Stereospecific catalyst

Stereospecific polymerization catalysts lead to the formation of stereoregular (tactic) polymers, that is, polymers where the centers of steric isomerism in the main chain are arranged in a regular fashion with respect to their configurations. Three factors determine the tacticity of a polymeric chain during its formation: (1) the kind of monomer approach to the growing chain end, (2) the kind of attack of the growing chain end on the double bond (cis or trans opening), and (3) the configuration in the initiation step. In addition to a regular head-to-tail configuration and absence of branching, reactions have to be assumed. The kind of monomer approach is strongly affected by electrical and stereochemical forces, and therefore changes in monomer structure and environment greatly influence polymer tacticity. The intermediate radical or ion can be assumed to have a planar or near-planar structure, and in an uncomplexed form it should be able to rotate freely around its axis with cis and trans addition being equally possible. Upon addition of the next monomer this carbon changes into a tetrahedral structure, thereby creating two isomeric forms, isotactic or syndiotactic. See also Asymmetric synthesis; Catalysis; Polymerization; Stereochemistry.