Ring-Opening Polymerization via CC Bond Opening - ACS Publications

Jun 1, 1977 - Department of Chemistry, University of Arizona, Tucson, AZ 85721 ... Ring-opening polymerization usually involves compounds containing ...
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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)

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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 -

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1

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

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:

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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:

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

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

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

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52

26

1.0

Mg(S-i-C » )

3

26

26

2

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-

-

26

26

1.0

2

7

Hg(S-i-C H )

3

13

s

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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

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-

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

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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

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Polymerization

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HALL ET AL.

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