Guo Yeshu¹, Lu Jianmei*^{1 2}, Wu Jianfei¹, Xia Xuewei¹, Zhu Xiulin^1
(1Department of Chemistry and Chemical Engineering, Suzhou University, Suzhou  China 215006; ²Jiangsu Polytechnic University, Changzhou,  China, 213016)

Received Sep. 22, 2003; Supported National Natural Science Foundation of China (No. 20076031) and Natrural Science Foundation of   Jiangsu Province  (No. BK2002042)

Microwave irradiation technology has been applied in the organic synthesis as an alternative of traditional heating method. In 1986, R.N. Gedye^[1] and R. J. Giguere^[2] reported some organic reactions prompted by microwave irradiation. Microwave irradiation has also been used in polymer synthesis and curing. Such a radical polymerization of 2-Hydroxy Ethyl Methacrylate^[3],Styrene^[4],MMA^[5],and curing of Epoxy resins and Polyurethane^[6-10]. The imidization of Polyamic Acids (PAA) under microwave irradiation ^{[11, 12]} also has been studied. In the previous papers, the author have also reported work related to the microwave irradiation prompted solid-state co-polymerization in Binary Maleic Anhydride (MAH) and AT system^[13], MAH and Dibenzyl Maleate system ^[14]. It also shows positive effect on carrying out the Metal-Organic reactions using microwave oven, e.g. for synthesis of some metal complex, only took minutes under microwave irradiation instead of several hours by traditional heating method.^[15] Mingos and his colleagues^[16] prepared self-assembled polymer-metal complex (PMC) in one step by utilizing microwave oven, which was the first example that self-assembled PMC was prepared based on coordinate bond and hydrogen bond in one step. The PMC, a new type of polymeric material with novel structure and performance, has both the characters of organic polymer and inorganic metal. Because of the attractive thermal, mechanical, photo, electrical and magnetic performance, PMC has probability to be applied to information storage and transmission, photoelectric fiber, photography, electrostatic printing, catalyst, semiconductor, superconductor, ceram etc.
    In this paper microwave irradiation was applied to synthesize PMC such as Cu-PDM,and the catalytic function of Cu-PDM was studied and the kinetics of MMA polymerization catalyzed by Cu-PDM was studied. Herein methacrylic 2-(dimethylamino) ethyl ester (DM) was used to synthesize PMC. During the polymerization, the C=C was opened and formed the main chain, while N was located on side chain, so the obtained PMC was side-chain metal complex.

1 EXPERIMENT SECTION
1.1 Base Material
Methacrylic 2-(dimethylamino) ethyl ester(DM), Methyl Methacrylate(MMA), methyl alcohol, N,N-dimethyl formamide (DMF), Chemical grade, cyclohexane, chloroform, tetrahydrofuran, sodium sulfite, blue vitriod and hydrochloric acid: analytical grade.
1.2 Device
Xianhua microwave, KS2163, the maximum output power is 850 W, refitted. The microwave oven electric current could be adjusted and read continuously by using voltage regulator, which installed outside of the oven. The reaction device is showed as Figure 1.

Figure1 Refitted microwave device

1.3 Procedure of experiment
1.3.1 Polymerization of DM
(1) Polymerization of DM under microwave irradiation
Weighted DM was added into three-necked bottle, put into refitted microwave device, repeated degassed and filled with N₂ for 30 minutes, then start the microwave irradiation to induce reaction under protection by N₂. The temperature was controlled at 85±1ºC, the irradiation reaction power was adjusted by the regulation of electric current. After certain set time, the mixture was taken out and treated with specified solvent ¹, washed by alcohol and dried up until to constant weight under vacuum at low temperature.
    [Note1] DM homo-polymer was poured into cyclohexane 20 times of the quantity of polymerization system. PDM was precipitated and filtered through the cyclohexane.
(2) Traditional heating polymerization of DM
Weighed DM was added into three-necked flask, pick up the suction and filled with N₂ for 30 minutes, and then put into the hot bath which has been heated to 85±1ºC, started the heating polymerization with protection by N₂ all the time, the temperature was still controlled at 85±1ºC,After set time, the mixture was took out and treated with specified solvents as process of Note 1. Finally, it was washed out, dried up until to constant weight under vacuum at low temperature.
1.3.2 Synthesis of Cu-PDM
CuSO₄ aqueous solution was added into the PDM prepared as part3 (1) in this paper, and the mixture was set for 24 hours to improve the coordination degree between the PDM and metallic ion, then it was separated through centrifugal and uncoordinated Cu²⁺ was removed by deionized water through repeated wash with deionized water. The product above mentioned was dried up until to constant weight under vacuum at low temperature.
1.3.3 Copper content analysis through atomic absorption spectroscopy
PDM-Cu(II) complex was washed by diluted hydrochloric acid(1:1), then filtered. The copper content in the filtrate was determined through atomic absorption spectroscopy. The result showed copper content in PDM-Cu(II) was 3.04%.
1.3.4 MMA polymerization in the presence of Cu-PDM
A certain weight of MMA was added into a conical flask, and then Cu-PDM/Na₂SO₃ aqueous solution was added in certain ratio. Methyl alcohol containing a certain concentration of hydrochloric acid was added into the flasks to precipitate the PMMA, which was then repeatedly filtered, washed with deionized water and dissolved in THF. A certain hydrochloric acid was added in order to separate the PMC, dissolving Cu²⁺, which was in PMC into water. PMMA was dried up to constant weight, and PMMA's viscosity and molecular weight was determined by using GPC.
1.3.5 Investigation of kinetics MMA polymerization catalyzed by Cu-PDM /Na₂SO₃ aqueous solution.
MMA was poured into 100ml round bottom flask placed in a bath at 45ºC, and 60 ml aqueous solution of 0.2g Cu-PDM and 0.2g Na₂SO₃ was added as catalyst. A dilatometer was installed onto the top of the flask.
1.3.6 Measurement
a. IR spectrum: determined by American Perkin – Elmer 577 (KBr pressed disc method))
b. ESR: ER2000-SRC (Bruker)
c. XPS:V.G.. Scientific ESCALAB Mk(II) (U.K.)
d. GPC: Waters 150c GPC.
e. Thermal analysis system: DELTA SERIES TGA7, American P-E. Temperature range: room temperature—950ºC; Sensitivity: 0. 1mg, heating Rate: 10. 0ºC/min.
f. Atomic absorption spectrometer: Hitachi 180-80 Polarized Zeeman Atomic absorption spectrophotometer.

2 RESULT AND DISCUSSION
2.1 The polymerization of DM under microwave irradiation
The reaction scheme was showed below:

2.1.1 Influence of microwave irradiation power on the inherent viscosity of polymer
As Figure 2 showed the inherent viscosity of PDM was found to deal with increasing microwave irradiation power. In the range of experiment, when the microwave irradiation power was controlled by electric current at 15mA, the inherent viscosity of polymer reached maximum. The inherent viscosity decrease was due to the increase of radical number result from increase of microwave irradiation energy.

Figure2 Relationship between the microwave irradiation time and inherent viscosity of PDM

2.1.2 Productive rate of PDM under microwave irradiation:
(1) Influence of microwave irradiation time to the yield of PDM:
As Figure 3 showed, at a certain microwave irradiation power, with electric current controlled at 15mA, the PDM yield increased with the increasing of irradiation time. In the range of this experiment, results show that the yield of polymerization can reach the maximum value 92%,when the polymerization time is belonged to 2.5 hours.

Figure3 Relationship between microwave irradiation time and conversion of PDM



Figure4 Relationship between microwave irradiation power and conversion of DM

Figure5 Comparison between heating polymerization and microwave irradiation polymerization of DM

2.2 Characterization of the Cu-PDM
2.2.1 IR spectrum
Figure 6 was the IR spectrum of PDM, and Figure7 was the IR spectrum of Cu-PDM complex. From the Figure7, it showed that a strong peak appeared at 1130.4 cm^-1, it should be migration of VC-N peak 1161.2cm^-1 resulted from the coordination of N of the co-polymer with Cu²⁺. The change of the IR peak was confirmed that coordination of Cu(II) with N which is in the co-polymer PDM and to form Cu-PDM^[17].
2.2.2 ESR spectrum of Cu-PDM
As Figure 8 showed, on the ESR curve of the Cu-PDM, three g value appeared. The first g=2.907, the second g=2.183 and the third g=2.036. The polymer itself has no ESR sign, and Cu²⁺ has a g=2.190^[18]. Since the change of ESR sign position or change of g value is due to the nature of chemical bond^[19], it could be inferred a new coordinate bond formed between Cu²⁺ and polymer.

Figure 8 ESR spectrum of Cu-PDM

    Figure 9 and figure 10 are XPS spectrum of Cu(2p) in CuCl₂ 2H₂O and PDM-Cu(II) respectively. We can see a strong peak of shake up satellite in Figure 9, which is due to a lone pair electron in 3d obital . While we can see that shake up satellite of PS-DM-Cu(II) decreased sharply. If a tested sample in XPS, uses dsp2 hybrid orbital, the shake-up satellite can not be observed in the spectrum of XPS. In condition that electron is in low-spin state, coordination number of Cu is 4^[20]. That is to say covalence bond in dsp2 hybrid orbital, 3d electron on the coordinated Cu (II) is excited to 4p orbital. In PS-DM-Cu(II) shake up satellite decrease sharply while not disappear. From this phenomenon we consider that there is majority of quadricovalent PDM-Cu(II).

Figure 9 Binding Energy (eV) of Cu2P in CuCl₂2H₂O

Figure 10 Binding Energy (eV) of Cu2P in PDM-Cu(II)

2.2.4 Thermal analysis
(1) Thermogravimetric analysis (TGA)
From the Figure11, the thermogravimetric curve showed that,from the start temperature to 270ºC the weight loss of Cu-PDM (about 6%) was not obvious. From about 226ºC to 450ºC there was an obvious decompose and the loss of weight about 21%. In the range of 450-525ºC, there was a comparatively stable phase, after which there followed another further decompose till to 735ºC (loss of weight was about 21%). The Thermogravimetric analysis displayed that there were two different phase existed in the Cu-PDM^[21].
(2) Differential scanning calorimetry (DSC)
From the Figure 12, there was an obvious exothermic peak of Cu-PDM at 285ºC, it was in accordance with its TGA curve, a compose of the complex occurred at this temperature, which resulted from the breaking of coordination bond.
    From the before analysis, it showed that the PDM coordinated with the Cu ²⁺.

Temperature (ºC) DELTA SERIES TGA	Temperature (ºC) DELTA SERIES DSC
Figure 11 TGA of Cu-PDM	Figure12 DSC of Cu-PDM

3 Polymerization of MMA initiated by Cu-PDM/Na₂SO₃
The polymerization of MMA was examined at room temperature in the presence of water. It was found that the MMA could not polymerize when only Na₂SO₃ or 5ml Cu-PDM aqueous solution was added alone into 10g MMA, while Na₂SO₃ and Cu-PDM existed at the same time MMA started to polymerize after about 3 minutes at room temperature.
(1) Polymerization kinetics of MMA initiated by Cu-PDM/Na₂SO₃ system
The polymerization mechanism of MMA initiated by Cu-PDM/Na₂SO₃ system Yang^[22]etal found that the copper polycarboxylate/sodium sulfite can catalyze polymerization of MMA in a complexation way. Cu-PDM/Na₂SO₃ system catalyzed MMA polymerization in a similar way, since the color of Cu-PDM/Na₂SO₃ did not change, from which we could exclude the oxidation-reduction mechanism. The reaction mechanism can be showed as Figure13.

Figure 13 The polymerization mechanism of MMA initiated by Cu-PDM/Na₂SO₃ system

the kinetics of polymerization could be formulated as follows ^[23].

At a lower monomer conversion, the concentration of free radicals was constant, so the formulation was described as:


[M₀] is the initial monomer concentration, [M] is monomer concentration at time t. K is a constant when temperature keep constant.
    Based on data obtained through dilatometer method, the next formula is deduced:

is the volume change of the polymerization system when the monomer conversion reached 100% and is volume change during the polymerization.
    In all, the formulation could be deduced as follows,

  Based on the kinetics determination of MMA polymerization initiated by Cu-PDM /Na₂SO₃ aqueous solution, it displayed that the polymerization was in accordance with free radical mechanism , R_p=68.5*10^-5mol/ls.

Figure 14 Kinetics determination of MMA catalyzed by Cu-PDM/Na₂SO₃ aqueous solution system

(2) Influence of the content of Cu-PDM and Na₂SO₃ to the polymerization of MMA
The influence of Na₂SO₃ content on of PMMA was showed in Figure 15. When the content of Na₂SO₃ was 0.2% of the weight of monomer, the of PMMA obtained was the maximum. The of PMMA decreased when Na₂SO₃ content increased, since the number of free radicals was increasing with the Na₂SO₃ content increase.

Figure 15 Relationship between content of Na₂SO₃ and Mw of PMMA

4 CONCLUSION
The homo-polymerization of DM can be prompted by the microwave irradiation, which shorten the reaction time and improved the yield of PDM. Microwave irradiation power decreased the inherent viscosity of PDM resulted from the increase of free radicals number. The PDM obtained by microwave irradiation can coordinate with the Cu²⁺ directly to form Cu-PDM. The Cu-PDM could heterogeneously catalyze MMA polymerization at room temperature in a free radical mechanism. The Cu-PDM/Na₂SO₃ system has a high catalytic activity when catalyzing MMA polymerization. The molecular weight of PMMA obtained by catalysis of Cu-PDM can reach 830,000, which was greater than that obtained by traditional initiator. The solid catalyst could be easily separated from the polymerization system.

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