Polar covalent sigma bond

Items 2 and 3 arise from the fact that both the "counterion" and the medium itself can markedly affect the nature of the growing chain end. Thus, the growing chain end may assume various forms that depend on the extent of electrical charge separation and range all the way from a polarized covalent (sigma) bond to a completely dissociated state of free ions. This characteristic presents the greatest distinction between the mechanisms of free-radical and ionic polymerization. 

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