Stereoregular and/or optically active

Syntheses of stereoregular and/or optically active poly(carbosilane)s and poly(carbosiloxane)s from optically active silicon compounds are also shown. 

Stereoregular and/or Optically Active Poly(carbosilane)s and Poly(carbosiloxane)s... 

These systems can yield high-molar-mass polyethers, which could not be obtained using conventional anionic polymerization except for poly(EO). The possibility to synthesize stereoregular and/or optically active polyethers with crystalline properties is also an important particularity of these coordination processes. 

The poly(propylene oxide) obtained at low conversion has an enantiomeric purity of around 30%, is not homogeneous from the point of view of stereoregularity (or stereoselectivity), and can be separated into fractions of different tacticity and optical activity, the latter being greater for polymers of higher stereoregularity. 

The stereochemistry of step polymerization is considered now. Bond formation during step polymerization almost never results in the formation of a stereocenter. For example, neither the ester nor the amide groups in polyesters and polyamides, respectively, possess stereocenters. Stereoregular polymers are possible when there is a chiral stereocenter in the monomer(s) [Oishi and Kawakami, 2000 Orgueira and Varela, 2001 Vanhaecht et al., 2001], An example would be the polymerization of (R) or (S)-H2NCHRCOOH. Naturally occurring polypeptides are stereoregular polymers formed from optically active a-amino acids. 

After the Natta s discovery of highly stereospecific polymerization processes, the interest in the preparation and properties of optically active polymers has greatly increased. In fact, the use of asymmetric catalysts or monomers to obtain optically active polymers may supply interesting informations on the mechanism of steric control in stereo-specific polymerization furthermore optical activity is an useful tool to study the polymer stereoregularity and the chain conformations of polymers in the molten state or in solution. 

In this case a considerable optical activity should be expected for completely stereoregular isotactic cis (V) or trans (VI) polymer, since all the tertiary carbon atoms of the main chains bound to a > CH2, -CH3 and -CH= group, have the same absolute configuration. 

Investigations were made on the I. R. spectra of poly-[(S)-l-methyl-propylj-vinyl-ether and poly-[(S)-2-methyl-butyl]-vinyl-ether (65) a band at 911 cm-1 related to stereoregularity was detected in the spectrum of the former while a crystallinity band at 827 cm-1 was found in the latter. The spectra of polymers obtained from an optically active or a racemic monomer did not reveal any remarkable difference. 

The failure in separating in fractions possessing optical activity of opposite sign the stereoregular polymers of racemic 5-methyl-l-heptene, polymerized in the presence of the same catalyst as that used to prepare polymers from racemic 3-methyl-l-pentene and 4-methyl-1-hexene (75), might be an indication that, in order to obtain prevailingly (R) and (S) separable polymers instead of random copolymers from racemic vinyl monomers, the asymmetric carbon atom of the monomer must be in a or in / position with respect to the double bond. 

Although many stereoregular polymers have a helical conformation in the solid state (5,96], the conformation is lost in solution in most cases, except in the case of some polyolefins with optically active side groups [12], because the dynamics of the polymer chain are extremely fast in solution. Therefore, isotactic polystyrene [15,16] and polypropylene [17] prepared with an optically active catalyst do not show optical activity due to a helical conformation. However, a helical conformation can be maintained in solution for some polymers having a rigid main chain or bulky side groups that prevent mutation to random conformation, and the conformation may... 

Kawakami and coworkers synthesized stereoregular and optically active polysiloxanes by polycondensation with deamination or dehydrogenation. Optically active disiloxane disilanol [173] and achiral bis(dimethylamino)si-... 

A further significant factor which comes to the fore in coordinate catalysis is stereoregular polymerization. A number of the catalyst systems are capable of producing isotactic, optically active and/or crystalline polymers. Except for Price s potassium t-butoxide system [21], this has not been observed in anionic or cationic polymerization. Thus, in addition... 

Stereoregularities of the resulting polymers depend on the sizes of the host channels. Moreover, the space effect in chirality was observed in asymmetric inclusion polymerization of trans- or cis-2-methy 1-1,3-pentadiene by using a pair of hosts, deoxycholic acid and apocholic acid. We obtained optically active polymers with predominant absolute configurations (R). Optical yields varied with the polymerization conditions and the hosts. A maximum optical yield of the trans monomer was 36% m the channel of apocholic acid. 

The synthesis of 1,6-diamino-l,6-dideoxy-2,3,4,5-tetra-0-methyl-D-mannitol and its L-iditol analogue from D-mannitol has been described [100], These diamines, containing a two-fold axis, gave stereoregular AABB polyamides on polycondensation with terephthaloyl dichloride and dipentachlorophenyl esters, or dichlorides of aliphatic dicarboxylic acids, in solution or under interfacial polycondensation conditions [99, 109]. In spite of the regioregularity present in the polymeric chains, these optically active polyamides could not be crystallized. 

In its original form the Higashimura et al. mechanism still seems to be a more reasonable way of explaining the stereoregular polymerization of benzofuran (Natta et al., 1961b Farina and Bressan, 1963). It was found that aluminum chloride, complexed with an optically active compound, such as /3-phenylalanine (— or 4-) or ( + )— 10-camphor, afforded an optically active... 

DSC analysis of the optically active poly-L-malolactone benzyl ester also showed multiple endotherms in the temperature range of 140-160regardless of the catalyst system used to prepare the polymer. The difference in the melting points of the optically active and racemic polymers could be due to differences in either the degree of stereoregularity or the molecular weights of the polymers. 

---