Biocatalytic Polytransesterification of Inulin with Divinyladipate

Enzymatic synthesis of polymers has attracted significant attention in recent years1 because of high inherent selectivity under mild reaction conditions. A wide range of polymers has been synthesized using purely enzymatic means, including polyphenols,2 polyesters,3 and polycarbonates.4 Although the vast majority of polymers have been prepared from rather simple monomers, enzymes offer the opportunity to incorporate complex polyfunctional compounds, such as sugars and polysaccharides, into polymer backbones,5 thereby extending the synthetic repertoire of polymer chemistry. In the current work, we report the enzyme-catalyzed polytransesterification of inulin with divinyladipate (DVA) in DMF to produce inulin polyesters. Inulin is composed by a mixture of oligomers and polymers containing 2 to 60 (or more) â 2-1 linked D-fructose molecules having a glucose unit as the initial residue.6 Six proteases and five lipases, all commercially available, were tested for their abilities to catalyze the polytransesterification of inulin with DVA in anhydrous DMF (Scheme 1), at 50 °C, for 72 h (Table 1).7 There was significant variation in the inulin conversion (based on consumption of native inulin) and molecular weight8 obtained as a function of the enzyme, but in all cases the products were water soluble. Proleather, an alkaline protease from Bacillus subtilis, showed the highest conversion.9 The number average molecular weight (Mn) of the polymer formed was also influenced by the source of the enzyme, and this was mainly due to the extent of reaction conversion, an expected finding given the mechanism of AA-BB polycondensation reactions.10 The relatively high polydispersities are expected with such a mechanism given the large size of the inulin “monomers” in the polymerization reaction. The polymer obtained using Proleather consisted of ca. 3-4 inulin molecules linked through adipate moieties, yet remained water soluble, indicating that it was not heavily crosslinked.