24 g (0.1 mol) of 4'-aminophenyl 4-aminobenzoate and 100 cm3 of N-methylpyrrolidone are introduced into the apparatus described in Example 18. A suspension is obtained, which is kept at between 5° and 10° C. whilst a solution of 51 g (0.2 mol) of the chloride of p-(vinyldimethy...