Poly[1-(trimethylsilyl)-1-propyne] (PTMSP) is known to have the highest permeability coefficients for glassy polymers. However, unlike other glassy membranes such as polycarbonates or polysulfones, the high permeability of PTMSP decays as a function of time. A series of measurements of the gas permeability coefficients were reported in this work to highlight the effect of the physical ageing on the permeability of N-2 gas molecules through PTMSP membranes. In order to develop the molecular design rules for this high permeability, and the molecular structural parameters mostly responsible fur the physical ageing, molecular dynamics techniques were used to investigate the effect of the presence of the double bond. the Si-atom, the bulky trimethylsilyl side group, the co-operative effect of the singular methyl and the trimethylsilyl side groups on the diffusion coefficients of nitrogen and argon through the polymer. It was shown that a combination of a high volume fraction, high torsion barriers around the single bonds in the main backbone and high specific electrostatic interactions due to the presence of the Si-atom are all necessary for high diffusion coefficients of gases through PTMSP membranes. This combination was also shown to provide the necessary voids fur the diffusion process as well as for characterising the polymer with low cohesive energy densities and extra rigidity resulting in an exceptionally high permeability. Loss in free-volume due to the physical ageing may thus lead to lower permeability coefficients.