CO2 copolymers from epoxides: catalyst activity, product selectivity, and stereochemistry control

Area of Science:

• Catalysis
• Polymer Chemistry
• Sustainable Chemistry

Background:

• Carbon dioxide (CO2) is a stable molecule, limiting its use as a feedstock.
• Existing CO2 conversion processes yield limited products like urea and hydroxybenzoic acid.
• Biodegradable polycarbonates are valuable materials, but their synthesis via CO2 copolymerization often produces byproducts.

Purpose of the Study:

• To develop highly active and selective cobalt(III)-based catalysts for CO2 and epoxide copolymerization.
• To achieve high yields of polycarbonates with minimal byproducts.
• To understand the catalytic mechanism for improved process control.

Main Methods:

• Development of binary catalyst systems: (salen)Co(III)X with nucleophilic cocatalysts.
• Design of one-component catalysts: (salen)Co(III)X with appended quaternary ammonium salts or Lewis bases.
• Utilized unsymmetric multichiral Co(III) complexes for enantioselective copolymerization.

Main Results:

• Binary systems achieved >99% carbonate linkages and ~95% head-to-tail regiochemical control under mild conditions (0.1 MPa CO2).
• One-component catalysts demonstrated high activity with low catalyst loading and/or low CO2 pressures.
• Enantioselective copolymerization yielded aliphatic polycarbonates with >99% head-to-tail content; terpolymerization of multiple monomers was also successful.

Conclusions:

• Co(III)-based catalysts offer high activity and selectivity for producing CO2-based polycarbonates.
• A cooperative monometallic mechanism involving epoxide activation and CO2 insertion drives the catalysis.
• These catalysts enable the synthesis of crystalline CO2-based polymers and perfectly alternating polycarbonates.