Amidines imprinting

Fig. 5 Complexes formed (a) between amidine groups in the functional monomer V,lV -diethyl-4-vinyl-benzamidine and carboxyl groups in the imprinting template IV-terephthaloyl-D-phenylglycine [82] and (b) between amidine groups in the imprinting template pentamidine and carboxyl groups in the functional monomer methacrylic acid [83]...

The result of this approach was a 100-fold increase in the hydrolytic activity of the imprinted polymer compared with the background at pH = 7.6. As a control, another polymer was made using a complex between amidine and benzoate, showing a surprisingly 20-fold increase in the hydrolysis of the substrate. The authors also reported a kinetic investigation of the TSA-imprinted and the benzoate-imprinted polymers, in addition to the free catalyst in solution. Although the ratio substrate/catalyst is not specified, and therefore the steady-state conditions could not be verified, the authors claimed for the two polymers a Michaelis-Menten kinetic behaviour, with a higher profile for the TSA-imprinted polymer. On the other hand, the free catalyst in solution showed, as expected, a linear dependence of the rate from the substrate concentration. The TSA also showed a moderate selectivity towards its own substrate. 

A further evolution of this system has recently been published by Liu and Wulff [45], in which one amidine unit was linked with on both sides of the trialkyl-amine, in order to have a complex coordinating a single atom of Cu(II) in close proximity to two molecules of substrate interacting with the amidines (67). As a consequence of this structural modification the imprinted polymer was able to accelerate the rate of the reaction by an extraordinary factor of 410,000 when compared with the background. This is the highest rate enhancement ever achieved for an imprinted polymer. 

Wulff and collaborators, for instance, reported the preparation of TSA imprinted beads for the hydrolysis of carbonate and carbamate [61, 62], exploiting the amidine (33) functional monomer previously developed by the same group and successfully applied to the bulk format [63]. The polymers were prepared using a suspension polymerisation that produced beads with sizes in the range 8-375 pm, depending on the polymerisation conditions. The pseudo-first order reaction rate of the imprinted beads (Tyrrp/ soin) was enhanced by a factor of 293 for the carbonate hydrolysis and 160 for the carbamate, when compared with the background. 

The usefulness of these novel amidine binding site monomers in molecular imprinting was demonstrated by their application in the preparation of polymers that were imprinted with optically active A -(4-carboxybenzoyl)-phenylglycine (15) the resulting materials could discriminate between the enantiomers of the template molecule with a values of up to 2.8 [12]. 

The special advantage of amidine-based stoichiometric non-covalent interactions is illustrated in Fig. 4.7. The templates 15 were removed from an imprinted polymer and then the polymer was re-offered different amounts of template 15 in methanol for rebinding. Figure 4.7 shows a nearly stoichiometric uptake with a reloading yield of about 99%. This is in marked contrast to non-stoichiometric non-covalent interactions in which only around 15% of the cavities can be reloaded. 

It is important to notice that the amidine moiety can act as a binding group and as a base at the same time. The importance of this dual nature will become apparent below, when we discuss the catalytic properties of imprinted polymers prepared using this functional group. 

At pH 7.6, the imprinted polymer accelerated the rate of hydrolysis of ester 18 by more than 100-fold compared to the reaction in solution at the same pH (see equation (c) and Table 4.9). Addition of an equivalent amount of monomeric amidine to the solution only slightly increased the rate. A control polymer prepared from the amidinium-benzoate salt gave a somewhat stronger rate enhancement. 

Wulff, G. Schonfeld, R. Polymerizable amidines— adhesion mediators and binding sites for molecular imprinting. Adv. Mater. 1998,10, 957-959. 

Ester hydrolysis is most conveniently used because (1) its reaction mechanism is well established, and (2) both substrate and transition state analogs are easy to obtain. In Fig. 8.8b, phosphonic acid (2) is used as a transition state analog of the hydrolysis of substrate 3 [26]. A vinyl monomer of amidine 1 is chosen as a functional monomer because it readily forms stable complexes with the carboxylic acid ester and the phosphonic acid monoester. The imprinted polymers are synthesized in THF from 1 (the monomer), 2 (the template), and ethylene glycol dimethacrylate (the cross-linker) by using AIBN as the radical initiator. 

M ). In very recent experiments [151] we could show that by optimizing the polymer structure the enhancement of reaction rate of the imprinted polymer compared to the polymer with statistically distributed amidine groups could be further improved. 

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