4-Trimethylsilyldiphenylacetylenes with methyl group or bromine atom (Me3SiC6H4C=CC6H4R, R = m-CH3, p-CH3, m-Br, p-Br, 1a-d, respectively) were polymerized with TaCl5/ n-Bu4Sn to afford poly(diphenylacetylene) derivatives (2a-d). The polymers (2a-d) had high molecular weight over 5x10(5), and gave free-standing membranes by solution casting method. Chlorination of the obtained poly[1-(3-methylphenyl)-2-(4-trimethylsilyl)phenylacetylene] was carried out by using sulfuryl chloride, and then substitution of polyethylene glycol was performed to give poly(diphenylacetylene) possessing polyethylene glycol moieties. Its carbon dioxide permeability (P-CO (2)) and permselectivity (P-CO (2)/P-N2) were 2,970 barrers and 9.0, respectively.