In this study, maleinized soybean oil triglycerides (SOMA) were reacted with epoxidized soybean oil triglycerides (ESO) to give plant oil-based thermoset polymers. To increase fracture toughness of the product, different amounts of SOMA was replaced by maleic anhydride grafted polypropylene (MMPP) (Epolene E43, maleate content 2.9%, M-n = 3900 and Polybond 3200, maleate content 1%, M-n = 1,10,000). The improvement in mechanical properties was monitored. The characterizations of the products were done by DMA, DSC, TGA, and IR spectroscopy. ESO-SOMA and ESO-SOMA-MMPP polymers are crosslinked rigid infusible polymers. ESO-SOMA-MMPP(E43) and ESO-SOMA-MMPP(PB3200) showed a phase change at 146 and 169 degrees C, respectively, probably due to the melting transition of the MMPP backbone. Storage moduli of the two polymers at 35 degrees C were 54.6 and 246.1 MPa, respectively. Storage moduli and the impact strength of the polymers increased with MMPP content and with MMPP molecular weight. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 3311-3317, 2010