1. Laboratory of Macromolecular Chemistry, ESA (5076) CNRS, Ecole Nationale Supérieure de Chimie de Montpellier, 8 Rue de l'Ecole Normale, 34296 Montpellier Cedex 5, France, and Center for Electron Beam Technology and Polymers, University of Bourgas, 8010 Bourgas, Bulgaria