Radical-initiated polymerizations are generally nonspecific, but this is not true for ionic initiators because the formation and stabilization of a carbonium ion or carbanion depend largely on the nature of the group R in the vinyl monomer CH

=CHR. For this reason, cationic initiation is usually limited to monomers with electrondonating groups that help stabilize the delocalization of the positive charge in the

π

-orbitals of the double bond. Anionic initiators require electron-withdrawing substituents (—CN,—COOH,—CH=CH

, etc.) to promote the formation of a stable carbanion, and when there is a combination of both mesomeric and inductive effects, the stability is greatly enhanced.