Here we report the reversible addition-fragmentation chain transfer (RAFT) polymerization of acrylated epoxidized soybean oil (AESO), a cross-linker molecule, to high conversion (>50%) and molecular weight (>100 kDa) without macrogelation. Surprisingly, gelation is suppressed in this system far beyond the expectations predicated both on Flory-Stockmeyer theory and multiple other studies of RAFT polymerization featuring cross-linking moieties. By varying AESO and initiator concentrations, we show how intra- versus intermolecular cross-linking compete, yielding a trade-off between the degree of intramolecular linkages and conversion at gel point. We measured polymer chain characteristics, including molecular weight, chain dimensions, polydispersity, and intrinsic viscosity, using multidetector gel permeation chromatography and NMR to track polymerization kinetics. We show that not only the time and conversion at macrogelation, but also the chain architecture, is largely affected by these reaction conditions. At maximal AESO concentration, the gel point approaches that predicted by the Flory-Stockmeyer theory, and increases in an exponential fashion as the AESO concentration decreases. In the most dilute solutions, macrogelation cannot be detected throughout the entire reaction. Instead, cyclization/intramolecular cross-linking reactions dominate, leading to microgelation. This work is important, especially in that it demonstrates that thermoplastic rubbers could be produced based on multifunctional renewable feedstocks.
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.
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