Catalyst-Free Vitrimers from Vinyl Polymers - ACS Publications

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Catalyst-Free Vitrimers from Vinyl Polymers Jacob J. Lessard, Luis F. Garcia, Charles P. Easterling, Michael B. Sims, Kyle C. Bentz, Scarlett Arencibia, Daniel A. Savin, and Brent S. Sumerlin* George & Josephine Butler Polymer Research Laboratory, Center for Macromolecular Science & Engineering, Department of Chemistry, University of Florida, Gainesville, Florida 32611, United States

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ABSTRACT: Cross-linked networks feature exceptional chemical and mechanical resilience but consequently lack recyclability. Vitrimers have emerged as a class of materials that feature the robustness of thermosets and the recyclability of thermoplastics without compromising network integrity. Most examples of vitrimers have involved new polymers with exchangeable bonds within their backbones. In pursuit of a more universal, commercially viable route, we propose a method utilizing commercially available and inexpensive reagents to prepare vitrimers from vinyl monomer-derived prepolymers that contain cross-linkable βketoester functional groups. Controlled radical copolymerization of methyl methracrylate and (2-acetoacetoxy)ethyl methacrylate afforded linear prepolymers that were converted into vitrimers in a single step by treatment with a trifunctional amine. These materials displayed the characteristic features and reprocessability of vitrimers over as many as six (re)processing cycles. Critically, the networks prepared through this process largely retain the chemical and thermal properties of their linear counterparts, suggesting this method holds significant utility as a user-friendly and commercially relevant approach to the rational design of vitrimers with diverse properties.

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allowing the material to flow and be reprocessed.25,26 This mechanism of cross-link exchange is essential to the unique properties of vitrimers. In a traditional dynamic network, a reversible cross-link must first break before finding a suitable partner to re-form the cross-link (“dissociative” exchange). In vitrimers, exchange occurs when pendent reactive groups dispersed within the network react directly with an existing cross-link to form a new cross-link and another reactive functional group capable of further exchange. This degenerate process conserves the number of cross-links at all times via “associative” exchange, as shown in Figure 1. Associative exchange results in a constant cross-link density, ensuring the fidelity of the network and not compromising the material properties during (re)processing. Many vitrimers require an exogeneous catalyst to activate cross-link exchange on relevant time scales.26−34 While this can produce very rapid and modifiable rates of exchange, the catalyst may be unstable or prone to leaching from the network, potentially undermining the recyclability of the material.35 Therefore, a number of catalyst-free vitrimers have been developed that rely on the exchange of silyl ethers,36 vinylogous urethanes,37−40 vinylogous ureas,40 trialkylsulfonium salts,41 dioxaborolanes,42 boroxines,43 imines,44 Meldrum’s acid derivatives,45 and hydroxyurethanes.46

INTRODUCTION Commodity plastics are traditionally classified according to two different categoriesthermoplastics and thermosets. Thermoplastics lack covalent interactions between chains and can therefore be readily repurposed or recycled. However, the properties of these materials are often compromised in harsh environments (e.g., high temperatures or exposure to solvents). In contrast, thermosets are covalently cross-linked, endowing them with significant thermal, mechanical, and chemical resistance, but the presence of a permanent network prohibits the use of traditional recycling methods. Recent advances have been made to bridge these two classes of materials by creating networks with cross-links that are dynamic under specific stimuli, enabling the preparation of materials that are recyclable while still retaining adequate physical properties. Many exchangeable cross-links have been explored, including both supramolecular interactions1−6 and dynamic−covalent bonds.7−20 However, many of these networks suffer from creep, stress relaxation, and/or loss of network integrity,3,21 and effective means of addressing these limitations are needed.22,23 Recently, a new class of networks with dynamic cross-links was developed to circumvent the drawbacks typically associated with many of the previous strategies toward recyclable thermosets. First reported by Leibler and coworkers, “vitrimers” are a class of covalent adaptable networks24 that behave as traditional thermosets at ambient temperature, while at elevated temperatures, the cross-links undergo dynamic exchange by an associative mechanism, © XXXX American Chemical Society

Received: November 21, 2018 Revised: February 7, 2019

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links were easily synthesized in a single step from the condensation of a primary amine with a β-ketoester.38,40 This approach allowed the vitrimer properties to be tuned by introducing acidic or basic additives.39 In these examplesas in most othersthe vitrimers were prepared by step-growth polycondensation of A−A and B−B monomers with B3 crosslinkers. Recently, Nicolaÿ, Leibler, and co-workers showed that commodity vinyl polymers such as poly(methyl methacrylate) (PMMA) and polyethylene could be converted into vitrimers through the incorporation of dynamic dioxaborolanes.42 Extending the vitrimer concept to existing commercially relevant polymers has obvious benefits. This approach allows the chemical composition (e.g., cross-link density) and physical properties of the materials to be tuned in a modular manner, though the reliance on organoboron compounds may prove challenging for large-scale production. In this report, we use only commercially available methacrylic monomers and a trifunctional amine to synthesize catalyst-free vitrimer networks utilizing the associative exchange of vinylogous urethanes. Copolymers of methyl methacrylate (MMA) and (2-acetoacetoxy)ethyl methacrylate (AAEMA), an inexpensive and commercially available monomer containing a reactive β-ketoester moiety, were prepared and then converted into networks via a condensation reaction with tris(2-aminoethyl)amine (TREN) in one step. Critically, this strategy decouples the network curing and polymerization steps, potentially allowing for significant variation in the linear segment molecular weight and functionality. The cured networks displayed the characteristic properties of vitrimers and exhibited high network fidelity over as many as six destruction−(re)processing cycles.

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RESULTS AND DISCUSSION Polymer Synthesis and Characterization. We targeted PMMA vitrimers in anticipation that the properties of the linear polymer, a critically important commodity plastic, would be reflected in those of the final material. We began our investigation by preparing statistical copolymers of MMA and AAEMA, an amine-cross-linkable unit, to be used as reactive

Figure 1. Schematic depiction of proposed amine exchange in vinylogous urethane vitrimer systems.

Du Prez and co-workers recently reported a promising class of vinylogous urethane vitrimers for which the dynamic cross-

Figure 2. RAFT copolymerization of MMA and AAEMA and subsequent thiocarbonylthio end-group removal: (A) general reaction scheme, (B, C) polymerization kinetics, and (D) GPC UV−vis trace of polymer before (blue) and after (orange) end-group removal with a visible color change of the polymer before (yellow) and after (white) end-group removal. B

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a solution of P(MMA-co-AAEMA) in tetrahydrofuran (THF) was treated with TREN at room temperature. The resultant organogel was evaporated to yield homogeneous, optically transparent vitrimer films (Figure 3B). Vitrimers with 5, 10, 15,

polymeric precursors (prepolymers) capable of network formation in the presence of multifunctional amines. Compositions containing 10 mol % AAEMA were targeted to maximize the content of MMA and achieve desirable thermal and mechanical properties while also retaining sufficient cross-link density to promote facile exchange. Conventional radical polymerization was initially used to prepare the prepolymers due to its ease of use, requiring no specialty reagents or stringent reaction conditions. However, we encountered challenges processing the networks derived from these prepolymers on viable time scales, presumably due to significant chain entanglement of the high molecular weight polymers produced by this route. Therefore, reversible addition−fragmentation chain transfer (RAFT) polymerization was adopted to target prepolymer molecular weights closer to the critical entanglement molecular weight (Me) of PMMA.47 Characterization of prepolymers by both gel permeation chromatography (GPC) and NMR spectroscopy confirmed that molecular weights remained below Mn of 11 kg/mol and that the incorporation of AAEMA was readily controlled by the initial comonomer feed (Figures S1−S6). We also investigated the polymerization kinetics to confirm the copolymerizations were controlled, as expected for a reversible-deactivation radical polymerization process (Figure 2 and Figure S1). As shown in Figure 2B, the growth of prepolymer molecular weight remained uniform with monomer conversion and dispersities were consistently low, indicating that side reactions that could produce ill-defined prepolymers were largely absent. Furthermore, analysis of monomer conversion over time revealed pseudo-first-order incorporation of both MMA and AAEMA throughout the reaction (Figure 2C) and that both comonomers were incorporated at similar rates (Figure S2), suggesting that reactive sites for cross-linking are uniformly distributed throughout the prepolymers. While well-defined prepolymers were obtained in a single polymerization step, polymers synthesized by the RAFT process are predominantly terminated by the thiocarbonylthio chain transfer agent moiety, which is prone to rapid aminolysis upon treatment with primary amines. In addition to consuming free amines that are necessary for associative bond exchange, this side reaction generates chain-end mercaptans that can further undergo oxidative chain coupling reactions. We therefore employed an end-group removal strategy to replace the reactive trithiocarbonate chain ends with inert cyanopropyl groups (Figure 2) by treatment of prepolymers with excess free radical initiator at 70 °C.48 Characterization by 1H NMR spectroscopy, GPC, and ultraviolet−visible (UV−vis) spectroscopy confirmed the success of the employed end-group removal strategy (Figure 2 and Figures S3−S7), as shown by the disappearance of resonances corresponding to the trithiocarbonate dodecyl group as well as disappearance of the absorption corresponding to the thiocarbonyl π → π* electronic transition in the prepolymer UV−vis spectrum (Figure S7). Additionally, complete removal of the trithiocarbonate end group was further confirmed by GPC analysis utilizing UV−vis detection corresponding to the thiocarbonyl n → π* electronic transition (420 nm, Figure 2D). Lastly, a distinct change in visual appearance of copolymer from yellow to white was observed following the removal of the thiocarbonylthio end group (inset of Figure 2D). Vitrimer Synthesis, Reprocessing, and Characterization. After end-group removal, prepolymers were converted to vitrimers in a straightforward solution cast procedure. First,

Figure 3. PMMA vitrimer network formation and processing: (A) network formation via condensation with trifunctional amine (TREN) and amine exchange as a route for reprocessability; (B) cured solution-cast network (C) before and (D-E) after compression molding at 160 °C (rheology sample); (F) processed rheology sample (round) and processed (left bar) and reprocessed (right bar) DMA samples.

and 20 mol % AAEMA incorporation were reprocessed via compression molding (Table S1). The lowest incorporation to afford reprocessesability, 10 mol % AAEMA, was chosen for the in-depth study. Formation of vinylogous urethanes was confirmed by FTIR spectroscopyafter initial processing, new absorbances appeared in the FTIR spectrum corresponding to N−H bending and CC stretching modes at 1655 and 1600 cm−1, respectively (Figure 4A).49 Thermal and mechanical properties of vitrimer films were then characterized by thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and rheological studies. According to TGA, networks were shown to exhibit similar thermal stability to a poly(methyl methacrylate) standard (12.6 kg/mol), with 50% degradation observed at a slightly higher temperature than the standard (Figure S9). Additionally, cured networks show little isothermal degradation over 6 h at 160 °C under nitrogen flow (Figure S10). DSC and DMA furthermore revealed glass transition temperatures (Tg) of 108 ± 8 and 110 ± 12 °C, respectively, which are in good agreement with observed Tg values for atactic PMMA.50 Finally, processed vitrimers exhibited a tensile storage modulus of up to 1800 MPa at room temperature, which again is in good agreement with observed PMMA commercial materials (Figures S34−S36).42 C

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Figure 4. Reprocessing and characterization of PMMA vitrimer networks: (A) FTIR spectra showing the appearance of vinylogous urethanes in the processed sample vs. prepolymer. (B) FTIR spectra of processed (P) and reprocessed (R) samples, showing retention of vinylogous urethane crosslinks. (C) Storage modulus from DMA over six compression cycles. (D) Tan δ from DMA over six compression cycles. (E) DSC thermograms over six compression cycles (cooling cycles).

believed the higher Ea could be attributed to limited diffusion of the reactive species due to polymer topology and stoichiometry.30 We investigated this possibility through the synthesis of a PMMA vitrimer with the same cross-link density but with 3 times the amount of free amine along the polymer (Figures S29−S33).The resulting material relaxed much faster than the original system; however, importantly, the Ea of the material (104 ± 3 kJ/mol) did not change, offering additional evidence that the nonexchangeable backbone diffusion is responsible for the increased Ea. The rate of flow for vitrimer materials is dictated by the exchange rate of the cross-linker. This is significant because the rate of cross-link exchange can afford a secondary transition temperature, the topology freezing transition temperature (Tv). Tv is the point in which the dynamic cross-link exchange rate increases enough to enable macroscopic flow (elastic solid to viscoelastic fluid) and, by convention, is the temperature in which the network reaches a viscosity of 106 MPa·s.25 With many exchange reactions occurring rapidly at low temperatures, this transition is rarely observed for high-Tg polymers due to the cross-links being effectively locked in place below the Tg: Tv is only experimentally relevant when greater than the Tg. However, a theoretical value can be extrapolated from the Arrhenius relationship as the temperature at which the viscosity reaches 106 MPa·s (eqs S2 and S3). The Tv for our

As a consequence of the dynamic nature of the cross-links, vitrimers feature the ability to be remolded or reprocessed into new materials after curing. To simulate a potential reprocessing approach, initial vitrimer films were either fragmented into small shards or ground into a powder before being compression molded at elevated temperatures to yield processed disks or bars for further testing (Figures S13− S15). Networks were reprocessed over five compression cycles (Figure 4) and characterized in a similar manner to the processed materials. As shown in Figure 4A, the characteristic peaks in the FTIR spectrum corresponding to vinylogous urethanes are retained over all reprocessing cycles, indicating that the cross-links remain stable under the thermal and mechanical stresses of destruction and reprocessing. Retention of network integrity was further proven through the lack of appreciable change in storage modulus (Figure 4B) and Tg (Figures 4C and 4D) after reprocessing. Stress-relaxation experiments were performed to obtain the activation energy (Ea) of cross-link exchange by the Arrhenius relationship (Figure 5 and eq S2). In this case, Ea is the energy required for macroscopic flow of the material, which was calculated to be 102 ± 8 kJ/mol, was higher than some values previously reported for small molecule vinylogous urethanes and vinylogous urethane-containing materials without catalyst (60−81 kJ/mol),38,39 but not unprecedented.51 We originally D

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Figure 6. Chemical recycling of vitrimers: (i) addition of excess monofunctional amine, (ii) precipitation of modified prepolymer, and (iii) reintroduction of TREN to yield rejuvenated networks.

TREN-derived cross-links, this process enabled networks to be successfully un-cross-linked at temperatures above Tv. The modified prepolymer was recovered by precipitation; no chain scission during reprocessing and limited residual TREN crosslinking were observed (Figure S44). By selecting a low boiling monofunctional amine, such as n-butylamine, TREN can be reintroduced to the modified prepolymer to again produce a network upon a successive curing cycles. The resulting network could not be dissolved with introduction of solvent and heat, indicating that cross-links were reestablished with the addition of the cross-linker.

Figure 5. Stress relaxation and Arrhenius relationship (A) stress relaxation at various temperatures and (B) Arrhenius relationship of the vitrimer network. The individual colors in (A) and (B) correspond to identical samples.

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PMMA vitrimers was extrapolated from the Arrhenius plot (Figure 5) to yield a value of 47 °C, which is comparable with previously reported values and, as expected, is much below the PMMA vitrimer Tg.38 Additionally, we also observed a high apparent Ea (91 kJ/mol) for high molecular weight (52 kg/ mol) PMMA from stress relaxation experiments, which may explain the difficulty processing high molecular weight PMMA vitrimers (Figure S38). The results from control reactions were consistent with the hypothesis that vinylogous urethane exchange being responsible for the vitrimer properties of the networks. To further probe the thermal stability of the PMMA vitrimer, a PMMA homopolymer was subjected to benzylamine at the operating temperature for (re)processing (160 °C). After 12 h of heating followed by dialysis, the benzylamine-treated PMMA showed no evidence of side-chain amidation by NMR spectroscopy or GPC (Figures S39−S42), agreeing with current literature.52 These results led us to believe that permanent cross-linking of the networks, if apparent, occurs on significantly longer time scales than that of material (re)processing (Figure S20) or at much higher temperatures, such as over 180 °C. Additionally, retardation of healing was observed for PMMA vitrimers exposed to air at high temperatures for long durations of time. In consideration of the lack of observable amine exchange, retardation of flow at elevated temperatures was attributed to free amine oxidation from exposure to air for long durations during testing. To design a more robust system, we devised an alternative form of recycling by the addition of excess monofunctional amine with heat (Figure 6 and Figure S43). By displacing

CONCLUSIONS Upcycling commodity polymers into next-generation materials is crucial for novel material development with industrial relevance. We have demonstrated a modular approach to synthesize catalyst-free vinylogous urethane vitrimers from vinyl monomer-derived prepolymers. Solution cast crosslinking of prepolymers and compression molding, using commercially available monomer and cross-linker, was used to synthesize vinyl-polymer-derived vitrimer networks. Nevertheless, networks showed retention of vinylogous urethane cross-links and healing properties over several processing and reprocessing cycles. Additionally, a secondary form of recycling was implemented by un-cross-linking the network with monofunctional amine to yield a modified prepolymer, which could be repurposed or re-cross-linked. This system demonstrates excellent reprocessability and tunability, and we envision this approach leading to development of similar systems and a variety of applications. By decoupling the formation of polymer backbone and cross-linking, a variety of networks formed from commodity monomers can potentially be modified in a simple fashion to yield materials with customizable functionality or properties.

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ASSOCIATED CONTENT

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02477. E

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Experimental details including materials, instrumentation, synthesis, characterization, images, and calculations (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]fl.edu. ORCID

Jacob J. Lessard: 0000-0003-2962-6472 Daniel A. Savin: 0000-0002-9235-517X Brent S. Sumerlin: 0000-0001-5749-5444 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation [DMR-1606410 (B.S.S.), DGE340 1315138 (M.B.S.), and NSF DMR-1808204 (S.A.)].

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DOI: 10.1021/acs.macromol.8b02477 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b02477 Macromolecules XXXX, XXX, XXX−XXX