Mo-2(O2CCH3)(4)-LA catalysts (LA = TiCl4, SnCl4, GeCl4, EtAlCl2) induce monosubstituted acetylene (phenylacetylene and tert-butylacetylene) to cyclotrimerize and/or to polymerize. The catalytic ability of these catalyst as well as the structure of polymers formed strongly depends on the Lewis acid and solvent. The systems with TiCl4 and GeCl4 trimerize phenylacetylene with high selectivity. The system with TiCl4 also cyclotrimerizes tert-butylacetylene, but SnCl4 and GeCl4 systems polymerize tert-butylacetylene with 100% conversion. In the latter system the polymerization of tert-butylacetylene follows a living mechanism. (C) 2002 Elsevier Science B.V. All rights reserved.