Hydrogen Effects in Propylene Polymerization with Ti-Based Ziegler-Natta Catalysts. Chemical Mechanism
The hydrogen activation effect in propylene polymerization reactions with Ti—based catalysts is usually explained in the literature as a consequence of the secondary insertion of a propylene molecule into the Ti—CH2CH(CH3)—Polymer bond resulting in the formation of a sleeping active center. This article proposes a different mechanism of the hydrogen activation effect. It is based on the assumption that propylene insertion into the Ti—H bond has poor regioselectivity, in contrast to its insertion into the Ti—C bond, which is highly regioselective. Ti—CH(CH3)2 species formed after the secondary insertion of propylene into the Ti—H bond is stable due to the ß—agostic interaction between the H atom of its CH3 group and the Ti atom. However, the Ti—H bond is restored in a reaction with hydrogen. Validity of this mechanism is demonstrated in the study of ethylene/α—olefin copolymerization reactions with two Ti—based catalysts. GC analysis of ethylene—propylene co—oligomers prepared in the presence of hydrogen showed that the probability of the secondary insertion of a propylene molecule into the Ti—H bond is only 3-4 times lower than the probability of its primary insertion.
KeywordsEthylene Polymerization Hydrogen Effect Copolymerization Reaction Propylene Polymerization Propylene Molecule
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