The ring-collapsed radical terpolymerization of pentamethylcyclopentaarsine (1a), hexaphenylcyclohexastibine (2), and dimethyl acetylenedicarboxylate (3) provided a periodic terpolymer containing both antimony atom and arsenic atom in the polymer chain. In the case of a 1:1:2 ratio of 1a:2:3, the obtained polymer has 1:1:2 structure of arsine, stibine, and vinylene unit. Employing excess 1a did not affect the periodic structure of the terpolymerization. On the other hand, employing excess 2 resulted in the terpolymer with more stibine units. The arsenic radical was more reactive toward 3 forming the vinyl radical, while the antimony radical was more reactive toward the vinyl radical forming the terpolymer. After the arsenic radicals were consumed, the antimony radical also reacted with 3 thereby affecting the periodic structure of the terpolymerization. Different reactivity of pnictogen radicals made it possible to construct the periodic vinylene-arsine-vinylene-stibine backbone.