Solvents Porogens

Aside from its dual role as a solvent and a pore-forming agent, the solvent in a covalent polymerization must also be judiciously chosen so that it simnltaneously maximizes the likelihood of complex formation between the template and the functional monomers. More specifically, the use of a highly thermodynamic solvent tends to result in polymers with well-developed pore structures and high specific surface areas, while the use of a thermodynamically poor solvent leads to polymers with poorly developed pore structures and low specific surface areas. Historically, chloroform has been used as a highly thermodynamic solvent other solvents that have been investigated include dimethyl 

Korotcenkov, Handbook of Gas Sensor Materials, Integrated Analytical Systems, 

Toluene/ethanol Ethyl cellulose (EC) poly(vinyl chloride-co-isobutyl vinyl ether) (PVC-IBVE) poly(4-tert-butyl styrene) (PTBS) polystyrene (PS) poly(vinyl methyl ketone) (PVMK) 

Tetrahydrofuran Poly(tetrafluor ethylene-co vinylidenfluorid-co-propylene) (PFE-VFP) poly(4-vinyl phenol) (PVPh) poly(vinyl chloride) (PVC) 

Chloroform Poly(styrene-co-acrylonitrile) (PSAN) cellulose acetate (CAc) polysulfone (PSu) poly(bisphenol A carbonate) (PC) poly(methyl methacrylate) (PMMA) 

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Polymers were prepared using MAA (A-K) or 4-vinylpyridine/MAA (1/1) (L) as functional monomers and EDMA as cross-linking monomer in the presence of the template and various solvents (porogens) as shown schematically in Fig. 5.2. (Chapter5). EtOAc = ethylacetate, THF = tetrahydrofuran, RT = room temperature. 

Solvent porogen effects for macroporous resins are often explained in terms of the degree of solvation imparted to the incipient polymer netwoik, the point at which phase separation takes place, and the resultant degree of in filling between primary particles [26]. This may play a role in some amorphous MOPs (for example, micro/ mesoporous PPV [13]) however other systems such as HCPs (Sect. 2.1) do not undergo phase separation in this way [21, 22]. This basic mechanistic difference also accounts for the apparent independence of surface area on monomer concentration for conjugated microporous PAE networks [ 19], for example, in comparison with macro-porous polymer resins where surface area may be strongly concentration dependent. 

As shown in Figure 5.12, the preparation of MIPs includes the polymerization of suitable monomers around a template in the presence of an appropriate cross-linker and solvent (porogen). Following polymerization, the template molecule is removed to leave cavities complementary in size, shape, and chemical functionality to the analyte. Accordingly, the polymer should be able to selectively rebind the target analyte. 

More recently [68] it has been shown that by crosslinking the polyimide matrix and adjusting the solvent (porogen), respectably porous spherical particulates can be achieved with surface areas up to m g" as measured by N2 BET. 

In the synthesis of these macromolecular system, many parameters involved can affect the information associated with the binding sites, such as functional monomers/polymers, crosslinkers and solvents/porogens. Thus, both the feasibility of imprinting and the proper preparation conditions need exploration for the preparation of efficient imprinted materials (Liu Z. et al., 2010). It is important to state that MIP can be obtained in different formats, depending on the preparation method followed. To date, the most common polymerizations for preparing MIPs involve conventional solution, suspension, precipitation, multi-step swelling and emulsion core-shell. There are also other methods, such as aerosol or surface rearrangement of latex particles, but they are not used routinely (Puoci et al., 2011). 

The dependence of the structure of polymers containing EDA and DETA groups on the size of the molecule of hydrocarbon (inert solvent, porogenic agent) was also investigated. The largest pores contain polymers obtained with the n-decane solvent (refs.9,11). 

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