Stereoelectivity, effect

Some cadmium compounds, including simple salts, were revealed to be excellent catalysts for the enantiosymmetric polymerisation of propylene sulphide [156,157], For instance, the proportion of isotactic diads in the polypropylene sulphide) sample obtained in polymerisation with the cadmium (7 )-tartrate catalyst was more than 95%, higher than the 69% which was characteristic of a polymer sample prepared using the zinc (i )-tartrate catalyst [158]. The superior stereoselectivity of the cadmium (i )-tartrate catalyst is also borne out by the more effective separation into fractions having opposite optical rotations of the polypropylene sulphide) yielded by cadmium tartrate, compared with that yielded by zinc (i )-tartrate. Note the quite different behaviour of these two catalysts in terms of their stereoelectivity in the polymerisation of propylene sulphide only very slight optical activity was found for the polypropylene sulphide) sample prepared using cadmium tartrate, whereas that associated with the polymer sample obtained with zinc tartrate was found to have a much higher value [158]. 

A similar scheme was reported by Pino, based on the stereoregulating effect of electron donors and on the stereoelectivity obtained in the polymerization of racemic a-olefins in the presence of chiral bases 109). 

Rebek, J., Jr., Duff, R. J., Gordon, W. E., and Parris, K. Convergent functional groups provide a measure of stereoelect ronic effects at carbonyl oxygen. J. Amer. Chem. Soc. 108, 6068-6069 (1986). 

Effect on stereoelectivity If one considers initiators prepared in homosteric cond itibhs (I < 1) it is possible to compare the efficiency of resolution depending on the chiral hydroxy ligand associated with the organometallic compound. 

Effect on stereoelectivity Let us consider optical yields obtained at half reaction with several thiiranes and oxi" ranes. 

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). 

Effect of the temperature on stereoelectivity Few results only were heported on the effect of the tiemperature on stereoelection. In the case of monomers of first class the stereoelectivity was not modified by changing the temperature, while the stereoselectivity of the process increased by lowering the temperature of polymerization as demonstrated in the case of methyl oxi-rane (25). 

The temperature showed a strong effect in the polymerization of t-butyl thiirane (26) The stereoelectivity Pj, doubled in va lue when temperature Towered from 20° to -3" and on the contrary Pn decreased with raising of T and at temperatures higher than 1T5° the choice of the enantiomer was inverted as shown in table IV. The limit value of the optical purity of monomer was also modified. 

Effect of solvents and additives The role of solvent may be important in such anionic-coordinated" polymerizations. It was shown for example in the case of methyl thiirane that addition of tetrahydrofuran decreased the stereoelectivity, a competition occuring between the monomer and the solvent for the coordination on the metallic atom (30). 

Another goal of this study was to determine the important parameters which determine the rates of polymerization of these chiral e-lactones. This study is presently directed at investigating the effect of reaction variables (solvent and counterion) on polymerization rate, and in the future, attempts will be made by rate studies to ascertain if stereoelection exists in this homogeneous, anionic polymerization reaction. 

Another effective stereoelective catalyst for both epoxide and episulfide polymerization is the reaction product of R-(or S-) t-butylethylene glycol with diethylzinc (33). 

Thus the chiral agents seem to promote two effects not necessarily correlated, firstly a modification of the chirality around the active sites leading to an increase of their stereoelectivity and secondly a more or less important modification of their selectivity. 

From monomers with two chiral centers, optically active or racemic, (both in the heterocycle and in the lateral chain) different diastereoisomeric polymers can be obtained by classical or stereoelective processes. OA polythiiranes with two OA centers have been synthesized by a stereoelective polymerization of (+) or (-)A -methyl,A -sec-butyl,A-thiiranylamine using ZnEt2/( ) dimethyl-3,3-butanediol-l,2. The presence of an R) or (S) lateral chain has no influence on the stereoelection. In this case the thioether chromo-phore near the asymmetric carbon atom of the main chain becomes optically active and its contribution to the ORD curves is preponderant with no special conformational effect their characterization is now in progress [1 lOd]. 

We shall examine in the next chapters the effects on stereoelectivity and stereospecificity of these various parameters. 

Typically, in a stereoelective polymerization one follows the optical activity of the isolated unreacted monomer at different steps of conversion. The sign of the optical ac- One can also follow in principle the optical activity of the obtained polymer, but one must be careful with solvent effects, fractionation, etc. 

Effect of the solvent in the stereoelective polymerization of methyl thiirane. (Initiator system ZnEtj —/ (-)-tBu—CHOH—CHj OH (1 1) prepared in situ.)... 

Effect of enantiomeric purity of methyl thiirane on stereoelectivity. ... 

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. 

Few results have been reported on the effect of temperature on stereoelection. Propylene oxide was polymerized at various temperatures. It was found that the overall stereoelectivi-ty is not modified when the temperature is changed from +80 to -8° [13]. However the rate of polymerization is decreased (by a factor of 10 ) and the distribution of different fractions in the polymer modified. With a lowering of the temperature the proportion of crystalline fraction is increased up to 70% (Figure 11). The behaviour of propylene sulfide is similar to that of propylene oxide, r being not modified between +30° and -30°, while the crystallinity was increased in the latter case [24]. 

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°. 

Effect of enantiomeric purity of initial monomer on properties of polymer obtained by, stereoelective process. 

An equilibrium of complexation of the enantiomers with the different sites was suggested as explanation of the stereoelection. However in that case an effect of the temperature on the equilibrium, i.e. ithe stereoelection, should be observed, which was not presently found in the polymerization of methyl oxirane and methyl thiirane. 

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