1. Contribution from the Institut für Polymerforschung Dresden, Hohe Strasse 6, Dresden, 01069 Germany, Department of Chemistry, Clarkson University, Potsdam, New York 13699-5614, Centre for Education and Research on Macromolecules (CERM), University of Liège, Sart-Tilman, B6a, 4000 Liège, Belgium, and Lawrence Berkeley National Laboratory, Berkeley, California 94720