Template polymerization structure

FIGURE 1.49 Principle of molecular imprinting.169 1 = functional monomers 2 = cross-linking monomer 3 = molecule whose imprint is desired (molecular template). In (A), 1 and 2 form a complex with 3 and hold it in position in (B), polymerization involving 1 2 occurs and the template (imprint molecule) is held in the polymeric structure in (C) and (D) the imprint molecule is removed leaving a cavity complementary to its size and shape into which a target analyte of similar dimensions can fit. (Reproduced with permission from Taylor Francis.)... 

A polymer prepared in the presence of a secondary force often possesses a structure different from that obtained in solution. Template polymerization is a typical example. Micelles and polymer micelles are formed under conditions of thermodynamic equilibrium, so that the structure of these aggregates are always quite fluid. If the aggregate structure is immobilized by polymerization, they will provide better models of enzymes. 

Template or matrix polymerization can be defined as a method of polymer synthesis in which specific interactions between preformed macromolecule (template) and a growing chain are utilized. These interactions affect structure of the polymerization product (daughter polymer) and the kinetics of the process. The term template polymerization usually refers to one phase systems in which monomer, template, and the reaction product are soluble in the same solvent. 

Spontaneous polymerization of 4-vinyl pyridine in the presence of polyacids was one of the earliest cases of template polymerization studied. Vinyl pyridine polymerizes without an additional initiator in the presence of both low molecular weight acids and polyacids such as poly(acrylic acid), poly(methacrylic acid), polyCvinyl phosphonic acid), or poly(styrene sulfonic acid). The polyacids, in comparison with low molecular weight acids, support much higher initial rates of polymerization and lead to different kinetic equations. The authors suggested that the reaction was initiated by zwitterions. The chain reaction mechanism includes anion addition to activated double bonds of quaternary salt molecules of 4-vinylpyridine, then propagation in the activated center, and termination of the growing center by protonization. The proposed structure of the product, obtained in the case of poly(acrylic acid), used as a template is ... 

Another type of multimonomer has been synthesized and examined by Juntas. Selecting an appropriate dilution, concentration of initiator, and temperature, even if initiation is random, the polymerization leads to the ladder-type structure of the product as shown in the example of template polymerization of multimethacrylates according to following reaction ... 

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. 

In radical template polymerization, when only weak interaction exists between monomer and template and pick-up mechanism is commonly accepted, the reaction partially proceeds outside the template. If macroradical terminates by recombination with another macroradical or primary radical, some macromolecules are produced without any contact with the template. In fact, such process can be treated as a secondary reaction. Another very common process - chain transfer - proceeds simultaneously with many template polymerizations. As a result of chain transfer to polymer (both daughter and template) branched polymers appear in the product. The existence of such secondary reactions is indicated by the difficulty in separating the daughter polymer from the template as described in many papers. For instance, template polymerization of N-4-vi-nyl pyridine is followed, according to Kabanov et aZ., by the reaction of poly(4-vinylpyridine) with proper ions. The reaction leads to the branched structure of the product ... 

Description of polymerization kinetics in heterogeneous systems is complicated, even more so given that the structure of complex formed is not very well defined. In template polymerization we can expect that local concentration of the monomer (and/or initiator) can be different when compared with polymerization in the blank system. Specific sorption of the monomer by macromolecular coil leads to the increase in the concentration of the monomer inside the coil, changing the rate of polymerization. It is a problem of definition as to whether we can call such a polymerization a template reaction, if monomer is randomly distributed in the coil on the molecular level but not ordered by the template. 

Production of materials in which the daughter polymer and the template together form a final product seems to be the most promising application of template polymerization because the template synthesis of polymers requiring further separation of the product from the template is not acceptable for industry at the present stage. Possible method of production of commonly known polymers by template polymerization can be based on a template covalently bonded to a support and used as a stationary phase in columns. Preparation of such columns with isotactic poly(methyl methacrylate) covalently bonded to the microparticulate silica was suggested by Schomaker. The template process can be applied in order to produce a set of new materials having ladder-type structure, properties of which are not yet well known. A similar method can be applied to synthesis of copolymers with unconventional structure. 

Many polymer-polymer complexes can be obtained by template polymerization. Applications of polyelectrolyte complexes are in membranes, battery separators, biomedical materials, etc. It can be predicted that the potential application of template polymerization products is in obtaining membranes with a better ordered structure than it is possible to obtain by mixing the components. The examples of such membranes from crosslinked polyCethylene glycol) and polyCacrylic acid) were described by Nishi and Kotaka. The membranes can be used as so-called chemical valves for medical applications. The membranes are permeable or impermeable for bioactive substances, depending on pH. 

The polycomplexes obtained by template polymerization of methacrylic acid or acrylic acid in the presence of poly(N,N,N, N - tetramethyl-N-p-xylene-ethylenediammonium dichloride) were used for spinning of fine fibers 5 to 50 pm in diameter. The fibers are soluble in water but become stable after thermal treatment at about 80°C. The polycomplex with regular structure, obtained by template polymerization, is expected to be of considerable interest for textile industry. 

Polymerization of monomers in the presence of polymem which can interact with monomers or newly formed polymers via secondary binding forces is called matrix polymerization (or template polymerization, replica polymerization) (Fig. 52). It is expected that matrix polymerization fundamentally affects the kinetic behavior and/or controls some structural details (for example, molecular weight and its distribution, tactidties, optical isomerism, etc.). However, a variation of structural details seems to be realized only when the geometry of the monomers firmly bound on the matrix polymers, which exhibit a regular arrangement of their structures, can be controlled. 

Dent Glasser et al. found that metal cations can affect the polymeric state of silicate ions, the polymerization rate and form of silicon and aluminum species, and the properties of the gel.[40 41] In addition, it has been confirmed by extensive experimental data that there is a tight correlation between the metal cations and small cage-like structural units of the resultant zeolites (see Table 5.11 for details), i.e., the size of the cations or hydrated cations matches that of these small cages. One type of cation could template different structures under different conditions as well. Studies indicate that metal cations will affect the structure of zeolites via their electropositivity, size, and geometric configuration. 

Molecularly imprinted polymer recognition units are based upon template polymerization techniques (Haupt and Mosbach 2000). The MIP recognition units are formed in the presence of a template molecule that is later leached out or extracted, thus leaving complementary cavities embedded in the Ii nal structure of the polymer. These polymers display high chemical-binding affinity for molecules with structural similarities to the template molecule. Hence, MIPs can be used to fabricate sensors... 

The formation of 2-D structures including functionalised parts. Mobility within the 2-D surfaces generated can optimise the positioning of the functionalised molecules with respect to the template. These structures can then be frozen by polymerising the vinylic surfactants used to form the layers [191]. In this way, Arnold et al. [87] prepared polymeric material function-... 

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