Growth via Macroanions

The classic anionic polymerization is an anionic polymerization with growth via macroanions. A new monomer molecule is added onto the growing polymer chain end in every propagation step, as the general form of (18-1) shows. It is immaterial how the macroanion has been produced, that is, if the start reaction was a two-electron reaction with formation of a monomer anion or zwitterion or whether it was a one-electron transfer. 

The polymerization rate depends very much on the proportion of free macroanions present. In the anionic polymerization of styrene with sodium as gegenion in THF, kinetic measurements (see below) gave the rate constants for polymerization via free macroanions as /C(- = 65000 dm mol s and fc(+) = 80 dm mol s for the polymerization via ion pairs (Table 18-2). According to the relation Vp = kp[P ] [M], the rate of the propagation reaction depends on the active species concentration [P ] as well as on the rate constant kp. But in this system, the dissociation equilibrium constant Kjy for ion pairs into free ions is only 10 mol/dm. If the polymerization is carried out in a solution with 10 mol/dm, then the proportion of free ions is consequently only (10 10 ) = 0.01, that is, 1 %, with 99% of the active species being ion pairs. Thus, despite a much lower rate constant, the polymerization rate is also determined to a considerable extent by the ion pairs. 

Here the solvent S does not solvate the free polystyryl anions, for otherwise a solvent dependence of the rate constant fe( ) would be seen. The rate constants for polymerization via contact ion pairs and via solvated ion pairs, related to the mean rate constant of ion pairs obtained 

The polymerization via zwitterions represents a special case of a polymerization with participation by ion pairs. The ion pairs can occur either as intramolecular macrocyclics or as chainlike associates  

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