Mechanism of the emulsion polymerization of methyl acrylate: 2. Kinetics and growth of the polymers

Abstract

The emulsion polymerization of methyl acrylate (MA) was investigated in a batch reactor at 50°C using potassium persulphate (KPS) as an initiator with sodium lauryl sulphate (NaLS) acting as a micelle generator at a low monomer/water ratio (m/w). The rates of polymerizations, number of latex particles per ml ( N), and colloid stability ( CS) of the latex particles have been measured, while the molecular weights of the polymers ( M ̄ w and M ̄ n ) have been estimated from the intrinsic viscosity (η) data, as a function of (m/w) ratio, detergent and initiator concentrations. It has been found that the steady state rate ( R p) obtained during the 10 to 40% conversions, may be written as when the (m/w) ratio is higher, i.e. monomer concentration is above its solubility (0.58 moles per litre at 50°C), and when the monomer concentration is less than 0.58 moles per litre. In the absence of detergents, the steady state rate ( R″ p) is given by when the monomer concentration is less than 0.58 moles per litre. The polymer phase was a very stable colloid in the aqueous medium even in the absence of detergents. At very high concentrations of detergents (i.e. above 3.5 × 10 −2 moles per litre) the rates of polymerizations were found to decrease. Injections of additional quantity of KPS late in a run did not alter the rates of polymerization, whereas injections of additional quantities of detergents late in a run always increased the polymerization rates. Injections of monomer late in a run increased the rates, if the (m/w) ratio was lower and detergent was present, and had no effect on the rates if the initial (m/w) ratio was higher. No experimental evidence was found in favour of the hypothesis that the chain termination in the latex particles would occur as a result of chain transfer to the monomer molecules. Injections of monomer late in a run always increased the polymer molecular weights. The ratio ( M ̄ w M ̄ n ) was found to be about 1.30 to 1.50, but at higher (m/w) ratios lower KPS concentrations and higher soap concentrations (above CMC) the molecular weight ratio was found to be very close to unity. In a given run, the average rate of polymerizations ran approximately parallel with the rise and fall of the cumulative molecular weights of the polymers with time or conversion. It has been found that stirring always decreased the polymerization rates in the absence of detergents, but increased the rates in the emulsion systems containing detergents. The Harkins-Smith-Ewart theory of the emulsion polymerization of styrene has been tested with the analytical data, and it has been found that the theory gives only a qualitative description of the emulsion polymerization of methyl acrylate.