Inexpensive and Recyclable Thermosets Jacob J

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Polystyrene-Based Vitrimers: Inexpensive and Recyclable Thermosets Jacob J. Lessard, Georg M. Scheutz, Rhys W. Hughes, and Brent S. Sumerlin*

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ABSTRACT: We report a straightforward and scalable method for the generation of polystyrene-based vinylogous urethane vitrimers using conventional radical polymerization. The copolymerization of the commercially available and inexpensive monomers styrene and (2-acetoacetoxy)ethyl methacrylate pro- duced β-ketoester-functional network precursors on a multigram scale, which could be cross-linked with diamines to yield thermally robust vitrimer materials. Vitrimers were (re)processed over three destruction/compression cycles with acid catalysis to overcome the eﬀects of backbone entanglements. Lastly, the viscoelastic properties were investigated, revealing a higher activation energy ﬂ for viscous ow (Ea) compared with previously prepared methacrylate-based analogs. KEYWORDS: covalent adaptable network, vitrimer, reprocessable thermosets, dynamic cross-links, recyclable networks

he cross-linked structure of a thermoset endows the effective tool for investigating the fundamental relationships T material with mechanical durability and chemical between network structure and flow behavior,14,27 the broader resistance. However, these benefits come at the cost of utility of simplified conventional radical polymerization for dramatically impeded reprocessability via conventional ther- industrial applications is undeniable.28 Despite this, in the field momechanical methods.1 To circumvent the static nature of of vitrimers, there are very few reports that take advantage of thermosets without entirely compromising their stability, the simplicity of conventional radical polymerization of vinyl network structures containing dynamic linkages have been monomers to generate vitrimer materials.8,29,30 Nishimura et developed to permit cross-link exchange and macroscopic al. synthesized vitrimers via silyl ether cross-linking of linear 2,3 flow. In particular, vitrimers have shown great promise in copolymers from styrene and 2-(4-vinyl-benzyloxy)-ethanol 2,4 terms of material properties and reprocessability. Thus far, prepared by conventional radical polymerization.8 However, Downloaded via UNIV OF FLORIDA on June 2, 2021 at 21:48:55 (UTC). most research has focused on the exploration of different cross- both the cross-linker and the reactive comonomer in this 5−9 10−13 link exchange chemistries, catalyst systems, and report required additional synthetic steps, potentially limiting 14−17 network structures. However, a critical hurdle for the scalability and cost efficiency. application of vitrimers lies in the translation from the

See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. Herein, we report a straightforward protocol for the scalable academic laboratory to industrially relevant materials and 2,18−20 synthesis of polystyrene-based vitrimers (VitPS) from commer- procedures. For example, vitrimer materials have been cially available and inexpensive reagents using conventional developed using readily available, inexpensive monomers or radical polymerization (Figure 1). Styrene and (2- postpolymerization modiﬁcation from commercially available 7,8,21−26 acetoacetoxy)ethyl methacrylate (AAEMA) were copolymer- polymers. Our group recently prepared vitrimeric ized at 70 °C, yielding linear vitrimer precursors. The poly(methyl methacrylate) mimetics that exhibited superior incorporation of the β-ketoester-bearing AAEMA monomer thermal and solvent stability while maintaining reprocessability allowed for facile network formation with xylylene diamine via through amine exchange of vinylogous urethane (VU) cross- 31 26 VU formation. The polymerization of the network precursors links. Moreover, we reported that the phase separation and the vitrimer formation were scalable (>15 g of polymer present in vitrimers derived from block copolymers leads to enhanced resistance to macroscopic deformation as compared with networks prepared from statistical copolymers of identical Received: May 18, 2020 compositions.14 The polymers in these two previous studies Accepted: June 29, 2020 were synthesized using reversible addition−fragmentation Published: July 9, 2020 chain transfer (RAFT) polymerization to limit the molecular weight and allow block copolymer formation, respectively. Although the control aﬀorded by RAFT polymerization is an

Figure 1. Polymerization scheme, vitrimer formation scheme, and reprocessing images for polystyrene-based vitrimers.

fi and >6 g vitrimer generated), and the networks could be Figure 2. Synthesis of poly(styrene-co-AAEMA). (A) Pseudo- rst- order rate plot of the averaged monomer consumption with time reprocessed over three deconstruction/compression cycles, showing a constant radical concentration throughout the course of the showing the retention of the network structure. polymerization. (B) Pseudo-first-order rate plot of each monomer Finally, we probed the viscoelastic properties of the resulting consumption showing a faster incorporation of AAEMA. (C) Percent polystyrene-based vitrimers, revealing a higher activation of unreacted AAEMA monomer compared with unreacted styrene fl energy for viscous ow (Ea) compared with previously monomer. (D) Gel permeation chromatography traces of poly- prepared methacrylate-based analogs.14,26 (styrene-co-AAEMA) polymerization over time, normalized using the The copolymerization of styrene with AAEMA (2 M total reaction solvent as an internal standard, show an increase in polymer concentration at a relatively constant molecular weight. monomer concentration, [AAEMA]o/[Sty]o 40:60) was conducted at 70 °C in toluene using azobis(isobutyronitrile) as a radical initiator (Figure 1). The copolymer molecular weight remained relatively constant with monomer conversion, as expected for conventional radical polymerizations (Figure 2A−D). The averaged monomer consumption followed a pseudo-first-order rate law, suggesting a constant radical concentration throughout the polymerization (Figure 2A). Notably, we found that AAEMA was consumed at a slightly faster rate than styrene, which is reflected in the decreasing monomer ratio of AAEMA to styrene and the steeper slope of the linear pseudo-first-order rate plot of AAEMA (Figure 2B,C). The differing rates of incorporation of AAEMA and styrene are indicative of a slight compositional drift during the copolymerization. In early stages, copolymers slightly enriched in AAEMA are generated, shifting toward styrene-rich copolymers in later stages as the reaction mixture becomes depleted of AAEMA (Figure 2D). Poly(styrene-co-AAEMA) (72.2 kg/mol, Đ = 1.62, 43% overall AAEMA incorporation) was isolated in multigram quantities through precipitation into methanol, followed by drying under reduced pressure (Figures β Figure 3. Formation of vitrimers by solution-casting poly(styrene-co- S1 and S2). The preservation of the -ketoester on the AAEMA) with a xylylene diamine cross-linker and p-toluenesulfonic polymer backbone was verified by Fourier transform infrared acid (TsOH) catalyst. (A) Stacked and normalized ATR-FTIR (FTIR) and 1H NMR spectroscopy (Figure 3A and Figure S3). spectra of poly(styrene-co-AAEMA) (orange) and vitrimer (black) Network formation was achieved by cross-linking poly- with a β-ketoester (purple) and vinylogous urethane (red) small- (styrene-co-AAEMA) with xylylene diamine (XDA, 0.86 mol molecule analog. (B) Differential scanning calorimetry traces of the equiv of diamine to AAEMA) and p-toluenesulfonic acid (0.1 polymer (orange) and vitrimer (black) showing an increase in Tg after mol % to XDA) as a catalyst via a solution-cast procedure cross-linking. The inset image shows the vitrimer material before and (Figure 3 and Figure S4).12 The resulting film was cured at 80 after processing. °C and compression-molded at 160 °C under a vacuum environment to yield the processed vitrimer material. The Additionally, the increase in the glass-transition temperature ffi fi ° e cient formation of VU cross-links was con rmed through (Tg) from 40 to 100 C was consistent with the formation of a ATR-FTIR spectroscopy by comparing the characteristic VU covalently cross-linked network structure (Figure 3B). ∼ −1 signals at 1650 and 1600 cm with carbonyl peaks from To validate the reprocessability of the networks, VitPS was 1700 to 1750 cm−1 in small-molecule analogs (Figure 3A). subjected to two additional destruction/compression cycles,

3045 https://dx.doi.org/10.1021/acsapm.0c00523 ACS Appl. Polym. Mater. 2020, 2, 3044−3048 ACS Applied Polymer Materials pubs.acs.org/acsapm Letter

Figure 4. Vitrimer reprocessing and analysis. (A) Image of the vitrimer material after reprocessed cycles (P, processed; R×1, first reprocess cycle; R×2, second reprocess cycle). (B) Storage (solid) and loss (dashed) modulus of (re)processing cycles showing a slight increase in the rubbery plateau modulus for R×2. (C) Tan(δ) for all three (re)processing cycles. (D) Differential scanning calorimetry traces of vitrimer materials after × (re)processing showing an increase in Tg for R 2 (heating, exotherm up). (E) Stacked and normalized ATR-FTIR spectra after (re)processing cycles showing the retention of the vinylogous urethane signal. (F) Stacked and normalized Raman spectra after (re)processing cycles showing the retention of characteristic peaks. (G) Dissolution study after (re)processing cycles in bulk benzyl amine before and after heating to 100 °C for 5 h. which we denote here as reprocessing cycles (Figure 4A). The constituting higher molecular weight backbones displayed destruction process consisted of the sample being fragmented inadequate reprocessability. We believe this was due to the into smaller shards/powder followed by compression at 160 combination of sluggish chain diffusion from polymer °C to yield the reprocessed material. The (re)processed entanglements and slow cross-link exchange at high molecular vitrimers were characterized by dynamic mechanical analysis weights. Therefore, in this study, the rate of cross-link ff (DMA), di erential scanning calorimetry (DSC), ATR-FTIR exchange was increased through excess reactive free amine spectroscopy, optical microscopy coupled to Raman spectros- groups and toluene sulfonic acid as a Brønsted acid catalyst.13 copy, and solubility tests (Figure 4 and Figures S5−S11). The viscoelastic properties of the VitPS were investigated Isochronic DMA traces show a slight increase in the rubbery using rheology in the form of stress relaxation studies at six plateau of the storage modulus and tan(δ) maximum for the different temperatures ranging from 135 to 160 °C(Figure 5, second reprocessing cycle, suggesting an increase in the cross- − link density and decreasing chain mobility of the system Figures S12 20,andTable S2). At all temperatures investigated, the materials displayed dynamic behavior and (Figure 4B,C). Additionally, the DSC traces show an increase ff in T , from 100 to 120 °C, further indicating an increase in the e ectively dissipated the stress induced by a 0.3% displacement g fl cross-link density (Figure 3B). The increasing cross-link (Figure 5A). To investigate the temperature-dependent ow of fl density could be explained by either the permanent cross- VitPS quantitatively, the energy of activation for viscous ow link formation of the backbone through amidation of the methacrylate esters or by the loss of the xylylene diamine during reprocessing under vacuum. However, we detected little to no change in the IR absorbance or Raman signal of the VU moiety over reprocessing cycles, other than a decreased signal- to-noise ratio, which we believe is due to increased surface defects (Figure 4E,F). Additionally, all (re)processed VitPS were insoluble in various solvents but dissolved when exposed to monofuctional amine and heat (Figure 4G, Figures S8−S10, and Table S1), which should induce the displacement of VU cross-links (Scheme S1). The solubility of the reprocessed vitrimers treated under these conditions suggests that the extent of permanent (i.e., irreversible) cross-link formation is relatively minimal. 26 Figure 5. Rheology of polystyrene vitrimers. (A) Vitrimer stress In our original report, vitrimers of low-molecular-weight relaxation at a constant 0.3% strain. (B) Arrhenius plot, with ∼ τ ( 10 kg/mol) poly(methyl methacrylate)-based backbones activation energies (Ea), where was obtained from a stretched were reprocessed without a catalyst; however, networks exponential fit of the stress relaxation.

3046 https://dx.doi.org/10.1021/acsapm.0c00523 ACS Appl. Polym. Mater. 2020, 2, 3044−3048 ACS Applied Polymer Materials pubs.acs.org/acsapm Letter

(Ea) was calculated from the stress relaxation experiments. Ea Author Contributions describes the sensitivity of viscous flow to temperature and can The manuscript was written through contributions of all be obtained from the slope of the Arrhenius plot of the authors. τ τ characteristic relaxation time ( ) versus temperature. Often, Notes fi is identi ed as the time when the normalized modulus in stress The authors declare no competing financial interest. relaxation experiments approaches the value of 1/e, which assumes relaxation via a single Maxwell element. However, ■ ACKNOWLEDGMENTS polymer networks usually contain multiple relaxation modes and are better fit by a stretched exponential that accounts for This material is based on work supported by the National multiple modes of relaxation.15 Therefore, the τ at each Science Foundation (DMR-1904631). We thank Anton Paar temperature was calculated from the stress relaxation fittoa for the use of the Anton Paar 702 rheometer through their VIP stretched exponential (Figure S12, Table S2) and plotted with academic program. We also thank Cameron D. Morley and ± Prof. Thomas E. Angelini for their assistance in the operation temperature to calculate an Ea of 165 7 kJ/mol (Figure 5B). This activation energy is higher compared with our previous of the rheometer. reports, which can perhaps be attributed to the higher polymer molecular weight and the different network environment.32,33 ■ REFERENCES In summary, we have demonstrated a straightforward and (1) Fortman, D. J.; Brutman, J. P.; De Hoe, G. X.; Snyder, R. L.; scalable method for the generation of polystyrene-based VU Dichtel, W. R.; Hillmyer, M. A. Approaches to Sustainable and β Continually Recyclable Cross-Linked Polymers. ACS Sustainable vitrimers using conventional radical polymerization. - − Ketoester-bearing copolymers with molecular weights consid- Chem. Eng. 2018, 6, 11145 11159. erably higher than the entanglement threshold were used to (2) Scheutz, G. M.; Lessard, J. J.; Sims, M. B.; Sumerlin, B. S. Adaptable Crosslinks in Polymeric Materials: Resolving the form vitrimer materials through VU cross-links. Such vitrimers Intersection of Thermoplastics and Thermosets. J. Am. Chem. Soc. were (re)processed over three destruction/compression cycles 2019, 141, 16181−16196. with acid catalysis to overcome the effects of backbone (3) Bowman, C. N.; Kloxin, C. J. Covalent Adaptable Networks: entanglements. We believe covalent adaptable networks are a Reversible Bond Structures Incorporated in Polymer Networks. crucial tool in the generation of innovative materials, and Angew. Chem., Int. Ed. 2012, 51, 4272−4274. uncomplicated, cost-effective strategies like this will be the (4) Van Zee, N. J.; Nicolay , R. Vitrimers: Permanently crosslinked platform for industrial applications. polymers with dynamic network topology. Prog. Polym. Sci. 2020, 104, 101233. ■ ASSOCIATED CONTENT (5) Brutman, J. P.; Delgado, P. A.; Hillmyer, M. A. Polylactide Vitrimers. ACS Macro Lett. 2014, 3, 607−610. *ı s Supporting Information (6) Denissen, W.; Rivero, G.; Nicolay , R.; Leibler, L.; Winne, J. M.; The Supporting Information is available free of charge at Du Prez, F. E. Vinylogous Urethane Vitrimers. Adv. Funct. Mater. https://pubs.acs.org/doi/10.1021/acsapm.0c00523. 2015, 25, 2451−2457.

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3048 https://dx.doi.org/10.1021/acsapm.0c00523 ACS Appl. Polym. Mater. 2020, 2, 3044−3048