Die Makromolekulare Chemie 94 (1965) 74-83
From the Department of Synthetic Chemistry, Kyoto University, Kyoto, Japan
Asymmetric- Seleetion Polymerization of Propylene Oxide by Diethylzinc and D- or g glut am ate System
By JUNJI FURUKAWA, TAKEO SAEGUSA, SEIMEI YASUI, and SEIICHI AKUTSU
(Eingegangen am 12. Mai 1965)
SUMMARY: Asymmetric polymerization of racemic mixtyre of d- and I-propylene oxide (PO) was
carried out by means of catalyst system of diethylzinc and glutamic acid ester. From the optical activity of the recovered monomer, the effect of unbalance between D- and L-gluta- mates in the catalyst system upon the selectivity of asymmetric consumption of d- and Z- monomers was examined. It has been shown that the catalyst system having excess diethyl D-glutamate (DG) over the L-counterpart (LG) favors the polymerization of 1(-)-PO. A directly opposite phenomenon of asymmetric consumption was observed in the poly- merization with catalyst system having excess LG. The unbalance degree of asymmetric consumption is almost linearly related to the unbalance between DG and LG in the catalyst system. The sign of optical rotation of produced polymer was opposite to that which is to be expected from the unbalance in monomer consumption. This peculiar phenomenon has been explained by the findings that partial inversion of the asymmetric carbon atom of PO monomer occurs in the course of propagation reaction, and that the extent of in- version of one monomer enantiomer is different from that of its antipode in the polymeri- zation with asymmetric catalyst.
ZUSAMMENFASSUNG: Die asymmetrische Polymerisation von Propylenoxid-Racemat rnit dem Katalysator-
system Diithylzink/Glutaminsaureester wurde untersucht. Aus der optischen Aktivitat des nicht umgesetzten Propylenoxids (PO) wurde der E i d u B des Verhaltnisses D- zu L-Glutamat im Katalysatorsystem a d den bevorzugten Verbrauch von d- oder I-PO be- stimmt. Bei einem uberflul3 an D-Glutamat (DG) gegeniiber L-Glutamat (LG) im Kataly- .satorsystem wird bevorzugt I(-)-PO polymerisiert, bei einem U'berschuB an LG d(+)-PO.
Das Verhiltnis von d- zu I-Monomerverbrauch, l(d-po)-(z-po)I ist dem VerhatGs DG (d-PO)+@-PO) '
zu LG im Katalysatorsystem, IDG-LGI anniihernd proportional. Das Vorzeichen der D G L G '
optischen Drehung des entstehenden Polymeren stimmt nicht mit dem aus dem Monomer- verbrauch erwarteten iiberein. Dies liegt daran, daB fiir den Fall der Polymerisation mit asymmetrischen Katalysatoren bei der Wachstumsreaktion teilweise Inversion des asym- metrischen CcAtoms am PO stattfindet und daB das Ausmal3 dieser Inversion bei den beiden Antipoden verschieden ist.
Isymmetric-Selection Polymerization of Propylene Oxide
There have been published several papers 1--8) upon the so-called "asym- metric selection" polymerization of racemic mixture of propylene oxide ( P O ) , in which a catalyst system containing an optically active compo- nent is employed. Consumption rate of one of two monomer enantiomers is larger than tha t of the antipode. Consequently, the unreacted monomer recovered from the polymerization system has optical activity, in which the amounts of two monomer enantiomers are not equal t o each other. Catalyst systems hither t o used for this polymerization are ZnEt,/opti- cally active a l~oho l l -~ ) , ZnEt,/H20/optically active ether5), (FeC1,-PO)/ H,O/d-bornyl ethyl ether6), and ZnEt2/optically active amine systems').
I n the present paper, diethyl esters of D- and L-glutamic acids were employed as the asymmetric component of catalyst, which were combined with ZnEt, in the asymmetric selection polymerization of d,l-PO. The function of glutamate as the asymmetric component has been examined from the optical activities of the recovered monomer and of the produced polymer. The mechanism of the ring-opening process of this asymmetric polymerization has also been studied.
2 . Experimental
Racemic propylene oxide (d,l-PO)
and then distilled, b.p. 34.5 "C. Commercial reagent was refluxed over KOH pellets for 2 hrs. and over CaH, for 5hrs.
Levorotatory propylene oxide (l(-)-PO) *) According to the procedure reported by PRICE et d9), I(-)-PO was prepared from mono-
chloroacetone through the fermentation process of acetol benzoate. l(-)-PO, b.p. 34.5 "C. [XI: = -14.2" in diethyl ether.
Dextrorotatory propylene oxide ( d ( + ) - P O )
diluted). d(+)-PO was prepared by the method of LEVENEIO), b.p. 34.5 "C., [a]: - +14.5 O (un-
*) It has been discussed8) that (-)-PO is in the L-series and has S-configuration. In this paper, however, the absolute configuration of the PO monomer is not taken into con- sideration.
J. FURUKAWA, T. SAEGUSA, S. YASUI, and S. AKUTSU
Diethyl L-glutamate (LG) and D-glutamate (DG) According to the procedure given by FISCE~ER~~) , L- and D-glutamic acids ([a13 = +31.8
and -30.3 O, respectively, both in 6 N HCl) were esterified, separately, with absolute ethyl alcohol in the presence of dry HCI. LG, b.p. 139-143"C./10 mm. Hg, [a]g = +7.0" in diethyl ether; DG, [a13 = -7.1 in diethyl ether.
b.p. 112-115C. Commercial reagent was purified by distillation under reduced nitrogen atmosphere,
Toluene Commercial reagent was refluxed over sodium metal for two days, distilled, and stored
over sodium wire.
used without purification. Commercial reagents of the highest purity of acetone, chloroform, and diethyl ether were
2.2. Polymerization procedure Under nitrogen atmosphere, toluene, ZnEt,, and diethyl glutamate were placed in a glass
tube and the mixture was heated a t a definite temperature. Then the mixture was cooled to -78 "C., and the PO monomer was added. The tube was sealed and was allow- ed to stand at 75 "C. for a definite time of polymerization.
After polymerization, the reaction mixture was subjected to vacuum distillation a t room temperature (first distillation). The volatile matter was condensed and collected in a trap kept a t -78"C., from which the unreacted PO monomer was isolated and purified by fractional distillation through a 400 x 10 mm. helices-packed column. The non-volatile residue of the first distillation was dissolved in benzene to an about 2% solution and the benzene solution was washed repeatedly with 10% HCl solution and finally with water. Crude PO polymer was isolated as a slightly yellow mass from benzene solution by freeze- drying technics, which was dissolved again in benzene and treated with active alumina in order to insure the removal of impurities including the glutamate component.
The crude PO polymer was then purified and fractionated into the crystalline and amorphous fractions by the following procedure: About 2% solution of the crude polymer in acetone was cooled to O"C., and the precipitated part (crystalline fraction) was isolated by centrifuge. Amorphous fraction was recovered from the acetone solution by evaporating acetone. Both crystalline and amorphous fractions were isolated as porous mass from their benzene solutions by freeze-drying technics.
2.3. Optical activity measurement Optical activity was measured at room temp. with sodium D light by using Rex photo-
electric polarimeter (Rex Optical Works Co., Nishinomiya, Japan), cell length 1 dm., measurement accuracy 0.003 O. The optical activity of the recovered monomer from poly- merization system was measured without diluent. The activity of PO polymer was meas- ured in benzene and in chloroform.
Asymmetric-Selection Polymerization of Propylene Oxide
3. Results and Discussion
The effect of unballance between D- and L-glutamates in the catalyst system upon the asymmetric-selection polymerization is illustrated in Table 1. It is shown that the catalyst system in which DG predominates over the counterpart LG favors the polymerization of Z(-)-PO. Here, the unreacted monomer recovered from the polymerization reaction mixture has a n optical activity of positive sign; the amount of d(+)-PO exceeds tha t of Z(-)-PO. A directly opposite phenomenon of asymmetric consumption of d,Z-PO was observed in the polymerization with catalyst system having excess LG over DG, where the recovered monomer showed minus sign of optical rotation.
When a racemic mixture of glutamate was used, the recovered mono- mer is of no optical activity. Therefore, it is obvious tha t the consump- tion rates of d- and I-PO are equal t o each other in this balanced poly- merization.
Assuming the first order dependency of polymerization rate upon the monomer concentration, the degree of unbalance of consumption rate between the two monomer enantiomers, ( k l - k d ) / ( k l + k d ) or (kd -k l ) / (kd + kl), was calculated from the optical activity of recovered monomer and the total conversion percent using the following equation6) :
- ( o r K ) kd-k , kl-kd = - - . UM 1 kd+k/ a; In ( l - X / 1 0 0 )
k,, k , : Consumption rate constants of d(+)-PO and Z(-)-PO, respectively. x: Total conversion percent.
x i :
Optical activity of recovered monomer. Optical activity of pure species of monomer enantiomer which is favored in asym- metric consumption.
The variation of the unbalance degree of asymmetric consumption be- tween d(+ ) - and Z(-)-PO monomers according t o the unbalance in the glutamate component is shown in Table 1 and Fig. 1. The so-called mir- ror-image relationship is depicted in Fig. 1. It is also shown tha t the unbalance degree of asymmetric consumption is almost linearly related to the unbalance between DG and LG in the catalyst system.