Stereoelective processes

In most of the cases the choice is not as perfect and one speaks of "stereoelective"process when a preferential polymerization of one of the enantiomers from a mixture is observed. 

A linear correlation was found between log p, and 1/T and the overall difference in energy of activation relative to the stereoelective process for both enantiomers could be calculated (5 Kcal/ mol). 

The synthesis of these polymers may be carried out starting from either optically active monomers of very high optical purity or racemic monomers. In the first case polymers having steric order in the side chains are obtained irrespective of whether the polymerization process is stereospecific or not. In the second case the polymers exhibit steric order in the side chains only if the process is such that each macromolecule predominantly derives from one of the antipodes of the monomer (stereoselective or stereoelective process). 

The main known features in such stereoelective processes were... 

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... 

The preliminary results show that a linear correlation exists between optical activity of initial polymers and their degradation products and thus, the latter reflect the structure of the polymer chain. From mechanistic point of view this study of cis monomers supports the important concept that the chiral initiator is not only able to distinguish between two enantiomeric molecules (stereoelective process) but is able to recognize an asymmetric carbon of parent configuration and to orientate the attack on the neighbouring one. This is the caracteristies of a "regioselective process and it opens a field of the study of differently disubstituted monomers leading to various diastereo-isomeric structures. 

From racemic mixtures of monomers containing one chiral center in the heterocycle and an achiral lateral chain an OA polymer is obtained with the asymmetric carbon atom in the main chain by a stereoelective process using asymmetric initiators (see Section... 

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]. 

Polymerization of racemic vinyl monomers with chiral stereospecific catalysts yields, in general, a mixture of macromolecules of different stereoregularity, also containing different types of sequences of d- and /-monomeric units as a result of stereoselective or stereoelective processes (see the review by Pino [106]. 

Until now, it was not possible to find a good fractionating system for polypropylene sulfide obtained by stereoelective processes (see paper of Dr Spassky etal). 

In the case of chiral cyclic monomers the prepared polymer is isotactic, while this is not a necessity in principle in the case of chiral olefins. In most of the cases however, the choice is not as perfect as defined and, generally, one speaks of stereoelective process when there is a preferential polymerization of one type of enantiomer from a mixture. Moreover the sites controlling the polymerization could be more or less stereospecific, so that heterotactic polymers could in principle be obtained with the predominance of one type of enantiomeric unit. 

If however only two types of highly stereospecific, e.g. enantiomorphic, sites are present in unequal number the stereoelective process is at the same time highly stereoselective and the optical activity of the polymer is due to a predominant amount of one type of enantiomeric macromolecules. 

In the stereoelective process we have considered the interaction between a racemic monomer and a chiral initiator, which produces an optically active polymer enriched in one of enantiomers while the unreacted monomer is enriched in the opposite antipode. Thus the enantiomorphic choice of the catalyst is the predominant element of this process and it could be characterized by its stereoelectivity and its stereospecificity. 

There is a linear correlation between logp and 1/T (Figure 12) and from the slope of the line one can determine the difference in energy of activation relative to the stereoelectivity processes for both enantiomers [28]. 

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

The reader should note the difference between the terms stereoselective and stereoelective. The former refers to process 2, the latter to process 3. 

It is part of the question wether man-made stereoelect ronic pattern could act not only as competitors, but might advance for substitutes and partners of the natives of the great process. 

A "stereoelective" (17) or "asymmetric-selective" (3) polymerization is a process in which a single stereoisomer oT a mixture is polymerized giving macromolecules containing one type of configurational base units. For example an optically active catalyst will choose one enantiomer from a racemic mixture and form a macromolecule containing only one type of enantiomeric units. Such an ideal reaction should stop at 50 % yield after consumption of the corresponding stereoisomer. 

Influence of the nature of the monomer Oxiranes and thiiranes could be polymerized by the same type of initiators which makes easy a way of comparison of their behaviour. We shall now use our standard homosteric initiator ZnEt2 (-)DMBD (1 1) and study the influence of the nature of the monomer on the stereoselectivity and the stereoelectivity of the process. 

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). 

On the contrary, in the case of monomers of the second group (t-butyl thiirane), the active centers are formed after complexation of the monomer on the initiator species. Thus, the stereoelectivity should depend on the temperature which is indeed observed in a significant way. The second order law could be explained by a two step process the complexation -a reversible step, then the propagation step. 

Stereoselective or stereoelective polymerization of racemic monomers. Stereoregular polymers have been obtained by stereoselective polymerization from many cyclic, racemic monomers (100,121,127-140). Some of these monomers are indicated in Table 2. The identification of the stereoselective character of a polymerization process, which leads to isotactic optically inactive polymers, requires a very accurate characterization of the polymer obtained. For the polymers reported in Table 2, elution chromatography on an optically active support, spectroscopic (IR, or NMR) measurements, comparison of the X-ray pattern with that of the corresponding optically active polymer, and enzymic degradation have been used. In many cases the stereoselectivity of the polymerization process has been confirmed by the stereo-electivity observed when using the catalyst in an optically active form. 

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. 

The stereoelective choice of monomers is however in agreement with configuration rules as seen from the sign of optical activity of unreacted monomer. Homosteric and antisteric processes are observed with considerable amount of a-scission (39). According to the chemical composition of these initiators a cationic character of the latter seems to be excluded and therefore this particular behaviour could be due to some steric reasons which are not yet completely understood. New studies are now in progress. 

In a previous paper presented by Dr, Leborgne and Spassky, from the Universite Pierre et Marie Curie, in Paris, it has been shown that it is now possible to prepare optically active polylactones by a "stereoelective" polymerization process. It is the purpose of the present paper to indicate important differences in properties between the optically active and the racemic poly(a-methyl-a-n-propyl-3-propiolactone) (PMPPL). Results will be presented concerning the crystallization and melting properties, as well as the dynamic and transient mechanical properties of these polymers. 

It can be concluded that studies of a subtle elements of propagation, particularly in the coordinated processes, are still needed. Perhaps the advanced quantum mechanical calculations methods would be very helpful for this purpose. The p-substituted p-lactones are still waiting for the stereospecific initiator allowing the stereoelective and/or stereoselective polymerizations to be carried out. Finally, initiators of the stereocontrolled polymerization operating in bulk, high-temperature processes are also wanted. 

The polymers described in this part possess monomeric units of a given configuration in excess over the opposite one because of preferential conversion of one of the two antipodes of the racemic monomers, by two fundamentally different processes stereoselection or stereoelection. This last one has been recently reviewed by Tsuruta [106,137], especially from the point of view of the mechanism, which is often problematical. 

Stereoelective polymerization of a racemic monomer is a sterical control polymerization process in which one of the monomeric antipodes is preferentially polymerized. If the conversion is not complete, both polymer and recovered monomer are optically active. It is synonymous with asymmetric-selection polymerization the yield can be simply evaluated from the optical activity of unreacted monomer. This theoretically important method of synthesis concerns, for the moment, mainly the polymerization of racemic three-membered heterocycles (oxiranes, thiiranes and aziridines) and of a-amino-7V-carboxy acid-anhydrides (or Leuch s salts). 

A stereoelective (4) or asymmetric selective (5) polymerization is a process in which a single stereoisomer of a mixture is polymerized giving macromolecules containing only one type of configurational base units. 

A shown in Figure 2 the stereospecificity and the stereoelectivity of a process are determined by the active species formed by the interaction between the monomer and the initiator. 

By extension one may call a homosteric type initiator an initiator which gives an homosteric stereoelection with a given monomer. Antisteric type process were observed in the case of methyl thiirane [19, 23, 24] and more recently in the case of methoxy methyl thiirane [25] and ethyl thiirane [26]. However, f-butyl thiirane did not give inversion with conventional antisteric initiators. This could be due to its high sterical hindrance. 

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