Kinetics and mechanism of the emulsion polymerization of styrene in the aqueous media at 50°C and at low monomer concentration initiated by persulfate. I. Initiation

Abstract

Abstract The mechanism of the water‐soluble persulfate‐initiated emulsion polymerization of styrene in the aqueous media at 50°C has been investigated kinetically by the conventional dilatometric and gravimetric methods at low concentration of the monomer (5% v/v). It has been found that the initial rate of polymerization V p is approximately proportional to initiator concentration [I] to the 0.50 power, i.e., V p ∝ [I] 0.50 , and the viscosity‐average molecular weight M v is approximately inversely proportional to the 0.50 power of the initiator concentration, i.e., M v ∝ [I] −0.50 . With the progress of the reaction, the initiator exponent of the reaction rate equation decreases gradually from 0.50 to 0.25, but that of the molecular weight (1) equation remains constant up to 20% conversion and thereafter begins to decrease. Since the kinetic data at zero conversion satisfy the steady‐state kinetics of the free‐radical‐initiated homogeneous vinyl polymerization, it is suggested that the initiation of emulsion polymerization of styrene is a two‐step process. It starts in the aqueous phase by the primary free radicals from the water‐soluble initiator or secondary free radicals derived from the soap molecules. The second step occurs in the monomer‐leaded micelles by the water‐soluble or water‐insoluble macroradicals or by radicals derived from the soap molecules. The latter are likely to be produced in the aqueous phase by the oxidation of soap with S 2 O 8 2− ions or SO 4 − radicals. It has been noted that the rate of thermal decomposition of persulfate increases by a factor of 6–8 times under different experimental conditions in the presence of soap.