Controlled Release Pesticides - ACS Publications

11 Synthesis, Characterization, and Release Mechanisms of Polymers Containing Pendant Herbicides CHARLES L. McCORMICK and MICHAEL FOOLADI Downloaded by EAST CAROLINA UNIV on April 22, 2017 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0053.ch011

Department of Polymer Science, University of Southern Mississippi, Hattiesburg, Miss. 39401

As the w o r l d ' s population r a p i d l y i n c r e a s e s , a g r i c u l t u r e i s faced with the demand f o r enhanced production u t i l i z i n g chemicals with l i t t l e or no detrimental e f f e c t on the surrounding e n v i r o n ment. P e s t i c i d e leaching i n t o drainage waters and subsequent t r a n s p o r t i n t o non-target areas i s o f growing e c o l o g i c a l concern. The immensity o f the problem i s apparent when c o n s i d e r i n g the t o t a l a g r i c u l t u r a l p e s t i c i d e a p p l i c a t i o n and the annual r u n - o f f f o r drainage a r e a s . The M i s s i s s i p p i Watershed area alone covers 1,244,000 square miles i n c l u d i n g vast s t r e t c h e s o f c e n t r a l U.S. farmland. The annual water discharge a t the mouth o f the M i s s i s s i p p i has been estimated to 7.8 χ 1 0 y d s and 2,000,000 tons o f sediment are c a r r i e d i n t o the sea per day. The average annual r a i n f a l l over t h i s area i s about 30 i n c h e s , o f which about one-fourth t r a v e l s to the G u l f o f Mexico by way o f the M i s s i s s i p p i River ( 1 , 2 ) . In 1974, nearly 1.4 b i l l i o n pounds o f organic p e s t i c i d e s were s o l d by U.S. companies, representing a growth o f 12% over 1973. I n s e c t i c i d e s accounted f o r 50.4% o f the t o t a l volume with the balance c o n s i s t i n g o f h e r b i c i d e s , f u n g i c i d e s , and p l a n t hormones (3). With some p e s t i c i d e systems, 70 - 80% of the useful chemical a c t i v i t y i s l o s t by various mechanisms i n c l u d i n g i n t e r a c t i o n with non-target organisms. S c i e n t i s t s have measured the rates o f loss o f a c t i v i t y o f various chemicals i n terms o f "persistence" levels. P e r s i s t e n t p e s t i c i d e s have been attacked i n environmental studies due t o t h e i r usual migration to nont a r g e t areas. However, i t should be pointed out that some degree o f persistence i s necessary t o y i e l d weed, i n s e c t or fungus c o n t r o l f o r a reasonable period o f time i n the t a r g e t area. Often the most p e r s i s t e n t chemicals are a l s o the most effective. Several f a c t o r s are known t o determine p e r s i s t e n c e i n the soil. These include (a) uptake and degradation by microorgan isms, (b) l o s s through p h y s i c a l processes o f v o l a t i l i z a t i o n and l e a c h i n g , and (c) chemical changes such as photo-decomposition and chemical reactions (4). 11

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Scher; Controlled Release Pesticides ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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The Environmental P r o t e c t i o n Agency i s imposing s t r i n g e n t requirements on several e f f e c t i v e and p r e v i o u s l y widely used pesticides. The t y p e , amount a p p l i e d , s p e c i f i c i t y , and p e r s i s t e n c e o f each p e s t i c i d e w i l l be under continuing s c r u t i n y . The pest c o n t r o l agents must not merely c o n t r o l t a r g e t organisms but must be harmless to humans, l i v e s t o c k , c r o p s , f i s h , w i l d l i f e , b e n e f i c i a l i n s e c t s , s o i l microorganisms, e t c . Dramatic improvements i n a n a l y t i c a l instrumentation have allowed claims o f d e t e c t i o n o f t r a c e amounts of organic chemicals i n non-target areas i n the parts per b i l l i o n range. T h i s a d vance, i n conjunction with the controversy generated by adverse p u b l i c i t y on i n s e c t i c i d e s such as DDT and Mi rex, has led to a f l u r r y o f experiments on n e a r l y every chemical manufactured i n the U.S. P a r t i c u l a r emphasis has been placed on chemicals having p o t e n t i a l impact on the aquatic environment. For example, B u t l e r ( 5 - 1 2 ) has reported growing evidence that "the c o n t i n u i n g use of p e s t i c i d e chemicals i s producing environmental changes or residues i n the food web that may cause reproductive f a i l u r e . . " Some organisms have been shown to accumulate or concentrate c e r t a i n p e r s i s t e n t p e s t i c i d e s at alarming r a t e s . The o y s t e r , f o r example, when continuously exposed to 0.1 ppb o f DDT, was r e ported to concentrate i n i t s t i s s u e s up to 7.0 ppm i n a month. It may be p r e d i c t e d that c h l o r i n a t e d h e r b i c i d e s w i l l soon come under attack ( 1 3 , 2 8 - 3 0 ) . S t r i n g e n t r u l e s and r e g u l a t i o n s (apparently subject to frequent m o d i f i c a t i o n ) have been imposed on a g r i c u l t u r a l chemical producers and consumers as a r e s u l t of environmental s t u d i e s . Many knowledgeable sources p r e d i c t an impending d i s a s t e r f o r the whole a g r i c u l t u r a l i n d u s t r y from the high costs o f l i c e n s i n g , r e g i s t r a t i o n , and production of new p e s t i c i d e s . In 1976, new p e s t i c i d e commercialization required an average of 2 . 5 years of research and development at a cost o f over $10,000,000.00. The a g r i c u l t u r a l i n d u s t r y has, i n g e n e r a l , responded to the e n v i r o n mental r e g u l a t i o n s by producing l e s s p e r s i s t a n t but often l e s s e f f e c t i v e p e s t i c i d e s , r e q u i r i n g more frequent a p p l i c a t i o n over an extended growing season. A more l o g i c a l and c e r t a i n l y more f r u i t f u l approach i s to attack the undesired " l e a c h i n g " or t r a n s p o r t of a given p e s t i c i d e r a t h e r than i t s " p e r s i s t e n c e . " A g r i c u l t u r a l chemical leaching and subsequent p e s t i c i d e t r a n s p o r t to non-target environments can be g r e a t l y reduced, p o s s i b l y e l i m i n a t e d , by c o n t r o l l e d - r e l e a s e systems based on macromolecules. Polymers can be synthesized which contain r e a c t i v e chemical bonds to common p e s t i c i d e s ; these bonds are subject to enzymatic or h y d r o l y t i c break-down at a c o n t r o l l a b l e rate. The macromolecular nature of these systems w i l l prevent d i s s o l u t i o n , leaching and t r a n s p o r t to non-target areas. Cont r o l l e d - r e l e a s e can a l s o reduce the number of a p p l i c a t i o n s and the q u a n t i t y of chemical required f o r pest c o n t r o l . A number of n a t u r a l l y o c c u r r i n g polymers ( 2 5 - 2 7 ) o f f e r e x c e l l e n t p o t e n t i a l as raw m a t e r i a l s f o r substrate preparation o f c o n t r o l l e d r e l e a s e systems. In a d d i t i o n , c e r t a i n polysaccharides decompose y i e l d i n g



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products b e n e f i c i a l to the s o i l . Development of a commercial h e r b i c i d e system must combine e f f e c t i v e n e s s , favorable economics, with l i t t l e adverse e n v i r o n mental impact. The chemical must: (1) c o n t r o l weeds at reason able dosages, (2) s e l e c t i v e l y c o n t r o l target organisms o n l y , l e a v i n g b e n e f i c i a l i n s e c t s , p l a n t s , and humans unharmed, (3) p e r s i s t f o r a reasonable time, (4) be inexpensive f o r l a r g e s c a l e usage, and (5) and be e a s i l y a p p l i e d , (preferably with conventional equipment). Provided the above c r i t e r i a are met, p o t e n t i a l b e n e f i t s d e r i v e d from properly formulated c o n t r o l l e d - r e l e a s e systems include: (1) enhanced a g r i c u l t u r a l p r o d u c t i o n , (2) fewer a p p l i c a t i o n s , (3) l e s s environmental p o l l u t i o n and (4) reduced production costs to the farmer. Macromolecular

Design

Polymeric systems f o r c o n t r o l l e d - r e l e a s e of p e s t i c i d e s may be assigned to two broad c a t e g o r i e s . In the f i r s t , the p e s t i c i d e i s p h y s i c a l l y d i s s o l v e d , entrapped, or dispersed i n a polymer matrix. Chemical release i s g e n e r a l l y based on d i f f u s i o n phenomena (14-19, 34-39); however, chemical or b i o l o g i c a l e r o s i o n of the polymer matrix i s a l s o p o s s i b l e . In the second category, the p e s t i c i d e i s chemically bound (pendant) to the macromolecular backbone. Release i s then dependent on the r a t e o f chemical or b i o l o g i c a l break-down of the p o l y m e r - t o - p e s t i c i de (20-24, 31-39). Polymers c o n t a i n i n g pendant p e s t i c i d e s can be prepared by two s y n t h e t i c methods. The f i r s t involves bonding (via covalent o r i o n i c chemical bonds) of a p e s t i c i d e to a ρre-formed polymer. T h i s approach requires macromolecules with pendant f u n c t i o n a l groups capable of r e a c t i o n with p e s t i c i d e s or t h e i r d e r i v a t i v e s . The nature of the chemical bond may be v a r i e d to y i e l d bonds with q u i t e d i f f e r e n t rates of cleavage i n the environment. Ad vantages of t h i s method i n c l u d e : (a) a v a i l a b i l i t y of r e l a t i v e l y inexpensive polymers with b i o d e g r a d a b i l i t y such as c h i t i n , c e l l u l o s e , e t c . , and (b) use of commercially a v a i l a b l e p e s t i c i d e s as s t a r t i n g m a t e r i a l s i n polymer s y n t h e s i s . The second approach involves polymerization of monomeric pesticides. The major advantages of t h i s method l i e i n the a b i l i t y to c o n t r o l the molecular design of the polymer and the pesticide/polymer weight r a t i o . b

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Experimental Preformed, hydroxy-containing polymers were s e l e c t e d f o r i n i t i a l study. Three polymers - p o l y v i n y l a l c o h o l , c h i t i n , and c e l l u l o s e were chose on the basis o f : (a) p o t e n t i a l biode g r a d a b i l i t y , (b) commercial a v a i l a b i l i t y , and (c) h y d r o p h i l i c i t y i n a d d i t i o n to having proper pendant f u n c t i o n a l i t y . The r e s u l t s o f the experiments on p o l y v i n y l alcohol are reported i n t h i s work



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Metribuzin was chosen as a model p e s t i c i d e based on: (a) a v a i l a b l e amine f u n c t i o n a l i t y , (b) high a c t i v i t y at r e l a t i v e l y low c o n c e n t r a t i o n s , (c) s e l e c t i v i t y , (d) lack o f p e r s i s t e n c e i n the environment, and (e) high m o b i l i t y . A s e r i e s of laboratory and commercial polymers o f p o l y v i n y l a l c o h o l (with varying r e s i d u a l amounts of unhydrolyzed v i n y l acetate) were c a r e f u l l y c h a r a c t e r i z e d . Isocyanate adducts of metribuzin were prepared and reacted with the pendant hydroxy! f u n c t i o n a l i t y o f the pre-formed polymers (Figure 1). It was p o s s i b l e to prepare copolymers with varying degrees o f s u b s t i t u t i o n on l i n e a r and h i g h l y c r o s s - l i n k e d c h a i n s . The isocyanate to hydroxy1 r a t i o s were v a r i e d over a wide range to prepare solvent s w o l l e n , c r o s s - l i n k e d g e l s . These were converted to microporous s o l i d s by a g i t a t i o n of the product i n the presence of a non-solvent ( s e l e c t e d from s o l u b i l i t y parameter data). Rates o f Release of Metribuzin Polymers with pendant metribuzin (0.100 g) were placed i n an Erlenmeyer f l a s k . 500 ml o f d i s t i l l e d water was added. At designated i n t e r v a l s , samples were taken to determine the c o n c e n t r a t i o n o f released m e t r i b u z i n . U l t r a v i o l e t Spectroscopic Method. A Gary 1756 Spectrophoto meter was used to determine released metribuzin l e v e l s i n water. A standard p l o t of absorbance v s . concentration was obtained using l e a s t squares a n a l y s i s . 3 ml samples were taken at d e s i g nated i n t e r v a l s and placed i n standard quartz c e l l s . The ab sorbance at 293.5 nm was monitored i n two types o f t e s t s . The f i r s t measured t o t a l concentration of released metribuzin over a time p e r i o d . The second t e s t was conducted as f o l l o w s : (a) 0.100 g samples were placed i n 500 ml o f d i s t i l l e d water f o r a predetermined time; (b) the samples were f i l t e r e d , d r i e d and again placed i n a second Erlenmeyer f l a s k c o n t a i n i n g 500 ml o f d i s t i l l e d water; (c) concentrations were measured d i r e c t l y from the f i l t r a t e . Gas Chromatographic Method. 2yl of aqueous s o l u t i o n were removed and e x t r a c t e d with 5.0 ml o f benzene. 1 μΐ o f the benzene phase was then i n j e c t e d i n t o the gas chromatograph (Micro-Tek 220 with e l e c t r o n capture d e t e c t o r ) . S o i l M o b i l i t y Studies T h i n - l a y e r p l a t e s were prepared by spreading a s o i l s l u r r y onto 20 X 20 cm glass p l a t e s to a thickness of 1.0 mm. Plates were d i v i d e d i n t o three equal s e c t i o n s by s c r i b i n g the s o i l layer. Metribuzin was a p p l i e d to one p l a t e by s t r e a k i n g 500 λ o f a 100 g/ml s o l u t i o n onto each s e c t i o n o f the p l a t e 2 cm from the bottom. Polymers c o n t a i n i n g pendant metribuzin were embedded i n



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the s o i l l a y e r on other p l a t e s which were a l s o d i v i d e d i n t o three sections. The p l a t e s were e l u t e d to 10 cm with water, a i r d r i e d , and 1-cm zones were removed from one of the three s e c t i o n s of each p l a t e . The p l a t e s were returned to the chamber and again e l u t e d with 10 cm of water, and the second zone was removed i n 1-cm s e c t i o n s . T h i s procedure was repeated with the t h i r d s e c t i o n of s o i l . The s o i l removed i n t h i s manner was e x t r a c t e d with 5 ml of hexane: acetone (3:1) by shaking. E x t r a c t was analyzed by gas chromatography.


Residual P h y t o t o x i c i t y Of Metribuzin From Polymers The polymers c o n t a i n i n g pendant metribuzin were added to the surface of a Bosket sandy loam s o i l contained i n 4" p l a s t i c pots i n a c o n t r o l 1ed-environment chamber. The a p p l i c a t i o n rates were 0, 0 . 1 , 0 . 2 , and 0.3 g of each formulation. A commercial formul a t i o n of metribuzin was a p p l i e d to other pots at 0.5 and 1.0 ppmw, and thoroughly mixed i n t o the s o i l . The s o i l s were b i o a s sayed over a period of 112 days with a mixture of weeds which are normally s u s c e p t i b l e to the h e r b i c i d e ; a f t e r growing two weeks, the weeds were harvested and f i r s t weights recorded. Results And Discussion Five polymers c o n t a i n i n g pendant metribuzin were chosen f o r study: 22-S, 23-S, 41-S, 45-S, and 50-S. 23-S, 4 5 - S , and 50-S were e s s e n t i a l l y l i n e a r polymers prepared from 99% hydrolyzed polyvinyl alcohol. 22-S and 41-S were h i g h l y c r o s s - l i n k e d m i c r o porous s o l i d s . These system r e q u i r e both h y d r o l y s i s of the urea bond and d i f f u s i o n from a water s w o l l e n , c r o s s - l i n k e d matrix f o r metribuzin r e l e a s e . P l o t s of s o l u t i o n concentration vs. time (Figures 2, 3) i n d i c a t e d that the l i n e a r polymers (23-S, 4 5 - S , and 50-S) r e leased h e r b i c i d e much more r a p i d l y than the c r o s s - l i n k e d systems. The 23-S, 45-S, and 50-S were c h a r a c t e r i z e d by a r a p i d i n i t i a l r e l e a s e i n the f i r s t few hours followed by a more gradual r a t e l a s t i n g several days. The c r o s s - l i n k e d systems 22-S and 41-S (Figure 4) had much lower r e l e a s e rates with l i t t l e i n i t i a l r e lease. T h i s could be p r e d i c t e d by the time required f o r s w e l l i n g o f the h y d r o p h i l i c polymer so that h y d r o l y s i s and d i f f u s i o n could occur. A f t e r s w e l l i n g , s l i g h t concentration increases were noted. The u.v. s p e c t r o s c o p i c data and the gas chromatographic data were i n t e r n a l l y c o n s i s t e n t . It should be noted that the u l t r a v i o l e t technique requires no e x t r a c t i o n and, t h e r e f o r e , o f f e r s less chance f o r e r r o r at small concentrations of metribuzin. S o i l t h i n - l a y e r chromatographic techniques showed metribuzin (Figure 5) moved as a normal chromatogram peak with each s u c c e s s i v e e l u t i o n moving the peak nearer the 10-cm zone. The chromatograms from 23-S (Figure 6) and 45-S (Figure 7) showed " s t r e a k i n g " continuously along the p l a t e i n d i c a t i n g a sustained r e l e a s e mechanism.· The c r o s s - l i n k e d f o r m u l a t i o n s , 22-S and 45-S,



Figure 2. Metribuzin release from polymers in water (ultraviolet spectroscopy)


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Figure 8. Phytotoxicity of metribuzin from polymer formulations as a function of time


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did not r e l e a s e enough metribuzin f o r a measurable rate i n these studies. Residual p h o t o t o x i c i t y of the four polymeric systems i s i l l u s t r a t e d i n Figure 8. Metribuzin at 1.0 ppmw had d i s s i p a t e d to a l e v e l which was e s s e n t i a l l y n o n - t o x i c a f t e r 78 days. Likew i s e , p h y t o t o x i c i t y from 41-S had diminished to a large extent by t h i s time. A r e l a t i v e l y low l e v e l of p h y t o x i c i t y was observed f o r 22-S i n i t i a l l y ; however, t h i s same l e v e l was maintained f o r over 78 days, then r a p i d l y decreased. The highest l e v e l of p h y t o t o x i c i t y was observed with 23-S and 45-S. These m a t e r i a l s were s t i l l showing p h y t o t o x i c i t y at our l a s t t e s t date of 112 days. It must be noted that p h y t o t o x i c i t y comparison t e s t s of polymeric c o n t r o l l e d - r e l e a s e formulations and commercially formulated h e r b i c i d e s must be i n t e r p r e t e d with c a r e . In the pendant polymeric systems, h e r b i c i d e s are not phytotoxic u n t i l bond cleavage has occurred. For t h i s reason the t o t a l h e r b i c i d e e v e n t u a l l y a v a i l a b l e i n the polymer cannot be compared to t h a t immediately a v a i l a b l e i n a commercial f o r m u l a t i o n . Conclusions Polymeric systems f o r c o n t r o l l e d release of metribuzin have been prepared using biodegradable s u b s t r a t e s . Properly s e l e c t e d macromolecular substrates were reacted with p e s t i c i d e adducts to y i e l d systems with l a b i l e p e s t i c i de-to-polymer bonds s u s c e p t i b l e to chemical or enzymatic h y d r o l y s i s . The metribuzin/polyviny1 alcohol system i n t h i s work i s adaptable f o r formation of a range of products with d i f f e r e n t degrees of c r o s s - l i n k i n g and, t h e r e f o r e , d i f f e r e n t rates of herbicide release. P h y t o t o x i c i t y , s o i l t h i n - l a y e r chromatography, u l t r a v i o l e t spectroscopy, and gas chromatography t e s t s showed sustained r e l e a s e c a p a b i l i t i e s of the polymeric systems. The p r e l i m i n a r y r e s u l t s of t h i s research p o i n t to the immense p o t e n t i a l of polymeric systems f o r c o n t r o l l e d - r e l e a s e of s e l e c t i v e h e r b i c i d e s which can: (1) reduce environmental p o l l u t i o n i n non-target areas by reducing p e s t i c i d e m o b i l i t y , (2) r e q u i r e fewer a p p l i c a t i o n s during the growing season, and (3) r e s u l t i n enhanced a g r i c u l t u r a l production a t , perhaps, lower c o s t to the farmer. Acknowledgements The authors would l i k e to express t h e i r thanks f o r the generous support of research conducted at the U n i v e r s i t y of Southern M i s s i s s i p p i Polymer Science Laboratories provided by Hopkins A g r i c u l t u r a l Chemical Company of Madison, Wisconsin. Research support was a l s o obtained from the USDA Weed Science Laboratories at S t o n e v i l l e , M i s s i s s i p p i , and from the M i s s i s s i p p i Alabama Sea Grant Program. The s o i l studies were conducted by


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PESTICIDES

Κ. E. Savage, Southern Weed Science L a b . , ARS, USDA, S t o n e v i l l e , MS, 38776.


Abstract Recently, there has been a growing interest in developing pesticide controlled release technology. Much of the impetus has resulted from demands for enhanced agricultural production at lower levels of environmental risk. Most of the activity has been directed toward formulations in which the pesticide is physically dissolved or dispersed in a polymer matrix. Polymers have been prepared in our laboratories which contain labile polymer to pesticide covalent bonds. These linkages are susceptible to aqueous and/or bacterial break-down, resulting in long-term release. Theoretically, the rate of herbicide release can be controlled by changing the nature of the labile bonds or by altering the cross-link density of the polymer. The synthe sized systems have been characterized by IR, NMR, U.V., GPC, etc. Release studies have been conducted in aqueous media using U.V. and gas chromatography. In addition, soil mobility and phyto toxicity studies are in progress. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Barrett, Β. Β., Louisiana Wildlife and Fisheries Commission Report, Cooperative Gulf of Mexico Estuarine Inventory and Study, Hydrology, p. 115, 1971. Christmas, J.Y., Ed., Cooperative Gulf of Mexico Estuarine Inventory and Study, Mississippi, Publishers: Gulf Coast Research Laboratory, p. 12, 1973. Chemical and Engineering News, July 28, 1975, pp. 18-31. Kearney, R.C. and Kaufman, D.D., Herbicides, Chemistry, Degradation, and Mode of Action, 2nd Edition, Marcel Dekker, Inc., 1975. Butler, P.A., Pesticide Monitoring Journal, Vol. 6(4) 238, 1973. Butler, P.Α., "Pesticides in the Estuary," Proc. of Marsh and Estuary Management Symposium (July 1967), Baton Rouge, LA, pp. 120-124, 1968. Butler, P.A., Proc. of National Symposium on Estuarine Pollution, (August, 1967) Stanford University, p. 107, 1968. Butler, P.Α., "Significance of DDT Residue in Estuarine Fauna," Chemical Fallout, Chapter 9, pp. 205-220, 1969. "Pesticides in the Marine Environment," Journal Appl. Ecology 3, (Supplement) pp. 253-259, 1966. "Problems of Pesticides in Estuaries," American Fish Soc., SPE Public No. 3, pp. 110-115, 1966. Firth, F.E., Ed., "Pesticides in the Sea," Encyclopedia of Marine Resources, pp. 513-516. Butler, P.Α., "Pesticides," U.S. Bureau of Commercial Fisheries Report: Contract No. 85, 1967.


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13. Andus, L.J., Ed., The Physiology and Biochemistry of Herbicides, Academic Press, New York, 1964. 14. U.S. Patent #3,074,845. 15. U.S. Patent #3,318,769. 16. U.S. Patent #3,737,521. 17. U.S. Patent #3,127,752. 18. U.S. Patent #3,400,093. 19. U.S. Patent #3,343,941. 20. Allan, G.C., et al. Nature, 234, 349 (1971). 21. Neogi, A.N., Ph.D., Thesis University of Washington, Seattle, Washington, (1970). 22. Jakube, H.D., Busch, E., F. Chem, 13 (3), p. 105 (1973). 23. Allan, G.C., et al. Int. Pest Control, 14 (2), p. 15 (1972). 24. Harris, F.W. and Post, L.K., Polymer Preprints, 16 (1), pp. 622-627, (1975). 25. Pariser, E.R. and Block, S., Chitin and Chitin Derivatives, Report No. MITSG 73-2, October 15, 1972, (A bibliography with 593 references). 26. Davidson, R.L. and Sittig, Marshall, Eds., Water-Soluble Resins, Van Nonstrand Reinhold Co., New York, 1968. 27. Bikales, N.M., Water-Soluble Polymers, Plenum Press, New York, 1973. 28. O'Brien, R.D., Insecticides, Action and Metabolism, Academic Press, New York, 1967. 29. White-Stevens, R., Pesticides in the Environment, Marcel Dekker, 1973. 30. Chemical Engineering, January 19, 1976. 31. Allan, G.C., Canadian Patent #846785. 32. Allan, G.C., Canadian Patent #863310. 33. Allan, G.C., Canadian Patent #855181. 34. Chemical and Engineering News, June 28, 1976. 35. Controlled Release Pesticide Symposium, The University of Akron, September, 1974. 36. Proceedings 1975 International Controlled Release Pesticide Symposium, Wright State University, September, 1975. 37. Proceedings 1976 Controlled Release Pesticide Symposium, The University of Akron, September, 1976. 38. Cardarelli, N., Controlled Release Pesticides Formulations, CRC Press, Cleveland, Ohio, 1976. 39. Paul, D.R. and Harris, F.W., Eds., Controlled Release Polymeric Formulations, ACS Symposium Series; 33, 1976.


Controlled Release Pesticides - ACS Publications

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