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 ΔGcrb –
ΔGepx value should be an effective
indicator for the catalytic activities [ΔGepx: dissociation energy of ethylene oxide from the epoxide-coordinating
metal complex; ΔGcrb: dissociation
energy of methyl carbonate from the metal–carbonate complex].
In addition, metal complexes with a subthreshold ΔGepx value were found to show low PPC/CPC selectivity.
The PPC/PPO selectivity was related to the ΔGalk – ΔGepx value
and steric environment around the metal center (ΔGalk: 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.