Asymmetric enantiomer-differentiating polymerization

Note 1 A stereoselective polymerization is defined as a polymerization in which a polymer molecule is formed from a mixture of stereoisomeric monomer molecules by the incorporation of only one stereoisomeric species [1]. Thus, an asymmetric enantiomer-differentiating polymerization is a stereoselective polymerization, in which all the polymer molecules are formed by the incorporation of only one type of stereoisomeric species. 

If the polymerization sites are not prefectly isoselective, the result is a modified asymmetric enantiomer-differentiating polymerization behavior with some cross-propagation of the two enantiomeric monomers and a decrease in the overall isotacticity of the reaction product. 

The extent of asymmetric enantiomer-differentiating polymerization in an isoselective process is evaluated by measuring the optical activity of unreacted monomer a as a function of the extent of reaction p [Zhong et al., 2003], The rates of reaction of R and S enantiomers are given by... 

A special case of asymmetric enantiomer-differentiating polymerization is the isoselective copolymerization of optically active 3-methyl-1-pentene with racemic 3,7-dimethyl-1-octene by TiCl4 and diisobutylzinc [Ciardelli et al., 1969]. The copolymer is optically active with respect to both comonomer units as the incorporated optically active 3-methyl-l-pentene directs the preferential entry of only one enantiomer of the racemic monomer. The directing effect of a chiral center in one monomer unit on the second monomer, referred to as asymmetric induction, is also observed in radical and ionic copolymerizations. The radical copolymerization of optically active a-methylbenzyl methacrylate with maleic anhydride yields a copolymer that is optically active even after hydrolytic cleavage of the optically active a-methylbenzyl group from the polymer [Kurokawa and Minoura, 1979]. Similar results were obtained in the copolymerizations of mono- and di-/-menthyl fumarate and (—)-3-(P-styryloxy)menthane with styrene [Kurokawa et al., 1982],... 

Configurationally chiral, optically active polymers having stereogerric centers in the side chain or main chain can be obtained by enantiomer-selective pol5nnerization (lUPAC nomenclature asymmetric enantiomer-differentiating polymerization). In enantiomer-selective polymerization, one antipode of a racemic chiral monomer is preferentially polymerized to afibrd an optically active polymer. In this process, kinetic resolution of the racemic monomer takes place. The first clear polymerization of this typ>e was reported for propylene oxide. 

In the literature concerning the ROP of heterocyclic monomers, the asymmetric, enantiomer-differentiating polymerization and racemate-forming enantiomer-differentiating polymerization are informally named... 

Stereoelective (IDRAG asymmetric enantiomer-differentiating polymerization)... 

The tendency of the sec-butyl group to chiral disordering led [30], however, to the possibility of achieving an absolute asymmetric polymerization. Racemic (1) is dimorphic. Crystalline form >1, which may be obtained from the melt, has space group Pi with Z= 1 and cell constants a = 13.35, b = 7.02, c = 5.42, a = 104°,i8 = 93°, y = 92°. These cell constants are similar to, but significantly different from, those of crystals of the resolved monomer. Thus crystallization of racemic (1) involves not spontaneous resolution but formation of a solid solution of the two enantiomers. Details of the X-ray diffraction patterns as also results by differential scanning calorimetry support this view. Thus we have an example of the behavior ... 

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