Anionic Copolymerization of Butadiene and Isoprene with

34

Anionic Copolymerization of Butadiene and Isoprene with Organolithium Initiators in Hexane

Downloaded by CORNELL UNIV on July 29, 2016 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0166.ch034

I.-C. WANG, Y. MOHAJER, T. C. WARD, G. L. WILKES, and JAMES E. McGRATH Chemistry and Chemical Engineering Departments, Polymer Materials and Interfaces Laboratory, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 In homopolymerization initiated by sec-butyl-lithium in hexane, isoprene is a more active monomer than butadiene (with k1=5.53 χ 10-5 sec - 1 vs. k1=0.98 χ 10 - 5 sec - 1 at 20°C). This is also true for reactions at 30° and 40°C. The apparent activation energy for both monomers has been found to be roughly the same, i . e . , 19.4 kcal/mole. In the case of copolymerization, butadiene reacts preferentially, but nonexclusively. Significant amounts of isoprene units are also incorporated in a rather random fash ion during the early stage of copolymerization. At 50 mole per cent isoprene or higher, one observes a second stage of the poly merization that is faster and essentially identical to that ob served for isoprene homopolymerization under similar conditions. Various methods have been used to estimate reactivity ratios. The average values at 20°C... are rB=2.64 and rI=0.40. Preliminary evidence suggests that the copolymerization becomes more selec tive at lower temperature where the inversion phenomenon is more significant. Copolymerization, of course, involves the simultaneous polymerization of a mixture of two (or more) monomers. k * ll * ~~~~M + Mj •, ~~~~Mi-M]. (1) k * 12 * H\

+ M2

,

M1-M2

(2)

M2* + M2

,

M2-M2*

(3)

~~~~2* + Mi , ~~~~M2~Ml* (4) In its most simplified form, one assumes a steady state and thus deals with the probabilities of an activated macromolecular chain end either adding another chemically identical unit, e.g., its own monomer, or "cross-initiating" the second monomer. The 0097-6156/81/0166-0529$06.75/0 © 1981 American Chemical Society McGrath; Anionic Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

530

ANIONIC POLYMERIZATION

l a t t e r s i t u a t i o n amounts to a copolymerization and is i l l u s t r a t e d s c h e m a t i c a l l y in Equations (2) and (4) above. T y p i c a l l y one d e f i n e s r e a c t i v i t y r a t i o s r and V2 basis that: o

r

-

kll k


1 2

k J

and

T2

=

n

t

n

e

22 k i

—;

2

The most s t u d i e d a c t i v a t e d c h a i n ends are the macromolecular f r e e r a d i c a l s . Vast numbers of monomers have been i n v e s t i g a t e d (1) in hopes of i d e n t i f y i n g new random ( s t a t i s t i c a l ) copolymers which might show a f a v o r a b l e balance of averaged p h y s i c a l properties. In the case of f r e e r a d i c a l intermediates, one must be very much concerned with both the r a d i c a l l i f e t i m e and the need f o r a f a c i l e c r o s s - i n i t i a t i o n . Otherwise, premature t e r m i n a t i o n can lead., to generation of ( u s u a l l y ) incompatible homopolymers which in t u r n possess r a t h e r u n a t t r a c t i v e p h y s i c a l p r o p e r t i e s . (2) In the case of a n i o n i c c o p o l y m e r i z a t i o n , the " l i v i n g end" must still make a choice between r e a c t i n g w i t h its own monomer or the second monomer. R e l a t i v e b a s i c i t i e s are an important c o n s i d e r a t i o n . C3,4) However, f o r organolithium i n i t i a t e d polymerizations of butadiene, isoprene or styrene in hexane, cyclohexane or benzene, it is p o s s i b l e to study homogeneous termination f r e e systems. The r e a c t i v i t y of the carbanion end would be expected to be dependent on s e v e r a l parameters such as the c o u n t e r i o n , s o l v e n t , temperature, e t c . I t is known f o r example, that c a r banions can e x i s t in " t i g h t " , " l o o s e " , or even " f r e e " s t r u c t u r e s as a f u n c t i o n of counter i o n and s o l v e n t . ( 5 , J 5 ) In hydrocarbons, one should expect organolithiums to e x i s t in more or l e s s t i g h t i o n - p a i r s . Perhaps the most i n v e s t i g a t e d a n i o n i c system is the organolithium 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 of the monomer p a i r s , styrene-butadiene and styrene-isoprene. (7) In hydrocarbon s o l v e n t s , it is the diene which dominates the i n i t i a l copolymeri z a t i o n to the v i r t u a l e x c l u s i o n of styrene, and one observes a rate n e a r l y i d e n t i c a l to that of the diene alone. (9,10) Only when the diene supply is n e a r l y depleted does styrene begin to be i n c o r p o r a t e d in the polymer c h a i n . I n t e r e s t i n g l y , a f a s t e r p o l y m e r i z a t i o n rate is then observed f o r the styrene segment. Thus in these systems, there is an apparent " r e v e r s a l of r e a c t i v i t y " of styrene and the diene, s i n c e f o r the homopolymerization s i t u a t i o n styrene is a much more r e a c t i v e monomer than e i t h e r diene. R e l a t i v e l y l i t t l e i n f o r m a t i o n is a v a i l a b l e f o r the copolym e r i z a t i o n of butadiene with isoprene. In an e a r l y paper by Rakova and Korotkov (8), it was concluded that in n-hexane with n - b u t y l l i t h i u m as the i n i t i a t o r , the r e a c t i v i t y r a t i o s f o r butadiene and isoprene were r g = 3.38 and r j = 0.47, r e s p e c t i v e l y . In other words, the butadiene monomer r e a c t i v i t y s i g n i f i c a n t l y d i f f e r s from isoprene but by a smaller f a c t o r than f o r a s t y r e n e diene system. Nevertheless, the phenomenon of " r e v e r s a l of

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


34.

WANG E T A L .

Butadiene

and Isoprene

531

r e a c t i v i t y " still r e p o r t e d l y occured in the copolymerization between these two dienes. We f e l t that if these values were e s s e n t i a l l y c o r r e c t that r e l a t i v e l y pure blocks of butadiene might w e l l be formed a t the e a r l y stage of r e a c t i o n by directly copolymer!zing butadiene and isoprene simultaneously. We a r e q u i t e i n t e r e s t e d in s y n t h e s i z i n g block-type thermoplastic e l a s tomers of t h i s type. The butadiene-isoprene-butadiene copolymer per se may not be i n f l u e n c e d by a r c h i t e c t u r e arrangement. However, in the t o t a l l y o r s e l e c t i v e l y hydrogenated d e r i v a t i v e s , one might expect major d i f f e r e n c e s if there are c r y s t a l l i z a b l e polyethylene blocks d e r i v e d from the 1,4 butadiene u n i t s in polyethylene-poly(ethylene-co-propylene) o r p o l y e t h y l e n e - p o l y isoprene block copolymer. By analogy w i t h the styrene-diene (7) systems, the c r y s t a l l i n e polyethylene end blocks in the t r i b l o c k should a s s o c i a t e t o form c r y s t a l l i n e tie-down p o i n t s which w i l l d e s i r a b l y r e i n f o r c e the s o f t i n t e r i o r block of polyethylene-copropylene o r polyisoprene in the r e s u l t i n g two hydrogenated copolymers. Our k i n e t i c i n v e s t i g a t i o n was thus p r i n c i p a l l y motivated by the d e s i r e t o l e a r n whether the p o s s i b l e " r e v e r s a l of r e a c t i v i t y " between butadiene and isoprene v i a a n i o n i c c o p o l y m e r i z a t i o n would be of s u f f i c i e n t magnitude t o produce c r y s t a l i z a t i o n sequences in the hydrogenated d e r i v a t i v e s . We were a l s o i n t e r e s t e d in i n v e s t i g a t i n g p o s s i b l e s u b t l e temperature or solvent e f f e c t s . High vacuum techniques were employed (12) with the view that t h i s work might improve on the research published two decades ago. (8) Our new r e s u l t s on the r e a c t i v i t y r a t i o s f o r butadiene and isoprene have a l s o been analyzed v i a both c o n v e n t i o n a l (13,14) and the more recent s t a t i s t i c a l methods. (15-18) Experimental Copolymerization Apparatus and Techniques The high r e a c t i v i t y o f a l k y l l i t h i u m compounds r e q u i r e s that these p o l y m e r i z a t i o n s be performed under extremely high p u r i t y c o n d i t i o n s . In order t o achieve t h i s we have u t i l i z e d a high vacuum system. The b a s i c design of a high vacuum apparatus and p u r i f i c a t i o n procedures has been described in d e t a i l e l s e where. (12) A c c o r d i n g l y , the r e q u i r e d techniques used f o r the p u r i f i c a t i o n of n-hexane, the monomers andthes e c - b u t y l l i t h i u m i n i t i a t o r have thus been performed. A t y p i c a l glassware r e a c t o r which a l s o permits purging o f the r e a c t i o n v e s s e l s is shown in Figure 1. The r e a c t o r s were always flamed at a constant pressure of 10~ mm Hg u n t i l the flame took on the c h a r a c t e r i s t i c sodium c o l o r . I t was then sealed o f f the vacuum system, purged w i t h n-butyllithium/hexane s o l u t i o n and r i n s e d with hexane by back d i s t i l l a t i o n a t l e a s t four times. The purging s e c t i o n was then sealed o f f the r e a c t o r . I n i t i a t o r which has p r e v i o u s l y been vacuum d i s t i l l e d and d i l u t e d with pure hexane was used. A 5


532


measured volume and known c o n c e n t r a t i o n was introduced from the attached ampoule i n t o the r e a c t o r , followed by c a r e f u l r i n s i n g again w i t h hexane. The purged r e a c t o r was then sealed onto the vacuum l i n e through one of the break s e a l s . The a d d i t i o n a l s o l v e n t and monomers were then q u a n t i t a t i v e l y d i s t i l l e d in. Volumes were recorded at s u i t a b l e low temperatures (e.g. -78°C...) where d e n s i t y v a l u e s were a v a i l a b l e . The values of 0.73 gm/cc f o r butadiene at -78°C... and 0.68 gm/cc f o r isoprene at 20°C... were used. I t should be s t r e s s e d that the s e a l i n g of all c o n s t r i c t i o n s or d i l a t o m e t e r s was performed with the s o l u t i o n f r o z e n or cooled down at -196C...or -78°C., r e s p e c t i v e l y .


K i n e t i c Study Four to s i x d i l a t o m e t e r s (2 mm inner bore) of 3-5 ml each were attached to one of the s i d e arms of the r e a c t o r shown in F i g u r e 1. The a c t u a l s o l u t i o n volume depended on the monomer c o n c e n t r a t i o n and was u s u a l l y adjusted to allow about a 10-15 cm drop at 100% c o n v e r s i o n s . A f t e r the contents of the r e a c t i o n were e q u i l i b r a t e d near room temperature, the d i l a t o m e t e r volume was u s u a l l y adjusted such that the stems were about h a l f - f u l l . To prevent any u n d e s i r a b l e d i s t i l l a t i o n and/or bumping which would cause c o n c e n t r a t i o n f l u c t u a t i o n s , the procedures o u t l i n e d by J u l i a n o (19) were c a r e f u l l y followed. Next, each detached d i l a t o m e t e r was s e c u r e l y clamped in a constant temperature bath which was maintained at 20, 30, or 40°C... as d e s i r e d . Readings were taken w i t h a cathetometer a f t e r the i n i t i a l thermal expans i o n to the bath temperature. The data were t r e a t e d as d e s c r i b e d by P e t t . (20) The d i l a t o m e t r i c treatment was a l s o used f o r those r e a c t i o n s where a c o n v e r s i o n versus time curve was r e q u i r e d f o r l a t e r e s t i m a t i o n of extent of conversion. The extent o f c o n v e r s i o n was cross-checked by p r e c i p i t a t i o n of the polymer i n t o methanol, followed by f i l t r a t i o n and d r y i n g at room temperature under mechanical pump vacuum (VL0~ T o r r ) u n t i l constant weight was a t t a i n e d . The composition of the copolymer was determined by e i t h e r NMR a n a l y s i s a t 90 MHz a c c o r d i n g to the equations d e r i v e d by Mochel (21) or by i n f r a r e d . (22) The agreement of these methods was ± 2% when a p p l i e d to copolymer taken to 100% c o n v e r s i o n . The r e a c t i v i t y r a t i o s were c a l c u l a t e d a c c o r d i n g to the Mayo-Lewis P l o t (13,15), the Fineman-Ross Method (14), or by the Kelen-Tudos equation.(16,17,18) The s t a t i s t i c a l v a r i a t i o n s r e c e n t l y noted by O ' D r i s c o l l (23), were a l s o c o n s i d e r e d . The t o t a l hydrogénation of the copolymers u t i l i z e d the d i i m i d e method (24), generated in s i t u from p - t o l u e n e s u l f o n y l h y d r a z i d e (TSH) in xylene f o r s i x hours a t r e f l u x temperature (132-134°C...). In general, 5.0 moles of TSH per 100 grams of p o l y mer were used. We have found (25) that the a d d i t i o n of a phenolic a n t i o x i d a n t such as Irganox 1010 e f f e c t i v e l y decreases the minor, but d e t e c t a b l e s i d e r e a c t i o n s . The s a t u r a t i o n of the 2



WANG ET AL.

Butadiene

and Isoprene

533

Figure 1. Copolymerization purging apparatus and reactor with attached dilatometers: initiator ampule (I); dilatometers (D); reactor (R); purging solution collector (P); n-butyllithium in hexane solution (N).


534


polydiene was determined to be complete e i t h e r by h i g h temperature H-NMR a n a l y s i s in hexachloro-1,4-butadiene using hexamethyd i s i l o x a n e as an i n t e r n a l standard, or by i n f r a r e d spectra of the polymer f i l m c a s t on a KBr p l a t e . Thickness of between 1/2 to 1 m i l was r e q u i r e d f o r a good s p e c t r a . The hydrogenated copolymers were f u r t h e r checked f o r c r y s t a l l i n i t y content on a Model 2 Perkin-Elmer d i f f e r e n t i a l scanning c a l o r i m e t e r (DSC) at a heating rate of 20°C.../minute. Results And D i s c u s s i o n


Homopolymerization of Butadiene

and Isoprene in n-Hexane

Homopolymerizations were first conducted to e s t a b l i s h a b a s i s f o r the copolymerization study. The p o l y m e r i z a t i o n of the diene monomers in n-hexane has been i n v e s t i g a t e d in the l i t e r a ture v i a two approaches: (a) by mixing all of the monomers and solvent directly with i n i t i a t o r , and (b) by premixing the i n i t i a t o r with a s l i g h t molar excess of the monomer to form a l i v i n g "seeded" polymer. The l a t e r method was p a r t i c u l a r l y employed with n - b u t y l l i t h i u m i n i t i a t o r (26) where the i n i t i a t i o n rate was slow enough that the sample organolithium p e r s i s t e d during a s u b s t a n t i a l p e r i o d of the p o l y m e r i z a t i o n process. In order to enhance the rate of i n i t i a t i o n r e l a t i v e to that of propagation, s e c - b u t y l l i t h i u m has evolved (27) as the p r e f e r r e d i n i t i a t o r , e s p e c i a l l y f o r k i n e t i c s t u d i e s . Branched a l k y l l i thiums have been reported to i n c r e a s e the i n i t i a t i o n rate of dienes in hydrocarbon s o l v e n t s . (28) When the i n i t i a t i o n rate is of the same order of magnitude as the rate of propagation, the homogeneous a n i o n i c p o l y m e r i z a t i o n s allows the s y n t h e s i s of polymers possessing a very narrow molecular weight d i s t r i b u t i o n . R e l a t i v e l y l i n e a r p o l y m e r i z a t i o n curves can be obtained f o r both butadiene and isoprene as shown f o r example in F i g u r e 2. This was true at three d i f f e r e n t temperatures, namely, 2 0 , 3 0 , and 40° C. The k i n e t i c r e s u l t s obtained by d i l a t o m e t r y f o r the polymeri z a t i o n of butadiene and isoprene are shown in Figures 3 and 4. As u s u a l , AH r e f e r s to the change in height at a given time. I t can be seen that the k i n e t i c s t u d i e s on dienes confirm that the propagation r e a c t i o n has a first-order dependence on the monomer c o n c e n t r a t i o n . These observations are to be expected s i n c e in the absence of a d v e n t i t i o u s i m p u r i t i e s the number of growing chains should remain constant, only the monomer concent r a t i o n decreases. The observed apparent first-order rate constants are l i s t e d in Table I. From the constants at d i f f e r e n t temperatures but at the same i n i t i a t o r c o n c e n t r a t i o n , it is p o s s i b l e to c a l c u l a t e an apparent a c t i v a t i o n energy f o r the propagation r e a c t i o n in each case. T h i s has been done, and the Arrhenius p l o t is shown in F i g u r e 5. I t is i n t e r e s t i n g to note the s i m i l a r i t y of these t



Figure 2.

Homopolymerization

rates of butadiene and isoprene in hexane.


>

I

Si S

& ft.

CO

>

M Η

Ο

McGrath; Anionic Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1981. Figure 3.

First-order plots for butadiene in hexane.


u>

δ

Η

se

ο

2

δ

>

ON

34.

WANG ET AL.

Butadiene

and Isoprene


1.6

0

100

200

300

400

Time (min.) Figure 4.

First-order plots for isoprene in hexane.


500

McGrath; Anionic Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1981. Figure 5.

The Arrhenius plot.


1

Ν > H

ta

ο

2

δ

00

34.

Butadiene

WANG ET AL.

and

Isoprene

539

Table I F i r s t - O r d e r Propagation Rate Constants (kj) of the Homopolymerizations of Butadiene and Isoprene


System

Temp.°C...

sec-BuLi mole/1. χ 10

Monomer mole/1.

k i , sec" χ I0

3

1

k

Butadiene -Hexane

20 30 40

1.27 1.27 1.27

1.00 1.00 1.00

0.0978 0.327 0.796

Isoprene -Hexane

20 30 40

1.26 1.26 1.26

1.00 1.00 1.00

0.553 1.48 4.56

a c t i v a t i o n energies, 19.2 kcal/mol, f o r p o l y m e r i z a t i o n of the two monomers in n-hexane. The value obtained is much higher that what has been reported f o r the case where THF was the s o l v e n t . (26) Our v a l u e is a l s o somewhat higher than the 14.3 k-cal/mole published by Worsfold and Bywater. (29) From the v i s c o s i t y s t u d i e s of Morton, e t . a l . , (30,31) one may assume that the a c t i v e p o l y d i e n y l l i t h i u m is a s s o c i a t e d in p a i r s in n-hexane, but no such a s s o c i a t i o n is observed in THF. On t h i s b a s i s , the high a c t i v a t i o n energy found f o r the propagation r e a c t i o n s in n-hexane should i n c l u d e the heat of d i s s o c i a t i o n of the a s s o c i a t e d i o n p a i r s if the dimer s p e c i e s do not r e a c t directly w i t h the incoming monomer. (32) The dependence of the propagation rate on the c o n c e n t r a t i o n of growing chains is i l l u s t r a t e d in F i g u r e s 6 and 7, and is l i s t e d in Table I I . The first-order rate constant from Table I I are p l o t t e d as a f u n c t i o n of the i n i t i a t o r concen t r a t i o n . Although the k i n e t i c s of organolithium p o l y m e r i z a t i o n in nonpolar s o l v e n t s have been subjected f o r i n t e n s i v e s t u d i e s , the r e s u l t s were still somewhat c o n t r o v e r s i a l . In view of the strong experimental evidence f o r a s s o c i a t i o n between the organo l i t h i u m s p e c i e s , the k i n e t i c order a s c r i b e d to t h i s phenomenon was p o s t u l a t e d (30,31) as shown in Equations (5) and (6). +

(RMj~Li ) RMj~Li

+

N

nRMj ~ L i

n

+. M

4

,

RMLi "

+

(5) (6)

T h i s assumes that only the d i s s o c i a t e d c h a i n ends are a c t i v e . A c t u a l l y , measurements on the s t a t e of a s s o c i a t i o n , i . e . , the v a l u e of η in Equation (5), have been c a r r i e d out f o r s t y r e n e ,



540 ANIONIC POLYMERIZATION


Butadiene

and Isoprene


WANG ET AL.


541


542 Table I I

E f f e c t of I n i t i a t o r Concentration of the Propagation Rate of Butadiene and Isoprene in Hexane


System

Temp.°C...

sec-BuLi mole/1, χ 10 3

Monomer mole/1.

k i , sec" χ 10

Butadiene -Hexane

20

0.683 1.27 2.54

1.00

0.0802 0.0978 0.121

Isoprene -Hexane

30

0.704 1.26 2.51

1.00

0.967 1.482 1.769

1

butadiene and isoprene. The η values were measured to be c o n s i s t e n t l y very c l o s e to 2. While t h i s could e x p l a i n the h a l f - o r d e r k i n e t i c s demonstrated by most i n v e s t i g a t o r s (12) in the case of styrene, it could not account f o r the lower orders (1/4 to 1/6) found f o r butadiene anH isoprene in v a r i o u s hydrocarbon concen t r a t i o n s . Our data (Fig6) f i t a 1/4-order type k i n e t i c s , e s p e c i a l l y f o r the case f o r butadiene at 20°C. The same k i n e t i c s data f o r isoprene at 30C...shown in F i g u r e 7 are not so p r e c i s e on the b a s i s of a 1/4-order dependence. Nevertheless, our r e s u l t s con f i r m the i d e a that the propagation r a t e s of the diene monomers in hydrocarbon are c e r t a i n l y a f r a c t i o n a l order type dependence. Whether 1/4-order o r something d i f f e r e n t order probably needs to be f u r t h e r d e f i n e d . I t appears that the propagation r e a c t i o n of these a s s o c i a t e d growing chains in nonpolar media may be more complicated than what is proposed in Equations (5) and (6), and probably i n v o l v e s a d i r e c t i n t e r a c t i o n between the monomer and the a s s o c i a t e d complex. (32) K i n e t i c Study on the Copolymerization of Butadiene in Hexane

and

Isoprene

I t has been emphasized in the copolymerization of styrene with butadiene or isoprene in hydrocarbon media, that the diene is p r e f e r e n t i a l l y i n c o r p o r a t e d . (7,9,10) The rate of copolymer i z a t i o n is i n i t i a l l y slow, being comparable to the homopolymer i z a t i o n of the diene. A f t e r the diene is consumed, the rate i n c r e a s e s to t h a t of the homopolymerization o f s t y r e n e . Ana l o g o u s l y our current i n v e s t i g a t i o n of the copolymerization of butadiene with isoprene shows s i m i l a r behavior. However, the



34.

WANG ET AL.

Butadiene

and

Isoprene

543

r e v e r s a l of r e a c t i v i t y is much l e s s . The degree of r e v e r s a l phenomenon among these two dienes can be c o r r e l a t e d with the r e a c t i o n temperature f o r a f i x e d monomer feed ratio. The d i l a t o m e t r i c data are p l o t t e d in F i g u r e s 8-10 f o r three d i f f e r e n t temperatures, and are summarized in Table I I I f o r the purposes of c l a r i t y and comparison. Curves 1 and 5 in these f i g u r e s r e p r e sent the homopolymerization r a t e s of isoprene and butadiene, respectively. In a d d i t i o n , data f o r three d i f f e r e n t i n i t i a l monomer feed r a t i o s are expressed by B/I's with 25/75, 50/50, and 75/25. The numbers denote the monomer ratio of butadiene content to isoprene f o r curve 2, 3, and 4, r e s p e c t i v e l y . The i n i t i a l propagation rate constants were measured and are l i s t e d in Table I I I , along with the homopolymerization r a t e s of butadiene and isoprene under the same c o n d i t i o n s . For higher content of butadiene ( i . e . with a mole f r a c t i o n of 0.75), the o v e r a l l c o p o l y m e r i z a t i o n rate is almost i d e n t i c a l to the homopolymerizat i o n rate of butadiene. T h i s behavior was observed at three d i f f e r e n t temperatures, 20°, 30°, and 40°C. Curve 4 shown in F i g u r e s 8-10 is almost p a r a l l e l to Curve 5, and shows no s i g n of the i n v e r s i o n phenomenon. A c c o r d i n g l y , during the copolymerizat i o n w i t h a butadiene-to-isoprene molar ratio of 75 to 25, there is a c o n s i d e r a b l e amount of isoprene i n c o r p o r a t e d in a r a t h e r random f a s h i o n w i t h the butadiene. The o v e r a l l rate is neverthel e s s c o n t r o l l e d by the slower rate-determining step of butadiene p o l y m e r i z a t i o n . The copolymer composition a t low conversion has been determined to be r i c h in butadiene, f o r example 83.5 to 88.5 mole % were found compared to 75 mole % in charge (cf a l s o Table I V ) . However, when a mixture of butadiene and isoprene with 25/75 or 50/50 molar ratio is polymerized, the i n i t i a l propagating rate is enhanced s l i g h t l y due to the i s o p r e n e . A l l the prematurely terminated copolymers w i t h butadiene mole f r a c t i o n from 0.20 to 0.80 in the feed are n e v e r t h e l e s s found to be r i c h in butadiene, as seen by the c o p o l y m e r i z a t i o n data l i s t e d in Table IV. Moreover, there is another i n t e r e s t i n g p o i n t observed w i t h the molar r a t i o s of butadiene to isoprene of 25/75 or 50/50. Here one can observe the occurance of the " i n v e r s i o n " phenomenon which is manifested by the i n f l e c t i o n p o i n t of the k i n e t i c Curves 2 and 3 of F i g u r e s 8-10 a t all three temperatures. The lower the c o p o l y m e r i z a t i o n temperature is, f o r example 20 C., the sharper the i n f l e c t i o n appears to become. T h i s behavior leads to the c o n c l u s i o n that the c o p o l y m e r i z a t i o n is more s e l e c t i v e at lower temperatures. Conversely, when temperature is i n c r e a s e d s e l e c t i v i t y is n o t i c e a b l y decreased. The d e r i v e d r e a c t i v i t y r a t i o s r e f l e c t t h i s trend and are shown in Table V. By measuring the k i n e t i c rate of second stage r e a c t i o n a f t e r i n f l e c t i o n , one can observe that rate is v e r y analogous to the homopolymerization rate of i s o p r e n e . The data are l i s t e d in Table I I I , and can a l s o be detected by the s t r a i g h t p o r t i o n of Curves 2 and 3 a f t e r i n f l e c t i o n . The " i n v e r s i o n " phenomenon can be e a s i l y explained by the f a c t t h a t , although the isoprene is



Figure 8.

Effect of comonomer feed ratio on the rate of polymerization in hexane at

Time (min.J


20°C.

H δ 25

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4*.


Figure 9.

Effect of comonomer feed ratio on the rate of polymerization in hexane at 30°C.

Time (min.)


in


546


0

100

200

300

400

Time (min.) Figure 10.

Effect of comonomer feed ratio on the rate of polymerization in hexane at 40°C.


500

34.

W A N G ET AL.

Butadiene

and

Isoprene

547

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