Batch Emulsion Polymerization Mediated by Poly(methacrylic acid

Article pubs.acs.org/Macromolecules

Batch Emulsion Polymerization Mediated by Poly(methacrylic acid) MacroRAFT Agents: One-Pot Synthesis of Self-Stabilized Particles Isabelle Chaduc,† Marion Girod,‡ Rodolphe Antoine,§ Bernadette Charleux,† Franck D’Agosto,*,† and Muriel Lansalot*,† †

Université Lyon 1, Université de Lyon, CPE Lyon, CNRS UMR 5265, Laboratoire de Chimie Catalyse Polymères et Procédés (C2P2), Equipe LCPP Bat 308F, 43 Bd du 11 Novembre 1918, F-69616 Villeurbanne, France ‡ CNRS, UMR5280, ISA, Université de Lyon, Villeurbanne Cedex, France § CNRS, UMR5579, LASIM, Université de Lyon, Villeurbanne Cedex, France S Supporting Information *

ABSTRACT: The present paper describes the successful onepot synthesis of self-stabilized particles composed of amphiphilic block copolymers based on poly(methacrylic acid) (PMAA) obtained by polymerization-induced selfassembly. First, controlled radical polymerization of MAA is performed in water using the RAFT process by taking advantage of our recent results showing the successful RAFT polymerization of MAA in water [Chaduc et al. Macromolecules 2012, 45, 1241−1247]. The so-formed hydrophilic macroRAFT agents are then chain-extended in situ with a hydrophobic monomer to form amphiphilic block copolymer chains of controlled molar mass that self-assemble into stable nanoparticles. Various parameters such as the pH, the molar mass and the concentration of the PMAA segments or the nature of the hydrophobic block have been investigated.

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INTRODUCTION

Focusing on the RAFT technique, the pioneering work of Ferguson et al.11,12 using oligomeric poly(acrylic acid)- (PAA-) based RAFT agents showed that good control over both the colloidal stability and the molar masses could be achieved. Various preformed water-soluble macromolecular RAFT (macroRAFT) agents were then used for the aqueous emulsion polymerization of hydrophobic monomers added under starved conditions such as PAA13,14,37 and poly(4-vinylpyridine).15 Later on, the same level of control was reached under batch conditions using poly(ethylene oxide) (PEO),16,17 poly(acrylic acid-co-poly(ethylene oxide) methyl ether acrylate),21,22 poly(methacrylic acid-co-poly(ethylene oxide) methyl ether methacrylate),23 poly(N,N-dimethyl acrylamide),19 polyacrylamide,18 poly(N-(4-vinylbenzyl)-N,N-dibutylamine hydrochloride)38 macroRAFT agents, or amphiphilic block copolymers PAA-b-PS with short PS block.14,25−28,37 In most cases the macroRAFT agents carried a trithiocarbonate chain end and were previously synthesized in an organic solvent. We put a special focus on simplifying the process to allow broad use of this strategy. This required adjustment of the polymerization conditions, and by further using monomers of industrial interest. In their original study, Ferguson et al. briefly reported on the synthesis of hydrophilic PAA macroRAFT

A pertinent combination of controlled radical polymerization (CRP) techniques and aqueous emulsion polymerization makes now possible the formation in water of amphiphilic block copolymers that self-assemble in situ to form self-stabilized particles.1 Indeed, water-soluble polymers carrying a CRPactive chain end can initiate the polymerization of a hydrophobic monomer in water. The simultaneous chain growth of the hydrophobic block from the water-soluble polymer and the self-assembly of the resulting amphiphilic copolymer chains lead to the in situ formation of self-stabilized particles ideally constituted of identical polymer chains with a predefined and narrowly distributed molar mass. This approach also called polymerization-induced self-assembly appears as an elegant way of getting rid of low molar mass surfactants which are known to have detrimental effects such as poor Latex stability upon freezing conditions or under high shear, or poor film properties when exposed to water or high conditions of humidity.2 This strategy has been successfully implemented using nitroxide-mediated polymerization (NMP),3−10 reversible addition−fragmentation chain transfer (RAFT),11−29 organotellurium-mediated radical polymerization (TERP),30−35 and reverse iodine transfer polymerization (RITP).36 Besides spherical particles unusual morphologies for emulsion polymerization such as vesicles or fibers have also been reported.6,10,21−23 © 2012 American Chemical Society

Received: April 30, 2012 Revised: June 22, 2012 Published: July 23, 2012 5881

dx.doi.org/10.1021/ma300875y | Macromolecules 2012, 45, 5881−5893

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Scheme 1. Particle Synthesis by Polymerization-Induced Self-Assembly in a One-Pot Process

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EXPERIMENTAL SECTION Materials. Methacrylic acid (MAA, Acros, 99.5%), methyl methacrylate (MMA, Acros, 99%), 4,4′-azobis(4-cyanopentanoic acid) (ACPA, Fluka, >98%), sodium hydrogen carbonate (NaHCO3, Aldrich, >99.7%), were used as received. Styrene (S, Aldrich, 99%) was purified by removing the inhibitor by filtration with aluminum oxide. n-Butyl acrylate (BA, Acros, 99%) was distilled under reduced pressure. Water was deionized before use (Purelab Classic UV, Elga LabWater). 4Cyano-4-thiothiopropylsulfanyl pentanoic acid (CTPPA) was obtained by reaction of ACPA with bis(thiobenzoyl) and bis(propylsulfanylthiocarbonyl) disulfide according to the literature.42 The fluorescent solvatochromic Nile Red dye was purchased from Invitrogen Molecular Probes (Cergy Pontoise, France). One-Pot Procedure for the Synthesis of Polymer Particles by Emulsion Polymerization in the Presence of PMAA MacroRAFT Agent. Step 1: Synthesis of PMAA MacroRAFT Agent in Water. The detailed study of MAA polymerization in water using CTPPA as a RAFT agent has been fully described in a recent paper from our group.41 In this work, PMAA segments were obtained in water using CTPPA as a chain transfer agent and ACPA as a radical initiator. Whatever the targeted molar masses, well-defined PMAA chains with controlled features were obtained with narrowly distributed molar masses (dispersity, Đ < 1.15). In a typical experiment (targeted number-average molar mass, Mn ca. 4000 g mol−1), the procedure is the following: in a two-necked round-bottom flask equipped with a condenser 149 mg of CTPPA (2.29 × 10−2 mol L−1) and 15 mg of ACPA (2.29 × 10−3 mol L−1) were dissolved in 4 mL of water and 2 g of MAA (0.99 mol L−1). 349 mg of 1,3,5-trioxane (1.65 × 10−1 mol L−1) was added as an internal reference for NMR analysis. Then 19.5 mL of water

agents in water which were then used for the RAFT polymerization of butyl acrylate continuously added in the reactor.11 But experimental conditions for which the synthesis of the water-soluble macroRAFT agent could be carried out in water and directly followed by the RAFT polymerization of the hydrophobic block after the simple addition of one shot of hydrophobic monomer would be very appealing. Indeed, the whole process would then take place in the same reactor. With an emphasis on the block copolymer synthetic aspects, we recently showed that amphiphilic block copolymers could effectively be synthesized in water using such a one-pot strategy under batch conditions (Scheme 1).39 Both the hydrophilic and hydrophobic segments are formed in water providing the opportunity to skip the often time-consuming step of preparation and purification of the hydrophilic macroRAFT agent in organic solvent. This simple, robust and eco-friendly strategy results in the improvement of both the kinetics and the copolymer quality. The colloidal aspects of particles resulting from polymerization-induced self-assembly of PAA- or P(MAA-coMPEG)-based block copolymer have already been depicted in the literature.12,24,25 On the other hand, PMAA macroRAFT agent has not been studied yet for the synthesis of selfstabilized Latex particles, although methacrylic acid is widely used in the coatings industry.40 We recently showed for the first time that well-defined PMAA could be obtained by RAFT directly in water up to high conversion.41 The aim of the present paper is to exploit this last feature to show the potential of the aforementioned one-pot strategy for the production of Latex particles stabilized by PMAA chains. Various parameters such as the pH, the molar mass and concentration of the PMAA segments or the nature of the hydrophobic block have been investigated in order to efficiently produce stable, welldefined amphiphilic block copolymer particles. 5882


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was finally added. After deoxygenation by nitrogen bubbling for 30 min, the resulting mixture was immersed in an oil bath thermostated at 80 °C, which corresponded to time zero of the polymerization. The regular withdrawal of samples allowed us to follow the monomer conversion as a function of time and the evolution of molar masses and molar mass distributions as a function of monomer conversion. The monomer conversion was determined by 1H NMR spectroscopy of the crude reaction medium diluted with D2O by the relative integration of the protons of 1,3,5-trioxane and the vinylic protons of the monomers. Different PMAA macroRAFT agents (PMAACTPPA) were synthesized for the purpose of the present study (Latexes 1−4, Mn = 4315 g mol−1, Đ = 1.12; Latexes 5− 7, Mn = 4020 g mol−1, Đ =1.12; Latex 8, Mn = 2390 g mol−1, Đ = 1.15; Latex 9, Mn = 7310 g mol−1, Đ = 1.10; Latex 10, Mn = 12990 g mol−1, Đ =1.06; Latex 11, Mn = 3925 g mol−1, Đ = 1.12; Latexes 12−13, Mn = 4385 g mol−1, Đ = 1.09; Latex 14, Mn = 4230 g mol−1, Đ = 1.10). Step 2: Emulsion Polymerization Procedure. Polymerization of styrene, BA (with 10 mol % of styrene in one case) or MMA was performed at 80 °C in a two-necked round-bottom flask equipped with a condenser. In a typical experiment (Latex 1), 2.50 g of styrene was added to a solution of previously synthesized PMAA-CTPPA (Mn = 4315 g mol−1) and water content was adjusted so that the final concentrations of styrene and macroRAFT agent were 2.4 and 5.56 × 10−3 mol L−1, respectively. Then, 1 mL (1.20 × 103) of an aqueous solution of ACPA (concentration = 3.4 mg mL−1, neutralized by 3.5 mol equiv of NaHCO3) was added to the reaction mixture. In some cases, the pH of the medium was adjusted with NaOH 1 M. The medium was purged with nitrogen during 30 min. The immersion of the round-bottom flask in an oil bath thermostated at 80 °C corresponded to time zero of the polymerization. For the synthesis of triblock copolymers (Latex 14), a PS seed was first prepared following the procedure described above. Then, 3.08 g of BA and 10 mL of water were added to the mixture, which was then stirred at room temperature overnight. The solution was purged with nitrogen during 30 min and immersed in an oil bath thermostated at 80 °C. For each experiment, samples were periodically withdrawn to follow the conversion by gravimetric analysis. The average molar masses (number-average molar mass Mn and weightaverage molar mass Mw) and the molar-mass dispersity (Đ = Mw/Mn) were determined, after methylation, by SEC in THF. Analytical Techniques. MAA conversion was determined by 1H NMR spectroscopy in D2O at room temperature (Bruker DRX 300). For the polymerizations in water of hydrophobic monomers (styrene, BA, MMA) in the presence of PMAA macroRAFT agent, monomer consumption was followed by gravimetric analysis of samples withdrawn from the polymerization medium at different times. Size exclusion chromatography (SEC) measurements were performed in THF at 40 °C at a flow rate of 1 mL min−1, using toluene as a flow rate marker. Before analysis, the polymers were modified by methylation of the carboxylic acid groups using trimethylsilyl diazomethane.43 They were analyzed at a concentration of 3 mg mL−1 after filtration through a 0.45 μm pore-size membrane. The separation was carried out on three columns from Malvern Instruments [T6000 M General Mixed Org (300 × 8 mm)]. The setup (Viscotek TDA305) was equipped with a refractive index (RI) detector (λ = 670 nm). Mn and Đ were derived from the RI signal by a calibration curve

based either on poly(methyl methacrylate) standards (PMMA from Polymer Laboratories) for the analysis of methylated PMAA or on polystyrene standards (PS from Polymer Laboratories) for the analysis of the block copolymers, except for Latex 12 for which a PMMA calibration was used. The particle size (hydrodynamic average diameter Dh) and the dispersity of highly diluted samples (Poly - the higher this value, the broader the size distribution) were measured by dynamic light scattering (DLS) (NanoZS from Malvern Instruments). The Latexes (diluted solution deposited on a carbon/Formvar-coated copper grid and allowed to evaporate) were observed by transmission electron microscopy (TEM) with a Philips CM120 microscope operating at an accelerating voltage of 80 kV (Centre Technologique des Microstructures (CTμ), Claude Bernard University, Villeurbanne, France). In order to preserve particle shape, PBA containing Latexes were observed in their natural hydrated environment using cryogenic TEM (cryo-TEM). According to the method described elsewhere,44 thin liquid films of the suspensions were formed on NetMesh lacy carbon films (Pelco International, USA) and quench-frozen in liquid ethane using a Leica EM CPC workstation (Leica Microsystems, Austria). The specimens were then mounted on a precooled Gatan 626 specimen holder, transferred in the microscope and observed as described previously. The number- and mass-average particle diameter (Dn and Dw, respectively) as well as the particle-diameter dispersity (Dw/Dn) were determined using AnalySIS software (Soft Imaging System). The number of particles Np per unit volume of aqueous phase (mL−1water) was calculated using the diameter obtained from TEM (Dn, nm) according to eq 1, with τ (g mL−1water) the solids content of the dispersed phase (τ = (mmacroRAFT + conversion × mstyrene)/Vwater, with mmacroRAFT and mstyrene the initial weight of PMAA-CTPPA and styrene, respectively, Vwater the initial volume of water) and ρ (g cm−3) the density of the polymer. Np =

6τ ρπDn3

(1)

The average number of radicals per particle ñ is estimated using Np and the slope of the linear part of the monomer conversion versus time curve according to the following eq 2. n ̃ = (slope × NA × [M]0 )/(k p × Np × [M]p )

(2)

In eq 2, NA is the Avogadro’s number, [M]0 the initial concentration of styrene per Lwater, kp the propagation rate constant of styrene (kp = 660 Lp mol−1 s−1 at 80 °C45) and [M]p the concentration of styrene inside the particles ([M]p = 5.5 mol Lp−1 at 50 °C for styrene46). The hydrolytic stability of the trithiocarbonate extremity of PMAA macroRAFT agent at pH = 8 and 3.4 was studied by UV analysis. An aqueous solution of PMAA-CTPPA (Mn = 4315 g mol−1, c = 5.56 × 10−3 mol L−1) was prepared in the same conditions as those of an emulsion polymerization experiment performed at pH = 8 or 3.4. Then the mixture was immersed in an oil bath thermostated at 80 °C. Samples were periodically withdrawn to follow the UV signal of the trithiocarbonate. UV analysis was performed using an UV/vis spectrophotometer (JASCO V-530) and quartz cells. The measurements were carried out at the wavelength of 310 nm (absorbance of the thiocarbonyl thio) at a concentration of 4.2 × 10−5 mol L−1. 5883


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4315 4315 4315 4315 4020 4020 4020 2390 7310 12 990 Latex 1 Latex 2 Latex 3 Latex 4 Latex 5 Latex 6 Latex 7 Latex 8 Latex 9 Latex 10

a All the experiments were performed at T = 80 °C with [NaHCO3]/[ACPA] = 3.5; [PMAA-CTPPA]/[ACPA] = 5. bGlobal solids content taking into account the amount of PS for the given conversion and the macroRAFT agent. cpH was adjusted by addition of NaOH 1 M to the medium, except for pH = 3.4 which was the natural pH of the starting emulsion. dObtained by SEC THF using PS calibration. eObtained by dynamic light scattering. fObtained by transmission electron microscopy. gCalculated from the number-average diameter obtained by TEM. hOn the basis of the total weight of polymer and macroRAFT agent.

8.1 4.1 2.3 3.2 7.5 8.1 9.4 6.0 6.2 13.7 × × × × × × × × × × 8.4 2.8 1.2 5.4 1.8 9.7 3.0 1.0 8.3 1.0 1.04 1.25 1.21 1.40 1.06 1.05 1.06 1.09 1.06 1.08 39 26 31 43 31 37 54 36 40 38 0.09 0.14 0.11 0.08 0.18 0.19 0.07 0.10 0.08 0.12 53 53 84 104 46 59 74 57 62 75 1.37 1.47 2.03 2.21 1.39 1.36 1.34 1.35 1.38 1.46 56 960 58 130 125 760 190 840 23 660 42 530 88 730 44 160 47 000 52 190 100 100 68 85 99 99 100 100 100 94 2 2.5 22 22 4 3.2 4 3 2.5 3 3.4 5.0 6.5 8.0 3.4 3.4 3.4 3.4 3.4 3.4 275 274 194 237 296 270 261 259 293 304 432 432 432 432 200 402 804 480 418 433

convn (%) time (h) pHc τ (g L‑1 water)b DPn,theo concn (mmol L‑1 water) Mn (g mol‑1) expta

PMAA-CTPPA

Table 1. Emulsion Polymerizations of Styrene using PMAA-CTPPA MacroRAFT Agents

Mn,expd (g mol‑1)

Mw/Mnd

Dh (nm)e

RESULTS AND DISCUSSION The synthesis of PMAA-stabilized nanoparticles was performed according to the one-pot procedure depicted in our previous work.39 Briefly, an aliquot of the medium coming out of the reactor where MAA was polymerized up to high conversions under RAFT control was directly diluted with the required amount of water and styrene to target a styrene concentration close to 20 wt %. ACPA was then added and the polymerization mixture heated to 80 °C. The influence of various parameters such as the pH, the length and concentration of the PMAA segments, the nature of the hydrophobic block have been studied. Influence of the pH. In order to assess the effect of the initial pH value on both the kinetics and the control of styrene emulsion polymerization in the presence of PMAA-CTPPA (Mn = 4315 g mol−1, Đ = 1.12), four experiments were carried out at pH = 3.4, 5.0, 6.5, and 8.0 (Latexes 1−4, Table 1) targeting 42 000 g mol−1 for the polystyrene (PS) block. As shown in Figure 1, the pH had a great impact on the polymerization kinetics. The syntheses were very fast with quantitative conversion reached in 90 min at pH = 3.4 and in 2 h at pH = 5.0 (Figure 1). In contrast, the conversion was limited to ca. 70% after 8 h at pH = 6.5 and remained extremely low at pH = 8.0 (less than 8% after 7.5 h). An inhibition period was observed in all cases and dramatically increased when the pH was higher than 5.0. Like the kinetics, the control of the polymerization was strongly affected by the pH (SEC in Figure 2). A very good control was obtained for pH = 3.4 with a fast and quantitative consumption of the initial macroRAFT agent and narrow molar mass distributions (with however a slight increase in the dispersity value at the very end of the polymerization likely due to chain termination by combination) together with molar masses that increased linearly with conversion (see Supporting Information, Figure S1). When the pH was raised to 5.0, a slower consumption of the macroRAFT agent was observed. At pH = 6.5, the situation got worse with still unreacted PMAA at 68% conversion and broader molar mass distribution, phenomena which were even more pronounced at pH = 8.0. Trithiocarbonates are known to be less subjected to hydrolysis than dithiobenzoate.47,48 However, a UV study performed at pH = 8.0 revealed that after only 30 min at 80 °C the absorbance of the PMAA macroRAFT agent at λ = 310 nm (corresponding to the maximum absorbance of the thiocarbo-

5.56 5.56 5.56 5.56 11.90 5.94 2.98 4.95 5.73 5.54

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10 1019 1019 1018 1019 1018 1018 1019 1018 1019

coag (wt %)h g

Dn (nm)f

Dw/Dnf

(L‑1

Np

water)

Fluorescence measurements of PMAA-CTPPA solutions were performed in solution using a spectrofluorometer FluoroMax 4 equipped with a 150 W ozone-free xenon arc lamp (Horiba Jobin Yvon, Longjumeau, France). The purpose was to evaluate the presence of hydrophobic domains as a function of pH. Aqueous solutions of PMAA-CTPPA and Nile Red at different pHs were placed in a 1-cm quartz cell. The excitation wavelength was 532 nm and the fluorescence emission spectra were recorded from 580 to 800 nm. The slit width was 2 nm. A stock solution of Nile Red at 2.1 × 10−4 mol L−1 was prepared in methanol. Solutions of Nile Red at a final concentration of 2.1 × 10−7 mol L−1 at different pHs values were prepared by diluting the stock solution with the PMAACTPPA solution ([PMAA-CTPPA] = 6 × 10−3 mol L−1water). The pH was adjusted using 5 mM NaOH solution (for pH = 8.0) and was measured using a 211R pHmeter equipped with a 4 mm-diameter microelectrode (Hanna Instruments, Woonsocket, RI, USA).

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indicating that the dye moved from less polar to more polar environment. The Stokes shifts of Nile Red can be related to the Lippert−Mataga polarity parameter Δf (eq 3).55,56 Δf =

ε−1 n2 − 1 − 2 2ε + 1 2n + 1

(3)

In eq 3, ε is the dielectric constant of the solvent and n is the refractive index of the solvent. Dielectric constants ε and refractive indices n of pure water and PMAA are 78.3, ≲ 10 and 1.33, ∼1.45, respectively.57 Considering that at pH = 11, the observed Stokes shift (i.e., 3.7 × 103 cm−1) is representative of Nile Red completely solvated by water, the expected Stokes shift of Nile Red solvated by PMAA would be: ΔvPMAA = ((Δf PMAA)/(Δf water)) × Δvwater ≈ 2.55 × 103 cm−1. This value is in qualitative agreement with the experimental Stokes shift of 2.74 × 103 cm−1 observed at pH = 2.9. These results suggest that at low pH Nile Red is mainly solvated by the PMAA chains due to its coiled structure, while Nile Red molecules are released into water at high pH due to the conformational change of PMAA chains to a water-swollen state. It is worth mentioning here that the fluorescence of Nile Red is known to be unaffected by pH between 4.5 and 8.5,51,58 but also that its protonated form is non fluorescent.59 However, the fluorescence intensities were the highest in our experiments for pH lower than 5, which is a strong indication that Nile Red was not protonated in that case. This supports the confinement of Nile Red in a hydrophobic environment. Besides the aforementioned PMAA conformation, a potential aggregation of the chains at this pH could also be consistent with these results and cannot be ruled out. In addition to their potential influence on the stabilization of the trithiocarbonate end-groups at pH = 3.4, the presence of hydrophobic domains in the initial medium of the emulsion polymerization would favor a fast chain growth by enhancing the local monomer concentration in the vicinity of the active PMAA chain end. This would lead to a rapid compartmentalization of the polymerization into organic domains formed by the growing PS segments swollen by styrene. As the compartmentalization rapidly takes place, the RAFT equilibrium is quickly moved to the inside of the formed hydrophobic domains where the chains continue to grow homogeneously. In contrast, the more opened water-swollen structure present at pH ≥ 6.5 offers not only higher chance to the trithiocarbonate chain ends to be hydrolyzed but also a less favorable environment for the growth of the PS segment with a lower local concentration of styrene. The situation at pH = 5 is likely to be intermediate. As mentioned above, an inhibition period was observed whatever the pH. However, in acidic conditions (pH = 3.4 and 5.0), this inhibition already observed in successful one-pot RAFT emulsion polymerization systems24 was very short (Figure 1). It is related to the critical molar mass that the hydrophobic block has to reach for the corresponding block copolymers to start to self-assemble and induce nucleation. When the pH was increased (6.5 and 8.0), charged PMAA became more hydrophilic and the molar mass of the PS block needed to be higher for the block copolymers to become amphiphilic. In addition, both the hydrolysis of the RAFT groups and the low local styrene concentration mentioned above led to a very long inhibition period. In these conditions, homogeneous nucleation may become a significant process for particle formation. In these cases, the particles would be

Figure 1. Evolution of monomer conversion versus time for the RAFT emulsion polymerizations of styrene performed at different pHs. Latex 1, pH = 3.4; Latex 2, pH = 5.0; Latex 3, pH = 6.5; Latex 4, pH = 8.0. See Table 1 for detailed experimental conditions.

nylthio function) decreased by 36% meaning that one-third of the chains had lost their RAFT chain end. After 5 h, only 14% of the chains still carried an active chain end. These results clearly explain the high amount of unreacted PMAA-CTPPA observed at this pH and the consequent poor control of the polymerization. By comparison, the hydrolytic stability of PMAA-CTPPA was significantly better at pH = 3.4 since a decay of only 4% was observed after 30 min at 80 °C and the proportion of living chains was still of 75% after 5 h (see Figure S2 in Supporting Information). In the emulsion polymerization performed at pH = 3.4, PMAA-CTPPA chains do not spend more than 45 min in the aqueous phase (Figure 1) and chain end hydrolysis can thus be neglected. Although these results are first to be ascribed to a direct influence of the pH on the hydrolytic stability, the protecting effect of the PMAA chain in acidic conditions should not be ruled out. Indeed a conformational transition from a hypercoiled structure to a water-swollen state is known to occur for PMAA between pH = 4.0 and pH = 6.0.49 The exact nature of this switch in conformation is still debated in the literature and to our knowledge none of these studies deal with short chain PMAA (i.e., lower than 5000 g mol−1). To check whether this change of conformation was still valid for our short PMAA polymers, and to evaluate the presence of hydrophobic environment at low pH, fluorescence studies of PMAACTPPA aqueous solutions were undertaken at different pHs using Nile Red as a solvatochromic dye. Nile Red was used as the fluorescence probe because its fluorescence is known to be sensitive to the polarity of the microenvironment50−52 and increases in hydrophobic environment such as micelle core.53 The peak wavelength of Nile Red shows a red shift and its quantum yield decreases when the solvent polarity increases because of a twisted intramolecular charge-transfer excitedstate.54 The fluorescence emission spectra of 2.1 × 10−7 mol L−1 Nile Red in 1.7 wt % aqueous solution of PMAA-CTPPA ([PMAA-CTPPA] = 6 × 10−3 mol L−1water) at different pHs were recorded (see Supporting Information, Figure S3). The maximum peak wavelength underwent a red shift with the increase of pH, from 628 nm at pH = 2.9 to 662 nm at pH = 11. When the pH was raised above 5.5, the fluorescence emission intensity decreased significantly (Figure 3a). Figure 3b shows the Stokes shift (Δv = va − vf = (1/λa) − (1/λf), where λa and λf are the wavelength of absorption and emission maxima of Nile Red, respectively) as a function of pH for 1.7 wt % aqueous solution of PMAA-CTPPA. The graphs (both fluorescence intensity and Stokes shift) clearly show the expected sharp conformational transition around pH 5.5, 5885


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Figure 2. Evolution of the size exclusion chromatograms versus conversion and TEM images of the final Latexes for the RAFT emulsion polymerizations of styrene performed at different pHs. Latex 1, pH = 3.4; Latex 2, pH = 5.0; Latex 3, pH = 6.5; Latex 4, pH = 8.0. See Table 1 for detailed experimental conditions.

generated by two simultaneous nucleation mechanisms: (i) the self-assembly of block copolymers in small particles and (ii) the homogeneous nucleation of particles poststabilized with the help of the same block copolymers, some of which may have lost their capability to grow. This is indeed well illustrated by the observed loss of control and broad particle size distributions (Figure 2 and Table 1). DLS analyses and TEM observations (which only reflect the PS core size) of the final Latexes illustrate well the phenomena described above (Table 1 and Figure 2). Whatever the pH, the

Latexes were stable. However, both the size and particlediameter dispersity increased with the pH, i.e. with the progressive loss of control of the polymerization (Table 1). In each case the formation of a small amount of coagulum (less than 10 wt %, amount decreasing when the pH was increased) was observed. At acidic pH the coagulum was found on the wall of the round-bottom flask. The SEC analysis of this coagulum (see Supporting Information, Figure S4) showed the same SEC profile as the polymer of the final Latex. The destabilization observed in this case was thus probably occurring at the very 5886


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Figure 3. (a) Relative fluorescence intensity and (b) Stokes shift of Nile Red in 1.7 wt % aqueous solution of PMAA-CTPPA as a function of pH. The excitation wavelength was 532 nm.

end of the polymerization. At pH = 8.0, the situation was different as the SEC analysis of the coagulum (solid polymer collected on the stir bar) showed two broad molar mass distributions. One distribution was identical to that of the final polymer obtained after 22 h of polymerization and the other one corresponded to a mixture of unreacted PMAA-CTPPA and chains with a slightly higher molar mass than that of PMAA-CTPPA (Figure S4, Supporting Information). The presence of this second molar mass distribution indicated that the destabilization occurred very early in the process, consistently with the loss of reactivity of the macroRAFT agent. In order to optimize the process, we investigated the possible poststabilization of the particles obtained at pH = 3.4 by the introduction of NaOH (1M) after high conversion was reached. Coagulum content was reduced from 8% to 5% by addition of NaOH at 93% conversion. In conclusion, the one-pot synthesis of PMAA-b-PS in water via polymerization-induced self-assembly was successfully performed at acidic pH and led to stable monodisperse particles with however a small amount of coagulum formed only at the end of the polymerization. The polymerization process performed at pH = 3.4 being robust, this system was further investigated by varying the concentration and the molar mass of the PMAA chains, the nature of the hydrophobic monomer and eventually it was evaluated for the synthesis of triblock copolymers. Influence of PMAA-CTPPA Concentration at pH = 3.4. In this series of experiments the same amount of styrene was used in the presence of different PMAA-CTPPA (Mn = 4020 g mol−1, Đ = 1.12) concentrations thus targeting various degrees of polymerization (DPn) for the PS block (Latex 5, DPn = 200; Latex 6, DPn = 402; Latex 7, DPn = 804; Table 1). As the molar ratio of PMAA-CTPPA over ACPA was kept constant and equal to 5, the initiator concentration was not the same for all the experiments. Very fast polymerizations were observed with inhibition periods of ca. 40 min and full conversions reached in less than 90 min (Figure 4a). For the three experiments, a very good control over the growth of PS segments was achieved with a linear increase of molar masses with conversions and low dispersities (Figure 4b and Figure S5 in the Supporting Information). The conversion versus time data given in Figure 4a allowed the slope of the linear part of the plots to be estimated (Table 2). It actually slightly decreased (Latex 5 > Latex 6 > Latex 7) when higher DPn was targeted, i.e., when lower concentration of the macroRAFT agent was used. This is consistent with a number of particles that decreased (increase of DnTable 2 and Figure S5 in the Supporting Information) from Latex 5 to

Figure 4. Evolution of (a) monomer conversion versus time, (b) number-average molar mass Mn (full symbols) and Đ = Mw/Mn (open symbols) versus conversion (straight lines are the theoretical evolution of molar masses with conversion) for the RAFT emulsion polymerizations of styrene carried out with various initial concentrations of the PMAA macroRAFT agent (Mn = 4020 g mol−1, Đ = 1.12): [PMAACTPPA] = 11.9 mmol L−1water (DPn = 200, Latex 5), (b) [PMAACTPPA] = 5.94 mmol L−1water (DPn = 402, Latex 6), (c) [PMAACTPPA] = 2.98 mmol L−1water (DPn = 804, Latex 7). See Table 1 for detailed experimental conditions.

Latex 7. Such an effect of the macroRAFT agent concentration has already been observed with other macroRAFT agents, namely copolymers of MAA and poly(ethylene oxide) methacrylate, employed in the emulsion polymerization of styrene at the same pH.24 It should however be noted that the decrease of Np is more pronounced than the concomitant decrease of the conversion versus time slope, and this can be related to the expected increase of ñ when the particles become larger (Table 2). The increase of the number of particles with the concentration of the macroRAFT agent indicates that the self-assembly process is identical in all cases and leads to particles with similar aggregation numbers (average number of block copolymers per particle). This is actually observed for Latex 5 (396) and Latex 6 (368) (see Table 2). In contrast, Latex 7 exhibits larger particles with also more chains per 5887


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Table 2. Kinetic Data for Styrene Emulsion Polymerizations Using PMAA-CTPPA as MacroRAFT Agents expt Latex Latex Latex Latex Latex Latex

5 6 7 8 9 10

Mn (g mol‑1) (Mw/Mn) 4020 (1.12) 4020 (1.12) 4020 (1.12) 2390 (1.15) 7310 (1.10) 12990 (1.06)

[PMAA-CTPPA]0 (mmol L‑1water) 11.9 5.94 2.98 4.95 5.73 5.54

Dna (nm) 31 37 54 36 40 38

slopeb (10‑4 s‑1) 9.7 8.1 6.9 9.6 11.2 8.1

τc (g L‑1water)

Npd (L‑1water)

ñe

NPMAA‑CTPPA/ particlef

APMAA‑CTPPAg (nm2)

296 270 261 259 293 309

× × × × × ×

0.02 0.03 0.09 0.04 0.05 0.03

396 368 595 294 414 329

8 12 15 14 12 14

1.8 9.7 3.0 1.0 8.3 1.0

19

10 1018 1018 1019 1018 1019

a

Obtained by transmission electron microscopy. bThe slope is given by d(conversion)/dt determined in the linear part of the monomer conversion versus time plot. cCalculated from the number-average diameter obtained by TEM. dCalculated from the average diameter obtained by TEM. eñ is the average number of radicals per particle determined according to eq 2, ñ = (slope × NA × [M]0)/(kp × Np × [M]p) with NA the Avogadro’s number, [M]0 the initial concentration of styrene per Lwater, kp the propagation rate constant of styrene (kp = 660 Lp mol−1 s−1 at 80 °C45), [M]p the concentration of styrene inside the particles ([M]p = 5.5 mol Lp−1 at 50 °C for styrene46). fCalculated average number of PMAA-CTPPA chains per particle, NPMAA‑CTPPA/particle = ([PMAA-CTPPA]0 × NA)/Np. gCalculated average area per PMAA-CTPPA chain at the core particle surface, APMAA‑CTPPA = (π × Dn2)/(NPMAA‑CTPPA/particle).

particle (595). This may be explained by a limited coalescence of the particles shortly after their formation by self-assembly. Influence of the Molar Mass of the PMAA-CTPPA MacroRAFT Agents at pH = 3.4. The influence of the molar mass of the hydrophilic PMAA segment was then studied. Four well-defined PMAA-CTPPA macroRAFT agents were synthesized in water (Mn = 2390 g mol−1, Đ = 1.15; Mn = 4020 g mol−1, Đ = 1.12; Mn = 7310 g mol−1, Đ = 1.10, Mn = 12990 g mol−1, Đ =1.06) and further used to grow by emulsion polymerization a similar polystyrene block with a polymerization degree of 480 (Latex 8), 402 (Latex 6), 418 (Latex 9), and 433 (Latex 10) respectively (Table 1). Again, the polymerizations were very fast (Figure 5). Apart from a different inhibition period observed in the four experiments, the same good control over the growth of the PS segment was observed with final dispersities of the corresponding block copolymer being lower than 1.5 (Figure 5b and SEC in Figure 6). As mentioned above, the inhibition period corresponds to the time required to reach the critical molar mass of the hydrophobic PS block before the self-assembly can occur. The different inhibition periods observed here were directly related to the molar mass of PMAA and increased when the latter decreased. This was particularly pronounced for the shortest PMAA segment. A closer look at the kinetic data given in Table 2 shows that the four polymerization systems exhibited similar Np, conversion versus time slopes, ñ, and aggregation numbers and thus behaved the same way. The slight differences observed are probably due to the slightly different experimental conditions (notably the slightly different starting PMAACTPPA concentrations) between Latexes 6, 8, 9, and 10. These results indicate that the increase of molar mass of the stabilizing block at a given concentration does not lead to an enhancement of the stabilized surface area in the system, which may reflect the less hydrophilic behavior of PMAA when its molar mass increases. The highest amount of coagulum (13.7 wt %) was actually observed with the longest PMAA block (Table 1, Latex 10). In the four cases, stable and monodisperse spherical particles were formed (Table 1 and TEM in Figure 6) and calculations from the TEM images revealed that the core size was similar (Dn comprised between 36 and 40 nm). The same spherical morphologies were also observed when using the poly(methacrylic acid-co-poly(ethylene oxide) methyl ether methacrylate) macroRAFT agents in the emulsion polymerization of styrene at pH = 3.5,24 while other morphologies were found when the pH was increased to 5.23,60

Figure 5. Evolution of (a) monomer conversion versus time and (b) number-average molar mass Mn (full symbols) and Đ = Mw/Mn (open symbols) versus conversion (dashed lines are the theoretical evolution of molar masses with conversion) for the RAFT emulsion polymerizations of styrene carried out with PMAA-CTPPA macroRAFT agent of various molar masses: Mn = 2390 g mol−1 (Latex 8), Mn = 4020 g mol−1 (Latex 6), Mn = 7310 g mol−1 (Latex 9), Mn = 12990 g mol−1 (Latex 10). The legend is valid for both plots. See Table 1 for detailed experimental conditions.

Influence of the Hydrophobic Monomer. The use of PMAA-CTPPA as a macroRAFT agent being efficient for the synthesis of stable and monodisperse PS particles, the hydrophobic monomer was varied in the next series of experiments (Table 3), and n-butyl acrylate (BA) or methyl methacrylate (MMA) were used instead of styrene under the same conditions as those of Latex 1 or Latex 6 in Table 1 (Latexes 11 and 12 in Table 3). As in the case of styrene polymerization, the kinetic profiles of the conversion versus time curves (Figure 7) showed inhibition periods followed by a very fast polymerization reaching full conversion in less than 2 h. The short inhibition period observed in the case of MMA is very likely related to its much higher water solubility compared 5888


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Figure 6. Size exclusion chromatograms versus conversion and TEM images of the final Latex for the RAFT emulsion polymerizations of styrene carried out with PMAA-CTPPA macroRAFT agent of various molar masses: Mn = 2390 g mol−1 (Latex 8), Mn = 4020 g mol−1 (Latex 6), Mn = 7310 g mol−1 (Latex 9), Mn = 12990 g mol−1 (Latex 10) . See Table 1 for detailed experimental conditions.

10), the temperature (70 or 80 °C), or BA content (24.7 or 14.2 wt %) did not significantly lower the extent of chain transfer reactions, and broad molar mass distributions were always obtained (see Table S1 and Figure S7 in the Supporting Information). An alternative consists in using a small fraction of styrene with BA (BA/styrene 90/10 molar ratio, Latex 13, Table 3). In this case, the reactivity ratios (rBA = 0.2, rSty = 0.7)62 at 80 °C are such that initially the concentration of styrene-terminated macroradicals is much higher than that of BA-terminated macroradicals, even at low styrene proportion. According to the reactivity ratio, 10% of the radicals would still be of the styrene type at high conversion (ca. 80%). Chain transfer reactions leading to branching are thus strongly reduced. This was indeed confirmed in Figure 8 that shows that much narrower molar mass distributions were obtained in this case (Mw/Mn = 1.52) without any detrimental effect on the conversion rate (Figure 7). The obtained Latex was stable and

to styrene and BA.61 The self-assembly process corresponding to the onset of nucleation is thus taking place within a much shorter time than in the case of styrene or BA. A complete consumption of PMAA-CTPPA was observed in the case of BA polymerization and the molar mass distributions of the formed block copolymers was shifted toward higher molar mass values with conversion (SEC in Figure 8). However, from the beginning of the polymerization, dispersities were high (Mw/Mn = 1.52 at 11% conversion) and finally reached very high values when quantitative conversion was obtained (Mw/Mn = 2.29). The well-known occurrence of irreversible chain transfer reaction to polymer in free radical polymerization of BA induces branching that broadens the molar mass distributions. This probably explains the high dispersity values observed in this experiment, although the control of the polymerization seemed to be operative (see Figure S6 in the Supporting Information). Varying PMAA-CTPPA/ACPA molar ratio (5 or 5889


5.6 6.1 5.9 5.6 4315 3925 3960 4280 styrene BA MMA BA/styrene Latex 1 Latex 11 Latex 12 Latex 13

Article a All the experiments were performed at T = 80 °C with [NaHCO3]/[ACPA] = 3.5; [PMAA-CTPPA]/[ACPA] = 5; [monomer(s)] = 2.4 mol L−1water; pH = 3.4. bGlobal solid content taking into account the amount of hydrophobic polymer for the given conversion and the macroRAFT agent. cObtained by SEC THF with a PS calibration except for Latex 12 for which PMMA calibration was used. e Obtained by dynamic light scattering. fObtained by transmission electron microscopy. gCalculated from the number-average diameter obtained by TEM. hOn the basis of the total weight of polymer and macroRAFT agent.

8.1 14.1 6.0 5.0 10 1019 1018 1018 39 38 38 48 432 392 432 428

275 328 264 328

2 2 1 2

100 99 100 100

56 963 59 245 44 200 78 393

1.37 2.29 1.48 1.52

53 92 69 71

0.09 0.16 0.13 0.12

1.04 1.17 1.03 1.11

8.4 1.1 8.8 5.6

× × × ×

18

Dn (nm)f Mn,expc (g mol‑1) convn (%) time (h) τ (g L‑1water)b DPn,theo water)

concn (mmol L‑1 Mn (g mol ) monomer expt

PMAA-CTPPA

‑1 a

Table 3. Emulsion Polymerizations of Hydrophobic Monomers Using PMAA-CTPPA MacroRAFT Agents

Mw/Mnc

Dh (nm)e

Polye

Dw/Dnf

Np (L‑1

water)

g

coag (wt %)h

Macromolecules

Figure 7. Evolution of monomer conversion versus time for RAFT emulsion polymerizations of various hydrophobic monomers carried out with PMAA-CTPPA macroRAFT agent (Mn ca. 4000 g mol−1): styrene (Latex 1), BA (Latex 11), MMA (Latex 12) and BA/styrene (molar ratio 90/10) (Latex 13). See Table 1 for detailed experimental conditions.

the particle diameter larger (Dh = 71 nm) than the one observed for the pure PS system (Dh = 53 nm, Table 3). In the case of MMA polymerization (Latex 12), a complete consumption of PMAA-CTPPA was rapidly observed and very well-defined block copolymers were formed (Mw/Mn < 1.5, Figure 8). A slight broadening of the molar mass distribution was observed after 80% conversion on the high molar mass side that possibly corresponded to a reduced efficiency of the transfer reaction between macromolecules, analogous to the gel effect often observed in the radical polymerization of MMA.63 The final block copolymer particles were very stable and the resulting particles exhibited a hydrodynamic diameter of Dh = 69 nm (Table 3). Synthesis of triblock copolymers. As mentioned in the Introduction part, the simultaneous chain growth of the hydrophobic blocks from the water-soluble polymer synthesized by RAFT and the self-assembly of the resulting amphiphilic copolymer chains that leads to the in situ formation of self-stabilized particles have been employed with different types of hydrophilic chains. Despite the very good control over the molar mass in some of these systems, the ability of the block copolymer particles to be reactivated for the growth of a second batch of hydrophobic monomer and thus the generation of triblock copolymers has only been scarcely studied.12,26,28,37 The robustness of our system based on PMAA-CTPPA demonstrated above under various conditions and for different hydrophobic monomers led us to consider the synthesis of triblock copolymers particles starting from PMAACTPPA. Well-defined PMAA-b-PS block copolymer nanoparticle seeds (Mn = 42 490 g mol−1, Đ = 1.38) were thus used in emulsion polymerization of BA for the generation of triblock copolymer nanoparticles PMAA-b-PS-b-PBA (Latex 14). Taking advantage of the presence of styrene in the polymerization of BA to avoid chain transfer reactions, we decided to add BA when a small amount of styrene was still present (98% of styrene conversion). After 5 h, 96% conversion was reached. Polymerization of BA was controlled and the formation of the desired triblock copolymer was confirmed by the shift of the molar mass distribution toward higher molar mass (Figure 9a). Final dispersity of 1.7 was obtained and consistent with the benefit provided by the presence of a small amount of styrene in the polymerization of BA. It is worth mentioning the presence of a very small population of chains of high molar mass. The final particles were quite monodisperse and exhibited original morphologies as shown by TEM (Figure 9b). The hollow-like structure is not due to the formation of voids but to 5890


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Figure 8. Size exclusion chromatograms versus conversion and TEM images (cryo-TEM for Latexes 11 and 13) of the final Latex for RAFT emulsion polymerizations of various hydrophobic monomers carried out with PMAA-CTPPA macroRAFT agent (Mn ca. 4000 g mol−1): BA (Latex 11), MMA (Latex 12), and BA/styrene (molar ratio 90/10) (Latex 13). See Figure 2 for Latex 1 and Table 1 for detailed experimental conditions.

Figure 9. (a) Size exclusion chromatograms versus conversion and (b) TEM image of the final Latex for RAFT emulsion polymerization of BA starting from a PMAA-b-PS-CTPPA seed (Latex 14).

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the inner PBA block that is less electron rich than the PS block,

CONCLUSION

Taking advantage of the successful synthesis of hydrophilic PMAA macroRAFT agents in water,41 the one-pot synthesis of self-stabilized polymer particles composed of amphiphilic block copolymers of controlled molar masses was evaluated with

and the PBA rich phase thus appears less contrasted than the PS one. This is another evidence of the successful formation of the targeted PMAA-b-PS-b-PBA triblock copolymer. 5891


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PMAA as hydrophilic block. First, MAA was polymerized up to high conversions under RAFT control using CTPPA as a chain transfer agent and ACPA as an initiator. The resulting hydrophilic macroRAFT agents were used directly for the polymerization of styrene in water. The conformation of the PMAA chains according to the pH was found to play a key role in the formation of the particles: at pH = 3.4 the hypercoiled conformation of the macroRAFT agent would provide an important local concentration of monomer in the vicinity of the active chain end allowing a rapid addition of styrene units on the PMAA segment and the formation of well-defined amphiphilic copolymer chains of controlled molar masses that self-assembled into stable nanoparticles. In contrast, the more opened water-swollen structure present at pH ≥ 6.5 and the trithiocarbonate end-group hydrolysis offered a less favorable environment for the growth of the PS segment with a lower local concentration of styrene. In these conditions, the particles could be generated by two nucleation mechanisms that strongly depended upon pH: (i) the self-assembly of block copolymers in small particles mainly operating in acidic conditions, and (ii) the homogeneous nucleation of particles poststabilized with the help of the same block copolymers which may have lost their capability to grow in basic conditions. The latter was well illustrated by the observed loss of control and broad particle size distributions. Whatever the pH, the obtained Latexes were stable, even if a small amount of coagulum (less than 10 wt %, amount decreasing when the pH increased) was observed. Spherical particles were always obtained, whatever the experimental conditions. The one-pot synthesis of PMAA-b-PS in water being successful at acidic pH, this system was further investigated by varying the concentration and the molar mass of the PMAA chains. In all cases, the system obeyed the mechanism of particle formation depicted above. Changing the nature of the hydrophobic monomer led to the formation of stable Latexes of PMMA or P(BA-co-styrene) with nevertheless slightly broader molar mass distribution at the end of the polymerization. Eventually stable, spherical particles composed of PMAA-b-PSb-PBA triblock copolymers were formed.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: (M.L.) [email protected]; (F.D.) dagosto@lcpp. cpe.fr. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the French National Agency for Research (ANR ALGIMAT 09-CP2D-02) and from the competitiveness cluster Axelera is acknowledged. The authors would like to thank Olivier Boyron and Pierre-Yves Dugas (C2P2, LCPP Team) for their help concerning SEC analyses and cryo-TEM, respectively, Marion Santacreu for her help in the fluorescence study and Malvern Instruments SA for technical support. B.C. thanks the Institut Universitaire de France for her nomination as senior member.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Evolution of Mn versus conversion for the RAFT emulsion polymerizations of styrene performed at different pH with PMAA-CTPPA; Evolution of the percentage of living chains versus time in a aqueous solution of PMAA−CTTPA macroRAFT agent at pH = 8.0 and pH = 3.4; Fluorescence emission spectra of Nile Red in aqueous solution of PMAACTPPA at different pH; SEC chromatograms of the coagulum obtained at the end of RAFT emulsion polymerizations of styrene carried out with PMAA-CTPPA at pH = 3.4 and pH = 8.0; Size exclusion chromatograms versus conversion for RAFT emulsion polymerizations of styrene carried out with various initial amount of PMAA-CTPPA; Evolution of Mn versus conversion for RAFT emulsion polymerizations of various hydrophobic monomers carried out with PMAA-CTPPA; Emulsion polymerizations of BA using PMAA-CTPPA (table of experiments and evolution of monomer conversion versus time, Mn and Đ = Mw/Mn versus conversion, and size exclusion chromatograms versus conversion). This material is available free of charge via the Internet at http://pubs.acs.org. 5892


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