Isotactic arrangement

Generally polymers involve bonding of the most substituted carbon of one monomeric unit to the least substituted carbon atom of the adjacent unit in a head-to-tail arrangement. Substituents appear on alternate carbon atoms. Tacticity refers to the configuration of substituents relative to the backbone axis. In an isotactic arrangement, substituents are on the same plane of the backbone axis that is, the configuration at each chiral center is identical. 

The desired form in homopolymers is the isotactic arrangement (at least 93% is required to give the desired properties). Copolymers have a random arrangement. In block copolymers a secondary reactor is used where active polymer chains can further polymerize to produce segments that use ethylene monomer. 

Polybutylene. Polybutylene is composed of linear chains having an isotactic arrangement of ethyl side groups along the chain backbone. 

By combining orientational isomerism ( forward or backward ) with isotactic arrangement of —CHR— groups in the main chain, as already discussed for the case of polyallenes (Scheme 10) and polyketenes (Schemes 11, 12). 

Later, Tieke reported the UV- and y-irradiation polymerization of butadiene derivatives crystallized in perovskite-type layer structures [21,22]. He reported the solid-state polymerization of butadienes containing aminomethyl groups as pendant substituents that form layered perovskite halide salts to yield erythro-diisotactic 1,4-trans polymers. Interestingly, Tieke and his coworker determined the crystal structure of the polymerized compounds of some derivatives by X-ray diffraction [23,24]. From comparative X-ray studies of monomeric and polymeric crystals, a contraction of the lattice constant parallel to the polymer chain direction by approximately 8% is evident. Both the carboxylic acid and aminomethyl substituent groups are in an isotactic arrangement, resulting in diisotactic polymer chains. He also referred to the y-radiation polymerization of molecular crystals of the sorbic acid derivatives with a long alkyl chain as the N-substituent [25]. More recently, Schlitter and Beck reported the solid-state polymerization of lithium sorbate [26]. However, the details of topochemical polymerization of 1,3-diene monomers were not revealed until very recently. 

Fig. 6. Polypropylene isotactic arrangement of methyl groups along the helix (described by a tape) the structure of syndiotactic polypropylene (drawing in the middle) is also regular the chain in atactic polypropylene (below) is statistically irregular ... 

Both the carboxylic acid and aminomethyl substituent groups are in an isotactic arrangement, causing an erythro-diisotactic configuration of the polymer chains. Indeed, the 1,4-addition induces two centers of chirality per monomer unit. However, due to the center of symmetry of the space group the unit cell contains a racemic mixture of the polymer. 

The vinyl groups may have either syndic- or isotactic arrangements. Depending on the method of preparation, the percentages of these different micro structures in a PB sample may vary. In industrially manufactured PB the cis-, A microstructure (>92%) dominates. 

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