Polymerization of Optically Active p-Substituted fbPropiolactones. IV.

p-1 ,l-Dichloroalkyl p-Propiohctones Polymerized with Aluminum Triisopropoxide

RICHARD VOYER and ROBERT E. PRUD'HOMME, Centre de Recherche en Sciences et Ing4nkr-k des Macroml&c&s, D4partement

de Chimi?, Uniuersitd Laud, Qdbec, Canuda GlK 7P4, and ROBERT JER6ME and PHILIPPE TEYSSIE, Labratoire de

Chi& Macroml&culaire et de Catulyse Organique, Uniuersitd de Lsge, Sart Tilman BS, 4000 Lsge, Belgigue

Synopsis

The polymerization of three optically active 8-1,l-dichloroalkyl 8-propiolactones has been investigated in toluene, at 55OC, using aluminum triisopropoldde (Al(oiPr),) as initiator in a range of monomer/initiator molar ratios smaller than 150. /3-1,1-dichloroethyl 8-propiolactone polymerizes according to a living mechanism. However, the ability to polymerize decreases with an increase in the length of the alkyl substituent. For instance, /3-l,l-dichloro-n-propyl /%pro- piolactone is obtained only in low yields, whereas /?-1,l-dichloro-n-butyl 8-propiolactone does not polymerize at all. Actually, each of the lactones investigated reacts with Al(oiPr), in an initiation step that obeys a coordination-insertion mechanism. However, the size of the chloroakyl substituent has a critical effect on the propagation: when the alkyl group contains more than two methylene units, the insertion of a second monomer becomes exceedingly slow.

INTRODUCTION

During the last two decades, a large number of /3-substituted /3-propiolac- tonesfCR,R,CH,C003;, have been polymerized with a variety of initiators. Table I summarizes the monomers and catalysts that have been investigated so far.'-l'

It is generally found that racemic and optically active monomers lead, respectively, to amorphous and semicrystalline materials, although some ini- tiators promote the formation of a tactic and semicrystalline fraction whatever the optical purity of the m ~ n o m e r . ~ * ~ - ~ ? ' ~

Anionic initiators were tested in the polymerization of this series of mono- m e r ~ ~ , ~ ~ * ~ ~ but were in general unsuccessful.'8 In contrast, cationic initiators and coordination catalysts give high molecular weight compounds. However, the control of the molecular weight of the polylactone is difficult, if not impossible, and the polymers obtained have a high degree of polydispersity, with one exception: the AlEt,Cl/TPPH, systern4y5 leads to the living poly- merization of &butyrolactone. However, this catalyst becomes less efficient with other substituted p-propiolactones with bulkier s~bstituents.'~

In view of the results mentioned above, there is a need for an efficient initiator for &substituted /3-propiolactones leading to controlled molecular

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 26, 117-129 (1988) 0 1988 John Wiley & Sons, Inc. CCC 0360-3676/88/010117-13$04.~

TAB

LE I

Poly

( B-s

ubst

itute

d /3

-pro

piol

acto

nes)

Rep

orte

d in

the

Lite

ratu

re

Subs

titue

nt

Opt

ical

ly ac

tive

(0) or

R

, R

, ra

cem

ic (r

ac) m

onom

er

Initi

ator

R

efer

ence

CH

, C

H,

CH

, C

H,

CH

,

C(C

H,),

CH

2CH

3 C

H(C

H3)

!2

CH

,CH

, C

H,C

H,

CC

l,

CC

l,

CH

,Cl

cc1,

CH

Cl,

cci,

CC

l, C

Clz

CH

, C

Clz

CH

ZCH

, C

Cl,C

H,C

H,C

H,

CF3

CF3

CH

,CH

,CO

OCH

,

H

H

H

H

H

H

H

H

H

H

H

H

o an

d ra

c ra

c ra

c ra

c

rac

rac

rac 0

rac

rac

o an

d ra

c

o an

d ra

c

o an

d ra

c ra

c

AlE

t,/H

,O

ZnEt

,, C

dM+,

and

AlE

t,-D

MB

D'

AlE

t &l/T

PPH

; (E

t, A

lOC

PhN

Ph),

AlE

t,Cl,

AlE

t,/H

,O

AlE

t,/H

,O/e

pich

lorh

ydri

n A

lEt,C

l, A

lCl,,

ZnC

1, Fe

Cl,,

NEt

,, C

H,C

OO

Na

AlE

t,Cl,

AlE

t,/H

,O

x-ra

y, y

-ray

A

lEt,,

AlE

t,/H

,O,

AlC

l,, Z

nEt,-

DM

BD

"

AlE

t,, A

lEt,/

H,O

, Zn

Et,/H

,O

AlC

l,, A

lEt3

/H,0

, Zn

Et ,/H

,O,

AlE

t ,C

l/TPP

H;

AlE

t,/H

,O,

ZnEt

,/H,O

192

3 495

6 7,8

$ 23 + r 3 9 10

11

8 12

13

14

15

16,1

7 ~~

~~~~

~

' DM

BD

= R

(-)3

,3-d

imet

hyl-l

, 2-b

utan

edio

l. bT

PPH

, =

a,

/3, y,

6-te

traph

enyl

porp

hine

.

POLYMERIZATION OF /3-PROPIOLACTONES. IV 119

weights and small polydispersities. In that context, it seems interesting to use metal alkoxides such as alumhum triisopropoxide (Al(OiPr)3), since these complerdng catalysts have been shown to promote the living polymerization of ~-caprolactone’~-~~ and methyl-substituted c-~aprolactones.~~

It is the purpose of this article to report on the polymerization of /3-1,1- dichloroethyl /3-propiolactone (CH3CCl 2-PL), /3-l,l-dichloro-n-propyl p-pro- piolactone (C,H,CCl,-PL), and /3-l,l-dichloro-n-butyl /3-propiolactone (C3H,CC1,-PL) with aluminum triisopropoxide. The polymers were char- acterized in solution by vapor pressure osmometry and steric exclusion chro- matography to obtain number and weight-average molecular weights, and the polydispersity index. The polymers were also characterized in the solid state by differential scanning calorimetry to obtain glass transition, melting, and decomposition temperatures and enthalpies of fusion. The “living character” of this polymerization and the mechanisms of ring-opening and propagation of the chain are discussed.

EXPERIMENTAL

Materials

AU chemicals used in this study were obtained commercially. Toluene (practical grade) was dried by refluxing over CaH, and distilled before use under a nitrogen atmosphere. Aluminum triisopropoxide (practical grade) was distilled under reduced pressure, dissolved in dry toluene, and kept under a nitrogen atmosphere.

The synthesis and analysis of the three monomers used were already described in a previous paper.14

METHODS A Beckman IR-4250 spectrophotometer and a Varian XL-200 NMR spec-

trometer were used to verify the purity of the chemicals used. Measurements of optical rotation, at a polymer concentration of 0.01 g/mL, in chloroform, were made with a Carl Zeiss polarimeter.

For molecular weight measurements, a Wescan Model 233 vapor pressure osmometer was used. The polymers were dissolved in chloroform, and the measurements done at 25 f 1°C; solutions of sucrose odoacetate were used for calibration. Molecular weights were also measured using a steric exclusion chromatography apparatus (SEC), Waters Models 590 (pump), 7125 (injector), and R-401 (refractometer) equipped with p-styragel columns. Solutions of 1 mg/mL of polymers in tetrahydrofuran (THF) were prepared; and measure- ments were performed at 27 f 1°C. Low polydispersity polystyrene standards were used for calibration.

A Perkin-Elmer DSC-4 apparatus, equipped with a 3600 TADS computer, was used for calorimetry measurements. The apparatus was calibrated with indium, and the measurements were run at a heating rate of 20°C/min.

Polymerization Polymerizations were conducted in toluene, at 55”C, under vacuum. The

monomer was first dried over CaH, during 48 h under a nitrogen atmosphere

120 VOYER ET AL. TABLE I1

Polymerization of Optically Active R( +)-CH,CCl,-PL in Toluene at 55OC Using Al(OiR), a~ Initiator and Characterization of the Polymem

Mn ( S W B Experiment [u]? M, (kg/mol)

No. (degree fl) M/C Calc? ESP? Exp.(SEC)' (kg/mol) Mu/&,

GII-38 44 51 2.1 2.3 1.6 1.8 1.1 GII-42 41 50 1.6 1.5 1.5 1.7 1.1 GII-37 43 100 2.3 2.2 1.7 1.9 1.1 R-111-40 43 102 4.4 - 3.8 4.1 1.1 R-111-44 35 134 6.3 - 7.0 7.7 1.1 GII-36 40 165 2.7 2.3 3.3 3.8 1.1

Y3EC measurements in THF using PS standards for calibration. bCalculated using the yield of the reaction and considering that one molecule of Al(OiR),

'Vapor pressure osmometry in CHCl, using sucrose octoacetate for calibration. initiates three polymer chains.

and then distilled in the reaction flask. It was degassed by successive freeze- pump-thaw cycles; Al(OiPr), was introduced with a syringe at - 78"C, under a dry nitrogen flow, and the tube was sealed under vacuum. In all cases, the monomer concentration was close to 5 mol/L. After the polymerization, the tube was opened, and chloroform was added. A few drops of HCl were also added in order to hydrolyze the active chain ends (aluminum &oxides). The resulting solution was poured into methanol and stirred for 30 min. The polymer suspension was centrifuged at lo00 rpm and 0°C for 10 min, and the polymer was dried under vacuum before its characterization.

Separation of the Product Resulting from the First Step of the Polymerization

A mixture of monomer and initiator in a molar ratio of 0.7 was stirred at 55°C for 2 days. The reaction product was hydrolyzed by adding a dilute HC1 aqueous solution and isolated by four successive extractions in toluene. The toluene solution was then neutralized with a standard NaCl aqueous solution and dried over magnesium sulfate. The product of the first insertion step was then isolated by distillation under reduced pressure and analyzed.

RESULTS Table I1 shows the results of the polymerization of optically active t3-1,l-

dichloroethyl 8-propiolactone (CH,CCl,-PLY [a]E6 = 18.5') in toluene and at 55°C using Al(OiPr), and different monomer/catalyst ([M]/[C]) molar ratios. This monomer is optically pure and has an R configuration.'* Yields of about 35 and 75% were obtained after 10 and 30 days, respectively, with Al(OiPr), concentrations close to 0.02 M in all cases.

The polymers thus obtained are optically active, with an average value of [a]E2 = 40.3 f 0.5", a value that is in agreement with those reported before for polymers prepared with ZnEtJH20.14 It was shown that this value of optical rotations corresponds to an optical purity close to 100%.

POLYMERIZATION OF 8-PROPIOLACTONES. IV 121

EXPERIMENTAL fin (kg/mol)

Fig. 1. Calculated molecular weight of optically active poly(CH,CCl,-PL)H as a function of the experimental molecular weight assuming that one catalyst molecule initiates three polymer chains. (m) Vapor pressure osmometry, (0) SEC.

The molecular weights of the polylactones were determined by vapor pressure osmometry or SEC. Table I1 shows that the molecular weights measured experimentally are in reasonable agreement with the values predic- ted for a living polymerization (Fig. 1) on the basis of the [M]/[C] ratio and the experimental yield. This conclusion is valid in the investigated range of [M]/[C] ratios smaller than 150. Table I1 and Figure 2 also show that the polydispersity is, in all cases, close to 1.1, which agrees with a living polymer- ization in which the initiation rate constant is much larger than the propa- gation one.

The polylactones prepared are semicrystalline; their glass transition (Tg), melting (Tm), and decomposition (Td) temperatures and enthalpy of fusion (AH) are given in Table 111, as well as those obtained previously for poly(CH,CCl,-PL) polymerized with ZnEtJH,O and AlEt,. These transi- tion temperatures increase with the molecular weight as shown in Figure 3. The melting temperature reaches a limiting value of about 230°C for very high molecular weights.

The polymerization of optically active 8-1,l-dichloro-n-propyl (C2H,CC1, -PL, [a]," = 22.5') and 8-1,l-dichloro-n-butyl (C,H,CCl,-PL, [a]," = 21.2") /3-propiolactones was also carried out with Al(OiPr), under similar conditions (Table 1%. No poly(C,H,CCl,-PL) was obtained after 10 days; after 65 days the yield was only 20%. No poly(C,H7CC1,-PL) was obtained after 30 days at two different [M]/[C] ratios. The results indicate that the polymerization of /3-1,1-dichloroalkyl 8-propiolactones slows down when in- creasing the size of the dichloroalkyl group. Finally, two experiments, carried out a t a [M]/[C] ratio of 0.7 (Table IV), indicate that the initiation of the

122 VOYER ET AL.

'S-36001 PS-9ooo

1 I " ' " ' 7 8 9 10 I I 12 I3 5 6 7 8 9 10 II

? (mi )

1 , . " " . 7 8 9 1011 12 13 5 6 7 8 9 1011

t(mlnf

Fig. 2. Steric exclusion chromatography curves of polystyrene standards and of some poly(CH3CC1 2 -PL)b.

TABLE 111 Calorimetric Analysis of Optically Active Poly(CH3CC1,-PL)'s

1.6 1.5 1.7 3.8 7.0 3.3 a

b b

84 83 79 89 92 96 110 110 115

142 140 151 163 203 148 165 217 237

160 160 170 183 234 190 200 217 237

31 30 20 12 29 15 10 22 29

aPolymerized with ZnEt2/H20; Ref. 14. bPolymerized with AlEt,; Ref. 13.

POLYMERIZATION OF /3-PROPIOLACTONES. IV 123

240

220

Tm -

___--- _***--*-- -

I I f

0 2 4 6 8 10 - Mn (kg/md)

Fig. 3. Glass transition (T‘), melting (T,) and decomposition temperatures (Td) of optically active poly(CH,CCl,--PL)’s as a function of molecular weight. All polymers used in this figure were prepared with Al(OiPr), as initiator.

TABLE IV Polymerization of Optically Active R(+)-C2H,CC12-PL and Fi(+)-C3H7CCl2-PL in

Toluene at 55°C Using Al(OiPr), as Initiator

Monomer ee concentration Polymerization Yield

Monomer Experiment (I%) (MI M/C time (days) (I%)

- [ GII-34 100 2 10 0 65 20

L-11-25 0 18, 6 200 68 0 L-11-49 84 190 0,7 2

R(+)-C,H,CCl,--PL 1,s 51 30 0 A-111-73 84 3,5 100 30 0

-

polymer chain occurs easily. These observations will be discussed in the following section.

DISCUSSION

It is thus obvious that the polymerizability of optically active /3-1,1-dichlo- roalkyl /3-propiolactones with Al(OiPr),, in toluene and at 55OC, strongly depends upon the size of the substituent. With an ethyl moiety, the polymer- ization does not suffer from any deleterious perturbation from the /3 substitu-

124 VOYER ET AL.

tion. The experimental results suggest the absence, or a least the very small extent, of transfer and/or termination reactions. Indeed, the experimental molecular weights agree with the calculated ones, at least in the investigated range of monomer over catalyst molar ratios (I 150) (Fig. 1). Furthermore, the rate of initiation is observed to be fast as compared with the rate of propagation, since the polydispersity of the samples is narrow (MJMn = 1.1). All these features were also reported in the literature for the polymerization of unsubstituted 8-propiolactines by soluble bimetallic p-o~o-alkoxides.~~ The similarity in the behavior exhibited by aluminum-zinc p-0x0-alkoxides and aluminum alkoxides is well known, especially for the ring-opening polymeriza- tion of c-caprola~tone.'~*~~ All these observations, from this study and from the literature, lead to the conclusion that the 8-1,l-dichloroethyl substituent has no significant effect on the course of the 8-propiolactone polymerization. However, a dramatic fall in the polymerization rate is reported when the size of the substituent is increased by one methylene unit (in toluene and at 55"C), whereas the addition of two extra methylene units makes the polymerization of the corresponding 8-substituted 8-propiolactone exceedingly slow, to the point where no .polymer is detected after 30 days of polymerization (Table Iv).

Although the effect of the length of the dichloroalkyl substituent is un- questionable, it is of interest to determine which polymerization step, i.e., initiation or propagation, is perturbed by the substituent. Since the initiation is faster than the propagation, it is feasible to limit the polymerization to the addition of only one monomer unit per catalytic site by using an excess of catalyst compared with monomer ([M]/[C] = 0.7, see Table IV). After the hydrolysis of the reaction medium and the elimination of water soluble by-products, the recovery of either the neat monomer or its addition product to the catalyst must provide a clear insight into the effect of the 8-substituent on the occurrence of the initiation step.

Whatever the length of the alkyl substituent (ethyl to n-butyl), the ad- dition product was recovered rather than the unmodified monomer. The composition and structure of that product have been characterized by IR and NMR spectroscopy. This analysis has been made easier thanks to the previous isolation and characterization of the first addition product of both c-caprolac- tone and 8-propiolactone to aluminum-zinc p-o~o-n-butoxide~~*~~ and of c-caprolactone to aluminum triisopr~poxide.'~ For most organometallic com- p o u n d ~ , ~ ~ the ring-opening polymerization of lactones is recognized to proceed by the insertion of the monomer into the metal-heteroatom bond (Al-0 with the catalysts here considered). Two situations can be observed depending upon the opening mechanism of the lactone: i.e., a selective acyl-oxygen (a) or alkyl-oxygen (b) cleavage of the lactone ring:

b A H+O-CO(CH,)&OR I 7% (CH2) n- 1

a - H $. O(CH,),CO OR 11

POLYMERIZATION OF fi-PROPIOLACTONES. IV 125

In the special case of the r-caprolactone polymerization promoted by aluminum-zinc p-oxo-n-butoxide [(RO),Al-0-Zn-0-Al(OR),, where OR is the n-butoxy group], the selective acyl-oxygen cleavage a has been de- finitely established by the synthesis of the hydroxyester (11) (where n = 5 and R = n-butyl), the elemental analysis, molecular weight, and spectral char- acteristics of which are identical to those of the insertion product correspond- ing to [M]/[C] = 1.,l These data and those reported for the ring-opening polymerization of c-caprolactone and /3-propiolactone by aluminum alkoxide or bimetallic p-oxo-alk~xides~~,~ fit in with the insertion of the lactone molecules into the Al-OR bonds of the catalyst involving a specific acyl- oxygen cleavage of the lactone.

Tables V and VI report the IR and NMR characteristics of the products resulting from the reaction of the three investigated p-1,l-dichloroalkyl &pro- piolactones with an excess of aluminum triisopropoxide. The comparison of these data with the spectral features previously reported for the insertion of unsubstituted lactones into Al-alkoxide bonds convincingly supports that the isolated products correspond to the initiation step of the polymerization process. Therefore, the bulkiness of the B-dichloroalkyl substituents does not interfere with the initiation step but plays a determining role on the course of the propagation step.

Similarly, the addition of optically active /3-l,l-dichloroalkyl 8-propiolac- tone to aluminum triisopropoxide is quite easy but leads to a new insertion site (- CH,-CH(CC1,-alky1)-0-Al<) in which steric effects owing to the substituent very rapidly have a critical importance when the size of the alkyl group increases.

In short, the three j3-1,l-dichloroalkyl 8-propiolactones investigated in this study are ring-opened by aluminum triisopropoxide in toluene at 55°C. This initiation step proceeds through the selective acyl-oxygen cleavage of the lactone ring in such a way that the monomer unit is attached to the catalyst through an alkoxide bond. This ring-opening mechanism involves the coordi- nation of the lactone to the metal, followed by a nucleophilic attack of the alkoxide anion to the activated carbonyl group (Fig. 4). This process is known to occur with the coordination of the carbonyl group as suggested by Ymashita et al.= When the new polymerization site resulting from the initiation step is not too perturbed by the bulkiness of the P-dichloroalkyl substituent, the propagation takes place independently of secondary reactions (transfer and/or termination). Since the opening mechanism of the lactone ring does not rely on the alkyl-oxygen cleavage, optically active polymers are expected, in agreement with the experimental results (Table 11).

It must be stressed that all three investigated /3-l,l-dichloroalkyl P-pro- piolactones give high molecular weight polylactones using ZnEtJH,O as initiator. However, with AlEt, or AlEt,/H,O, no poly(C,H,CCl,-PL) nor poly(C,H,CCl,-PL) were obtained even after 200 days of reaction.14 When those polymerizations were continued over a long period of time, a slight precipitate was observed after about 1 year. This result indicates that the initiation of the ring occurs, but that the propagation step is slow. This seems to be the case for several aluminum compounds used as initiators, i.e., AlEt,, AlEt,/H,O, and Al(OiPr),, where the propagation reaction is affected by the size of the ring substituent, becoming extremely slow with 1,l-dichloro-n-pro- pyl and 1,l-dichloro-n-butyl substituents on P-propiolactone.

TA

BL

E V

IR

Spe

ctru

m A

naly

sis of

the

Fir

st-s

tep P

rodu

ct in Ft(

+)-R

-CC

1,-P

L an

d c-

Cap

rola

cton

e'

Poly

mer

izat

ion

Initi

ated

by

Alu

min

um T

riiso

prop

oxid

e

Cap

rola

cton

e C

H,C

Cl,-

PL

C,H

,CC

l,-PL

C

,H,C

Cl,-

PL

8 !a 8 A

ssig

nmen

t (w

ave n

umbe

r, cm-')

(wav

e num

ber, cm

- )

(wav

e num

ber, an-')

(wav

e num

ber, cm-')

0-H

st

retc

hing

34

50 (

my

34

50

3430

3440

C-0

st

retc

hing

(-C

OH

) 10

50 (l

O50

)b

1050

1050

1050

C=O

str

etch

ing

1750

(173

0)b

1730

17

05

1710

C

-0

stre

tchi

ng (-O

C=O

) 11

90 (1

175)

b 11

70

1170

11

70

"Ref

. 19.

bV

alue

s rep

orte

d fo

r th

e re

late

d hy

drox

yest

er (H

O-(C

H,),

-CO

OC

4H,)

synt

hesi

zed

inde

pend

ently

.

TA

BL

E V

I N

MR

Spe

ctru

m A

naly

sis

of t

he F

irst

-ste

p Pro

duct

in R

( +)-R

-CC

1,-P

L

and

c-C

apro

lact

one P

olym

eriz

atio

n Ini

tiate

d by

Alu

min

um 'k

iisop

ropo

xide

R( +

)-CH

3CC

l,-PL

R

(+)-

C2H

,CC

l,-PL

R

( +)-C

3H,C

Cl,-

-PL

c-C

apro

lact

one

6 R

elat

ive

6 R

elat

ive

6 R

elat

ive

6 R

elat

ive

Ass

ignm

ents

(P

P@

Peak =-

(P

P4

Peak

=-

(PPm

) peak==

(PPn

-4 Pe

ak=-

CH3

CH3

-CH

Z-

1.50

5.8

>c<

1.20

6.0

1.23-1.31

6.1

1.23-1.33

9.0

1.23-1.33

9.0

-CH

2-C

H3

-

-

-

-

-

-

1.75

2.0

-

-

-

-

-

-

-

-

-

-

-

-

-

2.17

3.1

(7 C

H3-

C

- I

2.30

2.0

2.31

2 .o

Cl

0 I1 I I

-0-

C -C

H2-

- C -

OH

-CH

- ,-0 f H

)

-CH

-0

-

+H)

I

2.21

1.9

2.77

2.0

2.89

3.37

1 .o

3.61

1 .o

3.55

3.50

1.8

-

-

-

2.0

2.95

1.0

3.52

-

-

2.0

1.0 -

ClC

Cl

-

-

4.33

0.9

4.30

1 .o

4.37

1 .o

u

N

b

1 .o

5.09

1 .o

5.05

1 .o

5.13

w

1 .o

Y

128 VOYER ET AL.

R O R HS o+ I

O P r Pro-AI-OCH%-C-OPr A Fig. 4. Mechanism of ring-opening polymerization of 8-1, 1-dichloroalkyl 8-propiolactones

initiated by aluminum triisopropoxide.

Although the mechanisms of ring-opening polymerization using these ini- tiators are not well known, it is admitted that they are different using the ZnEt.JH,O system or aluminum derivatives. For example, important differ- ences in the polymerization process were observed by Araki et al. in the polymerization of #I-propiolactones carrying side-chain ester groups16* 1 7 s n and by Iida et al. in the study of the polymerization of #I-alkyl and B-chloroalkyl P-propiolactones.’ The molecular structure of the polymers obtained using these initiators may also be different.

In previous articles, L a v a k et al. reported melting temperatures of 217 and 237°C for optically active poly(CH,CCl,-PL) prepared with aluminum derivative initiator^,'^ whereas we reported values ranging between 163 and 171”C’* using the same initiators. In view of the curve shown in Figure 3, this difference could be explained by differences in molecular weights. However, for poly(CH,CCl,-PL) prepared with ZnEtJH,O, melting temperatures of about 165°C were observed for a molecular weight of 27.1 kg/mol.14 This discrepancy might be attributed to the different nature of the polymers prepared using zinc and aluminum initiators. With zinc derivatives, branching might occur, causing a decrease of the melting temperature of the polylactone as compared with that of the corresponding linear polymer ( 5 230°C accord- ing to Fig. 3). With aluminum derivatives, a more linear polymer is obtained, and the relationship between molecular weight and melting temperatures of Figure 3 is obeyed.

The authors thank the “Communautb franpise de Belgique” and the Department of Educa- tion of the Province of Quebec for a Belgium-Quebec grant that supported this work. R. J. and P. T. also thank the “Services de la programmation scientifique” (BrusseLs) and R. E. P. thanks the National Sciences and Engineering Research Council of Canada and the Department of Education of the Province of Quebec (FCAR program) for financial support.

POLYMERIZATION OF P-PROPIOLACTONES. IV 129

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Received October 29, 1986 Accepted February 5, 1987