An effect peculiar to free radical polymerization that goes by many names:
Autoacceleration only occurs during polymerizations with high concentrations of monomer (i.e., little or no solvent). The reaction proceeds normally for a while, then suddenly the rate of polymerization goes up dramatically. The molecular weight of chains that grow during the accelerated period is substantially higher than that of chains that grew earlier.
The following graph illustrates the effect for PMMA polymerization in benzene solution at various conecntrations. Note that at low concentreations (40% or more dilute in this example), the polymerization proceeds smoothly with no unusual effects. However, at higher concentrations, a pronounced rate acceleration after partial conversion of monomer to polymer. The more concentrated the solution, the earlier the acceleration occurs.
To explain the effect, recall that initiation, propagation, and termination are completely different chemical reactions with different responses to conditions. Termination involves the reaction between two chain ends. However, in concentrated solutions, the viscosity of the reaction mixture becomes high as polymer chains form. This high viscosity hinders the diffusion of chains because of entanglements, so the rate of termination slows considerably. However, the diffusion of small molecular monomers is hardly affected by viscosity, so propagation proceeds as before. In addition, initiator continues to add more free radicals to the system. The rates of initiation and propagation come out of balance. What was once a low, steady state concentration of radicals gives way to increasing concentration. Chains grow without termination, so the conversion is rapid and the MW is high.
In dilute solutions, the viscosity never builds up to the point where the diffusion of chains is slowed, so autoacceleration does not occur.
For neat monomer (i.e., 100% in the graph), often in cases where the polymer formed is a high Tg material, there can come a point at which even the diffusion of monomer is slow. The mixture has become a hard glass, and unreacted radicals become trapped inside. The reaction shuts down at less than 100% conversion, as depicted in the curve.
Note that autoacceleration can be dangerous because the exotherm of polymerization can be released suddenly, leading to a runaway reaction.
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