Metal-Free Catalysts for the Polymerization of Alkynyl-Based Monomers

: Novel polymerizations based on alkyne monomers are becoming a powerful tool to construct polymers with unique structures and advanced functions in the areas of polymer and material sciences, and scientists have been attracted to develop a variety of novel polymerizations in recent decades. Therein, catalytic systems play an indispensable role in the inﬂuence of polymerization efﬁciencies and the performances of the resultant polymers. Concerning the shortcomings of metallic catalysts, much of the recent research focus has been on metal-free polymerization systems. In this paper, metal-free catalysts are classiﬁed and the corresponding polymerizations are reviewed, including organobase-catalyzed polymerizations, Lewis-acid-catalyzed polymerizations, as well as catalyst-free polymerizations. Moreover, the challenges and perspectives in this area are also brieﬂy discussed. Sulfydryl-yne polymerizations A ) with the catalysts of organobases.


Introduction
The establishment of new polymerization techniques to easily synthesize functional polymeric materials is of vital importance to polymer and material sciences. Alkynes have been investigated as excellent monomers in recent decades. Because of their conjugated main chains, new polymers with intrinsic optoelectronic properties could be obtained [1]. With the continuous efforts of polymer scientists, many attractive alkynyl-based polymers, such as hydroxyl-yne, sulfydryl-yne, azide-alkyne, amino-yne click polymerizations, and various multicomponent or multi-group polymerizations have been established, and functional polymers have been prepared for exciting applications [2].
Catalysts can reduce the activation energy barrier and increase the reaction rate by providing an alternate pathway for a reaction to occur [3]. Metallic compounds or complexes have been widely used to catalyze alkyne-based polymerizations [4]. However, it is tedious to remove them completely from polymer products. The metallic catalysts used in the copper-mediated click polymerizations will form complexes, which are difficult to remove from the polymeric products. In a sense, the metal complexation is a crosslinking reaction, which often leads to a polymer product with a poor solubility [5]. Moreover, catalyst residues can deteriorate the photo-physical properties of the polymers; for example, their light emissions can be quenched by metallic traps. Thus, there is an urgent need to develop metal-free polymerization systems.
Compared with metallic catalysts, organobase-cataysts and Lewis-acid-catalysts have remarkable virtues, such as a good solubility in solvents, easy removal from polymers, and extremely low price. Furthermore, catalyst-free conditions might be a wiser strategy to thoroughly simplify the procedure of polymerization. Therefore, this article captures the remarkable progress in the development of the metal-free polymerization of strategy to thoroughly simplify the procedure of polymerization. Therefore, this article captures the remarkable progress in the development of the metal-free polymerization of alkynyl-based monomers. In this review, metal-free catalysts are classified and their corresponding polymerizations are reviewed, including organobase-catalyzed polymerizations, Lewis-acid-catalyzed polymerizations, as well as catalyst-free polymerizations, each of which has its own features and virtues. Moreover, the perspectives and challenges in this area are also briefly introduced.

Organobase-Catalysts for Hydroxyl-yne Polymerization
Tri-n-butylphosphine (n-Bu3P) is a nucleophilic organobase-catalyst with a simple structure (Scheme 1) and cheap price. In 1995, it was used to catalyze the polyadditions of ester-activated diynes 1 and diols 2 (Scheme 2A), and the amount of n-Bu3P used in the polymerizations reached 20 mol% [6][7][8]. Although the polymerization is less efficient, this is the first attempt at the polymerization of alkynes and alcohols. With the new employment of 4-dimethylaminopyridine (DMAP), the polymerization of keto-activated diynes 3 and diphenols 4 was firstly reported in 2017 [9]. As shown in Scheme 2B, in the presence of DMAP the two monomers are simply mixed at room temperature, and regio-and stereoregular poly(vinylene ether ketone)s P2 can be obtained with an excellent yield after 4 h. This DMAP-promoted polymerization has many prominent advantages-for instance, the avoidance of expensive transition-metal catalysts [10,11], the mild reaction conditions, the high efficiency, and no byproducts. Furthermore, the poly(vinylene ether ketone)s are chemically stable in neutral and basic conditions but decompose rapidly when the --lg c(HCl) value is lower than 3.2 ( Figure 1a). Thus, these poly(vinylene ether ketone)s prepared by DMAP-catalyzed phenol-yne polymerization are promising for application in biomedical and environmental fields. With the new employment of 4-dimethylaminopyridine (DMAP), the polymerization of keto-activated diynes 3 and diphenols 4 was firstly reported in 2017 [9]. As shown in Scheme 2B, in the presence of DMAP the two monomers are simply mixed at room temperature, and regio-and stereoregular poly(vinylene ether ketone)s P2 can be obtained with an excellent yield after 4 h. This DMAP-promoted polymerization has many prominent advantages-for instance, the avoidance of expensive transition-metal catalysts [10,11], the mild reaction conditions, the high efficiency, and no byproducts. Furthermore, the poly (vinylene ether ketone)s are chemically stable in neutral and basic conditions but decompose rapidly when the -lg c(HCl) value is lower than 3.2 ( Figure 1a). Thus, these poly(vinylene ether ketone)s prepared by DMAP-catalyzed phenol-yne polymerization are promising for application in biomedical and environmental fields. where I = peak intensity and I 0 = peak intensity at [PA] = 0 µg/mL. Inset: the chemical structure of PA and fluorescence images of P5-1 containing strips (a, b, c, d, and e represent strips after dipping into 0, 5000, 500, 50, and 5 µg/mL aqueous solutions of PA) upon irradiation with a hand-held lamp (365 nm). n-Bu 3 P or DMAP could only catalyze the diol-yne or phenol-yne polymerization, respectively, in the above two studies. Afterwards, researchers found that a more efficient organobase of bicyclo[2.2.2]-1,4-diazaoctane (DABCO) could catalyze both diol-yne and phenol-yne polymerizations in 2020 [12] (Scheme 2C). DABCO is a robust nucleophilic organobase-catalyst, and the polymerizations could be carried out under ambient conditions. After the systematic optimization of polymerization conditions, thermally stable poly(vinyl ether ester)s P3/P4 with high weight-average molecular weights (up to 71,000) were finally produced in excellent yields (up to 99%) with the promotion of DABCO (10 mol%). This work established an efficient DABCO-catalyzed hydroxyl-yne click polymerization; meanwhile, it enriches the family of click polymerizations.
When the alkalinity of organic base is notably enhanced, the monomers of alkynes do not even have to be activated by ester or ketone for the hydroxyl-yne polymerization. Phosphazene bases are organic superbases, and soluble in organic solvents [13,14]. The uncharged organic phosphazene superbase, particularly t-BuP 4 , has some remarkable merits, such as a highly enhanced basicity, good solubility in nonpolar solvents, hyposensitivity to oxygen and moisture, and high thermal stability [15]. Thanks to the super alkalinity of t-BuP 4 , inactivated diynes 5 and diols 2 were successfully polymerized (Scheme 2D). A series of soluble and regio-regular anti-Markovnikov additive poly(vinyl ether)s P5 with high molecular weights (M w up to 40,600) were obtained in high yields (up to 99%) [16]. The tetraphenylethene (TPE)-containing P5-1 showed characteristics of aggregation-enhanced emission (AEE), and its aggregates exhibited a super-amplification quenching effect to picric acid, which could be potentially used in explosive detection ( Figure 1b). Similar to poly(vinylene ether ketone)s, the poly(vinyl ether)s are degradable under acidic conditions, and thus may be used in the field of drug release.
Only two works of metal-catalyzed hydroxyl-yne polymerizations were reported in 2019 [17,18]. The catalysts used in the two works are Pd(PPh 3 ) 4 and Cs 2 CO 3 . Compared with noble metal complexes, organobase catalysts are more available at very low prices. Compared with inorganic salts, organobases can completely dissolve in solvents, forming a homogeneous catalytic system. Therefore, organobase-catalyzed hydroxyl-yne polymerizations have been well developed in recent years.

Organobase-Catalysts for Sulfydryl-yne Polymerization
Organic bases could also catalyze thiol-yne polymerization. In 1995, the Endo group [19][20][21] firstly reported the organocatalyzed polymerization of alkynes and thiols. Simply mixing the ester-activated diynes 6 and dithiols 7 in the presence of n-Bu 3 P at ambient conditions readily produced P6 with a high molecular weight (M n = 20,700) after 1 h (Scheme 3A). This polymerization catalyzed by an organic base has the favorable characteristics of simplicity, efficiency, and conditional mildness, promising a powerful polyaddition reaction.

Organobase-Catalysts for Sulfydryl-yne Polymerization
Organic bases could also catalyze thiol-yne polymerization. In 1995, the Endo group [19][20][21] firstly reported the organocatalyzed polymerization of alkynes and thiols. Simply mixing the ester-activated diynes 6 and dithiols 7 in the presence of n-Bu3P at ambient conditions readily produced P6 with a high molecular weight (Mn = 20,700) after 1 h (Scheme 3A). This polymerization catalyzed by an organic base has the favorable characteristics of simplicity, efficiency, and conditional mildness, promising a powerful polyaddition reaction. When the dithiols were replaced with diphenols, the organocatalyzed sulfydryl-yne polymerizations could also be established. In the presence of diphenylamine (DPA), the alkyne polyhydrothiolations of ester-activated diyne 8 and dithiophenol 9 proceed smoothly at room temperature, furnishing the sole anti-Markovnikov products of poly(vinylenesulfide)s P7 (Scheme 3B) with high molecular weights (Mw up to 32,300) and high stereoregularities (Z content up to 81.4%) in high yields (up to 98.2%) [22]. The poly(vinylene sulfide)s are optically transparent, allowing almost all visible and IR light to transmit through, and their films show high refractive indices (n = 1.73-1.70) and low optical dispersions (Dʹ down to 0.002) in the wavelength region of 500-1700 nm. With the promotion of DPA, a new synthetic route for sulfur-rich polymers has been developed.
Soon afterwards, Dove group realized a polymer-polymer coupling though an organocatalytic click addition of thiol 12 to ester-activated alkyne 10 (Scheme 3C) [23]. Compared with primary amine and secondary amine, the author found that Et3N is the best catalyst. Most of the selected amine catalysts have very similar PKA values, so the basicity of the catalysts is probably not the main thing that affects their reaction activity in these reactions. Although the basicity of tertiary amine diisopropylethylamine (DIPEA, pKa = 11.4) is higher than that of Et3N (pKa = 10.7), it does not show any catalysis, which indicates that steric hindrance is the main factor of catalyst activity in the polymer-polymer coupling reaction.
Based on the above research, the same group reported another organocatalyzed polymerization of ester-activated alkynes and thiols in 2016 [24]. In this work, ester-activated alkyne 12 was combined with hexane dithiol 13 in chloroform (CHCl3) in the presence of 1 mol% of 1,8-diazabicycloundecane (DBU) to promote the polymerization (Scheme 3D). After only 1 h, P9 with a high cis-content (80%) was obtained. Interestingly, changing the polarity of the reaction solvent could result in polymers with different cis/trans ratios (P9-P12). The resulting controllable double-bond stereochemistry defines When the dithiols were replaced with diphenols, the organocatalyzed sulfydryl-yne polymerizations could also be established. In the presence of diphenylamine (DPA), the alkyne polyhydrothiolations of ester-activated diyne 8 and dithiophenol 9 proceed smoothly at room temperature, furnishing the sole anti-Markovnikov products of poly (vinylenesulfide)s P7 (Scheme 3B) with high molecular weights (M w up to 32,300) and high stereoregularities (Z content up to 81.4%) in high yields (up to 98.2%) [22]. The poly(vinylene sulfide)s are optically transparent, allowing almost all visible and IR light to transmit through, and their films show high refractive indices (n = 1.73-1.70) and low optical dispersions (D down to 0.002) in the wavelength region of 500-1700 nm. With the promotion of DPA, a new synthetic route for sulfur-rich polymers has been developed.
Soon afterwards, Dove group realized a polymer-polymer coupling though an organocatalytic click addition of thiol 12 to ester-activated alkyne 10 (Scheme 3C) [23]. Compared with primary amine and secondary amine, the author found that Et 3 N is the best catalyst. Most of the selected amine catalysts have very similar pK a values, so the basicity of the catalysts is probably not the main thing that affects their reaction activity in these reactions. Although the basicity of tertiary amine diisopropylethylamine (DIPEA, pK a = 11.4) is higher than that of Et 3 N (pK a = 10.7), it does not show any catalysis, which indicates that steric hindrance is the main factor of catalyst activity in the polymer-polymer coupling reaction.
Based on the above research, the same group reported another organocatalyzed polymerization of ester-activated alkynes and thiols in 2016 [24]. In this work, ester-activated alkyne 12 was combined with hexane dithiol 13 in chloroform (CHCl 3 ) in the presence of 1 mol% of 1,8-diazabicycloundecane (DBU) to promote the polymerization (Scheme 3D). After only 1 h, P9 with a high cis-content (80%) was obtained. Interestingly, changing the polarity of the reaction solvent could result in polymers with different cis/trans ratios (P9-P12). The resulting controllable double-bond stereochemistry defines the crystallinity and the concomitant mechanical properties as well as enabling the synthesis of materials that retain their excellent mechanical properties through changing monomer compositions. Thus, the organocatalytic control of stereochemistry opened the realm to a new and easily scalable class of elastomers that will have unique chemical handles for functionalization and post synthetic processing.

Organobase-Catalysts for Alkyne-Azide Polymerization
Alkyne-azide polymerization is well known as the most famous click polymerization. Organobase-catalysts play an indispensable role in the development of alkyne-azide polymerization.
Recently, two interesting studies were reported in succession and showed that two different organobase-catalysts could catalyze the same polymerization [25,26]. As clearly shown in Scheme 4, the same monomers of diynes 5a and 5b and diazides 14 were polymerized by the catalysis of NMe 4 OH or t-BuP 4 , producing polytriazoles of P13a-N, P13b-N, P13a-P, and P13a-P. Although the polymers were all 1,5-regioisomers, the other performances of the polymerization catalyzed by NMe 4 OH or t-BuP 4 were totally different. The results are summarized in Table 1. Compared with the yields of NMe 4 OHcatalyzed polymerizations, the yields of t-BuP 4 -catalyzed ones are much higher. However, the NMe 4 OH-catalyzed ones excel in another evaluation index, such as M w , structural regularity, and conditional mildness.

Acid-Catalysts for Alkyne-Based Polymerization
Multicomponent polymerization (MCP) as a powerful and popular synthetic approach for functional polymers with a great structural diversity has been demonstrated to be a promising tool for the construction of novel materials. MCP enjoys a series of advantages, such as great structural diversity, various monomer combination strategies, high efficiency and convenience, mild conditions, and environmental benefits [27]. Lewis-acid often plays a crucial role in the catalyst system for MCP.
In 2017, the Hu group developed a facile AcOH-catalyzed MCP of activated internal alkyne 16, aromatic diamines 15/17, and formaldehyde 18 to synthesize structural-regulated and sequence-controlled polyheterocycles with high molecular weights (up to 69 800) in high yields (up to 99%) [28]. Noticeably, if the combination and feeding sequence of two diamine monomers 15/17 were changed, the four-component polymerization could afford sequence-controlled polymers P14-P17 (Scheme 5A). Through such MCP, polymers with in situ-generated multi-substituted tetrahydropyrimidines or dihydropyrrolones in the Based on the same methodology and the same AcOH-catalyzed system, four hyperbranched polymers with various topological structures as well as sequences of different functional groups in the polymer backbone were obtained (molecular weights up to 30,000 and yield up to 98%) [29]. The hyperbranched polymers were designed to be aggregationinduced emission (AIE)-active, which mean they emitted luminescence faintly in solution, while the luminescence was notably enhanced in the aggregated state. It is anticipated that the MCP may provide a synthetic platform for the construction of hyperbranched polyheterocycles with diverse structures and functionalities.
Another kind of acid-catalyzed MCP was reported in 2019 [30]. With a combined promotion of TFA/AcOH, a series of multifunctional polyheterocycles with AIE characteristics were generated by the MCPs of terminal diynes 5b-e, dialdehydes 19a and 16b, and ureas 20a-d. Most of the polymers were synthesized with high molecular weights (M w up to 49,900) in nearly quantitative yields and displayed intense solid-state emission and a remarkable fluorescence response to protonation and deprotonation. Thus, a sensitive and reversible fluorescent sensor for ammonia was developed to detect biogenic amines and seafood spoilage (Figure 2a,b). Besides this, the polymer-based nanoparticles showed excellent lysosome-targeting specificity in cell imaging (Figure 2c-j). These polyheterocycles have a specific pH-sensitive behavior and can respond in the wide range of pH 1-9, and thereby can be used for the physiological pH visualization of the gastrointestinal tract. The in vivo mapping of the freshwater Cladocera Moina macrocopa intestinal pH tract showed an increased pH gradient from approximately 4.2 to 7.8 along the foregut, midgut, and hindgut. Therefore, the polymers obtained through the acid-catalyzed MCPs show great potential in biological applications both in vitro and in vivo. Further, three-group polymerizations have been realized with Lewis-acid-catalysts by the ingenious design of monomers, such as alkyne-aldehyde monomer 21 and alkyne-nitrile monomer 23.
Under the catalysis of B(C 6 F 5 ) 3 [31], a novel synthetic route to polyquinolines P19 with 6-substituted quinoline as the structural unit was developed based on the polymerization of alkyne-aldehyde monomer 21 and aniline derivative 22 (Scheme 6A). The polymerization was conducted in dichloroethane at 100 • C for 36 h under air atmosphere, affording polyquinolines P19 with molecular weights of up to 13,100 and a good solubility in most organic solvents.
The in vivo mapping of the freshwater Cladocera Moina macrocopa intestinal pH tract showed an increased pH gradient from approximately 4.2 to 7.8 along the foregut, midgut, and hindgut. Therefore, the polymers obtained through the acid-catalyzed MCPs show great potential in biological applications both in vitro and in vivo. Further, three-group polymerizations have been realized with Lewis-acid-catalysts by the ingenious design of monomers, such as alkyne-aldehyde monomer 21 and alkynenitrile monomer 23.
Under the catalysis of B(C6F5)3 [31], a novel synthetic route to polyquinolines P19 with 6-substituted quinoline as the structural unit was developed based on the polymerization of alkyne-aldehyde monomer 21 and aniline derivative 22 (Scheme 6A). The polymerization was conducted in dichloroethane at 100 °C for 36 h under air atmosphere, affording polyquinolines P19 with molecular weights of up to 13,100 and a good solubility in most organic solvents. Similarly, a three-group [2+2+1] cycloaddition polymerization of alkyne-nitrile monomer 23, and O-atom for the regioselective assembly of highly substituted oxazole compounds has been achieved by the use of iodosobenzene (PhIO) 24 with the catalysis of trifluoromethanesulfonic acid (TfOH) [32], which is clearly shown in Scheme 6B. Polyoxazole with a satisfactory solubility, thermal stability, and relative molecular weight was obtained by optimizing the polymerization conditions. Moreover, the polyoxazoles with unreacted terminal alkynes and nitriles showed a good thermal stability, and thus have potential applications in modified resin.
Lewis acid could not only promote some of MCPs, but also perform as one component of monomers. In 2016, Tang's group [33] presented a BCl 3 -mediated MCP route for the construction of stereoregular functional polymers (Scheme 7A). The MCP of terminal diyne 5, monoaldehyde 25, and BCl 3 that proceeds at 0 • C for 1 h followed by stirring at room temperature for another 3 h afforded poly(dichloropentadiene)s with a predominant (E,Z)configuration in high yields (up to 92%) with high molecular weights (M w up to 174,300).
Similarly, a three-group [2+2+1] cycloaddition polymerization of alkyne-nitrile monomer 23, and O-atom for the regioselective assembly of highly substituted oxazole compounds has been achieved by the use of iodosobenzene (PhIO) 24 with the catalysis of trifluoromethanesulfonic acid (TfOH) [32], which is clearly shown in Scheme 6B. Polyoxazole with a satisfactory solubility, thermal stability, and relative molecular weight was obtained by optimizing the polymerization conditions. Moreover, the polyoxazoles with unreacted terminal alkynes and nitriles showed a good thermal stability, and thus have potential applications in modified resin.
Lewis acid could not only promote some of MCPs, but also perform as one component of monomers. In 2016, Tang's group [33] presented a BCl3-mediated MCP route for the construction of stereoregular functional polymers (Scheme 7A). The MCP of terminal diyne 5, monoaldehyde 25, and BCl3 that proceeds at 0 °C for 1 h followed by stirring at room temperature for another 3 h afforded poly(dichloropentadiene)s with a predominant (E,Z)-configuration in high yields (up to 92%) with high molecular weights (Mw up to 174,300). Significantly, totally different configurations of poly(dichloropentadiene)s with 1,5dihalo-(Z,Z)-1,4-pentadiene as dominated segments were achieved two years later [34]. When diyne 15a and BCl3 were allowed to react at 45 °C for 1 h prior to further coupling with aldehyde at 0 °C and room temperature, a high molecular weight polymer (P22, Mw = 119,700) with a mainly (Z,Z)-configuration was obtained in a satisfactory yield. The content of the (Z,Z)-isomeric unit of P22 was determined to be up to 92%. While the mediator of Scheme 7A was replaced from BCl3 to BBr3 (Scheme 7C), it also resulted in polymers with a predominant (Z,Z)-1,4-pentadiene unit (P23) in a good yield of 79% with a high Mw of 63,100. Therefore, the stereoselectivity of the polymerization was demonstrated to be highly dependent on the reaction conditions as well as employed boron trihalide species.

Catalyst-Free Polymerization
Compared with organobase-catalysts or Lewis-acid-catalysts, catalyst-free conditions might be a wiser strategy for the polymerization of alkynyl-based monomers, which Significantly, totally different configurations of poly(dichloropentadiene)s with 1,5dihalo-(Z,Z)-1,4-pentadiene as dominated segments were achieved two years later [34]. When diyne 15a and BCl 3 were allowed to react at 45 • C for 1 h prior to further coupling with aldehyde at 0 • C and room temperature, a high molecular weight polymer (P22, M w = 119,700) with a mainly (Z,Z)-configuration was obtained in a satisfactory yield. The content of the (Z,Z)-isomeric unit of P22 was determined to be up to 92%. While the mediator of Scheme 7A was replaced from BCl 3 to BBr 3 (Scheme 7C), it also resulted in polymers with a predominant (Z,Z)-1,4-pentadiene unit (P23) in a good yield of 79% with a high M w of 63,100. Therefore, the stereoselectivity of the polymerization was demonstrated to be highly dependent on the reaction conditions as well as employed boron trihalide species.

Catalyst-Free Polymerization
Compared with organobase-catalysts or Lewis-acid-catalysts, catalyst-free conditions might be a wiser strategy for the polymerization of alkynyl-based monomers, which extremely simplify the procedure of polymerizations. Here, photo-/thermoinitiated or even spontaneous polymerizations will be introduced below.

Catalyst-Free Azide-Alkyne Polymerization
As the most classical and earliest click polymerization, azide-alkyne polymerization has been soundly developed, and the exploration of catalyst-free azide-alkyne polymerizations has taken a great proportion. Many endeavors have been paid to the development of catalyst-free azide-aklyne polymerizations, some comprehensive reviews on this method-Catalysts 2021, 11, 1 9 of 16 ology have been published in recent years [4,35,36]. Synoptically, four reforms are of vital importance.
As the most classical and earliest click polymerization, azide-alkyne polymerization has been soundly developed, and the exploration of catalyst-free azide-alkyne polymerizations has taken a great proportion. Many endeavors have been paid to the development of catalyst-free azide-aklyne polymerizations, some comprehensive reviews on this methodology have been published in recent years [4,35,36]. Synoptically, four reforms are of vital importance.
In the last, the proximity was taken as the driving force for the catalyst-free azidealkyne polymerization (Scheme 8D) [50][51][52][53][54]. Because ethynyl and azide groups, the reaction units, are wrapped in the lattice, the structure is similar to their transition-state-like arrangement, which promotes cycloaddition in catalyst-free and solvent-free conditions.

Catalyst-Free Thiol-yne Polymerization
Besides the extensive reports on catalyst-free polymerizations of alkyne and azide, the research on catalyst-free thiol-yne polymerization has found growing interest in the last decade.
Since the pioneering work of Perrier in 2009 [55], the photo-/thermoinitiated thiolyne click polymerization of AB2-type monomers 27 has been systematically investigated In the last, the proximity was taken as the driving force for the catalyst-free azidealkyne polymerization (Scheme 8D) [50][51][52][53][54]. Because ethynyl and azide groups, the reaction units, are wrapped in the lattice, the structure is similar to their transition-state-like arrangement, which promotes cycloaddition in catalyst-free and solvent-free conditions.

Catalyst-Free Thiol-yne Polymerization
Besides the extensive reports on catalyst-free polymerizations of alkyne and azide, the research on catalyst-free thiol-yne polymerization has found growing interest in the last decade.
Since the pioneering work of Perrier in 2009 [55], the photo-/thermoinitiated thiolyne click polymerization of AB 2 -type monomers 27 has been systematically investigated (Scheme 7A). It is widely applied in the preparation of functional hyperbranched polymers with such versatile properties as self-assembly, drug-release, biodegradability, thermoresponse, photoluminescence, and liquid crystallinity [56][57][58][59][60][61][62]. Linear polymers P25 can also be prepared via the photo-/thermoinitiated thiol-yne polymerization of monoyne 28 and dithiol monomers 29 (Scheme 9B) [63]. Moreover, Voit et al. [64,65] found that by controlling the ratio of mercapto and ethynyl groups (Scheme 9C), poly(vinyl sulfide)s P26 was able to be prepared based on the selective and quantitative mono-addition reaction between sulfhydryl 29 and ethynyl 30. This novel thiol-yne polymerization method can also be used in the fields of organic light-emitting diodes and planar one-dimensional all-polymer photonic crystals. The synthesized hyperbranched poly(vinyl sulfide)s have unique properties such as processability, thermal stability, high transparency, high reflective indices, and low optical dispersion [66][67][68]. All these photo-/thermoinitiated thiol-yne polymerizations proceed via a free-radical mechanism.
(Scheme 7A). It is widely applied in the preparation of functional hyperbranched polymers with such versatile properties as self-assembly, drug-release, biodegradability, thermo-response, photoluminescence, and liquid crystallinity [56][57][58][59][60][61][62]. Linear polymers P25 can also be prepared via the photo-/thermoinitiated thiol-yne polymerization of monoyne 28 and dithiol monomers 29 (Scheme 9B) [63]. Moreover, Voit et al. [64,65] found that by controlling the ratio of mercapto and ethynyl groups (Scheme 9C), poly(vinyl sulfide)s P26 was able to be prepared based on the selective and quantitative mono-addition reaction between sulfhydryl 29 and ethynyl 30. This novel thiol-yne polymerization method can also be used in the fields of organic light-emitting diodes and planar onedimensional all-polymer photonic crystals. The synthesized hyperbranched poly(vinyl sulfide)s have unique properties such as processability, thermal stability, high transparency, high reflective indices, and low optical dispersion [66][67][68]. All these photo-/thermoinitiated thiol-yne polymerizations proceed via a free-radical mechanism. UV light or elevated temperature is needed for the aforementioned thiol-yne polymerizations, which greatly complicates the experimental operation and limits its application. In 2014, a facile and powerful thiol−yne click polymerization was developed, which could be carried out under very mild conditions without any catalyst or assistant [69]. Simply mixing the aromatic diynes 5f and dithiols 30 with an equivalent molar ratio in THF at 30 °C could readily produce regioregular functional poly(vinylene sulfide)s P27 with high molecular weights (Mw up to 85,200) in excellent yields (up to 97%) within 2 h (Scheme 10A). Furthermore, no double addition product of an ethynyl group was found. This spontaneous thiol−yne click polymerization has remarkably simplified the reaction conditions and will facilitate the preparation of functional materials applied in diverse areas. One year later, the same authors prepared multifunctional hyperbranched poly(vinylene sulfide)s by the spontaneous thiol−yne click polymerization for the first time [70]. When replacing 5f with 1,4-diethynyl triptycene, triptycene-containing poly(vinylene sulfone) derivatives were well prepared based this kind of spontaneous thiol−yne click polymerization in 2018 [71]. UV light or elevated temperature is needed for the aforementioned thiol-yne polymerizations, which greatly complicates the experimental operation and limits its application. In 2014, a facile and powerful thiol−yne click polymerization was developed, which could be carried out under very mild conditions without any catalyst or assistant [69]. Simply mixing the aromatic diynes 5f and dithiols 30 with an equivalent molar ratio in THF at 30 • C could readily produce regioregular functional poly(vinylene sulfide)s P27 with high molecular weights (M w up to 85,200) in excellent yields (up to 97%) within 2 h (Scheme 10A). Furthermore, no double addition product of an ethynyl group was found. This spontaneous thiol−yne click polymerization has remarkably simplified the reaction conditions and will facilitate the preparation of functional materials applied in diverse areas. One year later, the same authors prepared multifunctional hyperbranched poly(vinylene sulfide)s by the spontaneous thiol−yne click polymerization for the first time [70]. When replacing 5f with 1,4-diethynyl triptycene, triptycene-containing poly(vinylene sulfone) derivatives were well prepared based this kind of spontaneous thiol−yne click polymerization in 2018 [71]. In 2020, terminal alkynes were further replaced with ester-activated internal alkynes to improve the stereoregularity of polymers produced in thiol-yne polymerizations [72]. As shown in Scheme 10B, the polymerizations of ester-activated internal diyne 31 and dithiol 32 performed in N,N-dimethylformamide (DMF) at 60 °C in an open atmosphere, producing poly(β-thioacrylate)s P28 with high stereoregularities (Z content up to 84%).

Spontaneous Amino-yne Polymerization
Great progress was made in 2017. He B. et al. successfully realized catalyst-free amino-yne click polymerizations for the first time [73], which opened the door to catalystfree amino-yne click polymerizations. Without the assistance of catalyst or external stimuli, the polymerization of ester-activated diynes 1 and diamines 34 carried out smoothly at room temperature, and 100% E-poly(β-aminoacrylate)s P29 with high molecular weights (Mw up to 64,400) were readily generated in excellent yields (Scheme 11A). By In 2020, terminal alkynes were further replaced with ester-activated internal alkynes to improve the stereoregularity of polymers produced in thiol-yne polymerizations [72]. As shown in Scheme 10B, the polymerizations of ester-activated internal diyne 31 and dithiol 32 performed in N,N-dimethylformamide (DMF) at 60 • C in an open atmosphere, producing poly(β-thioacrylate)s P28 with high stereoregularities (Z content up to 84%).

Spontaneous Amino-yne Polymerization
Great progress was made in 2017. He B. et al. successfully realized catalyst-free aminoyne click polymerizations for the first time [73], which opened the door to catalyst-free amino-yne click polymerizations. Without the assistance of catalyst or external stimuli, the polymerization of ester-activated diynes 1 and diamines 34 carried out smoothly at room temperature, and 100% E-poly(β-aminoacrylate)s P29 with high molecular weights (M w up to 64,400) were readily generated in excellent yields (Scheme 11A). By incorporating the TPE moiety in the main chains, this spontaneous amino-yne polymerization could be applied to prepare AIE-active polymers, becoming a powerful tool in explosive detection and bioimaging. Compared with CuCl-catalyzed amino-yne click polymerization at 140 • C, which was set up by the same author in 2016 [74], the spontaneous reaction system at room temperature possesses incomparable advantages. In 2020 [75], the same authors also succeeded in developing a multicomponent tandem polymerization to transform ester-activated internal diynes 35 and diamines 34 into poly(aminomaleimide)s P32 (Scheme 11D) with unique nontraditional intrinsic luminescent properties without any catalyst. Then, a new kind of activated diynes of bis-(ethynylsulfone)s 33, in which the ethynyl groups are activated by the sulfonyl groups, was rationally designed [76]. By using the activated diynes, a novel synthetic route of spontaneous amino-yne click polymerization was successfully opened (Scheme 11B). Compared with the ester-activated diynes, sulfonyl-activated diynes could polymerize with all kinds of diamines including aliphatic and aromatic primary and secondary ones, and poly(β-aminovinylsulfone)s P30 with high molecular weights (Mw up to 160,000) and excellent regio-and stereoregularity (the ratio of E isomers up to 100%) were obtained in good yields. Due to the strong electron-withdrawing ability of sulfonyl groups, P30 can exchange amines and easily degrade after the addition of monoamines. In a word, a novel spontaneous amino-yne click polymerization with regio-and stereoregular rules emerged. The synthesized polymer is dynamically degradable and can be used to label bioconjugates.
Afterwards, it was reported that keto-activated diynes 3 could be employed in the amino-yne polymerization under ambient conditions without the addition of any catalysts (Scheme 11C) [77]. Poly(β-enaminone)s P31 could be produced in an excellent yield (up to 99%). More importantly, the polymerization of keto-activated diynes with different types of amines showed different stereoselectivities. When secondary amines were used, polymers with a sole E-configuration were obtained, whereas 100% Z-configuration polymers were constructed using primary amines. By employing DFT calculation, the extraordinary stereoselectivity of such reactions was well explained. Taking advantage of the dynamic bonds in P31, the degradation reaction of polymers generated by the click polymerization of secondary diamines and bis(aroylacetylene)s was systematically investigated, which could produce definite degradation products. Thus, this work provides an efficient amino-yne click polymerization for synthesizing degradable nitrogen-containing polymers.
The most remarkable example of spontaneous amino-yne polymerization occurred in living cells [78]. A lab-in-cell is illustrated in Figure 3. Keto-activated diyne 3-1 can Then, a new kind of activated diynes of bis-(ethynylsulfone)s 33, in which the ethynyl groups are activated by the sulfonyl groups, was rationally designed [76]. By using the activated diynes, a novel synthetic route of spontaneous amino-yne click polymerization was successfully opened (Scheme 11B). Compared with the ester-activated diynes, sulfonyl-activated diynes could polymerize with all kinds of diamines including aliphatic and aromatic primary and secondary ones, and poly(β-aminovinylsulfone)s P30 with high molecular weights (M w up to 160,000) and excellent regio-and stereoregularity (the ratio of E isomers up to 100%) were obtained in good yields. Due to the strong electronwithdrawing ability of sulfonyl groups, P30 can exchange amines and easily degrade after the addition of monoamines. In a word, a novel spontaneous amino-yne click polymerization with regio-and stereoregular rules emerged. The synthesized polymer is dynamically degradable and can be used to label bioconjugates.
Afterwards, it was reported that keto-activated diynes 3 could be employed in the amino-yne polymerization under ambient conditions without the addition of any catalysts (Scheme 11C) [77]. Poly(β-enaminone)s P31 could be produced in an excellent yield (up to 99%). More importantly, the polymerization of keto-activated diynes with different types of amines showed different stereoselectivities. When secondary amines were used, polymers with a sole E-configuration were obtained, whereas 100% Z-configuration polymers were constructed using primary amines. By employing DFT calculation, the extraordinary stereoselectivity of such reactions was well explained. Taking advantage of the dynamic bonds in P31, the degradation reaction of polymers generated by the click polymerization of secondary diamines and bis(aroylacetylene)s was systematically investigated, which could produce definite degradation products. Thus, this work provides an efficient amino-yne click polymerization for synthesizing degradable nitrogen-containing polymers.
The most remarkable example of spontaneous amino-yne polymerization occurred in living cells [78]. A lab-in-cell is illustrated in Figure 3. Keto-activated diyne 3-1 can spontaneously polymerize with TPE-containing primary diamine 34-1 inside cells, and polymer P33 with weight-average molecular weight of 7300 was yielded. By utilizing this in vivo amino-yne polymerization and taking advantage of the AIE feature of TPE, a "turn-on" cell imaging was realized, and the in situ killing of cells was also acquired by destroying the structures of actin and tubulin, which cannot be realized by a pre-prepared polymer. This strategy provided a useful platform and hold great promise in biochemistry and therapy applications.

Conclusions and Perspective
From the viewpoint of catalytic systems, the progress on metal-free polymerizations of alkynyl-based monomers has been classified and introduced. In conclusion, organic bases are frequently employed to catalyze click polymerizations, such as hydroxyl-yne, sulfydrylyne, and azide-alkyne ones, while Lewis-acids are usually used in the catalysis of multicomponent polymerizations or multi-group polymerizations. Catalyst-free polymerizations are normally photo-/thermo-initiated or even proceed spontaneously. All these kinds of metal-free polymerizations of alkynyl-based monomers enjoy incomparable advantages without the problem of catalyst-residual. Thus, these well-developed polymerization methodologies and corresponding polymers are very suitable for the requirements of biology and optoelectronics.
Up to now, almost all of the catalyst-free polymerizations of alkynyl-based monomers belong to various categories of click polymerizations. The family of photo-/thermoinitiated/spontaneous multicomponent polymerizations of alkynyl-based monomers is yet to be exploited and expanded. Although there was one report entitled "Catalyst-Free, Atom-Economic, Multicomponent Polymerizations of Aromatic Diynes, Elemental Sulfur, and Aliphatic Diamines toward Luminescent Polythioamides" in 2015 [79], the prospects in this aspect are broad. Meanwhile, the applications of these metal-free polymerizations keep up with the pace of polymeric methodologies, and more brilliant works will come into the view in the scientific world. We believe that this feature article will attract more and more talented practitioners to embrace these challenges of vast potential.