Highly robust supramolecular polymer networks crosslinked by a tiny amount of metallacycles

Supramolecular polymeric materials have exhibited attractive features such as self-healing, reversibility, and stimuli-responsiveness. However, on account of the weak bonding nature of most noncovalent interactions, it remains a great challenge to construct supramolecular polymeric materials with high robustness. Moreover, high usage of supramolecular units is usually necessary to promote the formation of robust supramolecular polymeric materials, which restrains their applications. Herein, we describe the construction of highly robust supramolecular polymer networks by using only a tiny amount of metallacycles as the supramolecular crosslinkers. A norbornene ring-opening metathesis copolymer with a 120° dipyridine ligand is prepared and self-assembled with a 60° or 120° Pt(II) acceptor to fabricate the metallacycle-crosslinked polymer networks. With only 0.28 mol% or less pendant dipyridine units to form the metallacycle crosslinkers, the mechanical properties of the polymers are significantly enhanced. The tensile strengths, Young’s moduli, and toughness of the reinforced polymers reach up to more than 20 MPa, 600 MPa, and 150 MJ/m3, respectively. Controllable destruction and reconstruction of the metallacycle-crosslinked polymer networks are further demonstrated by the sequential addition of tetrabutylammonium bromide and silver triflate, indicative of good stimuli-responsiveness of the networks. These remarkable performances are attributed to the thermodynamically stable, but dynamic metallacycle-based supramolecular coordination complexes that offer strong linkages with good adaptive characteristics.


Reviewers' Comments:
Reviewer #1: Remarks to the Author: Stang et al.This manuscript describes the design and syntheses of norbornene-based copolymers with small percentages of a pendant bipyridine unit, the complexation of these copolymers with Pt(II) to form metallacyclic crosslinks from the bipyridine pendant groups, and mechanical property tests to demonstrate their improvement via the metallacyclic crosslinks.The impressive results confirm that this constitutes a new approach to supramolecular crosslinking of polymers for property enhancement.Therefore, this paper will be of broad interest to supramolecular chemists, polymer scientists and material scientists; therefore, after the minor issues below have been adequately addressed, it should be published in Nature Communications.
• The pages of the ms and the SI should be numbered!!The SI needs a Table of Contents.
• Page 2: The authors use the term "strong enough".What is "strong enough" in terms of parameters they study?If this question cannot be answered satisfactorily, the term should be removed.
• It is suggested that in the text a table is used to indicate the mol % of dipyridyl monomer M1 used in the polymerization be indicated: 1.7, 0.55, 0.28 and 1.7% for CP-1, CP-2, CP-3 and CP-4.The Table could also contain the average molecular weights and polydispersities.
• Reference list: The use of "et al." in the list of author names is restricted by most journals; include the names of the coauthors!• SI, pages 19-22: There is confusion about the structure numbers.
• SI 1H NMR spectra, throughout: Unless special conditions are imposed during collection of the spectra, coupling constants can only be estimated to whole number precision.Thus, not J = 5.4, 3.0 Hz, but rather J = 5, 3 Hz.
These and language issues are noted in the "Track Changes" Word documents attached.
Reviewer #2: Remarks to the Author: In this manuscript, Li and coworkers described the construction of highly robust supramolecular polymer networks by using metallacycles as the supramolecular crosslinkers.It should be noted that construction of supramoecular polymeric materials form dynamic metallacycles and covalent polymers has evolved to be one of the most attractive topics within supramolecular chemistry and materials sciences.In this work, Li and coworkers presented the design and construction of tough supramolecular polymer networks crosslinked by a tiny amount of metallacycles.Importantly, after crosslinking, the yield strength, tensile strength, Young's modulus, and toughness could all increase to several times of the original ones.In addition, the obtained gel shows a multiple stimuli responsiveness behavior.The manuscript, with good novelty and scientific value and the data collected are interesting and the overall findings merit publication Thus, I would like to recommend to be published in Nature Communications after some revisions.
1. Crosslinking process usually produces some intriguing morphologies.Therefore, the morphologies of the supramoecular gel are recommended to be characterized by the microscopy techniques (e.g., SEM, TEM, AFM and so on), which will be helpful to understand the nanostructures of such soft materials.
We appreciate the reviewers' comments which have greatly improved our manuscript.The specific changes to the manuscript as per request are listed as following.

Response to Reviewer 1:
This manuscript describes the design and syntheses of norbornene-based copolymers with small percentages of a pendant bipyridine unit, the complexation of these copolymers with Pt(II) to form metallacyclic crosslinks from the bipyridine pendant groups, and mechanical property tests to demonstrate their improvement via the metallacyclic crosslinks.The impressive results confirm that this constitutes a new approach to supramolecular crosslinking of polymers for property enhancement.Therefore, this paper will be of broad interest to supramolecular chemists, polymer scientists and material scientists; therefore, after the minor issues below have been adequately addressed, it should be published in Nature Communications.
Reply: We appreciate the reviewer's positive comments on the work in this manuscript.

The pages of the MS and the SI should be numbered!! The SI needs a Table of Contents.
Reply: Thanks.We have numbered the pages of the main text and the Supplementary Information, and added a Table of Contents in the Supplementary Information.
Page 2: The authors use the term "strong enough".What is "strong enough" in terms of parameters they study?If this question cannot be answered satisfactorily, the term should be removed.

Reply:
As the reviewer pointed out, it is inappropriate to use the term "strong enough".We have changed it to "strong" on Page 2 in the main text.
It is suggested that in the text a table is used to indicate the mol % of dipyridyl monomer M1 used in the polymerization be indicated: 1.7, 0.55, 0.28 and 1.7% for CP-1, CP-2, CP-3 and CP-4.The Table could also contain the average molecular weights and polydispersities.
Reply: As the reviewer suggested, we have added a table that contains average molecular weights, polydispersities and mol% of dipyridyl monomer M1 used in the polymerization for CP-1, CP-2, CP-3 and CP-4 on Page S36 in the Supplementary Information.
Page 5: What is meant by the term "trammel"???
Reply: The term "trammel" is not suitable for use here, and we have changed it to "entanglement of polymer chains" on Page 5 in the main text.