Template Polymerizations

Lack of definition in the molecules of synthetic polymers may arise from a number of causes, two of the most important being the statistical nature of the polymerization process and the absence of specificity in the propagation reaction. The former leads to a distribution of molecular weights and the latter to distributions of iso- and syndiotactic placements of units along the chains and, in copolymerizations, to randomness in the sequence of different types of unit along the chains. 

Considerations of this kind have led to the view that precision in polymerization might be achieved by ordering the monomer molecules before polymerization, e.g. by adsorption on to a template, which in its simplest form would be a linear macromolecule. A detailed review of template polymerization has recently been published.  

Template-monomer complexation. (Template unit T, monomer M). 

for example, Polymerization of Organized Systems , ed. H-G, Elias Midland Macro-molecular Monographs, Gordon and Breach, 1977, vol. 3. 

A simple template process in which polymerization passes along a chain of adsorbed monomer molecules would be expected to lead to rate-enhancement, compared to the non-template reaction. However, rate-enhancement may arise from incidental causes not connected with propagation, e.g. reduction in the rate of chain termination. 

The interest in this area may be seen to stem from the biological area where the phenomenon is well know n and accounts for the regularity in the structure of natural proteins and polynucleotides. Such polymers are efficiently synlhesized by enzymes which arc capable of organizing monomer units within regularly structured molecular-scale spaces and exploiting weak forces such as hydrogen bonds and Van der Waal forces to control the polymerization process.. 

The literature distinguishes two limiting forms of template polymerization.  

The interaction of the template w itti monomer and/or the propagating radical may involve solely Van der Waals forces or it may involve charge transfer complexation, hydrogen bonding, or ionic forces (Section 8.3.5.1). In other cases, the monomer is attached to the template through formal covalent bonds (Section 

In 1972, Buter et reported that polymerization of MMA in the presence of isotactic PMMA leads to a greater than normal predominance of syndiotactic sequences during the early stages of polymerization. Olher investigations of Ibis 

The nature of the interaction between the monomer and the template is more obvious in cases where specific ionic or hydrogen bonding is possible. For example, A-vinylimidazole has been polymerized along a PMAA template and acrylic acid has been polymerized on a A -vinylpyrrolidone template. The daughter PAA had a similar degree of polymerization to the template and had a greater fraction of isotaclic triads than PAA formed in the absence of the template. 

Under conditions where bimolecular termination between propagating radicals becomes difficult because of the increased viscosity or heterogeneity, primary termination may become important or even the only mode of termination. The latter leads to Rp being second-order in [M] and zero-order in [I]. 

---

New templated polymer support materials have been developed for use as re versed-phase packing materials. Pore size and particle size have not usually been precisely controlled by conventional suspension polymerization. A templated polymerization is used to obtain controllable pore size and particle-size distribution. In this technique, hydrophilic monomers and divinylbenzene are formulated and filled into pores in templated silica material, at room temperature. After polymerization, the templated silica material is removed by base hydrolysis. The surface of the polymer may be modified in various ways to obtain the desired functionality. The particles are useful in chromatography, adsorption, and ion exchange and as polymeric supports of catalysts (39,40). 

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. 

Template polymerizations where the monomer is covalently bound to the template clearly have limitations if polymers of high molecular weight or large quantities arc required. However, their use offers much greater control over... 

Kammerer ei aL1(n m have conducted extensive studies on the template polymerization of acrylate or methacrylate derivatives of polyphenolic oligomers 22 with X n < 5 (Scheme 8.14). Under conditions of low "monomer" and high initiator concentration they found that X n for the daughter polymer was the same as X n for the parent. The possibility of using such templates to control microstructure was considered but not reported. 

A new form of template polymerization based on ring-opening polymerization of 4-methylcncdioxalane has been reported by Endo and coworkers (Scheme 8.15).220,221 For this system, the monomer is covalently bound and the daughter polymer is released from the template as a consequence of the polymerization process. 

Condensed DNA Particle Assembly Using Template Polymerization of Counterions... 

FIG. 3 Condensed DNA particle assembly using template polymerization of cations, (a) Principle of DNA condensation by counterion polymerization on DNA template (b) chemistry of polycation formation (c) e microscopy of condensed DNA particles, bar =100 nm. [(c) was reprinted from Ref. 97, copyright 1998 Oxford University Press.]... 

Our experiments indicated that template DNA remained intact and functionally active after template polymerization. The luciferase-encoded plasmid pCl Luc recovered from the reduced complexes after dialysis was able to express luciferase at levels comparable to pClLuc that had not undergone template polymerization but was also reduced and dialyzed. 

A polymer prepared in the presence of a secondary force often possesses a structure different from that obtained in solution. Template polymerization is a typical example. Micelles and polymer micelles are formed under conditions of thermodynamic equilibrium, so that the structure of these aggregates are always quite fluid. If the aggregate structure is immobilized by polymerization, they will provide better models of enzymes. 

Template polymerization (Sec. 3-10d-2) also involves self-assembly. 

Uezu K, Nakamura H, Kanno J, Sugo T, Goto M, Nakashio F. Metal ion-imprinted polymer prepared by the combination of surface template polymerization with postirradiation hy gamma-rays. Macromolecules 1997 30 3888-3891. 

---