Free-Radical Ring-Opening Polymerization

13 Free-Radical Ring-Opening Polymerization Use in Synthesis of Reactive Oligomers W I L L I A M J. BAILEY, BENJAMIN G A P U D , YIN-NIAN LIN, Z H E N D E NI, and SHANG-REN WU

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Department of Chemistry, University of Maryland, College Park, M D 20742

A new method was developed for the synthesis of reactive, end-capped oligomers which involved copolymerization of a vinyl monomer with a cyclic monomer which can undergo free radical ring-opening copolymerization followed by hydrolysis of the resulting copolymer. Since the free radical ring-opening polymerization makes possible the introduction of functional groups, such as esters, carbonates, amides, and thioesters, into the backbone of an addition polymer, hydrolysis of the copolymer will give oligmers terminated with various combinations of hydroxy1, amino, thiol, and carboxyl groups. For example, copolymerization of 2-methylene-l,3-dioxepane and styrene (r =0.021andr =22.6)produced a copolymer containing 10 mole-percent of ester-containing units with 100% ring opening. Hydrolysis of this copolymer gave an oligomer of styrene endcapped with a hydroxyl group on one end and a carboxyl group on the other end. Similarly copolymerization of ethylene with the cyclic ketene acetal gave a series of biodegradable copolymers which, upon hydrolysis, gave oligomers of ethylene capped with a hydroxyl arid a carboxyl group. 1

2

Even though f u n c t i o n a l l y terminated oligomers are commercially important, most oligomers are made by i o n i c addition polymerization or condensation reactions rather than the convenient and inexpensive free r a d i c a l process. One of the few exceptions i s the hydroxyterminated polybutadiene produced by a free r a d i c a l process by Arco Chemical Company. Also, the polymerization of butadiene and s u l f u r by a free r a d i c a l mechanism that involves a ring-opening of the S3 r i n g followed by reduction of the r e s u l t i n g polysulfide groups gives a mercapto-terminated polymer which has found limited use ( l ) . Since i t was shown that free r a d i c a l ring-opening polymerization (2) made i t possible to introduce functional groups, such as esters (3[7, carbonates (4), thioesters (5), and amides (6), into the backbone of an addition polymer, i t was reasoned that simple hydrolysis would produce the desired oligomers that could be terminated with various combinations of hydroxyl, amino, t h i o l , and carboxylic a c i d groups. 6.00/0 l l Society

Society Library St., Harris, N.W. F., et al.; In 1155 Reactive16th Oligomers;

ACS Symposium Series; American Chemical Society: Washington, DC, 1985. Washington, D.C. 20036


148

REACTIVE OLIGOMERS

The f a c t that examples i n the l i t e r a t u r e of free r a d i c a l ring-opening polymerization are quite rare i s rather s u r p r i s i n g i n view of the f a c t that the i o n i c ring-opening polymerization of heterocyclic compounds, such as ethylene oxide, tetrahydrofuran, ethylenimine, 3-propiolactone and caprolactam, as well as the Ziegler-Natta metathesis ring-opening polymerization of c y c l i c o l e f i n s , such as cyclopentene and norbornene, are quite numerous• The few examples of free r a d i c a l ring-opening polymerization that are reported i n the l i t e r a t u r e include derivatives of vinylcyclopropane (7,8), o-xylylene dimer (9), derivatives of bicyclo[1.1.0]butane (T07> and elemental sulfur (11). This i s probably r e l a t e d to the f a c t that simple unstrained f i v e - or six-membered carbocyclic rings do not undergo r a d i c a l r i n g opening r e a d i l y , and i n fact the open-chain radicals have been shown to undergo r i n g closure to the corresponding unstrained c y c l i c r a d i c a l . For example, Butler and Angelo (12) found that diallyldiraethylammonium bromide would undergo inter-Tntramolecular polymerization to produce a soluble polymer. Apparently the reaction i s k i n e t i c a l l y c o n t r o l l e d to form the five-membered r i n g rather than the thermodynamically favored six-membered r i n g .

+ CH2

CH2

R-CH?

1

y

I

11

CH*

I

CH

I

CH

I

CH9

/

(H

I

CH2 CH

(Ho

2

X

/CH

>

/

\

CH3

/ CH3

R-CHo-CH

I

I

CH

CHo

2

/\ CH3

CH

3

\

CH3

CH-CHo*

CH

3

CHo-CH repeat >

2

x

I

CH-CHo

I

CHo

CH

2

A CH

3

CH

3

Jn

The course of some of these ring-opening and r i n g - c l o s i n g polymerizations can be explained by the recent data of M a i l l a r d , Forrest, and Ingold (13). They studied the transformations i n the cyclopropylmethyl ancTthe cyclopentylmethyl series by electron spin resonance. In the case of the three-membered r a d i c a l the reaction involves ring opening since the energy i s favorable (5.94 kcal) and the rate of reaction i s very high (1.3 X 10$ s " ) . In the case of the f i v e membered r i n g system the reaction proceeds i n the d i r e c t i o n of ring-closure since the energetics of that reaction i s favorable (7.8 kcal) and the rate of the ring closure i s also moderately high (1.0 X 10 s " ) . In an e f f o r t to f i n d some mechanism other than r e l i e f of s t r a i n or aromatization to promote free r a d i c a l r i n g 1

5

1

In Reactive Oligomers; Harris, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

13.

BAILEY

Free-Radical Ring-Opening Polymerization

ETAL.

149

opening polymerization, we reasoned that since a carbon-oxygen double bond i s 40-50 kcal more stable than a carbon-carbon double bond, the introducation of an oxygen atom into an unsaturated c y c l i c monomer appeared to be a possible solution.


Free Radical Ring-Opening Polymerization For this reason, a reinvestigation of the c y c l i c ketene a c e t a l , 2-raethylene-l,3-dioxolane ( I ) , that had been prepared by McElvain and Curry (14; was undertaken. Although McElvain and Beyerstedt (15) reported that benzoyl peroxide had no appreciable e f f e c t on d i e t h y l ketene a c e t a l , no such study was reported (14) f o r the 2-methylene-l,3-dioxolane ( I ) . The synthesis was c a r r i e d out as follows (6):

/O-CH2-CH3

^0-ai

HO-CH2CH2-OH

Br-CH -OL

>

2

1

O-CH2-CH3

t-BuOK

H"

2

2

N

87%

2

y ^ 2 CH =C I 0-CH 2

62%

Br-CH -CH | 0-CH

X

2

Treatment of this monomer with benzoyl peroxide gave a high molecular weight polyester by a free r a d i c a l ring-opening polymerization which can be r a t i o n a l i z e d by the accompanying scheme. The structure of the polyester IV was established by analysis and hydrol y s i s as w e l l as infrared and NMR spectroscopy. benzoyl peroxide

0-CH CH < I 0-CH I y

2

0 y CH2-C

2

60°C

2

IV

0

0

1

1

CH2-CH2

"Oi?

,0-CH

2

R-Oi -Ct 2

II

->

R-CH -C

I

III

0-CH2

2

At 60°C only 50% of the rings were opened and a t 120°C, 87% of the rings were opened (6); high d i l u t i o n also favored the extent of ring opening. There was a competition between the d i r e c t addition of the intermediate r a d i c a l I I and i t s r i n g opening to the r a d i c a l I I I . An a l t e r n a t i v e method of analysis of the extent of ring opening was the basic hydrolysis of the copolymer IV, which


150

REACTIVE OLIGOMERS


cleaved the ester groups but l e f t the c y c l i c ketals i n t a c t . Gopolymerization of styrene and I gave a copolymer containing mostly ring-opened plus some non-ring-opened u n i t s . In a search f o r other c y c l i c acetals that would undergo quant i t a t i v e r i n g opening even a t room temperature we prepared the seven-membered ketene a c e t a l , 2-methylene-1,3-dioxepane (V), which underwent e s s e n t i a l l y complete r i n g opening a t roan temperature. This process can be considered a major breakthrough i n polymer chemistry which makes possible the quantitative introduction of an ester group i n the backbone of an addition polymer.

di-tert-butyl

,0-CH -CH2 2

0-CH -Oi 2

>

-•£H -C-0-(CH )4-2

2

2

peroxide 80°

VI

repeat

R-CH -C 2

I 0-CH -CH 2

VII

11 R-CH -C 2

2

N

-012-012 | 0-CH -Oi2 2

VIII

Models show that the seven-membered r i n g increases the s t e r i c hindrance i n the intermediate free r a d i c a l VII to eliminate pract i c a l l y a l l of the d i r e c t addition and a l s o introduces a small amount of s t r a i n so that the r i n g opening to the r a d i c a l VIII i s accelerated. The seven-membered ketene a c e t a l V was e a s i l y copolymerized with styrene, 4-vinylanisole, v i n y l acetate, ethylene, and v i n y l chloride, to give copolymers with ester groups i n the main chain, a l l with quantitative ring opening. For example, by the use of a large amount of styrene and a small amount of the ketene a c e t a l V, followed by hydrolysis, an oligomer of styrene was produced that was capped with a hydroxy 1 group and a carboxylic acid group.


13.

BAILEY E T A L .


/0-CH2-CH2 CH =C 2

X

I

(013)30-0-0-0(013)3 +

GH =CH

>

2

I

O-CH2-CH2

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151

120°C

0

—CH -C-0-(Ol2)4--CH2-CH—CH -C-0-(CH )4' CHo-CH— 2

2

2

NaOH then H*"

HD-(Oi ) 2

4

Oi -01—CH -C-0H 2

2

For the copolymerization of the ketene a c e t a l V with styrene, r ^ i s 0.021 and r2 i s 22.6 at 120°C. With a mixture containing about 80% V and 20% styrene, a copolymer containing 90 raole-% styrene and 10 mole-% ester-containing units was obtained. The hydrolysis of this copolymer gave an oligomer of styrene containing an average of about nine styrene units end capped with the hydroxy! and carboxylic a c i d groups. Thus a very general method has been developed f o r the synthesis of a wide variety of oligomers with any desired molecular weight range. Of course, since the copolymers are random, the molecular weight d i s t r i b u t i o n of the oligomers i s quite broad. However, these oligomers should prove quite useful f o r the synthesis of polyurethanes and block polyesters. In an e f f o r t to produce a biodegradable addition polymer/ the 2-raethylene-l,3-dioxepane (V) and ethylene were copolymerized a t 120°C f o r 30 minutes at a pressure of 1800 p s i to give a low conversion of copolymers with ester-containing units varying from 2.1 to 10.4 raole-%. Although most synthetic polymers are nonbiodegradable since they have not been on the earth long enough f o r microorganisms or enzyme systems to have evolved to u t i l i z e them as food, polyesters that are r e l a t i v e l y low molecular weight and rather low melting are biodegradable (16). This observation i s related to the f a c t that poly( 3-hydroxybutyric acid) occurs widely i n nature and many micro-organisms use this polyester to store energy i n the same way that animals use f a t . On the other hand, no synthetic addition polymer was known that was readily biodegradable. However, the ethylene copolymers containing the ester groups i n the backbone were i n fact biodegradable with the copolymers containing the high amount of ester groups being rapidly degraded and the copolymers containing


REACTIVE OLIGOMERS

152

only 2.1% coraonomer only slowly degraded (17). Apparently there are enzymes i n the micro-organisms that are capable of hydrolyzing the ester linkages i n the ethylene copolymer to produce the oligomers with terminal carboxylic a c i d groups; these oligomers are then degraded as analogs of f a t t y acids by the normal metabolic processes.

CH =CH2 2

,0-CH2-CH2 CH =C | 0-CH -CH2

+

2

peroxide * 120°C

V

2


V

-ra -C-0-(GH )4—£cH -CH^—C^-C-CKC^^^C^-C^^-2

2

2

OH"

-»

HQ- •(CH ) —Ôi -CH2J—CH -C-OH 2

4

2

2

then H* When the ethylene-2-methylene-l,3-dioxepane copolymer was hydrolyzed, i t gave oligomers which were capped with a hydroxyl group a t one end and a carboxylic acid group a t the other. For material with an ester-containing unit content of 2.1 mole-%, the value of n was approximately 47 and f o r material with a content of 10.4 mole-%, the value of n was approximately 9. The copolymers with 6 or less mole-% of the ester-containing units had melting points i n excess of 90°C. Related work had shown that the nitrogen analogs of the c y c l i c ketene acetals were readily synthesized and would polymerize with e s s e n t i a l l y 100% r i n g opening. For this reason their copolymerization with a variety of monomers was undertaken (6). 0 li

•°-CH CH =C I 2

2

Uc-o-)

N-CH9

I

2

-N-CH -CH -

V

2

80°C CH

CH3

3

2

0 II •CH2-C-N-CH2-CH2 -CH -C-N 2

L

CH

3

(100% r i n g opened)

IX Thus the amide linkage i s s u f f i c i e n t l y more stable than the ester group to greatly favor the ring opening. Copolymerization of IX with styrene proceeded with e s s e n t i a l l y quantitative r i n g opening (6). In the case of the copolymer with styrene and IX, the copolymer was r e a d i l y hydrolyzed to give an oligomer of styrene capped with an aminoraethyl group and a carboxylic a c i d group.


13.

BAILEY


ETAL.

153


Also i n a related study the s u l f u r analog of the c y c l i c ketene a c e t a l X was prepared and polymerized. However, since the r e s u l t i n g thioester i s apparently higher i n energy than the ordinary ester and therefore retards the extent of ring opening, even a t 120°C only 45% of the rings were opened. Nevertheless, copolymerization of X with styrene gave a copolymer containing some thioester groups and hydrolysis of this copolymer gave an oligomer capped with a ine reap tan and a carboxylic a c i d group.

/WH CH2=(\ | S-CH

120°C

2

+

2

CH =CH I

> (CH )3C-0-0-C(CH )3

2

3

0 II —CH -C-S-CH -CH -|-CH -CH' I 2

2

2

3

CH -CH2

2

S

0

I I CH -CH 2

I

2

2

CH -CH

CH -CH-

2

2

S

2

2

2

2

OH" then

H-S-CH -CH

-CH -C-S-CH -CH —

^

0 II -CH -C-OH 2

0

I I CH -CH 2

2

F i n a l l y , the unsaturated spiro ortho carbonates were been shown to undergo double r i n g opening i n a free r a d i c a l polymerization to introduce a carbonate group i n the backbone of an addition polymer (19).


REACTIVE OLIGOMERS

154

. P*2~G*2 ° * /0-CH, 2

RO-CH -C 2

,C OCH CH -0 0-ai

%

S

x

2

CH -0

2

2


-> R0-Oi -C

2

2

#

0-0H

2

0 ca y H RD—CH -C-(H -0-C-0-CH -C-CH -0CH

repeat >

2

2

2

2

2

2

J n Apparently, the d r i v i n g force f o r the ring opening i s the r e l i e f of the s t r a i n i n the spiro system and the formation of the stable carbonate double bond. The double r i n g opening i s probably a concerted process from the i n i t i a l r a d i c a l addition product to the open-chain r a d i c a l . Even though the spiro compound XI i s an a l l y l monomer, i t does copolyraerize with a wide variety of comonomers. For example, XI w i l l copolymerize with styrene to give a copolymer containing carbonate groups i n the main polymer chain ( 2 0 ) . Hydrolysis gives the oligomeric polystyrene capped with reactive hydroxyl groups (2). ,,CH -0 /D-G^x CH =G C .OCHo 2

2

peroxide

X

y

+ n CH =CH

x

CH -0 2

2

I

O-CH/

80°C

• XI CH

CH

2

CH

2

(H

2

2

II CH -C-CH -0-C-0-ai -C-ai -OJGH -CH-CH -G-(^ -(> 2

2

2

2

2

2

2

j J n

OH"

CH

GH

2

2

m-CH -c-ai -o-4 2" j 1—CH -C-CH -OH FFL

2

2

0-CH2-ai3 CH?=CH

I

+

(H =C 2

Oumene hydroperoxide

N

O-CH2-CH3

>

• 140° C

(1:1)

O-CH2-CH3 CH3-CH2—CH -CH2

0 CH -a*

CHo-C

2

-CH2-C-0-CH2-CH3

I O-CH2-CH3 (9:1 a t 29% conversion) Since the d i e t h y l ketene a c e t a l (XII) appears to be a moderat e l y e f f e c t i v e chain transfer agent as well as a coraonomer, a search


156

REACTIVE OLIGOMERS

was made to f i n d a ketene a c e t a l that would be a more e f f i c i e n t chain transfer agent. I t was reported e a r l i e r that the introduction of a phenyl group into the 2-methylene-l,3-dioxolane r i n g system would so s t a b i l i z e the ring-opened free r a d i c a l that the 4-phenyl-2methylene-l,3-dioxolane (XIII) would undergo 100% r i n g opening even at room temperature ( 2 2 ) .

,0-CH-cJ

25°C R"

+

CH =C 2

V

|


0-CH

0-CH

2

0

0 #

II Oi- ->

R-CH -C 2

2

0-CH

II >

|

X

CH -C-0-CH -CH 2

2

—

2

On this basis i t was reasoned that a benzyl group i n a ketene a c e t a l should greatly increase the extent of cleavage during polymerization and, therefore, should increase the e f f i c i e n c y of chain transfer. That i n f a c t i s what occurred when an equimolar mixture benzyl methyl ketene a c e t a l (XIV) and styrene was heated a t 120°C i n the presence of d i - t e r t - b u t y l peroxide; an oligomer with 8 0 % styrene units and capped with a carbornethoxy group was obtained.

(013)3-0-0-0-0(013)3

Ô-CHS CH =C 2

CH =CH 2

N

\>-CH-

120° C

2

XIV

(1:1)

0 ll

CH ~-CH -CH—CH -C-0-CH3 2

2

2

(at 2 5 % conversion)

Thus the a d d i t i o n a l s t a b i l i z a t i o n of the eliminated free r a d i c a l by the phenyl group promotes e s s e n t i a l l y quantitative cleavage. The mechanism of the production of the end-capped o l i gomer i s probably as follows ( 2 4 ) ;


13.


BAILEY E T A L .

•0-CH -

,0-012-* CH =C 2

157

2

->

X

R-CH -C: 2

0013

O-CH3 XIV

CH=CH-(j) > 2


->

n-1

R-CH2-C-OCH3

+

*CH

# 2

#

(j>-CH -CH -CH 2

XIV

CH =CH2

-CH —fCH -CH2

-CH -C; 2

-

2

n-1

2

2

>

*

0 II (J>-CH —CH -CH -CH -C-0-CH3 2

2

2

O-CH3

(})-CH* 2

Although there are other unsaturated compounds that w i l l undergo addition-elimination with free r a d i c a l s , the benzyl ketene a c e t a l XIV appears to be the most active double bond as f a r as rate of addition i s concerned and the most e f f i c i e n t as f a r as regards to the extent of elimination i s concerned. A comparison with the l i s t of chain transfer agents l i s t e d i n the Polymer Handbook (23) i n d i cated that only the sulfur compounds appear to be more e f f e c t i v e than XIV. Hydrolysis of the end-capped oligomer gives a macromer that i s terminated with a carboxylic a c i d group. Since functional oligomers were of prime interest, an e f f o r t was made to f i n d a way to u t i l i z e the chain transfer properties of the ketene acetals to give oligomers that are end-capped a t both ends with funtional groups without hydrolysis. For this reason di(p-hydroxymethylbenzyl) ketene a c e t a l (XV) was synthesized. Pre!Liminary studies show that copolymerization of XV with styrene gives an oligomer of styrene with hydroxyl-containing groups a t both ends.


REACTIVE

158

Ô-O^ÂX />CH -OH CH =C )=( 0-CH -^/ ^rCH -OH 2

2

2

2

+

CH =CH I 2

OLIGOMERS

(CH3)3C-(>-0-C(CH3)3 • 120°C

0

H>CH—

Free-Radical Ring-Opening Polymerization

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