Supramolecular Polymer Networks Made by Solvent-Free

Oct 30, 2015 - Supramolecular polymer networks based on polyacrylates with hydrogen bonding 2-ureido-4[1H]-pyrimidinone (UPy) side chains are of ...
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Supramolecular Polymer Networks Made by Solvent-Free Copolymerization of a Liquid 2‑Ureido-4[1H]‑pyrimidinone Methacrylamide Christian Heinzmann,† Iris Lamparth,‡ Kai Rist,‡ Nobert Moszner,‡ Gina L. Fiore,† and Christoph Weder*,† †

Adolphe Merkle Institute, University of Fribourg, Chemin des Verdiers 4, CH-1700 Fribourg, Switzerland Ivoclar Vivadent AG, Bendererstrasse 2, FL-9494 Schaan, Liechtenstein

‡

S Supporting Information *

ABSTRACT: Supramolecular polymer networks based on polyacrylates with hydrogen bonding 2-ureido-4[1H]-pyrimidinone (UPy) side chains are of considerable interest due to the stimuliresponsive nature imparted by the reversible cross-links formed by dimerized UPy groups. Previously reported UPy-containing acrylic monomers are solid and show limited miscibility with comonomers, and this has stifled their (co)polymerization in bulk. We here report the synthesis of a liquid 2-ureido-4[1H]-pyrimidinone methacrylamide (UPy-OPG-MAA), which was made by connecting the UPy motif and methacrylamide (MAA) via an amine-terminated oligo(propylene glycol) (OPG) linker. The new monomer was miscible with conventional methacrylates. This permitted the photoinitiated free-radical bulk copolymerization with hexyl methacrylate (HMA) to afford a series of copolymers (poly(UPyOPG-MAA-co-HMA)) in which the UPy-OPG-MAA content was varied between 0 and 20 mol %. The investigation of the mechanical properties of these copolymers by dynamic mechanical analysis and adhesion tests revealed that the introduction of the UPy groups leads to an increase of the stiffness in the glassy state, the formation of a rubbery plateau above the glass transition temperature, and a significant increase of the adhesive strength. Joints bonded with poly(UPy-OPG-MAA-co-HMA) could be debonded on demand using light or heat.

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INTRODUCTION The use of noncovalent interactions to control assembly processes in macromolecular materials is an increasingly used design tool and is now widely employed to create supramolecular polymers in which polymerization is achieved via metal−ligand interactions or hydrogen bonding motifs.1−4 The initial motivation to utilize reversible supramolecular interactions in the context of polymer technology was the promise of simpler processing5,6 and recyclability;7,8 however, the dynamic and reversible binding of supramolecular motifs can also be used to introduce stimuli-responsive properties,9,10 which can be harnessed to create advanced functions such as mechanochromism,11,12 healability,13 shape memory behavior,14 (de)bonding on demand,15,16 and others. The majority of current work in this domain relies on the quadruple hydrogen bond forming 2-ureido-4[1H]-pyrimidinone (UPy) motif originally developed by Meijer and co-workers.17 The high dimerization constant of UPy makes it a powerful tool to create supramolecular materials;18−20 however, it also limits the solubility of UPy-containing compounds in many (nonpolar) solvents.21,22 While this effect may be advantageous in the case of linear supramolecular polymers assembled from UPyterminated telechelics, in which the UPy groups not only dimerize but also phase-separate into a hard phase,23 the © XXXX American Chemical Society

synthesis of polymers functionalized with UPy side chains by free-radical (co)polymerization of UPy-containing monomers requires polar solvents such as dimethyl sulfoxide or dimethylformamide that are capable of forming hydrogen bonds with the UPy groups and limit dimerization of the latter.24,25 One way to circumvent the low solubility of UPycontaining compounds is to temporarily “mask” the hydrogen binding motif with a cleavable group so that the free-radical polymerization of such altered monomers can be carried out in apolar solvents such as toluene.26 A different approach to increase the solubility of UPy-containing small molecules is to modify the pendant group in the isocytosine ring at the 6position27 or by attaching highly soluble substituents via the urea group.28,29 This type of derivatization may increase the solubility not only via entropic contributions but also, as was shown for oligo(ethylene oxide) “tails”, by way of lowering the dimerization constant (Kdim) of UPy from >106 M−1 to around 104 M−1.29 However, to our best knowledge, so far all UPycontaining monomers reported in the literature are solid and their miscibility with acrylic comonomers is often limited, Received: September 21, 2015 Revised: October 21, 2015

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DOI: 10.1021/acs.macromol.5b02081 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Technologies instrumentation (series 1200 HPLC) using a Varian 5 μm mixed-C guard column and two SEC columns, a refractive index detector (Optilab REX interferometric refractometer, λ = 658 nm, 40 °C), and multiangle laser light scattering (MALLS, miniDawn TREOS laser photometer, λ = 658 nm, 25 °C) from Wyatt Technology Corporation. Data were analyzed with ASTRA software, and a singleinjection method with the assumption of 100% mass recovery from the columns was used to estimate the incremental refractive index (dn/dc). A Mettler-Toledo Stare system was used for thermogravimetric analysis (TGA); experiments were conducted with a heating rate of 10 °C/min under N2. Differential scanning calorimetry (DSC) measurements were conducted (unless indicated otherwise) with a heating/cooling rate of 10 °C/min under N2 on a Mettler-Toledo Stare system. The glass transition temperatures (Tg) correspond to the maximum of the first derivative of the second heating cycle. A Carver CE Press was used to produce polymer films by compression molding at a temperature of 80−100 °C and a pressure of 1 ton for 3 min, using spacers (200 μm for DMA experiments; 80 μm for adhesive experiments) to control the film thickness. Dynamic mechanical analysis (DMA) experiments were conducted on a TA Instruments DMA Q800 under N2 with a heating rate of 5 °C/min at a frequency of 1 Hz with a 15 μm amplitude, unless indicated otherwise. The collected mechanical data are averages of 3−5 independent experiments, and all errors are standard deviations. Adhesion measurements were performed with a Zwick/Roell Z010 tensile tester equipped with a 10 kN load cell at a strain rate of 1 mm/ min at room temperature, and the samples were gripped with mechanical clamps. Single lap joints were made with stainless steel substrates, and polymer films (10 × 10 mm2) were placed on the bonding area. The joints were mechanically fixed during the bonding process (3 min in a preheated oven at 80 °C) to prevent undesired changes of the bonding area. The resulting lap joints were tested within 1 h after bonding. Reported shear strength results are averages of 3−5 samples. Synthesis of UPy-OPG. N-(6-Methyl-4-oxo-1,4-dihydropyridin-2yl)-1H-imidazole-1-carboxamide (UPy-CDI), synthesized as previously reported27 (2.5 g, 11.4 mmol), was dispersed in Jeffamine D-400 (oligo(propylene glycol), OPG, Mn = 430 g/mol; 49.0 g, 114 mmol, 10 equiv under N2 atmosphere), and the mixture was stirred at 40 °C overnight. The clear liquid was allowed to cool to room temperature and poured into cold hexane. The yellow, oily liquid that separated as the lower portion of a two-phase system was collected, diluted with CHCl3 (30 mL), and washed with brine (3 × 50 mL) and deionized water (1 × 50 mL). The organic phase was dried over Na2SO4 and filtered, the solvent was removed by rotary evaporation, and the resulting oil was purified by flash column chromatography (diethyl ether/ethanol, gradient 0−100% ethanol) and dried under vacuum overnight to afford the title product as a clear, slightly yellow, viscous liquid (4.38 g, 66%). 1H NMR (300 MHz, CDCl3): δ = 11.81 (bs, 1 H, H11), 9.95 (bs, 1 H, H1), 5.76 (s, 1 H, H16), 3.99 (bs, 1 H, H2), 3.77−2.96 (m, 22 H, H2,3,6,7), 2.17 (s, 3 H, H20), 1.31−1.17 (m, 3 H, H4), 1.17−0.87 (m, 20 H, H4,9). 13C NMR (75 MHz, CDCl3): δ = 172.8, 156.1, 154.8, 148.2, 106.7, 104.2, 75.5, 75.4, 75.3, 73.3, 73.1, 73.0, 72.6, 72.3, 65.9, 47.1, 46.6, 46.3, 46.1, 30.4, 19.5, 18.6, 18.3, 17.4, 17.3, 17.2, 15.3. MS (ESI): n = 6: m/z = 574.2 (+ H+; calcd = 537.37). Synthesis of UPy-OPG-MAA. UPy-OPG (3.5 g, 6.03 mmol) was diluted with anhydrous CHCl3 (40 mL), stirred under N2 atmosphere, and cooled to 0 °C, before anhydrous triethylamine (0.60 mL, 8.28 mmol, 1.37 equiv) was added dropwise. After the addition was complete, the reaction mixture was stirred for another 10 min at 0 °C. Methacrylic anhydride (0.98 mL, 6.64 mmol, 1.1 equiv) was added dropwise, before the reaction mixture was allowed to warm to room temperature and was stirred overnight. The organic phase was collected, washed with brine (3 × 50 mL), dried over Na2SO4, and filtered, before the solvent was removed by rotary evaporation. The crude product was purified by flash column chromatography (diethyl ether/ethanol, gradient 0−20% ethanol) to yield the title compound as a clear, slightly yellow, viscous liquid (2.62 g, 67%). 1H NMR (300 MHz, CDCl3): δ = 13.09 (bs, 1 H, H14), 11.84 (bs, 1 H, H11), 9.98 (bs, 1 H, H1), 6.42−6.16 (m, 1 H, H8), 5.79 (s, 1 H, H24), 5.66 (s, 1

which prevented (co)polymerizations in bulk. We show here that a liquid UPy-methacrylamide can be obtained by linking the UPy group to a methacrylamide (MAA) via an amineterminated oligo(propylene glycol) (OPG) spacer (Figure 1).

Figure 1. Chemical structure of UPy-OPG-MAA (top), the copolymerization with methacrylic monomers to form cross-linked supramolecular polymers (middle), and pictures of the UPy-OPGMAA monomer and poly(UPy-OPG-MAA-co-hexyl MA) (bottom).

The resulting UPy-OPG-MAA was found to be miscible with conventional methacrylates, which allows for its photoinitiated free-radical copolymerization in bulk. These copolymers form supramolecular networks in which the UPy side chains serve as reversible noncovalent cross-linkers (Figure 1). Different reaction conditions were screened by varying the solvent, monomer type and composition, temperature, reaction time, and initiation method on the copolymerization behavior of UPy-OPG-MAA and tested the mechanical and adhesive properties of the resulting polymers.

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EXPERIMENTAL SECTION

Materials. Jeffamine D-400 (oligomeric propylene glycol, OPG, Mw = 430 g/mol, DP = 6.1) was donated by Huntsman AG (Switzerland). All other chemicals were purchased from SigmaAldrich. All methacrylic monomers were passed over basic alumina prior to use. Chloroform and dimethylformamide (DMF) were dried over molecular sieves (3 and 4 Å, respectively). Triethylamine was dried over 4 Å molecular sieves. Toluene was dried by passing over a basic alumina column. All other chemicals were used as received. Methods. 1H NMR (300 MHz) and 13C NMR (75 MHz) spectra were recorded on a Bruker Avance III spectrometer in CDCl3 (unless indicated otherwise), and the residual CHCl3 (δ = 7.26 ppm) was used as a reference to express chemical shifts. Size exclusion chromatography (SEC, THF, 40 °C, 1.0 mL/min) was carried out with Agilent B

DOI: 10.1021/acs.macromol.5b02081 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of UPy-OPG-MAA

Solution Polymerization with a Photoinitiator. UPy-OPGMAA (480 mg, 738 μmol) was diluted with anhydrous toluene (0.48 mL) in a round-bottomed flask equipped with a magnetic stir bar under N2 atmosphere, and 2-hydroxy-2-methyl-1-phenyl-propan-1-one (1.7 μL, 11.1 μmol) was added. After brief stirring, the reaction mixture was sparged with N2 for 10−15 min. The reaction mixture was subsequently stirred and irradiated with UV light (λ = 365 nm; 0.9 mW/cm2) for 1 min before it was poured into methanol (20 mL). After decanting the supernatant, hot DMF was added to swell the product, and the polymer was further dried under high vacuum for 2 days to yield a clear, slightly rubbery solid (46 mg, 10%). Details of other polymerization reactions based on this protocol are compiled in Table S3. Bulk Copolymerization with a Photoinitiator under Low Conversion Conditions. Hexyl methacrylate (HMA) (1.60 mL, 8.11 mmol) and UPy-OPG-MAA (586 mg, 902 μmol) were combined in in a round-bottomed flask equipped with a magnetic stir bar, and the mixture was stirred under N2 atmosphere until homogeneous. 2Hydroxy-2-methyl-1-phenyl-propan-1-one (20.6 μL, 135 μmol) was added, and after brief stirring, the reaction mixture was sparged with N2 for 10−15 min. The reaction mixture was subsequently irradiated with UV light (λ = 365 nm; 0.9 mW/cm2) until the solution gelled (ca. 8.5 min). At this point, the reaction mixture was poured into methanol (20 mL). After decanting the supernatant, the polymer was redissolved in chloroform (10 mL) and precipitated into methanol (150 mL). This dissolution/precipitation cycle was repeated once more. The product was separated by removing the supernatant solvent and dried under high vacuum for 2 days to yield a clear, rubbery solid (629 mg, 32%). Details of other polymerization reactions based on this protocol are compiled in Table S4. Bulk Copolymerization with a Photoinitiator to Full Conversion. Hexyl methacrylate (HMA) (2.96 mL, 15.0 mmol) and UPy-OPG-MAA (513 mg, 789 μmol) were combined in in a round-bottomed flask equipped with a magnetic stir bar, and the mixture was stirred under N2 atmosphere until homogeneous. 2Hydroxy-2-methyl-1-phenyl-propan-1-one (36.0 μL, 237 μmol) was added, and after brief stirring, the reaction mixture was sparged with N2 for 10−15 min. The reaction mixture was then poured into a PTFE mold (see Figure S6) placed under N2 atmosphere and irradiated with UV light (λ = 365 nm; 0.9 mW/cm2) until the reaction mixture was solid (25 min). The solid, rubbery samples were removed from the mold and were used as prepared (3.07 g, 99%). Details of other polymerization reactions based on this protocol are compiled in Table S5. Bulk Copolymerization with a Photoinitiator between Quartz Glass Single Lap Joints. The reaction mixtures were prepared as described above (and in Table S5), but the monomer mixtures were polymerized between two quartz glass slides (overlap 12 × 12 mm2) instead of a PTFE Petri dish. For one lap joint, two drops

H, H24), 5.28 (s, 1 H, H16), 4.23−3.88 (m, 2 H, H2,7), 3.76−3.27 (m, 20 H, H3,6), 2.20 (s, 3 H, H20), 1.94 (s, 3 H, H25), 1.28−1.04 (m, 23 H, H2,4,7,9). 13C NMR (101 MHz, CDCl3): δ = 172.83, 167.88, 156.22, 154.73, 148.10, 140.38, 119.17, 106.76, 75.54, 75.39, 75.25, 75.02, 73.38, 72.95, 72.57, 72.10, 71.95, 58.20, 53.50, 46.43, 46.15, 45.57, 45.39, 18.96, 18.78, 18.69, 18.46, 17.73, 17.44, 17.35, 17.25, 17.05. MS (ESI): n (number-average degree of polymerization of OPG) = 6: m/z = 641.39998 (found: 664.38905 (+ Na+; calcd = 664.38920)). EA: calculated for n = 6: C = 58.01; H = 8.64; N = 10.91. Found: C = 57.4; H = 8.9; N = 10.9. Polymerization Procedures. Typical examples of all (co)polymerization reactions are described in the following; more detailed information is provided in Tables S1−S6 of the Supporting Information. Bulk Copolymerization with a Thermal Initiator. UPy-OPGMAA (1.02 g, 1.57 mmol) was added to butyl methacrylate (2.24 mL, 14.1 mmol) in a round-bottomed flask equipped with a magnetic stir bar under N2 atmosphere, and azobis(isobutyronitrile) (AIBN; 12.9 mg, 78.6 μmol) was added. After the AIBN had dissolved, the reaction mixture was sparged with N2 for 10−15 min. The reaction flask was placed in an oil bath that had been preheated to 70 °C, and the reaction mixture was stirred at this temperature for 1.5 h under N2. The reaction was stopped by removing the flask from the oil bath and cooling to room temperature, before cold methanol was added to precipitate the polymer. After decanting the supernatant, CHCl3 was added (10 mL) to swell (or in some cases dissolve) the polymer, which was then precipitated into cold methanol (100 mL). After removing the supernatant, the polymer was dried under high vacuum for 2 days to yield a clear, rubbery solid (1.13 g, 37%). Details of other polymerization reactions based on this protocol are compiled in Table S1. Solution Copolymerization with a Thermal Initiator. UPyOPG-MAA (0.25 g, 385 μmol) and hexyl methacrylate (1.44 mL, 7.31 mmol) were dissolved in anhydrous toluene (6 mL) in a roundbottomed flask equipped with a magnetic stir bar under N 2 atmosphere, and AIBN was added (31.6 mg, 192 μmol). After the AIBN had dissolved, the reaction mixture was sparged with N2 for 10− 15 min. The reaction flask was placed in an oil bath that had been preheated to 80 °C, and the mixture was stirred at this temperature for 6 h under N2. The reaction was stopped by removing the flask from the oil bath and cooling to room temperature, before cold methanol was added to precipitate the polymer. The crude polymer was filtered off, dissolved in diethyl ether (10 mL), and precipitated again into cold methanol (150 mL). The product was separated by removing the supernatant solvent and dried under high vacuum for 2 days to yield a clear, sticky solid (1.04 g, 68%). Details of other polymerization reactions based on this protocol are compiled in Table S2. C

DOI: 10.1021/acs.macromol.5b02081 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 2. Copolymerization Conditions of UPy-OPG-MAA with Methacrylate Comonomers to Form Poly(UPy-OPG-MAACo-RMA), with R Being an Alkyl or an Aryl Group

(ca. 40 μL) of the monomer mixture was placed on one quartz glass slide and immediately covered by the second quartz glass slide and subsequently irradiated with UV light (λ = 365 nm; 0.9 mW/cm2) until the lap joint was bonded (see the reaction times in Table S6). To render the samples suitable for UV-light-triggered debond on demand experiments, a light−heat converter (2-(5-chloro-2H-benzotriazole-2yl)-6-(1,1′-dimethylethyl)-4-methylphenol) was optionally added as indicated to the monomer mixture (1% w/w), and the mixture was polymerized in the quartz glass single lap joint. Details of polymerization reactions based on this protocol are compiled in Table S6.

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(HEMA) (Figure S5). Thus, with respect to solubility (or better: miscibility), UPy-OPG-MAA does indeed display the behavior targeted through its molecular design. To explore the possibility to (co)polymerize UPy-OPGMAA under free-radical conditions, a broad range of conditions was screened (Scheme 2, Tables S1−S5). Besides using the above-mentioned comonomers, the polymerization behavior in bulk versus solutions in polar (DMF) and nonpolar (toluene) solvents was explored and initiators that were thermally (azobis(isobutyronitrile), AIBN) or photochemically (2hydroxy-2-methyl-1-phenyl-propan-1-one, HMPP) cleaved and permitted conducting the reactions at elevated (AIBN) or room temperature (HMPP). First, the solvent-free (bulk) (co)polymerization of UPyOPG-MAA with AIBN as a thermal initiator (azobis(isobutyronitrile); AIBN) was tested (Table S1). Attempts to homopolymerize UPy-OPG-MAA at 70−75 °C afforded insoluble and nonfusible products, even if the conversion was limited to a few percent. The copolymerization of UPy-OPGMAA with BnMA, BMA, HMA, and MA yielded products that were soluble in (hot) DMF and chloroform and could also be melt-processed, but only if the UPy-OPG-MAA content in the monomer feed was kept at 5 mol % or lower. Invariably, if the UPy-OPG-MAA content was increased above this value, insoluble and nonfusible products were obtained, indicative of covalent cross-linking. Attempts to conduct the thermally initiated copolymerization of UPy-OPG-MAA with BMA or HMA in DMF or toluene solutions yielded similar results (Table S2). Speculating that the insolubility of the (co)polymers made by thermally induced free radical polymerization may be at least in part be associated with covalent cross-links that are introduced by chain-transfer reactions involving the UPy motif, and that such side reactions might be reduced by conducting the reaction under conditions that favor UPy dimerization (low temperature and nonpolar environment21,31), we explored the (co)polymerization of UPy-OPG-MAA at room temperature in toluene, using the photoinitiator HMPP and HMA as comonomer (Table S3). The photopolymerization reactions were conducted at a low and a high concentration (20 and 53% w/w total monomer in toluene, respectively) and stopped at various times (0.5−45 min) to explore any possible influence of the conversion. Indeed, the (co)polymerization reactions yielded products that were melt-processable, although only the copolymer with 10 mol % UPy-OPG-MAA made in toluene was soluble. Building on this outcome, the photoinitiated copolymerization of UPy-OPG-MAA with HMA as comonomer was carried out in the bulk, using 0, 5, 10, 20, and 100 mol % UPy-OPGMAA and keeping the reaction times (2−15 min) and therewith the conversion (