Langmuir 1991, 7, 615-616
615
Rapid Hydrolysis of p-Nitrophenyl Diphenyl Phosphate Catalyzed by o-Iodosobenzoate in Cationic Latexes Warren T. Ford* and Hui Yu Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 Received January 7, 1991 In aqueous dispersions of cationic polymer latexes containing 0.01-0.41 mg/mL of particles, (0.40-5.0) 10-5M o-iodosobenzoate (IBA) catalyzes hydrolysis of 1.0 X 10-5 M p-nitrophenyl diphenyl phosphate at 25 "C and pH 8.0 with a pseudo-first-order rate constant of up to 0.064 s-1. The highest second-order rate constant kIBA of 1.35 X 103 M-1 s-1 exceeds by a factor of about 2 the maximum value reported for IBA in CTACl micelles, the previous fastest medium for IBA-catalyzed hydrolysis of phosphate esters. The most active latexes are swollen 2.4-10 times their dry volume in water. The 150 nm diameter ion exchange latexes were prepared by emulsion copolymerization of (chloromethyl)styrenes,(vinylbenzylb trimethylammonium chloride, divinylbenzene, and styrene and characterized by transmission electron microscopyand dynamiclight scattering. Reaction of the chloromethylgroups with trimethylamine created ion exchange latexes with quaternary ammonium groups inside as well as on the surface of the particles. X
Introduction Nucleophilic catalysts enable rapid hydrolyses of esters in the absence of strong acid or strong base. The need for efficient decontamination of organophosphate insecticides and nerve agents has led t o a variety of new nucleophilic catalysts. The most active are substituted o-iodosobenzoates, which exist as the cyclized 1-oxido-1,Zbenzoiodoxol-3(lH)-one.' The rates of IBA-catalyzed hydrolyses of phosphates such as p-nitrophenyl diphenyl phosphate (PNPDPP) (eq 1)are much faster in association colloids
A
\
0-
such as mi~roemulsions,~ and lyotropic liquid crystals4 than in aqueous solution. Silica gel,5 titanium dioxide,6 nylon-6,6 and ion exchange resinsZbv7modified with IBA groups also are active catalysts. Of these heterogeneous systems for IBA-catalyzed hydrolysis of phosphates, the most active until now has been micellar cetyltrimethylammonium chloride (CTAC1).'Y2 We report here that the rates are as fast or faster in dispersions of water-swollen quaternary ammonium ion latexes, which (1) (a) Moss, R. A.; Alwis, K. W.; Bizzigotti, G. 0. J.Am. Chem. SOC. 1983,105,681. (b)Moss, R. A.; Alwis, K. W.; Shin, J.-S. J. Am. Chem. SOC. 1984, 106, 2651. (c) Moss, R. A.; Kim, K. Y.; Swarup, S. J. Am. Chem. SOC.1986, 108,788. (2) (a) Katritzky, A. R.; Duell, B. L.; Durst, H. D.; Knier, B. L. J. Org. Chem. 1988.53.3972. (b) Hammond. P. S.: Forster. J. S.: Lieske. C. N.: Durst,H. D.'J. Am. Cheh: SOC.1989,111,7860. ( c ) Bunton, C. A.; Mhala; M. M.; Moffatt, J. R. J.Phys. Chem. 1989,93,854. (d) Leslie, D. R. Aust. J. Chem. 1989, 42, 2119. (e) Leslie, D. R.; Pantelidis, S. Aust. J.Chem. 1990,43, 937. (3) (a) Mackay, R. A,; Longo, F. R.; Knier, B. L.; Durst, H. D. J.Phys. Chem. 1987,91,861. (b) Burnside, B. A.; Knier, B. L.; Mackay, R. A.; Durst, H. D.; Longo, F. R. J. Phys. Chem. 1988, 92, 4505. (4) Ramesh, V.; Labes, M. M. J. Chem. SOC., Chem. Commun. 1988,
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are effectively colloidal anion exchange resins. The ion exchange particles bind the IBA anion without need for covalent attachment. A major advantage of latexes over larger polymer particles as catalyst supports is that their 0.15-pm diameter minimizes mass transfer and diffusional limitations to reaction rates, as shown previously for polymer-supported phase transfer catalysts.8
Results and Discussion A series of latexes containing 1-60 mol % of ionic repeat units was prepared by shot growthg emulsion copolymerizations of styrene, 1 wt % divinylbenzene, and chloromethylstyrenes using 1.05 wt 7% (vinylbenzy1)trimethylammonium chloride (from (chloromethy1)styrenes and trimethylamine) to create the charged particle surface. Subsequent treatment with trimethylamine converted the chloromethylstyrene groups throughout the particles to quaternary ammonium ions. The samples are described in Table I. In the notation XN+, X denotes the sum of weight percent (chloromethy1)styrene and weight percent (vinylbenzy1)trimethylammonium chloride in the copolymer latex before quaternization. The latexes were ultrafiltered and washed with water to remove any possible low molar mass solutes until the filtrate reached a constant low conductivity. Measurements of the diameters of dry particles by transmission electron microscopy and the hydrodynamic diameters by dynamic light scattering (DLS) are reported in Table I. The polydipersity indexes d,/d, by TEM were 1.01except for one sample. The larger sizes found by DLS than by TEM show that the particles were swollen in water. Moreover the most highly charged particles were more swollen in pure water than in 5.0 mM TAPS [N-tris(hydroxymethyl)methyl-3-aminopropanesulfonic acid] buffer a t pH 8.0, as expected for lightly cross-linked ion exchange resins.'O The latexes were used as supports for IBA-catalyzed hydrolysis of p-nitrophenyl diphenyl phosphate (PNPDPP) under the following typical conditions: 0-0.4 mg of latex particles containing 0-1.4 pmol of quaternary ammonium groups per mL of dispersion, 1.0 X M PNPDPP, (0-5.0) X 10-5 M IBA, and 5.0 mM TAPS at pH (8) Bernard, M.; Ford, W. T.; Taylor, T. W. Macromolecules 1984,17,
891. _.
1812.
(5) Moss, R. A,; Chung, Y.-C.; Durst, H. D.; Hovanec, J. W. J. Chem. SOC., Perkrn Trans. I 1989, 1350. (6) Moss, R. A.; Chung, Y.-C. J. Org. Chem. 1990, 55, 2064. (7) Moss, R. A.; Chung, Y.-C. Langmuir 1990, 6, 1614.
(9) Kim, J. H.; Chainey, M.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci.: Part A: Polym. Chem. 1989, 27, 3187. (IO) Helfferich, F. Ion Exchange; McGraw-Hill: New York, 1962; pp
0743-7463/91/2407-0615$02.50/0
100-125.
0 1991 American Chemical Society
Letters
616 Langmuir, Vol. 7,No. 4, 1991 Table I. Sizes of Ion Exchange Latex Particles diameter, nm sample
N+, mol 5%
1N+ 2N+ 5N+ 10N+ 25Nt 50Nt 75N+
0.63 0.55 1.27 6.2 16.9 34.1 60.4
TEMa dwb dnc 151.0 149.4 157.1 147.3 153.9 152.7 156.5 155.2 163.5 161.8 147.1 146.5 154.3 153.4
2ooo
1
DLS dhd
Vwet/Vdae
167.4 158.8 171.2 179.0 218 250 336
1.36 1.03 1.38 1.50 2.37 4.91 10.3
50 particles were measured on micrograph negatives taken at 1 0 OOOX magnification. Weight average diameter d, = [ E N i d i * / E N i d i 3 ] ' / 3 . N u m b e r average diameter d , = [ xNidb/ENi]1/3. Hydrodynamicequivalent diameter at 90" scattering angle in the single scattering limit obtained with an ALV5000 photon correlatormeasured in 5.0 mMTAPS buffer solution. e Ratio of particle volume by DLS in buffer to dry volume by TEM. a
8.0 and 25.0 "C. Kinetic runs were followed by the appearance of the UV absorbance of the p-nitrophenoxide ion (PNP) without prohibitive intereference from light scattering. Good pseudo-first-order kinetics were obtained over 4 or more half-lives for amounts of IBA both greater than and less than the amount of PNPDPP, showing that the latex-bound IBA is a true turnover catalyst. The halflife for appearance of PNP using 0.2 mg of 50N+particles/ mL and 5.0 X M IBA was 10.7 s, identical with the previous best result obtained by using IBA in CTACl micelles.' Catalysts supported on silica, titanium dioxide, nylon-6, and conventional ion exchange resins showed burst kinetics-rapid evolution of an amount of PNP equivalent to a fraction of the possible IBA sites followed by a slower turnover of the rest of the PNPDPP, which indicates that dephosphorylation of phosphorylated IBA was rate limiting."7 With the highly swollen latexes 50N+, all of the bound IBA appears to be active. For each of eight different weights of the 50N+particles at least four different IBA concentrations were employed, and from the rate equation (2),the second-order catalytic
rate constants k l B A were calculated as the slopes of plots of kobsd vs [IBA]. The ~ I B Awas linearly related to the amount of particles up to 0.2 mg mL-'. In the absence of latex particles, or in the absence of IBA, kobd was negligible compared with the catalytic rate constants. PNPDPP hydrolysis catalyzed by IBA in 0.2 mg mL-l of latexes 25N+ and 50N+ gave ~ I B A= 1510 and 1350 M-l s-l, respectively, compared with kIBA = 7.16 X lW5 M-l s-l in the absence of latex. The highest previous ~ I B A= 645 M-' s-1 was obtained with 0.32 mg mL-' of CTACl micelles.1b A l t h o u g h second-order rate constants in heterogeneous catalysis depend arbitrarily on the specific conditions chosen, three of the many micelle,'S2 microemulsion,3and other heterogeneous catalyst^^-^ reported for IBA-cata-
0 0.0
1 0.2
0.4
0.6
0.8
(a)mol fraction N+ in latex. (b) mequiv N+ per mL of swollen particles.
Figure 1. Dependence of k m on ~ composition of swollen latex particles.
lyzed hydrolysis of PNPDPP have much higher secondorder rate constants based on overall mixture concentrations than all others: CTACl micelles,Iban anion exchange resin in the presence, but not in the absence, of 5 X M CTACl (less than the ~ m c ) and , ~ the swollen anion exchange latexes reported here. The low charge density latexes 1N+, 2N+, and 5N+ consist approximately of a shell of hydrated charged repeat units surrounding a core of polystyrene. The higher charge density latexes 25N+, 50N+, and 75N+ consist of colloidal gel particles with binding sites throughout the core and on the surface. The kinetic results of Figure 1show that the higher the concentration of ion exchange sites on the basis of millimolesof charged sites per milliliter of swollen particles, the higher the activity. Concentration of ion exchange sites depends upon both mole fraction of quaternary ammonium repeat units and degree of swelling. The highest activity latex (25N+) has 0.17 mol fraction ionic groups, much less than 50N+and 75N+,but its lower degree of swelling provides the highest concentration of quaternary ammonium groups inside the particles. Regardlessof the degree of swelling of the particles under conditions of PNPDPP hydrolysis, the particle environment is less hydrophilic than the surrounding water. Consequently the lipophilic PNPDPP must be mostly or all in the particles. The IBA anion is less hydrophilic than chloride, but perhaps not less hydrophilic than the TAPS buffer anion, and so the concentration of IBA in the particles depends on the relative affinities of IBA and TAPS anions to the particles. The catalytic effect is due mainly to higher concentrations of IBA and PNPPDP in the particle phase than in water, and may also be affected by intrinsic reactivity in the particle phase. We are now evaluating the relative contributions of the concentration and intrinsic reactivity effects via determination of the concentrations of the reactive species in the particle phase. Acknowledgment. This research was supported by the U.S. Army Research Office. We thank B. J. Ackerson and U. Nobbmann for the particle size measurements by light scattering.
