1. The W.M. Keck Laboratories for Organic Synthesis, Department of Macromolecular Science, Case Western Reserve University, Cleveland, Ohio 44106-7202, Department of Engineering Materials and Center for Molecular Materials, University of Sheffield, Sheffield S1 3JD, U.K., and Maurice Morton Institute of Polymer Science, Department of Polymer Science, The University of Akron, Akron, Ohio 44325-3909