Absolute propagation rate constants for the radical polymerization of substituted styrenes

Abstract

AbstractAbsolute propagation rate constants were determined by application of the Smith‐Ewart kinetic theory to the emulsion polymerization of styrene, o‐methylstyrene, p‐methylstyrene, and “vinyltoluene” (Dow, 60:40 m‐:p‐methylstyrene). Conversion during polymerization was followed by dilatometry, and weight‐average molecular weights of the latex particles were determined by light scattering. The monomer–polymer equilibrium ratio was determined both dynamically as the inflection point of the rate curve and statically by equilibrating monomer with polymer. For styrene, the propagation rate constant (300 l./mole sec. at 50°C.) and its activation energy (17.6 kcal./mole) are in good agreement with literature values. Both rates and activation energies are lower for the other monomers (o‐methylstyrene, 60 l./mole sec. and 13.9 kcal./mole; p‐methylstyrene, 140 l./mole sec. and 7.7 kcal./mole; vinyltoluene, 220 l./mole sec. and 13.4 kcal./mol, all at 50°C.). The differences are attributed to increased radical stability from methyl group contributions to resonance structures and to steric hindrance of the ortho‐methyl group.