Mechanistic Aspects of the Copolymerization Reaction of Carbon Dioxide and Epoxides, Using a Chiral Salen Chromium Chloride Catalyst

The air-stable, chiral (salen)CrIIICl complex (3), where H2salen = N,N‘-bis(3,5-di-tert-butyl-salicylidene)-1,2-cyclohexene diamine, has been shown to be an effective catalyst for the coupling of
cyclohexene oxide and carbon dioxide to afford poly(cyclohexenylene carbonate), along with a small quantity
of its trans-cyclic carbonate. The thus produced polycarbonate contained >99% carbonate linkages and
had a Mn value of 8900 g/mol with a polydispersity index of 1.2 as determined by gel permeation
chromatography. The turnover number (TON) and turnover frequency (TOF) values of 683 g of polym/g of
Cr and 28.5 g of polym/g of Cr/h, respectively for reactions carried out at 80 °C and 58.5 bar pressure
increased by over 3-fold upon addition of 5 equiv of the Lewis base cocatalyst, N-methyl imidazole. Although
this chiral catalyst is well documented for the asymmetric ring-opening (ARO) of epoxides, in this instance
the copolymer produced was completely atactic as illustrated by 13C NMR spectroscopy. Whereas the
mechanism for the (salen)CrIII-catalyzed ARO of epoxides displays a squared dependence on [catalyst],
which presumably is true for the initiation step of the copolymerization reaction, the rate of carbonate chain
growth leading to copolymer or cyclic carbonate formation is linearly dependent on [catalyst]. This was
demonstrated herein by way of in situ measurements at 80 °C and 58.5 bar pressure. Hence, an alternative
mechanism for copolymer production is operative, which is suggested to involve a concerted attack of
epoxide at the axial site of the chromium(III) complex where the growing polymer chain for epoxide ring-opening resides. Preliminary investigations of this (salen)CrIII-catalyzed system for the coupling of propylene
oxide and carbon dioxide reveal that although cyclic carbonate is the main product provided at elevated
temperatures, at ambient temperature polycarbonate formation is dominant. A common reaction pathway
for alicyclic (cyclohexene oxide) and aliphatic (propylene oxide) carbon dioxide coupling is thought to be
in effect, where in the latter instance cyclic carbonate production has a greater temperature dependence
compared to copolymer formation.