Racemic monomers separation

A somewhat different situation arises in the copolymerization of a racemic monomer with an optically active monomer of similar structure in the presence of a conventional stereospecific (or stereoselective) catalyst (299, 321). Examples concern the copolymerization of racemic 3,7-dimethyl-1-octene with (5)-3-methyl-l-pentene and of racemic jec-butyl vinyl ether with various optically active vinyl ethers. In all cases there was preferential copolymerization of one of the two enantiomers of the racemic monomer with the second monomer and simultaneous formation of an optically active homopolymer containing predominantly the noncopolymerized antipode, according to Scheme 20. The two products are easily separated, due to their different solubilities. 

Chromatographic separation of polymers obtained from racemic monomers. 433... 

It is long since that the resolution of low-molecular-weight racemic compounds by elution chromatography on optically active supports was known (58) however only recently vinyl-polymers obtained from racemic monomers have been separated in fractions having optical activity of opposite sign by this technique (106). 

Beside the structure of the monomer, also the type of catalyst used should play an important role in favouring the synthesis of either prevailingly (R) and (S) separable polymers or random (R) (S) copolymers from racemic monomers until now all the separable polymers have been produced by heterogeneous coordination catalysts. 

The investigations on the mechanism of chromatograph ic separation and on the production of different types of polymers of racemic monomers should lead to interesting improvements in this method for the preparation of optically active polymers. 

There are several alternative methods for the synthesis of optically active polymers from achiral or racemic monomers that do not involve polymerization catalysts. Optically active polymers have been formed from achiral dienes immobilized in a chiral host lattices [ 106]. In these reactions, the chiral matrix serves as a catalyst and can be recovered following the reaction. For example, 1,3-penta-dienes have been polymerized in perhydrotriphenylene and apochoUc acid hosts, where asymmetric induction occurs via through-space interactions between the chiral host and the monomer [107,108]. The resultant polymers are optically active, and the optical purities of the ozonolysis products are as high as 36%. In addition, achiral monomers have been found to pack in chiral crystals with the orientations necessary for topochemical soHd-state polymerization [109]. In these reactions, the scientist is the enantioselective catalyst who separates the enantiomeric crystals. The oligomers, formed by a [27H-27i] asymmetric photopolymerization, can be obtained in the enantiomeric pure form [110]. 

From the economic point of view it would be highly desirable to polymerize a racemic monomer mixture followed by the resolution of the products. However, for this to be practical, one would expect the two enantiomorphs to polymerize at different rates and, in order for optically active products to be isolated, conversion in the process would have to be only partial. If the reaction rates were the same, the two polymers would have to be resolved. To be resolvable the two enantiomorphs should not copolymerize with each other but should substantially homopolymerize to separate polymers. 

Introduction of relatively weak functional groups, such as carbonyl, hydroxyl, nitro, amide, etc., in the nanochannels of PCPs would affect the monomer alignment, which may lead to precision control of stereoselectivity and regioselectivity of the resulting polymers. In particular, PCPs with either helical or chiral structures on the pore surface are of intense interest in chemistry and such porous solids are potentially useful to find applications in enantioselective sorption/separation and catalysis [34, 38 0, 42, 45]. Of considerable interest is the use of the chiral channels to affect asymmetric polymerizations such as asymmetric selective polymerization of racemic monomers as well as asymmetric polymerization of prochiral monomers, which may give helical polymer conformations. 

It is interesting to notice that poly(t-butyl thiirane) obtained in stereoelective experiments could be separated by selective solubility in two franctions, one of which was identified as pure poly R polymer m.p. 157 C a s5 = + 164 (CHCl,) and the other, as the racemate (poly R + poly S) m.p. = 204 (27)7 These fractions were compared with authentic samples prepared previously from pure levorotatory monomer and racemic monomer (15). 

A special type of stereoelectivity can be foreseen in a copolymerization of a racemic monomer with an optically active comonomer. Under the influence of the latter, one enantiomer of the racemic monomer can be preferentially incorporated in the polymer chains. Only a few systems of this type have been investigated. In the following sections we shall consider separately the synthesis of stereoregular polymers from monomers of high steric purity, and from racemic monomers. 

A(a)<- by stereoelection the polymerization of the racemic monomer R,S) gives a predominant R oi S polymer polypropylene oxide polypropylene sulfide poly-a-aminoacid A -carboxylic acid anhydride (initiator, aluminum alkyl, Ni carboxylate phosphine). A(b) by stereoelection one enantiomer of racemic mixture polymerises more easily than the other one racemic a-olefin B Chromatography of poly-racemic RRS/SSR oiRR/SS) just some examples are known (not separation but more exactly enrichment)... 

In the case A, optically active polymers have been synthesized starting with a single antipode and conventional catalysts as well as starting with racemic monomers and optically active catalysts, separation in fractions having optical rotation with opposite sign of polymer from racemic monomers has also been performed. 

SEPARATIONS OF POLYMERS FROM RACEMIC MONOMERS INTO FRACTIONS HAVING OPPOSITE OPTICAL ROTATION... 

The asymmetric selection stereoelective) polymerization of racemic monomers in the presence of optically active catalysts could be very useful, but the catalytic systems used up to now give a rather poor enantiomeric purity. On the other hand the separation of nonoptically active polymers of racemic monomers into fractions with opposite optical rotation, while possible in several cases, would need an improvement of separation techniques before being employed for preparative purposes. 

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-... 

Fig. 6-10. Influence of the number of basic interaction sites of the template versus the separation factor measured in chromatography for the corresponding racemate. The templates were imprinted using MAA as functional monomer by thermochemical initiation at 60/90/120 °C (24 h at each temperature) and using acetonitrile as porogen. (From Sellergren et al. [15].)...

The natural tendency of polypeptide chains to grow homochirally may suggest an alternative mechanism for the breaking of symmetry, based on macromolecules instead of monomers. The argument is that it should be easier to separate enantiomeric homochiral chains, rather than racemic low-molecular-weight monomers, from each other. It has been shown for example that when the NCA-polycondensation is performed on mineral support, the oligomeric product remained absorbed on the surface. The lower oligomers are, however, easily... 

These experiments, which were originally devised to establish whether the stereoregular polymers of racemic a-olefins were substantially random copolymers of (R) and (S) monomers or contained macromolecules prevailingly synthesized from one antipode that is prevailingly (R) and (S) separable polymers, have shown that by this technique it is possible to obtain, in one step only, fractions having optical purity of 40% or more. 

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. 

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