Langmuir 1993,9, 1698-1703
1698
Synthesis of Monodisperse Cross-Linked Polystyrene Latexes Containing (Vinylbenzyl)trimethylammonium Chloride Units Warren T. Ford,’ Hui Yu, Jeng-Jong Lee, and Hany El-Hamsharyt Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 Received February 11,1993. In Final Form: April 19,1993 Monodisperse latexes were synthesized by copolymerization of 23%8% by weight styrene and 0-76% vinylbenzyl chloride with 1% divinylbenzene and 1% (vinylbenzy1)trimethyla”onium chloride wing 2,2‘-azobis(N~‘-dimethyleneisobutyramide)dihydrochloride initiator at 60 O C . Particle size in the range 100-200nm in diameter was controlled via the amount of charged monomer uaed in the fllt stage of shot growth polymerization. Polydispersity indexes were D,/D, = 1.01. Treatment of the copolymers with trimethylamine produced latexes containing 1-60mol%of (vinylbenzy1)trimethylammoniumchloride repeat units.
Introduction Polymer colloids substituted with quaternary ammonium ions are highly active supports for catalysis of reactions of organic compounds in aqueous dispersions when one of the reactants or a catalyst is an anion that binds strongly to the partic1es.l-ll Their high surface areas provide for rapid maw transport of readants to and products from the active sites. Evaluation of the factors controlling catalysis in such dispersions requires colloidal particles of carefully controlled composition and structure. Catalytic reactions could occur on the surfaces of chargestabilized particles, a t ion exchange sites inside the particles, or in both environments. Particles containing only the quaternary ammonium groups used to stabilize the colloid during synthesis consist of polystyrene cores with essentially all of the ionic sites on the particle surface. Cross-linked particles containing a large fraction of ionic repeat units are gels swollen by water with ionic sites distributed throughout the particle ae well as on the surface. Since cationic micelles12 and polyelectrolyte^^^ ale0 can serveashighly active media for reactions of anions, the study of catalysis by polymer colloids requires dispersions containing negligibly small amounts of cationic surfactants and polyelectrolytes. Thus the method of synthesis of the particles should be free of nonpolymerizable surfactants, minimize production of water-soluble polymer, and still give particles with large numbers of binding sites for anions. Moreover, the particles should be asmonodisperse as possible to create a uniform catalytic environment.
In this paper we describe the synthesis and characterization of cross-linked poly(styrene-co-vinylbe~lchloride) (VBC) colloids that meet thew requirements. They contain high concentrations of quaternaryammonium ions, introduced without any surfactant by reactions of amines with the VBC repeat units, and they are more nearly monodisperse than any previously reported cationiclatexof which we are aware. Latexes prepared with ionic monomers have been reviewed recently.14 Cationic polymer latexes have been synthesized before using ionic monomers,7~10~11~1~~l~ cationic initiators,17Ja cationic surfa~tants,21~J8~~ and polymerizable cationic s u r f a ~ t a n t s ~ @ - ~ and via quatemization of copolymer latexes containing VBC units.2J2Ja20*26~26 Similarly, anionic polystyrene latexes prepared with the ionic monomer, sodium styrenesulfonate (NaSS), have high concentrations of ionic groups on the surface and are unusually resistant to coagulation in electrolyte s0lutions.~~-31Anionic polystyrene latexes also have been produced by emulsion polymerization of VBC followed by conversion of the chloromethyl groups to sodium sulfonate groups with
N&S03.29
(14)Pichot, C. Makromol. Chem.,Macromol. Symp. 1990,96/36,~7. (16)Liu, L.-J.; Krieger, I. M. J. Polym. Sci., Polym. Chem. Ed. 1981, 19.3013. (16)VM Streun,K. H.;Belt, W. J.; Piet, P.;German, A. L. Eur. Polym. J. 1991,27,931. (17)Goodwin, J. W.;Ottewill, R. H.; Pelton. R. Colloid Polym. Sci. 1979,257,61. (18)Weanling, R A. In Polymer Colloids: Churacterization, Stabilization and Application Properties; Poehlein, G. W., Ottewill, R. H., Goodwin, J. W., Eda.; Mnrtinua NijhoR Bonton, MA, 1983; Vol II. (19)Upeon, D. A. J. Polym. Sei., Polym. Symp. 1986,72,45. + Tenia University, Egypt. (20)Campbell, G. A.; Upeon, D. A. Macromol. Synth. 1990, IO, 1. (l)Ford, W. T.; Badley, R. D.; Chandran, R. S.; Hari Babu, S.; (21)Hamid, S.M.;Sherrington, D. C. Br. Polym. J. 1984,16,39. Hasaanein,M.;Sriniv~,S.;Turk,H.;Yu,H.;Zhu,W.Am.Chem.Soc. (22)Hamid, 8. M.;Sherriagton,D. C. Polymer 1987,!28,332. Symp. Ser. 1992,492,422. (23)Teaur, S.-L.; Fitch, R. M. J. ColloidInterfaceSci. 1987,115,450. (2)Bernard,M.;Ford, W. T.; Taylor, T. W. hhCrOmokCUk8 1984,17, (24) Choubal, M.;Ford, W. T. J. Polym. Sci., Part A Polym. Chem. 1812. 1989,27,1873. (3)Hassanein, M.;Ford, W.T. MaCrOm0kCUkS 1988,21,526. (25)Suen, C.-H.;Morawetz, H. ~ a c r o m o k c u ~ 1984,17, es 1800. (4)Haasanein, M.;Ford, W. T. J. Org. Chem. 1989,64,3106. (26) Verrier-Cbarleux,B.;Grillat,C.;Chevalier,Y.;Pichot,C.; Revillon, A. Colloid Polym. Sei. 1991,269,398. (5)Turk, H.; Ford, W. T. J. Org. Chem. 1988,53,460. (@Turk, H.; Ford, W. T. J. Org. Chem. lSSl,56,1253. (27)Juang, M.S.;Kreiger, I. M. J. Polym. Sci., Polym. Chem. Ed. (7)Ford, W.T.; Yu, H. Langmuir 1991,7,615. 1976,14,2089. (8) van Herk, A. M.; van Streun,K. H.; van Welzen, J.; German, A. L. (28)Schild, R. L.; E l - h r , M. S.;Poehlein,G. W.; Vanderhoff,J. W. Br. Polym. J. 1989,21,125. Emukbn,Latexes,andDkpersion;Becher,P.,Ed.;MarcelDekker: New (9)van Streun, K. H.; Tennebroek, R.; Piet, P.; German, A. L. York, 1978;p 99. Makromol. Chem. 1990,191,2181. (29)Chonde, Y.; Liu, L.-J.; Krieger, I. M. J. Appl. Polym. Sci. 1980, (10)Twigt, F.; Piet, P.; German,A. L. Eur. Polym. J. 1991,27,939. 25,2407. (11) van Streun,K. H.; Belt, W. J.; Schipper, E. T. W. M.; Piet, P.; (30)Kim, J. H.; Chainey, M.; E l - h r , M. S.; Vanderhoff, J. W. J. German, A. L. J. Mol. Catal. 1992,71,245. Polym. Sci., Part A: Polym. Chem. 1989,27,3187. (12)Bunton, C. A.; Savelli, G. Adu. Phys. Org. Chem. 1986,22,213. (31)Kim, J. H.; Chainey, M.; E l - h r , M. S.; Vanderhoff, J. W. J. (13)Iee, N.Acc. Chem. Res. 1982,15,171. Polym. Sci., Part A: Polym. Chem. 1992,90, 171. ~
,._,
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0 1993 American Chemical Society
Langmuir, Vol. 9,No. 7,1993 1699
Monodisperse Cross-Linked Polystyrene Latexes
Scheme I
Table I. Effects of Monomers and Surfaotant on Sizes of Latexes. monomers, CTAB,b D,, D., sample method g g n“ u &ID, LA14 LA2-1 LA2-2 LA3-1
batch batch batch batch
SG1-2 shot growthd a
styrene, 12.0 styrene, 12.0 styrene, 12.0 styrene, 9.6 VBC,2.4 styrene, 12.0
0 0.20 0.10 0
153 160 73 69 105 101 159 157
1.02 1.07 1.04 1.01
0
163 161
1.01
mlP 1 wt%
0-75 wl Yo
All batch and f i t shot mixtures contained 72 mg (0.6 wt %) of
N+monomer, 0.15 g of DVB,0.12 g of VA-044 initiator,and 108 mL of water in addition to the other components listed. Hexadecyltrimethylammoniumbromide. Measured by TEM. Second shot composition wm N+monomer 1.05 g, VBC 3.0 g, DVB 0.04 g, VA-044 0.04 g, and water 20 mL.
The key to our formation of monodisperse particles is the use of the water-soluble,non-micelle-formingmonomer (vinylbenzyl)trimethylammonium chloride and no added surfactant in a shot growth emulsion polymerization process. The method resembles the emulsifier-free copolymerizations of VBC using an amidine initiator reported by Verrier-Charleux and co-workers2s and is patterned aftar the two-stage shot-growth copolymerization of styrene and NaSS described by Kim,Chainey, ElAasser, and V a n d e r h ~ f Pin~which ~ ~ ~small amounts of the charged monomer were used to create surface active sites on the monodisperse particles without concurrent formation of soluble polyelectrolyte, and larger amounts of NaSS were used in the second ehot of monomers to create polystyrene latexes with unusually high concentrations of ionic repeat units. Our particles differ from theirs in three major respects (1)The ionic sites bound covalently to the polystyrene are positively charged quaternary ammonium ions rather than negatively charged sulfonate ions. (2)Conversion of VBC units to quaternary ammonium ions gives much higher concentrations of ionic groups in the final particles. The particles contain as much as 60 mol % of ionic repeat units. (3)The particles are crosslinked with divinylbenzene (DVB)to prevent dissolution of the polymers as polyelectrolytes. The more highly ionic polymers have the compositions of ion exchange resins, and having only 1 % DVB, they swell in water to form gels having up to 8 times the volume of the dry particles.
Results and Discussion The shot growth emulsion polymerization method of
Kim, Chainey, El-Aasser, and VanderhofPOtS1was modified for copolymerization of styrene, VBC, and DVB using (vinylbenzy1)trimethyla”onium chloride (N+monomer) to provide the surface active sites for latex particle formation. The initiator, 2,2’-azobis(N,”-dimethyleneisobutyramidine)dihydrochloride (VA-0441,provided additional cationic sites. First we explored the effects of styrene/VBC compositions and of added cationic surfactant (hexadecyltrimethylammoniumbromide, CTAB) on particle size. If no new particles are formed during the second stage of shot growth polymerization, the final particle size depends on the primary size determined during the first stage. The first stage is equivalent to a batch emulsion polymerization. The results shown in the first three lines of Table I show that surfactant in a batch polymerization markedly reducea particle size,as expected. Lines 1,4,and 5 of Table I show that replacement of some of the styrene in the monomer mixture by VBC has little effect on particle size. Without the second shot of monomers, the diameter of the particles of line 5 would
cr
‘h/k
0.540 mol YO
have been about D, = 163 nm,the same as that of lines 1 and 4 within the error limits expeded for repeated experiments. The amount of N+ monomer used also affected particle size, and from preliminary experiments with varied amounts of N+ monomer we decided to focus on 0.6 wt 9% as an amount that consistently gave particles about 160 nm in diameter in the absence of added surfactant. With guidance from the results of Table I we prepared a series of latexes having the same dry particle size and widely varied styrene/VBC composition. Quaternization with trimethylamine produced a series of latexes having as little as 0.6 and as much as 60 w t % of ionic repeat units, as indicated in Scheme I. Results of copolymerizations using varied amounts of VBC are reported in Table 11. The particle sizes measured by TEM were 140-160 nm in diameter. A major factor responsible for the variations in size is the method of measurement of the amount of the N+ monomer used in the fmt shot of monomers. The monomer is very hygroecopic and difficult to weigh accurately. Most of the experiments in Table I1 were carried out by weighing the N+ monomer. A few experimentswere performed by preparing a stock aqueous solution of monomer, assaying its concentration by titration of the chloride ion, and transferring the stock solution volumetrically to the reaction mixture. By use of the volumetric method, variation of the amount of N+ monomer in the first shot from 0.3 to 1.3 wt % gave variation in particle size from 103 to 192 nm (experiments VBCWJL1-5, in which the total amount of N+monomer in both shots was a constant 1.1 wt %). The particle size distributions of the samples listed in Table I1 are DdD, < 1.02 in all but two examples. The VBC copolymers were treated with aqueous trimethylamine to convert the chloromethyl groups to quaternary ammonium chloride groups and produce the latexes listed in Table 111. Most of the reactions were carried out in a sealed vessel at 4060 OC. In no case was there quantitative conversion, according to analyses of the chloride ion contents of the quaternized products. Aa shown in Table IV, in some cases yields were surprisingly low, particularly with the sampleshaving low VBC content in the first place. Incomplete reaction of VBC latexes has been attributed to incomplete penetration of the polymers by trimethylamine when the polymer Tgexceeds the
Ford et al.
1700 Langmuir,Vol. 9, No. 7, 1993
~~~
Table 11. Compositions of Styrene/VBC/DVB Copolymer Latexes weighta of monomers, Bo fmt shot second shot TEMb
eamplec VBClHYd VBC2HYd VBC5HYd VBClOHYd VBC25HYd VBC50HYd VBC75HYd VBC25JJLe VBC50JJLe VBC75JJL' VBC5OJJLlf8 VBC5OJJL2f8 VBC5OJJL3fa VBC50JJI.Ae8 VBCWJJL5'8 VBC25HEk VBC50HElh VBC75HElh VBC50HE2J.h VBC75HEP.h
styrene 12.00 12.00 12.00 12.00 11.00 7.00 3.00 11.00 7.00 3.00 14.00 14.00 14.00 7.00 7.00 22.00 14.00 6.00 14.00 6.00
VBC 0
0 0 0 1.00 5.00 9.00 1.00 5.00 9.00 10.00 10.00 10.00 5.00 5.00 2.00 10.00 18.00 10.00 18.00
N+ styrene VBC 0.072 4.00 0 0.072 3.80 0.20 0.072 3.30 0.70 1.50 0.072 2.50 0.072 1.00 3.00 0.072 1.00 3.00 0.072 1.00 3.00 0.072 1.00 3.00 0.072 1.00 3.00 0.072 1.00 3.00 6.00 0.146 2.00 6.00 0.146 2.00 0.203 2.00 6.00 0.036 1.00 3.00 0.166 1.00 3.00 6.00 0.144 2.00 6.00 0.144 2.00 6.00 0.144 2.00 0.144 2.00 6.00 6.00 0.144 2.00
N+ 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.203 0.203 0.146 0.139 0.008 0.20 0.20 0.20 0.20 0.20
D, D. 151 148 154 157 164 147 154 161 138 163 141 146 134 192 103 150
174 159 142 164
149 146 153 155 162 147 153 160 137 162 141 145 133 191 102 147 171 153 139 160
W samples contained 1wt % of initiator VA044 in each shot. Weight average and number average diameters of SO particles, except for samplea VBC25HE, VBC5OHE1, and VBC75HE1, which are from only 25 particlee, and VBC75JJL, which is the average of two TEM meaaurementa taken one year apart. c Notation: VBClHY meana 1% by weight of combined VBC and N+ monomer in the shot growth copolymer. HY designates the experimenter. d mHY samples contained 0.15 g of DVB and 108 mL of water in the fmt shot and 0.05gofDVBand20mLofwaterinthesecondshot. e TheVBCxrJJL and VBC5OJJL4-5 samples contained 0.15 g of DVB and 108 mL of water in the first shot and 0.050 g of DVB and 20 mL of water in the second shot. f The VBCtiOJJLl-3 samples contained 0.30 g of DVB and 216 mL of water in the fvst shot and 0.10 g of DVB and 40 mL of water in the second shot. The N+ monomer waa measured volumetrically from an aqueous solution whose concentration waa determined by titration with the chloride-selective electrode. All HE polymerization mixtures contained 0.30 g of DVB and 216 mL of water in the first shot and 0.10 g of DVB and 40 mL of water in the second shot.
*
4
Table 111. Composition# a n d Sizes of Monodisperse Quaternized Latexes TEM" sample QlHY Q2HY Q5HY
QlOHY QWHY Q5OHY Q75HY Q25JJL Q5OJJL Q75JJL
85HE Q25HE QSOHE1 Q75HEl QSOHE2c Q75HE2c
precursor VBClHY VBC2HY VBC5HY VBClOHY VBC25HY VBCSOHY VBc75HY VBc25JJL VBC5OJJL VBC75JJL VBCBHE VBC25HE VBC50HE1 VBC75HE1 VBCSOHE2 VBC76HE2
N+sites, mol %
D,,
D,,
DLSb
nm
nm
h,nm
0.6 0.6 1.3 6.2 17 34
151 148 155 156 176 170 183 185 172 214 131 177 224 200 177 217
149 146 154 155 175 168 181 184 169 212 128 175 220 195 174 212
166 159 172 183 217 274 379 225
80 14 39 61 0.9 15 33 54 23 41
300 423 156 198 284 324 312 508
a See footnote b of Table 11. The 650 and 6 7 5 particles are not spherical on the TEM grid, although their images are round when viewed with the electron beam normal to the grid. Hydrodynamic radius from dynamic light scattering in water. e These latexes were quaternized at 25 OC for 6 h and a t 40 OC for 18 h.
reaction temperature.18 In the case of copolymer VBC-
25JJL we analyzed the quaternized latex Q25JJL for residual chloromethyl groups by 13C NMR spectroscopy of particles swollen in chloroform and found no hy-
Table IV. Latex Quaternization reactant VBC75HY VBCSOHY VBC25HY VBClOHY VBCSHY VBC2HY a
VBC, mmol 29.3 19.6 9.81 3.92 1.96 0.78
25% MeZN, mL
product
21 14 7 7 7 7
Q75HY1 Q5OHY1 Q25HY QlOHY Q5HY Q2HY
?6 yield4
93
88 91 89 36 40
Of quaternary ammonium groups from chloromethyl groups. Table V. Analyses of Chloride Ion Contents of Quaternized Latexes
N+,mmoUg of dry latax method of analytical "ration method QSOHYl" Q50HYllb Q50HY12b 875HY none Volhard 0.90 1.24 1.44 electrode 1.95c 2.37, 2.47c 2.43 3.50 2.42 2.24 3.22d ulbafiiVolhard 1.94 tration electrode 1.94 2.40 2.43 3.31d centrifVolhard 2.41 ugation electrode 2.40 ultrafiielemental 1.88 2.63 tration, drying
Q5OHYl waa quatemized in a flask a t atmospheric pressure. QSOHYl1 waa quatemized in a sealed reactor. QSOHYll and QSOHYl2 are from different ultrafitrates of QSOHYl. e Thew titrations were done a t pH 7. The others were done at pH 2 aa in the Volhard method. Samples were not washed well due to dWiculty of ultrafitration. (I
droxymethyl peak at 65 ppm.32 We estimate that 5 mol % of hydroxymethyl repeat units could be detected by thismethod.32 Previously the degree of hydrolysis of VBC during similar emulsion polymerizations was detected by 13CNMR analysis of the product polymers and was found to correlate inversely with the rate of polymerization.26In our experiments the DVB increases the rate of polymerization and the monomers are >90% consumed in 90min, which probably accounts for the lack of detectable hydroxymethyl groups in the latexes. The quaternary ammonium chloride contents of the latexes were determined by titration using a chloride ionselective electrode, as shown in Table V. Attempts to determine chloride ion contents of latexes by the usual Volhard titration gave irreproducible results due to the difficulty of detecting the endpoint in a latex dispersion. Ultrafiltration or centrifugation of the latexes followed by Volhard titration of the ultrafiltrate or supernatant solution gave results in reasonable agreement with those from chloride ion-selective electrode titration, but the separation processes require hours to days of time. Direct titration of the latex is by far the most convenient method of analysis. The monodispersity of the copolymers is retained in the latexes. However, the sizes measured by TEM are incorrect for all of the quaternary ammonium latexes with high content of ionic groups (all samples denoted Q50 and Q75) for two reasons. (1)These gel particles deform on the TEM grid as shown in parta C and D of Figure 1.The particles appear round when viewed with the electron beam normal to the sample plane (Figure 10,and they appear to have a flat side like a drop of cookie dough when viewed with the electron beam at a 60° angle with respect to the sample. Thus the particle diameters of the Q75 and Q50 latexes measured by TEM with the electron beam normal to the grid plane are the diameters of flattened ellipsoids, not the diameters of spheres. (2) The uranyl acetate stain used for preparation of all TEM samples, except those (32) Ford, W. T.; Yacoub, S. A. J. Org. Chem. 1981, 46, 819.
Monodisperse Cross-Linked Polystyrene Latexes
Langmuir, Vol. 9, No. 7,1993 1701
Figure 1. Transmission electron micrographs of (A) copolymer VBC75JJL taken with the electron beam normal to the sample plane, (B) vBC75JJL at a 60° angle of the beam and the plane, (C) Q75JJL with the beam normal to the plane, and (D) Q75JJL at a 60° angle of the beam and the plane. All images are loooOX TEM and 1.66X photographic magnification. Table VI. Effect of Uranyl Acetate on Particle Sizes Measured by TEM sample VBC75JJL VBC75JJL Q75JJL Q75JJL
uranyl acetate no Yes no Yes
OW
Dn
165 163 196 214
164 162 194 212
indicated in Table VI, swells the particles with high quaternary ammonium content, according to measurements with the electron beam normal to the sample plane reported in Table VI. This may be due to ion exchange of chloride ion for the U02(0Ac), anion during TEM sample preparation. In contrast to the quaternized latex, the copolymer shows no evidenceof distortion on the TEM grid, as shown in Figure 1A,B, and no evidence of swelling by uranyl acetate, as reported in Table VI. Due to both the distorted particle shapes and the swelling by uranyl acetate, the absolute sizes of the high quaternary ammonium content particles in Table I11 measured by TEM should not be compared quantitatively with one another or with those of their copolymer precursors. We first suspected that the particles were deformed on the TEM grid when their measured diameters were 1836% larger than the diameters of their copolymer precursors. The predicted increase in diameter during quaternizationof the VBC75JJL latex to the Q75JJL latex, calculated from their densities (1.169 f 0.003 and 1.142 f 0.005 g cm3,respectively) and the ion exchange capacity of Q75JJL, is only 9%. Similarly, the predicted increase in diameter on conversion of a VBC50 latex to a Q50 latex is only 5 % . The reason for the distortion of the Q75 and Q50latexes, but not the correspondingVBC75 and VBC50 copolymers, is that the high ion content anion exchange latexes are gels that deform on the grid to reduce airparticle interfacial tension during TEM sample preparation, whereas the copolymer particles are rigid, having
T,much above room temperature (T,= 82 "C for polyVBC,18 and T, = 100 "C for polystyrene). The particles containing >30% of quaternary ammonium chloride repeat units are highly swollen by water. They can be described as ion exchange resins lightly crosslinked with 1%DVB or, alternatively, as cross-linked cationic polyelectrolytes. In independent experiments polyelectrolytes identical with Q75, Q50,and Q25, except lacking DVB, have been synthesized via solution copolymerization of styrene and VBC and quaternization with t r i m e t h ~ l a m i n e . ~The ~ ? ~chloride ~ forms of the Q75 and Q50 polyelectrolytes are water soluble and that of Q25 is insoluble. Particle diameters were also measured by dynamic light scattering in dilute, deionized aqueous dispersions. These measurements are reported as the hydrodynamic diameters Dh in Table 111. Dynamic light scattering measures the diffusion coefficient of a sphere, which in the high dilution limit of negligible particle-particle interactions is calculated from the Stokes-Einstein equation D = kT/ 67&,, in which D is the diffusion coefficient, k is the Boltzmann constant, T is absolute temperature, and 7 is the viscosity of the medium. The hydrodynamic radius of the particles, Rh, is the distance from the center of the particle to the shear plane a t the surface. The shear plane is assumed to be that of a nondraining particle. However, interpretation of the dymamic light scattering measurements of the sizes of swollen gel particles is not straightforward. More extensive dynamic light scattering and angulardependent static light scattering measurements of & and of the radii of gyration, R,, of the Q50 and Q75 particles reveal that the R h of these particles is much larger than (33) Yu, H. PbD. Dissertation, Oklahoma State University, 1993.
.(34) Ford, W. T.; Yu, H. Langmuir, in press.
Ford et al.
1702 Langmuir, Vol. 9, No. 7, 1993
would be predicted from R, and a hard sphere model.% This indicates decreasing density of polymer chain segments with increasing distance from the center of the particle, and the data have been fit numerically to a model in which the density of chain segments ha^ a Gaussian distribution about the center of a nondraining particle. Measurements of R h and R, as a function of electrolyte concentration Show marked decrease of R h at high NaCl concentrations (up to 1 M)and little change of Rpss The decrease of R h with increasing electrolyte concentration can be understood by a hairy ball model of the particles, in which the polymer chains at the surface are polyelectrolytes with tails and loops extending far into the surroundingsolution. Added NaCl screens the intrachain Coulombic repulsions of the polyelectrolyte hairs,causing decrease of the hydrodynamic size of the particle. In this model R, measures the core of the particle that is affected only little by external electrolyte. Conclusion We have synthesized the most monodisperse cationic latexes reported to date from styrene, VBC, DVB, a cationic monomer and a cationic amidine initiator. Quaternization of the VBC units with trimethylamine gives particles that are highly swellableand have light scattering properties best described by a hairy ball model of the particle structure. The particles comprise a new type of model polymer colloid. Experimental Section Materials. Styrene (Aldrich),divinylbenzene (DVB, 55%, Polysciences), and vinylbenzyl chloride (VBC, m / p 70/30 from
lH NMR spectra, Scientific Polymer Products or Dow) were distilled under vacuum and stored at 5 OC. Trimethylamine gas (Matheson)and 25 wt % aqueoustrimethylamine (Aldrich)were used as received, and 25 wt % trimethylamine in methanol (Eastman Kodak) was decolorized using a column of an active carbon. 2,2’-Azobis(NJV’-dimethyleneisobutyr~~e) dihyd d o r i d e (WakoChemicalsUSA, VA-044) was used asreceived. Deionized, glass distilled water with resistivity 1.4 X 109 n cm was used in all experiments. (m,pVinylbenzyl)trimethyla”onium Chloride. A 260mL three-neck round-bottom flask was equipped with a trimethylamineinlet,and a dry ice/acetonecondenserwith an outlet through an empty flask to a beaker of aqueous HCl. The flask was charged with 70 mL (74.9 g) of VBC and 50 mL of dry ethanol and was cooled in an ice/water bath. Trimethylamine gas was p d through a trap of NaOH pellets and an empty trap and bubbled slowly into the mixture with magnetic stirring at 0 O C for 3h. After standingat room temperature overnightthe mixture of solid and liquid was extractad with ethyl ether and acetone. GC analyses of the extracts showed no VBC. The solid product was dissolved by adding more ethanol,precipitated intoacetone, washed several times with acetone, and washed last with ethyl ether. The residual solvent was removed under vacuumto leave white, hygroscopic, needlelike crystale, 68.9 g (57 % 1, which were stored in the dark under nitrogen at -6 OC. ‘H NMR (300 MHz, CDCb) 8 3.42 [t, 9H, N(CHa)sl, 5.08 (d, 2H, ArCHaNI, 6.36 and 5.82 [2t,2H, CH==CH21,6.7 [q,lH, CH=CHzl, 7.4-7.7 Im, 4H, CsHdI. ‘W NMR (75 MHz, CDCb) 6 52.7 [CHal, 68.6 [CHzI, 116.0, 117.2,126.8, 129.0, 129.4, 130.8,132.4,133.3,136.8,138.6,
and 140.0 113 peaks, CH2and C&l. The m y aromatic ring carbon signals are due to meta and para isomers. Shot Growth Copolymerization. All of the emulsion polymerizations were carried out by the general procedure of the followingparagraph. Compositionsof the entire range of latexes are reported in Table 11. VBC7SHY Latex. A 250-mL three-neckround-bottom flask was equipped with an overhead stirrer with a Teflon blade, a (36)Ackerson, B. J.; Davis, K.; Jhbin, P.; Ford, W.T.;Yu,H.Paper in preparation.
Table VII. Conwenion of First Shot of Monomers time, h % conversion time, h % conversion 1.0 39.7 2.5 96.6 1.5 2.0
91.9 96.9
3.0
96.6
condenser, an argon inlet, and an addition funnelfor the second shot of monomers that was equipped with an overhead flexible shaft stirrer (Ace Glass Co., catalog number 8081) with a Teflm blade and an argon inlet. The flask was charged with 108 mL of water and 0.072 g of (m,p-vinylbenzyl)trimethyla”onium chloride (N+ monomer), stirred, and purged with argon for 20 min. The N+monomer was weighed by adding it to water in a tared vial on an analyticalbalance. (A more accuratemethod of measuring the N+ monomer is to prepare an aqueous solution, assay the C1- concentration, and transfer the solution to the reaction flask by volume.) A mixture of 3.0 g of styrene, 9.0 g of VBC, and 0.15 g of DVB was added. The mixture was stirred and heated for 20 min in a 60 OC oil bath, 0.12 g of VA-044 was added, and the mixture was stirred at 60 “C for 1.5 h before adding the second shot of monomers. The 1.5-h reaction time was determined independentlyto give about 90-95 % conversion of styrene and VBC as described below. The addition funnel was charged with 20 mL of water and 0.1 g of N+monomer, and the solution was stirred and purged with argon for 20 min. A mixture of 3.0 g of VBC, 0.05 g of DVB, and 1.0 g of styrenewas added. After 5 min 0.04 g of VA-044initiator was added. The stirring mixture in the funnel was transferred to the flask 1.5 h after initiation of the first batch of monomers. The mixture was stirred in a 60 O C oil bath for another 3 h. The latexwas filteredthrough a cottonplug to removea small amount of coagulum, usually
