# A Review on Styrene Substitutes in Thermosets and Their Composites
## 2 Traditionally Crosslinkable Monomers
### (Methyl)acrylate Monomers  
(Methyl)acrylate monomers are commonly obtained from unrenewable resources. Generally, they have highly reactive (methyl)acrylate C=C bonds and relatively low molecular weight (Table 2).  
(Methyl)acrylates have been widely used in the production of coatings, elastomers, adhesives, thickeners, amphoteric surfactants, fibers, plastics, textiles and inks. Butyl methacrylate (BMA) can not only be used as crosslinking agent to reduce styrene content, but also toughen the obtained composites for its soft long molecular chains [2]. The incorporation of BMA into hemp fibers reinforced UPE composite systems significantly increased the toughness of UPE matrices and thus the resulting composites. As a highly polar monomer, hydroxypropylacrylate (HPA) was usually used to control the thermal shrinkage of UPE resin, but its high affinity with organic agent caused insufficient crosslinking, thus negatively affecting the mechanical properties of resins [32]. In a novel family of UPE derived from 2,5-furandicarboxylic acid and itaconic acid, 2-hydroxyethylethacrylate (HEMA) was selected for copolymerizing with such newly biobased UPE to improve its fluidity and crosslinking degree; results indicated that the increase of HEMA content led to the increases of gel content and \(T_{\text{g}}\) of UPE resins [33; 34]. However, HEMA contributed to the increase in swelling capacity of the resins, which was likely due to its high hydrophilicity. It is worth to mention that HEMA could be applied in the preparation of devices used in biomedical applications due to its favorable biocompatibility [35]. In the work of [PERSON] et al. [36], HEMA, isobornyl methacrylate (IBOMA) and methyl methacrylate (MMA) were used as RDs for itaconic acid-based UPE, respectively; results showed that these styrene-free biobased UPE resins had properties equivalent to commercial styrene-based resins. Therefore, these methacrylate could not only replace styrene to reduce resin viscosity, but also improve resin crosslinking density and provide feasible coating properties. However, segments of such monomers may set bond into 3D network and is easy to break down at lower temperature, and thus the HEMA-resulting thermosets are less thermally stable than pure UPE resins [33; 34; 36]. Also, BMA, HPA and HEMA had low boiling points and thus might easily evaporate during processing.  
[PERSON] et al. [37] investigated the free-radical polymerization kinetics of a multifunctional acrylate monomer, i.e., trimethylolpropantiacrylate (TMPTA) with UPE. Furthermore, TMPTA was further used as a RD for curable acrylateated polyester based hybrid coatings [38]. It was found that the fracture toughness of the composites is the highest when 15% TMPTA was incorporated. With similar chemical structures, the impact of unsaturated sites of RDs on AESO resin has also been discussed by comparing 1,4-butanediol dimethacrylate (BDDMA) and trimethylolpropane trimethacrylate (TMPTMA) in bamboo fibers reinforced AESO composites [39]. The results indicated that the methacrylate RDs with more double bonds endowed the resulting resins with higher crosslinking density. These methacrylate had high molecular weight and thus boiling point, which exhibits almost no VOC emissions during processing.