## 1 Introduction  
Poly (4-methyl-2-pentyne) and poly (4-methyl-1-pentene) are prospective membrane materials for the separation of lower-hydrocarbon-containing gaseous mixtures. Both these polymers are usually abbreviated to PMP in the literature. Here, to distinguish one from another we will write them as PMPentyne and PMPentene accordingly throughout the article.  
PMPentene is a semicrystalline polymer with T\({}_{\mathrm{g}}\) \(\sim\) 30 \({}^{\circ}\)C and T\({}_{\mathrm{m}}\) \(\sim\) 235 \({}^{\circ}\)C. Physical density is \(\sim\)0.83 g/cm\({}^{3}\). Free volume fraction (FFV) is 0.186 [1]. The crystalline phase of PMPentene is permeable to gases, and it has practically the same physical density as the amorphous phase [2; 3; 4]. It occupies a special place among commercially available polymers [2; 5]. PMPentene is a medium-permeable polymer with a history of application as a membrane material. PMPentene-based membranes are used mostly for medical purposes in oxygenators [6]. Melt-spinning technology is used for hollow fiber production. It should be noted that previous production of PMPentene hollow fibers with homogenous walls was performed in Russia at NPO 'Khimvolokno' [7]. Currently, on an industrial scale PMPentene is manufactured by Mitsui Chemical, Inc. under the trademark TPX, including hollow fiber gas separation membranes. Note that PMPentene is one of the most permeable commercially used membrane polymers. The permeability of PMPentene towards several different gases is shown in Table 1.  
PMPentyne is a glassy hydrocarbon-based 1, 2-disubstituted polyacetylene. It has density of \(-0.8\) g/cm\({}^{3}\)[8] with high free volume fraction (FFV value is 0.28 [9]) and very high glass transition temperature (T\({}_{8}>250\)\({}^{\circ}\)C) [9]. Usually, thermodestruction of polymers of this type starts at temperatures lower than T\({}_{\rm g}\). This type of polymer is more permeable to larger hydrocarbons (e.g., butane) than to smaller molecules (e.g., nitrogen). PMPentyne can be used for laboratory-scale flat-sheet composite membranes [10]. The gas permeability characteristics of the polymer are shown in Table 1.  
Both these polymers are stable to lower hydrocarbons and can be used for hydrocarbon-containing gas mixture separation. It is known that PMPentene is characterized in terms of C\({}_{1}\)-C\({}_{4}\) permeability as a conventional glassy polymer (C\({}_{1}>\) C\({}_{2}>\) C\({}_{3}>\) C\({}_{4}\)) [13]. In contrast, PMPentyne shows the opposite permeance behavior (C\({}_{1}<\) C\({}_{2}<\) C\({}_{3}<\) C\({}_{4}\)) [14]. PMPentyne has a peculiarity, which is that its very high gas diffusivity makes direct experimental measurements of diffusion coefficients impossible. The current study is dedicated to combining experimental and theoretical approaches to the estimation of the permeability parameters for gas separation membranes based on glassy and rubber-like polymers. The paper demonstrates an example of the approaches combination for PMPentene and PMPentyne in order to be able to consider the full set of gas separation parameters. This combination allows such types of evaluation whether for homogeneous films of highly permeable polymers or for ultrathin membranes (the cases of very low time-lag values).