Stereochemistry of Polymerization

Isotactic and syndiotactic polymers are stereoregular structures obtained in the polymerization of propene and other carbon–carbon double bond monomers. Atactic polymer is the irregular structure. Stereoregular polymers are more crystalline, stronger, and more useful polymers than atactic polymers. Isotactic and syndiotactic polymers are produced in stereoselective polymerizations when coordination initiators (catalysts) are used, including the traditional Ziegler–Natta initiators such as TiCl 3 /Al(C 2 H 5 ) 3 , metallocenes, and post‐metallocene initiators. Stereoselectivity proceeds through either polymer chain control or catalyst site control, depending primarily on the symmetry (C 2n , C 2 , meso‐C S , C S , or C 1 ) of the initiator, but also on temperature, solvent and monomer. The kinetics and other characteristics of stereoselective polymerization are described with emphasis on the parameters that control polymer stereochemistry and molecular weight. Stereochemistry is also important in the polymerization of 1,3‐dienes where stereoregular cis‐ and trans‐structures are possibilities along with the isotactic and syndiotactic structures. Coordination initiators are important in the commercial production of polypropene, ethylene‐propene and various poly(1,3‐diene) elastomers, as well as high density polyethylene and linear low density polyethylene.