2-oxepane-1,5-dione: a precursor of a novel class of versatile semicrystalline biodegradable (co)-polyesters

2-oxepane-1,5-dione (OPD) was synthesized by Baeyer-Villiger oxidation of 1,4-cyclohexanedione and ring-opening polymerized by 1-phenyl-2-propanol / tin octoate with formation of a semicrystalline polyester of high melting temperature (Tm: 147°C). A crystallographic study showed that the polyester is orthorombic in the P212121 space with two molecules extended along the c-axis as is the case for poly(ε-caprolactone) (PCL). Dibutyltin dimethoxide and 2,2-dibutyl-2-stanna-1,3-dioxepane (DSDOP) proved to be efficient initiators for the random copolymerization of ε-caprolactone (εCL) and OPD. Random distribution of the comonomer units was confirmed by DSC, 1H and 13C NMR analyses. A single Tm was observed, which varies regularly and monotonically with the OPD content as result of a cocrystallisation phenomenon. The single glass transition temperature (Tg) obeys the Fox equation. It is noteworthy that a copolymer containing 30 mol % OPD exhibits a Tm of 90°C. It is stable up to 140°C under nitrogen for one hour, such that processing by extrusion is quite possible. The OPD decomposition is maximum at 223°C for a heating rate of 10°C/min. The mechanism of thermal degradation was investigated and cleared up. The ketone increases the copolymer hydrophilicity and thus sensitivity to hydrolysis. The mechanical properties are quite reminiscent of PCL of comparable molecular weight. Quite interestingly, OPD imparts sensitivity to photo-cross-linking to this novel class of materials.