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

Abstract

Bifunctional Al^III 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 (T_g) 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 (T_d¹⁰) 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 T_d¹⁰ value.