Two different approaches to the creation of phosphorus-containing soybean-oil copolymers were investigated. First, two phosphorus-containing styrene (ST) derivatives, diphenyl styryl phosphine oxide and dimethyl-p-vinylbenzylphosphonate (STP2), where tested as comonomers in the cationic copolymerization of soybean oil (SOY), ST, and divinylbenzene (DVB), to obtain heterogeneous systems in all cases. To overcome this drawback, the cross-metathesis reaction of methyl 10-undecenoate and STP2 was carried out to link the phosphorus moiety to the vegetable-oil derivative. This second approach permitted the synthesis of a new reactive phosphorus-containing plant-oil derivative, which was incorporated into the soybean oil, ST, and DVB system. The cationic copolymerization was investigated, and the structure, thermal stability, and mechanical and flame-retardant properties of the resulting copolymers were studied. Thermosets with moderate glass-transition temperatures were obtained; this showed that the cross-metathesis reaction is a convenient way to produce oil-compatible monomers able to undergo homogeneous polymerization reactions. The resulting thermosets with 1% phosphorus had limiting oxygen index values about 24.0; this indicated an improvement in the fire-retardant properties of the soybean-oil-based copolymers. (C) 2011Wiley Periodicals, Inc. J Appl Polym Sci 122: 1649-1658, 2011