Template polymerization mechanisms

Division of all processes leading to the polymer synthesis into the above classes is a simplification - convenient to present general mechanisms of template polymerization. 

The majority of papers published in the field of template polymerization deal with the systems in which both template and monomer are dissolved in a proper solvent and initiation occurs according to the chain mechanism.It is generally accepted that, for chain processes, there are at least three elementary processes initiation, propagation and termination. The mechanism of the addition radical polymerization can be schematically written as follows ... 

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. 

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 mechanism of template polymerization in this case seems to be complex. It involves both a stepwise and a chain reaction. However, experimental results show ... 

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 ). 

In fact, the first mechanism was accepted as a template polymerization by majority of... 

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 ... 

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 ... 

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. 

Zipping-up reaction is a well known method for ladder polymer production. The most important from the practical point of view is cyclization and carbonization of polyacrylonitrile. " The mechanism of the thermal conversion of polyacrylonitrile to carbonized product has been intensively studied and it is extremely complex.The reaction can be partially treated as a template polymerization proceeding according to the reaction ... 

Frequently the complexation of the monomer is weak, and only oligomers exceeding a critical degree of polymerization can become complexed to the template-chain (Figure 39). Because the polymerization simultaneously proceeds in the solution and along the template complex, a lower over-all rate enhancement is observed. If the number of monomer molecules exceeds the available complexation sites, the reaction rate is decreased. Virtually all investigated template polymerizations seem to obey this mechanism [484]. 

These template polymerizations suffer from three fundamental problems (i) In most cases the binding of the polymer to the template is stronger than the binding of the monomer due to the cooperativity of the interaction between the polymers. As a consequence the newly formed macromolecules are not released from the template and multiple replication is not possible without multiple separation steps, (ii) We lack the possibility to start the polymerisation reaction at the terminal group of the monomer-template complex, (iii) While a weak interaction between the template and the monomer is favourable to allow easy separation of the template and the newly formed macromolecule, it leads to incomplete complexation of the template and interraption of the polymerisation along the chain. A solution of these problems would require a relatively strong complexation of the monomers in combination with sufficient anticooperativity in the complexation of the polymer. The latter however would inevitably impede the polymerisation reaction and require therefore a living polymerisation mechanism which does not suffer from a slowed down rate of polymerisation. 

The polymerization in clathrates, which has been compared to the template polymerization of biological systems, illustrates the exact requirements which need to be fulfilled for topotactic polymerization via a free radical mechanism. The hopes to find a monomer-polymer crystal pair which meets those conditions seem rather dim indeed. 

Kinetic analyses in the absence of denaturants provided evidence that the conversion process can be separated into two stages, first the binding of PrP-sen to PrP and then a slower conversion of the bound PrP-sen to PrP-res (Bessen et al, 1997 DebBurman et al, 1997 Horiuchi et al, 1999). These observations, and the formation of amyloid fibril polymers by PrP-res, are consistent with an autocatalytic or templated nucleated polymerization mechanism. However, the fact that PrP-res usually induces the conversion of only substoichiometric quantities of PrP-sen in current cell-free reactions makes the reaction less continuous than typical nucleated polymerizations of proteins or peptides. This may be a technical problem rather than a fundamental limitation of the reaction mechanism. On the other hand, since PrP-res forms continuously for long periods of time in vivo, there may be important elements of the mechanism, such as cofactors or microenvironments, that remain to be elucidated (DebBurman et al, 1997 Saborio etal, 1999). 

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