Synthesis and Polymerization of a Lactone Building Block Derived From Cashew Nut Shells

The biorenewable feedstock cardanol, a phenol compound substituted with an unsaturated C15 hydrocarbon chain, was selectively transformed into a cyclohexanone derivative through partial hydrogenation. Subsequent conversion via Baeyer-Villiger oxidation (BVO) gave a lactone product (pentadecylcaprolactone, PDCL), which is a mixture of two regioisomers. For the BVO, two variants that are based on either the direct use of a perbenzoic acid as reagent or alternatively the use of a readily available aldehyde in the presence of molecular oxygen as an attractive oxidation reagent have been studied and turned out to be feasible. In addition, suitable work-up has been demonstrated that furnished a sample of the cardanol-derived PDCL-type lactone in 84% isolated yield with 99% purity. Using metal-free polymerization techniques, a PDCL sample has then been readily converted into its homopolymer, which displays a low crystallinity, the latter due to moderate side-chain crystallization. Importantly, in a statistical copolymer with ε-caprolactone (CL), PDCL was shown to be a useful agent to modulate crystallinity, resulting in copolyesters with reduced or very minor crystallinity and accordingly low melting points. For such an effect, typically expensive and fossil-based monomers such as δ-valerolactone (VL) have to be used, highlighting the potential of PDCL for practical application.