CATIONIC POLYMERIZATION

Many vinyl monomers readily polymerize in the presence of very small amounts of catalyst of the type used in Friedel-Crafts reactions. Examples of effective catalysts are AlCl₃, AlBr₃, BF₃, TiCl₄, SnCl₄, and in some cases strong acids such as H₂SO₄. All these catalysts are examples of Lewis acids with strong electron-acceptor capability. They usually require a co-catalyst, namely a Lewis base such as Water, acetic acid or alcohol:

BF₃ + H₂O ⇔ H⁺BF₃OH^-

Monomers that polymerize in the presence of these catalysts include isobutylene, styrene, alpha-methylstyrene, butadiene, vinyl alkyl ethers and many other monomers having electron-donating substituents that enhance the electron-sharing ability of the double bond of the vinyl monomers.

H⁺BF₃OH^- + CH₂=CHR → H₃C-C⁺HR + (BF₃OH)^-

They all can be readily polymerized to high-molecular weight polymers.

H₃C-C⁺HR + n CH₂=CHR → H(-CH₂-CHR-)_nCH₂-C⁺HR

However, some other monomers, such as propylene and other olefins, reach only low to medium molecular weights when polymerized with strong Lewis acids.

The cationic polymerization usually proceeds at high rates both at high and (very) low temperatures. For this reason, a uniform and constant reaction condition cannot be maintained during polymerization. For example, isobutylene at -100C in the presence of a strong Lewis acid polymerizes to high molecular weight polybutylene within a fraction of a second. To prevent excessive rise in temperature in the reaction vessel, an “internal refrigerant” is usually added to the mixture that dissipates the heat by evaporation of a portion of the liquid.

Both the rate of reaction and the molecular weight decreases with increasing temperature. For this reason, low temperatures are usually preferred. In fact, molecular weights obtained at room temperature are often much lower than those achieved by free-radical polymerization.