Direct Comparative Study of Ring-Opening Polymerization between Propylene Oxide and Cyclohexene Oxide: Geometric Control of Epoxide Polymerization Behaviors

Propylene oxide (PO) and cyclohexene oxide (CHO) are pivotal monomers for versatile polymer synthesis through ring-opening polymerization (ROP) and ring-opening copolymerization (ROCOP), yet direct comparative studies on their intrinsic ROP behavior remain underexplored due to the lack of well-defined catalysts capable of efficiently polymerizing both monomers. This work systematically elucidates the mechanistic divergences between PO and CHO under identical bifunctional organoborane catalysts, highlighting structural influences on polymerization kinetics and molecular weight control. Kinetic analyses reveal that CHO exhibits superior ROP activity (TOF: 7120 h–1 vs 2735 h–1 for PO) and follows first-order dependence with a lower activation energy (Ea = 39.7 kJ/mol), whereas PO displays quasi-zero-order kinetics with a higher Ea (66.8 kJ/mol), attributed to its preferential coordination to the catalyst and formation of a bridged arrest state. Despite its high activity, CHO suffers from inferior molecular weight control (Đ = 1.7 vs 1.1 for PO) due to sterically hindered initiation by bromide anions. Mechanistic insights from 11B NMR and MALDI-TOF MS demonstrate that CHO’s rigid six-membered ring structure suppresses bridged intermediate formation while weakening catalyst-monomer coordination, thereby accelerating propagation. This work establishes monomer geometry as a critical design parameter for controlling polymerization kinetics and chain fidelity, offering valuable insights for the design of advanced polymeric materials via ROP and ROCOP.