Softwood Lignin-Based Methacrylate Polymers with Tunable Thermal and Viscoelastic Properties

Softwood (guaiacylic) lignin-based methacrylate polymers (LBMPs) that exhibit excellent glass transition temperatures (T g’s), desirable thermal stabilities (greater than 100 °C above T g), and intermediate shear-flow resistances, in comparison to polystyrene and poly­(methyl methacrylate), are reported herein. Different R-groups (p-position hydrogen, methyl, ethyl, and formyl groups) in otherwise homologous LBMPs impart distinct characteristics to the flow behavior and thermal properties of these bio-based polymers, which permit the investigation of unique structure–property relationships. More specifically, the zero-shear viscosities (η0’s) for the LBMPs span nearly 2 orders of magnitude as the R-group is varied, while the characteristic degradation temperatures differ more modestly (by ≈50 °C over the same series of polymers), and the T g’s exhibit minimal, yet application relevant, variations between ≈110 and ≈130 °C. These property differences were probed independent of tacticity, molecular weight, and dispersity effects due to the nature of the well-controlled macromolecules generated via reversible addition–fragmentation chain-transfer polymerization. Furthermore, heteropolymers prepared from mixtures of the lignin-based monomers have composition-dependent T g’s and component-dependent thermal degradation temperatures, thermolysis rates, and η0’s. The multicomponent materials demonstrate the enhanced tunability inherent in LBMPs. Altogether, this versatile library of softwood lignin-based monomers, and the unique structure–property relationships intrinsic to the resulting polymers, provides a unique platform for building potentially low-cost, high-performance, and bio-based viscoelastic materials attractive for thermoplastic elastomer and binder applications.