The cationic copolymerization of native or conjugated fish oil (FO or CFO) with divinylbenzene (DVB), norbornadiene (NBD) or dicyclopentadiene (DCP) comonomers initiated by boron trifluoride diethyl etherate (BF3.OEt2) was conducted in an effort to develop useful biodegradable polymers with rationally designed structures from natural renewable resources. Polymers ranging from rubbers to hard plastics have been obtained. H-1 NMR,C-13 NMR, solid state C-13 MMR, differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA) have been used to characterize the structures and physical properties of the fish oil polymers. The results indicate that the products are typical thermosetting polymers with densely crosslinked structures. These polymer materials have densities of approximately 1000 kg/m(3) which appear to be independent of their compositions. The structure of the bulk polymer is composed of 40-85 wt% of crosslinked polymer networks plasticized by 15-60 wt% of unreacted free oil. The moduli of the FO (similar to 0.8 GPa) and CFO (similar to 1.1 GPa) polymers at room temperature are found to be comparable to those of conventional petroleum-based plastics. The glass transition temperatures of the polymers range from 50 to 150 degrees C with a broad transition region. Thermogravimetric analysis (TGA) indicates three distinct decomposition temperature regions, i.e. 200-400 degrees C, 400-560 degrees C, and 560-800 degrees C, which correspond to evaporation of the unreacted free oil, carbonization of the crosslinked polymer network, and oxidation of the carbon, respectively. The thermal stability of these materials is found to be largely dependent on the amount of unreacted free oil molecules present in the bulk polymers. The CFO polymers have higher moduli and thermal stability than the FO polymers due to the lower amount of free oil present in the CFO bulk polymers.