The Cossee–Arlman mechanism describes the growth of stereospecific polymers. This mechanism states that the polymer grows through alkene coordination at a vacant site at the titanium atom, which is followed by insertion of the C=C bond into the Ti−C bond at the active center. Termination processes On occasion, the polymer chain is disengaged from the active centers in the chain termination reaction. Several pathways exist for termination: Cp2−(CH2−CHR)n−CH3 + CH2=CHR → Cp2−CH2−CH2R + CH2=CR–polymer Another type of chain termination reaction called a β-hydride elimination reaction also occurs periodically: Cp2−(CH2−CHR)n−CH3 → Cp2−H + CH2=CR–polymer Polymerization reactions of alkenes with solid titanium-based catalysts occur at special titanium centers located on the exterior of the catalyst crystallites. Some titanium atoms in these crystallites react with organoaluminum cocatalysts with the formation of Ti–C bonds. The polymerization reaction of alkenes occurs similarly to the reactions in metallocene catalysts: LnTi–CH2−CHR–polymer + CH2=CHR → LnTi–CH2-CHR–CH2−CHR–polymer The two chain termination reactions occur quite rarely in Ziegler–Natta catalysis and the formed polymers have a too high molecular weight to be of commercial use. To reduce the molecular weight, hydrogen is added to the polymerization reaction: LnTi–CH2−CHR–polymer + H2 → LnTi−H + CH3−CHR–polymer Another termination process involves the action of protic (acidic) reagents, which can be intentionally added or adventitious. Commercial polymers prepared with Ziegler–Natta catalysts Polyethylene Polypropylene Copolymers of ethylene and 1-alkenes Polybutene-1 Polymethylpentene Polycycloolefins Polybutadiene Polyisoprene Amorphous poly-alpha-olefins (APAO) Polyacetylene References Further reading Coordination complexes Catalysts Polymer chemistry Industrial processes 1953 in science 1953 in Germany