The polymerizations of 1-(3-methylphenyl)-2-(4-trimethylsilyl)phenylacetylene (1a) and 1-(4-methylphenyl)-2-(4-trimethylsilyl)phenylacetylene (1b) were carried out with TaCl5-n-Bu4Sn to give relatively high-molecular-weight polymers (2a and 2b) (M-n > 5 x 10(5)). The obtained polymers were brominated by using benzoyl peroxide and N-bromosuccinimide first, followed by substitution reaction of three types of polyethylene glycol. When diethylene glycol was used as a reagent on substitution reaction of meta-substituted polymer, PEG-functionalized poly(diphenylacetylene) with the highest content of oxyethylene unit [4a(2)] was obtained, and the degree of substitution was 0.60. The degrees of substitution decreased to 0.15 and 0.08 when the polyethylene glycols with higher molecular weights were used. PEG-substitution reaction to the para-substituted polymers was difficult to proceed, and hence the degree of substitution was 0.18 even when diethylene glycol was used. The CO2/N-2 separation factor of PEG-functionalized polymer [4a(2)] was as large as 28.8, although that of 2a was 7.41. The other PEG-functionalized polymers also exhibited high CO2 permselectivity, and their CO2/N-2 separation factors were over 20. (C) 2009 Wiley Periodicals, Inc. J AppI Polym Sci 113: 3504-3509, 2009