DMC-Mediated Copolymerization of CO₂ and PO—Mechanistic Aspects Derived from Feed and Polymer Composition

• Main Authors: Sarah-Franziska Stahl, Gerrit A. Luinstra
• Format: Article
• Language: English
• Published: MDPI AG 2020-09-01
• Series: Catalysts
• Subjects: double metal cyanide catalysis; ring-opening polymerization; poly(propylene ether carbonate), PPEC; heterogeneous catalysis; propylene oxide; carbon dioxide
• Online Access: https://www.mdpi.com/2073-4344/10/9/1066

The influence of composition of liquid phase on composition of poly(propylene ether carbonates) in the copolymerization of CO₂ with propylene oxide (PO), mediated by a zinc chloride cobalt double metal cyanide, was monitored by FT-IR/CO₂ uptake/size exclusion chromatography in batch and semi-batch mode. The ratio of mol fractions of carbonate to ether linkages <i>F</i> (~0.15) was found virtually independent on the feed between 60 and 120 °C. The presence of CO₂ lowers the catalytic activity but yields more narrowly distributed poly(propylene ether carbonates). Hints on diffusion and chemistry-related restrictions were found underlying, broadening the distribution. The incorporation of CO₂ seems to proceed in a metal-based insertion chain process, ether linkages are generated stepwise after external nucleophilic attack. The presence of amines resulted in lower activities and no change in <i>F</i>. An exchange of chloride for nitrate in the catalyst led to a higher <i>F</i> of max. 0.45. The observations are interpreted in a mechanistic scheme, comprising surface-base-assisted nucleophilic attack of external weak nucleophiles and of mobile surface-bound carboxylato entities on activated PO in competition to protonation of surface-bound alkoxide intermediates by poly(propylene ether carbonate) glycols or by surface-bound protons. Basic entities on the catalyst may promote CO₂ incorporation.