Dipropynylbenzenes with alkyl groups (CH3C CRC6H2R CCH3, R=n-C6H13, n-C8H17, n-C10H21, 1a-c, respectively) were polymerized with Mo(CO)(6) to afford solvent-soluble poly(2,5-dialkyl-p-phenyleneethynylene)s (2a-c). The polymers (2a-c) had high molecular weight over 3x10(4), and gave free-standing membranes by solution casting method. According to thermogravimetric analysis (TGA), these poly(p-phenyleneethynylene)s showed high thermal stability (T-0 >= 380 degrees C). The densities of membranes of poly(2,5-dialkyl-p-phenyleneethynylene)s (2a-c) were 0.936-0.965, and their fractional free volume (FFV) were relatively large (ca. 0.14-0.15). The oxygen permeability coefficients (PO2) of membranes of 2a-c were 4.88, 7.06, and 16.6 barrers, respectively.