Exploring the Graft-To Synthesis of All-Conjugated Comb Copolymers

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Exploring the Graft-To Synthesis of All-Conjugated Comb Copolymers Using Azide−Alkyne Click Chemistry Nimrat K. Obhi, Denise M. Peda, Emily L. Kynaston, and Dwight S. Seferos* Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada S Supporting Information *

ABSTRACT: Copolymers with graft architectures possess interesting material properties distinct from their linear polymer counterparts. The effects of multidimensional architectures on the optoelectronic and physical properties of all-conjugated graft copolymers are not wellknown, thus providing a large incentive for their study. In order to readily access these materials (hypothesized to have “comb” architectures), it is extremely important to investigate the methods used in their synthesis. Here we study the graft-to synthesis of comb copolymers composed of polythiophene backbones and polyselenophene side chains and identify the opportunities and challenges associated with copolymer formation. Azide-functionalized polythiophene “backbones” and acetylene-terminated polyselenophene “side chains” were synthesized in a controlled fashion using Kumada catalyst-transfer polycondensation (KCTP) polymerization and grafted together using copper-catalyzed azide−alkyne click chemistry (CuAAC). 1 H NMR, GPC, and FTIR results confirm the attachment of polyselenophene side chains to the polythiophene backbone, resulting in comb copolymers with varying grafting densities. Low grafting density copolymers are readily synthesized using various backbone and side chain polymers. Midrange grafting density copolymers are more challenging but can be accessed when the availability of the graft sites on the polythiophene backbones is maximized. The synthesis of high grafting density combs remains challenging even when various modifications to the backbone and side chain polymers are implemented to improve the grafting efficiency. Problematic Glaser homocoupling of acetylene-terminated polyselenophenes was observed in certain conditions; however, this can be successfully prevented using an organic-soluble copper catalyst which is broadly applicable to many polymer−polymer CuAAC reactions. Ultimately, this investigation demonstrates a graft-to synthetic method that is useful for low- and midgrafting density all-conjugated comb copolymers, thus providing a means to further the study of these interesting multidimensional semiconducting materials.

■

INTRODUCTION Graft polymers are a subclass of materials with complex architectures, characterized by having multiple polymeric side chains appended to a polymeric backbone.1,2 The synthesis of graft copolymers presents an opportunity to study materials incorporating properties of two or more polymers that are functionally distinct from their linear copolymer relatives. Numerous examples of nonconjugated graft copolymers have been studied, with various applications ranging from the fundamentalstudies on mechanical and viscoelastic3−6 properties as well as phase separation and self-assembly7−11to the commercialstimuli-responsive materials,12,13 biomedical applications,14−17 and photonics.18−20 Three synthetic methods are used for their preparation: termed “graft-from”, “graft-through”, and “graft-to”.1 Graft-from methods involve the polymerization of side chains from an already-prepared polymeric backbone; graft-through methods utilize polymeric side chains attached to monomers that can be polymerized in order to obtain the backbone; and graft-to methods use prepared polymer side chains and polymer backbones which are coupled to form the final graft copolymer. The material properties of graft copolymers heavily depend on their grafting density, defined as the percentage of polymeric © XXXX American Chemical Society

side chains per polymer backbone. Variations in the synthetic procedures of these grafting methods can be employed to generate copolymers with well-defined grafting densities.2 Conjugated organic polymers are both interesting and feasible to study from a complex architecture standpoint. The combination of their optoelectronic properties and solution processability has allowed for their widespread use in electronic devices such as organic field-effect transistors (OFETs),21 organic light-emitting diodes (OLEDs),21−23 and organic photovoltaics (OPVs).24−26 A large incentive for the study of complex all-conjugated copolymers is to observe the effect of introducing multidimensional architectures on their optoelectronic properties. In addition to this multidimensionality, the overall increased rigidity of conjugated polymers compared to nonconjugated polymers is hypothesized to affect the material properties of the resulting graft copolymers. With this in mind, it is important that the synthesis of these copolymers is studied. Currently, and in contrast to nonconjugated graft copolymers, examples of the synthesis of all-conjugated graft Received: January 22, 2018 Revised: March 27, 2018

A

DOI: 10.1021/acs.macromol.8b00138 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Scheme 1. Synthesis of (a) Polythiophene Backbone Copolymers and (b) Polyselenophene Side Chains Using Kumada Catalyst-Transfer Polycondensation

Figure 1. MALDI-TOF MS spectra showing single distribution of end groups in the polymer side chain sample synthesized using external initiation, in contrast to small percentages of dicapped polymer present in nonexternally initiated samples.

systems.32−35 We favored the use of copper over ruthenium as a catalyst due to its selectivity in forming 1,4-triazoles instead of the sterically encumbered 1,5-triazoles formed by ruthenium.36 We used polymer backbones and side chains polymerizable in a controlled fashion using KCTP, allowing for installation of click-active azide and acetylene groups. Poly(3-alkylthiophene)s (P3AT) and poly(3-alkylselenophene)s (P3AS) have been studied by our group37−43 and by others,44−54 and so represented suitable materials for these polymers. We used azide-functionalized P3AT for the polymer backbones and acetylene-capped P3AS for the polymer side chains. P3AT and P3AS display distinct aromatic proton resonances from one another by proton nuclear magnetic resonance (1H NMR) spectroscopy, allowing for comparative analysis of the resulting copolymers. We hypothesized that the rigid nature of the P3AT−P3AS graft copolymers will force them to adopt “comb” architectures; henceforth, we refer to these graft copolymers as comb copolymers. In this report, we describe the challenging graft-to synthesis of a variety of comb copolymers, ultimately contributing to the understanding of the formation of multidimensionally architectured all-conjugated materials.

copolymers are rare. Two studies have been published using the graft-from27 and graft-through28 synthetic methods, resulting in the successful synthesis of all-conjugated donor−acceptor graft copolymers. The graft-to method is especially attractive for the study of all-conjugated graft copolymers, as well-defined pregraft conjugated polymers can be synthesized in a controlled fashion using pseudoliving Kumada catalyst-transfer polycondensation (KCTP) polymerizations.29,30 The first example of a graft-to method has recently been employed by Steverlynck et al.,31 where copolymers were synthesized with a chiral poly(phenyleneethynylene) (PPE) backbone and poly(3hexylthiophene) (P3HT) side chains grafted together using the copper-catalyzed azide−alkyne click chemistry (CuAAC) reaction. We noticed that the synthesis of these copolymers appeared to be challenging, where high grafting density copolymers were not obtained.31 Since understanding the synthesis of all-conjugated graft copolymers is vitally important to future studies on their material properties, more work is thus necessary to better understand these graft-to synthetic methods. To that end, the focus of this investigation is to study the efficacy of graft-to methods in synthesizing all-conjugated graft copolymers. We chose to use the copper-catalyzed azide− alkyne click (CuAAC) reaction as the grafting-to reaction, as it has been previously studied in the context of other copolymer B


Article

Macromolecules Scheme 2. General Scheme for Control Click Reactions Using Small Molecule 4-Ethynyl-α,α,α-trifluorotoluene

Scheme 3. CuAAC Click Reactions To Form Comb Copolymers: (a) First-Generation Combs, (b) Second-Generation Combs, and (c) Third-Generation Combs

■

o-tolyl(Ni(dppe)Cl)51,57 and quenched using ethynylmagnesium bromide to obtain side chains with only one distribution of o-tolyl/acetylene end groups by MALDI-TOF MS (Figure 1). Test Click Reactions. Two test reactions were performed to assess the click chemistry reaction conditions. In the first test reaction, we observed that the CuAAC click conditions did not affect the integrity of the azide groups on the polythiophene backbone, thus eliminating the possibility of azide crosslinking58 during the comb reactions. Specifically, a 14.5 kDa polythiophene backbone with 53% N3 (B1, Scheme 1) was subjected to the CuAAC click conditions in the absence of any alkyne-functionalized molecules. We used the CuAAC conditions most commonly used for polymer−polymer click reactions, involving the use of Cu(I)Br in conjunction with amine ligand PMDETA with the application of heat.59 There was no difference in the integration of the backbone −CH2N3 protons by 1H NMR before and after subjection to these conditions. In addition, the N3 stretch at 2100 cm−1 by FTIR was unchanged before and after the reaction, confirming that the azide groups remain intact in the presence of Cu(I)Br/ PMDETA (Figure S2). The second test reaction was performed to assess the efficiency of the grafting-to click procedure using small molecule alkyne 4-ethynyl-α,α,α-trifluorotoluene (ETFT) (Scheme 2). Here we obtained 100% grafting of ETFT to the 53% N3 backbone using the same CuAAC conditions described above. Here, the backbone (B1) was treated with a stoichiometric amount of the small molecule and 1.2 equiv of Cu(I)Br/PMDETA relative to the percentage of azide groups. The 1H NMR spectrum of the backbone copolymer after the reaction shows the absence of the peak corresponding to the backbone −CH2N3 protons, suggesting that complete con-

RESULTS AND DISCUSSION Starting Material Synthesis. The polythiophene backbones and polyselenophene side chains used in this investigation were synthesized using KCTP. This pseudoliving polymerization technique allows one to functionalize the polymers with click-active azide and acetylene functional groups (Scheme 1). The polythiophene backbones were synthesized from a copolymerization of 2,5-dibromo-3-(6bromohexyl)thiophene and 2,5-dibromo-3-hexylthiophene (Scheme 1a) according to literature procedure.55 Postpolymerization modification by displacement with NaN3 yielded the azide-functionalized copolymers B1−B3 in quantitative yield. Using this approach, we were able to synthesize a variety of polythiophene copolymers with varying azide-functionalized monomer ratios and Mn values (Table S1). The percentage of azide-functionalized monomers in the backbone copolymers was determined using 1H NMR by comparing the integration of the aromatic thiophene −C−H protons at 6.98 ppm to the alkyl −CH2N3 protons at 3.28 ppm (Figure S1). The polyselenophene side chains were also synthesized with a variety of Mn values using KCTP of either 2,5-dibromo-3-(2ethylhexyl)selenophene or 2,5-dibromo-3-hexylselenophene monomers prepared according to a literature procedure40 (Scheme 1b). Poly(3-(2-ethylhexyl)selenophene) (P3EHS) was terminated with an acetylene group by quenching with ethynylmagnesium bromide at the end of the polymerization.56 These polymers showed a major “monocapped” distribution of Br/acetylene end groups by MALDI-TOF MS; however, some “dicapped” acetylene/acetylene polymers were observed in minimal amounts (Figure 1 and Table S1). Poly(3hexylselenophene) (P3HS) was also externally initiated using C


Article

Macromolecules Table 1. Synthetic Conditions and Grafting Densities of All Polythiophene−Polyselenophene Comb Copolymers

a

reaction

polythiophene backbone

polyselenophene side chain

side chain equiv (to N3)

catalyst equiv (to N3)

ligand equiv (to N3)

time (days)

expected grafting density (%)

resultant grafting density (%)

reaction efficiencya (%)

Comb1a Comb1b Comb1c Comb2a Comb2b Comb2c Comb2d Comb3a Comb3b Comb3c

B1 B1 B1 B2 B2 B2 B2 B3 B3 B3

SC2 SC1 SC2 SC3 SC4 SC5 SC5 SC6 SC6 SC6

0.07 0.73 0.85 0.13 0.53 0.94 1.0 0.38 0.38 0.38

1.2 1.2 1.2 2.0 2.0 2.0 2.0 1.0 1.0 1.0

1.2 1.2 1.2 2.0 2.0 2.0 2.0 2.0 2.0 2.0

2 1 1 1 1 1 2 3 7 4

7 73 85 13 53 94 100 38 38 38

7 26 65 13 25 25 69 29 32 35

100 36 76 100 47 27 69 76 84 92

Calculated by dividing the resultant grafting density by the expected grafting density.

For the 26% grafted comb, the backbone −CH2N3 peak integration decreased from 1.06 to 0.78 after the click reaction, indicating that 26% of the azide sites are occupied by P3EHS side chains (Figure 2a). Additionally, the FTIR spectra for all

sumption of azide groups had occurred. The 1H NMR spectrum also shows the development of new aromatic protons corresponding to the small molecule. The 19F NMR spectrum of the backbone confirms the presence of the ETFT −CF3 fluorine atoms while FTIR spectra confirm complete azide consumption from the absence of the N3 stretch after the reaction (Figure S3). This confirms that complete attachment of the small molecule to the backbone had occurred, resulting in a 100% grafting densitydefined as 100% occupation of the azide sites on the backbone. These results demonstrate excellent efficiency of the grafting-to procedure using the Cu(I)Br/PMDETA click system with a small molecule and suggested that this grafting-to procedure would be appropriate for comb copolymer synthesis. Comb Copolymer Reactions. Three “generations” of comb copolymers using various polythiophene backbones and polyselenophene side chains were synthesized in this investigation (Scheme 3). With each generation, we observed that resultant grafting densities are heavily dependent on the reaction conditions and the percentage of azide sites on the polythiophene backbones. For each comb copolymer, the expected grafting densities were estimated based on the equivalents of polyselenophene side chains used in the click reactions (Table 1). The first generation of combs involved the graft-to synthesis of the 14.5 kDa 53% N3 polythiophene backbone (B1) and Br/acetylene-terminated 8.7 and 5.0 kDa P3EHS side chains (SC1 and SC2). We used polyselenophene side chains of approximately one-third to one-half the Mn of the polythiophene backbone to minimize sterics during the click reaction. Polyselenophene polymers with branched 2-ethylhexyl alkyl chains were used to maximize the solubility of the side chains in solution and so better facilitate the click reaction. Following the success of our test reaction with 100% grafting of ETFT, we anticipated that the use of the same CuAAC click conditions should yield high grafting density comb copolymers. Grafting P3EHS to polythiophene at low grafting densities occurs readily; however, high grafting densities are difficult to obtain. Three different combs were targeted with expected grafting densities of 7%, 73%, and 85% and were synthesized with resultant grafting densities of 7%, 26%, and 65%, respectively (Comb1a, Comb1b, and Comb1c), where a distinct difference between the expected and resultant grafting densities is observed for higher P3EHS side chain loadings. The grafting densities of all the comb copolymers were calculated by comparing the difference in 1H NMR integrations of the backbone −CH2N3 protons before and after the click reaction.

Figure 2. (a) 1H NMR spectrum of the 26% grafting density comb showing calculation of grafting densities according to relative proton integrations. (b) Normalized GPC spectrum of the 26% grafting density comb showing the formation of high molecular weight material indicated by an arrow and side chain dimers indicated by an asterisk.

comb copolymers show a decrease in the N3 stretch at 2100 cm−1, further indicating consumption of azides and supporting the formation of grafted materials (Figure S4). Finally, the formation of high molecular weight material was also confirmed by GPC (Figure 2b). The high molecular weight peak does not correspond to either the backbone or the side chain, suggesting that combs are formed and elute at lower retention volumes. This high molecular weight comb copolymer peak was also D


Article

Macromolecules

Figure 3. GPC spectra of second-generation comb copolymers and starting materials. The presence of homocoupled side chain dimers is shown by an asterisk.

generation combs, which we hypothesized would decrease the possibility of grafted side chains obscuring free azide sites. Externally initiated monocapped P3HS side chains were used to prevent the possibility of cross-linking polythiophene backbones during comb formation. In addition, more equivalents of Cu(I)Br/PMDETA were used to increase the concentration of copper in the CuAAC reaction.63 Similar to first-generation comb copolymers, it was feasible to synthesize combs with low grafting densities and challenging to synthesize combs with high grafting densities. The 13% expected grafting density comb was synthesized with a matching resultant grafting density postreaction. The normalized GPC spectrum of the 13% grafting density comb shows a similar retention volume to the polythiophene backbone, with small amounts of free P3HS side chain present (Figure 3). The 13% grafting density comb was expected to have a similar hydrodynamic radius to the polythiophene backbone due to the small Mn of P3HS side chains, and this accounts for their similar retention volumes. Two 25% grafting density combs were synthesized, despite large changes in their expected grafting densities (53% and 94%, respectively) and displayed similar GPC profiles (Figure 3). In order to obtain higher grafting density products, one reaction was allowed to proceed for 2 days instead of 1 day. The increase in reaction time led to a higher grafting density of 69%, similar to the highest grafting density comb obtained in first-generation syntheses. Despite lowering the percentage of azide monomers from 53% to 32%, it appears that the synthesis of high grafting density combs remains difficult. Changing the identity of the alkyl side chain on the polyselenophenes from 2-ethylhexyl to n-hexyl did not directly improve the resultant grafting densities between firstand second-generation comb copolymers. In addition, it appears that alleviating steric hindrance around the attachment

observed for the 65% grafting density comb sample (Comb1c, Figure S5). Although the formation of comb material is ubiquitous, it appears that the use of medium-Mn, highly soluble P3EHS side chains did not allow for the formation of high grafting density combs. With higher side chain loading, click reactions are less efficient, as evident in the 26% and 65% resultant grafting density combs. We hypothesized that the spatial distance between azide sites on the 53% N3 polythiophene backbone is too small to allow for the relatively bulkier branched P3EHS side chains to attach fully in proximity to one another. We also hypothesized that P3EHS side chain dimers formed by coppermediated Glaser terminal alkyne homocoupling60 are competitively synthesized alongside the CuAAC reaction. These polymers are observable by GPC in both the comb copolymer and P3EHS side chain sample (Figure 2b). While the increased solubility of P3EHS is advantageous for solution reactivity, in this instance it hampers the purification of comb copolymers. The similar solubility of polythiophene backbones and P3EHS side chains prevented purification procedures such as Soxhlet extractions and repeated precipitations from completely isolating the comb copolymers (Figure 2b). The second-generation comb copolymers were synthesized to both address these limitations and investigate a broader range of grafting densities (Table 1). In the second-generation system, the percentage of azide units on the polythiophene backbone was decreased to 32% (B2) to increase the spatial distance between attachment sites, and lower Mn (1.6 kDa) polyselenophenes with linear alkyl chains (SC3−SC5) were used to mitigate the potential steric constraints of the previous graft-to procedure. Because of the quinoidal nature of polyselenophene,61,62 short-chain P3HS possesses more rodlike character than the 8.7 and 5.0 kDa P3EHS used in firstE


Article

Macromolecules

prevention of Glaser alkyne−alkyne homocoupling during the synthesis of these comb copolymers. To the best of our knowledge, two mechanisms for the homocoupling reaction have been studied. In the first, the alkyne−alkyne homocoupling proceeds in the presence of Cu(II), while more recently, a DFT-based mechanism has been proposed that indicates Glaser coupling is likely promoted by Cu(I) in the presence of oxygen.60 First- and second-generation copolymers were synthesized in air-free conditions; however, the insolubility of Cu(I)Br in THF may have prevented complete oxygen removal from the catalyst during freeze−pump−thaw. Oxygen coordination to Cu(I) also remains a possibility when the amine ligand is weakly bound to the Cu(I) complex. The tris(benzyltriazolylmethyl)amine (TBTA) ligand has been shown to strongly bind Cu(I) during the CuAAC reaction, preventing coordination to oxygen.64,65 Additionally, its use is compatible with [Cu(CH3CN)4]PF6−, which is advantageously THFsoluble,66 unlike Cu(I)Br. Postreaction, homocoupled P3HS side chain was not observed for any comb samples by GPC (Figure 3). This suggested that Glaser coupling of terminal acetylene groups on the P3HS side chains was successfully prevented under conditions that use [Cu(CH3CN)4]PF6−/ TBTA and that the possibility of competing reactions is eliminated. Purification of Comb Copolymers. The separation of comb copolymers from excess polyselenophene side chains remains a limitation of the graft-to synthetic method. Separation would likely be best facilitated using a chromatography method such as preparative gel-permeation chromatography (GPC) or preparative size-exclusion chromatography (SEC); however, these purification methods were unable to be conducted on the polythiophene−polyselenophene comb copolymers due to the high temperatures (140 °C) required for effective polymer separation. We recommend that starting polymer materials with excellent solubilities in organic solvents at room temperature are used in graft-to syntheses to facilitate the usage of preparative GPC/SEC as a purification method. For the polythiophene−polyselenophene comb copolymer samples, we investigated a variety of alternative purification methods attempting to separate the combs from excess polyselenophene side chains. In all cases, separation was not possible and resulted in excess polyselenophene side chains present in all final comb copolymer samples. We first attempted the chemical isolation of second- and third-generation comb copolymers using both azide-functionalized magnetic TurboBeads and azide-terminated short chain (Mn = 1000 Da) poly(ethylene glycol) (PEG-N3) during the click reaction. The addition of excess TurboBeads or PEG-N3 prior to the termination of the reaction was anticipated to react with any excess acetylene-terminated polyselenophene side chains, which could then be separated using either a magnet for the TurboBead−polyselenophene or a silica column for the PEG−polyselenophene. In all cases, neither the TurboBead nor the PEG-N3 addition appeared to react with excess acetylene-terminated polyselenophene, and thus excess side chains were not removed from the crude copolymer mixture. This rendered the chemical isolation of comb copolymer samples an ineffective method. Separation based on solubility is used routinely for the purification of conjugated polymers; therefore, we sought to apply this to the comb copolymer mixtures. In first-generation comb copolymers, solid−liquid Soxhlet extractions using methanol, acetone, hexanes, dichloromethane, tetrahydrofuran,

sites by decreasing the size of the polyselenophenes did not allow for higher resultant grafting densities. We next hypothesized that further increasing the spatial distance between attachment sites on the polythiophene backbone by dramatically lowering the percentage of azide monomers would improve the grafting efficiency of the polyselenophene side chains. Third-generation comb copolymers were synthesized with the intent of eliminating alkyne−alkyne homocoupling reactions and targeting midrange grafting densities (Table 1). In these syntheses, we obtained grafting densities that were consistent with our expected grafting densities. We further decreased the percentage of azide monomers in the polythiophene backbone from 32% N3 to 15% N3, while also increasing the Mn to 18.0 kDa (B3) in order to maximize the availability of the azide sites. We used externally initiated P3HS polymers with Mn values one-sixth of the Mn of the polythiophene backbone (3.1 kDa, SC6). Targeting midrange grafting densities of 38% was expected to provide more information about the limitations associated with the graft-to syntheses. The 29% resultant grafting density comb copolymer was relatively consistent with the 38% expected grafting density based on polyselenophene side chain loading. By repeating these conditions using longer reaction times, two other samples were synthesized with grafting densities (32% and 35%, respectively) that now approach the 38% expected grafting density (Comb3b and Comb3c, Table 1). The evolution of expected high molecular weight peaks by GPC was associated with successful comb formation for all third-generation samples (Figure 4). Despite the use of higher Mn polyselenophene side

Figure 4. Normalized GPC spectra of third-generation comb copolymers. The arrow indicates the formation of midrange grafting density comb copolymers. Any homocoupled side chain dimers would be expected to elute at retention volume marked by an asterisk.

chains, the grafting efficiencies improved compared to secondgeneration comb copolymers, suggesting that the grafting efficiency is not dependent on polyselenophene side chain length. These results suggest that successful comb formation is limited by the steric availability of azide graft sites to the approaching P3HS side chains. Consistent with other generations of comb copolymers, excess free P3HS side chains were still present in the comb samples and were unable to be removed using chemical separations, Soxhlet extractions, repeated precipitations, or dialysis. It is important to note that the use of a [Cu(CH3CN)4]PF6−/TBTA copper catalyst/ligand system is ideal for the F


Article

Macromolecules

catalyst system promotes Glaser coupling of terminal acetylene groups, which may be a competing reaction with comb formation by CuAAC. Glaser coupling can be successfully prevented by using organic-soluble catalyst/ligand system [Cu(CH3CN)4]PF6−/TBTA. Third-generation comb studies additionally showed that midrange resultant grafting densities were successfully achieved when a polythiophene copolymer with low azide monomer percentage (15%) was used as the backbone. The synthesis of high-grafting density combs remains challenging using graft-to procedures and appears limited by the steric availability of graft sites on the chosen backbone polymer. Isolation of comb copolymers is an additional limitation of the graft-to synthesis, and starting polymer materials should be chosen with a purification method in mind. Ongoing work involving graft-from and graft-to synthetic studies is expected to discern the best method for the efficient synthesis of high grafting density species. Ultimately, this investigation shows that the graft-to synthetic method is useful for the accession of low- and midgrafting density allconjugated comb copolymers, thus providing a means to further the study of these interesting multidimensional semiconducting materials.

and chloroform fractions were performed after the click reaction. These solvents and their ordering were chosen based on studies on the fractional separation of P3HT.67,68 In all polymer-soluble solvent fractions (hexanes to chloroform), comb copolymers eluted in mixed quantities with excess P3EHS side chain, preventing the effective isolation of the combs. We attributed this to the similar solubilities of the polythiophene backbone and P3EHS side chains in solution; however, for second-generation copolymers made using lesssoluble P3HS, Soxhlet extractions were also unable to result in comb isolation. In these samples, Soxhlet fractions (hexanes to chloroform) also contained a mixture of comb copolymers and excess P3HS side chains. Isolation of second- and thirdgeneration comb copolymers was additionally attempted using a repeated precipitation procedure. The addition of methanol or 1,2-dichloroethane to the crude polymer mixture dissolved in chloroform did not result in the selective precipitation of comb copolymer samples; instead, coprecipitation of P3HS side chains and comb copolymers occurred in all cases. From these experiments, purification methods based on solubility are observed to be unsuitable for isolation of these comb copolymer samples. Lastly, we sought to investigate purification experiments based on size exclusion. We attempted separation on secondand third-generation combs using dialysis under several conditions using a variety of cellulose pore sizes and solvent mixtures. Studies on cellulose swelling in dialysis tubing69 indicate that aqueous and polar organic solvents are most appropriate to fully open pores to allow for the diffusion of polymers. This was confirmed through our experiments with tetrahydrofuran and chloroform mixtures, as no polymer diffusion was observed even with larger dialysis tubing pore sizes (14 kDa cutoff). We attempted to use a 50:50 mixture of dimethyl sulfoxide to chloroform in order to facilitate cellulose swelling; however, this mixture was not polar enough and additionally caused comb copolymers and P3HS side chains to coprecipitate out of solution. We concluded that separation of these largely nonpolar copolymers using dialysis is not feasible as the solvent mixtures required for separation are incompatible with regenerated cellulose tubing.

■

EXPERIMENTAL SECTION

General Considerations. 3-(2-Ethylhexyl)selenophene,40 2,5dibromo-3-hexylselenophene,40 and o-tolyl[Ni(dppe)Cl]57 were synthesized as previously reported. The detailed synthesis and corresponding 1H NMR data for thiophene and selenophene monomers can be found in the Supporting Information along with the detailed synthesis of all backbone, side chain, and comb copolymer samples. TBTA was purchased from Click Chemistry Tools and used as received. All other reagents were purchased from Sigma-Aldrich and used without further purification. All dry solvents were obtained using an Inert Technology solvent purification system. All reactions were performed using standard air-free techniques on a Schlenk manifold supplied with Ar gas. Instrumentation. 1H NMR was performed using a Varian Mercury 400 (400 MHz) spectrometer, and 19F NMR was performed using a Bruker Avance III (400 MHz) spectrometer. GPC measurements were carried out using a Malvern Viscotek 350 HT-GPC system at 140 °C with 1,2,4-trichlorobenzene stabilized with butylated hydroxytoluene (1 mL/min) and calibrated with narrow dispersity polystyrene standards. FTIR measurements were obtained using a PerkinElmer Spectrum 100 FTIR spectrometer. MALDI-TOF MS was performed using a Bruker AutoFlex Speed MALDI-TOF MS, with polymer end-group analysis completed using PolyTools software. General Procedure A for the Synthesis of P3HT-co-P3H(Br)T. 2,5-Dibromo-3-hexylthiophene and 2,5-dibromo-3-(6-bromohexyl)thiophene were loaded into a Schlenk flask and placed under vacuum for 20 min. Dry THF (0.15 M) was added under Ar, followed by dropwise addition of i-PrMgCl (1.0 equiv) and stirred for 1 h at room temperature. The solution was added all at once to a separate Schlenk flask containing Ni(dppp)Cl2 and stirred at room temperature for 20 min. The reaction mixture was quenched with 5% HCl, precipitated in methanol, filtered through a Soxhlet thimble, and extracted with methanol, hexanes and chloroform. The chloroform fraction was concentrated under reduced pressure to yield the polymer as a purple solid. 1H NMR in CDCl3: δ (ppm) = 6.99 (s), 3.43 (t), 2.81 (m), 1.90 (m), 1.73 (m), 1.50 (m), 1.37 (m), 0.93 (m). General Procedure B for the Synthesis of P3HT-co-P3H(N3)T. P3HT-co-P3H(Br)T was dissolved in dry THF (0.02 M) under Ar. A blast shield was set up and NaN3 (10 equiv to mmol Br), a catalytic amount of NaI, and phase-transfer agent TBAC or TBATFB (1.0 equiv to mmol polymer) were added all in one portion. The mixture was refluxed under Ar at 70 °C for 16−24 h and then precipitated in methanol. The purple solid was filtered into a Soxhlet thimble and extracted with methanol and chloroform. The chloroform fraction was

■

CONCLUSIONS In conclusion, we have employed a variety of conditions to assess the use of graft-to procedures for the formation of allconjugated comb copolymers. A library of well-defined polythiophene backbones and polyselenophene side chains with click-active azide and acetylene functional groups was synthesized using KCTP. Control test reactions showed excellent azide group tolerance to CuAAC click conditions, and successful synthesis of a 100% graft copolymer using a polythiophene backbone and small molecule ETFT was also performed under these conditions. Three generations of allconjugated comb copolymers using polyselenophene side chains were synthesized under a variety of conditions, where studies on each generation contribute to our understanding of the nature of these graft-to syntheses. First- and secondgeneration comb studies confirmed that low grafting density combs are readily synthesized, but high grafting densities are challenging to achieve. This is true even when the percentage of graft (azide) sites on the polythiophene backbone is reduced from 53% to 32% and when short (ca. 1.6 kDa) polyselenophene side chains are used to minimize steric interactions. The use of insoluble Cu(I)Br/PMDETA as a G


Macromolecules

■

reduced under concentrated pressure to yield the polymer as a purple solid. 1H NMR in CDCl3: δ (ppm) = 6.97 (s), 3.27 (t), 2.81 (s), 1.71 (m), 1.64 (m), 1.46 (m), 1.35 (m), 0.91 (m). General Procedure C for the Synthesis of AcetyleneTerminated P3EHS. 2,5-Dibromo-3-(2-ethylhexyl)selenophene was evacuated under vacuum for 20 min. Dry THF (0.14 M) was added under Ar, followed by the dropwise addition of i-PrMgCl (1.0 equiv) at room temperature. The monomer solution was stirred for 1 h and then injected all at once into a separate flask containing Ni(dppe)Cl2. The polymer solution was stirred for 15 min at room temperature, and then ethynylmagnesium bromide (15 mol %) was injected all at once and stirred for another 5 min. The mixture was then precipitated into methanol, filtered through a Soxhlet thimble, and washed at room temperature with methanol, hexanes, and chloroform. The hexanes fraction was concentrated under reduced pressure to yield the polymer as a dark red solid. 1H NMR in CDCl3: δ (ppm) = 7.09 (s, 1H), 3.67 (s, alkyne H), 2.67 (m, 2H), 1.70 (m, 1H), 1.31 (m, 8H), 0.88 (t, 6H). General Procedure D for the Synthesis of AcetyleneTerminated o-Tolyl(P3HS). 2,5-Dibromo-3-hexylselenophene was evacuated under vacuum for 20 min. Dry THF (0.15 M) was added under Ar, followed by the dropwise addition of i-PrMgCl (1.0 equiv) at room temperature. The monomer solution was stirred for 1 h. At the same time, o-tolyl[Ni(dppe)Cl] was evacuated under vacuum for 1 h in a separate flask. The activated monomer solution was injected into the catalyst flask and stirred for 15 min at room temperature; then excess ethynylmagnesium bromide was injected all at once and stirred for another 5 min. The mixture was then precipitated into methanol, filtered through a Soxhlet thimble, and washed at room temperature with methanol, hexanes, and chloroform. The hexanes or chloroform fraction was concentrated under reduced pressure to yield the polymer as a purple solid. 1H NMR in CDCl3: δ (ppm) = 7.11 (s, 1H), 3.69 (s, alkyne −H), 2.74 (m, 2H), 2.48 (s, o-tolyl −CH3), 1.68 (m, 2H), 1.56 (m, 2H), 1.33 (m, 2H), 0.90 (t, 3H). MALDI-TOF MS indicated repeat unit weight of 213.2 with 100% o-tolyl/acetylene end groups for all polymers. General Procedure E for Cu(I)Br Click Reactions. Polythiophene backbone and polyselenophene side chain were dissolved in dry THF and added to a flame-dried Schlenk flask under Ar. A stock solution of Cu(I)Br and PMDETA in dry THF was injected into the reaction flask, after which the reaction mixture was degassed using freeze−pump−thaw for three cycles. The reaction flask was stirred at 50 °C under Ar for 24−48 h before addition of deoxygenated azidefunctionalized (0.1 mmol N3/1 g beads) TurboBeads (1.0 equiv relative to polyselenophene, Comb2 samples only) and reacting for another 100 min before cooling. The cooled mixture was decanted and passed through a neutral alumina column to remove residual metal traces, concentrated, precipitated into MeOH, and purified by Soxhlet extraction. The 1H NMR spectra of Comb1a−c and Comb2a−d are provided in the Supporting Information. General Procedure F for [Cu(CH3CN)4]PF6− Click Reactions. Polythiophene backbone, polyselenophene side chain, TBTA, and DIPEA were added to a flame-dried Schlenk flask and evacuated under vacuum for 20 min. Dry THF was added under Ar, and the mixture was degassed by freeze−pump−thaw over three cycles. At the same time, [Cu(CH3CN)4]PF6− was evacuated under vacuum for 1 h in a separate flame-dried bomb flask. The bomb flask was backfilled with Ar and the degassed polymer mixture was added. The reaction flask was sealed under Ar and stirred at 50 °C for 3−7 days before addition of deoxygenated azide-functionalized PEG (Mn = 1.0 kDa, 1.0 equiv relative to polyselenophene) and reacting for another 24 h before cooling. The cooled mixture was passed through a neutral alumina column to remove residual metal traces, concentrated, precipitated into MeOH, and filtered. The solid was washed at room temperature with MeOH and CHCl3. The 1H NMR spectra of Comb3a−c are provided in the Supporting Information.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00138. Detailed syntheses and corresponding 1H NMR data for thiophene and selenophene monomers, detailed synthesis of all backbone, side chain, and comb copolymer samples, supporting table and additional figures, and 1H NMR spectra for all comb samples (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.S.S.). ORCID

Dwight S. Seferos: 0000-0001-8742-8058 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

This work was supported by the Natural Sciences and Engineering Research Council of Canada, the Ontario Research Fund, and the Canadian Foundation for Innovation. N.K.O. is grateful to the Tyler Lewis Clean Energy Foundation for the TLCERF Grant and the Natural Sciences and Engineering Research Council of Canada for NSERC PGS-D. We thank Prof. Mark Taylor for use of his FTIR instrument.

(1) Feng, C.; Li, Y.; Yang, D.; Hu, J.; Zhang, X.; Huang, X. WellDefined Graft Copolymers: From Controlled Synthesis to Multipurpose Applications. Chem. Soc. Rev. 2011, 40, 1282−1295. (2) Polymeropoulos, G.; Zapsas, G.; Ntetsikas, K.; Bilalis, P.; Gnanou, Y.; Hadjichristidis, N. 50th Anniversary Perspective: Polymers with Complex Architectures. Macromolecules 2017, 50, 1253. (3) Verduzco, R.; Li, X.; Pesek, S. L.; Stein, G. E. Structure, Function, Self-Assembly, and Applications of Bottlebrush Copolymers. Chem. Soc. Rev. 2015, 44, 2405−2420. (4) Neugebauer, D.; Zhang, Y.; Pakula, T.; Sheiko, S. S.; Matyjaszewski, K. Densely-Grafted and Double-Grafted PEO Brushes via ATRP. A Route to Soft Elastomers. Macromolecules 2003, 36, 6746−6755. (5) Pakula, T.; Zhang, Y.; Matyjaszewski, K.; Lee, H. il; Boerner, H.; Qin, S.; Berry, G. C. Molecular Brushes as Super-Soft Elastomers. Polymer 2006, 47, 7198−7206. (6) Cerrada, M. L.; De La Fuente, J. L.; Fernández-García, M.; Madruga, E. L. Viscoelastic and Mechanical Properties of Poly(butyl Acrylate-G-Styrene) Copolymers. Polymer 2001, 42 (10), 4647−4655. (7) Xenidou, M.; Beyer, F. L.; Hadjichristidis, N.; Gido, S. P.; Tan, N. B. Morphology of Model Graft Copolymers with Randomly Placed Trifunctional and Tetrafunctional Branch Points. Macromolecules 1998, 31, 7659−7667. (8) Zhu, Y.; Burgaz, E.; Gido, S. P.; Staudinger, U.; Weidisch, R.; Uhrig, D.; Mays, J. W. Morphology and Tensile Properties of Multigraft Copolymers with Regularly Spaced Tri-, Tetra-, and Hexafunctional Junction Points. Macromolecules 2006, 39, 4428−4436. (9) Cai, C.; Lin, J.; Chen, T.; Tian, X. Aggregation Behavior of Graft Copolymer with Rigid Backbone. Langmuir 2010, 26, 2791−2797. (10) Gitsas, A.; Floudas, G.; Mondeshki, M.; Butt, H. J.; Spiess, H. W.; Iatrou, H.; Hadjichristidis, N. Effect of Chain Topology on the Self-Organization and Dynamics of Block Copolypeptides: From Diblock Copolymers to Stars. Biomacromolecules 2008, 9, 1959−1966.

H


Article

Macromolecules (11) Gao, K.-J.; Li, G.; Lu, X.; Wu, Y. G.; Xu, B.-Q.; Fuhrhop, J.-H. Giant Vesicle Formation through Self-Assembly of Chitooligosaccharide-Based Graft Copolymers. Chem. Commun. 2008, 1449−1451. (12) Wang, M.; Zou, S.; Guerin, G.; Shen, L.; Deng, K.; Jones, M.; Walker, G. C.; Scholes, G. D.; Winnik, M. A. A Water-Soluble pHResponsive Molecular Brush of Poly (N,N -Dimethylaminoethyl Methacrylate) Grafted Polythiophene. Macromolecules 2008, 41, 6993−7002. (13) Balamurugan, S. S.; Bantchev, G. B.; Yang, Y.; McCarley, R. L. Highly Water-Soluble Thermally Responsive Poly(thiophene)-Based Brushes. Angew. Chem., Int. Ed. 2005, 44, 4872−4876. (14) Qiu, L. Y.; Bae, Y. H. Self-Assembled Polyethylenimine-GraftPoly(ε-Caprolactone) Micelles as Potential Dual Carriers of Genes and Anticancer Drugs. Biomaterials 2007, 28, 4132−4142. (15) Sato, Y. I.; Kobayashi, Y.; Kamiya, T.; Watanabe, H.; Akaike, T.; Yoshikawa, K.; Maruyama, A. The Effect of Backbone Structure on Polycation Comb-Type Copolymer/DNA Interactions and the Molecular Assembly of DNA. Biomaterials 2005, 26, 703−711. (16) Huang, N.-P.; Michel, R.; Voros, J.; Textor, M.; Hofer, R.; Rossi, A.; Elbert, D. L.; Hubbell, J. A.; Spencer, N. D. Poly(L-Lysine)-gPoly(ethylene Glycol) Layers on Metal Oxide Surfaces: SurfaceAnalytical Characterization and Resistance to Serum and Fibrinogen Adsorption. Langmuir 2001, 17, 489−498. (17) Lee, S.; Spencer, N. D. Adsorption Properties of Poly (LLysine)-Graft-Poly(Ethylene Glycol) (PLL-G-PEG) at a Hydrophobic Interface: Influence of Tribological Stress, pH, Salt Concentration, and Polymer Molecular Weight. Langmuir 2008, 24, 9479−9488. (18) Bowden, N. B.; Runge, M. B. Synthesis of High Molecular Weight Comb Block Copolymers and Their Assembly into Ordered Morphologies in the Solid State. J. Am. Chem. Soc. 2007, 129, 10551− 10560. (19) Rzayev, J. Synthesis of Polystyrene-Polylactide Bottlebrush Block Copolymers and Their Melt Self-Assembly into Large Domain Nanostructures. Macromolecules 2009, 42, 2135−2141. (20) Fenyves, R.; Schmutz, M.; Horner, I. J.; Bright, F. V.; Rzayev, J. Aqueous Self-Assembly of Giant Bottlebrush Block Copolymer Surfactants as Shape-Tunable Building Blocks. J. Am. Chem. Soc. 2014, 136, 7762−7770. (21) Sirringhaus, H. 25th Anniversary Article: Organic Field-Effect Transistors: The Path Beyond Amorphous Silicon. Adv. Mater. 2014, 26, 1319−1335. (22) Chavez, R., III; Cai, M.; Tlach, B.; Wheeler, D. L.; Kaudal, R.; Tsyrenova, A.; Tomlinson, A. L.; Shinar, R.; Shinar, J.; Jeffries-EL, M. Benzobisoxazole Cruciforms: A Tunable, Cross-Conjugated Platform for the Generation of Deep Blue OLED Materials. J. Mater. Chem. C 2016, 4, 3765−3773. (23) Adhikari, S.; Hopson, R. A. A.; Sedai, B. R.; McFarland, F. M.; Guo, S.; Nelson, T. L. Synthesis and Characterization of EumelaninInspired Poly(indoylenearylenevinylene)s and Poly(indoylenearyleneethynylene)s. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 457−463. (24) Günes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated Polymer-Based Organic Solar Cells. Chem. Rev. 2007, 107, 1324− 1338. (25) Dou, L.; Liu, Y.; Hong, Z.; Li, G.; Yang, Y. Low-Bandgap NearIR Conjugated Polymers/Molecules for Organic Electronics. Chem. Rev. 2015, 115, 12633−12665. (26) Dang, M. T.; Hirsch, L.; Wantz, G.; Wuest, J. D. Controlling the Morphology and Performance of Bulk Heterojunctions in Solar Cells. Chem. Rev. 2013, 113, 3734−3765. (27) Wang, J.; Lu, C.; Mizobe, T.; Ueda, M.; Chen, W. C.; Higashihara, T. Synthesis and Characterization of All-Conjugated Graft Copolymers Composed of n-Type or p-Type Backbones and poly(3-Hexylthiophene) Side Chains. Macromolecules 2013, 46, 1783− 1793. (28) Zeigler, D. F.; Mazzio, K. A.; Luscombe, C. K. Fully Conjugated Graft Copolymers Comprising a P-Type Donor-Acceptor Backbone and Poly(3-Hexylthiophene) Side Chains Synthesized Via a “Graft Through” Approach. Macromolecules 2014, 47, 5019−5028.

(29) Iovu, M. C.; Sheina, E. E.; Gil, R. R.; McCullough, R. D. Experimental Evidence for the Quasi-″Living” Nature of the Grignard Metathesis Method for the Synthesis of Regioregular Poly(3Alkylthiophenes). Macromolecules 2005, 38, 8649−8656. (30) Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. Catalyst-Transfer Polycondensation. Mechanism of Ni-Catalyzed Chain-Growth Polymerization Leading to Well-Defined Poly(3-Hexylthiophene). J. Am. Chem. Soc. 2005, 127, 17542−17547. (31) Steverlynck, J.; De Winter, J.; Gerbaux, P.; Lazzaroni, R.; Leclère, P.; Koeckelberghs, G. Influence of the Grafting Density on the Self-Assembly in Poly(phenyleneethynylene)-g-poly(3-Hexylthiophene) Graft Copolymers. Macromolecules 2015, 48, 8789−8796. (32) Johnson, J. A.; Finn, M. G.; Koberstein, J. T.; Turro, N. J. Construction of Linear Polymers, Dendrimers, Networks, and Other Polymeric Architectures by Copper-Catalyzed Azide-Alkyne Cycloaddition “Click” Chemistry. Macromol. Rapid Commun. 2008, 29, 1052−1072. (33) Li, Y.; Zheng, X.; Zhu, H.; Wu, K.; Lu, M. Synthesis and SelfAssembly of Well-Defined Binary Graft Copolymer and Its Use in Superhydrophobic Cotton Fabrics Preparation. RSC Adv. 2015, 5, 46132−46145. (34) Smith, K. A.; Lin, Y. H.; Dement, D. B.; Strzalka, J.; Darling, S. B.; Pickel, D. L.; Verduzco, R. Synthesis and Crystallinity of Conjugated Block Copolymers Prepared by Click Chemistry. Macromolecules 2013, 46, 2636−2645. (35) Arslan, M.; Gok, O.; Sanyal, R.; Sanyal, A. Clickable Poly(ethylene Glycol)-Based Copolymers Using Azide-Alkyne Click Cycloaddition-Mediated Step-Growth Polymerization. Macromol. Chem. Phys. 2014, 215, 2237−2247. (36) Johansson, J. R.; Beke-Somfai, T.; Said Stålsmeden, A.; Kann, N. Ruthenium-Catalyzed Azide Alkyne Cycloaddition Reaction: Scope, Mechanism, and Applications. Chem. Rev. 2016, 116, 14726−14768. (37) Li, L.; Hollinger, J.; Jahnke, A. A.; Petrov, S.; Seferos, D. S. Polyselenophenes with Distinct Crystallization Properties. Chem. Sci. 2011, 2, 2306−2310. (38) Hollinger, J.; Jahnke, A. A.; Coombs, N.; Seferos, D. S. Controlling Phase Separation and Optical Properties in Conjugated Polymers through Selenophene-Thiophene Copolymerization. J. Am. Chem. Soc. 2010, 132, 8546−8547. (39) Gao, D.; Hollinger, J.; Seferos, D. S. Selenophene-Thiophene Block Copolymer Solar Cells with Thermostable Nanostructures. ACS Nano 2012, 6, 7114−7121. (40) Hollinger, J.; Seferos, D. S. Morphology Control of Selenophene−Thiophene Block Copolymers through Side Chain Engineering. Macromolecules 2014, 47, 5002−5009. (41) Gao, D.; Hollinger, J.; Jahnke, A. A.; Seferos, D. S. Influence of Selenophene−Thiophene Phase Separation on Solar Cell Performance. J. Mater. Chem. A 2014, 2, 6058−6063. (42) Hollinger, J.; Sun, J.; Gao, D.; Karl, D.; Seferos, D. S. Statistical Conjugated Polymers Comprising Optoelectronically Distinct Units. Macromol. Rapid Commun. 2013, 34, 437−441. (43) Kozycz, L. M.; Gao, D.; Seferos, D. S. Compositional Influence on the Regioregularity and Device Parameters of a Conjugated Statistical Copolymer. Macromolecules 2013, 46, 613−621. (44) Palermo, E. F.; van der Laan, H. L.; McNeil, A. J. Impact of πConjugated Gradient Sequence Copolymers on Polymer Blend Morphology. Polym. Chem. 2013, 4, 4606−4611. (45) Palermo, E. F.; McNeil, A. J. Impact of Copolymer Sequence on Solid-State Properties for Random, Gradient and Block Copolymers Containing Thiophene and Selenophene. Macromolecules 2012, 45, 5948−5955. (46) Palermo, E. F.; Darling, S. B.; McNeil, A. J. Pi-Conjugated Gradient Copolymers Suppress Phase Separation and Improve Stability in Bulk Heterojunction Solar Cells. J. Mater. Chem. C 2014, 2, 3401−3406. (47) Hardeman, T.; Koeckelberghs, G. The Synthesis of Poly(thiophene-co-Fluorene) Gradient Copolymers. Macromolecules 2015, 48, 6987−6993. I


Article

Macromolecules (48) Smeets, A.; Willot, P.; De Winter, J.; Gerbaux, P.; Verbiest, T.; Koeckelberghs, G. End Group-Functionalization and Synthesis of Block-Copolythiophenes by Modified Nickel Initiators. Macromolecules 2011, 44, 6017−6025. (49) Steverlynck, J.; Monnaie, F.; Warniez, E.; Lazzaroni, R.; Leclère, P.; Koeckelberghs, G. Strategies toward Controlling the Topology of Nonlinear Poly(thiophenes). Macromolecules 2016, 49, 8951−8959. (50) Okamoto, K.; Luscombe, C. K. Controlled Polymerizations for the Synthesis of Semiconducting Conjugated Polymers. Polym. Chem. 2011, 2, 2424−2434. (51) Bronstein, H. A.; Luscombe, C. K. Externally Initiated Regioregular P3HT with Controlled Molecular Weight and Narrow Polydispersity. J. Am. Chem. Soc. 2009, 131, 12894−12895. (52) Jeffries-EL, M.; Kobilka, B. M.; Hale, B. J. Optimizing the Performance of Conjugated Polymers in Organic Photovoltaic Cells by Traversing Group 16. Macromolecules 2014, 47, 7253−7271. (53) Tsai, C.-H.; Fortney, A.; Qiu, Y.; Gil, R. R.; Yaron, D.; Kowalewski, T.; Noonan, K. J. T. Conjugated Polymers with Repeated Sequences of Group 16 Heterocycles Synthesized through CatalystTransfer Polycondensation. J. Am. Chem. Soc. 2016, 138, 6798−6804. (54) Qiu, Y.; Fortney, A.; Tsai, C. H.; Baker, M. A.; Gil, R. R.; Kowalewski, T.; Noonan, K. J. T. Synthesis of Polyfuran and Thiophene-Furan Alternating Copolymers Using Catalyst-Transfer Polycondensation. ACS Macro Lett. 2016, 5, 332−336. (55) Chen, Y.; Cui, H.; Li, L.; Tian, Z.; Tang, Z. Controlling MicroPhase Separation in Semi-Crystalline/Amorphous Conjugated Block Copolymers. Polym. Chem. 2014, 5, 4441−4445. (56) Jeffries-EL, M.; Sauvé, G.; McCullough, R. D. In-Situ EndGroup Functionalization of Regioregular Poly(3-Alkylthiophene) Using the Grignard Metathesis Polymerization Method. Adv. Mater. 2004, 16, 1017−1019. (57) Standley, E. A.; Smith, S. J.; Müller, P.; Jamison, T. F. A Broadly Applicable Strategy for Entry into Homogeneous Nickel(0) Catalysts from Air-Stable Nickel(II) Complexes. Organometallics 2014, 33, 2012−2018. (58) Mueller, C. J.; Klein, T.; Gann, E.; McNeill, C. R.; Thelakkat, M. Azido-Functionalized Thiophene as a Versatile Building Block to Cross-Link Low-Bandgap Polymers. Macromolecules 2016, 49, 3749− 3760. (59) Lutz, J. F. 1,3-Dipolar Cycloadditions of Azides and Alkynes: A Universal Ligation Tool in Polymer and Materials Science. Angew. Chem., Int. Ed. 2007, 46, 1018−1025. (60) Leophairatana, P.; Samanta, S.; De Silva, C. C.; Koberstein, J. T. Preventing Alkyne-Alkyne (i.e., Glaser) Coupling Associated with the ATRP Synthesis of Alkyne-Functional Polymers/Macromonomers and for Alkynes under Click (i.e., CuAAC) Reaction Conditions. J. Am. Chem. Soc. 2017, 139, 3756−3766. (61) Zade, S. S.; Zamoshchik, N.; Bendikov, M. Oligo- and Polyselenophenes: A Theoretical Study. Chem. Eur. J. 2009, 15, 8613−8624. (62) Patra, A.; Bendikov, M. Polyselenophenes. J. Mater. Chem. 2010, 20, 422−433. (63) Worrell, B. T.; Malik, J. A.; Fokin, V. V. Direct Evidence of a Dinuclear Copper Intermediate in Cu(I)-Catalyzed Azide-Alkyne Cycloadditions. Science 2013, 340, 457−460. (64) Presolski, S. I.; Hong, V.; Cho, S. H.; Finn, M. G. Tailored Ligand Acceleration of the Cu-Catalyzed Azide-Alkyne Cycloaddition Reaction: Practical and Mechanistic Implications. J. Am. Chem. Soc. 2010, 132, 14570−14576. (65) Chan, T. R.; Hilgraf, R.; Sharpless, K. B.; Fokin, V. V. Polytriazoles as Copper(I)-Stabilizing Ligands in Catalysis. Org. Lett. 2004, 6, 2853−2855. (66) Diez-Gonzalez, S. Well-Defined Copper(I) Complexes for Click Azide-Alkyne Cycloaddition Reactions: One Click Beyond. Catal. Sci. Technol. 2011, 1, 166−178. (67) Liu, J.; Loewe, R. S.; McCullough, R. D. Employing MALDI-MS on Poly(alkylthiophenes): Analysis of Molecular Weights, Molecular Weight Distributions, End-Group Structures, and End-Group Modifications. Macromolecules 1999, 32, 5777−5785.

(68) Schilinsky, P.; Asawapirom, U.; Scherf, U.; Biele, M.; Brabec, C. J. Influence of the Molecular Weight of Poly(3-hexylthiophene) on the Performance of Bulk Heterojunction Solar Cells. Chem. Mater. 2005, 17, 2175−2180. (69) Fidale, L. C.; Ruiz, N.; Heinze, T.; El Seoud, O. A. Cellulose Swelling by Aprotic and Protic Solvents: What Are the Similarities and Differences? Macromol. Chem. Phys. 2008, 209, 1240−1254.

J


Exploring the Graft-To Synthesis of All-Conjugated Comb Copolymers

Recommend Documents