Group 6 transition metal catalysts (MoCl₅, WCl₆, MoCl₅-Ph₄Sn, WCl₆-Ph₄Sn, Mo (CO) ₆-CCl₄-hν, W (CO) ₆-CCl₄-hν etc) polymerized not only monosubstituted acetylenes (e. g., HC≡C-t-Bu) but also disubstituted acetylenes (e. g., CH₃≡C__nPr, ClC≡CPh). Group 5 transition metal catalysts (NbCl₅, TaCl₅ etc), which selectively cyclotrimerize monosubstituted acetylenes, polymerized disubstituted acetylenes (e. g., CH₃C≡CPh, CH₃C≡CSi (CH₃) ₃). Thus these catalysts are especially effective to sterically hindered acetylenes for which Ziegler catalysts do not work. The structure and reactivity of monomers, polymer structure, and polymerization mechanism have been discussed. Interestingly, polymers with very high molecular weights (mot. wt. 10⁵-10⁶) were obtained from sterically rather hindered acetylenes (e. g., HC≡C-t-Bu, CH₃C≡C-n-Pr, CH₃C≡CPh, ClC≡CPh, CH₃C≡CSi (CH₃) ₃). These polymers are generally white (or light yellow), soluble, amorphous, air-stable, electrically insulating, non-paramagnetic solids ; all these properties give a striking contrast to those of polyacetylene.