Untersuchung der Kationischen Copolymerisation von Cyclischen Anhydriden mit Cyclischen Äthern

The preparation of linear polyesters by the well-known base-catalyzed copolymerization of carboxylic dianhydrides and cyclic ethers is possible only with epoxides or in the essentially slower reaction with 4-membered cyclic ethers. It is not possible with higher cyclic ethers. The reaction of 5-7-membered cyclic ethers (tetrahydrofuran, tetrahydropyran, hexamethylene oxide) and succinic and phthalic anhydride can be carried out with varied reaction conditions. Using complex Lewis acid/co-initiator type initiators, e.g., SbCl5-MeCOCl or Et3O+BF4- producing reactive cations polyesters with molecular weights up to 3200 are formed within 3 hr. in the melt at 120°C. with 70-95% yield. The products obtained contain ether bond units in the polymer chain. The ether content not only depends on the molar ratio of the reactants but is strongly influenced by the nature and especially by the concentration of initiator. On the basis of this fact a two-step reaction mechanism is postulated: (1) polymerization of the cyclic ether to the corresponding poly(alkylene oxide); (2) subsequent transacylation inserting the anhydride between the polyether chain units. The occurrence of the secondary reaction could be confirmed by short-time experiments. Similarly, polyethers and carboxylic dianhydrides are converted to the corresponding polyesters by complex cationic initiators. This further proves the two-step mechanism postulated above.