Stereoisomerism optically active polymers

Arcus, C. L. Stereoisomerism of Addition Polymers. Part I. The Stereochemistry of Addition and Configurations of Maximum Order. J. chem. Soc. [London] 1955, 2801. The Stereoisomerism of Addition Polymers. Part II. Configurations of Maximum Order from Altering Copolymerisation. The Requirements for Optical Activity in Polymers. J. chem. Soc. [London] 1957, 1189. 

Irregular differences have been found (108) in the stereoisomeric composition of the polymers obtained from racemic or optically active monomers in the presence of the same catalytic systems. 

Further experimental data are needed to draw reasonable conclusions about the differences in stereoisomeric composition or in solubility of polymers from optically active and racemic monomers. 

By enantioselective polymerization polymer chains, each containing only one configurational kind of monomeric unit, are produced from a mixture of stereoisomeric monomer molecules. The number of kinds of polymer chain generated therefore equals the number of various stereoisomers in the monomer mixture. In the course of propagation, the enantiomeric composition of the polymer and unreacted monomer remains identical to the intial composition. When optically active monosubstituted cyclic monomers are polymerized, stereoregular polymers are formed with both isotactic polyR and polyS chains... 

The polymers with trans-fused five-membered rings linked with a diisotactic head-to-tail sequence have chirality, although the polymers composed of the cis-fused ring are achiral. Scheme 10 summarizes the structures of the stereoisomeric polymers. The optically active zirconocene complex with a C2 symmetric structure catalyzes the enantioselective cyclopolymerization of 1,5-hexadiene (Eq. 20) [98, 99]. Although the polymer contains not only trans-fused ring but also cis-fused ring units (ca. 68 32), it shows optical rotation due to the main chain chirality. 

Optical Activity in Polymers Stereoisomerism in polymers is formally similar to the optical isomerism of organic chemistry. In a vinyl polymer with the general structure shown in (XIH) every other carbon atom in the chain, labeled C, is a site of steric isomerism, because it has four different substituents, namely, X, Y, and two sections of the main chain that differ in length (Rudin, 1982). 

Over the past several decades, polylactide - i.e. poly(lactic acid) (PLA) - and its copolymers have attracted significant attention in environmental, biomedical, and pharmaceutical applications as well as alternatives to petro-based polymers [1-18], Plant-derived carbohydrates such as glucose, which is derived from corn, are most frequently used as raw materials of PLA. Among their applications as alternatives to petro-based polymers, packaging applications are the primary ones. Poly(lactic acid)s can be synthesized either by direct polycondensation of lactic acid (lUPAC name 2-hydroxypropanoic acid) or by ring-opening polymerization (ROP) of lactide (LA) (lUPAC name 3,6-dimethyl-l,4-dioxane-2,5-dione). Lactic acid is optically active and has two enantiomeric forms, that is, L- and D- (S- and R-). Lactide is a cyclic dimer of lactic acid that has three possible stereoisomers (i) L-lactide (LLA), which is composed of two L-lactic acids, (ii) D-lactide (DLA), which is composed of two D-lactic acids, and (iii) meso-lactide (MLA), which is composed of an L-lactic acid and a D-lactic acid. Due to the two enantiomeric forms of lactic acids, their homopolymers are stereoisomeric and their crystallizability, physical properties, and processability depend on their tacticity, optical purity, and molecular weight the latter two are dominant factors. 

A second, and often more important, type of configurational variation is stereoisomerism (Flory, 1953 Lenz, 1967, pp. 252-260 Schultz, 1974), which arises from differences in symmetry between substituted carbons along a polymer chain. In small molecules, carbon atoms that are attached to four different groups are clearly asymmetric, and mirror image isomers and optical activity are observed. For example, assuming tetrahedral bonding to the carbon atom,... 

Other stereoregular polymers with asymmetric carbon atoms in the main chain as stereoisomerism sites have been obtained by ring-opening polymerization of optically active cyclic monomers. These monomers include epoxides (100-104), episulfides (105,106), aziridines (107-109), lactides(110), lactones (111-116), thiolactones (117), lactcunes (118-120), cyclic acetals (121), and N-carboxy cuihydrides (122). Some stereoregular polymers containing atropoisomeric units as stereoisomerism sites (VIII (123) and IX (124)) have also been synthesized. 

Finally, stereoregular polymers with asymmetric carbon atoms in the side chains as stereoisomerism sites have been produced by polycondensation (125) and by polymerization of optically active isocyanates (X (126)). As stressed in section 2.2. we do not consider the amide groups in the main chain of polyisocyanates as sites of steric isomerism. 

Let us remember that in vinylhomopolymers, two relative configurations of a dyad are possible although these structures are stereoisomeric, the asymmetric tertiary carbons of the main chain are optically inactive by internal compensation, except for the end group of the chain and its contribution to the optical activity of a high molecular weight polymer is negligible. 

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