The Reactivity of Polydiene Anions with Divinylbenzene - American

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The Reactivity of Polydiene Anions with Divinylbenzene M. K. MARTIN1, T. C. WARD, and JAMES E. McGRATH

Downloaded by MONASH UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0166.ch035

Chemistry Department and Polymer Materials and Interfaces Laboratory, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 Star-branched homopolymers of butadiene and isoprene have been synthesized utilizing anionic polymerization techniques. The commercial mixture of divinylbenzene was employed as the star linking agent for the "living" polydienyllithium anions. The nature of the star-branching reaction was studied with respect to: the molar ratio of divinylbenzene (DVB)/alkyllithium (RLi), reaction temperature, reaction time and the nature of the polydienyllithium chain end. In general, a higher number of arms is achieved via increased DVB/RLi ratio, reaction temperature and through the choice of polydienyllithium chain ends. The polymers were characterized by gel permeation chromatography (G.P.C.), membrane osmometry, intrinsic viscosity, and H-NMR spectroscopy. A Chromatix low-angle light scattering G.P.C. detector was also employed for the determination of the weight average molecular weight (Mw), as well as a sensitive detector, in order to assay absolutely the Mw versus elution volume profile for a series of star-branched polyisoprenes and polybutadienes. The results indicate that under optimum conditions a relatively well defined number of arms can be achieved with DVB linking. In 1965, Milkovich (I) reported that divinylbenzene could be utilized for the formation of star-branched macromolecules. Later, Rempp and coworkers (2, 3, 4) successfully applied this method for the synthesis of star-branched polystyrenes. Moreover, Fetters and coworkers (5-8) used this procedure for the synthesis of multi-arm star-branched polyisoprene homopolymers and poly' Current Address: Shell Development Company, P.O. Box 1380, Houston, TX 77001. 0097-6156/81/0166-0557$06.00/0 © 1981 American Chemical Society In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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styrene-polydiene block polymers. T h i s method o f s t a r - b r a n c h e d p o l y m e r f o r m a t i o n i n v o l v e s the s e q u e n t i a l p o l y m e r i z a t i o n o f monomer g i v i n g the "arm", f o l l o w e d by the a d d i t i o n o f d i v i n y l b e n z e n e . The p o l y m e r i z a t i o n o f d i v i n y l b e n z e n e t h e n r e s u l t s in a " m i c r o g e l " n u c l e u s c o n t a i n i n g p e n d a n t v i n y l g r o u p s w h i c h s e r v e as b r a n c h p o i n t s f o r the s t a r - s h a p e d p o l y m e r . T h i s p r o c e d u r e has l e d t o the f o r m a t i o n o f s t a r - b r a n c h e d p o l y i s o p r e n e s c o n t a i n i n g up t o 56 w e i g h t - a v e r a g e numbers o f arms ( J 5 , 6). The arms o f t h e s e s t a r m a c r o m o l e c u l e s p o s s e s s n a r r o w 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 due t o the t e r m i n a t i o n f r e e a s p e c t o f d i e n e p o l y m e r i z a t i o n s i n i t i a t e d by o r g a n o l i t h i u m s p e c i e s in h y d r o c a r b o n s o l v e n t s ( 9 ) . S t a r - b r a n c h e d b l o c k and homopolymers a r e o f i n t e r e s t b o t h f r o m a t h e o r e t i c a l and p r a c t i c a l v i e w p o i n t . S t u d i e s on the m e l t r h e o l o g y have shown the v i s c o s i t y t o be i n d e p e n d e n t o f the e x t e n t o f b r a n c h i n g , y e t d e p e n d e n t upon the arm m o l e c u l a r w e i g h t (6, 7, 8) . S t a r - b r a n c h e d 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 p p e a r t o h a v e i m p r o v e d m e c h a n i c a l p r o p e r t i e s and p r o c e s s a b i l i t y o v e r the l i n e a r s t y r e n e - b u t a d i e n e - s t y r e n e a n a l o g s ( 7 , 8). A high styrene content s t a r - s h a p e d b l o c k c o p o l y m e r i n t r o d u c e d by P h i l l i p s (K-resin) offers the rigidity, transparency, and good i m p a c t s t r e n g t h n e c e s s a r y f o r p a c k a g i n g and t h e r m o f o r m i n g a p p l i c a t i o n s ( 10) . H i g h b u t a d i e n e c o n t e n t s t a r b l o c k p o l y m e r s h a v e b e e n u s e d in h o t m e l t and 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 s ( 1 1 , 1 2 ) . Due t o t h i s g r o w i n g i n t e r e s t in s t a r - b r a n c h e d p o l y m e r b e h a v i o r , it became o f i n t e r e s t to study the nature o f the s t a r - b r a n c h i n g r e a c t i o n b e t w e e n p o l y d i e n y l l i t h i u m a n i o n s and the c o m m e r c i a l m i x t u r e o f d i v i n y l benzene. The s u b j e c t o f o u r r e s e a r c h d i s c u s s e d h e r e i n was to i n v e s t i g a t e the v a r i a b l e s t h a t i n f l u e n c e the s t a r formation p r o c e s s . T h e s e c e r t a i n l y i n c l u d e the m o l a r ratio o f D V B / R L i , the l i n k i n g r e a c t i o n t e m p e r a t u r e , r e a c t i o n t i m e , and the n a t u r e o f the " l i v i n g " d i e n e c h a i n end. Experimental S t a r - p o l y m e r s y n t h e s i s was a c h i e v e d by u t i l i z i n g an all g l a s s h i g h vacuum d e s i g n as shown in F i g u r e 1. Monomers and s o l v e n t s were p u r i f i e d by d r y i n g o v e r c a l c i u m h y d r i d e ; w i t h p e r i o d i c d e g a s s i n g , f o l l o w e d by d i s t i l l a t i o n u n d e r vacuum o v e r a s u c c e s s i o n o f s o d i u m m i r r o r s . The c o m m e r c i a l DVB monomer was f u r t h e r p u r g e d over dibutylmagnesium ( L i t h i u m C o r p o r a t i o n of America), which p r o d u c e d a b r i g h t - y e l l o w complex t h a t was i n d i c a t i v e o f d r y n e s s . S o l v e n t s and d i e n e monomer were f u r t h e r s t o r e d o v e r an o r g a n o m e t a l l i c p u r g e u n t i l needed. The DVB was s p l i t down i n t o ampoules in b e n z e n e o r h e x a n e t o f o r m 0.2N-0.8N s o l u t i o n s . This was n e c e s s a r y due t o the h i g h r e a c t i v i t y o f DVB. A t y p i c a l p o l y m e r i z a t i o n r e a c t o r is shown in F i g u r e 2. The r e a c t o r c o n t a i n s a s i d e - a m p o u l e u s e d t o i s o l a t e a p o r t i o n o f the " l i v i n g " l i n e a r p r e c u r s o r b e f o r e s t a r - l i n k i n g by the a d d i t i o n o f DVB. The r e a c t o r was p u r i f i e d by f l a m i n g w i t h a hand t o r c h w h i l e c o n t i n u o u s l y pumping u n t i l a p r e s s u r e o f 10" was r e a c h e d . The

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

MARTIN ET AL.

Divinylbenzene


35.

559

s

S 3 Ο

Si .00




560

Figure 2. Polymerization reactor.



35.

MARTIN ET AL.

Divinylbenzene

561

apparatus was then heat-sealed from the vacuum l i n e v i a the c o n s t r i c t i o n and the i n t e r i o r w a l l s of the r e a c t o r r i n s e d with nbutyllithium-hexane s o l u t i o n in order to remove any adsorbed i m p u r i t i e s on the i n t e r i o r w a l l s of the glassware. sec-Butyl l i t h i u m was used as the i n i t i a t o r to prepare the polydienes. The s e c - b u t y l l i t h i u m was p u r i f i e d by high vacuum d i s t i l l a t i o n as d e s c r i b e d in previous work (13K The pure secb u t y l l i t h i u m was then d i l u t e d with hexane and split-down i n t o ampoules c o n t a i n i n g b r e a k s e a l s . Each ampoule could be used f o r subsequent p o l y m e r i z a t i o n . A t y p i c a l split-down apparatus is shown in F i g u r e 3. I n i t i a t o r c o n c e n t r a t i o n s were determined by t i t r a t i o n with standard HC1 to a phenolphthalein endpoint. The diene p o l y m e r i z a t i o n s were g e n e r a l l y allowed to proceed f o r 48 hours, which ensured q u a n t i t a t i v e conversion. In the case of butadiene, the r e a c t o r was cooled p e r i o d i c a l l y to draw r e s i d u a l butadiene monomer i n t o the s o l u t i o n from the v o i d volume of the reactor. The star-shaped polymers were prepared by the a d d i t i o n of the DVB s o l u t i o n to the " l i v i n g " p o l y d i e n y l l i t h i u m hexane s o l u t i o n . P r i o r to the a d d i t i o n of d i v i n y l b e n z e n e (DVB), a p o r t i o n of the l i n e a r polymer s o l u t i o n was i s o l a t e d v i a the sidearm, which enabled the arm molecular weight to be determined. Upon a d d i t i o n of divinylbenzene the c h a r a c t e r i s t i c r e d orange c o l o r of the s t y r y l l i t h i u m type anion r e s u l t e d . A f t e r the star-polymer formation was complete, the s o l u t i o n s were terminated with degassed methanol. The polymer was p r e c i p i t a t e d in methanol and d r i e d to a constant weight under vacuum. Characterization techniques i n c l u d e d membrane osmometry, i n t r i n s i c v i s c o s i t y , GPC and s p e c t r o s c o p i c methods. A Wescan r e c o r d i n g osmometer was used at 43°C... with chlorobenzene as the s o l v e n t . The accuracy of the instrument was checked by the use of s e v e r a l s o l u t i o n s of known molecular weight p o l y s t y r e n e standards. The values of îïn were determined from measurements made on s o l u t i o n s of at l e a s t four c o n c e n t r a t i o n s , followed by the t y p i c a l (7T/c)l/2 versus conc e n t r a t i o n p l o t procedure. The g e l permeation chromatography a n a l y s i s was achieved u s i n g a Waters HPLC equipped with both r e f r a c t i v e index and U.V. d e t e c t o r s . The c a r r i e r solvent was t e t r a h y d r o f u r a n at 25°C., with a flow rate of 1.0 ml/min. The column arrangement c o n s i s t e d of four o n e - f t . columns o f micros t y r a g e l with a continuous p o r o s i t y range of 500 i*P, 10 10 and 10 X. The weight average molecular weight(Mw) measurements were determined by u t i l i z i n g the Chromatix low angle l a s e r light s c a t t e r i n g G.P.C. d e t e c t o r (14). The G.P.C. column set was s i m i l a r to those mentioned above and the c a r r i e r s o l v e n t was t e t r a h y d r o f u r a n at 25°C... with a flow rate of 1.5 ml/min. The (dn/dc) values of both polybutadiene and polyisoprene were determined in THF with the Chromatix KMX-16 d i f f e r e n t i a l r e f r a c t o m e t e r . 5




(I)

Figure 3.

The stc-butyllithium split-down apparatus.


35.

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Divinylbenzene

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Results and D i s c u s s i o n One would a n t i c i p a t e the l i n k i n g o f p o l y i s o p r e n y l l i t h i u m as w e l l as p o l y b u t a d i e n y l l i t h i u m anions with commercial DVB to be of a complex nature. The c o n t r o l o f s e v e r a l r e a c t i o n parameters could be expected to i n f l u e n c e the e f f i c i e n c y o f the l i n k i n g r e a c t i o n . Young and F e t t e r s (15) have studied the nature o f the l i n k i n g r e a c t i o n by observing the change in the u l t r a v i o l e t and v i s i b l e spectrum throughout the course of the r e a c t i o n . I t was concluded that meta-divinylbenzene was more e f f i c i e n t than paraDVB with respect to star-formation. These authors a l s o observed that s i g n i f i c a n t d i f f e r e n c e s e x i s t e d between p o l y b u t a d i e n y l l i t h ium and p o l y i s o p r e n y l l i t h i u m anions with respect to t h e i r c r o s s over r e a c t i o n with the DVB isomers. Q u a l i t a t i v e l y , we have observed that tha i n i t i a l crossover r e a c t i o n from the p o l y b u t a d i e n y l l i t h i u m anion to that o f the v i n y l b e n z y l anion ( I I ) (Reaction 1) was much slower than the corresponding r e a c t i o n i n v o l v i n g p o l y i s o p r e n y l l i t h i u m anions. This observation was in agreement with the Young and F e t t e r s U.V. - v i s i b l e a n a l y s i s (15).

CH=CH-CH ©Li© + ( Q )

K

l

2

, ~CH=CH-CH -CH 2

(I)

CtRi©(l)

2

(II)

Once the v i n y l b e n z y l a n i o n ( I I ) forms, it may add to another divinylbenzene molecule r e s u l t i n g in DVB "homopolymerization ' (Reaction 2). Likewise, the v i n y l b e n z y l a n i o n ( I I ) may attach a pendant v i n y l group of another polymer chain (Reaction 3) to give the a l k y l b e n z y l a n i o n ( I I I ) . 1

CH ÇlLi®

CH =CH

2

ocH CH-CH CI#Li®

2

2

(2)

2

(II) /v~,CH Cl£Li© + 2

e

CH CH-CH CH Li 2

@

v

, ' O C H Ç H C H Ç H L i ® (3)

2

2

N

2

CH -CHCH ClA i 2

2

f i

J

(III)


564


I n a n a l o g o u s f a s h i o n , a n uncapped p o l y b u t a d i e n y l l i t h i u m o r p o l y i s o p r e n y l l i t h i u m a n i o n may a t t a c k an u n r e a c t e d pendant v i n y l g r o u p o f a n o t h e r p o l y m e r c h a i n ( R e a c t i o n 4 ) t o g i v e the a l k y l b e n z y l anion ( I I I ) .

'

w

O H - l - C H p L i ®

( R = C H ,

A

H

2

C H C H

H )

3


+

(§)

C t p L i ®

2

C H C H C H C l P L i © (4)

-,

2

2

igj

(§)

~ ~ € H =

écH CH ClLi© 2

(III)

2

R e a c t i o n s ( 3 ) a n d ( 4 ) d e s c r i b e d above r e s u l t in the b e g i n n i n g o f s t a r - b r a n c h e d p o l y m e r f o r m a t i o n . Once the a l k y l b e n z y l a n i o n ( I I I ) has formed it may f u r t h e r r e a c t w i t h any a v a i l a b l e DVB monomer a s i l l u s t r a t e d in r e a c t i o n sequence ( 5 ) . L r I $ C H

2

- C H - C H

2

+

CH =CH

,

2

rf=li® CH =CH

Li®£fcHCH —CHCH 2

2

CH

2

(

5

)

/ X

cH cri3Li

The Reactivity of Polydiene Anions with Divinylbenzene - American

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