| Résumé | A new reaction procedure has been developed for the polycondensation of hexafluorobisphenol A (6F-BPA) with bis(pentafluorophenyl) ketone (BPK) in dimethylacetamide (DMAc). In this reaction, CaH2 has been used as a base to facilitate the formation of the phenolate; an alkali metal fluoride was used as a catalyst. Unlike the conventional polycondensation reactions, this procedure avoided producing water during the reaction, and hence, azeotropic distillation with the assistant of a low-polar co-solvent is not required. This improvement maintains the high polarity of the reaction medium; consequently, the selectivity of the reaction at the para-position of BPK is significantly enhanced and polymers with high molecular weights (Mn ca. 30 000 Da) can be obtained with very low branching contents (less than 3 mol-%) and free of any cross-linked gel particles. In addition to avoiding the formation of water in the reaction, the use of CaH2 also reduced the concentration of the fluoride ion in the solution. These effects minimize chain degradation reactions such as hydrolysis. The use of CaO instead of CaH2 for this reaction produces similar effects. In the case of CaO, traces of water can be introduced into the reaction. The presence of trace amounts of water increases the polycondensation rate, probably due to the enhanced ionization of the phenolate. However, it also causes a hydrolysis of polymer chains, resulting in the elimination of the ketone group in the polymer backbone. Fortunately, this side reaction is highly water and temperature dependent, and can be efficiently prevented by using CaH2 or conducting the reaction at a lower temperature (<65 °C). |
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