Polymerization with Incomplete Dissociation of Initiator

If the initiator used in anionic polymerization dissociates completely from the inactive form, CA, to the active form, A (or C + A ), before any propagation 

It may be noted that only propagation by ion pairs has been considered in the above scheme. In a more complete scheme, propagation by free ions [cf. Eq. 

Let us now consider an anionic polymerization where propagation and termination reactions occur simultaneously and polymerization follows in a manner similar to free-radical polymerizations. An example is the potassium amide initiated polymerization in liquid ammonia. This is one of the few anionic systems in which all active centers behave kineticaUy as free ions. The initiation step in this case 

Since the second step [Eq. (8.77)] is slow relative to the first [Eq. (8.76)], the rate of initiation is given by 

When KNH2 is used as the initiator and no external K is added, the concentrations ofK andNH2 areequal. Therefore, from Eq. (8.78),i = [NH2] /[KNH2], and Eq. (8.86) can thus be rewritten as 

If the initiator used in anionie polymerization dissociates completely from the inactive form, CA, to the active form, C A (or C + A ), before any propagation reactions take place, the kinetics, as we have seen earlier, assumes a very simple form. Some initiators (e.g., lithium alkyls and aryls), however, dissociate only partially and so also the anionic growing chains. In such a case, dissociation equilibria must be considered both for initiation steps, which may now be written as (Allcock and Lampe, 1990)  

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