# A Review on Styrene Substitutes in Thermosets and Their Composites
## 3 Novel Monomers Synthesized from Renewable Materials
### Grafting Methacrylate  
Methacrylic group is another functionality that was used to graft on renewable monomers. Methacrylic acid is one of the simplest approaches for equipping monomers with C=C bonds by esterification. The functionalization of epoxidized cardanol and soybean oil with methacrylic groups were directly conducted with methacrylic acid via an epoxy-acid esterification (Figure 4) [48]. The methacrylic monomers were copolymerized with methacrylated dicyclopentadiene prepolymer; the obtained VE networks presented a \(T_{\text{g}}\) from 100 to 130 \({}^{\circ}\)C. [PERSON] et al. prepared cardanol-based thermosetting resin by using methacrylated cardanol (MC) as RD for VE resin [49]. The increasing addition of cardanol-based RD from 0 to 50 wt % significantly decreased the viscosity of both commercial and synthesized fully biobased VE resins, but its long chains weakened the crosslink network and mechanical performance. Methacrylate anhydride (MA) is another commonly used monomer for esterification with alcohols. Take a simple example: similar to the route of using methacrylic acid, MA was utilized to graft double bonds onto cardanol for the synthesis of MC as a RD for commercial and synthesized VE (Figure 5a) [50]. [PERSON] et al. [51] synthesized sobrerol methacrylate (SobMet) from sobrerol with MA (Figure 5b), and the new monomer is an interesting alternative for styrene in UPE formulations. SobMet-based resins presented higher \(T_{\text{g}}\) and comparable storage modulus than styrene-containing resins. Hence, SobMet exhibited promising properties to be used as a low-volatile, biobased styrene substitute. Noteworthy, rigid isosorbide could be synthesized through cellulose and starch hydrolysis to glucose followed by hydrogenation to sorbitol and subsequent dehydration to isosorbide. In our previous research, isosorbide was methacrylated and copolymerized with flexible AESO to gain the resins with higher flexural properties, thermal properties, crosslinking density and curing efficiency (Figure 5c) [52]. The achieved resins were further used as matrices for hemp fibers and bamboo fibers to prepare green biocomposites [62]. Results indicated that the resulting composites obtained comparable performance to petroleum-based composites. Figure 4: Synthesis of biobased methacrylates from epoxidized cardanol and soybean oil with methacrylic acid [48]. Notably, as novel biobased RDs for thermosets, MA-methacrylated lignin derivatives (Figure 6) became very attractive during past 10 years. This is because lignin is the second most abundant polymer in nature, which is only less than cellulose. New catalytic methods for the efficient production of aromatic monomers from lignin are usually classified as: (1) isolation of lignin by lignocellulosic fractionation prior to catalytic treatment, (2) reductive catalytic fractionation using lignocellulosic in the presence of a catalyst, and (3) complete conversion of all lignocellulosic components by a one-pot catalytic process [63]. Especially, under the reductive depolymerization, the yield of the product is greatly increased; many monomers including phenol, guaiacol, eugenol and catechol could be achieved [64]. [PERSON] and his fellows used MA to modify vanillin, guaiacol, and eugenol to generate methacrylated lignin model compounds for VE resins. All the lignin-based resins had relatively high \(T_{\text{g}}\), low viscosity, and low VOC emission, which meets the requirements for liquid molding technique [53]. Furthermore, as a byproduct of esterification between MA and vanillin, methacrylic acid was further used to react with glycerol methacrylate for forming a liquid copolymerized crosslinking agent, i.e., glycerol dimethacrylate [65]. Additionally, a series of methacrylated lignin-based bio-oil mimics were comprised of methacrylated phenol, guaiacols and catechols and utilized as low viscosity VE resins and RDs in typical VE resins [54]. Further work was conducted on the influence of impurities in methacrylated phenolic compounds from lignin [66].