Effect of Fatty Acid Saturation and Side-Chain Length on the Thermal Behavior and Morphology of Plant Oil-Based Acrylic Polymers

In this work, saturated plant oil-based acrylic monomers (POBMs) derived from coconut oil (CCM), babassu oil (BBM), and fully hydrogenated soybean oil (HG SBM) were investigated as building blocks for semicrystalline biobased polymers. The effects of fatty acid saturation and side chain length on free radical polymerization, copolymerization, and polymer morphology were evaluated. Increasing fatty acid saturation suppressed allylic chain transfer reactions, resulting in lower chain transfer constants and higher polymer molecular weights than those of POBMs with higher degrees of unsaturation. Copolymerization of CCM with styrene yielded reactivity ratios of rCCM = 0.44 and rSt = 0.96 and Alfrey–Price parameters of Q = 0.49 and e = 0.03, demonstrating favorable reactivity of saturated POBMs toward conventional vinyl comonomers in chain copolymerization. Thermal and structural analyses revealed a pronounced dependence of polymer morphology on fatty acid side chain structure. Polymers containing predominantly C12 side chains exhibited limited molecular ordering, whereas highly saturated C16–C18 side chains produced pronounced melting transitions and substantially increased crystallinity. Specifically, semicrystalline morphologies were observed in HG SBM-based (co)polymers with degrees of unsaturation of approximately 4 mol% or lower. Poly(HG SBM) exhibited an enthalpy of fusion of 78 J g−1 and degree of crystallinity of 37%, compared with 67 J g−1 and 26%, respectively, for poly(HG SBM-co-BBM). These findings establish fatty acid saturation and side chain length as complementary molecular design parameters for controlling side chain packing, and morphology in POBM-based polymers. The ability to generate semicrystalline materials from plant oil-derived acrylic monomers provides a strategy for extending the POBM platform toward fully biobased thermoplastic materials.