Low-molecular-weight template

A low-molecular-weight template controls the synthesis of low-molecular-weight compounds. This principle is often used for the preparation of cyclic compounds, but many asymmetric syntheses also fall into this scheme. 

In this review article, templated syntheses of macromolecules and polymeric materials will be discussed in which a low-molecular-weight template controls the structure of macromolecules. The composition (e.g., the proportion of co-monomers), the sequence of co-monomers as well as the stereochemistry (including the chirality) of the newly formed stereogenic centers is controlled during the polymerization of double bonds. 

The two-dimensional information transfer from low-molecular-weight templates to polymers can be achieved on the surfaces, e.g., of wide-pore silica. We have attempted... 

Yoshikawa and coworkers [17-22,24,26-28] were using specifically synthesized polystyrene resins with peptide recognition groups, in ablend with a matrix polymer, for the membrane formation via a dry PI process. The resulting membranes seemed to be microporous. The permeability was much higher for the MIP as compared with the blank membranes hence, the low-molecular weight templates seemed to act also as a pore former. 

In light of the importance of the / -turn motif in peptide and protein recognition, and the design and synthesis of bioactive small molecules, / -turn mimetics has attracted considerable attention. Seebach and coworkers have shown recently that low molecular weight open-chain / - and y-peptides designed to promote turn formation can be used as templates for mimicking the a-peptide hormone somatostatin. 

Low molecular weight heparins, heparin fractions, or other sulfated polysaccharides still have the same or a similar complexity as heparin and are, therefore, not regarded as heparinoid mimetics only compounds with one defined carbohydrate backbone serving as a template for sulfates will be discussed in this context. 

In this case one monomer with groups x (e.g., COOH) can he ahsorhed on the template -T-T-. The second monomer with groups y (e.g., amine) reacts, forming a daughter polymer having groups xy and the template is available for further reaction. Low molecular weight product is not indicated in this scheme. 

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 study of template polymerization was preceded by examination of quaternary salts polymerization both in aqueous solution and in organic solvents. Examination of 4-vinylpyridine polymerization in water, induced by low molecular weight acids published by Salamone at al shows that in the first step, the following reaction occurs ... 

The authors thus concluded that the polymerization studied was affected by the template. The rules observed for polymerization of 4-vinylpyridine in the presence of low molecular weight acids and polyacids can be summarized as in the Table 4.1. ... 

Data presented in Table 8.2 for templates having low molecular weight (PVP and PDAMA with degree of polymerization of about 50) are an interesting exemption from this rule. The presence of both types of template - PVP or PDAMA - changes the rate constants, kp and kt, of elementary processes. In comparison with polymerization without template, kt for template polymerization is lower by a few orders of magnitude. Also, kp... 

It has already been reported that the weight loss of as-synthesized MMSs depends on the kind of the template used in the synthesis [17]. This is an obvious consequence of the fact that different templates decompose and thermodesorb at different temperatures. However, it was somewhat unexpected that the decomposition/desorption of the same kind of the template may be dramatically influenced by the framework composition of materials [4,10-14]. This can be understood as an influence of the framework structure on the process of Hoffmann elimination of alkylammonium to the corresponding alkene and low molecular weight amine [4,8], This decomposition process leads not only to the elimination of the electrostatic framework-template interactions but also to the formation of decomposition products of lower molecular weight than that of the surfactant. Thus, the framework-surfactant interactions are crucial factors determining the thermogravimetric behavior. 

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