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

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Abstract
A series of zinc β-diiminate (BDI) complexes and their solid-state structures, solution dynamics, and copolymerization behavior with CO2 and cyclohexene oxide (CHO) are reported. Stoichiometric reactions of the copolymerization initiation steps show that zinc alkoxide and bis(trimethylsilyl)amido complexes insert CO2, 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)ZnOiPr [(BDI-1) = 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)imino)-2-pentene] react with CO2 to form [(BDI-2)Zn(μ-OMe)(μ,η2-O2COMe)Zn(BDI-2)] and [(BDI-1)Zn(μ,η2-O2COiPr)]2, respectively. (BDI-2)ZnN(SiMe3)2 inserts CO2 and eliminates trimethylsilyl isocyanate to give [(BDI-2)Zn(μ-OSiMe3)]2. [(BDI-7)Zn(μ-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(μ,η2-OAc)(μ,η1-OCyOAc)Zn(BDI-7)]. Under typical polymerization conditions, rate studies on the copolymerization exhibit no dependence in [CO2], 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 CO2 at 50 °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.