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Apr 28, 2016 - Research Institute for Physical Chemical Problems, Belarusian State University, 14 Leningradskaya st., 220030 Minsk, Belarus. ‡. IMP,...
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New Insights into the Cationic Polymerization in Emulsion Catalyzed by Water-Dispersible Lewis Acid Surfactant Complexes: A Case Study with p‑Methoxystyrene Irina V. Vasilenko,† Francois Ganachaud,*,‡,§ and Sergei V. Kostjuk*,†,‡ †

Research Institute for Physical Chemical Problems, Belarusian State University, 14 Leningradskaya st., 220030 Minsk, Belarus IMP, CNRS, UMR5223, INSA-Lyon, 20 Boulevard Einstein, F-69621 Villeurbanne, France § Université de Lyon, 69003 Lyon, France ‡

S Supporting Information *

ABSTRACT: The process of cationic polymerization of pmethoxystyrene in emulsion using recently discovered waterdispersible Lewis acid surfactant complexes (LASCs) has been investigated in detail. These latter are prepared from specific branched sodium dodecylbenzenesulfonate and different metal salts (namely ytterbium, scandium, and indium). First, the reaction rate increases while changing the nature of Yb salt in the range Yb(OTf)3 > YbBr3 > YbCl3 > Yb(NO3)3. The nature of metal was also shown to influence the propagation rate, with a fair acceleration of the polymerization when using indium or scandium ions. Raising the temperature led also to an increase of the reaction rate, but the molar mass of obtained polymers decreased. It was also demonstrated that the use of an appropriate initiator (pentachlorophenol) allows controlling to some extent the molar mass of synthesized polymers. Polymer characterization by 1 H NMR spectroscopy revealed a significant fraction of olefinic end groups, indicating that the main chain-breaking process is via chain transfer reactions. All these results are consistent with a mechanism where polymerization proceeds inside the monomer droplets.

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3000 g mol−1).7,8 Using appropriate surfactants allowed to conduct cationic polymerization of pMOS either in direct9 or inverse emulsion,10 both producing similar molar masses.10,11 All these polymerizations were mistakenly classified as controlled, on the basis that Mn increased with monomer conversion at early stages of reaction (up to 25% of monomer conversion). Still, significant deviation of experimental values of Mn from theoretical line was observed above this conversion; moreover, changing the initiator content did not influence the Mn vs conversion curves.7,8,11 A true controlled cationic polymerization of pMOS12 in water-dispersed medium was recently reported, using an initiating system based on an aromatic alcohol (adduct of pMOS with water, pMOS-OH) and tris(pentafluorophenyl)borane (B(C6F5)3) as initiator and co-initiator, respectively. B(C6F5)3 does not decompose or dissociate in water,13 contrary to Yb(OTf)3 that totally releases its hydrated Yb3+ cation9 (a too weak Lewis acid to induce cationic polymerization of less reactive than pMOS monomers). It was also shown that the pMOS-OH/B(C6F5)3 pair successfully polymerizes p-hydroxystyrene,14 styrene,1 cyclo-

INTRODUCTION Cationic polymerization in aqueous media has emerged during the past decade as a new and attractive polymer synthesis method. It allows conducting the polymerization of various cationically polymerizable monomers in the cheap and environmentally benign water solvent.1 This approach solves in part the main problems of conventional cationic polymerization, i.e., the necessity to work at low temperatures (from −30 to −100 °C) and under strictly anhydrous conditions.2 p-Methoxystyrene (pMOS) was almost exclusively investigated in early studies due to its high reactivity in cationic polymerization and the high stability of the carbocations formed in the course of polymerization.3,4 In fact, cationic polymerization of pMOS in aqueous emulsion can be readily conducted using dodecylbenzenesulfonic acid (DBSA) as both initiator and surfactant to afford in a quantitative yield low molar mass polymers (Mn ∼ 1000 g mol−1) in about half a day at 60 °C.5 Ytterbium triflate (Yb(OTf)3), a water-tolerant Lewis acid,6 in conjunction with the adduct of pMOS with HCl (pMOS-HCl) or sulfonic acid-based initiators was also shown to induce slow suspension cationic polymerization of pmethoxystyrene at 30 °C (∼100% of monomer conversion in ca. 50−200 h) giving poly(p-methoxystyrene)s with slightly higher molar masses than those obtained with DBSA (typically © 2016 American Chemical Society

Received: February 21, 2016 Revised: April 13, 2016 Published: April 28, 2016 3264

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Scheme 1. (a) Polymerization Loci for Different Generations of Catalytic Systems in Aqueous Cationic Polymerization; (b) Mechanism of the Interfacial Cationic Polymerization in Aqueous Media [R = H, 1-(4-Methoxyphenyl)ethyl; A = C12H25(C6H4)SO3−, LA·OH−; M = Monomer]

pentadiene,1 and isoprene15 in aqueous suspension and/or emulsion. Despite these obvious progresses, the molar masses of synthesized polymers were never exceeding 3000 g mol−1 in all these studies.1 This is due to the interfacial character of polymerization: all reactions (initiation, propagation, and termination) proceed at the particle interface. Since the catalyst is typically hydrated by water, it is too hydrophilic to penetrate in the core of the monomer droplets (Scheme 1a). So long as the interface is saturated by water, the termination reaction via water occurs shortly after initiation to give low molar mass polymers. Another phenomenon is associated with the entry of polymer chains inside monomer droplets when they reach a certain length and become too hydrophobic. In this case, even if the reactivation of hydroxyl-terminated chains would be anticipated for some initiating systems, it does not occur since the catalyst invariably stays at the particle interface (see Scheme 1b). In order to produce long polymer chains by aqueous cationic polymerization, it seems necessary to displace the polymerization locus toward the core of monomer droplets.1 One of the possibilities to do so is the use of a cosolvent like acetonitrile. In this case, the Lewis acid B(C6F5)3 complexed with the solvent sank into the monomer droplets to promote living polymerization. The molar mass did not exceed 3000 g mol−1, however, since the polarity of the droplets was too low to accommodate the catalyst.12 Another possibility is the use of so-called Lewis acid surfactant combined catalysts (LASCs),6,16 creating a hydrophobic environment around the metal center. LASC prepared from ytterbium salts (YbX3, X = Cl, NO3, OTf) and sodium dodecyl sulfate were found insoluble in both organic and aqueous phases, so that only interfacial polymerization promoted by the acidified surfactant proceeded in this case.17 On the contrary, the LASC prepared from Yb(OTf)3 and an electrosteric surfactant (sodium polyoxyethylene (8) lauryl sulfate) showed a fair activity in emulsion polymerization of pMOS (few days reaction) and afforded high molar mass poly(p-methoxystyrene)s (Mn ∼ 40 000 g mol−1)18 (see Scheme 1). The very long inhibition period (ca. 100 h) and slow polymerization certainly arise from the difficulty to bring the LASC inside the monomer droplets, but the formation of high molar mass confirms the Lewis acid activity in this very hydrophobic environment. Note that the application of this

particular LASC was limited only to the polymerization of pMOS.18 Recently, we have communicated on a new family of LASCs prepared from commercially available branched dodecylbenzenesulfonate (DBSNa), which showed high activity toward cationic polymerization, not only of pMOS but also of (a priori less reactive) monomers such as styrene and isoprene.19 It was unambiguously shown that the micellization of LASC in water is a key prerequisite in achieving cationic polymerization.18,19 This is achieved by using branched surfactants that are more soluble in water than their linear counterparts20 and which structural disorder brought by alkyl branching in the hydrophobic chains stabilizes the interface.20 These encouraging preliminary results were however not sufficient to propose a full mechanism of polymerization. Here, we show an exhaustive study of the LASC-mediated cationic polymerization of pMOS, our preferred model monomer, performed to determine the optimal reaction conditions as well as to clarify the mechanism of unique LASC-mediated polymerization process. To do so, the effects of different parameters (YbCl3:surfactant ratio, nature of initiator, water-tolerant salt, etc.) on the kinetics and molar mass evolution with conversion during the polymerization of pMOS were studied and are presented in detail.

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EXPERIMENTAL SECTION

Materials. Unless otherwise stated, all reagents were purchased from Aldrich. Sodium dodecylbenzenesulfonate (branched, technical grade), YbCl3·6H2O (99.9%), YbBr3·xH2O (x = 5−7), Yb(NO3)3· 5H2O (99.9%), Yb(OTf)3·xH2O (x = 1−2), ScCl3·6H2O (99.9%), Sc(CH3CO2)3 (Reahim), and InCl3 (98%) were used as received. pMethoxystyrene (97%) was distilled over CaH2 under reduced pressure. 1-(4-Methoxyphenyl)ethanol (99%), pentachlorophenol (97%), CH2Cl2 (≥99.8%), CDCl3 (Euriso-top), D2O (Euriso-top), methanol (Fluka, 99.8%), and Milli-Q water were used as received. Methods. Size exclusion chromatography (SEC) was performed on a PL-GPC 50 integrated GPC system with two columns (PL gel, 5 μm, 300 mm, 500 and 100 Å) and one precolumn (PL gel 5 μm guard) thermostated at 30 °C. The detection was achieved by a differential refractometer as well as in some cases by a Wyatt DAWN EOS multiangle light scattering detector. THF was eluted at a flow rate of 1 mL min−1. The calculation of molar mass and polydispersity is based on polystyrene standards (Polymer Laboratories, Germany). 1H NMR (400 MHz) and 13C NMR spectra (100 mHz) were recorded in CDCl3 or D2O at 25 °C on a Bruker AC-400 spectrometer calibrated relative to the solvent peaks. Particle size measurements were carried 3265

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Macromolecules out by dynamic light scattering (DLS) using a Malvern Instruments Zetasizer nano series instrument and using the cumulant method. The aqueous solutions were prepared at 1 mg/mL, and at least 10 measurements were made for each sample. Equilibration times of 5 min were respected before each measurement. Polymerization. Polymerization of p-methoxystyrene was carried out in open air at temperatures between 40 and 60 °C. In a typical experiment, ytterbium chloride hexahydrate (0.21 g) and DBSNa (0.78 g) were placed in the flask and then dissolved in water (3.5 g) at room temperature. When the aqueous solution was isotropically clear, pMOS (1.5 g) was added through a pipet. The mixture was allowed to stir gently for 5−10 min. Then, the reaction mixture was poured into 5−6 test tubes equipped with a magnetic stirrer bar, which plunged in an oil bath to reach the temperature of reaction (typically 60 °C). After predetermined times, the polymerization was terminated by adding chilled methanol. The polymer was separated from the reaction mixture by centrifugation, rinsed with methanol, centrifuged, and dried under vacuum to constant weight. Monomer conversions were determined gravimetrically. For further analyses, polymers were dissolved in CH2Cl2 and precipitated in an excess of methanol and then dried under vacuum to constant weight.

surfactant, while very slow polymerization was observed at YbCl3:DBSNa 1:2 molar ratio (Figure 1a).21 At YbCl3:DBSNa 1:3 mol/mol, the formation of LASC takes place as confirmed by 1H NMR spectroscopy (Figure S1). Indeed, the interaction of YbCl3 with DBSNa is materialized by a downfield shift of the phenyl protons of surfactant, indicating the successful formation of LASC. Still, small signals corresponding to original surfactant were also observed in the 1H NMR spectrum (Figure S1). In addition, all signals show broad line widths consistent with ongoing exchange reactions between both LASC and DBSNa species and/or formation of micelles (see Figure 2a and discussion therein for details). At YbCl3:DBSNa 1:3 mol/mol ratio, the emulsion polymerization of pMOS proceeds smoothly with an induction period of ca. 7 h to reach 80% of monomer conversion in 1 day (Figure 1a). The use of an excess of DBSNa over YbCl3 in the course of LASC preparation (YbCl3:DBSNa 1:4 mol/mol) allowed to decrease the induction period, on the one hand, and to increase the reaction rate (kp,app = 2.8 × 10−5 and 7.2 × 10−5 s−1 for YbCl3:DBSNa = 1:3 and 1:4 mol/mol, respectively; see Figure S2), on the other hand. Further increase of DBSNa content (YbCl3:DBSNa 1:5 mol/mol) did not influence anymore the polymerization kinetics (kp,app = 6.9 × 10−5 s−1) (Figure 1a and Figure S2). It should be also noted that the first-order plots are linear (Figure S2), indicating that no irreversible termination, which would lead to the decrease of the concentration of active species, occurred here. As shown in Figure 1b, the number-average molar mass of synthesized polymers increased with monomer conversion for all YbCl3:DBSNa ratios investigated here, but higher Mns were obtained for the largest ratios. In addition, molar mass distribution (Đ) broadened throughout the polymerization for experiment performed at a YbCl3:DBSNa 1:3 mol/mol ratio and almost did not change for higher ratios. From those experiments, we can conclude that the YbCl3:DBSNa molar ratio of 1:4 is optimal in terms of reaction rate and molar mass of synthesized polymers; it was then used in all other experiments further described in this article. We also briefly investigated the colloidal properties of the dispersions obtained for YbCl3:DBSNa molar ratio 1:4 at different stages of the polymerization process by quasi-elastic light scattering (QELS) (Figure 2). LASC by itself generates micelles in the range of few nanometers,19 while the addition of the pMOS leads to the formation of droplets of around 400 nm

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RESULTS AND DISCUSSION Effect of YbCl3:DBSNa Ratio. In an initial series of experiments, we have investigated the influence of YbCl3:surfactant molar ratio on the polymerization of pMOS at 60 °C (Figure 1). No polymer was formed in the absence of

Figure 1. (a) Conversion vs time and (b) Mn, Đ vs conversion plots for pMOS polymerization at 60 °C and at different YbCl3:DBSNa molar ratios. Polymerization conditions: YbCl3·6H2O: 0.21 g; H2O; 3.5 g; pMOS: 1.5 g. YbCl3:DBSNa molar ratio: (1) no surfactant; (2) 2; (3) 3; (4) 4; (5) 5.

Figure 2. Evolution of particle size at YbCl3:DBSNa molar ratio 1:4 (run 4 in Figure 1): (a) particle size distribution after 15% (red line), 35% (green line), 78% (brown line), and 92% (blue line) of pMOS conversion; (b) average size distribution versus conversion for the QELS-visible size distribution. 3266

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Macromolecules in size and bigger reservoir droplets (see typical distribution in Figure 2a). In the absence of polymerization, these are metastable and cream after ca. 1 h without stirring. Once the induction period is over, the size of the smaller distribution constantly increases with conversion to level off at about 1 μm at 30% monomer conversion (Figure 2b), whereas the reservoir droplets disappear, most likely through monomer diffusion into the polymerizing particles. The large error in particle size measurement does not allow us to draw any mechanistic scheme concerning the physical chemistry of this system: for instance, we do not know at this stage if small monomer droplets are directly converted into particles or if new particles are nucleated in the course of the polymerization. We are currently concentrating our efforts on these questions. As already stated, the Mns of synthesized polymers progressively increase with monomer conversion, although the corresponding dependences are not linear (Figure 1b). In addition, we mentioned that the first-order plots are linear (Figure S2). An important question arising from these two sets of results is whether the polymerization is controlled or not. To rule on this case, SEC traces of various samples normalized by their conversion are shown in Figure 3. Evidently, the peak

Figure 4. (a) Conversion vs time and (b) Mn, Đ vs conversion plots for pMOS polymerization at 60 °C in the presence of various ytterbium salts. Polymerization conditions: H2O; 3.5 g; DBSNa: 0.78 g. Ytterbium salt: (1) Yb(NO3)3·6H2O (0.24 g); (2) YbCl3 × 6H2O (0.21 g); (3) YbBr3 × 6H2O (0.22 g); (4) Yb(OTf)3 × 2H2O (0.39 g).

HBr (pKa = −9) > HCl (pKa = −7) > HNO3 (pKa = −1.6).22 Since the hydrolysis constant is the same for all ytterbium salts studied here (because it depends exclusively on the nature of metal ion),23 the observed difference in the induction period and reaction rate could be explained by the different solubility of these acids in the monomer phase. Another explanation would be that the stronger the Brønsted acids, the more efficient initiators (vide inf ra). The nature of ytterbium salts has a similar effect on the molar mass of the synthesized poly(pMOS): the lowest final molar mass was obtained with Yb(OTf)3 and the highest ones with YbCl3 and YbBr3 (Figure 4b). In the line of discussion above, the molar mass depends on the nature of Brønsted acid (initiator) formed by partial hydrolysis of ytterbium salts: the strongest acid leads to the lowest molar mass. Basically, the molar mass distribution of synthesized polymers becomes broader through the reaction and levels off at Đ ∼ 2. One exception is the polymerization with LASC prepared using Yb(NO3)3 where polymers with somewhat lower Mn and broader MMD (Đ ∼ 2.5) were formed. To summarize, these results support the assumption that polymerization is initiated by acids generated by partial hydrolysis of ytterbium cation in water. The induction period, reaction rate, and molar mass depend on the nature and strength of the Brønsted acid generated in situ. Effect of the Nature of the Metal. In a second series of experiments, different water-tolerant metal chlorides, i.e. YbCl3, InCl3, and ScCl3, were first transformed into LASCs and further tested in the cationic polymerization of pMOS at 60 °C. As shown in Figure 5a and Figure S4, the induction period is considerably reduced when using indium- and scandium-based LASCs compared to ytterbium one (60, 78, and 440 min, respectively), and polymerization is faster (kp,app = 13.1 × 10−5, 11.3 × 10−5, and 7.2 × 10−5 s−1 for Sc-, In-, and Yb-based LASCs, respectively; see Figure S4). Oppositely, the molar masses of polymers synthesized with Sc- and In-based LASCs are lower than with Yb-based one (Figure 5b). It should be noted also that polymerization of pMOS in the presence of ScCl3-based LASC stopped at partial monomer conversion (Figure 5a), while full pMOS conversions were observed at lower temperatures, 50 or 40 °C (vide inf ra), or when polymerizing pMOS at 60 °C but starting from another scandium salt (e.g., Sc(CH3CO2)3, Figure S5). Such behavior is not clear at the present time and requires further investigations.

Figure 3. SEC traces normalized by conversion of poly(p-methoxystyrene)s synthesized at 60 °C: YbCl3·6H2O: 0.21 g; DBSNa; 0.78 g; H2O; 3.5 g; pMOS: 1.5 g. Monomer conversion (from bottom to top): 15%; 35%; 78%; 92%.

maximum shifts with conversion toward high molar mass region, but the distributions still overlap in the low molar mass region. In other words, relatively short polymer chains are generated at the beginning of reaction, which are not reactivated under the action of LASC. These “dead” polymer chains accumulate inside monomer droplets to increase their hydrophobicity; that leads to a decrease of rate of termination/ chain transfer via water and, in turn, to an observed increase of molar masses. Effect of the Nature of the Metal Ligand. First, a variety of LASCs prepared using different ytterbium salts (Yb(OTf)3, YbCl3, YbBr3, and Yb(NO3)3) were tested in the emulsion cationic polymerization of pMOS (Figure 4). The induction period increases, whereas the rate of polymerization decreases, following the series Yb(OTf)3 > YbBr3 > YbCl3 > Yb(NO)3 (Figure 4a and Figure S3). This ranking coincides with the acidity row of corresponding Brønsted acids, which are formed in course of hydrolysis of ytterbium salts: HOTf (pKa = −12) > 3267

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Figure 5. (a) Conversion vs time and (b) Mn, Đ vs conversion plots for pMOS polymerization at 60 °C in the presence of various water-tolerant salts: H2O: 3.5 g; DBSNa: 0.78 g. Salts: (1) YbCl3·6H2O (0.21 g); (2) ScCl3·6H2O (0.12 g); (3) InCl3 (0.12 g).

Figure 6. (a) Conversion vs time and (b) Mn, Đ vs conversion plots for pMOS polymerization at different temperatures in the presence of LASC prepared from ScCl3: H2O; 3.5 g; DBSNa: 0.78 g; ScCl3·6H2O 0.12 g. Reaction temperature: 60 °C (1), 50 °C (2), and 40 °C (3).

reaction through the partial hydrolysis of the salt in water also strongly influences the LASC-mediated cationic polymerization of pMOS. Indeed, the induction period is shorter and Mn is lower, while the reaction rate is higher for LASC prepared from ScCl3 compared to Sc(CH3CO2)3 (see Figure 4, Figure S4, and Figure S5). This confirms the trend observed earlier for the series of ytterbium salts with the strength of generated Brønsted acid (see Figure 4 and discussion therein). Effect of Temperature. LASCs prepared from ytterbium salts are efficient in the emulsion cationic polymerization of pMOS only at 60 °C, while In- and Sc-based LASCs can be used at lower temperatures.19 Taking into account these preliminary results, we investigated the influence of temperature on the kinetics of pMOS polymerization. As shown in Figure 6a and Figure S7, using ScCl3-based LASC, the reaction rate progressively decreases, while the induction period increases, when decreasing the reaction temperature from 60 to 40 °C. Since the reaction is quite slow at 40 °C (typically 2 days required for polymerization completion), we did not investigate the polymerization at lower temperatures. Molar masses evolve conversely to reaction rates: Mn increases significantly when decreasing the reaction temperature (from Mn ∼ 5000 g mol−1 at 60 °C to Mn ∼ 25 000 g mol−1 at 40 °C; see Figure 6b). The lowering of Mn with increasing temperature is evidently attributed to the increase of rate of chain transfer reaction to monomer and/or to water,

The higher activity as well as the shorter induction period of Sc- and In-based LASCs is in line with their hydrolysis constants: pKh(Yb3+) = 7.7 ≫ pKh(Sc3+) = 4.3 and pKh(In3+) = 4.023 (see Figure S6). A low value of pKh indicates that the corresponding salt is hydrolyzed in higher extents, thus providing higher concentrations of acids in both aqueous and organic phases. This leads to shorter induction period and faster polymerization, on the one hand, and to polymers with lower molar mass, on the other hand. Other unique features of InCl3 and ScCl3 should be also considered here to explain their exceptionally high reactivity toward pMOS polymerization. First of all, due to the smaller ionic radii of the central metal, InCl3 and ScCl3 (as well as LASCs prepared from them) exhibit higher Lewis acidity than YbCl3, and therefore they induce faster polymerization (ionic radii (Å): Sc(0.885) < In(0.940) < Yb(1.008)).11 The higher Lewis acidity in aqueous medium for Sc(OTf)3 and In(OTf)3 as compared to Yb(OTf)3 was confirmed recently by DFT calculations.24 Finally, TGA investigations showed that scandium p-toluenesulfonate dehydrates at lower temperature (40 °C) than the ytterbium one (56 °C).25 This would facilitate the penetration of LASC into hydrophobic monomer droplets/polymer particles that in turn, would lead to a decrease of the induction period and to an increase of the rate of polymerization. It should be emphasized that along with the nature of metal in the salt, the nature of Brønsted acids formed in the course of 3268

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pMOS-OH, especially at its highest concentration, can be attributed to an increase of the polarity of the droplets thanks to polar pMOS-OH accumulation. In turn, this would lead to faster chain transfer to water and to the formation of shorter polymer chains. We do not exclude pMOS-OH itself to act as a chain transfer agent, as we proved recently for the aqueous cationic polymerization of isobutyl vinyl ether with pMOSOH/BF3OEt2 initiating system.30 All in all, these data indicate that under investigated conditions pMOS-OH does not initiate the polymerization of pMOS but enhance transfer reactions. Introducing pentachlorophenol, the polymerization is approximately 2 times slower than in the absence of it (Figure 8a and Figure S9). Besides, no induction period is observed, and linear first-order plots pass through zero at high concentration, whereas at lower concentration, the induction period is still present though reduced (Figure S9). The addition of C6Cl5OH also resulted in a significant decrease of molar masses from Mn ∼ 35 000 to ∼15 000 g mol−1 (Figure 8b). Moreover, for the lowest initiator concentration, the experimental value of Mn at complete monomer conversion is close to calculated values (Mn,theor ∼ 13 400 g mol−1). A further increase of C6Cl5OH concentration, however, leads to only insignificant decrease of molar mass (Mn ∼ 13 000 and 10 000 g mol−1 against Mn,theor ∼ 5800 and 2900 g mol−1, respectively). These results indicate that C6H5OH indeed acts as an initiator, but the initiation efficiency is rather low; the final molar mass can be controlled only to some extent by the concentration of initiator. It should be noted also that polydispersity of poly(p-methoxystyrene)s synthesized in the presence of pMOS-OH and C6Cl5OH as initiators (Figures 7b and 8b) is quite similar (Đ ∼ 2.0) to those obtained in the absence of initiator. This confirms that both mechanisms of propagation and chain transfer/termination are the same for all these polymerizations. Polymer Characterization. The structure of synthesized polymers were exclusively characterized by NMR spectroscopy. A typical 1H NMR spectrum of poly(p-methoxystyrene) synthesized using Yb-based LASC (YbCl3:DBSNa molar ratio is 1:4) as catalyst and C6Cl5OH as initiator is assigned in Figure 9. Apart from the resonances of protons of main-chain methylene (a) methine (b), methoxy (c), and phenyl group (d), the less intensive signal of the CH3 group at the α-end at ca. 0.9 ppm is clearly visible. In addition, the broad signal at 3.0 ppm (ω1 + ω2) is attributed to the CH of the ω-end group close to double bond or indanyl structure, while signals at 6.0 and 7.1 ppm correspond to protons of double bond (ω′) and indanyl group (ω″) (Figure 9).5,17 The number-average functionality Fn(ω1 + ω2) calculated as 3I(ω1 + ω2)/I(α) is 0.73, indicating that the main chain-breaking processes are the chain transfer to monomer and intramolecular alkylation. One cannot exclude nevertheless the strong possibility of chain transfer to H2O: this leads to the formation of hydroxyl group at the chain end, which small peak cannot be integrated because it overlaps with the large methoxy signal. Besides, water elimination occurs at high temperature and under acid catalysis to generate the same ω′ and ω″ unsaturations.5 These data evidence that the polymerization of pMOS proceeds inside of monomer droplets rather than at the particle interface. The number-average molar mass calculated based on NMR as the peak intensity ratio of methyl proton (α) of headgroup to phenyl protons (d) of the main chain (Mn(NMR) = 6800 g mol−1) is lower than one determined by SEC (Mn = 9100 g mol−1; Đ = 1.70). This may indicate that intermolecular

which is consistent with a cationic mechanism of polymerization. Based on the obtained apparent rate constants of propagation (Figure S7a), an Arrhenius plot was constructed (Figure S7b) and an apparent energy of activation of propagation (Ea,app) was calculated as 98 kJ mol−1. Generally, negative Ea,apps are reported for cationic polymerization of various vinyl monomers due to higher ionization of the chain ends at lower temperature.26,27 The positive value of Ea,app obtained in this work can be ascribed to the formation of denser solvation cage around LASC at lower temperature which leads to an observed decrease of reaction rate. This explanation is in line with (though higher than) values of Ea,app ∼ 60 kJ mol−1 reported for the cationic polymerization of styrene using complexes of AlCl3 with dibutyl ether as co-initiator28 and pMOS in the presence of complexes of B(C6F5)3 with H2O and/or acetonitrile,29 respectively. Effect of Initiator. In order to confirm the polymerization mechanism, two potential cationic-active initiators, i.e., 1-(4methoxyphenyl)ethanol (pMOS-OH) and pentachlorophenol, were tested. The former molecule was selected due to its high efficiency in initiating an aqueous cationic polymerization catalyzed by B(C6F5)31,12,14,19 and since its mimics the poly(pMOS) chains ends terminated by H2O (possible dormant species). The latter molecule is a weak Brønsted acid (pKa = 4.5) that does not initiate the polymerization by itself but that was shown to be an efficient initiator in the first successful LASC-mediated cationic polymerization of pMOS.18 As shown in Figure 7a and Figure S8, the addition of pMOS-

Figure 7. (a) Conversion vs time and (b) Mn, Đ vs conversion plots for pMOS polymerization at 60 °C and in the presence of 1-(4methoxyphenyl)ethanol. Polymerization conditions: YbCl3·6H2O: 0.21 g; H2O; 3.5 g; pMOS: 1.5 g; DBSNa: 0.78 g. 1-(4methoxyphenyl)ethanol: (1) none; (2) 0.08 g; (3) 0.16 g.

OH to the system hardly affects the polymerization kinetics (kp,app = 7.2 × 10−5, 6.9 × 10−5, and 6.6 × 10−5 s−1 for polymerizations in the absence of initiator and with 0.08 and 0.16 g of pMOS-OH, respectively). Besides, the evolution of Mn with conversion is quite similar for polymerization experiments performed without the addition of initiator and in the presence of 0.08 g of pMOS-OH (Figure 7b). Only at relatively high pMOS-OH content (0.16 g), the Mn vs conversion profile was changed and synthesized polymers were characterized by lower molar mass (Mn ∼ 20 000 g mol−1) than genuine ones (Mn ∼ 35 000 g mol−1) (Figure 7b). However, for both pMOS-OH concentrations investigated here, the experimental values of Mn are much higher than theoretical ones calculated based on the assumption that one molecule generates one polymer chain (Mn,theor = 2860 and 1400 g mol−1 for 0.08 and 0.16 g of pMOS-OH, respectively). Therefore, some decrease of molar mass in the presence of 3269

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Figure 8. (a) Conversion vs time and (b) plots Mn, Đ vs conversion plots for pMOS polymerization at 60 °C and in the presence of C6Cl5OH. Polymerization conditions: YbCl3·6H2O: 0.21 g; H2O; 3.5 g; pMOS: 1.5 g; DBSNa: 0.78 g. Pentachlorophenol: (1) none; (2) 0.03 g; (3) 0.07 g; (4) 0.14 g.

Figure 9. 1H NMR spectrum of poly(p-methoxystyrene) obtained during pMOS polymerization at 60 °C and in the presence of C6Cl5OH as initiator: YbCl3·6H2O: 0.21 g; H2O; 3.5 g; pMOS: 1.5 g; DBSNa: 0.78 g; C6Cl5OH: 0.14 g. Conversion: 84%; Mn = 9100 g mol−1; Đ = 1.7.

knowledge of the general principles of cationic polymerization, the syndiotactic-rich polymer would be preferably generated at high polymerization temperatures, while isotactic sequences would predominate at low reaction temperatures.2a This is an additional proof that LASC-mediated polymerization of pMOS occurs via a cationic mechanism. Polymerization Mechanism. On the basis of the results obtained in this work, we propose the following mechanism describing the cationic polymerization of pMOS in aqueous emulsion mediated by water-soluble LASCs (Scheme 2). Upon dissolving the Lewis acid in water, the fast dissociation of salt and its hydration followed by slow partial hydrolysis forms an acidthe true initiator of polymerization. At the same time, a micellizable LASC is generated upon the addition of DBSNa to the metal ion, as confirmed by 1H NMR spectroscopy (Figure S1). LASC micelles in the range of few

alkylation leading to the formation of branched structures containing more than one headgroup per chain could also occur.31 To clarify this point, the molar mass of polymer was also determined using multiangle light scattering detector; it gives slightly higher Mn (Mn = 10 800 g mol−1; Đ = 1.56) than the one calculated based on RI data (see Figure S10 for corresponding SEC traces). This indicates that intermolecular alkylation is not significant under investigated conditions. In order to support the cationic mechanism, we have briefly investigated the microstructure of poly(p-methoxystyrene) by 13 C NMR spectroscopy. The signal of ipso-carbon atom shows the predominance of syndiotactic triads at 137.6 ppm ([rr] ∼ 57%; [mm] ∼ 14%; [mr] ∼ 29%) (Figure S11) that differs from the sample obtained by radical polymerization which is characterized by much intensive fraction of isotactic ([mm] ∼ 21%) and mixed ([mr] ∼ 32%) triads.18 Based on the 3270

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Scheme 2. (a) Formation of LASC and Acid (Initiator) in Situ; (b) Tentative Mechanism of LASC-Mediated Cationic Polymerization (M: Yb, Sc, In; X = OTf; Cl; Br; NO3; OAc)

Figure 10. (a) Induction period vs log P (predicted or measured33) and (b) kp,app vs pKa plots for pMOS polymerization at 60 °C in the presence of LASCs prepared from different ytterbium (YbX3, X = NO3; Cl, Br, OTf) and scandium (ScX3, X = Cl or CH3COO) salts and InCl3.

nanometers19 (Scheme 2a) turn into a milky emulsion upon the addition of pMOS with droplets of about 400 nm in size (Figure 2). The observed induction periods can be assigned to the slow penetration of Brønsted acid inside hydrophobic monomer droplets and the building of a sufficient acid concentration to initiate the polymerization. Indeed, even if YbCl3 is well-known to be a Lewis acid stable in water, it hydrolyzes to a low extent (hydrolysis constant (pKh) for Yb3+ is 7.7).23 The acidic reaction media at close to complete monomer conversion (pH ∼ 2) confirms the above-mentioned hypothesis.32 The length of induction period clearly depends on the partitioning of Brønsted acid between water and monomer droplets that can be tentatively correlated to their estimated log P data (Figure 10a). The slow initiation related to both low accessibility of metal center in LASC due to its bulkiness and low acidity of corresponding Brønsted acid is an another reason for the observed induction period. In addition, the relatively low stability of the obtained emulsions at zero monomer conversions can also influence the induction period. While penetrating inside monomer droplets, the acid formed in situ interacts with LASC to generate a proton associated with a bulky counterion ([LASCX]−). As is shown in Figure 10b, the

reaction rate depends on the strength of Brønsted acid. This observation is consistent with higher initiation efficiency of stronger acid as well as formation of less nucleophilic counteranion ([LASCX]−). It should be also noted here that for similar ligand a higher reaction rate is observed for metal salts with lower values of hydrolysis constants (see Figure S6b and Figure 10b). Indeed, salts with lower pKh are hydrolyzed to a higher extent and therefore provide higher acid concentration. In the particular case of a hydrophobic weak acid such as pentachlorophenol, this scheme also applies. C6Cl5OH is hydrophobic and is located exclusively inside the monomer droplets, so that the induction period is significantly reduced and even suppressed for acid concentration of typically 4 × 10−2 mol/L (see Figure 8a and Figure S9). However, since it is a weak acid, the reaction rate is lower than for the induced by strong Brønsted acids (Figure 10b) due to slower initiation and formation of more basic (nucleophilic) counteranion. Once a chain is initiated, it grows quickly up to chain transfer to the few H2O molecules present in the core of monomer droplets or to monomer. As we have shown above (see Figure 9 and discussion therein), the formation of olefin end group at the ω-end can be ascribed to transfer/water elimination reaction. This observation is an additional evidence that 3271

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Macromolecules polymerization takes place inside of monomer droplets and therefore is similar to conventional cationic polymerization. The above-mentioned chain breaking processes lead to the regeneration of initial active species (H+[LASCX]−), which can initiate a new chain (see Scheme 2). It should be emphasized that no irreversible termination takes place under investigated conditions since the first-order plots are linear (see Figures S2− S4, S7, and S8). All initiation/propagation/transfer reactions proceed inside of monomer droplets/polymer particles. The relatively high molar mass of polymers obtained under investigated conditions is consistent with the formation of so-called weakly coordinating counterion34 during the initiation stage due to the high bulkiness of LASC. Although the molar mass increases with monomer conversion, the polymerization is not controlled. The observed increase of molar mass is consistent with the decrease of H2O content inside of monomer droplets due to the formation of the hydrophobic polymer in the course of polymerization as well as due to the increase of size of particles with increasing monomer conversion (Figure 1c). Still, the average molar mass can be tailored to some extent by adding an initiator (pentachlorophenol) or playing with the Lewis acid nature and/or polymerization temperature.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (S.K.). *E-mail [email protected] (F.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the ANR project “SYNBIORUB” BLAN08-1_340665.

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REFERENCES

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CONCLUSIONS In this work, the emulsion cationic polymerization of pmethoxystyrene in the presence of water-dispersible LASC was thoroughly studied. Water-tolerant Lewis acids and specific branched sodium dodecylbenzenesulfonate were shown to ensure an efficient polymerization of pMOS, chosen as a model monomer. It was demonstrated that polymerization proceeds inside the monomer droplets, which allows synthesizing high molar mass polymers in aqueous media by cationic polymerization. The initiator is either a Brønsted acid formed by partial hydrolysis of water-tolerant Lewis acids or a purposely added weak hydrophobic acid. The observed induction period was attributed to the time necessary to build up a sufficient concentration of Brønsted acid inside of hydrophobic monomer droplets (estimated at 4 × 10−2 mol/L) as well as to create a proton through its interaction with bulky Lewis acid so that to initiate the polymerization. Propagation occurs in the bulk of monomer droplets up to chain transfer to monomer or water with the regeneration of initial active species. The formation of very bulky weakly coordinating counteranion (LASC + acid residue) is probably responsible for the formation of high molar mass polymers. These results open a new avenue in the field of cationic polymerization in aqueous media and will be used during the investigation of cationic polymerization of other, industrially relevant monomers in aqueous media (e.g., isoprene, butadiene, and isobutene).

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energy of activation (Ea,app), RI and LS SEC traces of poly(p-methoxystyrene) (PDF)

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00379. 1 H NMR spectra of surfactant and LASC, 13C NMR spectrum of poly(p-methoxystyrene), conversion vs time, Mn, Mw/Mn vs conversion plots, first-order plots, induction period vs pKh, kp,app vs pKh dependences for pMOS polymerization under different conditions, Arrhenius plot of ln(kp,app) vs 1/T to determine apparent 3272

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