Stable organoboron main-chain polymers were prepared by hydroboration polymerization of various diynes with mesitylborane. The polymerization gave selectively monohydroboration products regardless of the bulkiness of mesityl group, and no gelation was observed. In other words, the organoboron polymers having dialkenylborane repeating units in their main-chain were effectively produced. The resulting polymers were found to be more stable toward air-oxidation than the organoboron polymers prepared from thexylborane. The organoboron polymer prepared from diethynylbenzene and mesitylborane was subjected to reaction with iodine to form poly(phenylene-butadienylene). (C) 2000 Elsevier Science Ltd. All rights reserved.