Click chemistry constitutes a class of reactions broadly characterized by efficiency, selectivity, and tolerance to a variety of solvents and functional groups. By far the most widely utilized of these efficient transformation reactions is the Cu(I)-catalyzed azide-alkyne cycloaddition. This reaction has been creatively employed to facilitate the preparation of complex macromolecules, such as multiblock copolymers, shell or core cross-linked micelles, and dendrimers. This critical review highlights the application of click chemistry, in particular the Cu(I)-catalyzed azide-alkyne cycloaddition, to the synthesis of a wide variety of new materials with possible uses as drug delivery agents, tissue engineering scaffolds, and dispersible nanomaterials (83 references).

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 R,ω-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 Pt II . The Pt II catalyst demonstrated the highest catalytic activity relative to those of the other metals.

No abstract

Polyelectrolyte coatings significantly increase the mobility of nanoscale zerovalent iron (NZVI) in saturated porous media. The effect can be attributed to improved colloidal stability of NZVI suspensions, decreased adhesion to soil surfaces, or a combination of the two effects. This research explicitly examines how coatings control NZVI adhesion to model soil surfaces. NZVI was coated with three different polyeleotrolyte block copolymers based on poly(methacrylic acid), poly(methyl methacrylate or butyl methacrylate), and poly(styrenesulfonate) or with a poly(styrenesulfonate) homopolymer. SiO2 and a humic acid film served as model soil surfaces. The polyelectrolytes increased the magnitude of the electrophoretic mobility of NZVI over a broad pH range relative to unmodified NZVI and shifted the isoelectric point outside the typical groundwater pH range. Quartz crystal microgravimetry measurements indicated extensive adhesion of unmodified NZVI to SiO2. Polyelectrolyte coatings decreased adhesion by approximately 3 orders of magnitude. Adding 50 mM NaCL to screen electrostatic repulsions did not significantly increase adhesion of modified NZVI. Coated NZVI did not adhere to humic acid films for either 1 mM NaHCO3 or 1 mM NaHCO3 + 50 mM NaCl. The lack of adhesion even in a high ionic strength medium was attributed to electrosteric repulsion, as opposed to electrostatic double layer repulsion, between the polyelectrolyte-coated NZVI and the negatively charged surfaces. The lack of significant adhesion on either model surface was observed for all polymer architectures investigated.

Since the concept of highly efficient and selective “click” reactions was put forth by Sharpless and coworkers, this branch of chemical transformations has been subject to an astounding degree of applications. Although click chemistry encompasses a wide variety of reactions, the CuI‐catalyzed azide–alkyne cycloaddition has received the most attention. It has been increasingly employed in polymer functionalization and materials synthesis, especially in conjunction with controlled radical polymerization methods, such as Atom Transfer Radical Polymerization (ATRP). The CuI‐catalyzed azide–alkyne cycloaddition is utilized particularly well with ATRP, due to the ease of incorporating clickable functionality into polymers prepared by ATRP and the use of the same catalyst in each process. This minireview summarizes and analyzes recent developments in the field of CuI‐click chemistry as applied to ATRP, and how the combination of these two powerful techniques has greatly expanded the range of available materials and has contributed to fundamental understanding of this process.