Theory of Autoacceleration in Free Radical Polymerization
Abstract
This thesis develops a systematic theory of free -radical polymerization at high conversion based on modern developments in polymer physics. We calculate steady state rates of conversion and molecular weight distributions in the "autoacceleration" regime where dramatic increase in polymerization rates are observed experimentally; this is the celebrated "gel" effect. The starting point is recent theory of polymer-polymer reaction kinetics, according to which the termination rate constant between two macroradicals is dominated by the length of the shortest chain. At lower conversions, we show that it is justified to preaverage the rate constants over the living population; thus the classical Flory theory is a good approximation. Autoacceleration results from a transition to entanglement-dominated kinetics in which short chain domination is so strong as to be unintegrable. This invalidates preaveraging. Net termination rates are then dominated by short-long events; long chain mobility is so reduced by entanglements that short mobile chains provide the fastest termination mechanism despite their small numbers. This leads to very different results to those of the classical Flory theory. We compare these predictions with experimental measurements. In addition to the steady state analysis, a preliminary discussion of free radical polymerization dynamics is presented, as directly measured in electron spin resonance "post-effect" experiments. These experiments provide the most fundamental available experimental probe of free radical polymerization phenomena.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1996
- Bibcode:
- 1996PhDT........24Y
- Keywords:
-
- GEL EFFECT;
- Chemistry: Polymer; Physics: Molecular