Reaction Temperature and Solvent Influence Reactivity Ratios in the Copolymerization of Ethylene Oxide and Propylene Oxide

Statistical copolymers of ethylene oxide (EO) and propylene oxide (PO) are widely used in industry and academia. Despite their decade-long use, the influence of the polymerization conditions on reactivity ratios is underexplored, and surprisingly solution and bulk properties of the resulting polyether copolymers have not been reported in a systematic manner. In this study we examined the copolymerization of EO and PO in a variety of solvents (dimethyl sulfoxide, toluene, anisole) and at different temperatures (25-60 °C), correlating reaction conditions with the thermal and solubility properties of the resulting P-(EO-<i>co</i>-PO) copolymers. The copolymerization was monitored online by <i>in situ</i> ¹H NMR spectroscopy to determine the reactivity ratios for the full conversion range. The results show a temperature-dependent trend in reactivity ratios (<i>r</i>) for different solvents. In toluene, the reactivity ratios converge with increasing temperature, changing from <i>r</i> _PO = 0.26 and <i>r</i> _EO = 3.78 at 40 °C to <i>r</i> _PO = 0.31 and <i>r</i> _EO = 3.21 at 60 °C. A similar pattern is observed in anisole, with the reactivity ratios shifting from <i>r</i> _PO = 0.28 and <i>r</i> _EO = 3.52 at 40 °C to <i>r</i> _PO = 0.30 and <i>r</i> _EO = 3.32 at 60 °C, respectively. In contrast, the reactivity ratios in DMSO are generally slightly more similar, with <i>r</i> _PO = 0.32 and <i>r</i> _EO = 3.10 at 40 °C. Thermal characterization of the polyether copolymers revealed similar melting points of approximately 10 °C and enthalpies of around 40 J·g^-1. Cloud point measurements of the copolymers showed decreased aqueous solubility as the differences in reactivity ratios decreased. These findings demonstrate that the statistical EO/PO copolymerization reaction conditions affect the gradient and thereby significantly influence copolymer physical properties, highlighting the need to consider these parameters for applications.