Mechanism of the Alternating Copolymerization of Epoxides and CO2 Using β-Diiminate Zinc Catalysts:  Evidence for a Bimetallic Epoxide Enchainment

A series of zinc beta-diiminate (BDI) complexes and their solid-state structures, solution dynamics, and copolymerization behavior with CO(2) and cyclohexene oxide (CHO) are reported. Stoichiometric reactions of the copolymerization initiation steps show that zinc alkoxide and bis(trimethylsilyl)amido complexes insert CO(2), whereas zinc acetates react with CHO. (BDI-2)ZnOMe(2) (BDI-2) = 2-((2,6-diethylphenyl)amido)-4-((2,6-diethylphenyl)imino)-2-pentene and (BDI-1)ZnO(i)Pr (BDI-1) = 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)imino)-2-pentene react with CO(2) to form (BDI-2)Zn(mu-OMe)(mu,eta(2)-O(2)COMe)Zn(BDI-2) and (BDI-1)Zn(mu,eta(2)-O(2)CO(i)Pr)(2), respectively. (BDI-2)ZnN(SiMe(3))(2) inserts CO(2) and eliminates trimethylsilyl isocyanate to give (BDI-2)Zn(mu-OSiMe(3))(2). (BDI-7)Zn(mu-OAc)(2) (BDI-7) = 3-cyano-2-((2,6-diethylphenyl)amido)-4-((2,6-diethylphenyl)imino)-2-pentene reacts with 1.0 equiv of CHO to yield (BDI-7)Zn(mu,eta(2)-OAc)(mu,eta(1)-OCyOAc)Zn(BDI-7). Under typical polymerization conditions, rate studies on the copolymerization exhibit no dependence in CO(2), a first-order dependence in CHO, and orders in Zn(tot) ranging from 1.0 to 1.8 for (BDI)ZnOAc complexes. The copolymerizations of CHO (1.98 M in toluene) and 300 psi CO(2) at 50 degrees C using (BDI-1)ZnOAc and (BDI-2)ZnOAc show orders in Zn(tot) of 1.73 +/- 0.06 and 1.02 +/- 0.03, respectively. We propose that two zinc complexes are involved in the transition state of the epoxide ring-opening event.