Ring-Opening Polymerization

11 Ring-Opening Polymerizations: Mechanism of

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Polymerization of ε-Caprolactone R. H . YOUNG, M. MATZNER, and L. A. PILATO Union Carbide Corp., Bound Brook, NJ 08805

The first s y n t h e s i s o f Є-caprolactone was r e p o r t e d by C a r o t h e r s ( 1 ) . He a l s o i n v e s t i g a t e d its p o l y m e r i z a t i o n under t h e i n f l u e n c e o f h e a t and catalysts. S i n c e then t h e p o l y m e r i z a t i o n s o f this as w e l l as t h a t o f o t h e r l a c t o n e s were s t u d i e d by many r e s e a r c h e r s . Throughout t h e 1950's t o t h e 1970's t h e polymer f o r mation and its p r o p e r t i e s were t h e s u b j e c t o f s e v e r a l i n v e s t i g a t i o n s in o u r laboratories (2-5). Union Carbide is p r e s e n t l y t h e commercial p r o d u c e r o f t h e monomer and o f a s e r i e s o f polymers w h i c h range in m o l e c u l a r w e i g h t s from 500 t o 40,000. The starting Є-caprolactone is produced by t h e p e r a c e t i c a c i d o x i d a t i o n o f c y c l o h e x a n o n e as shown in E q u a t i o n (I).

In s p i t e o f t h e number o f investigations that were d e v o t e d t o t h e p o l y m e r i z a t i o n o f l a c t o n e 3, t h e e x a c t mechanism whereby t h e polymer is formed is still not entirely clear. I t is t h e purpose o f this paper t o present the various f a c t o r s that influence the r e a c t i o n and t o d e s c r i b e its c o m p l e x i t y when it is performed in the m e l t in t h e p r e s e n c e o f either a n i o n i c o r c o o r d i nation catalysts.

152

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

11.

YOUNG E T A L .

Polymerization

of

^-Caprolactone

153


Mechanisms ; In p r i n c i p l e , t h e p o l y m e r i z a t i o n o f a l a c t o n e s h o u l d f o l l o w mechanism(s) s i m i l a r t o t h e c a t a l y z e d r e a c t i o n s o f simple e s t e r s . The t r a n s f o r m a t i o n s that a r e o b s e r v e d a r e a f u n c t i o n o f t h e c a t a l y s t and c a n be s u b d i v i d e d i n t o (a) c a t i o n i c , (b) a n i o n i c , and (c) c o o r d i n a t i o n type. A s i m p l i f i e d d e s c r i p t i o n f o r the t h r e e mechanisms i s shown w i t h ^ - c a p r o l a c t o n e as an example· (a)

Cationic

I t was suggested (4,6,7) t h a t t h e c a t i o n i c c a t a l y z e d p o l y m e r i z a t i o n proceècTs v i a t h e s t e p s shown i n E q u a t i o n ( I I ) . F i r s t an e q u i l i b r i u m o f t h e c a t i o n i c species, w i t h t h e i n t e r m e d i a t e , Jô, i s e s t a b l i s h e d . T h i s i s f o l l o w e d by r i n g - o p e n i n g t o 7, which then p r o p a g a t e s u n t i l a h i g h polymer i s o b t a i n e d .

5

Ο II

Ο

3

6

7

(ID Monomer Polymer


154

RING-OPENING

(b)

POLYMERIZATION

Anionic


In t h i s c a s e , a t t a c k o f t h e base upon t h e c a r b o n y l group o f t h e c y c l i c e s t e r i s t h e c h a r a c t e r i s t i c f e a t u r e i n t h e i n i t i a t i o n p r o c e s s (£) ( E q u a t i o n ( I I I ) ) ,

(III)

Monomer 1 ¥

Ο Polymer

M

o

n

o

m

\

R

R £

(

C

H

9

Ο 0C(CHO

)

2

κ

0"

2 5

5

11 Once p r o d u c e d a n i o n i s then the p r o p a g a t i n g mediate u n t i l t h e f i n a l polymer i s formed. (c)

Coordination

inter

Type

The c o o r d i n a t i o n c a t a l y z e d p o l y m e r i z a t i o n i s de f i n e d f o r t h e purpose o f t h i s paper as one w h i c h i n volves a concerted i n s e r t i o n with concurrent cleavage o f a c o v a l e n t p o l y m e r - c a t a l y s t bond(£) . I t i s i l l u s t r a t e d i n Equation (IV).

ÇJ Ο II

R-M 12

+

0'' \ R

II

V

"I Ο il R-C-(CH )

-0-M

9

5

Z

(IV)

13 Monomer

Ο Monomer M *t Polymer ^ R-C-(CH ) - 0 - C - ( C H ) -OM 5. 0

2

0

5

2

Note t h a t t h e c a t i o n i c and a n i o n i c mechanisms as d e p i c t e d above a r e " l i m i t i n g " c a s e s . Depending upon the r e a g e n t s and e x p e r i m e n t a l c o n d i t i o n s t h e "whole spectrum" o f mechanisms i s o b s e r v e d ( F i g u r e 1 ) . As shown t h e c o o r d i n a t i o n mechanism i s b a s i c a l l y the " i n t e r m e d i a t e " c a s e between t h e two o t h e r modes o f reaction.


11.

Polymerization

YOUNG ET A L .

of

^-Caprolactone

155


Both the p o l y m e r i z a b i l i t y and the mechanism o f p o l y m e r i z a t i o n a r e o b v i o u s l y dependent on the r i n g s i z e o f the l a c t o n e :

ό ô ô

α *

An i n t e r e s t i n g g e n e r a l i z a t i o n i n t h i s r e g a r d was made by H a l e (8). He f i n d s t h a t (1) i n t h e c a s e o f t h e f i v e - and six-membered r i n g - c o n t a i n i n g monomers, t h e ease o f p o l y m e r i z a t i o n v a r i e s w i t h the c l a s s t o which t h e monomers b e l o n g ( i . e . , l a c t o n e , u r e a , i m i d e , a n h y d r i d e , l a c t a m , e t c . ) ; (2) monomers c o n t a i n i n g f o u r - , seven-, and eight-membered r i n g s appear t o p o l y m e r i z e i n a l l c a s e s ; and (3) a l k y l o r a r y l subs t i t u t i o n o f the r i n g has a d e l e t e r i o u s e f f e c t on the polymerization. Experimental

Approach

A c o r r e l a t i o n between t h e i n t r i n s i c v i s c o s i t y o f p o l y - £ - c a p r o l a c t o n e and i t s w e i g h t - a v e r a g e m o l e c u l a r w e i g h t has been r e p o r t e d p r e v i o u s l y ( 3 ) · A commonly known r e l a t i o n s h i p between the m e l t v i s c o s i t y a t e l e v a t e d t e m p e r a t u r e s and the w e i g h t - a v e r a g e m o l e c u l a r w e i g h t has been shown(SO t o a l s o h o l d f o r p o l y - £ c a p r o l a c t o n e (Equation V I ) . [n] = 9.9

χ 10"

5

μ=α M

Mw w

0

-

8

2

( i n benzene)

(V)

3.4

(VI)

Thus, an e x c e l l e n t m o n i t o r f o r f^cap^olactone p o l y m e r i z a t i o n s i s f o l l o w i n g the v i s c o s i t y as a f u n c t i o n of time. T h i s p r o c e d u r e was adapted and a t y p i c a l " r e a c t i o n p r o f i l e " ( a t 204°C, neat) i s shown i n F i g u r e 2. There a r e e s s e n t i a l l y t h r e e s t a g e s o f the reaction. F o r t h e time p e r i o d o f t to t ^ a r a p i d r i s e i n v i s c o s i t y i s o b s e r v e d , d e s i g n a t e d as p o r t i o n a o f the curve. A t time t i t h e v i s c o s i t y r e a c h e s i t s maximum v a l u e , v i . F o l l o w i n g t h i s , a d e c r e a s e o f the v i s c o s i t y t a k e s p l a c e , a l t h o u g h the change i s n o t as r a p i d ( p o r t i o n b o f the c u r v e ) . A t time t 2 , the v i s c o s i t y l e v e l s o f f t o a p r a c t i c a l l y c o n s t a n t v a l u e , V2 (por t i o n c of the c u r v e ) . 0


156

RING-OPENING

POLYMERIZATION

The shape o f t h e c u r v e which r e f l e c t s changes r e l a t e d t o M^ i s s i g n i f i c a n t and has a d i r e c t b e a r i n g on t h e r e a c t i o n mechanism. I t i s d i f f e r e n t from t h e results reported f o r solution polymerizations carried o u t a t lower temperatures (10) .


Kinetic

Considerations

The o v e r a l l mechanism o f an a n i o n i c - c o o r d i n a t i o n c a t a l y z e d p o l y m e r i z a t i o n o f £*-caprolactone i s depen dent upon a number o f f a c t o r s . The most i m p o r t a n t o f t h e s e a r e t h e type o f c a t a l y s t and whether a c o i n i t i a t o r i s u s e d . A v e r y l a r g e number o f b o t h have been r e p o r t e d (_2) . I f RjM r e p r e s e n t s t h e c a t a l y s t - i n i t i a t o r , R20H an a c t i v e hydrogen c o n t a i n i n g c o i n i t i a t o r , and CL the f - c a p r o l a c t o n e monomer, t h e f o l l o w i n g s t e p s have t o be c o n s i d e r e d i n o r d e r t o a r r i v e a t a m e a n i n g f u l k i n e t i c expression: (a) P r e e q u i l i b r i u m : K

R -M + R OH 1

(b)

2

l ^

R -H + R -OM

(1)

Ο H R..-C-(CH ) -OM 5

(2)

^1

1

2

Initiation: 2 R,-M + CL — = - ^ » ^ΚΖΓ" "2 κ

1

0

1

2

Ο Κ. R -0M + CL ^ - ^ R -0-C-(CH ) -OM 2 ^κΖΓ 5 "3 Ο Κ 4 » R--OH + CL — ^ S. » R ) -OH τ» -0-C-(CH r\ r% o

o

o

(3)

o

o

(4)

2

1

2

^7-

2

2

5

We have shown t h a t r e a c t i o n (4) i s v e r y slow i n comparison t o r e a c t i o n s (2) and ( 3 ) . When ζ-capro l a c t o n e i s h e a t e d w i t h an a l c o h o l i n t h e absence o f any c a t a l y s t under o u r normal e x p e r i m e n t a l c o n d i t i o n s no v i s c o s i t y change was o b s e r v e d .


Polymerization

11. Y O U N G E T A L .

of

^-Caprolactone

157

(c) P r o p a g a t i o n : 0

„

«

K

Rj-C-ÎCHp

-OM + CL 5

0

c

II

^ Rj-C-ÎCHp

"^5

-0-C-(CH >

Il

K

o

-OM + C L

o

5

° κ

O

II

6

N

e t c . (5)

5

O

R 0C-(CH )

-OM ^r>

2

5

O


0

II

R 0-C-(CH ) o

o

5

-6

II

- 0 - C - ( C H ) -OM — > o

5

e t c . (6)

(d) T e r m i n a t i o n : R

o

5

OM + R 0 R —

* R « 0 - C - ( C H ) > OH + R 0 M 5 η

o

o

η

o

(7)

(e) C h a i n T r a n s f e r : Il

M

R-(0-C-(CH ) > OM 5 η O 9

K

Q

+ R-(C-C-(CH ) )• OH 5 m K_ O

s

9

z

X

Q 8

R-fO-C(CH ) )· -OH + R-fO-C-(CH ) >· OM 5 η 5 m 2

2

(8)

(f) E s t e r I n t e r c h a n g e (both i n t r a - and i n t e r - m o l e c u l a r ) :

(9) The r e l a t i v e r a t e s o f t h e above p r o c e s s e s w i l l d e t e r m i n e t h e k i n e t i c s o f t h e p o l y m e r i z a t i o n and, c o n s e q u e n t l y , t h e m o l e c u l a r weight o f t h e polymer and i t s m o l e c u l a r weight d i s t r i b u t i o n . Needless t o say, t h i s i s a complex r e a c t i o n and t h e d a t a t h a t f o l l o w s must be c o n s i d e r e d i n t h a t c o n t e x t .


RING-OPENING POLYMERIZATION

158


RESULTS AND

DISCUSSION

Various authors (2^£, 11_) have suggested that the polymerization of ^-caprolactone proceeds v i a a " l i v i n g " mechanism. This should y i e l d polymers with very narrow molecular weight d i s t r i b u t i o n s r e f e r r e d to as a "Poisson d i s t r i b u t i o n " ( 3 , 1 2 ) . In t h i s case, the k i n e t i c s of the polymerization would be expected to be rather simple and s t r a i g h t forward. Figure 3 i l l u s t r a t e s three possible v i s c o s i t y p r o f i l e s f o r the polymerization of ξ-caprolactone at elevated temperatures. A Poisson molecular weight d i s t r i b u t i o n occurs only i f the following requirements are f u l f i l l e d : 1) the rate of i n i t i a t i o n i s much f a s t e r than the rate of polymerization; 2) propagation occurs by a d d i t i o n of the monomer to the polymer chain end; and 3) there i s no termination, chain t r a n s f e r or any other secon dary r e a c t i o n . I f the polymerization of Ç-caprolactone were to proceed i n t h i s manner (M /M =l) i t would be followed by ester interchange reactions u n t i l the establishment of the most probable d i s t r i b u t i o n . This would r e s u l t i n a continuing increase i n M and hence of the melt v i s c o s i t y (Figure 3). The polymerization of £-caprolactone does i n f a c t f u l f i l l the second requirement above. However, f u l f i l l m e n t of conditions one and three are questionable making two a l t e r n a t i v e polymerization p r o f i l e s p o s s i b l e . In t h e _ f i r s t a l t e r n a t i v e the "normal" d i s t r i b u t i o n of M /M =2 i s established during the r e a c t i o n . In that case, no further change i n M i s expected i r r e g a r d l e s s of the f a c t that ester-interchange may continue to occur. As a r e s u l t the melt v i s c o s i t y of the polymer a f t e r having reached a plateau would r e main e s s e n t i a l l y constant. Another a l t e r n a t i v e c o n s i s t s i n the polymerization reaching a molecular weight d i s t r i b u t i o n of >2. The subsequent ester interchange reactions should then r e s u l t i n a decrease i n and melt v i s c o s i t y u n t i l the normal d i s t r i b u t i o n i s reached. A t y p i c a l p r o f i l e f o r melt v i s c o s i t y as a function of time f o r a polymerization r e a c t i o n was shown i n Figure 2. The shape of the curve r e f l e c t s the k i n e t i c processes which are occuring during the polymerization. A l l of the reactions were c a r r i e d out neat, at A-»200°C. The v i s c o s i t y / t i m e p r o f i l e s that were observed with both anionic and coordination type c a t a l y s t s were e s s e n t i a l l y the same. The data do not f i t a normal " l i v i n g " mechanism, with no side r e a c t i o n s . There i s w

n

w

w

n

w


11.

Polymerization

YOUNG E T AL.

όRÔ--M

ι


R-C II ο

6+

R-0

ι O-R

159

of ^-Caprolactone

;

—>

ι

2

/

\

/ / /

Φ

/

\ \

NORMAL MW/MN=2

\ V

POISSON MW/MN=1

2

Time Figure 3.

Suggested melt viscosity profiles for polymerization of ^-caprolactone



160

RING-OPENING POLYMERIZATION

no doubt that i n addition to the usual i n i t i a t i o n and propagation steps, several other processes are occurring. These include ester interchange reactions, side-reactions i n v o l v i n g the c o - i n i t i a t o r , chain t r a n s f e r phenomena, formation of c y c l i c s i n depolymerization reactions, etc. As stated above, the v i s c o s i t y p r o f i l e had the same general shape both i n the presence and absence of a c o - i n i t i a t o r (Figure 4) and c a t a l y s t (Figure 5). The concentration of the c a t a l y s t has a s i g n i f i c a n t e f f e c t on both the rate of polymerization and on the rate of subsequent reactions r e s u l t i n g i n the decrease i n melt v i s c o s i t y (Figure 5). In order to r a t i o n a l i z e these r e s u l t s , molecular weight d i s t r i b u t i o n measurements were performed as a function of reaction time and the r e s u l t s are recorded i n Figure 6. The v a r i a t i o n i n molecular weight d i s t r i b u t i o n with time i n d i c a t e s that M i s i n i t i a l l y w

>2 and that M decreases with time. This i s consistent with the pattern predicted from v i s c o s i t y measurements (Figures 2, 4 and 5). In addition to ester interchange reactions, there i s a second post-polymerization equilibrium r e a c t i o n that occurs. I t was possible to show reformation of c y c l i c monomer as w e l l as the formation of other c y c l i c oligomers. Gas l i q u i d phase chromatography was c a r r i e d out during the course of the polymerization. I t was observed (Figure 7) that there i s a decrease i n monomer concentration to an equilibrium l e v e l (0.2%). Simultaneously the appearance of both c y c l i c dimer and c y c l i c trimer oligomers formed i n a depolymerization reaction was noted. w

0

0

It

0

»

II

^v^O-C-C-C-C-C-C-C-0-C-C-C-C-C-C-C-0-C-C-C-C-C-C-C-OH

Trimer

Dimer

Monomer



YOUNG E T AL.

Polymerization

of

^-Caprolactone

TIME

Figure 4. Viscosity-time relationship for the polymerization of ccaprolactone in the presence of an anionic catalyst with a hydroxyl containing co-initiator

' TIME

Figure 5. Polymerization t-caprolactone at constant co-initiator concentration (alcohol) at different levels of typical coordination catalysts


RING-OPENING

POLYMERIZATION


3.0r-

1.0»

»

1

1

1

1

TIME

Figure 6. Change in molecular weight distribution for the polymerization of e-caprohctone

23

CYCLIC DIMER CYCLIC TRIMER CYCLIC MONOMER

TIME

Figure 7. Concentration of cyclic oligomers as a function of time for high-temperature polymerizations of ^-caprolactone


11.

YOUNG ET AL.

Polymerization of ^-Caprolactone

163

I t i s i n t e r e s t i n g t h a t these depolymerization p r o c e s s e s have v e r y s i m i l a r appearance p r o f i l e s t o t h o s e o f polymer v i s c o s i t y and . However, t h e y do n o t o c c u r a t s u f f i c i e n t l y h i g h l e v e l s t o a f f e c t either M o r t h e o v e r a l l m e l t v i s c o s i t y o f the r e a c t i o n mixture. In s t e p r e a c t i o n (condensation) p o l y m e r i z a t i o n s , the m o l e c u l a r w e i g h t d i s t r i b u t i o n i s r e l a t e d t o the number o f p r o p a g a t i n g p o l y m e r i c b r a n c h e s . The mole c u l a r w e i g h t d i s t r i b u t i o n becomes narrower w i t h i n c r e a s i n g f u n c t i o n a l i t y , as shown i n the e q u a t i o n below:


w

M

w,/ w

= ι + 1±

where f i s t h e number o f p o l y m e r i c b r a n c h e s . The e f f e c t o f the number o f r e a c t i v e s i t e s o f t h e c o - i n i t i a t o r upon the m o l e c u l a r w e i g h t d i s t r i b u t i o n was e s t a b l i s h e d . T h i s was r e l a t i v e l y easy t o p e r f o r m by s i m p l y u s i n g mono- and d i h y d r o x y compounds as c o initiators. As p r e d i c t e d , a t the maximum M (νχ, F i g u r e 2) the v a l u e o f M ^-^ was h i g h e r when w

w

the monohydroxy i n i t i a t o r was used. I t i s c l e a r from o u r d a t a t h a t the h i g h tempera t u r e n e a t r e a c t i o n i s e x t r e m e l y complex. I t may be due i n p a r t t o the s e v e r e p o l y m e r i z a t i o n c o n d i t i o n s which were used. These r e s u l t s a r e n o t i n agreement w i t h t h o s e r e p o r t e d by T e y s s i e ( 1 0 ) . Under m i l d e r r e a c t i o n c o n d i t i o n s he has o b s e r v e d t h e f o r m a t i o n o f a polymer w i t h a m o l e c u l a r w e i g h t d i s t r i b u t i o n c l o s e t o one. He d e s c r i b e s the p o l y m e r i z a t i o n as a " p e r f e c t l y ' l i v i n g ' process". The d i f f e r e n c e i n r e a c t i o n c o n d i t i o n s c o u l d have caused t h e o b s e r v e d d i f f e r e n c e s . F u r t h e r work w i l l be r e q u i r e d t o make t h e s e d i s c r e pancies f u l l y understood. CONCLUSIONS The r i n g o p e n i n g p o l y m e r i z a t i o n o f Ç-caprolactone a t h i g h temperatures f o l l o w s a p a t t e r n w h i c h i s r a d i c a l l y d i f f e r e n t from t h e one o b s e r v e d i n s o l u t i o n under m i l d low temperature c o n d i t i o n s . I t i s p o s t u l a t e d t h a t the phenomenon i s b e s t e x p l a i n e d by assuming t h a t s e v e r a l secondary p r o c e s s e s o c c u r s i m u l t a n e o u s l y w i t h the p r i m a r y i n i t i a t i o n and p o l y m e r i z a t i o n r e a c t i o n s .


164

RING-OPENING

POLYMERIZATION


Literature Cited (1) VanNatta, F. J., Hill, J. W. and Carothers, W. E., J. Amer. Chem. Soc. (1934), 56, 455 . (2) Lundberg, R. D. and Cox, E. F. "Ring-Opening Polymerization", Ed. Frisch, K. C. and Reagen, S. L.; Marcel Dekker, Ν. Y. (1969), pp. 247-302. (3) Lundberg, R. D., Koleske, J. V. Wischman, Κ. B., J. of Poly. Sci. (1969), 7, 2915 . (4) Brode, G. L. and Koleske, J. V., J. Macromol. Sci. (1972), A6(6), 1109 . (5) Cox, E. F. and Hostettler, F. (to Union Carbide Corp.) U.S. Patents 3,021,309 (1962). (6) Ludwig, Ε. B. and Bebenbaya, B. G., J. Macrmol. Sci. (1974) A8 (4), 819 . (7) Sekiguchi, H. and Clarisse, C., Die Makromol. Chem. (1976), 177, 591. (8) Hall, H. K. and Schneider, A. K., J. Amer. Chem. Soc. (1958), 80, 6409 . (9) Jones, T. R., Union Carbide, unpublished results. (10) Ouhadi, T., Stevens, C. and Teyssie, P., Die Makromol. Chem. Suppl. (1975), 1, 191 . (11) Teyssie, P., Provate Communication. (12) Billmeyer, F. W., Jr., "Textbook of Polymer Science", 2nd Ed., Wiley, Interscience, New York (1971).


Ring-Opening Polymerization

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