The polymerization of phenylacetylene with the microheterogeneous Ti(OR)(4)-AlEt3 and homogeneous vanadium acetylacetonate/aluminum triethyl Ziegler-Natta catalyst systems was analyzed. The effects of some cocatalysts (e.g., pyridine and phenylacetylide) and the solvent, temperature, and time were analyzed. Both catalyst systems produced poly(phenylacetylene) (PPA) and a 1,2,4-triphenylbenzene (1,2,4-TPB)/1,3,5-triphenylbenzene (1,3,5-TPB) cyclotrimer mixture in various molar ratios. The titanium catalyst showed the lowest PPA/triphenylbenzene ratio. The 1,2,4-TPB/1,3,5-TPB molar ratio decreased with increasing PPA. On the basis of the spectroscopic data, PPA had a cis-transoidal stereoregular structure. The molecular mass of PPA was determined with vapor pressure osmometry and gel permeation chromatography. A mechanism for the polymerization reaction versus cyclotrimerization was proposed. (C) 2005 Wiley Periodicals, Inc.