Copolymerizations of 3.3-bis(chloromethyl)oxacyclobutane with β-propiolactone and γ-butyrolactone...

Die Makromolekulare Chemie 139 (1970) 153-164 (Nr . 3438)

From the Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Japan

Copolymerizations of 3.3-Bis(ehloromethyl)oxacyclobutane with b-Propiolactone and y-Butyrolactone by Lewis Acids:

Yl“o-Statess Polymerization Mechanism By KOICHI ITO, TOSHIHIDE INOUE, and YUYA YAMASHITA

(Eingegangen am 11. Mai 1970)

SUMMARY: A GLtwo-state” polymerization mechanism is proposed to explain the previously reported

abnormal sequence distributions in the copolymers from 3.3-bis(chloromethyl)oxacyclobutane (BCMO) and P-propiolactone (PL) prepared with SnC4 or BF30Etz as catalyst. Long BCMO sequences are formed by the normal cationic mechanism, whereas formation of long PL sequences is explained by the “coordinated” cationic mechanism which assumes the polymerization of PL molecules which are coordinated to the catalyst. The lactone monomers coordinate to the catalyst or to the propagating cation through the carbonyl oxygen atoms, and this is supposed to be an important factor for controlling the operation of the two mechanism. y-butyrolactone (BL) is particularly strongly coordinated and so produces alternating copolymers with BCMO.

ZUSAMMENFASSUNG: Zur Erklarung der bekannten abnormen Sequenzverteilung bei Copolymeren aus 3.3-

Bis(chlormethy1)oxacyclobutan (BCMO) und (3-Propiolacton (PL) mit SnC14 oder BF3 -O(CzHs)z als Katalysatoren wird ein sogenannter ,,Two State’’-Mechanismus vorgeschla- gen. Es wird angenommen, daB lange BCMO-Sequenzen durch einen normalen kationischen Mechanismus entstehen, wogegen die Bildung langer PL-Sequenzen durch einen ,,co- ordiniert&‘-kationischen Mechanismus erklart wird, bei dem nur solche PL-Molekiile poly- merisieren, die coordinativ an den Katalysator gebunden sind. Die Lactonmonomeren konnen sich an den Katalysator oder an das wachsende Kation iiber den Carbonylsauerstoff anlagern, und es wird angenommen, daB dieser Vorgang bestimmt, welcher der beiden Mechanismen jeweils zum Tragen kommt. y-Butyrolacton wird besonders stark coordinativ gebunden; das hat zur Folge, daB mit BCMO alternierende Copolymere entstehen.

Introduction

An abnormal sequence distribution in the copolymers of 3.3-bis(chloro- methy1)oxacyclobutane (BCMO) and P-propiolactone (PL) was deduced by TADA, SAEGUSA, and FURUKAWA from the hydrolysis of the copoly- niersl), and recently by us from high-resolution NMR analyses2). Notably, abndrmally high concentrations of PL-PL diad sequences were observed, in spite of the fact that the overall copolymer compositions show that the more basic BCMO is apparently far more reactive than the less basic PL, as expected for the normal cationic polymerization mechanism. There-

153

K. ITO, T. INOUE, and Y. YAMASHITA

fore, a different mechanism was suggested to account for the preferential polymerization of PL a t a PL chain end. TADA et aZ.3) ascribed the long PL sequences to some different catalytic species which might be formed during polymerization, possibly by reaction between the catalyst (BF30Et2) and the monomer.

However, no clear mechanism explaining these abnormal copolymerization results has been presented. I n this paper we propose such a mechanism by showing tha t the coordination of the lactone is an important factor in controlling the relevant copolymerization reactions. I n this work, y-butyrolactone (BL) was mainly used as a comonomer, because this monomer cannot homopolymerize, so that the characterization of the copolymers by NMR2) and its interactions with the catalyst are easy to examine.

Experimental Results

Polymerizations and the characterization of copolymers were carried out as described274).

(a) BL and BCMO were copolymerized to low conversions in toluene a t room temperature with SnC14 and HC104 (commercial aqueous per- chloric acid containing 7 0 % HC104). The results are shown in Fig. 1,

loo;

0

-- 50- c

q a, 0 - E

m -1

0 0 50 100

BL mole-% i n feed Fig. 1. Polymer-monomer composition data for the BCMO-BL copolymerization with the

catalysts: (a) SnC14 (2 mole-%), (b) BF30Et2 (0.5 mole-%), (c) HC104 (10 mole-o/o)

154

Copolymerizations of 3.3-Bis(chloromethyl)oxacyclobutane

together with the previous data obtained with BF30Et22). Alternating copolymers were also produced with SnC14 a t higher BL feeds, although a somewhat higher reactivity of BCMO was expected from the higher basicity of this monomer (pKb 5.65) as compared with that of BL (pKb 6.12)5). However, with HC104 only BCMO homopolymers were obtained in low yield. With different catalysts the apparent relative reactivity of BL decreases in the order: SnC14 > BF30Et2 > HC104.

(b) The I R spectra of equimolar mixtures of BL and the catalysts were measured in benzene a t room temperature, with the results shown in Fig. 2. The carbonyl stretching absorption of BL a t 1785 cm-1 was almost

1740 I

Fig. 2. Carbonyl stretching absorption spectra of the equimolar mixture of BL

and the catalysts : ( a ) SnC14, (b) BF30Et2, (c) HC104

completely shifted to 1665 cm-1 by complexing with SnC14, indicating the strong coordination of BL to the catalyst through the carbonyl group, as shown in structure I :

1

The shift of the carbonyl band to lower frequency is .well known for the complexes between LEWIS acids and various esters637). I n fact, 1 : 2 corn- plexes similar to (I) were isolated from SnC14 and &-caprolactams) and aromatic lactones such as phthalideg).

155


BL-SnC14 (equimolar)

A: (ppm)")

Similar, but somewhat weaker, complexing of BL is apparent with BF30Et2, because the carbonyl band is partly shifted to 1740 and 1665 cm-1. The absorption a t 1740 cm-1 may be due either to a complex of different stoichiometry (possibly 1 : l), or to ethylated BL, analogous to 11. The broad band a t 1740 cm-1 observed with HC104 is probably due to the protonated BL (11).

7.95 5 e 8.7 T 6.35 T (broad) (broad) (broad)

0.25 w o 0.1

(c) The coordination of BL through the carbonyl group was also supported by the NMR observation tha t the cc-CH2 group was more deshielded than the yCH2 group by complexing with SnC14 (Table l), because the reverse should be observed if the ether oxygen were coordinated t o SnC14. The ill-defined broad peaks observed in the BL-SnC14 system suggest that the mobility of BL is reduced by complexing.

Table 1. The chemical shifts of BL in benzene at room temperature

BL 8.2 T w 8.7 T 6.45 T

It is important to note that the reaction of BL with triethyloxonium tetrafluoroborate was also confirmed by NMR t o proceed as followslo).

Thus i t is reasonable t o conclude that lactones interact with cations and other electron-deficient species most readily by coordination of the carbonyl group.

(d) Some modified catalysts were prepared by the equimolar reactions of BF30Et2 and BCMO or BL, as described by TADA et aZ.3), and they

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Fraction Wt.-yo of BL mole-% in Fraction Wt.-% of N0.b) the fraction the fraction No. b, the fraction

1 8.4 32 5 15.2 2 9.2 33 6 5.7 3 20.6 31 7 11.9 4 9.1 33 8 19.8

were used for the copolymerization of BCMO and BL. However, the compositions of the copolymers obtained were almost the same as those obtained with BF30Et2 alone.

Also, a copolymer prepared with BF30Et2 was subjected to fractional precipitation from ethylene dichloride solution by adding ethanol. However, the results in Table 2 show that the copolymers are homo- geneous in composition.

BL mole-% in the fraction

-

35 34 36

These results are quite different from those for the PL-BCMO system reported by TADA E t aZ.3). They obtained considerable changes of copolymer composition by using the modified catalysts, and they also succeeded in the compositional fractionation of the copolymers. Therefore, the BL-BCMO copolymers appear to be formed through a single mechanism, in contrast to the PL/BCMO system where the very broad compositional distribution indicates that multiple polymerization mechanisms are apparently operating. This apparent discrepancy must be the conse- quence of using BL in place of PL. The reason will be discussed in detail later.

(e) Copolymerizations of BCMO with P L and BL were carried out in the presence of cc-methylstyrene (MS) which can easily polymerize by the normal cationic mechanism, but which does not copolymerize with PL4) nor with BCMO11). The results are given in Tables 3 and 4 .

It is important to note tha t the polymerization of MS by LEWIS acids is completely inhibited in the BL/BCMO system and that composition of the copolymer is not affected by the presence of MS. However, in the PL/BCMO system, MS polymerizes independently in high yield, and this enhances the incorporation of P L in the PL/BCMO copolymers as com-

157


Table 3. Copolymerization of BCMO and BL (mole ratio of 7 : 3) in toluene at room temp. : Effects of MS

1'69

2.25

2.25

[BCMOlo

(mole/l.)

3.94

5.25

3.94

5.25

SnC14 Oh3 (2.6 mole-%)

O (1.2 mole-%)

Oh3 (2.6 mole-%)

0

SnC14

BF30Etz

BF30Etz (0.5 mole-%)

0.47

1.02

- Poly- merization time

4.67 1.32 0.87

10.0 0 0.5

6.4 h

7.6 h

21 min

6 hr

Yield of MS homo- polymer

(wt.-%)"'

Yield of

copolymer BCMO-BL

(wt.-Y0)b'

7.9

9.0

2.4

1.8

BL in

copolymer

(mole-yo)

36

BCMO-BL

34

20

20

a) Yield based on the charged MS.

b) Yield based on the charged BCMO and BL.

Table 4. Copolymerization of BCMO and PL (mole ratio of 1 : 10) in toluene at room temp. : Effect of MS

I I I

SnC14 Poly- merization time

7.6 h

19 min

Yield of MS homo- polymer")

(wt.-%)b'

77.2

Yield of

copolymer BCMO-PL

(wt.-yo)"'

{I::; "2; 19.0

PL in

copolymer

(mole-yo)

70 89 58

BCMO-PL

a) MS homopolymers isolated by extraction with carbon tetrachloride from the benzene soluble part of the products.

b) Yield based on the charged MS. c ) Yield based on the charged BCMO and PL. d ) BCMO-PL copolymers soluble in benzene and insoluble in carbon tetrachloride. e ) BCMO-PL copolymers insoluble in benzene.

pared with that observed in the absence of MS. These results also suggest a single polymerization mechanism operating for the BL/BCMO system, and multiple mechanisms for the PL/BCMO system. Interestingly, the normal cationic polymerization of MS appears to be prevented by BL.

158

Copolymerizations of 3,3-Bis(chloromethyl)oxacyclobutane

Discussion

I n discussing the copolymerization mechanisms for the BCMOjPL and -BL systems, we must also consider the previous conclusion that the ring-opening polymerization of follows~"-14).

.Q II 0

lII

lactones occurs by acyl-oxygen fission as

M 0 --+ P-O(CH,),C-M

II 0

Here P is a preceding polymer or copolymer chain and M is the next cyclic monomer. Thus, for the propagation reaction to occur by acyl-oxygen fission, i t must be the ether oxygen, not the carbonyl oxygen, of the lactone tha t attacks the propagating cation.

Now the experimental results (b) and (c) suggest that the lactone most readily interacts with a cationic species through the carbonyl oxygen, thus forming a stable oxocarbonium ion IV. However, this species itself cannot propagate by addition of monomer, because this could only occur by alkyl-oxygen fission, contrary to the above conclusion.

0 + P-OC(CH,),-M I / 3" 0

P-QzC &\O

m (3)

Therefore we propose that this ion (IV) is a "dormant" species, which may be converted to an active species by isomerising to 111, or, more probably, by exchanging the last monomer unit for the other monomer to give the species V.

3 P-Q,C O'$o

m ZL

At each addition of PL to a growing chain, the probability of the ''dormant" ion IV being formed is much higher than that of the formation of the active ion 111, and an active ion I11 may be readily transformed to an inactive ion IV after addition of, a t most, two or three P L molecules. Thus the formation of long PL sequences cannot be expected as long as

159


we consider only these free cations and the free monomers in the polymerization system. Therefore in the following discussions we must also consider the coordination to the catalyst for explaining the observed formation of long PL sequences.

The interaction of lactones with catalysts (LEWIS acids) occurs through the coordination of the carbonyl oxygen as exemplified in I. It is very probable tha t BCMO coordinates with LEWIS acids through the oxygen atoms as shown in VI, although this is difficult t o examine directly because of the concurrent, facile polymerization of BCMO.

m

If we consider the polymerization of these coordinated monomers, then only the lactone would be susceptible to such polymerization, because the ether oxygen remains still free to attack the cationic species in the lactone complexes I, but this is not so for the BCMO complexes VI. Thus by considering this ,,coordinated” polymerization, we can expect the enhanced relative reactivity of lactones.

From the above considerations, we propose the following 6Ltwo-state” polymerization mechanism for the PL/BCMO system with LEWIS acids : (i) normal cationic mechanism + long BCMO sequences (ii) Licoordinated” cationic mechanism -+ long PL sequences.

The mechanisms (i) and (ii) refer, respectively, to the polymerization of free monomers and of the monomers coordinated to the catalyst. Accord- ing to the normal cationic mechanism, the more basic BCMO is expected to be far more reactive than the less basic PL, and thus to produce long BCMO sequences. Although the BCMO chain ends may be sometimes attacked by free PL, this occurs almost exclusively through the carbonyl oxygen to give the Lcdormant” species IV, as already explained.

P-”.3< + 0 ‘0’

P IF

Thus the polymerization of PL is very difficult by this mechanism.

160


However, the normal cationic mechanism is replaced by the ,,coordinated’’ mechanism when the coordinated PL is attacked by the propagating cation to give the species VlI.

The “coordinated” mechanism suggested here involves polymerization of the coordinated PL in the following manner :

LI

mr

P

- -0CHZ

I

rn The second step seems probable since the coordination of the cyclic lactone would be stronger than that of the open-chain carbonyl group of the penultimate unit. r An alternative “coordinated” mechanism involves the polymerization of the PL coordinated on the counterion, and in this mechanism the contact ion-pair VIII is the propagating species :

P P

P-OCH,CH,C =O

m where X is an initiator fragment which might be a hydroxyl group.

161

K. ITO, T. INOUE, and Y. YWSHITA

From both these “coordinated” mechanisms we can expect the formation of long PL sequences, because the polymerization of the coordinated BCMO is very difficult. The reaction may be stopped and changed to the normal cationic mechanism by the attack of free BCMO on the species VII or VIII.

These sequences of reactions satisfactorily account for the observed abnormally broad sequence distributions of the copolymers 1-3) if one assumes that the two mechanisms do not alternate very frequently with each other. The observed much higher reactivity of BCMO suggests that the polymerizations go largely by the normal cationic mechanism. This is probable since the propagating cation may be more easily attacked by the free BCMO than by the coordinated PL. The apparent relative reactivity of BCMO, as compared with PL, would be reduced by addition of cr-methylstyrene (MS) to the copolymerization system, as observed in the experimental result (e). This may be expected because MS clearly polymerizes by the normal cationic mechanism, and MS can not copolymerize with BCMO nor with PL. Thus the species IX or X, which may be easily formed by addition of BCMO or PL to an MS growing chain, cannot propagate by the ring-opening of the last cyclic monomer unit.

Ix X

Instead, these species would only propagate by the insertion of MS between the penultimate and ultimate monomer units, giving the same species but one unit longer. Therefore in the presence of MS, BCMO and PL would be more likely to copolymerize by the “coordinated” mechanism, thus leading to the enhanced relative reactivity of PL.

The proposed ‘(coordinated” mechanism would also be supported by showing that the incorporation of PL increases with the catalyst con- centrationl5). Although no data are available on the effect of the catalyst concentration in the PL/BCMO system, it was observed that only the alternating copolymers were obtained from the BL/BCMO system (3 :7 feed composition) with a very high SnC14 concentration (higher than that of BL).

The behaviour of the BL/BCMO system with LEWIS acids may also be accounted for by the same ‘6two-state” polymerization mechanism,

162


with the restriction that the alternations of the two mechanisms must occur very frequently during polymerization. This restriction may come from the difference in the reactivities of BL and PL. BL cannot homopolymerize because of the stability of the five-membered ring, but it would be very reactive toward a cationic species because of its high basicity (p& 6.12) as compared with PL (pKb 10.06)5). I n fact, the basicity of BL is very close to that of BCMO ( p & , 5.65), and it may even be expected that BL would be rather more reactive than BCMO toward a cationic species, since the “dormant” ion IV’ formed from BL is likely to be very stable.

Therefore the normal cationic mechanism leading to long BCMO sequences may be easily stopped in the presence of BL. This is clearly supported by the inhibition of the polymerization of MS, as described in the experimental result (e), indicating that the species X’ cannot propagate a t all because of its great stability.

C6H5

X’

Thus the polymerization in the presence of BL should occur almost entirely by the “coordinated” mechanism. However, the bbcoordinated” mechanism cannot, of course, give BL diads or longer sequences because of the inability of. BL to homopolymerize, so that after each addition of the coordinated BL the mechanism must switch over to the normal cationic propagation by addition of free BCMO.

I n this manner, highly alternating BL/BCMO copolymers would be formed and so i t would appear as if the propagation reactions proceeded through a single mechanism, in accord with the experimental results (a), (d), and (e). The higher alternating tendency observed with SnC14, as compared with BF30Et2, also accords with the stronger coordination of BL to the former catalyst (experimental results (a) and (b)). Similarly, a somewhat higher relative reactivity of P L was observed for the PL/ BCMO system with SnC14 than with BF30Et22). The formation only of

163


BCMO homopolymers in low conversions when HC104 was used would be accounted for by the “coordinated” mechanism being unable to ope- rate, since the protonated BL (11) itself would be too stable to react with another cationic species (propagating cation), and also i t is very unlikely that BL would coordinate to the perchlorate anion.

I n conclusion, we have proposed a “two-state” polymerization mechanism which is, a t least qualitatively, consistent with all the available data for the BCMOjPL and -BL systems with LEWIS acids. The coordination of the carbonyl group of the lactone to the catalyst or to the propagating cation was found to be an important factor controlling the propagation reactions. However, we must also note some important problems which still remain to be examined. For example, the exact ~ccoordinated~’ mechanism with SnC14 or BF30Et2 is uncertain, particularly in respect of the structure of the propagating or catalytic species. TADA et aZ.3) suggested that BF30Etz would be modified by reactions with monomers particularly a t higher conversions, which would corn- plicate the relevant problems. The extent of coordinstion of the respec- tive monomers to th2 catalyst should be examined so that a quantitative discussion of the relative importance of the two mechanisms can be constructed.

l) K. TADA, T. SAEGUSA, and J. FURUKAWA, Makromolekulare Chem. 71 (1964) 71. 2, Y. YAMASHITA, T. ASAKURA, M. OKADA, and K. ITO, Macromolecules 2 (1969) 613. 3) K. TADA, T. SAEGUSA, and J. FURUKAWA, Makromolekulare Chem. 102 (1966) 47. 4, K. ITO, K. UMEHARA, and Y. YAMASHITA, J. chem. SOC. Japan, ind. Chem. Sect. [Kagya

Kagaku Zasshi] 70 (1967) 2040.

(1966) 2121. 5 , Y. YAMASHITA, T. TSUDA, M. OKADA, and S. IWATSUKI, J. Polymer sci. 4, Part A-I

6 , M. F. L~PPERT, J. chem. SOC. [London] 1961, 817; 1962, 542. 7) J. J. MYHER and K. E. RUSSEL, Canad. J. Chem. 42 (1964) 1555. 8 ) A. J. AMASS and J. N. H ~ Y , Makromolekulare Chem. 103 (1967) 244. $1 C. €I. RUOF and H. C. HOWARD, J. Amer. chem. SOC. 76 (1954) 5565. 10) S. KOZAWA, Master’s thesis, Nagoya University, March 1970. 11) S. AOKI, Y. HARITA, T. OTSU, and M. IMOTO, Bull. chem. SOC. Japan 38 (1965) 1922. 12) H. CHERDRON, H. OHSE, and F. KORTE, Makromolekulare Chem. 56 (1962) 179. 13) T. TSUDA, T. SHIMIZU, and Y. YAMASHITA, J. chem. SOC. Japan, ind. Chem. Sect [Kagya

Kagaku Zasshi] 67 (1964) 1661, 2145; Makromolekulare Chem. 86 (1965) 304. 14) K. ITO, T. INOUE, and Y. YAMASHITA, Makromolekulare Chem. 117 (1968) 279; Y.

YAMASHITA, K. UMEHARA, K. ITO, and T. TSUDA, J. Polymer Sci. B 4 (1966) 241. 15) Suggested by Dr. P. H. PLESCH, University of Keele.

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Copolymerizations of 3.3-bis(chloromethyl)oxacyclobutane with β-propiolactone and γ-butyrolactone...

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