Anionic Polymerization: Some Commercial ... - ACS Publications

25 Anionic Polymerization: Some Commercial Applications H. L. HSIEH and R.C.FARRAR—Phillips Petroleum Co., Bartlesville, OK 74004

Downloaded by PURDUE UNIV on June 7, 2016 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0166.ch025

K. UDIPI—Monsanto Plastic & Resins Co., Indian Orchard, MA 01151

The phenomenal growth in commercial production of polymers by anionic polymerization can be attributed totheunprecedented cont r o l the process provides over the polymer properties. This cont r o l is most extensive in organolithium i n i t i a t e d polymerizations and includes polymer composition, microstructure, molecular weight, molecular weight distribution, choice of functional end groups and even monomer sequence distribution in copolymers. Furthermore, a judicious choice of process conditions affords termination and transfer free polymerization which leads to very efficient methods of block polymer synthesis. Commercial polymers from organolithium i n i t i a t o r systems cover a wide range, and P h i l l i p s commitment in this f i e l d on a world-wide basis is illustrated in Table I. TABLE I ORGANOLITHIUM BASED SOLUTION POLYMERIZATION PLANTS (Phillips) Plant

Date o f Commission

B o r g e r , USA Calatrava, Spain PACL, A u s t r a l i a Negromex, M e x i c o Petrochim, Belgium JEC, Japan ANIC, I t a l y 0

a. b. c.

December, 1962 A u g u s t , 1966 December, 1966 A u g u s t , 1967 J a n u a r y , 1968 A p r i l , 1969 S e p t e m b e r , 1972

Annual Capacity M e t r i c Tons 48,000 105,000 23,000 55,000 60,000 32,000 25,000

W h o l l y owned J o i n t venture Licensee

Work o n o r g a n o m e t a l i n i t i a t o r s y s t e m s f o r s o l u t i o n p o l y m e r i z a t i o n i n c r e a s e d s i g n i f i c a n t l y w i t h the d i s c o v e r y o f the t i t a n i u m based i n i t i a t o r s f o r o l e f i n p o l y m e r i z a t i o n by K a r l Z i e g l e r . 0097-6156/81/0166-0389$05.00/0 © 1981 A m e r i c a n Chemical Society

McGrath; Anionic Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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T h e s e e f f o r t s c o u p l e d w i t h the much earlier work on s o d i u m and l i t h i u m i n i t i a t e d p o l y m e r i z a t i o n s l e d t o a n a p p r e c i a t i o n o f the s t e r e o s p e c i f i c i t y o f the a l k y l l i t h i u m i n i t i a t o r s f o r d i e n e p o l y m e r i z a t i o n b o t h i n d u s t r i a l l y and a c a d e m i c a l l y . Polymerization of i s o p r e n e t o a h i g h c i s p o l y i s o p r e n e w i t h b u t y l l i t h i u m is w e l l known and the d e t a i l s have b e e n w e l l d o c u m e n t e d . i ~ 2 C o n t r o l o v e r p o l y b u t a d i e n e s t r u c t u r e h a s a l s o b e e n demonstrated.8. T h i s r e p o r t a t t e m p t s t o s u r v e y the u n i q u e f e a t u r e s o f a n i o n i c p o l y m e r i z a t i o n w i t h a n e m p h a s i s on the c h e m i s t r y and its c o m m e r c i a l a p p l i c a t i o n s and is n o t i n t e n d e d as a c o m p r e h e n s i v e r e v i e w .


Polydienes In a l k y l l i t h i u m i n i t i a t e d , s o l u t i o n polymerization of dienes, some p o l y m e r i z a t i o n c o n d i t i o n s a f f e c t the c o n f i g u r a t i o n s more than others. I n g e n e r a l , the s t e r e o c h e m i s t r y o f p o l y b u t a d i e n e and p o l y i s o p r e n e r e s p o n d t o the same v a r i a b l e s . 2 . T h u s , s o l v e n t h a s a p r o f o u n d i n f l u e n c e on the s t e r e o c h e m i s t r y o f p o l y d i e n e s when i n i t i a t e d with alkyllithium. P o l y m e r i z a t i o n o f i s o p r e n e in n o n p o l a r s o l v e n t s r e s u l t s l a r g e l y in c i s - u n s a t u r a t i o n (70-90 p e r c e n t ) w h e r e a s in the c a s e o f b u t a d i e n e , the p o l y m e r e x h i b i t s a b o u t e q u a l amounts o f c i s - and t r a n s - u n s a t u r a t i o n . A r o m a t i c s o l v e n t s s u c h as t o l u e n e t e n d t o i n c r e a s e the 1,2 o r 3,4 l i n k a g e s . P o l y m e r s p r e p a r e d in the p r e s e n c e o f a c t i v e p o l a r compounds s u c h a s e t h e r s , t e r t i a r y amines o r s u l f i d e s show i n c r e a s e d 1,2 ( o r 3,4 in the case o f i s o p r e n e ) and t r a n s u n s a t u r a t i o n . . , I t a p p e a r s t h a t the s o l v e n t i n f l u e n c e s the i o n i c c h a r a c t e r o f the p r o p a g a t i n g i o n p a i r w h i c h in t u r n d e t e r m i n e s the s t e r e o c h e m i s t r y . F i g u r e s 1 and 2 show the dependence o f p o l y m e r m i c r o s t r u c t u r e on the m o l e c u l a r w e i g h t o f the p o l y m e r and t h e r e f o r e on the i n i t i a l i n i t i a t o r c o n c e n t r a t i o n . The p o l y m e r i z a t i o n t e m p e r a t u r e a l s o h a s an e f f e c t on the m i c r o s t r u c t u r e a s can be s e e n in F i g u r e 3 f o r polybutadiene. The o v e r a l l h e a t a c t i v a t i o n e n e r g y l e a d i n g t o 1,2 a d d i t i o n is g r e a t e r t h a n t h a t l e a d i n g t o 1,4 addition.2., IZ I n summary, the s t e r e o c h e m i s t r y o f p o l y m e r i z a t i o n o f b u t a d i e n e and i s o p r e n e is s e n s i t i v e t o i n i t i a t o r l e v e l , p o l y m e r i z a t i o n temp e r a t u r e and s o l v e n t . The i n i t i a t o r s t r u c t u r e ( i . e . , o r g a n i c m o i e t y o f the i n i t i a t o r ) , the monomer c o n c e n t r a t i o n and c o n v e r s i o n have e s s e n t i a l l y no e f f e c t on p o l y m e r m i c r o s t r u c t u r e . F i r e s t o n e and S h e l l s t a r t e d c o m m e r c i a l p r o d u c t i o n o f c i s p o l y i s o p r e n e by the a n i o n i c p r o c e s s in the 1950's b u t t h e s e p l a n t s a r e no l o n g e r in o p e r a t i o n now. A b o u t the same t i m e , P h i l l i p s s t a r t e d the m a n u f a c t u r e o f p o l y b u t a d i e n e s b y the a n i o n i c r o u t e and e v e r s i n c e , t h e r e h a s b e e n a s t e a d y g r o w t h in t h e i r u s e , p a r t i c u l a r l y in the t i r e - t r e a d a s w e l l as t i r e - c a r c a s s f o r m u l a t i o n s . These s o l u t i o n p o l y b u t a d i e n e s , g e n e r a l l y , have low v i n y l c o n t e n t s b u t r e c e n t l y , P h i l l i p s h a s f o u n d some i n t e r e s t i n g a p p l i c a t i o n s f o r medium v i n y l p o l y b u t a d i e n e s as w e l l . 1 4 P o l y b u t a d i e n e s w i t h 50-55 p e r c e n t v i n y l c o n t e n t s b e h a v e l i k e e m u l s i o n p o l y m e r i z e d SBR in t i r e t r e a d f o r m u l a t i o n s and e x h i b i t v e r y s i m i l a r t r e a d


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INHERENT VISCOSITY AT 100% CONVERSION Journal of Polymer Science Figure 1.

Microstructure of polybutadiene initiated with alkyllithium in cyclohexane at 50°C... (9).

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Microstructure of polyisoprene initiated with alkyllithium in cyclohexane at 50°C... (9).

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The effect of temperature on microstructure of polybutadiene prepared in cyclohexane (9).



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w e a r , wet s k i d r e s i s t a n c e , low h e a t b u i l d - u p , e t c . A t h r e e - w a y b l e n d o f 45 p e r c e n t v i n y l p o l y b u t a d i e n e w i t h SBR and c i s - p o l y b u t a d i e n e (30/35/35) was e v e n b e t t e r in t h a t r e s p e c t t h a n a 65/35 b l e n d o f SBR and c i s - p o l y b u t a d i e n e . The medium v i n y l p o l y b u t a d i e n e s w e r e a l s o f o u n d t o be e f f e c t i v e as p a r t i a l o r c o m p l e t e r e p l a c e m e n t f o r SBR in a v a r i e t y o f n o n - t i r e a p p l i c a t i o n s . P o l y d i e n e s p r e p a r e d w i t h enough i n i t i a t o r t o g i v e a m o l e c u l a r w e i g h t o f 250,000 w i l l h a v e a d e s i r a b l e c o m b i n a t i o n o f v u l c a n i z a t e p r o p e r t i e s , b u t a l s o w i l l h a v e h i g h c o l d f l o w , h i g h compounded Mooney v i s c o s i t y and p o o r p r o c e s s a b i l i t y . I n c r e a s i n g the m o l e c u l a r w e i g h t does l o w e r the c o l d f l o w , b u t g r e a t l y d e c r e a s e s the processability. L o w e r i n g the m o l e c u l a r w e i g h t i m p r o v e s the p r o c e s s a b i l i t y but leads t o i n t o l e r a b l e c o l d flow problems. The s o l u t i o n t o t h e s e p r o b l e m s is u s u a l l y f o u n d by a l t e r i n g the m o l e c u l a r w e i g h t d i s t r i b u t i o n a n d / o r i n t r o d u c i n g b r a n c h i n g in the p o l y m e r m o l e c u l e . T h e r e a r e s e v e r a l ways t o b r o a d e n the m o l e c u l a r w e i g h t d i s tribution. An a l k y l l i t h i u m can be p i c k e d w h i c h w i l l g i v e a s l o w rate o f i n i t i a t i o n compared t o the rate o f p o l y m e r i z a t i o n G e n e r a l l y , t h i s r e s u l t s in a r e l a t i v e l y m i n o r b r o a d e n i n g as shown by the GPC c u r v e in F i g u r e s 4A and 4B. An i n i t i a t o r o f l i m i t e d s o l u b i l i t y can be used so t h a t it s l o w l y d i s s o l v e s and i n i t i a t e s p o l y m e r i z a t i o n t h r o u g h o u t the p r o c e s s . However, a more c o n t r o l l a b l e and p r a c t i c a l p r o c e d u r e is c o n t i n u o u s i n i t i a t o r a d d i t i o n to a b a t c h process By p r o g r a m m i n g the rate o f a d d i t i o n , a v a r i e t y o f m o l e c u l a r w e i g h t d i s t r i b u t i o n s can be o b t a i n e d . A v e r y common t e c h n i q u e f o r b r o a d e n i n g the m o l e c u l a r w e i g h t d i s t r i b u t i o n is c o n t i n u o u s p o l y m e r i z a t i o n ( s e e F i g u r e 5 ) . H e r e , all r e a g e n t s a r e added c o n t i n u o u s l y t o the r e a c t o r and the p r o d u c t is c o n s t a n t l y w i t h d r a w n a t the same rate. The m o l e c u l a r w e i g h t d i s t r i b u t i o n o f p o l y m e r f r o m s u c h a s y s t e m is q u i t e b r o a d w i t h b o t h v e r y l o w and v e r y h i g h m o l e c u l a r w e i g h t s p e c i e s . P r o c e s s a b i l i t y o f t h e s e b r o a d m o l e c u l a r w e i g h t p r o d u c t s is i m p r o v e d by the p r e s e n c e o f l o w m o l e c u l a r w e i g h t m a t e r i a l , b u t c o l d f l o w still can be u n a c c e p t a b l y h i g h . C o l d f l o w can be most e f f e c t i v e l y d e c r e a s e d and p e r f o r m a n c e o f the f i n a l p r o d u c t i m p r o v e d by b r a n c h i n g the p o l y m e r m o l e c u l e . A v a r i e t y o f t e c h n i q u e s h a v e b e e n used t o a c c o m p l i s h t h i s . B r a n c h i n g comonomers s u c h as d i v i n y l b e n z e n e can be used.16 The amount needed t o b r a n c h the p o l y m e r a d e q u a t e l y is g e n e r a l l y so low t h a t the r e a g e n t can h a r d l y be d e t e c t e d in the f i n a l p r o d u c t . A d d i t i v e s w h i c h c a u s e m e t a l a t i o n o f the p o l y m e r c h a i n s can be i n c l u d e d in the p o l y m e r i z a t i o n m i x t u r e o r added a f t e r p o l y m e r i z a t i o n t o c r e a t e new g r o w t h s i t e s a l o n g the p o l y m e r c h a i n . U Or, p o l y m e r can be h e a t s o a k e d by i n c r e a s i n g the r e s i d e n c e t i m e . W h i l e t h e s e methods r e s u l t in random b r a n c h i n g w i t h v e r y l i t t l e c o n t r o l o v e r p l a c e m e n t o r e x t e n t o f b r a n c h i n g in any p a r t i c u l a r m o l e c u l e , the p r o d u c t s can be q u i t e u s e f u l c o m m e r c i a l l y .


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L i n e a r p o l y m e r s can be b r a n c h e d by e m p l o y i n g a p o l y f u n c t i o n a l c o u p l i n g a g e n t a t the end o f p o l y m e r i z a t i o n . T h i s t e c h n i q u e h a s been used t o p r e p a r e model polymers t o a l l o w r h e o l o g i c a l s t u d ieslSiiS. o f s p e c i f i c t y p e s o f b r a n c h i n g and c o m m e r c i a l l y f o r some r u b b e r s p r o d u c e d by P h i l l i p s . By c o n t r o l l i n g the f u n c t i o n a l i t y o f the r e a g e n t , it is p o s s i b l e t o p r e p a r e n e a r l y p u r e t r i c h a i n , t e t r a c h a i n , s t a r - and comb-shaped b r a n c h e d p o l y m e r s . (see F i g u r e 6 ) . Use o f a p o l y f u n c t i o n a l i n i t i a t o r and b r a n c h i n g c o monomer in c o n j u n c t i o n w i t h t e r m i n a l c o u p l i n g w i l l r e s u l t in b o t h b r a n c h i n g and m o l e c u l a r w e i g h t b r o a d e n i n g . I n t r o d u c t i o n o f one o r two l o n g c h a i n b r a n c h e s i n t o a p o l y diene molecule t o form t r i - o r t e t r a c h a i n molecules l e a d s t o p r o f o u n d c h a n g e s in r h e o l o g i c a l b e h a v i o r . I t has b e e n foundlS t h a t in p o l y b u t a d i e n e , a t low m o l e c u l a r w e i g h t s , the N e w t o n i a n v i s c o s i t y is d e c r e a s e d r e l a t i v e t o a l i n e a r p o l y m e r o f same m o l e c u l a r w e i g h t ( s e e F i g u r e 7 ) . A t m o l e c u l a r w e i g h t s e x c e e d i n g 60,000 ( t r i c h a i n ) o r 100,000 ( t e t r a c h a i n ) , the N e w t o n i a n v i s c o s i t y r i s e s r a p i d l y above the c o r r e s p o n d i n g v a l u e f o r a l i n e a r p o l y b u t a d i e n e . F i g u r e 8 shows the r e l a t i o n s h i p b e t w e e n v i s c o s i t y (η) and m o l e c u l a r w e i g h t (Mw) a t h i g h e r s h e a r rate (s = 20 sec-1). A t t h i s s h e a r rate, the v i s c o s i t y o f the b r a n c h e d p o l y m e r s is u n i f o r m l y l o w e r t h a n t h a t o f the l i n e a r s a m p l e s o f i d e n t i c a l m o l e c u l a r weight. I n o t h e r w o r d s , the n o n - N e w t o n i a n b e h a v i o r o f b r a n c h e d p o l y m e r s becomes r a p i d l y more p r o n o u n c e d a t h i g h e r m o l e c u l a r weight. L o n g c h a i n b r a n c h e d p o l y m e r has h i g h e r r e s i s t a n c e t o f l o w a t l o w s h e a r rate ( i . e . , l o w c o l d f l o w ) and more f l o w a t m o d e r a t e and h i g h s h e a r rate ( i . e . , b e t t e r p r o c e s s i n g ) t h a n the c o r r e s ponding l i n e a r polymer. Diene-Styrene

Copolymers

I n the c o p o l y m e r i z a t i o n o f b u t a d i e n e o r i s o p r e n e and s t y r e n e , r e a c t i v i t y r a t i o s a r e i n f l u e n c e d by the t y p e o f s o l v e n t usedJLl ,1Ζ,2ύί T y p i c a l c o n v e r s i o n c u r v e s o f a 75/25 b u t a d i e n e / s t y r e n e c o p o l y m e r i z a t i o n in v a r i o u s h y d r o c a r b o n s o l v e n t s a r e shown in F i g u r e 9. I t is n o t i c e d t h a t the s o l v e n t s change o n l y the o v e r a l l r a t e s b u t the g e n e r a l s h a p e s o f the c u r v e s r e m a i n s i m i l a r . As one a n a l y z e s s a m p l e s a t v a r i o u s c o n v e r s i o n s ( F i g u r e 10) it is o b s e r v e d t h a t the s t y r e n e c o n t e n t s a r e i n i t i a l l y l o w e r t h a n in the monomer c h a r g e and g r a d u a l l y i n c r e a s e u n t i l the in f l e c t i o n p o i n t s o f the c o n v e r s i o n c u r v e s o f F i g u r e 9 a r e a t t a i n e d and t h e r e a f t e r i n c r e a s e v e r y r a p i d l y . F u r t h e r m o r e , in the a n a l y s i s o f the s a m p l e s by o x i d a t i v e d e g r a d a t i o n , polystyrene seg ments a r e r e c o v e r e d o n l y a f t e r the i n f l e c t i o n p o i n t s a r e r e a c h e d and the same i n c r e a s e t h e r e a f t e r ( s e e F i g u r e 1 0 ) . S i n c e the f i n a l p o l y m e r s a r e e s s e n t i a l l y homogeneous in c o m p o s i t i o n and in m o l e c u l a r w e i g h t , it f o l l o w s t h a t the p r o c e s s has r e s u l t e d in a t a p e r e d ( o r g r a d e d ) b l o c k c o p o l y m e r , one segment b e i n g a b u t a d i e n e - s t y r e n e c o p o l y m e r and the o t h e r , a p o l y s t y r e n e b l o c k w i t h an o v e r a l l B/S-S t y p e s t r u c t u r e . T h e s e r e s u l t s seem t o be in the


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Figure 7. Dependence of Newtonian viscosity on molecular weight: linear (Φ); trichain (HJ; tetrachain (A) (20).

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Polymerization of butadiene-styrene in different solvents at 50°C... (39).


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c o n t r a s t t o what is g e n e r a l l y known a b o u t a n i o n i c h o m o p o l y m e r i z a t i o n o f s t y r e n e , i . e . , s t y r e n e homopolymerizes f a s t e r than b u t a d i e n e a n d y e t , in a m i x t u r e , b u t a d i e n e p o l y m e r i z e s first. How e v e r , if one e x a m i n e s the c r o s s p r o p a g a t i o n r a t e s in h y d r o c a r b o n solvents,-Z, ΖΣ'11 the r e a s o n f o r t h i s b e h a v i o r is e v i d e n t .

k

ss

k

sb

k

bb

Where r is a b o u t 50 t i m e s l a r g e r t h a n r . The i n v e r s i o n phenomenon, a s one w o u l d e x p e c t f r o m a k i n e t i c p o i n t o f v i e w , is i n d e p e n d e n t o f p o l y m e r i z a t i o n t e m p e r a t u r e and is shown in Figure 11. O n l y a l i m i t e d number o f monomer p a i r s f o r m b l o c k c o p o l y m e r s in t h i s manner. E x a m p l e s a r e c o n j u g a t e d d i e n e s a n d v i n y l a r o m a t i c s t h a t h a v e s i m i l a r Q-e v a l u e s . The n a t u r e o f the a n i o n i c i n i t i a t o r , i . e . , the i o n i c c h a r a c t e r o f the c a r b o n - m e t a l b o n d p l a y s an i m p o r t a n t r o l e in b o t h the amount a n d s e q u e n c e o f b l o c k f o r m a t i o n . F o r i n s t a n c e , when p o t a s s i u m o r c e s i u m i n i t i a t o r s a r e used, styrene polymerizes first a s c a n be s e e n in F i g u r e 1 2 . A t a p e r e d b l o c k c o p o l y m e r c o n t a i n i n g 75 p e r c e n t b u t a d i e n e and 25 p e r c e n t s t y r e n e , m a r k e t e d a s S o l p r e n e 1 2 0 5 , was the first s o l u t i o n c o p o l y m e r p r o d u c e d c o m m e r c i a l l y b y P h i l l i p s in 1 9 6 2 . T h i s polymer has o u t s t a n d i n g e x t r u s i o n c h a r a c t e r i s t i c s , low water a b s o r p t i o n , l o w a s h a n d good e l e c t r i c a l p r o p e r t i e s . A s a r e s u l t , it is u s e f u l in s u c h d i v e r s e a p p l i c a t i o n s a s w i r e a n d c a b l e c o v e r i n g s , show s o l e s , f l o o r t i l e s , e t c . I t was a l s o d i s c o v e r e d a t PhillipsiLâ. t h a t the f o u r rate c o n s t a n t s d i s c u s s e d above c a n b e a l t e r e d b y the a d d i t i o n o f s m a l l amounts o f a n e t h e r o r a t e r t i a r y amine r e s u l t i n g in r e d u c t i o n o r e l i m i n a t i o n o f the b l o c k f o r m a t i o n . F i g u r e s 13 a n d 14 i l l u s t r a t e the e f f e c t o f d i e t h y l e t h e r on the rate o f c o p o l y m e r i z a t i o n and on the i n c o r p o r a t i o n o f s t y r e n e in the c o p o l y m e r . Indeed, random c o p o l y m e r s o f b u t a d i e n e a n d s t y r e n e o r i s o p r e n e a n d s t y r e n e c a n be p r e p a r e d by u s i n g a l k y l l i t h i u m as i n i t i a t o r in the p r e s e n c e o f s m a l l amounts o f a n e t h e r o r a t e r t i a r y a m i n e . Use o f t h e s e p o l a r r a n d o m i z e r s a l s o i n c r e a s e s the v i n y l u n s a t u r a t i o n in the c o p o l y m e r . B u t a d i e n e - s t y r e n e random c o p o l y m e r s c a n a l s o be prepared by a very slow and continuous a d d i t i o n o f s

monomers25. o r by an incremental addition o f butadiene t o a s t y r e n e - r i c h monomer m i x t u r e

during polymerization.

T h e s e two


398

ANIONIC POLYMERIZATION 25 •

A

20

* •

H ΕΧΑΝΕ C...IXLOHEXA NE Β ΕΝΖΕΝΕ Τ 3LUENE

TOTAL STYRE

15

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10 5

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0

20

40

60

80

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Syracuse University Press Figure

10.

Copolymerization of styrene from butadiene-styrene (75/25) at 50°C... (39).

~7

POLYMERIZATION TEMP.. C... • • * • Ο

0

/.

ou 50 100 107 121

/ •••Α...-

1

20

40

60

80

100

CONVERSION, % Syracuse University Press Figure 11.

Styrene incorporation from butadiene-styrene (75/25) in cyclohexane solutions at 30-121°C... (39).

70 60 Κ

50

BOUND STYRENE, %

40 30

ΙΤΙΑ

ι

r i A R f iP

BUTADIENE 75 STYR ENE 25

20 10 0

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10 20 30 40 50 60 70 80 90 100

CONVERSION, % Syracuse University Press Figure 12.

Styrene incorporation (39).



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ET AL.

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399

m e t h o d s , u n l i k e the e t h e r o r t e r t i a r y amine c o m p l e x i n g s y s t e m s , would produce copolymers o f low v i n y l u n s a t u r a t i o n < 1 0 % ) . Cons t a n t c o m p o s i t i o n c o p o l y m e r s c a n a l s o be p r o d u c e d by c o n t i n u o u s polymerization. T h e r e is y e t a n o t h e r g e n e r a l method t o p r e p a r e random c o p o l y mer. As s t a t e d earlier, when one u s e s p o t a s s i u m , r u b i d i u m o r cesium i n i t i a t o r , s t y r e n e polymerizes first, t o g i v e a S/B-B t y p e of tapered b l o c k polymer. B u t when one m i x e s an a l k y l l i t h i u m w i t h a p o t a s s i u m compound s u c h a s p o t a s s i u m t - b u t o x i d e , q u i t e a d i f f e r e n t s y s t e m is o b t a i n e d . 3 0 - 2 A l k y l l i t h i u m compounds a s w e l l a s p o l y m e r - l i t h i u m a s s o c i a t e not o n l y w i t h themselves b u t a l s o w i t h other a l k a l i m e t a l a l k y I s and a l k o x i d e s . I n a p o l y m e r i z a t i o n i n i t i a t e d w i t h combinations o f a l k y l l i t h i u m s a n d a l k a l i m e t a l a l k o x i d e s , dynamic t a u t o m e r i c e q u i l i b r i a b e t w e e n c a r b o n - m e t a l bonds a n d o x y g e n - m e t a l bonds e x i s t and l e a d t o p r o p a g a t i o n c e n t e r s h a v i n g the c h a r a c t e r i s t i c s o f both m e t a l s , u s u a l l y somewhere in b e t w e e n . T h i s way, one c a n p r e p a r e copolymer's o f v a r i o u s randomness a n d v a r i o u s v i n y l u n s a t u r a t i o n . T h i s r e a c t i o n is q u i t e g e n e r a l a s one c a n a l s o u s e s o d i u m , r u b i d ium o r c e s i u m compounds t o g e t d i f f e r e n t e f f e c t s . The s o l u t i o n random c o p o l y m e r g e n e r a l l y c o n t a i n s a b o u t 32 p e r c e n t c i s - , 41 p e r c e n t t r a n s - a n d 27 p e r c e n t v i n y l - u n s a t u r a t i o n compared t o 8 p e r c e n t c i s - , 74 p e r c e n t t r a n s - a n d 18 p e r c e n t v i n y l - u n s a t u r a t i o n in e m u l s i o n c o p o l y m e r o f the same monomer composition. The p r i n c i p a l e f f e c t o f s l i g h t l y h i g h e r v i n y l u n s a t u r a t i o n in s o l u t i o n c o p o l y m e r is a s m a l l i n c r e a s e in the g l a s s t r a n s i t i o n t e m p e r a t u r e (-58C...v e r s u s -62C...f o r the e m u l s i o n copolymer). However, b o t h s o l u t i o n a n d e m u l s i o n p o l y m e r i z e d c o p o l y m e r s e x h i b i t s a t i s f a c t o r y low temperature performance f o r g e n e r a l uses. I n h e r e n t l y , s o l u t i o n copolymers have v e r y narrow m o l e c u l a r weight d i s t r i b u t i o n s . T y p i c a l molecular weight d i s t r i b u t i o n c u r v e s f o r the s o l u t i o n a n d e m u l s i o n c o p o l y m e r s a r e shown in F i g u r e 1 5 . The low m o l e c u l a r w e i g h t component in the e m u l s i o n c o p o l y m e r h e l p s it p r o c e s s b e t t e r , b u t h a s a n u n d e s i r a b l e i n f l u e n c e on h y s t e r e s i s . The s o l u t i o n c o p o l y m e r on the o t h e r hand is c h a r a c t e r i z e d b y a s h a r p peak w i t h o n l y a s m a l l amount o f h i g h e r molecular weight f r a c t i o n . T h i s f e a t u r e l e a d s t o improved h y s t e r e s i s p r o p e r t i e s and g i v e s b e t t e r a b r a s i o n r e s i s t a n c e , b u t a l s o r e s u l t s in more d i f f i c u l t p r o c e s s i n g . The s o l u t i o n c o p o l y m e r s , l i k e p o l y d i e n e s , a r e a l s o known t o e x h i b i t c o l d f l o w a n d a s in the c a s e o f d i e n e s , the s o l u t i o n t o t h e s e two p r o b l e m s is found by a l t e r i n g the m o l e c u l a r w e i g h t d i s t r i b u t i o n a n d / o r b r a n c h i n g o f the p o l y m e r m o l e c u l e . B r a n c h i n g o f random c o p o l y m e r s c a n be accomplished as mentioned earlier f o r polybutadienes. In addit i o n , P h i l l i p s has developed a s e r i e s o f m u l t i c h e l i c i n i t i a t o r s which are e t h e r - f r e e , hydrocarbon s o l u b l e , m u l t i p l e - l i t h i u m i n i t i a t o r s t o produce both b r a n c h i n g and m o l e c u l a r weight broadeni n g in many o f o u r random c o p o l y m e r s b o t h l o w a n d medium v i n y l . T h e s e i n i t i a t o r s a r e b a s e d upon the r e a c t i o n o f an a l k y l l i t h i u m s u c h a s s e c - b u t y l l i t h i u m a n d a m u l t i f u n c t i o n a l v i n y l compound such as d i v i n y l b e n z e n e , 3 3 diisopropenylbenzene,34 t r i v i n y l p h o s -


400

ANIONIC POLYMERIZATION 100

"A CONVERSION, %

I

•

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CYCLOHEXANE DIETHYL ETHER BUTYLLITHIUM

Ύ Τ • / / /

60

1000 VARIABLE 0.13

120 180 240 300


TIME. MINUTES Figure 13.

Effect of amount of diethyl ether on rate of copolymerization at 50°C... (16).

25 Et 0 2

20 BOUND STYRENE, %

15

20

Figure

15.

•""

j

10 5

Figure 14.

—.

PARTS

i

I .—.

40 60 80 CONVERSION, %

100

Effect of amount of diethyl ether styrene incorporation at 50°C... (16).

Molecular weight distribution of anionically polymerized styrenebutadiene random copolymer and emulsion polymerized SBR.


25.

HSiEH E T A L .

Commercial

Applications

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Polymerization

phine,25,26 o r t e t r a v i n y l s i l a n e . 3 5 , 3 6 F u n c t i o n a l i t y o f these i n i t i a t o r s can be v a r i e d t o provide more o r l e s s branching and/or molecular weight broadening in these polymers. S o l u t i o n random copolymers prepared by the above procedures have performed w e l l in t i r e - t r e a d formulations.37 They r e q u i r e about 20 percent l e s s a c c e l e r a t o r (see Table II) as compared t o an emulsion SBR and give higher compounded Mooney, lower heat b u i l d up, increased r e s i l i e n c e and b e t t e r r e t r e a d abrasion index. TABLE I I


EVALUATION OF ALKYLLITHIUM INITIATED BUTADIENE-STYRENE RANDOM COPOLYMER (SOLPREN

1204) IN COMPOUNDED STOCK Solprene 1204

Rubber ISAF Black Highly Aromatic O i l Sulfur Accelerator Processing: Compounded Mooney, ML-4 Scorch a t 280°F E x t r u s i o n a t 250°F grams/minute Rating (12 is p e r f e c t ) M i l l Banding Laboratory T e s t s * 300% Modulus, p s i Tensile, psi Elongation, % Heat Build-up, °F Resilience, % Blowout, minutes Shore A Hardness Retread T i r e Test Performance Abrasion Index

SBR 1500

100 60 25 2.0 1.1

100 60 25 2.0 1.3

44 17

36 17

115 17 Good

105 17 Good

1120 3100 630 64 59 4 59

1080 3050 600 69 52 4 59

114

100

Compounds cured 30 minutes at 307°F. So f a r the d i s c u s s i o n was focused on copolymers d e r i v e d from a mixture o f styrene and a diene. In view o f the " l i v i n g nature of organolithium p o l y m e r i z a t i o n , it is a l s o p o s s i b l e t o synthesize b l o c k polymers in which the sequence and length o f the blocks are c o n t r o l l e d by incremental (or s e q u e n t i a l ) a d d i t i o n of monomers?-" T h i s general method o f preparing block polymers is r e a d i l y adaptable t o commercial production, and, indeed, a number of block c o polymers are manufactured t h i s way. Those that have r e c e i v e d the most a t t e n t i o n in recent years are the diene-styrene two-phase 1 1



402

s y s t e m s w i t h two d i s t i n c t l y d i f f e r e n t g l a s s t r a n s i t i o n t e m p e r a t u r e s . T h e s e p o l y m e r s h a v e t h r e e o r more b l o c k s and a r e c h a r a c t e r i z e d b y t h e i r h i g h raw s t r e n g t h , c o m p l e t e s o l u b i l i t y in com mon o r g a n i c s o l v e n t s a n d t h e r m o p l a s t i c i t y . They a r e r e f e r r e d t o as " t h e r m o p l a s t i c e l a s t o m e r s " b e c a u s e t h e y b e h a v e a s v u l c a n i z e d e l a s t o m e r s a t room t e m p e r a t u r e and y e t c a n be p r o c e s s e d a s t h e r m o p l a s t i c s a t elevated temperatures. The s i m p l e s t f o r m o f t h i s d i e n e b a s e d e l a s t o m e r is the l i n e a r t r i b l o c k p o l y m e r SBS o r S I S . Where S r e p r e s e n t s p o l y s t y r e n e , B, p o l y b u t a d i e n e a n d I , p o l y i s o p r e n e . The o t h e r v a r i a t i o n s a r e known as r a d i a l b l o c k p o l y m e r s and h a v e a c o n t r o l l e d number o f b r a n c h e s o f e q u a l l e n g t h and com p o s i t i o n , ( S B ) - X where X is a c o u p l e r r e s i d u e and η c a n be 3, 4, 5 o r h i g h e r T D e t a i l s o f r a d i a l b l o c k polymers and t h e i r p r e p a r a t i o n a r e d e s c r i b e d elsewhere.£2. T e l e b l o c k p o l y m e r s c a n a l s o be made, u s i n g m u l t i f u n c t i o n a l i n i t i a t o r s in a s i n g l e s t e p p o l y merization of a butadiene-styrene mixture. Some o f the m u l t i c h e l i c i n i t i a t o r s mentioned earlier y i e l d h i g h g r e e n s t r e n g t h if the i n i t i a t o r p r e p a r a t i o n is p r o p e r l y c o n d u c t e d . By v a r y i n g the monomer r a t i o s , it is a l s o p o s s i b l e t o v a r y the p r o p e r t i e s f r o m elastomeric to p l a s t i c . Thus, c e r t a i n s t y r e n e r i c h b l o c k polymers o f b u t a d i e n e and s t y r e n e w i t h r a t h e r s o p h i s t i c a t e d c h a i n a r c h i t e c t u r e s a r e f i n d i n g i n c r e a s i n g a p p l i c a t i o n s as c l e a r r e s i n s o f medium i m p a c t s t r e n g t h . 1 2 P r o d u c t i o n o f e l a s t o m e r i c r a d i a l b l o c k p o l y m e r s began in O c t o b e r , 1 9 6 7 , and a t p r e s e n t t h e r e a r e s e v e r a l in the m a r k e t t h a t b e l o n g t o the r a d i a l b l o c k p o l y m e r f a m i l y .


n

E l a s t o m e r i c b l o c k p o l y m e r s o f s t y r e n e and b u t a d i e n e o r i s o p r e n e and t h e i r p r o d u c t s o f hydrogénation a r e f i n d i n g i n c r e a s i n g use in a v a r i e t y o f f i e l d s . L i n e a r and r a d i a l b l o c k p o l y m e r s a r e u s e d e x t e n s i v e l y in i n j e c t i o n m o l d e d r u b b e r g o o d s , f o o t w e a r , p r e s s u r e s e n s i t i v e and h o t m e l t a d h e s i v e s a n d in m e c h a n i c a l rubber goods s u c h a s h o s e , t u b i n g , c o v e b a s e , t o y s , d r u g s u n d r i e s , r u b b e r bands, s t o p p e r s , e r a s e r s , e t c . The o u t s t a n d i n g f e a t u r e o f d i e n e - s t y r e n e b l o c k p o l y m e r s in p r e s s u r e s e n s i t i v e a d h e s i v e f o r m u l a t i o n s is t h e i r e x c e l l e n t r e s i s t a n c e to adhesive creep. I t is s t a t e d ! t h a t t h e s e a d h e s i v e s h a v e e s t a b l i s h e d new p e r f o r m a n c e h i g h s f o r s h e a r h o l d i n g when a p p l i e d t o s e l e c t e d m e t a l and p l a s t i c s u r f a c e s , w h i l e m a i n t a i n i n g a h i g h d e g r e e o f t a c k . R a d i a l b l o c k p o l y m e r s h a v e an a d v a n t a g e d 5 4 7 o v e r l i n e a r p o l y m e r s in t h a t a t e q u a l m o l e c u l a r w e i g h t s , the r a d i a l p o l y m e r s e x h i b i t l o w e r m e l t and s o l u t i o n v i s c o s i t i e s . T h i s is i m p o r t a n t b e c a u s e it a l l o w s the u s e o f r a d i a l p o l y m e r s o f h i g h e r m o l e c u l a r w e i g h t s w i t h a c o r r e s p o n d i n g improvement in s h e a r resistance. I s o p r e n e - s t y r e n e b l o c k p o l y m e r s h a v e an a d v a n t a g e o v e r b u t a d i e n e - s t y r e n e p o l y m e r s s i n c e in an o x i d a t i v e a t m o s p h e r e , p o l y i s o p r e n e d e g r a d e s by c h a i n s c i s s i o n r a t h e r t h a n g e l l i n g by c r o s s l i n k i n g r e s u l t i n g in b e t t e r t a c k r e t e n t i o n . S t y r e n e - b u t a d i e n e b l o c k p o l y m e r s a l s o f i n d a p p l i c a t i o n s in b l e n d s w i t h p o l y s t y r e n e and ABS p l a s t i c s . When m i n o r amounts o f r u b b e r y SBS o r ( S B ) - X b l o c k p o l y m e r s a r e b l e n d e d w i t h h i g h m o l e c u l a r w e i g h t p o l y s t y r e n e s u c h t h a t the p o l y s t y r e n e forms the n



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continuum, impact r e s i s t a n c e improves with i n c r e a s i n g styrene block l e n g t h . For good r e s u l t s , the styrene b l o c k lengths should be of the order of 20,000 as shown in a studyiS. where butadienestyrene d i b l o c k polymers (25% styrene) were blended with p o l y styrene (1:3) and cured with 0.1% dicumyl peroxide. When t r i b l o c k or branched m u l t i b l o c k polymers are used in blends with p o l y s t y r e n e , it is not necessary to c r o s s l i n k the rubbery domains£2 *ϋ2 although peroxide treatment does produce some a d d i t i o n a l improve ment in impact and t e n s i l e s t r e n g t h . Another use of butadienestyrene block copolymers is in blends with ABS. When ABS scrap is reprocessed, there r e s u l t s a l o s s in toughness which may be r e s t o r e d by the a d d i t i o n of s m a l l amounts (V,%) of a block polymer*I n t e r e s t i n g l y , SB d i b l o c k s are as e f f e c t i v e as SBS or ( S B ) - X m u l t i b l o c k polymers in t h i s a p p l i c a t i o n . It must be remembered that the butadiene-styrene d i b l o c k polymers are a l s o used as the source of e l a s t o m e r i c components in the p o l y m e r i z a t i o n of c e r t a i n types of ABS r e s i n s . Although very l i t t l e i n f o r m a t i o n is a v a i l able on t h i s a p p l i c a t i o n , it is conceivable that the d i b l o c k p o l y mers are sometimes p r e f e r r e d over polybutadienes because of the advantages in the processes and/or p r o p e r t i e s of the f i n a l p r o ducts . Blends o f butadiene-styrene b l o c k polymers with p o l y o l e f i n s , p a r t i c u l a r l y polypropylene are mentioned in l i t e r a t u r e ! to im prove the impact s t r e n g t h of the l a t t e r . Since s i m i l a r improve ments can be r e a l i z e d from the use of p o l y o l e f i n b l o c k polymers, the blends have not gained much r e c o g n i t i o n . However, butadienestyrene r a d i a l t e l e b l o c k polymers are blended i n t o p o l y e t h y l e n e f i l m , to i n c r e a s e the t e a r r e s i s t a n c e and t e n s i l e impact.£2 Butadiene-styrene b l o c k polymers with f u n c t i o n a l end groups such as carboxy terminated polymers have been developed5-3 by P h i l l i p s as impact m o d i f i e r s in sheet molding compounds based on f i b e r r e i n f o r c e d unsaturated p o l y e s t e r and s t y r e n e . These blends on c u r i n g , e x h i b i t a good balance of mechanical p r o p e r t i e s , improved impact s t r e n g t h , low shrinkage, no s i n k , e x c e l l e n t s u r face, and improved p i g m e n t a b i l i t y and p a i n t a b i l i t y . A r e l a t i v e l y new development which promises to g a i n in im portance in the f u t u r e is the m o d i f i c a t i o n of a s p h a l t by b u t a diene-styrene b l o c k polymers.M. The b l o c k polymers help reduce the low temperature b r i t t l e n e s s and impart r e s i s t a n c e to flow at e l e v a t e d temperatures. A p p l i c a t i o n s in m a s t i c s , automobile body undercoatings and waterproofing m a t e r i a l s such as high q u a l i t y r o o f i n g membranes are envisaged. A n i o n i c p o l y m e r i z a t i o n has a l s o been used to make t e l e c h e l i c polymers (Greek t e l o s , end, and c h e l e , claw), i . e . , polymers with r e a c t i v e t e r m i n a l g r o u p s . W e coined the term, t e l e c h e l i c in 1957 and it has been accepted ever s i n c e in t e c h n i c a l as w e l l as patent l i t e r a t u r e . L i q u i d carboxy- and hydroxy t e l e c h e l i c polybutadienes i n i t i a t e d with d i f u n c t i o n a l o r g a n o l i t h i u m i n i t i a t o r s are commercially produced s i n c e 1962. Some of the p h y s i c a l properties, p r o d u c t i o n d e t a i l s 5 2 and uses as in s o l i d rockets


404

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POLYMERIZATION


o f t h e s e p o l y m e r s h a v e b e e n d e s c r i b e d . £2. We a l s o developed55 s o l i d , t e l e c h e l i c e l a s t o m e r s o f w h i c h the p o l y m e r c h a i n ends inc o r p o r a t e i n t o the v u l c a n i z a t e n e t w o r k and t h u s o b t a i n s u p e r i o r physical properties. M e r c a p t o - , h y d r o x y - and a z i r i d i n y l t e l e c h e l i c e l a s t o m e r s when c u r e d w i t h p e r o x i d e o r s u l f u r a c c e l e r a t o r r e c i p e s e x h i b i t e d i m p r o v e d s t r e s s - s t r a i n and d y n a m i c p r o p e r t i e s in c o m p a r i s o n t o t h o s e o f the c o n t r o l s . The m e r c a p t o - and a z i r i d i n y l t e l e c h e l i c b u t a d i e n e - s t y r e n e c o p o l y m e r s a l s o showed o u t s t a n d i n g p r o p e r t i e s in t r e a d f o r m u l a t i o n s . However, t h e s e p o l y mers a r e n o t p r o d u c e d commercially. S e l e c t i v e l y h y d r o g e n a t e d random and b l o c k c o p o l y m e r s o f v i n y l a r o m a t i c monomers and d i e n e s a r e u s e d as v i s c o s i t y i n d e x i m p r o v e r s in m u l t i g r a d e l u b r i c a t i n g oils.âlrM The b l o c k c o p o l y mers a r e o f the SB t y p e w h e r e S is most commonly s t y r e n e a n d Β is b u t a d i e n e o r i s o p r e n e . H y d r o g é n a t i o n o f the d i e n e component imparts r e s i s t a n c e to o x i d a t i v e degradation. C r y s t a l l i n i t y is u n d e s i r a b l e in t h e s e a p p l i c a t i o n s and it is p r e v e n t e d by c o n t r o l o f m i c r o s t r u c t u r e and a random i n c o r p o r a t i o n o f s t y r e n e . Since the v i s c o s i t y o f a h e a v y l u b r i c a t i n g o i l s t o c k d e c r e a s e s w i t h inc r e a s i n g t e m p e r a t u r e more r a p i d l y t h a n t h a t o f a l i g h t e r s t o c k , s u c h a ( v i s c o s i t y i n d e x i m p r o v i n g ) p o l y m e r is a d d e d t o the l i g h t o i l t o m a t c h the v i s c o s i t y o f a h e a v i e r o i l a t h i g h t e m p e r a t u r e . The m o d i f i e d o i l t h o u g h t h i c k e n e d must still b e more f l u i d t h a n the h e a v y o i l a t low t e m p e r a t u r e s . I n o t h e r w o r d s , the d e s i r e d p r o p e r t y o f a V I i m p r o v e r is a r e l a t i v e v i s c o s i t y w h i c h r e m a i n s c o n s t a n t o r d e c r e a s e s as the t e m p e r a t u r e falls. An o u t s t a n d i n g p r o p e r t y o f t h e s e p o l y m e r s is t h e i r s h e a r stability. The s o n i c s h e a r s t a b i l i t y t e s t s & l i n d i c a t e t h a t t h e s e p o l y m e r s a r e s u p e r i o r t o some o f the c u r r e n t l y u s e d p o l y m e r s o f ethylene-propylene or methacrylate type. The e x c e l l e n t s t a b i l i t y o f the h y d r o g e n a t e d d i e n e - s t y r e n e p o l y m e r s is a t t r i b u t e d t o t h e i r r e l a t i v e l y low m o l e c u l a r w e i g h t and n a r r o w d i s t r i b u t i o n c o n s i s t e n t w i t h the e s t a b l i s h e d t h e o r y o f s h e a r d e g r a d a t i o n o f p o l y mers .èè The most r e c e n t d e v e l o p m e n t s in t h i s f i e l d a r e b l o c k p o l y m e r V I i m p r o v e r s w i t h d i s p e r s a n c y p r o p e r t i e s , b u i l t i n t o the m o l e c u l e by c h e m i c a l m o d i f i c a t i o n o f the r u b b e r b l o c k . e l I n c o n c l u s i o n , it is e v i d e n t f r o m the a b o v e d i s c u s s i o n t h a t a n i o n i c p o l y m e r i z a t i o n h a s emerged f r o m a l a b o r a t o r y c u r i o s i t y t o an i m p o r t a n t i n d u s t r i a l p r o c e s s in a r e l a t i v e l y s h o r t s p a n o f time. C u r r e n t l y , o v e r a m i l l i o n tons o f polymers a r e produced by the a n i o n i c r o u t e in a b o u t t w e n t y m a n u f a c t u r i n g p l a n t s a r o u n d the world. We a t P h i l l i p s a r e q u i t e p r o u d o f b e i n g one o f the p i o n e e r s a l o n g w i t h F i r e s t o n e and S h e l l in h a r n e s s i n g t h i s new technology t o commercial a p p l i c a t i o n s . The f a c t t h a t o u r polymers f i n d such wide r a n g i n g a p p l i c a t i o n s from t i r e t r e a d s t o i n j e c t i o n m o l d e d b l o o d f i l t e r s and f r o m l u b r i c a n t a d d i t i v e s t o s o l i d r o c k e t b i n d e r s bears ready testimony to t h i s .


25.

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Acknowledgements The authors wish to express their thanks to many of our colleagues at P h i l l i p s Petroleum Company who contributed so much to the success of this work.


Abstract Anionic polymerizations i n i t i a t e d with alkyllithium compounds enable us to prepare homopolymers as well as copolymers from diene and vinylaromatic monomers. These polymerization systems are unique in that they have precise control over such polymer proper ties as composition, microstructure, molecular weight, molecular weight distribution, choice of functional end groups and even co polymer monomer sequence distribution. Attempts have been made in this paper to survey these salient features with respect to their chemistry and commercial applications. Literature Cited 1. 2. 3. 4. 5. 6. 7.

8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

Stavely, F. W., et a l . , Ind. Eng. Chem., 48, 778 (1956). A l l i g e r , G., W i l l i s , J. Μ., Smith, W. Α., and Allen, J. J., Rubber World, 134, 549 (1956). Hsieh, H. L., and Tobolsky, Α. V., J. Polymer Science, 25, 245 (1957). Hsieh, H. L., Kelley, D. J., and Tobolsky, Α. V., J. Polymer Sci., 26, 240 (1957). Hsieh, H. L., and Glaze, W. Η., Rubber Chem. and Tech., 43, 22 (1970). Bywater, S., Adv. Polymer Sci., 4, 66 (1965). Forman, L. E., in J. P. Kennedy and E. Törnqvist, eds., Polymer Chemistry of Synthetic Elastomers, Wiley-InterScience, New York, Part II, 491. Short, J. N., Thornton, V., and Kraus, G., International Synthetic Rubber Symposium, London, 1967. Hsieh, H. L., J. Polymer Sci., A3, 153 (1965). Morita, Η., and Tobolsky, Α. V., J. Am. Chem. Soc., 79, 5853, (1957). Tobolsky, Α. V., and Rogers, C. E., J. Polymer Sci., 40, 73 (1959). Stearns, R. S., and Forman, L. E., i b i d . , 41, 381 (1959). Kuntz, I., and Gerber, Α., i b i d . , 42, 299 (1960). Railsback, Η. E., and Stumpe, Ν. Α., Rubber Age, 107, 12, 27 (1975). Farrar, R. C., IUPAC Meeting, Rio de Janerio, 1974. Zelinski, R. P., and Hsieh, H. L., (to P h i l l i p s Petroleum Company), U. S. Patent 3,280,084. Halasa, A. F., ACS Polymer Reprints, 13, No. 2, 678 (1972). Zelinski, R. P., and Wofford, C. F., J. Polymer Sci., A3, 93 (1965). Uraneck, C. Α., and Short, J. N., Rubber Chem. and Tech., 41, 1375 (1968). McGrath; Anionic Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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