Synthesis and Reactions of Halogenated Polyethers and Polysulfides

Chapter 5

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1

2

Melvin P. Zussman , Jenn S. Shih , Douglas A. Wicks, and David A. Tirrell Polymer Science and Engineering Department, University of Massachusetts, Amherst, M A 01003

Halogenated polyethers and polysulfides present a number of interesting possibilities in investigations of the reactivity of polymer chains and in the design of new reactive polymers. In this chapter, we explore the consequences of side chain extension, leaving group variation, and neighboring group participation in determining the stability and reactivity of these two classes of polymeric materials. P o l y e p i c h l o r o h y d r i n (PECH) i s well known as a r e a c t i v e e l a s t o m e r . D i s p l a c e m e n t at t h e c a r b o n - c h l o r i n e bond of PECH has been accom p l i s h e d with a wide v a r i e t y of n u c l e o p h i l i c r e a g e n t s , f o r t h e pur poses o f polymer m o d i f i c a t i o n , g r a f t i n g and c r o s s l i n k i n g ( l _ 2 ) . On the o t h e r hand, t h e PECH s t r u c t u r e (1) i s h a r d l y optimal from t h e p o i n t o f view of i t s r e a c t i v i t y as a~substrate f o r n u c l e o p h i l i c 9

1

substitution: c h l o r i d e i s modest i n i t s l e a v i n g group a b i l i t y , and t h e β-branch p o i n t ( i . e . t h e c h a i n backbone) would be expected t o d e p r e s s r e a c t i o n r a t e s by a f a c t o r o f 10 or so {3). W i t h i n t h e past s e v e r a l y e a r s , we have examined t h e s y n t h e s i s and r e a c t i o n s of s e v e r a l c l a s s e s of polymers r e l a t e d t o PECH. We have adopted t h r e e simple approaches t o t h e p r e p a r a t i o n of p o l y m e r i c s u b s t r a t e s more r e a c t i v e than PECH toward n u c l e o p h i l i c s u b s t i t u t i o n . We have: i ) . removed t h e β-branch p o i n t by e x t e n s i o n of the s i d e c h a i n , i i ) . r e p l a c e d t h e c h l o r i d e l e a v i n g group by a more r e a c t i v e bromide and i i i ) . r e p l a c e d t h e backbone oxygen atom by a s u l f u r atom t h a t o f f e r s s u b s t a n t i a l a n c h i m e r i c a s s i s t a n c e t o n u c l e o p h i l i c 1

2

Current address: Ε. I du Pont de Nemours and Company, Experimental Station, Wilmington, D E 19898

Current address: G A F Corporation, Wayne, N J 07470 0097-6156/88/0364-0060$06.00/0 © 1988 American Chemical Society

Benham and Kinstle; Chemical Reactions on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

5.

Halogenated Polyethers and Polysulfides

ZUSSMAN ET AL.

61

displacement. Each of these s i m p l e s t r u c t u r a l changes a f f o r d s e l a s t o m e r i c polymers more r e a c t i v e than PECH toward a t t a c k by nucleophilic species. We p r e s e n t i n t h i s c h a p t e r an overview of the s y n t h e s i s and r e a c t i o n s of t h e s e new r e a c t i v e p o l y m e r s . S i d e Chain

Extension

The r e p e a t i n g u n i t s t r u c t u r e of PECH would suggest t h a t t h e r e a c t i v i t y o f the polymer would be r a t h e r s i m i l a r t o t h a t of i s o b u t y l chloride. The e-oxygen atom would not be expected t o o f f e r s u b s t a n t i a l a n c h i m e r i c a s s i s t a n c e i n n u c l e o p h i l i c d i s p l a c e m e n t , and should perhaps be m i l d l y d e a c t i v a t i n g as a r e s u l t of a small i n d u c t i v e effect . In g e n e r a l , the r e a c t i v i t y of i s o b u t y l h a l i d e s toward n u c l e o p h i l i c reagents i s depressed i n comparison with the r e a c t i v i t y of primary, s t r a i g h t - c h a i n analogues. S t r e i t w i e s e r l i s t s compara t i v e data f o r nine r e a c t i o n s of i s o b u t y l and 1-propyl h a l i d e s {3); w i t h i n t h i s s e t , kp py-|/k-jsobutyl ranges from 4 . 3 ( f o r RBr + C l " i n acetone) t o 3 3 . 8 ( f o r RI + Br" i n a c e t o n e ) . The β-branch p o i n t p r e s e n t i n PECH i s absent i n polymers of h i g h e r l , 2 - e p o x y - a ) - c h l o r o a l k a n e s . Such polymers are r e a d i l y prepared by treatment of the neat monomers with the m o d i f i e d t r i ethylaluminum c a t a l y s t i n t r o d u c e d by Vandenberg (5_,6J ; r e s u l t s f o r ( 2 - c h l o r o e t h y l ) o x i r a n e , ( 3 - c h l o r o p r o p y l ) o x i r a n e and ( 4 - c h l o r o b u t y l ) o x i r a n e (2a-c) are summarized i n T a b l e I (7_,8). r0

2a:

δ, Ζ tr°i

η =2 η =3 η

Table m™™™ Monomer

a

(CH )CH C1 n-i

4

I.

2

Homopolymerization

Polymerization ^ (

p

a

y

s

D

ν

2

2

(Chloroalkyl)oxiranes

,· -ΐΛ

Yield

)

Λ

fo,\

(%)

*inh (

d

L

/

g

Tn(°r\ Tg( C)

D )

2a

14

89

4.12

2b

14

78

3.44C

-42

2c

10

77

1.21

-50

Polymerization

at room temperature

AlEt3/H20/acetylacetone c

3

of

(CH )CH C1 n-i

2

In CHC13,

c

-27

i n presence of 5 mol-%

(1/0.5/0.5)

0.5 wt %, 35°C

GPC peak m o l e c u l a r weight

> 10

6

In each c a s e , the polymer i s o b t a i n e d as a white e l a s t o m e r of high m o l e c u l a r weight (>10 i n some e x p e r i m e n t s ) . Each of t h e s e polymers i s s o l u b l e i n benzene, i n c h l o r i n a t e d hydrocarbons and i n dipolar aprotic solvents. F i g u r e 1 shows t h e k i n e t i c s of c h l o r i d e s u b s t i t u t i o n by tetra-n-butylammonium benzoate i n Ν , Ν - d i m e t h y l a c e tamide at 50°C, f o r PECH and f o r the polymers of 2 a - c . Under t h e s e c o n d i t i o n s , each of the h i g h e r homologues i s a b o u t ~ e q u a l l y r e a c t i v e , and a l l are c o n v e r t e d t o the benzoate more r a p i d l y than PECH. Each 6



CL

c ο

70

80

90

100

2

ι

1

1

2

1

12

1

n

1

1

η = 1

1

2

1

Time

14

(hr)

16

1

NBU4

DMAc, 5 0 *

PhC0

1

18

1

20

2

1

1

1

n

22

2

2

1

24

1

(CH ) 0 CPh

-CH -CH-O-

Kinetics of Substitution

10

2

(CH f CI

-CH -CH-0-

-τ

26

1

1

1

28

30

-

-

r-•>»—

F i g u r e 1. C o n v e r s i o n v e r s u s time f o r s u b s t i t u t i o n by tetrabutylammonium benzoate on p o l y ( e p i c h l o r o h y d r i n ) (Δ, A , s e p a r a t e r u n s ) ; p o l y [ ( 2 - c h l o r o e t h y l ) o x i r a n e ] ( O ) ; p o l y [ ( 3 - c h l o r o p r o p y l ) o x i r a n e ] ( V ) ; and p o l y [ ( 4 - c h l o r o b u t y l ) o x i r a n e ] ( • ) . (Reproduced w i t h p e r m i s s i o n from réf. 1. C o p y r i g h t 1982 W i l e y . )

Γ

Γ

P i * *

1 I / I

' 1 $

ι

W

ο *d Ο r *! S

ζ

Ο

u

>

w

>

n

s

w

5.

ZUSSMAN ET AL.


63

of t h e s e r e a c t i o n s shows n e g a t i v e d e v i a t i o n s from s e c o n d - o r d e r k i n e t i c s , but s e c o n d - o r d e r r a t e c o n s t a n t s can be e s t i m a t e d from the i n i t i a l reaction rates. T h i s procedure g i v e s a s e c o n d - o r d e r r a t e c o n s t a n t of 3.9 χ M" s " f o r p o l y [ ( 2 - c h l o r o e t h y l ) o x i rane] and a v a l u e of 0.63 χ 10" * M" s " f o r PECH. Thus the h i g h e r homologues are more r e a c t i v e than PECH by a f a c t o r of 6 , an o b s e r v a t i o n com p l e t e l y c o n s i s t e n t with known s t r u c t u r e - r e a c t i v i t y r e l a t i o n s f o r simple alkyl h a l i d e s . The r e a c t i v e c a r b o n - c h l o r i n e bond i n t h e s e p o l y e t h e r s a l l o w s t h e i r conversion to other i n t e r e s t i n g m a t e r i a l s . For example, q u a n t i t a t i v e c h l o r i d e d i s p l a c e m e n t by benzoate anion f o l l o w e d by basic methanolysis affords h y d r o p h i l i c polyether elastomers that are w a t e r - s o l u b l e or w a t e r - s w e l l a b l e , depending on s i d e c h a i n l e n g t h (9). Poly[(3-hydroxypropyl)oxirane] i s a c o l o r l e s s elastomer that can be c a s t i n t o t o u g h , c l e a r f i l m s from water or m e t h a n o l . Poly[ ( 4 - h y d r o x y b u t y l ) o x i r a n e ] i s i n s o l u b l e i n w a t e r , but q u i t e h y d r o p h i l i c ; a f i l m immersed i n water gains 46% i n weight i n 90 min at room t e m p e r a t u r e , w h i l e r e t a i n i n g good mechanical i n t e g r i t y . Uses f o r such m a t e r i a l s as a d h e s i v e s , as b i o m a t e r i a l s and as c o n t a c t l e n s e s may be a n t i c i p a t e d . 1

Bromide as L e a v i n g

1

1

1

1

Group

Perhaps the most s t r a i g h t f o r w a r d approach t o i n c r e a s i n g the r e a c t i v i t y of halogenated s u b s t r a t e s - - p o l y m e r i c or o t h e r w i s e - - i s t o e x p l o i t the well known o r d e r of l e a v i n g group a b i l i t y , i . e . , I > Br > Cl > F. For example, data p r o v i d e d by S t r e i t w i e s e r suggest t h a t replacement of c h l o r i d e by bromide w i l l l e a d t o r a t e enhance ments of 50- t o 2 0 0 - f o l d , depending on r e a c t i o n c o n d i t i o n s (3_). One cannot take t h i s approach too f a r i n the d e s i g n of r e a c t i v e p o l y m e r i c s u b s t r a t e s , however, s i n c e the l e a v i n g group must be i n e r t t o t h e c o n d i t i o n s of p o l y m e r i z a t i o n i f p r o t e c t i o n - d e p r o t e c t i o n schemes are t o be a v o i d e d . The i d e a l l e a v i n g group would be c o m p l e t e l y u n r e a c t i v e toward the n u c l e o p h i l i c s p e c i e s i n v o l v e d i n c h a i n growth, but r e a d i l y d i s p l a c e d i n subsequent polymer m o d i f i c a t i o n r e a c t i o n s . Bromide appears t o be u s e f u l i n t h i s r e g a r d . T a b l e II sum m a r i z e s our homopolymerization experiments on ( 2 - b r o m o e t h y l ) o x i r a n e , ( 3 - b r o m o p r o p y l ) o x i r a n e and ( 4 - b r o m o b u t y l ) o x i r a n e ( 3 a - c ) ( 1 0 - 1 2 ) .

3a:

η η =2

~b:

η η =3

c:

η =4

(CH )CH Br n-i 2

2

(CH )CH Br n-i 2

2


64

CHEMICAL REACTIONS ON POLYMERS

Table M^n^mo^ Monomer

a

b

c

II.

Homopolymerization

Γ*+*Ι„*>+

Catalyst

of

Polymeri z a t i on ^ e

(

h

p

)

(Bromoalkyl)oxiranes Yield ( % )

^inh ( ( j L / g )

3a

a

24

67

3.5

3a

b

24

63

0.6

3b

a

3

35

0.6

3b

b

24

74

0.5

3c

a

24

65

1.7

3c

b

24

68

0.6

c

/ \ Tg ( C) T r t

0 r

-35 -35 -34

Room t e m p e r a t u r e ,

6-7 mol-X A l E t / H 0 / a c e t y l a c e t o n e

(1/0.5/1)

Room t e m p e r a t u r e ,

6-7 mol-% A l E t / H 0 / a c e t y l a c e t o n e

(1/0.5/0.5)

3

3

2

2

In C H C 1 , 0.5 wt %, 30°C 3

P o l y m e r i z a t i o n of each of these o x i r a n e s i s e f f e c t i v e l y accom p l i s h e d by treatment of the neat monomer with the c h e l a t e d aluminum catalyst. It i s i n t e r e s t i n g t o note t h a t h i g h e r m o l e c u l a r weights are g e n e r a l l y r e a l i z e d by u s i n g the c a t a l y s t mixture t h a t c o n t a i n s a 1:1 r a t i o of t r i e t h y l a l u m i n u m and a c e t y l a c e t o n e ; t h i s may be a r e s u l t of c h e l a t i o n of the most a c i d i c c a t a l y s t s i t e s and a consequent r e d u c t i o n i n c h a i n t r a n s f e r a s s o c i a t e d with A l - a s s i s t e d i o n i z a t i o n o f the C-Br bond. The poly[(a>-bromoalkyl ) o x i r a n e ] s are s l i g h t l y t a c k y e l a s t o m e r s at room t e m p e r a t u r e ; a l l undergo a g l a s s t r a n s i t i o n at a p p r o x i m a t e l y - 3 5 ° C . F i g u r e 2 shows t h e k i n e t i c s of bromide s u b s t i t u t i o n by t e t r a - n butylammonium benzoate i n CDC1 at 45°C, f o r p o l y ( e p i b r o m o h y d r i n ) (PEBH) and f o r the polymers of 3 a - c . Although these r e s u l t s are not d i r e c t l y comparable t o those i n ~ F i g u r e 1 because of d i f f e r e n c e s i n s o l v e n t s and t e m p e r a t u r e s , q u a l i t a t i v e c o n s i s t e n c y i s a p p a r e n t . F i r s t of a l l , each o f the h i g h e r homologues i s about e q u a l l y r e a c t i v e , and a l l r e a c t more r a p i d l y than PEBH. The k i n e t i c s show n e g a t i v e d e v i a t i o n from s e c o n d - o r d e r b e h a v i o r , as b e f o r e , but the i n i t i a l s l o p e s of the s e c o n d - o r d e r p l o t s are again u s e f u l f o r com p a r a t i v e purposes. T a b l e I I I l i s t s the r a t e c o n s t a n t s o b t a i n e d i n t h i s way. As i n t h e c h l o r i d e s e r i e s , e x t e n s i o n of the s i d e c h a i n by a s i n g l e carbon atom removes the s-branch p o i n t and a c c e l e r a t e s the r e a c t i o n by a f a c t o r of about 10. (The s i g n i f i c a n c e of the apparent t w o - f o l d h i g h e r r e a c t i v i t y of the 3-bromopropyl s i d e c h a i n compared t o i t s 2- and 4-carbon homologues has not been d e t e r m i n e d . ) F i n a l l y , the bromides are indeed more r e a c t i v e than the c h l o r i d e s ; d e s p i t e the use o f CDC1 as the r e a c t i o n s o l v e n t , the bromides r e a c t more than an o r d e r of magnitude f a s t e r than t h e c h l o r i d e s , even though the l a t t e r were c o n v e r t e d i n DMAc, a s u p e r i o r s o l v e n t f o r n u c l e o p h i l i c s u b s t i t u t i o n s of t h i s k i n d . 3

3


5.

ZUSSMAN ET AL.


TIME

( hrs )

F i g u r e 2. C o n v e r s i o n v e r s u s time f o r s u b s t i t u t i o n by t e t r a b u t y l ammonium benzoate on p o l y ( e p i b r o m o h y d r i n ) (O) ; p o l y [ ( 2 - b r o m o e t h y l ) o x i r a n e ] ( 0 ) ; po1y[(3-bromopropyl)oxirane] (v, separate r u n s ) ; and p o l y [ ( 4 - b r o m o b u t y l ) o x i r a n e ] ( • , separate r u n s ) .


65

66

CHEMICAL REACTIONS ON POLYMERS Table

III. on

S i d e Chain Length

c

b

b

2

χ 10"

2

6.97

χ ΙΟ"

3

4.53

χ 10"

4

3.88

χ ΙΟ-

CDC1

(M"

0

0

8.57

3

3

3

3

1.74

χ ΙΟ"

1.40

χ ΙΟ"

9.11

χ ΙΟ"

8.10

χ ΙΟ"

s-i)

1

2

0.8

χ ΙΟ"

2

8.2

χ

10-3

19 χ Ι Ο "

3

7.9

3

3

χ ΙΟ"

3

3

3

Initial

Concentrations

Initial

slope

Anchimeric

[Benzoate] (M)

[-CH Br] (M)

1

a 45°C, b

K i n e t i c s of S u b s t i t u t i o n Poly(u)-bromoalkyl)oxiranes

of

second-order

Assistance

kinetic

by Backbone

plot

Sulfur

The a c c e l e r a t i o n of h a l i d e d i s p l a c e m e n t by n e i g h b o r i n g s u l f i d e s i s among the most f a m i l i a r examples of a n c h i m e r i c a s s i s t a n c e i n o r g a n i c chemistry. T a b l e IV i l l u s t r a t e s the magnitude of the e f f e c t f o r primary a l k y l c h l o r i d e s (4_).

Table

IV.

Relative

Rate C o n s t a n t s f o r H y d r o l y s i s of A l k y l C h l o r i d e s 3

CH CH CH CH C1

1.00

CH CH 0CH CH C1

0.18

3

3

2

2

2

2

2

2

CH CH SCH CH C1 3

1.00

a Aqueous d i o x a n e , relative

2750

2

0.77

3

Rate

2

CH CH SCH CH CH C1

3

b

2

CH CH 0CH CH CH C1

to

2

2

2

2

[H 0] 2

2

2

2

2

= 20 M, 100°C

1-chlorobutane

A 6 - s u l f i d e a c c e l e r a t e s the s o l v o l y s i s n e a r l y 3 0 0 0 - f o l d compared t o the s i m p l e a l k y l h a l i d e and a p p r o x i m a t e l y 1 5 , 0 0 0 - f o l d compared t o the β - c h l o r o e t h e r . On the o t h e r hand, the γ - s u l f i d e o f f e r s no a s s i s t a n c e ; a p p a r e n t l y c l o s u r e t o the four-membered c y c l i c s u l f o n ium ion cannot compete with d i r e c t d i s p l a c e m e n t by the e x t e r n a l nucleophile. These r e s u l t s suggest t h a t p o l y [ ( c h l o r o m e t h y l ) t h i i r a n e ] (PCMT, 4) s h o u l d be s u b s t a n t i a l l y more r e a c t i v e than p o l y e p i c h l o r o h y d r i n

4

toward n u c l e o p h i l i c s u b s t i t u t i o n s , and indeed i t i s ( 1 3 ) . In a d d i t i o n , the f o r m a t i o n of the i n t e r m e d i a t e e p i s u l f o n i u m ion by


5.

ZUSSMAN ET AL.

67


n e i g h b o r i n g group d i s p l a c e m e n t leads t o a rearrangement of the CMT r e p e a t i n g u n i t t o one (5) d e r i v e d from t h e r i n g - opening p o l y m e r i z a t i o n of 3 - c h l o r o t h i e t a n e (3CT, Scheme I) (13^17)·

Δ

{CH CH-S3- 2

CH

{CH CH-S} 2

^

k

- i

\ l

C 1

2

KH CH-S3^CH2CHCH -S} 2

2

CH C1

CH C1

4

k ,k_

2

Ù

rl

\ / CH

I

"

6

ÎCH ÇHCH -Sl

2

2

Cl

2

x

«

2

k.

2

CI

l 9

k2

5

Scheme I We have examined t h e e f f e c t s of c o n c e n t r a t i o n , t e m p e r a t u r e , s o l v e n t and added e l e c t r o l y t e on the k i n e t i c s of t h i s s t r u c t u r a l interconversion. In a l l i n s t a n c e s , the k i n e t i c s are well d e s c r i b e d by the r a t e law f o r a r e v e r s i b l e f i r s t - o r d e r r e a c t i o n [ E q u a t i o n 1 ] :

f ( 3 C T ) - foo(3CT)" 0

(K + K")t = In

(1)

f(3CT) - U 3 C T ) where f(3CT) i s the f r a c t i o n of 3 - c h l o r o t h i e t a n e r e p e a t i n g u n i t s i n t h e copolymer and the s u b s c r i p t s 0 and » r e f e r t o i n i t i a l and e q u i l i b r i u m copolymer s t r u c t u r e s , r e s p e c t i v e l y ; Κ and K~ are c o m b i n a t i o n s of the elementary r a t e c o n s t a n t s k , k , k . and k_ ( c f . Scheme I) such t h a t x

Κ =

,

2

L

, + k,

2

(2)

and k_,kk.i

+ k

(3) 2

The q u a n t i t y (K + K") i s thus o b t a i n e d as the s l o p e of a p l o t r i g h t s i d e of E q u a t i o n 1 v e r s u s t i m e , and because

f

(

3

C

T

)

»

=

τττ-


of the

( 4 )

68

CHEMICAL REACTIONS ON POLYMERS

the composite r a t e c o n s t a n t s K and K" can be determined i n d i v i d u ally. Each of t h e s e q u a n t i t i e s (K and K") may be viewed as a r a t e c o n s t a n t f o r c y c l i z a t i o n , m u l t i p l i e d by a f a c t o r t h a t d e s c r i b e s the p a r t i t i o n i n g of the s u l f o n i u m ion i n t e r m e d i a t e between the two i s o m e r i c p r o d u c t s of c h l o r i d e ion a t t a c k . T a b l e V summarizes the v a l u e s of (K + K") o b t a i n e d i n t h i s way:

Table

V.

K i n e t i c s of Repeating Unit I s o m e r i z a t i o n i n P o l y [ ( c h l o r o m e t h y l ) t h i i r a n e ] and Poly(3-chlorothietane)

Temperature (°C)

Solvent

48

none

40

none

35

none

20.5

none

35.5

none

35.5

CHC1

35.5

N0 Ph

35.5

CD C1

3

35.5

CD Cl c

0.55

3

b

c

d

e

Equilibrium Fraction 3CT U n i t s 0.685

3

0.690

3

0.692

3

0.700

0

2

15)

PCMT-E ( r e f

15)

P3CT ( r e f Calculated

3.6

d

± 0.9

0.34 ± 0.03

d

1.2

± .03

0.59

± .03

0.33 ± 0.04

0.62

± .03

0.47

0.55

± .02e

0.42 ± 0.06

± .02e

0.36 ± 0.07

b

2

PCMT-C ( r e f

6.3 ± 1.6

d

0.675 b 3

2

6

9.4 ± 0 . 9

± .04

3

Κ + K- ( 1 0 " s

2

2

± 0.14 ± 0.09

15) from r e s u l t

Based on rearrangement

at 48°C of

P3CT

These data reveal s e v e r a l i n t e r e s t i n g f e a t u r e s of the r e p e a t i n g unit isomerization. F i r s t of a l l , the r a t e c o n s t a n t s are of roughly t h e same o r d e r of magnitude as t h o s e observed i n s o l v o l y s e s and r e arrangements of B - c h l o r o s u l f i d e s of low m o l e c u l a r weight when d i f f e r ences i n s o l v e n t s and temperatures are taken i n t o a c c o u n t . T a b l e VI p r o v i d e s a summary of comparative data from the l i t e r a t u r e . We have a l s o determined the apparent r a t e c o n s t a n t f o r the analogous isom e r i z a t i o n of l - c h l o r o - 2 - e t h y l t h i o p r o p a n e (7) t o 2 - c h l o r o - l - e t h y l t h i o p r o p a n e (8) t o be 4 χ 1 0 s at 45°C Τη the absence of s o l v e n t . The l a t t e r i s ~ o f c o u r s e a composite r a t e c o n s t a n t of the kind d e f i n e d by E q u a t i o n s 2 and 3. _ 6

7

1

8


5.


ZUSSMAN ET AL.

Table VI.

Substrate

Temp

(°c)

Rates and A c t i v a t i o n E n e r g i e s o f R e a c t i o n s of 6 - C h l o r o s u l f i d e s Solvent

E (kcal/mol)

k(s"

a

40

EtOH/ Η,Ο

1.2 χ 10-5

18.6

PhSCH CH Cl

40

MeOH

2.5 χ ίο-**

18

CH SCH CH C1

55

AcOH

4 . 8 χ ίο- *

2-endo-chloro7-thiabicyclo[2.2.1]heptane

25

AcOH

1.8 χ 10"

CH PhSCH CH Cl 3

2

2

3

2

2

2

2

69

C1 CHCHC1

2

4.5 χ 1 0 "

threo-

C1 CHCHC1

2

9.2 χ 1 0 "

2

18 19 20

1

erythro70 Ph(Cl PhS)CHCHClCH

6 d

Ref

16.4

21

23

22

3

50

Ph(Cl PhS)CHCHClCH a

3

2

22

6 a

(6)

These are composite E q u a t i o n s 2 and 3.

rate constants

of the kind d e f i n e d by

A second p o i n t of i n t e r e s t i s the apparent a c t i v a t i o n energy of the isomen*zation. From an examination of the temperature depend ences of Κ and K" f o r r e a c t i o n i n the bulk polymer, we f i n d apparent a c t i v a t i o n e n e r g i e s of 22-23 kcal/mol f o r each of the forward and reverse processes. Given the composite nature of Κ and K , t h e s e are t r u e a c t i v a t i o n e n e r g i e s only i f the p a r t i t i o n i n g of the s u l f o n ium i o n i n t e r m e d i a t e i s i n s e n s i t i v e t o t e m p e r a t u r e . Nonetheless, t h e s i m i l a r i t y of these parameters t o those l i s t e d i n T a b l e VI argues f o r a common mechanism t h a t i n v o l v e s in a l l i n s t a n c e s r a t e d e t e r m i n i n g r i n g - c l o s u r e t o the e p i s u l f o n i u m i o n , as suggested by Scheme I . The s o l e e x c e p t i o n i n T a b l e VI i s the low a c t i v a t i o n energy f o r rearrangement of t h r e o - 6 ; t h i s b e h a v i o r appears t o be anomalous, and no e x p l a n a t i o n i s p r o v i d e d by the o r i g i n a l authors (22). The s o l v e n t dependence of the r e a c t i o n r a t e i s a l s o c o n s i s t e n t w i t h t h i s m e c h a n i s t i c scheme. Comparison of the r a t e c o n s t a n t s f o r i s o m e r i z a t i o n s of PCMT i n c h l o r o f o r m and i n n i t r o b e n z e n e shows a small ( c a . 40%) r a t e enhancement in the l a t t e r s o l v e n t . Simple e l e c t r o s t a t i c theory p r e d i c t s that n u c l e o p h i l i c s u b s t i t u t i o n s in which n e u t r a l r e a c t a n t s are c o n v e r t e d t o i o n i c p r o d u c t s s h o u l d be a c c e l e r a t e d i n p o l a r s o l v e n t s ( 2 3 ) , so t h a t a r a t e i n c r e a s e i n n i t r o b e n z e n e i s t o be e x p e c t e d . In f a c t , t h i s e f f e c t i s o f t e n very small ( 2 4 ) . For example, P a r k e r and co-workers (25) r e p o r t t h a t the Sjs|2 r e a c t i o n of methyl bromide and dimethyl s u l f i d e i s a c c e l e r a t e d by o n l y 50% on changing t h e s o l v e n t from 88% (w/w) methanol-water t o Ν , Ν - d i m e t h y l a c e t a m i d e (DMAc) at low i o n i c s t r e n g t h ; t h i s i s a f a r g r e a t e r change i n s o l v e n t p r o p e r t i e s than t h a t i n v e s t i g a t e d i n the p r e s e n t work. Thus a s m a l l , p o s i t i v e dependence of r e a c t i o n r a t e on s o l v e n t p o l a r i t y i s i m p l i c i t i n the s u l f o n i u m i o n mechanism. A f i n a l o b s e r v a t i o n c o n s i s t e n t with r a t e - d e t e r m i n i n g c y c l i z a t i o n i s t h a t the r e a c t i o n r a t e i s r e l a t i v e l y i n s e n s i t i v e t o added electrolyte. A d d i t i o n of 0.5 e q u i v a l e n t s of t e t r a - n - b u t y l a m m o n i u m c h l o r i d e or t e t r a - n - b u t y l a m m o n i u m a z i d e t o c h l o r o f o r m s o l u t i o n s of - 1


70

CHEMICAL REACTIONS O N POLYMERS

PCMT produces very s m a l l , and a p p r o x i m a t e l y e q u a l , i n c r e a s e s i n r a t e . A l t e r n a t i v e r e a c t i o n mechanisms t h a t invoke as r a t e - d e t e r m i n i n g s t e p s e i t h e r i ) . a t t a c k by c h l o r i d e i o n , o r i i ) . u n a s s i s t e d S^l d i s s o c i a t i o n o f t h e c a r b o n - c h l o r i n e bond a r e i n c o n s i s t e n t with t h i s result. Repeating

Unit

Isomerization

v s . Isomerization

Polymerization

Repeating unit i s o m e r i z a t i o n i s s i m i l a r i n several respects t o isomerization polymerization (26,27). Isomerization polymerization may be d e f i n e d as a p r o c e s s whereby a monomer o f s t r u c t u r e A i s c o n v e r t e d t o a polymer o f r e p e a t i n g u n i t s t r u c t u r e B, wherein t h e c o n v e r s i o n o f A t o Β r e p r e s e n t s a s t r u c t u r a l change which i s not a s i m p l e r i n g opening o r double bond a d d i t i o n : nA

— >

m

n

The product of an i s o m e r i z a t i o n p o l y m e r i z a t i o n i s thus determined by t h e r e l a t i v e r a t e s o f t h e p r o p a g a t i o n and i s o m e r i z a t i o n s t e p s ; i . e . , i t i s k i n e t i c a l l y determined. I f i s o m e r i z a t i o n i s much f a s t e r than p r o p a g a t i o n , t h e homopolymer o f Β i s o b t a i n e d ; c o m p e t i t i v e r a t e s w i l l l e a d t o A-B c o p o l y m e r s . We d e f i n e r e p e a t i n g u n i t i s o m e r i z a t i o n as a p r o c e s s subsequent t o p o l y m e r i z a t i o n , i n which an i n t r a m o l e c u l a r rearrangement o f t h e repeating unit leads to a thermodynamically p r e f e r r e d s t r u c t u r e : ηA

— > «A}

n

— >

-fAMB>y

Thus p r o p a g a t i o n must be much f a s t e r than i s o m e r i z a t i o n , and t h e product w i l l be determined by thermodynamics, r a t h e r than by r e a c tion kinetics. The net r e s u l t s o f t h e two p r o c e s s e s may be q u i t e s i m i l a r , however, i n t h a t polymers of unexpected s t r u c t u r e s may be o b t a i n e d , and copolymers may be prepared by p o l y m e r i z a t i o n o f a s i n g l e monomer. Acknowledgments T h i s work was s u p p o r t e d by g r a n t s from t h e Polymers Program o f t h e National Science Foundation. Support o f our r e s e a r c h by an NSF P r e s i d e n t i a l Young I n v e s t i g a t o r Award i s a l s o g r a t e f u l l y acknowledged.

Literature Cited 1. E. Schacht, D. Bailey and O. Vogl, J. Polym. Sci. Polym. Chem. Ed. 16, 2343 (1978) and references therein. 2. E.J. Vandenberg, in C.C. Price and E.J. Vandenberg, Eds. Coordination Polymerization, Plenum, 1983, p. 11. 3. A. Streitwieser, Jr., Solvolytic Displacement Reactions, McGraw-Hill, New York, 1962. 4. H. Bohme and K. Sell, Chem. Ber., 81, 123 (1948). 5. E.J. Vandenberg, J. Polym. Sci. 47, 486 (1960). 6. E.J. Vandenberg, J. Polym. Sci. A17, 525 (1969).


5.

ZUSSMAN ET AL.


71

7. J.S. Shih, J.F. Brandt, M.P. Zussman and D.A. Tirrell, J. Polym. Sci. Polym. Chem. Ed. 20, 2839 (1982). 8. J.S. Shih, Ph.D. Dissertation, Carnegie-Mellon University, 1984. 9. J.S. Shih and D.A. Tirrell, J. Polym. Sci. Polym. Chem. Ed. 22, 781 (1984). 10. J.S. Shih and D.A. Tirrell, J. Macromol. Sci. Chem. A21, 1013 (1984). 11. S. Vaze and D.A. Tirrell, J. Bioactive and Compatible Polym. 1, 79 (1986). 12. D.A. Wicks and D.A. Tirrell, Polym. Prepr. 27(2), 21 (1986). 13. M.P. Zussman and D.A. Tirrell, Macromolecules 14, 1148 (1981). 14. M.P. Zussman and D.A. Tirrell, Polymer Bull. 7, 439 (1982). 15. M.P. Zussman and D.A. Tirrell, J. Polym. Sci. Polym. Chem. Ed. 21, 1417 (1983). 16. M.P. Zussman, Ph.D. dissertation, Carnegie-Mellon University, 1982. 17. M.P. Zussman and D.A. Tirrell, J. Polym. Sci. Polym. Chem. Ed., in press. 18. R. Bird and C.J.M. Stirling, J. Chem. Soc. Perkin II, 1221, 1973. 19. F.G. Bordwell and W.T. Brannen, Jr., J. Am. Chem. Soc. 86, 4645 (1964). 20. I. Tabushi, Y. Tamaru and Z. Yoshida, Bull. Chem. Soc. Japan 47, 1455 (1974). 21. I. Tabushi, Y. Tamaru, Z. Yoshida and T. Sugimoto, J. Am. Chem. Soc. 97, 2886 (1975). 22. G.H. Schmid and V. Csizmadia, Can. J. Chem. 50, 2465 (1972). 23. R.W. Alder, R. Baker and J.M. Brown, Mechanism in Organic Chemistry, Wiley, New York, 1971, p. 43. 24. A.J. Parker, Chem. Rev. 69, 1 (1969). 25. Y.C. Mac, W.A. Millen, A.J. Parker and D.W. Watts, J. Chem. Soc. (B), 525 (1967). 26. J.P. Kennedy and R.M. Thomas, Makromol. Chem. 53, 28 (1962). 27. J.P. Kennedy in H.F. Mark, N.G. Gaylord and N.M. Bikales, Eds., Encyclopedia of Polymer Science and Technology, Wiley, New York, 1967, Vol. 7, p. 754. RECEIVED

September 11, 1987


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