Oligomeric models

Early studies of the condensation reaction on the monomer level did not give the full picture of this process and only in the 1980s was polycondensation of siloxanols studied by using oligomeric model compounds (76,77). These studies revealed that in the presence of strong protic acids three processes must be considered linear condensation (eq. 14), cyclization (eq. 15), and disproportionation (eq. 16). 

From UV/Vis investigations of this series of monodisperse, oligomeric model compounds (30) the effective conjugation length of the corresponding polymer... 

Pang, Li, and Barton indicated, based on viscosity data, that PAEs had a stiff structure in solution [43]. A PAE shows a Mark-Houwink constant as high as a= 1.92 [43], revealing that it has a very stiff structure, similar to rodlike poly-(pyridine-2,5-diyl) [65,66]. Liquid crystalline behavior of PAEs with a meso-gen has been reported [45]. The chemical properties of PAEs have also been investigated with oligomeric model compounds [67-76]. 

Aldrete, S., Worstell, J. H., Improved ethylene oligomerization modeling using Aspentech s polymers plus, in Proceedings of the AIChE Annual Meeting (16-21 Nov. 2003), AIChE, San Francisco, 2003. 

Spherical siloxanes (RSiOi 5), with polyhedral frameworks, also called silsesquioxanes, can be used as defined oligomeric models for surface-modified silica gels or polysiloxanes. Whereas silsesquioxanes with alkyl-, aryl-, hydrido-, and trimethylsiloxy groups are known for a long time [1], functionalized octa-[propyl-silsesquioxanes], [X-(CH2)3]g(SiOi.5)g, were synthesized for the first time in 1990 by Weidner, Zeller, Deubzer, and Frey [2]. Octa[(3-chloropropyl)-silsesquioxane], [Cl-(CH2)3]g(SiO, 5)g, could be obtained by hydrolysis of (3-chloropropyl)-trichlorosilane. 

Scheme 2. Conjugated polymers and the corresponding oligomeric model compounds. R is tert. butyl unless otherwise indicated...

In order to shed light on the polymerization mechanism, the attention of several research groups was focused on the structure of the living chain-ends. A major problem is that enolates are known for condensation at a rate that depends on solvent, temperature and structure of the ester group (f-Bu esters being less reactive than Me esters). This undesired reaction makes the structural analysis of the chain-ends more complex . In order to get rid of any contribution of the chain in the structural analysis, unimeric, dimeric and oligomeric models of the chain-ends were considered. 

Kwong PD, Wyatt R, Sattentau QJ et al (2000) Oligomeric modeling and electrostatic analysis of the gpl20 envelope glycoprotein of human immunodeficiency virus. J Virol 74 1961-1972... 

In the case of the model trimeric (H-EA-iBA-iBA ) and tetrameric radicals (H-EA-tBA-tBA-iBA ), their ESR spectra showed a clear temperature dependent change. The spectrum observed at -30 °C was veiy similar to that of the dimeric radical whereas the spectra observed at higher temperatures were totally different from that measured at lower temperatures. Interpretation of the higher temperature spectra suggested that a 1,5-hydrogen shift occurred in the uniform oligomeric model radicals. No other radicals except for the specific radicals at low and high temperatures were detected. 

Uniform dimeric, trimeric, tetrameric, and pentameric oligomeric model radical precursors of OdA were prepared by ATRP then separated and purified by repeated colurrm chromatography. 

Spectroscopic and oligomeric model compound studies suggest that the coordination is through skeletal nitrogen atoms, as shown in... 

Although phosphazene macromolecules have not yet been synthesized with outrigger chains of metal-metal-bonded polymers, a prototype reaction has been discovered at the oligomeric model compound level. Hexafluorocyclotriphosphazene, (NPF2)3, reacts with sodium cyclopentadienyldicarbonylferrate to yield compound XIV. This compound undergoes decarbonylation on photolysis to yield XV, which is a stable metal-metal-bonded species . Compounds XIV and XV were the first phosphazenes synthesized with P-metal side-group bonds. Related Ru derivatives have also been prepared. 

Thermodynamic properties of a system can also be obtained from the atomistic considerations. Molecular dynamics or Monte Carlo methods have been successfully used to smdy polymers. The success stems from the fact that many properties can be projected from dynamics of relatively simple, oligomeric models. Unfortunately, miscibility strongly depends on the molecular weight and so far it cannot be examined by these methods. 

Although the exact structure of eumelanin has still not been determined, there is strong evidence that it has a polymeric structure built up of 5,6-dihydroxyindole 1, 5,6-indolequinone 6 and semiquinone 7 monomers. Eumelanin has exceptionally good photoprotective properties and an absorption spectrum spanning a large part of the solar spectrum. These special properties have prompted a number of theoretical studies of 5,6-dihydroxyindoles, their oxidation products and dimers, and oligomeric models of eumelanin. 

Characteristics of macromolecules are often preserved In oligomeric model compounds. As one example, X-ray powder diffraction patterns (1) and the infrared spectra (2) of the oellodextrins beginning with cellotetraose are very similar to those of cellulose II. Such a similarity implies a close relationship in their molecular structures. While these model compounds should permit more accurate characterization because of their macroscopic, single-crystal domains (3), oellodextrins form non-centrosymmetric crystals and analysis of their diffraction data is a serious challenge. 

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