Homosteric

It may be noted that all of the homosteric catalysts were chiral zinc systems, the composition of which was expressed as [RZnOR ]x[R,OZnOR,]J,(v/y < 1). Antisteric catalysts were chiral zinc- or cadmium-based systems of a common compositional feature expressed as [RZnOR ]x[R OZnOR ]>, or [RCdOR ]x [R OCdOR ]) (x/y > 2). The nature of homosteric and antisteric stereoelection has not yet been elucidated fully at the molecular level because the structure of the operating species and the polymerisation mechanism with these catalysts are not clearly established [52]. 

Polymerization of chiral monomers was reviewed recently by Sigwalt430). Two situations may arise. The non-chiral initiator may interact randomly with either enantiomer, but its parity introduces a bias in the subsequent addition. For a homosteric bias, i.e. when the addition of one enantiomer favors the subsequent addition of the same kind, the resulting polymer is composed of sequences of blocks of one enantiomer followed by a block of the other. This is a typical example of stereoselectivity. For an antistericbias, i.e. the addition of, say R enantiomer is favored by the presence of terminal S enantiomer, or vice versa, the resulting polymer shows a bias for a simple alteration. 

Qualitative predictions of the theory Provided that L3 is not excessively small, the interpenetrational domain will determine the flocculation behaviour in heterosterically stabilized systems, just as it does in homosteric stabilization. In the limit of small L3, the interpenetrational-plus-compressional domain may well become important in predicting incipient instability. The elaboration of the general principles that govern heterosteric stabilization is then quite different. 

Summary. The foregoing analysis shows that for positive values of X23 corresponding to incompatible polymers, stability will normally be observed on mixing homosterically stabilized dispersions. Heteroflocculation is not possible without accompanying 2-2 and/or 3-3 homoflocculation. The selective flocculation of one type of particle in a mixture of different particles is predicted to be possible under suitable conditions, and has been demonstrated experimentally. 

If the dispersion medium is a good solvent for both polymers (xi 2. X13 < 2). then the system will be stable provided that Ix23l< 1—X12—Xi3- This apparently explains the heterosteric stabilization, alluded to above, that is ol rved on mixing aqueous polystyrene latices homosterically stabilized by poly(ethylene oxide) and polyacrylamide. 

The preceding discussion implies that compatible polymers should often induce heteroflocculation when particles homosterically stabilized by them are mixed. This should be especially evident with polymer pairs that form coprecipitates or coacervates, and in solvents that are not very good solvents for either polymer. Such behaviour contrasts markedly with the mixing of particles homosterically stabilized in the same dispersion medium by incompatible polymers, for which stability is predicted to occur. Selective flocculation seems feasible irrespective of the sign of Xz3, but will undoubtedly be more common in systems for which xzz is positive. 

The foregoing conclusions readily encompass mixtures that contain more than two types of homosterically stabilized particles since these can usually be treated in a pair-wise fashion. Of special importance is the removai of one type of coated particle in a mixture of particles by the addition of another type of coated particle that selectively heteroflocculates with it. 

In this equation, S =(1-I-Z-3/2L2+L2/2jI-3- I o(1/jI 2 + 1/ 3) + Ho I2L2L ), where Ho = minimum distance of separation of the surfaces of the particles. Of course, the Derjaguin integration procedure is only valid if a L2, Z-3. Note that if polymers 2 and 3 are identical, equation (14.12) reduces to the result derived previously for homosteric stabilization. 

Since the two hydrogen atoms of a methylene group in an isotactic diad are not nmr-spectroscopically equivalent, they have also been called (and also called heterosteric or diastereotopic). In analogy to this, the methylene group of a syndiotactic diad has been called racemic (and also called homosteric or enantiotopic). For these reasons the composition fractions of isotactic and syndiotactic diads are often given in the literature as (m) or (r) instead of asXi or The names racemic and meso are not equivalent... 

Conflguratlonarrelatlons could be established In several cases. If one considers the absolute configuration of the chiral ligand and that of the cyclic monomer, the choice of the initiator would correspond to an "homosteric" type process if the chosen enantiomer has the same configuration as the chiral ligand used in the initiator (18). Homosteric configurational relations are illustrated In the next scheme. 

We were able to isolate species of both types in the case of diethylzinc-(+) 3,3 dimethyl 2 butanol initiator system. The antisteric species has a composition close to Etf.Zn(0R)j, or Zn(0R) . (EtZnOR)g (-OR being the 3,3 dimethyl 2 butoxy group), while an homosteric initiator had a Zn(0R)j>. EtZnOR composition. Both species were soluble in benzene and were studied by H-NMR (18). It was possible to transform one specie into the other by ailing ZnEtp or by drying or heating (loss of ZnEt, and disproportionation . ... 

We shall now consider only homosteric type initiators for simplicity. 

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. 

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. 

Little or no activity was found when the zeolite cage size was too small to readily accommodate the catalyst, monomer, and growing chain. Two zeolite-like supports with larger pore sizes, 130 nm and 400 nm, were studied. These supported catalysts polymerized propylene much like the same catalyst in solution with respect to activity, molecular weight, and homosteric pentad population. 

In Figure 3.19(b) is shown the fully proton decoupled spectrum obtained in pyridine at 38 "C of a (MMA/MAA) copolymer of approximately 33% acid composition. While the spectrum resolves into a-CHj, CHj, OCH3 >C<, and C - O regions, the structural similarity of the comonomers results in resonance overlap (at 20 MHz) between ester and acid carbons in all regions but the carbonyl. The complete structure in the carbonyl region arises from the sensitivity of the carbons to the microstructural features of the copolymer chain, i.e., sequence and tacticity effects. To be sure that these effects do not lead to overlap of ester and acid carbonyl resonance, chemical shifts of both homopolymers and homosteric copolymers were examined with the result that the carbonyl resonance region does separate into distinct acid and ester regions in pyridine. 

We have demonstrated in the case o few thiiranes, oxiranes and recently for a,a disubstituted 3 propiolactones (36) that the type of the choice (homosteric or antisteric) is depending only on the x/y ratio i.e. the ratio of alkylalkoxide species over dialkoxide species. 

In some cases both type of species could be isolated under a soluble form. For example, when reacting diethylzinc with (+)3,3 dimethy1-2-butanol, antisteric species with a composition close to Et Zn (OR) were found (40) which are similar in composition to those reported for the methanol derivatives (13). The homosteric species corresponded to Zn(0R)2.EtZn0R co osition. 

In practice we have prepared the homosteric initiator by reacting at room temperature diethylzinc with (-) 3,3 dimethyl 1,2 butanediol and the antisteric initiator by reacting the latter diol with dimethylcadmium in the same conditions. 

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. 

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