1. Contribution from the School of Biological and Chemical Sciences, Birkbeck College (University of London), Gordon House, 29 Gordon Square, London WC1H 0PP, UK, Department of Chemistry, University of Southampton, Highfield, Southampton SO17 1BG, UK, and Institute of Polymer Chemistry, Polish Academy of Sciences, 34 M. Curie-Sklodowskiej, St., 41-800 Zabrze, Poland