Single-site Group VI Transition Metal Catalyst for the Copolymerization of CO2 and Epoxides.
Abstract: Global atmospheric carbon dioxide levels have reached a record high in the last century. Mitigating accumulation due to anthropogenic emissions will require a variety of approaches. Harnessing CO2 as a raw material source is one method of reclaiming CO2. In this context, the production of biodegradable polymers, such as polycarbonates, utilizing synthetic techniques which incorporate CO2 as a C1 feedstock holds significant interest. Ring-strained heterocycles (i.e. epoxides) are known to undergo chemical reactions with CO2, which is otherwise quite inert. The reaction of chromium(III) chloride with a tridentate Schiff-base ligand resulted a complex which exhibits catalytic activity toward the copolymerization of CO2 and cyclohexene oxide to form poly(cyclohexene carbonate). Typical polymerization reactions are carried out in bulk, utilizing a 25-ml stainless steel autoclave reactor, pressurized to 55 bar with CO2 and heated to 353 K in a temperature controlled oil bath. The catalyst Cr(III)(C24H29NO2)Cl exhibited good activity with a TOF = 11.32 hr-1 (s=2.64). Nevertheless, limited CO2 incorporation was observed, resulting in polymers that are ~40% polycarbonate at most. Introduction of a phosphine co-catalyst (tricyclohexylphosphine, PCy3) was observed to increase CO2 incorporations from 42% to as high as 78% with the addition of one equivalent of phosphine relative to chromium, based on 1H NMR analysis. However, GPC data indicated a linear decrease in number average molecular weight with increasing PCy3.
Mark Esposito*, Adrian Pap, Alexandra Muniz, James Woodyard, David R. Khan, and Jason C. Yarbrough
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