Lipase-catalyzed synthesis of an epoxy-functionalized polyester from the suberin monomer cis-9,10-epoxy-18-hydroxyoctadecanoic acid.

The synthesis of polymers with tailored reactive functionalities plays an important role in the development of various materials for industrial and biomedical applications. In addition, there is a need to develop chemistry that is based on the use of biodegradable and renewable resources.1 Appreciable amounts of natural polyesters occur in higher plants as structural components, for example, cutin in the cuticle that covers the aerial parts of plants and suberin in the cork cells present in the periderm of woody plants. Common aliphatic monomers in these polyesters are long-chain ω-hydroxyalkanoic acids and R,ω-alkanedioic acids, usually together with small amounts of substituted ω-hydroxyalkanoic acids such as cis9,10-epoxy-18-hydroxyoctadecanoic acid (1).2 The monomer composition is usually complex and differs between plant species, and this complexity probably explains why the use of cutin and suberin as a commercial source of “green” chemicals has not been well explored. One exception is the suberin in the outer bark of birch, where cis-9,10-epoxy-18-hydroxyoctadecanoic acid (1) is the principal monomer amounting to about 100 g/kg dry outer bark and 40% of the aliphatic suberin monomers in Betula Verrucosa.3 This epoxy acid can be isolated in high yield from alkali-hydrolyzed outer bark, a low value byproduct in the forest industry mainly used as fuel, by extraction followed by selective precipitation by acidification.4,5 Lipase-catalyzed polymerization may sometimes allow straightforward synthesis strategies for polyesters, which are difficult to prepare by more conventional polymerization processes, and has been used, for example, for the preparation of polyesters from epoxy-containing monomers. Activation of the substrate is often used to increase the yield and molecular weight in the polymerization. Common examples are activation of diacids by esterification and hydroxy acids by lactonization.6-8 However, this requires further reaction steps and must be considered as a less attractive alternative for commercial explorations. Recently, it was reported that suberin-related long-chain linear 9,10-epoxidized C18, C20, and C26 R,ω-dicarboxylic acid dimethyl esters can be polymerized with diols using Candida antarctica lipase B (Novozym 435), still keeping the epoxy groups intact.7 It has also been shown that linear C10 to C16 aliphatic ω-hydroxy acids can be polymerized efficiently without activation.9 In this study, we report the Novozym 435-catalyzed condensation polymerization of cis-9,10-epoxy-18-hydroxyoctadecanoic acid (1) isolated from the outer bark of Betula Verrucosa to form poly(9,10-epoxy-18-hydroxyoctadecanoic acid) (2) and compare the reaction with the corresponding polymerization of 10-hydroxydecanoic acid (Figure 1).