# A Review on Styrene Substitutes in Thermosets and Their Composites
## 3 Novel Monomers Synthesized from Renewable Materials
### Grafting Methacrylate
Results suggested that the substitutes on the aromatic ring of the RD have less effect, but the purity of the RD strongly influences the performance of materials. [PERSON] et al. used vanillin and vanillyl alcohol as feedstocks for reacting with MA. The obtained biobased and low viscosity monomers were blended with VE and maleinated AESO resins, respectively [55; 67]. The flexible long chains on AESO would compensate for the brittle drawback on methacrylated vanillin (MV) blended resins. The same method was used to synthesize methacrylated eugenol (ME) as a sustainable RD for maleinated AESO to produce resins with low viscosity, fast curing and superior mechanical performance that were suitable for pultrusion process [68; 69]. [PERSON] et al. [70] applied another lignin-based monomer, i.e., methacrylated guaiacol (MG), as a low emission crosslinking agent for curing with novel full biobased UPE resin. Due to the development of additive manufacturing, MG and vanillyl alcohol dimethacrylate are also used to mix with other acrylates for stereolithography 3D printing in recent years [71]. Figure 5: Synthesis of biobased reactive diluents (RDs) from (**a**) cardanol, (**b**) sobrerol, and (**c**) isosorbide via grafting with MA [50; 51; 52]. Glycidyl methacrylate (GMA) is a bifunctional monomer with epoxy group and C=C bond. The epoxy group of GMA gives the opportunity for endowing the target monomers unsaturation functionality through epoxy ring-opening reaction (Figure 7). Furan is a renewable monomer derived from pentose that is obtained from the hydrolysis of corn, oats, and hemicellulose. Both furfuryl alcohol and furoic acid were derived from furan. Furoic acid glycidyl methacrylate (FA-GM) was synthesized from GMA and furoic acid (Figure 7a) and then used as a RD for VE resins. Results indicated that the FA-GM monomer could improve the crosslinking density of resins, leading to admirable curing property with less usage of RD [56]. [PERSON] et al. [57] synthesized a UV-curable unsaturated monomer (IG) from itaconic acid with GMA (Figure 7b). IG could be used together with methylates and styrene to significantly improve the strength, modulus and thermal stability of AESO-based resins. Besides, IG was also used as RD for AESO and monomethyl itaconated epoxidized soybean oil for comparison with TMPTMA and styrene [72]. Figure 6: Novel RDs from lignin derivatives [53; 54; 55; 66]. Methacylated fatty acid (MFA) was obtained by reacting GMA with caprylic acid, caproic acid, oleic acid, linoleic acid and lauric acid from vegetable oil. Oleic acid was selected to present the typical reaction route as shown in Figure 7c. [PERSON] et al. [73] used methacylated fatty acid (MFA) as a RD for copolymerizing with maleinated AESO. However, due to the long chain characteristic of fatty acid, the \(T_{\text{g}}\) and storage modulus of the MFA-resulting resins were lower than those of styrene-based resins. Thus, it is still needed to blend with styrene to reduce resin viscosity and balance properties [74]. During the synthesis process of MFA, bromine atoms was grafted to form 9-10 dibromo stearic acid glycolidyl methacrylate (DSA-GMA), functionalizing the monomer with improved heating and flame resistance (Figure 7d) [3]. [PERSON] et al. reported that the MFA-based RDs were applied to improve the fluidity of resins, reduce the emission of volatile organic compound and keep the practicability of materials, which were gradually used in military application [75]. [PERSON] et al. also conducted a similar study to prepare MFA by changing the length of basic fatty acid chain [76]. It was found that the viscosity of VE/MFA resin increased with the increase of fatty acid chain length. Hydrogen bonds between MFA hydroxyl groups are affected by the chain length of fatty acids and play a crucial role in controlling the viscosity of the system. However, the \(T_{\text{g}}\) of VE/MFA resin increased with the decrease of fatty acid chain length, which was due to the decrease of free volume of polymer and effective molecular weight between crosslinks.