1. Contribution from the Laboratory of Physical Chemistry, ETH Hönggerberg, CH-8093 Zurich, Switzerland, Max-Planck-Institut für Polymerforschung, Postfach 3148, D-55021 Mainz, Germany, The Weizmann Institute of Science, Rehovot, 76100, Israel, and Institute for Organic Chemistry, University of Innsbruck, A-6020 Innsbruck, Austria