Functional Resin Composition for Regulated Polymerization Stress

Field of the Invention

[0001] This invention relates to "new composition of matter". Specifically, photo- polymerizable compositions for use as, for example, adhesive, cement, composite, hi particular, the use of the new composition of matter is suitable for medical and/or dental materials, as well as applications in other fields where reduction of polymerization stress is important for its durability. This is accomplished by incorporating a photopolymerizable and photocleavable resin based on novel arylketone oxime derivative. Through use of the new composition of matter, the radical polymerization kinetic/rate and thus the resultant polymerization stress is effectively regulated. In addition, the said composition is also capable of generating a new reactive species which in turn creates/initiates a new type of reaction or polymerization for even greater performance enhancement.

Background of the Invention

[0002] Highly cross-linked polymers have been widely studied as matrices for composites, foamed structures, structural adhesives, insulators for electronic packaging, etc. The densely cross-linked structures are the basis of superior mechanical properties such as high modulus, high fracture strength, and solvent resistance. However, these materials can also be irreversibly damaged by the high contraction stress that is a direct result of the network formation process. It is believed that such contraction or polymerization stress originates from polymerization shrinkage in combination with the limited polymer chain mobility. Consequently, internal.stress concentration is elevated and the trapped stress is ultimately released, causing microscopic damage in any weak zones, including within the bulk of the cured material, at the interfacial area, and into the

suiTounding structures. Macroscopically this effect is often observed as debonding, cracking, etc. This mechanism is observed in current adhesive dental restorations, wherein polymerization stress from restrained shrinkage build up during cure. The amount of stress within the material is highly dependent on the configuration of the restoration. Furthermore, non-homogeneous deformations resulting from stresses created during functional loading can damage the tooth/composite interface leading to loss of adhesion of the material to tooth structure.

[0003] Various approaches have been explored in an effort to reduce the overall polymerization stress generation either from modification of the restorative materials, and/ efforts to minimize the direct stress concentration at the restored interface. They include, for example, new resins, new resin polymerization chemistry, new initiators, new fillers, new coupling agents, new curing sources and processes, new bonding agents, and even new restoration procedures. Significant attention has been directed toward new resin matrix development for low polymerization shrinkage and shrinkage stress. For example, various structure and geometric derivatives of (meth)acrylate-based resin systems, non- (meth)acrylates resin systems, and non-radical-based resin system, etc., have been attempted. In addition, for light curable, low shrink dental composites, new filler and filler surface modifications have also been extensively explored. These include such approaches as fillers of varying particle size and size distributions, ranging from nanometer to micrometer, and different shapes, both irregular or spherical. In addition, it may be of various compositions, such as inorganic, organic (pre-polymerized), or combinations of both (hybrid). Although incremental improvements have been achieved using each approach, polymerization stress is still the biggest challenge in cured network systems.

[0004] Almost the entire prior art directed at low shrink and low stress are based on the minimization of the shrinkage and stress formation during the pre-gel phase. However, the shrinkage and stress development in cured network system occurs in two different stages: the pre-gel phase and the post-gel phase. Therefore, some approaches to reduce polymerization shrinkage were found ineffective in controlling over all stress

development in the post-gel stage. The immobility caused by the increasing cross-link density within the curing system leads to an increasing stress concentration within the matrix. Even worse, the trapped stress eventually is relieved by slow relaxation, which can create additional damage in the cured system. Therefore, the new approach is based on the concept that in the post-gel stage if some of this "closed network" of the cross- linked system can be broken selectively to promote stress relief, then the total stress concentration within the material will be substantially reduced. This is accomplished using a photopolymerizable and photocleavable resin of a general molecular composition described herein. Such a monomer system described in this work can be polymerized like any other system with the exception that the developed cross-linked network can be "triggered" to break upon the application of an additional energy source, such as additional light energy. The incorporation of the photocleavable monomer within the crosslinked network is believed to be a unique feature of the described new composition of matter.

[0005] Photocleavage is utilized in the synthesis of peptides. New peptides are created from an existing template through cleavage of the templates via light exposure. There is no chemical contamination with such a process. As another example, photolytically generated acids and bases can also be viewed as an extended application of photocleavage reactions. In these systems, acidic or basic components are temporally latent to avoid unwanted interaction with other components in the system; they are release on demand such as through light exposure to trigger the generation of the acid or base, which then acts as normal acidic or basic catalysts for secondary reactions. Recently, thermally or photo-chemically reversible materials have been developed in order to make the polymer network depolymerizable or degradable for applications such as easy removal of fill-in polymer in MEMS, thermally labile adhesives, thermaspray coatings and removable encapsulation etc. Most recently, photocleavable dendrimers have been explored in order to improve efficiency of drug delivery.

[0006] Previous work disclosed a new approach, which involved a new resin composition that underwent partial selective network photocleavage in the post-gel stage resulting in

reduction of polymerization stress. However, the photocleavage process required a relatively low UV wavelength (300nm or less), which is impractical for oral applications due to possible harmful effects to surrounding tissues. Nevertheless, a remarkable low polymerization stress was demonstrated from the curable compositions derived from such a photoactive resin.

[0007] It is still highly desirable to further reduce polymerization stress within a cured composite via an effective photocleavage method. Therefore, a new resin composition was explored consisting of a photopolymerizable and photocleavable moiety based on oxime derivatives, which can be cleaved more easily compared to the previous systems described in the prior patent application.

λ°- ^» λ

Photopolymeπzation

Scheme I: Illustration for P&P Resin and the Cured Networks Therefrom

[0008] Theoretically, if any environmentally sensitive moiety, such as a thermally cleavable or photo-labile linkage, were incorporated into polymerizable resin monomers, the resulting polymeric material would become thermally cleavable or photo-cleavable. The chemistry of some classical photo-initiators, which have been explored as polymerizable photoinitiator or macroinitiators could be adopted as basis for designing such photopolymerizable and photocleavable resin monomers. However, none of these photo-initiators has been incorporated into a polymer chain or polymeric network to render the polymeric chain or network breakable. Therefore one objective of this invention is to disclose an approach from which a cleavable polymer network is capable of being built up.

[0009] It is another objective of this investigation to develop a new resin system for the next generation low shrink and low stress restorative materials by incorporating a photocleavable or thermally liable moiety as part of a photopolymerizable resin monomer. It is expected that such an unusual approach would enable a conventional polymerized network to be selectively cleaved, thus dispersing stress post- polymerization, resulting in a "self stress-relief system and ultimately minimizing the overall stress concentration within the polymer network.

[0010] In order to make a polymerized network cleavable-on-command by light (photocleavable), a light responsive moiety must be incorporated into the polymer backbone that should be stable towards normal ambient visible light exposure processes until additional exposure to light with specific energy. In particular, such an energy source can be comprised of other wavelength light other than the standard visible blue light (400nm to 500nm). Near UV light (300nm to 400nm) would be among the many possible choices. Furthermore, it is expected that compounds derived from ortho- nitrobenzyl radicals or from D-hydroxyalkylphenone should be ideal candidates for this new class of resin monomers that can be photopolymerized by visible light and then triggered to be breakable by additional UV light.

[0011] The prior art disclosed in our previous US Patent Application (May, 2004/ June, 2006) described a photopolymerizable and photocleavable resin composition. This

approach involves designing a new resin composition that partially relieves the polymerization stress in post-gel stage via a subsequent, selective network cleavage. With this approach, the classic UV photoinitiator, Irgacure 2959, or 4-(2-hydroxyl-ethoxy) phenyl-2- (hydroxy-2-propyl) ketone (HP) was used as the core element in developing a photopolymerizable & photocleavable resin. This resin was used to formulate various dental compositions featuring remarkably low polymerization stress. However, the energy needed to promote the photocleavage falls in the 300nm (UV) range. Although incorporation of an additional photosensitizer was effective at shifting the wavelength to a more desirable range, there remains a need to create a modified photocleavable resin that would undergo photocleavage without the need for a separate, additional photosensitizer.

[0012] The prior art,US2005/0182148 (Aug. 18, 2005), disclosed a photosensitive composition in which an ortλo-nitrobenzyl moiety was incorporated for photocleavage. However, it did not address any property related polymerization stress reduction or other any reactive species generated from the cleavage process.

Scheme Ha: General Reaction Pathway towards D -hydroxyalkylphenone Oxime

R, R': H, a linear or brached C1-C18 alkyl or -O-C1-C18 alkyl residue, which can be interruped by one or more O atoms, PG-Y-R"-X- or a substituted or unsubstituted, aromatic C6 to C18 radiacl;PG; polymerizable group;

R": is absent or a linear or branched C1 to C18 alkylene radical, which can be interrupted by one or more O atoms;

R1 , R2, R3, R4: H, a linear or brached C1-C18 alkyl or -O-C1-C18 alkyl residue, which can be interruped by one or more O atoms, PG-Y-R'-X- or a substituted or unsubstituted, aromatic C6 to C18 radiacl;

X is absent, O or S; Y is absent, O, S an ester, ether, carboante, amide or urethane groups;

R5 or R6 is H, a linear or brached C1-C18 alkyl or -O-C1-C18 alkyl residue, which can be interruped by one or more O atoms, PG-Y-R'-X- or a substituted or unsubstituted, aromatic C6 to C18 radiacl;

HP or lrgacure 2959 Molecular Weight = 224.26 HPO Molecular Formula = C ₁₂ H ₁₆ O ₄ Molecular Weight = 239.27

Molecular Formula = C ₁₂ H ₁₇ NO ₄

Scheme lib: General Reaction for HP -based Oxime Derivative

[0013] Therefore, there is a desire to further refine the resin composition in order to improve the photocleavability and to better balance the overall mechanical performance along with total polymerization stress reduction. In this invention, a new photoreactive core element, arylketone oxime, was developed. A typical oxime derivative, 4-(2- hydroxylethoxy) phenyl-2-(hydiOxy-2-propyl) ketone oxime (HPO) was prepared from its parent ketone compound, 4-(2-hydroxylethoxy)phenyl-2-(hydroxy-2-propyl)ketone (HP) via a facile reaction with hydroxyl amine under mild condition as illustrated in Scheme lib.

[0014] A general resin composition based on such an oxime derivative is further illustrated in Scheme III. As one skilled in the art will recognize, the feasibility of this approach allows for a rapid exploration of a new class of resin monomers. Accordingly, a variety of polymerizable and photocleavable resin monomers has been successfully prepared with a wide range of compositions, as shown in Scheme IV, V and VI.

[0015] When di-isocyanate was reacted with HPO, different P&P resin based on HPO resulted due to the varying reactivity of the three different hydroxyl groups within HPO. Further, as illustrated below (Scheme IV, Scheme V and Scheme VI), it was found that

hyperbranched P&P resin and non-hyperbranched P&P resin could be effectively prepared via the proper reaction sequence.

[0016] Furthermore, different photoreactivity was demonstrated by the P&P resins prepared using slightly different reaction sequence. It appears that the non-hyperbranched P&P resins as showed in Scheme V and VI were more active towards the full spectrum of light than the hyperbranched version as showed in Scheme IV, evidenced by the relatively slow polymerization rate and overall lower polymerization stress that resulted. This can be attributed to the increased photocleavability of the "naked" Dhydroxyl group in non-hyperbranched P&P resin.

Scheme III: Typical Polymerizable and Photocleavable Resin Monomer based on D- hydroxyalkylphenone oxime (HPO)

Scheme IV: Typical Hyperbranched Resin Monomer based on D-hydroxyalkylphenone oxime (HPO)

Scheme V: Typical Non-hyperbranched Resin Monomer based on D- hydroxyalkylphenone oxime (HPO)

PAMA-TMDI-HPO-TMDI-PAMA Molecular Weight = 1088.27 Molecular Formula = C ₅₄ H ₈₁ N ₅ O ₁₈

Scheme VI: Typical Non-hyp erbranched Resin Monomer based on D- hydroxyalkylphenone oxime (HPO)

Scheme VII: Other Common D-hydroxylalkylphenone Oxime Derivatives

Scheme VIII: Resin Monomer based on Benzoin Oxime (BOX)

Scheme IX: Another Resin Monomer based on Benzoin Oxime (BOX)

Scheme X: Another Resin Monomer based on Benzoin Oxime (BOX)

[0017] Another important aspect in this invention is that in addition to the active ingredient (P &P resin) that is required to form a photocleavable network after conventional photopolymerization, other photopolymerizble resins should be blended into the composition to aid in establishing a permanent network. A typical activated resin blend contains about 10-90%, w/w, of the P&P resin and 90-10%, w/w, of conventional resins, including in-situ generated UDMA and/or extra diluent resin such as TEGDMA. In addition, some non-radically polymerizable resins, such as epoxy resin, can also be blended with the new P&P resin as a latent reactive diluent which can undergo a typical amine/epoxy addition reaction as a result of photocleavage or in situ amine generation. Thus, the addition or blending of additional monomer helps to establish an overall balanced performance of the resulting product, which would include proper viscosity, lower polymerization shrinkage and lower polymerization stress while maintaining good mechanical strength.

[0018] A typical composites can be formulated with \0-40%(w/w) of such an activated resin blend along with 60%-90%(w/w) of a filler material composed of a wide range of fillers having different sizes and/or size distributions in order to balance the paste's handling, consistency, and mechanical properties in the resulting composite. A variety of fillers can be used for making the filler blends, including fumed silica, nano filler, polymeric powders, inorganic filers, prepolymerized filler, hybrid filler, et al. For example, the filler blend may be composed as follows: 5-20 ⁰ Zo(WZwJ silanated BAFG (6- 9micron) 5-20%(w/w); silanated BAFG (0.6-0.8micron) 30-60%(w/w) and Aerosil OX- 50 fumed silica (0.01-0.04micron) 5-15%(w/w). Preferably up to 80-85% (w/w) of total filler blend can be loaded into the resin matrix.

[0019] The novel photocleavable and photopolymerizable resin exhibited a relatively slow polymerization rate, which allows lower polymerization stress buildup even without the photocleavage. In addition, this HPO-based P&P resin demonstrated easier photocleavability than the HP -based P&P resin. Unexpectedly, a further advantage of the systems described herein is the in situ amine generation, which triggered subsequently amine-assisted reactions, such as addition with epoxy resin, accelerator for redox initiator, etc.

[0020] Furthermore, resin and paste formulations with conventional resin diluent and proper filler composition also produced low shrink and low stress composites characterized by:

• Low polymerization shrinkage(l.0-2.0%, more than 50% lower than conventional composites, 2.8-3.2%);

• Extremely low shrinkage stress (1.0-1.5 MPa, more than 60-70% less than conventional composites, 2.5-3.0MPa);

• An excellent balance of mechanical strength and handling/delivery properties;

• A methacrylate chemistry that makes it compatible with most current bonding systems.

[0021] In addition, other physical properties demonstrated by these novel P&P resins resulted in a wide range of physical and mechanical properties that can not be regulated by standard P&P system. These include, but are not necessarily limited to:

• Heat, light, or acidic cleavage

• in situ amine generation, which would promote hybrid polymerization in addition to the radical polymerization and make it possible to formulate one-component, dual- cure compositions due to this unique capability to generate a reducing agent, amine, for redox reaction with peroxide.

• Preconditioning to make possible tuning of composite viscosity for better handling and better adaptation to the cavity contours.

[0022] Normally, there is an offsetting effect from polymerization stress and mechanical strength whereby enhancing mechanical strength is usually accompanied by increasing polymerization stress due to increasing cross-linking density. It is one purpose of this invention to provide a way to better balance polymerization stress and mechanical properties. Photocleavage during photopolymerization allows a pathway to rearrange covalent bonds. This is particularly significant in a gelling network where larger scale molecular mobility becomes severely restricted.

[0023] In the prior art, D-hydroxyalkylphenone is 4-(2-hydroxyethoxy)-phenyl-2- hydroxy-2-methyl-2-propanone, HP, was incorporated with methacylate, acrylate, vinyl ether etc., via a one or two-step reaction. Various photopolymerizable HP derivatives have been prepared via ester-ester, ester-carbonate, carbonate-carbonate, and urethane- urethane linkages. Among these, the urethane-urethane linkage offers the most robust process: easy reaction control in a non-solvent process. In addition, the new resin monomers were formulated with other conventional resin monomers such as BisGMA, TEGDMA, UDMA or experimental resin monomers of a macrocyclic nature in a variety of ratios to maximize the overall performance in the resulting composites. As shown in the following examples, remarkably low shrinkage, low stress and excellent mechanical properties, plus good handling characteristics, were demonstrated by those composites based on this new class of P&P resin monomers.

[0024] HP must be modified in order to further improve its photoreactivity for an effective photocleavage as discussed above. Here a facile method as described in Example I and Scheme 1 , for such a modification from HP to HPO was developed, hi addition, as shown in the example a series of P&P resins based on HPO monomer is easily prepared using essentially the same process for HP-based P&P resin. Depending upon the nature of the isocyanate, a one or two-step reactions was utilized to prepare the new P&P resin. IEM is the simplest polymerizable isocyanate. Unfortunately, its toxicity limits its application in biomedical materials. Thus, a simple two-step process was successfully developed to make a phopolymerizable and photocleavable resin: in step I, HPO is capped with diisocyanate to form new diisocyanate in a well-controlled sequence, which was then further reacted with any (meth) acrylate containing a hydroxyl group as shown in step II reaction. This procedure not only allows performing the reaction in one reactor, but also avoids IEM.

[00251 Suitable diisocyanates include the alkylene diisocyanates wherein the alkyleiie group ranges from 2 to about 18 carbon atoms and aryleiie and substituted arylene di-and polyisocyanates. Thus, exemplary diisocyanates and polyisocyanates include but are not limit to,

[0026] Alkaline diisocyanates: ethylene diisocyanate; propylene diisocyanate; tetramethylene diisocyanate; pentamethylene diisocyanate; hexamethylene diisocyanate(HDI); hexamethylene diisocyanate biuret; hexamethylene diisocyanate trimer(isocyanurate); octamethylene diisocyanate; decamethylene diisocyanate; undecamethylene diisocyanate; dodecamethylene diisocyanate; isophorone diisocyanate (IPDI); hydrogenated diphenyl ; methane diisocyanate (Hi ₂ MDI)

[0027] Arylene diisocyanate: xylylene-l,4-diisocyanate(p-XDI); xylylene-1,3- diisocyanate(m-XDI); m-pheylene diisocyanate; p-pheylene diisocyanate; toluene-2,6- diisocyanate(2,6-TDI); toluene-2,4-diisocyanate(2,4-TDI); mesitylene diisocyanate; durylene diisocyanate; benzidene diisocyanate; 1 -methyl phenylene-2,4-diisocyanate;

naphthylene-l,4-diisocyanate; 1,2,4-benzene triisocyanate; 4,4'-diisocyanato diphenyl methane(MDI); 3,3'-dimethyl-4,4'-diisocyanato diphenyl methane; 4,4'-diphenyl propane diisocyanate; dianisidine diisocyanate; m-tetramethylenexylene diisocyanate(TMXDI)

[0028] In addition, suitable (meth)acrylate containing a hydroxyl group include: 2- hydroxyethyl (meth)acrylate; 2-hydroxypropyl (meth)acrylate; 3-(acryloxyl)-2- hydroxypropyl (meth)acrylate; di ethylene glycol monohydroxyl (meth)acrylate; tiϊethylene glycol monohydroxyl (meth)acrylate; tetraethylene glycol monohydroxyl (meth)acrylate; polyethylene glycol monohydroxyl (meth)acrylate

[0029] Figure 1 shows polymerization stress as measured by ADA/NIST Tensometer and cured by standard QHT blue light or white light (the UV filter was removed for expanded spectrum and intensity). The novel P&P resins based on HPO, regardless of its structures, hyperbranched or non-hyperbranched type I or II, all exhibit slower curing kinetics which lead to reduced polymerization stress when compared to a conventional resin such as TPH resin. When the curing polymerization is broken into two parts, the stress developed during light irradiation (I) and the stress developed in post light irradiation (PI), slightly lower percentage of total stress is developed within the novel P&P resin system during part I when it is cured by blue light as compared to conventional resin systems such as TPH resin (e.g. 70% vs. 80%). However, when it is cured by white light (with UV light included, Table I), significantly different percentages of total stresses developed during and post light irradiation for the novel P&P resin vs. conventional resin. Thus, only 40-60% of total stress was developed using the P&P resin, whereas more than 70% of total stress was developed for TPH resin systems. Such dramatically different response towards blue and white light for the P&P resin and TPH resin confirms that the P&P resin does undergo special photosensitivity and it also explains the low polymerization stress under blue light curing as a synergetic effect from the same photo-responsive nature but presented in a different magnitude.

[0030] It was surprisingly discovered that type I non-hyperbranched P&P resin based on HPO, depending upon its composition as well (e.g. HPMA-HDI-HPO-HDI-HPMA), is

more readily degradable at room temperature (a kind of thermal cleavage) as evidenced by the dramatic decrease in viscosity, which was confirmed by NMR analysis. It was also found that such a molecular cleavage would not only lead to a significant change in the composite's consistency or rheological character, but also affected the polymerization rates and overall physical and mechanical properties as well. These may be attributed to the overwhelming presence of free amine that was generated in situ during the thermal cleavage process.

[0031] On the other hand, it was also discovered that some non-radically polymerizable resins, such as epoxy resin, can become polymerized in the presence of this P&P resin. This was evidenced by the significant mechanical property enhancement for the aged composite system as showed in Table III. The polymerization of the epoxy resin is attributed to the amine generated during cleavage as discussed above. Furthermore, no similar mechanical property enhancement could be found from those composite systems based on any other conventional resin/epoxy formulation because no amine is generated in that case.

[0032] Another finding from this cleavable resin system is that an aged P&P resin can be polymerized by addition of BPO alone, even though the aged P&P resin can not be polymerized by any type of light irradiation. This suggests that a one-component, dual- cure system is possible from this invention.

[0033] The present invention will now be described in detail with reference to the following examples that do not limit the scope of the invention.

Table I: Resin Composition based on P&P Resins and TPH Resins

Table II: Light Curing Effect on Featured Polymerization Stress Developing Kinetics:

P&P Resins vs. TPH Resins

Table III: Aging Effect on Mechanical Property for Composites: P&P Resins vs. TPH Resins

Example 1 : Preparation of HPO

[0034] A 1000ml jacketed, cylinder resin kettle equipped with a dropping funnel, mechanical agitator, dry air inlet and water-cooling condenser, through which 35 ⁰ C of heated water was circulated during the reaction, was charged 650ml 95% Ethanol, 110.0 grams of Irgacure 3959 (HP) and 60.0 grams of NH ₂ OH-HCl. After the solution turned clear, 120.0 grams of NaOAc was slowly added in portions into the system. It soon turned and remained cloudy through the reaction process, which lasted 2-3 hrs at 35 ⁰ C. Then it was filtered while the solution was wanned. Dilute aqueous HCl solution was added into the cooled filtrate. Most of ethanol was removed via Rotavapor. White crystals soon developed from the residue. The crystalline powder was filtered and collected, then air-dried and vacuum dried at 5O ⁰ C over night. A yield of more than 95% was received. ¹ H NMR in DMSO-d ₆ : D 10.45ppm (s, IH), D7.16-7.22ppm and D6.90- 6.96 (d, 4H), D3.72-3.78 and D3.98-4.04ppm (t, 4H) and D 1.24 (s, 6H). ¹³ C NMR in DMSO-d ₆ : D 161.715, 158.566, 130.667, 126.399, 113.986, 72.267, 70.036, 60.256, 29.212ppm, respectively. T ₁₁₁ : 131.6 ⁰ C (DSC).

Example 2: Preparation of the adduct Of (IEM) ₂ -HPO-IEM (AOl-16).

[0035] A 250ml three-necked flask equipped with a mechanical agitator, dry air inlet and water-cooling condenser was immersed in an oil-bath and charged with 55.8 grams of

IEM, 0.15gram of DBTDL, and 0.188 gram of BHT. Then it was charged in portions with 24.0 grams of HPO over 4hrs. The reaction was kept at 35 ⁰ C with the oil-bath temperature. The reaction took place for 2hrs before adding 11.5 grams of HPMA. The reaction ran for an additional 20hrs at this temperature. The total yield was 95%.

Example 3: Preparation of the adduct Of (HPMA-HMD) ₂ -HPO-HMDI-HPMA (AO 1-30)

[0036] A 500ml three-necked flask equipped with a powder addition funnel, mechanical agitator, dry air inlet and water-cooling condenser was immersed in an oil-bath and charged with 95.2 grams of HMDI and 0.21 gram of DBTDL. Then 30.1 grams of sieved HPO was slowly added in portions to the flask over a period of 3hrs. This slow addition took place in such a manner to avoid a rapid reaction heat increase, which might jeopardize the desired sequence of HMDI-HPO-HMDI. The reaction ran at 35 ⁰ C overnight.. Then 0.12gram of BHT was charged into the system. With a continuous purge of dry air into the reaction system, 112.5 grams of HPMA was added to the flask through a dropping funnel over a period of 2hrs. After HEMA addition, the reaction was allowed to proceed for an additional 3-4 hrs at 35 ⁰ C. Then 25.0 grams of TEGDMA was added as diluent into system and mixed for approximately 2 hours prior to discharge. The overall yield was about 97%. The resin had a viscosity of 160 Pa.s at 2O ⁰ C.

Example 4: Preparation of the adduct of HEMA-TMDI-HPO-TMDI-HEMA (AO 1-76)

[0037] A 500ml jacketed, cylinder resin kettle equipped with a powder addition funnel, mechanical agitator, dry air inlet and water-cooling condenser, through which 35 ⁰ C of heated water was circulated during the reaction, was charged with 96.8 grams of TMDI, 0.10 gram of BHT, and 0.22gram of DBTDL. 66.0 grams of HPMA was added into the system through a dropping funnel within a period of 2hrs. After an additional 2hrs of reaction at this temperature, 52.3 grams of sieved HPO was fed slowly in portions into the system over a period of 1.5hrs. Finally, 40.0 grams of TEGDMA was added into the system two hours later and then mixed and discharged with a yield of 97%. This resin showed a viscosity of 1600 Pa.s at 20 ⁰ C.

Example 5: Preparation of the adduct of HEMA-TMDI-HPO-TMDI-HEMA (AO 1-97)

[0038] A 500ml jacketed, cylinder resin kettle equipped with a powder addition funnel, mechanical agitator, dry air inlet and water-cooling condenser, through which 35 ⁰ C of heated water was circulated during the reaction, was charged with 96.8 grams of TMDI, 0.10 gram of BHT, and 0.22gram of DBTDL. 66.3 grams of HPMA was added into the system through a dropping funnel over a period of 3hrs. After an additional lhr of reaction at this temperature, 35.1 grams of sieved HPO was fed slowly in portions into the system over a period of lhr. Finally, 40.0 grams of TEGDMA was added into the system one hour later and then 24.0 grams of HEMA was added drop-wise into the reaction system over a one hour period. It was mixed overnight at 35°C and discharged with yield of 97%. This resin had a viscosity of 95 Pa.s at 2O ⁰ C.

Example 6: Preparation of the adduct of HPAMA-TMDI-HPO-TMDI-HPAMA (XJ6-53)

[0039] A 500ml jacketed, cylinder resin kettle equipped with a powder addition funnel, mechanical agitator, dry air inlet and water-cooling condenser, through which 35 ⁰ C heated water was circulated during the reaction, was charged with 96.8 grams of TMDI, 0.10 gram of BHT, and 0.22gram of DBTDL. 99.0 grams of HPAMA was added into the system through a dropping funnel within a period of 6hrs. After additional overnight reaction at this temperature, 40.0 grams of TEGDMA was charged into this system. Then 35.2 grams of sieved HPO was fed slowly in portions into the system over a period of 2hrs. Finally, 24.0 grams of HEMA was added drop-wise into the reaction system over a two-hour period. It was mixed overnight at 35 ⁰ C and discharged with yield of 97%. The viscosity was 195 Pa.s at 2O ⁰ C.

Comparative Example 1 : Preparation of the adduct of HEMA-TMDI-BOX-TMDI- HEMA (XJ6-64)

[0040] A 1000ml jacketed, cylinder resin kettle equipped with a powder addition funnel, mechanical agitator, dry air inlet and water-cooling condenser, through which 35 ⁰ C of

heated water was circulated during the reaction, was charged with 193.5 grams of TMDI, 0.15 gram of BHT, and 0.39gram of DBTDL. 132.6 grams of HPMA was added into the system through a dropping funnel within a period of 8hrs. After additional overnight reaction at this temperature, 68.2 grams of sieved benzoin oxime (BOX) was added slowly in portions into the system over a period of 1.5hrs. The reaction proceeded at this temperature for another three hours before 80 grams of TEGDMA was added into the system, which was then followed by adding 49.2 grams of HEMA for an additional two hours prior to stopping the reaction. 515 grams of clear resin resulted with a yield of 98%. After one week at room temperature, the resin was found partially gelled, and could not be further formulated to make any pastes.

Resin Formulation Example 1 to 11

[0041] Typical activated resin composition examples were formulated as described in Table I. Additional resin compositions were also formulated as listed in Table II, in which different types of P&P resin were formulated with a variety of other polymerizable but non-cleavable resin, such as TEGDMA or other non-radically-polymerizable resin, such as EHECH epoxy resin in presence/absence of its photocationic initiators, UVL6979, see AO1-79, 80, 81 , 82, AOl-101, 102, 103, and AO2-56, 57, 58, 59, respectively.

Comparative Resin Formulation Example 1 to 4

[0042] In addition, comparative resin compositions based on a conventional resin, a urethane-modified BisGMA (TPH resin) was also formulated in a way similar to those compositions as described above for P&P resin and listed in Table I as well, AO1-201, 202, 203 and 204.

Composite Formulation Example 1 to 11

[0043] Typical composite composition examples were accordingly further formulated as described in Table III, AO1-84, 85, 86, 87, AO1-105, 106, 107, AO2-60, 61, 62, and 63, respectively. Furthermore, some of these pastes were evaluated after several weeks' aging at room temperature in order to demonstrate the aging-induced nature for those new P&P resins.

Comparative Composite Formulation Example 1 to 4

[0044] In addition, comparative composite examples were accordingly further formulated as described in Table III as well, AO2-2, 3, 4, and 5, respectively. Again, some of these pastes were evaluated after several weeks' aging at room temperature in order to make a direct comparison on the aging effect on these non-cleavable systems to the novel P&P resin-based systems.