10 Polymers Containing Pendant Herbicide Substituents:
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Hydrolysis Studies II FRANK W. HARRIS, MARY R. DYKES, JIM A. BAKER, and ANN E. AULABAUGH Department of Chemistry, Wright State University, Dayton, Ohio 45431
The objective of this research has been the development of controlled-release herbicides that will afford extended control of aquatic weeds (1,2). One approach to these materials has been the synthesis of polymers that contain herbicides as pendent substituents (3-5). For example, homopolymers have been prepared that consist of over 80% 2,4-dichlorophenoxyacetic acid (2,4-D) or 2-(2,4,5-trichlorophenoxy)propionic acid (Silvex) as pendent side chains. It was postulated that the herbicide would be released from these systems by the slow, sequential hydrolysis of the herbicide-polymer bonds. Hydrolysis studies, however, showed that the homopolymers will not undergo hydrolysis under mildly alkaline conditions at 30° (6).
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The reactivity of substituents attached to the backbone of a polymer chain is known to be substantially increased by the presence of suitable neighboring groups (7). For example, the hydrolysis of pendent ester groups is enhanced by the incorporation of carboxyl groups along the backbone (8-11). In an attempt to similarly facilitate the hydrolysis of herbicide-polymer bonds, copolymers were prepared that contained pendent herbicide esters and carboxyl groups. Copolymers containing herbicides and hydrophilic aminimide residues were also prepared. Thus, the copolymer!zation of the 2-acryloyloxyethyl esters of 2,4-D (la) and Si 1 vex (lb) with methacrylic acid (II) and trimethyl ami ne methacrylimide (IV) afforded the polymers III and V. A preliminary hydrolysis study of polymer Ilia containing 20 mole percent methacrylic acid indicated that the copolymer undergoes relatively rapid hydrolysis at pH 8 and 30°. (A 1-g sample of the copolymer released 117 mg of 2,4-D in the first 6 days of the study.) Copolymers containing the aminimide residue, however, hydrolyzed slowly under these conditions. (A 1-g sample of Va containing 35 mole percent IV released 18 mg of 2,4-D in 32 days.) Decreasing the percentage of IV in Copolymer Va resulted in a decrease in the rate of hydrolysis (6). This communication describes the preparation and subsequent hydrolyses of samples of copolymer Ilia which contain reduced amounts of methacrylic acid. The final results of the study of the hydrolysis of copolymer Va are also presented. In addition, a copolymer of 2-methacryloyloxyethyl 2,4-dichlorophenoxyacetate and methacrylic acid has been prepared and subjected to mild hydrolysis conditions. Results and Discussion Copolymers of 2-Acryloyloxyethyl 2,4-Dichlorophenoxyacetate and Methacrylic Aci
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p a r t i c l e s , which e v e n t u a l l y prevented f u r t h e r s p e c t r o s c o p i c a n a l y s i s o f the s o l u t i o n s . In f a c t , the l a s t p o i n t on the h y d r o l y s i s curve o f the 90:10 2-acryoyloxyethyl 2 , 4 - d i c h l o r o phenoxyacetate: m e t h a c r y l i c a c i d copolymer may be high due to the c l o u d i n e s s o f the a n a l y t i c a l samples. One o f the r e p l i c a t e s o f t h i s copolymer was a c i d i f i e d and e x t r a c t e d with ether a f t e r 296 days. Approximately 290 mg o f pure 2 , 4 - D , which represents 87% o f the 2,4-D o r i g i n a l l y contained i n the polymer, was recovered. As can be seen i n Figure 1, the r a t e o f h y d r o l y s i s o f both copolymers increased with time. This i s s i m i l a r to the a u t o a c c e l e r a t i o n i n r a t e observed i n the h y d r o l y s i s o f polyacrylamides (12) and p o l y ( v i n y l acetate) (13). In these c a s e s , the a c c e l e r a t i o n has been a s c r i b e d to i n t r a m o l e c u l a r i n t e r a c t i o n s o f n e i g h boring groups generated during the h y d r o l y s e s . The r a p i d h y d r o l y s i s o f e s t e r groups incorporated along a p o l y ( a c r y l i c a c i d ) chain has a l s o been a t t r i b u t e d to a neighboring group e f f e c t (8). The enhanced h y d r o l y s i s i s thought to be due to an i n t e r n a l n u c l e o p h i l i c attack o f a neighboring carboxylate ion on the carbonyl carbon o f the e s t e r group, with the formation o f a
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six-membered a c i d anhydride intermediate being r a t e determining. Considerable subsequent work has s u b s t a n t i a t e d t h a t i n t e r m o l e c u l a r r e a c t i o n s are p r a c t i c a l l y n e g l i g i b l e i n comparison to the i n t r a molecular f u n c t i o n a l i n t e r a c t i o n s between an e s t e r group and a d i r e c t l y v i c i n a l a c i d or carboxylate group. S t e r i c hindrance, however, can reduce or prevent the neighboring group e f f e c t (14). In the present s t u d y , the i n t e r a c t i o n o f a neighboring carboxyl group should r e s u l t i n the generation o f 2-hydroxyethyl 2,4-dichlororophenoxyacetate (VI). Nearly q u a n t i t a t i v e amounts o f
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pure 2 , 4 - D , however have been e x t r a c t e d from the h y d r o l y s i s solutions. I t i s p o s s i b l e t h a t compound VI undergoes subsequent h y d r o l y s i s to a f f o r d 2,4-D and ethylene g l y c o l . Preliminary analyses o f the sample s o l u t i o n s f o r ethylene g l y c o l i n d i c a t e t h a t only 20 to 40% o f the t h e o r e t i c a l amount i s present. The h y d r o l y s i s o f compound VI a t pH 8 i s c u r r e n t l y being i n v e s t i g a t e d . Another p o s s i b l e explanation f o r the a c c e l e r a t i o n i n r a t e i n v o l v e s i n t r a m o l e c u l a r i n t e r a c t i o n s o f carboxyl and e s t e r groups t h a t are not l o c a t e d on neighboring residues i n the polymer chain. In t h i s c a s e , the r a t e o f h y d r o l y s i s r e f l e c t s the p r o b a b i l i t y o f chain conformations t h a t b r i n g the two i n t e r a c t i n g groups i n t o j u x t a p o s i t i o n and, hence, i s r e l a t e d to chain f l e x i b i l i t y and m o b i l i t y . The enhanced h y d r o l y s i s o f acrylamide terpolymers c o n t a i n i n g pendent e s t e r and c a t a l y t i c s u b s t i t u e n t s has been shown to be due to such i n t e r a c t i o n s (15). It has a l s o been demonstrated t h a t i n order f o r the c a t a l y s i s " to occur the e s t e r group must not be s t e r i c a l l y hindered. Thus, e s t e r groups t h a t are l o c a t e d several atoms away from the terpolymer backbone hydrolyze r a p i d l y while those attached d i r e c t l y to the backbone hydrolyze a t the normal i n t e r m o l e c u l a r r a t e . The polymers i n the present study underwent considerable s w e l l i n g and gradual d i s s o l u t i o n as the h y d r o l y s i s proceeded. Hence, the p r o b a b i l i t y o f i n t r a m o l e c u l a r i n t e r a c t i o n s between d i s t a n t e s t e r and carboxyl groups was i n c r e a s e d . Due to s t e r i c e f f e c t s , the carboxyl groups should only attack the 2,4-D e s t e r
c a r b o n y l s , which would r e s u l t i n the r e l e a s e o f pure 2,4-D. The presence o f more 2,4-D than ethylene g l y c o l i n the h y d r o l y s i s mixtures i n d i c a t e s t h a t considerable h y d r o l y s i s d i d occur at the 2,4-D-polymer e s t e r bond. It i s postulated t h a t the i n i t i a l h y d r o l y s i s o f the copolymers i s due to neighboring group i n t e r a c t i o n s with subsequent h y d r o l y s i s r e s u l t i n g from the i n t e r a c t i o n s o f d i s t a n t groups. Copolymer o f 2-Methacry1oyloxyethyl 2,4-Dichlorophenoxyacetate and M e t h a c r y l i c A c i d . The 2-methacryloyloxyethyl e s t e r
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o f 2,4-D (VII) was prepared from 2,4-dichlorophenoxyacetyl c h l o r i d e and 2-hydroxyethyl methacrylate by the p r e v i o u s l y described procedure (2). The copolymer!zation o f t h i s monomer with 10 mole percent m e t h a c r y l i c a c i d by the procedure described f o r the a c r y l i c e s t e r afforded a 65% y i e l d o f V I I I . The white copolymer was s o l u b l e i n c h l o r i n a t e d hydrocarbons and had an inherent v i s c o s i t y o f 0.14 (0.5 g/dl i n 2-butanone at 3 0 ° ) .
Samples o f VIII were a l s o irnnersed i n a pH 8 buffer maintained at 3 0 ° . The copolymer, however, d i d not undergo h y d r o l y s i s under these m i l d c o n d i t i o n s . T h i s decrease i n r e a c t i v i t y may be due to increased s t e r i c hindrance i n the formation o f the proposed c y c l i c - a n h y d r i d e i n t e r m e d i a t e . In the m e t h a c r y l i c copolymers, the six-membered r i n g i s t e t r a s u b s t i t u t e d i n the 1,1' and 3 , 3 ' p o s i t i o n s which r e s u l t s i n considerable
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s t e r i c hindrance between the two a x i a l s u b s t i t u e n t s . This argument has been used to e x p l a i n the f a c t that m e t h a c r y l i c a c i d methyl methacrylate copolymers hydrolyze about 10-12 times more slowly a t 110° than a c r y l i c acid-methyl a c r y l a t e copolymers with the same molar composition (16). Copolymer o f 2 - A c r y l o y l o x y e t h y l 2,4-Dichlorophenoxyacetate and Trimethylamine Methacrylimide. The p r e v i o u s l y described study o f the h y d r o l y s i s o f copolymer Va c o n t a i n i n g 35 mole percent trimethylamine methacrylimide was completed ( 6 ) . The data shown i n Figure 2 were obtained by employing the procedures used with the m e t h a c r y l i c a c i d copolymers. The study was terminated a f t e r 394 days at which time the copolymer had released 250 mg o f 2,4-D per 0 . 5 - g sample, which corresponds to
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90% o f the amount o r i g i n a l l y present. As can be seen from the F i g u r e , the r a t e o f h y d r o l y s i s increased during the f i r s t few days o f the study and then remained r e l a t i v e l y c o n s t a n t . Since the c o n c e n t r a t i o n o f e s t e r linkages decreases as the polymer i s h y d r o l y z e d , the r a t e constant f o r the h y d r o l y s i s must c o n t i n u a l l y increase. T h i s behavior, i n e f f e c t , provides a z e r o - o r d e r r a t e o f r e l e a s e , which i s i d e a l l y s u i t e d f o r c o n t r o l l e d - r e l e a s e applications. The increase i n the r a t e constant as the h y d r o l y s i s p r o ceeds may a l s o be due to i n t r a m o l e c u l a r c a t a l y s i s . Infrared data i n d i c a t e s t h a t amine acylimides have a reasonance s t a b i l i z e d s t r u c t u r e as shown (17). Hence, i t i s p o s s i b l e that the
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aminimide group acts as a c a t a l y s t i n the same manner as d e s c r i b e d f o r the carboxyl group. The mechanism by which c o polymer Va undergoes h y d r o l y s i s i s c u r r e n t l y being i n v e s t i g a t e d . Experimental U l t r a v i o l e t s p e c t r a were obtained with a Gary Model 14 spectrophotometer. Infrared spectra were obtained on t h i n f i l m s with a Perkin-Elmer Model 457 spectrophotometer. Viscosities were determined with a Cannon Number 75 viscometer. The aminimide monomer was furnished by Ashland Chemicals, Columbus, Ohio. General S o l u t i o n Copolymerization Procedure. Herbicide monomer, comonomer, 2-butanone (4 ml/g o f monomers), and 0.05% AIBN were thoroughly mixed and slowly heated under n i t r o g e n to 75°. A f t e r heating at 75° f o r 3 h r , the mixture was c o o l e d , d i l u t e d with 2-butanone, and p r e c i p i t a t e d i n hexane. The copolymer was c o l l e c t e d by f i l t r a t i o n and d r i e d under vacuum at 60° f o r 3 h r . Hydrolysis Studies. The copolymers were e x t r a c t e d with ether f o r 18 hr to remove unreacted monomer, d r i e d under vacuum, and then ground and sieved to a p a r t i c l e s i z e o f 125-400y. Three 0 . 5 - g samples o f each copolymer were placed i n 500-ml erlenmeyer f l a s k s c o n t a i n i n g 300 ml o f a b o r i c acid-sodium hydroxide buffer (pH = 8 . 0 8 ) . The f l a s k s were maintained a t 30 + 0.1° i n a constant temperature bath. The amount o f h e r b i c i d e r e l e a s e d from each copolymer was determined p e r i o d i c a l l y by spectrophotometric a n a l y s i s at 198 nm.
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Acknowledgement Support o f t h i s research by the Department o f the Army, U.S. Army Engineer Waterways Experiment S t a t i o n under Contract DACW39-86-C-0016 (Neg.) i s g r a t e f u l l y acknowledged. Appreciation i s a l s o expressed to AmChem Products, Inc. f o r f u r n i s h i n g the h e r b i c i d e s and to Ashland Chemicals Company f o r f u r n i s h i n g the aminimides used i n t h i s study.
Literature Cited 1. 2.
3. 4. 5.
6.
7. 8. 9. 10. 11. 12. 13. 14.
Harris, Frank W., Norris, Steve O., and Post, Larry Κ., Weed Science, (1973), 21 (4), 318. Harris, Frank W. and Post, Larry K. in "Proceedings 1974 International Controlled Release Pesticide Symposium," Cardarelli, Nate F., Ed., The University of Akron, Ohio, 1974. Harris, Frank W., and Post, Larry Κ., Am. Chem. Soc., Polym. Div., Preprints, (1975), 16 (2), 622. Harris, Frank W. and Post, Larry K., J. Polym. Sci., Polymer Letters Ed., (1975), 13, 225. Harris, Frank W., Feld, William Α., and Bowen, Bonnie, in "Proceedings 1975 International Controlled Release Pesticide Symposium," p. 334 Harris, F.W., Ed., Wright State University, Dayton, Ohio, 1975. Harris, Frank W., Aulabaugh, Ann Ε., Case, Robert D., Dykes, Mary Κ., and Feld, William Α., "ACS Symposium Series, No. 33, Controlled-Release Polymeric Formulations," p. 222, Paul, D.R. and Harris, F.W., Ed., American Chemical Society, Washington, D.C., 1976. Morawetz, Η., "Macromolecules in Solution," pp. 422-426, Wiley, New York, 1965. Morawetz, H., and Zimmering, P.E., J. Phys. Chem., (1954), 58, 753. Gaetjens, E. and Morawetz, J. Amer. Chem. Soc., (1961), 83, 1738. Morawetz, Η., and Westhead, E.W., J . Polym. S c i . , (1955), 16, 273. VanBeylen, M.M. in "Stereochemistry of Macromolecules," Vol. 3, p. 335, Ketley, A.D., Ed., Marcel Dekker, New York, 1968. Smets, G. and Hesbain, A.M., J. Polym. S c i . , (1959), 40, 217. Sakurada, I. and Sakaguchi, Y., Kobunshi Kagaku, (1956), 13, 441; Chem. Abst., (1957), 51, 17365 g. Smets, G. and VanHumbeeck, W., J . Polym. S c i . , (1963), 1, 1227.
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15. Goodman, N. and Morawetz, H., J . Polym. S c i . , Part C, (1970), 31, 177. 16. Smets, G., and DeLoecker, W., J . Polym. S c i . , (1960), 45, 461. 17. McKillip, W.J., Sedor, E.A., Culbertson, B.M., and Wawzonek, S., Chem. Rev., (1973), 73, 255.
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