RECYCLABLE THERMOSET RESINS The present invention relates to the field of recyclable thermoset resins, to polymers which may be used to make such materials, and to associated uses, methods and products. Recyclable thermoset resins are thermoset resins which can be recycled, i.e. can be recured by heating to reform into a thermoset resin, optionally one which has the same or similar properties to the original thermoset resin. Such recyclable thermoset materials may be considered to be a class of vitrimers. The recycling process may be facilitated by physically breaking down the resin (e.g. grinding it into powder) before reheating it to reform it into the further resin. Thermoset resins, also referred to as thermoset polymers or thermosets, are used in many fields, particularly in construction and engineering sectors, and often exhibit high strength. After use, they are typically not recycled because they are difficult to repurpose or process into other useful materials, and often end up in landfill. This is environmentally disadvantageous and also economically undesirable. The materials contain curable functional groups, which react under chosen curing conditions. The reactions which occur during curing typically entail the cross-linking of polymer chains together, and/or or the reaction of smaller molecules, oligomers or polymers to form larger entities, which are hardened, toughened or solidified materials. Several strategies are known for forming covalent linkages which result in cross-linked structures. Methods can include, for example, one or more of: using hardeners or curing agents, using activating chemistry to affect reactivity, using initiators, and/or applying heat or irradiation. The physical form of the cured thermoset product is typically not a thin coating, but rather is a 3D product, or part, which (in the case of recyclable thermosets) may be used again to form a further product. The recyclable thermoset product may be used as an adhesive (bonding two substrate surfaces), may be used in a composite where two or more materials with different properties are combined and a curing reaction allows the formation of large 3D structures (e.g. "large" could denote 5mm thick or larger), or may be used as a resin to cure to form a large 3D object (e.g., "large" could denote e.g. being 5mm or greater). In each case, the cured material (if recyclable) may be used to form a second object, which may be a large object. The recycling can entail recovering, grinding and reheating, but in some contexts recovering and grinding are not necessary or suitable; e.g. an adhesive joining together two large panels may be reheated to recure and secure the panels together. Depending on the application, durability, flexibility, insolubility, thermal resistance and/or solvent resistance may be important. UV stability can be important, and therefore in some instances the use of certain UV-absorbing components is avoided or controlled. Possible curable functional groups include epoxy functional groups, as well as other groups, including amine functional groups, carboxylic acid functional groups, hydroxyl groups and isocyanate groups. During curing, where the functional group is an epoxide, said epoxide is ring-opened and a covalent bond is formed to another moiety, thereby resulting in a cured, cross-linked structure. Polymers which contain curable epoxide groups are commonly referred to as epoxy resins. Some of the present discussion relates to epoxy functional groups, but it will be appreciated that many of the principles which apply to epoxy functional groups also apply to other types of functional group. Conventional epoxy resins often have a low molecular weight, in order to achieve sufficient epoxy groups to effect a good level of curing and hence effective thermosets with good physical properties. The “epoxy equivalent weight” of an epoxy resin is the weight of epoxy resin per epoxy group. The higher the epoxy equivalent weight, the fewer epoxy groups are present per unit weight of resin. In certain contexts, it is desirable to have a low epoxy equivalent weight, and this is easier to achieve with smaller molecules. In the context of polymers, as the molecular weight of the material increases, it is typically more difficult to maintain the same ratio of epoxy groups to weight of the material. This is because, conventionally, a polymer manufacture method may result in a set number of epoxy groups per polymer molecule, so for example a polymer chain with two terminal epoxy groups with a nominal molecular weight of 1000 may be compared to the same system where the nominal molecular weight is 2000: both may be chain extended polymers made by the same process, but the epoxy equivalent weight for the former is 500 whereas the epoxy equivalent weight for the latter is 1000. Thus, a higher molecular weight polymer may conventionally have a lower concentration of epoxy groups, which impacts on the curing reaction. With higher molecular weight materials, further complications can arise: syntheses are typically more complex, and the materials may be mixtures containing a range of components with a distribution of molecular weights. For step growth polymers, making “ultra high molecular weight” polymers is normally done by reacting monomers A2 + B2 to yield an intermediate AABBAA, and then using a second step to distil out the AA monomer to yield a long chain -AABB-. This negates the need to carefully balance the stoichiometry. These polymers are typically used for thermoplastic applications such as fibres and films. Performance is achieved via entanglement of the long chains. When producing intermediate molecular weight polymers, molecular weight is achieved by the stoichiometric balance of AA and BB monomers. In principle this is straightforward (if extremely sensitive), but in practice it is extremely difficult: the sensitivity to monomer ratio means any weighing errors result in off-spec product. Monomer purity compounds the problem, as monomers normally have some (small) fraction of mono and non-functional component. These lead to dead chain ends that cannot further react. The statistical nature of the reaction means that the final reaction mixture will contain a range of molecular weights from monomers, dimers, trimers up through higher species. Some of these can be cyclic species where a polymer A--------B has looped back on itself to react and close a loop. High molecular weight curable resins may provide considerable additional properties that are derived from the extended backbone of the resin and are difficult to build into a 3D structure during network formation through curing. However, conventionally, high molecular weight resins can be very viscous and have a very low concentration of curable functional groups. It would therefore be desirable to have access to epoxy resins which not only are high molecular weight materials but also have low epoxy equivalent weights, and ideally which avoid other problems known in the art. Conventionally, moving to higher molecular weight polymers results in an increase in Tg, especially with known oligomeric resins. High viscosity can lead to problems during curing. It would be desirable to have methodology which allows the epoxy equivalent weight to be controlled and tailored, regardless of the molecular weight of the epoxy resin. This would allow the extent of curing, and the physical properties of the resultant cured materials, to be controlled. Independent control of different characteristics, such as molecular weight and epoxy equivalent weight, would enable greater flexibility in the preparation of a range of different materials. For example, in some contexts it can be desirable to have a low epoxy equivalent weight whilst having high molecular weight polymeric materials and avoiding or reducing toxicity issues thereby complying with regulatory requirements in the context of REACH and other regulations. It can also be desirable to maintain a low viscosity of the resin. Furthermore, it would be desirable for these advantages to apply to not only epoxy resins but also resin which are curable by virtue of the presence of functional groups other than epoxides. The present invention relates to adhesives, composites and cured objects. Particular properties are relevant and desirable with such 3D objects. When moulding a 3D object and curing, one aim is for the structure to be the same size as the mould. Shrinkage is a known problem, and high molecular weight resins offer the opportunity to mediate or even prevent such shrinkage. Formation of polymers from low molecular weight materials can lead to a decrease in volume and therefore shrinkage. Higher molecular weight resins typically require fewer reactions to reach a gelled crosslinked network and have the potential to diminish the extent of shrinkage during cure. Lower molecular weight materials can also be associated with higher levels of toxicity. Types of backbone which may be used in curing systems include polyester backbones and polyurethane backbones. Curable materials with these types of backbone are effective, but conventionally they have been made by step-growth polymerisation methods. Step- growth chemistry imposes restrictions on the types of polyester architecture or polyurethane architecture which can be made, and on the positioning and frequency of curable groups present and how they are incorporated. It also is not compatible with certain other types of backbone. It would be desirable to have different methodology which is more flexible and addresses some of the problems of step-growth methods. For example, conventionally, after the primary structure of a polymer has been prepared by step-growth polymerisation, curable functional groups may be added, for example by reaction of -OH groups with epichorohydrin to add epoxide groups: this extra process step can be challenging in terms of yield and reaction rate, and epichlorohydrin is highly toxic. It is desirable to have a more effective method of providing curable groups within the polymer. Thus, although the prior art would suggest that thermoset materials could be prepared using step-growth chemistry and the curing of low molecular weight materials, these are associated with several disadvantages. It would be useful also to have a synthesis method that uses free radical chemistry, (which has the benefit of speed and reduces stoichiometry issues), which allows the preparation of high molecular weight resins, which facilitates control of curable group equivalent weight, and which facilitates recyclability. It would also be useful to have methodology which allows the preparation of thermoset materials which exhibit good properties and which are also recyclable. Surprisingly we have now found that a certain class of polymers is particularly well suited to these objectives. From a first aspect, the present invention provides a recyclable thermosetting composition comprising a polymer which is a branched vinyl polymer prepared by transfer-dominated branching radical telomerisation (TBRT) of multivinyl monomer(s) and optionally monovinyl monomer(s) in the presence of chain transfer agent(s), and wherein said polymer comprises curable functional group(s), wherein said curable functional group(s) is/are selected from: an epoxy group; an acid group or other reactive carboxyl or acyl functionality; a hydroxy group or activated hydroxy group; an isocyanate group or blocked isocyanate group; or an amine group. A thermosetting composition is a composition which can be cured to form a thermoset resin. In the present invention, the curing is effected by reaction of the above-mentioned curable functional group(s). The thermosetting composition is a recyclable thermosetting composition, i.e. said resin can be recycled, i.e. can be recured by heating to reform into a thermoset resin. Optionally the resin is physically broken down (e.g. ground into powder) before said heating. Optionally the reformed resin has the same or similar properties to the original thermoset resin. Optionally the resin can be recycled a plurality of times. It is surprising that polymers of this type react under curing conditions to form thermoset materials, and furthermore that said thermoset materials are recyclable. Branched polymers are globular and would not necessarily be assumed to entangle and readily form crosslinked networks. This is particularly applicable where the polymers are high molecular weight materials because of diffusion control issues and because isolated unreacted functional groups may have difficulties in reaching other functional groups with which to react. The present invention is particularly applicable in producing fully cured, insoluble, thermoset materials. The recycled materials are also homogeneous, cured, insoluble solid structures. Various types of chemical units may be present in the backbone of the polymer and in pendant groups, and these will be described in further detail below. Of particular interest are esters or urethane structural or pendant units. The skilled person will understand that the compositions may comprise, and indeed typically comprise, in addition to a curable resin, one or more additional component(s) selected from other reactive ingredient(s), pigment(s), filler(s), and/or additive(s). Accordingly, the composition of the present invention may comprise, in addition to the polymer, one or more additional component(s) selected from other reactive ingredient(s), pigment(s), filler(s), and/or additive(s). For example, the composition of the present invention may comprise, in addition to the polymer, other reactive ingredient(s). The present invention uses TBRT technology which allows control over the location of the curable functional groups (e.g. epoxy groups). It also provides more possibilities to alter equivalent weights of curable groups (e.g. epoxy equivalent weight). It is not reliant on step-growth syntheses, and can provide high molecular weight resins with controllable properties which are highly branched and so have low relative viscosities. It can do so without overly impacting on the Tg or viscosity of the polymer, which would not be possible using conventional routes, e.g. the route whereby an hydroxy-functional polymer converted to an epoxy-functional polymer by reaction with epichlorohydrin. Branched vinyl polymers produced by TBRT, which may be termed “branched vinyl TBRT polymers”, are a recognised group of polymers, as described in S. R. Cassin, P. Chambon and S. P. Rannard, Polym. Chem., 2020, 11, 7637-7649 and patent publications WO 2018/197885, WO 2018/197884 and WO 2020/089649. Following research into some properties and possible uses of said branched vinyl TBRT polymers, we have found that they are particularly well-suited for use in recyclable thermoset compositions when curable functional group(s) are present on the polymers. By way of background, the polymerisation of vinyl monomers using free-radical chain- growth chemistry is well known. Where a vinyl monomer has only one polymerisable vinyl group (i.e. is a monovinyl monomer), polymerisation of said monomer typically results in a linear polymer. Where a vinyl monomer has more than one polymerizable vinyl group (i.e. is a multivinyl monomer), e.g. has two polymerisable vinyl groups (i.e. is a divinyl monomer), polymerisation of said monomer typically results in a branched polymer, because each of, or some of, the multivinyl monomers can become part of more than one vinyl polymer chain. Depending on the reaction conditions, said branched polymer may be highly cross-linked and gelled. Several methods have been used to control this branching and avoid gelation. TBRT is a method for controlling branching during the free radical polymerisation of multivinyl monomers. It entails controlling the extent of propagation relative to the extent of chain transfer, and results in a hyperbranched polymer containing a large number of interconnected linear vinyl polymer chains, wherein the average length of each linear vinyl polymer chain is short. Limited propagation relative to chain transfer can be achieved by using suitable polymerisation conditions, and in particular by using a relatively large amount of chain transfer agent. TBRT is a type of telomerisation. Telomerisation is a method of polymerisation of a polymerizable monomer (a “taxogen”) having an unsaturated group, resulting in a chain of taxogen residues (“taxomons”) with fragments of a further molecule (a “telogen”) attached terminally to the chain of taxomons. This produces a product termed a “telomer” of formula Y(A) _n Z in which A is a taxomon, a residue of a taxogen, and Y and Z are fragments of a telogen, YZ. In TBRT in the field of vinyl polymerisation, the taxogen is a multivinyl monomer (often a divinyl monomer) and the telogen is a chain transfer agent (often a thiol RSH in which, according to the above terminology, Y is RS and Z is H). Advantageously, with the polymers according to this invention, there is access to a different viscosity-temperature curve enabling polymers to be formulated within the Tg constraint, but with better (e.g. lower) melt viscosities. This leads to better flow and appearance, and better molecular mobility resulting in faster reactions. A further advantage is that the present invention allows the use of lower curing temperatures, whilst achieving equivalent or similar appearances and degrees of cure, when compared to conventional materials cured at higher temperatures. Conventionally, thermosetting resins have typically been manufactured by step growth polymerisation methods, and it is conceptually very different to consider manufacturing them by chain growth methods, such as by free radical polymerisation. Conventionally, the creation of a thermoset resin requires the formation of a backbone polymer (polyester for example) whilst controlling monomer ratios so that the molecular weight is not high and so that the chain ends have the appropriate functionality – for example hydroxyl – to be derivatised. Hydroxyl groups can then be reacted with epicholorohydrin (ring opening the epoxy to form a hydroxyl-alkyl chloride) and subsequent ring closing reforms an epoxy ring. These processes are not quick and epichlorohydrin is highly toxic. In contrast, in the present invention, the free-radical polymerisation of vinyl groups allows rapid synthesis, avoids the complexities of step-growth monomer addition and ratios, and allows large-scale synthesis without condensation products. Furthermore, the use of functional vinyl monomers leads to the formation of the product in one reaction step at levels of curable functional group that are dictated by monomer feed, and functional group equivalent molecular weights that are not dependent on the molecular weight of the final polymer. This allows high molecular weight products, low functional group equivalent weights, options for different backbones that are not readily achievable by conventional methods, and economical production at scale by chain-growth chemistry. Nevertheless, free radical polymerisation methodology, such as TBRT methodology, whilst effective, can be unpredictable. The behaviour of TBRT materials can vary and, in part due to the complex and extensive branching, it might have been expected that side reactions would occur and that numerous outcomes might arise. Our investigations in this field have revealed a range of interesting and often unexpected properties. We have compared the methodology of the present invention against conventional thermoset systems and have found that branched vinyl TBRT polymers carrying curable functional groups are particularly effective curable materials and that they facilitate excellent properties whilst also bringing additional advantages including recyclability. The controlled free radical polymerisation methodology of the present invention is not only effective but also opens up new possibilities. Compared to some systems, the polymer system of the present invention may be catalyst- free, though catalysts may be employed if required. The polymer may be cured into a thermoset resin, optionally ground into powder, and then thermally recured into a thermoset without significant loss of properties. This can be demonstrated through two or more cycles and is highly unusual, reflecting the benefits of resins that are accessible through TBRT approaches. The resins may be functional resins. The resins may be of high molecular weight or very high molecular weight. The resins may be of low viscosity. Without wishing to be bound by theory, the recyclability of the resins may be due to bond exchange, i.e. the same or similar types of bond forming when the resin is recured. For example, recuring may result in the forming of an ester linkage where previously the resin had contained ester linkages but in different location(s), so in effect the recuring may include transesterification. Analogously, carbamate linkages may be reformed. Thus, the presence of functional groups within the cured resin may lead to interchange (e.g. transesterification or transcarbamoylation) of network bonding and allow the reforming of functional groups. For example, an ester or a carbamate at a first location and a hydroxyl at a second location may convert to a hydroxyl at said first location and an ester or carbamate at said second location. It may also be the case that esters/esters interchange and carbamates/carbamates interchange. The TBRT methodology of the present invention allows several curing options, for example: a) TBRT polymers with just epoxy groups (thermal epoxy-epoxy reactions); b) TBRT polymers containing different functional groups (eg epoxy and acid); c) TBRT polymers containing one type of functional group plus small multi functional molecules (B2 and higher) - eg epoxy TBRT polymer plus small molecule diacid; d) TBRT polymers containing one type of functional group plus a linear polymer (functional at its chain ends or other) - eg epoxy TBRT plus polyacid; acid TBRT plus polyester epoxy; e) combinations of any of the above - eg adding a TBRT polymer into a conventional resin formulation. The high molecular weight and branched architecture enabled by the present invention provides options that were not previously available, for example in terms of viscosity characteristics, Tg behaviour, and consequent properties and applications. Conventionally, high molecular weight leads to issues of epoxy equivalent weights (EEW), but the present invention provides the possibility of high molecular weight (if desired) and controlled low EEW (if desired) at the same time, and this offers beneficial options for formulation and property control. A further advantage of the present invention is the expansion of structural space in the thermosets area. The present invention provides and uses new and diverse chemistries that are not normally available to thermosets. These would be difficult to prepare or difficult to scale at low cost. Examples include the preparation of alkylparaphenylenes from divinyl benzene (DVB). Polymers produced by TBRT of multivinyl monomers exhibit interesting and unusual properties. They are formed by chain-growth polymerisation, yet may exhibit characteristics of step-growth polymers. Retrosynthetic analysis of some TBRT vinyl polymers can cause them to be viewed as comprising a mixture of polyfunctional step- growth monomer residues. Although they are vinyl polymers, many of them do not resemble vinyl polymers because their chemistry may be dominated by functional groups present in the parts of the multivinyl monomer which link the vinyl groups, and/or by functional groups in the chain transfer agent, and because the extent of reaction between vinyl groups is limited to low numbers such that the vinyl polymer segments within the polymer are short. TBRT methodology enables new architectures containing step-growth motifs which would be difficult or impossible to achieve using step-growth polymerisation. The methodology may also be understood when defined using different terminology to achieve the same or similar outcomes. Therefore, from second, third and fourth aspects, the present invention can be defined as follows. From a second aspect, the present invention provides a recyclable thermosetting composition comprising a polymer which is a branched polymer prepared by free radical vinyl polymerisation comprising residues of multivinyl monomer(s), residues of chain transfer agent(s), and optionally residues of monovinyl monomer(s), and wherein said polymer comprises curable functional group(s), wherein said curable functional group(s) is/are selected from: an epoxy group; an acid group or other reactive carboxyl or acyl functionality; a hydroxy group or activated hydroxy group; an isocyanate group or blocked isocyanate group; or an amine group. From a third aspect, the present invention provides a recyclable thermosetting composition comprising a polymer which is a branched polymer comprising vinyl polymer chains wherein the vinyl polymer chains comprise residues of vinyl groups of multivinyl monomers and optionally monovinyl monomers, wherein the longest chains in the polymer are not the vinyl polymer chains but rather extend through the linkages between double bonds of the multivinyl monomers, and wherein said polymer comprises curable functional group(s), wherein said curable functional group(s) is/are selected from: an epoxy group; an acid group or other reactive carboxyl or acyl functionality; a hydroxy group or activated hydroxy group; an isocyanate group or blocked isocyanate group; or an amine group. From a fourth aspect, the present invention provides a recyclable thermosetting composition comprising a polymer which is a step-growth polymer comprising a mixture of polyfunctional step-growth monomer residues formed by vinyl polymerisation, and wherein said polymer comprises curable functional group(s), wherein said curable functional group(s) is/are selected from: an epoxy group; an acid group or other reactive carboxyl or acyl functionality; a hydroxy group or activated hydroxy group; an isocyanate group or blocked isocyanate group; or an amine group. Regardless of whether the invention is understood according to the first aspect above or according to the second aspect above, or according to the third aspect above, or according to the fourth aspect above, the principles are the same and the description of the invention, including the description herein clarifying the features of the invention and specifying further, optional or preferred features, applies to all aspects. In each aspect, the recyclable thermosetting composition may comprise, in addition to the polymer, one or more additional component(s) selected from other reactive ingredient(s), pigment(s), filler(s), and/or additive(s). For example, the recyclable thermosetting composition of the present invention may comprise, in addition to the polymer, other reactive ingredient(s). In some cases the thermosets may be used externally and environmental resistance in that context is desirable, including one or more properties selected from UV-, weather-, salt water -, temperature- and mechanical abrasion/stress - resistance From further aspects, the present invention provides the use of the polymer defined in each of the first four aspects to form a recyclable thermoset. The extent of propagation may be controlled relative to the extent of chain transfer to prevent gelation of the polymer. The curable functional group(s) may be incorporated via the multivinyl monomer(s), via co- polymerised monovinyl monomer(s), or via chain transfer agent(s). Alternatively, or additionally, the curable functional group(s) may be incorporated by post-functionalisation, i.e. by reaction(s) after polymerisation, to add the curable functional group. For example, an alcohol or acid may be present on the polymer; such groups (and others) may act as curable functional groups in their own right; but alternatively they can be functionalised using other reagents to add on different curable functional groups. Advantageously, the present invention allows the incorporation of more than one type of curable functional group in the same polymer. As shown in the figures, it is possible, for example, to have hydroxyl groups and epoxy groups in the same polymer, or acid groups and alcohol groups in the same polymer. Furthermore the present invention allows the incorporation of curable functional groups with backbone chemistry that would not conventionally be compatible. For example, conventionally, when forming polyurethanes in the backbone, and/or polyesters in the backbone, there are limitations regarding other functional groups which may also be present. In contrast, the present invention allows the mixing of functionalities in ways that were not previously possible. The polymer may be defined by its curable functional group equivalent weight, i.e. the mass of polymer, in g, per mol of curable functional group. Where the curable functional group is an epoxide, the curable functional group equivalent weight is the epoxy equivalent weight (EEW). The curable functional group equivalent weight (e.g. the epoxy equivalent rate) may optionally be within the range of 100 to 10,000 g/ mol, or 100 to 5,000 g/ mol, or 100 to 3,000 g/ mol, or 100 to 2,000 g/ mol, or 100 to 1,500 g/ mol, or 100 to 1,000 g/ mol, or 100 to 900 g/ mol, or 100 to 850 g/ mol, or 250 to 5,000 g/ mol, or 250 to 3,000 g/ mol, or 250 to 2,000 g/ mol, or 250 to 1,500 g/ mol, or 250 to 1,000 g/ mol, or 250 to 900 g/ mol, or 250 to 850 g/ mol, or 500 to 10,000 g/ mol, or 500 to 5,000 g/ mol, or 500 to 3,000 g/ mol, or 500 to 2,000 g/ mol, or 500 to 1,500 g/ mol, or 500 to 1,000 g/ mol, or 500 to 900 g/ mol, or 500 to 850 g/ mol, or 1,000 to 10,000 g/ mol, or 1,000 to 5,000 g/ mol, or 1,000 to 3,000 g/ mol, or 1,000 to 2,000 g/ mol, or 2,000 to 10,000 g/ mol, or 2,000 to 5,000 g/ mol, or 2,000 to 3,000 g/ mol, or 2,000 to 3,000 g/ mol, or 2,000 to 3,000 g/ mol, or 2,000 to 3,000 g/ mol, or 3,000 to 10,000 g/ mol, or 3,000 to 5,000 g/ mol, or 4,000 to 10,000 g/ mol, or 4,000 to 5,000 g/ mol. The curable functional group equivalent weight (e.g. the epoxy equivalent rate) may optionally be up to 1,500 g/ mol, or optionally up to 1,000 g/ mol, or optionally up to 900 g/ mol, or optionally up to 850 g/ mol. The polymer, i.e. the thermoset precursor, may be analysed by triple detection size exclusion chromatography (SEC). The multivinyl monomer residue(s) may be divinyl monomer residue(s) and the polymer may comprise on average between 0.9 and 1.1 chain transfer agent residues per divinyl monomer residu The multivinyl monomer may have more than two vinyl groups, i.e. within the scope of the invention are polymers which may be made from not only divinyl monomers but also, for example, trivinyl and/or tetravinyl monomers. In such scenarios, the polymer may comprise on average between 0.9 and 3.3 chain transfer agent residues per multivinyl monomer residue. The polymer may comprise a multiplicity of vinyl polymer chain segments having an average length of between 1 and 3 multivinyl monomer residues. The resins may be solid, liquid or dissolved in a diluent for application benefits. The material may be suitable for injection moulding. Processability advantages and mitigation against shrinkage are brought about by the present invention. One of the major advantages of the present invention is that extremely branched polymers can be achieved via radical polymerisation that would be impossible, or challenging, or extremely dangerous, to make using step growth polymerisation. It is advantageous to be able to control the polymerisation to avoid gelled polymers. Conventional step growth polymerisation methods have been known to result in gelled branched polymers within industrial-scale reactors, which have taken weeks to recover; this can clearly be very costly and time-consuming. Curable functional group(s) The polymers comprise curable functional groups. These are typically incorporated by being present on one or more of the components used in the polymerisations, namely monovinyl monomers (where used), multivinyl monomers or chain transfer agents, or by post-functionalisation of chemistry which has been incorporated via one or more of said components. Therefore, multiple occurrences of the curable functional groups are present in the polymers, in amounts which can be controlled and tailored by controlling the amount of feedstocks (monovinyl monomer, multivinyl monomer or chain transfer agent). For example, where an epoxide-carrying monovinyl monomer is used, the number of epoxide groups incorporated will correspond to the number of residues of said monomer in the polymer. Suitable curable functional groups are sufficiently inert during storage and during preparation of the formulations, but reactive during curing conditions under reasonable timescales. They include the following groups: epoxy; carboxylic acid; carboxyl; amine; hydroxy; and isocyanate. They also comprise variants of the same, which variants also exhibit suitable cross-linking chemistries: these variants can control reactivity – e.g. activated functional groups (including activated alcohols) and blocked hardeners (e.g. blocked isocyanates). The curable functional group may be an epoxide. For example, a monovinyl monomer may comprise, in addition to a polymerisable double bond, an epoxide group. One example of a suitable monovinyl monomer is glycidyl methacrylate. The curable functional group may be an acid. For example, a monovinyl monomer may comprise, in addition to a polymerisable double bond, an acid group. One example of a suitable monovinyl monomer is methacrylic acid. The curable functional group may be a hydroxyl group. For example, a monovinyl monomer may comprise, in addition to a polymerizable double bond, a hydroxyl group. One example of a suitable monovinyl monomer is hydroxyethylmethacrylate. And/or a divinyl monomer and/or a chain transfer agent may comprise one or more hydroxyl group. During curing, the curable functional groups react. Curing can be effected by heat or by reaction with other reagents, or with each other, to form covalent linkages which can result in cross-linked network structures. Thus the following are examples of possible combinations of reagents which allow curing to take place to form thermoset materials which can be recycled. It may be that the recyclable thermosetting composition comprises a polymer comprising curable functional groups of one type which react with each other; for example, where the curable functional groups comprise epoxy groups, curing may occur by epoxy-epoxy reactions, e.g. thermal epoxy-epoxy reactions. It may be that the recyclable thermosetting composition comprises a polymer comprising curable functional groups of more than one type which react with each other; for example, where the curable functional groups comprise epoxy groups and acid groups, curing may occur by reaction between those groups. It may be that separate reactive ingredients are added to react with the curable functional groups on the polymers thereby effecting curing. Said reactive ingredients may be as follows. Where the recyclable thermosetting composition comprises a polymer comprising curable functional groups which comprise an epoxy group, said recyclable thermosetting composition may further comprise one or more reactive ingredient which reacts with said epoxy group, wherein said reactive ingredient comprises for example functionality selected from hydroxy, amine, carboxyl, or epoxy. Where the recyclable thermosetting composition comprises a polymer comprising curable functional groups which comprise an acid group or other reactive carboxyl or acyl functionality, said recyclable thermosetting composition may further comprise one or more reactive ingredient which reacts with said acid group or other reactive carboxyl or acyl functionality, wherein said reactive ingredient comprises for example functionality selected from hydroxy, or activated hydroxy. Where the recyclable thermosetting composition comprises a polymer comprising curable functional groups which comprise a hydroxy group or activated hydroxy group, said recyclable thermosetting composition may further comprise one or more reactive ingredient which reacts with said hydroxy or activated hydroxy group, wherein said reactive ingredient comprises functionality selected for example from epoxy, acid or other reactive carboxyl or acyl functionality, isocyanate or blocked isocyanate. Where the recyclable thermosetting composition comprises a polymer comprising curable functional groups which comprise an isocyanate group or blocked isocyanate group, said recyclable thermosetting composition may further comprise one or more reactive ingredient which reacts with said isocyanate group or blocked isocyanate group, wherein said reactive ingredient comprises for example hydroxy functionality. Where the recyclable thermosetting composition comprises a polymer comprising curable functional groups which comprise an amine group, said recyclable thermosetting composition may further comprise one or more reactive ingredient which reacts with said amine group, wherein said reactive ingredient comprises for example epoxy functionality. The above-mentioned reactive ingredients may be small molecules or may be polymers which carry said functionality. For example, where the curable functional group on the TBRT polymer is an epoxy group, the additional reactive ingredient may be a polymer which carries suitable reactive functionality (e.g. a polyacid), or by way of further example, where the curable functional group on the TBRT polymer is an acid, the additional reactive ingredient may be a polymer which carries suitable reactive functionality (e.g. a polyester epoxy). Alternatively, a TBRT polymer may be added into a conventional resin formulation. In all cases, the combination of TBRT polymer(s) with themselves or with other TBRT polymer(s) or with other reactive components gives a resulting thermoset which is recyclable, i.e. which contains groups which can undergo bond transfer or bond exchange thereby allowing the reforming of functional groups. This allows, for example, ester functionalities or carbamate (urethane) functionalities to be reformed at different locations, by transesterification or transcarbamoylation or by ester/ester interchange or carbamate/carbamate interchange. Possible embodiments include “ester groups + OH groups” or “carbamate groups + OH groups” or both “ester+ carbamate+OH”. Thus the present invention achieves recyclable vitrimer characteristics. One way of describing processes of relevance to the present invention is to refer to two different stages, which can be termed “stage 1” and “stage 2”. “Stage 1” is the initial cure of the resin. “Stage 2” is the recure (during recycling). Recycling may occur several times and therefore stage 2 may occur several times. Stage 2 may be facilitated: - either by functionality (e.g. OH) which has been generated during formation of the resin in the initial reaction (stage 1); - or by functionality (e.g. OH) which was present on one of more of the reagents (multivinyl monomer, monovinyl monomer or chain transfer agent) and which still remains unreacted after stage 1; - or both. Types of branched polymers The branched vinyl polymers may be considered to be scaffolds to which the curable functional groups are attached, and may comprise any suitable chemistry. Suitable types of branched vinyl polymers include branched polyesters, which may be made from the polymerisation of monomers which comprise vinyl groups as well as ester- containing or ester-forming functionality. Suitable monomers include methacrylates, acrylates and vinyl esters. Suitable divinyl and multivinyl monomers include dimethacrylates, diacrylates, divinyl esters, multimethacrylates, multiacrylates, and multivinyl esters. Said branched polyesters may be aliphatic polyesters, or may be aromatic polyesters if aromatic groups are also present in the monomer(s), or may be mixed aromatic/ aliphatic polyesters. Said branched polyesters may also contain other functional groups, for example by inclusion of certain chemical moieties within the monomers used in the feedstock. Therefore, further suitable types of branched vinyl polymers include branched poly(urethane-ester)s. These may be made from the polymerisation of monomers which comprise: (i) vinyl groups; (ii) ester-containing or ester-forming functionality; and (iii) urethane-containing or urethane-forming functionality. Suitable monomers include urethane dimethacrylate. Types of polymer which have been found to be particularly effective in accordance with the present invention include epoxy-functional branched polyesters and epoxy-functional branched poly(urethane-ester)s. Suitable types of polymer include but are not limited to those containing functionality selected from the following: carbonates; carbonate esters; amides; amide esters; urethanes; urethane esters; urethane amides; urethane carbonates; ureas. The branched polymer may be prepared by the free radical polymerisation of a multivinyl monomer, and optionally a monovinyl monomer, in the presence of a chain transfer agent, using a source of radicals. A curable functional group is present on the polymer and may be incorporated by being present on the multivinyl monomer, the monovinyl monomer and/or the chain transfer agent; or by post-functionalisation. The extent of propagation may be controlled relative to the extent of chain transfer to prevent gelation of the polymer. The term multivinyl monomer denotes monomers which have more than one free radical polymerisable vinyl group. One particular class of such monomers are those which have two such vinyl groups, i.e. divinyl monomers. Therefore, the branched polymer may be prepared by the free radical polymerisation of a divinyl monomer, and optionally a monovinyl monomer, in the presence of a chain transfer agent, using a source of radicals. A curable functional group is present on the polymer and may be incorporated by being present on the divinyl monomer, the monovinyl monomer and/or the chain transfer agent; or by post-functionalisation. The extent of propagation may be controlled relative to the extent of chain transfer to prevent gelation of the polymer. Thus, in contrast to some prior art methods, cross-linking and insolubility are avoided not by using a combination of a predominant amount of monovinyl monomer and a lesser amount of divinyl monomer, but instead by controlling the way in which a divinyl monomer, or other multivinyl monomer, reacts. In many cases no monovinyl monomer is present or required. The branched polymer may be prepared by the free radical polymerisation of a divinyl monomer, and optionally a monovinyl monomer, in the presence of a chain transfer agent, using a source of radicals, wherein propagation is controlled relative to chain transfer to achieve a polymer having a multiplicity of vinyl polymer chain segments wherein the average number of divinyl monomer residues per vinyl polymer chain is between 1 and 3. A curable functional group is present on the polymer and may be incorporated by being present on the divinyl monomer, the monovinyl monomer and/or the chain transfer agent. The branched polymer may be prepared by the free radical polymerisation of a multivinyl monomer, and optionally a monovinyl monomer, in the presence of a chain transfer agent, using a source of radicals, wherein propagation is controlled relative to chain transfer to achieve a polymer having a multiplicity of vinyl polymer chain segments wherein the average number of multivinyl monomer residues per vinyl polymer chain is between 1 and 3. A curable functional group is present on the polymer and may be incorporated by being present on the multivinyl monomer, the monovinyl monomer and/or the chain transfer agent. The branched polymer may be prepared by the free radical polymerisation of a trivinyl monomer, and optionally a monovinyl monomer, in the presence of a chain transfer agent, using a source of radicals, wherein propagation is controlled relative to chain transfer to achieve a polymer having a multiplicity of vinyl polymer chain segments wherein the average number of trivinyl monomer residues per vinyl polymer chain is between 1 and 2. A curable functional group is present on the polymer and may be incorporated by being present on the trivinyl monomer, the monovinyl monomer and/or the chain transfer agent. The branched polymer may be prepared by the free radical polymerisation of a tetravinyl monomer, and optionally a monovinyl monomer, in the presence of a chain transfer agent, using a source of radicals, wherein propagation is controlled relative to chain transfer to achieve a polymer having a multiplicity of vinyl polymer chain segments wherein the average number of tetravinyl monomer residues per vinyl polymer chain is between 1 and 1.7. A curable functional group is present on the polymer and may be incorporated by being present on the tetravinyl monomer, the monovinyl monomer and/or the chain transfer agent. Combinations of different types of multivinyl monomer may be incorporated, regardless of whether or not a monovinyl monomer is incorporated. For example, it may be that any combination of two or more of a divinyl monomer, a trivinyl monomer, a tetravinyl monomer, or other multivinyl monomer, are incorporated, and that optionally a monovinyl monomer may also be incorporated. Additionally or alternatively it may be that the polymer contains more than one of each type of monomer and/or more than one type of chain transfer agent. For example, where the polymer contains divinyl residues and monovinyl residues and chain transfer agent residues, the divinyl residues may be all the same or different, the monovinyl residues may be all the same or different, and the chain transfer agent residues may be all the same or different. By way of example, Figure 3 shows a product formed using two different divinyl monomers plus one monovinyl monomer. Copolymerisations using mixed multivinyl monomers, mixed monovinyl monomers, and/or mixed chain transfer agents therefore enhance the options for preparing copolymers with a wide range of structures. For example one way of preparing polymers containing urethanes and esters could be polymerisation of one type of monomer (e.g. a urethane dimethacrylate). Another way of preparing polymers containing urethanes and esters could be copolymerisation of one type of monomer (e.g. a diester multivinyl monomer) to incorporate ester functionality and another type of monomer (e.g. a urethane-containing multivinyl monomer which does not include, or react to form, ester linkages) to incorporate urethane functionality. The polymer is prepared by free radical polymerisation and any suitable source of radicals can be used. For example, this could be an initiator such as AIBN. A thermal or photochemical or other process can be used to provide free radicals. In contrast to some prior art methods, a large amount of initiator is not required; only a small amount of a source of radicals is required in order to initiate the reaction. To prepare the polymer, the skilled person is able to control the chain transfer reaction relative to the propagation reaction by known techniques. This may be done by using a sufficiently large amount of a chain transfer agent (CTA). The chain transfer agent caps the vinyl polymer chains and thereby limits their length. It also controls the chain end chemistry. Various chain transfer agents are suitable and of low cost, and impart versatility to the method and resultant product. The primary chains may be kept very short so that gel formation is avoided, whilst at the same time a high level of branching is achieved. An important advantage of the present invention is that the polymer may be prepared by industrial free radical polymerisation. This is completely scalable, very straightforward and extremely cost effective. In contrast, some prior art polymers are more complex and/or more costly and/or require the use of initiator systems or more complex purification procedures. Optionally the only reagents to prepare the branched polymer are one or more multivinyl monomer (for example a divinyl monomer), a chain transfer agent, a source of radicals, and optionally a solvent. Thus, in contrast to some prior art methods, the present invention relates to polymers which can be prepared by the homopolymerisation of multivinyl monomers. Monovinyl monomers are not required to prepare the polymer. The curable functional group may be present on a monovinyl monomer, but may alternatively be present on multivinyl monomer or chain transfer agent. Nevertheless, monovinyl monomers may be incorporated, i.e. optionally a copolymerisation may be carried out to produce the polymer. Monovinyl monomers are convenient means of introducing curable functional groups. For example, the polymer may incorporate not only a divinyl monomer but also an amount, optionally a lesser amount, of monovinyl monomer. The molar amount of divinyl monomer relative to monovinyl monomer may be greater than 50%, greater than 75%, greater than 90% or greater than 95%, for example. Optionally, the ratio of divinyl monomer residues to monovinyl monomer residues may be greater than or equal to 1:1, or greater than or equal to 3:1, greater than or equal to 10:1 or greater than or equal to 20:1. Alternatively, in some scenarios, more monovinyl monomer may be used. Optionally, the polymer may incorporate not only one or more divinyl monomer but also monovinyl monomer, wherein for example 20% or more, 30% or more, 40% or more, 50% or more, 60% or more, 70% or more, 80% or more, 90% or more, or 95% or more, of the vinyl monomers used are divinyl monomers. Optionally, the polymer may incorporate not only one or more divinyl monomer but also monovinyl monomer, wherein for example 20% or more, 30% or more, 40% or more, 50% or more, 60% or more, 70% or more, 80% or more, 90% or more, or 95% or more, of the vinyl monomers residues in the product are divinyl monomer residues. The possible incorporation of monovinyl monomers is applicable not just with divinyl monomers but also with other types of multivinyl monomers. Accordingly, the polymer may incorporate not only one or more multivinyl monomer but also monovinyl monomer, wherein for example 10% or more, 20% or more, 30% or more, 40% or more, 50% or more, 60% or more, 70% or more, 80% or more, 90% or more, or 95% or more, of the vinyl monomers used are multivinyl monomers. Optionally, the polymer may incorporate not only one or more multivinyl monomer but also monovinyl monomer, wherein for example 10% or more, 20% or more, 30% or more, 40% or more, 50% or more, 60% or more, 70% or more, 80% or more, 90% or more, or 95% or more, of the vinyl monomers residues in the product are multivinyl monomer residues. The polymer may comprise on average between 0.25 and 5 monovinyl monomer residues per multivinyl monomer (e.g. divinyl) residue, for example 0.25 to 4, or 0.25 to 3, or 0.25 to 2, or 0.25 to 1, or 0.25 to 0.5, or 0.5 to 5, or 0.5 to 4, or 0.5 to 3, or 0.5 to 2, or 0.1 to 1, or 1 to 5, or 1 to 4, or 1 to 3, or 1 to 2, or 2 to 5, or 2 to 4, or 2 to 3, or 3 to 5, or 3 to 4, or 4 to 5. Divinyl Monomer One type of multivinyl monomer residue which may be present in the polymer is a divinyl monomer. The divinyl monomer contains two double bonds each of which is suitable for free radical polymerisation. It may contain one or more other group which for example may be selected from, but not limited to: aliphatic chains; esters; amides; esters; urethanes; silicones; amines; aromatic groups; oligomers or polymers; or a combination of one or more of these; and/or which may optionally be substituted. For example there may be PEG groups or PDMS groups between the double bonds, or a benzene ring (e.g. as in the monomer divinyl benzene) or other aromatic groups. Each vinyl group in the divinyl monomer may for example be an acrylate, methacrylate, acrylamide, methacrylamide, vinyl ester, vinyl aliphatic, or vinyl aromatic (e.g. styrene) group. Due to the large amount of chain transfer agent in the reaction, the vinyl polymer chains in the final product are generally quite short and the chemistry of the longest chains in the polymer may be governed by the other chemical species in the monomer. Thus, for example, monomers which contain, in addition to two vinyl groups, ester linkages (e.g. dimethacrylates, such as EGDMA) polymerise to form polyester structures, wherein the longest repeating units comprise esters. Similarly, monomers which contain, in addition to two vinyl groups, amide linkages (e.g. bisacrylamides) polymerise to form polyamide structures, wherein the longest repeating units comprise amides. Thus the polymers may be polyesters, polyamides or other polymers. The monomer residues may comprise other linkages or moieties, (e.g. urethane units), thereby resulting in other types of polymer (e.g. polyurethanes). The monomer residues may contain more than one type of moiety, thereby resulting in hybrid polymers. For example, monomers which contain, in addition to two vinyl groups, ester linkages and urethane linkages (e.g. dimethacrylates containing urethane linkages, such as urethane dimethacrylate (UDMA)) polymerise to form polymers which may be termed poly(urethane-ester)s or polyurethane esters. The divinyl monomer may be stimuli-responsive, e.g. may be pH, thermally, or biologically responsive. The response may be degradation. The linkage between the two double bonds may for example be acid- or base-cleavable, for example may contain an acetal group. This allows the preparation of a commercial product which is a stimuli-responsive branched polymer. Alternatively, a further step of cleaving divinyl monomer may be carried to remove bridges in the polymer, to produce product in which the linkages between vinyl polymer chains have been removed or reduced. Optionally the polymer may be prepared using a mixture of divinyl monomers. Thus two or more different divinyl monomers may be copolymerised. Other types of multivinyl monomer Multivinyl monomers other than divinyl monomers may be used, for example, trivinyl monomers, tetravinyl monomers and/or monomers with more vinyl groups. Trivinyl monomers, in particular, are useful, as they can be sourced or prepared without significant difficulty, and allow further options for producing different types of branched polymers. The discussion, disclosures and teachings herein in relation to divinyl monomers also apply where appropriate, mutatis mutandis, to other multivinyl monomers. Chain transfer agent (CTA) Any suitable chain transfer agent may be used. These include thiols, including optionally substituted aliphatic thiols, such as dodecane thiol (DDT). Another suitable chain transfer agent is alpha-methylstyrene dimer. Another is 2-isopropoxyethanol. Other compounds having functionality which is known to allow the transfer of radical chains may be used. These can be bespoke to bring about desired functionality to the polymers. The chain-end chemistry can be tailored by the choice of CTA. Thus, hydrophobic/ hydrophilic behaviour and other properties can be influenced. Alkyl thiols can have quite different properties to alcohol-containing groups, acid-containing groups, or amine- containing groups, for example. Optionally, a mixture of CTAs may be used. Thus, two or more different CTAs may be incorporated into the product. Relative amounts of chain transfer agent and divinyl monomer The relative amounts of chain transfer agent and divinyl monomer can be modified easily and optimised by routine procedures to obtain non-gelled polymers without undue burden to the skilled person. The analysis of the products can be carried out by routine procedures, for example the relative amounts of chain transfer agent and divinyl monomer can be determined by NMR analysis. Regarding the reagents used, optionally at least 1 equivalent, or between 1 and 10 equivalents, or between 1.2 and 10 equivalents, or between 1.3 and 10 equivalents, or between 1.3 and 5 equivalents, or between 1 and 5 equivalents, or between 1 and 3 equivalents, or between 1 and 2 equivalents, or between 1.2 and 3 equivalents, or between 1.2 and 2 equivalents, of chain transfer agent may be used relative to divinyl monomer. The presence of a large amount of chain transfer agent means that on average the primary vinyl polymer chains react, and are capped by, chain transfer agent, whilst they are short. This procedure amounts to telomerisation, i.e. the formation of short chains with small numbers of repeat units. In the final product, there may be n+1 chain transfer agent moieties per n divinyl monomer moieties (thus tending to a 1:1 ratio as the molecular weight increases): this is based on a scenario where a theoretically ideal macromolecule of finite size is formed. Other scenarios are however possible, for example intramolecular loop reactions may occur or initiator may be incorporated: in practice, therefore, ratios other than (n+1):n are possible. Optionally, on average between 0.5 and 2 chain transfer agent moieties are present per divinyl monomer moiety, optionally between 0.7 and 1.5, optionally between 0.75 and 1.3, or between 0.8 and 1.2, or between 0.9 and 1.1, or between 1 and 1.05, or approximately 1. Relative amounts of chain transfer agent and trivinyl monomer Where the multivinyl monomer used is a trivinyl monomer, the following may optionally apply. Regarding the reagents used, optionally at least 2 equivalents, or between 2 and 20 equivalents, or between 2.4 and 20 equivalents, or between 2.6 and 20 equivalents, or between 2.6 and 10 equivalents, or between 2 and 10 equivalents, or between 2 and 6 equivalents, or between 2 and 4 equivalents, or between 2.4 and 6 equivalents, or between 2.4 and 4 equivalents, of chain transfer agent may be used relative to trivinyl monomer. In the final product, there may be 2n+1 chain transfer agent moieties per n trivinyl monomer moieties (thus tending to a 2:1 ratio as the molecular weight increases): this is based on a scenario where a theoretically ideal macromolecule of finite size is formed. Other scenarios are however possible, for example intramolecular loop reactions may occur or initiator may be incorporated: in practice, therefore, ratios other than (2n+1):n are possible. Optionally, on average between 1 and 4 chain transfer agent moieties are present per trivinyl monomer moiety, optionally between 1.4 and 3, optionally between 1.5 and 2.6, or between 1.6 and 2.4, or between 1.8 and 2.2, or between 2 and 2.1, or approximately 2. Relative amounts of chain transfer agent and tetravinyl monomer Where the multivinyl monomer used is a tetravinyl monomer, the following may optionally apply. Regarding the reagents used, optionally at least 3 equivalents, or between 3 and 30 equivalents, or between 3.6 and 30 equivalents, or between 3.9 and 30 equivalents, or between 3.9 and 15 equivalents, or between 3 and 15 equivalents, or between 3 and 9 equivalents, or between 3 and 6 equivalents, or between 3.6 and 9 equivalents, or between 3.6 and 6 equivalents, of chain transfer agent may be used relative to tetravinyl monomer. In the final product, there may be 3n+1 chain transfer agent moieties per n tetravinyl monomer moieties (thus tending to a 3:1 ratio as the molecular weight increases): this is based on a scenario where a theoretically ideal macromolecule of finite size is formed. Other scenarios are however possible, for example intramolecular loop reactions may occur or initiator may be incorporated: in practice, therefore, ratios other than (3n+1):n are possible. Optionally, on average between 1.5 and 6 chain transfer agent moieties are present per tetravinyl monomer moiety, optionally between 2.1 and 4.5, optionally between 2.25 and 3.9, or between 2.4 and 3.6, or between 2.7 and 3.3, or between 3 and 3.15, or approximately 3. Relative amounts of chain transfer agent and multivinyl monomer In summary, without wishing to be bound by theory, in certain idealised scenarios the number of CTA residues per n MVM residues in the final product may be as follows: Thus it can be seen that, as the valency of the monomer increases, more and more CTA is required to be present in the final product to cap the chains, unless some other mechanism (e.g. intramolecular reaction) does that. In general the following may optionally apply across the various types of multivinyl monomers discussed herein. Regarding the reagents used, optionally at least 1 equivalent, or between 1 and 30 equivalents, or between 1.2 and 30 equivalents, or between 1.3 and 30 equivalents, or between 1.3 and 15 equivalents, or between 1 and 15 equivalents, or between 1 and 9 equivalents, or between 1 and 6 equivalents, or between 1.2 and 9 equivalents, or between 1.2 and 6 equivalents, of chain transfer agent may be used relative to multivinyl monomer. In the final product, optionally, on average between 0.5 and 6 chain transfer agent moieties are present per multivinyl monomer moiety, optionally between 0.7 and 4.5, optionally between 0.75 and 3.9, or between 0.8 and 3.6, or between 0.9 and 3.3, or between 1 and 3.15, or between approximately 1 and approximately 3. The polymer may comprise on average between 0.9(x-1) and 1.1(x-1) chain transfer agent residues per multivinyl monomer residue, where “x” is the number of polymerizable vinyl groups on the multivinyl monomer. Where the multivinyl monomer is a divinyl monomer, x=2, and accordingly the polymer may comprise on average between 0.9 and 1.1 chain transfer agent residues per divinyl monomer residue. Extent of vinyl polymerisation We believe that one important feature of the present invention is that the average length of the vinyl polymer chains within the overall polymer is short. A typical polymeric molecule prepared as described herein will contain many vinyl polymer chains (each of which is on average quite short) linked together by the moiety which in the multivinyl monomer is between the double bonds. This is achieved by adjusting the conditions, including the amount of chain transfer agent, so that the rate of chain transfer competes with the rate of vinyl polymerisation to the desired extent. The identities of the multivinyl monomer and the chain transfer agent, as well as other factors, affect this balance, but the progress of the reaction can be easily monitored and the properties of the resultant polymer easily determined, by known, routine, techniques. Therefore there is no undue burden to the skilled person. The resulting chain length in this context is the kinetic chain length. Extent of vinyl polymerisation when using divinyl monomers The number of propagation steps (i.e. how many divinyl monomers are added) before each chain transfer (i.e. termination of the growing vinyl polymer chain) needs to be high enough to generate a branched polymer but low enough to prevent gelation. It appears that an average vinyl polymer chain length of between 1 and 3, between 1 and 2.5, between 1 and 2.2, between 1 and 2, between 1.3 and 2, between 1.5 and 2, between 1.7 and 2, between 1.8 and 2, between 1.9 and 2, or between 1.95 and 2, or of approximately 2, divinyl monomer residues, is suitable. Whilst the average may optionally be between 1 and 3, a small number of vinyl polymer chains may contain significantly more divinyl monomer residues, for example as many as 10, 15, 18, 20 or more. Optionally 90 % of the vinyl polymer chains contain fewer than 10 DVM residues, or 90% have a length of 7 or fewer, or 90% have a length of 5 or fewer, or 95% have a length of 15 or fewer, or 95% have a length of 10 or fewer, or 95% have a length of 7 or fewer, or 75% have a length of 10 or fewer, or 75% have a length of 7 or fewer, or 75% have a length of 5 or fewer, or 75% have a length of 4 or fewer, or 75% have a length of 3 or fewer. Without wishing to be bound by theory, the average vinyl polymer chain length, or kinetic chain length, in a scenario which assumes that there is no intramolecular reaction, can be calculated as follows. If, as discussed above there are n+1 chain transfer agent moieties per n divinyl monomer moieties, and one chain transfer agent per vinyl polymer chain, then, because there are 2n double bonds per n divinyl monomers, the number of double bond residues per chain will on average be 2n/(n+1) which will tend towards 2 as the molecular weight increases. The skilled person will understand that the process makes a range of products which, depending on the conditions, can include low molecular weight products (the smallest being the product containing just one DVM, i.e. wherein the vinyl chain length is 1) up to high molecular weight products. Whether the product mixture is purified, and how it is purified, will of course affect the composition of the product and accordingly the length of vinyl polymer chains present. Thus, in some scenarios, where lower molecular weight products are removed, the average vinyl polymer chain length in the resultant purified product may be higher. Optionally, the product may contain a large amount of divinyl monomer residues wherein one of the double bond residues is capped with a chain transfer agent (as opposed to being part of a chain), i.e. has a nominal chain length of 1. The other double bond residues of those divinyl monomer residues may be part of a longer chain. This may be the most common form of the vinyl residue in the product. Optionally the most common vinyl “chain” is that which contains only one divinyl monomer residue. Optionally the two most common vinyl chains are (i) the vinyl “chain” which contains only one divinyl monomer residue and (ii) a vinyl chain which contains an integer selected from between 2 and 8, e.g. between 2 and 7, e.g. between 2 and 6, e.g. between 3 and 8, e.g. between 3 and 7, e.g. between 3 and 6, e.g. between 3 and 5, e.g.4 or 5, e.g.5, divinyl monomer residues. Optionally the most common vinyl “chain” is that which contains only one divinyl monomer residue, and the second most common vinyl chain contains an integer selected from between 2 and 8, e.g. between 2 and 7, e.g. between 2 and 6, e.g. between 3 and 8, e.g. between 3 and 7, e.g. between 3 and 6, e.g. between 3 and 5, e.g.4 or 5, e.g.5, divinyl monomer residues. Optionally the distribution of chain lengths may be bimodal, e.g. the maxima may be at chain length 1 and at a second chain length which may optionally be between 3 and 8, e.g. between 3 and 7, e.g. between 3 and 6, e.g. between 3 and 5, e.g.4 or 5, e.g.5. Extent of vinyl polymerisation when using trivinyl monomers The number of propagation steps (i.e. how many trivinyl monomers are added) before each chain transfer (i.e. termination of the growing vinyl polymer chain) needs to be high enough to generate a branched polymer but low enough to prevent gelation. It appears that an average vinyl polymer chain length of between 1 and 2, between 1 and 1.8, between 1 and 1.7, between 1 and 1.5, between 1.1 and 1.5, between 1.2 and 1.5, between 1.25 and 1.5, between 1.3 and 1.5, between 1.4 and 1.5, or between 1.45 and 1.5, or of approximately 1.5, trivinyl monomer residues, is suitable. Whilst the average may optionally be between 1 and 2, a small number of vinyl polymer chains may contain significantly more trivinyl monomer (TVM) residues, for example as many as 5, 10, 15, 18, 20 or more. Optionally 90 % of the vinyl polymer chains contain fewer than 8 TVM residues, or 90% have a length of 5 or fewer, or 90% have a length of 4 or fewer, or 95% have a length of 10 or fewer, or 95% have a length of 8 or fewer, or 95% have a length of 5 or fewer, or 75% have a length of 8 or fewer, or 75% have a length of 6 or fewer, or 75% have a length of 4 or fewer, or 75% have a length of 3 or fewer, or 75% have a length of 2 or fewer. Without wishing to be bound by theory, the average vinyl polymer chain length, or kinetic chain length, in a scenario which assumes that there is no intramolecular reaction, can be calculated as follows. If, as discussed above there are 2n+1 chain transfer agent moieties per n trivinyl monomer moieties, and one chain transfer agent per vinyl polymer chain, then, because there are 3n double bonds per n trivinyl monomers, the number of double bond residues per chain will on average be 3n/(2n+1) which will tend towards 1.5 as the molecular weight increases. The skilled person will understand that the process makes a range of products which, depending on the conditions, can include low molecular weight products (the smallest being the product containing just one TVM, i.e. wherein the vinyl chain length is 1) up to high molecular weight products. Whether the product mixture is purified, and how it is purified, will of course affect the composition of the product and accordingly the length of vinyl polymer chains present. Thus, in some scenarios, where lower molecular weight products are removed, the average vinyl polymer chain length in the resultant purified product may be higher. Optionally, the product may contain a large amount of trivinyl monomer residues wherein two of the double bond residues are capped with a chain transfer agent (as opposed to being part of a chain), i.e. have a nominal chain length of 1. The other double bond residues of those trivinyl monomer residues may be part of a longer chain. This may be the most common form of the vinyl residue in the product. Optionally the most common vinyl “chain” is that which contains only one trivinyl monomer residue. Optionally the two most common vinyl chains are (i) the vinyl “chain” which contains only one trivinyl monomer residue and (ii) a vinyl chain which contains an integer selected from between 2 and 7, e.g. between 2 and 6, e.g. between 2 and 5, e.g. between 3 and 7, e.g. between 3 and 6, e.g. between 3 and 5, e.g.3 or 4, e.g.3 or e.g.4, trivinyl monomer residues. Optionally the most common vinyl “chain” is that which contains only one trivinyl monomer residue, and the second most common vinyl chain contains an integer selected from between 2 and 7, e.g. between 2 and 6, e.g. between 2 and 5, e.g. between 3 and 7, e.g. between 3 and 6, e.g. between 3 and 5, e.g.3 or 4, e.g.3 or e.g.4, trivinyl monomer residues. Optionally the distribution of chain lengths may be bimodal, e.g. the maxima may be at chain length 1 and at a second chain length which may optionally be between 3 and 7, e.g. between 3 and 6, e.g. between 3 and 5, e.g.3 or 4, e.g.3 or e.g.4. Extent of vinyl polymerisation when using tetravinyl monomers The number of propagation steps (i.e. how many tetravinyl monomers are added) before each chain transfer (i.e. termination of the growing vinyl polymer chain) needs to be high enough to generate a branched polymer but low enough to prevent gelation. It appears that an average vinyl polymer chain length of between 1 and 1.7, between 1 and 1.5, between 1 and 1.4, between 1 and 1.33, between 1.1 and 1.33, between 1.2 and 1.33, between 1.25 and 1.33, or between 1.3 and 1.33, or of approximately 1.33, tetravinyl monomer residues, is suitable. Whilst the average may optionally be between 1 and 1.7, a small number of vinyl polymer chains may contain significantly more tetravinyl monomer residues, for example as many as 3, 5, 10, 15, 18, 20 or more. Optionally 90 % of the vinyl polymer chains contain fewer than 6 tetravinyl monomer residues, or 90% have a length of 4 or fewer, or 90% have a length of 3 or fewer, or 90% have a length of 2 or fewer, or 95% have a length of 8 or fewer, or 95% have a length of 6 or fewer, or 95% have a length of 4 or fewer, or 95% have a length of 3 or fewer, or 75% have a length of 5 or fewer, or 75% have a length of 4 or fewer, or 75% have a length of 3 or fewer, or 75% have a length of 2 or fewer. Without wishing to be bound by theory, the average vinyl polymer chain length, or kinetic chain length, in a scenario which assumes that there is no intramolecular reaction, can be calculated as follows. If, as discussed above there are 3n+1 chain transfer agent moieties per n tetravinyl monomer moieties, and one chain transfer agent per vinyl polymer chain, then, because there are 4n double bonds per n tetravinyl monomers, the number of double bond residues per chain will on average be 4n/(3n+1) which will tend towards 1.33 as the molecular weight increases. The skilled person will understand that the process makes a range of products which, depending on the conditions, can include low molecular weight products (the smallest being the product containing just one tetravinyl monomer residue i.e. wherein the vinyl chain length is 1) up to high molecular weight products. Whether the product mixture is purified, and how it is purified, will of course affect the composition of the product and accordingly the length of vinyl polymer chains present. Thus, in some scenarios, where lower molecular weight products are removed, the average vinyl polymer chain length in the resultant purified product may be higher. Optionally, the product may contain a large amount of tetravinyl monomer residues wherein three of the double bond residues are capped with a chain transfer agent (as opposed to being part of a chain), i.e. have a nominal chain length of 1. The other double bond residues of those tetravinyl monomer residues may be part of a longer chain. This may be the most common form of the vinyl residue in the product. Optionally the most common vinyl “chain” is that which contains only one tetravinyl monomer residue. Optionally the two most common vinyl chains are (i) the vinyl “chain” which contains only one tetravinyl monomer residue and (ii) a vinyl chain which contains an integer selected from between 2 and 6, e.g. between 2 and 5, e.g. between 2 and 4, e.g. between 3 and 6, e.g. between 3 and 5, e.g.3 or 4, e.g.3 or e.g.4, tetravinyl monomer residues. Optionally the most common vinyl “chain” is that which contains only one tetravinyl monomer residue, and the second most common vinyl chain contains an integer selected from between 2 and 6, e.g. between 2 and 5, e.g. between 2 and 4, e.g. between 3 and 6, e.g. between 3 and 5, e.g.3 or 4, e.g.3 or e.g.4, tetravinyl monomer residues. Optionally the distribution of chain lengths may be bimodal, e.g. the maxima may be at chain length 1 and at a second chain length which may optionally be between 3 and 6, e.g. between 3 and 5, e.g.3 or 4, e.g.3 or e.g.4. Extent of vinyl polymerisation when using multivinyl monomers in general Numerical relationships and theoretical assessments have been presented above for each of divinyl monomers, trivinyl monomers and tetravinyl monomers. In summary, without wishing to be bound by theory, in certain idealised scenarios the average number of multivinyl monomer residues per vinyl polymer chain may be as follows, where the product contains n multivinyl monomer residues: Thus it can be seen that, as the valency of the monomers increases, the average vinyl chain length is required to decrease. In general the following may optionally apply across the various types of multivinyl monomers discussed herein. The average vinyl polymer chain length may contain the following number of multivinyl monomer residues: between 1 and 3, between 1 and 2.5, between 1 and 2.2, between 1 and 2, between 1.1 and 2, between 1.2 and 2, between 1.3 and 2, between 1.33 and 2, between 1.5 and 2, between 1.8 and 2, between 1.9 and 2, between 1.95 and 2, between 1.2 and 1.5, between 1.3 and 1.5, between 1.4 and 1.5, between 1.45 and 1.5, between 1.1 and 1.4, between 1.2 and 1.4, between 1.2 and 1.33, or between 1.3 and 1.33. Whilst the average may optionally be between 1 and 3, a small number of vinyl polymer chains may contain significantly more multivinyl monomer residues, for example as many as 3, 5, 8, 10, 15, 18, 20 or more. Optionally 90 % of the vinyl polymer chains contain fewer than 10 multivinyl monomer residues, or 90% have a length of 7 or fewer, or 90% have a length of 5 or fewer, or 90% have a length of 4 or fewer, or 90% have a length of 3 or fewer, or 90% have a length of 2 or fewer, or 95% have a length of 15 or fewer, or 95% have a length of 10 or fewer, or 95% have a length of 7 or fewer, or 95% have a length of 5 or fewer, or 95% have a length of 4 or fewer, or 95% have a length of 3 or fewer, or 75% have a length of 10 or fewer, or 75% have a length of 7 or fewer, or 75% have a length of 5 or fewer, or 75% have a length of 4 or fewer, or 75% have a length of 3 or fewer, or 75% have a length of 2 or fewer. Optionally, the product may contain a large amount of multivinyl monomer residues wherein all but one of the double bond residues in the multivinyl monomer residue is capped with a chain transfer agent (as opposed to being part of a chain), i.e. has a nominal chain length of 1. The remaining double bond residue of the multivinyl monomer residues may be part of a longer chain. This may be the most common form of the vinyl residue in the product. Optionally the most common vinyl “chain” is that which contains only one multivinyl monomer residue. Optionally the two most common vinyl chains are (i) the vinyl “chain” which contains only one multivinyl monomer residue and (ii) a vinyl chain which contains an integer selected from between 2 and 8, e.g. between 2 and 7, e.g. between 2 and 6, e.g. between 2 and 5, e.g. between 3 and 8, e.g. between 3 and 7, e.g. between 3 and 6, e.g. between 3 and 5, e.g.3, e.g.4 or e.g.5 multivinyl monomer residues. Optionally the most common vinyl “chain” is that which contains only one multivinyl monomer residue, and the second most common vinyl chain contains an integer selected from between 2 and 8, e.g. between 2 and 7, e.g. between 2 and 6, e.g. between 2 and 5, e.g. between 3 and 8, e.g. between 3 and 7, e.g. between 3 and 6, e.g. between 3 and 5, e.g.3, e.g.4 or e.g. 5, multivinyl monomer residues. Optionally the distribution of chain lengths may be bimodal, e.g. the maxima may be at chain length 1 and at a second chain length which may optionally be between 3 and 8, e.g. between 3 and 7, e.g. between 3 and 6, e.g. between 3 and 5, e.g.3, 4 or 5. Source of radicals The source of radicals may be an initiator such as azoisobutyronitrile (AIBN). Optionally the amount used relative to divinyl monomer may be 0.001 to 1, 0.01 to 0.1, 0.01 to 0.05, 0.02 to 0.04 or approximately 0.03 equivalents. In view of the presence of two double bonds per monomer this equates to 0.0005 to 0.5, 0.005 to 0.05, 0.005 to 0.025, 0.01 to 0.02 or approximately 0.015 equivalents relative to double bond. It has been found that the reactions proceed effectively when only small amounts of initiator are used. Reducing the amount of initiator means that the reactions may proceed more slowly but still at speeds which are industrially acceptable. Lower amounts of initiator are beneficial in terms of cost, residual effect in the product, and controlling the exotherm to enhance safety and facilitate manageable reactions even when scaled up. Other possible sources of radicals include peroxides, organo-boranes, persulfates or UV- initiated systems. Reaction conditions The polymers may be prepared under conventional industrial free radical polymerisation conditions. Optionally a solvent such as for example toluene may be used. As the reaction conditions become more dilute, the amount of CTA in the product can decrease. Without wishing to be bound by theory, this may be because at greater dilution intramolecular reaction is more likely, meaning that, effectively, reaction of the molecule with itself takes the place of reaction of the molecule with a CTA molecule. Accordingly, this can alter the numerical relationships discussed above, because these assume a theoretical situation in which there is no intramolecular reaction. This provides a further way of controlling the chemistry and tailoring the type of product and its properties. For example, whereas in some scenarios it may be desirable to have a large amount of CTA residue in the product, in other scenarios it is desirable not to, for example to reduce the amount of thiol residues. Furthermore, carrying out the same reaction at different dilutions can lead to different physical properties such that for example some products are solids and others are liquids. Ways of manipulating the glass-transition temperature and/or melting temperature can be useful for various applications. Residual vinyl content in the polymer The polymer is formed by vinyl polymerisation. Said polymerisation may have proceeded to the extent that the polymer product contains very little, substantially no, or no, residual vinyl functionality. Optionally, no more than 20mol%, no more than 10mol%, no more than 5mol%, no more than 2mol%, or no more than 1mol%, of the radically polymerizable double bonds of the multivinyl monomer, e.g. of the divinyl monomer, remain in the polymer. Nevertheless, as discussed above, where it is desired to incorporate unsaturated groups, e.g. vinyl functionality, and where there is no or substantially no residual vinyl content after polymerisation, this may be done by post-functionalisation. Further examples of suitable branched polymers The branched polymer may comprise divinyl monomer residues and chain transfer residues, wherein the molar ratio of chain transfer residues to divinyl monomer residues is between 0.5 and 2. The ratio is optionally between 0.7 and 1.5, optionally between 0.75 and 1.3, optionally between 0.8 and 1.2, optionally between 0.9 and 1.1, optionally between 1 and 1.05, optionally approximately 1. Some of the vinyl polymer chains may contain as many as 18, or 15, divinyl monomer residues. Only a small proportion are this long, however: the average, for high molecular weight materials, may be around 2. Optionally 90 % of the vinyl polymer chains contain fewer than 10 DVM residues, or 90% have a length of 7 or fewer, or 90% have a length of 5 or fewer, or 95% have a length of 15 or fewer, or 95% have a length of 10 or fewer, or 95% have a length of 7 or fewer, or 75% have a length of 10 or fewer, or 75% have a length of 7 or fewer, or 75% have a length of 5 or fewer, or 75% have a length of 4 or fewer, or 75% have a length of 3 or fewer). Thus the branched polymer product may comprise divinyl monomer residues and chain transfer residues, wherein 90 % of the vinyl polymer chains contain fewer than 10 DVM residues, or 90% have a length of 7 or fewer, or 90% have a length of 5 or fewer, or 95% have a length of 15 or fewer, or 95% have a length of 10 or fewer, or 95% have a length of 7 or fewer, or 75% have a length of 10 or fewer, or 75% have a length of 7 or fewer, or 75% have a length of 5 or fewer, or 75% have a length of 4 or fewer, or 75% have a length of 3 or fewer). During preparation of the polymer, it is possible that neither of the two carbon atoms of a vinyl group forms a bond to another vinyl group (instead they could form a bond to a CTA residue or hydrogen, or, in some cases, other moiety such as initiator residue or solvent residue), or it is possible that one of the two carbon atoms of a vinyl group forms a bond to another vinyl group, or it is possible that both carbon atoms of a vinyl group form bonds to other vinyl groups. Therefore, in the product, each vinyl residue may be directly linked to 0, 1 or 2 other vinyl residues as closest neighbours. Optionally the branched polymer comprises divinyl monomer residues and chain transfer residues, wherein each vinyl residue is directly vinyl polymerised to on average 0.5 to 1.5 other divinyl monomer residue. Optionally this may be 0.8 to 1.2, 0.8 to 1.1, 0.9 to 1, or approximately 1, on average. Thus the polymers are characterised by having a large amount of chain transfer agent incorporation, and also by having short distinct vinyl polymer chains. Whereas, conventionally, a vinyl polymer chain will normally comprise a long saturated backbone, during the preparation of the present polymers - even though they are built up using vinyl polymerisation - most of the double bonds only react with one other double bond, or react with no other double bonds, rather than react with two other double bonds. This means that the linkages between the two double bonds in the monomer, which linkages conventionally bring about branching between polymer chains in the prior art, instead form the backbone of the longest polymer chains in the present branched polymers. The branched polymer may comprise divinyl monomer residues and chain transfer residues, in which there is a multiplicity of vinyl polymer chain segments having an average length of between 1 and 3 divinyl monomer residues. The average length may be between 1 and 2.5, between 1 and 2.2, between 1 and 2, between 1.3 and 2, between 1.5 and 2, between 1.7 and 2, between 1.8 and 2, between 1.9 and 2, between 1.95 and 2, or approximately 2. The skilled person will understand how the number of double bond residues affects the carbon chain length of the resultant vinyl polymer segment. For example, where a polymer chain segment comprises 2 double bond residues, this equates to a saturated carbon chain segment of 4 carbon atoms. The incorporation of monovinyl monomers as well as divinyl monomers may affect the average vinyl chain length but does not affect the average number of divinyl monomer residues per chain. It can be a way of increasing the vinyl chains without increasing branching. The branched polymer may comprise divinyl monomer residues and chain transfer residues wherein the divinyl monomer residues comprise less than 20mol% double bond functionality. In other words, in such polymer products, at least 80% of the double bonds of the divinyl monomers have reacted to form saturated carbon-carbon chains. The residues may comprise less than 10mol%, or less than 5mol%, or less than 2mol%, or less than 1mol%, or substantially no, double bond functionality. Another way of defining the polymer is in terms of its Mark Houwink alpha value. Optionally, this may be below 0.5. The above description of polymer products relates in particular to those containing divinyl monomer residues. Analogously, polymer products containing other multivinyl monomer residues may include for example trivinyl monomer residues and/or tetravinyl monomer residues. Definitions and disclosures herein apply mutatis mutandis. The molar ratio, on average, of chain transfer residues to multivinyl monomer residues may optionally be: - for multivinyl monomers generally: between 0.5 and 6, between 0.7 and 4.5, between 0.75 and 3.9, between 0.8 and 3.6, between 0.9 and 3.3, between 1 and 3.15, or between approximately 1 and approximately 3; - for trivinyl monomers: between 1 and 4, between 1.4 and 3, between 1.5 and 2.6, between 1.6 and 2.4, between 1.8 and 2.2, between 2 and 2.1, or approximately 2; - for tetravinyl monomers: between 1.5 and 6, between 2.1 and 4.5, between 2.25 and 3.9, between 2.4 and 3.6, between 2.7 and 3.3, between 3 and 3.15, or approximately 3. Optionally: - for multivinyl monomers generally: 90 % of the vinyl polymer chains contain fewer than 10 multivinyl monomer residues, or 90% have a length of 7 or fewer, or 90% have a length of 5 or fewer, or 90% have a length of 4 or fewer, or 90% have a length of 3 or fewer, or 90% have a length of 2 or fewer, or 95% have a length of 15 or fewer, or 95% have a length of 10 or fewer, or 95% have a length of 7 or fewer, or 95% have a length of 5 or fewer, or 95% have a length of 4 or fewer, or 95% have a length of 3 or fewer, or 75% have a length of 10 or fewer, or 75% have a length of 7 or fewer, or 75% have a length of 5 or fewer, or 75% have a length of 4 or fewer, or 75% have a length of 3 or fewer, or 75% have a length of 2 or fewer; - for trivinyl monomers: 90 % of the vinyl polymer chains contain fewer than 8 TVM residues, or 90% have a length of 5 or fewer, or 90% have a length of 4 or fewer, or 95% have a length of 10 or fewer, or 95% have a length of 8 or fewer, or 95% have a length of 5 or fewer, or 75% have a length of 8 or fewer, or 75% have a length of 6 or fewer, or 75% have a length of 4 or fewer, or 75% have a length of 3 or fewer, or 75% have a length of 2 or fewer; - for tetravinyl monomers: 90 % of the vinyl polymer chains contain fewer than 6 tetravinyl monomer residues, or 90% have a length of 4 or fewer, or 90% have a length of 3 or fewer, or 90% have a length of 2 or fewer, or 95% have a length of 8 or fewer, or 95% have a length of 6 or fewer, or 95% have a length of 4 or fewer, or 95% have a length of 3 or fewer, or 75% have a length of 5 or fewer, or 75% have a length of 4 or fewer, or 75% have a length of 3 or fewer, or 75% have a length of 2 or fewer Optionally each vinyl bond is directly vinyl polymerised to on average: - for multivinyl monomers generally: 0.1 to 1.5, 0.2 to 1.2, 0.825 to 1.1, or approximately 0.3 to 1, other multivinyl monomer residue; - for trivinyl monomers: 0.2 to 1.3, 0.25 to 1.2, 0.3 to 1, 0.4 to 0.7, or approximately 0.5, other trivinyl monomer residue; - for tetravinyl monomers: 0.1 to 1, 0.2 to 0.8, 0.25 to 0.5, or approximately 0.3, other tetravinyl monomer residue. Optionally the branched polymer product comprises a multiplicity of vinyl polymer chain segments having an average length of: - for multivinyl monomers generally: between 1 and 3, between 1 and 2.5, between 1 and 2.2, between 1 and 2, between 1.1 and 2, between 1.2 and 2, between 1.3 and 2, between 1.33 and 2, between 1.5 and 2, between 1.8 and 2, between 1.9 and 2, between 1.95 and 2, between 1.2 and 1.5, between 1.3 and 1.5, between 1.4 and 1.5, between 1.45 and 1.5, between 1.1 and 1.4, between 1.2 and 1.4, between 1.2 and 1.33, or between 1.3 and 1.33 multivinyl monomer residues; - for trivinyl monomers: between 1 and 2, between 1 and 1.8, between 1 and 1.7, between 1 and 1.5, between 1.1 and 1.5, between 1.2 and 1.5, between 1.25 and 1.5, between 1.3 and 1.5, between 1.4 and 1.5, or between 1.45 and 1.5, or of approximately 1.5, trivinyl monomer residues; - for tetravinyl monomers: between 1 and 1.7, between 1 and 1.5, between 1 and 1.4, between 1 and 1.33, between 1.1 and 1.33, between 1.2 and 1.33, between 1.25 and 1.33, or between 1.3 and 1.33, or of approximately 1.33, tetravinyl monomer residues. The incorporation of monovinyl monomers as well as multivinyl monomers may affect the average vinyl chain length but does not affect the average number of multivinyl monomer residues per chain. It can be a way of increasing the length of the vinyl chains without increasing branching. Optionally a branched polymer product comprises multivinyl monomer residues and chain transfer residues wherein the multivinyl monomer residues comprise less than 20mol% double bond functionality. The residues may comprise less than 10mol%, or less than 5mol%, or less than 2mol%, or less than 1mol%, or substantially no, double bond functionality. Description of the Drawings The present invention will now be described in further non-limiting detail and with reference to the drawings in which: Figures 1 to 12 show fragments of branched polymers in accordance with the present invention, each of which is prepared by the co-polymerisation of one or more divinyl monomer and a monovinyl monomer using one or more chain transfer agent, wherein said monovinyl monomer carries a curable functional group; The example structures shown in figures 1 to 12 are the result of using: - as chain transfer agent: o dodecane thiol (DDT) (figures 1 to 5, 7, 8 and 10 to 12), o dodecane thiol in combination with thiolglycerol (TG) (figure 6), o cyclohexanethiol (CHT) (figure 9) - as divinyl monomer (DVM): o ethylene glycol dimethacrylate (EGDMA) (exclusively in figures 1, 4 and 9; and in combination with other divinyl monomers in figures 3, 8, 11 and 12), o a di- aromatic ring - containing dimethacrylate (exclusively in figure 2; and in combination with other divinyl monomer in figure 3), o urethane dimethacrylate (UDMA) (exclusively in figures 5 to 7, and in combination with other divinyl monomer in figure 8), o glycerol-1,3-dimethacrylate (exclusively in figure 10; in combination with other divinyl monomer in figure 11), o divinyl benzene, in combination with EGDMA (figure 12) - as monovinyl monomer: o an epoxy group – containing monomer (glycidyl methacrylate (GlyMA)) (figures 1 to 3, 5, 6 and 8 to 12), o an acid group – containing monomer (methacrylic acid (MA)) (figures 4 and 7). Figures 13 to 15 show IR spectra of resins in accordance with the present invention; Figures 16 to 20 show stress-strain curves for resins prepared in accordance with the present invention; Figure 21 shows reusable thermoset behaviour wherein a thermoset resin of the present invention is ground and recured twice; Figure 22 shows a cured thermoset resin of the present invention in the form of a disc; Figure 23 shows stress relaxation behaviour and an Arrhenius plot in respect of a product of the present invention; Figure 24 shows dog bone moulds which have been cured and tested, ground down, remoulded and retested; Figures 25 to 29 show flexible dog bone specimens, the manipulation of these, broken specimens, once-recycled specimens and twice-recycled specimens; Figures 30 to 32 show stress-strain curves for the specimens of Figures 25, 28 and 29 respectively, and Figures 33 to 37 show the shape memory behaviour of specimens. Examples Synthesis of polymers (branched polymers carrying curable functional groups) used in recyclable thermosets) An example of a branched polymer carrying curable functional groups in accordance with the present invention is an epoxy polymer (“Polymer 1”) prepared by the transfer- dominated branching radical telomerisation of a divinyl monomer in the presence of an epoxy-carrying monovinyl monomer and in the presence of a chain transfer agent. In this example, the divinyl monomer is urethane dimethacrylate (UDMA), the monovinyl monomer is glycidyl methacrylate (GlyMA or GMA), and the chain transfer agent is dodecane thiol, in a ratio of 1:1:1. An example fragment of this epoxy polymer is illustrated in figure 5. A related example (figure 7) uses methacrylic acid (MA) instead of glycidyl methacrylate. Copolymerisation of UDMA with GMA (to make epoxy polymer (Polymer 1)) or MA (to make corresponding carboxyl polymer) Diurethane dimethacrylate (1 g, 2.13 mmol), 1-dodecanethiol (0.43 g, 2.13 mmol), AIBN (0.0157 g, 0.096 mmol), glycidyl methacrylate (0.32 g, 2.13mmol) or methacrylic acid (0.18 g, 2.13mmol) and ethyl acetate (17.28 mL, if using glycidyl methacrylate; or 16.09 mL, if using methacrylic acid) were added to a 50 mL round-bottomed flask and purged with nitrogen for 15 minutes. The reaction flasks were heated at 70 ºC, stirred for 24 hours and then cooled. The reaction mixtures were concentrated by rotary evaporation to around 2.5 mL, and purified via precipitation into ten times the amount of methanol at room temperature which afforded a white powder solid (1.25 g, 87 %). A sample of the product was taken for ¹ H NMR spectroscopic analysis in CDCl ₃ . ¹ H NMR (400 MHz, CDCl3): δ ppm = 0.8-1.2 (m, 21H), 2.6-2.9 (d, 2H), 3.0-3.2 (m, 4H), 4.0-4.5 (s, br, 8H). Examples of curing the polymers, grinding down the resultant cured materials, and recuring the ground materials Polymer ‘Auto’ cure using MeltPrep Hot Melt Extrusion (curing due to epoxy-epoxy reaction of epoxy-carrying TBRT polymer) CHT/UDMA/GMA copolymer (0.5 g, 1.23 mmol) was ground in a pestle and mortar. The powder was added to a 20mm disc mould and cured samples were manufactured using a MeltPrep hot melt extrusion machine. The mould was heated to 180 ºC for 30 minutes and then heated to 250 ºC for a further 30 minutes. Cured disc were obtained and the extent of cure was monitored by FTIR. The cured discs were then broken and ground down to a powder using a coffee grinder and pestle and mortar. The discs were reformed using the 20 mm disc mould and MeltPrep by heating to 250 ºC for 30 minutes. The resulting vitrimer disc was submerged in THF for 24 hours to ensure the disc swelled and did not dissolve. Auto cured disc of CHT/UDMA/GMA copolymer is shown in Fig.22. Acid/ epoxy cure using MeltPrep Hot Melt Extrusion 1:1 by mol% of CHT/UDMA/GMA copolymer (0.5 g, 1.23 mmol) to DDT/UDMA/MA copolymer (0.84 g, 1.11 mmol) or DDT/EGDMA/MA copolymer (0.54 g, 1.11 mmol) or Sebacic acid (0.22 g, 1.09mmol) or EGDMA/3-Mercatopropionic acid copolymer (0.34 g, 1.12 mmol) was dissolved in THF, dried in 40 ºC vacuum oven overnight and resulting powder was ground in a pestle and mortar. The powder was added to a 20mm disc mould and cured samples were manufactured using a MeltPrep hot melt extrusion machine. The mould was heated to 180 ºC for 30 minutes and then heated to 250 ºC for a further 30 minutes. Cured disc were obtained and the extent of cure was monitored by FTIR. The 20mm discs described above were subjected to stress relaxation tests using a rheometer. Using a 20mm flat plate geometry, a constant strain of 1% was maintained at an initial force of 10N over temperature ranging from 210 ºC to 240 ºC until 37% stress relaxation has been reached. The time at 37% stress relaxation was plotted in an Arrhenius plot from which the activation energy could be calculated. Stress relaxation and Arrhenius plot shown in Fig.23 a) and b) respectively. Fig.23 a) shows normalized stress relaxation plotted against time (CHT/UDMA/GMA cured with 1:1 mol ratio of DDT/UDMA/MA. Specimen subjected to stress relaxation experiments, at 210, 220, 230 and 240 ºC.) The time at each temperature to reach 37% of stress relaxation was plotted in an Arrhenius plot (Fig.24 b)). From the gradient of the plot the activation energy can be calculated as 115.74 KJ/mol. Acid/epoxy cure and dog bone mould manufacture 1:1 by mol% of CHT/UDMA/GMA copolymer (0.5 g, 1.23 mmol) to UDMA/MA (0.34 g, 1.23 mmol) or EGDMA/MA (0.22 g, 1.23 mmol) or Sebacic acid (0.9 g, 1.23mmol) was ground in a pestle and mortar and added to a Teflon mould. The mould was sandwiched between two 20 x 20 cm sheets of stainless steel and put into a hot press at 200 °C for 20 minutes. The mould was allowed to cool and the specimen was retrieved. Once cured specimen had been tested for tensile strength, they were ground using a coffee grinder and a pestle and mortar. The ground particles were added again to the Teflon mould and put into the hot press at 250 °C for 30 minutes. Vitrimer specimens were allowed to cool before being tested for tensile strength. Images of dog bone moulds which have been cured and tested, ground down, remoulded and retested are shown in Fig.24. Thus, the epoxy-containing polymers were cured with an acid containing polymer, with methacrylic acid as monofunctional monomer or 3-mercaptopropionic acid as CTA or a small molecule diacid, sebacic acid, to form hydroxyesters which we believe then react with further epoxy groups resulting in complete cure of the network and full reaction of epoxy groups as visualised in FTIR. IR spectra indicated the reaction of curable functional groups after curing and also after re- curing. Figure 13 shows the disappearance of the epoxy peak after “auto” curing of the epoxy-functional polymer (Polymer 1). One of the traces refers to the neat uncured sample showing epoxy peak at 908 cm ^-1 . Two further traces relate to FTIR spectra performed during curing to check when full cure is reached, and a further trace (the lowest trace) indicates when that full cure has been reached, as visualized by disappearance of peak at 908cm ^-1 . Analogously, Figure 14 shows disappearance of the epoxy peak after curing of epoxy- functional polymer (Polymer 1) with Polymer 2, an acid functional polymer (DDT/UDMA/MA copolymer). The upper trace indicates before curing; the lower trace indicates after curing. Analogously, Figure 15 shows disappearance of the epoxy peak after curing of an epoxy- functional polymer (Polymer 3) (DDT/GDMA wherein GDMA is glycerol dimethacrylate). The hydroxy groups attacks epoxy to ring open to form insoluble cured network. The resins exhibited resuable thermoset behaviour. After curing, grinding they could be recured to materials with substantially the same properties. A further cycle of grinding and recuring retained substantially the same properties. Physical properties, including tensile strength, of the recyclable thermosets were investigated. Various systems were investigated, including those using GlyMA as epoxy-functional monovinyl monomer, CHT (cyclohexane thiol) as CTA, and EGDMA and/or UDMA as multivinyl monomer: For experimental details see acid/epoxy cure for dog bone molds above. M0 refers to the acid containing polymer DDT/EGDMA/MA described above. Stress-strain curves for the resultant resins are shown in Figure 16. Fig 16a refers to G0, Fig 16b refers to G50 and Fig 16c refers to G100. For each graph some traces represent the stress-strain curves for the cured dog bone mold and some traces represent the tested, ground and remolded dog bone mold. In these examples cyclohexanethiol is used as the CTA. Stress-strain curves are also shown for other resins in Figures 17 to 20. Figure 17 relates to the stress-strain curves of an epoxy/acid cure of DDT/UDMA/GMA (Polymer 1) and DDT/UDMA/MA (Polymer 4). This is a comparison to Fig 16c in that only UDMA is present in the polymer composition. As indicated in the legend, the solid lines refer to the stress-strain curves of the cured samples and the dashed lines refer to the stress-strain curves of the tested, ground and remolded samples of previous. This demonstrates that it is possible to measure the stress-strain of a cured sample of TBRT polymer, test it, grind it, remold and the mechanical properties are retained. In Figure 18, Polymer 5 refers to a polymer with composition DDT/TG/UDMA/GMA at molar ratios of 0.8:0.2:1:1 respectively. This was to investigate if it was possible to add hydroxy groups and form a cured polymer, Polymer 5, and then test, grind and remold it, dashed line. Figures 19 & 20 demonstrate stress-strain curves for the cured samples described above and made in accordance with the tables below but do not contain and stress-strain curves for remolded specimens. The resins corresponding to Figure 19, and the monomer systems used to make them, were as follows:

The resins corresponding to Figure 20, and the monomer systems used to make them, were as follows: In conclusion, the present results indicate physical property characteristics and demonstrate recycling possibilities. Evidence of bond exchange is seen in stress relaxation experiments with the variation of temperature allowing the behaviour to be studied under different conditions and bond energies (of the exchanging bonds) calculated from Arrhenius methods. Numerous polymer types have been prepared. Numerous cured materials have been prepared – these have been cured in different ways. Furthermore there is evidence of insolubility and complete reaction of epoxies (IR data) Properties of cured materials are shown in photographs of cured materials. These can be ground into powder, recured and ground/cured again. Tensile data show the recovery of properties and interesting strain-to-break values across cured and recured materials. Further experiments relating to flexible thermosets, multiple recycling and shape memory behaviour Further series of experiments were carried out. Flexible thermosets The recyclable thermoset resins of the present invention may optionally be flexible thermosets. A polymer with a composition of 1:1:1 of dodecanthiol: diurethane dimethacrylate: glycidyl methacrylate was prepared via TBRT. T _g was measured at -1 °C by DSC. The polymer was partially cured by heating via hotplate to 250 °C for -10 minutes to increase the T _g in order to add sample to vacuum compression mould. Polymer was added to vacuum compression mould and heated to 250 °C at 1 bar for 30 minutes to produce thermoset dog bone specimens (Fig.25). Dog bone specimens were flexible and could be easily manipulated (Fig.26). Dog bone specimens were then subjected to tensile strength measurements using a universal testing machine to ascertain Young's modulus and elongation at break (Fig.30). Recycling 1 Previous tensile strength test produced broken dog bone specimens (Fig.27). Broken specimens were re-inserted into dog bone mould and healed to 250 °C at 1 bar for 30 minutes using vacuum compression to heal broken specimens and regain dog bone (Fig.28). Recycled dog bone specimens were then subjected to tensile strength measurements using a universal testing machine to ascertain Young's modulus and elongation at break (Fig.31). Recycling 2 Broken specimens from previous recycled dog bones were again re-inserted into mould and heated to 250 °C at 1 bar for 30 minutes using vacuum compression to heal broken specimens and regain dog bone (Fig.29). Doubly recycled dog bone specimens were then subjected to tensile strength measurements using a universal testing machine to ascertain Young's modulus and elongation at break (Fig.32). Shape memory behaviour A cured specimen (Fig.33) was heated in a set conformation in a water bath at 50 °C for 20 minutes then left to cool to room temperature to produce temporary shape (Fig.34) The temporary shape was put back into 50 °C water bath and original shape (Fig.33) was regained after -5 seconds and left to cool to room temperature (Fig.35). Specimen was then heated in set conformation again to 200 °C for 20 minutes using a hotplate and allowed to cool to room temperature to produce a permanent shape (Fig 36). The permanent shape was then placed in 50 °C water bath for 5 minutes and left to cool to room temperature to produce (Fig.37).