Epoxidized soybean oil was reacted with cinnamic acid with triphenyl phosphine as a catalyst. Cinnamic acid reacted with 79% of the available epoxy groups, and this yielded cinnamate esters of epoxidized soybean oil (ESOCA). H-1-NMR, IR, and mass spectra of the new cinnamate derivatives confirmed the proposed structure. The mass spectra revealed that the average number of cinnamate groups per triglyceride molecule was 3.33. ESOCA could be photopolymerized with UV light. ESOCA could also be homopolymerized into a soft and insoluble polymer by free-radical initiation and copolymerized with styrene, vinyl acetate, and methyl methacrylate. A mixture of ESOCA with 25 wt % styrene had a viscosity of 410 cP and could be free-radically polymerized with benzoyl tert-butyl peroxide at elevated temperatures. Differential scanning calorimetry confirmed the formation of copolymers. The ESOCA homopolymer and its copolymers all showed a first-order transition by differential scanning calorimetry around -1.5degreesC that was attributable to side-chain relaxations of the triglyceride fatty acids. The styrene copolymer of ESOCA showed a tan delta peak at 66.6degreesC. (C) 2003 Wiley Periodicals, Inc.