Another possible angle of analysis stems from some similarities between high porosity glassy polymers such as polymers of intrinsic microporosity (PIMs) and crystalline porous materials such as zeolites and Metal Organic Frameworks (MOFs). PIMs have been considered as promising membrane materials for CO 2 /N 2 separation. Here, the structure consists of rigid units forming connected porous spaces. They can be seen as an amorphous counterpart to crystalline MOFs. From the theoretical considerations, nanoporous materials, such as zeolites and MOFs, should behave very differently from the dense polymers, and in fact for zeolites it has been shown that the overall selectivity increases with permeability -which is the opposite trend to the Robeson boundary as shown in Fig. . Based on these considerations, it appears that a comprehensive theoretical framework for adequately explaining sorption-driven separation in glassy polymers, analogous in terms of the level of theoretical rigour and insight to the Freeman's theory for kinetics-driven separation, is currently lacking. Without a clear understanding of the factors that determine the theoretical limits of sorptiondriven separation, it is not feasible to rationally design new polymer materials.