Huan
Wang
,
Baoqing
Shentu
* and
Zhixue
Weng
State Key Lab of Chemical Engineering, Department of Chemical and biological Engineering, Zhejiang University, Hangzhou 310027, China. E-mail: shentu@zju.edu.cn; Fax: +86 571 87951612; Tel: +86 571 87951612
First published on 5th November 2013
A one-pot synthetic approach for the preparation of poly(2,6-dimethyl-1,4-phenylene oxide)/polystyrene (PPO/PS) alloy in aqueous medium was developed. The method was based on the combination of the oxidative coupling polymerization of 2,6-dimethylphenol (DMP) to form PPO in the presence of a reactive swelling agent, styrene (St), and in situ reverse atom transfer radical polymerization (RATRP) of St initiated by 2,2′-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (AIBI) in alkaline aqueous solution. The complex of CuCl2 and 4-dimethylaminopyridine (DMAP) was utilized as the catalyst to catalyze the two types of polymerization mentioned above. Oxygen was employed as the sole oxidant to synthesize PPO. The introduction of St during the oxidative polymerization of DMP could increase the molecular weight of PPO. Finally thermodynamically compatible PPO/PS alloy was successfully prepared.
PPO has developed into important engineering thermoplastics in the short period of time since the discovery of the oxidative polymerization of phenols.2 The largest commercial usage of PPO is Noryl (Saudi Basic Industry Corp.) engineering resins, which are alloys of PPO and PS. Since PPO and PS are thermodynamically compatible over the complete composition range, a Noryl resin has came into the market for these days.
Synthesizing PPO alloy in water would be significance from the viewpoint of green chemistry, furthermore, PPO alloy can be separated from water easily due to its insolubility in water.3,4 While there have been only a few researches about the synthesis of PPO alloy in water. In our previous study, a one-pot synthetic method for preparing PPO/PS alloy in reactor containing aqueous medium was proposed.5 During the oxidative polymerization of DMP with potassium ferricyanide as the oxidant, the reactive styrene was used as swelling agent to increase the molecular weight of PPO. After the oxidative polymerization of DMP, styrene was in situ polymerized under the initiation of peroxide. However, during the process mentioned above, both the use of potassium ferricyanide as the oxidant and the large amount of sodium hydroxide engaged in the reaction were not adequate from the point of green process.
On the other hand, DMP could be oxidatively polymerized to give PPO in the presence of copper complex catalyst and oxygen in water medium, which would meet the demand of green chemistry.3,4 Moreover, in our previous studies of employing copper complex as catalyst in the reaction, only a little amount of NaOH was needed to synthesize PPO in water.6,7 However, bivalent copper salts are inhibitors of conventional radical polymerization, thus, a reverse atom transfer radical polymerization (RATRP) of St could be considered in this system. RATRP is based on transition metal-catalyzed atom transfer radical addition including the reversible formation of carbon–halogen bonds by transition metals in a high oxidation state, such as copper(II).8–14 The RATRP method was successfully applied to the living radical polymerization of St, MMA etc. And among all of the transition metals, copper appears to be the most robust because of its versatility. In this way, a copper complex could be selected to be the catalyst for the polymerizations of both DMP and St.
This paper reports a one-pot synthetic method of preparing PPO/PS alloy in water medium based on green chemistry. The reactive swelling agent, styrene (St) was added into the mixture to swell the polymer particles during the oxidative polymerization of DMP and increase the mobility of the oligomer chains and facilitate the oxidative polymerization at the later period, which would result in the increase of the molecular weight of PPO. And then in situ RATRP of St was initiated by 2,2′-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (AIBI). The polymerization of DMP and St were both catalyzed by the complex of CuCl2 and 4-dimethylaminopyridine (DMAP). Finally thermodynamically compatible PPO/PS alloy was successfully prepared.
13C NMR spectra were recorded on a 400 MHz Bruker AVANCE II nuclear magnetic resonance spectrometer (Bruker BioSpin Corporation, Switzerland) using CDCl3 as the solvent.
To analyze the composition of PPO/PS blend, the product was extracted firstly by water to eliminate the water-soluble impurities, and then acetonitrile to remove the byproduct DPQ, at last 2-butanone to separate PS, then the proportion of PS and PPO could be determined respectively by measuring the weights before and after each extraction. In addition, the yield of PPO and conversion of St could be calculated according to the weights of these two polymers and the initial amounts of two monomers.
To confirm that the polymerization of St followed the rules of RATRP, a chain extension reaction was conducted. The solution containing PS after extraction was distilled under reduced pressure, and the polymer was used as macroinitiator to initiate the conventional ATRP of St in the presence of CuCl and DMAP.
The molecular weight and polydispersity of PPO and PS were determined by gel permeation chromatography (GPC, Waters 1525/2414, Waters Instrument, Milford, Massachusetts) equipped with Waters Styragel HT4/HT3/HR1 columns and a refractive index detector at 30 °C. The mobile phase was toluene and maintained at a flow rate of 1.0 mL min−1. The molecular weight was calibrated with polystyrene standards.
The glass transition temperature (Tg) of blends were determined by differential scanning calorimetry (DSC) measurements performed on a PE DSC-7 (Perkin-Elmer). The sample was heated from room temperature to 250 °C at a rate of 10 °C min−1 under N2 atmosphere.
St content (wt%) | M n | M w | M w/Mn | PPO yield (%) | PS yield (%) | Impuritiesb (%) |
---|---|---|---|---|---|---|
a All the polymerizations of DMP were carried out in water under oxygen at 50 °C for 24 hours ([Cu(II)] = 1 mmol L−1, [DMP] = 0.25 mol L−1, [SDS] = 0.025 mol L−1, [NaOH] = 0.25 mol L−1, feeding time of St = 3 h). b Here impurities are consisted of the water-soluble impurities and the byproduct DPQ. | ||||||
0 | 4000 | 8800 | 2.2 | 88.63 | 72.55 | 0.34 |
10 | 4000 | 8900 | 2.2 | 88.96 | 73.46 | 0.33 |
20 | 4500 | 10![]() |
2.3 | 89.45 | 71.28 | 0.37 |
25 | 5800 | 13![]() |
2.4 | 89.99 | 73.65 | 0.40 |
30 | 8400 | 20![]() |
2.4 | 90.34 | 72.88 | 0.38 |
35 | 9300 | 26![]() |
2.8 | 91.20 | 71.97 | 0.30 |
40 | 9800 | 30![]() |
3.1 | 91.76 | 72.90 | 0.32 |
50 | 9800 | 30![]() |
3.1 | 92.00 | 74.10 | 0.29 |
Feeding time (h) | M n | M w | M w/Mn | PPO yield (%) | PS yield (%) | Impuritiesb (%) |
---|---|---|---|---|---|---|
a All the polymerizations of DMP were carried out in water under oxygen at 50 °C for 24 hours with St wt% = 50% ([Cu(II)] = 1 mmol L−1, [DMP] = 0.25 mol L−1, [SDS] = 0.025 mol L−1, [NaOH] = 0.25 mol L−1). b Here impurities are consisted of the water-soluble impurities and the byproduct DPQ. | ||||||
1 | 7200 | 20![]() |
2.8 | 91.47 | 74.36 | 0.32 |
1.5 | 7500 | 21![]() |
2.8 | 92.08 | 71.23 | 0.31 |
2 | 8500 | 26![]() |
3.1 | 92.10 | 70.99 | 0.36 |
3 | 9800 | 30![]() |
3.1 | 93.28 | 72.54 | 0.35 |
4 | 9200 | 25![]() |
2.8 | 92.90 | 71.85 | 0.44 |
5 | 8800 | 22![]() |
2.6 | 92.78 | 73.56 | 0.33 |
6 | 8400 | 20![]() |
2.4 | 91.84 | 73.66 | 0.29 |
8 | 7300 | 17![]() |
2.4 | 90.77 | 72.90 | 0.43 |
10 | 6600 | 15![]() |
2.3 | 90.14 | 73.80 | 0.33 |
12 | 5900 | 13![]() |
2.3 | 89.25 | 74.12 | 0.40 |
16 | 5100 | 11![]() |
2.3 | 89.00 | 73.09 | 0.34 |
18 | 4700 | 10![]() |
2.1 | 88.60 | 74.89 | 0.39 |
20 | 4900 | 10![]() |
2.0 | 88.78 | 72.11 | 0.38 |
The composition and molecular characteristics of the product were confirmed by FT-IR and NMR spectroscopies. Fig. 1 shows the FT-IR spectrum of PPO/PS alloy. The peaks at 3060 cm−1 and 3026 cm−1 indicate the stretching vibration of C–H bond in benzene ring, PPO and PS both possess the characteristic absorption peaks at 2924 cm−1 and 2850 cm−1 ascribed to the stretching vibration of aliphatic C–H bond and the peaks at 1601 cm−1, 1493 cm−1 and 1453 cm−1 due to the stretching vibration of benzene skeleton, the peak at 1376 cm−1 is the absorption caused by the deformation vibration of –CH3 in PPO and –CH–CH2– in PS, the peak at 1187 cm−1 is the stretching vibration of Ar–O bond, the peaks at 855 cm−1, 766 cm−1 and 697 cm−1 are assigned to the deformation vibration of C–H bond in benzene ring.15–17 The solid product contains the features of both PPO and PS, suggesting that PPO/PS alloy was obtained.
13C NMR spectrum of the polymer alloy in CDCl3 is shown in Fig. 2. The chemical shifts at 145.5, 128.2, 127.9, and 125.9 ppm are attributed to the carbons in benzene ring of PS, the signals at 154.8, 145.5, 132.3 and 114.6 ppm are assigned to the carbons in benzene ring of PPO, the peak around 46.5–41.0 ppm is ascribed to carbons in –CH2–, the peak at 40.2 ppm is attributed to the carbons in –CH–, and the chemical shift at 16.8 ppm is assigned to the carbons in –CH3 which are the substituents of benzene ring in PPO.18 All above indicate that PPO/PS alloy has been successfully synthesized.
After extracting the product with 2-butanone for 48 h, the polymer alloy was separated into two parts. The soluble part is supposed to be PS since PS dissolves well in 2-butanone and the insoluble part is ascribed to be PPO and grafted polymers (PPO–g–PS) that may be formed during the polymerization.1913C NMR spectrum in CDCl3 of the insoluble part is shown in Fig. 3. It is found that there are only characteristic signals of PPO but no signals of PS, meaning that no PPO–g–PS was generated during the preparation of PPO/PS alloy.
Since St could undergo polymerization slowly without catalyst, how to demonstrate that CuCl2/DMAP was a versatile catalyst in this system, i.e. CuCl2/DMAP did catalyze the homopolymerization of St? The resulting PS should be able to be used as a macroinitiator for chain extension using a conventional ATRP process if the polymerization of St followed the rule of RATRP during the preparation of PPO/PS alloy.10,14 The chain extension reaction of St was carried out in bulk at 100 °C in the presence of a conventional ATRP catalyst, i.e., CuCl/DMAP, and PS (Mn = 5100, Mw/Mn = 1.10) which extracted with 2-butanone and distilled under reduced pressure was added. The Mn of the resulting PS increased up to 19300 (Mw/Mn = 1.13), which can be essentially demonstrated by the GPC curves shown in Fig. 4. No additional peak appears in Fig. 4, confirming that higher-molecular-weight PS has been formed. These results indicate that the AIBI/CuCl2/DMAP initiation system did induce living polymerization via a RATRP process during the preparation of PPO/PS alloy and CuCl2/DMAP did act as a versatile catalyst for the homopolymerizations of both monomers.
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Fig. 4 GPC curves of PS before and after chain extension reaction (the mobile phase was toluene and flow rate = 1.0 mL min−1 at 30 °C). |
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Fig. 5 DSC curve of PPO/PS alloy (51 wt% for PPO with Mw = 30![]() ![]() |
According to Fox equation (eqn (1)) the weight percentage of PPO and PS in the PPO/PS alloy were calculated to be 51 wt% and 49 wt% based on Tg,PPO = 210 °C and Tg,PS = 100 °C,2,20 respectively, which is in good agreement with the weight measurement (51 wt% for PPO, and 49 wt% for PS).
![]() | (1) |
As reported before,5 the introduction of St could lower Tg of the PPO polymer particles, increase the mobility of the oligomer chains and facilitate the oxidative polymerization at the later period.
Two preformed polymers were blended with the same composition in comparison and similar characterization results (for FTIR, NMR, DSC) were obtained, indicating that the product synthesized using this method was exact the desired one.
The feeding time of St could also affect the molecular weight of PPO, as shown in Fig. 7, when adding St at 3 h after the polymerization of DMP beginning, the molecular weight of PPO reached the highest one. This phenomenon may be explained as follows. The oxidative polymerization of DMP is influenced by two factors, one is how difficulty the oligomers and monomers could be oxidized, and the other is the mobility of oligomer chain. Since the oxidative polymerization of DMP is quite similar to condensation polymerization for the monomers quickly formed oligomers during the early period (0.5 h) of the reaction, the amount of monomers is so little that it can be negligible. A significant decrease in the oxidation potential of oligomers could be achieved with the addition of base, which make oxidation of oligomers proceed easily, and during the early stage of polymerization, the reaction rate is mainly affected by the oxidation of oligomers. When adding St during the earlier part of the polymerization, the oligomers end with –OH swelled in St and the rest part end with –ONa remained in water. The oligomers swelled in St possess higher oxidation potential than the ones end with –ONa, which makes the oxidation of them more difficult. Thus, adding St too early during the polymerization of DMP would depress the reaction rate and could not obtain the high molecular weight in turn. On the other hand, the precipitation of PPO in water plays the dominant role in inhibiting the increase of molecular weight during the later period of polymerization of DMP. Therefore, since the addition of St could lower the Tg and improve the mobility of oligomers, the later the St is added, the shorter time for it to act as the swelling agent, and the molecular weight of PPO would decrease.
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