Click polymerizations with remarkable advantages have been nurtured into powerful polymerization techniques with widespread applications. In this overview, several types of click polymerizations and their applications are briefly summarized. Of these, azide–alkyne click polymerizations (AACPs) are the most widely used due to the easy availability of the monomers and the stability of the products. AACPs catalyzed by Cu(I) and Ru(II) can offer 1,4- and 1,5-regioregular PTAs, respectively. Meanwhile, 1,4-regioregular PTAs can also be obtained via metal-free click polymerizations of activated alkynes and azides or activated azides and alkynes. 1,5-regioregular PTAs can be produced by organic base-mediated AACP. Thiol-ene click polymerizations are versatile tools for the preparation of polythioethers with linear and hyperbranched structures. Similarly, thiol-yne click polymerizations, as the updated version of the former, can provide polythioethers and poly (vinyl sulfide)s (PVSs) with diverse structures. Novel reversible polymers can be yielded via Diels–Alder click polymerizations. Some new click polymerizations have also been researched for the synthesis of functional polymers with various structures. With these efficient polymerization techniques in hand, polymer scientists have prepared a large number of polymers with unique properties, such as luminescence, photonic patterning, adjustable light refractivity, optical nonlinearity, biodegradability, catalyst activity, self-assembly and self-healing.
1.1Introduction
Nowadays, our daily lives are more convenient and comfortable than before, owing to the extensive application of ubiquitous polymeric materials with a variety of functionalities in various areas. The widespread application of polymeric materials is inseparable from the development of polymer science. Polymerization reactions, the cornerstone of polymer science, have drawn intense attention from polymer scientists. Meanwhile, the exploration of new efficient polymerization reactions for the construction of novel functional polymer materials is an everlasting topic in the area of polymer science.1 Generally, most new polymerization reactions are developed from existing organic reactions of small molecules with such favorable features as high efficiency, moderate reaction conditions, accessible highly effective catalysts, and available multi-functionalized monomers.2
Click chemistry, coined by Sharpless and co-workers in 2001, is a concept proposed for a class of almost perfect reactions that are highly effective with high atom economy, wide in scope, and stereospecific (but not necessarily enantioselective), generate only inoffensive by-products that can be easily removed, and require only simple reaction conditions as well as readily available reactants and simple product isolation procedures.3 In the following year, two research groups led by Sharpless and Meldal, respectively, independently reported that Cu(I) species can catalyze the Huisgen 1,3-dipolar cycloaddition of alkynes and azides, producing 1,4-disubstituted 1,2,3-triazole derivatives in high yields. This new reaction perfectly fulfills the above criteria for click chemistry and is regarded as an archetypal click reaction.4,5 This Cu(I)-catalyzed azide–alkyne cycloaddition (CuAAC) enjoys remarkable characteristics, such as high efficiency, atom economy, and regioselectivity, great functionality tolerance, mild reaction conditions and simple product isolation, as well as commercially available reactants. Thus, it has found widespread applications in a number of fields, from the synthesis of bioconjugates and dendrimers to the modification of preformed polymers and surfaces.6–25 Meanwhile, CuAAC meets the aforementioned requirements well for an organic reaction to be developed into an efficient polymerization reaction. Indeed, with enthusiastic efforts made by polymer chemists, it has been developed into an effective polymerization technique, being referred to as Cu(I)-catalyzed azide–alkyne click polymerization (CuAACP).26–30
Compared with traditional polymerizations, click polymerizations not only enjoy the advantages of click reactions, but also have their own particular features. For instance, polymers with purer structures and high molecular weights can be obtained owing to the orthogonality of the click reaction. Furthermore, thanks to the great functionality tolerance of click polymerization, electron-rich heteroatoms, such as N and S, and polar groups can be easily incorporated into the architectures of the polymers, producing polymers with specific properties, such as unique optoelectronic properties, biocompatibility, photonic properties, and thermostability.27 With so many wonderful characteristics, click polymerizations have been applied in preparing a number of functional polymers with linear and hyperbranched structures, covering areas from biomaterials and optoelectronic materials to supramolecular materials and shape memory polymeric materials.26,28,31–33
Inspired by the great achievements of CuAACP, and with the rapid development of new click reactions of small molecules, polymer scientists have paid increasing attention to exploiting new click polymerizations. Therefore, new click polymerizations are booming and the family of click polymerizations is getting stronger and stronger. Nowadays, click reactions can be classified into four general categories: (1) cycloaddition reactions, commonly the azide–alkyne click reaction37–40 (2) thiol-click reactions, including thiol-ene/yne, thiol-epoxy, and thiol-isocyanate click reactions and the thiol-Michael addition reaction;41,42 (3) amino-click reactions, including the aza-Michael addition reaction43 and amino-epoxy ring-opening reaction;44 and (4) non-aldol-type carbonyl click reactions involving imine, hydrazine and oxime carbonyl-condensations.45 Besides the famous CuAAC, the cream of the crop of click chemistry, most of these click reactions have also bloomed into click polymerizations.46,47 Among these click polymerizations, the azide–alkyne click polymerizations (AACPs), thiol-ene/yne click polymerizations and DA polymerization are most notable.2,26–30,33,38,46,48–53 Other click polymerizations, by contrast, are rarely investigated or are still at an initial stage.54,55
In the past decade, click polymeri...
