Initiation of Polymerization - American Chemical Society

21

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 29, 2016 | http://pubs.acs.org Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch021

Factors Affecting the Isomeric Chain Unit Structure in Organolithium Polymerization of Butadiene and Isoprene MAURICE MORTON and J. R. RUPERT

1

The University of Akron, Institute of Polymer Science, Akron, OH 44325

Organolithium initiators are used extensively in the polymerization of butadiene and isoprene, because of their solubility in a variety of solvents. It is well known that non-polar media lead to polymers having a high 1,4 chain unit structure, while polar solvents can exert a dramatic effect in producing a high proportion of 1,2 or 3,4 structures. However, recent work, using the most accurate available NMR spectroscopy, has also shown that, even in nonpolar media, the concentration of initiator and the presence of solvent can have a very significant effect on the cis-l,4/trans-l,4 ratio, without noticeably influencing the proportion of side-vinyl units. Thus, at very low initiator concentrations (~10 M. ), and in the absence of any solvents, i t is possible to attain a cis-1,4 content of 96% for polyisoprene and 86% for polybutadiene. These chain structures have not been found to be noticeably affected by temperature or extent of conversion. -5

The intensive investigations that followed the discovery (l) that lithium and its compounds can lead to the polymerization of isoprene to a very high cis-1,4 configuration, close to that of natural rubber, showed that both the type of alkali metal and the nature of the solvent can have a profound effect on the chain structure of polydienes (2-8). The conclusions reached from these investigations are as follows: 1. In non-polar media, the 1,4 structure is highest for lithium and decreases with increasing electropositivity of the alkali metals. 2. Polar solvents (e.g., ethers, amines, etc.) lead to a higher side-vinyl content (1,2 or 3,4), especially 1

Current address: Monsanto Company, Akron, OH 44313. 0097-6156/83/0212-0283$06.00/0 © 1983 American Chemical Society In Initiation of Polymerization; Bailey, Frederick E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


284

INITIATION OF

POLYMERIZATION

i n t h e case o f l i t h i u m , a p p a r e n t l y b y s o l v a t i n g the m e t a l c a t i o n and thus i n c r e a s i n g t h e i o n i c c h a r a c t e r o f t h e carbon-metal bond. W i t h r e g a r d t o t h e e f f e c t s o f s o l v e n t s on t h e c h a i n m i c r o s t r u c t u r e o f p o l y d i e n e s , t h e most t h o r o u g h l y i n v e s t i g a t e d s y s t e m s h a v e b e e n those i n v o l v i n g o r g a n o l i t h i u m i n i t i a t o r s , s i n c e these operate i n a v a r i e t y o f s o l v e n t s , b o t h p o l a r and n o n - p o l a r , a n d c a n be con t r o l l e d t o u n d e r g o l i t t l e o r no s i d e r e a c t i o n s . Furthermore, i n n o n - p o l a r media, such systems y i e l d p o l y b u t a d i e n e s and p o l y i s o p r e n e s h a v i n g a v e r y h i g h (>90%) 1,4 u n i t c o n t e n t , a n d t h e s e a r e important s y n t h e t i c rubbers. M o s t o f t h e d a t a r e f e r r e d t o above w e r e o b t a i n e d i n e a r l i e r work, and were b a s e d o n i n f r a r e d s p e c t r o s c o p y . I n r e c e n t y e a r s , more r e l i a b l e d a t a were o b t a i n e d b y means o f NMR s p e c t r o s c o p y , using both H and C r e s o n a n c e s ( 9 - 1 4 ) . Some o f t h e s e i n v e s t i g a t i o n s suggested t h a t , a s i d e from the dramatic e f f e c t s o f p o l a r s o l v e n t s on t h e c h a i n s t r u c t u r e i n o r g a n o l i t h i u m systems, t h e r e were some s u b t l e e f f e c t s e v e n i n n o n - p o l a r m e d i a , e.g., c a u s e d b y i n i t i a t o r c o n c e n t r a t i o n and type o f non-polar s o l v e n t . S i n n and c o w o r k e r s ( 1 5 ) , f o r e x a m p l e , u s e d i n f r a r e d s p e c t r o s c o p y t o show a n e f f e c t o f b u t y l l i t h i u m c o n c e n t r a t i o n on t h e c h a i n s t r u c t u r e o f p o l y i s o p r e n e a n d p o l y b u t a d i e n e . Hence a n e x t e n s i v e s t u d y was c a r r i e d o u t r e c e n t l y ( 1 3 , 1 4 ) o n t h e i n f l u e n c e o f r e a c t i o n param e t e r s on the c h a i n structure" 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 prepared i n non-polar media. 1

1 3

Experimental A l l s p e c t r a were o b t a i n e d w i t h t h e V a r i a n HR-300 NMR S p e c t r o m e t e r , u s i n g H i n n o r m a l mode, w i t h o c c a s i o n a l u s e o f F o u r i e r t r a n s f o r m f o r v e r y h i g h m o l e c u l a r weight samples. Hexachlorobutad i e n e was u s e d a s s o l v e n t , w i t h 1% h e x a m e t h y l d i s i l o x a n e a s r e f e r e n c e . The t e m p e r a t u r e s u s e d were 110°C. f o r p o l y i s o p r e n e a n d 125°C. f o r p o l y b u t a d i e n e . The p o l y m e r s a m p l e s were p r e p a r e d w i t h s e c - b u t y l l i t h i u m a s i n i t i a t o r , u s i n g t h e h i g h vacuum t e c h n i q u e s d e s c r i b e d e l s e w h e r e (13). NMR p e a k a s s i g n m e n t s were a s f o l l o w s . For polyisoprene, t h e 3 , 4 - u n i t c o n t e n t was d e t e r m i n e d f r o m t h e o l e f i n i c m e t h y l e n e p r o t o n s a t 4.61 a n d 4.67 ppm, w h i l e t h e t r a n s - 1 , 4 u n i t s w e r e d e t e r m i n e d f r o m t h e m e t h y l p r o t o n r e s o n a n c e a t 1.54 ppm. The c i s - 1 , 4 c o n t e n t was t h e n c a l c u l a t e d b y d i f f e r e n c e . No 1,2 o l e f i n i c m e t h y l e n e p r o t o n s c o u l d be s e e n a t 5,4 ppm, where t h e y w o u l d b e e x p e c t e d . F o r p o l y b u t a d i e n e , t h e 1 , 2 - u n i t c o n t e n t was c a l c u l a t e d f r o m a c o m p a r i son o f t h e 1 , 2 - o l e f i n i c m e t h y l e n e p r o t o n s a t 4.δ ppm w i t h t h e 1,4m e t h i n e p r o t o n s a t 5.4 ppm. The c i s / t r a n s r a t i o was t h e n computed f r o m t h e 1 , 4 - m e t h y l e n e p r o t o n s a t 1.98 a n d 2.03 ppm. These methods made i t p o s s i b l e t o e s t i m a t e t h e c h a i n u n i t s t r u c t u r e s w i t h i n a b o u t 1%. 1

R e s u l t s and D i s c u s s i o n The r e a c t i o n v a r i a b l e s i n v e s t i g a t e d w e r e : a ) i n i t i a t o r c o n c e n t r a t i o n , b ) monomer c o n c e n t r a t i o n , c ) t y p e o f s o l v e n t ( i . e . ,

In Initiation of Polymerization; Bailey, Frederick E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

21.

MORTON AND RUPERT

Isomeric Chain

Unit

285

Structure

b e n z e n e , n-hexane o r c y c l o h e x a n e ) , d ) t e m p e r a t u r e , a n d e ) d e g r e e of conversion. P o l y m e r i z a t i o n s were c a r r i e d o u t g e n e r a l l y a t room t e m p e r a t u r e t o c o m p l e t e ( o r n e a r c o m p l e t e ) c o n v e r s i o n e x c e p t when t h e e f f e c t s o f t h e s e two v a r i a b l e s w e r e b e i n g s t u d i e d .


Polyisoprene E f f e c t o f I n i t i a t o r a n d Monomer C o n c e n t r a t i o n . The e f f e c t o f i n i t i a t o r c o n c e n t r a t i o n ( s - b u t y l l i t h i u m ) and s o l v e n t s on t h e c h a i n s t r u c t u r e o f p o l y i s o p r e n e i s shown i n T a b l e I . The f o l l o w i n g c o n c l u s i o n s c a n b e drawn f r o m t h e s e r e s u l t s : 1. The 3,4 c o n t e n t i s a f f e c t e d v e r y l i t t l e , i f a t a l l , b y t h e i n i t i a t o r o r monomer c o n c e n t r a t i o n , o r b y the type o f s o l v e n t present. 2. The p r e s e n c e o f s o l v e n t d e c r e a s e s t h e c i s - 1 , 4 c o n t e n t , an aromatic s o l v e n t l i k e benzene h a v i n g a g r e a t e r e f f e c t than an a l i p h a t i c solvent l i k e n-hexane. 3. A d e c r e a s e i n i n i t i a t o r c o n c e n t r a t i o n i n c r e a s e s the c i s - 1 , 4 c o n t e n t , i n t h e absence o f s o l v e n t , o r even i n t h e p r e s e n c e o f n-hexane. Table I E f f e c t o f I n i t i a t o r Concentration and Solvents on C h a i n S t r u c t u r e o f L i t h i u m P o l y i s o p r e n e ( 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 = 20°C) Solvent

M i c r o s t r u c t u r e - m.ol.%

[s-Ci+HgLi]

cis-1,4 Benzene* Benzene* n-Hexane* n-Hexane* None None *

9x10" 4x101x101x103x108x10"

3

5

2

5

3

6

69 70 70 86 77 96

trans-1,4

3,4

25 24 25 11 18 0

6 6 5 3 5 4

[Monomer] = 0.5 M.

Thus i t a p p e a r s t h a t l o w c o n c e n t r a t i o n s o f l i t h i u m , i n g e n e r a l , enhance t h e f o r m a t i o n o f t h e c i s - 1 , 4 s t r u c t u r e , a t t h e e x p e n s e o f the t r a n s - 1 , 4 s t r u c t u r e , and t h a t t h i s e f f e c t i s most n o t i c e a b l e i n t h e p o l y m e r i z a t i o n o f t h e u n d i l u t e d monomer. However, i n t h e p r e s e n c e o f b e n z e n e , no s u c h e f f e c t c a n b e s e e n , p r e s u m a b l y b e c a u s e t h e " s o l v e n t e f f e c t " o f t h i s a r o m a t i c compound c a n c o u n t e r the e f f e c t o f the l i t h i u m concentration. This i s discussed further i n a l a t e r section.


286

INITIATION OF

POLYMERIZATION

E f f e c t o f Conversion. The e f f e c t o f i n c r e a s i n g d e g r e e s o f conversion on the chain s t r u c t u r e i n p o l y m e r i z a t i o n o f u n d i l u t e d i s o p r e n e i s shown c l e a r l y i n T a b l e I I . A s c a n b e s e e n , t h e e x t e n t Table I I E f f e c t o f C o n v e r s i o n on C h a i n S t r u c t u r e of Lithium Polyisoprene


(No

s o l v e n t , p o l y m e r i z a t i o n temp. = 20°C. [ s - C ^ H g L i ] = 1 0 "

Conversion

(%)

M i c r o s t r u c t u r e - mol.

13

29 40 46 48 86

5

M. )

%

Cis-1,4

Trans-1,4

93 92 92 92 95 95

2 3 2 3

6

1 1

4 4

3,4

5 5 5

o f c o n v e r s i o n o f monomer t o p o l y m e r h a s no n o t i c e a b l e e f f e c t o n the chain s t r u c t u r e . Since t h i s i s a non-terminating chain growth r e a c t i o n , t h i s means t h a t e a c h c h a i n m a i n t a i n s a c o n s t a n t c h a i n s t r u c t u r e as i t grows. E f f e c t o f T e m p e r a t u r e . T a b l e I I I shows t h e e f f e c t o f p o l y m e r i z a t i o n temperature on the c h a i n s t r u c t u r e o f l i t h i u m p o l y i s o p r e n e , b o t h i n t h e c a s e o f u n d i l u t e d monomer a n d i n t h e p r e s e n c e o f n-hexane a s s o l v e n t . W i t h i n t h e r a n g e s shown, t h e r e does n o t appear t o be any i n f l u e n c e o f temperature o n the placement o f t h e various isomeric chain unit structures. Table I I I E f f e c t o f P o l y m e r i z a t i o n Temperature on Chain S t r u c t u r e o f L i t h i u m Polyisoprene Temp. ( ° C )

46 20 0 -25 40 -25

*

[s-Ci+HgLi]

1x10" 1x10" 1x10" 1x10" 1x10" 1x10"

5

5

5

5

3

3

Solvent

None None None None n-hexane* n-hexane*

M i c r o s t r u c t u r e - mol. Cis-1,4

Trans-1,4

3,4

95 95 95 93 76 78

1 1 2 3 18 18

4 4 3 4 6 4

[Monomer] = 2.5 M.


%

21.

MORTON AND RUPERT

Isomeric Chain

Unit

287

Structure

Polybutadiene E f f e c t o f I n i t i a t o r C o n c e n t r a t i o n and S o l v e n t s . In t h i s c a s e , t h r e e d i f f e r e n t s o l v e n t s were used t o c a r r y out t h e p o l y m e r i z a t i o n , i n a d d i t i o n t o t h e u s e o f u n d i l u t e d monomer, as shown i n T a b l e IV. I t i s w e l l known t h a t t h e o r g a n o l i t h i u m Table

IV


E f f e c t o f I n i t i a t o r C o n c e n t r a t i o n and S o l v e n t s on C h a i n S t r u c t u r e o f L i t h i u m P o l y b u t a d i e n e ( P o l y m e r i z a t i o n Temp. = 20°C) Solvent

Microstructure -

[s-Ci+HgLi]

Cis-1,4 Benzene* Cyclohexane* n-Hexane* n-Hexane* None None

*

[Monomer] = 0.5

8x10" 1x10" 2x103x107x10" 3x10"

6

5

5

2

6

3

52 68 56 30 86 39

mol. %

Trans-1,4

1,2 12 4 7 8 5 9

36 28 37 62 9 52

M.

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 does n o t y i e l d as h i g h a c o n t e n t o f c i s - 1 , 4 u n i t s as i n t h e c a s e o f i s o p r e n e . I n f a c t , under p r a c t i c a l p o l y m e r i z a t i o n c o n d i t i o n s i n i n d u s t r y ( [ R L i ] v L O " ) , the t r a n s - 1 , 4 c o n t e n t i s g e n e r a l l y g r e a t e r t h a n 50$. However, t h e d a t a i n T a b l e I V show e x a c t l y how t h e s e r e a c t i o n p a r a m e t e r s a f f e c t t h e c h a i n s t r u c t u r e , as f o l l o w s : 1. A g a i n , as i n t h e c a s e o f i s o p r e n e , t h e s i d e - v i n y l content ( 1 , 2 - u n i t s ) i s not g r e a t l y a f f e c t e d e i t h e r by the i n i t i a t o r c o n c e n t r a t i o n or the presence of solvent. I t does seem t o be i n c r e a s e d s l i g h t l y b y h i g h e r i n i t i a t o r l e v e l s , and a p p a r e n t l y i n c r e a s e s n o t i c e a b l y i n the presence of benzene. 2. Compared t o t h e c a s e o f i s o p r e n e , t h e c i s - 1 , 4 c o n t e n t changes d r a m a t i c a l l y as t h e i n i t i a t o r concentration i s decreased, e s p e c i a l l y i n the absence o f s o l v e n t s , where t h e c i s - 1 , 4 content i s shown t o r i s e f r o m 39$ t o 86$. This also happens i n t h e p r e s e n c e o f s o l v e n t s , a l t h o u g h n o t t o as g r e a t an e x t e n t . 3

I t i s i n t e r e s t i n g t o n o t e t h a t a r e v i e w o f t h e l i t e r a t u r e shows no r e f e r e n c e t o the s u c c e s s f u l s y n t h e s i s o f p o l y b u t a d i e n e , by o r g a n o l i t h i u m i n i t i a t o r s , w i t h a c i s - 1 , 4 c o n t e n t as h i g h as 86$


288

INITIATION OF POLYMERIZATION

a l t h o u g h t h e t r e n d t o w a r d h i g h e r v a l u e s t h a n 50$ h a s b e e n shown. T h i s i s p e r h a p s n o t s u r p r i s i n g , s i n c e t h e a t t a i n m e n t o f 86$ c i s 1,4- was o n l y f o u n d p o s s i b l e i n t h i s w o r k a t e x t r e m e l y l o w i n i t i a t o r c o n c e n t r a t i o n s (

Initiation of Polymerization - American Chemical Society

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