Dual Catalyst System forAsymmetric Alternating Copolymerizationof Carbon Dioxide and Cyclohexene Oxide with Chiral Aluminum Complexes:Lewis Base as Catalyst Activator and Lewis Acid as Monomer Activator.

Optically active aluminum complexes such as Schiff base,binuclearβ-ketoiminate, and bisprolinol complexes were found to promoteasymmetric alternating copolymerizations of carbon dioxide and cyclohexeneoxide. The aluminum Schiff base complexes–tetraethylammoniumacetate afforded isotactic poly(cyclohexene carbonate)s with low enantioselectivities.Lewis bases having two coordinating sites were utilized to enhanceactivity and selectivity based on the binuclear structure of the aluminumβ-ketoiminate clarified by X-ray crystallography. [2gAlMe]2–bulky bisimidazole produced the alternatingcopolymer with high enantioselectivity (62% ee). The polymerizationis considered to preferentially proceed at more crowded, enantioselectivesite owing to coordination of bulky Lewis bases to aluminums in lessenantioselective sites. 32AlMe–2-picolinealso exhibited a high enantioselectivity (67% ee). Methylaluminumbis(2,6-di-tert-butyl-4-methylphenoxide) was appliedto perform faster and more enantioselective copolymerizations at lowtemperature (82% ee). The asymmetric copolymerizations were foundto be significantly dependent on size of epoxide, temperature, andkind/amount of activators. [ABSTRACT FROM AUTHOR]