Polymerization by activated monomer mechanism

The active centre usually appears simultaneously as the end group of the growing chain. In some cases, the Gibbs energy of the active centre-monomer system is more negative for the chain-activated monomer pair 

The equilibrium in these cases is shifted to the right the monomer yields a formation carrying charge (energy), and only the monomer activated in this way is added to the end of the (usually) neutral chain. This type of addition is typical for the ionic polymerization of lactams. 

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