Poly(vinylidene chloride)-Based Amphiphilic Block Copolymers

Article pubs.acs.org/Macromolecules

Poly(vinylidene chloride)-Based Amphiphilic Block Copolymers Emilie Velasquez,†,‡ Gael̈ le Pembouong,† Jutta Rieger,*,† François Stoffelbach,*,† Olivier Boyron,‡ Bernadette Charleux,*,‡ Franck D’Agosto,‡ Muriel Lansalot,‡ Pierre-Emmanuel Dufils,§ and Jérôme Vinas∥ †

Laboratoire de Chimie des Polymères, UPMC Univ. Paris 6, Sorbonne Universités and CNRS, UMR 7610, 3 rue Galilée, 94200 Ivry, France ‡ C2P2 (Chemistry, Catalysis, Polymers & Processes), Team LCPP Bat 308F, Université de Lyon, Univ Lyon 1, CPE Lyon, CNRS, UMR 5265, 43 Bd du 11 Novembre 1918, 69616 Villeurbanne, France § High Barrier Polymers, SOLVAY, Avenue de la République, F-39500 Tavaux, France ∥ High Barrier Polymers, SOLVAY, Rue de Ransbeek 310, B-1120 Brussels, Belgium S Supporting Information *

ABSTRACT: The controlled/living free-radical copolymerization of vinylidene chloride (VDC) with methyl acrylate (MeA) or acrylic acid (AA) was studied by the reversible addition−fragmentation chain transfer (RAFT) technique using a trithiocarbonate RAFT agent. The reactions were performed in 1,4-dioxane solution at 30 °C and led to good control and high chain-end functionality. P(VDC-co-MeA)-bPAA, PAA-b-P(VDC-co-MeA), and PAA-b-P(VDC-co-AA) amphiphilic block copolymers were then prepared in the same conditions, starting either from a hydrophobic P(VDCco-MeA) macromolecular RAFT (macro-RAFT) agent or from a hydrophilic PAA one. The advantage of the first synthesis pathway relies on the very good transfer efficiency to trithiocarbonate-ended P(VDC-co-MeA) and on the rapid consumption of the latter even when low percentages (10 mol %) of MeA comonomer are incorporated in the macro-RAFT agent. In contrast, for the second approach a rapid consumption of the macro-RAFT agent is only reached with 30 mol % of MeA in the comonomer feed, whereas with 10 mol % of MeA the transfer constant was determined to be only close to 1. Finally, we demonstrated that PAA-b-P(VDC-co-AA) diblock copolymers might also be obtained with controlled features in a one-pot process.

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INTRODUCTION Poly(vinylidene chloride) (PVDC) is a specialty polymer wellknown for its barrier properties toward gases and water vapor, which is widely used in the pharmaceutical and food packaging industry.1−3 Because of its crystallinity, it is poorly soluble in most of the conventional solvents, and only copolymers are of commercial importance. A commonly used comonomer is methyl acrylate (MeA), which allows copolymers with improved processability, solubility, and thermal properties to be produced.1,2,4 When copolymerized by radical polymerization, the reactivity ratios r1 and r2 for VDC and MeA have been determined to be equal and close to 1, meaning that random copolymers form, which possess the same composition as the monomer feed at any monomer conversion.4,5 To achieve materials with enhanced properties, not only the chemical nature of the monomers but also the control over the molar mass, the molar mass distribution, the comonomer distribution, and the architecture (block copolymers, graft copolymers) of the polymers are crucial parameters.6 With the development of controlled/living free-radical polymerization (CRP) techniques,7−13 the synthesis of well-defined (co)polymers has become possible in very simple conditions that are transposable to industry. Among the available CRP © 2013 American Chemical Society

techniques, the reversible addition−fragmentation chain transfer (RAFT) method7,8,10,11 allows the controlled polymerization of a wide variety of monomers at convenient temperatures in various solvents. However, only very few studies on the controlled synthesis of VDC-based (block) copolymers have been reported so far, and they are mainly using the RAFT technique14−18 or the (reverse) iodine transfer polymerization ((R)ITP).18,19 Using RAFT, Boutevin et al.14 have tested different dithiobenzoates in the controlled radical copolymerization of VDC with MeA (20 mol %) in benzene. The polymerizations showed the features of a controlled system with acceptable dispersities (Đ = Mw/Mn) ranging from 1.5 to 1.7. The molar masses deviated from the theoretical values, which was ascribed by the authors to radical transfer reactions to VDC monomer. Note that a calibration based on polystyrene standards was used for the determination of the molar masses by size exclusion chromatography, making difficult the interpretation of the results. One of those dithiobenzoate RAFT agents was later employed for the Received: November 13, 2012 Revised: January 12, 2013 Published: January 31, 2013 664

dx.doi.org/10.1021/ma302339x | Macromolecules 2013, 46, 664−673

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Scheme 1. Synthesis Pathways toward PVDC-Based Block Copolymers (R = COOH or COOCH3)

of the diblocks was started either from a PVDC-based macromolecular RAFT agent or from a PAA one. The influence of the synthesis pathway and of the percentage of the acrylic comonomer in the hydrophobic PVDC block on the control of the polymerization and on the efficiency of the crossover (i.e., transfer) reaction have been thoroughly investigated.

preparation of terpolymers (composed of VDC, MeA and a phosphonated methacrylate or hydroxyethyl acrylate, HEA) in benzene.16 Again, the Đ values were fairly high (about 1.7). Only few publications briefly reported the synthesis of VDCbased diblock copolymers: they were prepared either by chain extension of iodine-terminated poly(n-butyl acrylate)20 by RITP of VDC with MeA in aqueous dispersion (the numberaverage molar mass, Mn, was 14 450 g mol−1 and Đ = 3.3) or by chain extension of P(VDC-co-MeA) macromolecular RAFT (macro-RAFT) agents with styrene (S)19 in bulk and with HEA16 or perfluorodecyl acrylate15 in benzene solution. In the latter case, no common solvent was found to dissolve and characterize the resulting copolymers. The P(VDC-co-MeA)-bPHEA and P(VDC-co-MeA)-b-PS copolymers could be dissolved in THF, and size exclusion chromatography clearly showed a slight shift of the initial SEC trace toward higher molar mass peaks, indicating the formation of diblock copolymers. Amphiphilic block copolymers, consisting of hydrophilic and hydrophobic blocks, have aroused increasing interest over the past decades in a variety of applications such as emulsifiers, dispersion stabilizers, wetting agents, or compatibilizers. However, despite the unique properties of PVDC-based copolymers, to the best of our knowledge only one PVDCbased amphiphilic block copolymer, P(VDC-co-MeA)-bPHEA,16 has been reported. The design of such copolymers using simple but nevertheless precise polymerization techniques may enlarge the scope of the existing applications involving PVDC or open the door to new ones. In the present study, we aimed at synthesizing new amphiphilic block copolymers composed of a PVDC-based hydrophobic block and a poly(acrylic acid) (PAA) hydrophilic one with well-defined structure, molar mass, and composition, using the RAFT technique. A trithiocarbonate RAFT agent, 2(dodecylthiocarbonothioylthio)-2-methylpropanoic acid) (TTCA, Scheme 1), was selected, as it is known to efficiently control the polymerization of a large variety of monosubstituted ethylenic monomers.21−25 Moreover, it is easy to synthesize and exhibits less unfavorable odor and color compared to most other RAFT agents. Only one example of RAFT copolymerization of VDC with n-butyl acrylate using a trithiocarbonate RAFT agent has been reported in the literature, but no information on the polymerization conditions was provided.17 In this context, we present here a detailed investigation of the RAFT synthesis of P(VDC-co-M) random copolymers using TTCA and of PAA-b-P(VDC-co-M) or P(VDC-co-MeA)-bPAA (with M = acrylic acid (AA) or MeA) diblock copolymers according to two strategies depicted in Scheme 1. The synthesis

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EXPERIMENTAL SECTION

Materials. Methyl acrylate (MeA, Aldrich, >99%) was vacuumdistilled before use. Vinylidene chloride (VDC, Fluka, 99.5%) was washed with a 25 wt % NaOH aqueous solution and dried on MgSO4 before use. Acrylic acid (AA, Aldrich, >99%), 4,4′-azobis-4cyanopentanoic acid (ACPA, Aldrich, >98%), 2,2′-azobis(isobutyronitrile) (AIBN, Fluka), 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile) (V70, Wako), 1,4-dioxane (VWR, Rectapur), 1,3,5trioxane (Aldrich, ≥99%), toluene (VWR, Rectapur), methanol (VWR, Rectapur), diethyl ether (Acros Organics, 99+% stabilized with 3,5-di-tert-butyl-4-hydroxytoluene, BHT), and tetrahydrofuran (VWR, Normapur, stabilized with BHT) were used as received. The 2(dodecylthiocarbonothioylthio)-2-methylpropanoic acid (TTCA) RAFT agent was prepared as described before.21 The synthesis22 and characterizations of poly(acrylic acid) macro-RAFT agents (PAATTC) are described in the Supporting Information. Characterization Techniques. Nuclear Magnetic Resonance (NMR). The purity of TTCA was determined by 1H and 13C NMR spectroscopy in CDCl3 at room temperature (300 and 500 MHz Bruker). The conversion of AA was determined by 1H NMR spectroscopy in DMSO-d6 by the relative integration of the internal reference (1,3,5-trioxane) peak at 5.11 ppm and the vinylic proton peaks of AA at 6.26, 6.07, and 5.86 ppm. Size Exclusion Chromatography (SEC). The number-average molar masses (Mn), the weight-average molar masses (Mw), and the molar mass distributions (Đ = Mw/Mn) were determined by SEC. Measurements were performed with a Viscotek TDAmax system from Malvern Instruments that consists of an integrated solvent and sample delivery module (GPCmax) and a Tetra Detector Array (TDA) including a right (90°) and a low (7°) angle light scattering (LS) detector (RALS/LALS), a four-capillary differential viscometer, a differential refractive index detector (RI), and a diode array UV detector. THF was used as the mobile phase at a flow rate of 1 mL min−1 and toluene as a flow rate marker. All polymers were injected (100 μL of solution) at a concentration of 10 mg mL−1 after filtration through a 0.45 μm pore-size membrane. The separation was carried out on three Polymer Laboratories columns [3 × PLgel 5 μm Mixed C (300 × 7.5 mm)] and a guard column (PL gel 5 μm). Columns and detectors were maintained at 40 °C. The OmniSEC 4.6.2 software was used for data acquisition and data analysis. The dispersities (Đ = Mw/ MnPS) were calculated with a calibration curve based on narrow polystyrene (PS) standards (from Polymer Standard Services), using only the RI detector. The absolute number-average molar masses, MnLS, were calculated using the RALS/LALS and RI signals. The 665


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Table 1. Experimental Conditions for the Synthesis of P(VDC-co-MeA) Copolymers in the Presence of TTCA and Their Molecular Characteristicsa expt

mol % MeA0b

wt % MeA0c

VM1 VM2 VM3 VM4 VM5 VM6 VM7 VM8i VM9i VM10j

10.7 11.2 17.2 30.1 50.0 74.7 100 11.2 21.7 11.1

9.7 10.1 15.3 27.1 47.0 72.4 100 10.1 19.8 10.0

wt % MeAEAd 9.3 30.0 48.3 73.7

11.7

[monomers]0e (mol L−1)

[monomers]0/ [TTCA]0

time (h)

convf (wt %)

Mnth (g mol−1)

MnPS (g mol−1)

MnLS (g mol−1)

Đ

dn/dC (mL g−1)

5.01 5.25 5.52 4.95 4.92 4.90 3.39 12.3 12.2 5.18

211 70 232 201 199 197 197 190 199 200

44.0 63.3 72.0 28.1 25.5 8.0 3.8 18.0 19.0 48.0

36.0 67.6 47.4 44.1 61.8 63.7 65.0 36.7 63.7 57.5

7 640 4 898 10 801 8 667 11 619 11 528 11 412 7 015 9 957 11 372

5 500 3 600 7 200 7 600 9 600 10 600 11 400 5 300 7 000 7 500

9 400 6 400 9 500 10 600 12 400 12 600 13 200 9 400 11 000 11 700

1.67 1.56 1.64 1.34 1.27 1.18 1.13 1.71 1.60 1.65

0.109g 0.110h 0.106h 0.103h 0.092h 0.080h 0.067h 0.109g 0.105g 0.109g

The polymerizations were performed in toluene at 30 °C with a [TTCA]0/[V70]0 ratio of 7. bInitial mol % of MeA in the mixture of monomers. Initial wt % of MeA in the mixture of monomers. dwt % of MeAEA in the copolymer (calculated on the basis of the monomer units, without the end groups) deduced from Cl weight percentage determinated by elemental analysis (see the Supporting Information). e[Monomers]0 = [VDC]0 + [MeA]0. fOverall monomer weight conversion determined by gravimetric analysis. gdn/dC was calculated from the wt % of MeA0 in the polymer (from eq 1: −4.7 × 10−4 × (wt % MeA0) + 0.114) (see the Supporting Information). hdn/dC was determined by size exclusion chromatography (see the Supporting Information). iThe polymerization was performed in bulk. jThe polymerization was carried out in toluene at 66 °C in a thermostated glass Parr reactor pressurized at 5 bar in the presence of AIBN as an initiator. a c

MnMHS values correspond to the number-average molar masses derived from the PS calibration considering the Mark−Houwink−Sakurada (MHS) parameters26 (for details see the Supporting Information). The refractive index increments (dn/dC) were measured with the online RI detector, as described in the Supporting Information. In all plots showing the evolution of Mn with monomer conversion, the straight line corresponds to the expected evolution of the theoretical number-average molar mass, Mnth, calculated by the product of the introduced mass of monomer and the conversion divided by the initial mole number of the (macro-)RAFT agent plus the molar mass of the latter. The overall monomer weight conversions were determined by gravimetric analysis. Polymers containing AA units have been modified by methylation of the carboxylic acidic groups using trimethylsilyldiazomethane before SEC analysis.26 Thermogravimetic Analyses (TGA). TGA were performed with TGAQ50 TA Instruments to quantify the weight percentage of remaining solvent in the used macro-RAFT agents. Elemental Analyses (EA). EA were performed at Service Central d’Analyse, CNRS, Solaize, France. Reversible Addition−Fragmentation Chain Transfer Copolymerization of VDC and MeA (or AA). Copolymers of VDC and MeA (or AA) are noted “VMx” (or “VAx”), where “x” stands for the experiment number. In a typical experiment (Table 1, entry VM3) 0.110 g (0.30 mmol) of TTCA was dissolved in 6.020 g of toluene in a 25 mL round-bottom flask. Then 0.014 g (0.05 mmol) of V70 was added, and the resulting mixture was purged with nitrogen for 20 min at 0 °C. After deoxygenation of the monomers by bubbling with nitrogen for 20 min, 5.624 g (58 mmol) of VDC and 1.036 g (12 mmol) of MeA were injected through the septum into the reaction mixture. The sealed round-bottom flask was immersed in a thermostated oil bath at 30 °C. Samples were periodically withdrawn to monitor the overall monomer weight conversion by gravimetric analysis and the number-average molar mass of the polymer by SEC. The polymerization was stopped after 72 h by cooling the flask at 0 °C (ice−water bath), and the P(VDC-co-MeA) polymer was recovered after precipitation in cold methanol. The procedure was the same for the copolymerization of VDC and AA, except that the P(VDC-co-AA) polymer was synthesized in 1,4-dioxane and recovered by precipitation in cold diethyl ether. Synthesis of Amphiphilic P(VDC-co-MeA)-b-PAA, PAA-bP(VDC-co-MeA), and PAA-b-P(VDC-co-AA) Block Copolymers. Block copolymers are noted “Cx”, where “x” stands for the experiment number and the number-average molar mass of the macro-RAFT agent in kg mol−1 is indicated as yK. P(VDC-co-MeA)-b-PAA, PAA-bP(VDC-co-MeA), and PAA-b-P(VDC-co-AA) were synthesized following similar protocols exemplified here for a PAA5K-b-P(VDC-

co-MeA) copolymer (Table 5, entry C4): 1.457 g (0.26 mmol) of PAA5K-TTC (MnMHS = 5300 g mol−1, Đ = 1.19, Table 4, entry A2) was dissolved in 9.56 g of 1,4-dioxane in a 25 mL round-bottom flask. 0.012 g (0.04 mmol) of V70 was added, and the resulting mixture was purged with nitrogen for 20 min in a cold water bath. After deoxygenation of the monomers by bubbling with nitrogen for 20 min, 2.704 g (28 mmol) of VDC and 1.197 g (14 mmol) of MeA were injected through the septum into the reaction mixture. The sealed round-bottom flask was immersed in a thermostated oil bath at 30 °C, and the polymerization lasted for 16.6 h. The polymer was recovered after precipitation in cold diethyl ether. “One-Pot” Synthesis of PAA-b-P(VDC-co-AA) Diblock Copolymer (Table 5, Entry C7). 0.090 g (0.25 mmol) of TTCA was dissolved in 16.96 g of 1,4-dioxane in a 25 mL round-bottom flask. 0.008 g (0.025 mmol) of V70 was added to the reaction mixture deoxygenated by bubbling with nitrogen for 20 min at 15 °C. After similar deoxygenation of AA, 3.078 g (42 mmol) of the latter was injected through the septum into the reaction mixture. The sealed round-bottom flask was immersed in a thermostated oil bath at 30 °C. After 2.8 h, the conversion of AA was determined by gravimetric analysis (66 wt %), and the average molar masses were determined by SEC (MnPS = 7300 g mol−1, MnMHS = 8000 g mol−1, and Đ = 1.16). At this moment, 2.911 g (30 mmol) of nitrogen-purged VDC was injected through the septum. After 26.8 h of reaction, the polymerization was stopped by cooling the flask at 0 °C (ice−water bath), and the overall monomer conversion (AA and VDC) was determined to be 30.5 wt % by gravimetric analysis for the second block. The polymer was recovered after precipitation in diethyl ether. MnPS = 13 800 g mol−1 and Đ = 1.27.

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RESULTS AND DISCUSSION 1. Synthesis of VDC-Based Copolymers. To evaluate the efficiency of TTCA as a chain transfer agent for the synthesis of well-defined PVDC-based copolymers, two series of experiments were performed with various proportions of either MeA or AA as a comonomer. RAFT-Mediated Copolymerization of VDC and MeA. MeA was first tested as a comonomer. A series of copolymers with different proportions of VDC and MeA were prepared at 30 °C using V70 as an initiator (the half-life time of V70 at 30 °C is 10 h27). The resulting polymers were characterized by size exclusion chromatography, SEC, and elemental analysis, EA (Table 1). 666


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Figure 1. (A) Evolution of the refractive index increment (dn/dC) of P(VDC-co-MeA) copolymers versus the initial weight percentage of MeA in the comonomer feed. (B) Evolution of the overall monomer weight conversion versus time for the RAFT copolymerization at 30 °C in toluene at various VDC/MeA molar ratios: 89/11 (◆, VM1), 70/30 (▲, VM4), 50/50 (■, VM5), and 25/75 (●, VM6) with [monomers]0 = 5 mol L−1, [TTCA]0 = 2.10−2 mol L−1, and [TTCA]0/[V70]0 = 7.

Figure 2. (A) Evolution of the size exclusion chromatograms with the overall monomer weight conversion and (B) evolution of the number-average molar mass: MnPS, MnLS (◇, ◆) and dispersities: Đ (△) versus overall monomer weight conversion for the RAFT copolymerization in toluene of VDC/MeA with [VDC]0 = 4.57 mol L−1, [MeA]0 = 0.95 mol L−1, [monomers]0/[TTCA]0 = 232, and [TTCA]0/[V70]0 = 7 (experiment VM3 in Table 1).

obtained, from which the dn/dC value (0.114 mL g−1) for a pure PVDC in THF was determined by extrapolation:

Elemental analyses performed on the various copolymers confirmed that the copolymerizations proceeded without variation of the copolymer composition. Indeed, the calculated weight percentages of MeA in the copolymers (calculated on the basis of the monomer units) determined by EA at any conversion remained close to the initial ones (values given in Table 1). This shows a very similar reactivity of both monomers and reactivity ratios close to one as discussed in the Introduction.4,5 For the first time, the absolute Mn values of the VDC-based copolymers were determined by SEC equipped with a static light scattering (SLS) detector in-line. For that purpose, it was necessary to determine the refractive index increment (dn/dC) of the copolymers at various compositions (detailed in the Supporting Information). A differential refractive index detector coupled to the SEC line at 40 °C was used (see Table 1). In Figure 1A the dn/dC values are plotted against the (co)polymer compositions (note that the initial monomer composition was used in this purpose, as it was shown above to be the same as the copolymer composition at any conversion). As expected,28 a linear relationship between dn/dC and the composition was

dn/dC = −4.7 × 10−4 × (wt % MeA) + 0.114

(1)

As shown in Figure 1B for similar [monomer(s)]0/[TTCA]0 molar ratios, the rate of polymerization increased when the proportion of MeA was increased as expected considering the propagation rate coefficient values of both monomers. Indeed, the rate constants of homopropagation are kp,MeA ≈ 15 000 L mol−1 s−1 at 20 °C (approximate value) for methyl acrylate in bulk,29 and kp,VDC = 37 L mol−1 s−1 at 35 °C for VDC in hexane solution (determined using the rotating sector technique).30 The average ⟨kp⟩ value for copolymerization under the terminal model hypothesis is given by ⟨kp⟩ = (f MeA/kp,MeA + f VDC/ kp,VDC)−1 when the reactivity ratios are both equal to 1,29 where f MeA and f VDC correspond to the molar fraction of the monomers in the comonomer mixture (f MeA + f VDC = 1). In such condition, an increase of f MeA will lead to an increase of ⟨kp⟩. Nevertheless, the copolymerization reactions remained slow for all proportions studied, by comparison with the homopolymerization of methyl acrylate (VM7), since the 667


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Table 2. Experimental Conditions for the Synthesis of the P(VDC-co-AA) Copolymers in 1,4-Dioxane Solution at 30 °C in the Presence of TTCA and the Molecular Characteristics of the Recovered Copolymersa expt

mol % AA0

[monomers]0b (mol L−1)

[monomers]0/ [TTCA]0

time (h)

convc (wt %)

Mnth (g mol−1)

mol % A Acalc

mol % AAEAd

dn/dCcalce (mL g−1)

dn/dCf (mL g−1)

MnLS g (g mol−1)

MnPS g (g mol−1)

Đ

VA1 VA2 VA3

10.0 20.1 40.3

5.89 5.82 5.93

299 306 301

51.8 39.0 27.4

48.8 52.4 70.8

14 168 15 104 18 891

14.7 27.1 46.5

13.2 25.7 53.7

0.108 0.103 0.090

0.106 0.109 0.093

16 400 19 600 19 600

11 800 15 000 16 200

1.46 1.30 1.23

a

The polymerizations were performed with a [TTCA]0/[V70]0 ratio of 7. b[Monomers]0 = [VDC]0 + [AA]0. cOverall monomer weight conversion determined by gravimetric analysis. dAcrylic acid molar percentage deduced from Cl weight percentage determinated by elemental analysis. edn/ dCcalc of the methylated copolymers was calculated with the eq 1: dn/dCcalc = −4.7 × 10−4 × (wt % MeA) + 0.114 and from the wt % of MeA deduced from the wt % of AAEA in the recovered copolymer (see the Supporting Information). fdn/dC of the methylated copolymers was determined by size exclusion chromatography. gMnLS and MnPS determined by SEC for the methylated copolymers and recalculated for the nonmethylated ones (see the Supporting Information). MnLS was determined using the experimental dn/dC values.

Figure 3. (A) Evolution of the overall monomer weight conversion versus time for the RAFT copolymerization at 30 °C in dioxane of various VDC/ AA mixtures at initial molar ratios of 90/10 (◆, VA1), 80/20 (▲, VA2), and 60/40 (■, VA3) with [monomers]0 = 6 mol L−1, [TTCA]0 = 2 × 10−2 mol L−1, and [TTCA]0/[V70]0 = 7. (B) Evolution of the size exclusion chromatograms with the overall monomer weight conversion (experiment VA2 in Table 2).

thus the control, by introducing secondary carbon radicals at the chain end. To study the influence of the solvent and of the temperature on the control, the copolymerization with 11 mol % of MeA was also performed in bulk at 30 °C (experiment VM8) and in toluene at higher temperature (66 °C; experiment VM10) under 5 bar pressure (Table 1). In both cases, the results were similar to those obtained in toluene solution at 30 °C. Compared to similar studies reported earlier and conducted with dithiobenzoate RAFT agents in benzene,14 lower Đ and a better concordance of the experimental molar masses with the theoretical values were obtained in the present work. RAFT-Mediated Copolymerization of VDC and AA. The efficiency of the TTCA RAFT agent in controlling the copolymerization of VDC with different percentages of acrylic acid as a hydrophilic comonomer (10, 20, and 40 mol % of AA, Table 2) was tested at 30 °C in 1,4-dioxane solution. Like in the case of MeA, by keeping the same initial [monomers]0/[TTCA]0 molar ratio, the rate of polymerization increased when the proportion of AA was increased due to a large kp value for the homopolymerization of AA32 (Figure 3A). The reactivity ratios r1′ and r2′ for VDC and AA have been reported to be 0.46 and 1.26, respectively, at 50 °C,33 meaning that gradient copolymers form with this comonomer. As summarized in Table 2, the mol % of AA in the resulting copolymers (mol % AAcalc with respect to the monomer units) calculated at the final conversions from these reactivity ratios by using the terminal model and numerical simulation were close to those determined experimentally by EA.

average rate constant of propagation is mainly governed by the value of the “slow” monomer, i.e., VDC here.29 This discussion, based on the rate constant of propagation only, does not take into account the additional influence of the rate constant of termination kt, which should also affect the kinetics via the kp/ √kt ratio. Throughout the polymerizations, the SEC traces of the polymers were narrow and shifted toward higher molar masses with increasing monomer conversion (Figure 2A). The absolute number-average molar mass values, MnLS, increased linearly with monomer conversion and were close to the theoretical ones for all copolymer compositions (Figure 2B), a feature for controlled radical polymerization. For a given conversion, Đ decreased with increasing concentration of MeA; i.e., the control was better for the copolymerizations with the highest MeA contents (Table 1, 1.67 for 10.7 mol % of MeA to 1.18 for 74.7 mol % of MeA). All these results demonstrate that TTCA is an effective RAFT agent for the synthesis of random copolymers based on VDC and MeA. It should be mentioned here that the structure of the RAFT agent is not well suited for the polymerization of 1,1-disubstituted ethylenic monomers23,25,31 (i.e., leading to tertiary carbon-centered radicals) such as VDC (the homopolymerization was however not performed due to the high crystallinity and hence poor solubility of PVDC). This might be explained by a low chain transfer constant of TTCA, resulting from a slow release of the initiating radical by fragmentation of the intermediate radical in the pre-equilibrium stage. In consequence, the copolymerization with an acrylic ester improves the initial transfer step, and 668


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During the copolymerization process, the SEC traces of the polymers (e.g., Figure 3B) shifted toward higher molar masses with increasing monomer conversion accounting for the efficiency of the chain transfer reaction. The molar masses of the final samples after precipitation were determined by SEC using conventional PS calibration and SLS after quantitative methylation26 of the carboxylic acid groups yielding VDC/MeA copolymers (the refractive index increment was calculated with eq 1 from the wt % of AA with respect to the monomer units determined by elemental analysis in the recovered copolymer (see the Supporting Information)). The values were generally in the same range as the theoretically expected ones (the experimental MnLS being slightly higher, see Table 2). As already observed for the VDC/MeA copolymers, the narrowest molar mass distributions were obtained in the presence of the highest initial percentage of acrylic comonomer, AA (Đ = 1.23 for 40 mol % of AA vs 1.46 for 10 mol % of AA), due to a better transfer efficiency for an AA terminated propagating macroradical compared to a VDC-terminated one, as discussed above for MeA. 2. Synthesis of P(VDC-co-MeA)-b-PAA and PAA-bP(VDC-co-M) Amphiphilic Block Copolymers (with M = MeA or AA). In the previous section, we demonstrated the controlled character of the copolymerization of VDC with MeA or AA by using a RAFT agent of the trithiocarbonate type. To synthesize an amphiphilic block copolymer containing a hydrophilic block of PAA and a PVDC-based hydrophobic block, two pathways can be pursued: the first one starts with a PVDC-based macro-RAFT agent, P(VDC-co-MeA)-TTC, which is extended by acrylic acid, and the second one relies on the copolymerization of a mixture of VDC with MeA or AA in the presence of a PAA macro-RAFT agent, PAA-TTC (Scheme 1). It is known that the quality of the chain extension of a preformed first block used as controlling agent for the RAFT polymerization of a second monomer is related to the chemical nature of the macromolecular leaving group. In fact, the chemical nature of the latter has an important impact on the transfer efficiency in the polymerization. It must therefore be carefully chosen with respect to the monomer to be polymerized.34 It should also be mentioned thataccording to the pathway selectedthe hydrophobic alkyl chain originating from the TTCA RAFT agent (Scheme 1) will be located at the chain end of either the hydrophobic or the hydrophilic block, which certainly alters the amphiphilic properties of the copolymers. As already reported, the presence of such a hydrophobic “sticker” end group may also be desired. If necessary, it may be removed by established methods.35,36 First Pathway: Polymerization of AA in the Presence of P(VDC-co-MeA)-TTC. Two P(VDC-co-MeA)-TTC macroRAFT agents containing 11 and 22 mol % (10 and 20 wt %) of MeA (VM2 and VM9, respectively; for their synthesis see Table 1) have been extended with AA to obtain amphiphilic block copolymers. As shown in Figure 4, the SEC curves obtained for the methylated polymers for different conversions were narrow, showing no significant tailing toward the lower molar mass side. This demonstrated a good blocking efficiency and no residual macro-RAFT agent. As reported in Table 3, the experimental MnLS values matched quite well the theoretical ones, indicating the formation of well-defined amphiphilic P(VDC-co-MeA)-b-PAA diblock copolymers, when P(VDC-coMeA) macro-RAFT agents containing more than 10 mol % of MeA were used.

Figure 4. Chain extension of P(VDC-co-MeA)-TTC (VM2) with AA (experiment C1 in Table 3). Evolution of the size exclusion chromatograms with monomer conversion.

Second Pathway: Copolymerization of VDC and MeA (or AA) in the Presence of PAA-TTC. Preparation of Poly(acrylic acid)-Based Macro-RAFT Agents. In a first step, the PAA-TTC macro-RAFT agents were prepared by homopolymerization of AA in 1,4-dioxane solution in the presence of TTCA at 60 or 70 °C. The polymerizations were stopped below 70% monomer conversion to guarantee the recovery of a large proportion of chains terminated by a trithiocarbonate group. Three PAA-TTC macro-RAFT agents differing in the Mn of the PAA chain have been synthesized (Table 4). As discussed in detail in the Supporting Information, a good agreement between the theoretical Mn values and the experimental ones along with a high degree of TTC endfunctionality was confirmed by 1H NMR and MALDI-TOF mass spectrometry. Determination of the Chain Transfer Constants of PAATTC in the Copolymerizations of VDC with Different mol % of MeA. To prepare well-defined PAA-b-P(VDC-co-MeA) amphiphilic diblock copolymers, we first examined the influence of the VDC/MeA molar ratio on the ability of the PAA trithiocarbonate macro-RAFT agent (PAA-TTC) to be extended by a mixture of VDC/MeA. Indeed, like TTCA, the PAA-TTC macro-RAFT agents do not exhibit an appropriate leaving-group structure for the controlled polymerization of 1,1-disubstituted monomers. Therefore, it was expected that an increased proportion of MeA would improve the transfer reactions, and hence the crossover efficiency, as discussed above. For this purpose, PAA6K with an Mn of 6100 g mol−1 was used (Table 4, entry A3). We demonstrated that all chains were end-capped by a trithiocarbonate functional group (see the Supporting Information, Figure SI-1), which was confirmed by a chain extension reaction until high conversion (Figure SI-2). The experimental conditions for the determination of the chain transfer constants are summarized in Table SI-1 of the Supporting Information (experiments CT1, CT2, and CT3). Figure 5A shows the conversion versus time plots for copolymerizations of VDC with MeA at three different molar ratios of VDC/MeA (89/11, 79/21, and 69/31) in the presence of PAA-TTC. Keeping the same [monomers]0/[PAA-TTC]0 ratio, the rate of polymerization again increased with increasing the initial concentration of MeA. Figures 5B−D show the size 669


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Table 3. Experimental Conditions for the Synthesis of the Amphiphilic PVDC-Based Block Copolymers and Their Molecular Characteristicsa expt C1 C2

macro-RAFT agent

[AA]0 (mol L−1)

[AA]0/ [RAFT]0

time (h)

convb (wt %)

Mnth (g mol−1)

dn/dCc (mL g−1)

MnLS d (g mol−1)

MnPS d (g mol−1)

Đ

2.07

447

1.4

60.5

25 906

0.078

25 100

14 800

1.45

2.04

721

1.5

54.2

39 134

0.078

34 400

18 600

1.40

P(VDC0.89-coMeA0.11)e P(VDC0.78-coMeA0.22)f

The polymerizations were performed at 70 °C with a [TTCA]0/[ACPA]0 ratio of 4. bMonomer conversion determined by gravimetric analysis. dn/dC is calculated from the wt % of MeA in the methylated copolymer with the eq 1: dn/dCcalc = −4.7 × 10−4 × (wt % MeA) + 0.114 (see the Supporting Information). dMnLS and MnPS determined by SEC for the methylated copolymers and recalculated for the nonmethylated ones (see the Supporting Information). eVM2: MnLS = 6400 g mol−1, Đ = 1.56. fVM9: MnLS = 11 000 g mol−1, Đ = 1.60. a c

Table 4. Experimental Conditions for the Synthesis of the Hydrophilic Macro-RAFT Agents PAA-TTC in 1,4-Dioxane and Their Molecular Characteristicsa expt

macro-RAFT agent

[AA]0 (mol L−1)

[AA]0/ [TTCA]0

T (°C)

time (min)

convb (mol %)

Mnth (g mol−1)

MnPS c (g mol−1)

MnMHS c (g mol−1)

Mn,NMR c (g mol−1)

Đ

A1 A2 A3

PAA3K PAA5K PAA6K

2.25 2.14 2.05

46 136 129

70 70 60

60 65 165

63.6 42.0 45.9

2489 4487 4612

2400 4800 5600

2700 5300 6100

2160 4900 5040

1.12 1.19 1.16

a

The polymerizations were performed with a [TTCA]0/[ACPA]0 ratio of 10. bMonomer conversion determined by 1H NMR. cMn recalculated for the nonmethylated polymers (see the Supporting Information).

Figure 5. Radical copolymerization of VDC with MeA in the presence of PAA-TTC (MnMHS = 6100 g mol−1, Đ = 1.16) in toluene at 30 °C. (A) Evolution of the overall monomer weight conversion versus time for various molar ratios of VDC/MeA: 89/11 (◆, CT1), 79/21 (▲, CT2), and 69/31 (■, CT3) with [monomers]0 = 3.7 mol L−1, [PAA-TTC]0 = 3.3 mmol L−1, and [PAA-TTC]0/[V70]0 = 3.4. (B−D) Evolution of the size exclusion chromatograms with the overall monomer weight conversion at constant trithiocarbonate chain-end concentration equal to PAA-TTC concentration of the first SEC sample: (B) 0.40, (C) 0.61, and (D) 0.55 mmol L−1.

exclusion chromatograms of samples taken from the reaction medium at different monomer conversions for the three studied molar ratios. It clearly appears that the formation of block copolymers was much less efficient than observed using the first

synthetic route. Indeed, in all cases a shoulder on the low molar mass side of the SEC traces indicates the presence of nonconverted PAA-TTC macro-RAFT agent, for monomer conversions as high as 31.0% in the experiment with the lowest 670


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fragmentation of the intermediate radical resulting from the addition of a MeA terminated oligomeric radical onto PAATTC leads to a more efficient release of the PAA macroradical. Consequently, the introduction of a higher amount of MeA in the polymerization medium enhances the transfer reaction to the macro-RAFT agent (PAA-TTC) and allows high transfer efficiency to be reached. As reported in the following section, well-defined PAA-b-P(VDC-co-MeA) copolymers were thus prepared in the presence of about 30 mol % of MeA. Polymerization of VDC with MeA (or AA) in the Presence of PAA-TTC and Implementation of a One-Pot Process. As summarized in Table 5, a series of PAA-b-P(VDC-co-M) (with M = MeA or AA) copolymers were prepared with PAA macroRAFT agents PAA3K and PAA5K (Table 4) in 1,4-dioxane solution at 30 °C. In all experiments the molar percentage of the comonomer “M” was of about 30% to guarantee Ctr ≫ 1 and the formation of well-defined diblock copolymers. In all cases, the SEC analyses confirmed the high efficiency of the transfer reaction, and the Đ values were consequently below 1.4. The molar masses determined by SEC using an SLS detector were generally a little higher than the theoretical values, which might be explained by the limitation of SLS to detect molecules of low molar mass (Table 5). The experimental refractive index increments matched well the calculated theoretical values (using eq 1 with the AA wt % calculated by numerical simulation at the final conversion using the terminal model as mentioned above). Finally, a one-pot process to synthesize PAA-b-P(VDC-coAA) was also investigated (Table 5, entry C7). Here, the polymerization was performed in two steps, both performed consecutively in the same reactor. For the first step, corresponding to the synthesis of the hydrophilic PAA block, the polymerization of acrylic acid was carried out at 30 °C. After 2.8 h, a sample was withdrawn from the polymerization mixture (the conversion of AA was 66% and the obtained PAATTC had a MnMHS = 8000 g mol−1 and Đ = 1.16), and the VDC monomer was injected into the reactor to extend the in situ formed PAA macro-RAFT agent by a second polymer block composed of AA and VDC. The copolymerization was led for further 24 h. The efficiency of the chain transfer reaction was confirmed by chromatograms that were completely shifted toward higher molar masses with increasing conversion (Figure 7). The overall weight conversion of AA and VDC for the second block was 31%. It has been determined from the individual conversions of AA (42%) and of VDC (27%) calculated using the terminal model that this second block contained 43 mol % of AA. The MnLS of the final block copolymer was 18 900 g mol−1 with a low dispersity (1.27). It can thus be concluded that well-defined PAA-b-P(VDC-coM) amphiphilic diblock copolymers can be prepared in 1,4dioxane solution at 30 °C starting from PAA macro-RAFT agents, e.g., in mild and simple conditions, without the use of an autoclave reactor. This pathway is interesting from a process point of view since recent advances in CRP in dispersed media have indeed shown the ability of hydrophilic macro-RAFT agents to control the polymerization of hydrophobic monomers and to stabilize the resulting latexes.38 Furthermore, it is possible to prepare PAA-b-P(VDC-co-AA) amphiphilic diblock copolymers by a simple one-pot process, which can easily be transferred to industrial scale. Such amphiphiles may then find applications as additives in PVDC-based formulations. The study of their emulsifying properties and their use as stabilizers

proportion of MeA. The chain transfer constant of the PAATTC macro-RAFT agent was thus supposed to be quite low and was determined for the three different initial monomer compositions using the SEC technique reported by Fukuda et al.37 For this purpose, a fixed volume of the reaction solution was analyzed by SEC to keep constant the concentration of PAATTC (converted or not to the diblock copolymer). In this way, the amount of unreacted PAA-TTC as a function of monomer conversion can be determined by deconvolution of the SEC curves (the procedure for deconvolution is detailed in the Supporting Information). As reported earlier,37 the chain transfer constant Ctr can be determined from the slope of the plot of ln(S0/S) versus ln(1/(1 − convM)): ⎛ ⎞ ⎛S ⎞ 1 ln⎜ 0 ⎟ = C tr ln⎜ ⎟ ⎝S⎠ ⎝ 1 − convM ⎠

(2)

where Ctr = ktr/kp, S is the area of the peak corresponding to the macro-RAFT agent (S0 corresponds to S at t = 0), and convM is the monomer conversion (VDC copolymerizes randomly with MeA as described above, and hence the overall monomer weight conversion corresponds to the overall molar conversion in this case). The slopes of the curves presented in Figure 6 give access to the three Ctr values. With 11 mol % of MeA, which is a typical

Figure 6. Chain extension of PAA-TTC (MnMHS = 6100 g mol−1, Đ = 1.16) in toluene at 30 °C with VDC/AMe: ln(S0/S) versus ln(1/(1 − convM)) for various molar ratios of VDC/MeA: 89/11 (◆, CT1), 79/ 21 (▲, CT2), and 69/31 (■, CT3). S0 and S are the deconvoluted RI area of the PAA-TTC in SEC at t = 0 and at a given conversion, respectively.

composition for a commercial vinylidene chloride copolymer, Ctr was close to 1; i.e., the macro-RAFT agent and the monomers were consumed at a similar relative rate, resulting in broad chain distributions. It has been demonstrated that a Ctr > 2 is generally needed to reduce the dispersities and that Ctr values above 10 lead to polymers presenting the characteristics often associated with living/controlled polymerizations (very narrow molar mass distributions and molar masses that increase linearly with conversion).11 The transfer constant increased when the proportion of MeA was increased (up to 6 for 31 mol % of MeA). This may be explained as follows: (i) the propagating radicals with a methyl acrylate terminal subunit undergo faster addition reaction to the PAA-TTC than the propagating radicals with a VDC terminal subunit; (ii) 671


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Table 5. Experimental Conditions for the Synthesis of Amphiphilic PVDC-Based Block Copolymers in 1,4-Dioxane at 30 °C and Their Molecular Characteristicsa expt

macroRAFT agentb

M

C3 C4 C5 C6 C7i

PAA3K PAA5K PAA3K PAA5K PAA8K

MeA MeA AA AA AA

mol % [monomers]0c [monomers]0/ [RAFT]0 M0 (mol L−1) 35.8 33.3 33.3 34.1 32.4

3.38 3.28 3.44 3.46 2.40

45 163 46 176 181

time (h)

convd (wt %)

Mnth (g mol−1)

dn/dCcalce (mL g−1)

dn/dCf (mL g−1)

MnLS g (g mol−1)

MnPS (g mol−1)

Đ

16.4 16.6 16.4 16.5 26.8

84.0 39.6 72.9 39.3 30.5h

5722 11 064 5201 11 327 12 955

0.086 0.085 0.084 0.082 0.079

0.085 0.082 0.082 0.082 0.082

6100 13 800 5400 12 000 18 900

5300 9500 5000 9700 13 800

1.27 1.27 1.34 1.30 1.27

a The polymerizations were performed with a [PAA-TTC]0/[V70]0 ratio of 7. bPAA3K, PAA5K, and PAA8K denote the PAA-TTC RAFT agents with molar masses of 3000 g mol−1 (MnMHS(PAA-TTC) = 2700 g mol−1, Đ = 1.13), 5000 g mol−1 (MnMHS(PAA-TTC) = 5300 g mol−1, Đ = 1.19), and 8000 g mol−1 (MnMHS(PAA-TTC) = 8000 g mol−1, Đ = 1.12), respectively. c[Monomers]0 = [VDC]0 + [M]0. dMonomer conversion was determined by gravimetric analysis. edn/dCcalc was calculated according to the method presented in the Supporting Information. fdn/dC of the methylated polymer was determined by size exclusion chromatography. gMnLS was obtained with the experimental dn/dC. hOverall weight conversion of AA and VDC for the second block. iThe polymerization was performed by a one-pot process.

■

ASSOCIATED CONTENT

S Supporting Information *

Synthesis and characterization of poly(acrylic acid)-based macro-RAFT agents; determination of the chain transfer constants of PAA-TTC; determination of the number-average molar mass of the macro-RAFT agent PAA-TTC using Mark− Houwink−Sakurada (MHS) parameters. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: (J.R.) [email protected], (F.S.) francois. stoff[email protected], (B.C.) [email protected]. Notes

Figure 7. Two steps “one-pot” synthesis of a PAA-b-P(VDC-co-AA). Evolution of the size exclusion chromatograms with monomer conversion in the first and in the second polymerization step (Table 5, entry C7).

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank the French Agence Nationale de la Recherche (ANR-2010-RMNP-005-02 ASAP) for financial support of this work. Laurent Bouteiller (UPMC), Joël Belleney (UPMC), Ludovic Dubreucq (UPMC), and Ourdia Larfi (UPMC) are acknowledged for technical support. B.C. thanks the Institut Universitaire de France for her nomination as senior member.

in the emulsion polymerization will be the subject of a forthcoming paper.

■

■

CONCLUSION

Amphiphilic PVDC/PAA-based diblock copolymers were prepared with good control by the RAFT technique using a trithiocarbonate RAFT agent. It was shown that their synthesis is possible in 1,4-dioxane solution starting from either a hydrophobic P(VDC-co-MeA) macro-RAFT agent or a hydrophilic PAA macro-RAFT agent. The interest in the first synthesis pathway relies on the very good transfer efficiency of P(VDC-co-MeA)-TTC for acrylic monomers and the rapid consumption of the latter even when low percentages (10 mol %) of MeA comonomer are incorporated in the macro-RAFT agent. In contrast, for the second approach a rapid consumption of the macro-RAFT agent is only reached with 30 mol % MeA, whereas with 10 mol % of MeA the transfer constant was determined to be only close to 1. Finally, we demonstrated that PAA-b-P(VDC-co-AA) amphiphilic diblock copolymers might also be obtained with controlled features in a one-pot process. Future work will deal with their use as stabilizers in the emulsion polymerization of VDC.

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