Catalyst- and solvent-free "click" chemistry: a facile approach to obtain cross-linked biopolymers from soybean oil.

A series of highly cross-linked biopolymers (1-10) was obtained by the copper-catalyzed and the thermal polyaddition of alkynated and azidated soybean oil with suitable diazides and diynes, respectively. Thermal polymerization (heating at 100 °C), which requires no catalyst and no solvent, was observed to be a superior approach, yielding polymers (6-10) with more homogeneous cross-linking. The temperature of decomposition of 6-10 was narrower (∼170 °C) than that of the polymers (1-5) obtained by the copper-catalyzed method (∼210 °C). The glass-transition temperatures of 1-5 were higher (Tg ranging from 9 to 80 °C) than those of the comparable polymers obtained thermally (Tg ranging from -13 to 45 °C) because of the catalyst entrapped in the networks of 1-5. Furthermore, the thermal approach requires less time and is higher yielding, establishing the suitability and ease of polymerization of vegetable oil-derived alkynes or azides through thermal "Click" chemistry. The effects of the structure of the monomers and the nature of the linkers on the thermal properties of 1-10 (e.g., Tg and decomposition temperatures) are detailed.