Anionic Polymerization of the Strontium Salt of One-Ended Living

10

Downloaded by UNIV ILLINOIS URBANA on March 16, 2013 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0166.ch010

Anionic Polymerization of the Strontium Salt of One-Ended Living Polystyrene in Tetrahydrofuran and Tetrahydropyran C. D E SMEDT and M . V A N

BEYLEN

Laboratory of Macromolecular and Organic Chemistry, University of Leuven, Celestijnenlaan 200 F, B-3030 Heverlee, Belgium

ABSTRACT Similarly to the previously reported barium salt, the strontium salt of one-ended l i v i n g polystyrene (SrS ) was prepared by converting on a strontium mirror dibenzylmercury into dibenzylstrontium. The latter was reacted with α-methylstyrene and the resulting oligomer was converted into the one-ended polystyryl salt by reaction with styrene. Conductance measurements i n THF and THP i n the temperature range of respectively -70°C and -40°C to +20°C indicated that i n analogy with the barium salt, two equilibria are simultaneously established i n both solvents: 2

SrS

2

+

=

+

(SrS)

-

+

S

(K ) 1

-

and 2SrS = (SrS) + (SrS ) (K ) 2

3

2

The ionic dissociation of strontium tetraphenylboride SrB

2

+

-

(SrB =SrB + B ) was also investigated in THF in the temperature range of -70°C to +20°C and provided the respective Λ' s and KdSrB . Viscometric measurements on mixtures of SrS 2

o

2

2

and strontium tetraphenylboride (SrB ) i n THF clearly indicate the formation of a mixed salt SrSB, which dissociates according to SrSB = ( S r S ) + B (K ) 2

+

-

b

Conductance measurements on the mixed salt provided an accurate value for λ SrS and for K . +

+

o

b

0097-6156/81/0166-0127$06.50/0 © 1981 American Chemical Society

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

ANIONIC POLYMERIZATION

128

K i n e t i c measurements i n THF i n the absence and i n the presence of v a r i a b l e amounts of added SrB2 i n d i c a t e d that, s i m i l a r l y to B a S , propagation occurs mainly v i a the f r e e S~ anions, the r e a c t i v i t y of the other s p e c i e s being n e g l i g i b l e . A d d i t i o n of S r B slows down and e v e n t u a l l y i n h i b i t s the propagation through formation of the mixed s a l t SrSB, the d i s s o c i a t i o n of which can no longer g i v e f r e e S" anions. Even i n THP where the i o n i c d i s s o c i a t i o n of S r S and t h e r e f o r e a l s o the amount of f r e e S"~ anions was shown to be very small, propagation i n v o l v i n g the f r e e S~ anions s t i l l accounts f o r the observed k i n e t i c s and no propagation by s p e c i e s , other than S" anions, c o u l d be detected e x p e r i m e n t a l l y . 2

2


2

The p o l y m e r i z a t i o n k i n e t i c s of a l k a l i s a l t s of l i v i n g v i n y l polymers i n e t h e r e a l s o l v e n t s , such as t e t r a h y d r o f u r a n (1.), tetrahydropyran (2), dimethoxyethane (1), oxepane (4) and dioxane (5) have been s t u d i e d and d i f f e r e n t comprehensive reviews or books d e a l i n g w i t h a n i o n i c p o l y m e r i z a t i o n have been published (6) . In a l l cases reported, except i n the l e a s t p o l a r s o l v e n t s l i k e dioxane, both i o n p a i r s M~, C a t and f r e e ions M~ were found to c o n t r i b u t e to the propagation. In some cases even t r i p l e ions were shown to p a r t i c i p a t e i n the a n i o n i c propagation process (la,I). In e t h e r e a l s o l v e n t s the a l k a l i n e - e a r t h s a l t s of l i v i n g polymers (one-ended as w e l l as two-ended) show a k p ( k b s d / C ) i n v e r s e l y p r o p o r t i o n a l to the carbanion c o n c e n t r a t i o n . B. F r a n c o i s and coworkers proposed the formation of i n a c t i v e aggregates of i o n p a i r s QS~* M^"*\ S 7 ) and a propagation proceeding through f r e e ions ( 8 , 9 , 1 0 ) . F o r the p o l y m e r i z a t i o n of the one-ended l i v i n g B a ^ , ( p o l y - S " ) i n THF B. De Groof, M. Van Beylen and M. Szwarc presented another mechanism which f u l l y accounts f o r the observed k i n e t i c s and conductances and which assumes the formation of M^ , ( p o l y - S " ) 3 t r i p l e ions next to M^ , (poly-S~)~and f r e e poly S" i o n s and a propagation mainly by the f r e e p o l y s t y r y l anions, the r e a c t i v i t y of other species being n e g l i g i b l e or at l e a s t not observable ( Π ) . Other systems were however, r e p o r t e d i n which the r e a c t i v i t y of s p e c i e s , other than f r e e i o n s , cannot be neglected (12,13). In the present communication the strontium s a l t of one-ended l i v i n g p o l y s t y r e n e ( S r S ) was s t u d i e d i n t e t r a h y d r o f u r a n (THF) and tetrahydropyran (THP), i n order to check the v a l i d i t y of the t r i p l e i o n mechanism. The I o n i c d i s s o c i a t i o n of S r S i n THP was expected to be s m a l l e r than i n THF and t h e r e f o r e i t was thought that perhaps a c o n t r i b u t i o n to the propagation from s p e c i e s , other than the f r e e S" anions, would be d e t e c t a b l e . The marked d i f f e r e n c e i n behavior between two-ended and one-ended l i v i n g polymers of a l k a l i n e - e a r t h metals needs to be s t r e s s e d . Whereas the r a t e of p o l y m e r i z a t i o n seems to be independent of the degree of p o l y m e r i z a t i o n i n the case of one-ended polymers, such a dependence was d i s c l o s e d i n the case of the two-ended l i v i n g polymers ( 8 , 9 ) . +

e

a p

0

n

+

2

+

+

2

2


10.

DE SMEDT AND VAN BEYLEN

One-Ended

Living

Polystyrene

129

Experimental


1.

P r e p a r a t i o n of the strontium s a l t of one-ended l i v i n g p o l y s t y r e n e (SrS?)

The p r e p a r a t i o n of the strontium s a l t was c a r r i e d out under high vacuum f o l l o w i n g the procedure d e s c r i b e d p r e v i o u s l y f o r the barium s a l t (11), with t h i s m o d i f i c a t i o n that a g r e a t e r excess ( 9 . 5 f o l d ) of α-methylstyrene (α-MeS) was used a t a temperature of 31°C. A f t e r 22 hr the remaining α-MeS was evaporated a t the vacuum l i n e together w i t h the THF (10). A f t e r r e d i s s o l v i n g i n THF, a 6 0 - f o l d excess of styrene was added i n 6 s t e p s . An approximate molecular weight (M ) of 6000 was determined f o r the s i n g l e "S" arm by vapor-pressure osmometry a f t e r p r o t o n a t i o n of the s a l t , i n good agreement with the c a l c u l a t e d molecular weight (M i = 6 3 0 0 ) . From GPC data a value of M /M =1.36 was obtained. To o b t a i n the SrS2 i n THP, the THF was removed and the s a l t r e d i s s o l v e d i n THP. T h i s procedure was repeated once to ensure complete removal of THF (14). The same ε-values as found i n THF (see T a b l e I ) were used. n

n i C a

c

w

n

Table I e

The main absorption maxima of the strontium salts in THF at 20 C Sr(X)

λ , max

2

4

ε χ 10~

nm 2

Sr *(PhCH ~) 2

2

Sr *(PhCH

2

(XMeS")

2

2

2+ Sr

(poly-S )

2+

2

332

1.46

this work

322

1.2

a

334

1.27

this work

348

1.23

this work

2

Sr (PhCH CH CPh ~) 2

(mol of x")"

Ref. 1

2

2

348

b

350

c

447

d

(a) K. Takahashi, Y. Kondo, R. Asaroi, J.Chem.Soc.(Perkin II), 577 (1978); (b) C. Mathis, L. Christmann-Lamande, B. François, J.Polymer Sci.(Pol.Chem.Ed.)

\S

t

1285 (1978); (c) C. Mathis, B. François,

C.R.

Acad.Sci.(Sér.C) 288, 113 (1979); (d) Preparation of SrS, by addi2+ tion of styrene to the adduct of 1,1-diphenylethylene to Sr (PhCHO2+ led to the same λ » 348 nm, though not a l l of the Sr (PhCH-CH. max l l CPh ) was converted into SrS . 2

2

2


130

2.


P r e p a r a t i o n of s t r o n t i u m t e t r a p h e n y l b o r i d e Two

methods were used:

a)

Strontium t e t r a p h e n y l b o r i d e ( S r B ) was prepared by the method o u t l i n e d i n r e f . 15. Since S r B i s l e s s s o l u b l e than BaB , the p r e c i p i t a t e d s a l t on the f i l t e r was a l s o recovered and p u r i f i e d . A second method c o n s i s t e d of making f i r s t diphenyl strontium ( S r ( P h ) ) by r e a c t i n g diphenyl mercury on a S r - m i r r o r i n THF. A f t e r removal of the excess of Sr and Hg, the S r ( P h ) s o l u t i o n was reacted w i t h an equimolar amount of B(Ph>3 i n THF. The S r B formed p r e c i p i t a t e d partially. I t should be pointed out that conductance measurements with S r B prepared a c c o r d i n g to method a or b l e d to the same r e s u l t s . 2

2

2

b)


2

2

2

2

3.

P u r i f i c a t i o n of s o l v e n t s and monomer

T e t r a h y d r o f u r a n and tetrahydropyran were r e f l u x e d f o r 24 hr over Na-K a l l o y and d i s t i l l e d onto f r e s h a l l o y . Before use the s o l v e n t s were d i s t i l l e d once more onto S r S and subsequently d i s t i l l e d under vacuum i n t o ampules. An a l l - g l a s s apparatus equipped w i t h breakseals was used f o r the l a t t e r o p e r a t i o n . Styrene and α-methylstyrene were d i s t i l l e d under vacuum and d r i e d twice over CaH under high vacuum. Styrene was f u r t h e r p u r i f i e d by d i s t i l l i n g i t i n the presence of S r S , w h i l e a-methylstyrene was f u r t h e r d r i e d over Na-K a l l o y . 2

2

2

4.

Conductance and k i n e t i c measurements

A l l measurements were c a r r i e d out under high vacuum. The d e t a i l s of the procedure and the apparatus used are g i v e n i n r e f . 11. R e s u l t s and D i s c u s s i o n Conductance measurements 1.

Strontium s a l t of one-ended l i v i n g p o l y s t y r e n e ( S r S ) i n THF 2

The conductance measurements cover a c o n c e n t r a t i o n range from 8 χ 10"^ up to 7.5 χ 10~^M and a temperature range from -70°C t o +20°C. Adopting the procedure of Kraus and Bray (16) one f i n d s a pronounced d e v i a t i o n from l i n e a r i t y , r e s u l t i n g i n a constant value of Λ a t high c o n c e n t r a t i o n s ( f i g . 1 ) . In analogy w i t h the B a - s a l t t h i s phenomenon can be e x p l a i n e d by a d m i t t i n g t h a t the i o n i z a t i o n of S r S i s due not only to i t s d i s s o c i a t i o n i n t o S r S c a t i o n s and f r e e S" anions (eq. 1) but a l s o to the 2

+



10.

One-Ended

Living

Polystyrene

131

formation of u n i l a t e r a l t r i p l e ions SrS3~ (eq. 2 ) , both e q u i l i b r i a being simultaneously maintained: SrS ^ (SrS)+ + S" K

ionic

±

2

(1)

x

2 SrS ^ (SrS)+ + (SrS3>~ K (2) Thus, i f the degree of d i s s o c i a t i o n (eq. 1) can be neglected (at s u f f i c i e n t l y high nominal c o n c e n t r a t i o n s C), the f r a c t i o n of s a l t converted i n t o t r i p l e ions becomes independent of C. Treatment of the data by the r e l a t i o n d e r i v e d by Wooster (12) f o r systems i n v o l v i n g u n i l a t e r a l t r i p l e ions v i z . CA Λ Κ χ + ( 2 Λ λ - A ) K C , where Λ i s the sum of the l i m i t i n g conductances of S r S and S" and λ the sum of those of S r S and SrS3~ r e s p e c t i v e l y , y i e l d s a s t r a i g h t l i n e ( f i g . 2 ) , c o n f i r m i n g the formation of u n i l a t e r a l t r i p l e i o n s . The constants Kj and K were c a l c u l a t e d r e s p e c t i v e l y from the i n t e r c e p t and the slope of t h i s s t r a i g h t l i n e . A l t e r n a t i v e l y , as o u t l i n e d i n r e f . 11, the v a l u e of K may a l s o be c a l c u l a t e d from the r e l a t i o n A i [ K / / ( l + Κ / ) ] λ which reduces to Αχ - Κ / · λ i f K / « l and where Λ\ i s the e q u i v a l e n t conductance i n the c o n c e n t r a t i o n range where the f r a c t i o n of t r i p l e ions and the e q u i v a l e n t conductance remain constant and independent of C, whereas Κ χ may a l s o be d e r i v e d by determining, as i n r e f . Π , the c o n c e n t r a t i o n C f o r which [SrS3~] - [ S ~ ] , s i n c e then Κχ - K Cj[. The Κχ-values d e r i v e d from the Wooster p l o t and the K - v a l u e s obtained u s i n g the equation K * / Λ χ / λ are the most r e l i a b l e ones ( Π ) . The r e s p e c t i v e Κ χ and K values are t a b u l a t e d i n T a b l e I I . Κ χ obtained u s i n g the value of and K d e r i v e d from the Wooster p l o t are given i n parentheses. The λ and Λ values needed f o r these c a l c u l a t i o n s are c o l l e c t e d i n T a b l e I I I and were d e f i n e d as follows: 2

2

2

2

0


88

2

0

0

0

2

0

+

+

0

2

s

2

1

2

1

2

2

1

2

0

2

1

2

±

0

2

2

2

2

2

β

2

0

2

2

0

0

+ A

o>SrS

s

*o,S~

0 2

+

X

s

S~

( = s 2 1

'°

a

t

2

0

°

C

)

s

e

e

f

u

r

t

h

e

r

part 4 conductance of SrSB

+ X ,triple = 0

*o>SrSf

β

X

X ^ s 0

0fS

Sr

+

3

"/1.26

X

0 > S r

S

+

β

8

( 1 ·

8

at 20°C)

where

(18)

To o b t a i n the values of Λ and λ a t d i f f e r e n t temperatures we used the f o l l o w i n g equation, r a t h e r than the Walden product which i s known to decrease w i t h temperature ( l b , 1 9 ) . 0

0

β

λ ~(20 Ο ο

β

λ "(τ Ο 0

X

+ o

x

> N a

+(20°C)

+ 0

e

> N a

+(T c)

A Van't Hoff p l o t p r o v i d i n g the corresponding thermodynamic parameters f o r the i o n i c d i s s o c i a t i o n of S r S (eq. 1) i s shown i n 2


132



Figure 2.

Plot of equivalent conductance vs. concentration of Sr *, (poly-S') in THF at 20°C (a); Wooster conductance plot in THF at 20°C (b). 2


2

10.


One-Ended

Living

Polystyrene

133

f i g u r e 3. (ΔΗχ° - -1.87 kcal/mol and ASx° » -46.4 e.u.). A curvature of the Van't H o f f p l o t such as observed f o r the t r i p l e i o n formation constant K has been r e p o r t e d i n many other cases and may r e f l e c t the decrease i n exotherraicity of the d i s s o c i a t i o n a t lower temperatures as expected on the b a s i s of the Increase of the d i e l e c t r i c constant with decreasing temperature. Why t h i s i s not the case f o r Κχ i s d i f f i c u l t to say i n cases where s p e c i f i c s o l v a t i o n occurs next to solvent p o l a r i z a t i o n . I f i t were p o s s i b l e to go to lower temperatures a s i m i l a r decrease of the exotherraicity might e v e n t u a l l y a l s o show up f o r Κχ.


2

Table II

Z+

—

S r , ( p o l y - S ) in THF and THP. Equilibrium constants Kj and

(THF)

2

and K- (THP) at different temperatures. T, C Kj χ 10 , M K χ 10 9

e

5

2

(THF)

(THF)

K

χ 10

6

2

(THP)

20

1.6 (1.6)

3.3 (3.0)

1.2 (1.4)

15

1.9 (1.8)

3.4 (3.0)

1.2 (1.3)

10

2.0 (2.0)

3.5 (3.1)

1.2 Π.4)

5

2.2 (2.1)

3.7 (3.3)

i.2 a . 4 ;

0

2.3 (2.1)

4.1 (3.7)

1.2

a.4)

-5

2.4 (2.3)

4.0 (3.6)

1.2

a.4;

-10

2.6 (2.6)

4.3 (3.9)

1.2 r i . * ;

-15

2.8 (2.7)

4.6 (4.1)

1.1 (1.3)

-20

3.1 (3.0)

4.9 (4.3)

1.1

-25

3.3 (3.2)

5.1 (4.S)

1.1 (1.3)

-30

3.6 (3.7)

5.6 (4.9)

1.1

-35

3.9 (3.9)

5.8 (5.0)

1.1 f i . 3 ;

-40

4.1 (3.7)

5.9 (4.9)

0.92fl.i;

-45

4.4 (4.2)

6.0 (5.1)

-50

4.8 (4.7)

6.1 (5.2)

-55

5.4 (S.3)

6.9 (5.9)

-60

6.2 (S.6)

7.0 (5.8)

-65

6.5 (S.9)

7.1 (5.8)

-70

7.3 (6.6)

7.5 (6.1)

u.s;

a.s;

The S r S - s a l t i s more d i s s o c i a t e d i n THF than the B a S - s a l t (11,15), probably due t o g r e a t e r s o l v a t i o n of the s m a l l e r S r 2

2

2 +


134

ANIONIC

POLYMERIZATION

z +

c a t i o n i n the S r S 2 " s a l t than of the B a c a t i o n i n the corresponding s a l t . For a l k a l i ions the s o l v a t i o n energy ( s p e c i f i c and/or through solvent p o l a r i z a t i o n ) exceeds the Coulombic i n t e r a c t i o n energy which binds the ions i n t o i o n p a i r s , and t h e r e f o r e i n THF the d i s s o c i a t i o n of i o n p a i r s i s exothermic ( l b ) . T h i s i s a l s o the case with the SrS2 s a l t , though the s o l v a t i o n of the ( S r S ) c a t i o n i s much smaller than e.g. that of the N a c a t i o n . T h i s i s confirmed by comparing the corresponding AHi°-values (-1.87 and -8.2 kcal/mol r e s p e c t i v e l y ) . In the case of BaS2 i t was already shown that the g a i n i n s o l v a t i o n energy of the ( B a S ) i o n i s smaller than the Coulombic i n t e r a c t i o n energy (ΔΗχ » +0.9 kcal/mol) (11). Furthermore, the entropy change àS\° f o r the d i s s o c i a t i o n of the a l k a l i n e e a r t h p o l y s t y r e n e s a l t s i s s u b s t a n t i a l l y l e s s f l°,SrS ~ * -5 l ° , B a S ~ 4 0 e»u* ( L I ^ t h a n f o r t h e +

+


+

0

AS

s

4 6

e

u

A S

a

2

2

+

0

d i s s o c i a t i o n of the Na ,S~ contact i o n p a i r (AS = - 60 e.u.) (ytjlb)T h i s smaller decrease i s a t t r i b u t e d to the f a c t that upon d i s s o c i a t i o n of the M , ( S - ) 2 ~ s a l t s , a S" anion remains bound to the M c a t i o n preventing the M * to be reached from a l l s i d e s by the s o l v e n t molecules. The d i f f e r e n c e between A S i , g g ^ and ASi°,B s^also accounts f o r the g r e a t e r s o l v a t i o n of the smaller ( S r S ) c a t i o n formed upon d i s s o c i a t i o n . 2+

2 +

2

e

r

a

+

2.

Strontium

s a l t of one-ended polystyrene In

THP

In THP, which has a lower d i e l e c t r i c constant than THF, and In which conductance measurements were c a r r i e d out at temperatures ranging from -40°C to +20°C, the constant value of the e q u i v a l e n t conductance was maintained throughout the whole c o n c e n t r a t i o n range s t u d i e d (3x10"^ to 8xlO"^M) ( f i g . 4 ) . From these constant h\ v a l u e s , K2 was obtained (see T a b l e I I ) . Since the i n t e r c e p t of the Wooster p l o t ( f i g . 4 ) i s i n d i s t i n g u i s h a b l e from zero, i t was impossible to determine Κχ· The slope of t h i s p l o t gives K2 (values given i n parentheses)· The r e l e v a n t A and λ values are l i s t e d i n T a b l e I I I and were obtained from those i n THF using the Walden r u l e . The v i s c o s i t i e s of THF (19) and THP (20) were taken from the l i t e r a t u r e . Our measurements i n THP thus confirm the mechanism proposed i n the equations (I) and (2) and c l e a r l y i n d i c a t e the lower d i s s o c i a t i o n ( e q . l ) of SrS2 i n THP than i n THF. 0

0

3.

Strontium

t e t r a p h e n y l b o r i d e i n THF

Conductance measurements on strontium t e t r a p h e n y l b o r i d e (SrB2) were performed over a c o n c e n t r a t i o n range of 5 x 1 0 " % to 2xlO~ M and at temperatures extended from -70°C to +20°C. In F i g u r e 5 the r e s u l t s are g r a p h i c a l l y presented as a 1/Λ vs. CA p l o t g i v i n g an accurate i n t e r c e p t , as a r e s u l t of the high degree of d i s s o c i a t i o n of SrB2 i n THF. In t h i s way the values of 6


One-Ended

Living

Polystyrene

135



Figure 4. Plot of equivalent conductance vs. concentration of Sr *, (poly-S~) in THP at 20°C (a); Wooster plot for Sr *, (poly-S) in THP at 20°C (b). 2

2

2


2




10.


One-Ended

Living

Polystyrene

137

Table III Limiting equivalent conductances A and \ in THF and THF at different Q

q

temperatures.


Λ

0

X ,triple

λ .triple ο

A ,SrS

(THF)

(THF)

(THP)

(THP)

20

21.0

18.8

12.4

11.1

15

19.6

17.6

11.2

10.1

10

18.4

16.5

10.3

9.2

5

17.2

15.4

9.3

8.3

0

16.0

14.3

8.4

7.5

-5

15.0

13.5

7.6

6.9

-10

Τ e

c

V

S r S

2

Q

Q

2

14.0

12.6

6.9

6.2

-15

13.0

11.7

6.2

5.6

-20

12.0

10.8

5.5

5.0

-25

11.2

10.0

5.0

4.4

-30

10.2

9.1

4.4

3.9

-35

9.4

8.4

3.7

3.3

-40

8.8

7.9

3.5

3.1

-45

8.0

7.2

-50

7.3

6.6

-55

6.5

5.7

-60

5.8

5.2

-65

5.2

4.7

-70

4.6

4.1

and the corresponding SrBo ^

values f o r the d i s s o c i a t i o n were (SrB) + B~ K (3) SrB +

d

a

2

determined. They are summarized i n T a b l e IV. In a d d i t i o n , Table IV shows the r e l e v a n t thermodynamic parameters. The enthalpy of d i s s o c i a t i o n of the contact i o n p a i r of d i f l u o r e n y l strontium ( S r F l 2 ) was reported to be Δ Η ^ -14.4 kcal/mol a t 20°C, that of the mixed t i g h t - l o o s e Ion p a i r being much l e s s negative (ΔΗ° in THF 2

ΔΗJ - - 0.69 kcal/mol AS! - - 23 e.u.

kcal/mol (15) r e s p e c t i v e l y , we may conclude that these s a l t s are a t l e a s t of the mixed t i g h t - l o o s e type, f o r which there i s but l i t t l e change i n the s o l v a t i o n s t a t e upon d i s s o c i a t o n . Moreover, s i n c e the entropy of d i s s o c i a t i o n f o r the contact i o n p a i r s i s of the order of -50 to -60 e.u., we may again say that the t e t r a p h e n y l b o r i d e s a l t s of Sr and Ba, showing an entropy change of r e s p e c t i v e l y -23 and -22 e.u., are a l s o f o r entropie reasons not of the contact type. In f a c t , the A +-value of ( S r B ) being 12.1 and that of ( B a ^ 26.1 may be taken to i n d i c a t e a double solvent separated i o n p a i r s t r u c t u r e f o r the S r - s a l t and a mixed c o n t a c t - s o l v e n t separated p a i r s t r u c t u r e f o r the BaB2» The double s o l v a t e d +

0


10.


One-Ended

Living

139

Polystyrene

2+

c a t i o n of the S r s a l t represented as B ~ 7 / S r / / w i l l upon d i s s o c a t i o n indeed be l e s s mobile than the smaller B"\Ba2+// c a t i o n . The assumption o u t l i n e d above i s a l s o r e f l e c t e d i n the greater K B (=3.05xl0~" M) i n comparison with K (=1.65x10-%) a t 20°C. In c o n c l u s i o n , the f o l l o w i n g e q u i l i b r i a might be w r i t t e n f o r the d i s s o c a t i o n of S r B and BaB , r e s p e c t i v e l y : 5

d

S r

d

2

2

B-//Sr +//B-—

B

2

B-

+

2+

//Sr //B-

K

(4)

d

SrB 2+

B-,Ba //B- ς = ± Β " +


a

2

2

2+

//Ba ,B-

K

2

(5)

d

BaB 4.

B

2

The mixed s a l t SrSB i n THF a t 20°C

When mixed w i t h S r B , the strontium s a l t of one-ended l i v i n g p o l y s t y r e n e S r S , l i k e the barium s a l t BaS w i t h added BaB ( L a ) , forms a mixed s a l t SrSB a c c o r d i n g to the e q u i l i b r i u m : 2

2

SrS

2

+

2

SrB

2 SrSB

2

K

2

(6)

a

The formation constant K o f the mixed s a l t SrSB was determined by measuring the v i s c o s i t y of a s o l u t i o n o f such mixture. Indeed the l e n g t h of the S r S molecule i s twice as h i g h as that o f SrSB (and a l s o of dead poly-S) so that we may consider SrSB e q u i v a l e n t to dead poly-S. To a s c e r t a i n a s u f f i c i e n t v i s c o s i t y , a S r S s a l t o f molecular weight of about 58,000 p e r "S"-arm was used (M^/Mn 1.26). Adopting the same method as o u t l i n e d i n r e f . 15, a c a l i b r a t i o n curve was then c o n s t r u c t e d ( f i g . 6 , T a b l e V ) , g i v i n g the s p e c i f i c v i s c o s i t y of mixtures of l i v i n g and r i g o r o u s l y d r i e d dead p o l y s t y r e n e , obtained by t e r m i n a t i o n o f some o f the i n v e s t i g a t e d S r S , as a f u n c t i o n of t h e i r composition r ( r S r S / t o t a l poly-S) f o r a constant t o t a l weight c o n c e n t r a t i o n of p o l y s t y r e n e (83.5 g r / 1 ) . Subsequently the v i s c o s i t y of a mixture of l i v i n g S r S and dead p o l y s t y r e n e of the same t o t a l c o n c e n t r a t i o n was measured without and w i t h a d d i t i o n of S r B . E q u i v a l e n t amounts of l i v i n g S r S and S r B , as were used i n the a

2

2

β

e

2

2

2

2

2

2

case of barium, were not taken i n t h i s case, i n view of the lower s o l u b i l i t y of S r B . I t should a g a i n be s t r e s s e d that the molecular weight d i s t r i b u t i o n of any mixture of l i v i n g and dead polymer i s the same as that of the corresponding mixture of S r S and S r B of the same s p e c i f i c v i s c o s i t y , thus j u s t i f y i n g the method used t o determine the K value f o r any molecular weight d i s t r i b u t i o n . The mixtures of S r S and terminated S used i n determining the c a l i b r a t i o n curve have the same molecular weight d i s t r i b u t i o n as the mixture of S r S and SrSB r e s u l t i n g from the a d d i t i o n of S r B . Indeed samples of the same stock s o l u t i o n were used t o c a r r y out these experiments. Thus, i t was avoided that changes i n molecular weight d i s t r i b u t i o n would a f f e c t the 2

2

2

a

2

2

2



140


8.00

5.00

1

« 0.2 8

0

ί0.5

Figure 6. Viscosity of a solution containing SrS and the terminated dead poly-S at various r values (τ = \SrS ]/total poly-S) but at constant weight concentration of total polystyrene (whether as SrS or dead poly-S). The dashed line corresponds to a mixture of living strontium polystyrene ([SrS ] = 4.3 X 10~ M), dead poly styrene ([2(terminated S)] = 2.9 Χ /0~ M), and a certain amount of SrB (= 2.4 χ 10 Μ). 2

2

2

2

4

4

2

4


10.


Table Viscosity

One-Ended

Living

V

data of the study in

THF

of the mixed β

a t 20


spec

(b

a )

5.42

>

5. 16

0.09

5.60

0.33

5.76

0.50

6.36

0.79

6. 69

( c )

r

SrSB

t?

e x

+

K

b

= SrB '

K l

« K

K

(9)

i > e x

b

. K d

Β"

+

8.50xl0

6

c

2

j u s t i f y i n g our assumption

that K

b

»

K

Comparing the two d i s s o c i a t i o n s SrB2 ^

c

+

( S r B ) + B" and SrSB

^

(SrS)+ + B " c h a r a c t e r i z e d by the d i s s o c i a t i o n constants K

Anionic Polymerization of the Strontium Salt of One-Ended Living

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