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SynopsisAnalytical grade sodium metabisulfite (NazS205) has been found to initiate the polymerizations of methyl methacrylate (MMA) and ethyl methacrylate (EMA) in the aqueous media in the presence and absence of detergents, and of styrene in the presence of detergents only, but it fails to initiate the polymerization of methylacrylate (MA) at low concentrations of the initiator and of acrylonitrile (AN) in the absence of cationic detergent micelles. If a mixture of AN (2.0%, v/v) and metabisulfite (l.60%, w/v) is kept for 16 h at 5OoC in the presence of nitrogen, no polymerization occurs, but if a little ferric chloride (0.001%, w/v) is added to this mixture in nitrogen atmosphere, the initiation of AN polymerization is found to occur. MA can be polymerized partly by adding metabisulfite to an aqueous solution of MA and a cationic detergent (above CMC) in the presence and absence of air. Very little polymer is found under similar conditions with AN. No polymerizations were found to occur with any of the above-mentioned monomers if hydroquinone was present in the system. In the Na2S205-MMA and Na2S205-EMA systems, the average rates of the aqueous polymerizations were found to decrease with the increase of the initiator concentrations (from 1.316 X to 2.63 X lo-' m n ) at 50°C in the presence of nitrogen, and to be approximately inversely proportional to the square root of the initiator concentrations. It is suggested that the bisulfite (produced by the reaction of S20;-ions with water) adds to vinyl monomers as well as initiating polymerization reactions by the reduction activation of the monomers in the presence of nitrogen. The presence of bulky groups such as methyl, phenyl, etc., at the 0-position of the ethylenic double bond of the monomer, probably prevents or slows down the bisulfite addition reactions due to the steric hindrance, and so the polymerization reactions will predominate in the system of MMA, EMA, and styrene-like monomers. The complex species formed due to the interactions of the cetyltrimethyl ammonium bromide (CTAB) micelles and free CTAB cations with HSO; and Sz0;-ions initiate the polymerizations of MA and of AN in the presence of nitrogen or air. Cationic detergent micelles protect the monomers from the direct attack of the HSO;/S20;-ions.

Sodium metabisulphite (Na2S2O5) has been found to initiate the aqueous polymerisation of methyl methacrylate (MMA) at 35°C in phosphate buffer solutions of pH 6.85 and a constant ionic strength (μ) of the media in an inert atmosphere of pure nitrogen. The reaction has a well defined induction period which is a function of the concentrations of the initiator, of the monomer and also of temperature. The polymerisation is signalled by the sudden appearance of a haze at the end of the induction period in a given run, and the polymer separates out as a coarse precipitate during the progress of the polymerisation reactions. When the conversion is over 50 per cent complete, the polymerisation media looks like a thick curd if the monomer concentration is relatively high. The rate of polymerisation is found to decrease with conversion or time in a given run, and the initial rate (vp), obtained by extrapolating the linear yield/time versus time curves to zero time, keeping the conversion below 10 percent, is found as where (I) = initiator concentration in the range, (0.26 to 3.94) × 10−3 (mol dm−3), and (M) = monomer concentration in the range 0.019 to 0.141 (mol dm−3). At high initiator concentrations, the rate of polymerisation is found to decrease. In a given run, the viscosity average molecular weights (M v) of the polymers is found to increase quickly with a conversion of up to 25 to 30 percent, and then slowly with the further increase in conversion. (Mv) however is found to decrease with the increase of the initiator concentrations at a given conversion but increases with the increase of the monomer concentrations. Hydroquinone inhibits the polymerisation reactions, whereas air is found to increase the induction period, but later enhances the polymerisation rate in the same run. It has been shown that the bisulphite addition reaction of MMA is not important under the experimental conditions, and the polymerisation occurs by the free radical mechanism. The rate constant (k2) of the reaction, has been estimated from the analytical data as, k2 = 14.62 × 10−2 (dm3 mol−1 s−1) at 35°C.

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