Activity and mechanism of rare earth solid superacid for initiating ring-opening polymerization of chloromethyl thiirane

Rare earth solid superacid, SO42-/TiO2/Ln(3+), was successfully developed f rom SO42-/TiO2 by modifying with the rare earth ion. It showed a good behav ior in initiating ring-opening polymerization of chloromethyl thiirane (CMT ). A striking characteristic of this polymerization was that poly(CMT) prod uced in fine yield possessed a molecular weight of 40 000-50 000, rather hi gher than previously reported. The conditions for the preparation of rare e arth solid superacid and for CMT polymerization were thoroughly studied. It was found that the increase of Nd3+ concentration in the solution, in whic h the solid was immersed, enhanced the amount of sulfur absorbed on the sur face of TiO2, which is essential for generation of strong acidity, and so i mproved the catalytic activity. Therefore, it is reasonable to suggest that the role of Ln(3+) is mainly to retard the loss of sulfur in the calcinati on treatment. Heating at 300-350 degreesC in vacuum almost deactivated the rare earth solid superacid. However, its activity was restored successfully by adding a small suitable amount of water. This clearly indicates that th e catalytic center is a Bronsted acid site other than a Lewis one. A chelat ing bidentate structure for superacid center and a cationic process for CMT polymerization were proposed. (C) 2001 Elsevier Science Ltd. All rights re served.