Molecular dynamics simulation is used to model the structure and thermodynamic properties of a novel rubbery polymer with promising membrane properties for hydrocarbon separation. A realistic united atom force field is developed based on extensive density functional theory quantum mechanics calculations for a model dimer and volumetric data at various temperatures and pressures. Both a constant bond length and a flexible bond length model are examined. Well-equilibrated structures of the polymer melt at various conditions are used to evaluate numerous thermodynamic properties, such as the isothermal compressibility, thermal expansion coefficient, and cohesive energy density, and structural properties, including intra- and intermolecular distribution functions and the static structure factor. The microscopic structure of the free volume of the polymer matrix and its evolution with time affects the diffusion of penetrant molecules considerably; they are calculated accurately using the Greenfield and Theodorou geometric analysis. The solubilities of various n-alkanes from methane to n-hexane at 300 and 400 K are calculated using the Widom test particle insertion technique. In all cases, simulation results are in good agreement with literature experimental data for the volumetric properties of the polymer melt and the solubility coefficients of n-alkanes in the polymer. In a forthcoming publication, the transport properties of these systems and the underlying molecular mechanisms will be examined.