Facile estimation of catalytic activity and selectivities in copolymerization of propylene oxide with carbon dioxide mediated by metal complexes with planar tetradentate ligand.

Mechanistic studies were conducted to estimate (1) catalytic activity for PPC, (2) PPC/CPC selectivity, and (3) PPC/PPO selectivity for the metal-catalyzed copolymerization of propylene oxide with carbon dioxide [PPC: poly(propylene carbonate); CPC = cyclic propylene carbonate; PPO: poly(propylene oxide)]. Density functional theory (DFT) studies demonstrated that the ΔG(crb) - ΔG(epx) value should be an effective indicator for the catalytic activities [ΔG(epx): dissociation energy of ethylene oxide from the epoxide-coordinating metal complex; ΔG(crb): dissociation energy of methyl carbonate from the metal-carbonate complex]. In addition, metal complexes with a subthreshold ΔG(epx) value were found to show low PPC/CPC selectivity. The PPC/PPO selectivity was related to the ΔG(alk) - ΔG(epx) value and steric environment around the metal center (ΔG(alk): dissociation energy of alkoxide ligand from the metal center). Based on the mechanistic studies, two metal complexes were designed and applied to the copolymerization to support validity of these indicators. The results presented here should be useful for brand-new catalyst candidates since these indicators can be easily calculated by DFT method without computing transition states.