Anti-Arrhenius reactions in the aqueous polymerizations of methyl methacrylate (MMA) initiated by the Fe(II)-persulphate redox system

898-903The (initial) rates of aqueous polymerization of methyl methacryslate (MMA) initiated by the redox pair, Fe2+-S2O ions, increase with increase in temperature from 5 to 10°C, and then decrease with further rise in temperature from 10 to 20°C, giving a rate minimum at 20°C. The rate increases again with increase in temperature from 20 to 40°C, giving a second maximum at 40°C, and decreases monotonously with further rise in temperature from 40 to 65°C. The Arrhenius activation energy (Ea) for polymer formation thus varies differently over the different zones of temperature, giving Ea =27.63, - 63.78, 24.36 and - 12.14 kJ/mol, over the temperature zones of 5 to 10°, 10 to 20°, 20 to 40° and 40 to 65°C respectively. The physical appearance of the polymer phase during the process also changes with temperature, viz.: (i) milky white precipitate below 10oC, (ii) a light yellow unstable colloid partly mixed with a related precipitate from 10 to 20°C, and (iii) stable yellow colloid above 20°C. The stability of the colloid and the intensity of its yellow colour increase with increase in temperature from 20 to 65°C. The observed odd anti-Arrhenius kinetics is suggested to be due to some temperature dependent side reactions of SO free radicals, and to odd changes in the physical state of the polymer formed with change in temperature