thumb|340x340px|Free radicals in solvent can potentially react with a monomer within the solvent cage or diffuse out.
In chemistry, the cage effect (also known as geminate recombination) describes how the properties of a molecule are affected by its surroundings. First introduced by James Franck and Eugene Rabinowitch in 1934, the cage effect suggests that instead of acting as an individual particle, molecules in solvent are more accurately described as an encapsulated particle. The encapsulated molecules or radicals are called cage pairs or geminate pairs. In order to interact with other molecules, the caged particle must diffuse from its solvent cage. The typical lifetime of a solvent cage is 10 seconds. Many manifestations of the cage effect exist.
In free radical polymerization, radicals formed from the decomposition of an initiator molecule are surrounded by a cage consisting of solvent and/or monomer molecules. This can be described by the following reaction:
:<math>
R\!-\!R
\;\;\underset{k_c}{\overset{k_1}{\rightleftharpoons\;\;
\underset{\text{cage pair{(R^{\,\bullet},^{\bullet}\!R)}
\;\;\underset{k_D}{\overset{k_d}{\rightleftharpoons\;\;
\underset{\text{free radicals{2R^{\,\bullet
\;\rightarrow\;
\text{Products}
</math> and heme proteins. In the solid state, geminate recombination has been demonstrated with small molecules trapped in noble gas solid matrices and in triiodide crystalline compounds.
Cage recombination efficiency
The cage effect can be quantitatively described as the cage recombination efficiency F<sub>c</sub> where:
:<math>F_c = k_c/(k_c + k_d) </math> It is reported that the cage effect will increase with an increase in radical size and a decrease in radical mass.
Initiator efficiency
In free radical polymerization, the rate of initiation is dependent on how effective the initiator is.