Chiral molecules vinyl polymers

Scientists who joined the Polymer Research Institute at Brooklyn Polytechnic were encouraged by Herman Mark to carry out research in areas they might have been familiar with in their studies of small molecules. Professor Mark knew from his scientific experience that there existed no boundaries between the principles of the science that depended on molecular size. In 1980 therefore when I first met him on joining Polytech he asked about my background and, on discovering that I had an interest in stereochemistry and chirality, he suggested that I look at the stereochemical properties of the vinyl polymers. This led me to read the work of two... 

In small molecules, the absence of optical activity at some wavelength for chiral molecules is almost always ascribed to the fact that the ensemble of molecules contains equal number of both enantiomers - a racemic mixture. However, in a chain longer than a hundred or so units, statistical considerations demonstrate that the presence of mirror image isomers of enantiomeric chains, and therefore for racemic states, is virtually impossible, which leaves the absence of optical activity in atactic vinyl polymers an open question. The answer turned out to be one that never arises in small molecule stereochemistry an ensemble of polymer chains of an atactic polymer is a mixture of diastereomeric chains, each one chiral but without the enantiomeric chain present in the ensemble. If a single chain could be studied by a method that could reveal chiral optical properties, optical activity should be observed. However, each chain in the ensemble (a very large number of chains) would exhibit a different optical activity, even of differing sign. The optical activity properties of a sample of an atactic polymer would arise as the sum... 

The polymerization of monosubstituted vinyl compounds that give polymers like PS and PP produces polymer chains that possess chiral sites on every other carbon in the polymer backbone. Thus, the number of possible arrangements within a polymer chain is staggering since the number of possible isomers is 2" where n is the number of chiral sites. For a relatively short chain containing 50 propylene units the number of isomers is about 1 x lO. While the presence of such sites in smaller molecules can be the cause of optical activity, these polymers are not optically active since the combined interactions with light are negated by other similar, but not identical, sites contained on that particular and other polymer chains. Further, it is quite possible that no two polymer chains produced during a polymerization will be exactly identical because of chiral differences. 

Chiral polymers have been applied in many areas of research, including chiral separation of organic molecules, asymmetric induction in organic synthesis, and wave guiding in non-linear optics [ 146,147]. Two distinct classes of polymers represent these optically active materials those with induced chirality based on the catalyst and polymerization mechanism and those produced from chiral monomers. Achiral monomers like propylene have been polymerized stereoselectively using chiral initiators or catalysts yielding isotactic, helical polymers [148-150]. On the other hand, polymerization of chiral monomers such as diepoxides, dimethacrylates, diisocyanides, and vinyl ethers yields chiral polymers by incorporation of chirality into the main chain of the polymer or as a pedant side group [151-155]. A number of chiral metathesis catalysts have been made, and they have proven useful in asymmetric ROM as well as in stereospecific polymerization of norbornene and norbornadiene [ 156-159]. This section of the review will focus on the ADMET polymerization of chiral monomers as a method of chiral polymer synthesis. 

The various regular polymers that can be produced by polymerization of butadiene and isoprene are summarized in reactions (4-3) and (4-4). In addition to the structures shown in these reactions, it should be remembered that 1, 4 polymerization can incorporate the monomer with cis or trans geometry at the double bond and that the carbon atom that carries the vinyl substituent is chiral in 1,2 and 3,4 polymers. It is therefore possible to have isotactic or syndiotactic polybutadiene or polyisoprene in the latter cases. Further, these various monomer residues can alt appear in the same polymer molecule in regular or random sequence. It is remarkable that all these conceivable polymers can be synthesized with the use of suitable catalysts comprising transition metal compounds and appropriate ligands. 

A solution phase chiral auxiliary for 1,3-dipolar cycloaddition of isomunchnones with vinyl ethers has been adapted for solid phase synthesis by attaching both enantiomers of the precursor a-hydroxyvaline to benzhydrylamine resin (Scheme 12.12) [13,19]. The auxiliary 22 was then functionalized by acylation and diazotiza-tion to provide diazoimide resin 23. Rhodium(II)-catalyzed nitrogen extrusion and cycloaddition in the presence of different vinyl ethers afforded, after detachment from the polymer, various bicydic molecules (24) in 49-65% yield and provided high degrees of selectivity (93-95% ee). 

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