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A Trommsdorff effect, also known as gel or Trommsdorff-Norrish effect, which occurs during polymerization at high conversions increases the rate of reaction will be referred to.

Cause of this effect is decreasing with increasing rigidity of the growing polymer chains probability of chain termination by recombination of the reactive chain ends, which increases their concentration. Thus increasing the rate of the exothermic reaction causes the temperature to rise, so that the decomposition of the radical initiator is accelerated and the number of reactive species is increasing. At the same time the dissipation of heat from the reaction by further increasing viscosity becomes difficult.

This cycle can lead to an explosion, which is why the reaction must be effectively cooled or stopped previously.

Named is the Trommsdorff effect after the German chemist Ernst Trommsdorff.