Autoacceleration

In polymer chemistry, autoacceleration (gel effect) is a dangerous reaction behavior that can occur in free-radical polymerization systems.

Without proper precautions, autoacceleration of polymerization systems could cause metallurgic failure of the reaction vessel or, worse, explosion.

[2] To avoid the occurrence of thermal runaway due to autoacceleration, suspension polymerization techniques are employed to make polymers such as polystyrene.

Norrish and Smith, Trommsdorff, and later, Schultz and Harborth, concluded that autoacceleration must be caused by a totally different polymerization mechanism.

This decrease in termination rate, kt, is caused by the raised viscosity of the polymerization region when the concentration of previously formed polymer molecules increases.

[2][3] The Brownian motion of the larger molecules in the polymer "soup" is restricted, therefore limiting the frequency of their effective (termination) collisions.

With termination collisions restricted, the concentration of active polymerizing chains and simultaneously the consumption of monomer rises rapidly.

The decrease of termination reactions also allows radical chains to add monomer for longer time periods, raising the mass-average molecular mass dramatically.