1. Laboratory of Macromolecular Chemistry (UMR CNRS 5076), Ecole Nationale Supérieure de Chimie de Montpellier, 8 Rue de l'Ecole Normale, F-34296 Montpellier Cedex 5, France, and Department of Organic Chemistry, Institute of Chemical Technology, Technickà 5, 166 28, Prague 6, Czech Republic