Shape of the imprints

Polyakov was first to create complementary cavities on the surface of silica gel . He removed water from silica gel in atmospheres of benzene, toluene, or xylene vapors and found that the pore structure of the resulting silica gel is influenced by the size and shape of the imprinting molecules in the gaseous atmosphere. Such silica gel dried in a benzene atmosphere... 

Mandatory requirements for successful imprinted polymers include (Srebnik, 2003) (1) The polymerization process should be a soft one as to avoide destmction or deformation of the imprinting molecule. (2) The polymer should be sufficiently rigid so that it will retain the shape of the imprint long after its removal from the polymer. (3) The polymer should... 

This is the most straightforward method of imprinting. Sol-gel polymerization of the silicate or other inorganic oxide takes place in the presence of an inert molecule that does not participate directly in the polymerization process. Removal of the guest molecule by extraction, thermal evaporation, or calcinations yields empty pores with the shape of the imprint. This is essentially a direct descen-dent of an approach that was introduced by Dickey [79,80] for the preparation of... 

Templates made of surfactants are very effective in order to control the size, shape, and polydispersity of nanosized metal particles. Surfactant micelles may enclose metal ions to form amphiphilic microreactors (Figure 11a). Water-in-oil reverse micelles (Figure 11b) or larger vesicles may function in similar ways. On the addition of reducing agents such as hydrazine nanosized metal particles are formed. The size and the shape of the products are pre-imprinted by the constrained environment in which they are grown. 

Ratios of turnover frequencies (TOFs) corresponding to the degree of enhancement of the reaction rates by the imprinting revealed that the imprinted Rh-dimer catalyst (10) showed size and shape selectivities for the alkenes as shown in Figure 8.10. Selectivity for the alkene hydrogenation on the Rh2imp catalyst (10) depends on the size and shape of the template cavity as reaction site in... 

Figure 8.10 Shape and size selectivity of the imprinted Rh-dimer catalyst in alkene hydrogenation.

The problem that reduction of the plate heights will not considerably improve the peak shapes on MIP phases has perhaps not been recognized immediately when MIP CEC was developed. A recent (2006) review by Ch. Nilsson and S. Nilsson [13] concludes, however So far all verified imprint-based CEC separations showed peak tailing, due to polydispersity of the imprinted receptor sites, resulting in different affinity and poor mass transfer. Although we do not agree with the exact wording (because the isotherm may be nonlinear even if all sites are the same... 

In a subsequent report, in 2005 [55], the same group described the preparation of imprinted polymer capable of oxidising alcohols and alkanes with 2,6-dichlor-opyridine /V-oxide (86) without mineral acid activation. The polymer was imprinted with a ruthenium porphyrin complex (87) using the diphenylmethana-mine (88) as pseudo-substrate template in order to achieve a shape of the cavity complementary to the substrates, diphenylmethane (89) and diphenylmethanol (84). The reaction, carried out with the imprinted polymer on the diphenylmethanol as substrate, showed a rate enhancement 2.5 higher than with the non-imprinted polymer. In the same conditions, but with diphenylmethane and... 

When the indentation hardness is measured, a small sphere or the tip of a pyramid-shaped crystal of a material which is harder than the one to be tested is pressed in the surface. The depth of the imprint is a measure for the hardness. Especially in the case of porous material it is vital that the determination is carried out carefully to avoid that... 

The imprinting of polymer supports is an exciting development in the immobilization of transition metal complexes. The process involves the copolymerization of an inorganic or an organic template into a crosslinked polymer network. In a subsequent step, the template is chemically removed leaving an imprint of molecular dimensions in the resin. Ideally, the imprint retains chemical information related to the size and shape of the template. This approach has been used to prepare chiral imprints in otherwise achiral polymer networks. The method is outlined in Scheme... 

Flexibility of the polymer network is crucial to allow the template to enter the cavity and rebind to the binding sites. However, flexibility is also crucial to maximise the degree of complementarity towards the template that the crosslinker is able to provide. Even in solution, prior to polymerisation, the crosslinker associates with the template. Hence flexibility of the crosslinker allowing it to model the shape of the template is essential for selective imprinting. EGDMA has so far proven to be the optimum choice, the best compromise between flexibility/rigidity and length [1, 74, 75, 87, 98, 104]. 

With these template monomers very careful investigations on the mechanism of the imprinting procedure have been performed by Shea s group. The influence of the cooperativity of binding sites, of the shape of the cavity, and of the occurrence of one- or two-point binding has been taken into consideration. 

The effect of the CAP-MR concentration on the response to injections of CAP was examined and, in contrast to results obtained using the bulk-imprinted polymer, the response did not display a maximum value but rose gradually from 0.3 to 0.6 g/mL then increased dramatically from 0.6 to 0.8 jUg/mL CAP-MR. However, the shape of the peaks became highly asymmetric at the higher concentrations, so a compromise of 0.5 pgjraL was adopted. As with the bulk-imprinted polymer, lower flow rates produced dramatically greater responses, as a consequence of the increased time available for the template/analyte to interact with the polymer matrix. In order to facilitate comparison with the former study a flow rate of 2 mL/min was employed. 

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