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

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/CO 2 copolymerization . The boron active sites within the p- Hexa B , m- Hexa B , and o- Hexa B catalysts were like two stretched “arms” presenting the cooperative intramolecular structures as para-, meta-, and ortho-locations, respectively. For propylene oxide (PO)/CO 2 copolymerization, the catalysts exhibited > 99% polymer selectivity under 25 – 60 ℃. The p- Hexa B catalyst provided the highest carbonate linkages (88%) in the poly(propylene carbonate)s while the o- Hexa B catalyst generated much more ether linkages as 45%. The o- Hexa B catalyst performed higher activity in cyclohexane oxide (CHO)/CO 2 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/CO 2 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- Hexa B catalysts within the unique cooperative structure, leading to the PO homopolymerization in the copolymer with high ether linkages. Meanwhile, the o- Hexa B catalyst with enough steric space exhibited higher activity than p- Hexa B and m- Hexa B catalysts during the copolymerization.