Emulsion Polymerization

11 Aqueous-Phase Polymerization of Butadiene Initated by Cobalt (III) Acetylacetonate D. C. BLACKLEY and W. F. H . BURGAR

Downloaded by UNIV OF NORTH CAROLINA on June 20, 2013 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0024.ch011

National College of Rubber Technology, The Polytechnic of North London, Holloway, London N7 8DB, England

The purpose o f t h i s paper i s to g i v e a b r i e f summary o f an i n v e s t i g a t i o n which has been c a r r i e d out i n t o the aqueous-phase p o l y m e r i s a t i o n of butadiene i n i t i a t e d by c o b a l t ( I I I ) a c e t y l a c e t o n ate i n the presence o f s u r f a c t a n t s such as sodium dodecylbenzenesulphonate and sodium d e c y l sulphate. I t i s intended that f u l l e r d e t a i l s o f t h e i n v e s t i g a t i o n w i l l be p u b l i s h e d elsewhere i n due course. S e v e r a l papers have appeared i n the l i t e r a t u r e i n recent years showing that c e r t a i n metal a c e t y l a c e t o n a t e s can f u n c t i o n as i n i t i a t o r s f o r the p o l y m e r i s a t i o n o f v i n y l and diene monomers i n bulk and s o l u t i o n (1 - 12). R e s u l t s f o r the k i n e t i c s o f b u l k and s o l u t i o n p o l y m e r i s a t i o n a r e c o n s i s t e n t w i t h the view that the r e a c t i o n occurs by a f r e e - r a d i c a l mechanism. The u s u a l f r e e r a d i c a l k i n e t i c s a r e o p e r a t i v e , but an unusual f e a t u r e i s t h a t , i n some cases, c e r t a i n a d d i t i v e s such as c h l o r i n a t e d hydrocarbons have an a c t i v a t i n g e f f e c t upon the r e a c t i o n by inducing more r a p i d decomposition of the i n i t i a t o r (2,11,12,13). Other a d d i t i v e s which have been r e p o r t e d as promotors f o r the p o l y m e r i s a t i o n i n c l u d e p y r i d i n e ( 1 4 ) and aldehydes and ketones(15). The complexity o f the r e a c t i o n i n the presence of such a d d i t i v e s i s evident from the f a c t that chloroform has been reported to be an i n h i b i t o r f o r the p o l y m e r i s a t i o n (3) . K a s t n i g and h i s co-workers(2) appear to have been the f i r s t to r e p o r t that metal a c e t y l a c e t o n a t e s a c t i v a t e d by c h l o r i n a t e d hydrocarbons a r e capable o f a c t i n g as i n i t i a t o r s f o r the emulsion p o l y m e r i s a t i o n o f butadiene. Unfortunately the p o l y m e r i s a t i o n r e c i p e s and r a t e s o f p o l y m e r i s a t i o n were not p u b l i s h e d , but the data given f o r the s t r u c t u r e o f the polybutadienes obtained i n d i c a t e that some degree of s t e r e o r e g u l a t i o n can be e f f e c t e d by t h i s type o f i n i t i a t ing system. T h i s c o n c l u s i o n has not been confirmed by subsequent investigations. B l a c k l e y and Matthan(16) have reported that i r o n ( I I I ) a c e t y l acetonate i s a very e f f e c t i v e i n i t i a t o r f o r the aqueous-phase p o l y -

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In Emulsion Polymerization; Piirma, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


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m e r i s a t i o n of butadiene using sodium dodecylbenzenesulphonate as s u r f a c t a n t , but that l i t t l e p o l y m e r i s a t i o n occurs when isoprene i s used as the monomer. No promoters were d e l i b e r a t e l y added to these r e a c t i o n systems. Blackley(17) has reported that c o b a l t ( I I I ) a c e t y l a c e t o n a t e i s a l s o an e f f e c t i v e i n i t i a t o r f o r the aqueousphase p o l y m e r i s a t i o n of butadiene i n the absence of added promoters. In each case, the polymer obtained had e s s e n t i a l l y the micros t r u c t u r e which would be expected i f the r e a c t i o n occurred by nonstereoregulated f r e e - r a d i c a l polymerisation. In the case of the polybutadienes obtained using c o b a l t ( I I I ) a c e t y l a c e t o n a t e , the m a t e r i a l appeared to be e x t e n s i v e l y c r o s s l i n k e d . At the commencement of the i n v e s t i g a t i o n to which t h i s present paper r e l a t e s , the i n t e n t i o n was to study i n much g r e a t e r d e t a i l than h i t h e r t o the c a t a l y s i s of the aqueous-phase p o l y m e r i s a t i o n of butadiene by i r o n ( I I I ) a c e t y l a c e t o n a t e . I t was soon found, however, that t h i s i s i n f a c t a most u n s u i t a b l e system f o r p r e c i s e study. The c a t a l y t i c a b i l i t y of i r o n ( I I I ) a c e t y l a c e t o n a t e seems to depend very much upon the method of p r e p a r a t i o n , and, indeed, c o n s i d e r a b l e batch-to-batch v a r i a t i o n s have been observed between m a t e r i a l s made by nominally i d e n t i c a l procedures and from the same s t a r t i n g m a t e r i a l s . Attempts to p u r i f y t h i s compound by techniques such as r e c r y s t a l l i s a t i o n and vacuum s u b l i m a t i o n have l e d to even g r e a t e r variations i n catalytic activity. I t should be added that the many samples of the c h e l a t e which have been prepared have f r e q u e n t l y d i f f e r e d i n appearance from one another, and t h a t a broad c o r r e l a t i o n has been observed between appearance and c a t a l y t i c a c t i v i t y . I t appears that the darker, reddish-mauve v a r i a n t s of the compound appear to be more a c t i v e than the l e s s h i g h l y - c o l o u r e d v a r i a n t s . I t seems that B l a c k l e y and Matthan(16) had f o r t u i t o u s l y used one o f the more a c t i v e v a r i a n t s i n the e a r l i e r i n v e s t i g a t i o n . In view of the d i f f i c u l t i e s encountered u s i n g i r o n ( I I I ) a c e t y l a c e t o n a t e , a t t e n t i o n was turned to the c o b a l t analogue. T h i s compound was known to show some c a t a l y t i c a c t i v i t y . Furthermore, a simple p r e p a r a t i v e procedure i s a v a i l a b l e which seems to g i v e a w e l l - d e f i n e d c r y s t a l l i n e product which i s r e p r o d u c i b l e from batch to batch, both as regards appearance and c a t a l y t i c a b i l i t y . For t h i s reason, a d e t a i l e d i n v e s t i g a t i o n of systems c o n t a i n i n g t h i s i n i t i a t o r has been undertaken as, i n e f f e c t , a "model" f o r aqueousphase diene p o l y m e r i s a t i o n s u s i n g t h i s c l a s s of i n i t i a t o r . In the i n i t i a l stage of the i n v e s t i g a t i o n which t h i s paper summarises, a range of experiments was c a r r i e d out u s i n g beverage b o t t l e s as r e a c t i o n v e s s e l s . I t was d u r i n g the course of these experiments that the v a r i a b i l i t y of i r o n ( I I I ) a c e t y l a c e t o n a t e was d i s c o v e r e d , and a l s o the e f f e c t i v e n e s s of the c o b a l t analogue. Other i n d i c a t i o n s from t h i s e a r l y s e r i e s of experiments were as follows : (1) P o l y m e r i s a t i o n i n the monomer phase occurs to a n e g l i g i b l e extent. Most of the p o l y m e r i s a t i o n takes p l a c e i n the aqueous phase, the presence of which i s e s s e n t i a l f o r the occurrence of r e a c t i o n .


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(2)

The p o l y m e r i s a t i o n r a t e depends very much upon the i n i t i a l pH o f the aqueous phase. (3) The p o l y m e r i s a t i o n r a t e depends very much upon the nature of the s u r f a c t a n t used. In p a r t i c u l a r , r e a c t i o n occurs more r e a d i l y i n the presence of sodium dodecylbenzenesulphonate than i n the presence o f potassium o l e a t e . (4) I n i t i a t i o n i s not due to the presence o f a d v e n t i t i o u s i m p u r i t i e s , such as peroxides, which might form a redox couple w i t h the metal compound. (5) A d d i t i o n o f mercaptans i n an endeavour t o suppress g e l formation leads to severe r e t a r d a t i o n of p o l y m e r i s a t i o n . These p r e l i m i n a r y experiments e s t a b l i s h e d that systems comprising e s s e n t i a l l y butadiene, water, sodium dodecylbenzenesulphonate and c o b a l t ( I I I ) a c e t y l a c e t o n a t e were s u i t a b l e f o r d e t a i l e d study. L a t e r work showed that i t was d e s i r a b l e t o r e p l a c e the a l k y l benzenesulphonate s u r f a c t a n t by sodium dodecyl s u l p h a t e . Materials 1. C o b a l t ( I I I ) a c e t y l a c e t o n a t e . C o b a l t ( I I I ) a c e t y l a c e t o n a t e was prepared by r e a c t i n g c o b a l t ( I I ) carbonate w i t h a c e t y l a c e t o n e i n the presence o f hydrogen peroxide a t 90 - 95°C. The green s o l i d which formed was f i l t e r e d o f f , washed w i t h water, and then d r i e d f o r t e n minutes a t 110°C. The compound was p u r i f i e d by d i s s o l v i n g i n b o i l i n g benzene, f i l t e r i n g , adding petroleum e t h e r , and then c o o l i n g t o -20°C. Deep green c r y s t a l s formed; these were f i l t e r e d o f f and d r i e d under vacuum a t 40°C. F i n a l l y , the compound was r e c r y s t a l l i s e d from methanol, d r i e d , and then s t o r e d i n the dark under n i t r o g e n . The product melted w i t h decomposition a t 205 207°C. I t s composition agreed w i t h the t h e o r e t i c a l e x p e c t a t i o n . 2· Butadiene. Butadiene was obtained from two sources, namely I.C.I, and A i r Products L t d , i n both cases the p u r i t y being approximately 98%. I t was subjected t o two d i s t i l l a t i o n s b e f o r e being polymerised, once at room temperature and once a t approximately -40°C. 3. Sodium dodecylbenzenesulphonate. The m a t e r i a l s u p p l i e d by Marchon Products L t d under the name "Nansa HS 85/S" was used. This i s s t a t e d to c o n t a i n a mixture o f sodium alkylbenzenesulphonates, the average chain l e n g t h of the a l k y l group b e i n g about twelve carbon atoms. The p r o p o r t i o n o f a c t i v e m a t e r i a l i s s a i d t o be about 85%, the remainder comprising approximately 12% o f sodium c h l o r i d e and sulphate, 1% of hydrocarbon o i l , and 3% of water. The crude m a t e r i a l was p u r i f i e d by a lengthy procedure i n which the s u r f a c t a n t was d i s s o l v e d i n a mixture o f i s o p r o p a n o l and water a t 50°C, the s o l u t i o n t r e a t e d w i t h sodium m e t a b i s u l p h i t e t o destroy p e r o x i d i c i m p u r i t i e s , and then r e f l u x e d w i t h saturated


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sodium carbonate s o l u t i o n . On c o o l i n g , the mixture separated i n t o two l a y e r s , the lower o f which contained the i n o r g a n i c i m p u r i t i e s and was i n consequence d i s c a r d e d . Treatment w i t h sodium carbonate was repeated twice. The a l k a l i n e top l a y e r was then n e u t r a l i s e d with the a p p r o p r i a t e alkylbenzenesulphonic a c i d , s u p p l i e d by Marchon Products L t d . A f t e r d i l u t i o n w i t h water, petroleum s p i r i t was added i n order to e x t r a c t hydrocarbon o i l i m p u r i t i e s . The aqueous s o l u t i o n o f s u r f a c t a n t was then evaporated t o dryness, and the f i n a l t r a c e s o f water, petroleum s p i r i t and i s o p r o p a n o l were removed by h e a t i n g i n a vacuum oven a t 50°C t o constant weight. 4. Sodium dodecyl s u l p h a t e . The m a t e r i a l s u p p l i e d by Marchon Products L t d under the name "Empicol LX" was used. T h i s i s s t a t e d to c o n t a i n some 85% o f a c t i v e m a t e r i a l . The crude m a t e r i a l was p u r i f i e d by f i r s t d i s s o l v i n g i n hot methanol t o form a 5% s o l u t i o n , and f i l t e r i n g to remove i n o r g a n i c s a l t s . The f i l t r a t e volume was then reduced t o h a l f by evaporation, under n i t r o g e n . The s o l u t i o n was cooled to -30°C i n order to separate the s u r f a c t a n t as a s o l i d phase, which was then f i l t e r e d o f f , washed thoroughly w i t h methanol, acetone and d i e t h y l ether i n order t o remove f a t t y m a t e r i a l s . The s o l i d was then powdered and d r i e d i n a vacuum oven a t 40°C f o r t e n minutes. Experimental

Procedure F o r P o l y m e r i s a t i o n s

P o l y m e r i s a t i o n s were c a r r i e d out under vacuum i n d i l a t o m e t e r s c o n t a i n i n g a glass-covered a g i t a t i o n element which was actuated by e x t e r n a l r e c i p r o c a t i n g magnets. The r e a c t i o n system comprised two phases, one o f which was p r i n c i p a l l y water and the other p r i n c i p a l l y monomer. The i n i t i a t o r p a r t i t i o n e d i t s e l f between these two phases. Although a s u r f a c t a n t was present i n the aqueous phase, the r a t e o f a g i t a t i o n w i t h i n the d i l a t o m e t e r s was i n s u f f i c i e n t t o maintain a s t a b l e monomer emulsion. I t was, however, s u f f i c i e n t to ensure that the aqueous phase remained s a t u r a t e d w i t h monomer, as judged by the c r i t e r i o n that the r a t e o f p o l y m e r i s a t i o n was i n d e pendent o f the r a t e o f a g i t a t i o n under the c o n d i t i o n s o f a g i t a t i o n employed. The p o l y m e r i s a t i o n temperature was g e n e r a l l y 50°C. The f o l l o w i n g procedure was adopted f o r f i l l i n g the d i l a t o meters: The aqueous phase, which had p r e v i o u s l y been purged with n i t r o g e n , was introduced i n t o the d i l a t o m e t e r s and then out-gassed to 10~~-*mm mercury pressure. Out-gassed butadiene was then d i s t i l l e d i n t o the d i l a t o m e t e r s . F i n a l l y , the d i l a t o m e t e r s were purged w i t h n i t r o g e n , evacuated to a pressure o f 10""^mm mercury, and then s e a l e d i n the usual way. I t w i l l appear from r e s u l t s summarised below that c o n s i d e r a b l e care i s necessary i n p r e p a r i n g the r e a c t i o n systems i f r e p r o d u c i b l e r e s u l t s are to be obtained. The compositions o f the v a r i o u s r e a c t i o n systems used a r e given w i t h the r e s u l t s to which they r e f e r .


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Experiments

1. E f f e c t of storage time and presence of oxygen. P r i o r to c a r r y i n g out the p o l y m e r i s a t i o n r e a c t i o n , the r e a c t i o n v e s s e l s were s t o r e d f o r v a r y i n g lengths of time at -30°C. E a r l y experiments using d i l a t o m e t e r s which had been f i l l e d out and out-gassed i n the presence of r e s i d u a l a i r r a t h e r than n i t r o g e n , r e v e a l e d unexpected v a r i a t i o n s i n the r a t e of p o l y m e r i s a t i o n . I t was found that the r a t e of p o l y m e r i s a t i o n appeared to c o r r e l a t e w i t h the time of storage at -30°C b e f o r e the r e a c t i o n was i n i t i a t e d . A s e r i e s of experiments was t h e r e f o r e undertaken to i n v e s t i g a t e t h i s e f f e c t s y s t e m a t i c a l l y . The r e s u l t s are shown i n F i g u r e 1, from which i t i s seen that the r a t e of p o l y m e r i s a t i o n i n c r e a s e s s h a r p l y w i t h storage time over a p e r i o d of days, e v e n t u a l l y r e a c h i n g a p l a t e a u a f t e r about ten days storage. The r e a c t i o n r a t e i n d i l a t o m e t e r s f i l l e d under r e s i d u a l n i t r o g e n r a t h e r than r e s i d u a l a i r i s somewhat higher than the r a t e e v e n t u a l l y a t t a i n e d i n d i l a t o m e t e r s f i l l e d under a i r ; i t a l s o appears that i n t h i s case the r a t e i s very much l e s s dependent upon storage time. The i n d i c a t i o n s are that oxygen a c t s as an i n h i b i t o r or r e t a r d e r f o r t h i s type of ρ οlymeris at ion, and t h a t , i n order f o r p o l y m e r i s a t i o n to proceed at an a p p r e c i a b l e r a t e , i t i s f i r s t necessary f o r r e s i d u a l oxygen to be removed by r e a c t i o n w i t h some component of the r e a c t i o n system during the storage time. That oxygen has a severe r e t a r d i n g e f f e c t upon the p o l y m e r i s a t i o n i s confirmed by the r e s u l t s summarised i n F i g u r e 2. For t h i s experiment, a number of d i l a t o m e t e r s c o n t a i n i n g a given r e a c t i o n system were out-gassed i n the c o n v e n t i o n a l manner, and the d i l a t o m e t e r s and vacuum l i n e to which they were attached then f i l l e d w i t h oxygen and slowly re-evacuated. The d i l a t o m e t e r taps were c l o s e d i n t u r n , so that each r e a c t i o n system contained oxygen at a d i f f e r e n t , but known pressure. The d i l a t o m e t e r s were then s e a l e d . Two d i l a t o m e t e r s c o n t a i n i n g the same r e a c t i o n system were thoroughly purged w i t h n i t r o g e n before out-gassing and s e a l i n g . The r e s u l t s obtained show c l e a r l y the s e n s i t i v i t y of the r e a c t i o n r a t e to r e s i d u a l oxygen, and a l s o give an i n d i c a t i o n of the r e p r o d u c i b i l i t y i n r e a c t i o n r a t e which i s a t t a i n a b l e when the r e a c t i o n system i s purged w i t h n i t r o g e n before being out-gassed. The r e s u l t s reported here provide the reason f o r adopting the r a t h e r e l a b o r a t e procedure d e s c r i b e d above f o r the p r e p a r a t i o n of most of the r e a c t i o n systems used i n t h i s i n v e s t i g a t i o n . I t was considered e s s e n t i a l to e l i m i n a t e the complex e f f e c t s a s s o c i a t e d w i t h the presence of a d v e n t i t i o u s oxygen, and the evidence presented here suggests that t h i s o b j e c t i v e can be a t t a i n e d by using the procedure d e s c r i b e d . 2. E f f e c t of aqueous phase: monomer r a t i o . A s e r i e s of experiments was c a r r i e d out to i n v e s t i g a t e the e f f e c t upon p o l y m e r i s a t i o n r a t e of v a r y i n g the r a t i o of aqueous phase to monomer phase, the composition of both phases being kept nominally constant.


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f i l l e d under nitrogen


f i l l e d under oxygen

Storage-time (days) Figure 1. Dependence of the polymerization rate, R , on "storage time" (time elapsing between sealing the dilatometer and placing it in the water bath at 50° C to start polymerization). Recipe: 500 g of butadiene and 10 g of C (HI) (acac) per 1000 g aqueous solution, the latter containing 25 g/l. sodium alkyl benzenesulfonate and with the pH adjusted to 10.1 with NaOH. p

0

s

f i l l e d under nitrogen

ο

cx

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0.02

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oxygen pressure Figure 2. Dependence of the polymerization rate, R , on the pressure of oxygen in the dilatometer. Recipe same as Figure 1. p



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It i s necessary to i n s e r t the q u a l i f i c a t i o n "nominally", because the a c t u a l s i t u a t i o n i s to some extent complicated by the p a r t i t i o n i n g of the i n i t i a t o r between the two phases. The r e s u l t s are given i n F i g u r e 3. They show that, except a t very low aqueous phase: monomer r a t i o s , the r a t e o f p o l y m e r i s a t i o n per u n i t q u a n t i t y o f aqueous phase i s e s s e n t i a l l y independent o f the phase r a t i o . T h i s i m p l i e s that the t o t a l r a t e o f conversion of butadiene to polybutadiene i s d i r e c t l y p r o p o r t i o n a l to the volume o f the aqueous phase, i f the compositions of the two phases are kept constant. This o b s e r v a t i o n i s c o n s i s t e n t w i t h t h e view that the p o l y m e r i s a t i o n takes p l a c e w i t h i n the aqueous phase o f the system. The r e d u c t i o n i n r a t e per u n i t q u a n t i t y o f aqueous phase which occurs a t low r a t i o s of aqueous phase to monomer phase may be due to s e r i o u s d e p l e t i o n o f i n i t i a t o r i n the aqueous phase. The i n i t i a t o r i s c'onsiderably more s o l u b l e i n the butadiene phase than i n the aqueous phase, and t h e r e f o r e may have been present l a r g e l y i n the monomer phase i n those systems which contained l a r g e volumes of butadiene. 3

· E f f e c t o f pH. The profound e f f e c t which the i n i t i a l pH o f the aqueous phase has upon the r a t e o f p o l y m e r i s a t i o n i s i l l u s t r a t e d by t h e r e s u l t s summarised i n F i g u r e 4. L i t t l e polymerisa t i o n i s observed when the pH i s below about n i n e . As pH i s i n c r e a s e d above ten, so the r e a c t i o n r a t e i n c r e a s e s r a p i d l y , p a s s i n g through a maximum i n the r e g i o n pH 11 - 12. As the pH i s f u r t h e r i n c r e a s e d , so t h e r e a c t i o n r a t e f a l l s somewhat. Although the r e s u l t s given i n F i g u r e 4 a r e f o r a system which contained sodium dodecyl sulphate as the s u r f a c t a n t and a l s o a c e r t a i n q u a n t i t y of an i n o r g a n i c e l e c t r o l y t e (sodium s u l p h a t e ) , very s i m i l a r behaviour has been observed i n systems which c o n t a i n sodium dodecylbenzenesulphonate as s u r f a c t a n t and no added i n o r g a n i c electrolyte. I n these l a t t e r systems, some p o l y m e r i s a t i o n appeared to occur a t pH's below e i g h t , although the r a t e was low. As when sodium dodecyl sulphate was used as s u r f a c t a n t , the r a t e was observed to i n c r e a s e s h a r p l y with i n c r e a s i n g pH, and to pass through a d e f i n i t e maximum i n the r e g i o n o f pH 11. As expected, the pH o f the l a t e x produced by t h e r e a c t i o n c o r r e l a t e s c l o s e l y w i t h the i n i t i a l pH o f the r e a c t i o n system. However, r a t h e r unexpectedly i t was found (see F i g u r e 4) that the f i n a l pH i s always about two u n i t s lower than the i n i t i a l pH. S i m i l a r e f f e c t s a r e observed i n persulphate - i n i t i a t e d emulsion p o l y m e r i s a t i o n s , where the r e d u c t i o n i n pH i s a t t r i b u t e d t o the formation of b i s u l p h a t e ions by r e a c t i o n between water molecules and sulphate r a d i c a l ions formed by decomposition o f the p e r sulphate. No such ready e x p l a n a t i o n i s a v a i l a b l e i n t h i s i n s t a n c e . I t should be noted that the r e a c t i o n systems to which these r e s u l t s r e f e r were unbuffered. Many o f the l a t e r r a t e measurements were made upon systems which contained pH b u f f e r s . I t i s t h e r e f o r e presumed that no s i g n i f i c a n t drop i n pH o c c u r r e d i n the course o f



BLACKLEY

AND

BURGAR

Polymerization

ratio-weight aqueous phase:

of

Butadiene

weight butadiene phase.

Figure 3. Dependence of the polymerization rate, R , on the ratio-weight of aqueous phase: weight of butadiene. Recipe as in Figure 1 only butadiene levels are varied between 300 and 1760 g and pH was 10.4. p

ίο

I

PH Figure 4. Dependence of the polymerization rate, R , on the pH of the aqueous phase at constant surfactant concentration. Recipe analogous to that of Figure 1 except the aqueous solution contained 10 g/l. Na SO „ and the pH varied. p

2

f


170

EMULSION

POLYMERIZATION


these l a t t e r r e a c t i o n s , although no check has been made to establ i s h i f t h i s i s i n f a c t the case. 4. E f f e c t of s u r f a c t a n t l e v e l . Much o f the e a r l i e r work on t h i s r e a c t i o n system was c a r r i e d out using sodium dodecylbenzenesulphonate as the s u r f a c t a n t . In the course o f experiments which were intended to i n v e s t i g a t e the e f f e c t upon p o l y m e r i s a t i o n r a t e o f v a r y i n g the s u r f a c t a n t l e v e l , i t was d i s c o v e r e d that p u r i f i e d sodium dodecylbenzenesulphonates a r e apparently a b l e to a c t as i n i t i a t o r s o f f r e e - r a d i c a l emulsion p o l y m e r i s a t i o n i n the absence of other added i n i t i a t i n g substances. Furthermore, as the r e s u l t s summarised i n F i g u r e 5 show, the r a t e o f p o l y m e r i s a t i o n i n the absence o f added i n i t i a t o r i s d i r e c t l y p r o p o r t i o n a l to the concent r a t i o n o f sodium dodecylbenzenesulphonate i n the aqueous phase. The reason f o r t h i s behaviour i s obscure; i t i s not apparently due to the presence o f peroxides i n the s u r f a c t a n t . However, i t i s perhaps s i g n i f i c a n t that an emulsion p o l y m e r i s a t i o n r e a c t i o n i n which the r a t e of p o l y m e r i s a t i o n i s f i r s t - o r d e r w i t h respect to s u r f a c t a n t l e v e l i s c o n s i s t e n t w i t h Smith-Ewart "Case 2" k i n e t i c s f o r a system i n which the s u r f a c t a n t f u n c t i o n s as an i n i t i a t o r as w e l l as a m i c e l l e generator. Because o f p o s s i b l e complications a r i s i n g from the s u r f a c t a n t a c t i n g as an i n i t i a t o r , subsequent experiments were c a r r i e d out using sodium dodecyl sulphate i n p l a c e o f sodium dodecylbenzenesulphonate. T h i s s u r f a c t a n t does not appear to a c t as an i n i t i a t o r . F i g u r e 6 summarises the v a r i a t i o n o f r a t e o f p o l y m e r i s a t i o n w i t h l e v e l o f sodium dodecyl sulphate a t v a r i o u s pH l e v e l s . Two e f f e c t s are immediately apparent from these r e s u l t s . In the f i r s t p l a c e , i t i s c l e a r that the r a t e a t any given s u r f a c t a n t l e v e l r i s e s s h a r p l y with i n c r e a s i n g pH over the range pH 9 - 11. But secondly, i t appears t h a t , as the s u r f a c t a n t l e v e l i s r a i s e d , so the r a t e o f p o l y m e r i s a t i o n tends to pass through a shallow maximum. The e f f e c t i s e s p e c i a l l y n o t i c e a b l e a t high pH. T h i s r e s u l t was unexpected, but has been observed i n s e v e r a l s e r i e s o f experiments. I t i s b e l i e v e d that the maximum a r i s e s from the o p e r a t i o n o f two c o n f l i c t i n g tendencies. Both tendencies a r e b e l i e v e d to a r i s e from the i n c r e a s e d number o f m i c e l l e s which a r e present as the s u r f a c t a n t c o n c e n t r a t i o n i s i n c r e a s e d . On the one hand, an increased number of m i c e l l e s i m p l i e s an i n c r e a s e d number o f p o t e n t i a l r e a c t i o n l o c i and t h e r e f o r e an enhanced r a t e o f p o l y m e r i s a t i o n . On the other hand, as w i l l appear from s o l u b i l i t y and decomposition s t u d i e s to be d i s c u s s e d below, the amount o f i n i t i a t o r s o l u b i l i s e d w i t h i n the m i c e l l e s , and thereby rendered i n a c t i v e , w i l l i n c r e a s e as the number o f m i c e l l e s i n c r e a s e s . The r a t e o f i n i t i a t i o n i s t h e r e f o r e reduced as the s u r f a c t a n t c o n c e n t r a t i o n i n c r e a s e s . I t i s b e l i e v e d that the maximum i n the v a r i a t i o n o f p o l y m e r i s a t i o n r a t e w i t h s u r f a c t a n t c o n c e n t r a t i o n a r i s e s from the balance o f these two tendencies. I t i s perhaps s i g n i f i c a n t t h a t , a t a given pH and low s u r f a c tant c o n c e n t r a t i o n , the v a r i a t i o n r a t e o f p o l y m e r i s a t i o n w i t h



BLACKLEY

AND

Polymerization

BURGAR

of

Butadiene

Surfactant (g 1

)

Figure 5. Dependence of the polymerization rate, R , on the surfactant concentrations, S, in the absence of cobalt (III) acetylacetonate. Recipe as for Figure 1 except the surfactant concentration is varied at pH 11.3. p

1

Surfactant ( g l " ) Figure 6. Dependence of the polymerization rate, R , on surfactant concentration, S, at constant pH (values shown alongside curves). Recipe as in Figure 1 except the aqueous solution contained 10 g/l. Na SO , and 2.1 g/l. NaHCO.,. The NaOH and surfactant content varied. p

2

f


172

EMULSION

POLYMERIZATION


s u r f a c t a n t c o n c e n t r a t i o n i s c o n s i s t e n t with an order o f r e a c t i o n o f 0.6 (see F i g u r e 7 ) . T h i s order o f r e a c t i o n i s i n t u r n c o n s i s t e n t with a m i c e l l a r i n i t i a t i o n mechanism of t h e type envisaged by the Smith-Ewart theory. 5. E f f e c t of adding i n o r g a n i c e l e c t r o l y t e . One of the p r i n c i p a l i m p u r i t i e s l i k e l y t o be a s s o c i a t e d w i t h s u r f a c t a n t s such as sodium dodecyl sulphate and sodium dodecylbenzenesulphonate i s i n o r g a n i c e l e c t r o l y t e such as sodium s u l p h a t e . I t was t h e r e f o r e considered d e s i r a b l e to i n v e s t i g a t e the e f f e c t o f such s a l t s upon the r a t e of p o l y m e r i s a t i o n . The r e s u l t s o f t h i s s e r i e s o f experiments a r e summarised i n F i g u r e 8. Sodium dodecyl sulphate was the s u r f a c t a n t and sodium sulphate the a d d i t i v e . The r e s u l t s show that the r e a c t i o n r a t e r i s e s s h a r p l y w i t h i n c r e a s i n g e l e c t r o l y t e a d d i t i o n u n t i l a p l a t e a u i s reached. T h e r e a f t e r , f u r t h e r a d d i t i o n s o f e l e c t r o l y t e have l i t t l e e f f e c t upon the r a t e o f polymerisation. I t t h e r e f o r e appears t h a t , i f i t i s d e s i r e d to o b t a i n r e p r o d u c i b l e r a t e s o f p o l y m e r i s a t i o n w i t h t h i s type o f r e a c t i o n system, i t i s p r e f e r a b l e t o c o n t r o l b o t h the pH and the i o n i c s t r e n g t h o f the aqueous phase. These two o b j e c t i v e s a r e most r e a d i l y achieved by adding b u f f e r s t o the system. A b u f f e r comp r i s i n g sodium carbonate and sodium b i c a r b o n a t e i n an a p p r o p r i a t e r a t i o has been found to be s u i t a b l e . 6. E f f e c t o f i n i t i a t o r l e v e l . The e f f e c t o f i n c r e a s i n g amounts of c o b a l t ( I I I ) a c e t y l a c e t o n a t e upon the r a t e o f p o l y m e r i s a t i o n i s complex. W h i l s t the r a t e always appears to i n c r e a s e as the i n i t i a * * t o r l e v e l i n c r e a s e s ( i n c o n t r a s t to the behaviour observed when the s u r f a c t a n t l e v e l i s i n c r e a s e d ) , the order o f r e a c t i o n w i t h r e s p e c t to i n i t i a t o r depends upon the c o n c e n t r a t i o n o f s u r f a c t a n t i n the aqueous phase. The r e s u l t s summarised i n F i g u r e 9 show t h a t the order i s approximately the Smith-Ewart v a l u e o f 0.4 a t h i g h s u r f a c t a n t c o n c e n t r a t i o n s , whereas i t f a l l s markedly as the s u r f a c t a n t l e v e l i n the r e a c t i o n system i s lowered. F i g u r e 10 i l l u s t r a t e s the r a t h e r s u r p r i s i n g o b s e r v a t i o n that the order o f r e a c t i o n w i t h respect to i n i t i a t o r appears to vary l i n e a r l y w i t h the l o g a r i t h m o f the s u r f a c t a n t c o n c e n t r a t i o n . 7. E f f e c t o f temperature. The r e s u l t s f o r the determination o f r a t e s o f p o l y m e r i s a t i o n a t v a r i o u s temperatures a r e summarised i n F i g u r e 11, from which i t i s seen that the temperature v a r i a t i o n o f r a t e i s o f the Arrhenius type. The s l o p e o f the r e l a t i o n s h i p between the l o g a r i t h m o f the r a t e o f p o l y m e r i s a t i o n and the r e c i p r o c a l o f the absolute temperature corresponds to an o v e r a l l a c t i v a t i o n energy of p o l y m e r i s a t i o n o f 15.8 k c a l mole . This value i s very c l o s e to that which has been reported f o r the aqueous -phase p o l y m e r i s a t i o n of butadiene i n i t i a t e d by rhodium c h l o r i d e ( 1 6 ) . However, i n view o f the marked d i s s i m i l a r i t y i n r e a c t i o n product, and t h e r e f o r e presumably i n r e a c t i o n mechanism, i t seems l i k e l y that the correspondence i s f o r t u i t o u s .



BLACKLEY

AND

Polymerization

BURGAR

s

0.6

of

( g

173

Butadiene

i-1,0.6

Figure 7. Dependence of the polymerization rate, R , on S . Recipe as in Figure 1 but with 10 g/l Na SO 2.1 g/l. NaHC0 at pH 10.9, with surfactant content varied between 0-100 g/l. 06

p

2

h

3

Figure 8. Dependence of the polymerization rate, R , on added sodium sulfate concentration, at constant pH and surfactant concentration, S. Recipe as in Figure 1 with the aqueous solution containing 0-50 g/l. Na S0 . p

10

Na S04 2

20

(g

30

2

4


EMULSION

POLYMERIZATION


0.4

0.1

20

40

60

80

100

s (gr ) 1

Figure 9. Dependence of t, the order with respect to initiator on surfactant concentration, S. Recipe: buta diene 500 g and CO (III) (acac) 1-10 g per 1000 g aqueous solution of 25-60 g/l. surfactant, 10 g/l. Na SO,„ 2.1 g/l. NaHCO at pH 10.6. y

2

s

0.1

ι

ι

«

1.4

1.6

1.8 1

,

1~

2-0

1

log S ( g " ) Figure 10. Dependence of t on log S. Recipe as in Figure 9.


11.

BLACKLEY

AND

BURGAR

Polymerization

of

175

Butadiene


Other Experiments In an endeavour to seek a deeper understanding of some of the f e a t u r e s of what i s undoubtedly a r a t h e r complex r e a c t i o n system, a number of a n c i l l a r y experiments have been c a r r i e d out. These i n c l u d e the f o l l o w i n g : (1) s t u d i e s of the k i n e t i c s of the decomposition of c o b a l t ( I I I ) a c e t y l a c e t o n a t e i n v a r i o u s media; (2) s t u d i e s of the s o l u b i l i t y of c o b a l t ( I I I ) a c e t y l a c e t o n a t e i n aqueous media, and of i t s p a r t i t i o n i n g between aqueous and hydrocarbon media; ^ (3) experiments u s i n g C labelled cobalt(III) acetylacetonate. 1. Decomposition k i n e t i c s of c o b a l t ( I I I ) a c e t y l a c e t o n a t e . The decomposition of c o b a l t ( I I I ) a c e t y l a c e t o n a t e i n v a r i o u s media has been s t u d i e d by f o l l o w i n g the change i n c o n c e n t r a t i o n by means of u l t r a v i o l e t spectrophotometry. The f o l l o w i n g f a c t s have been e s t a b l i s h e d concerning the decomposition of t h i s compound at 50°C; (a) L i t t l e decomposition of c o b a l t ( I I I ) a c e t y l a c e t o n a t e occurs i t i t i s d i s s o l v e d i n a non-polar s o l v e n t such as hexane. I t appears that the presence of p o l a r molecules such as water i s e s s e n t i a l i f s i g n i f i c a n t decomposition i s to occur. I t t h e r e f o r e seems l i k e l y that the aqueous-phase p o l y m e r i s a t i o n of butadiene i s i n i t i a t e d through the decomposition of c o b a l t ( I I I ) a c e t y l a c e t o n a t e which i s d i s s o l v e d i n the true aqueous phase. That f r a c t i o n of the i n i t i a t o r which i s d i s s o l v e d i n the monomer phase i s presumably i n e f f e c t i v e . So a l s o , presumably, i s the f r a c t i o n which i s s o l u b i l i s e d w i t h i n the s u r f a c t a n t m i c e l l e s , s i n c e the i n t e r i o r s of the l a t t e r are e s s e n t i a l l y non-polar i n nature. (b) In aqueous media, decomposition occurs at a r a t e which i s f i r s t order with respect to the c o n c e n t r a t i o n of cobalt(III) acetylacetonate. Furthermore, the r a t e constant f o r the decomposition depends upon both the pH of the aqueous medium and the s u r f a c t a n t c o n c e n t r a t i o n . The e f f e c t of pH i s shown i n F i g u r e 12, and that of s u r f a c t a n t c o n c e n t r a t i o n i n F i g u r e 13. In both cases, log c / c i s p l o t t e d as a f u n c t i o n of t , where C denotes the i n i t i a l c o n c e n t r a t i o n of c o b a l t ( I I I ) a c e t y l a c e t o n a t e , and c denotes i t s c o n c e n t r a t i o n a f t e r a time t . The l i n e a r i t y of these p l o t s shows that the decomposition of the i n i t i a t o r i s f i r s t order over a wide range of r e a c t i o n c o n d i t i o n s . F i g u r e 12 shows that the f i r s t - o r d e r r a t e c o e f f i c i e n t f o r the decomposition r i s e s s h a r p l y as the pH i s i n c r e a s e d from about 9.5 to 11.6. This observation i m p l i e s that the i n i t i a l step of the i n i t i a t o r - d e c o m p o s i t i o n r e a c t i o n i s probably i n t e r a c t i o n with hydroxy1 i o n s . I t seems that here i s the most l i k e l y e x p l a n a t i o n f o r the pronounced e f f e c t which the i n i t i a l pH of the t

q

t


EMULSION

POLYMERIZATION


1.3

3.0

3.1 Τ"

1

3.2

χ ΙΟ

3

Figure 11. Flot of log R against 1/T for the determina tion of the overall activation energy of polymerization, E . Recipe differs from Figure 1 in that the aqueous solution contains 2.1 g/l. NaHCO and pH 10.6. P

A

s

1.0 l 0

1

ι

1

10

20

30

Time (hr)

Figure 12. Plot of log C /C against time as a function of pH varied with buffer solutions; surfactant = 0 g/l. ; temperature = 60.5°C. The lines are associated, from top to bottom, with the following pH and k Χ 10 (sec' ) values: 11.62 and 18.6, 10.84 and 14.0, 10.5 and 10.9, 10.28 and 8.0, 9.56 and 4.1. t

c

0

6

1


BLACKLEY

—


Folymerizotion

A N D BURGAR

of

Butadiene

coupon point

_a

1

ι.

20

• ι

«

40

*

*-

60

80

Time (hr) Symbol

0

S (gl-1)

0

k

6

d

1

χ 10 (sec" )

3.31

a

V

χ

•

60

100

1.37

0.94

10

25

40

3.20

2.01

1.60

Δ

Figure 13. Plot of log C /C against time as a function of surfactant concentration, S. Decomposition in buffered surfactant solutions; pH = 11.62; temperature =49.7°C. t

0



178

(c)

EMULSION

POLYMERIZATION

aqueous phase has upon the r a t e at which the aqueousphase p o l y m e r i s a t i o n of butadiene takes p l a c e . Figure 13 shows that the e f f e c t of s u r f a c t a n t i s to r e t a r d the decomposition of the i n i t i a t o r . The most obvious explan a t i o n f o r t h i s e f f e c t i s that an increased c o n c e n t r a t i o n of s u r f a c t a n t causes an increased amount of s u r f a c t a n t to be s o l u b i l i s e d w i t h i n the s u r f a c t a n t m i c e l l e s , and t h e r e f o r e to be rendered u n s u s c e p t i b l e to decomposition. C e r t a i n l y , the r e s u l t s of s o l u b i l i t y measurements which are reported below support the view that the s u r f a c t a n t does s o l u b i l i s e a p p r e c i a b l e q u a n t i t i e s of i n i t i a t o r . Whether or not t h i s i s the true explanation f o r the r e t a r d a t i o n of i n i t i a t o r decomposition by s u r f a c t a n t , the f a c t that the s u r f a c t a n t behaves i n t h i s way e x p l a i n s why the r a t e of the aqueous-phase p o l y m e r i s a t i o n of butadiene' passes through a maximum as the l e v e l of s u r f a c t a n t i n the system i s increased, The p r e c i s e manner i n which the f i r s t - o r d e r r a t e constant for the decomposition of c o b a l t ( I I I ) a c e t y l a c e t o n a t e i n aqueous media at 50°C v a r i e s w i t h the pH and the s u r f a c t a n t c o n c e n t r a t i o n i s complex. E m p i r i c a l l y , i t has been found that the j o i n t v a r i a t i o n can be w e l l represented by the equation «, q

k, - 1.28

X 10~

3

n

[0H7

Λ

ς

1 - o.i5s

n

u,:)

i

[OHT

-I

where k^ denotes the f i r s t - o r d e r r a t e c o e f f i c i e n t i n sec , and S denotes the s u r f a c t a n t c o n c e n t r a t i o n i n grams per litre. I t has not so f a r been p o s s i b l e to deduce any mechanistic i m p l i c a t i o n s from t h i s equation, other than the q u a l i t a t i v e i m p l i c a t i o n s which have been discussed i n the previous paragraph. Results f o r the v a r i a t i o n of k^ w i t h temperature are shown i n F i g u r e 14. Data are given f o r decomposition i n aqueous media at s e v e r a l pH values i n the absence of s u r f a c t a n t , and a l s o f o r an aqueous medium at pH 10.84 to which sodium dodecyl sulphate had been added. I t i s c l e a r that the v a r i a t i o n of k^ w i t h temperature f o l l o w s the Arrhenius equation i n a l l the cases s t u d i e d , and t h a t , i n c o n t r a s t to k^ i t s e l f , the energy of a c t i v a t i o n i s r e l a t i v e l y i n s e n s i t i v e to both pH and the presence of s u r f a c t a n t . There i s some i n d i c a t i o n that the energy of a c t i v a t i o n increases s l i g h t l y w i t h i n c r e a s i n g pH, and that the a d d i t i o n of s u r f a c t a n t at a given pH reduces the energy of a c t i v a t i o n . However, taking the r e s u l t s as a whole, they are c o n s i s t e n t w i t h an energy of a c t i v a t i o n of 32.9 + 1.6 k c a l mole . This appears to be approximately the energy r e q u i r e d f o r the s c i s s i o n of the Co-0 bond. Spectroscopic evidence suggests that the i n i t i a l product of the thermal decomposition of c o b a l t ( I I I ) acetylacetonate i n aqueous media i s c o b a l t ( I I ) a c e t y l a c e t o n a t e , and t h e r e f o r e (presumably) an acetylacetone l i g a n d a l s o . The presence of the l a t t e r has not been detected, and so i t i s b e l i e v e d that i t undergoes r a p i d decom p o s i t i o n . The presence of c o b a l t ( I I ) acetylacetonate has been


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Figure 14. An Arrhenius plot of log k against 1/T for ithe determination of the activation energy of decomposition of cobalt (III) acetylacetonate in aqueous and surfactant solutions d


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detected, but t h i s compound seems to decompose more r a p i d l y than does the c o b a l t ( I I I ) analogue. 2 . P a r t i t i o n and s o l u b i l i t y measurements on c o b a l t ( I I I ) a c e t y l a c e t o n a t e . The f o l l o w i n g procedure was adopted f o r i n v e s t i g a t i n g the approximate manner i n which c o b a l t ( I I I ) a c e t y l a c e t o n a t e p a r t i t i o n s between butadiene and an aqueous phase c o n t a i n i n g sodium dodecyl s u l p h a t e : Known weights of aqueous phase and c o b a l t ( I I I ) a c e t y l a c e t o n a t e were p l a c e d i n weighed 1-cm diameter g l a s s tubes to the open end of which were attached g l a s s cones. The tubes were attached to a vacuum l i n e , the aqueous phase out-gassed, and then the r e q u i r e d volumes of butadiene d i s t i l l e d i n . The tubes were then s e a l e d , re-weighed, and immersed i n a water-bath at 50°C. The tubes were shaken and allowed to e q u i l i b r a t e f o r ten minutes. They were then plunged i n t o l i q u i d n i t r o g e n to f r e e z e the contents, and the tubes broken to l i b e r a t e the f r o z e n c o n t e n t s . The butadiene f r a c t i o n was removed by c u t t i n g through the f r o z e n c y l i n d e r w i t h a k n i f e . The aqueous phase was melted i n t o a beaker, d i l u t e d to a known extent w i t h d i s t i l l e d water, and the c o n c e n t r a t i o n of c o b a l t ( I I I ) a c e t y l a c e t o n a t e i n the d i l u t e d aqueous s o l u t i o n d e t e r mined by u l t r a v i o l e t spectrophotometry. I t was then a simple matter to c a l c u l a t e the c o n c e n t r a t i o n of the compound i n the u n d i l u t e d aqueous phase, and hence, knowing the weight o f the aqueous phase, the t o t a l amount i n that phase. By d i f f e r e n c e , i t was p o s s i b l e to know the t o t a l amount i n the butadiene phase, and hence, from the known weight of the butadiene phase, the concent r a t i o n i n that phase. The r e s u l t s given i n F i g u r e 15 are f o r two aqueous phases, one of which had a pH o f 8.5 and the other a pH of 11.1, and both of which contained the same c o n c e n t r a t i o n of sodium dodecyl sulphate. Two p o i n t s are immediately apparent from these r e s u l t s : (a) the normal p a r t i t i o n law i s obeyed, a t l e a s t over the range of c o n c e n t r a t i o n s i n v e s t i g a t e d , and (b) the p a r t i t i o n c o e f f i c i e n t i s independent of the pH of the aqueous phase. The f i r s t of these observations i m p l i e s that c o b a l t ( I I I ) a c e t y l a c e t o n a t e has the same molecular complexity i n both the aqueous and the butadiene phases. These r e s u l t s a l s o show that the i n i t i a t o r p a r t i t i o n s i n such a way that i t s c o n c e n t r a t i o n i n the butadiene phase i s c o n s i d e r a b l y greater than that i n the aqueous phase; i n f a c t , the p a r t i t i o n c o e f f i c i e n t f o r t h i s p a r t i c u l a r aqueous phase at 50°C i s a p p r o x i mately 6.54. The e f f e c t of s u r f a c t a n t c o n c e n t r a t i o n upon the s o l u b i l i t y of c o b a l t ( I I I ) a c e t y l a c e t o n a t e i n b u f f e r e d aqueous phases at 50°C was also investigated spectroscopically. Saturated s o l u t i o n s were prepared by shaking excess finely-powdered c o b a l t ( I I I ) a c e t y l a c e tonate w i t h s o l u t i o n s of sodium dodecyl sulphate f o r approximately one hour at 50°C. I t appeared that one hour was s u f f i c i e n t time f o r the attainment of e q u i l i b r i u m . The r e s u l t s of these experiments are shown i n F i g u r e 16, from which i t i s apparent that the s a t u r a t i o n s o l u b i l i t y of the i n i t i a t o r



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ο

0.5

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Figure 15. Plot of Co(III) (acac) concentration in butadiene, C , against the Co (III) (acac) concentration in the total aque ous phase, C , for the determination of the partition coeffi cient, P, at 50°C. The aqueous phase contained sodium dodecyhulfate (25 g/l). The line was obtained by linear regression. Circles at pH = 11.1 and squares at pH = 8.5. 3

B

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Aq



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i n c r e a s e s as the s u r f a c t a n t c o n c e n t r a t i o n i s i n c r e a s e d . T h i s i s most n a t u r a l l y i n t e r p r e t e d as solubilisâtion of the i n i t i a t o r w i t h i n the s u r f a c t a n t m i c e l l e s . In order f u r t h e r to i n t e r p r e t these r e s u l t s , i t i s necessary to attempt to d i s t i n g u i s h the weight of c o b a l t ( I I I ) a c e t y l a c e t o n a t e which i s present i n the s u r f a c t a n t m i c e l l e s (denoted by i n F i g u r e 16) from that which i s present i n s o l u t i o n i n the t r u e aqueous phase (denoted by i n F i g u r e 16). The approach adopted was to assume that i s p r o p o r t i o n a l to the volume of the t r u e aqueous phase (which was c a l c u l a t e d by s u b t r a c t ing the estimated m i c e l l a r volume from the t o t a l volume of the aqueous phase), and then to o b t a i n by d i f f e r e n c e between and the t o t a l weight of c o b a l t ( I I I ) a c e t y l a c e t o n a t e known to be present i n u n i t volume of the aqueous phase. The i n t e r e s t i n g o b s e r v a t i o n has been made that i s approximately p r o p o r t i o n a l to the square root of the s u r f a c t a n t c o n c e n t r a t i o n . The r e s u l t s are given i n F i g u r e 17. Two s e t s of data are shown i n t h i s graph. The f i r s t set (the p o i n t s ) are f o r as a f u n c t i o n of the t o t a l s u r f a c t ant c o n c e n t r a t i o n i n the aqueous s o l u t i o n . The second s e t (the points ) are f o r W as a f u n c t i o n of what we have termed the " e f f e c t i v e " s u r f a c t a n t c o n c e n t r a t i o n , that i s , the a c t u a l s u r f a c t a n t c o n c e n t r a t i o n minus the minimum c o n c e n t r a t i o n of s u r f a c t a n t which i s a p p a r e n t l y necessary f o r s o l u b i l i s a t i o n to occur at a l l . (For reasons which are not understood, t h i s c o n c e n t r a t i o n i s c o n s i d e r a b l y i n excess of the c r i t i c a l m i c e l l e concentration.) C l e a r l y , the p r o p o r t i o n a l i t y between W and the square root of the s u r f a c t a n t c o n c e n t r a t i o n i s obeyed more exactly i f the l a t t e r has been c o r r e c t e d by f i r s t s u b t r a c t i n g that c o n c e n t r a t i o n of s u r f a c t a n t which apparent c o n t r i b u t e s nothing to the s o l u b i l i s i n g power of the s o l u t i o n . As f a r as i s known, t h i s i s the f i r s t o c c a s i o n upon which d i r e c t p r o p o r t i o n a l i t y between the c o n c e n t r a t i o n of substance s o l u b i l i s e d and r o o t c o n c e n t r a t i o n of s o l u b i l i s i n g s u r f a c t a n t has been reported. Various models have been i n v e s t i g a t e d i n an attempt to e x p l a i n t h i s r e l a t i o n s h i p , but without success. The s o l u b i l i s a t i o n of c o b a l t ( I I I ) a c e t y l a c e t o n a t e by aqueous s o l u t i o n s of sodium dodecyl sulphate c e r t a i n l y appears to c o n t r a s t w i t h that o f , say, hydrocarbon l i q u i d s by s o l u t i o n s of f a t t y - a c i d soaps, where the r e l a t i o n s h i p between soap c o n c e n t r a t i o n and amount of l i q u i d s o l u b i l i s e d i s more n e a r l y one of d i r e c t p r o p o r t i o n a l i t y . ( 1 8 ) M

14 3. Experiments using C - l a b e l l e d c o b a l t ( I I I ) a c e t y l a c e t o n a t e . P o l y m e r i s a t i o n experiments using C - l a b e l l e d c o b a l t ( I I I ) a c e t y l acetonate as i n i t i a t o r have f a i l e d to produce polymers which c o n t a i n s i g n i f i c a n t amounts of C ^ a c t i v i t y , w h i l s t the observa t i o n s which have been made are not e n t i r e l y c o n c l u s i v e , they suggest that the i n i t i a t i o n mechanism does not r e s u l t i n the a c e t y l a c e t o n e l i g a n d becoming c h e m i c a l l y bound to the polymer. 1

P o s s i b l e Mechanisms f o r I n i t i a t i o n of P o l y m e r i s a t i o n As has been shown above, c e r t a i n f e a t u r e s of the aqueous-phase


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25 L

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s (gl"~* of surfactant solution) Figure 16. Dependence of the saturated solubility of Co (III) (acac) , W , in buffered surfactant solutions on the surfactant concentration. The concentration of Co (III) (acac)s soluble in the true aqueous phase, W , is illustrated by the dotted line. W is the concentration of Co(HI) (acac) soluble in the surfactant micelles. The pH values are as follows: Δ , 11.62; +, 10.84; •, 10.28; and 0,9.56. 3

T

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p o l y m e r i s a t i o n of butadiene i n i t i a t e d by c o b a l t ( I I I ) a c e t y l a c e t o n ate can be i n t e r p r e t e d i n the l i g h t o f the r e s u l t s f o r the decomposition k i n e t i c s of the i n i t i a t o r and o f i t s s o l u b i l i t y behaviour i n aqueous s o l u t i o n s o f s u r f a c t a n t s . There s t i l l remains the problem of the b a s i c mechanism whereby the p o l y m e r i s a t i o n r e a c t i o n i s i n i t i a t e d . There seems to be no reason f o r d i s s e n t i n g from a mechanism which was proposed by one o f the present authors some years ago(17) and which agrees i n c e r t a i n r e s p e c t s w i t h the mechanisms which have been proposed f o r the i n i t i a t i o n o f b u l k and s o l u t i o n f r e e - r a d i c a l p o l y m e r i s a t i o n s by metal a c e t y l a c e t o n a t e s . The f r e e - r a d i c a l a c t i v i t y i s conceived as b e i n g generated by an " i n t e r n a l redox" r e a c t i o n , i n which the c e n t r a l metal i o n i s reduced to a lower valency s t a t e ( i n t h i s case c o b a l t ( I I ) ) by an e l e c t r o n t r a n s f e r from the l i g a n d , t h e l a t t e r then a c q u i r i n g a f r e e - r a d i c a l s i t e by e l e c t r o n i c rearrangement. P o l y m e r i s a t i o n then proceeds by propagation from the f r e e - r a d i c a l s i t e so generated. In the case o f the a c e t y l a c e t o n a t e s , a d r i f t o f e l e c t r o n s from the l i g a n d moiety to the c e n t r a l metal i o n i s presumably f a c i l i t a t e d by the presence of a " q u a s i - a r o m a t i c " system o f conjugated u n s a t u r a t i o n w i t h i n the l i g a n d . Even assuming that the " i n t e r n a l redox" r e a c t i o n i s the primary source o f the r a d i c a l a c t i v i t y through the agency o f which the p o l y m e r i s a t i o n r e a c t i o n o c c u r s , there remain many p o s s i b l e pathways by which i n i t i a t i o n i s e f f e c t e d i n an emulsion system. These p o s s i b i l i t i e s a r i s e because of the presence o f s p e c i e s , such as water and s u r f a c t a n t , w i t h which the primary r a d i c a l can i n t e r a c t . Three p o s s i b i l i t i e s a r e i l l u s t r a t e d i n F i g u r e 18. In each case, the r a d i c a l a c t i v i t y i s a s s o c i a t e d i n the f i r s t i n s t a n c e w i t h a s p e c i e s i n which c o b a l t ( I I I ) has been reduced t o c o b a l t ( I I ) and the a c e t y l a c e t o n a t e l i g a n d has rearranged to g i v e a f r e e r a d i c a l on the methylenic carbon atom. In the f i r s t p o s s i b i l i t y , the monomer r e a c t s d i r e c t l y w i t h t h i s s p e c i e s , and propagation then proceeds i n the normal way. The consequence of such a mechanism would be that the polymer produced would c o n t a i n both c o b a l t ( a l b e i t perhaps more l o o s e l y bound than i n an a c e t y l a c e t o n a t e ) and a moiety d e r i v e d from a c e t y l a c e t o n e . In the second p o s s i b i l i t y , the s p e c i e s which r e s u l t s from the i n t e r n a l redox r e a c t i o n i n t e r a c t s w i t h another molecule i n the r e a c t i o n system (such as water) i n such a way that the r a d i c a l - b e a r i n g e n t i t y i s d i s p l a c e d from the metal complex. The d i s p l a c e d r a d i c a l - b e a r i n g e n t i t y then r e a c t s w i t h monomer t o i n i t i a t e the p o l y m e r i s a t i o n . The consequence o f such a mechanism would be that an a c e t y l a c e t o n e moiety, but not c o b a l t , would become c h e m i c a l l y i n c o r p o r a t e d i n the polymer produced. In the t h i r d p o s s i b i l i t y , the r a d i c a l - b e a r i n g e n t i t y i s d i s p l a c e d as i n the second p o s s i b i l i t y , but undergoes a t r a n s f e r r e a c t i o n w i t h , say, water molecules to produce, say, h y d r o x y l r a d i c a l s which then become the e f f e c t i v e i n i t i a t i n g s p e c i e s . Such a mechanism i s c o n s i s t e n t w i t h the p r o d u c t i o n o f a polymer which c o n t a i n s n e i t h e r c o b a l t nor a c e t y l a c e t o n e r e s i d u e s . To the extent that our e x p e r i ments u s i n g C - l a b e l l e d c o b a l t ( I I I ) a c e t y l a c e t o n a t e g i v e any


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11.

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Figure 18. Possible mechanisms which cobalt (III) acetylacetonate initiates aqueuosphase polymerization


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i n d i c a t i o n s a t a l l , they suggest that a mechanism o f the t h i r d i s probably o p e r a t i v e .

type


Acknowledgements One o f us (W F Η Β) wishes to thank the Inner London Education A u t h o r i t y and The P o l y t e c h n i c o f North London f o r g r a n t i n g a p e r i o d of study leave during which some of the work d e s c r i b e d i n t h i s paper was c a r r i e d out. Thanks a r e a l s o due to Dr Ε W Duck (The I n t e r n a t i o n a l S y n t h e t i c Rubber Company) f o r h e l p f u l d i s c u s s i o n s during i t s progress.

Abstract A description is given of the main features of the aqueous -phase polymerisation of butadiene initiated by cobalt(III) acetyl acetonate in the presence of surfactants such as sodium dodecyl -benzenesulphonate and sodium dodecyl sulphate. It is shown that the polymerisation rate increases as the amount of oxygen in the system decreases, and that, i f oxygen is present, the rate depends upon the length of time for which the reaction system has been stored. The rate of polymerisation is directly proportional to the quantity of aqueous phase. Initial pH has a profound effect upon the reaction rate. The dependences of rate upon the concentrations of initiator and surfactant are complex. Results are presented for the decomposition of cobalt(III) acetylacetonate in aqueous solution in the presence and absence of surfactant, and also for the partitioning of the initiator between hydrocarbon and aqueous phases and its solubility in various aqueous phases. The information obtained from these ancillary investigations has consdierably enhanced understanding of some of the features of the polymerisation reaction.

Literature Cited 1. Arnett, Ε M, Morris, A, and Mendelsohn, M A, J. Am. chem. Soc., (1962) 84, 3821. 2. Kastnig, E G, Naarman, H, Reis, H, and Berding, C, Agnew. Chem. Internat. Edn., (1965) 4, 322. 3. Izawa, S, Shimizu, A, and Kubo, M, Sci. Rep. Toyo Soda Co., (1966) 10, 123. 4. Bamford, C H, and Lind, D J, Chem. Comm., (1966) 792. 5. Otsu, T, Minamii, N, and Nishikawa, Y, J Macromol. Sci. - Chem., (1968) 42, 905. 6. Isawa, Z, and Shibamiya, T, J. Polym. Sci. Part B, (1968) 6, 721. 7. Allen, Ρ Ε M, and Goh, Τ Η, Eur. Polym. J., (1969) 5, 419. 8. Allen, Ρ E M, Ayling, D, and Goh, Τ Η, Eur. Polym. J., (1970) 6, 1587.



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9. Ghanem, Ν A, Yehia, A A, Moustafa, A B, and Rizk, Ν A, Eur. Polym. J., (1971)7,717. 10. Zafar, M M, Yousufzai, Α H Κ, Mahmood, R, and Hussain, S A, J. Polym. Sci. Part A-1, (1972) 10, 1539. 11. Urehara, K, Nishi, T, Matsumura, T, Tamura, F, and Murata, N, Kogyo Kagaku Zasshi, (1967) 70, 191. 12. Urehara, K, Nishi, T, Matsumura, T, Tamura, F, and Murata, N, Kogyo Kagaka Zasshi, (1967) 70, 755. 13. Kaeriyama, K, and Yamazaki, Y, Kogyo Kagaku Zasshi, (1971) 74, 1718. 14. Uehara, K, Kataoka, Y, Tanaka, M, and Murata, N, Kogyo Kagaku Zasshi, (1969) 72, 754. 15. Kaeriyama, K, Bull. Chem. Soc. Japan, (1970) 43, 1511. 16. Blackley, D C, and Matthan, R K, Br. Polym. J., (1970) 2, 25. 17. Blackley, D C, Proceedings of 4th International Rubber Symposium, (1969) 15. 18. Stearns, R S, Oppenheimer, H, Simon, E, and Harkins, W D, J. Chem. Phys., (1947) 15, 496.


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