DFT-Based Mechanistic Study of Bifunctional Al-Porphyrin Catalyst for CO 2 /PO and TsNCO Polymerizations toward Polycarbonates and Polyurethanes

Abstract

Density functional theory (DFT) calculations were used to clarify the mechanisms behind Al(III)-porphyrin-catalyzed copolymerization of propylene oxide (PO) with CO 2 and p-toluenesulfonyl isocyanate (TsNCO). Our research showed distinct reactivity patterns that influence polymer microstructure and selectivity. PO is activated by nucleophilic ring opening, generating a metal-alkoxide intermediate that then inserts either CO 2 or TsNCO.TsNCO insertion is both kinetically and thermodynamically preferred over CO 2 , thereby guiding the system toward the formation of a polyurethane-poly(propylene carbonate); PU-PPC copolymer. Importantly, carbonate-linkage formation in PPC is the rate-limiting step, while imidocarbonate formation during PU growth occurs more quickly. Backbiting and cyclic by-product formation are minimized along the TsNCO pathway due to the decreased nucleophilicity of the imidocarbonate intermediate and the stabilizing effect of the tethered quaternary ammonium (QA) cation. These factors explain the high selectivity toward PU-PPC copolymers and the low occurrence of side reactions observed experimentally. Overall, this study offers the first DFT-based mechanistic insight into TsNCO polymerization, emphasizes the cooperative role of bifunctional catalysts in controlling linkage selectivity, and provides a framework for designing catalysts that enable efficient and highly polymerizable systems.