1538 Macromolecules 2005,38,1538-1541 A"Living"Radical ab Initio Emulsion Fluorinated xanthate Agent ant than 10. and if the c loss than 10 PDI ng ho for example 1.3(Cu.RAPT=3.8)and 2 (Cu.RAPT 1)can A ever.what should be an easy transition fror Austro men solutio d to an agents fo D.Box that there was a loss of collid eceed Octobe d January 6.2005 )Ab initi 10n Intro ue allows the desig of polymer e phas phas e here is tures are w al diag 整路 and n cr em sities 1.5 wh anhitectureandg in ength It shouldals versely,the us of less reactive raft of polyr Its from compart en has pro 02 ducing the amount of bim 2 and 16 100 nm the e PDI er into the 1 stem) haooeoncentationa e first,and ly the er using RAFT that init tor must be as hic for styrene using nd.the png agent should be and should theoreticall result in a PDI of 1.3 00 agent re polyme T erage molecula 20% 340m n and a PDI of1 im o: con b ini em r criteria.we ould als have to in our raf n poly make high ults will als al ow mechanistic co region ns tos t olled by the PDI)is largely Results and Discussion.In a typical RAFT-medi 1).75 gofst ne wa oa ton of water (17 o whom correspondence should be sen e-mail m.monteiro mixture was deoxygenated with nitrogen for 20 min and 10.1021/ma0478567
A “Living” Radical ab Initio Emulsion Polymerization of Styrene Using a Fluorinated Xanthate Agent Michael J. Monteiro,*,† Monique M. Adamy,‡ Bastiaan J. Leeuwen,‡ Alex M. van Herk,‡ and Mathias Destarac§ School of Molecular and Microbial Sciences, Australian Institute of Bioengineering and Nanotechnology, University of Queensland, Brisbane QLD 4072, Australia; Department of Polymer Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands; and Rhodia Recherches, Centre de Recherches d’Aubervilliers, 52, rue de la Haie Coq, 93308 Aubervilliers Cedex, France Received October 17, 2004 Revised Manuscript Received January 6, 2005 Introduction. Emulsion polymerization is a unique technique for the synthesis of nanoscale polymer particles. The technique allows the design of polymer nanocomposites with a wide range of morphologies (e.g., core-shell,1 salami,2 hemisphere3), particle sizes, and particle size distributions.4 Such structures are widely used in a plethora of industrial applications,5 varying from protective coatings to biomedical diagnostic tests. This highlights the synthetic versatility and commercial importance of such a technique. “Living” radical polymerization using reversible addition-fragmentation chain transfer (RAFT)6-9 in principle should vastly increase the versatility of emulsion polymerization in the production of new nanostructures with controlled polymer architecture and chain length.10-12 It should also provide a means of producing these nanostructures at a much faster rate of polymerization and with much better control of the molecular weight distribution than in solution or bulk.13,14 This results from compartmentalization of radicals within the polymer particles, reducing the amount of bimolecular termination, and lowering the amount of dead polymer. Compartmentalization in the case of styrene occurs when the particle size is small (<100 nm in diameter), in which only one or zero radicals can exist in any particle (a “zero-one” system).4 There are a couple of criteria that need to be followed when synthesizing a polymer using RAFT. The first, and most important, is that the ratio of RAFT agent to initiator must be kept as high as possible to minimize radical coupling that forms dead polymer chains. Second, the ratio of monomer to RAFT agent should be chosen such that the desired number-average molecular weight (Mn) is reached. To increase the rate of polymerization in a solution experiment, the initiator concentration could be simply increased. However, according to our criteria, we would also have to increase our RAFT agent proportionately, and this restricts the synthesis of polymer chains in the low Mn region. Therefore, emulsion polymerization should allow us to carry out rapid polymerizations to high Mn’s with less formation of dead polymer. The polydispersity (PDI) is largely controlled by the Ctr,RAFT value14-16 () ktr,RAFT/kp, where ktr,RAFT is the rate constant for transfer of propagating radicals to RAFT and kp is the rate constant for propagation to monomer). If the Ctr,RAFT value is greater than 10, a low PDI (<1.1) is found at high conversions, and if the Ctr,RAFT is less than 10, PDIs ranging between, for example, 1.3 (Ctr,RAFT ) 3.8) and 2 (Ctr,RAFT e 1) can be produced.17 However, what should be an easy transition from solution to an ab initio emulsion polymerization has proved to be quite difficult,18-21 especially for highly reactive RAFT agents (see refs 18, 19, and 22 for a detailed mechanistic description for this process). The results18 showed that there was a loss of colloidal stability, retardation in rate, and loss in the control of the molecular weight distribution (MWD). Ab initio emulsion polymerization involves the emulsification of monomer with surfactant in a continuous aqueous phase, in which three phases are present: monomer droplets, swollen monomer micelles, and the water phase containing residual monomer. There is now literature on the techniques to obviate the use of ab initio emulsion polymerization using RAFT to obtain polymer particles with controlled MWDs. These are miniemulsion,21 seeded emulsion,19 and self-aggregation,23 all of which have other limiting factors, such as little control of the particle size distribution, special solvent removal to localize the RAFT agent in the seed particles, and poor control of the MWD of butyl acrylate (i.e., polydispersities close to 1.5 when PDI’s of less that 1.1 should be expected), respectively. Conversely, the use of less reactive RAFT agents (i.e., xanthates or MADIX agents24-27sethyl 2-(O-ethylxanthyl)propionate which has a Ctr,RAFT of 0.68 for styrene17) has proved to be quite successful in producing polymers with controlled MWDs, fast rates of polymerization, and controllable particle size distributions.10,14,28 Although the MWD could be predicted, the PDI’s for styrene13 and n-butyl acrylate14 were 2 and 1.6, respectively. It is possible to reduce the PDI using xanthates by feeding the monomer into the reaction at a slow rate to keep the local monomer concentration as low as possible.28 Although this worked well for n-butyl acrylate, theoretically the time required to reduce the PDI for the styrene system using this procedure is not practical. The Ctr,RAFT value (3.8)17 for styrene using a fluorinated xanthate (ethyl 2-(O-trifluoroethylxanthyl)propionate)29 is much higher and should theoretically result in a PDI of 1.3 at 100% conversion. However, the solution polymerizations using this RAFT agent resulted in a conversion of only 20% after 340 min and a PDI of 1.8. The aim of this work is to use this fluorinated xanthate in a styrene ab initio emulsion polymerization to make high conversion polymer (close to 100%) with PDI’s close to 1.3. The results will also allow mechanistic conclusions about the complex partitioning and transportation of the RAFT agent in a multiphase system to be made. Results and Discussion. In a typical RAFT-mediated ab initio emulsion polymerization (expt 4 in Table 1), 75 g of styrene was added to a solution of water (175 g), sodium dodecyl sulfate (SDS, 1.11 g, which is above its critical micelle concentration), sodium bicarbonate (NaHCO3, 25 mg), and RAFT agent 1-(O-trifluoroethylxanthyl)ethyl propionate (F-MADIX, 0.693 g). The mixture was deoxygenated with nitrogen for 20 min and † University of Queensland. ‡ Eindhoven University of Technology. § Centre de Recherches d’Aubervilliers. * To whom correspondence should be sent: e-mail m.monteiro@ uq.edu.au. 1538 Macromolecules 2005, 38, 1538-1541 10.1021/ma0478557 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/09/2005
Macromolecules,Vol.38.No.5,2005 Communications to the Editor 1539 Table 1.Concentrat expt [SPS](mol dm-3) [F-MADIX](mol dm) [styrenel/[F-MADIX] d(nmy 公 ×10 0.00 2×10 001 All conce ntatogserecaleulatedromthetotalreactionvolnm age dis am ref 17 for more details). H 0.8 0.6 0 CF 04 米X ethyl-2-(O-trifluoroethylxanthyl)propionate 02 米名 (F-MADIX 50 100 Time min)200 260 300 heated to the reaction temperature of 70 C.A water time alefrashini were va using size SEC Furthe graphy Figure 1 shows the conversion (x)vs time curves for ble 1 also number-at e diameter (nm into mul aler wher he (expts 25)show more micel oncent ation thereTh retardatio micelles to form above.The significant diffe mce in the rates of polym alone.The end of the r of a solution out at a hig nd n double the 10 s that compart zation h,no ew particle will be for Fron nd Dand much hi cals per h partly due to the nt simulations.The simulatians a coe a lowering the rate ate radic whic the majo and not nerizatio h 538 e nstant for p the water phs tant t in simulating the and little nge in requ detern evolution dominant process at this high initiator concentration for the F-MADIX does not show the typical profile
heated to the reaction temperature of 70 °C. A watersoluble initiator, sodium persulfate (SPS, 0.0614 g), was then added to start the polymerization. The conversion of monomer to polymer was determined by gravimetric analysis, and the MWD at each conversion was determined using size exclusion chromatography (SEC). Further experiments were carried out by maintaining the same recipe but changing the concentrations of SPS and F-MADIX (Table 1). Table 1 also gives the number-average diameter (nm) as determined from capillary hydrodynamic fractionation (CHDF).30,31 The diameter decreases from 88.1 (no F-MADIX, expt 1) to 69 nm (expt 5) with an increase in the F-MADIX concentration. This suggests that exit of the leaving group on F-MADIX enters more micelles during the nucleation stage (interval I, in which radicals enter micelles to form growing particles) and thus increases the number of particles relative to initiator alone. The end of the nucleation period (end of interval I and start of interval II) occurs when the surfactant concentration drops below its critical micelle concentration.4 During interval II, in principle if the particle number (or Nc, the number of particles per unit volume) is high enough, no new particle will be formed.32 From the rate of polymerization during interval II, the average number of radicals per particle, nj, can be calculated.4 The value of nj decreased with increasing F-MADIX concentration. This could be partly due to the increase in Nc with F-MADIX concentration; i.e., the greater number of particles will result in a lower entry rate coefficient, and a lowering the rate. However, an increase in Nc will only be a minor contribution, and the major contribution to retardation is most probably from exit of the leaving group on the F-MADIX agent out of the particles to terminate with radicals in either the water phase or through reentering particles that terminate with growing polymer chains.13,18,19,33 If we compare expts 4 and 6, the increase in initiator concentration by a factor of 5 results in less than a 2-fold increase in nj and little change in Nc, which suggests that water phase termination reactions become the dominant process at this high initiator concentration. Figure 1 shows the conversion (x) vs time curves for the six ab initio emulsion and one solution experiments given in Table 1. The highest rate was found when an ab initio emulsion polymerization is carried out with no F-MADIX agent (expt 1), in which full conversion was reached in only 150 min. The addition of F-MADIX (expts 2-5) showed that with an increased F-MADIX concentration there is a greater extent of retardation in rate due to exit. The reason for this was discussed above. The significant difference in the rates of polymerization between these emulsion experiments to that of a solution experiment17 carried out at a higher temperature (80 °C) and with double the initiator concentration (expt 7; 20% conversion after 310 min) shows that compartmentalization in our styrene emulsion system provided a unique advantage in obtaining much faster rates and much higher conversions. The Mn and PDI profiles of the ab initio expts 2-6 are given in parts a and b of Figure 2, respectively. The symbols represent the experimental data points, and the solid lines represent simulations. The simulations are based on solving the moment equations (similar to those solved by Wang and Zhu34 but assuming no intermediate radical termination and fast fragmentation, which is reasonable since no retardation in rate is observed) for a solution and not an emulsion polymerization. The rate parameters used in the simulations are Ctr,RAFT equals 3.8, rate constant for propagation, kp, equals 403 L mol-1 s-1, 35 and an average termination rate constant, 〈kt〉, of 3.4 × 108 L mol-1 s-1. 36 The latter rate constants kp and 〈kt〉 are not important in simulating the Mn and PDI, since Ctr,RAFT is the only necessary parameter that is required to determine the evolution of molecular weight distribution with x. The evolution of Mn with x for the F-MADIX does not show the typical profile Table 1. Concentrations of Sodium Peroxydisulfate (SPS) and 1 Used in All Styrene ab Initio Emulsion Polymerizations at 70 °C at Constant Sodium Dodecyl Sulfate (SDS) Concentration (1.5 mol dm-3) expt [SPS] (mol dm-3) [F-MADIX] (mol dm-3) [styrene]/[F-MADIX] dp (nm)b Nc c njd 1 0.001 0 no F-MADIX 88.1 1.13 × 1018 0.185 2 0.001 0.004 731 75.4 1.80 × 1018 0.038 3 0.001 0.0055 501 70.2 2.22 × 1018 0.030 4 0.001 0.01 289 71 2.17 × 1018 0.029 5 0.001 0.04 72 69 2.36 × 1018 0.012 6 0.005 0.01 290 70.3 2.23 × 1018 0.054 7e 0.002 0.0257 252 a All concentrations were calculated from the total reaction volume. b Number-average diameter dp measured by capillary hydrodynamic fractionation (CHDF). c The number of particles per unit volume Nc was calculated from dp. d The average number of radicals per particle nj was calculated during interval II. e Solution polymerization in toluene initiated with 2,2′-azobis(isobutyronitrile) (AIBN) at 80 °C (see ref 17 for more details). Figure 1. Conversion-time profile for ab initio emulsion polymerizations of styrene carried out at an sodium dodecyl sulfate (SDS) concentration of 1.5 mol dm-3 at 70 °C for expts 1-6: both the concentrations of F-MADIX and sodium peroxydisulfate (SPS) were varied (see Table 1). Expt 7 was a solution experiment carried out in toluene using 0.002 M 2,2′- azobis(isobutyronitrile) (AIBN) at 80 °C. Macromolecules, Vol. 38, No. 5, 2005 Communications to the Editor 1539
1540 Communications to the Edito Macromolecules,Vol.38.No.5.200 00 8E-04 6000 6E.0 3) 24000 4E0 2E04 0.E+0 02 04 0.8 0 01 0.2 0.3 0.4 05 B d a of 1.5 mol dmat 70C (SDS and the to m F-MADIX 02 04 0.6 0.8 1s m that it 2 of F-MADIX t70 A)Number-av ater ph erage ar olydi r that the hydr rophobicity of the F-MADIX,which hould beha tion support i.e.,a react onclusion.F-MADIX has been successfully used in ulated curves and dep ase with to can also be lled by simpl changing the F-MADLX water solubility on heF-ND Again.th M and PDI profiles could be f particles terval Ian no por sting in ace rda ciment i ce with the umed e iation a below 1.5 have n prepared 1.360 version, val through the domina of radical radical edged. ting Info mation Available:Exr cope of droplets and particles References and Notes aniannpnghepaidesoihm pa s the char 3) plets during interval II.In the th number of moles decreases in accord with the rate of
associated with highly reactive RAFT agents,7 i.e., a linear increase of Mn with x according to the well-known equation (Mn ) [monomer]M0x/[RAFT], where M0 is the molecular weight of a monomer unit). However, our results are consistent with the profile for a RAFT agent with a Ctr,RAFT of 3.8 (see simulated curves).17 The PDI profiles for all of these experiments are very similar. They start close to 2 and decrease with x to PDI values close to 1.5. This is the first time that such low PDI’s and predictable Mn’s for the synthesis of polystyrene using a living radical technique have been observed in an ab initio emulsion polymerization. Again, these PDI profiles are expected from a RAFT agent with a Ctr,RAFT of 3.8. The simulations are based on solution experiments and do not take into account transport of F-MADIX from the droplets to the particles. Remarkably, the fit with experiment is excellent, suggesting that the F-MADIX agent enters the particle at the rate at which it is consumed. There is however deviation at full conversion, in which the theoretical PDI value is 1.3 but experimental values are close to 1.5. This discrepancy could be the result of oligomeric species that are formed at very high conversions (in interval III) through the dominance of radical-radical termination over radical entry into particles. There could also be a myriad of other possibilities, all of which are subtle and are well outside the scope of this work. On the basis of this, we can determine the concentrations of F-MADIX in droplets and particles during interval II, which is the interval where the droplets act as reservoirs in supplying the particles with monomer, such that the monomer concentration within the particles remains constant. Figure 3 shows the change in the number of moles of F-MADIX within the particles and droplets during interval II. In the droplets, the number of moles decreases in accord with the rate of consumption of F-MADIX. In the particles, the number of moles changes to maintain an equal ratio of monomer to F-MADIX to give the experimental Mn at x. This shows that the diffusion of F-MADIX from the droplets is much faster than its rate of reaction with radicals and that its Ctr,RAFT of 3.8 is sufficiently low to avoid the deleterious kinetic effects of highly reactive RAFT agents (Ctr,RAFT > 100) that result in oligomeric formation in droplets and/or the water phase during interval II, after which the red layer is formed.18 The results also show that the hydrophobicity of the F-MADIX, which should be greater than for their nonfluorinated xanthate analogues,28,33 has little or no effect on the transportation through the aqueous phase and supports our hypothesis that diffusion of the F-MADIX from droplets to the particles is faster than its consumption in the reaction. Conclusion. F-MADIX has been successfully used in an ab initio emulsion polymerization to produce polystyrene with predictable Mn and PDI. The particle size can also be controlled by simply changing the F-MADIX concentration. We believe that by changing the leaving group’s water solubility on the F-MADIX agent the particle size can be altered in a desired fashion. The Mn and PDI profiles could be fit using solution-based simulations (using a Ctr,RAFT of 3.8). This suggested that the transport of F-MADIX from droplets to the growing particles is not rate determining during interval II and enters the particles at the same rate as being consumed in accordance with the twin-films theory,37 which has been discussed previously.18,28,33 This is the first time polystyrene with PDI’s below 1.5 have been prepared using an ab initio emulsion polymerization. Acknowledgment. The financial support by Rhodia PPMC (France) for this work is gratefully acknowledged. Supporting Information Available: Experimental details. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Morgan, L. W. J. Appl. Polym. Sci. 1982, 27, 2033. (2) Okubo, M. Makromol. Chem., Macromol. Symp. 1990, 35/ 36, 307. (3) Dimonie, V. L.; El-Aasser, M. S.; Vanderhoff, J. W. Polym. Mater. Sci. Eng. 1988, 58, 1104. (4) Gilbert, R. G. Emulsion Polymerization: A Mechanistic Approach; Academic: London, 1995. Figure 2. Ab initio emulsion polymerizations of styrene carried out at an sodium dodecyl sulfate (SDS) concentration of 1.5 mol dm-3 at 70 °C for expts 1-6: both the concentrations of F-MADIX and sodium peroxydisulfate (SPS) were varied (see Table 1). (A) Number-average molecular weight (Mn) as a function of conversion (x). (B) Polydispersity (PDI) as a function of conversion. The solid lines are simulations using solution conditions. Figure 3. Change in moles of F-MADIX in (a) droplets, (b) particles, and (c) total, for ab initio emulsion polymerizations of styrene carried out at an sodium dodecyl sulfate (SDS) concentration of 1.5 mol dm-3 at 70 °C for expt 3. These profiles were calculated on the basis of the fit of Mn vs conversion from solution simulations and the swollen diameter (see Table 1). 1540 Communications to the Editor Macromolecules, Vol. 38, No. 5, 2005
Macromolecules,Vol.38,No.5,2005 Communications to the Editor 1541 PolyerColoids:Martinus Nibot 23) 2003,36,960 (24) (25) (8)( 品8WaPm&盆脂aht Chem.Soc.Chem (10) -M: (28)1 s W:Monteiro M I Ma lecules2004.37, (29)D Pichot (32)4 ,B.R:Peach,S.;Leube 33 amnimM (19) rdo,E.;Gilbert,R.G. (20) 3.M ouwer.H:Monteiro.M J.App 21)d 034 s2000.33.9239 avalas,J.G;Schork,F MA0478557
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