Abstract
The emulsion polymerization of methyl acrylate (MA) has been carried out under two different experimental conditions so as to establish the mode of chain initiation. In the first system, the initiator was Fenton's reagent (FeSO 4 and H 2O 2 redox pair) in acid solution at 20°C in the presence of a cationic detergent, cetyltrimethyl — ammonium bromide (CTAB), while in the second system the initiator was potassium persulphate (K 2S 2O 8) at 50°C in the presence of an anionic detergent, sodium lauryl sulphate (NaLS). The polymerization was followed by the conventional dilatometric and gravimetric methods. Initial rates were determined by keeping the conversion below 10% and by plotting the (yield/time) versus time, and extrapolating the straight line to zero time. It was found that in either case, Initial rate ( v) ∝ (Initiator) 0.50 while the viscosity average molecular weight ( M v ) of the polymers up to 20% conversion was found to be approximately inversely proportional to the square root of the initiator concentration ( I), i.e. ( M v) ∝ (I) −0.50 . The rate of polymerization up to 10% conversion was found to be approximately linear with the monomer concentration (from 0.5 to 5.0—, v/v) in the presence of detergents (above CMC) [critical monomer concentration] under the experimental conditions. Since the kinetics of the emulsion polymerization of MA at zero conversion are the same as that of the homogeneous polymerization of MA initiated by the free radicals, it is suggested that the initiation of the emulsion polymerization of MA takes place in the aqueous phase with little or no initiation in the detergent micelles containing monomer. If the polymerization is carried out within the solubility range of MA in the aqueous phase, then the initiation will take place entirely in the aqueous phase. Assuming homogeneous polymerization at zero conversion, the value of the termination rate constant ( k t ) has been extracted from the experimental data at different initiator efficiencies.
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