Racemic, stereospecific polymerization

For instance, in the case of the stereospecific polymerization of racemic 3-methyl-l-pentene in the presence of catalysts capable of yielding isotactic polymers, the disposition of the asymmetric carbon atoms of the obtained isotactic polymer can be, in principle, XVIII, XIX, XX ... 

Thus the stereospecific polymerization of a racemic monomer will yield optically active polymers only if it is accomplished stereoelectively, namely in the presence of catalysts capable of polymerizing preferentially one of the two antipodes of the racemic monomer. 

An isotactic stereospecific polymerization arises essentially from the favored complexation of one prochiral face of the a-olefin, followed by a stereospecific process. The stereospecific insertion process and the stereospecific polymerization of racemic a-olefins giving isotactic polymers may be expected to be stereoselective whenever the asymmetric carbon atom is in an a- or /3-position relative to the double bond, and when the interaction between the chirality center of the olefin and the chiral catalytic site is negligible. 

Heterobicyclic compounds often display specific behavior in their ring-opening polymerization. This is attributed to their rigid and bulky structures which contain, in most cases, two or more asymmetric carbon atoms. Sometimes, stereoelectronic effects involving heteroatoms also play an important role in regulating polymerization processes. For example, racemic bicyclic acetals such as 6,8-dioxabicy-clo[3.2.1]octane and its derivatives often undergo stereospecific polymerization even in the presence of conventional Lewis acid initiators. 

Table 5. Stereoselectivity in the stereospecific polymerization of racemic a-olefins of the type CH2=CH-(CH2) (catalytic system TiCl3/AlR3) (185)...

Enantiomorphic sites concept The stereospecific polymerization of racemic methyloxirane produces generally a polymer with some cristallinity which can be fractionated by selective solubility (using acetone for example) in a crystalline fraction and an amorphous one (an intermediate "semi-crystalline fraction can be also isolated). 

As already indicated it is well known that stereospecific polymerization of racemic vinyl monomers or heterocycles can yield a mixture of macromolecules with varying proportions of block sequences of one or the other enantiomeric monomeric units such... 

A different situation arises when one considers a stereospecific catalyst which is endowed with optical activity and which favors therefore a specific configuration. Such a catalyst, if highly stereospecific, should form polymers, for example of all d configuration with an occasional inclusion here and there of l units. Of course if a racemic mixture of such a catalyst is used, then formation of a racemic mixture of polymers is expected, each polymeric molecule having an all d configuration incrusted with l units or an all l configuration incrusted with d units. 

These experimental findings suggest that the poly-a-olefins obtained in the presence of the conventional stereospecific catalysts should have the same optical purity as the monomers. Therefore, in the polymer of a (S) monomer having high optical purity, the remarkable differences in optical activity observed in the fractions having different stereoregularity cannot be attributed to the presence of different amounts of (R) asymmetric carbon atoms in the lateral chain, formed by racemization of the monomer during the polymerization. 

In the field of the stereospecific heterogeneous polymerization of a-olefins, the stereoselective (106,118) and stereoelective (103) polymerization of racemic monomers, having an asymmetric carbon atom in a. 

Recently a simplified process was developed for incorporating l-methionine directly into soy proteins during the papain-catalyzed hydrolysis (21). The covalent attachment of the amino acid requires a very high concentration of protein and occurs through the formation of an acyl-enzyme intermediate and its subsequent aminolysis by the methionine ester added in the medium. From a practical point of view, the main advantage of enzymatic incorporation of amino acids into food proteins, in comparison with chemical methods, probably lies in the fact that racemic amino acid esters such as D,L-methionine ethyl ester can be used since just the L-form of the racemate is used by the stereospecific proteases. On the other hand, papain-catalyzed polymerization of L-methio-nine, which may occur at low protein concentration (39), will result in a loss of methionine because of the formation of insoluble polyamino acid chains greater than 7 units long. 

The above Cp2MQ2 species are immediate precursors for highly active Ziegler-Natta catalysts for die preparation of isotactic polypropylene with methylalumoxane as a cocatalyst die racemic mixtures can be directly employed in stereospecific propylene polymerizations (see Section IV,D,2) (225,227,228) the enantiomers are employed in asymmetric hydrooligomerizations ofa-olefins... 

The above mechanisms for cis-1,4 polymerization of isoprene or isotactic polymerization of acrylates assume that the configuration of each unit is fixed at the moment of reaction and that no racemization occurs between additions of monomer molecules. Little evidence for the validity of the mechanisms was available when suggested. Recently it has been possible to obtain information, from NMR studies, on the reaction path. This evidence is of two types and depends on the polymerization of stereospecifically deuterated monomers to determine the mode of approach of monomer molecules and on direct observations of NMR spectra of the terminal monomer unit in the polymer. 

The coordinate type catalysts are also effective for thiirane polymerizations. The types of systems used are also similar. Thus diethylzinc and in particular diethylzinc/water mixtures have been studied [44]. Other studies made using triethylaluminium and diethylcadmium indicated that these metal alkyls all behave similarly. The reactions seem to be rather complex, and, as also was the case with the epoxides, no well defined kinetic studies have appeared. The polymers produced are of high molecular weight and are often crystalline. Thus stereospecific polysulphides have been reported. Again the bulk of the studies involve PS. Stereoselective polymerization of racemic monomer has been accomplished [45, 46] using a catalyst prepared from diethylzinc and (+) borneol. The marked difference between PO and PS in their polymer-... 

The X-ray structures of ethylenebis(indenyl) and bis(tetrahydroindenyl) hafnium dichloride have been determined and compared to Zr and Ti complexes. The ligand-metal distances decrease in the order Zr > Hf Ti. The hafnium complex activated by methylaluminoxane catalyzes the polymerization of propene at 50-80 °C. Indenyl complexes are more active and give higher molecular weight products, but are less stereospecific than tetrahydroindenyls. Racemic zirconium... 

Lactic acid reacts with diacid or diol to form telechelic polylactic acid, then through further a linking reaction it forms high-molecular-weight lactic acid copolymers [33, 44, 45]. Polymerization of a racemic mixture of L- and D-lactides usually leads to the synthesis of poly-DL-lactide (PDLLA)which is amorphous. Use of stereospecific catalysts can result in heterotactic PLA which is found to show crystallinity [46, 47]. The degree of crystallinity and many associated properties are greatly controlled by the ratio of D to l enantiomers in the polymer [48]. 

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