Novel thermosetting copolymers, ranging from tough and ductile to very soft rubbers, have been prepared by the cationic copolymerization of regular (SOY) and 100% conjugated soybean oils (C100SOY) with dicyclopentadiene (DCP) catalyzed by Norway fish oil (NFO)-modified and SOY- and C100SOY-diluted boron trifluoride diethyl etherate (BFE). The gelation time of the reactions varies from 4 to 991 min at 110 degrees C. The yields of the bulk copolymers are essentially quantitative, while the yields of the cross-linked copolymers remaining after Soxhlet extraction with methylene chloride range from 69% to 88%, depending on the monomer stoichiometry and the catalyst used. H-1 NMR spectroscopy and Soxhlet extraction data indicate that these copolymers consist of a cross-linked soybean oil-DCP network plasticized by certain amounts of methylene chloride- soluble linear or less cross-linked soybean oil-DCP copolymers, unreacted oil, and some low molecular weight hydrolyzed oil. The molecular weights of these soluble fractions are in the range from 400 to 10 000 g/mol based on polystyrene standards. The bulk copolymers have glass transition temperatures ranging from -22.6 to 56.6 T, while their tan delta peak values range from 0.7 to 1.2. Thermogravi metric analysis (TGA) indicates that these soybean oil-DCP copolymers are thermally stable below 200 degrees C, with 10% and 50% weight loss temperatures ranging from 280 to 372 degrees C and 470-554 degrees C, respectively. These properties suggest that these biobased thermosets may prove useful alternatives to current petroleum-based plastics and find widespread utility.