Trivalent cobalt complex mediated formation of stereoregular CO2copolymers from phenyl glycidyl ether

Abstract

Trivalent cobalt-based catalyst systems have proven to be efficient for the coupling of CO₂ with epoxides to afford the corresponding polycarbonates with perfectly alternating nature. Particularly, the single-component Co(III)–Salen complexes with an intramolecular cocatalyst exhibit excellent activity and selectivity for the formation of polycarbonates from various epoxides even at very low catalyst loading. Herein, we report the synthesis of CO₂ copolymers from phenyl glycidyl ether using the Co(III)–Salen complex with an appended 1,5,7-triazabicyclo[4.4.0] dec-5-ene (designated as TBD) as a catalyst. The resulting polymers have more than 99% carbonate linkage and possess unprecedented head-to-tail content. Comparative kinetic studies were performed via in situ infrared spectroscopy as a function of temperature to evaluate the activation barriers for the production of polycarbonate versus cyclic carbonate involving two Co(III)-based catalyst systems. The energies of activation determined for cyclic carbonate and copolymer formation in the coupling reaction of CO₂ and phenyl glycidyl ether catalyzed by the binary 1/PPNDNP system are 72.8 and 39.2 kJ mol⁻¹, respectively, compared to the corresponding values of 104.6 and 30.2 kJ mol⁻¹ with the single-component catalyst 3. The big difference in the energies of activation for cyclic carbonate versus copolymer formation accounts for the excellent selectivity for copolymer formation in the single-component catalyst systems even at elevated temperatures. Furthermore, with the use of enantiopure phenyl glycidyl ether, highly stereoregular polycarbonate with >99% isotacticity and >99% head-to-tail content could be obtained. Importantly, the completely isotactic copolymer is a typical semicrystalline thermoplastic, which possesses a melting point of 75 °C.