Kinetics and mechanism of potassium persulphate-initiated aqueous polymerization of methacrylonitrile at 50°C in an inert atmosphere of nitrogen gas

Abstract

The mechanism of persulphate-initiated aqueous polymerization of methacrylonitrile (MAN) has been investigated kinetically at 50°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 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 p∝[ I] 0.50±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 to 7), viz. reactions C and D, which would then compete with the well established reactions A and B: S 2 O 8 2→2 SO − 4( A) S 2 O 8 2+ H 2 O→2 HSO 4 −+ 1 2 O 2 at pH 5 to 7( B S 2 O 8 2+ M→ . O− SO 3 −+ SO 4 .− ( C) S 2 O 8 2+( M j .) w→ M j− O− SO 3 −+ SO 4 .− j = 1 to 10 (D) where (M · j ) w 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 ± 0.2 at 50°C in the presence and absence of electrolytes ([K 2SO 4] = 7.5 × 10 −3 mol dm −3). The termination rate constants in the aqueous phase ( k tw) and the polymer phase ( k tp) have been estimated as k tw = 1.29 × 10 5 dm 3mol −1s −1 and k tp = 1.90 × 10 2 dm 3mol −1s −1, based on the literature value of k p of MAN as 26 dm 3mol −1s −1 at 50°C. The locus of polymerization is partly in the aqueous phase and partly in the polymer phase.