Employing exchange reactions involving hypervalent iodine compounds for the direct synthesis of azide-containing linear and branched polymers

Abstract

A method for the preparation of azide-containing linear and branched polymers was developed that takes advantage of the efficient exchange of the acetoxy “ligands” in (diacetoxyiodo)benzene PhI(O₂CCH₃)₂ (Ph = phenyl) with azides, for instance in the presence of sodium azide. The in situ generated azide-containing hypervalent iodine(III) compounds PhI(N₃)(O₂CCH₃) and PhI(N₃)₂ decomposed rapidly, even at temperatures close to ambient, to the monovalent iodine compound (iodobenzene) and azide (and – in the former case – also acetoxy and/or methyl) radicals. The reactive azide radicals initiated the polymerization of methyl methacrylate at 40–100 °C, yielding polymers with an azide functionality at the α-terminus. Fast termination by coupling of the propagating radicals with azide radicals and possibly azide transfer from PhI(N₃)_x(O₂CCH₃)_2−x (x = 1, 2) were responsible for the formation of low-molecular-weight linear polymers with an azide group at the ω-end. When the polymerizations were carried out in the presence of divinyl compounds (crosslinkers), highly branched polymers were produced prior to gelation. The network formation was delayed in these systems until moderate to high monomer conversions were reached due to the aforementioned termination and/or transfer, which limited the size of the interconnected chains and therefore the average number of incorporated pendant vinyl groups per chain. The use of the PhI(O₂CCH₃)₂–NaN₃ initiating system proved to be an efficient one-pot route to mono- and diazide-capped linear or multi-azidated branched polymers that could be easily functionalized, e.g., under azide–alkyne “click” coupling conditions with alkynes, such as propargyl 4-(1-pyrenyl)butyrate.