Highly active bifunctional dual-arm organoboron catalysts bearing cooperative intramolecular structures for the copolymerization of CO2 and epoxides

Abstract

A series of bifunctional hexanuclear organoboron catalysts with dual-arm structures were facile prepared via a two-step procedure in ~100% yields featuring six boron centers and two quaternary ammonium bromides for the epoxides/CO2 copolymerization. The boron active sites within the p-HexaB, m-HexaB, and o-HexaB catalysts were like two stretched “arms” presenting the cooperative intramolecular structures as para-, meta-, and ortho-locations, respectively. For propylene oxide (PO)/CO2 copolymerization, the catalysts exhibited > 99% polymer selectivity under 25 – 60 ℃. The p-HexaB catalyst provided the highest carbonate linkages (88%) in the poly(propylene carbonate)s while the o-HexaB catalyst generated much more ether linkages as 45%. The o-HexaB catalyst performed higher activity in cyclohexane oxide (CHO)/CO2 copolymerization, obtaining fully alternating poly(cyclohexane carbonate)s with > 99% polymer selectivity under 25 – 150 ℃. For electron-withdrawing epichlorohydrin (ECH) monomer, the catalysts showed highly selective copolymerization (>99% polymer selectivity) and enhanced activity in comparison with tetraboron catalysts, providing poly(chloropropylene carbonate)s with > 99% carbonate linkages under 25 – 40 ℃. A novel bilaterally sequential copolymerization mechanism for alternating epoxide/CO2 insertion was proposed and verified by density functional theory (DFT) calculations. Bearing dual-arm intramolecular structures, the catalysts withstood the simultaneous growth of two polymer chains, achieving the enhanced activity for the copolymerization. Thus, up to seven effective boron pairs were found in the o-HexaB catalysts within the unique cooperative structure, leading to the PO homopolymerization in the copolymer with high ether linkages. Meanwhile, the o-HexaB catalyst with enough steric space exhibited higher activity than p-HexaB and m-HexaB catalysts during the copolymerization.