JOURNAL OF POLYMER SCIENCE: PART C NO. 23, PP. 711-719 (1968)

The Diethylzinc-L-a-Amino Acid Systems as Catalysts for the Asymmetric-Selection Polymerization of

Propylene Oxide

JUNJI B'URUKAWA, YOSHIYUKI IiUMATA, KOJI YAMADA, and TAKAYUKI FUENO," Department of Xynthetic Chemistry,

Kyoto University, Yoshida, Kyoto, Japan

synopsis The binary systems consisting of equimolar amounts of diethylzinc and L-a-amino

acids were found to be effective as catalysts for the asymmetric-selection polymerization of propylene oxides. Various L-a-amino acids were compared, in order to investigate the influence of the spatial character of the asymmetric center of catalysts on their efficiencies for asymmetric selection. It was found that all the L-a-amino acids investigated allow ( - )-propylene oxide to polymerize preferentially over the ( +) enantiomer, irrespective of the sign of their specific rotations, [ol]D. For each amino acid the [a ]D value of recovered monomer was proportional to -In (1 - x), where x is the extent of polymerization. The proportionality factor, which may be regarded as a measure of the relative asym- metric-selection efficiencies of catalysts, was found to parallel the bulkiness of the alkyl group attached to the asymmetric carbon atom of the amino acids used.

INTRODUCTION

Several classes of monomers are known to be polymerized into optically active high polymers by catalytic systems having a configurationally asymmetric center. Some of these monomers have no asymmetric carbon atoms and undergo an asymmetric-induction polymerization, whereas others, which contain asymmetric carbon atoms, are brought into the so-called asymmetric-selection polymerization. Propylene oxide is one of the monomers that belong to the latter class. The catalytic systems hitherto used for its asymmetric polymerization include the binary system consisting of diethylzinc and an optically active or amine4 and the ternary mixture comprising the ferric chloride-propylene oxide complex, water, and bbornyl ethyl ether.5

Although it is generally believed that the asymmetric catalysts coor- dinate with monomers prior to the propagation reaction, direct evidence has been scanty. One of us (J. F.) and his collaborators5 demonstrated previously that the asymmetric polymerization of propylene oxide by the above-mentioned ferric chloride catalyst is governed entirely by its selective

* Present address: Faculty of Engineering Science, Osaka University, Toyonaka, Osaka.

711

712 J. FURUKAWA ET AL.

adsorption of (+) monomer. It was also shown that such a selectivity was varied drastically through a chemical modification of the catalyst, which was effected by the use of varying amounts of water.

Yet the effect of the catalyst structure on the monomer selection still remains little understood. In particular, the relation between the absolute configuration of catalysts and the mode of the monomer selection is ambiguous.

The present study has been aimed at a systematic examination of how the spatial character of the asymmetric center of catalysts may influence the asymmetric polymerization. Racemic propylene oxide has been poly- merized by the diethylzinc-L-a-amino acid systems. The amino acids used are L-(+)-alanine, L-( +)-valine, L-( +)-isoleucine, L-( -)-leucine, and L-( -)-P-phenylalanine, which are all in the identical absolute configuration but have different signs of optical activities toward the sodium D line. The reason for the choice of amino acids as the optically active cocatalyst is, first, that they are a readily available family of substances having analogous structures and, second, that they mostly have only one asym- metric carbon, unlike some natural alcohols such as borneol and menth01.~

EXPERIMENTAL

Materials

The a-amino acids were of guaranteed grade and were used without further purification.

Propylene oxide of commercial grade was refluxed over potassium hydroxide pellets for 5 hr. and over calcium hydride for 3 hr. and then distilled under a dry nitrogen atmosphere.

Toluene of guaranteed grade was distilled from sodium ketyl of benzo- phenone under dry nitrogen.

Nujol was deaerated a t high vacuum and stored under dry nitrogen.

Polymerization Procedure

A typical procedure was as follows. In a polymerization tube of about 60 ml. was placed 2.2 mmole of amino

acid. After having been evacuated and flushed with dry nitrogen, the tube was further charged with 5 ml. of toluene and a 2.2 ml. portion of 1M toluene solution of diethylzinc. The catalyst system in the tube was aged for 20 hr. at 100 to llO"C., until evolution of a gas ceased. It was then cooled down to room temperature. A desired amount of toluene and 440 mmoles of propylene oxide were added, and the tube was sealed. Polymerization was run for a specified time interval in a rotatory bath kept at 70°C.

Measurements of Optical Activities After polymerization unreacted monomer and solvent were recovered in

a cold trap under reduced pressure at room temperature and fractionally

I'OLYR.1ERIZATION OF PIZOPYLENE OXIDE 713

distilled by means of a distillation column 3 mm. in diameter and 30 cm. in height. The optical rotations of the recovered monomer were measured by using a Rex photoelectric polarimeter (Rex Optical Works Co., Nishi- nomiya, Japan) : cell length, 1 dm., and measurement accuracy, ~ 0 . 0 0 3 " .

To remove the catalyst, the crude polymer was dissolved in a suitable amount of benzene containing a small amount of methanol, centrifuged, and then dried by the freezing technique. Measurements of the optical rotations of the polymers were poor in accuracy because of their poor solubility (about 1?&) in chloroform.

Characterization of Catalyst System

Characterization of the catalyst system was attempted by choosing glycine as the amino acid component.

The amount of the ethane gas evolved by the reaction of 1 mmole of glycine with 1.1 mmole of diethylzinc in 5 ml. of toluene at 100°C. was measured by vapor-phase chromat,ography. The catalytic residue was dried under vacuum, to remove the solvent and unreacted diethylzinc. To determine the ethyl group still remaining in the catalyst, the residue was hydrolyzed, and the gas evolved was measured in a similar way.

The infrared spectrum of the catalyst residue was recorded on a Hitachi Model EPI-G double-beam spectrometer. A Nujol mull of the sample was prepared and pressed between potassium bromide plates in a dry box under a dry nitrogen atmosphere. The specimen was immediately sub- jected to the infrared-absorption measurement. A comparative infrared measurement of an oxidized sample confirmed that this procedure caused no damage of the catalyst sample.

RESULTS AND DISCUSSION

Asymmetric-Selection Efficiencies

Preliminary experiments with varying molar ratios of a-amino acids to diethylzinc indicated that the catalytic ability of the binary systems was maximum when the molar ratio of the two components was 1 : 1, as can be seen in Fig. 1. The situation implies the possibility that a 1 : 1 reaction product of the two components is an effective catalyst for the polymeriza- tion. This view is supported by the fact that practically one mole of ethane was evolved during the thermal aging of the mixture of one mole each of the components.

Table I summarizes the results of polymerization with the various 1 : 1 diethylzinc-a-amino acid systems as catalysts. Bicarboxylic a-amino acids, such as glutamic and aspartic acids, did not react with diethylzinc and, as a result, showed no catalytic ability a t all.

As can be seen in Table I, all the L-a-amino acids used here allow (- )-propylene oxide to polymerize preferentially over the (+) enantiomer, irrespective of the sign of their specific rotations, [ a ] ~ . The resultant

714 J. FURUKAWA ET AL.

TABLE I Asymmetric-Selection Polymerization of Racemic Propylene Oxide

by the Diethylsinc-a-Amino Acid Systems5

[ a ] D of polymerb

Po1ymsn.l Polym. [ a ] D of Cryst. Amorph. Run time, yield, recovered [TI, no. hr. '% monomer dl./g. B C B C

L-( - )-pPhenylalanine 11 2.75 12 4.5 13 3.5 14 4.75 15 3.5 16 6.0 17 6.5 18 7.5 19 19'

- -

L-( - )-Leucine 21 1 .o 22 2.25 23 3.75 24 3.75 25 4.00 26 5.25 27 6.25

I,-( +)-Isoleucine 31 2.75 32 5.5 33 7.75 34 11.0 35 15.25 36 16.75

L-( +)-Valiie 41 0.75 42 1.5 43 1.5 44 2.25 45 4.0 46 3.5

L-( +)-Alanine 51 15.0 52 22.5 53 28.75 54 40.0 55 52.5

8.5 10.7 13.1 13.5 15.3 18.8 20.0 25.7 43.0 73.3

4.8 5.5 8.6 9.9

22.4 36.6 60.8

6.4 11.0 15.4 20.5 24.1 27.7

14 26 31 45 52 74

5.0 7.8

17.8 30.9 60.0

f O .09 0.19 0.17 0.20 0.22 0.27 0.32 0.35 0.73 1.63

f O .04 0.08 0.10 0.17 0.30 0.48 1.06

t0 .02 0.07 0.14 0.21 0.26 0.29

f0.02 0.05 0.07 0.11 0.16 0.23

f O ,006 0.013 0.03 0.06 0.11

4.8

2.4 3.75 4.1 5.5 +6.4 -13.0 6.4 +3.9 -13.2 5.4 +7.4 -11.1

- 3.5 +1.4 -2.1 +7.2 - 5.1 +3.6 -1.0

- 4.1 +2.1 -1.4 +7.6 - 1.3 +2.1 -1.3

9.7 +0.6 - 2.7 +1.6 -2.1

- 4.2 +1.5 - 2.5 +0.6

[Monomer] = 8.00 mole/liter. [ZnEtn] = [amino acid] = 0.50 mole-yo of monomer. Polymerization temper- Catalysts aged for 20 hr. a t 100-110°C.

ature, 70°C. Solvent, toluene.

b B, in benzene; C, in chlorofsrm.

POLYMERIZATION OF PROPYLENE OXIDE 715

0 0 10 2D

E d

Fig. 1. Influence of the [leucine]/[ZnEt~] ratio on the polymer yield. [Monomer] = Catalyst aged for 2 hr. at 80°C. 8.00 mole/liter.

Polymerization time, 16.5 hr. Temperature, 70°C. Solvent, toluene. [ZnEtz] = 1.9 mole-% of monomer.

polymers showed [ a ] D values negative in sign when dissolved in chloroform, while the values were positive in sign when benzene was used as solvent. The last observation is consistent with the trend already reported for the polymer prepared from pure (+) monomer6 as well as for the d,Z copolymer which is rich in the (-) monomer units.12

Table I also shows that in all cases the [a]D values of the recovered monomer increase with the increase in polymer yield. The efficiencies of the various catalytic systems in the asymmetric monomer selection may best be compared by the use of the relationship:5

where [ a ] D and [cz]DO are the specific rotations of the monomer recovered and the optically pure (+) enantiomer, respectively, r is the ratio of the first-order rate constant k- for the (-) monomer consumption to a similar constant, k+, for the (+) monomer, and x is the polymer yield expressed in decimal fraction. Equation (1 ) has been derived under the assumption that the asymmetric selection of the enantiomorphic monomers is governed by the catalyst alone. If this assumption holds, plots of the [a] , values against -In (1 - 2) should give a straight line for each catalyst system. The value of (T - l) /(r + 1) obtained from the slope will then be a measure of the asymmetric-selection efficiency of the catalyst.

Such linear relations as described above have been substantiated over fairly wide ranges of x for all the amino acids investigated, as can be seen in Fig. 2 . The values of (T - l)/(r + l), which were calculated from the slopes by assuming [a300 = +15.0°,6 are listed in Table 11.

It is to be expected that the relative efficiencies of any given homologs of compounds in a given type of asymmetric reaction may be related to the degrees of steric hindrance with which substituents may influence the reaction. This is probably also true of the catalysis in polymerization. As a possible measure of such a steric hindrance in our present case, we

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have chosen Taft’s steric substituent constant,g E,, of the a-alkyl group, R, of the amino acids. However, no correlation mas observed between E, and ( r - l ) / ( ~ + 1) .

Included in Table I1 are various bulkiness parameters” of the ~u-alkyl groups, R. Such parameters are: Biltz’ molar volumes V , the van der WaaIs second virial coefficients b, and the Lennard-Jones collision diameters c of the hydrocarbons RH and, in addition, the molar refractions [RJ of the corresponding alkyl chlorides, amines, arid alcohols. The magnitudes of these parameters were either taken directly from the literature or com- puted from the composite physical constants. It is seen in Table I1 that all these bulkiness parameters run roughly parallel to the value of (T - I)/@ + 1).

Chemical Structure of the Catalysts

As has already been mentioned, evolution of a gas was noticed during the course of the catalyst preparation. Vapor-phase chromatography of the gas identified it as ethane. When a mixture of 1.0 mmole each of glycine and diethylzinc in toluene was kept a t 100°C. until completion of the reaction, the collected ethane gas amounted to 0.93 mmole. The resulting catalyst species gave additionally 0.82 mmole of ethane on hydrolysis. From these results it may be concluded that only one ethyl group of diethylzinc had been consumed by the reaction with glycine during the catalyst preparation.

The infrared-absorption spectra of the catalyst prepared and of its starting materials are compared in Fig. 3. In the spectrum of the catalyst there are observed two peaks in the region of the N-H stretching vibration: one (3160 cm.2) may be assigned to a symmetric vibration, and the other (3270 em.-’) to an antisymmetric one.

718 J. FURUKAWA ET AL.

&O-' 28b0 ' 2460 ' 2000 1600 1200 k ' lodo 800 ' ' 600 -4" WAVE NUMBER (ern-')

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Fig. 3. Infrared absorption spectra of diethylzinc, glycine, and the reaction product: (a ) diethylzinc in heptane; ( b ) glycine, KBr disk; ( c ) reaction product, Nujol.

NOTE: It has been reported by Sweeny et a1.I0 that glycinatoziric hydrate, Zn(Gly)s,HzO, shows two absorption peaks at 3270 and 3450 cm.-l, which may be assignable to the N-H stretchings. According to our experiment, however, a single crystal of the same hydrate had absorp- tion maxima at 3260, 3300, and 3440 cm.-', whose transmittances were 29, 28, and 46%, respectively. Moreover, Zn(0H) 2 showed strong absorptions at 3450 and 3500 cm.-l. Thus, it appears that the bands to be ascribed to the N-H bond stretchings in the hydrate are those appearing at 3260 and 3300 cm.-l and that the peak which Sweeny et al. observed at 3450 em.-' is probably due to the 0-H bond of the water of crystallization.

In addition, these peaks are shifted to a longer wavelength than those of typical N-H bond stretchings (3300-3500 cm.-'). These situations may be taken to indicate that the catalyst involves a primary amino group in which the nitrogen atom is positively charged because of the release of its lone-pair electrons for complexation.

Figure 4.

POLYMERIZATION OF' PROPYLENE OXIDE 719

Several absorption peaks characteristic of the carboxylate structure were observed at 610, 690, 1400, and 1600 cm.-'. These may be assigned respectively to scissoring, wagging, and symmetric and antisymmetric stretching vibrations of the carboxyl group.

The absorption due to the symmetric stretching vibration of the C-C-N bonding present in isolated glycine disappeared in our catalyst. According to the literature," anhydrous glycinatozinc has the structure (I) shown in Fig. 4.

In the light of the results obtained we may assume that our asymmetric catalysts are of the form (11).

(11)

Unfortunately, however, these catalysts were hardly soluble in any conventional organic solvent, and hence determination of their degree of aggregation was unsuccessful. Besides, the number of the active catalytic sites estimated from the polymer yield was extremely small. Under these circumstances, much cannot be said at the present stage about the behavior of our catalysts in controlling the stereoregulation in polymerization. Rather more significant in the present work is its demonstration that the asymmetric selection of monomers is indeed controlled systematically by the spatial character of the asymmetric centers present in the catalysts of the form (11).

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3 . S. Inoue, Y. Yokota, N. Yoshida, and T. Tsuruta, Makromol. Chem., SO, 131

4. S. Akutsu, T. Saegusa, and J. Furukawa, paper presented a t the 16th Annual

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8. J. 0. Hirschfelder, C. F. Curtiss, and R. B. Bird, Molecular Theory of Gases and

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