Letter Cite This: ACS Macro Lett. 2018, 7, 604−608
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Polymer Constructed Through the Formation of Carbon−Carbon Triple Bonds: Reductive Coupling Polymerization of Bis(benzylic gem-tribromide)s Limei Ren, Xiaoyan Xu, and Qi Wang* MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, P. R. China S Supporting Information *
ABSTRACT: Ethynylene-bridged polymers are currently synthesized by alkyne metathesis polymerization or Pd-catalyzed coupling between diethynylarene and dihaloarene. We report the formation of CC linkages in reductive coupling polymerization of bis(benzylic gem-tribromide)s promoted by Cu/polyamine under mild conditions. The polymer backbone was constructed through cascade formation of (Br)CC(Br) bonds and CC bonds. This protocol provides a new method for synthesis of ethynylene-bridged polymers using monomers without alkynyl groups.
P
polymerization (ROAMP) (Scheme 1B). ADIMP of a series of dipropynylated benzenes produces various types of poly(aryleneethynylene)s (PAEs) in quantitative yields and high purity catalyzed by [(tBuO)3WCtBu]7 and Mo(CO)6/ phenols.8−10 The related progress has been documented in several reviews.6,11−14 ROAMP of strained cyclic alkynes15−20 generates ethynylene-bridged nonconjugated polymers. Due to the sensitivity of the catalyst, most of the alkyne metathesis polymerizations generally require moisture-free media and inert atmospheres. Although alkyne metathesis polymerizations generate polymer chains via CC bond formation, the protocol just entails the redistribution of the monomer’s alkynyl moiety by the scission and regeneration of CC bonds. Strictly speaking, no extra CC bonds are formed in alkyne metathesis polymerization, and half of the valuable C C bonds are wasted in ADIMP. Alternatively, ethynylene-bridged polymers can also be prepared by Pd-catalyzed coupling of diethynylarene to suitably substituted dihaloarene13 (Scheme 1C) or chain-growth condensation polymerization following a catalyst-transfer mechanism,21,22 in which only C−C single bonds are formed. In principle, the CC bonds can be generated from C−C bonds, CC bonds, and some reductive coupling reactions. The first two protocols are based on the elimination of substituted CC or even C−C bonds. Since the elimination reaction is a process of creating a new bond upon an existing backbone, it is not applicable in polymerization, at least for the construction of polymer backbones. Coupling reactions
olymers are constructed by formation of new bonds between small monomers. Polymerization can be classified according to the bond order of the formed linkage. Compared with a large amount of methods for construction of polymer by the formation of single and double bonds, efficient synthetic protocols for the formation of CC bonds are so limited. Alkyne metathesis is a chemical transformation to generate new CC bonds from existing CC bonds and has been widely used in the synthesis of a variety of organic compounds and polymers.1−6 Ethynylene-bridged polymers can be prepared by acyclic diyne metathesis polymerization (ADIMP) (Scheme 1A) and ring-opening alkyne metathesis Scheme 1. Ethynylene-Bridged Polymers Prepared by Different Pathways: (A) Acyclic Diyne Metathesis Polymerization, (B) Ring-Opening Alkyne Metathesis Polymerization, (C) Pd-Catalyzed Coupling Polymerization, (D) Reductive Coupling of Benzotribromide, and (E) Reductive Coupling Polymerization of Bis(benzylic gemtribromide)s
Received: April 7, 2018 Accepted: May 10, 2018
© XXXX American Chemical Society
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DOI: 10.1021/acsmacrolett.8b00257 ACS Macro Lett. 2018, 7, 604−608
Letter
ACS Macro Letters
Table 1. Polymerization Results of 2,2-Bis(4-tribromomethyl phenyl)hexafluoropropane (1a) in the Presence of Cu/Ligands under Different Conditions linkage composition (mol %)c polymerization conditionsa
run 1 2 3 4 5 6
[1a]:[Cu]:[L] = 1:5:5, [1a] = 0.025 M, 60 °C, 5 h
7 8 9 10 11 12 13 14
[1a]:[Cu]:[Me6TREN] =
[1a]:[Cu]:[Me6TREN] =
[1a]:[Cu]:[Me6TREN] = 60 °C, 5 h [1a]:[Cu]:[Me6TREN] =
[1a]:[CuBr]:[Me6TREN]
L = Me6TREN L = TPMA L = PMDETA 1:5:5, [1a] = 0.025 M 0.5 h (30 °C) 0.5 h (30 °C) + 1.5 h (40 °C) 0.5 h (30 °C) + 1.5 h (40 °C) + 3 h (60 °C) 1:5:5, 60 °C, 5 h [1a] = 0.01 M [1a] = 0.04 M 1:x:x, [1a] = 0.025 M, x=4 x=7 1:5:5, [1a] = 0.025 M 1 h (30 °C) + 4 h (60 °C) 1 h (40 °C) + 4 h (60 °C) 5 h (60 °C) = 1:6:6, [1a] = 0.025 M, 60 °C, 5 h
Mn/Đb
[CC]/[Z-CC]/[E-CC]
yield (%)d
23/2.1 24/3.4 12/2.6 17/2.5 20/2.2 26/2.1
100/0/0 60/40/0 47/53/0 0/61/39 55/45/0 100/0/0
98 97
23/2.0 22/1.9 22/2.1 22/2.2 25/2.4 25/2.4 23/2.1 8.4/3.8
78/22/0 100/0/0 94/6/0 100/0/0 100/0/0 100/0/0 100/0/0 88/12/0
98 98 96 92 98 -
a
All polymerizations were conducted in THF. bThe number-average molecular weight (Mn in KDa) and dispersity (Đ) of polymer were measured by GPC with polystyrene as standard. cMeasured by 1H NMR. dTheoretical yield was calculated based on the formation of poly(yne) only.
between two molecules can also generate alkynes. For example, gem-trihalogen compounds can be reductively coupled to 1,2disubstituted acetylene compounds by different metals or metal complexes in good yields.23−28 The yields of the above organic reactions are far less than 100%, so they cannot be applied in polymer synthesis according to the mechanism of stepwise polymerization. We have reported reductive coupling polymerizations of both bis(benzylic bromide)s and bis(benzylic gem-dibromide)s promoted by Cu/polyamine, which afford polymers through formation of C−C and CC bonds, respectively.29,30 Recently, we have reported quantitative construction of the CC bond via reductive coupling of various benzotribromides promoted by Cu/polyamine31 (Scheme 1D). Here we present the first example of synthesizing ethynylene-bridged polymers through CC bond formation via reductive coupling polymerization of bis(benzylic gem-tribromide)s (Scheme 1E). Two difunctional monomers shown in Scheme 1E, bis(benzylic gem-tribromide)s (1a and 1b), were prepared by bromination of aromatic substrates with NBS (see SI). 2,2Bis(4-tribromomethylphenyl)hexafluoropropane (1a) reacted with 4−7 equiv of Cu/polyamines at 30−60 °C for 0.5−5 h in THF. After polymerization, the reaction mixture was diluted with CH2Cl2 and filtered. The solution was precipitated with methanol, and light yellow solid polymer was obtained. The effect of ligand on the polymerization was first investigated because we found it strongly affected the reductive coupling of benzotribromides.31 According to runs 1−3 given in Table 1, polymer containing only CC bonds was produced when tris(2-dimethylaminoethyl)amine (Me6TREN) was employed as ligand, while polymers obtained by tris(2-pyridylmethyl)amine (TPMA) and N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) contained CC and (Br)CC(Br) bonds, which were evidenced by 1H NMR (Figure 1 and Figure S3 in SI). The same result was found in the reductive coupling of benzotribromide.31 Since Me6TREN as a ligand shows stronger capacity in debromination of Z-1,2-dibromo stilbene than the other two ligands,31 it is selected as the ligand for further study. As shown in Figure 1, the peak at 90.14 ppm
Figure 1. NMR spectra of polymer (run 1 in Table 1) prepared from 2,2-bis(4-tribromomethyl phenyl)hexafluoropropane (1a) promoted by Cu/Me6TREN.
detected in the 13C NMR spectrum of polymer 2a is a characteristic signal attributed to CC bonds. The multiple peaks centered at 64.84 ppm are related to the quaternary carbon (g) of the hexafluoroisopropylidene moiety, which appears as a septet due to two CF3 groups. A quartet at 119.9, 122.8, 125.7, and 128.5 ppm related to the CF3 group is found as well. The stretching vibration of symmetrical disubstituted alkyne (−CC−) is infrared inactive, but it is easily identified in Raman spectra. A sharp peak at 2223 cm−1 and a small shoulder at 2168 cm−1 are detected in the Raman spectrum of 605
DOI: 10.1021/acsmacrolett.8b00257 ACS Macro Lett. 2018, 7, 604−608
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ACS Macro Letters polymer 2a (Figure S10), which are characteristic of carbon− carbon triple bonds. Polymer 2a was claimed to be prepared by cross-coupling of 2,2-bis(4-trifluoromethanesulfonylphenyl)hexafluoropropane and bis(trimethylsilyl)acetylene in DMF catalyzed by CuCl/Pd(PPh3)4.32 The yield of this polymerization ranged from 32% to 63%, and the number-average molecular weights (Mn) of polymers were less than 10 kDa. However, we have obtained polymers 2a with excellent yields, and the Mns are more than 22 kDa. The 1H NMR of run 1 is consistent with the literature data, but the 13C NMR is different from the literature,32 which was not correctly assigned. The different linkages generated by different ligands during the polymerization (runs 1−3 in Table 1) suggest that the formation of CC bonds in reductive coupling polymerization of 1a must proceed via at least two steps, which is shown in Scheme 2. The first step is formation of a (Br)CC(Br) bond, Scheme 2. Evolution of Linkages during the Polymerization of Bis(benzylic gem-tribromide)s
Figure 2. 13C (A) and 1H NMR (B) spectra of polymers (runs 4−6 in Table 1) prepared from 2,2-bis(4-tribromomethyl phenyl) hexafluoropropane (1a) at different temperatures (* CHCl3).
and the second one is transformation of a (Br)CC(Br) bond to the CC bond via debromination. The two-step process is consistent with the formation of a CC bond in reductive coupling of benzotribromides.31 The evolution of linkage formation during the polymerization was tracked by three tandem polymerizations, which were (1) 0.5 h at 30 °C, (2) 1.5 h at 40 °C, and (3) 3 h at 60 °C, respectively. The resulting products were analyzed by NMR. Figure 2 presents the NMR spectra of polymers obtained by different steps. Polymer (run 4 in Table 1) obtained by the first step contains E- and Z(Br)CC(Br) bonds, which is evidenced by 13C NMR signals derived from two kinds of vinylene carbons (a1 and a2). The disappearance of E-vinylene carbon (a1) and the appearance of ethynylene carbon (a3) suggest that polymer (run 5 in Table 1) obtained by steps 1 and 2 contains Z-(Br)CC(Br) and CC bonds. Polymer (run 6 in Table 1) containing only CC bonds was obtained after all three steps. No more than two different linkages were formed in runs 4−6. This helps simplify the assignments of 1H NMR spectra, although the signals derived from E-(Br)CC(Br) and CC bonds are overlapped. The composition of three kinds of linkages can be calculated from 1H NMR spectra of polymers, which are given in Table 1. In other words, runs 4−6 obtained under different temperatures and time are three kinds of polymers: two kinds of copolymers composed of Z- and E-(Br)CC(Br) bonds or Z-(Br)CC(Br) and CC bonds, respectively, and one kind of homopolymer composed of CC bonds. The variation of linkages of polymer confirms that the polymer backbone is
constructed by formation of (Br)CC(Br) bonds followed by in situ generation of CC bonds via debromination of (Br)CC(Br) bonds. Dehalogenation of poly(diiododiacetylene) with Lewis base affording polyyne at room temperature under very mild conditions has been reported.33 The Mns of runs 5 and 6 are higher than that of run 4, which is probably due to the linkage transformation from (Br)CC(Br) bonds to CC bonds rather than the increment of Mn. It is well-known that the molecular weight of stiff macromolecules is overestimated by GPC. The effects of monomer concentration and the amount of Cu/ligand on the polymerization were also studied. The monomer concentration had less effect on Mn and Đ of polymer according to runs 7, 1, and 8 in Table 1. At low monomer concentration, longer reaction time was required to achieve full conversion of (Br)CC(Br) linkages to CC linkages, so the Z-(Br)CC(Br) bond was detected (run 7 in Table 1). The amount of Cu and ligand had a similar influence on the type of linkage according to runs 9, 1, and 10 in Table 1. When the [1a]:[Cu]:[ligand] ratio was 1:4:4 (run 9 in Table 1), 6% of (Br)CC(Br) bonds were detected after polymerization at 60 °C for 5 h. When more copper and ligand were used, all bonds formed were CC bonds. As shown in Scheme 2, ethynylene-bridged polymer is produced by two stages, which are construction of the backbone via formation of (Br)CC(Br) bonds and in situ 606
DOI: 10.1021/acsmacrolett.8b00257 ACS Macro Lett. 2018, 7, 604−608
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ACS Macro Letters
formation of carbon−carbon triple bonds. The UV−vis absorption spectrum acquired in CHCl3 of 2b (Figure S6) exhibits two major bands with λmax at about 290 and 307 nm, which are very close to the reported data.35 Polymer 2b is one kind of poly(phenyleneethynylene), which is currently prepared by ADIMP of dipropynylated benzene or the Pd-catalyzed coupling of diethynylarene to suitably substituted dihaloarene.13 In conclusion, we present the first successful example of construction of CC linkages in polymer by reductive coupling polymerization of bis(benzylic gem-tribromide)s. The linkage is constructed by cascade formation of (Br)C C(Br) and CC bonds, which can be manipulated by reaction temperature. The current method provides a new pathway for synthesis of attractive ethynylene-bridged polymers using easily available and cheap reagents without alkynyl groups.
generation of CC bonds via debromination of (Br)C C(Br) bonds. Thus, the Mn of polymer is determined by the first stage, and the type of linkages is determined by the second stage. We carried out two-stage polymerization at different temperatures. The first stage was conducted at different low temperatures trying to adjust the Mn of polymer and the second stage at the same high temperature aiming to form CC bonds. As shown by runs 11−13 in Table 1, the polymerizations started at 30 and 40 °C afforded polymers with higher Mn than polymer obtained at 60 °C. All polymers contained only CC bonds due to the second polymerization stage at 60 °C. CuBr was generated in redox reaction between Cu/ polyamine and alkyl bromide. The in situ formed CuBr has proven to be an efficient reductant for benzyl bromide, benzal bromide, and benzotribromide in the presence of polyamine by our previous results,29,31,34 which generates C−C, CC, and CC bonds, respectively. Polymerization was carried out in the presence of CuBr/Me6TREN, and polymer with high Mn was also obtained (run 14 in Table 1), which proves that in situ formed CuBr promotes the reductive coupling reaction as well. 1,3-Bis(tribromomethyl)-5-tert-butylbenzene (1b) was also polymerized under conditions, such as [1b]:[Cu]:[Me6TREN] = 1:7:7, [1b] = 0.025 M, 2 h (20 °C) + 7 h (60 °C). The 1H and 13C NMR spectra of polymer 2b (Mn = 18 kDa, Đ = 2.4, Figure S5) shown in Figure 3 demonstrate that the linkages formed between monomers are CC bonds. Polymer 2b was previously obtained by the Sonogashira method.35 The signals in both 1H and 13C NMR spectra are consistent with the corresponding polymer,35 which proves the linkage is a CC bond. A sharp peak at 2214 cm−1 is detected in the Raman spectrum of polymer 2b (Figure S11), which confirms the
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00257. Experimental procedures, synthesis of monomers and polymers, and their spectral data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Qi Wang: 0000-0002-5829-0077 Author Contributions
All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (21174123) is appreciated. REFERENCES
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DOI: 10.1021/acsmacrolett.8b00257 ACS Macro Lett. 2018, 7, 604−608
