Controlled Release Pesticides

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9 Controlled Release of Herbicides from Biodegradable Substrates G. G. ALLAN,* J. W. BEER, and N. J. COUSIN

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College of Forest Resources, University of Washington, Seattle, Wash. 98195

The controlled release concept now seems to have gained a firm foothold in pesticide technology as evidenced by the plethora of symposia which have been organized around the U.S. in recent years (1-3). Although many different controlled release techniques have been presented at these gatherings actually few have progressed beyond laboratory evaluation. Among those which have i s a con­ t r o l l e d release herbicide primarily intended as a reforestation aid (4). This is based on the chemical linkage of a herbicide to a biodegradable substrate by means of a hydrolyzable bond. After t h i s chemical combination is placed on the forest f l o o r , herbicide is slowly released to suppress the growth of competitive vegetation in the vicinity of the seedling for a period of time, measured in months or years. However, if this type of controlled release herbicide which has been successful in forestry is to be used in agriculture shorter periods of release w i l l be more appropriate. The design of such systems is now underway and t h i s paper describes an assessment of the effect of the degree of substitu­ tion on the rate of hydro l y t i c degradation of α-cellulose 2,4dichlorophenoxyacetate both in v i t r o and in νivo. Results and Discussion The α-ce I Iulose 2,4-d ichlorophenoxyacetates studied in t h i s project contained 4.1, 4.9 or 21.5$ by weight of combined herbi­ cide. The rates of hydrolysi s for these biocides in water at varying pH values are quite d i fferent. The data in Figure I shows that the release is dependent on both the degree of substitution and the acid ity or a l k a l i n i t y of the hydrolysis medium. Moreover, since the hydro Iysi s samples contained the same quantity of com­ bined active ingredient i t is clear that a faster release of *Address inquiries to Professor G. G. A l l a n , University of Wash­ ington, AR-10, Seattle, WA 98195. 94

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

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9.

ALLAN ET AL.

Herbicide

Release from

Riodegradables

95

h e r b i c i d e was o b t a i n e d w i t h t h e α-cellulose e s t e r h a v i n g t h e lower d e g r e e o f s u b s t i t u t i o n . T h i s p a t t e r n o f h e r b i c i d e r e l e a s e c a n be e x p l a i n e d i n t e r m s o f t h e m i c r o - s t r u c t u r e o f t h e α-cellulose f i b e r w h i c h c o n t a i n s d i s p e r s e d c r y s t a l l i n e (60%) and amorphous (40%) regions. In n o n p o l a r n o n s w e l I i n g r e a c t i o n media t h e l a t t e r p r o ­ v i d e s e s s e n t i a l l y a l l o f t h e r e a d i l y a v a i l a b l e r e a c t i v e hydroxy groups. I η u n s w o l I en n a t i v e e e l l u l o s e t h o s e amount t o on I y 0.4% o f t h e t o t a l f i b e r m a t e r i a l ( 5 ) . T h i s v a l u e i s i n c r e a s e d t o 0.54% as a r e s u l t o f t h e i s o I a t i o n p r o c e d u r e s wh i c h g e n e r a t e a b o u t o n e t h i r d more s u r f a c e a r e a (j3). T h u s , i t c a n be a n t i c i p a t e d t h a t e s t e r i f i c a t i o n r e a c t i o n s w i l l b e g i n r a n d o m l y on a c c e s s i b l e amor­ phous s u r f a c e s and s u b s e q u e n t l y s p r e a d a l o n g t h e p o l y s a c c h a r i d e chains. As a r e s u l t , t h e d e n s i t y and p a t t e r n o f s u b s t i t u e n t s w i l l v a r y t h r o u g h o u t t h e α-cellulose s u b s t r a t e ( I ) . T h i s v a r i a n c e i s compounded by t h e unequaI reactiν i t i e s o f t h e h y d r o x y g r o u p s o n each a n h y d r o g l u c o s e u n i t towards e s t e r i f i c a t i o n . For example, t o s y I a t i o n e x p e r i m e n t s have shown t h a t t h e C O , C 2 and C 3 h y d r o x y groups a r e e s t e r i f i e d i n t h e r a t i o of 2 1 5 : 3 3 : 1 r e s p e c t i v e l y (1). F u r t h e r m o r e , a k i net i c s t u d y d e m o n s t r a t e d t h a t t h e p r i m a r y hydroxy group r e a c t s f i f t y - e i g h t t i m e s f a s t e r than i t s secondary neighbors (8).

As a r e s u l t o f a l l t h e s e f a c t s , i t c a n be c o n c l u d e d t h a t e s t e r i f i c a t i o n r e a c t i o n s o f α-cellulose a t low d e g r e e s o f s u b s t i ­ t u t i o n wt I I random I y o c c u r a I most e x e l u s i v e l y on t h e f i b e r s u r ­ f a c e s a s s o c i a t e d w i t h t h e amorphous r e g i o n a t t h e p r i m a r y h y d r o x y g r o u p s o f t h e C§ c a r b o n atom o f e a c h g l u c o s e u n i t . It therefore f o l l o w s t h a t i n t h e α-ce I I u l o s e d e r i v a t i v e t h e r e w i l l be g r o u p s o f more o r l e s s c l o s e l y l o c a t e d e s t e r u n i t s s e p a r a t e d by a r e a s h a v i n g on I y an o c c a s i o n a I e s t e r s u b s t i t u e n t and hence a h y d r o p h i I i c c h a r ­ a c t e r c o m p a r a b l e t o t h e o r i g i n a l amorphous r e g i o n o f t h e α-cellu­ lose. Of c o u r s e , a s t h e d e g r e e o f s u b s t i t u t i o n i s i n c r e a s e d , t h e s i z e o f t h e s e h y d r o p h i I i c a r e a s must d e c r e a s e . Ultimately, the h y d r o p h i I i c i t y c a n become s o s m a l l t h a t w a t e r c a n n o t p e r m e a t e t h e poIymer m a t r i x ( 9 ) t o e f f e c t h y d r o l y t i c r e I e a s e o f t h e h e r b i c i de (10, II). T h i s s t r u c t u r a I p i c t u r e t h e r e f o r e p r o v i des a s a t i s f y i n g expIanation f o r h y d r o l y t i c r e l e a s e data i n Figure I s i n c e t h e more h y d r o p h i I i c α-cellulose e s t e r c a n be e x p e c t e d t o h y d r o l y z e a t a f a s t e r r a t e t h a n t h e more h y d r o p h o b i c p o I y m e r i c e s t e r . Ob­ v i o u s l y a def i η i ng mi η imum l i m i t must e x i s t where f u r t h e r d e c r e a s e s i n t h e amount o f p e n d a n t e s t e r l i n k a g e s w i l l n o t l e a d t o a f a s t e r r a t e o f h y d r o l y s i s . T h i s l e v e l r e m a i n s t o be e s t a b I i s h e d . In s p i t e o f t h e c l a r i t y o f t h e s e h y d r o l y t i c e x p e r i m e n t s , t h e a c t u a l b e h a v i o r o f t h e α-cellulose e s t e r i n s o i l w i l l be somewhat more c o m p l e x . C e r t a i n l y h y d r o l y t i c c l e a v a g e o f t h e p e n d a n t e s t e r w i l l o c c u r i n t h e s o i I b u t t h i s s c i s s i o n w i I I be s u p p I e m e n t e d by m i c r o b i a l and e n z y m i c a t t a c k on t h e α-cellulose d e r i v a t i v e . I t i s we I I known ([2) t h a t rap î d d e t e r i o r a t i o n o f e e l I u l o s e o c c u r s i n s o i l and t h a t t h e t e n s i l e p r o p e r t i e s o f r e g e n e r a t e d r a y o n , f o r example, a r e compIeteI y l o s t a f t e r f o u r t e e n days o f s o i l b u r i a l .

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

CONTROLLED RELEASE

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HO

PESTICIDES

μ

10

*

IS

TIME

2C

25

JO

35

IN DAYS

Figure 1. The cumulative release of 2,4-D from its α-cellulose esters where A, B, and C are the 4.1% w/w combinations exposed to solutions of pH 4, 7, and 10, respectively, and D, E, and F are the corre­ sponding treatments of the 21.5% w/w combinations subjected to the same treatments

I

I 0.5

I 1.0

I 1.5

LOG (APPLICATION LEVEL) Figure 2. The duration of herbicidal effectiveness provided by various application levels of a-cellulose 2,4-dichlorophenoxyacetate (D.S. 0.05)

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

9.

ALLAN ET AL.

Herbicide

Release from

Biodégradables

97

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However, t h e α-cellulose r e l e a s e m a t r i x need n o t be d e s t r o y e d w i t h i n t h i s s h o r t p e r i o d o f t i m e because o f t h e p r o t e c t i v e e f f e c t o f t h e e s t e r i f y i ng h e r b i c i de a c i d . T h u s , i n c o n t r a s t t o t h e une s t e r i f i e d c e l l u l o s e f i b e r s , t h e physicaI properties of t h e i r m o n o a c e t y l a t e d c o u n t e r p a r t s were n o t a d v e r s e l y a f f e c t e d by e x p o ­ s u r e t o s o i I b u r i a I f o r s i x months ( 1 3 ) . T h e s e e n z y m i c and m i c r o b i a I p a r a m e t e r s c a n b e s t be e x p l o r e d u n d e r a c t u a I u s e cond i t i o n s , t h a t i s , by e x p e r i m e n t s us i ng s o i l . However, t o o b t a i n r e p r o d u c i b I e r e s u l t s i n a b i o a s s a y i t i s n e c ­ e s s a r y t o c a r e f u I I y c o n t r o I t h e cond i t i o n s . Under s u c h precî s e cond i t i o n s t h e r a t e o f s u r f a c e h y d r o Iys i s f o r an e s t e r o f a w a t e r i n s o I u b I e poIymer i n a moi s t u r e î n u n d a t e d s o i I c a n be w r i t t e n a s