Synthesis of Arborescent Polymers by “Click” Grafting

ABSTRACTA novel method was developed for the preparation of arborescent (dendritic graft) polymers, by successive grafting reactions of linear chain segments using alkyne-azide “click” chemistry coupling. A linear polystyrene substrate was thus randomly functionalized with acetylene functionalities, by acetylation and further reaction with propargyl bromide in the presence of potassium hydroxide and 18-crown-6 in toluene. The anionic polymerization of styrene was achieved with 6-tert-butyldimethylsiloxy-hexyllithium to obtain polystyrene with a protected hydroxyl chain end. Deprotection of the hydroxyl group, followed by conversion into tosyl and azide functionalities yielded the material serving as side chains in the grafting reactions. Coupling of the azide-terminated side chains with the acetylene-functionalized substrate in the presence of a Cu(I) catalyst proceeded in up to 93% yield. Additional cycles of substrate functionalization and side chain coupling led to arborescent polymers of generations G1 and G2, with low polydispersity indices (Mw/Mn≈ 1.1), in 60-84% yield. These polymers are characterized by a very compact structure, and molecular weights increasing geometrically over successive generations. A similar methodology was also shown to work for the synthesis of arborescent polybutadiene systems, using azide-functionalized substrates and alkyne-terminated side chains. The coupling reaction proceeded in up to 76% yield under optimized conditions for these systems.