Biopolymers from Vegetable Oils via Catalyst- and Solvent-Free “Click” Chemistry: Effects of Cross-Linking Density

New monomers were prepared by introducing the azide groups in castor, canola, corn, soybean, and linseed oils. Polymerization of the azidated oils with alkynated soybean oil under thermal “click” chemistry conditions (without using a solvent or a catalyst) yielded fully cross-linked elastomers (1–5) of almost the same density (1.05 × 10–3 kg/m3). The degree of cross-linking gradually increased from the castor-derived polymer (220 mol/m3) to the linseed-derived polymer (683 mol/m3). A systematic correlation between the degree of cross-linking and the thermal and mechanical properties was observed in these biopolymers. Tensile strength (0.62–3.39 MPa) and glass transition temperature (−5 to 16 °C) increased and the linear thermal expansion coefficient decreased in the series from the canola-derived polymer (2) to the linseed-derived polymer (5). The castor-derived polymer (1) that possesses an additional hydroxyl group per fatty acid chain behaved differently.