Solution Processing of Cross-Linked Porous Organic Polymers

CONSPECTUS: Porous organic polymers (POPs), essentially including polymers with intrinsic microporosity (PIMs), conjugated microporous polymers (CMPs), covalent organic frameworks (COFs), hyper-cross-linked polymers (HCPs) and so on, have recently attracted broad interest in many application areas because of their structural diversity and functional tunability. However, except for linear PIMs that can dissolve in organic solvents for solution processing into membranes, most POPs are highly cross-linked (hereafter termed CPOPs) and are synthesized as insoluble and unprocessable powders, which prevent CPOPs in many applications. Developing methodologies for solution processing CPOPs to high-quality membranes, monoliths, and (aero)gels has been a major challenge in this field because of the following issues. First, the inherently cross-linked structures and the strong framework-framework interactions in CPOPs give rise to very weak solvation of the frameworks, leading to easy aggregation and precipitation in solutions. Next, to date, several methods for preparing CPOP membranes have been proposed, but their conditions vary with different systems, and there lacks a general strategy for membrane formation of most CPOPs (or at least CPOPs of the same category). Additionally, CPOP-based monoliths and (aero)gels are rarely reported, and it has been considered difficult to control the hierarchical porosity to form the monoliths and (aero)gels during the CPOP syntheses. Last, the effects of the forms of membranes/ (aero)gels on the transport (electron, ion, and mass) properties have not been intensively investigated for the lack of suitable systems. Therefore, since it was first announced accompanied by the birth of CPOPs, research studies regarding solution-processed CPOPs have been underexplored for a long time without significant advances being achieved. To break the unprocessable shackles of CPOPs, our group started to make contributions to this field in 2018. We developed two general strategies, namely, "charge-induced dispersion (CID) " and "thermal hyper-cross-linking (THC) " strategies, to produce high-quality CPOP membranes and (aero)gels, respectively. For the CID strategy, we found that the introduction of plenty of charges to the frameworks of CPOPs substantially enhanced their interactions with polar solvents, rendering the transparent, stable, and solution-like CPOP sols which could be further processed into membranes. For the THC strategy, we intensively investigated the gelation mechanism and found that this system was synthetically controllable to produce CPOP (aero)gels and could serve as a platform for hybridization with many porous materials to achieve a molecular-level entanglement. Moreover, we successfully demonstrated that the transport properties in the CPOP membranes and gels were largely promoted by 1-2 orders of magnitude compared to their powder forms, thereby expanding the use of CPOP membranes and gels in the fields of electronic conduction, proton conduction, iodine adsorption, and molecular separation with superior performance. In this Account, we summarize our above contributions, including (i) three detailed methods in CID strategy to produce CPOP membranes, (ii) THC strategy and the gelation mechanism, and (iii) transport properties in CPOP membranes/gels and the structure-function relationship. Overall, our studies not only provide an unprecedented paradigm of solution processing of previously unprocessable materials but also broaden the opportunities for future applications for CPOPs.