Measuring Template Effects

Determination of the reaction rate from calorimetric measurements, using DSC technique, is very useful and was applied with success for many template polymerization systems and for blank polymerizations.Two types of calorimetric measurements were described isothermal and scanning experiments. The heat of polymerization can be measured by DSC method, measuring thermal effect of polymerization and ignoring the heat produced from decomposition of the initiator and heat of termination. In isothermal experiments sample is placed at a chosen temperature and thermogram is recorded versus time. Assuming typical relationship... 

Template Polymers. Template effects in chelating polymers constitute an interesting development in the field of metal containing polymers. The Template effects are interpreted by the fact that the small molecule is templating a pattern in the macromolecule which can be recognized by the same molecule in a subsequent process. The idea is to prepare a polymer from the metal-chelated monomer, to remove the metal ion, and then to measure the selectivity of the prepared polymer for the metal ion of the template [36]. Typical examples of template systems are 4-vinyl-4 -methylbipyridine (Neckers [36]) and 1-vinyl-imidazole (Tsuchida [37]). These are polymerized in presence of divinylbenzene [36] and appropriate metal salts (Co2+, Cu2+, Ni2+, Zn2+). The template metal ions are removed by acid leaching and the polymer subsequently used for metal ion absorption studies (Fig. 16). 

Naturally, quantum chemical approaches to molecular recognition are usually employed for selected systems since the complexity of these systems requires a system-specific analysis which makes it difficult to extract results of general validity for examples, see Refs. [13-16] for studies of molecular tweezers. Further examples are mentioned in a review article by Schatz considering ab initio calculations on calixarenes and calixarene complexes [17]. Schatz concludes that although the systems are quite big, useful contributions have been made by ab intio calculations. However, a general model is needed in order to make host-guest processes and template-assisted reactions accessible to a comparison of quantitative measurements and calculations, which may finally provide the basis for rational host design and for the prediction of template effects (compare the recent attempt by Hunter [18]). 

Table 4-1 shows that pyrazine is the best template and is a million times better than the poorest measurable template, iV-methyl-2-pyrrolidinone (NMP) [14]. Also, considering the series of suitable guest molecules, the selectivity is unusually high, where small perturbations in guest structure can lead to large differences in templating abilities [14]. For example, methyl acetate is 10000 times better than ethyl acetate [14]. A similar template effect has also been observed in the formation of carceplex Ic guest [15]. 

Scheme 8-22 Bis-lactonization experiments to measure the effective initiator-terminator gap in the (NBD)3-derived template.

Interestingly, an unambiguous assessment of the thermodynamic template effect (tte) offered by a saturated solution of AgN(Cp3S02)2 in chloroform at 25°C to the dimer 14 can be given by the ratio in Eq. [33]. The tte turns out to be a pure number that gives a measure of the strength of the template in amplifying the selected receptor... 

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