Understanding the limitations of the solvent-free enzymatic synthesis of sorbitol-containing polyesters

\u3cp\u3eEnzymatic catalysis is an attractive approach toward the synthesis of sustainable polyesters, which also provides advantages in terms of selectivity compared to conventional methods. Furthermore, the use of immobilized enzymes allows for solvent-free, ecofriendly polycondensation routes, but also leads to some limitations in terms of applicability to certain systems. A systematic study has been performed on the synthesis of close to linear aliphatic polyesters from biobased, commercially available sorbitol, 1,10-decanediol, and dimethyl adipate. Polycondensation reactions were carried out in the melt using SPRIN liposorb CALB (trade name for the immobilized form of Candida antarctica lipase B) as catalyst, targeting a number-average molecular weight between 4 and 6 kg/mol, and an amount of pendant and terminal hydroxyl groups within the range commonly used for coating applications. The efficacy with which the increasing amounts of sorbitol were built into the polyester backbone was studied in detail via \u3csup\u3e13\u3c/sup\u3eC NMR spectroscopy. In addition, the particular selectivity for primary vs secondary hydroxyl groups of the biocatalyst was confirmed via \u3csup\u3e31\u3c/sup\u3eP NMR spectroscopy. Extensive structural characterization was carried out via MALDI-ToF-MS analysis, which also provided further insights into limitations of the system related to sorbitol incorporation. Differential scanning calorimetry and X-ray diffraction analysis revealed that the melting temperature and crystallinity of the materials are lower when increased amounts of sorbitol are incorporated into the polyesters.\u3c/p\u3