Chapter 9
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Cross-Linked Polymers in Ionic Liquids: Ionic Liquids as Porogens 1
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Peter Snedden, Andrew I. Cooper , Yaroslav Z. Khimyak , Keith Scott , and Neil Winterton 2
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Liverpool Centre for Materials and Catalysis, Department of Chemistry, University of Liverpool, Liverpool L 6 9 7 Z D , United K i n g d o m Department of Chemical and Process Engineering, M e r z Court, University of Newcastle, Newcastle-upon-Tyne N E 1 7 R U , United K i n g d o m 2
Free-radical copolymerization o f 4-vinylpyridine (VP) with >5% divinylbenzene (DVB) in [omim]N(SO CF ) , and homopolymerizations of the cross-linking monomers D V B or trimethylolpropane trimethacrylate (TRIM), led to gel-like composite materials. V P copolymers with D V B showed a low degree of permanent porosity in the dry state. However, poly(DVB) and poly(TRIM) (after Soxhlet extraction) have bulk densities, intrusion volumes, B E T surface areas and morphology (from S E M studies) which demonstrate the porogenic character of ionic liquids. Poly(DVB) prepared in ionic liquid was characterized by solid-state N M R and showed a higher degree o f cross-linking compared with material prepared in toluene. 2
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© 2005 American Chemical Society In Ionic Liquids in Polymer Systems; Brazel, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
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Introduction Interest in ionic liquid-polymer composites (/) stems from the possibility that they may enable a serious constraint on the use of ionic liquids as reaction media in large-scale chemicals production to be circumvented. Compared with conventional molecular solvents, ionic liquids, generally, have significantly higher viscosity (2) and their distribution in polymer composites may improve contacting (3) of reactants and catalysts. Other approaches to limit such mass transfer constraints include the use of biphasic (4) or multiphasic (5) systems, immobilisation o f ionic liquids on inorganic supports (6) or in inorganic (7) and organic (8) porous materials. Assembly of composites o f polymer, ionic liquid and catalyst (in our case, for transition-metal catalysed hydrogenation o f light alkenes (8,9,10)) may be achieved either by introducing a catalyst-ionic liquid solution into a polymeric membrane having permanent porosity or by in situ formation of a polymer gel, for example, by polymerization in/phase-separation from an ionic liquid. In either case, there must be compatibility between the components, the requisite degree of co-processibility in subsequent fabrication as well as essential mechanical and chemical robustness of the composite to the conditions of use. Fine control may, in principle, be achieved over the physical and chemical nature o f the composite materials formed by in situ reactioninduced phase separation, offering a route to 'moulded' composites. In this case residual monomer may remain in the composite after reaction. However, since the ionic liquids chosen have essentially zero vapour pressure, it should be possible to remove traces o f most monomers by evacuation at elevated temperatures. To further these investigations, we have prepared and characterised a number of cross-linked polymeric materials using ionic liquids as the reaction medium (/), describing the first materials in which permanent porosity is achieved using the ionic liquid, [omim]Tf N, as porogenic solvent. (Conventional salts, such as [ N H ] [ H C 0 ] and NaCl, may lead to pore formation (//) by gas evolution on heating or by leaching the solid.) 2
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Polymers and polymerizations in ionic liquids Polymerizations are among the earliest chemical reactions (12) to have been investigated in ionic liquids. While the interactions between polymers and ionic liquids are poorly understood, benefits have been reported (13,14) associated with increased rates o f propagation and reduced rates o f termination in [bmim]PF compared with the molecular solvent benzene. The range o f monomers polymerized in ionic liquids (12,15) is shown in Figure 1, and 6
In Ionic Liquids in Polymer Systems; Brazel, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
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includes those with a polymerisable functionality incorporated into the ionic liquid itself. A wide spectrum of polymerization processes has been reported (12,15), including addition, condensation and ring-opening processes, involving electrode, enzymatic, Lewis acid, cation, co-ordination and free-radical, atomtransfer, reversible addition fragmentation chain-transfer mechanisms. The majority of these studies has focussed on homopolymerizations and some copolymerizations, with very few (16-18) devoted to cross-linked polymers (Figure 2). None has reported the generation of porous materials in which the ionic liquid can act as a porogen (19).
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