title: Design and Synthesis of Aliphatic Monomer-Based Porous Organic Polymers
Creators: name: Li, Boyu
Publisher: name: University of Liverpool Repository
Year: 2026

description: Porous organic polymers (POPs) are a class of porous network materials that are constructed from organic building blocks via covalent linkages. They have diverse geometries/topologies and have gained a lot of attention because of their high surface areas, tunable pore shapes/sizes, amenability to design/functionalisation, and relatively high framework robustness. They also have useful applications in gas storage/separation, carbon capture and storage etc. Although unambiguous classification is challenging for POPs, they can be generally divided into two categories, amorphous polymers (e.g., amorphous POPs) and crystalline frameworks (e.g., covalent organic frameworks, COFs) based on the periodicity of their structures. 

Most POPs reported so far are aromatic due to the rigidity, availability and applications associated with π-electrons of polyaromatic monomers. While the construction of POPs from aromatic building blocks has been extensively explored, their aliphatic counterparts remain largely underrepresented as they often have too much conformational flexibility and lack the rigidity required for constructing materials with permanent microporosity. Moreover, the frameworks constructed from aliphatic monomers could be synthetically challenging due to the side reactions associated with the aliphatic aldehydes with enolisable α-hydrogen, or the lack of π-interaction to stabilise the imine linkage that is prone to hydrolysis. Despite these challenges, aliphatic monomers do have advantages such as higher flexibility, higher environmental degradability, higher reactivity and more diverse node geometries. To our knowledge, only very few examples of fully aliphatic imine POPs have been reported to date, preventing the experimental investigation of their unique structures and potential useful applications. 

The aim here was to design and synthesize a series of semi- and fully aliphatic frameworks to expand the diversity of chemical structures of POPs and to investigate how the incorporation of aliphatic monomers into frameworks could influence the structure and porosity properties of the frameworks. To this end, we constructed two semi-aromatic COFs by employing a novel aliphatic monomer triformylmethane (TFM). This presents the first successful use of the simplest aliphatic tri-aldehyde, TFM, in the synthesis of COFs. It was found that replacing the conventional aromatic units (e.g.,1,3,5-triformylbenzene) with TFM resulted in 2D layered COFs with smaller pore sizes and narrower band gaps compared with their aromatic counterparts. The intricate tiling patterns of these materials were observed and were attributed to the lower symmetry of the TFM monomer. Beyond the semi-aliphatic frameworks constructed from TFM to show that fully aliphatic POPs can be made, we designed two fully aliphatic POPs based on functionalised adamantane cores and successfully synthesized them. Although attempts to crystallise them were unsuccessful, preventing us from understanding if their degree of interpenetration is lower, the successful development of fully aliphatic POPs, alongside semi-aromatic COFs, demonstrates the potential of aliphatic monomers to generate porous materials with interesting features such as structural isomerism, smaller pore sizes and narrower band gaps. This work opens doors to future research of using more environmentally degradable aliphatic monomers, such as various substituted malondialdehydes and linear ethylenediamines, in the field of POPs and COFs, providing alternatives to the conventional polyaromatic monomers. The challenges in terms of stability and crystallization of these materials also need to be considered due to the side reactions associated with enolisable aldehyde monomers and the acid-base reaction between the acidic catalyst and the basic amine monomers.