Lu Jianmei, Ji Shunjun, Wu Jianfei, Zhu Xiulin
(Department of Chemistry and Chemical Engineering, Suzhou University, Jiangsu 215006,China)

Received Feb. 25, 2001; Supported by the National Natural Science Foundation of China (No.20076031).

Abstract The microwave irradiation polymerization of the dibutyltin maleate (DBTM) and allyl thiourea (AT) system^[1] or the DBTM and stearic acid vinyl ester (SAVE) system^[2] showed that the copolymerization can go on wheels by microwave irradiation whereas there is no polymer formed by thermal polymerization at the same temperature in 7h. It may be not only the simple dielectric heating but also the "the microwave effect". In order to verify the effect of microwave, eight systems were selected for investigation to compare their thermal polymerization with the microwave polymerization.
Keywords thermal effect, microwave effect, microwave irradiation, thermal polymerization

Microwave ovens have been used in chemical laboratories for moisture analysis ^[3] and wet ashing procedures of biological and geological materials ^[4] for a number of years. In recent years, microwave irradiation was widely used in the chemical field ^[5,6]. In organic chemistry, the use of microwave heating results in short reaction time compared with conventional heating ^[5,7], ease of workup after a reaction. Microwave ovens provide a clean and cheap alternative to conventional oil baths. The microwave irradiation was also used in the polymerization reactions recently ^[8]. In the field of synthetic polymer chemistry, microwave energy has been utilized for the radical polymerization of vinyl monomers such as 2-hydroxy ethyl methacrylate^[9], methyl methacrylate (MMA)^[10], and styrene^[11], during the course of our studies, we have already reported the microwave irradiation solid state copolymerization in binary maleic anhydride and dibenzyl maleate system^[12], maleic anhydride (MAH) and AT system in solid state^[13], on the other hand, the curing of polymer such as eposxy resins and polyurethane^[14], as well as the imidization of polyamic acids (PAA)^[15] has also been studied under microwave irradiation. However the mechanism of their homo-polymerizations or copolymerizations under microwave irradiation is still need to be studied. Here, the effect of microwave irradiation on some monomers for their copolymerizations was studied. In comparison with conventional copolymerization, microwave irradiation obviously had decisive function in the copolymerization, e.g. some copolymerization reactions occurred easily, and the inherent viscosity was greater either and it also showed that the microwave not only had thermal effect, but also had more important nonthermal effect. The reasons of nothermal effect were also discussed. This work also provides a new approach to studying copolymerization.

1. EXPERIMENT SECTION
1.1 Base material
Stearic acid vinyl ester (SAVE) , analytical reagent; allyl thiourea(AT), acrylamide (AM), itaconic acid (IA), maleic anhydride, N,N- methylene biacryamide (NBA), dibutyltin maleate(DBTM), chemically pure; sodium acrylate (AANa), self-made by acrylic acid.
1.2 Procedure of polymerization
1.2.1 Solid-phase polymerization under microwave irradiation
Solid-phase polymerization under microwave irradiation was conducted in a special vial, the monomer(note 1) was degassed with N₂ for 30 minutes，and the same original temperature was required. The mixed monomers reacted at the optimum temperature (note 2) while adopting intermittent irradiation, keeping under N₂. After a certain time, the mixture was poured into precipitating agent (note 3) and the polymer was separated out. Filtering , washing, drying at lower temperature under vacuum until constant weight. It was weighed and worked out the percent conversion of polymerization.
[note 1]：The carrier was ground with monomer together if the polymerization system contained it .
[note 2]：The optimum temperature is the temperature closest to melting point of system while the monomer will not melt.
[note 3]：Precipitating agent for DBTM-SAVE system is ethyl acetate methanol solution about 1 : 1, for the other polymerization systems is methanol .
1.2.2 Solution polymerization under microwave irradiation
The monomers were dissolved in a small quantity of DMF. The solution was poured into reactor and was degassed under N₂ for 30 minutes. The same original temperature was required and the intermittent irradiation method was adopted. The temperature of solution was controlled as the same as the one in 1.2.1 keeping under N₂. After a certain time, the mixture was poured into precipitating solvent and the polymer was separated out. Filtering, washing, drying at lower temperature under vacuum until constant weight. It was weighed and worked out the percent conversion of polymerization.
1.2.3 Conventional thermopolymerization
The monomers were ground thoroughly before setting into a special vial , degassed under N₂ for 30 minutes, set into the oven in which the temperature had been set (the temperature is the same as the optimum temperature as before). It was heated for 7h. The final treatment was as before.
1.3 Measurement
1.3.1 Determination of inherent viscosity
The polymer inherent viscosity was determined by one-point method separately at 30.0±0.5℃，conditions as follows： DBTM AT system，inner diameter of U-viscometer is 0.35mm，chloroform-acetic acid solution 20:1；DBTM-SAVE system, the inner diameter is 0.7mm, benzyl alcohol-chloroform solution 3:1.
1.3.2 Determination of reactivity ratio
The composition of the copolymer was determined by liquid chromatography (Shimadzu model LC-6A. Chromatographic column condition: filler-spherisorb CN 5m, size-4.0mm×150mm, velocity of flow-0.8ml/min). The reactivity ratios were obtained by the Lewis and Mayo equation ^{[1, 13,16]}.

2. RESULTS AND DISCUSSION
2.1. The copolymerization of DBTM and AT
2.1.1 Reactivity ratios of two monomers under microwave irradiation
The Lewis and Mayo composition equation is represented as follows:
                                       (1)
M₁ the concentration of AT in the system，
M₂ the concentration of DBTM in the system，
r₁ reactivity ratio of AT,
r₂ reactivity ratio of DBTM.
If b = [M₁] / [M₂]，a = d [M₁] / d [ M₂]，then the equation (1) could be changed as:
          or                                     (2)
    Various composition of monomers (value of b ) were copolymerized by the microwave (controlling conversion ＜10%). The relative value of "a" is
                            (3)
Here, [M₁]₀ and [M₂]₀ are the concentrations of AT and DBTM respectively before reaction, which represents the compositions of monomers. [M₁] and [M₂] are the concentrations of AT and DBTM respectively after reaction, which were measured by liquid chromatography.
    The line (Fig.1-Fig5) was obtained by (a-1)b/a as longitudinal coordinates, b² / a as horizontal ordinate. The linear slope was r₁，the linear intercept was -r₂ . The detailed results showed as Figure1 - 5 and Table 1.

Table 1 Reactivity ratio of copolymerization system DBTM and AT ^*

initiator: 0.4% (wt) of monomers, carrier: 160%(wt) of monomers

   
Fig.1 Solid phase and no initiator, no carrier         Fig.2 Solid phase and no initiator, had carrier

    
Fig.3 Solid phase and no initiator, SiO₂ as carrier       Fig.4 Solid phase and no initiator, Al₂O₃ as carrier

   From the figures and table, the results showed that in all of five reactions r₁r₂ <1，that was to say DBTM and AT were easy to copolymerize under microwave irradiation, but the reactive ratio was different in a wide range because of different polymerization systems.
2.1.2 The influence of the microwave irradiation energy
In the solid phase copolymerization of DBTM which has initiator or not, and intermittent irradiation method was adopted, white polymer powder was obtained. The influence resulted by the change of microwave irradiation energy showed as Figures 6, 7.
    The power and the time of microwave irradiation are important operation parameters. In this work, the influence of irradiation energy (product of power and time) to the percent conversion and inherent viscosity of polymer was studied.
    From the Figure 6, it can be seen that the percent conversion had an increasing trend while the irradiation energy increased. This is identical to the free radical polymerization mechanism, that is to say, when the irradiation power is certain, the percent conversion increased with the increase of reaction time. At the same time, when the irradiation energy is certain, the percent conversion in the system having initiator is higher than that in the system having no initiator. It displayed that the microwave irradiation can initiate the polymerization of this system, and the active center of reaction system increased when the initiator was added, which resulted in the increase of percent conversion.

Fig.5 Solution polymerization, no initiator	Fig.6 Relationship between microwave irradiation energy and percent conversion AT : DBTM = 1 : 1; initiator was 0.4%(wt) of monomers

Fig.7 Relationship between microwave irradiation energy and inherent viscosity of polymer
         AT : DBTM = 1 : 1; initiator was 0.4%(wt) of monomers

    From the Figure 7, it can be seen that，the inherent viscosity had a decreasing trend while the microwave irradiation energy increased, which was the same as the ordinary free radical polymerization. On the one hand, because of the increasing of irradiation energy it increased the chemical activity of chain free radical and resulted in the decreasing of the molecular weight; and on the other hand, since there were no initiator and no solvent，the termination of chain radical may be only coupled termination of diradical by itself. The microwave can cause the high speed vibration of molecules, in other words, the chain radicals of system vibrate on high speed too; the more of irradiation energy, the stronger the vibration is, and the chain radicals collided with each other, the chance of coupled termination of diradical increased, the molecular weight decreased.
    The solution copolymerization of DBTM and AT under microwave irradiation can also be carried out. The product was white powder.
2.1.3 Conventional copolymerization of DBTM and AT.
The experimental results showed that: when the system had no initiator, and the solid-phase was heated for 7 hours, there was no any polymer formed; when the system had initiator, there was only trace polymer formed. In comparison with conventional copolymerization, microwave irradiation obviously had decisive function in the copolymerization, and at the same time, it also showed that the microwave not only had thermal effect , but also had more important nonthermal effect.
2.2 The copolymerization of SAVE and AM
The polymer was got after 2 minutes solid phase polymerization of SAVE and AM by microwave. Although ,the polymer was also got after 12h heating polymerization at the same temperature as microwave temperature 30°C, but the inherent viscosity of this copolymer is less than the one of microwave polymerization.

Fig.8 Relation between polymer inherent viscosity and carrier quantity 
          m-carrier weight       M-total monomer weight

        
Fig.9 Relation between polymer inherent viscosity and initiator quantity

Fig.10 Relation between polymer inherent viscosity and ratio of  monomer
          M₁-AM       M₂-SAVE

2.3 The copolymerization of SAVE and MAH
The polymer was obtained separately after 2 min. microwave irradiation or 12h thermal polymerization of SAVE and MAH, but the inherent viscosity of the polymer by adopting microwave irradiation is greater than the one by thermal polymerization.

Fig.11 Relation between polymer inherent viscosity and  ratio of  monomer
          M₁-maleic, anhydride       M₂-SAVE

Fig.12 Relation between polymer inherent viscosity and  ratio of  monomer in the presence of SiO₂ carrier
          M₁-maleic, anhydride       M₂-SAVE

Fig.13 Relation between polymer inherent viscosity and  ratio of  monomer
            M₁-IA       M₂-SAVE

Fig.14 Relation between polymer inherent viscosity and  ratio of  monomer on the presence of SiO₂ carrier
           M₁-IA       M₂-SAVE

Table 2 Contrast between microwave irradiation and thermal polymerization of other systems

3. CONCLUSION
From the systems studied, there is the special active effect of microwave nonthermal effect besides the heating of dielectric.
The nonthermal effect comes into being because of three cases as follows probably:
3.1 The increasement of reaction rate because of the "overheat spot"
The overall temperature of mixture by microwave irradiation can not be looked as the reaction condition^[17，18] , similarly to the “overheat spot” of phonochemistry .The activity of microwave is due to the overheat spot formed by dielectric relaxation within molecular size .that is to say ,the selective absorption of active polar molecule to microwave picks up the reaction speed. While in the conventional heat, the whole molecule must be heated before reaction.
3.2 The increasement of reaction rate because of the stirring effect
The microwave can make the molecular dipole of compound shift swiftly. The hindrances of intermolecular bond to the rotate of dipole cause the dipole lag after electromagnetic radiation. And it shows the reason why the thermal effect was observed after microwave irradiation. This process can also be thought as molecular stirring or agitation. The productive rates of the several researched reaction systems in this paper are all greater than thermal polymerization. So high productive rate is the result of the stirring of intramolecular dipole because of the increasing of temperature resulted by microwave. The viewpoint that molecular stirring or agitation caused by the microwave is thought as the nonthermal effect of microwave either.
3.3 The increasement of reaction rate because of the increasement of molecular migration property
In the researched systems ,the slowest step of the solid phase reaction is the interdiffusion process of reactant though the inert medium (Al₂O₃ or SiO₂）. Since the reaction rate depends on the slowest step, the enhancement of interdiffusion process is important. At the same temperature, the microwave irradiation can strengthen the interdiffusion property of reactants more easily than that in thermal reaction ^[19，20] .

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