Acryl nitrile is polymerised under anion or cationic conditions. Explain your choice.

Anionic polymerization is the best method for the preparation of polyacrylonitrile, in this initiation can also involve the transfer of an electron from the alkali metal to the monomer to form an anion-radical. Initiation occurs on the surface of the metal, with the reversible transfer of an electron to the adsorbed monomer, which is shown by following mechanism: