Enzymatic synthesis of bio-based polyesters derived from vanillin

Aromatic polymers are known for their thermal stability as well as their good mechanical properties, but most of these polymers are synthesised from fossil resources. Vanillin is one of the few aromatic chemicals that is currently commercially produced from biomass and can be derivatised to make it suitable for polycondensation reactions. In this work a vanillin-derived bio-based diol was synthesised exploiting the potential of more benign reagents to replace toxic dimethylformamide. The resulting monomer was utilised in a series of enzymatic polycondensation reactions with different diesters including dimethyl succinate (DMSu), dimethyl adipate and dimethyl sebacate (DMSe), and the aromatic monomers diethyl terephthalate diethyl isophthalate diethyl pyridine-2,5-dicarboxylate (PD25) and diethyl pyridine-2,4-dicarboxylate (PD24) using a lipase to produce semi-aromatic polyesters. The molecular weight of the resulting polyesters increased as the chain length of the diester decreased, with a number average molecular weight of 21.8 kDa for polyesters achieved based on the dimethyl succinate (using diphenyl ether (DPE) as the organic media). For semi-aromatic diesters, para-substituted monomers yielded higher molecular weight polymers compared to the corresponding meta-substituted structure. Several different green solvents were also investigated to carry out this reaction with anisole that resulted to be a good alternative to diphenyl ether with similar number average molecular weights obtained at certain conditions.