Controlled Radical Polymerization of Crude Lignin Bio-oil Containing Multihydroxyl Molecules for Methacrylate Polymers and the Potential Applications

In this work, methacrylate polymers with different thermal and viscoelastic properties were synthesized from red oak lignin bio-oil. The bio-oil, also called pyrolytic lignin (PL), consisted of various phenolic monomers and oligomers with average hydroxyl content of 3.04 mol/mol. The PL was first esterified with different amounts of methacryloyl chloride and acetyl chloride to form PL methacrylates and then subjected to reversible addition–fragmentation chain transfer polymerization. Polymerization of fully methacrylated PL caused gelation to yield a cross-linked polymer. On the other hand, gel-point suppression occurred in the polymerization of partially methacrylated PL to yield a thermoplastic polymer with glass transition temperature (T g) of 161 °C and thermal decomposition temperature (T d) of 241 °C. In comparison, the functionalization of PL by partial methacrylation and subsequent acetylation resulted in a polymer with T g of 130 °C and T d of 250 °C. Unlike other biobased methacrylate polymers that cannot withstand high temperatures and volatilize, the polymers produced from this study retained 25–28% mass when pyrolyzed to 1000 °C. The latter polymer was also melt-spinnable and demonstrated highly attractive properties as an ideal carbon fiber precursor. Other than its narrow molecular weight distribution and high isothermal stability, this lignin-based polymer also had a linear molecular orientation that is critical in producing high-quality carbon fiber.