Kinetics template polymerization

CHEMICAL KINETICS TEMPLATE CHALLENGE METHOD PROCESSIVITY Template-directed assembly, MICROTUBULE ASSEMBLY Template-directed irreversible polymerization,... 

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 ... 

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 kinetics of template polymerization depends, in the first place, on the type of polyreaction involved in polymer formation. The polycondensation process description is based on the Flory s assumptions which lead to a simple (in most cases of the second order), classic equation. The kinetics of addition polymerization is based on a well known scheme, in which classical rate equations are applied to the elementary processes (initiation, propagation, and termination), according to the general concept of chain reactions. 

In contrast to template polycondensation or ring-opening polymerization, template radical polymerization kinetics has been a subject of many papers. Tan and Challa proposed to use the relationship between polymerization rate and concentration of monomer or template as a criterion for distinguishing between Type I and Type II template polymerization. The most popular method is to examine the initial rate or relative rate, Rr, as a function of base mole concentration of the template, [T], at a constant monomer concentration, [M]. For Type I, when strong interactions exist between the monomer and the template, Rr vs. [T] shows a maximum at [T] = [M]q. For type II, Rr increases with [T] to the critical concentration of the template c (the concentration in which template macromolecules start to overlap with each other), and then R is stable, c (concentration in mols per volume) depends on the molecular weight of the template. 

A kinetic model for template polymerization in dilute template systems has been published by Tan and Challa. It is assumed that polymerization takes place in surrounding medium (free solution) and on the template. The presented scheme was as follows ... 

P is a partition coefficient for radicals between two parts of the system - template and surrounding medium. The kinetic scheme of template polymerization is more complicated than that for simple radical polymerization. For many systems (monomer-template-solvent) general kinetic equation was applied ... 

Application of classical type of kinetic equations to the template polymerization was demonstrated by Kabanov at al It was shown that 4-vinylpyridine, in the presence of poly(methacrylic acid), poly(acrylic acid), poly(l-glutamic acid), and polyphosphate, polymerizes according to the classical equation and the order of reaction with respect to the monomer is 2 as demonstrated in the Figure 8.1. In log-log coordinates, for the all sets of polymerizations, experimental points fit straight lines. In the same paper dependence of the initial rate on the molar ratio of acid to monomer was examined. This relationship is shown on the Figure 8.2. The rate of polymerization in the presence of the poly(acrylic acid) is much higher than that for the low molecular analogue (acetic acid). The polymerization rate riches its maximum for the molar ratio [acid]/[monomer] 2. The authors found kinetic equation for template polymerization of 4-vinylpyridine in the presence of different polyacids in the form ... 

In order to estimate kinetic constants for elementary processes in template polymerization two general approaches can be applied. The first is based on the generalized kinetic model for radical-initiated template polymerizations published by Tan and Alberda van Ekenstein. The second is based on the direct measurement of the polymerization rate in a non-stationary state by rotating sector procedure or by post-effect in photopolymerization. The first approach involves partial absorption of the monomer on the template. Polymerization proceeds according to zip mechanism (with propagation rate constant kp i) in the sequences filled with the monomer, and according to pick up mechanism (with rate constant kp n) at the sites in which monomer is outside the template and can be connected by the macroradical placed onto template. This mechanism can be illustrated by the following scheme ... 

Further understanding of the kinetic of template polymerization needs consideration of the process entropy. Applying a well known lattice model, it is easy to see that entropy changes, AS, in free polymerization and the template polymerization, differs considerably. According to the principles of statistical thermodynamics, the entropy of mixing is given by the equation ... 

Change in chemical composition of the solvent used can also change the velocity of polymerization. Viscosity of the examined system is another very important parameter which should be taken into account. Templates, as any macromolecular compounds, change viscosity in comparison with the viscosity of polymerizing system in a pure solvent. It is well known that the increase in viscosity can change the rate constant of termination and eventually the rate of polymerization. In many systems, an insoluble complex is formed as a product of template polymerization. It is obvious that the character of polymerization and its kinetics change. 

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. 

All these ohservations lead to the conclusion that it is very difficult to judge whether the examined case is template polymerization or not on the basis of kinetic effects alone. It is especially true if the form of kinetic equation (exponents n and m) is different for template and blank polymerization since more complicated mechanism of reaction can he expected. 

Many experimental techniques were used to examine polymerization kinetics and products of template polymerization. In kinetic measurements, many conventional methods of determination of monomer concentration were applied, very often UV spectrometry or bromometric titration. For many systems examined, bromometric titration gives results comparable with the results obtained by other methods. However, systems were found in which the method successful for blank reaction gives results incomparable with another analytical methods. Perhaps some specific reaction with the complex formed affects the analytical procedure." ... 

IR spectrometry is a convenient method of examination of template copolymerization and polymerization kinetics. For instance, IR spectroscopy was applied in order to examine kinetics of template polymerization of multiacrylate according to the reaction ... 

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