Factors affecting the syntheses of high-molecular-weight poly(2,5-dialkyl-1,4-phenylene vinylene) by the acyclic diene metathesis polymerization of 2,5-dialkyl-1,4-divinylbenzenes [alkyl = n-octyl (2) and 2-ethylhexyl (3)] with a molybdenum or ruthenium catalyst were explored. The polymerizations of 2 by Mo(N-2,6-Me2C6H3) (CHMe2 Ph)[OCMe(CF3)2]2 at 25 °C was completed with both a high initial monomer concentration and reduced pressure, affording poly(p-phenylene vinylene)s with low polydispersity index values (number-average molecular weight = 3.3–3.65 × 103 by gel permeation chromatography vs polystyrene standards, weight-average molecular weight/number-average molecular weight = 1.1–1.2), but the polymerization of 3 was not completed under the same conditions. The synthesis of structurally regular (all-trans), defect-free, high-molecular-weight 2-ethylhexyl substituted poly(p-phenylene vinylene)s [poly3; degree of monomer repeating unit (DPn) = ca. 16–70 by 1H NMR] with unimodal molecular weight distributions (number-average molecular weight = 8.30–36.3 × 103 by gel permeation chromatography, weight-average molecular weight/number-average molecular weight = 1.6–2.1) and with defined polymer chain ends (as a vinyl group, CHCH2) was achieved when Ru(CHPh)(Cl)2(IMesH2)(PCy3) or Ru(CH-2-OiPr-C6H4)(Cl)2(IMesH2) [IMesH2 = 1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene] was employed as a catalyst at 50 °C. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 6166–6177, 2005