Kinetics and Mechanism of Potassium Persulphate-Initiated Aqueous Polymerization of Methacrylonitrile at 50 deg C in an Inert Atmosphere of Nitrogen Gas

The mechanism of persulphate-initiated aqueous polymerization of methacrylonitrile (MAN) has been investigated kinetically at 50 deg C in an inert atmosphere of nitrogen gas by the conventional gravimetric method within the solubility range of the monomer. It has been found that the rate of polymerization (R sub p ) of MAN is proportional to the square root of the initiator concentration \I\ and the 1.6 power of the monomer concentration \M\, i.e. R sub p proportional to \I\ exp 0.50 plus/minus 0.05 \M\ exp 1.6 plus /minus 0.05 . It has been suggested that both the monomer and the water-soluble monomeric/oligomeric free radicals react with the persulphate ions in aqueous solutions (pH 4-7), i.e. reactions C and D, which would then complete with the well established reactions A and B: (A) S sub 2 O sub 8 exp 2-- - > 2SO sub 4 exp -- ; (B) S sub 2 O sub 8 exp 2-- + H sub 2 O - > 2HSO sub 4 exp -- + 0.5 O sub 2 at pH 5-7; (C) S sub 2 O sub 8 exp 2-- + M - > M--O--SO sub 3 exp -- + SO sub 4 exp -- ; and (D) S sub 2 O sub 8 exp 2-- + (M sub j ) sub 2 - > M sub j --O--SO sub 3 exp -- + SO sub 4 exp -- , j = 1 to 10, where M sub j ) sub 2 is a water-soluble monomeric /oligomeric free radical. The separating polymer phase was in the aqueous solution as a stable colloid. The distribution coefficient of MAN between the polymer and the aqueous phase was found to be 1.8 plus/minus 0.2 at 50 deg C in the presence and absence of electrolytes (\K sub 2 SO sub 4 \ = 7.5 x 10 exp --3 mol dm exp --3 ). The termination rate constants in the aqueous phase (k sub tw ) and the polymer phase (k sub tp ) have been estimated as k sub tw = 1.29 x 10 exp 5 dm exp 3 mol exp --1 s exp --1 and k sub tp = 1.90 x 10 exp 2 dm exp 3 mol exp --1 s exp --1 , based on the literature value of k sub p of MAN as 26 dm exp 3 mol exp --1 s exp --1 at 50 deg C. The locus of polymerization is partly in the aqueous phase and partly in the polymer phase. Graphs.