Vitrimers Via Transalkylation of Trialkylsulfonium Salts Benjamin Hendriks, Jelle Waelkens, Johan M

This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Letter pubs.acs.org/macroletters Poly(thioether) Vitrimers via Transalkylation of Trialkylsulfonium Salts Benjamin Hendriks, Jelle Waelkens, Johan M. Winne,* and Filip E. Du Prez* Department of Organic and Macromolecular Chemistry, Polymer Chemistry Research Group and Laboratory of Organic Synthesis, Ghent University, Krijgslaan, 281, S4-bis, B-9000 Ghent, Belgium *S Supporting Information ABSTRACT: Vitrimers are permanently cross-linked organic polymers that can be reshaped, molded, and recycled without loss of network integrity. Herein, we report poly(thioether) networks, prepared through a straightforward thiol−ene photopolymerization, that can be turned into catalyst-free vitrimer materials by partial alkylation of the thioethers (1− 10%) to the corresponding trialkylsulfonium salts. Based on a classical SN2-type substitution, the resulting polyionic networks can be reshaped upon heating via swift transalkylation reactions. This novel exchange reaction for the design of vitrimers was studied both on low MW model compounds as well as on a material level. In addition, we demonstrated the recycling of these networks without significant loss of mechanical properties. he desirable but antipodal characteristics of thermosetting T and thermoplastic polymers, respectively, resistance to deformation or dissolution, and thermal plasticity for processing and recycling can be achieved by the introduction of exchangeable chemical bonds in a polymer network. Networks containing exchangeable cross-links are also known as covalent adaptable networks (CANs).1 A subset of CANs rely on an associative (or concerted) bond exchange reaction, rather than the more common dissociative exchange reactions in which bonds are first broken and then reformed.2 Uniquely, associative bond exchanges permit network reorganizations and Figure 1. Schematic overview of transesterification3 (A), C−N 13 stress relaxation while maintaining a constant cross-linking transalkylation (B), and transalkylation of sulfonium salts (C) used density. At higher temperature, the viscosity of such materials is for the design of vitrimer materials. solely controlled by the rate of the chemical exchange reactions. 14 Thermally triggered associative CANs have been classified as electrolytes, due to the ionic nature of the cross-links. In this vitrimers by Leibler and co-workers and combine excellent work, we have explored an alternative chemistry platform for mechanical properties at service temperatures and malleability transalkylating vitrimer materials, based on a classical SN2-type and reprocessing by heating without precise control of reaction of sulfonium salts with thioether nucleophiles (Figure 3 1C). In fact, the possibility of an exchange reaction between temperature. fi Vitrimer materials, or vitrimer behavior of organic polymers, sul des (thioethers) and sulfonium salts has been previously were first discovered by Leibler and co-workers, in polyesters explored as a reversible propagation step in the cationic ring- fi opening polymerization of cyclic thioethers, in seminal work by that can undergo transesteri cation exchange by addition of a 15 suitable catalyst.4 Next to Leibler’s initial vitrimers (Figure 1A), Goethals et al. From these and other studies, benzylic a limited but growing number of other exchange reactions have sulfonium salts and thioethers are known to readily exchange been explored and found to give vitrimer-like properties to alkyl chains at room temperature without requiring any polymers.5 Examples of other suitable chemistries include additives or catalysts. At the outset of the work reported exchange reactions of vinylogous urethane bonds,6 carbamate herein, we wanted to investigate the potential of this exchange bonds,7 siloxane bonds,8 disulfide bonds,9 imine bonds,10 olefin reaction in catalyst-free polythioether-based vitrimers. For this, bonds,11 dioxaborolane ester bonds,12 and 1,2,3-triazolium alkyl bonds.13 The latter exchange reaction is the first reported Received: July 7, 2017 transalkylation reaction to be used as vitrimer materials (Figure Accepted: August 10, 2017 1B), which also shows potential for applications in solid Published: August 15, 2017 © 2017 American Chemical Society 930 DOI: 10.1021/acsmacrolett.7b00494 ACS Macro Lett. 2017, 6, 930−934 ACS Macro Letters Letter we envisaged the hereunto little-known transalkylation of less alkylation reaction could be easily monitored using 13C NMR activated aliphatic sulfonium salts and thioethers. Poly- (Figure S1, Supporting Information). From the NMR (thioether) networks are readily prepared by the photoinduced integration values, the remaining fraction of a butyl group in thiol−ene reaction of alkene and thiol monomers in bulk.16 the sulfonium salt is plotted versus the reaction time (Figure Exchangeable trialkylsulfonium bonds should be easily 2b). Notwithstanding the limited accuracy of 13CNMR introduced by partial alkylation of the thioether functions, integration, approximative reaction half-lives can be determined resulting in poly(thioether-sulfonium salt) networks (PTESSs). at each temperature, ranging from a few hours (at 130 °C) to Indeed, we have found PTESS materials that are insoluble and just a few minutes (at 160 °C). From the fitted curves, an show short relaxation times at elevated temperatures. We also activation energy of 108 ± 4 kJ mol−1 was calculated for the report the simple recyclability of PTESSs by grinding and exchange reaction (Figure S2, Supporting Information). This remolding them repeatedly, whereas nonalkylated poly- activation energy is around 30 kJ mol−1 lower compared to the thioethers cannot undergo this recycling and thermoforming transalkylation of 1,2,3-triazolium bromide salts, in line with the process. higher nucleophilicity in an SN2 reaction expected for a To evaluate the exchange kinetics of simple aliphatic thioether compared to a triazole.13 trialkylsulfonium salts, a model study on low MW compounds With the possible exchange of alkyl groups between trialkyl was performed. First, model compound 1 (Figure 2a) was sulfonium salts and thioethers established in our model study, we then set out to implement this chemistry in poly(thioether) networks. However, cross-linking of linear poly(thioether) chains with bifunctional alkylating agents proved to be a hard to control process in bulk, due to solubility and phase separation issues. Working with solvents, it proved hard to reach high cross-linking conversions. Finally, an alternative strategy proved much more successful, in which cross-linked materials are directly prepared in a thiol−ene photopolymerization using multifunctional monomers (f > 2). Exchangeable bonds were readily introduced into these static networks by treatment with an alkylating agent. We initially explored alkylation of solvent- swollen networks, but as the alkylation reaction requires significant heating for its activation, we found that optimal results are obtained when alkyl brosylates are first physically incorporated into the networks by simply adding them to the monomer mixture before the photo-cross-linking polymerization is performed at low temperature. A thermal cure then covalently incorporates this additive through alkylation of the network thioether functions. Following a brief screening of possible network building blocks, the completely aliphatic octanedithiol 5, octadiene 6, and the more rigid trithiol17 7 were identified as suitable and readily available generic aliphatic monomers. The butyl brosylate 8 was chosen as a simple alkylating agent (Figure 3). All compounds (monomers 5, 6, Figure 3. Monomers used for production of vitrimer materials. A small Figure 2. (a) Exchange reaction on low MW compounds for a kinetic excess of diene (1.02 equiv) was used to limit the number of free thiols study. For convenience, the butyl hexyl sulfide and hexyl dibutyl after polymerization. Compound 8, butyl brosylate, was used as sulfonium salt as exchange products were not drawn. (b) alkylating agent. Disappearance of the butyl side chain as a salt of compound 1 by exchange reactions as a function of time for different temperatures − ° and 7, the initiator (DMPA), and the alkylating agent 8) were (130 160 C). mixed, giving a homogeneous liquid mixture that was then injected between two glass plates separated by a spacer. The prepared by the alkylation of butyl sulfide with butyl p- thiol−ene radical polymerization is initiated by UV irradiation bromobenzenesulfonate (brosylate). Brosylates are stable, non- (365 nm). After 1 h of UV irradiation, the samples were nucleophilic counterions, and alkylbrosylates also proved to be submitted to a thermal cure of 90 min at 140 °C, giving defect- the most promising alkylating reagents in a small screening free samples that should have undergone complete alkylation, process. In each exchange experiment, the tributylsulfonium salt as expected from the low MW model alkylation reactions (see 1 is treated with 5 equiv of dihexyl sulfide 2 without solvent, Figure S3, Supporting Information). The conversion of butyl and the resulting mixtures were heated at 130, 140, 150, and brosylate in the polymer matrix was also verified using FT-IR 160 °C. NMR spectra were recorded at different time intervals. analysis of the materials showing the complete disappearance of 1H NMR was not suitable for a quantitative study of the

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