Chain 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  

Termination can be realized both by macroradicals on the template (template-template termination) or by recombination of radicals on the template with macroradicals or oligoradicals not connected with the template (cross-termination). For some systems, it is difficult to decide whether they are type I or type II. The intermediate systems can also exist. 

The synthesis of biopolymers in vivo leads to macromolecules with a defined sequence of units. This effect is very important for living organisms and is different in comparison with random copolymerization in which sequences of units are distributed according to stochastic rules. On the other hand, the predicted sequence of units can be achieved by a set of successive reactions of respective monomer molecule addition. In template copolymerization, the interaction between comonomers and the template could pre-arrange monomer units defining sequence distribution in the macromolecular product. 

There is far less information in the scientific literature about template copolymerization than about template homopolymerization. As in the case of template homopolymerization, template copolymerization can be realized according to different types of reaction stepwise (template polycondensation), copolyaddition, radical or ionic polymerization, ring-opening copolymerization, etc. 

Only a few publications have appeared in the literature on template copolycondensation, in spite of the fact, that the process is very important to understand the mechanisms of processes similar to natural synthesis of biopolymers. General mechanism of this reaction can be considered in terms of the examples of template step homopolymerization. A few published systems will be described in the Chapter 5. 

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The most complete model of chain template polymerization was published by Tan and Alberda van Ekenstein. Assuming that the polymerization goes also outside the template (blank reaction) and onto the template, the process consists of... 

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

In both parts of the system the rate constant k is the same. Accepting, however that any reaction involving the polymer chain in proximity of template molecules, during at least part of its lifetime, may be called template or matrix polymerization. Polymerization proceeding outside the template is the secondary reaction. It is also convenient to generalize this definition to the step reactions, including in the template polymerization such cases in which polymerization proceeds only partially on the template. 

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 ... 

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. 

The composition of nucleotide polymers at any locale depended upon the relative concentration of the nucleosides G, C, A or T, in the medium and may have varied as a function of time, ionic environment and other factors. The first chains that polymerized without a template produced variety whereas subsequently chains were duplicated more or less accurately by complementarity, and that process gave rise to families of nucleic acid polymers that formed the basis for similarity (clones) and diversity among organisms (Chapter 4). 

The template polymerization of MAOT in the presence of stereoregular poly-MAOA was never accelerated. A strong tendency to cause self-association of adenine bases observed along the polyMAOA chain32) as well as in poly-VAd17,24) appears to inhibit the complex formation between a growing chain and polyMAOA and may result in a depression of the template effect of the polymerization. 

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. 

It is well known that rates of polymerizations can increase markedly w ith the degree of conversion or with the polymer concentration. Some workers have attributed this solely or partly to a template effect. It has been proposed that adventitious template polymerization occurs during polymerizations of AA, MAA and AN, and that the gel or Norrish-Trommsdorff effect observed during polymerizations of these monomers is linked to this phenomenon. However, it is difficult to separate possible template effects from the more generic effects of increasing solution viscosity and chain entanglement at high polymer concentrations on rates of termination and initiator efficiency (Section 5.2.1.4). 

R. P. Scaringe and S. Perez,/. Phys. Chem., 91,2394 (1987). A Novel Method for Calculating the Structure of Small-Molecule Chains on Polymeric Templates. 

Template polymerization for polypeptide synthesis was reported by Ballard and Bamford (63) and described in detail in Reference 2. The substrate for the synthesis was V -carboxy-a-amino acid anhydride (NCA). The first step consists in the reaction of NCA with secondary amine on the end of the polypeptide template. The ring-opening process and the elimination of CO2 then proceeds as a chain reaction. Valuable experimental material concerning the polymerization of many different N-carboxyanhydrides initiated by many different polypeptides (64-66) as well as by poly(vinyl pyridine) (67,68) was collected by Imanishi and co-workers. 

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