Copolyampholytes Produced from RAFT Polymerization of Protic Ionic

Article Cite This: Macromolecules XXXX, XXX, XXX-XXX

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Copolyampholytes Produced from RAFT Polymerization of Protic Ionic Liquids Céline C. J. Fouillet,†,‡ Tamar L. Greaves,† John F. Quinn,‡ Thomas P. Davis,‡,§ Jozef Adamcik,∥ Marc-Antoine Sani,⊥ Frances Separovic,⊥ Calum J. Drummond,*,† and Raffaele Mezzenga*,∥ †

School of Science, College of Science, Engineering and Health, RMIT University, GPO Box 2476, Melbourne, VIC 3001, Australia ARC Centre of Excellence in Convergent Bio-Nano Science and Technology, Drug Delivery, Disposition and Dynamics Theme, Monash Institute of Pharmaceutical Sciences, Monash University, Parkville, VIC 3052, Australia § Department of Chemistry, University of Warwick, Gibbet Hill, Coventry CV4 7AL, England ∥ ETH Zurich, Department of Health Sciences and Technology, Schmelzbergstrasse 9, 8092 Zürich, Switzerland ⊥ School of Chemistry, Bio21 Institute, University of Melbourne, Melbourne, VIC 3010, Australia ‡

S Supporting Information *

ABSTRACT: Polyampholytic copolymers have been synthesized via reversible addition−fragmentation chain transfer (RAFT) polymerization of protic ionic liquid monomers. The monomers were prepared by acid−base proton exchange between acid and basic precursor conjugates, each containing one or more vinyl pendant groups. The polymerization, which was carried out without additional solvents present, led to high molecular weight glassy polymers, which were stable in the form of bulk viscous liquid ionic complexes. Various polyampholytes belonging to both linear and cross-linked families were prepared by judiciously varying the molecular structure of the acid precursor. Furthermore, by using solid-state NMR, the molecular arrangement of the polymeric backbone was identified, highlighting the presence of copolymers with both random and alternating copolymer chains which in the latter case involves a regularly alternating acid monomer, whereas the base occurs as a random sequence with the average and most probable number of monomers dictated by the stoichiometry used. The structural and mechanical properties of the resulting copolyampholytes were characterized by atomic force microscopy, peak-force quantitative nanomechanical analysis, differential scanning calorimetry, and small-angle X-ray scattering. These showed that the final polymers were essentially glassy and amorphous, with weak compositional fluctuations of the order of a few monomers and Young moduli in the range 1−3 GPa.

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water treatment, and as smart polymers.2−5 Polyampholytes have charge balance and maximum electrostatic interactions at their isoelectric point (IEP). Modifying the pH from the IEP leads to increased electrostatic repulsions and a swelling of the polymer network. Similarly, the addition of salt causes charge screening, and the subsequent reduction of electrostatic interactions leads to a swelling of the polymer.5,6 We have identified a novel route for the synthesis of ampholytic polymers via ionic liquid (IL) monomers, where both the cation and anion of the ionic liquid contain a polymerizable group. We have focused on using protic ILs as monomers due to the simplicity of synthesis of the protic ILs enabling a broad

INTRODUCTION Ampholytic polymers contain both positive and negative charges within the polymer structure on different monomer units.1−6 They are closely related to zwitterionic polymers which contain cationic and anionic charges on the same unit and polyelectrolytes which have either cationic or anionic charges present.1,7 Proteins are the most common type of polyampholytes, though there are also a broad range of synthetic ampholytic polymers which are useful in a broad range of applications, particularly due to their being stimuli responsive.1−6 Their stimuli response can be modified through use of weaker or stronger basic/acidic groups and having the ions present as random or block copolymers.4 The properties of these polyampholytes have led them to have many potential applications, including use as nonfouling and bacteria-resistant membranes, in drug delivery and controlled release, as tissue engineering scaffolds, in oil recovery, in © XXXX American Chemical Society

Received: August 16, 2017 Revised: October 23, 2017

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DOI: 10.1021/acs.macromol.7b01768 Macromolecules XXXX, XXX, XXX−XXX

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phosphate (A5). All the acids contain a vinyl group, which can undergo free radical polymerization.

variety of combinations to be explored. Previously there have been limited reports on the polymerization of protic ILs,8,9 though they have been used to give proton conductivity to polymers, such as by being doped or adsorbed onto polymers used in proton conducting membranes for fuel cells.8,9 A benefit of polymerizing protic ILs to form polyampholytes is that the proton conducting properties may be retained, without leaching of either ion and without requiring a separate polymer matrix. In addition, the desirable properties of protic ILs have the potential to be retained within the polymer, such as miscibility with many solvents, tailorability, and ability to form hydrogenbonded networks.10,11 Previously there have been substantial reports of polymerizable ILs, but to the best of our knowledge, this has only involved having either the cation or anion polymerized, with the other ion present as a counterion or in the form of zwitterionic pendant groups off the polymer backbone.12−19 The only polyampholytes made from ILs that we are aware of were reported by Ohno et al. and consist of a copolymer produced from a mixture of an IL with a polymerizable cation and a lithium salt with a polymerizable anion.20,21 The resulting polymers contain cation and anion groups within the backbone, with cationic and anionic counterions present.20,21 Polymerizable ILs have been developed for many applications, such as for CO2 capture22 and as multiresponsive polymers with stimuli responsive functional groups present,15 but predominantly they have been developed as polyelectrolytes for their use in energy storage devices due to their ionic conductivity.12,13,17,23 In this work we selected commercially available Brønsted acids and Brønsted bases which contained a polymerizable vinyl group suitable for free radical polymerization. The acid−base pairs were constrained to have an aqueous pKa difference of at least 5, as an indication that there was likely to be good proton transfer from the acid to the base. By use of this criterion, the Brønsted base of vinyl-1,2,4-triazole was selected along with four Brønsted acids, namely acrylic acid, vinylphosphonic acid, vinylbenzoic acid, and bis[2-(methacryloyloxy)ethyl] phosphate. Protic ILs were used as the monomer due to their simple synthesis, which is through an acid−base proton exchange reaction. The vinyl groups present on both the selected cations and anions are able to undergo free radical polymerization when an initiator is added. The resulting polymers are likely to have a broad range of molecular weights present and, if polymerized in bulk without the addition of any chain transfer agent, to exhibit a potentially hazardous exotherm. In this work we have controlled the polymerization process through the addition of reversible addition−fragmentation chain transfer (RAFT) agents, which govern how many live polymers there are and hence control the molecular weight and polydispersity. RAFT agents have previously been used successfully in the polymerization of ILs.24,25 We have prepared polyampholytes from protic ILs using RAFT agents to control the free radical polymerization.26,27 A total of nine polymers were prepared from five protic ILs through trialling three different RAFT agents.

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Other bases, such as 2-aminoethyl methacrylate, N-vinylcarbazole, or vinylformamide, were also tested but were not selected because the difference in pKa between the bases and the acids may not be sufficient for complete proton exchange. I.1. Synthesis of Protic Ionic Liquid (Protic IL) Monomers. The chemicals were all used as received: vinyl-1,2,4-triazole (B1) (SigmaAldrich, ≥97%), acrylic acid (A2) (Sigma-Aldrich, 99%), vinylphosphonic acid (A3) (Sigma-Aldrich, 97%), vinylbenzoic acid (A4) (Sigma-Aldrich, 97%), and bis[2-(methacryloyloxy)ethyl] phosphate (A5) (Sigma-Aldrich, liquid stored at 2−8 °C). The protic ILs were synthesized through slow addition of the acid into a round-bottom flask containing the base. The temperature of solution was maintained at 3−8 °C using an ice bath and was constantly stirred during the reaction and for 30 min after acid addition to ensure complete reaction. The protic ILs were dried using a rotary evaporator. The round-bottom flask was protected against light during the reaction and drying process, and the final protic ILs were stored in the refrigerator with light protection until the RAFT polymerization process was to be initiated. All protic ILs were characterized by IR spectroscopy before and after drying by rotary evaporation in order to verify the integrity of the terminal double bonds (Figures S14−S21 in the Supporting Information). 1H NMR was used to verify the acid−base proton exchange, which was particularly visible after drying with a large characteristic hydrogen shift (see Figure S1) from 7−8 ppm to 10−12 ppm, for the protic ILs dissolved in deuterated dimethyl sulfoxide compared to the precursor base. I.2. RAFT Polymerization. In order to maintain unchanged the dielectric constant of the protic ionic liquid monomer precursors, RAFT polymerization was carried out in a solvent-free, bulk environment: this prevented changing the Bjerrum length of fluid, preserving the strength of the ionic coupling among acid and base throughout the polymerization process. All the reversible addition−fragmentation chain transfer (RAFT) polymerizations were initiated by 2,2′-azobis(2-methylpropionitrile) (AIBN), without solvent at 70 °C and in the presence of a RAFT agent (R), selected among 2-cyanoisoprop-2-yl-benzodithioate (2CBD), 2-cyanobutanyl-2-yl 3,5-dimethyl-1H-pyrazole-1-carbodithioate (DTC), or 4-cyano-4-(((dodecylthio)carbonothioyl)thio)pentanoic acid (TTC). Each protic IL was initially loaded into a hermetic tube, followed by dissolving the initiator without additional solvent and last the transfer agent. The ratio of acid:RAFT-agent:initiator was 500:5:1 for all combinations. Where there was slow dissolution of the solid initiator or RAFT agent, the tubes (13 mm o.d.) were heated up to 40−50 °C, allowing faster dissolution with minimal radical generation. When all compounds were totally dissolved, the tube was deoxygenated with

EXPERIMENTAL METHODS

The primary selection of acids and bases was based on having a sufficiently different pKa between the two species to enable proton transfer to occur. After testing reactions between different acids and bases, our interest was focused particularly on one base, vinyl-1,2,4triazole (B1), and several acids: acrylic acid (A2), vinylphosphonic acid (A3), vinylbenzoic acid (A4), and bis[2-(methacryloyloxy)ethyl] B


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I.3. Analytical Methods. a. NIR Spectroscopy. The near-infrared (NIR) experiments were conducted at 70 °C, allowing a series of NIR spectra to be acquired as a function of time and the total conversion to be monitored via the progressive decrease in intensity of the vinyl group peaks. NIR spectroscopy has been extensively employed by other researchers29−31 to follow the evolution of conversion with polymerization time. To this end, additional RAFT polymerization was also carried out directly inside small near-infrared (NIR) tubes of 8 mm o.d. as shown in Figures S2 (panel a) in the Supporting Information, avoiding transfer of material and eventual contamination by solvent (water). After the same nitrogen degassing procedure, the NIR tubes were placed inside the NIR spectrometer (Shimadzu IR-Prestige 21 spectrometer equipped with a tungsten light source, CaF2 beam splitter, InGaAs detector, and sample heating stage from Pike Technologies) to start the polymerization. b. Solid-State NMR Spectroscopy. Solid-state NMR experiments were conducted at 298 K on a Varian Inova (Palo Alto, CA) 600 MHz NMR equipped with HXY BioMAS 3.2 mm probe (Varian) using Torlon inserts. The spinning speed was maintained at 10 kHz. All spectra were processed using NMR Pipe.32 13 C Solid-State NMR. 13C cross-polarization (CP) magic angle spinning (MAS)33 was carried out using a 1H excitation pulse of 94 kHz, followed by a CP contact time of 1 ms using a 10% ramped field on 1H and 45.5 kHz field on 13C and a 1H SPINAL64 decoupling at 94 kHz during acquisition, a spinning rate of 10 kHz, and a recycle delay ranging from 5 to 30 s. The FIDs were zero-filled to 8K points, and 75 Hz line

nitrogen gas for 15−20 min in order to remove oxygen and thereby avoid possible reactions between the formed radicals and oxygen which would inhibit the polymerization. The viscous solution was then heated at 70 °C on a hot plate28 (Supporting Information, Figure S2, panel b). During 20−24 h of magnetic agitation, the mixture color changed, going from transparent to yellow in the presence of reagents TTC or DTC and passing from red/pink to orange with the reagent 2CBD, indicating that polymerization had occurred. Monitoring of the polymerization was not possible by this first procedure.

Table 1. General Results from Near-Infrared (NIR) Spectroscopy (Conv in %), Gel Permeation Chromatography (GPC), and Differential Scanning Calorimetry (DSC) Analyses (Tg in °C) on Pol-9, Pol-6, and Pol-5, with Corresponding Structures from Protic IL Precursor Monomersa

a

Molecular weight data were obtained by GPC with PEO calibration (all others polymers are detailed in Table S1). C


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(20 keV) was used, and scattering data were acquired at 20 °C over a scattering vector, q, between 0.03 and 1.92 Å−1 with a 1 s acquisition time. f. Atomic Force Microscopy (AFM) and Peak Force Quantitative Nanomechanical (PF-QNM) Technique. Samples were fixed on a metallic support and ultramicrotomed in order to obtain a relatively flat surface of the synthesized polymer for AFM experiments. AFM and PF-QNM measurements were performed by using a MultiMode VIII scanning probe microscope (Bruker, USA) operated in intermittent mode under ambient conditions at a scan rate of 1 Hz. The microscope was covered with an acoustic hood to minimize vibrational noise. The AFM cantilevers (Bruker, USA) were calibrated on the calibration samples (Bruker, USA)typically low-density polyethylene and polystyrenecovering the following ranges of Young’s moduli: from 100 MPa to 2 GPa (for low-density polyethylene) and from 1 to 20 GPa (for polystyrene). The analysis of the Derjaguin−Mueller−Toporov (DMT) modulus was performed by the software Nanoscope Analysis.37

broadening was applied. The spectra were externally referenced to 38.5 ppm using adamantane. 31 P Solid-State NMR. 31P MAS experiments were performed with a direct 55 kHz excitation, a spectral width of 125 kHz, 4096 complex points, 94 kHz SPINAL decoupling, and 50 Hz line broadening and externally referenced to 0 ppm using H3PO4. 2D homonuclear z-filtered DARR (dipolar-assisted rotational resonance)34 correlation 31P−31P solid-state MAS NMR spin-diffusion with experiments was performed with a mixing time τmix of 30 ms. During the DARR mixing period, the 1H RF field strength (10 kHz) was set to the n = 1 rotational resonance condition. A spectral width of 125 kHz, 4096 complex points in t2, and 128 points in t1 were used during acquisition, 50 Hz line broadening was applied on each dimension, and experiments were externally referenced to 0 ppm using H3PO4. Dynamic Nuclear Polarization (DNP) NMR. DNP NMR measurements were carried out on a 400 MHz Bruker Avance III spectrometer equipped with a 263 GHz gyrotron and a cryogenic triple-resonance 3.2 mm MAS probe at a spinning rate of 8 kHz and temperature of 114 K. An equal volume to mass ratio of 10 mM polarizing agent AMUPol35 (15-{[(7-oxyl-3,11-dioxa-7-azadispiro[5.1.5.3]hexadec-15yl)carbamoyl][2-(2,5,8,11-tetraoxatridecan-13-ylamino)}-[3,11-dioxa7-azadispiro[5.1.5.3]hexadec-7-yl])oxidanyl) was added to Pol-5 and Pol-6 and packed into a 3.2 mm rotor. DNP-enhanced 13C and 15 N CPMAS NMR measurements were carried out using a 1H excitation pulse of 102 kHz, followed by a CP contact time of 1.5 ms (13C) or 3 ms (15N) using a 50% ramped field on 1H and 62 kHz field on 13C or 40.5 kHz on 15N and a 1H SPINAL64 decoupling at 102 kHz during acquisition, a spinning rate of 8 kHz, and a recycle delay of 15 s. DNP-enhanced 2D 1H−13C and 1H−15N heteronuclear correlation (HETCOR) spectra were collected using frequency-switch Lee− Goldberg (FSLG) homonuclear decoupling scheme36 at an effective field of 102 kHz, followed by CP irradiation using similar fields on 1 H and 13C or 15N as in the 1D CPMAS experiments for 0.15 and 1.5 ms (13C) or 1.5 and 5.5 ms (15N). 64 t1 points and 4 repetitions were collected per experiment using similar 1H decoupling and recycle delay values as in the CPMAS experiments. c. Gel Permeation Chromatography (GPC). GPC was performed on a Waters Alliance system equipped with an Alliance 2695 separations module (integrated quaternary solvent delivery, solvent degasser, and autosampler system), a Waters column heater module, a Waters 2414 RDI refractive index detector, a Waters PDA 2998 photodiode array detector (210−400 nm at 1.2 nm), and 2× Agilent PL-AquaGel-OH columns (Mixed H, 8 μm), each 300 mm × 7.8 mm2, providing an effective molar mass range of 100−107). After testing the dissolution of all polymers in different solutions (Table S2), aqueous buffer was prepared containing 0.2 M NaNO3 and 0.01 M Na3PO4 in Milli-Q water with 200 ppm of Na3N, adjusted to pH 8, and filtered through a 0.45 μm filter. The filtered aqueous buffer used as an eluent with a flow rate of 1.0 mL/min at 30 °C. Number- (Mn) and weight-average (Mw) molar masses were evaluated using Waters Empower-3 software. The GPC columns were calibrated with low-dispersity PEO (poly(ethylene oxide)) standards (Polymer Laboratories) ranging from 238 to 969 000 g mol−1, and molar masses are reported as PEO equivalents. A third-order polynomial was used to fit the log Mp vs time calibration curve, which was near linear across the molar mass ranges. d. Differential Scanning Calorimetry (DSC). The DSC analysis of the polymers was carried out on a Mettler Toledo 821e system under a nitrogen atmosphere with a flow rate of 20 mL/min using 8−20 mg of each polymer enclosed in an aluminum pan. The glass transition temperature was identified by extrapolating the linear evolution of the heat capacity as a function of temperature on both the glassy and rubbery branches of the heat scan. The temperature scan window used for rubber/viscous liquid polymers (Pol-2/3/5/6/9 shown in Table S1 and Table 1) was from −100 to +130 °C while the temperature window for glassy polymers (Pol-1/4/7/8) ranged from 0 to +130 °C. The glass transition temperatures (Tg) of all the polymers are given in Table S1 and exemplified by Figure S11 for Pol-5 and Pol-9. e. Small-Angle X-ray Scattering (SAXS). SAXS was performed on all polymers at the small- and wide-angle X-ray scattering (SAXS/WAXS) beamline of the Australian Synchrotron. A wavelength of 0.620 Å

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RESULTS AND DISCUSSION The five protic IL monomers were synthesized with the same base vinyl-1,2,4-triazole (pKa = 9.43) and four acids. For one case, a different stoichiometry acid:base was used (see Table S1). By referring to the chemical structures in the Experimental Methods, the acids A2, A4, and A5 can be seen to bind with one hydroxide group each to the base B1 by respective ionic interactions, while the acid A3 can have two ionic bindings with its two hydroxide groups. As known from the literature,38 the proton from the carboxylic acid is preferentially transferred to the most electronegative atom (N4 here) on the triazole π-ring, with delocalized electrons and the net positive charge delocalized on its center. This results in a molar ratio choice of (acid:base) of 1:1 for acids A2, A4, and A5 with base B1 and 1:2 for the acid A3 with the same base. In terms of vinyl groups, this can be interpreted as a ratio of acid vinyl groups vs base vinyl groups (vinyla:vinylb of 1:1 for acids A2 and A4 with base B1; 1:2 for acid A3 with base B1; and 2:1 for acid A5 with base B1). All the protic ILs led to solid amorphous random or alternating copolymers after RAFT polymerization when using TTC or DTC as RAFT agent (Table S1), while only rubbery/gel compounds were obtained with a “more” reactive agent such as 2CBD. This is likely due to the thiocarbonyl group on the dithiobenzoate group being more active to radical addition and hence retarding or inhibiting the polymerization. The polymerization occurs in a first step (illustrated in Scheme S1, based on previous literature39,40) by an interaction between the p-orbital of free planar radical (I•) and the π-orbitals (C2 position) of the two vinyl carbons. New attacks of the formed radical add randomly other protic IL cation or anion monomers to the main molecular chain, which elongates in copolyampholyte polymer radicals maintained by ionic bindings. Secondly, chain transfer during the RAFT pre-equilibrium allows for freeing the formed copolyampholyte polymer by interaction with the sulfur of the RAFT agent, where the radical is transferred, as shown in part 2 of Scheme S1 (exemplified here by agent TTC for Pol-3 and Pol-8). The rearrangement process with the RAFT agent generates new free radicals, which in part 3 (Scheme S1) reinitiate the polymerization process of the protic ILs, leading to a reversible RAFT main equilibrium producing different lengths of chain, and ending in part 4 by the irreversible step of termination yielding the final form of nonradical copolyampholyte polymers. All the protic IL and RAFT combinations used are included in Table S1. D


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counterions were selected for their energetic properties38 and were not part of the polymer backbone. We focus the following discussion on three characteristic copolyampholyte polymers, which represent all classes of the nine polymers investigated and shown in Scheme 1: “linear random”, “linear random A-alternating”, and “cross-linked alternating”. Physical properties of conversion, polydispersity, weightaverage molecular weight (Mw), molecular weight of the highest peak (MP), number-average molecular weight (Mn), and glass transition temperature for all nine polymers synthesized in this work were obtained. The values corresponding to polymers Pol-5, Pol-6, and Pol-9 are provided in Table 1, and the data for the other six polymers are given in Table S1, with the protic IL monomer structures provided in the left column. The physical appearance and RAFT agents are included in these tables. All chemical structures of the polymers are provided in Scheme 1, but we restrict here the discussion to Pol-9, Pol-6, and Pol-5 as these polymers offer illustrative examples of the different molecular architectures obtained via RAFT polymerization of the various protic ILs. All NIR spectra were deconvoluted using Mathematica in order to calculate the conversion (%) of the polymers from the NIR spectra. Two representative deconvolution procedures are shown for Pol-5 in Figure 1, with the original, corresponding NIR spectra given in the Supporting Information (Figures S2 and S3 for Pol-9). In Figure S2, the NIR spectra of the precursors for Pol-5 have two main peaks at 6010 and 6168.2 cm−1, decreasing from time zero (spectrum 0) to 210 min (spectrum 7), corresponding to the terminal olefin specific wavenumbers. The spectra were acquired periodically every 30 min. After 3.5 h (spectrum 7), the appearance of spectral curves stayed identical, indicating the end of polymerization. Conversion of the polymerization was calculated in several steps. First, a maximum of eight closest peaks to the evolving olefin peaks, the latter included in the eight peaks, were located using the secondderivative method on the corresponding NIR spectra at time zero. Then, the peaks were deconvoluted using Lorentzian distribution functions (or Cauchy distribution). The area under each peak was calculated by integrating between ±∞ wavenumbers, and the normalized total content of initial olefin groups was obtained by dividing the sum of the areas of the olefin peaks vs the sum of the areas of the reference, constant peaks, yielding a ratio A(0). The same procedure was repeated on the NIR spectra at the end of polymerization (t = ∞) by performing a fit with an identical number of Lorentzian curves positioned at identical wavenumbers as for time zero, but with the other parameters (intensity and width) left free to evolve. The same ratio of sum of areas was recalculated in this way to be A(∞). The final conversion was then obtained as α = 1 − A(∞)/A(0). Figure 1 shows a representative example for Pol-5: according to the deconvolution procedure a conversion of 77% was calculated. It should be mentioned at this stage that conversions of the order of 100% cannot be reached: upon polymerization, the length of the chains increases and the glass transition increases accordingly. When the glass transition of the growing polymer approaches the temperature of the RAFT polymerization, the polymers turn glassy, and transport phenomena, including polymerization, are quenched. In view of the above, the conversion of 77% is remarkable. Deconvolution results for Pol-6 and Pol-9 yielded 62% and 61%, respectively (Table 1). Details are shown in Figures S5 and S6. It is worth mentioning that, in addition to having the highest conversion of 77%, Pol-5 is a very stiff solid, glassy amorphous,

Vinyl-1,2,4-triazole has previously been used to make proton conducting polymer membranes through combining poly(vinyl1,2,4-triazole) with other polymers, such as poly(vinylphosphonic acid)41 and poly(styrenesulfonic acid),42 or by adding in acids such as triflic acid,43 p-toluenesulfonic acid,44 nitrilotri(methyltriphosphonic acid),45 or phosphoric acid.46 Copolymers of poly(vinyl-1,2,4-triazole) have been reported with 2-acrylamido-2-methyl-1-propanesulfonic acid,47 vinylphosphonic acid,48 1-vinyl-4,5,6,7-tetrahydroindole,49,50 N-vinylcarbazole,51 and diisopropyl-p-vinylbenzyl phosphonate.52 However, none of these copolymers had the monomers present in an ionic form. Energetic polymer salts of vinyl-1,2,4-triazole in a salt form with inorganic or organic acids have been reported, where the E


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Macromolecules Scheme 1. Structures of All Polyampholytes Synthesized via RAFT from Protic ILs and Indicated in Table S1a

a The structures of the polymeric ionic complexes are idealized and do not account for residual unreacted IL monomers. In Pol-5, only 4-fold connections arising from both methacrylate groups being converted by addition to two different living chains are shown; 3-fold connections arising from the conversion of the two acid vinyl groups by both addition to a living chain and initiation by the initiator are not shown, as their frequency is estimated to be less than 1%.

and was the only polymer whose volume shrunk considerably in the NIR tube during the polymerization, a fingerprint of its cross-linked copolymer nature, as opposed to the “linear” nature

of Pol-6 and Pol-9, which showed little obvious shrinking. The 2-(methacryloyloxy)ethyl group on Pol-5 seems to play a key role in imparting a compact steric conformation of the polymer F


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find an optimal and compact stereochemical organization, diminishing the entropy of the atomic system and thus generating a smaller specific volume. The NIR and deconvolution of all polymers are given in the Supporting Information. As discussed previously, when using 2CBD as transfer (RAFT) agent, only rubbery/gel compounds were obtained while solid amorphous copolymers were obtained for DTC and TTC RAFT agents. Using the time evolution of the NIR profiles, the kinetic curves on Pol-8 and Pol-9, synthesized with the same compounds and quantities, but different RAFT agents, 2CBD and TTC, respectively, are compared in Figure S7 and confirmed the better efficiency of the RAFT agent TTC for the polymerization of these protic ILs. Dithiobenzoates such as 2CBD have previously been associated with potential inhibition in the early stages of polymerization,53 and with rate retardation at longer polymerization times,54 and as such, the superiority of the trithiocarbonate for these systems is unsurprising. The molecular weight distributions of the polymers were determined via GPC. This presented a significant challenge in most of the polymers because the ionic complex nature of the copolyampholytes made them poorly soluble in water and most organic solvents. Consequently, buffers were necessary to screen electrostatic attractive charges to enable the polymers to dissolve. Thus, all GPC curves were run in buffer conditions, allowing dissolution of the ionic complexes (see Experimental Methods). Polydispersities for Pol-5, Pol-6, and Pol-9 are provided in Table 1, and the polydispersities for the other polymers are provided in Table S2. The majority of the polymers were found to have polydispersities between 1 and 2.5 with a broad range of molecular weights, depending on the protic ILs and RAFT agents used. These are substantially higher than would typically be observed for RAFT polymerization performed in solvent with a suitably chosen RAFT agent for the monomer in question. However, given that the polymerizations of the present report were essentially conducted in bulk, proceed to relatively high conversions, and involve copolymerization of two unsaturated groups with different electronic and steric configurations, the results are not unexpected. It is noticeable that the protic ILs with A3 or A5 as an acid precursor formed polymers with significantly higher polydispersity

Figure 1. NIR deconvolution of Pol-5 synthesis: the spectra describing the polymerization from time zero (upper panel) to time infinity (end of polymerization, lower panel) are shown.

by strong inter- and intramolecular interactions. We have attributed this feature to it containing a longer chain with several degrees of freedom, being less rigid than a vinyl aromatic ring and more flexible/longer than a simple olefin. The atoms can possibly

Figure 2. Characteristic GPC results for (A) Pol-6 and (B) Pol-9. G


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Figure 3. (A) Direct excitation 31P MAS NMR spectrum of Pol-9 (solid line) and Lorentzian fit (dashed line). (B) 13C CPMAS spectrum of Pol-9. (C) 2D 31P−31P spin diffusion with DARR (30 ms) spectrum. (D) Chemical shift assignment. All experiments were performed under 10 kHz MAS at 298 K.

phosphorus signals at ca. 7 and −6 ppm, indicating that the phosphonic acid experienced at least two environments (Figure 3A). However, the broad line shape of the two blue peaks may suggest heterogeneous environments within the two distinct populations. The population at 7 ppm accounted for ca. 60% with a line width of 7.6 ppm (fwhm) while the population at −6 ppm accounted for ca. 40% and displayed a line width of 6 ppm. The 13 C CPMAS spectrum displayed the expected carbon chemical shifts (Figure 3B) with no distinguishable second set of resonances. However, a signal at 130 ppm was observed, likely arising from vinyl carbons of unreacted polymers. Interestingly, the 31P−31P spin diffusion experiment (Figure 3C) showed no correlation between the two phosphorus populations, which remain as two distinct well-isolated spots in the 2D NMR diagram of Figure 3C. This rules out the presence of neighboring phosphorus monomers in the final backbone and thus a homogeneous A-alternating polymer arrangement of the linear backbone is deduced (“A” for acid), where the alternating monomer is only the acid, whereas the base can occur as one, two, three, or more monomer sequences (where two is the most probable sequence by stoichiometry). We therefore refer to Pol-9 as linear random “A-alternating” copolymer. The presence of two uncorrelated environments is attributed to the different valences of the phosphorus in the final polymer (−2 and −1), which are to be expected in view of the 2:1 triazole:vinylphosphonic acid stoichiometry used for Pol-9. The investigation by 31P MAS of Pol-5 also showed two phosphorus signals: one at ca. −0.2 ppm with a dominant population accounting for ca. 90% and another at −11.5 ppm (Figure 4A), barely visible in the spectrum, and accounting for less than 10%. The 13C CPMAS of Pol-5 displayed the expected carbon

and/or molecular weights. Both of these acid precursors contain a phosphorus atom, which lowers the pKa of the acid and is likely to have led to stronger proton transfer to the base. The vinylphosphonic acid A3 is diprotic, with two hydroxyl groups (OH−) (pKa1/2 = 3.48 and 8.54). The results of Pol-3 and Pol-9 showed that the RAFT polymerization was improved by doubling the number of base molecules (acid:base = 1:2 vs 1:1), indicating the neutral protic IL precursor was preferable for this polymerization reaction. Pol-3 only yielded monodisperse oligomers (see Table S1 and Figure S6), whereas Pol-9 had a higher average molecular weight (Mw = 72 445 g mol−1, Mn = 28 639 g mol−1) and a large polydispersity (2.5) (Table 1). Moreover, the corresponding curve of GPC plots for Pol-9 (Figure 2B) showed a small discrete bimodal overlap, suggesting that the polymerization with acid:base = 1:2 generated two coexisting populations. For comparison, Pol-6, synthesized from protic IL precursor of vinylbenzoic acid A4 neutralized with vinyl-1,2,4-triazole B1, had a polydispersity of 2.05, with average molecular weights of Mw = 24 324 g mol−1 and Mn = 11 853 g mol−1 and showed in Figure 2A a regular molecular weight distribution. This data, together with the NIR conversion (61−62%), indicates successful polymerization for both Pol-6 and Pol-9. Attempted GPC analysis of Pol-5 confirmed the high conversion and cross-linked nature of the polymer: only fragments with a polydispersity around 1, and very low molecular weight averages (Mw = 1860 g mol−1, quasi-nondissolvable in water) could be extracted from the buffer as a soluble residue of the successfully cross-linked Pol-5. The microscopic arrangement of Pol-9 was investigated by 31 P and 13C solid-state NMR. The 31P MAS spectrum showed two H


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Figure 4. (A) Direct excitation 31P MAS NMR spectrum of Pol-5 (solid line) and Lorentzian fit (dashed line). (B) 13C CPMAS spectrum of Pol-5. (C) 2D 31P−31P spin diffusion with DARR (30 ms) spectrum. The peak at −31.1 ppm was low intensity and almost within the noise level. (D) Chemical shift assignments. All experiments were performed under 10 kHz MAS at 298 K.

chemical shifts (Figure 4B) with no obvious duplicated set, though the signal-to-noise was poor. A signal at 128 ppm was observed, similar to Pol-9, which was attributed to vinyl carbons of unreacted monomers (present as counterions). The 31P−31P spin diffusion experiment (Figure 4C) showed no correlation between the two phosphorus populations for Pol-5, in which the minority population is barely visible in the 2D diagram, again ruling out the presence of neighboring phosphorus−phosphorus atoms in the polymer. Overall, the NMR data suggest that Pol-5 has a primarily homogeneous structure, ruling out the presence of random copolymers. The high conversion (77%) implies polymerization of both the acid and the base and, therefore, infers a homogeneous “alternating” copolymer structure of the cross-linked backbone, where the alternating monomer is only the acid, whereas the base can occur as one, two, or more monomer sequences, where one is the most probable sequence by stoichiometry. This, together with the cross-linked structure of Pol-5 and the bifunctional nature of A5, allows the following important conclusions to be drawn. Since each A5 can carry two radicals, if both radicals lead to further polymerization (and cross-linking), each A5 must have at least two B1 triazole monomers as neighbors because no neighboring A5−A5 (P−P) occurs according to NMR. This, however, violates the 1:1 feeding stoichiometry, and any sequence with more than one contiguous B1 triazole monomers would lead the polymer to depart from the average feeding stoichiometry even further. Thus, for each cross-link, the alternating and bifunctional nature of the A5 leads to an enrichment of the network in the B1 base content, i.e., depletion of the triazole and enrichment of the dimethacrylate in the residual

monomer. This will also mean the RAFT mechanism breaks down at longer reaction times due to the lower number of the methacrylate units in the final polymer and the better leaving group ability of methacrylyl radicals with respect to radicals having a terminal vinyl 1,2,4-triazole unit. It can then be inferred from the above facts that Pol-5 implies (i) imperfect crosslinking (i.e., some A5 monomer where only one methacrylate reacts, therefore leaving an unsaturated pendant vinyl in the polymer structure), (ii) an alternating structure, and (iii) more dimethacrylate in the unreacted monomer. Hence, we can refer to Pol-5 as cross-linked alternating copolymer. One last comment is needed about the connectivity of the cross-link in Pol-5, as this can be both 3-fold and 4-fold, depending on whether one of the two vinyls of A5 is converted by addition to a living chain and the other by initiation (3-fold), or if both are converted by addition to two different living chains (4-fold). Considering that there are at most 0.014 equiv of initiator radicals (i.e., 0.01 equiv of initiator radicals from the leaving group of the RAFT agent plus 2 × 0.002 equiv from the AIBN) for every 1 equiv of acid monomer (i.e., 2 equiv of methacrylate groups), and assuming that the initiator radical adds equally efficiently to the vinyl triazole as to the methacrylate, this yields ≈0.009 equiv of initiator radicals added to the methacrylate. Since the reaction goes to more than 70% conversion, about 0.5−1% of methacrylate are expected to have only one adjacent triazole. For these methacrylates the other vinyl may be unreacted, reacted by an initiator fragment (i.e., giving a linear sequence), or be incorporated into a propagating polymer chain (giving the 3-fold situation). This leads to 4-fold connectivity as the most frequent case. I


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Macromolecules

Figure 5. DNP-enhanced (A) 13C CPMAS and (B) 15N CPMAS NMR spectra of Pol-6 with (upper spectrum) or without (lower spectrum) microwave irradiation. (C) 1H−13C HETCOR spectra of Pol-6 with 0.15 ms (black lines) or 1.5 ms (red lines) contact time. (D) 1H−15N HETCOR spectra of Pol-6 with 1.5 ms (black contours) or 5.5 ms (red contours) contact time. The triazole protons (dotted line) and the benzoic acid protons (dashed line) chemical shifts are marked in the 2D spectra. All experiments were performed at 114 K and a MAS frequency of 8 kHz using 10 mM AMUPol in D2O/ H2O (80:20) polarizing solution. (E) Chemical shift assignment.

Figure 6. Synchrotron 1-D SAXS plots of Pol-5 (A), Pol-6 (B), and Pol-9 (C).

The dominating environment emerging from 31P MAS (Figure 4) is attributed to the deprotonated status of the phosphorus in the final polymer (valence −1) and the protonated one (valence 0) accounting for less than 10% of the occurrence. It is interesting to note that the presence of an acid with two protons as in Pol-9 led to two nearly equally contributing populations of the valency

(60:40), highlighting two equilibrium constants, whereas a single proton acid as in Pol-5, shifts almost entirely the population to the deprotonated state. Because of the poor 13C signal of Pol-5 and the absence of 31 P in Pol-6, dynamic nuclear polarization (DNP) NMR was used to investigate the microscopic arrangement of Pol-5 (Figure S9) J


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Macromolecules

Figure 7. AFM height images of (a) Pol-5, (b) Pol-6, and (c) Pol-9. AFM 2D images of (d) Pol-5, (e) Pol-6, and (f) Pol-9. 3D AFM images of (g) Pol-5, (h) Pol-6, and (i) Pol-9.

indicate the presence of multiple environments for the triazole, suggesting a “random” copolymer arrangement of the linear backbone. Insight into the solid-state structure of the synthesized polyampholytes was obtained through small-angle X-ray scattering (SAXS) at the Australian Synchrotron. The SAXS patterns for polymers Pol-5, Pol-6, and Pol-9 are shown as representative examples in Figure 6. These contain similar features of a peak at a scattering vector qM ≈ 1.4 Å−1 and a slope corresponding to ∼q−3 at small angles, i.e., for scattering vectors of q < 0.1 Å−1. The feature at qM ≈1.4 Å−1 was attributed to the characteristic correlation distance between ionic pairs of opposite charges present in polyampholytes. The corresponding correlation length scale was estimated using Bragg’s law to be ≈2π/qM ≈ 4.5 Å. The low q feature, q < 0.1 Å−1, provides information about local interfaces. The slope of q−3 indicates these polymers are in between a microphase-segregated system, where Porod decay is q−4, and a fully disordered system, in which density fluctuations of segmental chemical differences of the polymer backbone would lead to an ∼q−2 decay of the intensity.55 Thus, this slope may point to some possible weak segregation at q < 0.1 Å−1, which corresponds to length scales larger than 6 nm. Indeed, ∼q−3 is consistent with the fractal surface model,56 which predicts ∼q−(6−Ds) where Ds, the fractal dimension of the surface, is 2 for perfectly sharp surfaces (Porod limit) or ≥2 for blurred surfaces. In the present case Ds = 3, which indicates distorted interfaces.

and Pol-6 (Figure 5) wetted with a solution of 10 mM AMUPol in D2O/H2O (80:20 vol). The macroscopic structure of both polymers was preserved, and the 13C CPMAS spectra were identical as obtained for the dry samples at 298 K (Figure S10). A small 15N signal enhancement (εDNP) of ca. 5 was observed, which allowed performing of 15N CPMAS experiment at natural abundance. Pol-5 showed only one environment for the triazole nitrogens, supporting the predominantly single environment observed in the 31P DARR spectrum. For Pol-6, a greater εDNP, for 13C and 15N, of 88 was measured under microwave irradiation (Figure 5A,B). Interestingly, six 15N signals were observed instead of the expected three signals from the triazole ring, indicating more than one environment, such as different polymer arrangement or unreacted vinyl-1,2,4-triazole B1. The 13C chemical shift at 103 ppm was assigned to the vinyl carbon of vinyl1,2,4-triazole B1 as the vinyl carbon from the vinyl benzoic acid A4 is predicted around 114 ppm. 2D heteronuclear correlation (HETCOR) 1H−13C and 1H−15N experiments for Pol-6 were performed to identify the proximity of the triazole and benzoic acid moieties (Figure 5C,D). Although the 1H signals were broad, correlations between the carbon at 153 ppm and protons at 9 and 8.2 ppm were observed even at short contact time, suggesting proximity between the triazole carbon and the benzoic acid protons (8.2 ppm). Similarly, the 15N signal at ca. 220 ppm showed correlation with both triazole and benzoic acid protons. Taken together, the results K


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Macromolecules Additionally, all three polymers had a weak peak at ≈0.3 Å−1 in their SAXS spectra, which suggests possible microsegregation on length scales of 2 nm or less. This weak segregation is consistent with the random and alternating copolyampholytes molecular structure suggested by solid-state NMR. Further insight into the structure and mechanical properties of the synthetic polyampholytes was obtained by atomic force microscopy (AFM), run in tapping mode and using the peak force quantitative nanomechanical (PF-QNM) technique. The polymer surfaces after ultramicrotoming were imaged in height mode. The structural analysis revealed a possible irregular nanostructure with distorted interfaces, consistent with the SAXS analysis and the random molecular structure of the polyampholytes. Figure 7 shows a selection of AFM height as well as 2D and 3D images for Pol-5, Pol-6, and Pol-9. The nanomechanical mapping of the surface obtained by nanoindentation enabled Young moduli histograms to be collected across areas of 5 μm × 5 μm surfaces for each glassy polymer, as shown in Figure 8 for Pol-5, Pol-6, and Pol-9, along

can be reached by performing a second-phase polymerization at higher temperatures. By combining information from solid-state NMR and infrared spectroscopy, it was possible to conclude that the backbone of the polymers synthesized in this work occurs as both an alternating and random copolymer architecture. In the alternating architecture, the regularly alternating monomer is the acid, whereas the base can occur as a sequence of one, two, three, or more monomers. By stoichiometry, the most probable sequence of the base monomers can be inferred. In this case, a most probable sequence of one and two base monomers was deduced to occur between acid monomers for Pol-9 and Pol-5, respectively. Small-angle X-ray scattering and atomic force spectroscopy revealed that the microstructure of the polymers is essentially amorphous. There were indications of weak, disordered microphase segregation occurring at nanometer length scales, which can be ascribed to compositional fluctuations or molecular segregation of the residual unreacted monomers arising from the bulk polymerization process. This is consistent with the molecular architecture deduced from solid-state NMR analysis, indicating weak segregation without significant long-range order (as expected for block copolymers). Finally, nanoindentation by atomic force spectroscopy revealed a Young modulus of the order of 1−3 GPa, benchmarking the rigidity range of most glassy organic polymers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01768. Scheme of material for protic IL preparation; general results (values) of NIR, GPC, and DSC analyses with polymers structures from protic ILs; structures of the polyampholytes synthesized via RAFT from protic ILs; material and method for preparing polymers; NIR deconvolution procedures and spectra describing the polymerization from zero to time infinite (end of polymerization); evolution of the NIR absorption intensities of the vinyl groups as a function of time for polymer reaction (Pol-8, Pol-9) with different RAFT agents (TTC, 2CBD); curves representing GPC traces of Pol-1−9; DNP-enhanced 13C and 15N CPMAS spectra and 13C CPMAS of Pol-5; DSC curves of Pol-5 and Pol-9 with extraction of the glass transition Tg; Young modulus measurement for all the glassy polymers synthesized in this work as measured by atomic force microscopy nanoindentation (peak force quantitative nanomechanical analysis) (PDF)

Figure 8. Young modulus for the glassy polymers Pol-5, Pol-6, and Pol-9 synthesized in this work as measured by atomic force microscopy nanoindentation (peak force quantitative nanomechanical technique).

with their average values obtained by a Gaussian fit of the histogram distributions. The Young moduli histograms for the other polymers are provided in Figure S12 All glassy polymers synthesized in this work were found to have Young’s moduli in the range 1.4−2.2 GPa, which is a typical range for synthetic polymers that are below their glass transition temperature and in a glassy state.

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CONCLUSIONS We have shown that protic ILs obtained by acid−base exchange of acid and base precursors can be used as efficient monomers for solvent-free, bulk RAFT polymerization into polyampholytic copolymers of various molecular conformations and architectures. By tuning the number of vinyl pendant groups in the acid, we designed both linear and cross-linked polymers. In both cases, the polymeric backbone was polyampholytic in nature, containing both acid and base groups. The polymers had high conversion of the vinyl groups and a glassy nature, with glass transition temperatures in the range of the polymerization temperature. Thus, during polymerization, the glass transition of the living polymer increases with the growing molecular weight, and when it approaches the RAFT polymerization temperatures, it quenches molecular transport turning the polymer into a glass and arresting the polymerization. Higher conversions when needed, but not pursued in this work,

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (C.J.D.). *E-mail raff[email protected] (R.M.). ORCID

John F. Quinn: 0000-0002-4593-1170 Thomas P. Davis: 0000-0003-2581-4986 Frances Separovic: 0000-0002-6484-2763 Calum J. Drummond: 0000-0001-7340-8611 Raffaele Mezzenga: 0000-0002-5739-2610 Notes

The authors declare no competing financial interest. L


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Macromolecules

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ACKNOWLEDGMENTS This research was undertaken in part on the SAXS/WAXS beamline at the Australian Synchrotron, Melbourne, Australia. Solid-state NMR spectroscopy was undertaken at the Bio21 NMR Facility, University of Melbourne. We thank Lisa Famularo at CSIRO Clayton (Melbourne, Australia) for access to the GPC facility, and Stephan Handschin is kindly acknowledged for the microtoming preparation of the samples for the AFM experiments. C.C.J.F. and R.M. acknowledge the Visiting Fellowship Support from RMIT University, Melbourne, Australia.

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Copolyampholytes Produced from RAFT Polymerization of Protic Ionic

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