Chiral molecules optically active polymers

Until now I have discussed the methods of synthesis of optically active polymers from chiral monomers. As is well known in organic chemistry, it is also possible to produce chiral molecules with one preferred configuration by reaction of achiral molecules in the presence of some chiral influence. These reactions are known as asymmetric syntheses (36, 323-325) when an unsatuiated compound is involved, the term enantioface-differenriating reaction is often used (281). 

The opposite case is also worthy of consideration. cis-2,3>Epoxybutane is a meso compound but the two halves of the molecule, and particularly the two O—CH(CH3) bonds, are not equivalent but enantiotopic. Ring opening polymerization occurring selectively on one of the bonds converts the R, S) monomer into a succession of monomer units (R, / )—(/ , R)— and so on, or —(5, S)—(S, S)— and so on. A chiral initiator can effect an enantiotopic differentiation (281) and thus give rise to an optically active polymer with an excess of (R, R) or (S, S) units (81, 82). 

Figure 1(b) shows what kinds of CSPs has been used in HPLC for analyzing chiral compounds. There are two types of CSPs one is prepared from small molecules with a chiral recognition ability and the other from optically active polymers. It is clear that the... 

The formation of the soIute-CSP diastereomeric complexes in these CSPs usually requires the insertion of an aromatic moiety on the solute into the chirality of the optically active polymer. Thus, the solutes should contain an aromatic moiety near or at the stereogenic center. Enantiomeric molecules containing the necessary aromatic moiety and one of the following functionalities have been resolved on these CSPs alcohol, amide, ester, ether, and ketone (9-11). 

Many optically active polymers exist in nature. Polysaccarides, proteins, enzymes, nucleic acids, polypeptides are some examples. Derivatives of such materials may also exhibit optical activity. This activity is usually preserved throughout the derivatization reactions, provided the reactions do not change the nature of the asymmetric carbon atoms that conferred the chirality to the molecule. 

PDBSMA " " and its derivatives, 2PyDBSMA and SPyDBSMA, afford nearly complete isotactic polymers by radical polymerization regardless of the reaaion conditions. The high isotactic specificity implies that the polymer obtained is an equimolar mixture of completely right- and left-handed helical molecules, suggesting that introduction of a nonracemic chiral influence to the polymerization reaction could result in the production of a single-handed helical, optically active polymer with an almost complete isotactic stmcture. 

A prochiral monomer molecule is converted to a polymer with chiral groups in prochiral stereospecific or chirality-producing stereospecific polymerizations. An example of this is the polymerization converting benzo-furan to optically active polymers with the catalyst RAlCl2/optically active phenyl alanine. 

The different examples presented show how complex is the interpretation of chiral property changes of optically active polymers. However, the situation can be simplified if one can consider chemical modifications of chromophores as the major factor affecting ORD and CD i.e. when monomeric units behave as non-rigid small molecules in concentrated solutions. 

Many optically active polymers have been tested for the selective extraction of chiral polymers or small molecules from their racemic mixtures or as chiral catalysts. 

Investigations on synthetic or natural chiral macromolecules making use of rotatory power, ORD or CD techniques, to study conformational or interactional problems. The narrow field of Synthetic Optically Active Polymers has progressively expanded during the past decade. Evaluations and confrontations of results and interpretations are now needed with polymers of the purely synthetic world and with those which nearly imitate nature and are effective model molecules. 

Oligomer (Section 14 15) A molecule composed of too few monomer units for it to be classified as a polymer but more than in a dimer trimer tetramer etc Oligonucleotide (Section 28 6) A polynucleotide containing a relatively small number of bases Oligosaccharide (Section 25 1) A carbohydrate that gives three to ten monosacchandes on hydrolysis Optical activity (Section 7 4) Ability of a substance to rotate the plane of polanzed light To be optically active a sub stance must be chiral and one enantiomer must be present in excess of the other... 

The trick used in asyrmnetric inclusion polymerization is to perform the reaction in a rigid and chiral environment. With more specific reference to chirality transmission, the choice between the two extreme hypotheses, influence of the starting radical (which is chiral because it comes from a PHTP molecule), or influence of the chirality of the channel (in which the monomers and the growing chain are included), was made in favor of the second by means of an experiment of block copolymerization. This reaction was conducted so as to interpose between the starting chiral radical and the chiral polypentadiene block a long nonchiral polymer block (formed of isoprene units) (352), 93. The iso-prene-pentadiene block copolymer so obtained is still optically active and the... 

The primary motivation for the development and application of vibrational optical activity lies in the enhanced stereochemical sensitivity that it provides in relation to its two parent spectroscopies, electronic optical activity and ordinary vibrational spectroscopy. Over the past 25 years, optical rotatory dispersion and more recently electronic circular dichroism have provided useful stereochemical information regarding the structure of chiral molecules and polymers in solution however, the detail provided by these spectra has been limited by the broad and diffuse nature of the spectral bands and the difficulty of accurately modeling the spectra theoretically. 

A polymerization of a bulky methacrylate ester (e.g. trityl methacrylate) using an optically active anionic initiator can give an isotactic polymer, poly 1-methyl-1-[(trityloxy)carbonyl]ethylene of high optical activity owing to the formation of helical polymer molecules with units of predominantly one chirality sense. 

Polymerization of tert-butyl isocyanide using an optically active initiator gives an optically active product comprising helical polymer molecules with units of predominantly one chirality sense. 

Example 3.4 Polymerization of racemic tra 5-2,3-dimethylthiirane (DMT) using an optically active initiator may proceed by reaction of only one of the two enantiomers to give stereoregular but optically inactive, non-chiral polymer molecules as a result of inversion of the configuration of the attacked carbon atom. 

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. 

Complexes between chiral polymers having ionizable groups, and achiral small molecules become, under certain conditions, optically active for the absorption regions of the achiral small molecules. Dyes such as acridine orange and methyl orange have been used as achiral species, since they are in rapport with biopolymers through ionic coupling. This phenomenon has been applied to the detection of the helix chirality in poly-a-amino acids, polynucleotides, or polysaccharides when instrumental limitations prevent direct detection of the helices. 

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