The need for more-efficient and environmentally-friendly CO2 separation techniques is becoming more and more evident for a carbon-constrained world. High performance membrane materials, such as Polymers of Intrinsic Microporosity (PIMs) and specifically PIM-1, can promote faster growth of the technology of membrane gas separation. However, despite the potential of high free volume polymers such as PIM-1, the decay in their performance over time has inhibited a full exploitation of these materials and this has been the motive for the current project. This research explores a variety of methods to improve the initial and/or the long term performance of PIM-1-based membranes for CO2 separation. Expanding on the current studies on PIM-1-based membranes, it has been tried to focus on novel approaches for fabrication of membranes and development of materials. The explored routes in this research include: in situ PIM-1 synthesis, i.e. integration of PIM-1 synthesis and thin film composite (TFC) membrane fabrication, synthesis of novel crosslinked materials for incorporation into composite membranes based on PIM-1 and grafting reactions to improve the compatibility between the two phases of PIM-1-based composite membranes. In situ synthesis of PIM-1 and highly crosslinked network-PIM-1 was investigated on porous organic and inorganic supports with high thermal and chemical stability. Interfacial polymerisation of an octafluoro and a tetrahydroxy monomer led to promising CO2/CH4 selectivity (~ 15) for the in situ generated highly crosslinked TFC membrane. A highly-crosslinked network variant of PIM-1 with (ultra-)microporous nanoplatelet structure was successfully synthesized here for the first time. A noticeable improvement in the CO2/CH4 mixed-gas (1:1, v:v) separation performance of PIM-1 membranes at 2 atm transmembrane pressure difference and 298 K was obtained upon incorporation of network-PIM-1 nanoplatelets in PIM-1 continuous matrix. Low-crosslink-density (LCD) network-PIM-1 was also synthesised and was incorporated into PIM-1-based Mixed Matrix Membranes (MMMs). Conventional mixing/casting method or grafting PIM-1 onto LCD network-PIM-1 was explored as two separate approaches for the preparation of MMMs. The obtained MMMs based on grafting PIM-1 onto LCD network-PIM-1 showed a noticeable improvement in the aging behaviour, compared to PIM-1 membranes and the conventionally-fabricated MMMs.