General Value in Polymerisation Systems

Very often these initiations avoid the difficulties presented by the more common protonic and aprotonic add catalysts, in that they are chemically well defined and allow a quantitative initiation reaction to proceed without complicating side reactions or the need for co-catalysts and promoters. Furthermore, since these salts are stable crystalline solids, physicochemical analysis of their solutions can be undertaken, in parallel with kinetic investigations of polymerisation. For example, ultraviolet/visible absorption spectrophotometry can be used to monitor directly the concentration of species such as Ph3C+ in solution (19—23). 

Proton NMR spectroscopy can similarly be used to monitor the concentrations of such salts, and has the advantage of being able to elucidate structures simultaneously 24,25), Independent conductimetric measurements can provide immediate information on the state of ionic aggregation in solution, in particular providing quantitative data concerning the proportion of free solvated ions to those existing as ion paired entities (26). 

While the latter measurements, strictly speaking, evaluate the state of dissociation of the initiator ions only, they also provide at least a guide as to the degree of dissociation of the propagating polymeric ion pairs. In the case of vinyl polymerisations, where no living cations have been observed to date, direct evaluation of the dissociation constant Kd, of the growing ion pair is not possible. However, in a number of cyclic monomer polymerisations living characteristics are observed, and direct measurements have been possible (27). 

Finally, highly reactive salts such as triphenylmethyl hexachloroantimonate provide very convenient scavenging agents for the removal of the last traces of nucleophilic impurities in polymerisation solvents. Penczek (35) has taken this further in developing a spectroscopic technique for the determination of trace quantities of residual water by reaction of the latter with known concentrations of triphenylmethyl salts, according to the following equilibria  

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