Template polymerization, radical

Further discussion on the effects of the reaction media and Lewis acids on lacticily appears in Section 7.2. Attempts to control laciicily by template polymerization and by enzyme mediated polymerization are described in Section 7.3. Devising effective means for achieving stereochemical control over propagation in radical polymerization remains an important challenge in the field. 

The template processes can be realized as template polycondensation, polyaddition, ring-opening polymerization, and ionic or radical polymerization. These types of template polymerization are fundamentally treated in the separate chapters below. 

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 ... 

The template polymerization was carried out at GO C in dimethyl sulfoxide/ethylene glycol mixture using AIBN as radical initiator. It was found that under these conditions interaction between adenine and uracil groups is remarkable. 

From the examples of template polymerization carried out under special condi-tions, we can see that intermolecular polymerization can be neglected. Indeed, if the concentration of monomer used is very low, and concentration of initiating radicals is... 

A very special type of template polymerization was presented by a group of Japanese scientists. The method used was based on the observation that during radical polymerization of 2,2-diphenyl-4-methylene-l,3-dioxolane, elimination of benzophe-none occurs according to 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 ... 

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. 

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 ... 

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 ... 

Template polymerization as seen in replicative biopolymer synthesis has recently received attention. From this point of view, vinyl polymerization has been studied in the presence of polymers that were expected to serve as templates. These template polymerizations, however, do not appear to be strictly selective because interactions between the monomeric or polymeric spedes and the template polymers may not readily be realized. It seems, however, to be one of the most attractive problems if template polymerization can be followed by suitable monomer-polymer pair formation with complementary nucleic acid bases. This section deals with the free-radical polymerization of methyacryloyloxy type monomers with pendant bases in the presence of template polymers with complementary bases41,42). 

It is known that interactions between polynucleotides also depend on temperature the complex formation is favored at lower temperatures39. For the free-radical copolymerization of MAOA with MAOT, the relative rate is increased as the polymerization temperature is lowered37. In the case of template polymerization, however, a reverse temperature dependency has been observed (Fig. 12) the relative conversion tends to increase with rising temperatures. 

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. 

All kinds of polymerization (radical, anion, cation, and condensation) can be employed for molecular imprinting. The only requisite is that the polymerization can satisfactorily occur under the conditions where all the components (the templates, the crosslinking agents, non-cova-... 

Figure 12. Template polymerization by radical ring-opening polymerization.

The radical ring-opening elimination polymerization of 4-methylene-l,3-dioxolane stimulated us to construct a novel template polymerization (3). The concept is that polymers bear polymers. Polymer-supported monomer, which had a structure of 2,2-dipheny 1-4-methylene-1,3-dioxolane, reacted with radical species to afford polyketone and copolymer of styrene with vinylbenzophenone as newborn polymer and template, respectively (Figure 12). These polymers were easily separated by fractional precipitation without any particular chemical treatment after polymerization. On the other hand, common template polymerization requires annoying procedures for the separation of obtained polymers form template. On this point, our novel template polymerization system differs from conventional template polymerization. 

R. N. Haward, ed., Dev. Polym., Vol. 2, Free Radical, Condensation, Transition Metal, and Template Polymerization, Appl. Sci. Publ., London, 1979. 

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