Intrinsically chiral polymers

In an earlier discussion (254) polymers in which the chirality depends only on the presence of chiral side groups were said to be nonintrinsically chiral, in contrast with intrinsically chiral polymers where the chirality is independent of the internal structure of the substituent. Substituted carbon atoms in the polymers described in the next paragraphs are often indicated as true or classic asymmetric carbon atoms. In this way one can distinguish between carbon atoms whose four substituents are constitutionally different in the proximity of the atom under consideration, from the tertiary atoms of vinyl isotactic polymers. For these, only the different length of the two chain segments and/or the stmcture of tire end grmips make all the ligands different from each other. 

Figure 11. Reaction scheme for synthesis of intrinsically chiral polymer phase (CSP) and coated onto silica.

Crystallization of polymers in chiral crystals, even in the case of achiral polymers, is quite frequent and strictly related to the occurrence of helical conformations of the chains. The crystallizable polymer consists of a regular sequence of a chemical repeating unit which can be chiral if it presents an asymmetric center or achiral. On the contrary, helical conformations assumed by the polymer chains in the crystalline state are intrinsically chiral, even though the chemical repeat is achiral. Three possible cases can be distinguished ... 

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). 

The hypothesis of stereochemical control linked to catalyst chirality was recently confirmed by Ewen (410) who used a soluble chiral catalyst of known configuration. Ethylenebis(l-indenyl)titanium dichloride exists in two diaste-reoisomeric forms with (meso, 103) and C2 (104) symmetry, both active as catalysts in the presence of methylalumoxanes and trimethylaluminum. Polymerization was carried out with a mixture of the two isomers in a 44/56 ratio. The polymer consists of two fractions, their formation being ascribed to the two catalysts a pentane-soluble fraction, which is atactic and derives from the meso catalyst, and an insoluble crystalline fraction, obtained from the racemic catalyst, which is isotactic and contains a defect distribution analogous to that observed in conventional polypropylenes obtained with heterogeneous catalysts. The failure of the meso catalyst in controlling the polymer stereochemistry was attributed to its mirror symmetry in its turn, the racemic compound is able to exert an asymmetric induction on the growing chains due to its intrinsic chirality. 

The role of supramolecular chemistry in materials is perhaps expressed most impressively in liquid crystals, in which slight variations of chiral content can lead to dramatic influences in the properties of the mesophases. The helical sense of these mesophases is determined not only by intrinsically chiral mesogens but also by the use of dopants which more often than not interact with achiral host LCs to generate chiral phases (Fig. 7). These phenomena are important both scientifically and technologically, most notably for the chiral smectic and cholesteric liquid crystal phases [68-71]. These materials—as small molecules and as polymers [72,73]—are useful because their order... 

A more elegant approach towards lipase-catalyzed synthesis of chiral polymers is to take advantage of the intrinsic ability of lipases to discriminate between ena-tiomers. Chiral polymers can be formed when one of the two enantiomers of a chiral monomer preferentially reacts during the polymerization reaction. This is referred to as a kinetic resolution polymerization (KRP) and allows the optically active polymer to be directly procured from a racemic mixture, albeit in a maximum of 50% yield. 

Hilker et al (44) combined dynamic kinetic resolution with enzymatic polycondensation reactions to synthesize chiral polyesters from dimethyl adipate and racemic secondary diols. The concept offered an efficient route for the one-pot synthesis of chiral polymers from racemic monomers. Palmans at al (18,43) generalized the approach to Iterative Tandem Catalysis (ITC), in which chain growth during polymerization was effected by two or more intrinsically different catalytic processes that were compatible and complementary. 

TTie extension of tandem catalysis to polymer chemistry is, however, not trivial. In order to reach high molecular weight polymers, each reaction has to proceed with almost perfect selectivity and conversion. Obviously, combining different catalytic reactions limits the choice of suitable reactions since they must also be compatible with each other. We recently introduced Iterative Tandem Catalysis (ITC), a novel polymerisation method in which chain growth during polymerisation is effectuated by two or more intrinsically different catalytic processes that are both compatible and complementary. If the catalysts and monomers are carefully selected, ITC is able to produce chiral polymers from racemic monomers, as was shown by us for the ITC of 6-MeCL and the DKR polymerisation of sec-diols and diesters. ... 

An early approach toward chiral heterogeneous catalysts was the deposition of the catalyticaUy active metal or metal oxide particles onto intrinsically chiral supports such as quartz [24], cellulose [25], or synthetic chiral polymers [26-28]. Hydrogenation and dehydration reactions were tested, but enantioselective performance was found to be poor. In a recent review, Mallat et al. [29] attributed this poor enantioselectivity to the fact that only a small fraction of the metal atoms would... 

Related to the process of crystallization is the growth of a polymer from a pool of available molecules in a racemate. There are many studies of the spectacular generation of intrinsically chiral supramolecular structures such as hehces, often from achiral materials seeded by a small amount of a chiral additive [6-9], but these are not preparative methods for the separation of enantiomers in any meaningful sense, even though the helices can themselves be stable, chiral structures that are separable by chromatography - these are not the enantiomers of primary interest... 

As far as polymer stereochemistry is concerned, a controversial issue is what should be defined as stereoblock-isotactic Isotactic polypropylene is usually obtained as a result of site control (i.e., the preference of an intrinsically chiral transition metal active species to react with one of the two enantiofaces of the prochiral monomer). In the case of a simple C2-symmetric single-center catalyst with homotopic active sites, if we denote as o the probability that the monomer inserts with a given enantioface at an active site of given chirotopicity, the fractions [m] and [r] of meso and racemo diads in the polymer are given by Equations 8.1 and 8.2... 

As a result of the particularities of the MAO activation method noticed so far, it has been concluded that, besides chain back-skip, a different mechanism occurs when using this cocatalyst. One possible explanation might be a reversible chain transfer reaction between the cocatalyst and the active species. As a result of the intrinsic chirality at the metal center, the catalytic system consists of two enantiomers (S and/ . Scheme 9.3). Under different polymerization conditions (i.e., different Al/Zr ratios), the coordination and insertion of the monomer can take place at the metal center of either of the two enantiomers. At higher Al/Zr ratios, a unidirectional transfer of polymer chains from Zr (enantiomer / , for example) to aluminum can be suggested, because reduced molecular weights of the polymer products have been found. Relocation of the chain from aluminum to the other enantiomer of the Ci-symmetric catalyst species (enantiomer 5, Scheme 9.3) and then back... 

Among the several liquid crystal polymers that have been studied in recent years those containing intrinsically chiral elements with a prevalent chirality hold a particular position. Some of these, in fact, by virtue of their structural characteristics, assume a spatial array with nematic planes stacked in a superhelical structure characterized by a prevalent screw sense and are known as cholesteric phase. This kind of order can be controlled by either concentration in solution (lyotropic systems) or temperature in bulk (thermotropic systems). 

In general, the chiral ligands are water-soluble variants of those already studied in purely organic solvents (e.g., the sulfonated chiraphos, A, cyclobutane-diop, C, BDPP, F, MeOBIPHEP-TS, Q, BIFAPS, R and the quaternary ammonium derivatives of diop, D, BDPP, E). Solubility in water could also be achieved by attaching the parent phosphine molecule to a water-soluble polymer (J, M, P). The chiral phosphinites and phosphines derived from carbohydrates (e.g., K and L) have intrinsic solubility in water. During studies of one-phase... 

Another approach to CPL is the synthesis of conjugated polymers with intrinsic chiro-optical properties. A variety of polymers with CPPL have been synthesized so far. Most of them are based upon well-known conjugated polymers such as poly(thiophene)s [4,111], polyphenylene vinylene)s [123], poly(thienylene vinylene)s [124], ladder polymers [125], PPPs [126], polyphenylene ethynylene)s, [127] and poly(fluorenes) [128]. All of them have been modified with chiral side-chains, which induce the chiro-optical properties. 

In addition to the classification of liquid chromatographic enantioseparation methods by technical description, these methods could further be classified according to the chemical structure of the diverse CSPs. The chiral selector moiety varies from large molecules, based on natural or synthetic polymers in which the chirality may be based on chiral subunits (monomers) or intrinsically on the total structure (e.g., helicity or chiral cavity), to low molecular weight molecules which are irreversibly and/or covalently bound to a rigid hard matrix, most often silica gel. 

A host of further issues complicate catalyst performance for ROMP reactions. Intrinsic polymer characteristics are not just dependent on the nature of the monomer and/or comonomer, but are also highly dependent on the cis, Irons sequence of double bonds along the polymer chain, as well as on the tacticity of the polymer if a chiral or prochiral monomer is used, since the latter reflects the stereochemical sequence by which the chiral centres are linked. [See Chapter 7 and J. G. Hamilton in Handbook of Metathesis, Volume 3 , R. H. Grubbs ed., Wiley-VCH, Weinheim, 2003]. 

Various PFs [61-63] and their pyrrole analogues [64] functionalized with chiral side chains exhibit NLO character and it is not clear how the chirality, helicity and NLO character are related. Because PF2/6 has become a model test system in the study of helical PFs, fully understanding its intramolecular structure can serve as a reference point for further comparisons. The molecular level heterogeneity intrinsic in the single chain structure of PF2/6 insures a high level of frustration and thus it represents a model polymer for paracrystals and paracrystallinity [24]. 

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