A “transient-comonomer” strategy for polycondensation of lignin-derived AB-type aromatic polyesters to high molecular weight

In the active exploration of novel biobased semicrystalline polyesters to replace petrochemical plastics, AB-type aromatic condensation polymerizations are relatively rarely investigated but gain growing attention due to the straightforward synthesis of AB-type monomers from diverse asymmetric biobased feedstocks. However, achieving high molecular weights through AB-type melt polycondensations is often challenging due to premature crystallization and limited thermal stability of the bio-based monomers. Herein, we investigate the polycondensation of methyl 4-(2-hydroxyethoxy)benzoate (MEB), a biobased aromatic hydroxy-ester monomer (AB-type), via the classical two-stage melt transesterification polycondensation process. We demonstrate an efficient “transient-comonomers” strategy to enhance the molecular weight of the obtained polymer PEB corresponding to an intrinsic viscosity of 1.15 dL g−1 by adding a small amount of an aliphatic AA/BB-type comonomer pair at the beginning of the polymerization. These comonomers suppress premature solidification during the initial stages of the melt polycondensation, and are partially depleted during the later stages of the reaction. In addition, we investigated the effects of various metal-based catalysts, including a kinetic analysis of the transesterification steps, which revealed that titanium-based catalysts produced a faster reaction rate compared to other catalysts based on antimony, tin, manganese, and zinc. Furthermore, the catalyst also influenced the thermal properties of the resulting polyester, particularly the crystallinity and crystallization kinetics. Overall, this study provides new insights into achieving high-molecular-weight biobased aromatic polyesters via AB-type polycondensation and offers a promising approach to broaden the synthesis approaches for sustainable condensation polymers.