Bootsma, J.

de Bruin, B.

de Zwart, F.J.

English

The development of universal methods for surface modification and crosslinking of polymeric materials is highly desirable. Mechanical properties of natural and synthetic materials rely on the ability of the polymeric chains to form a three-dimensional (3D) network, for example, through cross-linking. On page 875 of this issue, Lepage et al. show that a rationally designed bis-diazirine inspired by the wellknown 3-aryl-3-(trifluoromethyl)-3H-diazirine motif can be used for the cross-linking of nonfunctionalized saturated hydrocarbon polymers

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UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl)UvA-DARE (Digital Academic Repository)Cross-linking polyethylene through carbenesde Zwart, F.J.; Bootsma, J.; de Bruin, B.DOI10.1126/science.aaz7612Publication date2019Document VersionFinal published versionPublished inScienceLicenseArticle 25fa Dutch Copyright ActLink to publicationCitation for published version (APA):de Zwart, F. J., Bootsma, J., & de Bruin, B. (2019). Cross-linking polyethylene throughcarbenes. Science, 366(6467), 800. https://doi.org/10.1126/science.aaz7612General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s)and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an opencontent license (like Creative Commons).Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, pleaselet the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the materialinaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letterto: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. Youwill be contacted as soon as possible.Download date:09 Mar 2023INS IGHTS   |   PERSPECTIVESsciencemag.org  SCIENCEGRAPHIC: C. BICKEL/SCIENCEBy Felix J. de Zwart, Johan Bootsma, Bas de BruinThe development of universal methods for surface modification and cross-linking of polymeric materials is highly desirable. Mechanical properties of natural and synthetic materials rely on the ability of the polymeric chains to form a three-dimensional (3D) network, for example, through cross-linking (1, 2). On page 875 of this issue, Lepage et al. (3) show that a rationally designed bis-diazirine inspired by the well-known 3-aryl-3-(trifluoromethyl)-3H-diazirine motif can be used for the cross-linking of nonfunctionalized saturated hydrocarbon polymers.In natural macromolecules such as chitin and synthetic materials such as Kevlar, hydrogen-bond formation between amide groups form nonco-valent cross-links. Many oil-based polymer coatings used in paintings and to protect wooden structures de-rive their 3D structure and strength from covalent cross-links formed in radical-based processes. Other exam-ples of covalent cross-linking, such as vulcanization of rubber and the drying of glues, rely on reactive func-tional groups already present in the polymer.Polymers such as polyethylene (PE) and polypropylene (PP) lack reactive cross-link-able groups, but cross-linking can be essential to tune mechanical properties. Copolymeriza-tion of ethylene or propylene with function-alized olefins can introduce reactive groups, but the scope of suitable comonomers is lim-ited, and important material properties such as microstructure and molecular weight can become difficult to control (4, 5). It is espe-cially challenging to synthesize high–molecu-lar weight copolymers with tunable amounts of functional groups (5). Mildly efficient cross-linking strategies that break carbon-hydrogen (C−H) bonds by using high-energy radiation or peroxides exist for PE, but these methods degrade PP (6). High-energy reac-tants tend to react through hydrogen-atom abstraction (HAA) and related radical-type pathways that damage the polymer.Free (singlet) carbenes and transition-metal carbene complexes can undergo di-rect C−H insertion instead of HAA to form covalent carbon-carbon bonds even from unactivated C−H bonds. Carbenes are used for polymer postfunctionalization, as polym-erization catalysts and as polymer backbone (4, 5, 7). Tri- and tetravalent photoactivatable azides and diazirines can cross-link polyeth-ylene glycol, but efficient generic cross-link-ers for the strengthening of unfunctionalized polyolefins have not been developed (8).Lepage et al. now report a carbene cross-linker containing two diazirine motifs. Free carbenes can be generated upon photochemi-cally or thermally induced nitrogen (N2) loss (see the figure). A drawback of molecules containing N2 expelling motifs (such as di-azirines) is explosion risk, and the authors abandoned known bis-diazirine compounds because of their volatile and explosive nature. They rationally designed a bis-diazirine cross-linker that finely balances risk and reactivity and generates two free carbenes that insert into C−H bonds of the polymer. The tethered structure forms cross-links between different polymer chains, even those of PE and PP. The molecular weight of PP increased with the amount of cross-linker added, corroborating successful cross-linking of the substrate. The bis-diazirine also cross-linked several other polymers, such as polyvinylalcohol (PVA). For PVA, whether carbene insertion occurred through C−H or O−H bonds was not inves-tigated, but the latter seems most likely (9). This cross-linking methodology could find applications in upgrading biopolymers in which O−H bonds are ubiquitous.The authors also explored application of the cross-linker as an adhesive for high-density PE (HDPE). Adhesion to this low–surface-energy material is difficult—it failed for a control sample with a commercial adhe-sive (Super Glue). However, the bis-diazirine cross-linker showed high performance in adhesion tests, and analysis of ripped sam-ples revealed a cohesive failure mechanism indicative of cross-linking to the HDPE sur-face. The cross-linker was also an effective synthetic fabric strengthener.Lepage et al. assume that in their reactions, singlet carbenes are generated that insert in a concerted manner into C−H bonds, which makes sense given the relatively controlled reactivity to-ward cyclohexane. But formation of triplet carbenes that react through radical processes cannot be fully ex-cluded (10). Radical-type pathways could damage polymer integrity, so examining the electronic structure of the formed carbenes is needed. This could also aid in developing carbenes that can be generated at lower tem-peratures, possibly through transi-tion-metal catalysis (4).The cross-linker developed by Lep-age et al. not only has potential ap-plications as an adhesive, but similar reagents could be used in paints and coatings. Prospective bio-based bind-ers such as cellulose and lignin lack readily cross-linkable groups such as isocyanates, ke-tones, or alkenes. The cross-linking method of Lepage et al. may help turn biomass-based macromolecules into functional materials. jREFERENCES AND NOTES 1.  D. Wang, T. P. Russell, Macromolecules51, 3 (2018).  2.  A. Nojoomi et al., Nat. Commun. 9, 3705 (2018).  3.  M. L. Lepage et al., Science366, 875 (2019). 4.  M. M. Díaz-Requejo et al., Macromolecules 38, 4966 (2005).  5.  N. M. G. Franssen et al., Chem. Soc. Rev. 42, 5809 (2013).  6.  K. S. Whitely et al., Polyolefins: Ullman’s Encyclopedia of Industrial Chemistry (Wiley-VCH, 2000). 7.  C. W. Bielawski, R. H. Grubbs, Prog. Polym. Sci.32, 1 (2007).  8.  A. Welle et al., Synthesis44, 2249 (2012).  9.  J. Brunner et al., J. Biol. Chem. 255, 3313 (1980).  10.  J. Wang, R. S. Sheridan, Org. Lett.9, 3177 (2007). ACKNOWLEDGMENTSWe thank the ARC-CBBC (projects 2016.001.UvA and 2018.015.C) for financial support.10.1126/science.aaz7612Van ’t Hoff Institute for Molecular Sciences (HIMS), University of Amsterdam (UvA), Science Park 904, 1098 XH Amsterdam, Netherlands. Email: b.debruin@uva.nlUnlocking carbenesRelease of N2 creates carbenes (C:) that bond to polyethylene chains.A strong adhesiveCarbene-glued polyethylene (PE) sheets are difcult to rip apart.Light or heatreleases N2Covalentcross-linkPolyethylenechainPENF3C CF3N N NF3C CF3CCPECarbene-generatinggluePOLYMER CHEMISTRYCross-linking polyethylene through carbenesA carbene-forming molecule can glue various polymers, even ones lacking functional groupsGetting a grip on polyethyleneLepage et al. designed a molecule that generates highly reactive carbenes that can cross-link unreactive alkyl polymers.800    15 NOVEMBER 2019 • VOL 366 ISSUE 6467Published by AAASon January 30, 2020 http://science.sciencemag.org/Downloaded from Cross-linking polyethylene through carbenesFelix J. de Zwart, Johan Bootsma and Bas de BruinDOI: 10.1126/science.aaz7612 (6467), 800.366Science ARTICLE TOOLS http://science.sciencemag.org/content/366/6467/800CONTENTRELATED http://science.sciencemag.org/content/sci/366/6467/875.fullREFERENCEShttp://science.sciencemag.org/content/366/6467/800#BIBLThis article cites 9 articles, 2 of which you can access for freePERMISSIONS http://www.sciencemag.org/help/reprints-and-permissionsTerms of ServiceUse of this article is subject to the  is a registered trademark of AAAS.ScienceScience, 1200 New York Avenue NW, Washington, DC 20005. The title (print ISSN 0036-8075; online ISSN 1095-9203) is published by the American Association for the Advancement ofScience Copyright © 2019, American Association for the Advancement of Scienceon January 30, 2020 http://science.sciencemag.org/Downloaded from 

Cross-linking polyethylene through carbenes

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Cross-linking polyethylene through carbenes

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