Structure–Property Relationships in Polyesters from UV-Initiated Radical Ring-Opening Polymerization of 2-Methylene-1,3-dioxepane (MDO)

Radical ring-opening polymerization (RROP) of 2-methylene-1,3-dioxepane (MDO) leads to a polymer that is structurally similar to poly(ε-caprolactone) (PCL). Despite the similarities, polymerized MDO (PMDO) is known to exhibit little or no semicrystallinity, which is often explained by the branching of PMDO caused by the free-radical polymerization. Branching in RROP can arise from intramolecular 1,7-H-transfer to result in short-chain branching or from intermolecular H-transfer, resulting in long-chain branching. In this study, we performed UV-initiated RROP to isolate several PMDO samples with varying amounts of branching while keeping the molar mass below 30,000 g/mol and the dispersity below 2.0. A detailed analysis by NMR spectroscopy clarified which signals belong to the ring-retaining structure of PMDO and that branching originated from the α-carbon next to the carbonyl unit of the ester group. A deeper insight into the viscosity of PMDO solutions following a size exclusion chromatography analysis suggested that both long-chain branching and short-chain branching existed but could not discriminate between both of them. It did, however, show that the Kη and the α values of the Kuhn–Mark–Houwink–Sakurada (KHMS) plot vary systematically with the branching density of PMDO. Thermal analysis by differential scanning calorimetry exposed a semicrystalline behavior with more than one melting peak for all investigated samples. Both the degree of crystallization and the melting temperature of the investigated melting peaks again correlated with the branching density of PMDO. Altogether, this study shed light on how macroscopic properties like viscosity and thermal behavior correlate with the branching density of PMDO and thus provides insights into the structure–property relationships of this polymer. It lays the basis to synthesize branched PCL analogues with fine-tuned stability for biomedical applications.