Acidity and Polarity – Overcoming Challenges of the Enzyme Catalyzed Polycondensation of Adipic Acid and 1,4-Butanediol

Using enzymes in polycondensation reactions poses several advantages in terms of sustainability like reduced temperatures and limited side reactions which allows the synthesis of new polymers. The prediction of the enzymes’ performance is challenging due to its sensitivity to monomer selection and reaction conditions. Since reported results on short-chain dicarboxylic acids and diols often showed low conversions without conclusive explanations, a systematic investigation of the apparently simple polycondensation between adipic acid and 1,4-butanediol was carried out. Screening of reaction temperatures revealed that the performance of Candida antarctica lipase B immobilized on Immobead 150 increased from 30 to 50 °C until deactivation took place at 60 °C. While the diol component did not significantly influence the reactions of short-chain adipic acid esters, the solubility of the free adipic acid in the diol increased with decreasing diol length and rising temperatures. Enzyme deactivation was thus primarily ascribed to the acidity of the reaction medium which is both polarity- and temperature-dependent. Due to low molecular masses after 24 h reaction time using isothermal conditions, the equilibrium was shifted by temperature elevation and pressure reduction in a second step. The formation of oligoesters, in which the monomers were consumed, resulted in less acidic media thus preventing enzyme deactivation even at 60 °C. Increasing the reaction temperature from 40 to 60 °C combined with reducing the pressure led to poly(butylene adipate) samples with average weight molecular weights above 20,000 g mol ⁻¹ using a 1:0.95 ratio of diol and dicarboxylic acid.