The connection between structure formation and the kinetics of the methyl methacrylate radical polymerization over ZnCl 2 at high conversions

Abstract

The structural-rheological features of the polymer and of model systems were compared in this study with the kinetic parameters of the methyl methacrylate (MMA) radical polymerization over ZnCl 2 at high conversion and in the presence of a chain transfer agent. From the intensity of the light scattered by the reaction systems during polymerization it was established that the characteristic conversion q 1, associated with the change from a dilute polymethylmethacrylate (PMMA) solution in its monomer to a moderately concentrated solution, in which a fluctuating structural network starts to form, q 2, which coincides with the onset of spontaneous acceleration of the polymerization, q a , will shift towards smaller polymer concentrations in the reaction system when the ZnCl 2 content is increased. The rheological measurements carried out on model systems containing the hydrogenated monomer analogue, namely methyl isobutyrate (MIB) indicated a considerable increase in viscosity of the PMMA concentrate as a result of increase in ZnCl 2 concentration. The change from a slightly structurated system to a planar, steric, fluctuating network, observed during these rheological determinations as a kink on the log η = f (log c 2) plot and the effect of the ZnCl 2 additions on the position of this kink, correlated well with the q 2 and q a values determined from light scattering and the polymerization kinetics. The results are explained by the structuring effect of the ZnCl 2 on the PMMA macromolecules. The two coordination bonds of ZnCl 2 are saturated by reaction with the carbonyl groups in a single or in two adjacent macromolecules, which increases the PMMA chain rigidity, or they form ligands between the chains. Both these factors will give rise to the formation of supermolecular structures and of a bulky steric network in the polymerizing and in model systems.