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Fast and selective ring-opening polymerizations by alkoxides and thioureas

Nature Chemistry volume 8, pages 1047–1053 (2016)Cite this article

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Abstract

Ring-opening polymerization of lactones is a versatile approach to generate well-defined functional polyesters. Typical ring-opening catalysts are subject to a trade-off between rate and selectivity. Here we describe an effective catalytic system combining alkoxides with thioureas that catalyses rapid and selective ring-opening polymerizations. Deprotonation of thioureas by sodium, potassium or imidazolium alkoxides generates a hydrogen-bonded alcohol adduct of the thiourea anion (thioimidate). The ring-opening polymerization of L-lactide mediated by these alcohol-bonded thioimidates yields highly isotactic polylactide with fast kinetics and living polymerization behaviour, as evidenced by narrow molecular weight distributions (Mw/Mn < 1.1), chain extension experiments and minimal transesterifications. Computational studies indicate a bifunctional catalytic mechanism whereby the thioimidate activates the carbonyl of the monomer and the alcohol initiator/chain end to effect the selective ring-opening of lactones and carbonates. The high selectivity of the catalyst towards monomer propagation over transesterification is attributed to a selective activation of monomer over polymer chains.

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Figure 1: Bifunctional organocatalysts.
Figure 2: MALDI-TOF MS of PLAs generated by various initiator systems with a target DP of ~50 and conversion of ~90%.
Figure 3: Calculated energies and reaction coordinate.

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Acknowledgements

This material is based on work supported by the National Science Foundation (NSF-CHE-1306730). X.Z. acknowledges a Stanford Graduate Fellowship.

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Authors and Affiliations

  1. Department of Chemistry, Stanford University, Stanford, 94305-5080, California, USA

    Xiangyi Zhang & Robert M. Waymouth

  2. IBM Research – Almaden, 650 Harry Road, San Jose, 95120, California, USA

    Gavin O. Jones & James L. Hedrick

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Contributions

X.Z., R.M.W. and J.L.H. designed the experiments. X.Z. performed the experiments. G.O.J. performed the DFT calculations. All authors analysed the results and co-wrote the manuscript.

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Correspondence to Robert M. Waymouth.

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The authors declare no competing financial interests.

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Zhang, X., Jones, G., Hedrick, J. et al. Fast and selective ring-opening polymerizations by alkoxides and thioureas. Nature Chem 8, 1047–1053 (2016). https://doi.org/10.1038/nchem.2574

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