COPOLYMERIZATION OF CO2 AND EPOXIDES CATALYZED BY A SCHIFF-BASE CHROMIUM(III) COMPLEX
Global atmospheric carbon dioxide (CO2) levels have reached a record high in the last century and this is suspected to be a major contributor to climate change due to its effects as a greenhouse gas. Notwithstanding the 200 gigatons (Gt) of CO2 recycled through the natural carbon cycle each year, there is an excess of approximately 7 Gt of annual, anthropogenic CO2 emissions. Mitigating this accumulation will require a variety of approaches. One such approach is the capture and utilization of CO2 as raw material source which may have potential economic viability. 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, aziridines, etc.) are known to undergo chemical reactions with CO2, which is quite stable and fairly inert. On this basis, we sought to design single-site catalysts which facilitate the production of polycarbonates via copolymerization of epoxides and CO2. Since tetradentate catalysts comprise the great majority of studies and publications in this field, the basis of this study was to synthesize and investigate two tridentate Schiff bases. 2-[(2-Hydroxy-2-phenylethylimino)methyl]-4,6-bis(tert-butyl)phenol (H2LP) and (1S,2R)-1-[(3,5-Di-tert-butyl-2-hydroxybenzylidene)amino]-2-indanol, (H2LI). Addition of these ligands with an equal amount of CrCl3·3THF complex yielded catalysts LPCrIIICl and LICrIIICl respectively. Of these catalysts, LICrIIICl proved to be most effective in the coupling of CO2 and cyclohexene oxide (CHO) to produce poly(cyclohexene carbonate) (PCHC). Typical polymerization reactions were carried out in bulk, utilizing a 25-ml stainless steel autoclave reactor, pressurized to ~52 bar with CO2 maintained at 353K in a temperature-controlled oil bath. Under these conditions, catalyst LICrIIICl produced PCHC with a number average molecular weight (Mn) of 3578 Daltons (Da), polydispersity index (PDI) of 5.1, and a turnover frequency (TOF) of 13.82 hr-1. Initial PCHC was observed to have a CO2-incorporation of approximately 40% based on 1H NMR spectroscopy. Addition of a neutral phosphine cocatalyst (tricylcohexylphosphine, PCy3) proved effective at increasing CO2 incorporation to approximately 78%. Nevertheless, increasing PCy3 levels resulted in diminished TOF and a linear decrease in Mn to as low as 609 Da. In addition, Schiff base CrIII catalysts have been documented for the asymmetric ring opening of epoxides and aziridines. Catalyst LICrIIICl produced PCHC which is atactic as determined by 13C NMR spectroscopy. As such, Catalyst LICrIIICl proved effective for the copolymerization of CO2 and CHO at moderate TOF.