Selective synthesis of oligo(carbonate-ether) diols from copolymerization of CO₂ and propylene oxide under zinc-cobalt double metal cyanide complex

Colorless oligo(carbonate-ether) diols were selectively synthesized in high efficiency from copolymerization of CO₂ and propylene oxide (PO) using Zn₃[Co(CN)₆]₂-based double metal cyanide complex (DMC) as catalyst and different molecular weight polypropylene glycols (PPGs) as chain transfer agent. The catalytic activity was related to carbonate unit content and molecular weight of target oligo(carbonate-ether) diols, for oligo(carbonate-ether) diol with number average molecular weight of 6.4 kg/mol and carbonate unit content of 34.3 %, it reached 10.0 kg oligomer/g DMC catalyst during 10 h of copolymerization. Generally, the number average molecular weight of the oligo(carbonate-ether) diol was tunable between 1.8 kg/mol and 6.4 kg/mol, and the molecular weight distribution was controllable between 1.14 and 1.83. Moreover, the carbonate unit content in the oligo-diols can be adjusted between 15.3 % and 62.5 %, lower temperature and higher CO₂ pressure were favorable for higher carbonate content. Better selectivity of oligo(carbonate-ether)diol over propylene carbonate(PC) was realized, where the weight ratio of PC (W_PC) was controlled less than 8.0 wt%. We also found that the alkali metal ion residue may play an important role in PC formation, in some cases this effect may be more significant than backbiting process, removing the residual alkali metal ion should be meaningful in the future to further reduce the PC formation.