Ring-Opening Polymerization

methyl-α-propyl-β-propiolactone showed that two crystalline forms were possible ... quite different. In the as-prepared form, after precipitation fr...
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15 Rate and Stereochemistry of the Anionic Polymerization

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of α,α-Disubstituted-β-propiolactones ROBERT W. LENZ, CHRISTIAN G. D'HONDT, and EBRAHIM BIGDELI Polymer Science and Engineering Program, Chemical Engineering Department, University of Massachusetts, Amherst, MA 01003

Previous investigations in this laboratory (1) and else­ where (2) have shown that polyesters prepared from chiral α,αdisubstituted-β-propiolactones by the following reactions:

are crystalline even when the two substituents, R1 and R2, are considerably different in size (e.g., R1 = CH3, R2 = C3H7). This observation is surprising because the anionic polymerization re­ action used is homogeneous in character, and no heterogeneous, stereoregular catalysts are required to achieve the polymer crys­ tallinity observed. Furthermore, crystalline structure deter­ minations by wide-angle x-ray diffraction analysis of poly-αmethyl-α-propyl-β-propiolactone showed that two crystalline forms were possible depending upon sample preparation and treatment. These two forms consisted of unit cells in which the polymer was present as either a 21 helix or as a fully-extended, planar zig­ zag conformation (1). One purpose of the present investigation was to obtain some additional information on structure-crystallinity relationships in this family of polymers by the preparation of stereoregular isotactic polyesters from a single asymmetric isomer of the chiral monomer; that is, from an o p t i c a l l y - a c t i v e α,α-disubstituted-βpropiolactone. Because the polymerization r e a c t i o n mechanism operates through s c i s s i o n of the alkyl-oxygen bond and does not i n v o l v e bond r e o r g a n i z a t i o n s a t the asymmetric c e n t e r , i t was f u l l y expected that polymerization of the o p t i c a l l y - a c t i v e monomer

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15. LENZ ET AL.

Polymerization

of a,a-Disubstituted-fi-propiofoctones

211

would occur with complete r e t e n t i o n to y i e l d an i s o t a c t i c polymer i n which the m a j o r i t y o f the repeating u n i t s had the same abso­ l u t e c o n f i g u r a t i o n depending upon the o p t i c a l p u r i t y of the mono­ mer. A comparison o f the c r y s t a l l i n e p r o p e r t i e s o f that polymer with the one prepared from the racemic monomer should help to ex­ p l a i n the s t r u c t u r a l basis f o r the c r y s t a l l i n i t y observed f o r the latter (1). Another goal o f t h i s study was to determine the important parameters which determine the r a t e s o f polymerization of these c h i r a l 3-lactones. This study i s p r e s e n t l y d i r e c t e d a t i n v e s t i ­ gating the e f f e c t o f r e a c t i o n v a r i a b l e s (solvent and counterion) on polymerization r a t e , and i n the f u t u r e , attempts w i l l be made by r a t e s t u d i e s to a s c e r t a i n i f s t e r e o e l e c t i o n e x i s t s i n t h i s homogeneous, a n i o n i c polymerization r e a c t i o n . Polymerization o f O p t i c a l l y - A c t i v e Lactone α - P h e n y l - α - e t h y l - 3 - p r o p i o l a c t o n e , PEL, was chosen as the monomer f o r i n v e s t i g a t i o n because the intermediate amino e s t e r had p r e v i o u s l y been resolved (3). Both the racemic and o p t i c a l l y - a c ­ t i v e monomers were converted i n t o t h e i r p o l y e s t e r s i n homogeneous systems using tetraethylammonium benzoate as the i n i t i a t o r i n tetrahydrofuran solvent a t room temperature. The p o l y m e r i z a t i o n of PEL was q u i t e slow under these c o n d i t i o n s and several days were r e q u i r e d to achieve high conversions o f the monomer. Because t h i s i s a " l i v i n g polymer" system, high r e a c t i o n conversions were r e ­ quired f o r the formation o f high molecular weight polymers. Polymers obtained from PEL monomers o f d i f f e r e n t o p t i c a l pu­ r i t y were c h a r a c t e r i z e d as f o l l o w s : (1) f o r c r y s t a l l i n e proper­ t i e s by d i f f e r e n t i a l scanning c a l o r i m e t r y (DSC) and wide-angle x ray d i f f r a c t i o n ; (2) f o r r e l a t i v e molecular weights by s o l u t i o n v i s c o s i t y ; (3) f o r s t r u c t u r e by IR and NMR spectroscopy; and (4) f o r c h i r o p t i c a l p r o p e r t i e s i n s o l u t i o n by o p t i c a l r o t a t o r y d i s p e r ­ s i o n (ORD) and c i r c u l a r dichroism (CD). Molecular weight and melting point data f o r both the racemic and o p t i c a l l y - a c t i v e PEL polymers are c o l l e c t e d i n Table I. Table I. P r o p e r t i e s o f Poly-a-Phenyla-Ethyl-3-Propiolactone Polymer P r o p e r t i e s M

n

T| ,°C tl

50

50

9500

110

27

73

7000

116

90

10

7700

260

Saegusa and Goethals; Ring-Opening Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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RING-OPENING POLYMERIZATION

The l a r g e d i f f e r e n c e between the melting points i n Table I o f the h i g h l y o p t i c a l l y pure, i s o t a c t i c PEL polymer, on the one hand, compared to those o f e i t h e r the racemic or p a r t i a l l y o p t i ­ c a l l y pure PEL polymer, on the o t h e r , must be a d i r e c t r e s u l t of the e f f e c t of d i f f e r e n c e s i n s t e r e o r e g u l a r i t i e s o f the two types o f polymers on c r y s t a l l i n e p r o p e r t i e s . Molecular weight was ap­ p a r e n t l y not an important f a c t o r f o r t h i s property because e q u i v ­ a l e n t values were obtained f o r a l l three polymers i n Table I. U n f o r t u n a t e l y , the t a c t i c i t y d i f f e r e n c e s expected f o r these p o l y ­ mers could not be determined q u a n t i t a t i v e l y by e i t h e r the IR spectra of polymer f i l m s or the NMR spectra a t 90 MHz of the p o l y ­ mers i n s o l u t i o n . However, wide-angle x - r a y d i f f r a c t i o n measure­ ments showed c o n s i d e r a b l y d i f f e r e n t c r y s t a l l i n e patterns and 2θ values f o r the o p t i c a l l y - a c t i v e PEL polymer o f high o p t i c a l p u r i t y on the one hand, as compared to e i t h e r the racemic polymer o r that with low o p t i c a l p u r i t y on the o t h e r , as shown by the data in Table II, i n d i c a t i n g that the c r y s t a l l i n e s t r u c t u r e s o f these were quite d i f f e r e n t . In the as-prepared form, a f t e r p r e c i p i t a t i o n from s o l u t i o n , both types o f polymers appeared to be h i g h l y c r y s t a l l i n e by t h i s method o f a n a l y s i s , and both were comparable i n t h i s property to p o l y p i v a l o l a c t o n e , which i s known to be a very h i g h l y c r y s t a l l i n e polyester. In a d d i t i o n , both the o p t i c a l l y - a c t i v e and racemic polymers had c o n s i d e r a b l y higher degrees o f c r y s t a l l i n i t y than those p r e v i o u s l y observed f o r racemic poly-a-methyl-α-propyl-3p r o p i o l a c t o n e Q). Also of importance, i n a d d i t i o n to the d i f ­ f e r e n t x - r a y d i f f r a c t i o n patterns of the racemic and o p t i c a l l y a c t i v e polymers, was that the racemic polymer d i d not r e a d i l y c r y s t a l l i z e from the melt i n the DSC c h a r a c t e r i z a t i o n while the o p t i c a l l y - a c t i v e polymer of high o p t i c a l p u r i t y d i d . Hence, the higher s t e r e o r e g u l a r i t y a l s o imparts a more f a v o r a b l e r a t e o f c r y s t a l l i z a t i o n to the polymer as would be expected. It seems l i k e l y that the observed d i f f e r e n c e s i n c r y s t a l l i n e p r o p e r t i e s between the o p t i c a l l y a c t i v e and racemic PEL polymers c l e a r l y i n d i c a t e s that the c r y s t a l l i n e regions i n the l a t t e r are not simply formed from p h y s i c a l l y - s e p a r a t e d blocks o f R u n i t s and S units. Nevertheless, such block arrangements of c h i r a l u n i t s could s t i l l be present i n the polymer and form a d i f f e r e n t type of c r y s t a l l a t t i c e than the separate R or S polymers as was found i n the c r y s t a l s t r u c t u r e determination of i s o t a c t i c , racemic p o l y ( t butylethylene o x i d e ) ( 4 ) . Another p o s s i b i l i t y to account f o r the d i f f e r e n t c r y s t a l l i n e p r o p e r t i e s o f the racemic polymer i s t h a t , because o f strong asym­ metric s e l e c t i v e e f f e c t s r e s u l t i n g from s t e r i c i n t e r a c t i o n s , the polymerization o f the racemic monomer favors the enchainment of a l t e r n a t i n g R and S u n i t s ; t h a t i s the formation o f a h i g h l y synd i o t a c t i c polymer. Both x - r a y d i f f r a c t i o n and k i n e t i c studies are i n progress to attempt to e l u c i d a t e t h i s q u e s t i o n , but the high degree o f c r y s t a l l i n i t y and ease o f r e c r y s t a l l i z a t i o n o f the 80% o p t i c a l l y - a c t i v e polymer does not support t h i s p o s s i b i l i t y be-

Saegusa and Goethals; Ring-Opening Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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Polymerization

of «,a-Disubstituted-fi-propiolactones

213

cause an a l t e r n a t i n g asymmetric s e l e c t i v i t y would reduce the s t e r e o r e g u l a r i t y o f that polymer. Indeeed, the s u p e r i o r c r y s t a l ­ l i n e p r o p e r t i e s o f the o p t i c a l l y - a c t i v e polymer are s u r p r i s i n g f o r a monomer o f t h i s o p t i c a l p u r i t y , even i f i t i s assumed that propagation i s n o n - s e l e c t i v e or B e r n o u l l i a n i n c h a r a c t e r . That i s , t h i s degree of monomer o p t i c a l p u r i t y should lead to a p o l y ­ mer o f only approximately 73% i s o t a c t i c t r i a d content (or 82% isotactic diads). Rate I n v e s t i g a t i o n s The r a t e o f polymerization o f racemic PEL was determined i n two d i f f e r e n t s o l v e n t s , tetrahydrofuran (THF) and dimethyl s u l ­ f o x i d e (DMSO), a t two d i f f e r e n t temperatures with tetraethylammo­ nium benzoate as i n i t i a t o r , and the r e s u l t s are c o l l e c t e d i n Table III, The r e a c t i o n s were followed by i n f r a r e d spectroscopy based upon the carbonyl group absorption i n t e n s i t i e s f o r the monomer and polymer, and the data was treated according to the f o l l o w i n g p s e u d o - f i r s t order r a t e equation: *n[M] = * n [ M ]

0

-

k t a

The absolute propagation r a t e constant, kp of Table III, was c a l ­ c u l a t e d from the apparent r a t e constant, ka, by d i v i d i n g by the i n i t i a t o r concentration. The k i n e t i c r e s u l t s f o r the racemic PEL monomer reveal that the value o f kp i s somewhat higher i n THF than i n DMSO a t 35°C. The average value f o r kp i n THF was 9.5 M-l min-1 compared to 8.3 M-l min-1 i n DMSO a t t h i s temperature, i n d i c a t i n g a s i g n i f i ­ c a n t l y lower a c t i v a t i o n energy f o r the former r e a c t i o n . Very s i m i l a r r e s u l t s were obtained p r e v i o u s l y in t h i s l a b o r a t o r y f o r the a n i o n i c polymerization of α-methyl-α-butyl-3-propiolactone (5), and the present authors have a l s o confirmed the e x i s t e n c e of t h i s solvent e f f e c t i n the e q u i v a l e n t polymerization r e a c t i o n s of the α-ethyl and α-propyl monomers of t h i s s e r i e s . The cause o f t h i s unexpected solvent e f f e c t i s not y e t known, but i t may be r e l a t e d to s p e c i f i c s o l v a t i o n c h a r a c t e r i s ­ t i c s o f the ion p a i r endgroups (5J. That i s , i t i s p o s s i b l e that the lower r a t e constant f o r the solvent o f higher p o l a r i t y , DMSO, may be an i n d i c a t i o n o f the formation of a s t r u c t u r e d ion p a i r between t h i s s o l v e n t , the c a r b o x y l a t e anion and the ammonium c o u n t e r i o n , as f o l l o w s :

V

CM, CH

3

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Table II. Peak Angles and I n t e n s i t i e s o f X-ray D i f f r a c t i o n Spectra o f Poly-aPhenyl-o-Ethyl-e-Propiolactone Racemic Polymer Angle

O p t i c a l l y - A c t i v e Polymer

Intensity

7.8 9.2

m s

14.5° 15.0°

s m

Angle

Intensity

9.5° 14.5°

s m

15.3° 16.0° 19.5° 21.3°

17.5° 19.5° 21.4°

m w

w m w

Table III. SOLVENT

THF II II DMSO II II THF II II DMSO II II

Rate Constants f o r the P o l y m e r i z a t i o n o f ot-Phenyl-o-Etnyl-B-Propiolactone [M] [Ι] k k T, C » u J -1 -l .-1 M M min M min 0

0

e

P

M

24 II II 24 II II 35 II II 35 II II

0.0123 0.1254 0.1147 0.1410 0.1320 0.1382 0.2076 0.2153 0.2412 0.2307 0.2152 0.2417

0.0057 0.0059 0.0068 0.0076 0.0098 0.0087 0.0056 0.0068 0.0056 0,00698 0.00696 0.0082

0.0243 0.0233 0.0279 0.0322 0.0379 0.0341 0.0551 0.0633 0.0533 0.0551 0.0614 0.068

4.3 4.0 4.1 4.2 3.9 3.9 9.8 9.3 9.5 7.9 8.8 8.3

Saegusa and Goethals; Ring-Opening Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

15. LENZ ET AL.

Polymerization

of