Ring-Opening Polymerization

20 Ring-Opening Polymerization via C-C Bond Opening H. K. HALL, JR., H. TSUCHIYA, P. YKMAN, J. OTTON, S. C. SNIDER, and A. DEUTSCHMAN, JR. (1)

Downloaded by HARVARD UNIV on October 25, 2015 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0059.ch020

Department of Chemistry, University of Arizona, Tucson, AZ 85721

Ring-opening polymerization usually involves compounds con taining strained C-O, C-N, or C-S single bonds. Polymerizations involving strained C-C single bonds are less familiar. Cyclo propane and cyclobutane do not give clean results, because reagents sufficiently vigorous to open the ring also attack the resulting chain. Two types of strained bicyclic compound undergo ring-opening polymerization via C-C bond opening. The first group consists of compounds with a strained polycyclic structure. Ex amples include a variety of bicyclobutanes 1, (2) bicyclopentane[2.1,0]carbonitrile 2 (3), benzocyclopropenecarDonitrile 3 (4), benzocyclobutene 4 (5,6), two tetracyclooctanes 5 and 6, (7,8), and 1,3-dehydroadamantane 7 (9,10).

285

In Ring-Opening Polymerization; Saegusa, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

SOO

RING-OPENING POLYMERIZATION

Two factors account for the ability of such polycyclic com pounds to polymerize. First, the strain energy is very high, and may be relieved on polymerization. Secondly, the compact cage molecule shows low steric hindrance to attack. Breaking a C-C single bond always encounters hindrance from substituents on the adjacent carbons, but this repulsion is minimized in compact ring structures. The second group o f monomers c o n t a i n s those i n which a double bond i s "conjugated" w i t h a s t r a i n e d s i n g l e bond. A number o f v i n y l c y c l o p r o p a n e s 8 (11,12) belong i n t h i s category, as do 1 - v i n y l b i c y c l o b u t a n e 9 (2) and [2,4]-spiroheptadiene 10 (13) . For these compounds, the double bond o f f e r s a p o i n t oi r e a c t i o n f o r the growing polymer c h a i n . Again, the p r o j e c t i n g 2 S P o f f e r s minimal s t e r i c hindrance. U n l i k e other ring-opening p o l y m e r i z a t i o n s , most C-C s i n g l e bond p o l y m e r i z a t i o n s have been c a r r i e d out by f r e e r a d i c a l i n i t i a t i o n , even though examples o f c a t i o n i c , a n i o n i c , and c o o r d i n a t i o n p o l y m e r i z a t i o n s have been presented. The b i c y c l o b u t a n e monomers, such as b i c y c l o b u t a n e - l - c a r b o n i t r i l e ( l a X = CN), are as r e a c t i v e i n f r e e r a d i c a l p o l y m e r i z a t i o n as v i n y l monomers ( 2 ) . A n i o n i c P o l y m e r i z a t i o n o f Bicyclobutane-1- a r b o n i t r i l e We i n q u i r e d whether a n i o n i c p o l y m e r i z a t i o n s o f ]La could a l s o be c a r r i e d out. The most s u c c e s s f u l a n i o n i c p o l y m e r i z a t i o n s o f m e t h a c r y l o n i t r i l e (the v i n y l analog o f l a ) , have been those o f Joh and h i s c o l l e a g u e s (14,15), who used dialkylmagnesium and magnes ium d i a l k y l a m i d e i n i t i a t o r s . Therefore we u t i l i z e d them w i t h bicyclobutane-l-carbonitrile. B i c y c l o b u t a n e - l - c a r b o n i t r i l e polymerized v e r y r e a d i l y w i t h these organomagnesium i n i t i a t o r s (Table I ) . The magnesium amides and " a t e " compounds gave h i g h e s t y i e l d s , w i t h the mercaptides c l o s e behind. Dialkylmagnesiums gave lower y i e l d s . Dioxane and toluene as s o l v e n t s gave the highest y i e l d s , and the polymer p r e c i p i t a t e d from these media. U n s t i r r e d , m a g n e t i c a l l y s t i r r e d , and m e c h a n i c a l l y s t i r r e d p o l y m e r i z a t i o n s gave comparable r e s u l t s . Homogeneous p o l y m e r i z a t i o n s were performed i n dimethylformamide, s u l f o l a n e and tetramethylene s u l f o x i d e s o l u t i o n . The y i e l d s under these c o n d i t i o n s were v e r y low or zero. P o l y b i c y c l o b u t a n e c a r b o n i t r i l e obtained i n t h i s way was a white powder, u n l i k e the f i b r o u s m a t e r i a l obtained by f r e e r a d i c a l i n i t i a t i o n . The inherent v i s c o s i t i e s i n dimethylforma mide were u s u a l l y about 0.1 d l . g . " and r a r e l y exceeded 0.5 d l . g." . The nmr s p e c t r a resembled those of the r a d i c a l - i n i t i a t e d polymer. When magnesium d i ( i s o p r o p y l m e r c a p t i d e ) was used as the i n i t i a t o r , isopropylmercapto end groups were v i s i b l e i n the nmr s p e c t r a . The i n f r a r e d s p e c t r a a l s o resembled those of the r a d i c a l - i n i t i a t e d polymer, and supported the 1-cyano-1,3-cyclob u t a n e d i y l s t r u c t u r e . One a b s o r p t i o n which d i d not conform t o t h i s s t r u c t u r e was v i s i b l e i n every i r spectrum. T h i s a b s o r p t i o n a t 1700 cm" i s a s c r i b e d to a ketone carbonyl group. T h i s a s s i g n ment was confirmed by s t i r r i n g the polymer w i t h sodium borohydride overnight i n s u l f o l a n e - w a t e r (4:1), whereupon t h i s band d i s a p -


C H

=

r o u

1

1

1


20.

HALL ET AL.

Polymerization

via C-C

Bond

Opening

287

eared. The a b s o r p t i o n i n t e n s i t y o f t h i s carbonyl band c o r r e l a t e d i n v e r s e l y w i t h inherent v i s c o s i t y , i n d i c a t i n g t h a t i t was i n v o l v e d i n the termination reaction. We propose the f o l l o w i n g mechanism:


I n i t i a t i o n - A d d i t i o n o f o r g a n o m e t a l l i c RM t o the s t r a i n e d 1,3 bond i s known t o occur even f o r t h e more s t e r i c a l l y crowded

3 - m e t h y l - 1 - b i c y c l o b u t a n e c a r b o n i t r i l e ( 1 6 ) , and i s supported by de t e c t i o n o f the corresponding end groups i n the NMR s p e c t r a o f polymers i n i t i a t e d by magnesium d i ( i s o p r o p y l m e r c a p t i d e ) (and other i n i t i a t o r s ) . Propagation -

Termination -

We s t u d i e d a model system i n an e f f o r t t o determine whether or not a t t a c k o f a propagating a-cyanocylobutyl anion on n i t r i l e groups can take p l a c e q u i c k l y enough under our r e a c t i o n c o n d i t i o n s to represent a p l a u s i b l e t e r m i n a t i o n s t e p . Of a number o f strong bases s t u d i e d , o n l y triphenylmethylsodium c l e a n l y a b s t r a c t e d t h e a-H o f c y c l o b u t a n e c a r b o n i t r i l e t o g i v e the carbanion:



13 13

13 13

1.1.1

HgCn-C H ) (NC H ). MgCC^g)

9

7

5

10

5

5

10

13

0.2

2

Kg(s-i^: H )

7

7

3

7

Mg(S-i-C H )

3

3

2

13

0.2S

2

Mg(S-i-C H )

7

65

-

26

1.0

2

2

?

52

26

1.0

Mg(S-i-C » )

3

26

26

2

-

-

-

26

26

1.0

2

7

Hg(S-i-C H )

3

13

s

?

13

9

3

1.1.1

4

2

n-C H Li, Hg{C^i ) . i-C H SH

5

13

2

13

9

1.1.1

4

n-C H U. Mg(C H ) , i-C H SH

5

n - C ^ l i , MgCC^)^ HNC H

4

3

2

10

-

26

26

1.0

Mg(S-i-C H )(NC H )

9

2

-

13

26

2.0

Mg(S-i-C3H )(NC H )

4

5

l0

10

13

13

1.0

Mg(N-C H )

l0

7

-

13

26

2.0

2

5

Mg(NC H )

s

«WJAI

-

13

13

1.0

10

9

l0

Mg(NC H ) , HNC H

4

2

26

26

1.0

Mg(n-C H ) (NC H )

5

26

26

5

10

1.0

4

9

Mg(n-C H )(NC H )

26

26

1.0

10

Mg(C2H )(NC H )

5

-

13

13

1.0

MgCC^HNCjH^) (d)

5

Lewis Acid (a)

M/1

M moles Monomer, M

M noies Initiator, I

Initiator

64

0.23 0.18

28 23

0.15 Dioxane

20 Dioxane-DMF (2:5)

Toluene

58 Toluene

0.074

0.11 0.20

69 Dioxane-DMF (2:5) Dioxane

0.074

0.20 89 84

0.22 100

0.076

0.096

0.036

0.086

0.23

0.28 (e)

0.105

Dioxane

Dioxane

THF

36

90

Dioxane Dioxane-DMF (1:1)

47

Dioxane

51

88

Dioxane

75

Dioxane

44

0.21

73

(c

inh >

0.41

n

95

% Yield

Dioxane

Toluene

Dioxane

Oioxane

Solvent (b)

Table I. Selected Polymerizations of 1-Bicyclobutanecarbonitrile


Polymerization


HALL ET AL.

c

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r»»

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via C-C

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Opening

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β

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ι

Ο (Μ

w

Γ Μ Μ

3» 2» > C?» 3> 3*

g * 2 g 2 g

Ring-Opening Polymerization

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