Facile Access to Poly(N-vinylpyrrolidone)-Based Double Hydrophilic

Dec 16, 2013 - A new redox initiator pair employing sodium sulfite as reducing agent was proposed to perform aqueous ambient RAFT/MADIX polymerization...
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Facile Access to Poly(N‑vinylpyrrolidone)-Based Double Hydrophilic Block Copolymers by Aqueous Ambient RAFT/MADIX Polymerization Aymeric Guinaudeau,† Olivier Coutelier,† Aurélie Sandeau,† Stéphane Mazières,† Hong Diep Nguyen Thi,† Viviane Le Drogo,‡ David James Wilson,‡ and Mathias Destarac*,† †

Laboratoire Hétérochimie Fondamentale et Appliquée, Université Toulouse 3 Paul Sabatier, UMR-CNRS 5069, 118 route de Narbonne, 31062 Toulouse, Cedex 9, France ‡ Solvay Novecare, Research and Innovation Centre Paris, 52 rue de la Haie Coq, 93308 Aubervilliers, Cedex, France S Supporting Information *

ABSTRACT: A new redox initiator pair employing sodium sulfite as reducing agent was proposed to perform aqueous ambient RAFT/MADIX polymerization of N-vinylpyrrolidone (NVP) in the presence of a xanthate chain transfer agent. An efficient control of the polymerization with no formation of monomer byproducts was obtained regardless of the concentration of water in the medium. This system was applied to the aqueous synthesis of PVP-based double hydrophilic block copolymers through the polymerization of NVP at room temperature with several hydrophilic macro-chain-transfer agents based on poly(acrylamide), poly(acrylic acid), poly(sodium 2-acrylamido-2-methylpropanesulfonate), and poly(3-acrylamidopropyltrimethylammonium chloride). The diblock nature of the copolymers was established by DOSY NMR in all cases when satisfactory SEC analysis conditions could not be established due to the strong adsorption properties of the copolymers.

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formation of polyion complex (PIC) micelles16−18 and other triggered polymer assemblies for drug delivery19−23 and polymer-assisted-mineralization.24,25 For instance, poly(ethylene oxide)-block-poly(N-vinylpyrrolidone) (PEO-b-PVP) copolymers were prepared by Pound et al.26 via RAFT/MADIX polymerization of NVP starting from a PEO−xanthate macrochain-transfer agent. Keddie et al.27 reported the use of an acid/ base “switchable” RAFT agent to synthesize poly(N,Ndimethylacrylamide)-block-poly(N-vinylpyrrolidone) (PDMAb-PVP) copolymer. PVP−PNiPAm diblock copolymers were also synthesized by RAFT/MADIX by means of a O-ethyl xanthate.19 Gao and co-workers20,23 used RAFT to produce a series of PVP-based diblock copolymers for the preparation of PIC micelles with a PVP corona. Poly(N-vinylpyrrolidone)block-poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PVP-b-PAMPS)21−23 and poly(N-vinylpyrrolidone)-blockpoly(2-(dimethylamino)ethyl methacrylate) (PVP−PDMAEMA)20,23 were synthesized with a trithiocarbonate RAFT agent. Among the examples of mineral formation using a PVP template, Zhang et al. controlled the morphology of CaCO3 by means of poly(N-vinylpyrrolidone)-block-poly(methacrylic acid) (PVP-b-PMAA) copolymer synthesized by RAFT using 1phenylethyl dithiobenzoate.24 An interesting biomimetic synthesis of needle-like nano-hydroxyapatite was successfully achieved with all PVP-based triblock copolymer synthesized in

INTRODUCTION The possibility of synthesizing poly(N-vinylpyrrolidone)1 (PVP) with precisely controlled number-average molar mass, dispersity, end-groups, and architecture is of great importance because PVP is widely used in industry as a formulation additive in coatings, industrial production, home and personal care, and pharmaceutical and medical areas. The versatility of PVP can be explained by its diverse properties including its solubility in a broad range of liquid media and, in particular, water, strong chemical and thermal resistance, biocompatibility and unique wetting, binding, and film-forming properties. Since the origins of reversibledeactivation radical polymerization (RDRP), several techniques like organo-cobalt-mediated radical polymerization (OCMRP),2 organo-heteroatom-mediated radical polymerization (OHMRP),3 RAFT/MADIX polymerization,4−9 and in one single report atom transfer radical polymerization (ATRP)10 allowed RDRP of NVP. By far, xanthate RAFT/MADIX agents were the most widely employed among all the RDRP options available. The most recent examples of RAFT/MADIX polymerization of NVP and applications of the derived polymers deal with thermoresponsive PNIPAm-g-PVP copolymers,11 surfaceactive end-fluorinated PVP,12 heterotelechelic PVP for bioconjugation and targeted therapeutic delivery applications,13 and nanoparticles made of poly(ε-caprolactone)-block-PVP copolymer for drug delivery.14 Another emerging field of interest is the synthesis and applications of RAFT-derived double hydrophilic block copolymers (DHBCs)15 based on PVP. Because of the biocompatibility of PVP, most of the works related to DHBCs which contain at least one PVP block are motivated by the © 2013 American Chemical Society

Received: August 29, 2013 Revised: November 25, 2013 Published: December 16, 2013 41

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Scheme 1. Aqueous Ambient RAFT/MADIX Polymerization of NVP and Resulting Double Hydrophilic Block Copolymers

two steps with dibenzyltrithiocarbonate RAFT agent.25 The general structure of the polymer was PVP-b-P(VP-alt-MA)-bPVP (MA = maleic anhydride). In all the reported studies mentioned above, the polymerization of NVP was carried out either in bulk or in organic solvents like toluene, acetonitrile, THF, 1,4-dioxane, DMF, ethanol, or fluorinated alcohols. From an industrial perspective for waterborne applications, the use of bulk conditions or organic media for NVP polymerization would imply additional costly and energy-consuming process steps like drying, precipitation/ redissolution, or solvent replacement. Therefore, the controlled polymerization of NVP by direct polymerization in water has, for some time, been a significant scientific and industrial challenge. Over the past decade, aqueous RDRP has gained significant interest and progress for synthesizing well-defined water-soluble macromolecular architectures. As introduced earlier, in addition to the obvious “green” character of water as a polymerization medium, the possibility of setting up polymerization conditions for which the final product could be advantageously applied directly in its aqueous solution or latex form represents an important advantage for industry. Among the different RDRP strategies that may be considered, RAFT/MADIX polymerization28−30 appears to be the most versatile approach for designing precisely controlled copolymers in water with the broadest range of monomer reactivity and functionality offered, as supported by the works of Mc Cormick31 and ourselves32,33 among others for aqueous homogeneous processes or those of Charleux34,35 and Armes36,37 in dispersed media using hydrophilic macro-RAFT agents as reactive stabilizers. Although for some time now most of the classes of monomers have been efficiently polymerized in aqueous media using the RAFT/ MADIX technology, only recently we reported the first RDRP of NVP in water.38,39 This finding was rather unexpected since NVP is known to undergo many side reactions when polymerized in water by radical means.8 The well-established thermal40 and hydrolytical8 instabilities of NVP−O-ethyl xanthate terminal groups in RAFT/MADIX-derived PVP chains also represented significant hurdles for the development of an aqueous-based thermally initiated RAFT/MADIX process. Taking all these criteria into account, we screened many different oxidant/

reducing agent pairs for the redox initiation of aqueous RAFT/ MADIX polymerization of NVP at room temperature,41 among which we identified the tert-butyl hydroperoxide/ascorbic acid (t-BuOOH/AscAc) redox pair as an efficient system to control the growth of the PVP chains leading to controlled numberaverage molar masses (Mn) and low dispersities (Đ < 1.3) in high yields.39 At this stage, it became possible to consider the direct synthesis in water of all-hydrophilic block copolymers and other more complex architectures based on PVP. In particular, as Oethyl xanthates are known to be efficient RAFT/MADIX agents for both more activated monomers (MAMs) like acrylates42 and acrylamides43 and less activated monomers (LAMs) like Nvinylamides,4−9,11−14,39−41,44 vinyl phosphonates,32 and diallyl monomers,33 the access to a straightforward RAFT/MADIX process of NVP in water offers numerous perspectives. In the present study, we identify several limitations in the use of the tBuOOH/AscAc redox initiator that we first reported, the main one being the generation of a significant amount of NVP byproduct upon dilution of the reaction medium. Sodium sulfite (Na2SO3) is proposed as a much more versatile reducing agent than ascorbic acid, and several double-hydrophilic PVP-based block copolymers derived from the t-BuOOH/Na2SO3 initiator pair are proposed for the first time (Scheme 1).

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EXPERIMENTAL SECTION

Materials. N-Vinylpyrrolidone (NVP, Acros, 98%) was dried over anhydrous magnesium sulfate and distilled under reduced pressure. 2,2′Azobis(isobutyronitrile) (AIBN, Fluka, 98%) was recrystallized three times from methanol. 4,4′-Azobis(4-cyanovaleric acid) (ACVA, Fluka, >98%), 2,2′-azobis(2-methylpropionamidine)dihydrochloride (V-50, Acros, 98%), ethanol (EtOH, Normapur), tert-butyl hydroperoxide (tBuOOH, Aldrich, 70 wt % in water), sodium sulfite (Na2SO3, Acros, 98.5%), L-(+)-ascorbic acid (AscAc, Acros, 99%), O-ethyl-S-(1methoxycarbonyl)ethyldithiocarbonate (XA1, Rhodixan A1, Solvay), 3acrylamidopropyltrimethylammonium chloride (APTAC, Aldrich, 75 wt % in water), sodium 2-acrylamido-2-methylpropanesulfonate (AMPS, Aldrich, 50 wt % in water), acrylic acid (AA, Acros, 99.5%), and acrylamide (Am Cu, SNF, 50 wt % in water) were used as received. Analytical Techniques. Monomer conversions were determined by 1 H NMR spectroscopy in D2O at room temperature with a Bruker AMX 300 spectrometer. 42

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acrylamide prepolymer but using the t-BuOOH/AscAc redox initiator system was previously reported by our group.39 PAA-b-PVP, PAMPS-b-PVP, and PAPTAC-b-PVP. Step 1: Synthesis of XA1-Capped PAA, PAMPS, and PAPTAC Prepolymers. PAA macroRAFT/MADIX agent: XA1 (1 g, 4.2 × 10−4 mol), AA (4 g, 5.6 × 10−2 mol), ethanol (2 g, 4.3 × 10−2 mol), distilled water (1.5 g, 8.3 × 10−2 mol), and ACVA (25 mg, 8.9 × 10−5 mol) were introduced at room temperature in a 25 mL round-bottom flask. The reaction mixture was degassed by purging with argon for 30 min, heated, and stirred at 60 °C for 3 h. The polymerization mixture was then cooled, dried under vacuum, dissolved in ethanol, and precipitated into diethyl ether. The obtained powder was dried an extra day under vacuum. AA conversion = 98%. Mn,th = 1020 g/mol. Mn,NMR = 700 g/mol, PAMPS macro-RAFT/ MADIX agent: XA1 (1 g, 4.2 × 10−4 mol), AMPS (8 g of a 50 wt % solution in water, 1.7 × 10−2 mol), ethanol (4 g, 8.7 × 10−2 mol), and ACVA (25 mg, 8.9 × 10−5 mol) were introduced at room temperature in a 25 mL round-bottom flask. The reaction mixture was degassed by purging with argon for 30 min and placed in an oil bath at 60 °C. The reaction was stopped after 3 h of stirring. The prepolymer was isolated according to the same procedure described above for PAA macroRAFT/MADIX agent. AMPS conversion = 97%. Mn,th = 1035 g/mol. Mn,NMR = 770 g/mol. PAPTAC macro-RAFT/MADIX agent: XA1 (1 g, 4.2 × 10−4 mol), APTAC (4 g of a 75 wt % solution in water, 1.5 × 10−2 mol), ethanol (4.5 g, 9.2 × 10−2 mol), distilled water (3 g, 1.7 × 10−2 mol), and V-50 (17 mg, 6.3 × 10−5 mol) were introduced at room temperature in a 25 mL round-bottom flask. The polymerization mixture was degassed by purging with argon for 30 min. The flask was placed in oil bath at 60 °C and stirred for 3 h. The prepolymer was isolated according to the same procedure described above for PAA macro-RAFT/MADIX agent. APTAC conversion = 100%. Mn,th = 1010 g/mol. Mn,NMR = 800 g/mol. Step 2: RAFT/MADIX Redox Block Copolymerization of NVP in Water. The prepolymer macro-RAFT/MADIX agent (PAA-XA1, PAMPS-XA1, or PAPTAC-XA1 (1 g × 10−3 mol)), NVP (1 g, 9.0 × 10−3 mol), distilled water (2 g), and t-BuOOH (20 mg, 1.5 × 10−4 mol) were placed in a 15 mL Schlenk flask. The polymerization mixture was degassed by purging with argon for 30 min. Na2SO3 (35 mg, 2.7 × 10−4 mol) was then added under argon, and the reaction mixture was stirred at room temperature for 24 h. Block copolymerization of NVP from each macro-RAFT/MADIX agent was confirmed by DOSY NMR spectroscopy due to the different diffusion coefficients (D) values between the prepolymer and the corresponding diblock copolymer of PVP (DPAA = 203 μm2/s, DPAA‑b‑PVP = 89 μm2/s, DPAMPS = 260 μm2/s, DPAMPS‑b‑PVP = 66 μm2/s, DPAPTAC = 204 μm2/s, DPAPTAC‑b‑PVP = 63 μm2/ s).

DOSY NMR analysis were performed on a Bruker Avance 300 MHz spectrometer equipped with a QNP Z-GRAD 5 mm probe. The DOSY experiment was acquired using the sequence “ledbpgp2s” which applies field gradients in a 2-dimensional sequence allowing the virtual separation of molecules in solution as a function of their diffusion coefficients. Before performing the DOSY experiment, optimization of acquisition parameters was performed, yielding the following conditions for this study: number of experiments = 48; number of acquisitions per experiment = 32; pulse angle = 90° corresponding to 12.3 μs; acquisition time (AQ) = 1.638 65 s; range of gradients employed = 1−99% of the maximum gradient range available, incremented linearly; length of gradient pulse P30 = 1.5 ms; diffusion delay D20(Δ) = 200 ms. The DOSY raw data were treated using NMRNotebook software from NMRTech. The results are displayed as a 2D chart: diffusion coefficients are plotted on one axis and 1H NMR in the other (chemical shifts: δ). Number-average molar masses (Mn) and dispersities (Đ = Mw/Mn) were determined by size-exclusion chromatography (SEC) with three different systems depending on the analyzed polymer. Measurements of PAm, PVP, PAm-b-PVP, and PAA-b-PVP were performed in DMF/LiCl (0.1 N) at 40 °C with a flow rate of 1 mL/min on a SEC system comprising two Shodex K-805L columns (8 mm × 300 mm, 13 μm) and a refractive index (RI) detector. Toluene was used as flow marker. This system was calibrated with narrow poly(methyl methacrylate) standards (PMMA) ranging from 900 to 625 000 g/mol. PAm, PAA, and PAMPS samples were analyzed in water/NaNO3(0.1 N)/NaN3(100 ppm) at 25 °C with a flow rate of 1 mL/min. The separation was carried with three Shodex OH-pack SB-806 M HQ columns (8 mm × 300 mm, 13 μm), and the apparatus was equipped with a RI detector. Calibration was based on narrow poly(ethylene oxide) standards (PEO) ranging from 1080 to 950 000 g/mol. PAPTAC samples were eluted through three SB 806 M HQ Shodex columns in a 1 M NH4NO3 solution of water/ acetonitrile 80/20 wt % containing 10 ppm PDADMAC (flow rate: 1 mL/min). A PEO calibration curve was used to determine Mn and Đ. Matrix-assisted laser desorption and ionization-time-of-flight mass spectrometry (MALDI-TOF MS) measurements were performed on an applied Biosystems Voyager System 4243. Positive-ion spectra were acquired in the reflectron mode. The polymer sample and the matrix 4(4-nitrophenylazo)resorcinol were dissolved in THF and premixed in a 1:10 volume ratio. Aqueous RAFT/MADIX Polymerization of NVP. A typical polymerization was performed as follows: XA1 (150 mg, 7.3 × 10−4 mol), NVP (2 g, 1.8 × 10−2 mol), distilled water (1 g, 5.6 × 10−2 mol), and t-BuOOH (35 mg, 2.7 × 10−4 mol) were introduced in a 15 mL Schlenk flask. The polymerization mixture was degassed by purging with argon for 30 min. Na2SO3 (35 mg, 2.7 × 10−4 mol) was then added under argon. The reaction was left under stirring at room temperature for 24 h. PVP was isolated by three precipitations into a large excess of diethyl ether after dissolution of the sample in dichloromethane. The polymer was dried under vacuum at room temperature for 10 h. NVP conversion = 81%. Theoretically expected molecular weight Mn,th = 2430 g/mol. Mn,NMR = 2480 g/mol. Mn,SEC = 4000 g/mol, Đ = 1.18 (in DMF/LiCl, PMMA standards). Similar aqueous RAFT/MADIX polymerization of NVP initiated by the t-BuOOH/AscAc redox couple was previously reported.39 Synthesis of PVP-Based Double Hydrophilic Block Copolymers in Water. PAm-b-PVP. An acrylamide prepolymer (PAm-XA1) was first synthesized according to the protocol recently described by our group:39 Mn,NMR = 1200 g/mol, Mn,SEC = 3600 g/mol, Đ = 1.07 (in DMF/LiCl, PMMA standards); Mn,SEC = 500 g/mol, Đ = 1.41 (in H2O, 0.1 M NaNO3, PEO standards). The block copolymerization of NVP was performed as follows: PAm-XA1 (120 mg, 1.0 × 10−4 mol), NVP (1 g, 9.0 × 10−3 mol), distilled water (2 g, 1.1 × 10−1 mol), and t-BuOOH (20 mg, 1.5 × 10−4 mol) were placed in a 15 mL Schlenk flask. The polymerization mixture was degassed by purging with argon for 30 min. Na2SO3 (20 mg, 1.5 × 10−4 mol) was then added under argon. The reaction mixture was stirred at room temperature and stopped after 24 h of reaction. NVP conversion = 100%. Mn,th = 10 500 g/mol. Mn,SEC = 9300 g/mol, Đ = 1.59 (in DMF/LiCl, PMMA standards). Similar aqueous block copolymerization of NVP controlled by the same

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RESULTS AND DISCUSSION Aqueous Ambient RAFT/MADIX Polymerization of NVP. In a recent study from our group, we pointed out that there are several important criteria to fulfill in order to control aqueous RAFT/MADIX polymerization of NVP. First, the reaction must be carried out at a reasonably low temperature in order to avoid the elimination of the xanthate end and loss of control during polymerization.40,41 If redox initiation is chosen, the oxidizing agent potential should be low enough to prevent the direct oxidation of NVP. Also, as side reactions between NVP and water are catalyzed in acidic mediamainly forming the N(α-hydroxyethyl)pyrrolidone byproduct 1 (see Scheme 2)it is crucial to control pH and avoid the presence of strong acids in the redox couple. Moreover, both oxidizing and reducing agents should not produce acidic protons by homolysis or redox reaction. Very importantly, the redox potential and efficiency of the oxidant/reducing agent pair must be well chosen in order to obtain a sufficient rate of initiation at low temperature.41 Indeed, provided that a RAFT/MADIX agent of suitable reactivity is selected to ensure the reversible capping of growing PVP chains, the overall rate of polymerization must be faster than those of 43

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percentage of byproduct 1 at the end of the polymerization was calculated relative to the initial NVP concentration and determined by 1H NMR spectroscopy. Its value increases significantly with dilution and reaches more than 20 mol % for an initial NVP concentration of 20 wt %. In order to avoid the formation of byproduct 1, the polymerization was performed under slightly basic conditions (pH = 8.2) with an initial NVP concentration of 66 wt %. Although Mn control was still obtained and no formation of byproduct 1 was observed at basic pH as expected, the increase of pH led to a decrease of NVP conversion by about half (48% after 24 h reaction). This may be ascribed to the lower reducing power of ascorbate compared to ascorbic acid. Confronted with the limitations of the t-BuOOH/AscAc initiator, we identified Na2SO3 as an alternative reducing agent, which appeared to offer more flexibility in terms of NVP concentration. Indeed, by initiating the polymerization of NVP with the new t-BuOOH/Na2SO3 redox couple and varying NVP initial concentration in water from 66 to 20 wt % with corresponding pH > 9 did not lead to the formation of byproducts (not detectable by 1 H NMR, Figure 1a). Furthermore, NVP conversion remained unaffected around 80%. In comparison with ascorbic acid, the basic nature of the sodium sulfite reducing agent prevents the acid-catalyzed side reactions of NVP in water by keeping the polymerization reaction at a pH above 9 for all dilutions throughout the polymerization. Therefore, we demonstrated that the combination of the sodium sulfite reducing agent with t-BuOOH enables one to greatly improve the conditions of aqueous NVP polymerization over a broad range of dilution of the polymerization medium. The effects of AscAc and Na2SO3 on the overall rate of polymerization and macromolecular characteristics of the obtained PVPs were compared. Both polymerizations were deliberately conducted at 20 wt % of NVP in water, conditions for which 20 mol % of byproduct 1 were detected for the tBuOOH/AscAc pair. Figure 2a shows the evolution of Mn and dispersities Đ with NVP conversion. Like for any RDRP study, the theoretical Mn value (Mn,th) was calculated by taking into account the initial concentrations of NVP and XA1 and NVP conversion (see legend of Figure 2a for details). In the case of AscAc, a corrected expression of theoretical Mn (Mn,th,corr) was defined as NVP is reacted through both polymerization and formation of byproduct 1 ([NVP]reacted = [NVP]polymerized + [1]t). In order to simplify the calculation of Mn,th,corr, we approximated [1]t to the final concentration of 1 that we determined by 1H NMR. For both initiator systems, Mn values, based on SEC analyses, increase linearly with conversion throughout polymerization, as expected for a controlled chain growth with fast consumption of XA1. However, whereas Mn is very close to Mn,th for sodium sulfite, a significant downward deviation is observed for AscAc for which the Mn evolution profile is relatively close to that of Mn,th,corr. This can be mainly explained by the cumulated loss of ∼20% of NVP through formation of byproduct 1. Dispersity values are very low (1.08 < Đ < 1.23) and suggest a high exchange constant for the xanthate between dormant and growing chains. In addition, under identical polymerization conditions, sodium sulfite induces faster rates of NVP polymerization compared to AscAc as shown in Figure 2b, which represents the semilogarithmic plot of monomer conversion versus time. The slower polymerization observed for AscAc may be explained by the fact that ascorbic acid in combination with tBuOOH generated fewer efficient radicals than the system using sodium sulfite as a reducing agent. The important difference of

Scheme 2. Potential Side Reactions during Aqueous Radical Polymerization of NVP in Acidic Conditions8

byproduct formation and elimination of the xanthate terminal group. In this context, the t-BuOOH/AscAc redox couple appeared to be the most efficient initiator and allowed the formation of controlled PVP at room temperature in aqueous medium with high conversion.39 The controlled character of the polymerization was confirmed by MALDI-TOF mass spectrometry, SEC, and 1H NMR analyses. No side reactions and no degradation of the xanthate moiety were detected at a high initial NVP concentration (6.2 mol/L, which corresponds to 66 wt %) and a pH of 6.5. However, when considering an initiator system for aqueous RAFT/MADIX polymerization, ideally it should be as versatile as possible in terms of reaction conditions. In particular, its performances have to be insensitive to process parameters of primary importance like dilution or pH. In this work we investigate the range of usage of the tBuOOH/AscAc initiator by performing polymerization of NVP in a much wider range of dilution conditions than previously reported. We varied the initial NVP concentration in water from 66% down to 20 wt %. We observed that when the NVP concentration is lower than 66 wt %, byproduct 1 (Scheme 2), resulting from the hydrolysis of NVP in acidic aqueous medium, starts to be formed during polymerization (Figure 1) and no longer becomes negligible relative to NVP polymerization. The

Figure 1. Aqueous RAFT/MADIX polymerization of NVP performed at 25 °C with two different redox initiator systems: t-BuOOH/AscAc and t-BuOOH/Na2SO3. Effect of dilution on the presence of NVP byproduct 1. n(NVP)0 = 1.8 × 10−2 mol, n(t-BuOOH)0 = n(AscAc)0 = n(Na2SO3)0 = 2.8 × 10−4 mol, n(XA1)0 = 7.1 × 10−4 mol, t = 24 h. pH = 3.4 with AscAc and 9.3 with Na2SO3 at the beginning of the polymerization. 44

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Figure 2. Comparison of t-BuOOH/AscAc and t-BuOOH/Na2SO3 redox initiator couples for aqueous RAFT/MADIX polymerization of NVP performed at 25 °C and 20 wt % of NVP in water. [NVP]0 = 1.8 mol/L, [t-BuOOH]0 = [AscAc]0 = [Na2SO3]0 = 2.8 × 10−2 mol/L, [XA1]0 = 2.1 × 10−2 mol/L, t = 7 h. (a) Evolution of Mn and Đ with NVP conversion. Mn,th = (([NVP]0 − [NVP]t)/[XA1]0) × MNVP + MXA1 (solid line). Mn,th,corr = (([NVP]0 − [NVP]t − [1]t)/[XA1]0) × MNVP + MXA1 (dotted line). (b) ln(1/(1 − x)) vs time, x = NVP conversion. pH = 3.4 with AscAc and 9.3 with Na2SO3 at the beginning of the polymerization.

Figure 3. MALDI-TOF mass spectrum and identified populations A−E of a PVP-XA1 sample initiated by t-BuOOH/Na2SO3 in water at 25 °C at 40 wt % of NVP. Mn,th = 2700 g/mol, Mn,SEC = 3000 g/mol, Đ = 1.11.

reactions over a much broader range of medium dilution with faster polymerization, while offering a much better control of Mn and keeping low dispersities. Nevertheless, it is worth mentioning that when generated with AscAc, byproduct 1 does not lead to alteration of the reversible transfer process imposed by the xanthate, the only drawback being the loss of a significant fraction of polymerizable monomer and formation of an impurity that needs to be removed before further use of PVP. MALDI-TOF analysis also confirmed the controlled nature of polymer chain ends (Figure 3). The distribution is symmetrical and shows one main and four minor populations. Each series are 111.1 g/mol apart which corresponds to the molecular weight of NVP. The main structures A and B correspond to the population of chemical structure CH3OCOCH3CH−(NVP)n−CHCH(C4H6NO) with sodium and potassium adduct, respectively. The xanthate moiety of the polymer end was cleaved in favor of a

pH measured at the beginning of the polymerization (3.4 with AscAc and 9.3 for Na2SO3) should not influence propagation rate coefficient during NVP polymerization as reported by Stach et al.45 Under conditions of Figure 2, we observed that the decrease of the initial xanthate concentration by half led to doubled Mn values throughout the course of the polymerization, as expected, and no noticeable effect on dispersities (see Figures S1 and S2). Very interestingly, the decrease of the initiator pair concentration by a factor of 4 compared to Figure 2 still led to fast polymerization with NVP conversion higher than 80% after 7 h. This resulted in slightly better controlled Mn and significantly lower dispersities with Đ = 1.07 after 83% conversion (see Figures S3 and S4). To conclude, the replacement of an acidic reducing agentAscAcby the Na2SO3 base allows the aqueous RAFT/MADIX polymerization of NVP to proceed at pH > 9 at room temperature without detectable formation of side 45

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water comprising one PVP block and various other hydrophilic blocks of interest. PVP-Based Double Hydrophilic Block Copolymers. Among the few examples of DHBCs based on PVP that we described in the Introduction, none of the employed synthetic methodologies used water as the polymerization medium. In addition, in most cases authors19−24 decided in a rather unexpected fashion to first polymerize NVP and then a conjugated monomer in the second block, which is contrary to the classical established rules for an optimal diblock synthesis by RAFT.1 Except in one case where the PVP-b-PNiPAM structure was established based on reliable SEC data,19 the other reported analytical data to support the formation of diblock copolymer with high purity are generally very limited and not conclusive. From this overview of the literature of RAFT/MADIX-derived DHBCs based on PVP, it appeared to us that not only should “greener” conditions and improved control be found for some of the copolymers previously reported, but also several important block combinations with PVP were never proposed, for instance poly(acrylic acid) (PAA) and a strong cationic polyelectrolyte like poly(3-acrylamidopropyltrimethylammonium chloride) (PAPTAc). In addition, polyacrylamide (PAm) and PAMPS were chosen as supplementary non-PVP blocks in order to propose a varied selection of DHBCs comprising neutral and polyelectrolyte blocks. Nonsymmetrical block copolymers with short first block and much longer second block were targeted for facilitated analysis of diblock character. Four hydrophilic RAFT/MADIX macromolecular chain transfer agents (PAm-XA1, PAA-XA1, PAMPSXA1, and PAPTAC-XA1) of Mn = 1000 g/mol were first prepared and used to mediate the aqueous polymerization of NVP. Am was polymerized at ambient temperature with the t-BuOOH/ Na2SO3 redox initiator couple in water with a minimum of ethanol as cosolvent in order to solubilize the hydrophobic Rhodixan A1. The PAA-XA1, PAMPS-XA1, and PAPTAC-XA1 prepolymers were synthesized via thermally initiated RAFT/ MADIX polymerization of the corresponding monomers in the presence of Rhodixan A1 in hydroalcoholic media at 60 °C. As shown in Table 1, all monomers conversions were nearly quantitative and the polymerizations were well-controlled with experimental molar masses determined by 1H NMR close to the theoretical values (entries 1−4). For all polymers, Mn was determined by comparing the relative integration of the peak corresponding to the methine proton of the terminal monomer unit bonded to the S-xanthate fragment to that of specific protons of the main chain. Mn of the polyelectrolyte samples (entries 1− 3, Table 1) determined by aqueous SEC show some deviations from theory which is not surprising considering that neutral PEO standards were employed for calibration. However, dispersities were relatively low (Đ < 1.5) in all cases and were in the range expected given the moderate exchange chain transfer constant (1.3 < Cex = kex/kp < 5.9) determined for the considered range of monomers.48 The prepared macro-RAFT/MADIX agents were then used to mediate NVP polymerization with a targeted Mn value of 10 000 g/mol. The NVP polymerization was performed in water at room temperature with the t-BuOOH/Na2SO3 redox-initiator couple. NVP conversions were 99% minimum (Table 1, entries 5−8). SEC analysis of PAm-XA1 and PAm-b-PVP samples was possible due to the solubility of both PAm precursor and diblock copolymer in DMF. The SEC chromatogram of the diblock PAm-b-PVP was shifted to the higher molar mass region compared to the PAm-XA1 precursor (Figure 5). Moreover the

double bond due to fragmentation by the MALDI-TOF laser, as previously reported by ourselves46 for polystyrene and more recently for PVP.39,41 Populations D and E correspond to PVP chains terminated by a double bond and initiated by a tert-butoxy fragment. The population C, corresponding to the chemical structure CH 3 OCOCHCH 3 −(NVP) n −S(CSO)OCH2CH3, with a sodium adduct (m/z = M(R) + n·M(NVP) + M(Z) + 33 + M(Na+)), is thought to be due to the oxidation of sulfur into sulfine then in thioester in polymers by peroxides contained in THF used as solvent for MALDI-TOF MS analysis.47 Populations C, D, and E, although minor, were absent from a previous MALDI-TOF analysis we reported36 for a PVP initiated by t-BuOOH/AscAc with Rhodixan A1.39 We attributed these changes to differences in solvent quality and possible increase of initiator-derived chains. In order to further establish the chain end fidelity of the xanthate, a chain extension experiment was performed from a PVP-XA1 precursor. The first polymerization step took place at an initial NVP concentration of 66 wt % in water with 81% conversion and Mn,SEC= 2170 g/mol and Đ = 1.12 for a theoretical Mn of 1850 g mol−1. The polymer issued from the first step was purified by precipitation and then used as a macroRAFT/MADIX agent in NVP polymerization in order to extend the polymer chain up to Mn = 10 000 g/mol. The chain extension occurred efficiently as shown in Figure 4 with a NVP conversion

Figure 4. Chain extension experiment with NVP from a PVP-XA1 precursor in water at room temperature with the t-BuOOH/Na2SO3 initiator.

of 85%. SEC analysis showed a shift of the chromatograms toward higher molar masses with a Mn,SEC = 10 900 g/mol slightly above the theoretical value of 8060 g/mol. Dispersity increased to Đ = 1.72 at the end of the polymerization, which may be attributed to a loss of a fraction of xanthate end-groups over time. A detailed study is underway in our group in order to better understand this observation. To conclude this part, the combination of the Na2SO3 reducing agent with t-BuOOH as oxidant gave rise to a very efficient redox initiator system for RAFT/MADIX polymerization of NVP in water. Used with the Rhodixan A1 MADIX agent, it allowed fast controlled polymerization up to high conversions from dilute to more concentrated conditions, while preventing the formation of NVP byproducts. We took advantage of these results to synthesize diblock copolymers in 46

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which are related to hydrodynamic radius. It displays a 2D map with conventional chemical shift spectra in one dimension and spectra of diffusion coefficients in the other. The most interesting point is that this two-dimensional technique is a noninvasive method to obtain the pure spectrum for each component, providing an alternative to chromatographic analysis. Therefore, DOSY was shown to be an efficient technique for polymer chemists in order to detect byproducts or impurities such as unreacted monomers, residual homopolymers, or degradation products. Johnson and co-workers first demonstrated that this technique was useful for polymer chemists to characterize the molecular weight distributions of poly(ethylene oxide) in D2O.50 Very recently, Grubbs and co-workers51 reported the determination by DOSY of weight-average molecular weight of polymers produced by various controlled polymerizations. The polydispersity of a polymer chain could even be estimated for linear polymers using such diffusion experiment. This approach was based on the differential diffusion profile observed for the main polymer chain signal versus the extremity signal.52 Another recent application of DOSY was reported for the determination of the critical micelle concentration (cmc) of amphiphilic block copolymers.53 In order to shed light on the efficient formation of the DHBCs, DOSY NMR appeared to be a relevant technique capable of differentiating the macro-CTA (low hydrodynamic radius/high diffusion coefficient D value) from the diblock copolymer (lower D value). The comparison of the different DOSY NMR spectra is reported in Figure 6. The left-hand figures display the DOSY spectra obtained from the first block macro-CTA while those on the right show the corresponding spectra obtained after NVP chain extension. In all cases, the later spectra show that the chain extension with NVP completely consumes the macro-CTA and that the diblock copolymers exhibit diffusion coefficients which are systematically between 2.3 and 3.7 times lower than their macro-CTAs coherent with their expected increase in hydrodynamic volume. For example, the macro-CTA of PAA(1K) has a diffusion coefficient of 203 μm2/s while the corresponding diblock copolymer PAA(1K)−PVP(9K) presents a D value of 89 μm2/s. Furthermore, characteristic signals from the macro-CTA and PVP are observed at the same diffusion coefficient, confirming the diblock nature of the polymers. For example, in the case of PAPTAC(1K)-b-PVP(9K), the unique population at approximately 63 μm2/s displays a signal at 2.9 ppm characteristic of APTAC methyl and methine protons in the vicinity of nitrogen (β to nitrogen) as well as a peak at 3.2 ppm corresponding, majorly, to PVP methylene and methyne protons β to nitrogen. In this case and for the PAMPS-b-PVP polymers, DOSY NMR proved to be the discriminating technique that allowed confirmation of the successful and highly efficient synthesis of double hydrophilic cationic or anionic MAM/NVP block copolymers and attests the interest of the technique for the characterization of such systems.

Table 1. Hydrophilic Prepolymers (Entries 1−4) Mediating the Aqueous RAFT/MADIX Polymerization of NVP at 25 °C and Resulting PVP-Based Double Hydrophilic Block Copolymers (Entries 5−8)e entry

polymer

convb (%)

Mn,thc (g/mol)

Mn,NMRb (g/mol)

Mn,SEC (g/mol)

Đ 1.07 1.41 1.29 1.33

1

PAm-XA1

99

950

1200

2 3

PAA-XA1 PAMPSXA1 PAPTACXA1 PAm-b-PVP PAA-b-PVP PAMPS-bPVP PAPTAC-bPVP

98 97

1020 1035

700 770a

3600d 500e 3700e 3000e

100

1010

800a

1550e

1.49

99 100 99

10500 10600 10000

ND ND ND

9300d 9050d ND

1.59 1.25 ND

99

10300

ND

ND

ND

4 5 6 7 8

[NVP]0 = 2.9 mol/L, [t-BuOOH]0 = [Na2SO3]0 = 6.0 × 10−2 mol/L, t = 24 h. bBy 1H NMR in D2O. c Mn,th = conv × Mmonomer × ([monomer]0/[CTA]0) + MCTA. dBy SEC in DMF/LiCl with PMMA calibration. eBy aqueous SEC with PEO standards. ND = not determined. a

Figure 5. Size exclusion chromatograms (RI response) of PAm first block and PAm-b-PVP.

experimental Mn value of the diblock (9300 g/mol) was very close to the theoretical estimation (10 600 g/mol) supporting the controlled character of the polymerization, even though an unexpected broad (Đ = 1.59) and nonsymmetrical MWD potentially due to chromatographic effects was observed. Although PAA-XA1 could only be eluted in water and not in DMF, the low Mn and low fraction of PAA (∼10 wt %) in the PAA-b-PVP copolymer allowed its SEC analysis in DMF. A very good control of Mn was established (entry 6, Table 1) with low Đ value of 1.21. Suitable SEC conditions could not be found for PAPTAC-b-PVP and PAMPS-b-PVP copolymers because PAPTAC and PAMPS require aqueous SEC for relevant determination of average molar masses whereas SEC of PVP cannot be performed in water in a reliable manner. DOSY NMR was employed instead. DOSY (diffusion-ordered spectroscopy) was developed by Johnson in the early 1990s49 and was rapidly developed as one of the most powerful experimental NMR techniques to investigate solution state structures and interactions of macromolecules. This technique can provide diffusion coefficients of molecules

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CONCLUSIONS In summary, the conditions for aqueous ambient RAFT/MADIX polymerization of NVP mediated with Rhodixan A1 were greatly improved with the newly proposed t-BuOOH/Na2SO3 redox initiator. In contrast to ascorbic acid that we considered in an earlier study, sodium sulfite is a reducing agent that allows faster polymerization to proceed at a pH close to 9, thereby preventing the formation of N-(α-hydroxyethyl)pyrrolidone, which is the main NVP byproduct generated in acidic water. Most importantly, the quality of NVP polymerization (Mn, Đ, and 47

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Figure 6. DOSY 1H NMR spectra of PAA(1K) (a), PAMPS(1K) (b), PAPTAC(1K) (c), hydrophilic RAFT/MADIX precursors, and their resulting PVP-based double hydrophilic diblock copolymers PAA(1K)-b-PVP(9K) (a), PAMPS(1K)-b-PVP(9K) (b), and PAPTAC(1K)-b-PVP(9K) (c).

byproduct) is not affected by the concentration of water in the reaction medium. Complementary techniques to SEC like MALDI-TOF mass spectrometry and a chain extension

experiment contributed to strengthen our conclusions on the efficiency of this aqueous initiation and polymerization control. The versatility of Rhodixan A1 for polymerizing both hydrophilic 48

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conjugated acrylic and acrylamido monomers, on the one hand, and nonconjugated NVP, on the other hand, was successfully applied to the synthesis of PAm-b-PVP, PAA-b-PVP, PAMPS-bPVP, and PAPTAC-b-PVP copolymers in water at room temperature. The limitations encountered with SEC analysis for some of the diblock copolymers were circumvented thanks to DOSY NMR. It is believed that this study will be beneficial for users of DHBCs based on PVP looking for simpler and more environmentally friendly preparation protocols and new analytical solutions like DOSY NMR for the characterization of these physicochemically complex macromolecules. Original DHBCs such as PAA-b-PVP and PAPTAC-b-PVP were proposed which opens new perspectives in the field of PIC micelles and polymer templating for inorganic synthesis. Because of the high chemical and thermal resistance of PVP combined with its unique interfacial properties and biocompatible character, it is expected that many applications of the DHBCs presented in this work will flourish in the future.

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental results including the influence of RAFT/MADIX agent and initiator concentration of the macromolecular characteristics of PVP. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Solvay Novecare for permission to publish this work and for financial support. REFERENCES

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