Dipropynylbenzene with branched alkoxy and alkyl groups [CH3C CRC6H2RC CCH3, R = 2-methylpropoxy (1a), 3-methylbutoxy (1b), 4-methylpentoxy (1c), cyclohexylmethoxy (1d), 2-ethylhexoxy (1e), 2-octoxy (1f), 2-ethylhexyl (1g), and 2-octyl (1h)] were polymerized with Mo(CO)(6) in the presence of 4-(trifluoromethyl)phenyl to afford poly(2,5-di(alkoxy or alkyl)-p-phenyleneethynylene)s (2a-h). Polymer 2a was insoluble in any solvents, but the other polymers (2b-h) were soluble in common organic solvents. The polymers with relatively long side chains (2e-h) had high molecular weight over 1.6 x 10(4) and gave free-standing membranes by solution-casting method. The densities of membranes of 2e-h were 0.914-0.998, and their fractional-free volume values were relatively large (0.094-0.158). The oxygen permeability coefficients of membranes of 2e-h were 18.4, 12.7, 4.85, and 19.3 barrers, respectively. It was found that poly(p-phenyleneethynylene) with 2-octyl side groups, which have the branch at the nearest position from main chain, exhibited the highest gas permeability.