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Addition consumes the carbon–carbon double bond of vinyl chloride and retains chlorine as a side substituent. The interior repeat of A is thereforenamely polyvinyl chloride. Benzoyl peroxide supplies initiating free radicals; head-to-tail addition gives the usual backbone connectivity. End groups and tacticity are not uniquely specified by this repeat formula.

Vinyl chloride is chloroethene, $\mathrm{C}{\mathrm{H}}_2=\mathrm{CHCl}$. It is a small vinylic halide whose carbon-carbon double bond can participate in addition polymerization.

Polymerization converts many vinyl chloride molecules into polyvinyl chloride, with chlorine-bearing repeating units along the carbon backbone. The resulting polymer has physical properties very different from those of the molecular monomer.

A vinylic halide has a halogen atom bonded directly to a carbon of a carbon-carbon double bond. Vinyl chloride is the simplest common example.

Ordinary substitution at a saturated carbon cannot be transferred unchanged to this geometry and bonding. The distinction between a vinylic halide and an allylic halide is therefore mechanistically useful, even when both molecules contain an alkene and a halogen.