Gas permeabilities of a systematic series of glassy, isopropyl-based poly(dialkylacetylenes) for He, H-2, N-2, O-2, CH4, CO2, C2H6, and C3H8 were determined at temperatures from 25 to 55degreesC. Poly(4-methyl-2-pentyne) [PMP] has the highest fractional free volume (FFV) and, therefore, the highest gas permeability of all known isopropyl-based poly(dialkylacetylenes). For example, the nitrogen permeability of PMP at 35degreesC is 1250 Barters (1 Barrer = 1 x 10(-10) cm(3) (STP) cm/cm(2) s cmHg), one of the highest permeabilities reported to date. The high gas permeabilities of PMP result from very poor polymer chain packing due to alternating rigid double bonds along the main chain and a very bulky, sterically hindering isopropyl side-chain. Here, we report on the influence of the side-chain length on the pure- and mixed-gas permeation properties of two related isopropyl-based poly (dialkylacetylenes), namely, poly(5-methyl-2-hexyne) [P5M2H] and poly(6-methyl-2-heptyne) [P6M2H]. The gas permeabilities in this series of acetylene-based polymers decrease significantly as the length of the alkyl side-chain increases. For example, the nitrogen permeabilities of P5M2H and P6M2H at 35degreesC are 13- and 24-fold lower than that of PMP, respectively. An increase in side-chain length also has significant effects on the temperature dependence of gas permeation and the mixed-gas permeation properties of the polymers. An increase in side-chain length leads to a significant increase in the activation energy of permeation, indicating that the chain-packing density in long-chain isopropyl-based poly(dialkyl acetylenes) is higher than that in PMP. This trend is further reflected in the mixed-gas permeation properties of the branched side-chain poly(dialkylacetylenes); both the n-butane permeability and the n-butane/methane selectivity decrease as the length of the alkyl side-chain increases. (C) 2004 Elsevier B.V. All rights reserved.