Mixtures of cationic and anionic surfactants form insoluble salts and are not useful. Unlike molecular compounds, salts dissociate in solution into anionic and cationic components. Some salts of cationic and anionic metal carbonyls are soluble in water or lower alcohols. It can be further divided into anionic polymerization and cationic polymerization. Zwitterionic (amphoteric) surfactants have both cationic and anionic centers attached to the same molecule. The reactive center can be radical, anionic or cationic. Free-radical polymerization is a type of chain-growth polymerization, along with anionic, cationic and coordination polymerization. There are two main types of emulsions with different affinity for aggregates, cationic and anionic. Polyelectrolytes that bear both cationic and anionic repeat groups are called polyampholytes. Manufacturers often use a mixture of cationic and non-ionic surfactants as an emulsifier. Subsequent investigators reported iodine-like, cationic, or amphoteric behavior. Lewis acids are the most common compounds used for initiation of cationic polymerization. One common substrate is glass fibre coated with polybrene, a cationic polymer. The retention agent is a polymer with high cationic, positively charged groups. Pgp is known to transport organic cationic or neutral compounds.