Unexpected Random Copolymerization of Propylene Oxide with Glycidyl Methyl Ether via Double Metal Cyanide Catalysis: Introducing Polarity in Polypropylene Oxide

Abstract

The synthesis of amorphous, polar aliphatic polyethers based on the copolymerization of propylene oxide (PO) and glycidyl methyl ether (GME) is described. Copolymers with Mn of 1.9–4.5 kg mol–1, with moderate to low dispersities (D̵ < 1.29) and up to 45 mol % GME content, were obtained via double metal cyanide (DMC) catalysis. An in-depth investigation of the solvent-free copolymerization was conducted by pressure monitoring, in situ 1H NMR spectroscopy, and 13C NMR triad analysis. Surprisingly, the results reveal an almost ideally random copolymerization of both epoxides (rPO = 1.40 ± 0.01, rGME = 0.71 ± 0.01). This observation is in pronounced contrast to the well-known preferential incorporation and generally high reactivity of PO in DMC catalysis in comparison to other epoxide monomers as well as the considerably lower reactivity of PO in the anionic ring-opening polymerization compared to glycidyl ethers. The reactivity ratios were evaluated at both 60 and 80 °C, demonstrating the reproducibility of the utilized solvent-free in situ measurement, showing also the temperature independence of the reactivity ratios within this range. Supplementary 13C NMR triad analysis further supports an almost ideally random copolymerization, confirming an evenly distributed incorporation of polar GME units in the hydrophobic PPO backbone. Turbidimetric measurements demonstrate tunable thermoresponsive behavior and hydrophilicity of the synthesized copolymers with lower critical solution temperatures between 19 and 35 °C. Furthermore, the increase of hydrophilicity is illustrated by contact angle measurements. The random copolymerization of PO and GME by DMC catalysis renders the resulting flexible polyethers an alternative to established ethylene oxide/PO copolymers for flexible polyol components in soft polyurethane foams.