(TERP) in Emulsion Polymerization - ACS Publications - American

Jun 17, 2015 - A novel method has been developed for organotellurium-mediated radical polymerization (TERP) in emulsion, herein implemented for styren...
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An Innovative Approach to Implementation of OrganotelluriumMediated Radical Polymerization (TERP) in Emulsion Polymerization Yusuke Sugihara,† Shigeru Yamago,*,‡ and Per B. Zetterlund*,† †

Centre for Advanced Macromolecular Design (CAMD), School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia ‡ Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan ABSTRACT: A novel method has been developed for organotellurium-mediated radical polymerization (TERP) in emulsion, herein implemented for styrene using the low molecular weight TERP agent ethyl-2-butyltellanyl-2-methyl propionate (BTEE) at 90 °C in the absence of a conventional radical initiator. The method is based on the use of relatively high concentrations of the two surfactants polyoxyethylene(20) oleyl ether (Brij98; nonionic surfactant) and tetradecyltrimethylammonium bromide (TTAB; cationic surfactant) in the presence of toluene. This is a one-pot approach that does not require high energy mixing nor the use of a water-soluble macroTERP agent, but instead relies on a high initial monomer fraction being located in surfactant micelles (importantly, the system is not a microemulsion). Good control/livingness was obtained in conjunction with satisfactory colloidal stability and relatively narrow particle size distributions with particle diameters mainly in the range well below 100 nm.

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INTRODUCTION The implementation of controlled/living radical polymerization (CLRP) in dispersed systems is of crucial importance for the development of commercially viable syntheses involving CLRP, as well as for synthesis of various polymeric nano-objects with a wide range of potential applications.1,2 Organotelluriummediated radical polymerization (TERP)3,4 is a powerful CLRP technique, one of its main attractions being high monomer versatility and suitability for block copolymer synthesis. To date, TERP has been implemented in aqueous dispersed systems as miniemulsion polymerization5 as well as ab initio emulsion polymerization based on self-assembly.6,7 Miniemulsion polymerization is ideally suited for CLRP because it relies on monomer droplet nucleation, thus alleviating the requirement for monomer/control agent diffusion through the aqueous phase from monomer droplets to polymer particles. The drawback is that typically high energy mixing approaches are employed to generate the initial miniemulsions,8 although various low energy alternative approaches have been reported.9−13 The self-assembly emulsion polymerization approach (initially developed for RAFT systems14,15) is attractive as it does not involve the extra step of generating a miniemulsion, however, it is associated with the additional synthesis of a water-soluble macroTERP agent, and as such can only be used for synthesis of block copolymers. The most attractive aqueous dispersed system for TERP (or CLRP in general) would be a one-step one-pot process that does not require any special process requirements such as high energy mixing. Microemulsion polymerization,16 which has been implemented in connection with CLRP16−18 (but not © XXXX American Chemical Society

TERP) typically represents such a system, although a significant drawback is that very high levels of surfactant are required. The approach of implementing TERP as an ab initio emulsion polymerization described in this paper is based on the fact that certain mixtures of nonionic surfactant/water/vinyl monomer can form emulsions in a spontaneous manner (i.e., in the absence of high energy mixing) with a high fraction of the monomer in the system located within micelles as opposed to as a separate monomer phase (which would typically comprise micron-sized monomer droplets in the aqueous phase). Urbani et al.19 demonstrated that for polyoxyethylene(20) oleyl ether (Brij98)/water/St = 1/20/2.5 by weight, an emulsion forms spontaneously with >99% of the St located within micelles. It is important to note that such systems do phase separate with time (i.e., monomer not located within micelles forms a layer on top“creaming”), and contain markedly less surfactant than the thermodynamically stable microemulsions. RAFT polymerizations of St were conducted using such systems,19 revealing that good control/livingness was obtained for conditions favoring high nucleation rates of monomer-swollen micelles, i.e., high RAFT agent concentration and high initiator concentration. Such conditions minimize effects of superswelling,20 which leads to heterogeneity in terms of [RAFT]/ [monomer] ratio between different monomer-swollen micelles/particles. In the present work, we have developed a one-pot ab initio emulsion polymerization method utilizing a mixed surfactant Received: May 10, 2015 Revised: June 4, 2015

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DOI: 10.1021/acs.macromol.5b00995 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Polymerization Recipes Employed in the Present Study (All at 90 °C; No Radical Initiator Added) run

St (g)

toluene (g)

[St]/[BTEE]

Brij98 (g)

TTAB (g)

water (g)

comments

1 2 3 4 5 6

2 2 1 1 1 1

− − 1 1 1 1

400 400 38−300 100 100 100

0.2 0.8 0.8 1 0.1 2

0.1 0.1 0.2 1 − −

20 20 20 20 20 20

coagulation coagulation good control/livingness, no coagulation, wide particle size distribution good control/livingness and no coagulation, narrow particle size distribution poor colloidal stability poor colloidal stability

colloidal issues at low conversion), rendering conversion measurements by gravimetry inaccurate. However, good control over the molecular weight distribution was achieved with dispersities (Mw/Mn) < ∼1.2. Diluting the organic phase with toluene (keeping the total organic phase weight constant in the overall recipe) is equivalent to reducing the monomer conversion in the sense that with St:toluene = 1:1 (w/w), 100% conversion corresponds to 50% conversion in terms of weight fraction of polymer in the organic phase. Runs 3 and 4 showed that dilution with toluene results in high conversion, good control/livingness and no coagulation−polymerizations under these conditions will be described in more detail below. Before polymerization (runs 3 and 4), phase separation occurs immediately in the absence of stirring resulting in an upper yellow organic layer (due to BTEE; Figure 1 shows run

system for implementation of TERP in an aqueous dispersed system based on simple reagents. This approach does not require synthesis of a specific macroTERP agent, nor does it require high energy mixing (typically associated with miniemulsion polymerization).

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EXPERIMENTAL PART

Materials. Styrene (>99%, Sigma-Aldrich) was deinhibited by passing through a column of activated basic alumina. Ethyl-2butyltellanyl-2-methyl-propionate (BTEE; Otsuka Chemicals Ltd. (Japan); prepared based on a procedure previously published21), tetrahydrofuran (THF, anhydrous, 98%, Sigma-Aldrich), polyoxyethylene(20) oleyl ether (Brij98; Sigma-Aldrich) and tetradecyltrimethylammonium bromide (TTAB; Sigma-Aldrich; 99%) were used as received. Deionized (DI) water was produced by a Milli-Q reverse osmosis system and had a resistivity of 19.6 mΩ cm−1. Polymerizations. In a typical experiment, BTEE was dissolved in degassed St (by N2 bubbling) in a glovebox (TERP agents are sensitive to oxygen). Surfactant (Brij98 and/or TTAB) was added as a solid to a two-necked Schlenk flask, followed by flushing with nitrogen gas. Subsequently, under nitrogen gas flow, degassed water (by N2 bubbling) was added, followed by the St/BTEE solution using a gastight syringe. Polymerizations were conducted for prescribed times under a nitrogen atmosphere at 90 °C.22 Measurements. Monomer conversions were determined by gravimetry. Molecular weights and molecular weight distributions were determined by gel permeation chromatography (GPC) with a Shimadzu modular system with THF as eluent at 40 °C at a flow rate of 1.0 mL/min with injection volume of 100 μL. The GPC was equipped with a DGU-12A solvent degasser, a LC-10AT pump, a CTO-10A column oven and an ECR 7515-A refractive index detector, and a Polymer Laboratories 5.0 μm bead-size guard column (50 × 7.8 mm) followed by four 300 × 7.8 mm linear Phenogel columns. The system was calibrated against polystyrene standards ranging from 500 to 106 g/mol. Theoretical number-average molecular weights (Mn,th) were calculated from Mn,th = ([styrene]0/[BTEE]0)αMW, where brackets indicate initial concentrations, α is the fractional monomer conversion, and MW the molar mass of styrene. Polymer particle sizes were measured using dynamic light scattering (DLS; Malvern NanoZS) at the backscattering angle of 173° at 25 °C. TEM micrographs were obtained using a JEOL 1400 transmission electron microscope at the voltage of 100 V. The TEM samples were prepared by casting a diluted aqueous solution of miniemulsions (2−3 g/dm3 of polymer) onto a Formvar coated copper grid.

Figure 1. Photographs of polymerization mixture (run 3 in Table 1): (a) before polymerization in the absence of stirring; (b) shaken before polymerization; (c) 5 min after polymerization has commenced; (d) same as part c, but mixture turned to reveal opalescence; (e) after long polymerization time, just after stirring stopped; (f) Same as part e, but some time after stirring stopped, revealing a clear upper layer.

3). After approximately 5 min of polymerization, a milky white emulsion has formed. Also after the polymerization has begun, even at high conversion, cessation of stirring results in phase separation and formation of a now colorless thin upper layer comprising presumably mainly toluene (given that this upper layer forms also at or near 100% conversion). The extent of this phase separation is less significant for run 4 than run 3 (due to much higher surfactant content in run 4). It is important to note that no coagulum is formed under these conditions, unlike in the absence of added toluene. It thus appears of crucial importance to keep the polymer fraction in the dispersed phase below ∼50 wt % to avoid coagulation. Polymerizations with Brij98 as the sole surfactant (runs 5 and 6) resulted in poor colloidal stability. This may be related to the cloud point of Brij98 being reported as 87 °C,23 i.e. slightly below the current polymerization temperature of 90 °C. It is thus interesting to note that in the presence of TTAB (Run 3), this does not

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RESULTS AND DISCUSSION Initial Experiments. TERP of St in emulsion was conducted at 90 °C using various recipes based on the surfactants Brij98 and/or trimethyl(tetradecyl)ammonium bromide (TTAB) and the TERP agent ethyl-2-butyltellanyl-2methyl-propionate (BTEE).21 The amount of organic phase was 10 wt % relative to water, and the overall surfactant content was in the range 5−100 wt % relative to the organic phase. For polymerizations with the organic phase comprising only St and BTEE (runs 1 and 2 in Table 1), significant coagulation occurred at intermediate and high conversion levels (no B

DOI: 10.1021/acs.macromol.5b00995 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules appear to be an issue. Two features of the polymerization recipe are thus key to achieving satisfactory control/livingness and absence of coagulation: (i) the use of a mixed surfactant system of Brij98 and TTAB and (ii) dilution of the organic phase (St) with toluene. St/Toluene Mixed Surfactant System. The most successful system (runs 3 and 4 in Table 1) was examined in more detail. This type of system features the use of two surfactants, Brij98 and TTAB, and the presence of toluene (St:toluene = 1:1 w/w) in the organic phase. Polymerizations were conducted for a range of target molecular weights (various [St]:[BTEE] ratios) at two different overall surfactant contents of 50 (run 3; Table 1) and 100 wt % (run 4) relative to the organic phase. For run 3, polymerizations were performed with [St]:[BTEE] = 38, 75, 150, and 300. Figure 2a shows

Figure 3. Molecular weight distributions for emulsion TERP of St at 90 °C at various [St]/[BTEE] = 38 (a), 75 (b), 150 (c), and 300 (d) using the dual surfactant system of Brij98 and TTAB based on conditions in Table 1 (run 3). The numbers indicate monomer conversions.

Figure 2. Conversion−time data and first-order plots for emulsion TERP of St at 90 °C at various [St]/[BTEE] = 38 (black circles), 75 (green squares), 150 (blue triangles), and 300 (red tilted squares) using the dual surfactant system of Brij98 and TTAB based on conditions in Table 1 (run 3).

conversion−time data, revealing how the polymerization rate (Rp) increases markedly with a reduction in the target molecular weight (i.e., a reduction in [St]:[BTEE], which is achieved by increasing [BTEE]). It is also noteworthy that essentially complete conversion was reached for both [St]: [BTEE] = 38 and 75 within the polymerization times investigated. Figure 3 shows how the molecular weight distributions shifted to higher molecular weights with increasing conversion, although quite significant low molecular weight tailing is apparent for [St]:[BTEE] = 300. The Mn values were in general in good agreement with Mn,th, especially for [St]:[BTEE] = 38 and 75 (Figure 4). The control of the molecular weight distributions were good, with Mw/Mn values well below 1.5 in all cases except for the highest target molecular weight of [St]: [BTEE] = 300 at conversions beyond 20%. In systems based on

Figure 4. Number-average molecular weight (Mn) and dispersity (Mw/ Mn) data for emulsion TERP of St at 90 °C at various [St]/[BTEE] = 38 (black circles), 75 (green squares), 150 (blue triangles), and 300 (red tilted squares) using the dual surfactant system of Brij98 and TTAB based on conditions in Table 1 (run 3). The broken straight lines indicate Mn,th.

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prove, a miniemulsion-type mechanism where the vast majority of monomer is located in monomer droplets/polymer particles that act as loci of polymerization. This suggests that after the initial stage of polymerization, a very significant fraction of the monomer is located in monomer-swollen micelles/polymer particles as opposed to in larger monomer droplets (the latter being the case in a typical emulsion polymerization). Activation in TERP can occur via degenerative transfer and/ or thermal dissociation, although the former mechanism is the predominant one under typical polymerization conditions.3 The kinetics of TERP of St using PSt-TeMe under bulk conditions have been extensively investigated.27,28 In the temperature range 40−100 °C, the presence of PSt-TeMe has no influence on Rp, consistent with the contribution of thermal dissociation being insignificant. As such, one would not anticipate that variation in the TERP agent concentration would influence Rp (aside from relatively minor effects of chainlength dependent termination29). In the present emulsion system, however, Rp increases very markedly with increasing initial BTEE concentration in the overall organic phase (Figure 2). The slope of the first-order plot (=Rp/[M], where [M] is the monomer concentration in the organic phase and Rp = kp[M][P•] ([P•] is the propagating radical concentration in the organic phase)) increases close to linearly with [BTEE]. On the basis of the results from previous studies that thermal dissociation of the TERP agent is not a significant event under these conditions,27 it appears likely that some aspect of the system specific to its heterogeneity is causing this effect. As described above, the particle size decreases with decreasing [St]:[BTEE], and as such the effect on Rp may be associated with the change in particle size. RAFT polymerization in miniemulsion has been shown both experimentally and theoretically to proceed more rapidly for smaller particles due to the segregation effect on termination (compartmentalization).1,30,31 It thus appears likely that the present TERP system, which also proceeds via degenerative transfer, behaves similarly in this regard. Another possible explanation is photochemical activation of dormant species. Organotellurium chain transfer agents and organotellurium-living polymers exhibit characteristic UV−vis absorption accompanied by radical generation (activation).32−34 Photo-TERP has been shown to proceed with control/livingness under irradiation of low-intensity light. Such behavior would cause a dependence on [TERP] as qualitatively observed in the present study (note that the polymerizations cited above26,27 in regards to the absence of thermal dissociation were conducted under no influence of light, whereas the present polymerizations were not). In order to improve the control over the particle size distribution, the surfactant content was increased significantly to 100 wt % relative to the organic phase (run 4). Figure 6 displays conversion vs time data, revealing that also in this case near full conversion was reached. The control/livingness was good−the molecular weight distributions shifted to higher molecular weights with increasing conversion (Figure 7a), the Mw/Mn values remained below 1.3 (Figure 7b), although Mn was slightly higher than Mn,th (Figure 7c). The TEM images in Figure 8 show that the particle size distribution was much narrower than in run 3, both at 66 and 93% conversion. DLS measurements (Figure 9) revealed a largely monomodal particle size distribution with dn = 55 (dv/dn = 1.21; note that Figure 9 shows particle size distributions based on volume, not number) with a very minor particle population in the micron-size range at 66% conversion (this population of larger

degenerative transfer such as TERP and RAFT, the number of dead chains is directly corresponding to the number of chains generated from the added initiator and/or spontaneous generation of radicals (in the present system, only the latter− see the discussion below).24,25 However, the deterioration in control when increasing [St]:[BTEE] from 150 to 300 is too dramatic for this to be the sole explanation, given that the level of control is quite similar for [St]:[BTEE] = 38, 75, and 150. TEM images (Figure 5) reveal that the particle size decreased with decreasing [St]:[BTEE], with sizes mainly in the

Figure 5. TEM images for emulsion TERP of St at 90 °C using the dual surfactant system of Brij98 and TTAB based on conditions in Table 1 (run 3) at various values of [St]/[BTEE]: (a) 38, 100% conversion; (b) 75, 100% conversion; (c) 150, 81% conversion; (d) 300, 35% conversion.

approximate range 40−90 nm for [St]:[BTEE] = 38. The particle size distributions were quite wide, with particles smaller than 40 nm also present for [St]:[BTEE] = 38, 75, and 150. For [St]:[BTEE] = 300, the particles were ill-defined and nonspherical (separately confirmed not to be caused by the lower conversion of that TEM image), which may also be related to the poor control over the molecular distribution in this case. In all cases, larger particles in the micron-scale were also present (not visible in Figure 5). In RAFT emulsion polymerization using unusually high surfactant concentration (generating a microemulsion-like system), the number of particles has been reported to increase with increasing RAFT concentration as a result of increased nucleation efficiency.26 Exit of the expelled R radical from the RAFT agent leads to subsequent entry into another non-nucleated monomerswollen micelle, thus increasing the number of polymer particles. One may speculate that an analogous mechanism may be at play in the present system, whereby degenerative transfer results in formation of a small radical that exits, eventually leading to nucleation of another monomer-swollen micelle resulting in overall smaller particles. Figure 2b displays first-order plotsthe rationale behind a first-order plot is that all unreacted monomer is present at the polymerization locus (as is the case in a homogeneous system). In an emulsion polymerization, which contains a discrete monomer phase where polymerization does not occur, this is not the case. The fact that the first-order plots of the present system are close to linear is thus consistent with, but does not D

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Figure 8. TEM images at various magnifications at (a) 66 and (b) 93% conversion for emulsion TERP of St at 90 °C at [St]/[BTEE] = 100 using the dual surfactant system of Brij98 and TTAB based on conditions in Table 1 (run 4). Figure 6. Conversion−time data and first-order plots for emulsion TERP of St at 90 °C at various [St]/[BTEE] = 100 using the dual surfactant system of Brij98 and TTAB based on conditions in Table 1 (run 4).

particles is not visible in the number distribution (not shown)). At 93% conversion, this larger particle population had become more significant, with dn = 68 (dv/dn = 1.36). Mechanistic Discussion. As outlined in the Introduction, it is believed that the present system operates based on a mechanism similar to that in work previously reported by Urbani et al.,19 whereby a system that forms an emulsion in a spontaneous fashion is exploited. Before polymerization, a significant fraction of the total monomer would be expected to reside in micelles. It is important to emphasize that the present system does not contain a radical initiator, contrary to common TERP systems. However, it is well-known that spontaneous radical generation occurs in bulk styrene,35−37 and it has also been reported that aqueous styrene emulsions exhibit higher rates of radical generation than pure styrene.38 As such, it is anticipated that spontaneous radical generation plays an important role in this work, considering the polymerization temperature is 90 °C. Radical generation may thus occur throughout the system (in aqueous phase and in micelles/ monomer phase). Radical generation within a monomerswollen micelle, or radical entry into such a micelle from the aqueous phase, would eventually result in formation of a polymer particle. Non-nucleated monomer-swollen micelles would presumably act as monomer reservoirs, feeding monomer to nucleated micelles similar to in a microemulsion polymerization.16,39

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CONCLUSIONS A novel one-pot ab initio emulsion polymerization method has been developed for implementation of TERP in an aqueous dispersed system based on simple reagents. Importantly, the method does not involve synthesis of a specific macroTERP agent, nor does it require high energy mixing which is typically associated with miniemulsion polymerization. Furthermore, the method yields pure homopolymer−there is no requirement to use a water-soluble macroTERP type species (e.g., poly(acrylic acid)-TERP species) or amphiphilic macroTERP agent as when relying on particle formation via diblock copolymer selfassembly. Emulsification and particle formation occurs by

Figure 7. Molecular weight distributions and the corresponding Mn and Mw/Mn data for emulsion TERP of St at 90 °C at [St]/[BTEE] = 100 using the dual surfactant system Brij98/TTAB based on conditions in Table 1 (run 4). The numbers indicate monomer conversions. The straight broken line indicates Mn,th. E

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Figure 9. Particle size (diameter) distribution by volume by DLS for emulsion TERP of St at 90 at [St]/[BTEE] = 100 using the dual surfactant system of Brij90 and TTAB based on conditions in Table 1 (run 4) at (a) 66% and (b) 93% conversion. The different colors indicate repeat measurements.

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ACKNOWLEDGMENTS The authors are grateful to Mr. T. Kameshima of Otsuka Chemical Co., Ltd., Japan, for supplying TERP agents. P.B.Z. is grateful for a Future Fellowship from the Australian Research Council.

simple mixing and heating of the polymerization mixture. TERP of styrene was conducted in emulsion using the low molecular weight TERP agent ethyl-2-butyltellanyl-2-methylpropionate (BTEE) at 90 °C in the absence of a conventional radical initiator based on a mixed surfactant system comprising the two surfactants polyoxyethylene(20) oleyl ether (Brij98; nonionic surfactant) and tetradecyltrimethylammonium bromide (TTAB; cationic surfactant). The method relies on a high initial monomer fraction being located in surfactant micelles, but the system is not a microemulsion (it is not thermodynamically stable). The main source of radicals in the system is believed to be spontaneous radical generation involving styrene - it has been reported that aqueous styrene emulsions exhibit higher rates of radical generation than pure styrene. An important feature of the system is the presence of a significant amount of toluene in the organic phase. In the absence of toluene, colloidal stability issues lead to the occurrence of coagulation. However, using a 1:1 ratio (w/w) of styrene/ toluene ensures colloidal stability, relatively narrow particle distributions with average diameters well below 100 nm, as well as good control/livingness with Mw/Mn values generally below 1.3.

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REFERENCES

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Corresponding Authors

*E-mail: [email protected]; Tel: +61 2 9385 4331 Fax: +61 2 9385 6250. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. F

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