Chirality/Chiral monomers

Based on chiral functional monomers such as (15), MICSPs can be prepared using a racemic template. Thus, using racemic A-(3,5-dinitrobenzoyl)-a-methylbenzy-lamine (16) as template, a polymer capable of racemic resolution of the template was obtained [67]. Another chiral monomer based on L-valine (17), was used to prepare MIPS for the separation of dipeptide diastereomers [68]. In these cases the configu-... 

Figure 4 Sketch of two possible stereochemical arrangements for a chiral monomer. P represents the polymer chain, R represents a vinyl substitutent on a carbon, H represents hydrogen, (a) Linear sketch showing one conformation and two configurations (bracketed and unbracketed). The apex of bonds is a tetrahedrally bonded carbon atom (solid and dashed circles), (b) Newman projection of the same monomer showing the free rotation about the C-C bond.

Monolithic columns with the chiral anion exchange-type selectors incorporated into the polymer matrix obtained through in situ copolymerization process of a chiral monomer (in situ approach) [80-83,85] or attached to the surface of a reactive monolith in a subsequent derivatization step (postmodification strategy) [84], both turned out to be viable routes to enantioselective macroporous monolithic columns devoid of the limitations of packed columns mentioned earlier. 

In one approach, polymethacrylate-type monoliths have been fabricated by copolymerization of the chiral monomer 0-9-[2-(methacryloyloxy)ethylcarbamoyl]-10,11-dihydroquinidine 1 or 0-9-(tert-butylcarbamoyl)-ll-[2-(methacryloyloxy) ethylthio]-10,ll-dihydroquinine 2 (see Figure 1.34a), the comonomer 2-hydroxyethylmethacrylate (HEMA), the crosslinker ethylenedimethacrylate (EDMA) in presence of the binary porogenic solvent mixture cyclohexanol and 1-dodecanol, directly in a single step within fused-silica capillaries. Initiation of the polymerization by either thermal treatment or UV irradiation yielded microglobular polymer morphologies, such as those well known from their corresponding nonchiral... 

Synthetic polymers This type of chiral selectors (polyacrylamides, polymethacrylamide, etc.) was first developed by the group of Blaschke by polymerization of chiral monomers with cross-linking agents to form a three-dimensional polymer network. 

Lammerhofer, M., Tobler, E., Zarbl, E., Lindner, W., Svec, E, and Frechet, J. M. J. (2003). Macroporous monolithic chiral stationary phases for capillary electrochromatography new chiral monomer derived from cinchona alkaloid with enhanced enantioselectivity. Electrophoresis 24, 2986-2999. 

B. Synthesis of Optically Active Polymers from Chiral Monomers. 72... 

The polymerization of racemic mixtures of chiral monomers gives different results depending on the structure of the monomer and the type of polymerization process. For monomers containing only one asymmetric carbon atom we can distinguish three processes ... 

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). 

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. 

Polymerization in which a tactic polymer is formed. However, polymerization in which stereoisomerism present in the monomer is merely retained in the polymer is not to be regarded as stereospecific. For example, the polymerization of a chiral monomer, e.g., R)-propylene oxide ((i )-methyloxirane), with retention of configuration is not considered to be a stereospecific reaction however, selective polymerization, with retention, of one of the enantiomers present in a mixture of R)- and (S)-propylene oxide molecules is so classified. 

Note 2 A polymerization in which, starting from the racemate of a chiral monomer, two types of polymer molecules, each containing monomeric units derived from one of the enantiomers, form in equal amounts is termed racemate-forming enantiomer-differentiating polymerization . The resulting polymer is optically inactive (see Note 4 of Definition 2). 

CSP, beads chiral monomers (from amino acids) analytical to preparative fair, moderate to low... 

Helical polymers, formed as they usually are from chiral monomers, are themselves chiral. If an attempt is made to stack them in a regular parallel array, there will be a tendency for successive layers to be gradually twisted round with respect to the original layer. It is possible that the chiral effect is responsible for the fact that the polyglutamates tend to deposit at an angle of 35° to the dipping direction. A study of a racemic mixture of one of these materials would clarify this point. 

Table 13. Cholesteric LC copolymers containing chiral monomer units... 

Copolymer No. Structure of the copolymer unit3 Content of chiral monomer units in the copolymer, mole- % T Ag °C Tcl, QC Xb, max (nm)... 

As described in the preceding sections, asymmetric amplification is generally a consequence of the formation of aggregates (i.e., dimers or oligomers that are homochiral or heterochiral) of a chiral catalyst. However, even a racemic catalyst can be used as a chiral catalyst with the aid of chiral additives (a simple model consisting of dimers is depicted in Scheme 9.17). If a chiral additive (R)-B is selectively associated with (S)-A in the racemic catalyst, the remaining (R)-A could operate as the chiral monomer catalyst (asymmetric deactivation). Conversely, the chiral additive (/ )-B can be selectively associated with (/ )-A in racemic catalyst to generate an active dimeric catalyst (asymmetric activation). 

Chirality induction can be achieved in homo- and copolymerization of vinyl monomers based on chiral monomer structure [1,3,8,9]. The first example of this type of polymerization was the copolymerization of (S)-a-methylbenzyl methacrylate with maleic anhydride the polymerization product showed [a]D +23° after removal of the chiral side group [73]. For another example, the copolymerization of an optically active styrene derivative (39) with N-phenylmaleimide (17, R = -Ph) followed by removal of the optically active side group and deboronation gave an optically active N-phenylmaleimide-styrene copolymer [74]. 

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