Polymerization with uncontrolled propagation

It is apparent that, as a result of the extremely rapid propagation, if aU chain ends were ionized and grew simultaneously, monomer would disappear at such a high rate that the polymerization would be uncontrollable. In hving cationic polymerization, therefore, a dynamic equilibrium must exist between a very small amount of active ionic and a large pool of inactive dormant species. The expression of controlled polymerization is sometimes used to describe, perhaps questionably, such polymerizations with reversible deactivation of the chain carriers. 

For a long time, cationic polymerization has been considered to be very difficult to control to obtain polymers of narrow molecular-weight distribution. Usually, molecular weights are unpredictable and M /M is far from one. An active species at the propagating polymer end is a carbocation or an onium ion, which reacts with olefin monomers extremely rapidly. The active polymer end is also highly unstable and readily participates in chain-transfer reactions by loss of p-protons leading to uncontrollable molecular-weight distributions. 

The keLone is more reactive than MMA and reacts with some of the propagating MMA anions Lo form the less reactive anions. The alkoxide coordinates with the propa-gaLing species to affect their stereospecificity and reactivity. Therefore, the carbonyl attack makes the polymerization reaction complex and uncontrollable. 

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