Photosensitive alkoxyamine 2,2,6,6-tetramethyl-1-(1-phenylethoxy)piperidin-4-yl quinoline-2-carboxylate (PE-TEMPO-Q) was synthesized. Photochemical properties of PE-TEMPO-Q were studied to develop photoinduced nitroxide-mediated polymerization of methyl methacrylate (MMA). Rapid and facile polymerization at ambient temperature with PE-TEMPO-Q as an initiator was confirmed to proceed in a controlled mechanism based on the linear growth in molecular weight combined with relative narrow polydispersity index (1.4–1.8) of the resulting polymers. The stereochemistry of obtained polymers was also investigated, and the syndiotacticity slightly increased compared with the typical photopolymerization. Dual-controlled photopolymerization of MMA was achieved in the presence of synthesized alkoxyamine.
The design and preparation of well-defined polymers at the molecular level has been one of the major concerns in synthetic polymer chemistry over the past several decades. Controlled/living radical polymerization (CRP) methods, which mainly include nitroxide-mediated polymerization (NMP) [
A further target in radical polymerization is the attempt to synthesize polymers by controlling the molecular weight and tacticity [
UV irradiation, as an easy and effective activation method, offers a mild technique for dual control. Contrary to thermally based applications, typical photochemistry can be performed in mild temperatures, indicating its suitability for thermally sensitive monomers and substrates [
The combination of stereocontrol and molecular weight control of MMA, which is one of the most applied monomers, has been achieved by ATRP and RAFT polymerizations using various Lewis acids or polar solvents [
MMA (Tianjin Damao Chemical Reagent Factory, China) was distilled under vacuum before use. 4-Hydroxy-2,2,6,6-tetramethyl-piperidinyl-1-oxy [TEMPO-OH, 98%, Aladdin Industrial Corporation (AIC), USA] was purified by recrystallization from cyclohexane. Quinaldic acid (98%, AIC), 1-bromoethyl benzene (97%, AIC), 2,2′-bipyridyl, (bpy, 99%, AIC), CuBr (99%, AIC), Cu(0) powder (99.9%, AIC), N,N′-dicyclohexylcarbodiimide (DCC, 99%, AIC), 4-dimethylaminopyridine (DMAP, 99%, AIC), and 2,2-dimethoxy-2-phenyl acetophenone (DMPA, BASF) were used as received. All solvents were commercially available and used without further purification.
FT-IR spectra were measured between 4000 cm−1 and 500 cm−1 for 32 scans at 4 cm−1 resolution in KBr pellets using a Nicolet Magna 360 spectrophotometer. 1H NMR spectra were recorded on a Bruker spectrometer (400 MHz) with DMSO-
The photosensitive alkoxyamine PE-TEMPO-Q was prepared by a two-step reaction sequence. First, a mixture of TEMPO-OH (1.293 g, 7.50 mmol), Cu(0) powder (0.480 g, 7.50 mmol), CuBr (1.080 g, 7.50 mmol), 1-bromoethyl benzene (0.925 g, 5.00 mmol), and bpy (1.170 g, 7.50 mmol) in benzene (50 mL) was mixed in a flask and stirred at 70°C for 8 h under an inert atmosphere. The reaction mixture was then filtered and successively washed thrice with 1 mol/L CuSO4 solution and distilled water. After drying with magnesium sulfate and evaporating benzene, the crude product was purified by silica gel column chromatography (ethyl acetate/ether = 1/20) to obtain a white solid. Analytical data are as follows: Yield: 70.26%. Melting point: 98.9−99.6°C. Found: C, 73.61; H, 9.81; N, 5.05%. Calc. for C17H27NO2: C, 74.01; H, 9.79; N, 5.04%. 1H NMR (DMSO-
Then, the obtained PE-TEMPO-OH (2.770 g, 10.0 mmol), quinaldic acid (1.903 g, 11.0 mmol), and DMAP (0.122 g, 1.0 mmol) in CH2Cl2 (50 mL) were charged to a flask and stirred under an inert atmosphere. The DCC (2.060 g, 10.0 mmol) in CH2Cl2 (10 mL) was slowly dropped to the mixture for 1 h at −10°C and allowed to react for 24 h after returning to room temperature. The reaction mixture was then filtered and washed thrice with 1 mol/L HCl and distilled water. After drying with magnesium sulfate and evaporating CH2Cl2, the crude product was purified by silica gel column chromatography (ethyl acetate/ether = 1/50) to obtain a transparent sticky liquid. Analytical data are as follows. Yield: 62.87%. Found: C, 74.97; H, 7.46; N, 6.48%. Calc. for C17H27NO2: C, 75.05; H, 7.62; N, 6.13%. 1H NMR (DMSO-
The quantitated PE-TEMPO-Q was generally dissolved in MMA (1 mL) and placed in a quartz tube. The reaction mixture was deoxygenated by nitrogen bubbling and stirred for 30 min. Afterward, the tube was sealed and irradiated by a photoreactor (Mejiro Genossen CHG-200) equipped with a Hg lamp. Light intensity was 20 mW/cm2 measured by a UV radiometer (Photoelectric Instrument Factory of Beijing Normal University, China). The resulting polymers were precipitated in methanol and then dried at 40°C for 12 h under reduced pressure. Conversion of the monomer was directly monitored by weighing method. The number-average molecular weights and polydispersity index (PDI) were determined by GPC.
It has been proved that photosensitive alkoxyamine with a chromophore could be applied to the control/living free radical polymerization according to similar observations from the unimolecular thermal alkoxyamines [
Synthesis of photosensitive alkoxyamine PE-TEMPO-Q.
The results of the structural characterization of obtained products are shown in the supporting information (Figures
1H NMR spectra of (a) PS-TEMPO-OH and (b) PS-TEMPO-Q.
FT-IR spectra of (a) TEMPO-OH, (b) PS-TEMPO-OH, and (c) PS-TEMPO-Q.
The UV-visible absorption spectra of the investigated nitroxides in CH2Cl2 (
(a) UV absorption spectra. (b) Fluorescence spectra,
The photoinduced NMP of MMA was investigated in the presence of PE-TEMPO-Q at ambient temperature. The control of the molecular weight of MAA was first investigated with photosensitive alkoxyamine as a unimolecular initiator. The mechanism is proposed to occur as shown in Scheme
Polymerization mechanism of MMA in the presence of PE-TEMPO-Q.
Figure
Photopolymerization of MMA in the presence of photosensitive alkoxyamine, light intensity = 20 mW/cm2, [MMA] = 9.42 mmol, [PE-TEMPO-Q] = (●)
Experiment data of PMMA by photopolymerization.
Initiator |
|
Time (h) | Conv. (%) |
|
|
PDI | Tacticity ( |
|
---|---|---|---|---|---|---|---|---|
DMPA |
|
— | 17.07 | — | 18900 | 2.20 | 27.0/27.9/45.1 | 40.95/59.05 |
2 | — | 53.17 | — | 28300 | 2.74 | 27.4/26.1/46.5 | 40.45/59.55 | |
3 | — | 77.52 | — | 43500 | 2.63 | 24.4/27.8/47.8 | 35.80/60.20 | |
|
||||||||
PE-TEMPO-Q |
|
2 | 11.31 | 5317 | 17700 | 2.09 | 8.7/31.2/60.1 | 24.30/75.70 |
5 | 3 | 21.34 | 9650 | 28500 | 1.86 | 6.8/32.1/61.1 | 22.85/77.15 | |
6 | 4 | 27.49 | 12307 | 35300 | 1.74 | 4.9/34.7/60.4 | 22.25/77.75 | |
7 | 5 | 45.51 | 20092 | 43000 | 1.69 | 4.8/34.5/60.7 | 22.05/77.95 | |
8 | 6 | 57.52 | 25280 | 49800 | 1.59 | 4.5/34.1/61.4 | 21.55/78.45 | |
|
||||||||
PE-TEMPO-Q |
|
2 | 9.11 | 1743 | 10500 | 1.84 | 9.9/32.6/57.5 | 26.20/73.80 |
10 | 3 | 18.87 | 3149 | 14200 | 1.74 | 7.1/34.3/58.6 | 24.25/75.75 | |
11 | 4 | 28.52 | 4539 | 17300 | 1.62 | 5.9/35.9/58.2 | 23.85/76.15 | |
12 | 5 | 41.10 | 6351 | 19600 | 1.57 | 4.9/34.9/60.2 | 22.35/77.65 | |
13 | 6 | 48.23 | 7377 | 22500 | 1.49 | 4.4/35.4/60.2 | 22.10/77.90 | |
14 | 8 | 58.52 | 8859 | 27000 | 1.45 | 4.4/34.7/60.9 | 21.76/78.25 | |
15 | 10 | 67.13 | 10098 | 28700 | 1.42 | 4.5/34.5/61.0 | 21.75/78.25 | |
16 | 12 | 74.38 | 11142 | 30000 | 1.38 | 4.5/34.6/61.1 | 21.80/78.40 |
(a) Light intensity was 20 mW/cm2,
The evolution of molecular weight and distribution versus conversion is shown in Figure
Previous study has shown a significant penpenultimate group effect on the homopolymerization of PMMA, resulting in the change of tacticity [
The tacticity of PMMA was determined by analyzing
1H NMR spectra of
In this study, the photoproperties of alkoxyamine PE-TEMPO-Q bearing a quinolone group are investigated. The dual-controlled polymerization of MMA is determined in the presence of PE-TEMPO-Q under mild UV irradiation. A linear growth in the polymer chain combined with a comparatively narrow molecular weight distribution in the range of 1.4–1.8 is observed. The syndiotactic content of PMMA derived from the photoinduced NMP is approximately 59%–61% and higher than the ones obtained by typical photopolymerization. These results indicate that PE-TEMPO-Q affected stereocontrol in the polymerization of MAA to produce syndiotactic-rich polymers. To the best of the authors’ knowledge, this study is the first to report the use of an efficient photosensitive alkoxyamine to achieve dual-controlled polymerization under UV irradiation.
The authors declare that they have no competing interests.
The authors acknowledge the financial support by the Natural Science Foundation of China (NSFC-51403041).