Photoinitiated Cationic Copolymerization of Cyclohexene Oxide and Cyclopentene Oxide: Monomer Reactivity Ratios and the Composition Dependence of the Glass Transition Temperature

Abstract The photoinitiated cationic copolymerization of cyclohexene oxide (CHO) and cyclopentene oxide (CPO) was investigated at 25°C using (4-phenylthiophenyl)diphenylsulfonium hexafluorophosphate (PTPDS PF 6 − ) as the photoinitiator and 366 nm irradiation. Dilatometric measurements on the homopolymerizations showed that CHO polymerizes approximately 1.8 times faster than CPO and with a shorter induction period. Copolymer compositions, determined by 13 C NMR spectroscopy for ten feed compositions at low conversion, were consistently richer in CHO than the corresponding feeds. The Kelen–Tüdős method gave monomer reactivity ratios of r CHO = 2.45 and r CPO = 0.69, and nonlinear least-squares fitting of the Mayo–Lewis equation gave r CHO = 2.65 ± 0.25 and r CPO = 0.77 ± 0.08 ( r CHO r CPO ≈ 2.0). The copolymers are therefore statistical, with preferential incorporation of CHO and a modest tendency towards CHO homosequences. Each copolymer exhibited a single glass transition temperature ( T g ), which decreased from 337.3 K (64.2°C) for poly(CHO) to 254.9 K (− 18.3°C) for poly(CPO). The T g –composition curve showed a pronounced negative deviation from both linear additivity and the Fox equation, reaching − 18.5 K at only 5.4 mol% CPO, whereas the number-average molar mass (16–20 kg mol − 1 ) was essentially independent of composition. This behaviour is attributed to the increased segmental mobility that results when isolated cyclopentane units separate neighbouring cyclohexane rings along the polyether backbone, an interpretation supported by the contrasting 13 C NMR line shapes of the two homopolymers. The data provide composition–property relationships for the design of cationically UV-cured coatings and adhesives based on cycloaliphatic epoxides.