A Lewis Pair as Organocatalyst for One-Pot Synthesis of Block Copolymers from a Mixture of Epoxide, Anhydride, and CO2

Ring-opening copolymerization (ROCOP) of epoxide/anhydride/CO2 for the synthesis of CO(2-)based copolymers is a promising process, because CO2 is an abundant, nontoxic, and low-cost monomer and has been considered as one of the most attractive renewable C1-resources. Although considerable progress has been made in the past decade, synthesis of block copolymers from mixed monomers through a one-pot polymerization is still a great challenge. Selective terpolymerizations of epoxide/anhydride/CO2 have been exclusively catalyzed by metal-based catalysts. There has been no success in using organocatalysts that can bridge ROCOP of epoxide/CO2 and ROCOP of epoxide/anhydride to produce CO2-based block copolymers, although a few organocatalysts have been independently used for either ROCOP. In this contribution, the PPNCl/TEB Lewis pair was selected as the organocatalyst to conjoin these two processes for one-pot ROCOP of epoxide/anhydride/CO2. NMR investigations of cyclohexene oxide (CHO)/phthalic anhydride (PA)/CO2 copolymerization revealed that polyester production was much faster than polycarbonate production and that polycarbonate was not formed until PA monomer was almost fully consumed (>= 95% of conversion). Therefore, diblock copolymers composed of poly(PA-alt-CHO) and PCHC blocks with very little tapering were synthesized in this one-pot and one-step route under metal-free conditions. The pre-rate-determining mechanism has been proposed for this chemoselectivity, which was further verified by DFT calculations. Subsequently, poly(PA-alt-CHO)-b-PCHC diblock copolymers without any tapering were successfully synthesized by sequential ROCOP of CHO/PA and ROCOP of CHO/CO2. NMR and GPC analysis of the resultant polymers demonstrated the formation of well-defined poly(PA-alt-CHO)-b-PCHC diblock copolymers with unimodal and narrow molecular weight distribution.

Automated summary

The ROCOP of epoxide/anhydride/CO2 is a promising process using CO2 as a renewable resource. However, the synthesis of block copolymers from mixed monomers through a one-pot polymerization remains a great challenge. This study successfully synthesized CO2-based block copolymers using an organocatalyst, presenting a potential direction for future research.