Review on flexible frameworks: Phosphazene functional polymers powering the future of membranes

The poly(organo)phosphazenes (POPs) are inorganic-organic hybrid polymeric frameworks with a -P=N- skeleton and replaceable organic side groups. Due to their remarkable physical and chemical characteristics, the POPs have attracted significant attention for thermal stability and flame retardancy, chain flexibility, biocompatibility, radiation resistance, and biodegradability. Importantly, the POPs' P=N backbone comprises high flexibility for substitution reactions to accommodate various functional organic moieties with a range of incorporated benzene-containing functional groups (-OH, -NH₂, -SH, and others). These miscellaneous functional characteristics of POPs make them significant research candidates in diverse applications, including applications in sustainable membrane technologies for organic solvent nanofiltration (OSN) membranes, hydrogen/oxygen membrane barrier (H/OMB), ultrafiltration (UF), nanofiltration (NF), and gas separation (GS). The creation of novel membranes with adjustable characteristics and the ability to tolerate adverse mechanical, chemical, and thermal conditions is essential for the success of membrane-based processes in their commercialization. Recently, the POPs have made an important contribution to the preparation of highly stable membranes. In addition to outlining recent developments in their use in the fields of UF, NF, and GS, this review summarizes the primary synthesis pathways, properties, structure changes, and structure-property-relationships of POPs membranes. Finally, we discuss the prospects and challenges associated with the POPs in membrane technology.