Gas permeation, sorption, and structural properties of two highly branched polyacetylenes, poly [1-phenyl-2-[p-(triphenylsilyl)phenyl] acetylene] (PPhSiDPA) and poly [1-phenyl-2-[p-(triisopropylsilyl)phenyl] acetylene] (PPrSiDPA), are reported. Structurally, both polymers have much in common; however, their transport properties are quite different. PPhSiDPA has dramatically lower permeability coefficients than PPrSiDPA. For example, the Oz permeability coefficients of PPhSiDPA and PPrSiDPA are 12 x 10(-10) and 230 x 10(-10) cm(3) (STP) cm/(cm(2) s cmHg), respectively at 22 degreesC. Gas solubility is very high in PPrSiDPA, similar to that observed in poly(1-trimethysilyl-1-propyne) (PTMSP), the most permeable polymer known. Gas solubility coefficients of PPhSiDPA are 2-3 times lower than those of PPrSiDPA. Free volume size and size distribution were characterized using positron annihilation lifetime (PAL) spectroscopy. Results from these studies (i.e., a bimodal size distribution of free volume elements and large free volume element size) are consistent with the observed transport and sorption properties of PPrSiDPA. In contrast, the PAL spectrum of PPhSiDPA is similar to that of conventional low free volume glassy polymers.