Free Radical Ring-Opening Polymerization - American Chemical Society

4 Free Radical Ring-Opening Polymerization W I L L I A M J. BAILEY

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

Free radical ring-opening polymerization has pre viously been quite rare with the only examples being cyclopropane derivatives and o-xylylene dimer. This fact is surprising in view of the fact that ionic ring -opening polymerization is very common. Since a carbon -oxygen double bond is about 50 kcal more stable than a carbon-carbon double bond, it was found that intro ducing an oxygen atom into an unsaturated cyclic monomer would permit free radical ring-opening poly merization. Thus it was shown that cyclic ketene acetals, cyclic ketene aminals, cyclic vinyl ethers, unsaturated spiro ortho carbonates, and unsaturated spiro ortho esters, would a l l undergo such polymeriza tion. Furthermore, a l l of these monomers would copoly merize with a wide variety of vinyl monomers with the introduction of functional groups, such as esters, thioesters, amides, ketones, and carbonates, into the backbone of the addition polymers. This copolymerization makes possible the synthesis of biodegradable polymers, functionally terminated oligomers, polymers with enhanced thermal stability, and monomer mixtures which expand upon polymerization. In a research program to find monomers which expand upon polymeriza tion i t was desirable to have available monomers which would undergo double ring-opening polymerization by a free radical mechanism. However, a search of the literature revealed that there were very few examples of any free radical ring-opening polymerization. For example Takahashi (1) reported that during the free radical poly merization of vinylcyclopropane the cyclopropane ring opened to give a polymer containing about 80% 1,5-units and about 20% of undeter mined structural units but no cyclopropane rings. Apparently the radical adds to the vinyl group to give the intermediate cyclopropylmethyl radical which opens at a rate faster than the addition to the double bond of another monomer. The driving force for the poly merization is the relief of the strain of the three-membered ring. Somewhat similar results were obtained with the chloro derivatives. Very recently, Cho and Ann (2) studied the related ma Ionic ester derivative, which underwent Tree radical ring-opening polymerization to produce a high molecular weight polymer containing only the 1,5-units. 0097

AmeBtcaffe6ute9f9^/o

© 1985 ^ e T J ç a D . C h d t e ^ ^ o c i e t y

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

RING-OPENING POLYMERIZATION

48

AIBN CH9-CH-CH-Œ9

\/ CH

Oi -CH=CH-CH -CH 2

2

z

χ 2

(80% 1,5-units; 20% unknown units)

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]

R-CHo-CH-CH-CHo \ / CH

-> R-CH -CH=CH

^CH

2

^CH

2

e 2

2

Hall and coworkers (3) demonstrated that derivatives of bicyclo[1.1.0]butane would polymerize by free radicals by cleavage of the highly strained central bond. R

e

.00 CH 2

3

00 CH 2

3

Errede (4) showed that the dimer of o-xylylene would undergo free radical ring-opening polymerization to give the corresponding poly-o-xylylene· CH9

Œ 4-CH 2

2

,CH — 2

repeat R

I

CH

2

In this case the driving force for the ring-opening step is the for mation of the aromatic ring. Finally the ring-opening polymeriza tion of S3 has been postulated to involve free radicals ( 5 ) .


4.

BAILEY

Free Radical Ring-Opening Polymerization

49


The dearth of examples of free radical ring-opening polymeri zation is rather surprising in view of the fact that the ionic ringopening polymerization of heterocyclic compounds, such as ethylene oxide, tetrahydrofuran, ethylenimine, 3-propiolactone, and caprolactam, as well as the Ziegler-Natta ring opening of cyclic olefins, such as cyclopentene and norbornene, are quite common. One explana tion is that unstrained five- and six-membered carbocyclic rings usually are involved in ring-closing reactions rather than ring opening. For example, Butler and Angelo (6) in 1957 found that, when diallyldimethylammonium bromide was polymerized by a free radi cal mechanism, a soluble polymer containing five-membered rings was obtained by an inter-intramolecular polymerization. R* + CH2

CH

ι

R-CH

II

CH

I I

CH

CH*

CH

XHo

CHo

2

I

2

2

^y

/ \ I CH

CHo

/\ (H

3

2

2

^y

CH

R-CH-CH

CH

u CH I I

CH

3

3

e

CH-CHo

CH

3

R+

I 2

^CH

/\ CH CH 3

repeat

2

3

Apparently the reaction is kinetically controlled to form the five-membered ring rather than the t^rraodynamically favored sixmembered ring. The recent data of Maillard, Forest and Ingold (7) can be used to explain the course of some of these ring-opening and ring-closing polymerizations. When they studied the transformations in the cyclopropylmethyl and the cyclopentylmethyl series by electron spin resonance, they found that in the case of the threemembered radical the reaction involves ring-opening since the energy is favorable by about 6 kcal and the rate of the reaction is very high. In the case of the five-membered ring system they found that the reaction proceeds in the direction of ring-closure since the energetics of that reaction is favorable by about 8 kcal and the rate of the ring closure is also moderately high. Free Radical Ring-Opening of Cyclic Ketene Acetals Since the carbon-oxygen double bond is at least 50-60 kcal/mole more stable than the carbon-carbon double bond (8), we estimated that the introduction of an oxygen atom in place of "the carbon atom in the cyclopentylmethyl radical would favor the reverse reaction or the



50

ring opening. In other words the ring-opening reaction would be favored by at least 40 kcal/mole by producing the more stable carbonyl double bond. A search of the literature revealed several ring systems containing an oxygen atom that would undergo a ring-opening reaction in the presence of free radical catalysts. One such case was the cyclic formal, 1,3-dioxolane, which Maillard, Cazaux, and La lande (9) found rearranged to ethyl formate when heated at 160° C. CH?

'CH

/ \ Downloaded by UNIV OF GUELPH LIBRARY on July 12, 2012 | http://pubs.acs.org Publication Date: August 16, 1985 | doi: 10.1021/bk-1985-0286.ch004

0

R*

0

1 2

0

1

I

0

I

>

CH -CH 2

ο I

2

0

I CH -CH

2

2

CH /w

CH -CH 2

/ \\

0

I

160°C

2

0

0

— »

I

CH -CH

CH

/ \

e

2

CH

/\

ο

0

0

1

I

Œ2-Œ3

CH2-CH The reaction could be rationalized as indicated where the driving force for the ring-opening step in the chain reaction was the formation of the stable carbon-oxygen double bond in the final ester. With the knowledge that such a ring system would undergo cleavage, i t seemed to be a fairly straight forward process to synthesize a monomer that would undergo ring-opening polymerization by introducing a double bond at the carbon atom flanked by the two oxygens· The monomer desired for this ring-opening polymerization had indeed been prepared by McElvain and Gurry (10) in 1948. Although 0-CH

2

0-CH

2

2

di-tert-butyl peroxide

0 H - - C H - C-0-CH 0 -CH -

5

CH < 2

2

2

2

160°C

I

II repeat R

e

•/ΜΉ R-CH -Cv I 0-CH

0 u

2

R-CH -C

2

III

2

2

•f

2

V

0-CH

2

IV

Johnson, Barnes, and McElvain (11) had treated diethyl ketene aceta1 with peroxide and had reported Tnat there was no reaction, no such


4. BAILEY


51


study was reported for the 2-methylene-l,3-dioxolane (I). A rein vestigation of the cyclic ketene acetal I was therefore undertaken. This polyester II is difficult to synthesize with high molecular weight from the γ-hydroxybutyric acid because of the stability of the competing lactone. When the polymerization is carried out at lower temperatures, the ring opening is not complete. Thus at 60° C only 50% of the rings are opened to give a random copolymer of the following struc ture (12):

Qi

2

0

0

ι I 1 1 CH2-Œ2

m

Even at 120° C only 87% of the rings are opened. The uno pened radical III apparently can add directly to the monomer I in competition with the ring-opening process to form the open chain /0-Œ CH «C I 0-CH

2

v

2

0-CH R-CH-C-CH-CC I / \ 0-CH 0 0

2

2

2

2

2

1

0-CH R-CH -C^ 0-CH

2

I

CH -CH

2

2

2

2

III

0 » CH R-CH -C I 0-CH #

2

x

lSO

2

S

2

IV radical IV. High dilution was found to favor the ring-opening pro cess since the addition of III to the monomer I is a second order reaction while the conversion of III to the open chain radical is f i r s t order. The extent of ring opening is kinetically controlled with a direct competition between the rate of direct addition, k;Q, and the rate of ring opening, k i . In a program to find other cyclic ketene acetals that would undergo quantitative ring-opening even at room temperature we pre pared the seven-membered ketene acetal, 2-methylene-l,3-dioxepane (V), which underwent essentially complete ring-opening at roan tem perature (13-15). This process makes possible the quantitative introduction "OF an ester group in the backbone of an addition polymer. s 0



52

0 di-tert-butyl » -|Œ2-G-0-(CH2)4f-. peroxide 120°C VI

ι

O-CHo-CHo CH < I 0-Oi -CH2 2

2


R-

repeat

.,0-CH -CH R-CH -C I O-Œ2-CH9 VII 2

2

e

» CH -CH R-CH -C I

2

2

N

1

2

1

2

K

VIII

z

z

Apparently the seven-membered ring increases the steric hindrance in the intermediate free radical VII to eliminate practically a l l of the direct addition and also introduces a small amount of strain so that the ring-opening to the radical VIII i s accelerated. Additional cyclic ketene ace ta Is (16-18) that have been studied have included the 4-phenyl-2-methyIene-l,3-dioxepane (IX) which undergoes quantitative ring-opening to give the polyester X. Apparently the ring-opening step from XI to XII is greatly enhanced

O-Qi-φ CH =C I 0-CH 2

»

N

-+CH -C-OCH -CH2

2

2

IX

R-

repeat

. .O-CH-φ R-CH -C\ I 0-CH 2

2

XI

-> R-CH -C 2

I OCH

N

2

XII

by the formation of the relatively stable benzyl radical in XII even though XL is a five-membered ring analogous to the radical III. Nitrogen and Sulfur Analogs of Cyclic Ketene Acetals Since an amide group is more stable than an ester group, the nitro gen analog XIII of the cyclic ketene acetal was synthesized and polymerized to give the polyamide XIV.


4. BAILEY


0 M 0 Ν (φΟ0-) * -N-Œ -Œ -+Œ -C-N-(H -Œ 4-CH -C-N-80° C Œ CH CH

0-CH CH =C; I N-CH

2

2

2

2

2

I

53

2

2

2

2

3

3

3

2

(100% ring opened)

CH

3

XIV


XIII

repeat

0 O-CH R-CH -C^ I N-CH v

II

2

2

2

N-CHo

2

CH

'CH I

R-CH -C.

2

I

3

CH

3

XV XVI In contrast with the 2-methylene-1,3-dioxolane (I) the nitrogen ana log XIII undergoes essentially quantitative ring opening even at roan temperature. Although the sulfur analog of the cyclic ketene acetal I was prepared and polymerized, apparently the resulting thioester i s higher energy than the ordinary ester and therefore retards the extent of ring opening. Even at 120°C only 45% of the rings were opened (19-20). 0-CH CH

Free Radical Ring-Opening Polymerization - American Chemical Society

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