Anionic Polymerization - American Chemical Society

33 Anionic Polymerization. VII

Downloaded by PENNSYLVANIA STATE UNIV on July 18, 2012 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0166.ch033

Polymerization and Copolymerization with Lithium-Nitrogen-Bonded Initiator TAI CHUN CHENG Central Research Laboratories, The Firestone Tire and Rubber Co., Akron, OH 44317

Polymerization of butadiene with lithium morpholinide, an i n i t i a t o r with a built-in microstructure modifier, has been carried out in hexane. In general, the v i n y l content of the polymers prepared with this i n i t i a t o r is dependent on the i n i t i a t o r concentrations and on the polymerization temperatures. This dependence is i d e n t i c a l to that observed in a THF-modified lithium diethylamide polymerization i n i t i a t o r system. A comparison of these initiator systems for polymerization of butadiene is presented. In addition, a study of the effect of metal alkoxides on the v i n y l content of lithium morpholinide i n i t i a t e d butadiene polymerization is included. Copolymerization of butadiene and styrene in hexane with a number of initiators, such as lithium morpholinide, lithium dialkylamide, lithium piperidinide, etc., has also been examined. In general, the microstructure and styrene content of the polymers are dependent on the type of i n i t i a t o r and the polymerization conditions. Detailed results including a postulated mechanism for these polymerizations are discussed. Lithium diethylamide has been shown to be an effective i n i t i a t o r for the homopolymerization of dienes and styrene (1*2). I t is also known that such a polymerization process is markedly affected by the presence of polar compounds, such as ethers and amines (2,3). However, there has been no report of the use of a lithium amide containing a built-in polar modifier as a diene polymerization initiator. This paper describes the preparation and use of such an initiator, lithium morpholinide. A comparison between polymerization with this i n i t i a t o r and lithium diethyl amide, with and without polar modifiers, is included. Furthermore, we have examined the effects of lithium-nitrogen i n i t i a t o r s on the copolymerization of butadiene and styrene. 0097-6156/81/0166-0513$05.00/0 © 1981 American Chemical Society In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

514

ANIONIC POLYMERIZATION


Experimental A. Preparation of I n i t i a t o r s . The l i t h i u m amides were prepared by the r e a c t i o n of n - b u t y l l i t h i u m with the corresponding amines in hexane with the amounts shown in Table I. In general, the n - b u t y l l i t h i u m in hexane (15%) was added to the amine-hexane s o l u t i o n slowly because a strong exotherm is a s s o c i a t e d with the r e a c t i o n . The l i t h i u m amides p r e c i p i t a t e d from the s o l u t i o n in the form of c r y s t a l l i n e compounds. The l i t h i u m amides were i s o l a t e d by means of f i l t r a t i o n and were then washed four times with a t o t a l of two l i t e r s of hexane. The m a t e r i a l s were d r i e d in a vacuum oven at room temperature and then suspended in hexane s o l u t i o n f o r use as p o l y m e r i z a t i o n i n i t i a t o r s . A GC-mass s p e c t r o s c o p i c method was used to v e r i f y the p u r i t y of t h i s m a t e r i a l , and a t i t r a t i o n (11) was used f o r the determination of t o t a l a l k a l i n i t y . B. Polymerization Procedure. Butadiene was used as a blend in hexane. Neat butadiene was t r e a t e d with mercuric s u l f a t e , washed with c a u s t i c and water, d i s t i l l e d and d r i e d by passage through molecular s i e v e s . At t h i s p o i n t , the butadiene was blended with hexane, and the blend was passed through molecular s i e v e d r y i n g columns. Precautions were taken to avoid the presence of water, oxygen and other i m p u r i t i e s which would i n h i b i t the p o l y m e r i z a t i o n or r e a c t with the i n i t i a t o r . The homopolymerizations were all run in 28 oz. beverage b o t t l e s . The b o t t l e s were baked f o r at l e a s t 24 h r s . and then capped with crown, three-hole caps and rubber l i n e r s . C o o l i n g of the b o t t l e s was e f f e c t e d while purging with n i t r o g e n . A f t e r c o o l i n g , the b o t t l e s were charged with the butadiene blend, the heterogeneous i n i t i a t o r - h e x a n e suspension and m o d i f i e r s . The b o t t l e s were then placed in a constant temperature bath equipped with an a g i t a t i o n device f o r 24 h r s . or more. The copolymerizations were performed with a butadiene/ styrene/hexane blend. T h i s blend was prepared by a procedure i d e n t i c a l to the above method f o r the butadiene blend, with the exception t h a t d i s t i l l e d styrene was blended with the butadiene and hexane p r i o r to f i n a l d r y i n g . Polymers were i s o l a t e d by c o a g u l a t i o n in methanol, d i s s o l v i n g in toluene, f i l t e r i n g and r e c o a g u l a t i o n in methanol. The polymer was then d r i e d in an oven at 40°C... and 60-70 mm pressure. P u r i f i e d butadiene polymers and butadiene-styrene copolymers were analyzed by i n f r a r e d and nmr spectroscopy(4,5,6). C. Preparation of the 1;1 Adduct of Lithium Diethylamide and Butadiene. Butadiene (4.9 g, 9.07 mmole) was d i s s o l v e d in 400 ml of p u r i f i e d diethylamine. n - B u t y l l i t h i u m (9% mmole) in hexane was added dropwise over a p e r i o d of one hour at 0°C... The r e a c t i o n mixture was then s t i r r e d two hours at room temperature and heated a t 50°C... f o r 48 hours. Upon c o o l i n g , a mixture

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

33.

o

CHENG

Lithium-Nitrogen-Bonded

Initiator

515

f

c i s - a n d t r a n s - l - d i e t h y l a m i n o - 2 - b u t e n e was i s o l a t e d by means o f d i s t i l l a t i o n (b.p. 134-6°C...). The y i e l d o f the p r o d u c t b a s e d on b u t a d i e n e is 65-70% (7.5-8.0 g ) . The ratio o f c i s t o t r a n s by G.C. is 1 t o 2. I n a d d i t i o n , a s m a l l amount o f 1,2 p r o d u c t was o b s e r v e d ( A 5 % ) .


R e s u l t s and D i s c u s s i o n Homopolymerization. L i t h i u m M o r p h o l i n i d e . T h i s compound was s y n t h e s i z e d by the r e a c t i o n o f n - b u t y l l i t h i u m a n d m o r p h o l i n e in hexane s o l u t i o n . The l i t h i u m m o r p h o l i n i d e p r e c i p i t a t e d f r o m the s o l u t i o n in the f o r m o f a c r y s t a l l i n e compound. The r e a c t i o n was a c c o m p a n i e d by the l i b e r a t i o n o f g a s ( b u t a n e ) . The f o r m e r was d e t e c t e d by g a s c h r o m a t o g r a p h y and mass s p e c troscopy. The l i t h i u m m o r p h o l i n i d e powder was i s o l a t e d by means of f i l t r a t i o n . I n o r d e r t o be s u r e t h a t t h e r e was no b u t y l l i t h i u m o r m o r p h o l i n e in the d r y p o w d e r , G.C.-mass s p e c t r o s c o p i c a n a l y s i s o f D O - q u e n c h e d powder was p e r f o r m e d . No d e u t e r a t e d b u t a n e o r u n d e u t e r a t e d m o r p h o l i n e was f o u n d . The 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 w i t h l i t h i u m m o r p h o l i n i d e was c a r r i e d o u t in hexane a t s e v e r a l d i f f e r e n t t e m p e r a t u r e s . The d a t a p r e s e n t e d in T a b l e I I show t h a t the 1,2 c o n t e n t o f the p o l y b u t a d i e n e is d e p e n d e n t upon i n i t i a t o r l e v e l a n d p o l y m e r i z a t i o n temperature. A t 30 C., a s the c o n c e n t r a t i o n o f the i n i t i a t o r i n c r e a s e s f r o m 3.25 m o l e s t o 13 mmoles, the 1,2 c o n t e n t o f the p o l y b u t a d i e n e i n c r e a s e s f r o m 24.5% t o 3 6 . 3 % , respectively. However, when the t e m p e r a t u r e was i n c r e a s e d f r o m 30°C... t o 80°C., the 1,2 c o n t e n t o f the p o l y b u t a d i e n e d e c r e a s e d f r o m 24.5% t o 1 2 . 8 % , r e s p e c t i v e l y a t c o n s t a n t i n i t i a t o r c h a r g e (3.2 m m o l e s ) . The e f f e c t o f the a d d i t i o n o f a l k a l i m e t a l J.,-butoxides o n the l i t h i u m m o r p h o l i n i d e - i n i t i a t e d h e t e r o g e n e o u s b u t a d i e n e p o l y m e r i z a t i o n was a l s o s t u d i e d a t 30°C... in h e x a n e . The d a t a in T a b l e I I I show t h a t the 1,2 c o n t e n t o f the p o l y b u t a d i e n e is s e n s i t i v e t o the t y p e o f a l k a l i m e t a l u s e d . F o r example, by c h a n g i n g m e t a l a l k o x i d e s f r o m L i t o Na t o K, the 1,2 c o n t e n t o f p o l y b u t a d i e n e c h a n g e s f r o m 46.0% t o 58.2% t o 55.4%, r e s p e c t i v e l y , a t an i n i t i a t o r c o n c e n t r a t i o n o f a b o u t 10 mmoles. However, the 1,2 c o n t e n t o f the p o l y b u t a d i e n e s is u n a f f e c t e d by the m e t a l a l k o x i d e c o n c e n t r a t i o n (1,2 c o n t e n t , 42-46% f o r L i O t B u , 55-58% f o r NaOtBu, 53-55% f o r KOtBu) when the a l k o x i d e / m o r p h o l i n i d e ratio is g r e a t e r t h a n 1:1. L i t h i u m D i e t h y l a m i d e . T h i s compound h a s been u s e d a s a n i n i t i a t o r f o r the p o l y m e r i z a t i o n o f d i e n e by V i n o g r a d o r a n d B a s a y e v a ( 1 ) . I n o r d e r t o compare t h i s i n i t i a t o r w i t h l i t h i u m m o r p h o l i n i d e (a l i t h i u m - n i t r o g e n i n i t i a t o r w i t h a b u i l t - i n p o l a r m o d i f i e r ) , we h a v e p r e p a r e d l i t h i u m d i e t h y l a m i d e a c c o r d i n g t o the p r o c e d u r e d e s c r i b e d by V i n o g r a d o r a n d B a s a y e v a (1) a n d u t i l i z e d it a s a n i n i t i a t o r f o r T H F - m o d i f i e d b u t a d i e n e polymerizations.


516


TABLE I


PREPARATION OF INITIATORS No.

mmole o f n B u L i

1 2 3 4 5 6 7 8 9

349.0 5.0 5.0 5.0 154.0 154.0 154.0 154.0 112.0 and 224.0

Amine Type

mmole

Morpholine Dimethylamine Diethylamine Di-ji-butylamine Di-i-propylamine Piperidine 2,6-Dimethylpiperidine 2,2,6,6-Tetramethylpiper i d i n e Piperazine

382.0 5.0 5.0 5.0 154.0 154.0 154.0 154.0 112.0

A l l the a m i n e s were p u r c h a s e d f r o m e i t h e r MCB o r A l d r i c h C h e m i c a l Co. L i t h i u m t - b u t o x i d e and j i - b u t y l l i t h i u m (15%) in hexane were p u r c h a s e d f r o m L i t h c o a C o . Sodium and p o t a s s i u m t - b u t o x i d e s were p u r c h a s e d f r o m MSA R e s e a r c h C o r p .

TABLE I I POLYMERIZATION OF BUTADIENE WIT] LITHIUM MORPHOLINIDE I N HEXANE

No. 1 2 3 4 5 6 (a) (b)

P z n Temp °C...

mmole Initiator

30 30 30 50 70 80

3.25 6.5 13.0 3.25 3.25 3.25

M i c r o s t r u c t u r e , mole % C i s 1.4% T r a n s 1.4% 1.2% % Y i e l d 27.6 25.9 22.0 28.4 35.7 41.1

47.9 44.6 41.6 54.7 50.3 46.0

24.5 29.5 36.3 15.9 14.0 12.8

W e i g h t o f b u t a d i e n e 70 g (1.30 m o l e s ) . By I n f r a r e d method.


71.4 78.6 75.7 76.0 75.5 70.2

33.

CHENG


Initiator

517

The 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 w i t h l i t h i u m d i e t h y l a m i d e was c o n d u c t e d a t s e v e r a l d i f f e r e n t t e m p e r a t u r e s . In general, the c o n v e r s i o n t o p o l y m e r was r e a s o n a b l e ( 7 5 - 8 9 % ) , a n d the m i c r o s t r u c t u r e was 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 s and i n i t i a t o r l e v e l s o v e r the r a n g e i n v e s t i g a t e d . T h e s e r e s u l t s a r e shown in T a b l e I V . I t is i n t e r e s t i n g t o n o t e t h a t the m i c r o s t r u c t u r e o f the p o l y b u t a d i e n e p r e p a r e d b y u s a t 30°C... is d i f f e r e n t f r o m the m i c r o s t r u c t u r e r e p o r t e d b y V i n o g r a d o r a n d


Basayeva

(1) (1,2%:10-11.9

vs 27-29, c i s 1,4%; 30.7-32.3 vs

6-16). C u r r e n t l y , we d o n o t have a n e x p l a n a t i o n f o r t h i s discrepancy. The e f f e c t o f THF o n the m i c r o s t r u c t u r e o f l i t h i u m d i e t h y l amide i n i t i a t e d 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 - 1 , 3 was s t u d i e d . The 1,2 c o n t e n t o f the p o l y m e r s is d e p e n d e n t o n the ratio o f THF to l i t h i u m as expected. T a b l e V shows t h a t a s the ratio o f THF t o l i t h i u m d i e t h y l a m i d e v a r i e s f r o m 0 / 1 . 6 2 t o 3 0 . 5 / 1 . 6 2 , the 1,2 c o n t e n t c h a n g e s f r o m 1 1 % t o 5 1 . 9 % , r e s p e c t i v e l y a t 3 0 C. T h u s , the l e v e l o f THF a t c o n s t a n t l i t h i u m d i e t h y l a m i d e l e v e l s c o n t r o l s the o v e r a l l 1,2 c o n t e n t o f the p o l y b u t a d i e n e f o r m e d . A p o s s i b l e e x p l a n a t i o n f o r the o b s e r v e d d e p e n d e n c e o f p o l y m e r m i c r o s t r u c t u r e o n the l e v e l o f l i t h i u m m o r p h o l i n i d e is t h a t the p r e s e n c e o f the o x y g e n atom in m o r p h o l i n e a c t s a s a modifier. S u c h a m o d i f i e r , a s shown b e l o w , c a n be d e p i c t e d in terms o f +

CH

2

L i

: 0 , N - R

where R=polymer o r L i — ' c o m p l e x a t i o n b e t w e e n o x y g e n a n d p o l y b u t a d i e n y l l i t h i u m . The r e s u l t o f e l e c t r o n d o n a t i o n b y the m o d i f i e r is a n enhancement o f i o n i c c h a r a c t e r o f the p o l y b u t a d i e n y l l i t h i u m . Thus, i n c r e a s i n g the c o n c e n t r a t i o n o f l i t h i u m m o r p h o l i m i d e w i l l i n c r e a s e the l e v e l o f c o m p l e x e d i n i t i a t o r . C o n s e q u e n t l y , one w o u l d e x p e c t the i n c r e a s i n g o f 1,2 c o n t e n t o f p o l y b u t a d i e n e in the p o l y m e r i z a t i o n . T h i s e x p l a n a t i o n h a s been s u p p o r t e d b y the use o f THF a s a m o d i f i e r in l i t h i u m d i e t h y l a m i d e i n i t i a t e d p o l y m e r i z a t i o n o f butadiene (Table V ) . Similar r e s u l t s using THF a s a m o d i f i e r in a l k y l l i t h i u m c a t a l y z e d b u t a d i e n e p o l y m e r i z a t i o n have been r e p o r t e d b y A n t k o w i a k a n d h i s a s s o c i a t e s (2). The t e m p e r a t u r e d e p e n d e n c y o f 1,2 c o n t e n t shown in T a b l e I I is a l s o c o n s i s t e n t w i t h c o m p l e x f o r m a t i o n b e t w e e n p o l y b u t a d i e n y l l i t h i u m a n d the o x y g e n atom in the l i t h i u m m o r p h o l i n i d e moleculre. One c a n v i s u a l i z e an e q u i l i b r i u m b e t w e e n noncomp l e x e d and c o m p l e x e d m o l e c u l e s w h i c h w o u l d be i n f l u e n c e d b y temperature. Higher temperatures would f a v o r d i s s o c i a t i o n o f the c o m p l e x ; a n d , t h e r e f o r e , the 1,2 c o n t e n t o f the p o l y m e r w o u l d be l o w e r t h a n t h a t f r o m the l o w t e m p e r a t u r e p o l y m e r i z a tion. T h i s e x p l a n a t i o n is s u p p o r t e d b y the 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 w i t h l i t h i u m d i e t h y l a m i d e , in w h i c h the m i c r o s t r u c t u r e o f the p o l y b u t a d i e n e r e m a i n s c o n s t a n t r e g a r d l e s s o f 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 ( T a b l e I V ) . T h i s is p r e s u m a b l y due t o the f a c t t h a t t r i a l k y l a m i n e s a r e known t o be p o o r


518


TABLE I I I EFFECT OF METAL T-BUTOXIDE ON LITHIUM MORPHOLIΝID] INITIATED BUTADIENE POLYMERIZATION AT 30°C... I N HEXANE Modifier Type mmole


No. 1 2 3 4 5 6 7 8 9 10

Cis

25.9 15.7 13.2 15.5 3.6 7.0 9.3 12.3 8.0 9.1

,

LiO-t-Bu

NaO-t-Bu

KO-t-Bu

(b) M i c r o s t r u c t u r e , mole % 1.4% T r a n s 1.4% 1.2% % Yield

6.8 10.6 18.4 10.0 20.0 30.0 2.6 10.6 21.2

44.6 42.2 40.8 37.8 34.8 38.3 36.6 34.6 36.6 35.7

29.5 42.1 46.0 46.7 58.2 55.5 55.2 53.1 55.4 55.2

78.6 75.5 73.2 72.5 76.3 70.2 69.5 75.5 72.5 67.3

(a)

70 g (1.30 m o l e s ) o f b u t a d i e n e and 0.578 g (6.8 mmoles) o f l i t h i u m m o r p h o l i n i d e were u s e d .

(b)

By I n f r a r e d method.

TABLE I V POLYMERIZATION OF BUTADIENE WITH LITHIUM DIETHYLAMIDE I N HEXANE Pzn mmole Temp C... I n i t i a t o r C i s o

No. 1 2 3 4 5 6 7 8 g(c)

30 30 50 50 70 70 80 80 30

2.70 3.78 1.62 2.70 1.62 2.70 1.62 2.70

-

(a)

64 g (1.19

(b)


(c)

See r e f e r e n c e 1.

( b )

M i c r o s t r u c t u r e , mole % % Yield 1.4% T r a n s 1.4% 1.2%

31.2 30.9 32.2 32.3 31.3 32.0 31.1 30.7 6-16

57.9 58.3 58.0 57.7 57.7 57.0 57.1 57.4 55-57

10.9 10.8 9.8 10.1 11.0 11.0 11.7 11.9 27-29

75.0 89.0 84.3 85.9 87.5 89.0 84.3 85.9

m o l e s ) o f b u t a d i e n e monomer was u s e d .


-

33.

CHENG


Initiator

519


m o d i f i e r s f o r a l k y l l i t h i u m s y s t e m (7). A s i m i l a r complexed s p e c i e s c a n a l s o be p o s t u l a t e d in the b u t a d i e n e p o l y m e r i z a t i o n w i t h metal a l k o x i d e s m o d i f i e d l i t h i u m morpholinide system. S u c h c o m p l e x a t i o n i n v o l v i n g a l k y l m e t a l s ( L i , Na a n d K) a n d m e t a l a l k o x i d e s ( L i , Na and K) h a s been p r e v i o u s l y r e p o r t e d f r o m o u r l a b o r a t o r i e s (8,9,10;). The i n i t i a t i o n s t e p f o r the l i t h i u m d i e t h y l a m i d e c a t a l y z e d b u t a d i e n e p o l y m e r i z a t i o n c a n be p o s t u l a t e d in t e r m s o f the following equations:

•Li + C H

(I)

2

= CH-CH = C H

2

(II)

The compound ( I ) t h e n r e a c t s w i t h a d d i t i o n a l b u t a d i e n e monomer t o form a polymer. I n o r d e r t o v e r i f y t h i s h y p o t h e s i s , we have i s o l a t e d a m i x t u r e o f l - d i e t h y l a m i n o - 2 - b u t e n e in 65-70% o f y i e l d by the r e a c t i o n o f e q u i m o l a r amounts o f l i t h i u m d i e t h y l a m i d e a n d b u t a d i e n e in d i e t h y l a m i n e . This product c o n s i s t s o f a mixture o f the c i s a n d t r a n s o l e f i n s in a ratio o f 1 t o 2. I n a d d i t i o n , we h a v e a l s o o b s e r v e d a b o u t 5% o f l - d i e t h y l a m i n o - 3 - b u t e n e b y G.C. a n a l y s i s . A l lthes a m p l e s have been i d e n t i f i e d by NMR, mass s p e c t r o s c o p y , I.R. a n d G.C. W i t h t h i s i n f o r m a t i o n in m i n d , it is b e l i e v e d t h a t o t h e r s y s t e m s , s u c h a s l i t h i u m morpholinide i n i t i a t o r , l i t h i u m morpholinide-metal alkoxide, e t c . , f o l l o w the same p o l y m e r i z a t i o n mechanism. Copolyme r i ζ a t i o 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 s t y r e n e w i t h a l k y l l i t h i u m i n i t i a t o r h a s drawn c o n s i d e r a b l e a t t e n t i o n in the l a s t d e c a d e b e c a u s e o f the i n v e r s i o n phenom enon (12) a n d c o m m e r c i a l i m p o r t a n c e (13) . I t h a s been known t h a t the rate o f s t y r e n e h o m o p o l y m e r i z a t i o n w i t h a l k y l l i t h i u m is more r a p i d t h a n b u t a d i e n e h o m o p o l y m e r i z a t i o n in h y d r o c a r b o n s o l v e n t . However, the s t o r y is d i f f e r e n t when a m i x t u r e o f b u t a d i e n e a n d s t y r e n e is u s e d . The p r o p a g a t i n g p o l y m e r c h a i n s a r e r i c h in b u t a d i e n e u n t i l l a t e in r e a c t i o n when s t y r e n e c o n t e n t s u d d e n l y i n c r e a s e s . T h i s phenomenon is c a l l e d i n v e r s i o n b e c a u s e o f the rate o f b u t a d i e n e p o l y m e r i z a t i o n is now f a s t e r t h a n the s t y r e n e . A s a r e s u l t , a b l o c k c o p o l y m e r is o b t a i n e d in t h i s s y s t e m . However, the c o p o l y m e r i z a t i o n c h a r a c t e r i s t i c is c h a n g e d if a s m a l l amount o f p o l a r s o l v e n t


520



is u s e d as a m o d i f i e r . The r e s u l t i n g c o p o l y m e r is e s s e n t i a l l y r a n d o m i z e d (14,15,17). We f e l t it w o u l d be i n t e r e s t i n g t o s t u d y i n i t i a t o r s w i t h b u i l t - i n m o d i f i e r f o r r a n d o m i z i n g the styrene. L i t h i u m M o r p h o l i n i d e . L i t h i u m m o r p h o l i n i d e was the first one c h o s e n as an i n i t i a t o r f o r 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 b e c a u s e o f the b u i l t - i n o x y g e n . I t was f e l t t h a t the p r e s e n c e o f o x y g e n in the i n i t i a t o r may l e a d t o a c o p o l y m e r with unusual p r o p e r t i e s . 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 - s t y r e n e w i t h t h i s i n i t i a t o r was c a r r i e d o u t h e t e r o g e n e o u s l y in h e x a n e . The d a t a in T a b l e V I show t h a t the s t y r e n e c o n t e n t is h i g h l y d e p e n d e n t on the p e r c e n t o f c o n v e r s i o n . I t a p p e a r s t h a t a b l o c k s t y r e n e w o u l d be o b t a i n e d if 100% c o n v e r s i o n w e r e r e a c h e d . A detailed d i s c u s s i o n r e g a r d i n g t h i s s u b j e c t w i l l be m e n t i o n e d l a t e r . From thé T a b l e I V , it a l s o shows t h a t the l o w s t y r e n e c o n t e n t in the c o p o l y m e r may r e l a t e t o the p o l y m e r i z a t i o n temperature. As 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 was i n c r e a s e d f r o m 5 to 70 C., the s t y r e n e c o n t e n t o f the 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 d e c r e a s e d f r o m 21.7% t o 9.1%, respectively. The d e c r e a s i n g in s t y r e n e c o n t e n t a t h i g h e r t e m p e r a t u r e is c o n s i s t e n t w i t h the p a p e r r e p o r t e d by Adams and h i s a s s o c i a t e s (16) f o r thermal s t a b i l i t y of " l i v i n g " p o l y m e r - l i t h i u m system. I n Adams' p a p e r , it was c o n c l u d e d t h a t the f o r m a t i o n o f l i t h i u m h y d r i d e f r o m p o l y s t y r y l l i t h i u m and p o l y b u t a d i e n y l l i t h i u m d i d o c c u r a t h i g h t e m p e r a t u r e in h y d r o c a r b o n s o l v e n t . The t h e r m a l s t a b i l i t y o f p o l y s t y r y l l i t h i u m in c y c l o h e x a n e is p o o r e r t h a n p o l y b u t a dienyllithium. From t h e s e r e s u l t s , it a p p e a r s t h a t the d e c r e a s i n g in s t y r e n e c o n t e n t in l i t h i u m m o r p h o l i n i d e i n i t i a t e d c o p o l y m e r i z a t i o n a t h i g h e r t e m p e r a t u r e is b e l i e v e d t o be a s s o c i a t e d w i t h the f o r m a t i o n o f l i t h i u m h y d r i d e . S i n c e the r e v e r s a l o f a c t i v i t y o f b u t a d i e n e w i t h r e s p e c t t o s t y r e n e in a l k y l l i t h i u m s y s t e m has been o b s e r v e d (12), it w o u l d be o f i n t e r e s t t o f i n d o u t w h e t h e r the i n v e r s i o n phenomenon still h o l d s in the c a s e o f the l i t h i u m m o r g h o l i n i d e s y s t e m . F o u r t e m p e r a t u r e s , n a m e l y 3 0 , 4 0 , 50 and 60C...were c h o s e n f o r t h i s s t u d y . A t 30 C... 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 the c u r v e is c h a r a c t e r i s t i c o f b l o c k c o p o l y m e r i z a t i o n when one p l o t s p e r c e n t bound s t y r e n e v s p e r c e n t c o n v e r s i o n ( F i g , 1). Initially, a s m a l l amount (/,/3%) o f s t y r e n e is p o l y m e r i z e d . T h i s is f o l l o w e d by a b l o c k o f b u t a d i e n e . The r e m a i n i n g s t y r e n e is t h e n p o l y m e r i z e d a f t e r all the b u t a d i e n e is consumed. T h i s r e s u l t is i d e n t i c a l t o the a l k y l l i t h i u m i n i t i a t e d c o p o l y m e r i z a t i o n . Thus it a p p e a r s t h a t the o r d e r o f r e a c t i v i t y f o r l i t h i u m m o r p h o l i n i d e system again is: butadiene styrene I d e n t i c a l r e s u l t s have a l s o been o b s e r v e d f o r 4 0 , 50 and 60 C... 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 . T h i s is a l s o shown in F i g . 1. I t a p p e a r s t h a t 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 has no e f f e c t on the rate o f p o l y m e r i z a t i o n e v e n w i t h the p r e s e n c e o f b u i l t - i n o x y g e n as a m o d i f i e r in the i n i t i a t o r . However, it d o e s c h a n g e Q


CHENG


Initiator


33.


521

522



the styrene content in the copolymer as mentioned p r e v i o u s l y . Nevertheless, the r e s u l t d i d suggest the formation of block copolymer in l i t h i u m morpholinide i n i t i a t e d c o p o l y m e r i z a t i o n . In a d d i t i o n , we have a l s o found that the styrene content of butadiene-styrene copolymer is independent of the c a t a l y s t c o n c e n t r a t i o n over the range s t u d i e d . For example, a t 30 C., the styrene content of the butadiene-styrene copolymer is v i r t u a l l y unchanged between 6.5 mmoles and 9.75 mmoles of c a t a l y s t c o n c e n t r a t i o n (19.2-21.6%). S i m i l a r r e s u l t s were a l s o obtained at 5 and 50C...(Table V I ) . I t is b e l i e v e d that such phenomenon is due to the heterogeneous nature of the c a t a l y s t . Lithium d i a l k y l a m i d e . Four c a t a l y s t s , namely l i t h i u m dimethylamide, l i t h i u m diethylamide, l i t h i u m di-jn-butylamide and l i t h i u m d i - i - p r o p y l a m i d e , have been prepared by the r e a c t i o n of r i - b u t y l l i t h i u m and the corresponding amines. C h a r a c t e r i z a t i o n of these amides was performed by G.C.-Mass Spectroscopy a f t e r D 0 quenching of the amides. Further a n a l y s i s f o r the t o t a l a l k a l i n i t y was performed by a t i t r a t i o n (11). A f t e r quenching with 2°' deuterobutane or undeuteriated amines were observed. The copolymerizations of butadiene and styrene with these four amides were c a r r i e d out in hexane with the i n s o l u b l e i n i t i a t o r s at s e v e r a l d i f f e r e n t temperatures. In g e n e r a l , the styrene content in the l i t h i u m diethylamide or l i t h i u m d i - n butylamide i n i t i a t e d butadiene-styrene polymerizations is higher than in the l i t h i u m dimethylamide or l i t h i u m d i - i propylamide system (Table V I I ) . A high styrene content (23.4% to 25.3%) in the l i t h i u m diethylamide and the l i t h i u m d i - n butylamide systems was obtained at 50°C... p o l y m e r i z a t i o n temp e r a t u r e . From t h i s r e s u l t , it appears that the best cop o l y m e r i z a t i o n temperature f o r these systems is 50 C. In l i t h i u m dimethylamide and l i t h i u m di-i-propylamide systems, the styrene content in the copolymer is g e n e r a l l y low (7-15%) even though the i n i t i a l change of styrene is 25%. This phenomenon is more c l e a r l y in case of l i t h i u m d i - i - p r o p y l a m i d e system. One explanation f o r the low styrene content could be a t t r i b u t e d to the poor s o l u b i l i t y of the i n i t i a t o r s in s o l u t i o n . As a r e s u l t , it takes a long time to i n i t i a t e the p o l y m e r i z a t i o n . Such hypothesis has a l s o been suggested by Angood and h i s a s s o c i a t e s when they polymerized styrene with l i t h i u m diethylamide i n i t i a t o r (2). Another e x p l a n a t i o n could be the slow a c t i n g i m p u r i t i e s terminated the few a c t i v e chains before reaching the q u a n t i t a t i v e conversion. As a r e s u l t , the rate of copolymerization w i l l be slow. In other words, the i n d u c t i o n p e r i o d f o r t h i s i n i t i a t o r w i l l be longer due to the presence of l e s s a c t i v e i n i t i a t o r s . Nevertheless, all the i n i t i a t o r s are c l a s s i f i e d to be an a c t i v e c a t a l y s t f o r the p o l y m e r i z a t i o n of butadiene-styrene (Table V I I ) . 2

n

o


33.

CHENG


523

Initiator

TABLE V EFFECT OF THF ON LITHIUM DIETHYLAMIDE I N I T I A T E D BUTADIENE POLYMERIZATION AT 30°C... I N HEXANE Downloaded by PENNSYLVANIA STATE UNIV on July 18, 2012 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0166.ch033

3

NO.

mmole THF

1 2 3 4

M i c r o s t r u c t u r e , mole % 1.2% T r a n s 1.4% C i s 1.4% 31.7 20.8 17.4 13.5

6.17 12.3 30.5

57.7 49.8 45.3 34.5

% Yield 87 85 91 88

11.0 29.4 37.3 51.9

(a)

70 g (1.3 m o l e s ) o f b u t a d i e n e monomer a n d 1.62 mmole o f l i t h i u m d i e t h y l a m i d e were u s e d .

(b)


TABLE V I COPOLYMERIZATION OF BUTADIENE/STYRENE WITH LITHIUM MORPHOLINIDE I N HEXANE 3

NO. 1 2 3 4 5 6 7

Pzn Temp °C...

mmole Initiator

5 5 30 30 50 50 70

13.6 20.4 6.5 9.75 3.25 6.5 3.25

M i c r o s t r u c t u r e , mole % < , C i s 1.4% T r a n s 1.4% 1.2% % S t y b

14.1 12.1 26.0 19.2 34.7 33.8 41.4

39.4 38.3 49.7 52.5 49.4 47.0 45.2

46.6 49.5 19.6 22.2 16.0 19.2 13.3

18.7 21.7 19.2 21.6 12.8 13.9 9.1

%

Yield 78 79 79 84 76 71 63

(a)

64.2 g (1.2 m o l e s ) o f b u t a d i e n e and 16.8 g (0.162 m o l e s ) o f s t y r e n e were u s e d .

(b)



524



TABLE VII EFFECT OF ALKYL GROUP OF LITHIUM DIALKYLAMIDE ON MICROSTRUCTURE AND STYRENE CONTENT OF BUTADIENE/STYRENE COPOLYMER

NO.

Pzn Temp °C...

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

30 30 50 50 80 30 30 30 50 50 50 80 30 30 50 50 70 70 50 50 50 80 80

M i c r o s t r u c t u r e , mole % Trans 1.2% % Sty % Y i e l d 1.4% ( b )

Initiator Alkyl mmole

Cis 1.4%

Methyl

21.5 18.9 20.4 22.3 22.2 29.1 28.0 29.3 28.5 27.1 29.3 27.6 30.8 34.4 28.9 30.0 29.5 27.7 30.9 28.4 26.7 27.3 30.8

9.6 19.2 II 9.6 It 19.2 II 9.6 Ethyl 2.7 II 9.6 II 19.2 II 1.62 II 9.6 II 19.2 II 1.62 n-Butyl 0.9 II 1.5 II 0.9 II 1.5 II 0.6 II 0.9 i - p r o p y l 2.8 II 5.6 II 11.2 II 2.8 II 5.6 II

56.2 53.7 62.3 57.7 63.2 60.2 61.1 59.2 61.5 62.4 60.1 60.1 60.4 57.5 61.6 60.6 59.5 61.0 57.7 59.2 58.6 60.4 57.3

22.2 27.4 17.3 19.9 14.6 10.7 10.8 11.6 10.0 10.5 10.6 12.3 8.8 8.1 9.5 9.5 11.0 11.3 11.4 12.4 14.8 12.4 11.8

14.8 14.6 13.6 12.4 10.9 19.0 11.8 6.5 24.4 24.6 23.4 11.3 10.7 17.3 25.3 24.1 21.3 21.6 7.7 8.1 8.7 8.7 6.9

75 70 73 70 69 90 78 70 92 93 95 80 83 80 94 93 92 93 68 67 71 70 70

(a)

64.2 g (1.2 moles) o f butadiene and 16.8 g (0.162 moles) of styrene were used (21% styrene c o n t e n t ) .

(b)



33.

CHENG


Initiator

525


At t h i s stage, we do not know whether the styrene i n c o r porated in polymer c h a i n is a block styrene or not. However, it is speculated t h a t the block styrene w i l l be r e s u l t e d based on the l i t h i u m morpholinide system. More work along t h i s l i n e is in need in order t o q u a l i f y t h i s statement. L i t h i u m P i p e r i d i n i d e . Since the butadiene-styrene copolymer prepared from l i t h i u m d i - i - p r o p y l a m i d e contained low styrene content (Λ*7%) r e g a r d l e s s o f the p o l y m e r i z a t i o n temperature and i n i t i a t o r c o n c e n t r a t i o n , it would be i n t e r e s t i n g to know if there is a good reason t o e x p l a i n it. One p o s s i b l e e x p l a n a t i o n c o u l d be based on the s t e r i c e f f e c t s o f the i - p r o p y l group besides the problem o f i n s o l u b i l i t y . To t e s t t h i s theory, we prepared i n i t i a t o r s , namely l i t h i u m p i p e r i d i n i d e , l i t h i u m 2 , 6 - d i m e t h y l p i p e r i d i n i d e and l i t h i u m 2,2,6,6t e t r a m e t h y l p i p e r i d i n i d e , in hexane. The c o p o l y m e r i z a t i o n o f butadiene and styrene was c a r r i e d out in hexane a t 30C...a t s e v e r a l d i f f e r e n t i n i t i a t o r c o n c e n t r a t i o n s using these i n s o l uble i n i t i a t o r s . The data presented in Table V I I I show t h a t the styrene content o f the copolymers is g e n e r a l l y high (20-24%) in e i t h e r the l i t h i u m p i p e r i d i n i d e or the l i t h i u m 2 , 6 - d i m e t h y l p i p e r i d i n i d e systems r e g a r d l e s s o f the i n i t i a t o r level. The conversion o f the polymer based on i n i t i a l monomer change is a l s o high (80-90%) over the range 3 mmole t o 10.4 mmole o f i n i t i a t o r . However, the styrene content ( Λ 9 . 6 % ) and c o n v e r s i o n (

Anionic Polymerization - American Chemical Society

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