CHAPTER 1: Overview of Click Polymerization

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 reaction^21,34–36  and Diels–Alder (DA) click reaction;^37–40  (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 polymerizations have made considerable progress. This overview intends to give a brief summary of click polymerizations and focus on the advances in this area. In particular, the AACPs and thiol-ene/yne click polymerizations, the most common click polymerizations, will be highlighted and introduced in detail. Meanwhile, some new click polymerizations will be mentioned. Furthermore, the prospects of click polymerizations will also be discussed.

CuAAC, the flagship of click chemistry, has attracted considerable interest owing to its click characteristics since it was independently reported by Sharpless and Meldal in 2002.^4,5  Besides the post-functionalization of preformed polymers, polymer chemists have also tried to use CuAAC to prepare polymers directly. The first attempt was reported in 2004 by Voit and co-workers who tried to obtain hyperbranched polytriazoles (hb-PTAs) via polymerization of AB₂-type monomer 1 in the presence of the catalytic system of CuSO₄/sodium ascorbate (SA) in a mixture of dimethylformamide (DMF) and water (2 : 1, v/v) (Scheme 1.1).⁵⁶  Unfortunately, they only got a rubbery substance that was insoluble in organic solvents. Although unprosperous, this work still represented the beginning of click polymerization. However, the research in the area of click polymerization was stagnant for several years, probably because of the insolubility of the products. In consideration of the difficulties in the synthesis and stockpiling of AB₂-type monomers, Katritzky and co-workers took an A₂ + B₃ strategy to prepare hyperbranched polytriazoles (PTAs) in an aqueous medium.⁵⁷  Disappointingly, an insoluble product was obtained again, which meant that this strategy failed too.

As the saying goes, every cloud has a silver lining. With the problem of solubility being overcome in 2008, the development of click polymerization enjoyed a turnaround.²⁶  Being a research group focused on the exploration of new alkyne polymerizations,^58,59  our group is committed to converting alkynes into a kind of useful building block for the construction of linear and hyperbranched polymers with functional properties. Naturally, we set about endeavoring to develop CuAAC into an efficient polymerization technique and tried to prepare hyperbranched polytriazoles (hb-PTAs) by taking an A₂ + B₃ monomeric strategy.⁶⁰  Firstly, we carried out the polymerization under the typical Sharpless conditions using CuSO₄/SA as the catalyst system in a mixture of tetrahydrofuran (THF) and water with a volume ratio of 5 : 1. However, the obtained products were only soluble in highly polar solvents such as dimethyl sulfoxide (DMSO) and DMF, which indicated that the typical conditions for CuAAC were unsuitable for the preparation of processable hb-PTAs. Considering that the poor solubility of the product was presumably caused by the incompatibility of the growing species with the aqueous reaction medium, we chose bromotris(triphenylphosphine) copper(i) (CuBr(PPh₃)₃), an organosoluble catalyst, for the polymerization. As expected, 1,4-regioregular hb-PTAs with good solubility in common organic solvents were obtained in DMF at 60 °C (Scheme 1.2).

Besides the strategy of using organosoluble CuBr(PPh₃)₃ as a catalyst, Shi et al.^61,62  obtained hb-PTAs with nonlinear optical (NLO) activity and good solubility in common solvents such as cyclopentanone, cyclohexanone, DMSO and DMF via CuSO₄/SA-catalyzed AACP by minimizing the amount of water in the mixture of DMF and water. Right after this, Li et al.⁶³  also reported the preparation of NLO PTAs with good processability by polymerizing AB₂-type monomers in the presence of CuSO₄/SA in a DMF/H₂O mixture with a small amount of water (20 : 1 v/v), which confirmed that water in the reaction system will affect the solubility of the resultant products (Scheme 1.3).

With the problem of insolubility being solved, lots of hyperbranched PTAs with unique properties were prepared by CuAACPs.^64–85  Meanwhile, due to the lack of cross-linking, CuAACPs of diazides and diynes (A₂ + B₂ strategy) or azidoacetylenes (AB-type monomer) have also been widely investigated and used to synthesize linear PTAs with novel structures and versatile properties, since the first example was reported by Matyjaszewski et al. in 2005 (Schemes 1.4 and 1.5).^86–122 

As for the catalyst system, with the development of CuAACP in recent years, various Cu(i) species have been applied to catalyze click polymerizations. Besides the above-mentioned CuSO₄/SA catalyst system and the organosoluble CuBr(PPh₃)₃, Cu(i) compounds and complexes, such as CuBr, CuI, CuCl, CuOAc, and copper(i) iodide triethylphosphite (CuIP(OEt)₃) have also been used as catalysts for click polymerizations.^123–154  Meanwhile, microwave assistance and ultrasonic irradiation have been reported to be able to accelerate the polymerization rates.^155–157  Furthermore, by taking advantage of the photochemical reduction of Cu(ii) to Cu(i), photoinitiated CuAACP has been nurtured into an efficient polymerization technique for the preparation of PTAs. As shown in Scheme 1.6, there are two approaches to generating Cu(i) in situ by photoreduction of a Cu(ii) source, namely direct photolysis of Cu(ii) and indirect reduction of Cu(ii) using a photoinitiator. The latter is more effective and commonly used owing to the higher efficiency of charge transfer between photochemically produced electron donor radicals and Cu(ii). Thanks to the photoinitiated character, temporal and spatial control of click polymerization could be realized by control of the irradiation time and location, indicating that photoinitiated CuAACP has the potential to be developed into a controlled click polymerization. Up to now, with this special technique in hand, polymer scientists have prepared a lot of novel functional polymer materials, such as patterned devices,¹⁵⁸  shape memory materials,¹⁵⁹  dental resins¹⁶⁰  and others.^161–166 

Although great success in the construction of functional PTAs with diverse structures has been achieved via the mentioned CuAACPs, these polymerizations still have some weaknesses, such as the lack of recyclability of the catalysts and copper residues in the resultant polymers. It is difficult to completely remove the copper residues in the polymeric products because of the strong coordination between the copper species and the formed triazole rings. However, the copper residues are harmful to the electronic and optical properties of the resultant polymers, restricting their applications in the optoelectronics field. Moreover, PTAs with cytotoxic copper residues lack biocompatibility, which limits their applications in the biological field.^167–169  A supported Cu(i) catalyst for CuAACP is an alternative to reduce the copper residues in the resultant polymers because of the strong coordination between the Cu(i) and the supporting substrate. In addition, a supported Cu(i) catalyst as a heterogeneous catalyst is recyclable and reusable, as well as easily separated from the reaction system by simple filtration or centrifugation processes.¹⁷⁰  Our group is committed to reducing the copper residues in the generated PTAs. A strategy we reported recently is using recyclable and reusable supported Cu(i) catalysts during CuAACP.^171,172  One of the catalysts we used is CuI@A-21, which is synthesized by immobilizing CuI onto dimethylamino-grafted cross-linked polystyrene (Amberlyst® A-21 resin). CuI@A-21, as a heterogeneous catalyst, enjoys the advantage of recyclability and can be reused for 4 cycles with negligible loss of its activity. Moreover, as we expected, the products of CuI@A-21-catalyzed AACPs have much lower copper residue content when compared with those prepared by homogeneous Cu(i)-catalyzed AACPs. These results indicate that supported Cu(i)-catalyzed AACP is indeed an alternative for the reduction of the copper residues in the resultant polymers.

CuAACPs have been widely applied to prepare linear and hyperbranched PTAs with various functions; in the meantime, remarkable progress has also been made in the methodology of click polymerization. As we known, CuAACPs generally follow a step-growth mechanism. However, Gao and co-workers recently reported a novel chain-growth CuAACP that could be used to synthesize hb-PTAs with high molecular weights, narrow polydispersity (Đ) and a high degree of branching (DB) in a one-pot process.^173–178  As can be seen in Scheme 1.7, this CuAACP can proceed according to a chain-growth mechanism, of which the key points are the minimization of the amount of Cu(i) species and the ingenious design of the monomers. These studies reported by Gao et al. shine a light on the development of controlled click polymerization.

A tiny difference in the structure of a compound might totally change its properties. Accordingly, polymers with different regio-structures may show different properties. As aforementioned, CuAACPs can solely produce 1,4-regioregular PTAs. It is therefore necessary to develop a new click polymerization to prepare 1,5-regioregular PTAs to gain an understanding of the structure–property relationship. Inspired by Jia and co-workers’ work,¹⁷⁹  which showed that ruthenium-catalyzed azide–alkyne click reaction (RuAAC) could afford exclusively 1,5-disubstituted 1,2,3-triazole derivatives, our group developed a RuAACP and 1,5-regioregular hb-PTAs with high molecular weights were obtained.⁶⁰  Interestingly, not only Cp*Ru(PPh₃)₂Cl (Cp* = 1,2,3,4,5-pentamethylcyclopentadiene), but also its precursor [Cp*RuCl₂]_n, which can be easily prepared by refluxing an ethanol solution of Cp*H and RuCl₃·nH₂O for several hours, can be used as an effective catalyst for AACPs to produce 1,5-regioregular hb-PTAs. The synthetic accessibility of [Cp*RuCl₂]_n affords potential for the widespread application of RuAACP. Meanwhile, there are indeed some differences between 1,4- and 1,5-regioregular hb-PTAs. For instance, 1,4-regioregular hb-PTAs show a redder emission when compared with the relevant 1,5-regioregular hb-PTAs. Moreover, RuAACP has also been applied by Tyler et al. to produce linear PTAs containing Mo–Mo bonds in their backbones, which could not be prepared via CuAACP due to the disproportionation of the Mo–Mo bonds (Scheme 1.8).¹⁸⁰ 

A large number of functional PTAs with diverse structures have been prepared by transition metal-catalyzed AACPs. However, as mentioned above, the metal residues, which are difficult to completely remove from the resultant PTAs, will greatly limit the applications of these PTAs. For example, copper residues are cytotoxic and they can weaken and even quench the light-emission of the products, limiting their applications in the optoelectronic and biological fields.^167–169  Although the amount of metal residue in the products can be partially reduced by using supported transition metal catalysts, new AACPs are still expected to completely solve the problem of metal residues.^199,200  An alternative to completely circumvent the problem is metal-free click polymerizations of azides and alkynes (MFCPs).

Our previous research indicated that aroylacetylenes show high activity.^181,182  Thus, a new MFCP of bis(aroylacetylene)s and diazides was successfully developed.¹⁸³  PTAs with high F_1,4 (the fraction of 1,4-isomers, up to 92%) and high weight-average molecule weights (M_w) were obtained in high yields (Scheme 1.9). It is worth noting that this polymerization is insensitive to oxygen and moisture, which benefits its widespread application. Afterwards, we prepared a series of PTAs with various structures and functional properties, such as aggregation-induced emission (AIE) features, photonic patterning, adjustable light refractivity, optical nonlinearity and self-healing.^183–187 

In consideration of the difficult, trivial and environmentally unfriendly preparation procedures of aroylacetylenes, it is necessary to exploit activated alkyne monomers with facile preparation procedures for MFCP in order to extend its applications in diverse areas.^188,209,210  Propiolate derivatives, which have ethynyl groups directly connected with ester groups, have similar structures to aroylacetylenes. However, the preparation procedures of propiolate derivatives are very facile and they can be easily obtained by simple esterification reactions of commercially available propiolic acid and diols or triols. The MFCP of propiolate and azide monomers was systematically investigated and it was found that the performance of this MFCP was very similar to that of aroylacetylenes and azides.¹⁸⁹  PTAs with high molecular weights (M_w up to 35 700) and regioregularities (F_1,4 up to 94.3%) could be produced in high yields. By taking advantage of its great functional tolerance, this MFCP has been applied to prepare many functional PTAs, which can be used in the areas of sensitive detection of explosives, ready generation of soft magnetic ceramics and facile preparation of breath figures with high resolution.^189–195 

Moreover, MFCP based on activated azides (perfluorophenyl azides) was also investigated and established.¹⁹⁶  The MFCP/metal-free polycycloaddition of perfluorophenyl azides and alkynes efficiently (yields up to 93%) produced soluble 1,4-regioregular (F_1,4 up to 87.2%) PTAs with high molecular weights (M_w up to 166000).^196–198  In addition, the regioregularity of the products could be fine-tuned by the diyne monomers and reaction solvents. With this versatile polymerization technique in hand, a series of functional PTAs with versatile properties, such as aggregation-enhanced emission (AEE) characteristics, are generated.

Although great success has been achieved in the preparation of functional PTAs with linear and hyperbranched structures by MFCPs of activated monomers, these MFCPs still have certain limitations. For instance, activated monomers are necessary for these polymerizations, the regioregularity is not unity, and 1,5-regioregular PTAs have not been produced. Inspired by the organobase-mediated click reaction of aromatic azides and terminal alkynes, which exclusively produces 1,5-disubstitued 1,2,3-triazoles,¹⁹⁹  our research group established a new AACP mediated by the organic base NMe₄OH in DMSO at room temperature, and 1,5-regioregular PTAs (with the fraction of 1,5-isomer being up to 100%) with high molecular weights (M_w as high as 56 000) were produced in excellent yields (up to 96%) (Scheme 1.10).²⁰⁰  It is noteworthy that this organobase-mediated AACP is an alternative for the synthesis of 1,5-regioregular PTAs without the metal residue problem in the products.

As a common thiol-based click reaction, the thiol-ene click reaction has found widespread applications, such as in the construction of dendrimers^201,202  and polymer networks,^203–205  preparation of hydrogels and nanoparticles,^206–215  and post-functionalization of polymers.^216–219  In the past few years, polymerization based on the thiol-ene click reaction, i.e., thiol-ene click polymerization, has also been applied for the preparation of soluble linear and hyperbranched polymers with diverse functional properties. For instance, Feng and co-workers²²⁰  prepared silicon-containing hyperbranched polymers by a convenient and efficient photoinitiated thiol-ene click polymerization of AB_n-type (n = 2 or 3) monomers instead of platinum-catalyzed hydrosilylation, which has limited applications due to its harsh reaction conditions (Scheme 1.11). The AB_n-type monomer was polymerized in the presence of the photoinitiator DMPA (2,2-dimethoxy-2-phenyacetopheone) under a UV lamp in THF for 20 min, producing a soluble hyperbranched organosilicon polymer with number-average molecular weight (M_n) of 5800 and a Đ of 1.78. Meanwhile, the produced polymer with abundant allyl groups around the periphery could be further post-functionalized.

By taking advantage of its click characteristics, photoinitiated thiol-ene click polymerization has also been used to prepare linear polythioethers. In 2013, Du Prez et al.²²¹  prepared linear polythioethers, which could be transformed into polysulfones with improved thermal stability through oxidization by hydrogen peroxide, by photo- or thermal-initiated thiol-ene click polymerizations of AB-type monomers (Scheme 1.12). The generated polythioethers have M_n values as high as 40 200, however, the Đ values of the products were very high (up to 30). Using a similar AB strategy, Cádiz et al.²²²  synthesized linear polyurethane by photoinitiated thiol-ene click polymerization of a thiol-ene carbamate monomer, avoiding the use of toxic isocyanates. The obtained polymer had a high M_n of 14 300 and a low Đ of 1.78.

In consideration of the self-oligomerization of the AB-type monomers during storage, an A₂ + B₂ strategy has also been applied to prepare linear polymers (Scheme 1.13). Recently, Tew et al. prepared a series of redox-active polymers with various and dense functional groups, such as carbonate and zwitterionic phosphocholine in the backbone and pendent hydroxyl groups, by photoinitiated thiol-ene click polymerization.²²³  These polythioethers could be selectively oxidized to sulfoxides or sulfones by treatment with hydrogen peroxide or mCPBA (m-chloroperoxybenzoic acid), respectively. Meanwhile, they also prepared a series of lithium polymer electrolytes by the photoinitiated thiol-ene click polymerizations of dienes and dithiols²²⁴  or thiol-norbornene click polymerizations.²²⁵  Furthermore, photoinitiated thiol-ene click polymerization can be combined with thiol-Michael addition to produce sequence-ordered polymers in one pot.²²⁶ 

As a complement of thiol-ene click polymerization, thiol-yne click polymerization not only enjoys the advantages of the former, but also has its own remarkable features. For example, polymers with high cross-link densities and sulfur content can be obtained because one ethynyl group can react with two thiols.²²⁷  Meanwhile, as shown in Scheme 1.14, thanks to the diversity of the mono-additive products of click reactions between thiols and alkynes, polymers with new structures can be generated by changing the catalyst system.⁴⁸  Furthermore, it is relatively easy to prepare alkynes with diverse structures.²²⁸  With so many remarkable advantages, thiol-yne click polymerization has drawn intense attention from polymer chemists and has found extensive applications in the area of polymeric materials. According to the mechanism, thiol-yne click polymerization can be divided into three categories: free-radical (including photo-/thermo-initiated and spontaneous), amine-mediated and transition metal-catalyzed processes.

In 2009, Perrier et al. reported the preparation of hyperbranched polymers by photoinitiated click polymerizations of AB₂-type monomers bearing a thiol and an alkyne in which each π-bond is a B unit, with DMPA as the photoinitiator (Scheme 1.15).²²⁹  Hyperbranched polymers with very high DBs (close to 1) were obtained. Meanwhile, owing to the 2 : 1 ratio of π bonds to thiols, there were many ethynyl groups in the structure of the hyperbranched polymers, which enabled them to be further functionalized by many click reactions.

After Perrier's work, photoinitiated thiol-yne click polymerization was systematically investigated and extensively applied in the preparation of functional hyperbranched polymers with such properties as luminescence,^230,231  self-assembly,²³²  biodegradability,²³³  temperature-responsivity,²³⁴  liquid crystallinity and drug release,²³⁴  and so on. Moreover, Perrier and co-workers also investigated the photoinitiated thiol-yne click polymerizations of AB₂-type macromonomers.²³⁵  It was found that the size and the initial concentration of the macromonomers played an important role in the polymerization process.

Owing to the step-growth nature of the thiol-yne click polymerization, it is difficult to control the molecular weights and Đ values of the produced polymers. Recently, hyperbranched polymers with high DBs and low Đ values were prepared by Perrier and co-workers via photoinitiated thiol-yne click polymerization.²³⁶  These results were realized by slowly adding AB₂-type monomers into multifunctional core molecules under the irradiation of UV light in the presence of a photoinitiator. Thus, this work provided a useful approach to prepare hyperbranched polymers with high DBs and well-controlled molecular weights and low Đ values.

The AB₂-type monomers are noncommercial and have a tedious preparation procedure. Gao and co-worker reported a novel and universal couple-monomer methodology (CMM), which combined the thiol-halogen click reaction and thiol-yne click polymerization for the preparation of hyperbranched polymers without the risk of gel formation.²³⁷  As shown in Scheme 1.16, thiol-halogen click reactions between commercially available A₂-type monomers and CB₂-type monomers were carried out in the presence of a KOH catalyst, producing AB₂-type monomers. Sequentially, thiol-yne click polymerizations of AB₂-type monomers were performed in the presence of the photoinitiator DMPA under UV irradiation, yielding hyperbranched polymers with high molecular weights and DBs, which could be controlled by altering the polymerization time and conversion. Hyperbranched polymers with relatively low DBs were obtained by the relevant thermo-initiated thiol-yne click polymerizations. Similarly, Gao and co-workers also reported the synthesis of hyperbranched polymers by sequential thiol-ene click reaction and thermo-initiated thiol-yne click polymerization via a CMM protocol.^238,239 

Thanks to the two-step addition of thiols to ethynyl groups, cross-linked polymers and hyperbranched polymers can easily be prepared via radical-initiated thiol-yne click polymerizations. By controlling the structures of the monomers, linear polymers can be obtained too (Scheme 1.17). In 2012, Meier et al. synthesized functional linear polymers with diverse side groups via the photoinitiated thiol-yne click polymerizations of monoynes and dithiols.²⁴⁰  The structures of the alkyne monomers exert a significant effect on the polymerization. Using a similar strategy, Gao et al. also prepared sequence-controlled functional linear polymers by radical-initiated thiol-yne click polymerizations.²⁴¹ 

The aforementioned radical-initiated thiol-yne click polymerizations, which are bis-addition processes, can only afford polythioethers. By controlling the ratio of ethynyl groups and thiol groups, Voit and co-workers reported a novel click polymerization, which is a variation of traditional radical-initiated thiol-yne click polymerization (Scheme 1.18).^242,243  Only when the amount of thiol groups is much higher than that of ethynyl groups can bis-addition of thiols to alkynes be observed. Meanwhile, owing to the influence of steric hindrance and electronic properties, thiols show different reactivity (alkylthiol > thiophenol ≫ thioacetic acid) when reacting with diphenylacetylene. Thanks to the high content of aromatic rings and sulfur, the PVSs prepared by this polymerization showed high reflective indices. It was noted that compared with the linear products, the hyperbranched polymers showed better optical properties. Using this novel click polymerization, Voit and co-worker synthesized a series of hyperbranched PVSs with such remarkable features as excellent solubility and processability, high thermal stability, high transparency, very high reflective indices and low optical dispersions, and applied them in the fabrication of organic light-emitting diodes and planar one-dimensional all-polymer photonic crystals.^244–247 

As aforementioned, propiolate derivatives show higher reactivity than common alkynes and can be reacted with azides without the assistance of metal catalysts. In 2010, a powerful amine-mediated thiol-yne click polymerization based on propiolate derivatives was also established by our group.²⁴⁸  As shown in Scheme 1.19, the polymerizations of propiolate derivatives and thiols were carried out in DMF at room temperature in the presence of diphenylamine, efficiently generating exclusively anti-Markovnikov products of PVSs with M_w as high as 32 300 and high stereoregularities (Z content up to 81.4%) in excellent yields (up to 98.2%) after 24 h. It is noteworthy that moisture and oxygen had a negligible influence on the polymerization. The produced PVSs showed good solubility in common organic solvents and high transparency for visible light and IR light. Benefitting from the high content of sulfur in the polymers, they showed high refractive indices, which could be further improved by metal complexation. Moreover, these PVSs could be cross-linked upon UV irradiation and could be applied in fabricating fluorescent photopatterns.

Thanks to the endeavors of chemists in the past half century, it has been found that thiol-yne reactions can afford regio- and stereo-isomers with branched and linear structures via Markovnikov and anti-Markovnikov addition pathways (Scheme 1.14).^249–251  Besides the aforementioned radical- and amine-mediated thiol-yne click reactions, transition metal-catalyzed thiol-yne reactions have also been extensively investigated and been found to produce vinyl sulfides in a regio- and stereo-selective fashion. Various transition metal catalysts, such as Rh, Ir, Ni, Pd, Pt, Au and Zr complexes, have been proven to be effective catalysts for this kind of thiol-yne click reaction.^252–254  Among these catalysts, the most impressive ones are Rh complexes owing to their high efficiency and excellent ability to catalyze the reaction to proceed in a regio- and stereo-selective manner. It is worth mentioning that the transition metal-catalyzed thiol-yne reactions generally occur though a Migratory-Insertion mechanism.²⁵²  Inspired by these previous works, our group chose Rh complexes as catalysts and successfully established an Rh-catalyzed thiol-yne click polymerization.²⁵⁵ 

As shown in Scheme 1.20, diynes and dithiols were polymerized in the presence of Rh(PPh₃)₃Cl under mild conditions, producing solely anti-Markovnikov additive products of PVSs with high molecular weights (M_w up to 31 500) and high stereoregularities (E content as high as 100%) in high yields (up to 95.2%). In particular, the control of the product stereostructures was achieved by adjustment of the addition sequence of the monomers during polymerization and post-processing via UV irradiation. The produced PVSs showed good solubility, high thermo-stability and high optical transparency. Thanks to the great functionality tolerance of this polymerization, many functional groups, such as ferrocene and silole, could be incorporated into the polymer structures endowing them with such unique properties as ceramization capability and AEE features. Meanwhile, these PVSs could be used to fabricate fluorescent photopatterns owing to their thermal and UV curability. Furthermore, these polymers with a high content of polarizable aromatic rings, ester groups and sulfur atoms, as well as metal elements, showed very high refractive indices with low optical dispersions that could be fine-tuned via UV irradiation.

More excitingly, our group established a spontaneous thiol-yne click polymerization without the assistance of UV, heat, amines or transition metals, which was different from the aforementioned thiol-yne click polymerization and dramatically simplified the preparation procedure of PVSs.²⁵⁶  Simply mixing and stirring aromatic diynes and aromatic dithiols with equivalent molar ratios in THF without additional catalysts and external stimuli at 30 °C for as short as 2 h readily furnished exclusively the anti-Markovnikov additive products of soluble PVSs with M_w as high as 85 200 in high yields (up to 97%) (Scheme 1.21). The simple process indicated that this polymerization proceeded in a spontaneous manner and further investigation showed that it occurred via a radical mechanism. These PVSs also have high refractive indices due to their high content of polarizable aromatic rings and sulfur atoms. Owing to the good functionality tolerance, AIE characteristics could be endowed to the resultant PVSs by incorporating TPE units into the polymer structures. With this powerful and facile tool in hand, we also prepared multi-functional hyperbranched PVSs.²⁵⁷ 

As one of the most attractive members of click chemistry, DA reactions, which include straightforward [4 + 2] cycloaddition reactions between electron-rich dienes (such as furan, 1,3-cyclopentadiene and their derivatives) and electron-poor dienophiles (such as vinyl ketone, maleic acid and its derivatives) to form cyclohexene adducts, have attracted more and more attention and have found widespread applications in the area of polymer chemistry.^{38,47,53,258–261}  Therefore, the furan/maleimide DA reaction of which the cyclohexane adduct can be decoupled under a relatively low temperature via a retro-DA reaction is the most appealing one due to its thermal reversibility and the renewability of the monomer precursors.⁵³  This kind of DA reaction has been widely applied in the construction of dendrimers^262–264  and polymer networks^265–273  as well as the modification of surfaces.²⁷⁴  Meanwhile, thanks to its reversibility, the furan/maleimide DA reaction has shown interesting applications in the fields of recyclable networks and self-healing materials.^273,275  Since the first investigation of polymerization based on the furan/maleimide DA reaction was reported by Tesoro et al. in 1986,²⁷⁶  this polymerization has attracted increasing attention and has been applied to prepare functional polymers.

There are two entirely different research directions in the field of the preparation of linear polymers by furan/maleimide DA click polymerization (Scheme 1.22).⁵³  One is the preparation of highly thermo-stable polymers by furan/maleimide DA click polymerization combined with subsequent aromatization of the produced polymers.^276–281  The thermal stability of the polymers was enhanced drastically by irreversible chemical modification. The decomposition temperatures of the aromatized polymers reached up to 502 °C, depending on the DA monomers used. However, because the high temperatures needed for aromatization would trigger retro-DA depolymerization and radical polymerization of maleimide, this area remained silent after those studies in the 1990s, which were doubted by some scientists.⁵³ 

Another more attractive direction is taking advantage of the thermoreversibility to prepare functional polymers with such remarkable features as self-healing, recyclability and temperature responsiveness.^47,53  The groundbreaking work in this direction reported by Kuramoto et al.²⁸²  investigated the preparation of a thermoreversible polymer via thermoreversible DA polymerization between difurfuryl adipate and N,N-bismaleimidodiphenylmethane and the polymerization/depolymerization cycles. Afterwards, diverse strategies including the AB strategy and A₂ + B₂ strategy were applied to prepare linear polymers.^283–288  Because some monomers can be obtained from renewable sources, DA click polymerization can be applied in the area of green chemistry.

Since the pioneering work of Stille and co-workers,²⁸⁹  many hyperbranched polymers have been synthesized by DA polymerization.²⁹⁰  There are two types of DA polymerization based on the furan/maleimide DA reaction²⁹¹  or cyclopentadienone/alkyne DA reaction,^292–294  including the AB₂ and A₂ + B₃ approaches (Scheme 1.23). The former can offer hyperbranched polymers with thermoreversible features and the latter can produce soluble and high thermal stability hyperbranched polyphenylenes, which have been applied to fabricate nanotubes and show potential in the areas of advanced coatings and microelectronics.

Besides the common click polymerizations mentioned above, some other click polymerizations have also been preliminarily investigated and applied to synthesize polymers. For instance, polymerizations based on click reactions of nitrile oxides and alkynes have been applied to produce 3,5-regioregular linear poly(isoxazole)s (Scheme 1.24a).^54,55  Other thiol click reactions such as thiol-halogen and thiol-epoxy click reactions have also been used to construct polymers. Lowe et al. reported the preparations of functional multiblock and hyperbranched polymers by polymerizations based on thiol-bromo click reactions (thiol generated in situ) (Scheme 1.24b).²⁹⁵  Thiol-epoxy click reactions, which have been widely applied to prepare hyperbranched polymers, networks and hydrogels, have extensive applications in the fields of shape-memory materials, self-healing materials, optical materials and biomaterials (Scheme 1.24c).^296–301  Michael additions, including thiol-Michael addition and aza-Michael addition, have been used to synthesize linear, hyperbranched, and cross-linked polymers with various functions, such as drug carriers, cell imaging and silicone rubbers (Scheme 1.24d and e).^43,302–304  Self-assembly hyperbranched polymers, multiple stimuli-responsive hyperbranched polymers and multifunctional polymer particles can be prepared via polymerization based on epoxy-amine click reaction (Scheme 1.24f).^44,305,306  Thanks to their high efficiency, dynamic covalent properties, and environmentally friendly and bioorthogonal nature, non-aldol carbonyl click reactions including imine, hydrazine and oxime carbonyl-condensations have drawn increasing attention in the area of polymer materials and have been applied for the preparation of degradable polymers, hydrogels, and self-healing materials (Scheme 1.24g–i).^45,307–313 

In 2016, an efficient Cu(i)-catalyzed amino-yne click polymerization, which proceeds in a regiospecific and stereoselective manner, was established for the preparation of polyenamines with diverse properties, such as excellent solubility, high thermal stability and strong UV-light shielding efficiency (Scheme 1.24j).³¹⁴  Although this first reported amino-yne click polymerization can exclusively afford Markovnikov additive nitrogen-containing polymers in very high yields, it is still imperfect owing to the fact that the Z-isomeric content of the products cannot reach 100%. Very recently, in order to obtain polyenamines with straightforward structures, a fire-new amino-yne click polymerization with regio- and stereo-specificity, as an upgrade of the former, has been established through such endeavors as design of the monomer structures and optimization of the polymerization processes (Scheme 1.24k).³¹⁵  Most importantly, this polymerization with 100% atom efficiency can be carried out at room temperature by simply stirring the monomers in solution, implying that this is a spontaneous process. Thanks to its regio- and stereo-specificity, solely anti-Markovnikov additive poly(β-aminoacrylate)s with 100% E-isomer were generated in excellent yields. Meanwhile, by taking advantage of the great functionality tolerance, tetraphenylethene moieties were incorporated into the backbones of the products to yield AIE-active polymers, which could be used as explosive detectors and biosensors, giving beneficial inspiration for the design of functional polymers. This fire-new click polymerization exploits an avenue for the synthesis of functional polymeric materials.

In this overview, we give a brief summary of click polymerizations, including their types and applications. As the most popular click polymerizations, AACPs have attracted most of the attention. AACPs catalyzed by Cu(i) and Ru(ii) can offer 1,4-regioregular and 1,5-regioregular PTAs, respectively. The former can also be obtained via MFCPs using activated alkynes and azides or activated azides and alkynes as monomers. Meanwhile, the latter can be produced by organobase-mediated AACP. Thiol-ene click polymerization has been applied in the preparation of polythioethers with linear and hyperbranched structures. Similarly, thiol-yne click polymerization, as an updated version of the former, can provide polythioethers and PVSs with diverse structures. DA click polymerization can furnish reversible linear and hyperbranched polymers. There are many other click polymerizations that have been studied for the synthesis of various functional polymers. Multifarious monomers, especially commercially available monomers, have been adopted as monomers for these click polymerizations, which are tremendously conducive to the widespread application of click polymerizations. Using these efficient and useful polymerization techniques, polymer chemists have prepared a large number of functional polymers with various structures and unique properties, such as luminescence, photonic patterning, adjustable light refractivity, optical nonlinearity, biodegradability, self-assembly and self-healing.

Although remarkable progress has been achieved in the field of click polymerizations, the field is still full of challenges, which also signifies plentiful opportunities, and unremitting efforts are necessary to further develop click polymerizations and expand their scope. Primarily, the exploration of new-style click polymerizations, which can radically enlarge the territory of click polymerization, is always encouraged and welcome. For instance, the recently reported amino-yne click polymerization with commercially accessible amines as monomers can effectively yield nitrogen-containing polymers. Meanwhile, new click polymerizations generally stem from click reactions of organic small molecules. New click reactions such as the azide-acetonitrile reaction have the potential to be nurtured into new click polymerizations for the preparation of polymers with new architectures, such as 5-amino-1,4-disubstituted PTAs.

As for the existing click polymerizations, further endeavors should focus on four major directions: monomer design, new catalyst exploration, controlled click polymerization development and new functions of the produced polymers. The preparation of new polymers with tailor-made architectures and properties can be achieved by the design of new monomers. The exploitation of new green accessible catalysts with high efficiency and selectivity is beneficial for the widespread application of click polymerizations. The development of controlled click polymerizations is of importance for the preparation of polymers with controlled molecular weights, polydispersities and well-defined structures. Through the deft design of monomers with consideration of the interaction between the catalysts and formed triazoles, Gao et al. established a chain-growth CuAACP which shines a light on the area of controlled click polymerizations.¹⁷³  As mentioned above, click polymerization with great functionality tolerance can be applied to produce linear and hyperbranched polymers with diverse functionalities. It is meaningful to exploit new applications of the polymer products, such as their use as dynamic materials. Thanks to the enthusiastic efforts of polymer scientists, click polymerizations have great potential to be developed into more powerful and useful polymerization techniques for the preparation of polymers with explicit structures and various functional properties.

This work was financially supported by the National Natural Science Foundation of China (21525417 and 21490571), the key project of the Ministry of Science and Technology of China (2013CB834702), The National Program for Support of Top-Notch Young Professionals, the Fundamental Research Funds for the Central Universities (2015ZY013), and the Innovation and Technology Commission (ITC-CNERC14S01). A. J. Q. and B. Z. T. acknowledge the support from the Guangdong Innovative Research Team Program (201101C0105067115).