A-Level Structural isomerism and stereoisomerism: worked solution

Question

With a suitable catalyst, C2H2 polymerises to polyacetylene, -[CH=CH]n-, an organic semiconductor whose conductivity rises on reaction with iodine (n HC#CH -> -[CH=CH]n-). Two isomers arise on polymerisation. Draw both, showing at least two repeat units each, and state their isomeric relationship.

Polymerising the C=C-containing chain gives a backbone of alternating double bonds, and each C=C can have the chain continuing on the same side (cis) or on opposite sides (trans). Trans repeat units (chain groups on opposite sides across each C=C): ...=CH-CH=CH-CH=... drawn zig-zag with the continuing chain on opposite sides of each double bond. Cis repeat units (chain groups on the same side across each C=C): ...=CH-CH=CH-CH=... drawn with the continuing chain on the same side of each double bond (a bent chain). Both have the same molecular formula and connectivity but differ in the arrangement about the restricted C=C double bonds, so they are cis-trans (geometric) isomers.