انجمن شیمیدانان جوان

Comparison of Free Radical and Ionic Polymerizations

Ionic vinyl polymerizations are very similar to free radical vinyl polymerizations. The only difference is the "flow" of the electrons during propagation.

Recall that a double bond equals a single bond plus two more electrons.

In free radical vinyl polymerization, the electrons in the pi bond split up. One combines with the unpaired electron in the initiator (or growing chain end) to form the new bond, and the second ends up on the chain end, reproducing the attacking species.

In anionic vinyl polymerization, the electrons in the pi bond more together instead of separately. The initiator (or growing chain end) attacks with a pair of electrons, used to form the new bond. The pi-bond electron pair "flows" away from the attacking species, reproducing the anion at the chain end.

Cationic vinyl polymerization is exactly the same mechanism, except that the initiator (or chain end) lacks a pair of electrons. The electron "flow" is simply in the oposite direction, leaving behind a positive charge at the chain end to continue the process.

One important difference: ionic polymerizations necessarily carry along a counterion, and their rates are much more sensitive to reaction conditions (e.g., solvent polarity, temperature).