Catalyst performance in "click" coupling reactions of polymers prepared by ATRP: Ligand and metal effects

Cu-I-catalyzed azide-alkyne cycloadditions were conducted in organic media under various conditions. The effects of several parameters (ligand, solvent, reducing agent, metal) on these reactions were studied using the step-growth click coupling of low-molecular-weight alpha,omega-diazido- terminated polystyrene prepared by atom transfer radical polymerization (ATRP). These reactions were typically conducted in DMF, monitored by size exclusion chromatography (SEC), and semiquantitatively analyzed by Gaussian multipeak fitting and subsequent peak integration. Both the electronic properties of the ligand and the number of coordinating atoms had significant influence on the rates of the click coupling reactions. Aliphatic amine ligands led to significantly faster rates as compared to pyridine-based ligands. Faster rates were also observed with tridentate vs tetradentate ligands. A further rate enhancement was observed when the reactions were conducted in a noncoordinating solvent (toluene) vs a coordinating solvent (DMF). Despite the typical susceptibility of Cu-I complexes to oxidation, the addition of excess hydrazine as a reducing agent allowed click reactions to be conducted under limited amounts of air with decreased catalyst concentrations. A pronounced rate enhancement was observed during reactions conducted in the presence of hydrazine, which could be due to the basicity of hydrazine. Finally, azide-alkyne cycloadditions were successfully catalyzed by oxidatively stable metal complexes, including those of Ni-II, Pd-II, and PtII. The PtII catalyst demonstrated the highest catalytic activity relative to those of the other metals.