Enantiomer-differentiating polymerization

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 

The isoselective polymerization of a racemic mixture of monomers can proceed in two ways depending on initiator, monomer, and reaction conditions. Racemate-forming enantiomer-differentiating polymerization involves both the R and 5 monomers polymerizing at the same rate hut without any cross-propagation [Hatada et al., 2002]. A racemic monomer mixture polymerizes to a racemic mixture of all-5 and all-5 polymer molecules [Pino, 1965  

Sigwalt, 1976, 1979 Tsuruta, 1972]. This is consistent with the mechanism for isoselective polymerization that attributes steric control to the initiator. The initiator contains R and 5 enantiomeric polymerization sites in equal numbers such that R sites polymerize only R monomer and S sites polymerize only S monomer. If the isoselectivity of R and S sites is less than complete, there is some cross-propagation of the two enantiomeric monomers and a decrease in the overall isotacticity of the reaction product. 

As discussed in Sec. 8-14a, an optically active polymer sample, composed of either all-5 or all-5 polymer molecules, can be synthesized by isoselective polymerization of a pure enantiomer, the pure R or pure 5 monomer, respectively. The direction of optical rotation of the polymer is usually the same as the corresponding monomer. 

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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. 

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). 

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],... 

Similar effects are found in the enantiomer-differentiating polymerization of oligomers (ji r 30) of chiral, sterically crowded methacrylic acid esters E. Yashima, Y. Okamoto, K. Hatada, Macromolecules 1988, 21, 854. 

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)... 

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