Fabrication of low-dimensional network polymers with thermoresponsive properties using MOF scaffolds

Materials with ultimately thin (2D) or narrow (1D) structures have gained significant attention due to their exceptional properties. However, decreasing the dimensionality of soft polymer materials has been a formidable challenge due to the lack of rational synthetic methodology. Here, we performed cross-linking polymerization inside metal-organic frameworks (MOFs) as nanoporous scaffolds to afford poly(N-isopropylacrylamide) (PNIPAm) with unprecedented 1D and 2D network topologies: double strands and monolayer sheets. Remarkably, these polymer networks exhibited unique thermoresponsive properties in water that were strongly correlated with their specific topologies. Note that the transition temperature of double-stranded PNIPAm is among the lowest of known PNIPAm materials. The monolayer PNIPAm sheets exhibit a markedly slow thermal response over a wide temperature range. The dimensional constraint imposed on cross-linking by MOF-templated polymerization enables precisely controlling the chain orientation and proximity, providing new insights into the mechanism of the PNIPAm phase transition.