Kinetics and mechanism of 2-ethoxy-2-oxo-1,3,2-dioxaphospholane polymerization initiated by stannous octoate

The ring-opening polymerization (ROP) of the cyclic phosphoester 2-ethoxy-2-oxo-1,3,2-dioxaphospholane (EEP) in tetrahydrofuran solution with co-initiation of stannous octoate and dodecanol yields poly(ethylene ethlyl phosphate) (PEEP) with defined linear molecular structure. NMR analyses reveal every molecule of dodecanol starts growth of one PEEP chain and each polymer chain bears one hydroxyl end group. Kinetics studies for different co-initiator ratios and temperatures reveal that the ROP is a first-order reaction with respect to EEP monomer and suggest the formation of active center stannous alkoxide and a coordination-insertion polymerization mechanism. Side chain transfer leading to branched polymeric structure was observed particularly when the reaction time was extended after monomer consumption reaching equilibrium due to the pentavalent nature of phosphorus. By controlling the polymerization conditions, such side reaction can be suppressed to synthesize polyphosphoester with confined structure. This polymerization procedure is expected to facilitate the synthesis of polyphosphoesters with defined molecular architectures and properties for further biomedical applications.