Polymerization with Two Active Species

Because of the low degree of dissociation, the concentration of ion pairs is practically always identical to the total initiator concentration, i.e., [NaP] [C ]. With this relation and the Ostwald dilution law,f equation 

The equilibrium dissociation constant X iss is difficult to obtain with a sufficient degree of accuracy from conductivity measurements, but it can be found from reaction kinetics measurements by adding a foreign salt with the same gegenion. With Kdiss = [P ] /[C ], equation (18-80) becomes 

The dissociation of growing macroions is greatly reduced by the addition of foreign salts (for example, Kalignost in the polymerization of styrene in tetrahydrofuran started by sodium naphthalide)  

The slope of a plot of measured rates Up against the reciprocal of the total sodium gegenion concentration gives /C(-.)Xdiss- Since, according to equation (18-80), /C(-)Xdis s is obtained for measurements in the absence of foreign salts, the individual values of k( ) and X iss can be obtained from these data. 

In ionic polymerization via macroions without termination or transfer, the number-average molecular weight is clearly fixed by the yield and the equilibrium concentrations of monomer and initiator, since the formation of ions from the initiator is always very rapid compared to the propagation steps. However, the resulting molecular-weight distribution also depends on the fact that dimeric diions (in a one-electron reaction) do all initiate polymerization simultaneously. If the initiator solution is added to the monomer solution dropwise, then the initiator ions in the first drops will have started the polymerization before those of the subsequent drops can begin. Thus the chains will be of different lengths. 

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