Effect of solvent on the termination rate constant in initial stages of free radical polymerization

Abstract

The effect of solvent on the termination rate constant Kt, in the initial stages of free radical polymerizations has been estimated by considering its effect on the viscosity of the medium and on the overall dimensions of the macroradicals. The expression derived predicts that Kt is inversely proportional to the viscosity of the reaction medium, η0, and that Kt, increases as the overall dimensions of the radicals decrease in poorer solvents. The effect of solvent on the η0Kt, product depends on the average size and concentration of polymer in the system. For relatively high concentrations of high‐molecular‐weight polymers η0Kt can be greater in a better solvent than in a poorer solvent. This trend would be reversed for low concentrations and/or low polymer molecular weights. Good agreement has been found between experimental and estimated values of η0Kt.

Bimolecular Free-Radical Termination at Low Conversion

“…For a given polymer-solvent system, F 1 is a constant function, and the chain-length dependence of k t is thus fully contained in F 2 . 17-19 were found to be in close agreement with experimental results for the dependence of k t on solvent quality in the initial stages of polymerization (82,83), for the description of SIP data (100,149) and pulse radiolysis data (100), to account for the different k t values of a series of methacrylates (207), and the dependence of k t on pressure (206,208). Similar to the model by Horie et al, this model also contains no adjustable parameters, and all quantities involved in F 1 and F 2 are experimentally accessible.…”

“…The predictions resulting from the above equation were consistent with all experimental observations, and the model has successfully been applied to several experimental data sets (82)(83)(84)101). In θ solvents, the value of δ approaches zero; hence, the concentration dependence of k t as a result of coil shrinkage becomes insignificant.…”

“…Mahabadi and O'Driscoll also studied the effect of dissolved polymer on the termination rate coefficient (82,83). They derived a theoretical relationship for the dependence of k t on conversion (82) based on a previously derived segmental diffusion model (100) that allowed for a concentration-dependent linear expansion coefficient and polymer-solvent interactions.…”

Radical Chain Polymerization

“…It is possible to observe a ''crossover'' at somewhat higher conversion whereby the k t in the good solvent is larger than k t in the poor solvent. This occurs because coil size decreases with increasing polymer concentration, but the decrease is steeper in better solvents [Mahabadi and Rudin, 1979]. Many of these predictions for the course of polymerization in stage I have been verified [Abuin et al, 1978;Dionisio and O'Driscoll, 1980;Ludwico andRosen, 1975, 1976].…”

“…Segmental diffusion and translational diffusion are expected to be affected differently with conversion [Dionisio and O'Driscoll, 1980;Mahabadi and O'Driscoll, 1977a,b;Mahabadi and Rudin, 1979]. With increasing conversion the polymerization medium becomes a poorer solvent because of the increased polymer concentration.…”