Copolymerization of epoxides and carbon dioxide. Evidence supporting the lack of dual catalysis at a single metal site.
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abstract
The chromium tetramethyltetraazaannulene catalyst system, Cr(tmtaa)Cl in the presence of a quaternary organic salt, has been employed to further investigate the mechanism of polycarbonate formation between cyclohexene oxide and carbon dioxide. Although the features of the proposed mechanism are generally well-accepted, the question of whether polymer chain propagation occurs from one side of the catalyst in a mono catalytic single-site fashion or from both sides of the N(4)-ligand plane in a dual catalytic manner. To probe this behavior, the ligand architecture was altered by the addition of a sterically encumbering strap (Cr(stmtaa)Cl), which effectively blocks one side of the complex and limits polymer chain growth to a single side of the catalyst. For direct comparison, an electronically similar catalyst, Cr(s(m)tmtaa)Cl, was prepared to mimic the strap complex while not possessing the site-blocking steric restraints of the full strap. Infrared spectroscopy demonstrated an uptake of 2 equiv of PPNN(3) cocatalyst by the strap mimic catalyst, while only 1 equiv of azide was able to bind to the catalyst containing the full strap, supporting the design function of both complexes. Monitoring the formation of poly(cyclohexylene carbonate) from the reaction of cyclohexene oxide and carbon dioxide via in situ infrared spectroscopy for both Cr(stmtaa)Cl and Cr(s(m)tmtaa)Cl, under identical conditions, revealed copolymer formation at essentially equivalent rates. Analysis of the polycarbonate products found that Cr(stmtaa)Cl and Cr(s(m)tmtaa)Cl produced copolymers with turnover frequencies of 806.6 and 797.3 h(-1), molecular weights of 11,431 and 12,003 Da, and polydispersities of 1.108 and 1.048, respectively. These results strongly support the idea that this and other catalysts systems presumed to operate by a similar process, such as Cr(salen)X, catalyze the copolymerization of epoxides with carbon dioxide through a mono catalytic single-site mechanism.
