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.

Several CRP reactions have been developed over the past years, some of which capable of producing well-defined polymers with narrow molecular weight distributions.

[5] In many cases, CMRP exploits the weak cobalt(III)-carbon bond to control the radical polymerization reaction.

Cobalt is unusual in that it can reversibly reform a covalent bond with the carbon radical terminus of the growing chain.

When the monomer lacks protons that can be easily abstracted by the cobalt radical, (catalytic) chain transfer is also limited and the RP reaction becomes close to ‘living’.

The method involves adding a catalytic chain transfer agent to the reaction mixture of the monomer and the radical initiator.

Because the large polymer chains diffuse much slower than the small organic radicals, and thereby terminate much slower via 2nd order radical-radical coupling or disproportionation, long chains effectively build-up at cobalt while the small radicals keep terminating.

DT-CMRP is an associative process, which for CoIII(por)(alkyl) species implies formation of a 6-coordinate intermediate or transition state.