Template monomer

Fig. 25. Schematic representation of imprinting (a) cross-linking polymerization ia the presence of a template (T) to obtain cavities of specific shape and a defined spatial arrangement of functional groups (binding sites. A—C) (b) cross-linked polymer prepared from the template monomer and ethylene...

Lubke C, Lubke M, Whitcombe MJ, Vulfson EN. Imprinted polymers prepared with stoichiometric template-monomer complexes efficient binding of ampiciUin from aqueous solutions. Macromolecules 2000 1433 5098-5105. 

In the case of template processes, this mechanism must be completed by terms accounting for interaction between template, monomer, and polymer. This subject is discussed in more detail in Chapter 8. Intermolecular forces lead to absorption of the monomer on the template or, if interaction between monomer and template is too weak, oligoradicals form complexes with the template. Taking into account these differences in interaction, this case of template polymerization can be divided into two types. In Type I, monomer is preadsorbed by, or complexed with, template macromolecules. Initiation, propagation and perhaps mostly termination take place on the template. The mechanism can be represented by the scheme given in Figure 2.7. 

A few examples illustrate the interaction between monomer and template groups and the nature of forces operating in the system template-monomer (Table 3.1). 

The lower the initiator concentration the higher the molecular weight of polymer formed. Moreover, the effect of the ratio template/monomer concentration is more pronounced for low initiator concentration. In addition, it was found that tacticity of the template does not influence the suppression of degradative addition nor the tacticity of the poly(vinyl imidazole) obtained by the template process. 

The template polymerization of methacrylic acid at 60 C in DMF was studied with atactic poly(vinyl acetate) M =66,400 used as a template. The effect of template, monomer, and initiator (AIBN) concentration on the kinetics of polymerization was studied dilatometrically. Viscometric measurements showed that complexation between poly(vinyl acetate) and poly(methacrylic acid) was maximized when the template to polymer ratio was 1 1, and for the same ratio of the monomer to the template, the rate of template polymerization also reached the maximum. The overall energy of activation was the same (115 kJ/mol) in the presence and absence of the template. The polymerization follows mechanism II ( pick up mechanism ). 

Secondary reactions usually proceed in addition to template polymerization of the system template-monomer-solvent. They influence both kinetics of the reaction and the structure of the reaction products. Depending on the basic mechanism of reaction, typical groups of secondary reactions can take place. For instance, in polycondensation, there are such well known reactions as cyclization, decarboxylation, dehydratation, oxidation, hydrolysis, etc. In radical polymerization, usually, in addition to the main elementary processes (initiation, propagation and termination), we have the usual chain transfer to the monomer or to the solvent which change the molecular weight of the product obtained. Also, chain transfer to the polymer leads to the branched polymer. 

The initial state (entropy So) can be realized by various ways of placing the monomer units then the final state (entropy Si). In this case, So > Si and for the process AS < 0. With template unit denoted by T, and monomer unit, M, connected with the template by covalent bonding or by strong interactions denoted by / , the system template- monomer can be represented as follows ... 

Let us divide the overall volume of the reacting system, V, into two parts first Vi filled up with template coils being in contact with each other (possesses the critical concentration c in g/L) and the second part V2 free from the template. Monomer is divided into these two parts in such a way that concentrations are [M]i and [M]2, respectively. The overall rate of the polymerization Rqv is a sum of the rate in the first part, Ri, and in... 

A good imprinting effect also relies on the existence of stable assemblies between the template molecule and the functional monomers in the course of the polymerisation process therefore, any preparation method for beaded MIPs must preserve such template-monomer assemblies. 

Another possible way of overcoming the limitations posed by the presence of water in the suspension polymerisation process is to substitute the continuous water phase with alternative solvents that could still act as dispersing medium for the monomer mixture but better preserve noncovalent interactions in the template-monomer assembly. For example, liquid fluorocarbons are chemically inert and do not affect interactions which are used in noncovalent imprinting. Use of such solvents for the preparation of MIP microbeads has been demonstrated already in 1996 by Mayes and Mosbach [16,17]. A range of MIPs were prepared using Boc-l-phenylalanin as the template, MAA as the functional monomer and different kinds and amounts of crosslinkers and porogenic solvents. The resulting MIP microbeads... 

Fig. 5 Functional monomer and template-monomer assembly employed by the group of Resmini. Adapted from [79]...

Application Polymerisation Template Monomers + solvent (S) Sample separation Ref. 

Template Monomer/crosslinker Tracer Dynamic range Detection limit Sample Ref. 

Template Monomer/ Porogen Tracer crosslinker Dynamic range Ref. 

Template Monomers Porogen Enzyme Dynamic range Detection limit Ref. 

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