Microwave energy polymerization reactions

This study revealed that under microwave conditions polymerization phenomena such as polymerization selectivity, polymerization temperature shift, and polymerization temperature shift as a result of the microwave power setting, can be observed when products are compared with those obtained under conventional conditions. To explain these phenomena it was proposed that a new dipole partition function is present in the microwave field, so values of thermodynamic properties such as internal energy and Gibbs free energy of materials with permanent dipole moments change under microwave conditions, which in turn leads to shifts in the reaction equilibrium and kinetics compared with conventional conditions at the same temperature [46]. 

The effect of microwave power on the polymerization process under microwave irradiation (domestic microwave oven) was studied by Zhang et al. (2004). They found that molecular weight of the polymer was increased and reached a maximum, when 90% conversion was reached up to 255 W. When a higher microwave energy (510 W) was applied, the molecular weight first increased and subsequently decreased in time due to transesterification reactions. 

In the thermal-catalytic method a peroxide catalyst is usually used to initiate the free radical chain reaction. The main disadvantages are the higher temperatures required for carrying out the polymerizations, the potential hazard of explosion on addition of catalyst to the monomer, and disposal of excess catalyzed monomer after impregnating. Combinations of heat, radiation, and catalyst have been experimented with to reduce the radiation and catalyst requirements and to increase the rate of polymerization. In thermal polymerization a muffle furnace, infrared heating, and microwave heating can be used to provide the thermal energy. 

The bulk polymerization of styrene at two different power levels - 300 and 500 W - conducted in a multimode microwave cavity was investigated by Chia et al. [38]. The reactions were run in 2-ml sample vials of 10 mm diameter, in which 23.0 mg of AIBN was placed together with 455 mg of styrene. The conversion profiles of the microwave polymerization were significantly different from that of the thermal cure at the same temperature of 80 °C. The thermal cure was characteristic of a gradual gel effect at 30-50% conversion while, with the microwave cure at 300 W and 500 W, a sharp and large gel effect was recorded at conversions 20-69% and 20-65%, respectively (Fig. 5). Moreover, the comparison of thermal and microwave polymerization under similar conditions showed a reaction rate enhancement of 190% for 500 W and 120% for 300 W. Similar to the microwave polymerization of MMA [35], the limiting conversion of styrene decreased from 72% for conventional thermal conditions down to 69 % at 300 W and 65 % at 500 W of microwave irradiation power. Finally, it was stated that comparison of kinetic results of microwave induced reactions should consider the temperature as well as the power of micro-wave irradiation due to different energy supplied to the reaction system [38]. 

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