Discussion
How many initiator fragments are present in a polymer chain formed by coupling termination in addition polymerization ?
Answer: B
The chain contains 2 initiator fragments.
In free-radical addition polymerization each growing chain starts from one initiator radical, such as a benzoyloxy or phenyl fragment from benzoyl peroxide, so each living chain carries one initiator fragment at its starting end. In termination by coupling (combination), two growing chains meet and their radical ends join into one dead chain, which therefore carries an initiator fragment at each end.
One fragment per chain is the result of termination by disproportionation, where a hydrogen atom is transferred and the two chains stay separate. Three fragments would need a branching event, and zero would mean the chain never started from an initiator.
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