Correlation of Enzymatic Depolymerization Rates with the Structure of Polyethylene-Like Long-Chain Aliphatic Polyesters

Abstract

Long-chain aliphatic polyesters are emerging sustainable materials that exhibit polyethylene-like properties while being amenable to chemical recycling and biodegradation. However, varying polyester chemical structures results in markedly different degradation rates, which cannot be predicted from commonly correlated bulk polyester properties, such as polymer melting temperature. To elucidate these structure-degradability relationships, long-chain polyesters varying in their monomer composition and crystallinity were subjected to enzymatic hydrolysis, the rates of which were quantified via detection of formed monomers. Copolymers with poorly water-soluble, long-chain diol monomers (e.g., 1,18-octadecanediol) demonstrated strongly reduced depolymerization rates compared to copolymers with shorter chain length diol monomers. This was illustrated by, e.g., the 20× faster hydrolysis of PE-4,18, consisting of 1,4-butanediol and 1,18-octadecanedicarboxylic acid monomers, relative to PE-18,4. The insoluble long-chain diol monomer released upon hydrolysis was proposed to remain attached to the bulk polymer surface, decreasing the accessibility of the remaining ester bonds to enzymes for further hydrolysis. Tuning of polyester crystallinity via the introduction of branched monomers led to variable hydrolysis rates, which increased by an order of magnitude when crystallinity decreased from 72% to 45%. The results reported enables the informed design of polyester structures with balanced material properties and amenability to depolymerization.

Scheme 1. Synthesis of (a) Branched and (b) Linear Polyesters

Figure 1

Enzymatic hydrolysis of branched aliphatic polyester films incubated in phosphate buffer (pH = 8.5) with Humicola insolens Cutinase at 37 °C. Amounts of degradation were quantified by determinations of C₁₈-dicarboxylate monomer concentrations using LC-MS. The enzymatic hydrolysis of PE-18,18_25br after 5 days (2.7 g m^–2) corresponds to a hydrolysis of 18 wt % of the overall material.

Figure 2

Enzymatic hydrolysis by Humicola insolens Cutinase of linear aliphatic polyester films at 37 °C in phosphate buffer (pH = 7.2). Amounts of degradation were determined by quantification of monomers using LC-MS or HPLC-RI, depending on the monomer detected.