Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 1 / 1 / a / iii / Solution 2026-10-20
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Predominantly head-to-tail poly(3-hexylthiophene) is formed, with thiophene rings connected through their 2 and 5 positions. The hexyl groups occupy equivalent 3 positions along the conjugated polymer.
Selective iodine–magnesium exchange first generates an organomagnesium monomer while retaining the carbon–bromine bond. Nickel-catalyzed Kumada coupling then joins the organomagnesium end of one ring to the brominated end of another. Transfer of the nickel center to the new chain end permits catalyst-transfer polymerization and favors head-to-tail regioregularity. Organic side products of the activation include isopropyl iodide; magnesium halides accompany the coupling. The repeat unit itself contains neither iodine nor bromine.
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