Constructing star polymers via modular ligation strategies

Branched polymers result in a more compact structure in comparison to linear polymers of identical molecular weight, due to their high segment density which affects the crystalline, mechanical, and viscoelastic properties of the polymer. Star polymers constitute the simplest form of branched macromolecules where all of the chains-or arm segments-of one macromolecule are linked to a centre defined as the core. Over recent years, modular ligation reactions-some of which adhere to click criteria-have enabled the synthesis of a variety of star polymers via efficient polymer-polymer conjugations. While the modified Huisgen [3 + 2] dipolar copper catalyzed azide and alkyne cycloaddition (CuAAC) has been widely employed for macromolecular star synthesis, Diels-Alder and hetero Diels-Alder reactions offer alternative pathways which allow for similarly efficient macromolecular conjugations. Moreover, combinations of these protocols afford the synthesis of more complex star polymer structures which previously had not been achievable. Over recent years, modular ligation reactions-some of which adhere to the click criteria-have enabled the synthesis of a variety of star polymers via efficient polymer-polymer conjugations. The copper catalyzed azide-alkyne cycloaddition (CuAAC), Diels-Alder (DA), and Hetero Diels-Alder (HDA) reactions are reviewed here in detail for the facile generation of various macromolecular star topologies.