Stereospecific polymerizations comparisons

The catalysis of the stereospecific polymerization of conjugated dienes is of considerable interest from both the scientific and the industrial points of view [1,2]. From butadiene and isoprene, as the industrially most important 1,3-dienes, in comparison with the polymerization of olefins many more structurally different stereoregular polymers can be derived cf the structures of the stereoregular polybutadienes and polyisoprenes given in Scheme 1 [106]. 

This review article is concerned with chemical behavior of organo-lithium, -aluminum and -zinc compounds in initiation reactions of diolefins, polar vinyls and oxirane compounds. Discussions are given with respect to the following five topics 1) lithium alkylamide as initiator for polymerizations of isoprene and 1,4-divinylbenzene 2) initiation of N-carboxy-a-aminoacid anhydride(NCA) by a primary amino group 3) activated aluminum alkyl and zinc alkyl 4) initiation of stereospecific polymerization of methyloxirane and 5) comparison of stereospecific polymerization of methyloxirane with Ziegler-Natta polymerization. A comprehensive interpretation is proposed for chemistry of reactivity and/or stereospecificity of organometallic compounds in ionic polymerizations. 

Comparison of Stereospecific Polymerization of Methyloxirane with Ziegler-Natta Polymerization... 

The interest for such stereospecific polymerization started beginning of 50 S when crystalline polypropylene oxide of h.m.w. was synthesized. Numerous researches performed and still under investigation in the field of oxiranes were summarized in several reviews (1-5). The study of stereospecific polymerization of thi-iranes started later, early 60 S, and the main results were also recently reviewed (6-8). The comparison of different aspects of polymerization of oxiranes and thiiranes were examined in few publications (9-12). Practically no work have been succeeded in the field of coordination type polymerization of aziridines, although the cationic polymerization was extensively studied. 

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. 

By comparison, intramolecular chelation can be expected to reduce the degree of association of the catalyst. Addition of water results in increased association after hydrolysis of the ferric alkoxide. This may explain the effect of promoting stereoregularity by addition of water. The ferric alkoxide catalyst can also be made highly stereospecific by partial hydrolysis and still remain soluble in ether, the polymerization medium. This led to a suggestion that the catalyst may contain active Fe-O-Fe bonds. Such bonds would be formed from condensation of partially hydrolyzed alkoxide derivative. The monomer insertion between the iron-oxygen bonds can be illustrated as follows ... 

From the obtained polymerization data we have evaluate the activity of the catalysts employed. Table 1 lists the activity of the catalyst by us prepared (FOT cat.) compared with that exhibit by the commercial catalyst and the molecular weights of the obtained polymers. It is to be outlined that both these polymerizations were carried out in absence of hydrogen and external Lewis bases. Furthermore the polymerization promoted by the FOT catalyst was carried out also in absence of the internal base. As the presence of these components influences both the activity and stereospecificity of the catalysts, a comparison with the features shown by the commercially produced polypropylene it is not possible. 

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