Of achiral monomers

In 1978 Lahav et al. (Weizmann Institute, Israel) succeeded in an absolute asymmetric syntheses of chiral oligomeric crystals by the four-center polymerization of achiral monomer. 

Fig. 7 Acetylene-functionalized peptides. Inclusion of only 12% of the l and d chiral monomers (2a and 2c) into the polymerization of achiral monomers 2b and 2d resulted in the same helix content as for the homopolymers of 2a and 2c [37]...

Here, attention is confined especially to the asymmetric transformations of achiral monomers within crystalline or quasi-crystaUine architectures. In particular, the early studies of asymmetric polymerizations performed within enantiomorphous... 

No optically active centers are produced in the polymerization of achiral monomers to dissymmetric polymers. Consequently, the smallest configurational unit is the configurational diad from two monomer units. 

The stereoelectivity ratio was not modified when running a copolymerization reaction in the presence of achiral monomer e.g, ethylene sulfide. This confirms again the strong catalyst control process involved in these polymerization. 

Chain-end controlled isospecificity and syndiospecificity for 1-alkene polymerizations at low temperatures with achiral metallocenes have also been reported.2,163 81131135 The polymerization with these catalysts is highly regio-specific in favor of primary monomer insertion. 

Diastereoisomeric transition states calculated for propene primary insertion in a model of the Ewen achiral metallocenes are shown in Figure 1.20. The two possible diastereomeric transition states correspond to si (Figure 1.20a) and re (Figure 1.20b) insertions of the monomer for the case of a si chain (i.e., a growing chain in which the last monomeric unit has been obtained by a cis addition of a -coordinated monomer molecule) and are suitable for like (isotactic) and unlike (syndiotactic) propagations, respectively.142,143... 

Moller and co-workers co-polymerized dichlorodi- -pentylsilane with either dichloro-bis-(d )-2-methylbutylsilane or dichloro-(d )-2-methylbutyl- -pentylsilane in various ratios and found a linear dependence of optical activity on mole fraction of chiral co-monomer.313 On the other hand, studies by Fujiki on co-polymers 109 formed by the copolymerization of achiral (racemic) dichlorohexyl-2-methylbutylsilane and chiral dichlorohexyl-(d )-2-methylbutylsi-lane or dichlorohexyl-(l )-2-methylbutylsilane have shown that a preferential helical screw sense can be induced by even as little as 0.6 mol% of chiral co-monomer, and that at 5 mol%, the helicity, as gauged by the gabs value, is essentially the same as that of the chiral homopolymer, as shown in Figure 40. This indicates a positive non-... 

Optically active polymers may be obtained by polymerization of optically active, racemic, or achiral monomers (255) (disregarding methods where chirality is introduced into the macromolecular compound after polymerization, e.g., by attachment of chiral substituents to preexisting reactive groups). Each class may be further subdivided according to the stracture of the monomer and polymer. 

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). 

In the polymer field, reactions of this type are subject to several limitations related to the structure and symmetry of the resultant polymers. In effect, the stereospecific polymerization of propylene is in itself an enantioface-diflferen-tiating reaction, but the polymer lacks chirality. As already seen in Sect. V-A there are few intrinsically chiral stractures (254) and even fewer that can be obtained from achiral monomers. With two exceptions, which will be dealt with at the end of this section, optically active polymers have been obtained only from 1- or 1,4-substituted butadienes, fiom unsaturated cyclic monomers, fiom substituted benzalacetone, or by copolymerization of mono- and disubstituted olefins. The corresponding polymer stmctures are shown as formulas 32 and 33, 53, 77-79 and 82-89. These processes are called asymmetric polymerizations (254, 257) the name enantiogenic polymerization has been recently proposed (301). 

Copolymers between an achiral monomer and an enantiomer of a stracturally similar monomer sometimes have optical activity higher than that derived by a simple additive mle (376). Thus, in the copolymer between 4-methylpentene and (5)-4-methylhexene the monomer units of the first type are forced to assume a conformation analogous to those of the second and contribute to the optical rotation of the polymer. 

A method for obtaining optically active polyiminomethylenes from achiral monomers was recently devised by Nolte, Drenth and co-workers (420). It consists in the copolymerization of an achiral monomer (e.g., phenyl isocyanide) with an optically active isocyanide endowed with a low tendency to polymerize. The chiral monomer is incorporated in one of the two helices and, due to its low reactivity, stops or slows down its growth. The other helix is unaffected by this phenomenon and continues to grow, permitting the almost complete conversion of the achiral monomer into an optically active polymer. 

The extension of DKR to polymer chemistry is not trivial in practice since side reactions that are relatively unimportant in DKR (dehydrogenation, hydrolysis) have a major impact on the rate of polymerization and attainable chain lengths because the stoichiometry of the reactants is an important issue. As a result, the reaction conditions and catalyst combinations used in a typical DKR process will not a priori lead to chiral polymers from racemic or achiral monomers with good molecular weight (>10kDa) and high ee (>95%). 

Isocyanide Polymers Optically active polyisocyanides with excess helix sense are obtained from optically active monomers by polymerization with NiCI-,. The polymers obtained from (R)-(CH3)2CHCH(CH,)NC, (fl)-(CH3)2CHCH2CH(CH3)NC, and (R)-n-C6H13CH(CH3)NC have M helical sense with screw-sense excesses of 62%. 56%, and 20%, respectively [189]. The copolymerization of achiral phenyl isocyanide with optically active... 

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