Stereoelectivity ratio

Stereoelective polymerization of enantiomerically unbalanced (i.e., partly resolved) mixtures of oxiranes and thiiranes has given suq)rising results (277, 318, 322). The stereoelectivity ratio, r, is greatly dependent on the enantio-... 

IC are global rate constants relative to active species tne stereoelectivity ratio relative to R choice. r ... 

Influence of the enantiomeric composition of the monomer. Super stereoe lect ive processes. The enantiomeric composifTon of the initial monomer may have a strong effect on the stereoelectivity. It was shown on the example of methyl thiirane that the value of the stereoelectivity ratio (r) could be raised up to 7 when using enriched monomers (28). The same phenomenon was recently observed with ethyl thiirane (23)and methoxymethyl thiirane (29). 

The stereoelectivity ratios were found to be constant throughout the polymerization, therefore equations (5), (6) and (7) can be integrated and after introducing the optical purity of residual monomer a/a and the conversion x, they give respectively the following general equations applied when the starting mixture is racemic ... 

The stereoelectivity ratio was not modified when running a copolymerization reaction in the presence of achiral monomer e.g, ethylene sulfide. This confirms again the strong catalyst control process involved in these polymerization. 

The main caracteristics of this process is that it is strongly depending on the temperature. The stereoelectivity ratio P is doubled in value when the temperature lowers from 25 to - 7 C and on the contrary a raising in temperature decreases p and even inverted the enantiomeric choice (p < 1) as reported below Table 3. Dependence of stereoelectivity ratio p on the temperature of polymerization. 

Polymethyloxirane samples obtained with the same catalyst can be fractionated by the usual method in crystalline and amorphous parts. It was found (46) that the stereoelectivity ratio corresponding to crystalline part and amorphous part are quiet different, respectively equal to r = 2.6 and r = 1.6, this supporting again the concept of a large spectra of enantiomorphic sites of different stereospecificities present in the initiator. 

With enantiomerically enriched monomers, the stereoelectivity ratio r, calculated from an equation similar to that proposed for stereoelective process of racemic monomer, is constant during all the course of the polymerization. This result is consistent with the assumption that the active sites are formed in the presence of the monomer in an irreversible way at the first beginning of the reaction. For methyl- and ethyl-thiiranes, the stereoelectigity a linear function of the initial... 

With racemic methylthiirane, the stereoelectivity ratio r obtained with the ZnEt2/R( ) DMBD system modified with chiral additives such as (S) 2-methylbbutyl disulfide, (R) 1,2-dimethoxy 3,3-dimethyl butane and sparteine of good optical purities, is practically the same for all additives and equal to 4, i.e. almost the double of that obtained for unmodified initiator (49). In limonene, as chiral medium, r depends on the relative enantiomeric composition of the limonene used. It was found that the variation of r versus the composition II of(+) limonene pas-... 

We have found, however, that when using an initiator system in which i (-)-3,3-dimethyl-1,2-butanediol was reacted with dimethyl cadmium, the chosen enantiomer was surprisingly 5-propylene sulfide with a stereoelectivity ratio rs as high as 1.9 [23]. The experimental data again fitted well with the theoretical relation established previously (Figure 6). 

Using recent experimental data it was found that the stereoelective curve has a different shape from that proposed previously and correlated with a first order type stereoelectivity ratio r=2.8[12]. 

We have polymerized propylene sulfide and propylene oxide of different enantiomeric composition with the same initiator system, diethylzinc-/ (-)-3,3-dimethyl-1,2-butanediol (1 1). Starting from a non racemic mixture we have to use Equation (1) for the determination of stereoelectivity ratio r. 

In the case of propylene oxide, the introduction of one enantiomer in excess decreases the overall stereoelectivity ratio. However R enantiomer is still preferentially chosen even if S enantiomer is in excess. 

Influence of enantiomeric composition of initial monomer on the stereoelectivity ratio. (Chiral initiator ZnEtj—R(-) -tBu—CHOH — CH OH (1 1).)... 

The polymerization of monomers with unbalanced enantiomeric composition was performed using achiral initiators in the case of propylene oxide [13, 29] and propylene sulfide [24, 29]. In both cases it was shown that the optical activity of nonpolymerized monomer was identical to that of the initial monomer introduced. This seems to eliminate, at least in these cases, a possible chain effect. Furthermore, in the copolymerization of propylene oxide [30] and propylene sulfide [31] with achiral monomers, such as ethylene oxide and ethylene sulfide using chiral initiators, the stereoelectivity ratio is not affected by the achiral comonomer. If an end-chain effect exists, a decrease of stereoelection should be observed with the incorporation of achiral ethylene oxide or ethylene sulfide units. 

The temperature showed, however, a strong effect on the polymerization of f-butyl thiirane. As shown in Table VIII the lowering of temperature from 20° to -3° doubled the stereoelectivity ratio and increased therefore both the optical yield at half reaction and the value of at the limit. On the contrary, an increase in temperature lowered the stereoelectivity and, at temperatures higher than 115—120° the choice of the enantiomer was inverted as shown by the experiment carried out at 135°. 

The examination of polymers prepared at different conversions showed that the crystalline and the amorphous fractions fitted well on theoretical curves corresponding to two different stereoelectivity ratios [13]. 

As shown in Figure 13 the stereoelectivity ratio corresponding to the crystalline fraction (r=2,6) is much higher than that of the amorphous fraction (/ = .6). The crystalline fraction is purely isotactic, as shown by C NMR spectra and therefore according to optical activities obtained (optical purity 40% at low conversion) this fraction should be an unbalanced mixture of enantiomeric poly-i and poly-5 chains. Tsuruta [33] and Furu-kawa [34] showed that the separation of such mixtures is in principle possible. An increase in crystalline fraction could be obtained by lowering the temperature of polymerization, as pointed in sections 2-7. We found that the partial stereoelectivity ratios were somewhat decreased in this case, r going from 1.6 to 1.2 for the amorphous fraction. 

In general such a separation is not possible. We have seen previously that the optical activity of polymers in stereoelective reactions is the highest at low conversions and at the very early beginning corresponds to a distribution directly described by the stereoelectivity ratio. 

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