Modulating Two Copper(I)-Catalyzed Orthogonal “Click” Reactions for the One-Pot Synthesis of Highly Branched Polymer Architectures at 25 °C
(4814–4827) Macromolecules 44 #12 (2011)
Bell et al of the University of Queensland, Australia, found that using the model polystyrene (PSTY) building block system, third generation dendrimers could be formed divergently, convergently or in parallel through modulating the Cu(I) activity for the copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) and nitroxide radical coupling (NRC) reactions.  The parallel approach was the fastest, generating a G3 dendrimer in under 30 min, and the next fastest was the divergent pathway, followed by the very slow (24 h) convergent pathway.  The resulting G3 dendrimer could be cleaved at the alkoxyamine sites back to linear polymers by heating the reaction mixture at 120 °C in the presence of an excess hydroxyl nitroxide. The synthetic utility of this method was further extended to coupling linear telechelic polymer building blocks, consisting of PSTY, PtBA, PEG, and PNIPAM, to form a range of dendrimers in high yields. Preparative SEC was used to fractionate excess starting reactants and intermediate polymer species from the product dendrimer.  (RDC 6/22/2011)