Polymers of intrinsic microporosity (PIMs)

 

 

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Polymers of intrinsic microporosity (PIMs) were developed by Peter Budd and Neil McKeown. They are polymers which behave like molecular sieves in the solid state, because they have rigid, contorted molecular structures that are unable to pack efficiently. We are developing both insoluble network materials and soluble PIMs that may be precipitated from solution as powders or cast into a membrane form.

One example of a network PIM incorporates phthalocyanine units and allows access to catalytically active transition metal ions within the central cavity of the macrocycle. In contrast, the catalytic activity of a precipitated low molar mass phthalocyanine is limited because of the tendency for the macrocycles to associate.

Two examples of soluble PIMs are PIM-1 and PIM-7. These polymers have no single bonds in the backbone about which rotation can occur, but they incorporate spiro-centres that introduce sharp bends into the chain. In the solid-state they exhibit high apparent surface areas by nitrogen sorption (700-900 square metres per gram). They are amorphous and remain glassy up to their decomposition temperatures (>350 ºC). In membrane form, they perform extremely well for gas separations such as oxygen/nitrogen. In powder form, they act as selective sorbents, e.g. for the removal of phenolic compounds from aqueous media. The phenazine unit in PIM-7 can act as a ligand for the coordination of metal ions, giving rise to catalytically active materials.

 

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(a) PIM-1; (b) PIM-7; (c) Molecular model of PIM-1 fragment showing its rigid, contorted structure.