“…Cyclopolymerization (CP) of α,ω -diyne derivatives is a powerful tool for preparing substituted polyacetylenes. − Early studies regarding CP used ill-defined catalysts, including Ziegler–Natta, MoCl 5 , and WCl 6 catalysts, − until the first breakthrough by the Schrock group and later by the Buchmeiser group demonstrating living CP using well-defined Mo-based olefin metathesis catalysts. − Subsequently, the utility of CP broadened when user-friendly Ru-based catalysts were shown to promote living CP. − Extensive studies have been conducted using 1,6-heptadiyne monomers, yielding five- or six-membered rings (in a random or highly regioselective fashion) in the conjugated backbone via α - ,− or β -addition, ,− respectively. Moreoever, recent studies have expanded the scope of CP where 1,7-octadiyne − and 1,8-nonadiyne derivatives were used to successfully produce polyacetylenes containing six- and seven-membered rings via α -addition (Scheme ). However, in contrast to the cyclization to form thermodynamically stable five- to seven-membered rings, which is relatively easy, cyclization to make cyclobutenes is far more challenging due to their extremely high ring strain energy (29.5 kcal/mol), making the process thermodynamically disfavored.…”