Radical Template Copolymerization

General mechanism of template radical copolymerization was described in Chapter 2. From general consideration, it is clear that two monomers in copolymerizing system can interact with the template in a different manner. Generally, we have two groups in the systems  

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In principle, template copolymerization can be realized according to different reaction types as template copolycondensation, template copolyaddition, ring-opening copolymerization, and radical or ionic copolymerization. However, radical template copolymerization is slightly more elaborated. The process of template copolymerization can be classified from the point of view of stmcture of the template. We can make a distinction between two cases whether the template is a homopolymer or a copolymer. An important role in the process of template copolymerization plays a difference in interactions between the template and two comonomers. 

Deviations are also observed in some copolymerizations where the copolymer formed is poorly soluble in the reaction medium [Pichot and Pham, 1979 Pichot et al., 1979 Suggate, 1978, 1979]. Under these conditions, altered copolymer compositions are observed if one of the monomers is preferentially adsorbed by the copolymer. Thus for methyl methacrylate (M1 )-/V-vinylcarbazole (M2) copolymerization, r — 1.80, r2 = 0.06 in benzene but r — 0.57, > 2 0.75 in methanol [Ledwith et al., 1979]. The propagating copolymer chains are completely soluble in benzene but are microheterogeneous in methanol. /V-vinylcarba-zole (NVC) is preferentially adsorbed by the copolymer compared to methyl methacrylate. The comonomer composition in the domain of the propagating radical sites (trapped in the precipitating copolymer) is richer in NVC than the comonomer feed composition in the bulk solution. NVC enters the copolymer to a greater extent than expected on the basis of feed composition. Similar results occur in template copolymerization (Sec. 3-10d-2), where two monomers undergo copolymerization in the presence of a polymer. Thus, acrylic acid-2-hydroxyethylmethacrylate copolymerization in the presence of poly(V-vinylpyrrolidone) results in increased incorporation of acrylic acid [Rainaldi et al., 2000]. 

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. 

Copolymerization can be conducted stepwise (template copolycondensation), copolyaddition, radical or ionic copolymerization, ring-opening copolymerization, etc. 

Cyclopolymerization of bifunctional monomers is an effective method of chirality induction. Optically active vinyl homopolymers and copolymers have been synthesized by using optically active distyrenic monomers (41) based on a readily removable chiral template moiety. Free-radical copolymerization of 41a with styrene and removal of the chiral template moiety from the obtained copolymer led to polystyrene, which showed optical activity ([Oc]365 -0.5-3.5°) (Scheme 11.6) [84], The optical activity was explained in terms of chiral (S,S)-diad units generated in the polymer chain through cyclopolymerization of 41a [85], Several different bifunctional monomers have been synthesized and used for this type of copolymerization [86-90]. 

Using this approach, hydrophilic (neutral or ionic) comonomers, such as end-captured short polyethylene oxide (PEO) chains (macromonomer), l-vinyl-2-pyrrolidone (VP), acrylic acid (AA) and N,N-dimethylacrylamide (DMA), can be grafted and inserted on the thermally sensitive chain backbone by free radical copolymerization in aqueous solutions at different reaction temperatures higher or lower than its lower critical solution temperature (LCST). When the reaction temperature is higher than the LOST, the chain backbone becomes hydrophobic and collapses into a globular form during the polymerization, which acts as a template so that most of the hydrophilic comonomers are attached on its surface to form a core-shell structure. The dissolution of such a core-shell nanostructure leads to a protein-like heterogeneous distribution of hydrophilic comonomers on the chain backbone. 

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 monomer 1 was copolymerized by free radical initiation in the presence of an inert solvent with a large amount of a bifunctional crosslinking agent. Under these conditions, macroporous polymers were obtained which possessed a permanent pore structure, a high inner surface area, and good accessibility. Additionally, low polymer swellability would imply limited mobility of the polymer chains. From a polymer of this type, 40 to 90% of template molecules can be split off by treatment with water or alcohol (see Figure 3 ). If this polymer is... 

Monomer 1 is subjected to radical copolymerization with large amounts of a cross-linking agent such as ethylene dimethacrylate in the presence of an inert solvent (which acts as a porogen to give a porous structure). This yields macroporous polymers with a large inner surface area and a permanent pore structure. Up to 95 % of the templates can be split off again from polymers of this type by treatment with water or methanol (Scheme 2). 

The BAP functional monomer was prepared by 2,6-diamino pyridine and acryloyl chloride in triethylamine-chloroform solution (54% yield) [37]. Cyclobarbital-imprinted copolymers were prepared by the following procedure BAP (Immol), template cyclobarbital (0.5 mmol) and EGDA cross-linker (20 mmol) were dissolved in CHCI3. In the presence of a radical initiator, 2,2-azo (2,4-dimethylvaleronitrile), radical copolymerization was performed in N2 atmosphere with a two-step process for 6 h at 40°C and then for 3 h at 90°C. The obtained polymer was ground and sieved in the range of 26-63 pm. Selectivity of the cyclobarbital-imprinted polymer was assessed by chromatographic analysis for 5-barbiturates. They also used BAP monomer to imprint antitumor active compound of 5-fluorouracil (5-FU) [36]. The 5-FU imprinted polymers showed a higher afihnity for 5-FU than for 5-FU derivatives. The BAP imprinted polymer was prepared in the presence of a co-monomer, 2-(trifluoromethyl)-acrylic acid. 

In the covalent approach (Figure 2), a polymerizable derivative of the template is obtained by linking the template with a vinyl functional monomer via a strong reversible covalent bond (e.g., boronate esters, Schiff bases, or ketal). The derivatized print molecule is then radical copolymerized with an excess amount of a cross-linking agent, using either thermal or photochemical radical initiation. Reaction conditions are... 

If the reactor fluid contains two different monomers Mi and M2, both monomers can react with radical sites to form copolymer radicals. If there is no template for the monomer preference to react with the radical site, then the sequence of monomer addition will be based on monomer reactivity rules. Description of copolymerization kinetics differs from that in Fig. 1.3.1 (homopolymerization kinetics) during chain propagation, as shown in Fig. 1.3.4... 

MIPs are highly linked polymers, which can be synthesized by a typical three-step imprinting process, depicted in Figure 11. First, an imprint molecule or template forms a complex with functional monomers. The template is usually a molecule with the same or similar structure as analyte. Then, the complex is copolymerized with cross-linkers to form a polymer matrix typically via free radical polymerization. The functional monomer and crosslinker usually have vinyl or acrylic groups, which are commercially available. Finally, the template is removed from the matrix to create binding sites for analytes. 

In the presence of a homogeneous high-molecular-weight poly(2-vinyl-pyridine) template, the free-radical copolymerization of styrene and... 

These equations indicate that the reactivity ratios are dependent on the concentration of template used [T], as well as on the interactions between the templates and monomers (Ka and Kb). The interactions are dependent on the solvent used in the process of copolymerization. It seems that this statement can be generalized. Consequently, the template influences the average number of units in a sequence Ia and Ig. From the general mles concerning radical copolymerization ... 

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