1. Institut für Makromolekulare Chemie und Freiburger Materialforschungszentrum der Albert-Ludwigs-Universität Freiburg, Stefan-Meier-Strasse 21, D-79104 Freiburg, Germany, DSM Research, PO box 18, 6160 MD Geleen, The Netherlands, and DSM Venturing & Business Development, PO box 18, 6160 MD Geleen, The Netherlands