Mizoroki-Heck coupling polymerization of 1,4-bis[(2-ethylhexyl)oxy]-2-iodo-5-vinylbenzene (1) and its bromo counterpart 2 with a Pd initiator for the synthesis of poly(phenylenevinylene) (PPV) was investigated to see whether the polymerization proceeds in a chain-growth polymerization manner. The polymerization of 1 with (Bu3PPd)-Bu-t(Tolyl)Br (10) proceeded even at room temperature when 5.5 equiv of Cy2NMe (Cy=cyclohexyl) was used as a base, but the molecular weight distribution of PPV was broad. The polymerization of 2 hardly proceeded at room temperature under the same conditions. In the polymerization of 1, PPV with H at one end and I at the other was formed until the middle stage, and the polymer end groups were converted into tolyl and H in the final stage. The number-average molecular weight (M-n) did not increase until about 90% monomer conversion and then sharply increased after that, indicating conventional step-growth polymerization. The occurrence of step-growth polymerization, not catalyst-transfer chain-growth polymerization, may be interpreted in terms of low coordination ability of H-Pd(II)-X((Bu3P)-Bu-t) (X=Br or I), formed in the catalytic cycle of the Mizoroki-Heck coupling reaction, to -electrons of the PPV backbone; reductive elimination of H-X from this Pd species with base would take place after diffusion into the reaction mixture. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 543-551