Two-Dimensional and Related Polymers: Concepts, Synthesis, and their Potential Application as Separation Membrane Materials

Although two-dimensional polymers (2DPs) and related materials are very attractive subjects for research because they are expected to function as ideal gas separation membrane materials, we need to answer some basic questions about them. For example, "Why are they not a common or popular topic, especially in textbooks?"; "How can they be synthesized?"; and "Whatever are 2DPs? and "How can we define 2DPs?" In this review, we try to answer these questions. First, the significance of 2DP as molecular-sieving membrane materials is explained in Section 1. Second, the definitions and classifications are proposed that take into account their first-order and high-order structures in Section 2. Third, syntheses and function of a few real 2DPs and many related polymers described in the literature are reviewed, including such materials as covalent organic frameworks(COFs), carbonized molecular sieving membranes(CMSMs), polymers of intrinsic porosity(PIM), and our recent results on the synthesis and function of 2DPs and related polymers in Sections 3 and 4. Finally, in Section 5 we evaluate the two-dimensional (2D) characters of 2DPs described in Section 3 using some 2D characteristic values defined in Section 2, and the future of 2DPs is discussed in Section 6.