Cobalt-mediated radical polymerization
Cobalt based catalysts, when used in radical polymerization, have several main advantages especially in slowing down the reaction rate, allowing for the synthesis of polymers with peculiar properties. As starting the reaction does need a real radical initiator, the cobalt species is not the only used catalyst, it is a mediator.
Source: Wikipedia — Cobalt-mediated radical polymerization (CC BY-SA 4.0)