Butyl Acrylate in a

Article pubs.acs.org/IECR

Miniemulsion Photo-Copolymerization of Styrene/Butyl Acrylate in a Continuous Tubular Reactor Radmila Tomovska,†,‡ José C. de la Cal,† and José M. Asua*,† †

POLYMAT and Departamento de Química Aplicada, Facultad de Ciencias Químicas, University of the Basque Country UPV/EHU, Joxe Mari Korta zentroa, Tolosa Etorbidea 72, Donostia-San Sebastián 20018, Spain ‡ IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain S Supporting Information *

ABSTRACT: The 40 wt % solids content styrene/butyl acrylate miniemulsion photopolymerization was successfully carried out in a continuous tubular reactor. The effect of the type and concentration of photoinitiator (PI), the incident light irradiance (ILI), and the residence time on polymerization kinetics and polymer microstructure was investigated. An optimal value for the ILI that maximizes monomer conversion was found. The shape of the molecular weight distribution (monomodal versus bimodal) can be varied by modifying the particle size and the type of photoinitiator.

1. INTRODUCTION The use of photoinitiators in free radical polymerization is appealing because it allows better control of the initiating process. The reported photopolymerizations in dispersed media include works in emulsion,1−6 miniemulsion,7−13 and microemulsion.14−17 However, most of these works were carried out in batch reactors, where the irradiation is not efficiently used because of the limited penetration depth of the light in the dispersed media. Tubular reactors overcome this limitation and in addition offer the advantage of an easy removal of the polymerization heat. Tubular reactors have been used to synthesize hybrid polyurethane/acrylics pressure sensitive adhesives by miniemulsion polymerization12 and ultrahigh molecular weight polystyrene by emulsion polymerization.6 However, these works were performed at solids content (20 wt %), which are too low for commercial use. Higher solids contents are prone to suffer coagulation caused by the preferential interaction of the organic phase with the reactor wall.13 A way to overcome this limitation is to modify the reactor wall for the first part of the reactor (until reaching about 45% monomer conversion).13 In spite of these advances, further investigation is needed to fully exploit the possibilities of this technique. In this work, the photoinitiated miniemulsion copolymerization of styrene and butylacrylate carried out in a tubular reactor is studied. A relatively high solids content (40 wt %) was used and the effects of the photoinitiator type and concentration, incident light irradiance (ILI), and residence time (τ) on the polymerization kinetics and polymer microstructure were investigated.

buffer. All of them were used as received. As SA is not completely insoluble in water, it may diffuse from small to large droplets, reducing its efficiency as a costabilizer. Therefore, polystyrene (PS, Mw = 280.000 g/mol, Aldrich) was added to increase the miniemulsion stability. The role of PS was to minimize the diffusion of SA, which in turn minimizes droplet degradation by monomer diffusion.18 Nonbleaching (2,2dimethoxy-2-phenyl acetophenone (DMPA), ε ∼ 35 L/mol cm, Aldrich) and photobleaching (bis acyl phosphine oxide (BAPO), ε ∼ 300 L/mol cm, BASF; 2,4,6,-trimethylbenzoyldiphenylphosphine oxide (MBPO), ε ∼ 520 L/mol cm, BASF) oil-soluble photoinitiators were used as received. Oxygen-free grade nitrogen was used for purging the feed. Double deionized water (DDI) was used throughout this study. 2.2. Formulation. The formulation of the 40 wt % solids S/ BA miniemulsion used in the study is given in Table 1. All polymerizations were carried out at 60 °C. Table 1. Formulation of the St/BA Miniemulsion 40 wt % solids content St/BA miniemulsion component organic phase

aqueous phase

a

2. EXPERIMENTAL SECTION 2.1. Materials. Technical grade monomers, styrene (S, Quimidroga), and butyl acrylate (BA, Quimidroga) were used as received. To prepare the miniemulsions, Dowfax 2A1 (alkyl diphenyloxide disulfonate, Dow Chemicals), was used as a surfactant, n-octadecyl acrylate (SA, Aldrich) as a reactive costabilizer, and sodium bicarbonate (NaHCO3, Aldrich) as a © 2013 American Chemical Society

S BA SA PS PI DDI-water Dowfax 2A1 NaHCO3

amount (g) 35 35 2.8 1.4 0.35−1.05 105 3.206 0.23

weight % a

50 50a 4a 2a 0.5−1a 2(45 wt % active)a 0.05 Mb

Weight based on monomer weight (wbm). bBased on water weight.

Special Issue: John Congalidis Memorial Received: Revised: Accepted: Published: 7313

August 23, 2013 November 12, 2013 December 12, 2013 December 12, 2013 dx.doi.org/10.1021/ie402779y | Ind. Eng. Chem. Res. 2014, 53, 7313−7320

Industrial & Engineering Chemistry Research

Article

2.3. Miniemulsion Preparation. 40 wt % solids content miniemulsions were prepared by mixing the organic and aqueous phases (Table 1) under magnetic stirring (15 min at 1000 rpm) and subjecting the resulting coarse emulsion to sonication (15 min, at 9 output control and 80% duty cycle) with a Branson 450 instrument (Danbury, CT). The diameter of the miniemulsion droplets as measured by dynamic light scattering was around 140 nm. 2.4. Reactor Setup. A continuous tubular reactor composed of 11 quartz tubes connected with each other with 10 semicircular silicone bends with 2 mm inner diameter was used throughout this study. Each quartz tube had 400 mm length, 1 mm inner diameter, and 3 mm outer diameter. The total length of the reactor was 4.67 m. The reactor was placed in a water bath that was inside an UV chamber (Dr. Gröbel UV-Elektronik GmbH, model BS 03), equipped with 20 UV lamps (wavelength range from 315 to 400 nm with a maximum intensity at 368 nm). A radiometer UV sensor was used to measure the incident light irradiance (ILI), which was varied in the range 2.5−7 mW/cm2. A gear pump (Gilson, model 305) was utilized to control the miniemulsion flow rate (0.14−0.43 mL/min). All runs were carried out under laminar conditions (Re ≤ 61). The feeding tank of the miniemulsion was purged with nitrogen, under magnetic stirring at 450 rpm for 30 min prior to starting the reaction. The reaction samples were collected after reaching the steady-state conditions (>4 residence times). In preliminary experiments, it was determined that (i) the irradiation of the miniemulsion without PI did not lead to any monomer conversion and (ii) no polymerization of the monomers took place in the presence of initiator at room temperature without UV irradiation. 2.5. Characterization. Stability of the miniemulsion at 25 and 60 °C was measured by recording the backscattering signal of the dispersion (TurbiscanLAB equipment) every 15 min for 5 h. The evolution of the backscattering signal over time gives an indication of the miniemulsion stability: no change in the backscattering signal over time is the fingerprint of a stable miniemulsion. It has been recently demonstrated that reactor clogging occurs when the interfacial tension monomer/wall (γms) is lower than that of the aqueous phase/wall (γws).13 Under these conditions, monomer diffuses to the reactor wall where it is polymerized. Accumulation of polymer at the reactor wall results in plugging. Therefore, the ratio γws/γms provides a facile test to check if a given miniemulsion will lead to plugging in the quartz tubular reactor. This is assessed by measuring the contact angle of a droplet of an aqueous solution of surfactant surrounded by the monomer mixture on a quartz cuvette. If the contact angle is less than 90°, then γws/γms < 1 and no reactor clogging is expected.13 To measure the miniemulsion droplet and polymer particle average diameters, dynamic light scattering was used. Measurements were carried out in a Zetasizer Nano Z (Malvern Instruments) by diluting one drop of latex or miniemulsion in deionized water. The z-average diameters reported are an average of two measurements, and each of them was analyzed in 11 runs of 30 s. The monomer conversion was followed gravimetrically. In order to determine the polymer microstructure, the gel content and sol molecular weight distribution were measured. The gel content was determined by Soxhlet extraction. 10−15 latex drops were deposited on glass fiber square pads (weight W1) and dried 12 h at 60 °C (weight W2), followed by 24 h of

Soxhlet extraction under THF reflux. After the extraction, the pads were dried overnight (weight W3). The gel content was calculated as the ratio of the dry polymer remaining after the extraction and the initial amount of dry polymer W 3 − W1 gel content (wt %) = ·100 (1) W 2 − W1 Size exclusion chromatography (SEC) was utilized for measuring the MWD of the sol part obtained in the Soxhlet extraction. The system consisted of a pump (Waters 2410), UV (Waters 2487) and RI (Waters 2410) detectors, and three columns in series (Styragel HR2, HR4, and HR6; with a pore size of 102−106 Å). The analyses were performed at 35 °C with THF as the carrier at a flow rate of 1 mL/min. The equipment was calibrated using polystyrene standards (fifth order universal calibration), and therefore, the molecular weights presented in this study were referred to polystyrene. 2.6. Particle Separation Procedure. A 20 mL portion of latex was centrifuged in 38.5 mL polyallomer tubes (Beckman), in a TFT 70.38 fixed-angle rotor using a Centrikon T-2190 centrifuge (both Kontron Instruments, Milano, Italy) at 4 °C for 2 h at 15 000 rpm. Four color distinctive fractions were obtained. The samples for MWD determination were collected from the top and bottom fractions, namely, from the fractions with the highest differences in particle diameter.

3. RESULTS AND DISCUSSION 3.1. Miniemulsion Stability and Clogging Test. Figure 1 shows the evolution of the light backscattered by the

Figure 1. Time evolution of light backscattered by the S/BA miniemulsion.

miniemulsion at 60 °C. It can be seen that almost no variation was observed in 5 h, showing that the miniemulsion was stable at least for this period of time. Figure 2 shows that the contact angle of the aqueous solution of surfactant surrounded by the monomers on quartz was less than 90°; therefore, γws/γms < 1 and no reactor clogging was expected. 3.2. Reaction Kinetics. The kinetics of the S/BA miniemulsion photopolymerization in the quartz tubular reactor was investigated varying the PI type and concentration, ILI, and residence time. The photolysis of DMPA, BAPO, and MBPO is presented in Scheme 1. DMPA is oil soluble, is not photobleaching, and has an extinction coefficient of 32 L mol−1 cm−1 at 365 nm. After light absorption, benzoyl and methyl radicals are generated (see Scheme 1a).19 BAPO and MBPO are oil soluble photobleaching PIs with high extinction coefficients (ε ∼ 300 and 520 L mol−1 cm−1, respectively, at 7314

dx.doi.org/10.1021/ie402779y | Ind. Eng. Chem. Res. 2014, 53, 7313−7320


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ing the irradiation from 3.5 to 7 mW cm−2 had a very modest effect on polymerization rate at low values of residence time and led to lower conversions at higher residence times. Actually, for the three different concentrations of DMPA used, the monomer conversion achieved at τ = 30 min showed a maximum for intermediate values of ILI (Figure 4). The occurrence of a maximum in the conversion versus ILI curves was explained as follows. An increase of ILI accelerates the photolysis of the initiator and consequently the formation of radicals. The result is that plenty of radicals are formed in the first part of the reactor, which accelerates the polymerization, but the PI is exhausted sooner and the polymerization stops. While the rate of decomposition of PI is such that it does not disappear for the residence time considered (30 min in Figure 4), monomer conversion increases with ILI, but for ILIs that lead to the complete consumption of the PI at residence times lower than 30 min, monomer conversion decreases with ILI. An increase in the concentration of DMPA results in a less pronounced maximum. Also, as the DMPA concentration increases, the relative differences in monomer conversion are lower, because the penetration depth of UV light decreases with PI concentration and at high conversions there is less room for conversion increase. In order to check if the maximum in the conversion versus ILI curve is particular for DMPA or it is a characteristic feature of photopolymerization, the other two PIs with much higher extinction coefficient were selected (MBPA and BAPO) and the influence of ILI on conversion was investigated. Figure 4 shows that the maxima were even more pronounced, indicating that the occurrence of the maximum is a characteristic of photopolymerization in tubular reactors. 3.3. Polymer Microstructure. None of the latex obtained contained gel. The reason was the relatively high S/BA ratio used in the experiments. In acrylic containing latexes, gel is formed by intermolecular chain transfer to polymer followed by termination by combination.21 Styrene is not prone to suffer chain transfer to polymer because it does not have labile

Figure 2. Drop of aqueous solution of surfactant in the monomer mixture on quartz substrate.

365 nm), which after cleavage of the C−P bond produce benzoyl and phosphinoyl radicals (Scheme 1b and c). These radicals are efficient in initiating the polymerization; the phosphinoyl radicals have a higher rate coefficient than the benzoyl radicals for addition to carbon−carbon double bonds.20 The phosphinoyl radical formed from BAPO can suffer a second photolysis (Scheme 1b). The double radical formed has been used to synthesize ultrahigh molecular weight polymers in a tubular reactor.6 The kinetics of the S/BA miniemulsion photopolymerization initiated with DMPA is presented in Figure 3. For most of the cases (Figure 3c and d), a decrease of the particle size with the residence time was observed. This decrease cannot be only explained by the shrinkage due to the higher density of the polymer, and it is likely due to secondary homogeneous nucleation occurring mainly at higher residence times. On the other hand, monomer conversion increased with initiator concentration and residence time. However, intensify-

Scheme 1. Photolysis of (a) DMPA, (b) BAPO, and (c) MBPO Photoinitiators

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Figure 3. Effect of residence time and ILI on monomer conversion and particle size for different concentrations of DMPA (a, c) 0.23 mol % and (b, d) 0.5 mol %.

hydrogen. In addition, styrene radicals are not particularly active to abstract the tertiary hydrogen of the BA units in the polymer.22 Figure 5 shows the effect of the residence time on the MWD for different ILIs. It can be seen that a bimodal MWD was obtained at low residence times and that the relative intensity of the low molecular weight peak decreased as the residence time increased. In addition, the bimodality was more marked at high ILI. The presence of the two peaks indicates the existence of two mechanisms or two polymerization environments. Differences in the mechanisms may be caused by monomer composition drift due to the differences in the reactivity ratios (rS = 0.88:rBA = 0.223). As styrene is more reactive, one may speculate that the low molecular weight peak, which is more evident for low residence times, is richer in styrene. The lower kp of styrene24,25

Figure 4. Effect of ILI on monomer conversion for different types and concentrations of PI at a residence time of 30 min.

Figure 5. Effect of the residence time on the MWD with 0.23 mol % DMPA for (a) ILI = 7 mW/cm2 and (b) ILI = 3.5 mW/cm2. 7316



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Figure 6. Comparison of MWD curves obtained by UV and RI detectors: (a) DMPA = 0.23 mol %, ILI = 3.5 mW cm−2, 20 min residence time; (b) DMPA = 0.7 mol %, ILI = 3.5 mW cm−2, 30 min residence time.

Figure 7. MWD in the particles with different size: (a) 3.5 mW cm−2, DMPA = 0.23 mol %, 10 min residence time; (b) 7 mW cm−2, DMPA = 0.5 mol %, 30 min residence time.

and the absence of chain transfer to polymer would justify the smaller molecular weight. In order to check this point, the MWDs determined with refractive index (signal proportional to the total mass) and UV (able to detect only S units) detectors were compared (Figure 6). In polymers with unimodal or slightly bimodal MWD (Figure 6a), no differences were observed, indicating that the styrene units were homogeneously distributed in the whole sample. However, in samples with bimodal distributions (Figure 6b), although the MWDs determined by both detectors were still bimodal, the relative contributions of both peaks changed. Figure 6b clearly shows that the polymers with low molecular mass contain a fraction of styrene units larger than the polymers with higher molecular weights. The inhomogeneous distribution of styrene units indicates that the composition drift may contribute to the bimodal MWD, but this effect was not sufficient to explain the clear separation between the two peaks of the bimodal MWDs. Therefore, attention was turned toward the number of radicals. Because particle size (dp) plays a major role in determining the number of radicals per particle, the effect of dp on the MWD was studied by separating the particles by centrifugation. Figure 7 shows that indeed the MWD depended on the particle size. Figure 7a shows that the MWD evolved from bimodal for the large particles (180 nm) to monomodal for the small particles (88 nm). The MWDs obtained for the intermediate sizes presented in Figure 7b give additional proof for the evolution from monomodal to bimodal MWD as particle size increases.

In this process, as oil soluble initiators are used, radicals are formed in pairs within the polymer particles. Thus, benzoyl and methyl radicals are created by photolysis of DMPA (Scheme 1a). In the small particles, these radicals will undergo fast termination, unless one of them desorbs from the particle.26 Rapid termination will lead to a small amount of low molecular weight polymer, difficult to detect in SEC. On the other hand, radical desorption will strongly increase the molecular weight. Considering that styryl radicals have a non-negligible rate of desorption,27 the less hydrophobic and more mobile methyl radicals are expected to desorb more rapidly. Even benzoyl radicals may contribute to radical desorption. Radical desorption is largely controlled by the competition between the diffusion and propagation. The rate of addition of the first BA unit to the benzoyl radical has been determined experimentally to be 1.8 × 105 L mol−1 s−1 at 25 °C.28 For a BA concentration of 3.4 mol L−1, this gives a pseudo-first-order propagation rate coefficient of 6 × 105 s−1. Comparison of this value with the rate of desorption of benzoyl radicals from sodium dodecyl sulfate micelles (1.7 × 106 s−1)29 shows that desorption of benzoyl radical may be significant even though lower desorption rates are expected for polymer particles (due to the size effect on radical desorption26). Therefore, one of the initiator radicals may desorb from the small polymer particles and the remaining one may grow to give high molecular weight polymer chains. In large particles, the radical desorption is less likely, and hence, the amount of high molecular weight polymers is less. On the other hand, bimolecular radical 7317



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Figure 8. Molecular weight distribution (a) evolution with residence time, for 1% of MBPO and ILI 2.5 mW/cm2; (b) effect of ILI for 1% BAPO and τ = 30 min.

termination is also less likely than in small particles, and the radicals may grow to a significant length before terminating, leading to molecular weights smaller than those produced by radicals that grow alone. The result is that a bimodal MWD is produced in large particles. The bimodality becomes less defined as polymerization advances, and for the final sample, it is reduced to a shoulder. A possible reason for this behavior is that, as the reaction advanced, the termination rate decreased, increasing the lifetime of the radicals, and hence the polymer resulting from bimolecular termination had a molecular weight intermediate between the small and large peaks of the initial MWD. Figure 8a shows that MBPO presented an evolution of the MWD similar to that of DMPA (Figure 5), likely due to the similar decomposition mechanism, which seems to counteract the differences in the extinction coefficient and the fact that MBPO is photobleaching, whereas DMPA is not. The effect of the decomposition mechanism is highlighted in Figure 8b, where the effect of ILI on the MWD in the polymerization initiated with BAPO is presented. It can be seen that, even at τ = 30 min, this photoinitiator led to a much more pronounced bimodal distribution for the whole range of ILI used. Even more, two peaks can be distinguished in the high molecular weight mode. The effect of the particle size on the MWD when BAPO was used is presented in Figure 9. It can be seen that, for both small (113 nm) and large particles (178 nm), a bimodal distribution was obtained, although the relative area of the modes varied with the particle size. For large particles, the low molecular weight peak was the higher one, whereas the opposite occurred for the smaller particles. Comparison with the case of DMPA (Figure 7a) shows that, for similar particle sizes, BAPO yielded a more prominent small molecular weight peak and higher molecular weights for the second mode. The more prominent small molecular weight peak indicates that bimolecular termination is more frequent in the case of BAPO likely due to lower desorption rates of the radicals generated by photolysis of the initiator. The presence of a high molecular weight peak shows that some radical desorption occurred. On the other hand, the peak of high molecular weight comprises two distributions, the large one likely formed from the double phosphynoil radical.

Figure 9. Effect of particle size on the MWD of the latex synthesized with BAPO and following conditions (3.5 mW cm−2, PI = 0.23 mol %, τ = 10 min).

4. CONCLUSIONS In this work study, the high solids S/BA miniemulsion photopolymerization carried out in a continuous tubular reactor was investigated. The effect of the PI type and concentration, ILI, and residence time on reaction kinetics and polymer microstructure was investigated. It was found that monomer conversion increased with PI concentration and residence time, but the effect of ILI was more complex, showing an optimum value that maximizes monomer conversion. At low ILIs, monomer conversion at τ = 30 min increased with ILI, but at higher ILIs, it decreased with ILI because the PI was exhausted in the first part of the reactor of the monomers. The effect is less pronounced at higher PI concentrations. Photopolymerization led to bimodal MWDs. The bimodality was mainly caused by the effect of particle size, with a small contribution of the monomer composition drift due to the different reactivities of the monomers. It was found that, for 7318



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(10) Hoijemberg, P. A.; Chemtob, A.; Croutxé-Barghorn, C.; Poly, J.; Braun, A. M. Radical photopolymerization in miniemulsions. Fundamental investigations and technical development. Macromolecules 2011, 44, 8727. (11) Hoijemberg, P. A.; Chemtob, A.; Croutxé-Barghorn, C. Two routes towards photoinitiator-free photopolymerization in miniemulsion: acrylate self initiation and photoactive surfactant. Macromol. Chem. Phys. 2011, 212, 2417. (12) Daniloska, V.; Tomovska, R.; Asua, J. M. Hybrid miniemulsion photo polymerization in a continuous tubular reactor  A way to expand the characteristics of polyurethane/acrylics. Chem. Eng. J. 2012, 184, 308. (13) Daniloska, V.; Tomovska, R.; Asua, J. M. Designing tubular reactors to avoid clogging in high solids miniemulsion photopolymerization. Chem. Eng. J. 2013, 222, 136. (14) Jain, K.; Klier, J.; Scranton, A. B. Photopolymerization of butyl acrylate-in-water microemulsions: polymer molecular weight and endgroups. Polymer 2005, 46, 11273. (15) Peinado, C.; Bosch, P.; Martin, V.; Corrales, T. Photoinitiated polymerization in bicontinuous microemulsions: fluorescence monitoring. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 5291. (16) Wan, T.; Hu, Z. W.; Ma, X. L.; Yao, J.; Lu, K. Synthesis of silane monomer-modified styrene−acrylate microemulsion coatings by photopolymerization. Prog. Org. Coat. 2008, 62, 219. (17) Zhang, L.; Zang, L.; Zhang, H.; Guo, J. Synthesis of fluorinecontaining latexes with core−shell structure by UV-initiated microemulsion polymerization. Iran. Polym. J. 2013, 22, 93. (18) Asua, J. M. Miniemulsion Polymerization. Prog. Polym. Sci. 2002, 27, 1283. (19) Groenenboom, C. J.; Hageman, H. J.; Overeem, T.; Weber, A. J. M. ″Photoinitiators and photoinitiation. 3. Comparison of the photodecompositions of alpha-methoxy-and alpha, alpha-dimethoxydeoxybenzoin in 1,1-diphenylethylene as model substrate. Macromol. Chem. Phys. 1982, 183, 281. (20) Rutsch, W.; Dietliker, K.; Leppard, D.; Köhler, M.; Misev, L.; Kolczak, U.; Rist, G. Recent developments in photoinitiators. Prog. Org. Coat. 1996, 27, 227. (21) Plessis, C.; Arzamendi, G.; Leiza, J. R.; Schoonbrood, H. A. S.; Charmot, D.; Asua, J. M. Seeded Semibatch Emulsion Polymerization of n-Butyl Acrylate. Kinetics and Structural Properties. Macromolecules 2000, 33, 5041. (22) Plessis, C.; Arzamendi, G.; Leiza, J. R.; Schoonbrood, H. A. S.; Charmot, D.; Asua, J. M. Kinetics and polymer microstructure of the seeded semibatch emuslion copolymerization of n-butylacrylate and styrene. Macromolecules 2001, 34, 5147. (23) Chambard, G.; Klumperman, B.; German, A. L. Dependence of chemical composition of styrene/butyl acrylate copolymers on temperature and molecular weight. Polymer 1999, 40, 4459. (24) Buback, M.; Gilbert, R. G.; Hutchinson, R. A.; Klumperman, B.; Kuchta, F. D.; Manders, B. G.; O’Driscoll, K. F.; Russell, G. T.; Schweer, J. Critically evaluated rate coefficients for free-radical polymerization, 1. Propagation rate coefficient for styrene. Macromol. Chem. Phys. 1995, 196, 3267. (25) Asua, J. M.; Beuermann, S.; Buback, M.; Castignolles, P.; Charleux, B.; Gilbert, R. G.; Hutchinson, R. A.; Leiza, J. R.; Nikitin, A. N.; Vairon, J. P.; van Herk, A. M. Critically Evaluated Rate Coefficients for Free-Radical Polymerization, 5a,b. Propagation Rate Coefficient for Butyl Acrylate. Macromol. Chem. Phys. 2004, 205, 2151. (26) Autran, C.; de la Cal, J. C.; Asua, J. M. (Mini)emulsion Polymerization Kinetics Using Oil-Soluble Initiators. Macromolecules 2007, 40, 6233. (27) Asua, J. M.; de La Cal, J. C. Entry and Exit Rate Coefficients in Emulsion polymerization of Styrene. J. Appl. Polym. Sci. 1991, 42, 1869. (28) Colley, C. S.; Grills, D. C.; Besley, N. A.; Jockusch, S.; Matousek, P.; Parker, A. W.; Towrie, M.; Turro, N. J.; Gill, P. M. W.; George, M. W. Probing the Reactivity of Photoinitiators for Free Radical Polymerization: Time-Resolved Infrared Spectroscopic Study of Benzoyl Radicals. J. Am. Chem. Soc. 2002, 124, 14952.

initiators that yield two radicals upon photolysis, the MWD shifted from monomodal for small particles to bimodal for large particles. The peak of small molecular weights was attributed to chains formed by bimolecular termination of the active chains created from the radicals formed from the same photoinitiator molecule, whereas the high molecular weight peak was attributed to the polymer formed from initiator radicals whose pair had desorbed from the polymer particle. The bimodality was more pronounced in the case of a photoinitiator (BAPO) able to yield four radicals (two of them on the same molecule) upon suffering two photolysis.

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ASSOCIATED CONTENT

S Supporting Information *

Figures showing particle size distributions of different latex fractions, separated by centrifugation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Diputación Foral de Gipuzkoa, University of Basque Country (UFI 11/56), Basque Government (GVIT373-10 and Etortek Nanoiker IE11-304), and Ministerio de Economiá y Competitividad (CTQ2011-25572) are gratefully acknowledged for their financial support.

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REFERENCES

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