Terpolymerization of Oxetane, CO₂, and Isothiocyanates with Bifunctional Catalysts: Synthesis and Degradability of Poly(thioimidocarbonate-trimethylene carbonate)s

Bifunctional AlIII porphyrins catalyzed the terpolymerization reactions of oxetane, CO2, and isothiocyanates to synthesize poly(thioimidocarbonate-trimethylene carbonate)s, PTIC–PTMC, for the first time. In addition to the target polymers, three interesting sulfur-containing cyclic byproducts were obtained and fully identified by spectroscopic methods including X-ray crystallography. These byproducts were useful for the understanding of reaction behaviors of reactive intermediates, such as the O/S exchange reaction and the backbiting reaction. The time course of the terpolymerization reaction indicated that phenyl isothiocyanate was consumed much faster than CO2. This trend is opposite to our previous observation that CO2 reacted with cyclohexene oxide faster than phenyl isothiocyanate. This opposite trend is due to the difference in the selectivity-determining step in the catalytic cycles. The ring-opening of oxetane is the selectivity-determining step owing to the small ring strain of oxetane in the present case, while the reaction of an alkoxide ion with heteroallene (CO2 or isothiocyanates) is the selectivity-determining step in the previous case with the epoxide. UV light irradiation, heat treatment, or acid treatment selectively degraded the PTIC units without decomposing the PTMC units, giving a cyclic S-thiocarbamate compound, 3-phenyl-1,3-thiazinan-2-one. The polymers with different substituents had a glass transition temperature (Tg) in the range of −11 to 30 °C. It is likely that most terpolymers had a gradient character in composition, although some of them had block-like structures. The polymers with different substituents had a 10% weight loss temperature (Td10) in the range of 221 to 264 °C, among which the terpolymer with the p-tolyl group possessing the highest PTMC content showed the highest Td10 value.