Kinetic evidence of a matrix effect in the bulk polymerization of acrylonitrile

The kinetics of the γ-ray-initiated polymerization of acrylonitrile in bulk are reexamined in broad ranges of temperatures and radiation dose rates. The discussion of the results coupled with an analysis of earlier data indicates that the polymerization of acrylonitrile proceeds by different mechanisms depending on the reaction temperature. Above 600C the precipitated growing chains recombine readily; therefore, the autoaccelerated conversion curves cannot be accounted for by an occlusion effect. It is suggested that autoacceleration is caused by a fast propagation taking place in oriented monomer aggregates which result from dipole-dipole association of the monomer with the polymer chains formed in the early stages of the reaction (matrix effect). Below 100C the precipitated growing chains are buried in the dead polymer and monomer diffusion toward the occluded chain ends is very limited (occlusion effect). Between 10 and 600C the system gradually changes from one dominated by occlusion to one where the matrix effect determines the kinetic behavior. The conclusion based on kinetic data is in agreement with results obtained from studies of the postpolymerization in these various systems. 3 figures, 1 table