Enantiomeric copolymers

Thermo-Sensitive Gels Biodegradable Hydrogels from Enantiomeric Copolymers of Poly(lactide) and Poly(ethylene glycol)... 

Fujiwara T (2012) Thermo-responsive gels biodegradable hydrogels fi om enantiomeric copolymers of poly(lactide) and poly(ethylene glycol), vol 1114, ACS symposium series degradable polymers and materials. ACS, Washington, DC, pp 287-311... 

DUactide (5) exists as three stereoisomers, depending on the configurations of the lactic acid monomer used. The enantiomeric forms whereia the methyl groups are cis are formed from two identical lactic acid molecules, D- or L-, whereas the dilactide formed from a racemic mixture of lactic acid is the opticaUy iaactive meso form, with methyl groups trans. The physical properties of the enantiomeric dilactide differ from those of the meso form (6), as do the properties of the polymers and copolymers produced from the respective dilactide (23,24). 

Another significant cooperativity effect in preferential helical screw sense optically active copolymers is the majority rule phenomenon.18bl8q In this case, the screw sense of a helical main chain with unequal proportions of opposite chirality enantiopure chiral side groups is controlled by the enantiomeric excess only. Since this phenomenon was first reported from poly-a-olefins made of vinyl co-monomers bearing nonenantiopure chiral moieties by Green et al.8b and Pino et al.,16b this majority rule has been established in... 

From the NMR spectrum of copolymers produced from cyclohexene oxide and carbon dioxide it is difficult to assess low levels of asymmetric induction, i.e., low degrees of desymmetrization in the epoxide ring-opening step. In order to determine the extent of asymmetric induction it is necessary to hydrolyze the copolymer leading to the tra s-cyclohexane-l,2,-diol and examine the enantiomeric excess (4) [22]. Figure 4 shows the NMR spectrum in the carbonate region of atactic copolymer produced from cyclohexene oxide and CO2 using an achiral (salen)CrX catalyst. 

Similarly, good results were obtained with poly-(i)-alanine, poly-(L)-leucine and poly-(i)-isoleucine, whereas with poly-(i)-vahne or random copolymers reduced chemical yields as weU as asymmetric induction were obtained. Chemical and optical yields are closely related in all cases. The enantiomeric excesses obtained increased as the average chain length of the catalyst increased varying from 10 to 30 amino acids (ee values between 50 to 99%). For high ee values it seems to be essential that the polymer chain is at least 10 units. The degree of asymmetric induction decreases as the temperature is raised. The amount of catalyst only influences the chemical yield, not the optical yield of... 

Only two types of polymers are considered here. These are isotactic poly(5-methyl-hexene-l) (P5MH1), with a non-chiral side chain (for the sake of comparison) and mainly isotactic poly(4-methyl-hexene-l) (P4MH1). The side chain of the latter polymer is chiral since the two substituents of the carbon in the p position are a methyl and an ethyl group. Polymers that are made only of the S or the R conformers - in other words the true chiral polymers (P(S)4MH1 and P(R)4MH1), the racemic copolymer of the (R) and (S) monomers (P(R, S)4MH1) and of course the racemic blend of the two enantiomeric polymers - are available. 

Preliminary results obtained with the racemic P(R,S)4MH1 (the random copolymer of the R and S enantiomeric monomers) and of the racemic blend of P(S)4MH1 and P(R)4MH1, indicate a more conventional crystallization behavior crystallization of the tetragonal form takes place in the upper Tc range ( 183 and % 191 °C, respectively), presumably because both left- and right-handed helices (or helical stretches) are available at that temperature, whereas melting occurs at 211 and 214 °C. 

From the copolymer composition dependence of the molar ratio of the d- and L-enantiomeric units of 15e in the copolymer, the rate of reaction of the growing chain end of 21 with the D-enantiomer of 15e was estimated to be about four times faster than that with the L-enantiomer. Such asymmetric selection is mainly ascribable to the steric and electronic interactions between the asymmetric environment created by the bulky terminal unit of 21 and the rigid bicyclic monomer having three asymmetric centers and a polar bulky bromine substituent (Scheme 6, [23]). 

ECH elastomers have more than 97% of head-to-tail sequences, they are amorphous (no cristallinity detected by DSC or x-ray analysis) and with a stereorandom distribution of enantiomeric units. These polymers are thus atactic. In ECH/EO copolymer the mole ratio of both monomers was kept to approximately I. In the terpolymer the amount of AGE is small (around 2.5%). 

Examination of the NMR spectrum of vinyl acetate and vinyl propionate copolymers with Eu(hfc)3 or Pr(hfc)3 indicated that linkages within the polymer were racemic in nature. The H (400 MHz) and (100 MHz) NMR spectra of 3-amino-l,2-dicarba-c/oio-dodecaboranes exhibited enantiomeric discrimination in the presence of Eu(hfc)3. The resonances broadened in the presence of the shift reagent, but it was still possible to observe signal separation for the two enantiomers. 

Slivniak, R. Domb, A.J. Stereocomplexes of enantiomeric lactic acid and sebacic acid ester-anhydride triblock copolymers. Biomacromolecules 2002, 3, 754-760. 

The use of an achiral HMMS butyl acrylate-butyl methacrylate-methacrylic acid copolymer (BBMA) sodium salt was also investigated for enantiomer separations with CDs or as a CD-MEKC mode. A better enantiomeric resolution of Dns-DL-AAs was obtained by a /3-CD-BBMA-MEKC system than an /3-CD-SDS-MEKC system. 

A number of acrylonitrile - Cinchona alkaloid copolymers have been prepared in which the vinyl group of the alkaloid has been modified, and the ability of these copolymers to act as asymmetric catalysts in the Michael addition of methyl vinyl ketone to methyl indan-l-one-2-carboxylate has been studied.In twelve cases, good enantiomeric yields, ranging from 41 to 56% ee, were obtained. 

The best value was obtained for ligand 58 which yielded a very active system that within 15 min gave a 62% yield of the desired chiral monomer (out of a total conversion of 67%). The enantiomeric excess which at this point was of 78.5%. However, a significant isomerisation takes place upon longer reaction time which increases the amount of the undesired achiral copolymer. Still, this isomerisation occurs with a kinetic resolution, thus increasing the enantiomeric excess of the remaining chiral compound to 92%. 

Sarhan (112) used a copolymer (with 4-vinylpyridine as one comonomer) imprinted with the D-mandelic ester monomer 6 for deprotonation and selective protonation of the racemate of mandelic acid. An enantiomeric excess of several per cent for one enantiomer was observed. 

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