Ring-Opening Polymerization - American Chemical Society

16 Polymerization of Substituted Oxiranes, Epoxy Aldehydes, and Derived Oxacyclic Monomers Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 21, 2015 | http://pubs.acs.org Publication Date: August 16, 1985 | doi: 10.1021/bk-1985-0286.ch016

Z. J. JEDLIŃSKI, M . BERO, J . K A S P E R C Z Y K , and M . K O W A L C Z U K Institute of Polymer Chemistry, Polish Academy of Sciences, Curie-Sklodowskiej 34, 41-800 Zabrze, Poland

This paperisa review of studies on the ring-opening polymerization of cyclic ethers, e.g., styrene oxide, phenyl glycidyl ethers, epoxy aldehydes and derived oxacyclic monomers. Model reactions involving ring -openingprocesses occurring in those compounds have also been discussed.

Many papers have been p u b l i s h e d concerning the s t r u c t u r e o f the a c t i v e centers i n anionic and c a t i o n i c ring-opening polymerization reactions of o x a c y c l i c monomers. Recently, a t t e n t i o n has been paid i n our l a b o r a t o r y t o the i n f l u e n c e of the s t r u c t u r e of complex carbonium s a l t i n i t i a t o r s , e s p e c i a l l y of the dioxolanylium s a l t s used f o r i n i t i a t i n g t h e c a t i o n i c p o l y m e r i z a t i o n r e a c t i o n s o f t r i o x a n e , t e t r a h y d r o f u r a n and d i o x o l a n e , on the course o f t h e polymerization (1_). In the present report some examples of the influence of monomer structure, e s p e c i a l l y of s t e r i c hindrance and e l e c t r o n i c e f f e c t s , on the mechanism o f p o l y m e r i z a t i o n and on the nature of the a c t i v e centers formed i n the anionic p o l y m e r i z a t i o n o f c e r t a i n o x a c y c l i c monomers are discussed. Polymerization of Styrene Oxide In t h e p o l y m e r i z a t i o n of s u b s t i t u t e d o x i r a n e s , t h e d i r e c t i o n o f opening o f t h e epoxy r i n g i s o f great importance because i t 0097-6156/85/0286-O205$06.00/0 © 1985 American Chemical Society

In Ring-Opening Polymerization; McGrath, James E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

206

RING-OPENING POLYMERIZATION

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 21, 2015 | http://pubs.acs.org Publication Date: August 16, 1985 | doi: 10.1021/bk-1985-0286.ch016

determines both the k i n d of a c t i v e c e n t e r s present i n t h e polymerization reaction and the molecular structure and properties of the p o l y e t h e r s formed. I n t h i s regard, three d i f f e r e n t epoxyr i n g - o p e n i n g r o u t e s a r e p o s s i b l e : (1) β-ring opening; t h a t i s , opening of the O-CH2 bond; (2) α-ring opening, t h a t i s , opening o f the 0-CHR bond; and (3) combined a- and β-ring opening, as shown below:

POLYMER:

SCISSION: RRR...

..sss...

isotactic or atactic

\

CH — CHn

irregular

** Ο (R) • (S)

1. inversion 2. retention

or

)

isotactic SSS. .1 isotactic

.RRR.. + .

SSRSRSS. 3. racemization. .SSRSRSS.

atactic atactic

As a r e s u l t o f the β-ring opening, the c o n f i g u r a t i o n o f t h e asymmetric carbon atom remains unchanged. However, the α-ring opening may take place with e i t h e r an i n v e r s i o n or retention of the configuration, depending on reaction conditions. C e r t a i n g e n e r a l r u l e s determining the conditions f o r a- or βring-opening processes have been established from the i n v e s t i g a t i o n s of C. C. P r i c e (2^) and E. Vandenberg (3) and the g e n e r a l view i s t h a t i n the a n i o n i c p o l y m e r i z a t i o n o f epoxides, t h e r i n g opening occurs at the β p o s i t i o n , while f o r c a t i o n i c i n i t i a t o r s , both a- and β-ring opening take place simultaneously with the formation of both c y c l i c oligomers and l i n e a r oligomers containing i r r e g u l a r head-tohead and regular h e a d - t o - t a i l sequences. This l a t t e r t y p i c a l course f o r these p o l y m e r i z a t i o n r e a c t i o n s has been found t o occur i n the case of phenyl g l y c i d y l ethers polymerized by Lewis acids, and a l s o quite s u r p r i s i n g l y , polymerized by aluminum alkoxides (4). There a r e , however, numerous e x c e p t i o n s t o those r u l e s . F o r i n s t a n c e , Tsuruta showed t h a t t - b u t y l o x i r a n e p o l y m e r i z e d i n t h e presence of BF3 gave a polymer with regular h e a d - t o - t a i l sequences, by an almost e x c l u s i v e l y β-ring-opening process (5). Such a course f o r the polymerization reaction r e s u l t s from the considerable s t e r i c hindrance provided by the bulky t - b u t y l substituent. In recent studies of styrene oxide polymerization reactions we found the phenyl substituent t o have a s i g n i f i c a n t i n f l u e n c e on the course of the polymerization process, too. In our p a r t i c u l a r case, however, the influence i s due not only to s t e r i c f a c t o r s , but a l s o t o the i n d u c t i v e e f f e c t s o f t h e phenyl r i n g , which i n f l u e n c e s d i r e c t l y the course of the oxirane ring-opening reaction. 9


16.

Polymerization of Cyclic Ethers: Review

JEDLINSKI ET AL.

207


Anionic Polymerization by Sodium Methoxide Catalyst The d i r e c t i o n of the ring opening i n the anionic polymerization of styrene oxide was determined both by analyzing the products of model reactions i n v o l v i n g the addition of a l c o h o l s to the monomer and by s t r u c t u r a l s t u d i e s of the polymers and o l i g o m e r s obtained. The model addition reactions of a l c o h o l s to styrene oxide, catalyzed by the sodium alkoxide, showed that the oxirane ring opens i r r e g u l a r l y , both i n the a- and β-positions according to the a l c o h o l i n v o l v e d . The r e s u l t s of t h i s study are c o l l e c t e d i n T a b l e I. In the p o l y m e r i z a t i o n r e a c t i o n , polymer with a number average molecular weight of a p p r o x i m a t e l y 3,000 was obtained with CRÔNa i n i t i a t o r c o n c e n t r a t i o n of about 2 mole %. T h i s polymer was found to be a t a c t i c with a regular head-to-tail chain sequence as determined by i t s C NMR spectra i n Figure 1. With a high c o n c e n t r a t i o n of t h i s i n i t i a t o r (25 mole % ) , the formation of low molecular mass oligomers resulted. These oligomers c o n t a i n e d the l i n e a r dimers C and D shown below, i n about 30% by weight, and higher l i n e a r oligomers, E, i n about 70% by weight: 1 3

A

C

ι

E

ι

CKpCH^CHOH

957.

ι

CHÔCH^CHOCH^CHOH 95,3*

The dimer C i s formed by an i n i t i a l α-ring-opening r e a c t i o n followed by a β-ring-opening one, while the dimer D i s formed by a double β-ring-opening process. The reaction mixture was a l s o found to c o n t a i n about 1% by weight of the monomeric a l c o h o l s , A and B, formed by both a- and β-ring-opening processes. From these findings, and from r e s u l t s of the model reactions i n T a b l e I, which showed t h a t the b u l k i e r the s u b s t i t u e n t i n the a l c o h o l , the greater the p a r t i c i p a t i o n of the β-ring opening, i t i s p o s s i b l e to propose the f o l l o w i n g mechanism of i n i t i a t i o n and propagation f o r the p o l y m e r i z a t i o n of s t y r e n e oxide by the sodium methoxide:

0

^7 / R

0

CH-CH - O - C H j - C H - O *

Γ°

0

0 ~ C H2 - C H - 0

I

0, R-CH.-CH-O 1

I

0

0

v β

R-CH-CH-0-CH-CH-0*

ι

0

ι 0


208



ί Ί Γ «•CH -CM Of

9

h

4CH-CMJO^

m r I I

Figure 1.

13c-NMR 20 MHz s p e c t r a of p o l y ( s t y r e n e oxide) o b t a i n e d from the following polymerization reactions: (a) R,S - styrene oxide i n i t i a t e d with CRÔNa as i n i t i a t o r (b) R,S - styrene oxide catalyzed with Al(0iPr)3 as i n i t i a t o r (c) R(+) - styrene oxide i n i t i a t e d with OfÔNa as i n i t i a t o r (d) R(+) - styrene oxide catalyzed with Al(0iPr)3 as i n i t i a t o r


16.

JEDLINSKI ET AL.


209

According to these experimental r e s u l t s , the proposed reaction mechanism f o r the formation of p o l y ( s t y r e n e oxide) with a r e g u l a r chain structure by anionic polymerization i n v o l v e s the oxirane ring opening e x c l u s i v e l y at the β position. However, two kinds of a c t i v e c e n t e r s , A and Β i n the r e a c t i o n s above, occur i n the i n i t i a t i o n step. The a c t i v e center A, formed by α-ring opening, adds to a monomer m o l e c u l e i n the next step, but i n the second step the oxirane ring i s opened at the β position.


Polymerization by Aluminum Alkoxides Catalyst The polymerization of monosubstituted oxiranes catalyzed by aluminum a l k o x i d e s i s of p a r t i c u l a r i n t e r e s t from the p o i n t of view of the s t e r e o c h e m i s t r y of the ring-opening r e a c t i o n . The oxiranes which have been s t u d i e d to date, i n c l u d i n g propylene oxide and phenyl g l y c i d y l ethers, were found to polymerize with aluminum alkoxides to y i e l d polymers with an i r r e g u l a r c h a i n s t r u c t u r e c o n t a i n i n g both head-to-head and t a i l - t o - t a i l linkages (6). That i s , these polymers were formed as a r e s u l t of the oxirane r i n g opening at both the α and β positions, and they had the same chain microstructure as those obtained by the p o l y m e r i z a t i o n r e a c t i o n s i n i t i a t e d by standard c a t i o n i c c a t a l y s t s , such as Lewis acids. The polymerization of styrene oxide by such cationic i n i t i a t o r s as BF3H2O, SnCl4, FeCl3, e t c . does not l e a d to the formation of polymers with high m o l e c u l a r mass, but o n l y low m o l e c u l a r mass oligomers, both l i n e a r and c y c l i c , are formed. On the other hand, p o l y m e r i z a t i o n r e a c t i o n s c a r r i e d out with aluminum i s o p r o p o x i d e r e s u l t i n the formation of both an oligomeric f r a c t i o n and a polymer of higher molecular mass (FT 2500). The r e s u l t s of spectroscopic s t u d i e s i n d i c a t e t h a t the polymer i s both r e g u l a r and a t a c t i c , a c c o r d i n g to the spectrum i n F i g u r e l b . The model r e a c t i o n s of a d d i t i o n of a l c o h o l s to the styrene oxide i n the presence of s u i t a b l e aluminum a l k o x i d e s showed that the oxirane r i n g opens almost e x c l u s i v e l y at the α p o s i t i o n as seen by the data i n T a b l e II. I t was considered of i n t e r e s t , t h e r e f o r e , to i n v e s t i g a t e the mechanism of p o l y m e r i z a t i o n and the k i n d of a c t i v e c e n t e r s which g i v e r i s e to the formation of the r e g u l a r , a t a c t i c p o l y ( s t y r e n e oxide). O p t i c a l l y a c t i v e monomer was prepared and polymerized for t h i s purpose. The polymerization of R(+)-styrene oxide by Al(0iPr)3 lead to the formation of i s o t a c t i c poly(styrene oxide), as indicated i n F i g u r e Id, w i t h a p o s i t i v e o p t i c a l r o t a t i o n , w h i l e the p o l y m e r i z a t i o n of R(+)-styrene oxide by CH3UNa gave an i s o t a c t i c polymer, Figure l c , with a negative rotation. As indicated e a r l i e r , n

i n the a n i o n i c p o l y m e r i z a t i o n by sodium methoxide, p o l y ( s t y r e n e oxide) i s formed e x c l u s i v e l y by a β-ring-opening reaction i n which the c e n t e r of asymmetry and the c o n f i g u r a t i o n of the asymmetric carbon atom remain unchanged. Therefore, the p o l y m e r i z a t i o n of R(+)-styrene oxide by CH3ÛNa r e s u l t e d i n the formation of an i s o t a c t i c R(-)-polymer, while the dextrarotatory poly(styrene oxide) obtained with A l ( 0 i P r ) 3 had an S c o n f i g u r a t i o n of the asymmetric carbon atoms. Consequently the α p o s i t i o n of the oxirane r i n g opened and an inversion of configuration of the center of asymmetry took p l a c e f o r the l a t t e r c a t a l y s t , w h i l e β-ring opening with retention occurred for the former i n i t i a t o r , as shown below:


210


Table I. Products obtained by the addition of alcohols to styrene oxide under the influence of sodium alkoxides


Products Initiator

Alcohol

β-Opening

α-Opening

CH 0Na

CH 0H

2-methoxy-2-2 phenylethanol 35 mol %

2-methoxy-l-l phenylethanol 65 mol %

i-PrONa

i-PrOH

2-isopropoxy2-phenylethanol 12 mol %

2-isopropoxy 1-phenylethanol 88 mol %

t-BuONa

t-BuOH

2-t-butoxy-2phenylethanol 6 mol %

2-t-butoxy-lphenylethanol 94 mol %

3

3

Table I I .

Products obtained by the addition of alcohols to styrene oxide under the influence of aluminum alkoxides

Products Initiator

Ak*0Et)

3

Alcohol

α-Opening

EtOH

2-ethoxy-2-2 phenylethanol 100%

β-Opening

-

Al(0iPr)

3

i-PrOH

2-isopropoxy-2phenylethanol 95 mol %

2-isopropoxy-lphenylethanol 5 mol %

Al(0tBu)

3

t-BuOH

2-t-butoxy-2phenylethanol 91 mol %

2-t-butoxy-lphenylethanol 9 mol %


16.

JEDLINSKI ETAL.

211


0

I

-f-O-CH-CH^ poly R (-)

0 \

CH-CH

V

[m]

s7e

-133,2

e

2

Ring-Opening Polymerization - American Chemical Society

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