reaction rate, and initial concentrations of the silicone resins in this study was analysed, presenting a good fitting (R=0.97) of a power-law relation with an exponent of 0.548. This value aligned well with the literature reports ⁶ suggesting that the ratio of the propagation to chain transfer of the thiol-norbornene reaction is approximately 1.0 and the overall reaction rate is half order with respect to the thiol concentration.

Similar to recent studies ^2,15 , the mechanical properties of thiol-norbornene PDMS network reached optimal moduli at an off-stoichiometric ratio instead of a balanced stoichiometry. Figure 4D indicates that the storage modulus was maximum (122 kPa) with an ene-to-thiol ratio of 1:1.5, whereas the 1:1 ratio resulted in the lowest storage modulus (13 kPa). The frequency sweep of the cross-linked PDMS (Figure 4C) also displays a strong response of the 1:1 network to the oscillation frequency, which in turn suggests the presence of a significant number of defects in the material. The incomplete reactions at low thiol content (1:1) which lead to a large number of dangling or looping segments from the free ene-terminated chains might account for this observation. In contrast, increasing the thiol content beyond the stoichiometry optimum (1:1.5) caused a slight decrease of the storage modulus, which can be as a result of the lower cross-link density per chain. Interestingly, the optimal network property was achieved at lower thiol content for the thiol-norbornene system in this study than the condition described in the previous work with vinyl-terminated silicone (at vinyl :thiol of 1:2). This difference is likely due to a combination of the higher reactivity, which resulted in less incomplete reaction and consequently lower number of defects, and shorter siloxane chains of the norbornene telechelic PDMS (average M _n of 18,000 g/mol for norbornene vs 28,000 /mol for vinyl- molecules).

Versatile and efficient with different initiating systems.

2,2-Dimethoxy-2-phenylacetophenone (DMPA) was selected as a model photoinitiator throughout this investigation but the thiol-ene reaction, in fact, can be initiated by both classes of photoinitiators, namely Norrish Type I or Type II ¹¹ . With a broad interest of application, we attempted to explore the possibility to apply the thiol- norbornene silicone over a wide range of initiating systems. The thiol-ene mixtures (norbornene:thiol= 1:2) were additionally prepared with Phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO), and 4,4'- Bis(diethylamino)benzophenone (DEABP) at the molar ratio between thiol and photoinitiator of 1:0.05. DMPA and BAPO are both Type I photoinitiators, which generate initiating radicals by a unimolecular cleavage reaction, but the acyl phosphine oxide such as BAPO shows enhanced absorption in the near UV/visible range making it an attractive candidate for the applications towards the visible range of activation light. DEABP, on the other hand, is a derivative of the classical benzophenone, which is a Type II photoinitiator and undergoes bimolecular hydrogen abstraction during the initiation reaction. Unlike other analogues of Michler's ketone, DEABP represents a complete initiating system as its molecule contains both ketone and amine functional groups which also display good biocompatibility ¹⁶ ’ ¹⁷ .

The kinetics and mechanical properties data of different initiating systems were presented in Figure 5. Both type I photoinitiators (DM PA and BAPO) demonstrated very fast reaction with gelation time less than Is whilst DEABP took up to 30s to reach the gelation point and the evolution of storage modulus was relatively moderate. Since both of initiating radicals generated from DMPA and BAPO when excited by UV light can participate in the hydrogen abstraction with thiol ^{16 7} , it is not surprising that the reactions catalysed by these photoinitiators were significantly faster than that promoted by DEABP. Also, BAPO exhibits strong maximum absorption of UV light at 370nm with the tail of absorption spectrum expanding up to 450nm ¹⁸ , which resulted in the highest speed of the reaction observed in Figure 5B (322 ± 0.3ms with BAPO as compared to 565 ± 1.4 ms with DMPA). Despite of the large difference in the reaction kinetics, the mechanical properties of all initiating systems were similar (Figure 5D), indicating an efficient development of silicone network by using the thiol-norbornene cross-linking reaction.

Considering the fast and highly efficient cross-linking of this thiol-norbornene silicone system, a visible light source was used to replace the UV lamp in an endeavour to tailor our silicone system towards applications requiring less harsh curing condition such as biomedical or consumer products. The photocurable thiol-norbornene silicone mixture was formulated with BAPO as the photoinitiator (thiokBAPO = 1:0.5) due to its photosensitivity at longer wavelength near the visible range (absorption tail around 450nm) ¹⁸ . Although the absorptivity of BAPO in the visible range is relatively low, the cross-linking profiles presented in Figure 6 indicates a high speed of reaction with gelation times varying between 6-15s when changing the light intensity (from 55mW/cm ² to 10mW/cm ² ). It must be mentioned that the intensity of a phone LED is approximately llmW/cm ² at a distance of 10mm.

Extremely efficient cure in a dual-cure system with moisture-cure silicone

Moisture-cure silicone such as Room Temeperature Vulcanisation (RTV) use condensation reaction which depends largely on the level of humidity and surrouding temperature. The moisture cure also tends to be fairly slow. Overnight curing is often needed before a full cure can be achieved. These characteristics put severe limitations on the range of applications and products design, espcially with the requirement of deep or fast cure. Thus, a dual-cure mechanism has gained a strong attention to overcome the aforementioned limitations whilst exploing the merits of each cure chemistry. The moisture curable materials are typically manufactured by a, w-silanol terminated silicones encapped with various crosslinkers such as alkoxysilanes, oximinosilanes, acetoxysilanes, aminosilanes, and other silanes with hydrolyzable groups attached to the silicon atom(s). In this study, we obtained the readily formulated moisture-cure silicone base from our industrial partner employing a titanium-based organocatalyst. The counterpart UV curable resin was prepared with norbornemthiol of 1:2 and thiokphotoinitiator (DMPA) of 1:0.05. The two materials were blended by a mixer at different ratios by weight to investigate the effect of blending ratios on each cure chemistry and the multi-step cure process.

The moisture curable and UV curable silicones were first blended at a balanced ratio (50:50) and the impact of this bimodal system on the curing is addressed in Figure 7. It has been shown that the presence of either moisture curable or UV curable part did not influence the curing process of both chemistries (fast gelation with UV irradiation and moderate cure with exposure to moisture). Interestingly, both of the chemistries can be activated after the other curing process without any retardance to reach the final properties of the silicone networks. For examples, the storage modulus reached about 200kPa after Ghr curing in air via moisture-cure pathway (Figure 7A) and rapidly increase for another order of magnitude of modulus once irradiated with UV light. On the other hand, the complete silicone network can be mostly achieved when starting the curing process with UV irradiation (gelation within l-2s) and further toughened with moisture-cure for about 3hr (to reach more than 90% of the maximum storage modulus). These results highlight that there was no interference on each cure chemistry in the present dual-cure system.

In the case of moisture-cure only (Figure 8A), the presence of the UV curable resin resulted in a decrease of starting moduli (G' and G") as a result of the plasticising effect from lower viscosity of the UV- curable part. It also caused a longer reaction to reach the gelation point and lower maximum moduli as clearly demonstrated in Figure 8B. In contrast, Figure 8C indicates that the fast curing feature of the UV curable resin still retained in the blend when applying UV irradiation first (gelation time of 1.5s with only 25wt% of UV- cure part as compared to that of 0.4s with the pristine UV resin). Noteworthily, the mechanical properties of the blending mix were slightly higher than those obtained from the single-cure approach. It is believed that the vinyl- functional groups of from the moisture crosslinker also participated in the UV curing via thiol-ene reaction with the UV part, which resulted in higher cross-linker density of the silicone network. Figure 8D illustrates the sequential cross-linking after UV irradiation via condensation reaction of moisture-cure mechanism for each of the blending ratios. The formation of silicone network was shown to be mostly achieved via the first UV cure when incorporating more than 50wt% of the UV curable resin. This novel system, therefore, could lead to the development of a new product that is able to set quickly under UV light (using a lamp, or even sunlight) and toughen further within a few hours.

Example of using with light-scattering filler system and extrusion-based 3D printability

Silicone elastomers are currently of commercial and research interest in a number of areas because of their extraordinary properties. In the wave of the additive manufacturing (AM) technologies, researchers have been trying to develop methodologies that can directly print silicone elastomer, for examples, stereolithography (SLA), material extrusion, or material jetting ^19,20 . However, these printing methods still confront many challenges because of the limited availability of suitable silicone materials and chemistries. High viscosities of pre-elastomer ink and long setting time (e.g. around 5min for high temperature vulcanisation (HTV) elastomer) are common hindrances to industrial adoption of silicone elastomers in AM ¹⁹ . The fast and highly oxygen-tolerant silicone system presented in this study is particularly a promising candidate for the development of 3D printable elastomer. In order to impart an appropriate storage modulus of the matrix and yield stress, the thixotropic nano fumed silica, which is a rheological modifier and popular filler used in many industrial applications, was added into our thiol-norbornene resins. The UV curable silicone resin was prepared with norbornene:thiol ratio of 1:2 and DMPA as photoinitiator (thiol:DMPA of 1:0.1), and all the photorheology measurements were carried out under a constant UV irradiation of 94mW/cm ² . The impact of fumed silica (loading up to 10wt%) on the kinetics and network properties is shown in Figure 9. First, it is clearly demonstrated that the silica-incorporated silicones still posses extremely fast reactions (less than Is) with only 200-220 ms slower in gelation time than the pristine resin. This small and negligible variation is likely due to the light scattering effect of the fumed silica, which reduced the penetration of UV light into the samples. Second, despite the vast difference in moduli of the uncured materials (i.e. 5 order of magnitude, from viscous to waxy-like form), the mechanical properties of the final silicone networks are relatively similar (Figure 9D). The instantaneous curing, none of the surface tackiness (as a result of remarkable tolerance to oxygen inhibition) after UV cross-linking, and great potential to use in biomedical applications (because of the inertness of PDMS) make this thiol-norbornene silicone system highly suitable for the development of new material chemistry for 3D printing. A UV curable silicone added with 5wt% of silica was transferred into a cartridge and printed with a 3DDiscovery bio 3D printer. The recorded video MovieSl demonstrated the printing process of this material, indicating a nice formation of silicone filament at a moderate extruding pressure and the instant cure under a UV LED (365nm). The green strength of the material is substantial enough to retain its shape before getting irradiated for setting. A nice resolution of the printing is also illustrated in Figure 10A with a testing structure including corners with different radiuses (1mm, 2mm, 3mm, and right angle 90°), typical zigzag lines with a narrow gap (200pm), circles with different diameters, and areas with different thickness. The width of printed filament and layer thickness can be easily manipulated by adjusting the feed rate of material, or speed of printing, and the pressure of extrusion. Importantly, the SEM image in Figure 10B highlights the ability to print the overhanging part, which is often a major issue of printing complex design, in a discrete and continuous structure. Therefore, the cross-linking chemistry and the materials described here are particularly useful for further use in the field of additive manufacturing that should find application in many areas including bio-structure manufacturing, microfluidic devices fabrication, and rapid prototyping.

Example of using with light-absorbing filler system (Graphene oxide)

Graphene oxide (GO) is obtained commercially from Graphene. PDMS being hydrophobic while GO is hydrophilic, a functionalization step is required to disperse GO in a PDMS matrix. This can be achieved by various methods ²¹ . Here we adapted a method described in the literature using a commercially available Poly[dimethylsiloxane-co-(3- aminopropyl)methylsiloxane]. The functionalized GO is easily dispersed in the PDMS mixture described above. Although GO is typically an absorbing material, its optical density allowed us to cure lOOmicron thick films in less than a second with up to 3wt.% of GO (Figure 11A,B).

Adding graphene to the PDMS base formulation enhances the functionality of the materials. For instance, we demonstrated that the stiffness was increased up to 80 times with only 5 wt.% of GO (Figure 11C). While GO is electrically insulative, in its reduced form it conducts electrons. Taking advantage of the good thermal stability of PDMS, we heat treated our cured composites for 3h00 at 200°C in air. This annealing step, allowed to partially reduce the GO to graphene, thereby increases its conductivity. Our results showed that as little as 0.5 wt.% GO was enough to change the transport behaviour of the PDMS from insulating to conductive.

Photocurable silicones based on side-chain norbornene PDMS

Photocurable silicone materials were formulated from different side-chain norbornene PDMS (molecular weight of spacing between norbornene groups ranging from 550 g/mol to 18,000 g/mol). [(4-6% mercaptopropyl) methylsiloxane)-dimethylsiloxane] copolymer (Thiol 4-6) was used initially to reduce the viscosity of final photocurable resins for 3D printing with SLA. In addition, the molar ratio between ene- and thiol- functionalities was mostly at 1:2 throughout the present work because recent studies suggested an excess of thiol would be necessary to achieve optimal network properties. Molar ratio of 1:4 between norbornene and thiol groups was also studied in one series of formulations. 2,2-dimethoxy-2-phenylacetophenone (DMPA) was used as the UV photoinitiator at a constant molar ratio of 1 : 0.1 (Thiol : photoinitiator). Figure 20 summarises the compositions of formulation investigated in this work. Figure 14 highlighted that the cross-linking of silicone elastomers prepared from the sidechain norbornene PDMS still retained very fast curing speed (gelation time less than Is). The thiol-norbornene silicones also exhibited complete surface curing indicating their great tolerance to oxygen inhibition.

In order to achieve the mechanical properties that would be useful for SLA 3D printing application, for example a Young's modulus of about 2MPa and around 100% Elongation, several strategies were applied to tune the formulation of thiol-norbornene silicones. Firstly, the impact of structure of side-chain norbornene PDMS on the photocuring of silicone resins was explored by altering the spacing between norbornene functional groups. It is clearly indicated in Figure 14B and Figure 14C that the design of PDMS-N B plays an important role in determining the mechanical properties of the cross-linked silicone network. PDMS NB550 (shortest distance between norbornene groups) gave the highest Young's modulus whilst increasing the molecular weight of starting PDMS-OH (correspondingly larger distance between norbornene groups) resulted in greater stretchability of the materials. The silicones prepared with NB2500 and NB4200 are particularly of interest due to their good mechanical properties and elongations.

Secondly, the impact of thiol-functionalised PDMS on kinetics and mechanical properties of silicone networks were studied using NB2500 (side chain norbornene PDMS synthesised from PDMS-OH MW of 2,500). The kinetic data shown in Figure 15A indicated that the gelation time was not affected by the structure of PDMS-thiol, which is due to the constant ene:thiol ratio used. However, Young's modulus of silicone elastomer prepared with Thiol-100 (homopolymer PDMS-thiol) was higher than those prepared with copolymer PDMS-thiols (Thiol 4-6 and Th iol- 15) as well as blending of Thiol 4-6 and Thiol-100 (Figure 15C). This effect is likely to result from the shorter distance between cross-links when using Thiol-100, which also decreases its elongation (to c.a. 80%). Further investigation was carried out to incorporate 10wt% of Pentaerythritol tetrakis(3-mercaptopropionate) (Tetra-thiol) into the silicone materials prepared from Thiol 4-6 and Thiol-100 with NB2500 norbornene PDMS (Formulation 13 and 14, Figure 20). The storage moduli obtained from photorheology, however, were found to be lower in the presence of Tetra-thiol (Figure 15E). The drop in mechanical properties of silicone network could be due to lower miscibility of tetra-thiol in the silicone-based resins.

It has been shown so far that formulation based on NB550 resulted in the highest Young's modulus (3.6MPa) while NB18000 silicone led to an elastomer with elongation up to approximately 250% (Formulation 5, ene:thiol ratio of 1:2). Thus, a series of blending different PDMS-NB were explored to tune the silicone's properties close to the required target (Young's modulus of 2MPa, 100% elongation). Figure 16 presented the kinetic and mechanical properties of resins formulated from different blends between single PDMS-N B (NB550+NB18000, NB550+N B2500, NB2500+NB18000) in comparison with formulations based on the single PDMS-NB.

In a similar context, a new generation of random block copolymers was synthesised from blends of PDMS-OH having different molecular weights resulting in silicone structures bearing different lengths between norbornene groups. Copolymer PDMS-NB synthesised from PDMS-OH blends between OH550 and OH18000 showed a significant improvement of mechanical properties as compared to blending single PDMS-NB in the formulation. Young's modulus of Formulation 18 based on NB550+NB18000 is about 0.04MPa (Figure 16B) whilst Formulation 24 (using block copolymer NB550-18000) gave rise of modulus by almost an order of magnitude up to 0.34 MPa (Figure 17). It was also shown in Figure 17 that reducing the number of large spacing segments between norbornene groups (i.e. segments of 18,000g/mol) in the copolymer structure would help increase the mechanical properties of cross-linked silicone network. Formulation 22 (using block copolymer NB550-18000 synthesised from OH blend consisting of 80% OH550 and 20% OH18000) resulted in 1.15MPa MPa and 47 % of Young's modulus and elongation, respectively.

OH2500 and OH4200 were also included in the PDMS-OH blends with OH550 at molar ratio of 1:1 to synthesise norbornene functionalised block PDMS copolymers (NB550-2500 and NB550-4200). Formulations formulated from these two PDMS-NB (Formulation 25 and 26) showed relatively good mechanical properties (Young's modulus of 1.03MPa) whilst achieving elongation close to 100% after UV cross-linking.

Figure 21 presents photocurable silicone formulations based on side-chain norbornene PDMS that could be utilised for different applications. Formulation 18, 25 and 26 are particularly of interest for their uses in 3D printing as a result of its relatively strong network (Young's modulus around IMPa) and good stretchability (elongation close to 100%). These formulations were based on blending of two norbornene side-chain PDMS copolymers or copolymer containing different chain lengths between norbornene pedant groups. On the other hand, the presence of very long chain length (i.e. 18,000 g/mol) between norbornene functional groups in copolymers resulted in silicone materials (Formulation 5, 10, 20) displaying very high stretchability (elongation up to 250 %).

Examples of using side-chain norbornene PDMS in SLA 3D printing

The SLA 3D printing of silicone was conducted on the LC Precision 3D printer (Photocentric), which was equipped with a single wavelength LED light source (402nm) and able to irradiate at an intensity of 1.9mW/cm ² (measured by Thorlabs USB photometer). The photocurable silicone systems were mainly studied with UV light source during the development phase of formulation. To prepare an appropriate resin for the printing trial on this model of SLA 3D printer, Formulation 25 was modified to replace DMPA by other visible light photoinitiators including Bis(2,4,6- trimethylbenzoyl) -phenylphosphineoxide (BAPO) and Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO). The cross-linking kinetics of visible light formulations are shown in Figure 18, which indicates faster cure speed of silicone using TPO as phototoinitiator. Formulation based on TPO also exhibited a similar curing profile with a benchmark visible light resin (Daylight model resin, Photocentric). Thus, TPO was selected the as the photoinitiator for the visible light system of our thiol-nobornene silicone. In addition, a dye, 2,5-Bis(5-tert-butyl-benzoxazol-2-yl)thiophene (BBOT), was added to the silicone resin to help prevent deep cure when using the materials on SLA 3D printer. The data in Figure 18 shows that the presence of BBOT only had a slight impact on the cure speed without affecting the mechanical properties of silicone network.

It must be noted that the LC Precision 3D printer (Photocentric) can only provide very low irradiance intensity (1.9mW/cm2) at the wavelength of 402nm. This low irradiance, however, required a modification in the silicone formulation, which employed a dual photoinitiating system and higher loading of photoinitiators. A trial formulation was prepared based on NB550-2500 (synthesised from a 50:50 blend of PDMS-OH with molecular weight of 550 and 2500 g/mol), Thiol-100 (norbornene:thiol of 1:2), TPO (15wt%) and BAPO (5wt%) as photoinitiators, and BBOT(0.1wt%) as a dye to prevent deep cure. lOOOppm MEHQ was also added to stabilise the silicone. Figure 19 illustrates examples of printed parts (a microfluidic chip designed by the inventors' lab, and a standard dumbbell for tensile testing).

Materials and Methods

(Bicycloheptenyl)ethyl terminated Polydimethylsiloxane (M _n =12, 000-16, 000 g/mol, Viscosity = 1,300- l,800cSt), Poly[(mercaptopropyl)methylsiloxane] (Mn=4000-7000g/mol; mercaptopropyl siloxane unit content is 100%) (Thiol 100), [(Mercaptopropyl)methyl siloxane]dimethylsiloxane copolymer (Mn=6000-8000 g/mol, 120-170 cSt, mercaptopropyl siloxane unit content is 4-6%) (Thiol 4-6) [(Mercaptopropyl)methyl siloxane]dimethylsiloxane copolymer (Mn=3000-4000 g/mol, 100-200 cSt, mercaptopropyl siloxane unit content is 15%) (Thiol 15) were obtained from Gelest, Inc. 2,2-Dimethoxy-2-phenylacetophenone (DMPA), Phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide (BAPO), 4,4'-Bis(diethylamino)benzophenone (DEABP), Diphenyl(2,4,6- trimethylbenzoyl)phosphine oxide (TPO), 2,5-Bis(5-tert-butyl-benzoxazol-2-yl)thiophene (BBOT), Dichloromethane (DCM), and fumed silica were purchased from Sigma Aldrich. The poly[(mercaptopropyl)methylsiloxane] and (Bicycloheptenyl)ethyl terminated Polydimethylsiloxane are referred to thereafter as PDMS-thiol and PDMS-NB, respectively. All chemicals and solvents were used as received without any further purification. Photoinitiators were kept in the fridge and away from any source of lights.

Preparation of UV curable silicone resin

The thiol-norbornene UV curable silicone formulations consisted of a multifunctional Poly[(mercaptopropyl)methylsiloxane (PDMS-thiol), a (Bicycloheptenyl)ethyl terminated PDMS (PDMS-NB), and a photoinitiator (depending on the formulation studied). The chemical structures of these materials are shown in Figure 1. Each formulation was prepared by first adding the PDMS-thiol and PDMS-NB in a glass vial before diluted with Dichloromethane (DCM) to assist the mixing process. To the silicone mixture, the equivalent amount of photoinitiator stock (in DCM) was added at a thiokphotoinitiator ratio of 1:0.05 and mixed thoroughly for 15min. The solvent was finally removed from the mixture by rotovap and high vacuum. Samples were kept away from any source of light during the preparation, and all equivalent calculation was based on the molecular weights of starting materials given by the manufacturers.

Preparation of UV curable silicone composites filled with silica

Formulated mixture consisting of PDMS-NB, PDMS-thiol and PDMA as photoinitiator was added with fumed silica (0.5%, 1%, 5%, 7.5%, and 10%;). Dichloromethane was used to homogenise the mixture with the aid of stirring of 20 mins before being removed under reduced pressure.

In-situ Photorheology

Rheological measurements were conducted using a TA instruments Discovery Hybrid Rheometer (DHR-3) equipped with a 20 mm parallel top plate and a specially designed UV chamber. The fully formulated silicone mixtures were placed between the top and bottom plates of the UV cure cell at a fixed gap of 250 pm, and irradiated with a UV light (Omnicure S2000, Lumen dynamics) through a flexible light guide. The intensity of the UV light was calibrated by a photometer (Silver Line UV Radiometer, 230-410nm) and controlled by the advanced TRIOS software in all experiments. Oscillations were set at a fixed strain of 1% that was found to stay within the linear viscoelastic region of the present silicone obtained from amplitude sweeps. The in-situ time sweep was performed at 25Hz (fast sampling mode) to capture the rapid rates of the reactions while controlling the deviation of axial force less than 0.1N. For the experiments with visible light, a 150W Halogen Fiber Optic Illuminator (Thorlabs, 400-1300nm) was connected to the rheometer light chamber via a special design of adapter and light guide working in the range of 340-880nm. All experiments were performed at room temperature (25 °C) and repeated in triplicate.

Conductivity measurement

Electrical conductivities of graphene/silicone composites were obtained using a picoammeter (Keithley 6485) DC voltage source (Agilent 6614C) with the two-point probe configuration. The silicone composite was painted with a silver paint at the cross-section side and adhered to a copper tape to eliminate the resistive contribution from the contact point between the electrodes and the samples. Current-voltage sweeps were performed between 0 and 5 V and the conductivity was calculated from the slope of the l-V plots and the dimension of the specimens.

Mechanical tensile measurement

Tensile properties were determined by using an Instron 5586 universal tensile testing frame equipped with a load cell of 2.5 N at room temperature. Testing samples in rectangular shape was subjected to a constant strain rate of 10% until failure. The initial region of low extension (0-10%) in the stress-strain plot was used to determine the tensile properties of samples.

Extrusion-based 3D printing of photocurable silicone composite

3D printing of silicone elastomers was performed on a RegenHU 3DDiscovery Bioprinter equipped with a direct dispenser printhead. The wavelength of the UV LED light is 365 nm and the exposure time was set to be 3s after for each printed layer. The design and layer components were made in the BioCad software supplied by the manufacturer. 3D structures were generated before loaded to the machine software. Once the layer was printed, the printhead moved to a higher level according to the thickness set in the design. The photocurable silicone was added into a black cartridge to protect the material against the UV exposure, and removed bubbles and void by placing it in a vacuum desiccator for 15min.

Stereolithography (SLA)-based 3D printing of photocurable silicone

3D printing of silicone resin was performed on a LC Precision 3D printer (Photocentric) equipped with a single wavelength LED (402nm). The designs of printed objects were made in AutoCad, which were then sliced using Photocentric Studio software supplied by the manufacturer. The thickness of each layer was 25pm and irradiation intensity was measured to be 1.9mW/cm2 by Thorlabs USB photometer. The photocurable resin was poured into the resin basin which was covered by an amber shield to protect the material against room light exposure.

Comparison of the present invention with curable silicone compositions comprising vinyl As stated above, Nguyen et al., 2016, Polym. Chem., 7:5281-5293 describes a fast diffusion-controlled thiol-ene based crosslinking of silicone elastomers with tailored mechanical properties for biomedical applications. However, the systems described in that paper suffer from incomplete reactions, exhibiting as an oily residue after curing. The curing reaction also does not take place with sufficient speed for some applications. The inventors compared the system of Nguyen et al., 2016 with the photocurable compositions of the present invention. These results are provided below.

Improvements on such photo-curable silicone compositions are therefore required, in particular ones that provide a more complete reaction in combination with a fast reaction speed. To date, such photocurable silicone systems have not been possible.

The present invention provides a faster reaction time (below 1 second) compared to Nguyen et al. A comparison of the gelation time for the two systems using different irradiation intensities and ene:thiol ratios is provided in Figure 12. The present invention provides consistently faster gelation times and is able to achieve a gelation time of less than 1 second, which is never achieved by the system of Nguyen et al.

The present invention also provide a more complete reaction compared to the system of Nguyen et al. when the reaction takes place in air. Figure 3C illustrates a comparison of silicones cured in air for vinyl-based formulation (as in Nguyen et al.) and norbornene-based ( N B) formulation with unreacted oily residue observed for the vinyl sample shown on the left half of the figure. No such oily residue is present of the system according to the present invention.

Comparison of the present invention with compositions described by Muller et al.

The present invention also provides advantages over those systems described in Muller et al. The two systems can be compared as follows

A further comparison between the components providing the ene functionality in the two systems is shown below:

As shown in Figure 13, At a stoichiometric ratio or 1:1, there is no gelation of the resin formulated according to Muller et al. whilst the homopolymer thiol according to the invention results in fast curing with gelation of 0.63s. The networks of thiol formulation according to Muller et al developed with reaction time but were not sufficient to reach the crossover point of moduli (the formation of the three-dimensional network). This highlights the efficiency of the present invention, in particular when using the homopolymer thiol (ThiollOO). Furthermore, the approach proposed by Muller et al (a shorter ene component, such as NB-PDMS) would not result in sufficient compositional freedom to allow reproducible curing of materials with sufficiently strong elastomeric properties, in contrast to the present invention.

Reaching a gelation time below Is and complete cure on the silicone surface would not have been predictable quantitatively based on current knowledge, prior to the advent of the present invention. In addition, the present invention introduces a higher degree of functionalisation of the PDMS thiol, which allows to reach stiffer materials and introduce multi-NB PDMS, which is cheaper and easier to synthesise and allows to strengthen mechanical properties.

REFERENCES

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EMBODIMENTS

The invention will now be further described by reference to the following numbered embodiments.

1. A photocurable silicone composition comprising a. an olefin polysiloxane comprising one or more side chains comprising a strained cyclic functional group and/or one or more terminations comprising a strained cyclic functional group, wherein the strained cyclic functional group of the one or more side chains and/or the one or more terminations comprises a carbon-carbon double bond; and b. a mercapto polysiloxane having a plurality of thiol functional groups.

2. The photocurable silicone composition of embodiment 1, wherein the olefin polysiloxane is polydimethylsiloxane.

3. The photocurable silicone composition of any preceding embodiment, wherein the olefin polysiloxane is dinorbornene polydimethylsiloxane. 4. The photocurable silicone composition of any preceding embodiment, wherein the olefin polysiloxane has a molecular weight of at least 5 kDa and/or a kinematic viscosity of at least 0.001 m ² /s.

5. The photocurable silicone composition of any preceding embodiment, wherein the olefin polysiloxane comprises one or more side chains comprising the strained cyclic functional group and has a molecular weight of at least 5 kDa

6. The photocurable silicone composition of any preceding embodiment, wherein the olefin polysiloxane comprises one or more terminations comprising the strained cyclic functional group and has a molecular weight of at least 15 kDa

7. The photocurable silicone composition of any preceding embodiment, wherein there is an excess of thiol groups from the mercapto polysiloxane compared to the carbon-carbon double bonds from the olefin polysiloxane.

8. The photocurable silicone composition of any preceding embodiment, wherein the ratio of thiol groups from the mercapto polysiloxane to carbon-carbon double bonds from the olefin polysiloxane is from about 1:1 to about 1:10.

9. The photocurable silicone composition of any preceding embodiment, wherein the olefin polysiloxane component is substantially free from molecules corresponding to the side chains comprising a strained cyclic functional group and/or the terminations comprising a strained cyclic functional group.

10. The photocurable silicone composition of any preceding embodiment, wherein the strained cyclic functional group is a bicyclic functional group.

11. The photocurable silicone composition of any preceding embodiment, wherein the strained cyclic functional group is substituted with an alkyl, a carboxylic acid, an ester, an amide, an amine, an anhydride, a heteroalkyl, an aryl, a heteroaryl, an arylalkyl, or a heteroarylalkyl.

12. The photocurable silicone composition of any preceding embodiment, wherein the strained cyclic functional group is optionally substituted with an Ci-Cs alkyl, a carboxylic acid, ester or amide.

13. The photocurable silicone composition of any preceding embodiment, wherein the strained cyclic functional group is attached to the siloxane backbone of the polysiloxane via a linker.

14. The photocurable silicone composition of embodiment 13, wherein the linker is an alkane, a cycloalkane, a cyclohexane, an ester, an ether, a thioether, an amine, an imine, an oxime or an amide linker.

15. The photocurable silicone composition of embodiment 14, wherein the linker is a Ci-Ce alkyl linker. 16. The photocurable silicone composition of any preceding embodiment, wherein strained cyclic functional group is a heterocyclic functional group.

17. The photocurable silicone composition of any preceding embodiment, wherein the strained cyclic functional group is norbornene, a norbornene derivative, nadimide, or a nadimide derivative.

18. The photocurable silicone composition of any preceding embodiment, wherein the strained cyclic functional group is norbornene.

19. The photocurable silicone composition of any preceding embodiment, wherein the olefin polysiloxane comprises a strained cyclic functional group at each end of the polymer as terminator groups.

20. The photocurable silicone composition of any preceding embodiment, wherein the olefin polysiloxane comprises one or more strained cyclic functional groups as one or more side chains.

21. The photocurable silicone composition of any preceding embodiment, wherein the olefin polysiloxane polymer is PDMS and comprises a strained cyclic functional group as a terminator group according to the following structure:

Formula I wherein y is absent or is a linker, and R is a strained cyclic functional group comprising a carbon-carbon double bond.

22. The photocurable silicone composition of embodiment 21, wherein R is a bicyclic alkene.

23. The photocurable silicone composition of embodiment 21, wherein R is: wherein z is H, C1-C6 alkyl, a or a carboxylic acid, ester or amide and X is CH2 or O, or wherein R is: wherein x is CH2 or oxygen. The photocurable silicone composition of any one of embodiments 1 to 20, wherein the olefin polysiloxane polymer is PDMS and has the following structure:

Formula II wherein y is absent or is a linker, and R is a strained cyclic functional group comprising a carbon-carbon double bond. The photocurable silicone composition of embodiment 24, wherein R is a bicyclic alkene. The photocurable silicone composition of embodiment 24, wherein R is wherein z is H, C1-C6 alkyl, a or a carboxylic acid, ester or amide and X is CH2 or O, or wherein R is: wherein x is CHz or oxygen. The photocurable silicone composition of any one of embodiments 1 to 20, wherein the olefin polysiloxane polymer has a structure according to formula II, except one or both terminal hydroxyl groups is replaced with a terminator strained cyclic functional group R. The photocurable silicone composition of embodiment 27, wherein R is a bicyclic alkene The photocurable silicone composition of embodiment 27, wherein R is wherein z is H, C1-C6 alkyl, a or a carboxylic acid, ester or amide and X is CH2 or O, or wherein R is: wherein x is CH2 or oxygen; and optionally wherein the one or more terminator strained cyclic functional groups are attached to the polymer via a linker. The photocurable silicone composition of any preceding embodiment, wherein each strained cyclic functional group in the olefin polysiloxane is the same. The photocurable silicone composition of any preceding embodiment, wherein the olefin polysiloxane is a copolymer. The photocurable silicone composition of any preceding embodiment, wherein the olefin polysiloxane is a blend of copolymers. The photocurable silicone composition of embodiment 32, wherein the olefin polysiloxane is about 1:1 blend of 2 different copolymers. 34. The photocurable silicone composition of any one of embodiments 1 to 30, wherein the olefin polysiloxane is a block copolymer.

35. The photocurable silicone composition of embodiment 34, wherein the block copolymer comprises 2 different blocks.

36. The photocurable silicone composition of any preceding embodiment, wherein the olefin polysiloxane comprises a plurality of side chains comprising a strained cyclic functional group, wherein the molecular weight of the chain between adjacent side chains is at least about 500 g/mol.

37. The photocurable silicone composition of any preceding embodiment, wherein the olefin polysiloxane comprises a plurality of side chains comprising a strained cyclic functional group, wherein the molecular weight of the chain between adjacent side chains is at from about 500 g/mol to about 20,000 m/mol.

38. The photocurable silicone composition of any preceding embodiment, wherein the olefin polysiloxane comprises a plurality of side chains comprising a strained cyclic functional group, wherein the molecular weight of the chain between adjacent side chains is at from about 500 g/mol to about 4,500 m/mol.

39. The photocurable silicone composition of any preceding embodiment, wherein the composition comprises a blend of olefin polysiloxane polymers having a plurality of side chains comprising the strained cyclic functional group, wherein the blend comprises a first olefin polysiloxane having a plurality of side chains comprising the strained cyclic functional group in which the molecular weight of the chain length between adjacent side chains is from about 500 to about 600 g/mol, and a second olefin polysiloxane having a plurality of side chains comprising the strained cyclic functional group in which the molecular weight of the chain length between adjacent side chains is from about 2,500 to about 20,000 g/mol.

40. The photocurable silicone composition of any preceding embodiment, wherein the composition comprises a blend of olefin polysiloxane polymers having a plurality of side chains comprising the strained cyclic functional group, wherein the blend comprises a first olefin polysiloxane having a plurality of side chains comprising the strained cyclic functional group in which the molecular weight of the chain length between adjacent side chains is from about 500 to about 600 g/mol (for example about 550 g/mol), and a second olefin polysiloxane having a plurality of side chains comprising the strained cyclic functional group in which the molecular weight of the chain length between adjacent side chains is from about 2500 to about 4,500 g/mol.

41. The photocurable silicone composition of any preceding embodiment, wherein the composition comprises a blend of olefin polysiloxane polymers having a plurality of side chains comprising the strained cyclic functional group, wherein the blend comprises a first olefin polysiloxane having a plurality of side chains comprising the strained cyclic functional group in which the molecular weight of the chain length between adjacent side chains is from about 500 to about 600 g/mol (for example about 550 g/mol), and a second olefin polysiloxane having a plurality of side chains comprising the strained cyclic functional group in which the molecular weight of the chain length between adjacent side chains is about 2,500 g/mol. The photocurable silicone composition of any preceding embodiment, wherein the composition comprises a blend of olefin polysiloxane polymers having a plurality of side chains comprising the strained cyclic functional group, wherein the blend comprises a first olefin polysiloxane having a plurality of side chains comprising the strained cyclic functional group in which the molecular weight of the chain length between adjacent side chains is from about 500 to about 600 g/mol (for example about 550 g/mol), and a second olefin polysiloxane having a plurality of side chains comprising the strained cyclic functional group in which the molecular weight of the chain length between adjacent side chains is about 4,200 g/mol. The photocurable silicone composition of any preceding embodiment, wherein the composition comprises a blend of olefin polysiloxane polymers having a plurality of side chains comprising the strained cyclic functional group, wherein the blend comprises a first olefin polysiloxane having a plurality of side chains comprising the strained cyclic functional group in which the molecular weight of the chain length between adjacent side chains is about 2,500 g/mol, and a second olefin polysiloxane having a plurality of side chains comprising the strained cyclic functional group in which the molecular weight of the chain length between adjacent side chains is about 18,000 g/mol. The photocurable silicone composition of any preceding embodiment, wherein the composition comprises a blend of olefin polysiloxane polymers having a plurality of side chains comprising the strained cyclic functional group, wherein the blend comprises a first olefin polysiloxane having a plurality of side chains comprising the strained cyclic functional group in which the molecular weight of the chain length between adjacent side chains is from about 500 to about 600 g/mol (for example about 550 g/mol), and a second olefin polysiloxane having a plurality of side chains comprising the strained cyclic functional group in which the molecular weight of the chain length between adjacent side chains is about 18,000 g/mol. The photocurable silicone composition of any one of embodiments 39 to 44, wherein the polymers are blending in a ratio of about 1:1. The photocurable silicone composition of any preceding embodiment, wherein the olefin polysiloxane is a block copolymer, in which the molecular weight of the chain length between adjacent side chains is from about 500 to about 600 g/mol (for example about 550 g/mol) in a first set of blocks, and the molecular weight of the chain length between adjacent side chains is from about 2,500 to about 20,000 g/mol in a second set of blocks. The photocurable silicone composition of any preceding embodiment, wherein the olefin polysiloxane is a block copolymer, in which the molecular weight of the chain length between adjacent side chains is from about 500 to about 600 g/mol (for example about 550 g/mol) in a first set of blocks, and the molecular weight of the chain length between adjacent side chains is from about 2500 to about 4500 g/mol in a second set of the blocks. The photocurable silicone composition of any preceding embodiment, wherein the olefin polysiloxane is a block copolymer, in which the molecular weight of the chain length between adjacent side chains is from about 500 to about 600 g/mol (for example about 550 g/mol) in a first set of the blocks, and the molecular weight of the chain length between adjacent side chains is about 2,500 g/mol in a second set of the blocks. The photocurable silicone composition of any preceding embodiment, wherein the olefin polysiloxane is a block copolymer, in which the molecular weight of the chain length between adjacent side chains is from about 500 to about 600 g/mol (for example about 550 g/mol) in a first set of the blocks, and the molecular weight of the chain length between adjacent side chains is about 4,200 g/mol in a second set of the blocks. The photocurable silicone composition of any preceding embodiment, wherein the olefin polysiloxane is a block copolymer, in which the molecular weight of the chain length between adjacent side chains is about 2,500 g/mol in a first set of the blocks, and the molecular weight of the chain length between adjacent side chains is about 18,000 g/mol in a second set of the blocks. The photocurable silicone composition of any preceding embodiment, wherein the olefin polysiloxane is a block copolymer, in which the molecular weight of the chain length between adjacent side chains is from about 500 to about 600 g/mol (for example about 550 g/mol) in a first set of the blocks, and the molecular weight of the chain length between adjacent side chains is about 18,000 g/mol in a second set of the blocks. The photocurable silicone composition of any one of embodiments 46 to 51, wherein the two different block types are present in a ratio about 1:1. The photocurable silicone composition of any preceding embodiment, wherein the mercapto polysiloxane is a homopolymer. The photocurable silicone composition of any one of embodiments 1 to 52, wherein the mercapto polysiloxane is a copolymer. The photocurable silicone composition of any one of embodiments 1 to 52, wherein the mercapto polysiloxane is selected from the group consisting of (mercaptopropyl)methylsiloxane homopolymer and (mercaptopropyl)methylsiloxane dimethylsiloxane copolymer. The photocurable silicone composition of embodiment 55, wherein the (mercaptopropyl)methylsiloxane dimethylsiloxane copolymer is (4-6% mercaptopropyl)methylsiloxane dimethylsiloxane copolymer. 57. The photocurable silicone composition of any preceding embodiment, wherein at least 50% of the silicone atoms in the mercapto polysiloxane polymer comprise a mercaptopropyl functional group.

58. The photocurable silicone composition of any preceding embodiment, further comprising one or more photoinitiators.

59. The photocurable silicone composition of embodiment 58, wherein the photoinitiator is a Type I photoinitiator, for example a Type I photoinitiator selected from the group consisting of a phosphine oxide, an a-hydroxyketone, an a-aminoketone, a benzyl ketal and a benzoin.

60. The photocurable silicone composition of embodiment 59, wherein the photoinitiator is 2,2-Dimethoxy-2- phenylacetophenone (DMPA).

61. The photocurable silicone composition of embodiment 58, wherein the photoinitiator is a Type II photoinitiator, for example a Type II photoinitiator selected from the group consisting of a benzophenone and a xanthone.

62. The photocurable silicone composition of embodiment 61, wherein the photoinitiator is bis-acylphosphine oxide (BAPO) or 4,4'-bis(N,N-diethylamino) benzophenone (DEABP).

63. The photocurable silicone composition of any one of embodiments 58 to 62, wherein the photoinitiators are present in an amount of up to about 10 wt% of the total weight of the composition, or up to about 5 wt% of the total weight of the composition.

64. The photocurable silicone composition of embodiment 63, wherein the photoinitiators are present in an amount of from about 0.01 wt% of the total weight of the composition.

65. The photocurable silicone composition of embodiment 63, wherein the photoinitiators are present in an amount of from about 0.01 to about 5 wt% of the total weight of the composition.

66. The photocurable silicone composition of any preceding embodiment, further comprising one or more solvents.

67. The photocurable silicone composition of embodiment 66, wherein the solvent is selected from the group consisting of dichloromethane, ethylbenzene, chloroform, tetrahydrofuran, cyclohexane and hexane.

68. The photocurable silicone composition of any preceding embodiment, further comprising a filler.

69. The photocurable silicone composition of embodiment 68, wherein the filler is selected from the group consisting of metal particles (such as silver particles), metal-coated particles (such as nickel-coated particles), magnetic particles (such as iron oxides (FesC^), ferrites (e.g. SrFeizOig or BaFeizOig), alnico, samarium cobalt (SmCo)), carbon black, graphene oxide, nano clay, and fumed silica.

70. The photocurable silicone composition of embodiment 68, wherein the filler is fumed silica.

71. The photocurable silicone composition of embodiment 68, wherein the filler is fumed graphene oxide.

72. The photocurable silicone composition of any preceding embodiment, wherein the silicone composition is curable in the visible range of light and/or in UV light.

73. The photocurable silicone composition of any preceding embodiment, wherein the silicone composition is curable in the range of light having electromagnetic wavelength of from about 200nm to about 700 nm.

74. The photocurable silicone composition of any preceding embodiment, wherein the silicone composition is curable in air.

75. The photocurable silicone composition of any preceding embodiment, wherein the silicone composition is curable at room temperature.

76. The photocurable silicone composition of any preceding embodiment, wherein, once photocured, the composition has a storage modulus of at least about lOkPa.

77. The photocurable silicone composition of any preceding embodiment, wherein, once photocured, the composition has a storage modulus of from about lOkPa to about lOMPa.

78. The photocurable silicone composition of any preceding embodiment, wherein, once photocured, the composition has a Young's modulus of from about 30 kPa to about 9 MPa.

79. The photocurable silicone composition of any preceding embodiment, wherein the olefin polysiloxane comprising one or more side chains comprising a strained cyclic functional group is selected from the group consisting of olefin polysiloxane polymer based on blocks of at least about 500 g/mol), a mixture of olefin polysiloxane polymers based on blocks of at least about 500 g/mol, and olefin siloxane copolymers comprising blocks of different chain lengths based on blocks of at least about 500 g/mol.

80. The photocurable silicone composition of any preceding embodiment, wherein the mercapto polysiloxane is (4-6% mercaptopropyl)methylsiloxane dimethylsiloxane copolymer and the olefin polysiloxane comprises one or more side chains comprising a norbornyl strained cyclic functional group based on about 18,000 g/mol blocks of di-hydroxy-PDMS with a final molecular weight in the range of from about 18,000 to about 90,000 g/mol, with an ene:thiol ratio of about 1:2 to about 1:10, optionally wherein the cured composition displays an elongation at break of at least about 150%. The photocurable silicone composition of any preceding embodiment, wherein the mercapto polysiloxane is (4-6% mercaptopropyl)methylsiloxane dimethylsiloxane copolymer and the olefin polysiloxane comprises one or more side chains comprising a norbornyl strained cyclic functional group based on about 18,000 g/mol blocks of di-hydroxy-PDMS with a final molecular weight of about 30,000g/mol with an ene:thiol ratio of about 1:2, optionally wherein the cured composition displays an elongation at break of about 159%. The photocurable silicone composition of any preceding embodiment, wherein the mercapto polysiloxane is (4-6% mercaptopropyl)methylsiloxane dimethylsiloxane copolymer and the olefin polysiloxane comprises one or more side chains comprising a norbornyl strained cyclic functional group based on about 18,000 g/mol blocks of di-hydroxy-PDMS with a final molecular weight of about 30,000 g/mol with an ene:thiol ratio of about 1:4, optionally wherein the cured composition displays an elongation at break of about 251%. The photocurable silicone composition of any preceding embodiment, wherein the mercapto polysiloxane is poly(mercaptopropyl)methylsiloxane homopolymer and the olefin polysiloxane comprises a combination of olefin polysiloxanes comprising one or more side chains comprising a norbornyl strained cyclic functional group based on about 2,500 g/mol and about 18,000 g/mol blocks of di-hydroxy-PDMS with final molecular weights in the range of from about 5,000 to about 90,000 g/mol with an ene:thiol ratio of about 1:2, optionally wherein the cured composition displays an elongation at break of about 140%. The photocurable silicone composition of any preceding embodiment, wherein the mercapto polysiloxane is poly(mercaptopropyl)methylsiloxane homopolymer and the olefin polysiloxane comprises a combination of olefin polysiloxanes comprising one or more side chains comprising a norbornyl strained cyclic functional group based on about 550 g/mol and about 2,500 g/mol blocks of di-hydroxy-PDMS with final molecular weights in the range of from about 5,000 to about 50,000 g/mol with an ene:thiol ratio of about 1:2, optionally wherein the cured composition displays a Young's modulus of about 0.62MPa and an elongation at break of about 80%. The photocurable silicone composition of any preceding embodiment, wherein the mercapto polysiloxane is poly(mercaptopropyl)methylsiloxane homopolymer and the olefin polysiloxane comprises a block olefin siloxane copolymer comprising one or more siloxane blocks comprising one or more side chains comprising a norbornyl strained cyclic functional group based on about 550 g/mol and about 4,200 g/mol blocks of di- hydroxy-PDMS with a final molecular weight in the range of from about 5,000 to about 90,000 g/mol with an ene:thiol ratio of about 1:2, optionally wherein the cured composition displays a Young's modulus of about 0.62MPa and an elongation at break of about 80%. The photocurable silicone composition of any preceding embodiment, wherein the mercapto polysiloxane is poly(mercaptopropyl)methylsiloxane homopolymer and the olefin polysiloxane comprises a block olefin siloxane copolymer comprising one or more siloxane blocks comprising one or more side chains comprising a norbornyl strained cyclic functional group based on about 550 g/mol and about 2,500 g/mol blocks of di- hydroxy-PDMS with a final molecular weight in the range of from about 5,000 to about 90,000 g/mol with an ene:thiol ratio of about 1:2, optionally wherein the cured composition displays a Young's modulus of about 0.62MPa and an elongation at break of about 80%.

87. A method for curing a silicone composition, comprising contacting: a. an olefin polysiloxane comprising one or more side chains comprising a strained cyclic functional group and/or one or more terminations comprising a strained cyclic functional group, wherein the strained cyclic functional group of the one or more side chains and/or the one or more terminations comprises a carbon-carbon double bond; with b. a mercapto polysiloxane having a plurality of thiol functional groups; to provide a mixture, and then irradiating the mixture with light to cure the silicone composition.

88. The method of embodiment 87, wherein the method further comprises contacting the olefin polysiloxane and the mercapto polysiloxane mixture with a solvent.

89. The method of embodiment 87 or 88, further comprising contacting the olefin polysiloxane and the mercapto polysiloxane mixture with a filler.

90. The method of embodiment 89, wherein the filler is fumed silica or graphene oxide.

91. The method of any one of embodiments 87 to 90, wherein the method comprises mixing the components for at least about 5 mins.

92. The method of any one of embodiments 87 to 91, wherein the method comprises a step of removing solvent from the mixture after the mixing step.

93. The method of any one of embodiments 87 to 92, wherein the method further comprises contacting the olefin polysiloxane and the mercapto polysiloxane mixture with a photoinitiator.

94. The method of any one of embodiments 87 to 93, wherein the light has an electromagnetic wavelength of from about 200 to about 700 nm.

95. The method of any one of embodiments 87 to 94, wherein the step of irradiating the mixture with light comprises irradiating the mixture with from 1 to 1000 mW/cm ² .

96. The method of any one of embodiments 87 to 95, wherein the method takes place at room temperature.

97. The method of any one of embodiments 87 to 96, wherein the method takes place in air. 98. The method of any one of embodiments 87 to 97, wherein the method is not carried out in an inert atmosphere.

99. The method of any one of embodiments 87 to 98, wherein the method is carried out by additive manufacturing.

100. The method of embodiment 99, wherein the additive manufacturing is stereolithography, material extrusion, or material jetting.

101. The method of any one of embodiments 87 to 100, wherein the method is a method of photocuring the photocurable silicone composition of any one of embodiments 1 to 86.

102. A cured, cross-linked silicone composition comprising: a. an olefin polysiloxane comprising one or more side chains comprising a cyclic functional group and/or one or more terminations comprising a cyclic functional group; and b. a mercapto polysiloxane having a plurality of thiol functional groups wherein the olefin polysiloxane and the mercapto polysiloxane are crosslinked to each other via sulphide bonds.

103. A cured, cross-linked silicone composition obtainable according to the method of any one of claims 87 to 101.

104. A method of additive manufacturing to provide a 3D product, comprising: a. providing a print cartridge comprising a mixture of an olefin polysiloxane comprising one or more side chains comprising a strained cyclic functional group and/or one or more terminations comprising a strained cyclic functional group, wherein the strained cyclic functional group of the one or more side chains and/or the one or more terminations comprises a carbon-carbon double bond; and a mercapto polysiloxane having a plurality of thiol functional groups; b. depositing a portion of the mixture on to a build platform by printing using a 3D printing apparatus to provide a layer of the 3D product; c. irradiating the deposited portion of the mixture with light; d. repeating steps (b) and (c) to provide the 3D printed product.

105. The method of embodiment 104, wherein the mixture further comprises a filler.

106. The method of embodiment 105, wherein the filler is a thixotropic filler, such as fumed silica.

107. The method of any one of embodiments 104 to 106, wherein the mixture inside the cartridge is not exposed to light prior to printing. 108. The method of any one of embodiments 104 to 107, wherein the print cartridge comprises a photocurable composition of any one of embodiments 1 to 86.

109. A 3D product made according to a method of any one of embodiments 104 to 108.

110. A product incorporating a photocured silicone composition, wherein prior to curing the silicone composition comprised: a. an olefin polysiloxane comprising one or more side chains comprising a strained cyclic functional group and/or one or more terminations comprising a strained cyclic functional group, wherein the strained cyclic functional group of the one or more side chains and/or the one or more terminations comprises a carbon-carbon double bond; and b. a mercapto polysiloxane having a plurality of thiol functional groups;

111. A product incorporating a photocured silicone composition, wherein the photocured silicone composition comprises a. an olefin polysiloxane comprising one or more side chains comprising a cyclic functional group and/or one or more terminations comprising a cyclic functional group; and b. a mercapto polysiloxane having a plurality of thiol functional groups wherein the olefin polysiloxane and the mercapto polysiloxane are crosslinked to each other via sulphide bonds.

112. A product incorporating a photocured silicone composition prepared according to the method of any one of embodiments 87 to 101.

113. A 3D printed product comprising a photocured silicone composition prepared according to the method of any one of embodiments 87 to 101.

114. A 3D printer cartridge comprising a chamber, the chamber comprising a mixture of an olefin polysiloxane comprising one or more side chains comprising a strained cyclic functional group and/or one or more terminations comprising a strained cyclic functional group, wherein the strained cyclic functional group of the one or more side chains and/or the one or more terminations comprises a carbon-carbon double bond; and a mercapto polysiloxane having a plurality of thiol functional groups.

115. The 3D printer cartridge of embodiment 114, wherein the cartridge further comprises an indicator providing information on the amount of mixture contained within the chamber.

116. The 3D printer cartridge of embodiment 114 or 115, wherein the cartridge further comprises an interface unit. The 3D printer cartridge of embodiment 116, wherein the interface unit interfaces between the cartridge and a 3D printing apparatus when the cartridge is installed in the 3D printing apparatus. The 3D printer cartridge of embodiment 116 or 117, wherein the interface is operable to cause the release of at least a portion of the mixture from the chamber. The 3D printer cartridge of any one embodiments 116 to 118, wherein the interface is operable to cause the release of at least a portion of the mixture from the chamber via a release mechanism. The 3D printer cartridge of any one of embodiments 114 to 119, wherein the chamber further comprises a valve, wherein the valve provides for the deposition of the mixture when the cartridge is used in a 3D printing apparatus. The 3D printer cartridge of any one of embodiments 114 to 120, wherein the chamber is a flexible chamber, such as a bag. The 3D printer cartridge of any one of embodiments 114 to 120, wherein the chamber is rigid. The 3D printer cartridge of any one of embodiments 114 to 122, wherein the chamber is opaque. The 3D printer cartridge of any one of embodiments 114 to 122, wherein the chamber is impervious to light. The 3D printer cartridge of any one of embodiments 114 to 124, wherein the chamber is sealed. The 3D printer cartridge of any one of embodiments 114 to 125, wherein the chamber comprises a fill port. The 3D printer cartridge of any one of embodiments 114 to 126, wherein the mixture is not exposed to light prior to printing. The 3D printer cartridge of any one of embodiments 114 to 127, further comprising a filler. The 3D printer cartridge of any one of embodiments 114 to 128, wherein the 3D printer cartridge comprises the photocurable silicone composition of any one of embodiments 1 to 86. A kit, comprising: a. an olefin polysiloxane comprising one or more side chains comprising a strained cyclic functional group and/or one or more terminations comprising a strained cyclic functional group, wherein the strained cyclic functional group of the one or more side chains and/or the one or more terminations comprises a carbon-carbon double bond; and b. a mercapto polysiloxane having a plurality of thiol functional groups.

131. The kit of embodiment 130, further comprising a photoinitiator. 132. The kit of embodiment 130 or 131, further comprising a filler.

133. The kit of any one of embodiments 130 to 131, further comprising instructions for use.

134. The kit of any one of embodiments 130 to 133, wherein the olefin polysiloxane is as defined any one of embodiments 1 to 86.

135. The kit of any one of embodiments 130 to 134, wherein the mercapto polysiloxane is as defined any one of embodiments 1 to 86.