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Cyclic polymers from alkynes

Nature Chemistry volume 8, pages 791–796 (2016)Cite this article

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Abstract

Cyclic polymers have dramatically different physical properties compared with those of their equivalent linear counterparts. However, the exploration of cyclic polymers is limited because of the inherent challenges associated with their synthesis. Conjugated linear polyacetylenes are important materials for electrical conductivity, paramagnetic susceptibility, optical nonlinearity, photoconductivity, gas permeability, liquid crystallinity and chain helicity. However, their cyclic analogues are unknown, and therefore the ability to examine how a cyclic topology influences their properties is currently not possible. We have solved this challenge and now report a tungsten catalyst supported by a tetraanionic pincer ligand that can rapidly polymerize alkynes to form conjugated macrocycles in high yield. The catalyst works by tethering the ends of the polymer to the metal centre to overcome the inherent entropic penalty of cyclization. Gel-permeation chromatography, dynamic and static light scattering, viscometry and chemical tests are all consistent with theoretical predictions and provide unambiguous confirmation of a cyclic topology. Access to a wide variety of new cyclic polymers is now possible by simply choosing the appropriate alkyne monomer.

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Figure 1: Improved catalyst preparation.
Figure 2: Molecular structure of 4 with ellipsoids drawn at the 50% probability level and disordered THF atoms and lattice solvent molecule (pentane) removed for clarity.
Figure 3: Proposed ring-expansion polymerization with catalyst 4 and formation of macrocyclic polyenes.
Figure 4: Comparison of r.m.s. radii of linear (blue) and cyclic (orange) poly(phenylacetylene) samples reported in Table 1 in dimethylacetamide with 0.05 M LiCl over a wide range of molecular masses.
Figure 5: Mark–Houwink plot that compares intrinsic viscosities (η) over a range of molecular masses for the linear and cyclic polyphenylacetylene samples reported in Table 1 in THF at 35 °C.
Figure 6: GPC traces of partially hydrogenated cyclic and linear poly(phenylacetylene) samples after ozonolysis for 30 seconds (orange) and 16 minutes (grey).
Figure 7: GPC traces that compare fully hydrogenated cyclic poly(styrene) samples (cyclic PS) with well-defined linear poly(styrene) (linear PS) standards.

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Acknowledgements

A.S.V. thanks the University of Florida (UF) for financial support of this project. This material is based on work supported by the National Science Foundation (NSF) CHE-1265993. K.A.A. thanks the UF and the NSF CHE-0821346 for funding the purchase of X-ray equipment. The authors thank B. S. Tucker and C. P. Kabb for their assistance with GPC characterization, and M. R. Hill for assistance in acquiring DLS data. B. S. Sumerlin is thanked for insightful discussions.

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Authors and Affiliations

  1. Department of Chemistry, University of Florida, Center for Catalysis, PO Box 117200, Gainesville, 32611, Florida, USA

    Christopher D. Roland, Hong Li, Khalil A. Abboud, Kenneth B. Wagener & Adam S. Veige

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  1. Christopher D. Roland
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  2. Hong Li
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  3. Khalil A. Abboud
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Contributions

C.D.R. synthesized and characterized catalyst 4 and all the cyclic polyenes. C.D.R. co-wrote the paper. H.L. hydrogenated the polymers, characterized the hydrogenated the polymers and executed the ozonolysis reactions. H.L. edited the paper. K.A.A. performed single-crystal X-ray experiments and solved the solid-state structure for complex 4. K.B.W. analysed the polymer characterization data, and edited the paper. A.S.V. conceived the experiments, analysed the characterization data and co-wrote the paper.

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Correspondence to Adam S. Veige.

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Crystallographic data for compound 4. (CIF 1150 kb)

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Roland, C., Li, H., Abboud, K. et al. Cyclic polymers from alkynes. Nature Chem 8, 791–796 (2016). https://doi.org/10.1038/nchem.2516

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