How do alkenes react to form polymers in polymerisation?

1 Answer
anor277
Feb 4, 2017

Typically ethylene inserts into a #""^(+)M-R# bond, where the #R# group starts as methyl.......

Explanation:

The typical olefin polymerization reaction starts with a metal dimethyl species, #L_2M(CH_3)_2#, where #(i)# #M# is a metal such as titanium or zirconium, and #(ii)# #L# is some ligand that stabilizes and solubilizes the metal centre.

Addition of a Lewis acid abstracts one of the methyl groups to yield a species conceived to be #L_2M^(+)(CH_3)#:

#L_2M(CH_3)_2 + "aluminum reagent"rarrL_2M^(+)(CH_3) + "methylated aluminium reagent"#

Ethylene can bind to the cationic metal centre:

#L_2M^(+)(CH_3)+H_2"C=C"H_2rarrL_2M^(+)(CH_3)(eta^2-H_2"C=C"H_2)#

The methyl group bound to the metal centre can migrate to the #eta^2-"bound"# olefin to give a propyl group. And such insertion generates an open coordination site on the metal centre, to which another ethylene can bind, and this reinserts into the #M-"alkyl"# residue. Long chain alkyl residues, polyethylenes, can be built up this way.

This will be much better explained in your given text.

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