= Solution
Addition consumes the <carbon>–<carbon> double bond of <vinyl chloride> and retains <chlorine> as a side substituent. The interior repeat of A is therefore
$$
\boxed{[-\mathrm{CH_2-CH(Cl)}-]_n},
$$
namely <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.
\Image[/past-exam-of-the-chemistry-course-of-the-year-3-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-3-vinyl-copolymers.png]
{title=Representative backbone segments of the chloride polymer and the two copolymers}
{description=Original labelled segments. The copolymer rows illustrate one possible sequence, not regularly repeating three-to-one blocks; the feed ratio alone does not determine incorporated composition.}
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