Epoxidized soybean oil (ESO) triglycerides were reacted with maleinized polybutadiene (MMPBD) to give plant-oil-based thermoset polymers. MMPBD samples were of two different molecular weights [high-molecular-weight maleinized polybutadiene (MMPBD-H), maleate content = 10%, number-average molecular weight (M-n) = 9000, and low-molecular-weight maleinized polybutadiene (MMPBD-L), maleate content 15%, M-n = 5000]. To increase the crosslink density of the product, a free-radical initiator, benzoyl peroxide, was added to this mixture to further crosslink MMPBD through its double bonds. The characterizations of the products were done by dynamic mechanical analysis, differential scanning calorimetry, thermogravimetric analysis, and IR spectroscopy. The ESO-MMPBD polymers were crosslinked rigid infusible polymers. ESO-MMPBD-H-1 : 1 and ESO-MMPBD-L-1 : 1 showed glass-transition temperature values at -23, 78 and -17, 64 degrees C, respectively, whereas the storage moduli of the two polymers at 25 degrees C were 13 and 16 MPa, respectively. The storage moduli of the polymers remained the same or decreased with the addition of a free-radical initiator. The storage moduli also decreased with increasing ESO concentration above a 1 : 1 epoxy-to-anhydride molar ratio. The surface hardness increased dramatically, and the equilibrium swelling ratio decreased with the addition of free-radical initiator. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 116-123, 2011