Kinetics of precipitation polymerization of vinyl chloride

Abstract

AbstractThe polymerization of vinyl chloride in solution in tetrahydrofuran, and in cyclohexane, with AIBN initiator, has been studied over the monomer concentration range 3–14 mole/1. at 40 and 50°C., via a dilatometric technique. In tetrahydrofuran (a relatively good solvent for the polymer), polymerization rate is second order in monomer, and 0.7 order in initiator; DP is very low (∼100) at monomer concentrations below 10 mole/1. and relatively insensitive to monomer or initiator concentration. Below 10 mole/1. the reaction is either homogeneous, or the polymer separates as a solvated gel without evidence of autoacceleration. Above 10 mole/1., however, the polymer separates as a granular solid, autoacceleration is observed, and DP increases several fold. In cyclohexane (a poor solvent), polymer precipitates from the outset of the reaction, and autoacceleration is observed at all monomer concentrations. Polymerization rate is first order in monomer, half order in initiator, first order in polymer at low polymer concentration, and two‐thirds order in polymer at high polymer concentrations. DP is high, and essentially linear in monomer concentration. Results in tetrahydrofuran are interpreted as indicating that the solvent functions as a retarder, forming (by transfer from monomer) relatively unreactive radicals. In cyclohexane (as in bulk monomer), normal homogeneous kinetics govern the reaction in solution, but precipitated polymer particles function as secondary loci for propagation. A simplified moving‐boundary treatment of diffusion of radical activity into and out of a thin surface layer of a growing polymer particles satisfactorily accounts for the observed rate dependency on polymer concentration. Kinetic differences between the precipitation polymerization of vinyl chloride and acrylonitrile are believed attributable to the extremely high frequency of radical transfer to monomer in the former system.