Enzymatic synthesis of reversibly crosslinkable polyesters with pendant mercapto groups

Reversibly crosslinkable aliphatic and aromatic polyesters with free pendant mercapto groups were prepared by lipase-catalyzed polycondensation of various diols and dimethyl mercaptosuccinate. The reactions were carried out using immobilized lipase from Candida antarctica (CALB) in an organic solvent at 70 °C to produce the polyesters with Mw = 17000-27000 g mol−1 in high yields without any reaction at the mercapto group. The polyesters were easily crosslinked to form gels or films by air oxidation of the pendant free mercapto groups in dimethyl sulfoxide, which constructed disulfide bonds. The crosslinked polyesters were readily decrosslinked by reduction with tributylphosphine to regenerate polyesters with free mercapto groups almost quantitatively. For chemical recycling, the polyesters with free mercapto groups were depolymerized into cyclic oligomers, which were repolymerized to form polyesters with the same Mw and polymer structure as the initial polymers. Some physicochemical properties of the crosslinked polyesters were evaluated.