Sigma bond radical reactions with

In contrast with the usually slow progress of condensation polymerization the second major classification, addition, or vinyl-type polymerizations, usually proceed very rapidly, so rapidly that they are referred to as chain reaction polymerizations. This method of producing synthetic polymers uses the potential dual functionality present in a carbon-carbon double bond. The process is initiated by the use of radical or charged initiator species to form new sigma bonds from the carbon-carbon double bonds of the monomer, to link the monomer units (Eq. 20.6). 

Following are four reactions that result in homolytic cleavage to give radicals. Note that in the first three cases, the bond being broken is between atoms with at least two lone pairs. Lone pair repulsion is one factor that weakens sigma bonds. BDEs of these reactions from Appendix 3 are shown at the right of each equation. 

The presence of heteroatoms with non-bonding n electrons favors the localization of charges. When the radical and/or the charge are localized, this influences the fragmentation and the reaction mechanisms can be classified as sigma electron ionizations, localized charge initiations, radical site initiations, and rearrangements. 

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