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 from SO42-/TiO2 by modifying with the rare earth ion. It showed a good behavior in initiating ring-opening polymerization of chloromethyl thiirane (CMT). A striking characteristic of this polymerization was that poly(CMT) produced in fine yield possessed a molecular weight of 40 000-50 000, rather higher than previously reported. The conditions for the preparation of rare earth solid superacid and for CMT polymerization were thoroughly studied. It was found that the increase of Nd3+ concentration in the solution, in which the solid was immersed, enhanced the amount of sulfur absorbed on the surface of TiO2, which is essential for generation of strong acidity, and so improved 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 calcination 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 the catalytic center is a Bronsted acid site other than a Lewis one. A chelating bidentate structure for superacid center and a cationic process for CMT polymerization were proposed. (C) 2001 Elsevier Science Ltd. All rights reserved.