Asymmetric-selection polymerization of propylene oxide by diethylzinc and D- or L-glutamate system

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  • 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

    1. Introduction

    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

    2.1. Materials

    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.

    75

  • 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.

    Diethylzinc (ZnEt,)

    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.

    Other reagents

    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.

    76

  • 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 )

    where

    k,, k , : Consumption rate constants of d(+)-PO and Z(-)-PO, respectively. x: Total conversion percent.

    CLM:

    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.

    77

  • J. FURUKAWA, T. SAEGUSA, S. YASUI, and S. AKUTSU

    Table 1. Asymmetric-selection polymerization of propylene oxide by ZnEtz/diethyl gluta- mate system*) : Effect of unbalance in glutamate component

    Unbalance in glutamate componentb)

    Polymer yield

    (Yo) 21 26 23 25 21 24 20

    Cry st. c) Part

    of polymer (Y") 7.6 7.0 7.0 6.8 7.1 7.0 7.2

    [alD of recovered monomer

    +0.30 +0.21 +0.08

    0.00 -0.09 -0.22 -0.28

    Unbalance degreed) of consumption

    rate ~

    0.082

    0.026 I 0.053 kd+kl 1 0.083 kd--kl

    a) Polymerization conditions: d,Z-PO, 30 ml.; ZnEt,, 21,6 mmole (4.8 mole-% o monomer); combined amount of LG (diethyl L-glutamate) and DG (diethyl D-glutamate), 21.6 mmole; toluene, 30 ml.; catalyst system was treated for 40 min. at 105-110C. before the addition of monomer; polymerization a t 75 "C. for 6 days.

    b) The composition of the glutamate isomers was varied from pure DG to pure LG, the combined amount of them being kept constant.

    C) Insoluble part deposited from 2 yo solution of polymer in acetone a t 0 "C. The optical activity of amorphous part was negligibly small.

    d) Degree of unbalance in consumption rate between two monomer enantiomers was calculated by Eq. (1).

    e) Racemic mixture of glutamate was used.

    100 67 33 0 33 67 100 DG 4 - LG

    Fig. 1. Asymmetric-selection polymerization of propylene oxide by ZnEtz/diethyl gluta- mate system. Relationship between unbalance in monomer consumption rate (ordinate)

    and unbalance in glutamate component in catalyst (abscissa) DG: Diethyl D-ghtamate, LG: Diethyl L-glutamate

    78

  • Asymmetric-Selection Polymerization of I'ropylene Oxide

    Gluta- mate

    compo- nent

    The optical activities of produced polymers in the d,Z-PO polymeri- zations with ZnEt,/DG and with ZnEt,/LG catalysts are given in Table 2, The stereoregularity of produced polymer and the selectivity of catalyst system were different from those given in Table 1. The difference may be ascribed to the difference in the condition of catalyst preparation.

    Polym. Cryst. [ a ] ~ of yield part b) recovered

    (A) monomer (%) (oh) an1 (obsd.)

    Table 2. Asymmetric-selection polymerization of propylene oxide by ZnEt,/diethyl gluta- mate system") : Relationship between optical activities of recovered monomer and produced

    polymer

    LG LG DG

    13.0 4.5 -0.30 11.0 4.3 -0.18 9.3 3.5 +0.14

    +1.2 +2.6 -2.0

    -1.0 -0.045 -0.18, -2.4 -0.103 -0.14, +1.8 +0.068 f0.12,

    a) Polymerization conditions: d,l-PO, 30 ml.; ZnEt,, 21.6 mmole (4.8 mole-% to mono- mer); LG or DG, 21.6 mmole; toluene, 30 ml.; catalyst system was treated at 8OoC. for 10 min. before the addition of monomer; polymerization at 75 "C. for 4 days.

    b) Insoluble part deposited from 2 yo solution of polymer in acetone a t 0 "C. C) Amorphous part of polymer has no optical activity. d) Optical activity of total produced polymer consisting of crystalline and amorphous parts,

    ap = (A) x (B), because the optical activity of amorphous pait is negligibly small. e, aM (calcd.) has been given from ap according to Eq. (14).

    In Table2, a somewhat curious phenomenon is seen, i.e., the sign of optical rotation of produced polymer is opposite to that which is to be expected from the unbalance of monomer consumption. The catalyst system ZnEt,/LG prefers the consumption of d(+)-PO to that of I(-)-PO (see the first two runs of Table 2). If the configuration of asymmetric carbon atom of monomer is not inversed in the polymerization, d(+)-PO monomer should give a polymer whose optical activity is (+) in CHCl, and (-) in C,H, 12). The produced polymers in these runs of Table 2, however, showed (-) sign in CHCl, and (+) in C,H,. The peculiar phe- nomenon was also observed in the polymerization with ZnEt,/DG system, in which the optical rotatory directions of recovered monomer and pro- duced polymer were exactly opposite to those in the polymerization with ZnEt,/LG system.

    This apparently curious phenomenon has been explained by assuming that partial inversion of the asymmetric carbon atom of PO monomer

    79

  • J. FURUKAWA, T. SAEGUSA, S. YASUI, and S. Aau~su

    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 polymerization with the asymmetric catalyst.

    In the course of polymerization, d-monomer gives not only d-unit (UD) but also 2-unit (UL), where UD has the carbon atom having the same con- figuration as that of the original d-monomer and UL, on the other hand, contains the carbon atom having the opposite configuration. The mode of conversion of d-monomer is so with that of 1-monomer.

    retention 17) UD in polymer d-monomer-

    inversion

    retention + UL inpolymer IT I-monomer -l

    Inversion ___) UD in polymer

    1LfL

    (4)

    Here fD is the retention factor of d-monomer, which is defined as the fraction of UD resulting from d-monomer in the combined amount of UD and UL from d-monomer. Consequently, (1-fD) is the fraction of UL in UD + UL in the units from d-monomer. Similarly, fL is the retention factor of 1-monomer.

    For the polymerization of d,l-PO with partial inversion of the monomer configuration, the following equations can be derived concerning with the material balance.

    (6 ) UD = fDDp + (1-fL)Lp

    UD + UL = Dp + L p (=P) (8 )

    80

  • Asymmetric-Selection Polymerization of Propylene Oxide

    Dp + Lp = 100-(D + L) (9)

    D + L = M

    where D, and L, are...

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