Ziegler–Natta Catalyst - Mechanism of Ziegler-Natta Polymerization

Mechanism of Ziegler-Natta Polymerization

The structure of active centers in Ziegler–Natta catalysts is firmly established only for metallocene catalysts. A metallocene complex Cp₂ZrCl₂ reacts with MAO and is transformed into a metallocenium ion Cp₂Zr+-CH₃. A polymer molecule grows in length by numerous insertion reactions of C=C bonds of 1-alkene molecules into the Zr–C bond in the ion:

Cp₂Zr+−CH₃ + n CH₂=CHR → Cp₂Zr+−(CH₂−CHR)_n−CH₃

Many thousands of alkene insertion reactions occur at each active center resulting in the formation of long polymer chains attached to the center. On occasion, the polymer chain is disengaged from the active centers in the chain termination reaction:

Cp₂Zr+−(CH₂−CHR _n−CH₃ + CH₂=CHR → Cp₂Zr+−CH₂−CH₂R + CH₂=CR–Polymer

Another type of chain termination reaction called β-hydrogen elimination reaction also occurs periodically:

Cp₂Zr+−(CH₂−CHR _n−CH₃ → Cp₂Zr+−H + CH₂=CR–Polymer

Polymerization reactions of alkene with solid Ti-based catalysts occur at special Ti 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:

L_nTi–CH₂−CHR–Polymer + CH₂=CHR → L_nTi–CH₂-CHR–CH₂−CHR–Polymer

The two chain termination reactions occurs 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:

L_nTi–CH₂-CHR–Polymer + H₂ → L_nTi-H + CH₃-CHR–Polymer

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 Ti atom, which is followed by insertion of the C=C bond into the Ti-C bond at the active center.