Unexpected effect of the monomer concentration on the rate of a radical polymerization

The radical polymerization of vinyl monomers is usually initiated by physical and chemical means. After an increasing polymerization rate, R_p, at low monomer concentrations, some reactive systems show an unexpected minimum for R_p at high enough monomer concentrations. The radical polymerization of methyl methacrylate (MMA) initiated by the redox system D -glucose–ceric ion at varying MMA concentration is discussed. The peculiar behaviour of R_p is explained by the presence of two circumstances: the initiation rate from D -glucose radicals does not depend on MMA concentration when most of the D -glucose radicals formed react by adding to monomer, and the radical chains initiated by D -glucose radicals undergo mutual termination with a portion of the radical chains initiated by monomer radicals. Some information about the nature of the polymer end-groups is reached from the mechanistic approach.