Click Chemistry reactions include a variety of chemical transformations that are modular, wide in scope, stereospecific, versatile in nature, high yielding and without by-products. These criteria were defined by Barry Sharpless and co-workers as the characteristics of click reactions. Within a relatively short period of time since its discovery in 2002, click chemistry has substantially influenced and found applications in a wide range of disciplines including organic chemistry, bioconjugates , drug discovery, polymers, materials science and many multitude of related areas.
Copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) is the prime example of click reactions among other commonly used methods being thiol-ene/yne and Diels-Alder etc. The ability to on-demand control of the click reactions has enabled chemists to construct new molecular architects in a controlled manner, especially in the polymer community this has been a great opportunity to tailoring macromolecules.
A group of researchers led by Prof. Yagci in Istanbul has recently reviewed the concept of controlling click reactions by means of various external stimuli for the synthesis of polymers. The paper published in journal Progress in Polymer Science by Tasdelen, Kiskan and Yagci provides a comprehensive view of the utility of various click reactions in constructing polymer structures triggered by external means. It is shown that externally stimulating click reaction is advantageous in many aspects. For this purpose various means have utilized to control and trigger the click reaction including thermal, photochemical , electrochemical, microwave and ultrasound means.
Light in particular is utilized as a green external stimulus to trigger a variety of click reactions including CuAAC, thiol-ene and many others. In CuAAC, which is catalyzed by copper(I) species, light can be efficiently used to activate the copper catalyst by in situ photoreduction of copper(II) complexes and generating active copper(I) catalyst species. In thiol-ene/yne reactions, however, a radical photoinitiator is mainly used to initiate the click process.
Detailed mechanistic explanations are given for respective photochemical reactions realizing click transformations. In addition, potential applications of such methods have been described in detail with an emphasis on macromolecular tailoring. Polymers can be availed of in a step-growth manner using multifunctional monomers with specific functionalities to carry out click reactions that result in click polymers. Or by introducing suitable sites in various positions of a given polymer, it can be decorated, modified, or labeled to serve a special purpose. Light-induced click reactions are particularly applicable in surface functionalization and patterning in a spatially controlled manner.
Read more on this:
Externally stimulated click reactions for macromolecular syntheses, Prog. Polym. Sci., 2016, 52, 19-78.