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Light-induced step-growth polymerization
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Light-induced step-growth polymerization Gorkem Yilmaz a , Yusuf Yagci a,b,∗ a
Istanbul Technical University, Department of Chemistry, Maslak, 34469 Istanbul, Turkey Center of Excellence for Advanced Materials Research (CEAMR) and Department of Chemistry, Faculty of Science, King Abdulaziz University, 21589 Jeddah, Saudi Arabia b
a r t i c l e
i n f o
Article history: Available online xxx Keywords: Light Photopolymerization Step-growth polymerization Polyesters Polyamides Polyurethanes
a b s t r a c t There has been a growing interest in discovering new approaches for the preparation of step-growth polymers as they hold a large margin of polymer manufacture since the beginning of the synthetic polymer era. After their distinctive advantages have been realized, light induced processes have been adapted to almost all kinds of polymerization processes including step-growth polymerizations. This mini review focuses mainly on the new developments in the field of step-growth polymerizations activated by light induced processes. The new discoveries are generally based on light induced pericyclic reactions, ketene chemistry and radical coupling processes. In comparison to the conventional strategies, the advantages of these particular approaches are also discussed. © 2019 Elsevier B.V. All rights reserved.
Contents 1. 2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Strategies for light-induced step-growth polymerizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.1. Step-growth polymerizations by cycloaddition reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.2. Step-growth polymerizations by ketene-ol addition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.3. Step-growth polymers by click reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.4. Step-growth polymers by nitrile imine-mediated tetrazole-ene cycloaddition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.5. Step-growth polymers by Passerini reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.6. Step-growth polymers by electron transfer and radical coupling reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.7. Step-growth polymers by atom transfer radical addition reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.8. Step-growth polymers by atom transfer radical coupling reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.9. Other potential strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Declaration of interests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
1. Introduction Abbreviations: AFM, atomic force microscopy; ATRC, atom transfer radical coupling; CuAAC, copper catalyzed azide-alkyne cycloaddition; EPR, electron paramagnetic resonance; L, ligand; Mn2 (CO)10 , manganese decacarbonyl; On+ X-, onium salts; PAC, polynuclear aromatic compound; PEC, poly(N-ethylcarbazole); PI, photoinitiator; SEM, scanning electron microscopy; UV, ultraviolet; UV-vis, utraviolet-visible. ∗ Corresponding author at: Istanbul Technical University, Department of Chemistry, Maslak, 34469 Istanbul, Turkey. E-mail address: [email protected] (Y. Yagci).
Step-growth polymers have been of a major industrial interest and extensively used as commodity plastics since the beginning of the introduction of synthetic polymers [1,2]. The first example of step-growth polymer is Bakelite, which was synthesized by the condensation phenol with formaldehyde. It is considered to be the beginning of the truly synthetic plastic era and of the plastic industry [3,4]. The discovery of phenoplastics promoted other
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Scheme 1. Transition metal catalyzed radical polyaddition of ester-linked monomers.
developments namely aminoplasts, which were obtained by the reaction formaldehyde with urea and melamine [5]. Following the manufacture of aminoplasts, unsaturated polyesters derived from the reaction between an acid and an alcohol were industrialized. Following World War II, the major expansion of polycondensate production commenced especially after the development of polyesters and polyamides. They found practical applications in the textile industry and as moldable thermo-plastic elastomers with high tensile strength. Step-growth polymerizations do not require an initiator, which is the most important element of a chain polymerization. In general, a step-growth polymerization is performed by the reaction of a difunctional monomer or equimolar concentrations of two different monomers with antagonist functions to form polymers. The molecular weight increase at the early stages of the polymerization process is very slow and there is only a single reaction mechanism that repeatedly occurs for the formation of polymer. Thus, unlike a chain-growth process, steps of initiation, propagation and termination are disregarded. In general, step-growth polymers are named after the conventional organic reactions applied. For example, esterification reactions yield to polyesters whereas amidations yield polyamides. However, the latest studies for the synthesis of these polymers offer novel approaches through an activation mechanism. Yokozawa and Yokoyama utilized a unique strategy by activating the polymer end group by substituent effects changed between monomer and polymer for the synthesis of well-defined polycondensates [6,7]. This so called “chain-growth polycondensation” is based on the reaction of the monomer specifically with an initiator and the propagating group favored by substituent effect to artificially regulate the chain lengths. The developments in the field of step-growth polymerization is not limited to this particular example. Recently, Kamigaito and co-workers presented a novel strategy for the preparation of polyesters using atom transfer radical addition polymerization [8]. It requires certain inorganic mediators with two accessible oxidation states that are differentiated by one electron and reasonable halogen affinity (i.e. CuCl, FeCl2 ), surrounded by a multidentate ligand (L). Using these activators together with specifically designed monomers [CH2 =CH-R−OC(O)CH(CH3 )Cl] that bear a reactive CCl and an unconjugated double bond via an ester linkage produce novel aliphatic polyesters. Chromatographic evidences indicate the polymerization follows a step-growth mechanism as demonstrated below (Scheme 1). The strategy was also applied to the synthesis of various copolymers and block copolymers possessing polyester or polyamide and polyvinyl segments with interesting physicochemical properties [9–12].
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Scheme 2. Step-growth polymerization by light induced [2 + 2] cycloaddition using cinnamate derivatives.
2. Strategies for light-induced step-growth polymerizations Obviously, new developments for the preparation of stepgrowth polymers is not limited to these particular examples. Light induced strategies have gained growing interest especially after their advantages have been realized [13]. These advantages include temporal and spatial control over the processes and materials produced, often no need for use of volatile organic solvents giving rise to environmental and health benefits. Contrary to thermal processes, photochemical protocols offer several advantages, which can be desirable, especially in an industrial setting [14–17]. These advantages including low energy requirement and high reaction rates underline the importance of light induced processes as an example of a “green” process, which is such an important concept in the 21st century as we move toward a more sustainable and less wasteful world [18]. Therefore, many polymerizations as well as polymer modifications have been adapted to light induced processes to benefit from these unique advantages [19–26]. Although conventional approaches are in advanced state for step-growth polymer preparation, there are promising advances to adapt photoinduced reactions to step-growth polymerizations. 2.1. Step-growth polymerizations by cycloaddition reactions Cinnamates are known to undergo light induced [2 + 2] cycloaddition reactions. The application of this chemistry to polymer science are mainly based on polymer modifications and crosslinked network formation [27,28]. However, Hasegawa et. al. expanded the use of this chemistry to step-growth polymerizations by using specifically designed monomers bearing cinnamate units [29–31]. For example, it was demonstrated that irradiation of 2, 5-distyrylpyrazine leads to a crystalline linear high polymer by following a photocyclopolymerization as shown below (Scheme 2). Typical light induced [2 + 2] cycloadditions can also be achieved by using dibenzazepines, which can also be used for a variety of polymer synthesis [32,33]. Yagci and co-workers applied this chemistry to step growth polymerization by using specific monomers Light-induced
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Scheme 3. Step-growth polymerization by light induced [2 + 2] cycloaddition using dibenzazepine derivatives.
Scheme 4. Step-growth polymerization by light induced [4 + 2] cycloaddition using AB-type monomers.
having internal bisphenol-A unit and terminal dibenzazepines as shown below (Scheme 3) [34]. More recently, Barner-Kowollik and co-workers demonstrated specially designed AB-type photomonomers can undergo a light induced [4 + 2] Diels-Alder cycloaddition reaction. The mechanism follows a keto-enol tautomerization triggered by light irradiation producing active monomers bearing both the diene and dienophile functionalities required for the reaction, and thus polymerization [35]. The overall polymerization mechanism is demonstrated below (Scheme 4). Similar Diels-Alder strategy was also applied for the synthesis of well-defined high molar mass segmented copolymers, employing a unique combination of step-growth and reversible addition–fragmentation chain transfer polymerizations [36]. Synthesis of step-growth polymers by light induced reactions was also demonstrated on the example of [4 + 4] cycloaddition reactions using anthracene derivatives [37]. For this purpose, bisanthracene functionalized monomer involving perfluorophenyl ester moiety as internal functionality was prepared and polymerized under UV irradiation. The corresponding polymer was functionalized by simple amidation reactions for the preparation of polyesters having specific moieties at the side chain as shown below (Scheme 5). 2.2. Step-growth polymerizations by ketene-ol addition The ketene functional group was first discovered in 1905 by Hermann Staudinger, who is widely considered to be the father of polymer chemistry. He succeeded in synthesizing and characterizing the first ketene, which served as the basis of his pioneering Please cite this article as: G. Yilmaz and Y. https://doi.org/10.1016/j.progpolymsci.2019.101178
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work on its polymerization. Since then, ketene chemistry has been neglected over a century of time. Very recently though, ketene chemistry has been revitalized as a useful tool for synthetic polymer chemistry. New developments in light induced step-growth polymerizations are not limited to the application of pericyclic reactions as exemplified above. Dioxinones are known to yield ketene intermediates upon light irradiation or heating depending on the structures. These reactive intermediates have recently been used for polymerizations as well as modifications [38–41]. Typical example concerns synthesis of a benzodioxinone derivative bearing hydroxyl functions and its use as bifunctional monomer. Under irradiation at proper wavelengths, this monomer releases benzophenone and concomitantly generated ketene groups readily react with the hydroxyl group of another monomer (ketene-ol addition) to yield macromolecules as shown in Scheme 6 [42]. Due to the shielding effect of the benzophenone released, the molecular weight increase is restricted to certain degrees, but a great deal of research has still been conducted for enhancement. 2.3. Step-growth polymers by click reactions Click reactions are modular and versatile reactions, which can be realized under mild reaction conditions with almost quantitative efficiencies [43–45]. In addition to Diels-Alder reactions that considers the reaction of a diene with a dienophile, copper catalyzed azide-alkyne cycloaddition (CuAAC) and thiol-ene reactions are also honored as click reactions [46,47]. They have been used for a number of synthetic applications including preparation of complex macromolecular architectures including block, graft, Light-induced
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Scheme 5. Step-growth polymerization by light induced [4 + 4] cycloaddition using monomers having bis-anthtracene functionality.
Scheme 6. Step-growth polymerization by light induced ketene-ol addition using benzodioxinone derivatives.
Scheme 7. Photoinduced CuAAC and thiol-ene reactions.
star copolymers and telechelic polymers [48–51]. Both CuAAC and thiol-ene reactions can be utilized under light irradiation, which have positive impact on the processes by means of reducing air sensitivity and catalyst type used [52]. Traditional CuAAC reactions require Cu(I) catalyst, which is air sensitive. On the other hand, light induced CuAAC are realized in the presence of Cu(II), which is simultaneously reduced to Cu(I) by the aid of photochemical reactions. Thiol-ene reactions can also be used together with a radical photoinitiator (PI), which enables the generation of thiyl radicals responsible for addition step (Scheme 7) [21,53,54]. In general, they have been used for the preparation of crosslinked networks by step-growth polymerization. For this purpose, multifunctional azides and acetylenes or thiols and alkenes are used in CuAAC and thiol-ene reactions, respectively [55–57]. Linear polymers by thiol-ene reactions are limited to oligomeric materials due to the lower efficiency of this reactions in high molar mass compounds [58,59]. Notably, disulfide oligomers were also shown to be polymerizable under UV irradiation by self-photopolymerization [60]. 2.4. Step-growth polymers by nitrile imine-mediated tetrazole-ene cycloaddition Nitrile Imine-Mediated Tetrazole-Ene Cycloaddition (NITEC) reactions are established fast and catalyst free reactions where even non-reactive alkenes can be used reactants. It has recently been used for polymer modifications and ligations. In a recent study, bismaleimide linker was reacted with bistetrazole compounds in Please cite this article as: G. Yilmaz and Y. https://doi.org/10.1016/j.progpolymsci.2019.101178
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an efficient manner to produce step-growth polymers containing pyrazoline groups capable of fluorescent emission (Scheme 8). Thus, the fluorescence of the step-growth polymers is directly correlated with the number of ligation points in the polymer, forming an ideal self-reporting sensor system. The internal RAFT agent group was also utilized for the initiation of styrene, which acted as a macromonomer for typical polycondensation [61]. 2.5. Step-growth polymers by Passerini reaction Step growth polymers can also be attained by certain specific reactions. In a very recent study, Kowollik and co-workers demonstrated the possibility of obtaining high molar mass chains by additive-free visible light induced Passerini reaction [62]. In place of classical aldehydes (or ketones), highly reactive, in-situ photogenerated thioaldehydes are exploited along with isocyanides and carboxylic acids. Applying optimum reaction conditions to eliminate side reactions, it was shown that step-growth polymers can be easily prepared under mild reaction conditions, which mechanistically follows a series of addition and rearrangement steps (Scheme 9). 2.6. Step-growth polymers by electron transfer and radical coupling reactions Light induced radical coupling processes have also been employed for step-growth polymerizations. Initial studies from our laboratory showed that polythiophenes can be synthesized by Light-induced
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Scheme 8. Step-growth polymerization by light induced nitrile imine-mediated tetrazole-ene cycloaddition.
photochemical means through a mechanism similar to electropolymerization. It is well established that upon direct irradiation, onium salts undergo decomposition as presented below on the example of diphenyliodonium salt [63]. The iodophenyl radical cations (PhI+. ) thus formed undergo single electron transfer reaction with thiophene to form thiophene radical cation. Successive proton release and coupling reactions of the monomer and oligomers essentially leads to the formation of polythiophene (Scheme 10). Apart from diphenyliodonium salts, other onium salts such as triphenylsulfonium and N-alkoxy pyridinium salts were found to initiate the polymerization. However, laser flash photolysis and electron paramagnetic resonance studies revealed that iodonium salts were the most efficient initiators for the described photoinduced electron transfer reactions. The rate constants for the reactions of the corresponding radical cations (Ph2 I+. and Ph3 S+. ) with thiophene were found to be 1.26 × 1010 and 1.7 × 105 M-1 s-1 , respectively. The difference in rate constants can be attributed to the redox potentials of the onium salts, which is in favor of Ph2 I+ PF6 - [64].
Scheme 10. Photoinduced step-growth polymerization of thiophene.
Although both electro and photo polymerizations follow an oxidative mechanism, the morphology of the obtained polymers are different. While electropolymerizations produce polymers with cauliflower like morphologies, polymers from photopolymerizations exhibit wrinkled morphology as a result of favorable chain growth (Fig. 1). The more ordered morphology in the case of photoinduced polymerization can be explained by the difference in the mechanism of polymer growth [63]. In order to extend this approach to less harmful long wavelengths, sensitized systems were reported. Certain polynuclear aromatic compounds (PACs) are known to undergo an electron transfer reaction with onium salts under light irradiation where the onium salt is transparent [65]. Upon irradiation, excited state
Scheme 9. Step-growth polymerization by light induced Passerini reaction.
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Fig. 1. SEM images of polythiophene obtained by electropolymerization (a) and photoinduced step-growth polymerization (b). [64], Copyright 2005. Reproduced with permission from John Wiley and Sons).
Scheme 11. Photoinduced electron transfer from anthracene to diphenyliodonium hexafluorophosphate.
PACs form excited state complexes (exciplexes) with the onium salt. There is a single electron transfer from the PAC of high electron density to the onium salt of high electron affinity to generate highly reactive species as depicted in Scheme 11 on the example of anthracene and diphenyliodonium hexafluorophosphate. This process is generally used for photoinitiated cationic polymerizations where the PACs act as sensitizers [66–75]. Such long wavelength induced electron transfer reactions were also utilized for stepgrowth polymerization, which considers the use of the conjugated structures as monomers as discussed below. Recently, it has been shown that conjugated thiophene derivatives can be used as sensitizers for the cationic polymerizations of epoxides and vinyl ethers [50–52]. In a following study, certain conjugated thiophene derivatives were demonstrated to form polymers when used together with equimolar concentrations of iodonium ions [76]. Detailed mechanistic analyses reveal that after the electron transfer step, two equivalents of radical cations thiophene derivative couple each other while releasing Bronsted acid to give its dimeric form. Following analogous steps of photoinduced electron transfer and radical coupling reactions, polythiophenes are eventually formed (Scheme 12). Due to the extension of conjugation during polymerization, there is a clear color change, which was also observable by UV analysis (Fig. 2). Yagci and co-workers recently applied similar strategy to polymerize N-ethylcarbazole [77]. There has been a great deal of interest in the synthesis of conjugated polymers, which find various practical applications especially in electronics and optic devices. Main chain polycarbazoles are generally prepared by coupling reactions, and the polymer growth can be controlled by functionalization of the possible reactive sites on the aromatic rings. In the aforementioned work, it was shown that commer-
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Fig. 2. UV–vis spectral change during light induced step-growth polymerization of the thiophene derivative. [77], Copyright 2009. Reproduced with permission from the Royal Society of Chemistry.
cially available N-ethylcarbazole was readily polymerized in the presence of diphenyliodonium hexafluorophosphate under light irradiation absorbed only by the monomer. Consequently, poly(Nethylcarbazole) (PEC) is formed with a similar mechanism as discussed above (Scheme 13). Light-induced
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Scheme 12. Photoinduced step-growth polymerization of highly conjugated thiophene derivatives.
Scheme 13. Photoinduced step-growth polymerization of N-ethylcarbazole.
Despite having reactive sites both in 2,3 and 5,6 positions and there are multiple possibilities of macromolecular growth, detailed analyses show that the polymerization proceeds from the 3,6 positions of the monomer. When the atomic force microscopy of the polymer films both after doping and dedoping on the glass surface are investigated, the polymers were found to exhibit agglomerated and partly uniform grains arising from helical 3,6-linkage between carbazole moieties of the polymer backbone. The grain growth occurs with changing the orientation of carbazole moieties on the polymer backbone. Notably, before dedoping, polymer films
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exhibit uniform structure due to PF6 - ions, which have ion-dipole attractions with the glass surface. As expected, the dedoping process gives rise to the formation of more perforated surfaces due the absence of the above-mentioned attractions (Fig. 3). In a recent study from our laboratory, it was shown that PEC can be photochemically prepared without monomer and additional oxidant. Phenacyl ethylcarbazolium salt, originally used as photoinitiator for free radical and cationic polymerization [78], undergoes photoinduced decomposition to form PEC (Scheme 14). Similar proton release and coupling reactions pro-
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Fig. 3. AFM images of PEC thin film on the glass surface before (a) and after dedoping process (b). [78], Copyright 2015. Reproduced with permission from the American Chemical Society.
Scheme 14. Photoinduced synthesis of poly(N-ethylcarbazole) by using phenacylium carbazole salt.
posed for the above mentioned system were shown to be operative. [79] 2.7. Step-growth polymers by atom transfer radical addition reactions Iodine transfer radical polymerizations have recently attracted increasing interest as reflected by recent publications [80–82]. The photosensitivity of alkyl iodides provides the possibility of generation of alkyl radicals under visible light irradiation in a reversible Please cite this article as: G. Yilmaz and Y. https://doi.org/10.1016/j.progpolymsci.2019.101178
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manner. In the presence of vinyl monomers, atom transfer radical polymerization was shown to be operative even in the absence of catalysts. This unique chemistry has been adapted to light induced step growth polymerization, which follows atom transfer radical addition mechanism. To stabilize the iodide radical and prevent the formation of elemental iodine, generally ruthenium catalysts and ascorbic acid are used. Xu et al. presented the use of this chemistry for the preparation of step-growth polymers by reacting difunctional alkenes with diiodides [83]. This typical polymerization was
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Scheme 15. Synthesis of step-growth polymers by photoinduced step transferaddition and radical-termination polymerization.
named as step transfer-addition and radical-termination (START) (Scheme 15). Another detailed study demonstrated the positive effect of water contamination on the START polymerization [84].
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ally photoinduced processes has been applied for systems involving Mn2 (CO)10 . In a more recent study, we have reported a facile visible light induced synthetic method for preparing a series of conventional polycondensates, namely polyesters, polyurethanes and polyamides. For this purpose, bisbenzylbromide compounds possessing ester, urethane and amide groups in the inner structure were synthesized and employed as monomers. Irradiation of these monomers in the visible range using Mn2 (CO)10 as a radical generator by halide abstraction and subsequent radical coupling reactions yielded the corresponding polycondensates (Scheme 16) [100]. 2.9. Other potential strategies
2.8. Step-growth polymers by atom transfer radical coupling reactions The first synthetic utility of atom transfer radical coupling (ATRC) processes was reported by Yagci and co-workers for the preparation of telechelic polymers [85]. Following this report, ATRC was further used for polymer synthesis as well as modifications [86–93]. It appeared that such coupling reactions can be realized through manganese decacarbonyl (Mn2 (CO)10 ) photochemistry which eliminates the use of copper catalyst. Mn2 (CO)10 chemistry has been utilized for various polymerizations, polymer modifications and coupling processes [23,94–99]. Although thermal induction was shown to be also effective, gener-
Although not demonstrated for step-growth polymerization, photoinduced decomposition of N-alkoxy pyridinium salts offers an alternative route to the formation of polyurethanes and polyesters. Photolysis of pyridinium salts in the presence of hydrogen donor solvents leads to the formation of hydroxyl groups according to Scheme 17. Irradiation of bifunctional alkoxy pyridinium salts in the presence of selected diacidchlorides and diisocyanates would lead to the formation of polyesters and polyurethanes, respectively. The possibility of such step-growth polymerization was demonstrated by the end group functionalization of pyridinium ion terminated polymers via photoinduced urethane formation [101].
Scheme 16. Synthesis of step-growth polymers by light-induced radical coupling using Mn2 (CO)10 .
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Scheme 17. Photoinduced formation of hydroxyl functional molecules.
3. Conclusion Step-growth polymers gain a substantial research interest as they hold a large margin of polymer manufacture since the beginning of the synthetic polymer era. Therefore, there has been a revitalized interest in developing new synthetic strategies for the preparation of step-growth polymers. After their distinctive advantages have been realized, light induced processes have been adapted to almost all kinds of polymerizations including stepgrowth polymerizations. In this article, we have discussed the recent studies on step-growth polymerizations by photochemical means. The advantages of these particular approaches compare to the conventional strategies have also been discussed. The research field of photoinduced step-growth polymerization has a very bright future due to the ease of use combined with its less harmful and energetically favorable conditionals, and the possibility of spatiotemporal control. Declaration of interests None. Acknowledgements The authors thank Istanbul Technical University for financial support. References [1] Kricheldorf HR. Polycondensation history and new results. Berlin: Springer-Verlag Gmb H; 2016, 291 pp. [2] Rempp P, Merrill EW. Polymer synthesis. Basel: Hüthig & Wepf; 1991, 344 pp. [3] Crespy D, Bozonnet M, Meier M. 100 years of bakelite, the material of a 1000 uses. Angew Chem Int Ed 2008;47:3322–8. [4] Hirano K, Asami M. Phenolic resins-100 years of progress and their future. React Funct Polym 2013;73:256–69. [5] Seymour RB. Pioneers in polymer science. Dordrecht: Kluwer Academic Publishers; 1989, 272 pp. [6] Yokoyama A, Yokozawa T. Converting step-growth to chain-growth condensation polymerization. Macromolecules 2007;40:4093–101. [7] Yokozawa T, Yokoyama A. Chain-growth condensation polymerization for the synthesis of well-defined condensation polymers and -conjugated polymers. Chem Rev 2009;109:5595–619. [8] Mizutani M, Satoh K, Kamigaito M. Metal-catalyzed radical polyaddition for aliphatic polyesters via evolution of atom transfer radical addition into step-growth polymerization. Macromolecules 2009;42:472–80. [9] Mizutani M, Satoh K, Kamigaito M. Metal-catalyzed simultaneous chainand step-growth radical polymerization: marriage of vinyl polymers and polyesters. J Am Chem Soc 2010;132:7498–507. [10] Mizutani M, Satoh K, Kamigaito M. Construction of vinyl polymer and polyester or polyamide units in a single polymer chain via metal-catalyzed simultaneous chain- and step-growth radical polymerization of various monomers. Aust J Chem 2014;67:544–54. [11] Satoh K, Ishizuka K, Hamada T, Handa M, Abe T, Ozawa S, et al. Construction of sequence-regulated vinyl copolymers via iterative single vinyl monomer additions and subsequent metal-catalyzed step-growth radical polymerization. Macromolecules 2019;52:3327–41. [12] Satoh K, Ozawa S, Mizutani M, Nagai K, Kamigaito M. Sequence-regulated vinyl copolymers by metal-catalysed step-growth radical polymerization. Nat Commun 2010;1:1–6. [13] Yagci Y, Jockusch S, Turro NJ. Photoinitiated polymerization: advances, challenges, and opportunities. Macromolecules 2010;43:6245–60.
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Progress in Polymer Science xxx (xxxx) xxx
Contents lists available at ScienceDirect
Progress in Polymer Science journal homepage: www.elsevier.com/locate/ppolysci
Light-induced step-growth polymerization Gorkem Yilmaz a , Yusuf Yagci a,b,∗ a
Istanbul Technical University, Department of Chemistry, Maslak, 34469 Istanbul, Turkey Center of Excellence for Advanced Materials Research (CEAMR) and Department of Chemistry, Faculty of Science, King Abdulaziz University, 21589 Jeddah, Saudi Arabia b
a r t i c l e
i n f o
Article history: Available online xxx Keywords: Light Photopolymerization Step-growth polymerization Polyesters Polyamides Polyurethanes
a b s t r a c t There has been a growing interest in discovering new approaches for the preparation of step-growth polymers as they hold a large margin of polymer manufacture since the beginning of the synthetic polymer era. After their distinctive advantages have been realized, light induced processes have been adapted to almost all kinds of polymerization processes including step-growth polymerizations. This mini review focuses mainly on the new developments in the field of step-growth polymerizations activated by light induced processes. The new discoveries are generally based on light induced pericyclic reactions, ketene chemistry and radical coupling processes. In comparison to the conventional strategies, the advantages of these particular approaches are also discussed. © 2019 Elsevier B.V. All rights reserved.
Contents 1. 2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Strategies for light-induced step-growth polymerizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.1. Step-growth polymerizations by cycloaddition reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.2. Step-growth polymerizations by ketene-ol addition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.3. Step-growth polymers by click reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.4. Step-growth polymers by nitrile imine-mediated tetrazole-ene cycloaddition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.5. Step-growth polymers by Passerini reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.6. Step-growth polymers by electron transfer and radical coupling reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.7. Step-growth polymers by atom transfer radical addition reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.8. Step-growth polymers by atom transfer radical coupling reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.9. Other potential strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Declaration of interests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
1. Introduction Abbreviations: AFM, atomic force microscopy; ATRC, atom transfer radical coupling; CuAAC, copper catalyzed azide-alkyne cycloaddition; EPR, electron paramagnetic resonance; L, ligand; Mn2 (CO)10 , manganese decacarbonyl; On+ X-, onium salts; PAC, polynuclear aromatic compound; PEC, poly(N-ethylcarbazole); PI, photoinitiator; SEM, scanning electron microscopy; UV, ultraviolet; UV-vis, utraviolet-visible. ∗ Corresponding author at: Istanbul Technical University, Department of Chemistry, Maslak, 34469 Istanbul, Turkey. E-mail address: [email protected] (Y. Yagci).
Step-growth polymers have been of a major industrial interest and extensively used as commodity plastics since the beginning of the introduction of synthetic polymers [1,2]. The first example of step-growth polymer is Bakelite, which was synthesized by the condensation phenol with formaldehyde. It is considered to be the beginning of the truly synthetic plastic era and of the plastic industry [3,4]. The discovery of phenoplastics promoted other
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Scheme 1. Transition metal catalyzed radical polyaddition of ester-linked monomers.
developments namely aminoplasts, which were obtained by the reaction formaldehyde with urea and melamine [5]. Following the manufacture of aminoplasts, unsaturated polyesters derived from the reaction between an acid and an alcohol were industrialized. Following World War II, the major expansion of polycondensate production commenced especially after the development of polyesters and polyamides. They found practical applications in the textile industry and as moldable thermo-plastic elastomers with high tensile strength. Step-growth polymerizations do not require an initiator, which is the most important element of a chain polymerization. In general, a step-growth polymerization is performed by the reaction of a difunctional monomer or equimolar concentrations of two different monomers with antagonist functions to form polymers. The molecular weight increase at the early stages of the polymerization process is very slow and there is only a single reaction mechanism that repeatedly occurs for the formation of polymer. Thus, unlike a chain-growth process, steps of initiation, propagation and termination are disregarded. In general, step-growth polymers are named after the conventional organic reactions applied. For example, esterification reactions yield to polyesters whereas amidations yield polyamides. However, the latest studies for the synthesis of these polymers offer novel approaches through an activation mechanism. Yokozawa and Yokoyama utilized a unique strategy by activating the polymer end group by substituent effects changed between monomer and polymer for the synthesis of well-defined polycondensates [6,7]. This so called “chain-growth polycondensation” is based on the reaction of the monomer specifically with an initiator and the propagating group favored by substituent effect to artificially regulate the chain lengths. The developments in the field of step-growth polymerization is not limited to this particular example. Recently, Kamigaito and co-workers presented a novel strategy for the preparation of polyesters using atom transfer radical addition polymerization [8]. It requires certain inorganic mediators with two accessible oxidation states that are differentiated by one electron and reasonable halogen affinity (i.e. CuCl, FeCl2 ), surrounded by a multidentate ligand (L). Using these activators together with specifically designed monomers [CH2 =CH-R−OC(O)CH(CH3 )Cl] that bear a reactive CCl and an unconjugated double bond via an ester linkage produce novel aliphatic polyesters. Chromatographic evidences indicate the polymerization follows a step-growth mechanism as demonstrated below (Scheme 1). The strategy was also applied to the synthesis of various copolymers and block copolymers possessing polyester or polyamide and polyvinyl segments with interesting physicochemical properties [9–12].
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Scheme 2. Step-growth polymerization by light induced [2 + 2] cycloaddition using cinnamate derivatives.
2. Strategies for light-induced step-growth polymerizations Obviously, new developments for the preparation of stepgrowth polymers is not limited to these particular examples. Light induced strategies have gained growing interest especially after their advantages have been realized [13]. These advantages include temporal and spatial control over the processes and materials produced, often no need for use of volatile organic solvents giving rise to environmental and health benefits. Contrary to thermal processes, photochemical protocols offer several advantages, which can be desirable, especially in an industrial setting [14–17]. These advantages including low energy requirement and high reaction rates underline the importance of light induced processes as an example of a “green” process, which is such an important concept in the 21st century as we move toward a more sustainable and less wasteful world [18]. Therefore, many polymerizations as well as polymer modifications have been adapted to light induced processes to benefit from these unique advantages [19–26]. Although conventional approaches are in advanced state for step-growth polymer preparation, there are promising advances to adapt photoinduced reactions to step-growth polymerizations. 2.1. Step-growth polymerizations by cycloaddition reactions Cinnamates are known to undergo light induced [2 + 2] cycloaddition reactions. The application of this chemistry to polymer science are mainly based on polymer modifications and crosslinked network formation [27,28]. However, Hasegawa et. al. expanded the use of this chemistry to step-growth polymerizations by using specifically designed monomers bearing cinnamate units [29–31]. For example, it was demonstrated that irradiation of 2, 5-distyrylpyrazine leads to a crystalline linear high polymer by following a photocyclopolymerization as shown below (Scheme 2). Typical light induced [2 + 2] cycloadditions can also be achieved by using dibenzazepines, which can also be used for a variety of polymer synthesis [32,33]. Yagci and co-workers applied this chemistry to step growth polymerization by using specific monomers Light-induced
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Scheme 3. Step-growth polymerization by light induced [2 + 2] cycloaddition using dibenzazepine derivatives.
Scheme 4. Step-growth polymerization by light induced [4 + 2] cycloaddition using AB-type monomers.
having internal bisphenol-A unit and terminal dibenzazepines as shown below (Scheme 3) [34]. More recently, Barner-Kowollik and co-workers demonstrated specially designed AB-type photomonomers can undergo a light induced [4 + 2] Diels-Alder cycloaddition reaction. The mechanism follows a keto-enol tautomerization triggered by light irradiation producing active monomers bearing both the diene and dienophile functionalities required for the reaction, and thus polymerization [35]. The overall polymerization mechanism is demonstrated below (Scheme 4). Similar Diels-Alder strategy was also applied for the synthesis of well-defined high molar mass segmented copolymers, employing a unique combination of step-growth and reversible addition–fragmentation chain transfer polymerizations [36]. Synthesis of step-growth polymers by light induced reactions was also demonstrated on the example of [4 + 4] cycloaddition reactions using anthracene derivatives [37]. For this purpose, bisanthracene functionalized monomer involving perfluorophenyl ester moiety as internal functionality was prepared and polymerized under UV irradiation. The corresponding polymer was functionalized by simple amidation reactions for the preparation of polyesters having specific moieties at the side chain as shown below (Scheme 5). 2.2. Step-growth polymerizations by ketene-ol addition The ketene functional group was first discovered in 1905 by Hermann Staudinger, who is widely considered to be the father of polymer chemistry. He succeeded in synthesizing and characterizing the first ketene, which served as the basis of his pioneering Please cite this article as: G. Yilmaz and Y. https://doi.org/10.1016/j.progpolymsci.2019.101178
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work on its polymerization. Since then, ketene chemistry has been neglected over a century of time. Very recently though, ketene chemistry has been revitalized as a useful tool for synthetic polymer chemistry. New developments in light induced step-growth polymerizations are not limited to the application of pericyclic reactions as exemplified above. Dioxinones are known to yield ketene intermediates upon light irradiation or heating depending on the structures. These reactive intermediates have recently been used for polymerizations as well as modifications [38–41]. Typical example concerns synthesis of a benzodioxinone derivative bearing hydroxyl functions and its use as bifunctional monomer. Under irradiation at proper wavelengths, this monomer releases benzophenone and concomitantly generated ketene groups readily react with the hydroxyl group of another monomer (ketene-ol addition) to yield macromolecules as shown in Scheme 6 [42]. Due to the shielding effect of the benzophenone released, the molecular weight increase is restricted to certain degrees, but a great deal of research has still been conducted for enhancement. 2.3. Step-growth polymers by click reactions Click reactions are modular and versatile reactions, which can be realized under mild reaction conditions with almost quantitative efficiencies [43–45]. In addition to Diels-Alder reactions that considers the reaction of a diene with a dienophile, copper catalyzed azide-alkyne cycloaddition (CuAAC) and thiol-ene reactions are also honored as click reactions [46,47]. They have been used for a number of synthetic applications including preparation of complex macromolecular architectures including block, graft, Light-induced
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Scheme 5. Step-growth polymerization by light induced [4 + 4] cycloaddition using monomers having bis-anthtracene functionality.
Scheme 6. Step-growth polymerization by light induced ketene-ol addition using benzodioxinone derivatives.
Scheme 7. Photoinduced CuAAC and thiol-ene reactions.
star copolymers and telechelic polymers [48–51]. Both CuAAC and thiol-ene reactions can be utilized under light irradiation, which have positive impact on the processes by means of reducing air sensitivity and catalyst type used [52]. Traditional CuAAC reactions require Cu(I) catalyst, which is air sensitive. On the other hand, light induced CuAAC are realized in the presence of Cu(II), which is simultaneously reduced to Cu(I) by the aid of photochemical reactions. Thiol-ene reactions can also be used together with a radical photoinitiator (PI), which enables the generation of thiyl radicals responsible for addition step (Scheme 7) [21,53,54]. In general, they have been used for the preparation of crosslinked networks by step-growth polymerization. For this purpose, multifunctional azides and acetylenes or thiols and alkenes are used in CuAAC and thiol-ene reactions, respectively [55–57]. Linear polymers by thiol-ene reactions are limited to oligomeric materials due to the lower efficiency of this reactions in high molar mass compounds [58,59]. Notably, disulfide oligomers were also shown to be polymerizable under UV irradiation by self-photopolymerization [60]. 2.4. Step-growth polymers by nitrile imine-mediated tetrazole-ene cycloaddition Nitrile Imine-Mediated Tetrazole-Ene Cycloaddition (NITEC) reactions are established fast and catalyst free reactions where even non-reactive alkenes can be used reactants. It has recently been used for polymer modifications and ligations. In a recent study, bismaleimide linker was reacted with bistetrazole compounds in Please cite this article as: G. Yilmaz and Y. https://doi.org/10.1016/j.progpolymsci.2019.101178
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an efficient manner to produce step-growth polymers containing pyrazoline groups capable of fluorescent emission (Scheme 8). Thus, the fluorescence of the step-growth polymers is directly correlated with the number of ligation points in the polymer, forming an ideal self-reporting sensor system. The internal RAFT agent group was also utilized for the initiation of styrene, which acted as a macromonomer for typical polycondensation [61]. 2.5. Step-growth polymers by Passerini reaction Step growth polymers can also be attained by certain specific reactions. In a very recent study, Kowollik and co-workers demonstrated the possibility of obtaining high molar mass chains by additive-free visible light induced Passerini reaction [62]. In place of classical aldehydes (or ketones), highly reactive, in-situ photogenerated thioaldehydes are exploited along with isocyanides and carboxylic acids. Applying optimum reaction conditions to eliminate side reactions, it was shown that step-growth polymers can be easily prepared under mild reaction conditions, which mechanistically follows a series of addition and rearrangement steps (Scheme 9). 2.6. Step-growth polymers by electron transfer and radical coupling reactions Light induced radical coupling processes have also been employed for step-growth polymerizations. Initial studies from our laboratory showed that polythiophenes can be synthesized by Light-induced
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Scheme 8. Step-growth polymerization by light induced nitrile imine-mediated tetrazole-ene cycloaddition.
photochemical means through a mechanism similar to electropolymerization. It is well established that upon direct irradiation, onium salts undergo decomposition as presented below on the example of diphenyliodonium salt [63]. The iodophenyl radical cations (PhI+. ) thus formed undergo single electron transfer reaction with thiophene to form thiophene radical cation. Successive proton release and coupling reactions of the monomer and oligomers essentially leads to the formation of polythiophene (Scheme 10). Apart from diphenyliodonium salts, other onium salts such as triphenylsulfonium and N-alkoxy pyridinium salts were found to initiate the polymerization. However, laser flash photolysis and electron paramagnetic resonance studies revealed that iodonium salts were the most efficient initiators for the described photoinduced electron transfer reactions. The rate constants for the reactions of the corresponding radical cations (Ph2 I+. and Ph3 S+. ) with thiophene were found to be 1.26 × 1010 and 1.7 × 105 M-1 s-1 , respectively. The difference in rate constants can be attributed to the redox potentials of the onium salts, which is in favor of Ph2 I+ PF6 - [64].
Scheme 10. Photoinduced step-growth polymerization of thiophene.
Although both electro and photo polymerizations follow an oxidative mechanism, the morphology of the obtained polymers are different. While electropolymerizations produce polymers with cauliflower like morphologies, polymers from photopolymerizations exhibit wrinkled morphology as a result of favorable chain growth (Fig. 1). The more ordered morphology in the case of photoinduced polymerization can be explained by the difference in the mechanism of polymer growth [63]. In order to extend this approach to less harmful long wavelengths, sensitized systems were reported. Certain polynuclear aromatic compounds (PACs) are known to undergo an electron transfer reaction with onium salts under light irradiation where the onium salt is transparent [65]. Upon irradiation, excited state
Scheme 9. Step-growth polymerization by light induced Passerini reaction.
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Fig. 1. SEM images of polythiophene obtained by electropolymerization (a) and photoinduced step-growth polymerization (b). [64], Copyright 2005. Reproduced with permission from John Wiley and Sons).
Scheme 11. Photoinduced electron transfer from anthracene to diphenyliodonium hexafluorophosphate.
PACs form excited state complexes (exciplexes) with the onium salt. There is a single electron transfer from the PAC of high electron density to the onium salt of high electron affinity to generate highly reactive species as depicted in Scheme 11 on the example of anthracene and diphenyliodonium hexafluorophosphate. This process is generally used for photoinitiated cationic polymerizations where the PACs act as sensitizers [66–75]. Such long wavelength induced electron transfer reactions were also utilized for stepgrowth polymerization, which considers the use of the conjugated structures as monomers as discussed below. Recently, it has been shown that conjugated thiophene derivatives can be used as sensitizers for the cationic polymerizations of epoxides and vinyl ethers [50–52]. In a following study, certain conjugated thiophene derivatives were demonstrated to form polymers when used together with equimolar concentrations of iodonium ions [76]. Detailed mechanistic analyses reveal that after the electron transfer step, two equivalents of radical cations thiophene derivative couple each other while releasing Bronsted acid to give its dimeric form. Following analogous steps of photoinduced electron transfer and radical coupling reactions, polythiophenes are eventually formed (Scheme 12). Due to the extension of conjugation during polymerization, there is a clear color change, which was also observable by UV analysis (Fig. 2). Yagci and co-workers recently applied similar strategy to polymerize N-ethylcarbazole [77]. There has been a great deal of interest in the synthesis of conjugated polymers, which find various practical applications especially in electronics and optic devices. Main chain polycarbazoles are generally prepared by coupling reactions, and the polymer growth can be controlled by functionalization of the possible reactive sites on the aromatic rings. In the aforementioned work, it was shown that commer-
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Fig. 2. UV–vis spectral change during light induced step-growth polymerization of the thiophene derivative. [77], Copyright 2009. Reproduced with permission from the Royal Society of Chemistry.
cially available N-ethylcarbazole was readily polymerized in the presence of diphenyliodonium hexafluorophosphate under light irradiation absorbed only by the monomer. Consequently, poly(Nethylcarbazole) (PEC) is formed with a similar mechanism as discussed above (Scheme 13). Light-induced
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Scheme 12. Photoinduced step-growth polymerization of highly conjugated thiophene derivatives.
Scheme 13. Photoinduced step-growth polymerization of N-ethylcarbazole.
Despite having reactive sites both in 2,3 and 5,6 positions and there are multiple possibilities of macromolecular growth, detailed analyses show that the polymerization proceeds from the 3,6 positions of the monomer. When the atomic force microscopy of the polymer films both after doping and dedoping on the glass surface are investigated, the polymers were found to exhibit agglomerated and partly uniform grains arising from helical 3,6-linkage between carbazole moieties of the polymer backbone. The grain growth occurs with changing the orientation of carbazole moieties on the polymer backbone. Notably, before dedoping, polymer films
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exhibit uniform structure due to PF6 - ions, which have ion-dipole attractions with the glass surface. As expected, the dedoping process gives rise to the formation of more perforated surfaces due the absence of the above-mentioned attractions (Fig. 3). In a recent study from our laboratory, it was shown that PEC can be photochemically prepared without monomer and additional oxidant. Phenacyl ethylcarbazolium salt, originally used as photoinitiator for free radical and cationic polymerization [78], undergoes photoinduced decomposition to form PEC (Scheme 14). Similar proton release and coupling reactions pro-
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Fig. 3. AFM images of PEC thin film on the glass surface before (a) and after dedoping process (b). [78], Copyright 2015. Reproduced with permission from the American Chemical Society.
Scheme 14. Photoinduced synthesis of poly(N-ethylcarbazole) by using phenacylium carbazole salt.
posed for the above mentioned system were shown to be operative. [79] 2.7. Step-growth polymers by atom transfer radical addition reactions Iodine transfer radical polymerizations have recently attracted increasing interest as reflected by recent publications [80–82]. The photosensitivity of alkyl iodides provides the possibility of generation of alkyl radicals under visible light irradiation in a reversible Please cite this article as: G. Yilmaz and Y. https://doi.org/10.1016/j.progpolymsci.2019.101178
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manner. In the presence of vinyl monomers, atom transfer radical polymerization was shown to be operative even in the absence of catalysts. This unique chemistry has been adapted to light induced step growth polymerization, which follows atom transfer radical addition mechanism. To stabilize the iodide radical and prevent the formation of elemental iodine, generally ruthenium catalysts and ascorbic acid are used. Xu et al. presented the use of this chemistry for the preparation of step-growth polymers by reacting difunctional alkenes with diiodides [83]. This typical polymerization was
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Scheme 15. Synthesis of step-growth polymers by photoinduced step transferaddition and radical-termination polymerization.
named as step transfer-addition and radical-termination (START) (Scheme 15). Another detailed study demonstrated the positive effect of water contamination on the START polymerization [84].
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ally photoinduced processes has been applied for systems involving Mn2 (CO)10 . In a more recent study, we have reported a facile visible light induced synthetic method for preparing a series of conventional polycondensates, namely polyesters, polyurethanes and polyamides. For this purpose, bisbenzylbromide compounds possessing ester, urethane and amide groups in the inner structure were synthesized and employed as monomers. Irradiation of these monomers in the visible range using Mn2 (CO)10 as a radical generator by halide abstraction and subsequent radical coupling reactions yielded the corresponding polycondensates (Scheme 16) [100]. 2.9. Other potential strategies
2.8. Step-growth polymers by atom transfer radical coupling reactions The first synthetic utility of atom transfer radical coupling (ATRC) processes was reported by Yagci and co-workers for the preparation of telechelic polymers [85]. Following this report, ATRC was further used for polymer synthesis as well as modifications [86–93]. It appeared that such coupling reactions can be realized through manganese decacarbonyl (Mn2 (CO)10 ) photochemistry which eliminates the use of copper catalyst. Mn2 (CO)10 chemistry has been utilized for various polymerizations, polymer modifications and coupling processes [23,94–99]. Although thermal induction was shown to be also effective, gener-
Although not demonstrated for step-growth polymerization, photoinduced decomposition of N-alkoxy pyridinium salts offers an alternative route to the formation of polyurethanes and polyesters. Photolysis of pyridinium salts in the presence of hydrogen donor solvents leads to the formation of hydroxyl groups according to Scheme 17. Irradiation of bifunctional alkoxy pyridinium salts in the presence of selected diacidchlorides and diisocyanates would lead to the formation of polyesters and polyurethanes, respectively. The possibility of such step-growth polymerization was demonstrated by the end group functionalization of pyridinium ion terminated polymers via photoinduced urethane formation [101].
Scheme 16. Synthesis of step-growth polymers by light-induced radical coupling using Mn2 (CO)10 .
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Scheme 17. Photoinduced formation of hydroxyl functional molecules.
3. Conclusion Step-growth polymers gain a substantial research interest as they hold a large margin of polymer manufacture since the beginning of the synthetic polymer era. Therefore, there has been a revitalized interest in developing new synthetic strategies for the preparation of step-growth polymers. After their distinctive advantages have been realized, light induced processes have been adapted to almost all kinds of polymerizations including stepgrowth polymerizations. In this article, we have discussed the recent studies on step-growth polymerizations by photochemical means. The advantages of these particular approaches compare to the conventional strategies have also been discussed. The research field of photoinduced step-growth polymerization has a very bright future due to the ease of use combined with its less harmful and energetically favorable conditionals, and the possibility of spatiotemporal control. Declaration of interests None. Acknowledgements The authors thank Istanbul Technical University for financial support. References [1] Kricheldorf HR. Polycondensation history and new results. Berlin: Springer-Verlag Gmb H; 2016, 291 pp. [2] Rempp P, Merrill EW. Polymer synthesis. Basel: Hüthig & Wepf; 1991, 344 pp. [3] Crespy D, Bozonnet M, Meier M. 100 years of bakelite, the material of a 1000 uses. Angew Chem Int Ed 2008;47:3322–8. [4] Hirano K, Asami M. Phenolic resins-100 years of progress and their future. React Funct Polym 2013;73:256–69. [5] Seymour RB. Pioneers in polymer science. Dordrecht: Kluwer Academic Publishers; 1989, 272 pp. [6] Yokoyama A, Yokozawa T. Converting step-growth to chain-growth condensation polymerization. Macromolecules 2007;40:4093–101. [7] Yokozawa T, Yokoyama A. Chain-growth condensation polymerization for the synthesis of well-defined condensation polymers and -conjugated polymers. Chem Rev 2009;109:5595–619. [8] Mizutani M, Satoh K, Kamigaito M. Metal-catalyzed radical polyaddition for aliphatic polyesters via evolution of atom transfer radical addition into step-growth polymerization. Macromolecules 2009;42:472–80. [9] Mizutani M, Satoh K, Kamigaito M. Metal-catalyzed simultaneous chainand step-growth radical polymerization: marriage of vinyl polymers and polyesters. J Am Chem Soc 2010;132:7498–507. [10] Mizutani M, Satoh K, Kamigaito M. Construction of vinyl polymer and polyester or polyamide units in a single polymer chain via metal-catalyzed simultaneous chain- and step-growth radical polymerization of various monomers. Aust J Chem 2014;67:544–54. [11] Satoh K, Ishizuka K, Hamada T, Handa M, Abe T, Ozawa S, et al. Construction of sequence-regulated vinyl copolymers via iterative single vinyl monomer additions and subsequent metal-catalyzed step-growth radical polymerization. Macromolecules 2019;52:3327–41. [12] Satoh K, Ozawa S, Mizutani M, Nagai K, Kamigaito M. Sequence-regulated vinyl copolymers by metal-catalysed step-growth radical polymerization. Nat Commun 2010;1:1–6. [13] Yagci Y, Jockusch S, Turro NJ. Photoinitiated polymerization: advances, challenges, and opportunities. Macromolecules 2010;43:6245–60.
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