The initiators for continuous activator regeneration (ICAR) atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) were investigated in microemulsion under light irradiation with ethyl 2-bromoisbutyrate (EBiB) as an ATRP initiator, ferric chloride hexahydrate (FeCl3·6H2O) as a catalyst, N,N,N′,N′-tetramethyl-1,2-ethanediamine (TMEDA) as a ligand, and azobisisobutyronitrile (AIBN) as an initiator and reducing agent. The linear dependence of ln([M]0/[M]) on polymerization time was observed, indicating that the polymerization was living polymerization. Furthermore, the number-average molecular weights (Mn), gel permeation chromatography (GPC) and molecular weight distributions (MWD) of the PMMA obtained from GPC were controlled. The product was characterized by 1H nuclear magnetic resonance (NMR) and GPC. The living features were verified by chain-extended with MMA with the obtained PMMA as macroinitiator.
Since the introduction of controlled/living radical polymerization (C/LRP), C/LRP has revolutionized the fields of many new advanced materials preparation with a precisely controlled molecular architecture (1–3). The basic conception of CRP is the reversible activation/deactivation of the polymer chains (4). To date, three main living polymerization techniques have been extensively reported: namely nitroxide-mediated radical polymerization (NMP) (5, 6), atom transfer radical polymerization (ATRP) (7–9), reversible addition-fragmentation chain transfer (RAFT) (10–12). Among them, atom transfer radical polymerization (ATRP), also known as metal-mediated radical polymerization, which was independently discovered by Matyjaszewski and Wang (7, 8) and Sawamoto (9) in 1995, is a living radical polymerization that has been attractive in the academic and industrial fields (13, 14). The ATRP technique is a powerful tool for the preparation of (co)polymers with predictable molecular weights and narrow MWD. controlled chain functionality, composition and morphology. In the classic ATRP method, a dynamic exchange amongst the active and the inactive species.
ATRP techniques have been suitable for the polymerization of a wide range of monomers, including styrenes (15, 16), acrylates (17, 18), methacrylates (19, 20), acrylonitrile (21–23) and (meth)acrylamide (24–26). Nevertheless, there are two drawbacks within the conventional ATRP, the first is sensitive to oxygen, which is essential to remove thoroughly the oxygen, the other is higher concentration of catalyst employed. Recently, a new modification of the original ATRP process has been developed by Matyjaszewski and coworkers (27), which was called initiators for continuous activator generation (ICAR) ATRP, In the ICAR ATRP process, a tiny amount of initiator is used to as a reducing agent to generate activators coming from accumulated deactivating species.
Many of the transition metals have been reported in ATRP in complexes of various nitrogen and phosphine-based ligands (28). Copper catalyst is one of the most widely studied transition metal because of its low price, abundant and versatility. Comparing with copper catalyst, iron catalyst is also attractive since its low toxicity, environmentally friendly, biocompatibility, and potential application in biomaterials, and there are many excellent works in the field of iron-catalyzed ATRP reported in ATRP by Zhu and coworkers (29–35).
Miniemulsion polymerization is an ideal method to synthesize nanoscale materials and polymer particles with desirable copolymer composition. The droplets in a miniemulsion are typically between 100 and 500 nm in diameter. In addition, lower viscosity of the medium, polymerization of very hydrophobic monomers, and reduced environmental issues are the general preferences of miniemulsion polymerization. The process of conducting an ATRP in emulsion or miniemulsion provides many advantages and would be an efficient route in commercial production of living free radical polymers. ATRP conducted in miniemulsion or emuclsion has been extensively investigated by the Matyjaszewski group and other groups (36–38). To date, numerous Cu-mediated ATRP performed in miniemulsion have been reported (39, 40). However, only a few reports about iron catalyst have been reported.
There are many potential applications for photo-induced reactions due to no heat needed and good environmental friendly, especially to functional groups and materials that decompose at high temperatures. Photo-induced living polymerizations have been successfully applied to prepare the polymer (41–43). According to our understanding, No reports about photo-induced ATRP in miniemulsion system has been reported.
Herein, we report the preparation of ICAR ATRP of MMA in miniemulsion with ethyl 2-bromoisbutyrate (EBiB) as initiator, FeCl3·6H2O as catalyst, TMEDA as ligand, and AIBN as initiator and reducing agent. Poly(methyl methacrylate)s (PMMA) with well-defined Mn,GPC and narrow MWD were obtained in this system.
MMA was provided by Tianjin Fuchen Chemical Reagents Factory (Tianjin, China) and then distiled under reduced. Ethyl 2-bromoisobutyrate (EBiB, Alfa Aesar Chemical Co., Tianjin, China, 99%) was used directly. AIBN was purchased from Shanghai 4th factory of chemicals, 99%, Shanghai, China) and recrystallized twice from methanol. TMEDA was provided by JinJinLe Industry Co., Ltd. (Shanghai, China). FeCl3·6H2O was provided by Shanghai Qingfeng Chemical Factory (Shanghai, China). Hexadecane (HD), obtained from Aladdin (Shanghai, China). Polyoxyethylene oleyl ether (brij 98), was purchased from Sinopharm Chemical Reagent (Shanghai, China). The other reagents were used as received.
2.2 Typical procedure for miniemulsion polymerization
The ATRPs were performed according to the following typical procedure. Taking XX as an example, the preparation of organic phase was added hexadecane (HD, 1.8 g, 0.008 mol), AIBN (0.0049 g, 0.00003 mol) and MMA (5 g, 0.05 mol). The composition of aqueous phase consisted of brij 98 (1.5 g, 3 wt.-% vs. monomer), FeCl3·6H2O (0.0027 g, 0.00001 mol), TMEDA (0.0023 g, 0.00002 mol) in deionized water (30 ml). Then the mixing homogeneous oil phase was placed into the above water phase dropwise with vigorous mechanical agitation for 10 min. Then the initiator EBiB (0.0195 g, 0.0001 mol) was added. The miniemulsion was obtained with sonication at 30 W power (ultrasound cell disruputer, Ninbo Xinzhi Bio-tech Co.) for 30 min via treatment of the above reaction mixture. The miniemulsion was purged with N2 for 1 h. Finally, EBiB was added to initiate polymerization. Then the polymerizations were performed under a 500-W, high-pressure mercury vapor lamp, whose distance was about 15 cm at 25°C. At regular intervals, samples were withdrawn with a syringe to measure the conversion, Mn,GPC and MWD.
2.3 Chain extension experiment
The Br-terminated polymer was dissolved in MMA (~5 g) in a 150 ml flask. The molar ratio of [MMA]0/[PMMA-Br]0/[FeCl3·6H2O]0/[TMEDA]0/[AIBN]0 was kept 500:1:0.1:0.2:0.3. The flask were irradiated by the light from 500 W high-pressure mercury vapor lamp, whose distance was 15 cm at 25°C. The flask was charged with ultra-high-purity nitrogen for a few minutes. Then it was placed in a oil bath, and the magnetic stirring speed was kept 300 rpm/min. The flask was taken out after light irradiation for some time and cooled to room temperature. After the polymerization was completed, the product was obtained by centrifugation and treated with water and methanol several times, finally dried under vacuum overnight.
2.4 Characterization of PMMA polymer
Monomer conversions were determined gravimetrically. The number-average molecular weights (Mn,GPC) and molecular weight distribution (MWD) were accessed by Waters 1515 gel permeation chromatography (GPC). The GPC consisted of refractive index detector which was equipped with HR1, HR3, and HR4 column. The eluent was tetrahydrofuran (THF) and the flow rate was 1.0 ml/min and the column teperature was 35°C. Narrowly distributed poly(methyl methacrylate) standards were employed to calibrate the GPC column.
1H NMR spectra were recorded on a Bruker 400 MHz spectrometer with CDCl3 as deuterated solvent. Polymers were dissolved in an appropriate deuterated solvent to give a viscous polymer solution in an NMR tube. The used internal standard was tetramethylsilane (TMS).
3 Results and discussion
3.1 Photoinduced ICAR ATRP of MMA in miniemulsion
The first trial polymerizations for the photo-induced ICAR ATRP of MMA were completed at 25°C in miniemulsion using EBiB/FeCl3·6H2O/TMEDA initiation system. the results were shown in Figure 1 when the molar ratio of [MMA]0/[EBiB]0/[FeCl3·6H2O]0/[TMEDA]0/[AIBN]0 was 500:1:0.1:0.2:0.3,
The results shows that the semilogarithmic plot of ln[M]0/[M] on time for the ICAR ATRP of MMA in the presence of the FeCl3·6H2O/TMEDA catalyst system is linear. Suggesting constant radical concentration throughout the polymerization process. 76.5% conversion was obtained in miniemulsion within 16 h. The apparent rate constants (kapp) obtained from Figure 1 was 2.52×10-5 s-1, which is lower than that in bulk and is higher than that in solution polymerization reported by Zhu and coworkers (44). Furthermore, induction period have been observed in above system. It is probably that ICAR ATRP of MMA was performed in different system.
Figure 2 shows the plot of the experimental number average molecular weights (Mn,GPC) and molecular weight distributions (Mw/Mn) with percentage monomer conversion for the photo-induced ICAR ATRP synthesis of PMMA in miniemulsion. The Mn,GPC of the PMMA samples obtained in this systems increased linearly with percentage monomer conversion and are close to the theoretical values (Mn,theo). The Mw/Mn values are <1.35 during the polymerization process. Based on the above discussion, it can be concluded that the photo-induced ICAR ATRP of MMA was living fashion.
3.2 Effect of molar ratio of [FeCl3·6H2O]/[AIBN] on photo-induced ICAR ATRP of MMA
In a typical ICAR ATRP system, the molar ratio of [FeCl3·6H2O]/[AIBN] exerts a vital role in ICAR ATRP. The activators (lower oxidation state transition metals) are continuously generated by in-situ reduction between the deactivators (higher oxidation state transition metal) and the conventional thermal radical initiator (such as azobis(isobutyronitrile) (AIBN)). A serials of experiments were performed at 25°C in order to investigate the effect of [FeCl3·6H2O]/[AIBN] on photo-induced Fe-mediated ICAR ATRP in miniemulsion. The molar ratio of [MMA]0/[EBiB]0 was kept 500:1 with varied molar ratios of [FeCl3·6H2O]0/[AIBN]0. The results were listed in Table 1.
|Run||Rb||Time (h)||Conversion (%)||Mn,theo(g·mol-1)||Mn,GPC(g·mol-1)||Mw/Mn|
aMiniemulsion conditions: wBrij 98=1.5 g, wHD=1.8 g.
cThe mixtures were irradiated with 500 W high-pressure mercury vapor lamp at a distance of 15 cm at 25°C.
Seen from Table 1, the conversion increased from 18.4% to 51.3% when the molar ratio of [FeCl3·6H2O]0/[AIBN]0 changed from 0.01:0.3 to 0.3:0.3. The Mn,GPC of the resulting PMMA were in good agreement with the theoretical values. Furthermore, the Mw/Mn values were <1.35, which indicated that the photoinduced ICAR ATRP of MMA proceeded in a living fashion in miniemulsion. However, the Mw/Mn value was beyond 1.5 when the molar ratio of [FeCl3·6H2O]0/[AIBN]0 was 0.005:0.3, indicating that the polymerization was out of control.
3.3 Effect of concentration of FeCl3·6H2O on photo-induced ICAR ATRP of MMA
The advantage of ICAR ATRP over AGET ATRP is that less amount of catalyst is needed. Thus, the minimum concentration of FeCl3·6H2O required to be determined. The results are shown in Table 2.
|Run||Rb||FeCl3·6H2O (ppm)||Time (h)||Conversion (%)||Mn,theo(g·mol-1)||Mn,GPC(g·mol-1)||Mw/Mn|
aMiniemulsion conditions: wBrij 98=1.5 g, wHD=1.8 g.
cThe mixtures were irradiated with 500 W high-pressure mercury vapor lamp at a distance of 15 cm at 25°C.
Table 2 shows the concentration of FeCl3·6H2O on the photo-induced Fe-mediated ICAR ATRP of MMA in miniemulsion. The concentration of FeCl3·6H2O changed from 45 ppm to 27 ppm, the polymerization proceeded in a controlled/living process. The number average molecular weights (Mn,GPC) increased with monomer conversion and were in well agreement with the theoretical values with the molecular weight distribution (Mw/Mn) between 1.32 and 1.41. When the concentration of FeCl3·6H2O was <20 ppm, the Mw/Mn values became broader indicating out of control of polymerization. This can be attributed to a lower concentration of deactivator and longer transient radical lifetime (45).
3.4 Effect of light irradiance
The light irradiance played an important role in photoinduced polymerization. The effects of light irradiance on photoinduced ICAR ATRP of MMA were also investigated. The light intensity was varied from 20 to 100 W. The molar ratio was kept to be 500:1:0.1:0.2:0.3. The results were shown in Figures 3 and 4.
As shown in Figure 3, first-order kinetics with respect to monomer conversion was linear at different light intensities, indicating that the concentrations of propagating radical are almost constant throughout the polymerization. However, the induction periods were observed in all cases with different light intensities 100 W, 50 W and 20 W used, respectively. At higher irradiance, the induction period became shorter because the generation of more initiating radicals can be generated. Furthermore, 69.3% was reached within 30 h at 100 W and 33.6% was reached within 30 h at 20 W. The apparent rate constant (kapp) obtained from Figure 3 were, 1.16×10-5 s-1, 7.20×10-6 s-1 and 4.39×10-6 s-1, respectively.
The evolution of Mn,GPC and MWD with conversion for the photo-induced ICAR ATRP of MMA at different light intensities were investigated. Mn,GPC increased linearly with monomer conversion in all experiments as seen from Figure 4, Mn,GPC values were close to the theoretical values when higher light intensities were employed. However, when lower light intensities were employed, the Mn,GPC values are slightly higher than the theoretical values. The Mw/Mn values of PMMA were between 1.29 and 1.48. It was further verified that this is sufficiently effective photolysis. It can be conclude that light irradiance had a marked effect on Mn,GPC and Mw/Mn values.
3.5 The effect of light on the photoinduced Fe-mediated ICAR ATRP of MMA in miniemulsion
The photo-induced ICAR ATRP of MMA was carried out at 25°C under the conditions of alternating light “ON” and “OFF” environment when keeping [MMA]0/[EBiB]0/[FeCl3·6H2O]0/[TMEDA]0/[AIBN]0=500:1:0.1:0.2:0.3, The results were plotted in Figure 5. It can be seen that the initiation period of polymerization was finished when 14.79% conversion was fetched within 2 h under light irradiation and the sufficient radical was obtained. The light irradiation was stopped when irradiation time was in the period of 2–3 h, 4–5 h, 6–7 h, 8–9 h, 10–11 h and 12–13 h.
In addition, it must be demonstrated the polymerization stopped in the absence of light, indicating that the concentration of active species was neglected during the period. However, the polymerization rate increased remarkably in the presence of light. Furthermore, the polymerization rate was the same as the above results mentioned according to the slope. It implied that periodic light on-off process exerted a vital role in this process and demonstrated temporally controlled polymerization.
Seen from Figure 6, the GPC curves shifted to higher molecule weights in the presence of light. Furthermore, the curves were unimodal, symmetric and relatively narrow (Mw/Mn~1.3) until higher conversion. The GPC curves did almost not shift in the absence of light. These results indicate that the polymerization was controlled by light and the light induces the generation of radicals in miniemulsion.
3.6 Analysis of chain end and chain extension
The PMMA prepared by photo-induced Fe-mediated ICAR ATRP of MMA were also chain extended in miniemulsion using FeCl3·6H2O/TMEDA as the mediator. The mixtures were irradiated with 500 W high-pressure mercury vapor lamp, whose distance was 15 cm at 25°C. The [MMA]0/[PMMA-Br]0/[FeCl3·6H2O]0/[TMEDA]0/[AIBN]0 was 500:1:0.1:0.2:0.3. The macroinitiators was characterized by 1H NMR spectroscopy (Figure 7). The chemical shift at 0.84–1.21 ppm (a in Figure 7) was contributed to the methyl protons. The shift at 1.42–2.06 ppm (b in Figure 7) was attributed to the protons of methylene groups, 3.41–3.80 ppm (c in Figure 7) was attributed to methoxy groups, 3.78 ppm (d in Figure 7) proved the presence of the end group, as reported by Sawamoto (46). Seen from Figure 8, the Mn,GPC of macroinitiators increased significantly from 11,200 g/mol to 283,000 g/mol. MWD of chain-extended PMMA were broad (Figure 8). Active chain end further verified by the successful chain extension in this system.
The Fe-mediated ICAR ATRP of MMA using FeCl3·6H2O as catalyst in miniemulsion has been carried out successfully. It was found that the photo-induced ICAR ATRP of MMA obeyed first order kinetics in miniemulsion and the molecular weights increase linearly with the increasing of monomer conversion. The macroinitiator structure was characterized by 1H NMR spectroscopy and the living nature was verified by chain extension experiment.
This work was supported by the National Natural Science Foundation of China (no. 51374043).
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