Three naturally occurring conjugated aldehydes, (1R)-(-)-myrtenal, (S)-(-)-perillaldehyde, and beta-cyclocittal, were cationically copolymerized with isobutyl vinyl ether using the EtSO3H/GaCl3 initiating system in the presence of 1,4-dioxane as an added Lewis base. Alternating copolymerization proceeded exclusively via 1,2-carbonyl addition of the aldehydes. In addition, controlled alternating copolymerization was achieved under appropriate reaction conditions, producing copolymers with controlled molecular weights and narrow molecular weight distributions. The relationships between the copolymerization behaviors and the cyclic side group structures of the aldehydes suggested that conjugated and bicyclic structures were important factors for controlled alternating copolymerization. However, too much bulkiness around the carbonyl group resulted in termination of copolymerization. The resulting alternating copolymers were stable under neutral and basic conditions. In sharp contrast, mild acidic conditions degraded the alternating copolymers almost selectively to conjugated aldehydes with low molecular weights as nearly single products.