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Polymer Chemistry View Article Online PAPER View Journal | View Issue Reversible crosslinking and fast stress relaxation in dynamic polymer networks via transalkylation Cite this: Polym. Chem., 2021, 12, 3640 using 1,4-diazabicyclo[2.2.2] octane† Eveline E. L. Maassen, a,b Johan P. A. Heuts *a and Rint P. Sijbesma *a Dynamically crosslinked polymers offer the possibility to design materials that have the robustness of thermosetting materials and the reshapability of thermoplastic materials. A reversible crosslinker based on transalkylation chemistry using 1,4-diazabicyclo[2.2.2]octane (DABCO) was developed. The kinetics of the exchange reaction of benzyl bromide with DABCO was studied in detail by varying molar ratios and temp- erature. The system showed efficient exchange without addition of a catalyst. Reversibility in a polymer was studied by the crosslinking of a linear, benzylic bromide functionalized polyacrylate with DABCO. The Received 3rd March 2021, resulting network was insoluble in common solvents, but it readily dissolved in the presence of DABCO. Accepted 2nd June 2021 Rheological measurements showed that the crosslinks are dynamic, with a strong temperature dependent Creative Commons Attribution 3.0 Unported Licence. DOI: 10.1039/d1py00292a relaxation that at 140 °C is faster than for other transalkylation based dynamic networks reported in the rsc.li/polymers literature. over the past years, some examples are transesterification Introduction – – reactions,7,22 26 boronic ester exchange,27 29 disulfide Thermosetting polymers are covalently crosslinked networks exchange,30 controllable reversible addition–fragmentation – with excellent properties such as solvent resistance and high reactions,31 33 transcarbonation exchange,34 and (vinylogous) This article is licensed under a – thermal and dimensional stability.1,2 Although these pro- urethane exchange.35 38 Because nucleophilic substitutions are perties are highly advantageous for a wide range of appli- usually considered to be non-reversible, they have often been cations such as in the coatings,3 adhesives,4 construction,5 overlooked for use in CANs.15 However, the reversibility of 6 39 Open Access Article. Published on 02 June 2021. Downloaded 9/24/2021 6:10:40 AM. and automotive industries, recycling or reprocessing these specific nucleophilic substitutions has been known for years, products is very challenging because they are unable to flow and has recently been employed in CANs.40 Kulchat and Lehn upon heating. By introducing exchangeable crosslinks, investigated the transalkylation of quaternary ammonium enhanced properties such as reshapability,7 recyclability,7,8 salts,41 and showed their potential for the development of (self-)healing,9,10 or stress relaxation can be obtained while dynamic covalent chemistry. Drockenmuller and coworkers retaining the advantageous properties like enhanced thermal were the first to report a dynamic polymeric network based on and chemical stability.11,12 Networks containing specially transalkylation exchange of C–N bonds.42 Network polymers designed reversible covalent crosslinks are also known as containing 1,2,3-triazolium salts as reversible crosslinks were – covalent adaptable networks (CANs).8,13 19 Depending on the synthesized and could be recycled by compression molding. nature of the exchangeable bond, a stimulus such as light,20 After two recycling cycles, samples with an extension at break temperature,21 or pH triggers the reversibility of the bond.15 similar to the starting material were obtained, indicating Many dynamic motifs for bond exchange have been developed efficient network reformation. Furthermore, efficient stress relaxation was observed in rheological experiments. At a temp- erature of 200 °C, the characteristic relaxation time τ was only a a few seconds. Following this work, Konkolewicz and co- Supramolecular Polymer Chemistry group, Department of Chemical Engineering and Chemistry, and Institute for Complex Molecular Systems, Eindhoven University workers designed acrylic polymeric materials which were dyna- 43 of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands. mically crosslinked with anilinium salts. These dynamic E-mail: [email protected], [email protected] polymeric materials show excellent malleability at 60 °C and b Brightlands Materials Center (BMC), P.O. Box 18, 6160 MD Geleen, The moreover could be quantitively healed by mild heating to Netherlands 60 °C or 80 °C. A more detailed kinetic study elucidated the †Electronic supplementary information (ESI) available: Synthetic procedures, kinetic analysis, calculation of activation energy, reaction mechanisms material exchange mechanism and it was found that although the characterization and supplementary figures. See DOI: 10.1039/d1py00292a exchange follows a dissociative pathway, they have low degrees 3640 | Polym. Chem.,2021,12,3640–3649 This journal is © The Royal Society of Chemistry 2021 View Article Online Polymer Chemistry Paper of de-crosslinking and therefore the networks have properties of 103 and 106 respectively)52 will transalkylate much faster resembling those of associative CANs.44 Du Prez and coworkers than DMAP derivatives. An additional advantage is that one developed a reversible network via C–S transalkylation of DABCO molecule can form a crosslink between two polymer trialkylsulfonium salts.45 With kinetic studies they showed chains, because it contains two tertiary amine functional that alkylbrosylates are efficient transalkylation agents to groups. Finally, DABCO can easily be alkylated at low tempera- convert thioesters into trialkylsulfonium salts. These salts tures, making the alkylation process compatible with (meth) showed complete relaxation at elevated temperatures in rheolo- acrylates.53 gical stress relaxation experiments, and remolding by com- The work reported here investigates the potential of DABCO pression molding yielded defect free samples. Recycled in transalkylation reactions to create dynamically crosslinked samples showed no significant change in mechanical pro- acrylate networks. The kinetics of the exchange reaction of perties when measured in DMTA and tensile tests. Zhang and benzyl bromide with DABCO was studied in detail using a coworkers utilized similar C–S transalkylation reactions to small molecule model system. Additionally, we demonstrate recycle sulfur-cured rubber.46 the formation of a dynamic covalent network by crosslinking a Inspired by these systems, we aimed at developing a trans- linear benzylic bromide functionalized methacrylic prepolymer alkylation system that can be implemented in (meth)acrylate in a single step with DABCO. The cured network can be repro- polymers. Multifunctional (meth)acrylate polymers are a cessed and shows efficient stress relaxation in rheological typical example of thermosetting polymers and due to their measurements. fast curing behavior and compatibility with many different functionalities,47 they have many industrial applications e.g. in dentistry,48 coatings,3,49 liquid-crystal displays,50 inks and adhesives.4,51 The use of a two-step approach of linear Experimental polymerization followed by reversible crosslinking offers Materials Creative Commons Attribution 3.0 Unported Licence. opportunities for the introduction of dynamic behavior into All commercial chemicals and solvents were used as received, commercially available methacrylic polymers.38 A number of unless stated otherwise. 4-(Bromomethyl)benzoic acid and different tertiary amines were considered as reversible cross- 2-hydroxyethyl methacrylate were obtained from TCI EUROPE. linking agent. 1,4-Diazabicyclo[2.2.2]octane (DABCO) was All other chemicals for the synthesis were purchased from selected as the most promising candidate for fast exchange Sigma Aldrich. Solvents were obtained from Biosolve. reactions (Scheme 1). Compared to the popular nucleophilic catalyst 4-dimethylaminopyridine (DMAP), a pyridine derivate, the bicyclic aliphatic amine DABCO is a significantly better Characterization methods ff nucleophile, yet also a much better leaving group (by a factor Di erential scanning calorimetry (DSC). Tg of the polymers This article is licensed under a was measured using a TA Instruments Q2000 differential scan- ning calorimeter equipped with an RCS90 cooling accessory using aluminum hermetic pans. 5–10 mg of sample was used Open Access Article. Published on 02 June 2021. Downloaded 9/24/2021 6:10:40 AM. per measurement. The general method consisted of four cycles in which a sample was measured from −50 to 200 °C with − heating and cooling rates of 10 °C min 1 (cycles 1 and 2), − − 20 °C min 1 and 40 °C min 1. The first heating run was dis- carded. Universal Analysis TRIOS software was used for data acquisition, midpoint at half height was used to determine Tg. Gel fraction. The mass fraction of the crosslinked part of the material is defined as the gel fraction. Approximately 250 mg of crosslinked material was weighed and immersed in 10 mL THF for 75 hours at room temperature. The samples were fil- tered and dried in a vacuum oven at 60 °C in a vacuum oven. The gel fraction was determined using the following equation:54 m Gel fraction ¼ dry ð1Þ minitial where minitial is the initial weight of the sample, and mdry is the mass of the sample after extraction and subsequent drying. Matrix assisted laser absorption/ionization-time of flight mass spectra (MALDI-TOF). Molar masses of the synthesized Scheme 1 Schematic representation of the topology rearrangements small molecules were determined