Electron Pushing for Radical Additions

A radical chain mechanism for the addition of HBr to alkenes. I = initiator. 

In a chain mechanism, three types of reactions occur initiation, propagation, and termination. Initiation always creates a radical that starts the chain by creating a propagating species. Propagation involves an intermediate with an unpaired electron that undergoes a reaction to create a different species with an unpaired electron. The radical reactant in one step of the chain is necessarily a radical product in another step of the chain. Termination occurs whenever two radicals react with each other to produce closed-shell species. This removes some of the propagating species from the system, and hence terminates a chain. While initiation and termination are parts of a chain process, the propagation steps are what account for the conversion of reactants to products. 

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