Aprotic dipolar protophilic solvents

In this section we shall consider the state of protonic acids in the pure state and in solutions of three classes of solvents (i) amphiprotic-protogenic (mineral and carboxylic acids), (ii) aproticdipolar-protophobic (e.g., acetonitrile and nitromethane), and (iii) aprotic-inert (aliphatic and aromatic hydrocarbons and their haloderivatives). While classes (ii) and (iii) represent the only two families of solvents relevant to cationic polymerisation (with the possible exception of the polymerisation of N-vinylcarbazole, wdiich can be carried out in certain dipolar-protophilic solvents), class (i) is interesting because it represents the interaction between two Br nsted acids, the initiator and the solvent, as a direct source of protonating species. Althou the latter combination has not been used in cationic polymerisation, we will discuss its potentials and possible drawbacks. 

Amphiprotic protogenic solvents have higher acidic properties, but lower basic ones (always in comparison to water). Examples are formic and acetic acid. Amphiprotic protophilic solvents have lower acidity and higher basicity than water, with formamide or ethanolamine as examples. Aprotic dipolar solvents have low acidity and (occasionally) basicity as well, with A,A-dimethylformamide and dimethylsulfoxide as examples for protophilic dipolar solvents and acetonitrile for a protophobic dipolar solvent. 

Water has high permittivity and moderate acidity and basicity. Thus, in water, many cations and anions are easily solvated (hydrated) and many electrolytes are highly soluble and dissociate into ions. Water has fairly wide pH and potential ranges and a convenient liquid temperature range. Of course, water is an excellent solvent. However, as in Table 1.7, the reaction environment can be expanded much wider than in water by use of a solvent of weak acidity and/or basicity. This is the reason why dipolar aprotic solvents, which are either protophilic or protophobic, are used in a variety of ways in modern chemistry. 

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