Question

How do alkenes react to form polymers in polymerisation?

Answer 1

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.