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Highly branched and loop-rich gels via formation of metal–organic cages linked by polymers

Abstract

Gels formed via metal–ligand coordination typically have very low branch functionality, f, as they consist of 2–3 polymer chains linked to single metal ions that serve as junctions. Thus, these materials are very soft and unable to withstand network defects such as dangling ends and loops. We report here a new class of gels assembled from polymeric ligands and metal–organic cages (MOCs) as junctions. The resulting ‘polyMOC’ gels are precisely tunable and may feature increased branch functionality. We show two examples of such polyMOCs: a gel with a low f based on a M2L4 paddlewheel cluster junction and a compositionally isomeric one of higher f based on a M12L24 cage. The latter features large shear moduli, but also a very large number of elastically inactive loop defects that we subsequently exchanged for functional ligands, with no impact on the gel's shear modulus. Such a ligand substitution is not possible in gels of low f, including the M2L4-based polyMOC.

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Figure 1: Design of polyMOCs with variable junction structures from isomeric polymer precursors.
Figure 2: Solution self-assembly of junctions not bound to a polymer.
Figure 3: PolyMOC assembly and characterization.
Figure 4: Room-temperature rheology of polyMOCs.
Figure 5: Loop-defect exchange in polyMOCs.

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Acknowledgements

J.A.J. thanks the National Science Foundation (NSF) (CHE-1334703 and CHE-1351646), the MIT Energy Initiative and the Deshpande Center for Technological Innovation for their support of this work. R.G.G. MAS NMR spectroscopy is supported through the National Institutes of Health, EB-002026. A.V.Z. thanks the Department of Defense National Defense Science and Engineering Graduate program and Intel for graduate fellowships in support of this work. V.K.M. is grateful to the Natural Sciences and Engineering Research Council of Canada and the Government of Canada for a Banting Postdoctoral Fellowship. This work made use of the DCIF Shared Experimental Facilities at the MIT (National Institutes of Health, 1S10RR013886–01; NSF, CHE-0234877), the MIT X-Ray Facility (NSF, CHE-0946721) and Shared Experimental Facilities supported in part by the Materials Research Science and Engineering Center program of the NSF (DMR-1419807). We acknowledge the support of the National Institute of Standards and Technology (NIST), US Department of Commerce, in providing the neutron research facilities used in this work. This work utilized facilities supported in part by the NSF under Agreement No. DMR-0944772. This manuscript was prepared under cooperative agreement 70NANB12H239 from NIST, US Department of Commerce. The statements, findings, conclusions and recommendations are those of the authors and do not necessarily reflect the views of NIST or the US Department of Commerce. We thank P. Müller for X-ray crystallography and M. MacLeod for assistance in processing the crystal structure data, S. Trauger for ESI-TOF-MS., E. Dreaden for cryo-TEM, T. M. Swager and G. Gutierrez for the use of a fluorimeter and N. Holten-Andersen, S. Grindy, K. Kawamoto and M. Glassman for helpful discussions.

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A.V.Z. and J.A.J. conceived the idea. A.V.Z. conducted the synthesis and characterization experiments. A.V.Z. and M.Z. conducted the mechanical testing experiments. A.V.Z., E.G.K. and V.K.M. conducted the MAS NMR experiments. J.E.P.S. and D.J.P. conducted the SANS experiments and analysed SANS data. M.J.A.H. provided the SANS model. A.P.W. developed the simulations. All authors analysed data. A.V.Z. and J.A.J. wrote the paper.

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Correspondence to Jeremiah A. Johnson.

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Crystallographic data for compound (L2)4Pd(II)2 (CIF 6896 kb)

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Zhukhovitskiy, A., Zhong, M., Keeler, E. et al. Highly branched and loop-rich gels via formation of metal–organic cages linked by polymers. Nature Chem 8, 33–41 (2016). https://doi.org/10.1038/nchem.2390

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