In this study, three different silicone rubbers with alkyl side groups were synthesized by using the recently proposed in-situ technique that combines two hydrosilylation reactions: the introduction of side alkyl group followed by polymer chains cross-linking in the same reaction mixture. The membranes can be obtained in one step by using commercially available materials such as polymethylhydrosiloxane as well as corresponded alpha-olefin and diolefin. The completeness of polyallcylmethylsiloxane modification was confirmed by FTIR spectroscopy. As a result, dense membranes made of polyhexylmethylsiloxane (PHexMS), polyoctylmethylsiloxane (POMS) or polydecylmethylsiloxane (PDecMS) were fabricated for further investigation of their mechanical, structural and gas transport properties. In this work membranes from PDecMS were obtained and their gas transport properties were studied for the first time. For the first time, PHexMS and PDecMS were studied for their applicability in the separation of C-3(+) hydrocarbons from methane. Among studied membrane materials, PDecMS demonstrates the highest selectivity for n-C4H10/CH4 gas pair separation (alpha = 27). An attempt was made in this work to explain from the point of view of the relationship structure-property the influence of side chain substituent in polyalkylmethylsiloxanes on gas permeabilities and selectivities of membrane materials based on them.