1. Contribution from the Laboratoire de Chimie des Polymères Organiques, Ecole Nationale Supérieure de Chimie et Physique de Bordeaux-Université Bordeaux-1-CNRS 16, Avenue Pey Berland, 33607 Pessac Cedex, France, and Laboratory for Biomolecular Materials Research, Department of Surgery, Emory University School of Medicine, Atlanta, Georgia 30322