Oligomers chemical modification

Recent work on the synthesis, structure and some properties of macromolecules bearing furan rings is discussed. Two basic sources of monomers are considered, viz. furfural for monomers apt to undergo chain polymerization and hydroxymethylfurfural for monomers suitable for step polymerization.Within the first context, free radical, catiomc and anionic systems are reviewed and the peculiarities arising from the presence of furan moieties in the monomer and/or the polymer examined in detail. As for the second context, the polymers considered are polyesters, polyethers, polyamides and polyurethanes. Finally, the chemical modification of aU these oligomers, polymers and copolymers is envisaged on the basis of the unique reactivity of the furan heterocycle. 

Finally, the use of oligonucleotides as substrates may provide a mechanism for preferentially replicating RNA templates. The oligomers in a prebiotic mix that contained a larger proportion or ribose residues should have annealed more readily to an RNA template than those oligonucleotides that contained fewer ribose-linked bases. Duplexes formed solely from RNA are more stable than duplexes in which one of the strands contains even only a minor chemical modification, namely, ribose to deoxyribose.65 In addition, acyclic residues are found to decrease significantly the melting temperature of DNA duplexes in which they are included.66... 

Extraction of lignocellulosic materials with dioxane has been used for quite some time as a method for lignin isolation at atmospheric pressure (1) or, as recently reported, at high pressure and with supercritical C02 mixtures (2-5). In this case, it was possible to extract from wood lignin oligomers with a low degree of chemical modification, hemicelluloses were also depolymerized and extracted, but cellulose remained without significant mass losses. 

The reactivity of conjugated double bonds is significantly different from that of isolated double bonds in the polymer backbone of polydienes. This difference can be used for selective chemical modification of the dienes with reactions such as the Diels—Alder reaction. With maleic anhydride as an enophile, selective addition to the terminal pair of conjugated double bonds in a chloroprene oligomer is complete in a few hours to give 102.391... 

The present chapter will concentrate on these two types, including more or less close modifications of their structure, while cyclic oligomers derived from pyrocatechol which are [l ]orthocyclophanes and various other cyclooligomers for which meanwhile the prefix calix is used will be excluded. The main emphasis will be also on the synthesis and the chemical modification of calixarenes and some basic properties, such as their conformational behaviour. Their host properties towards cations, anions or neutral guests... 

Modifier, substance to be bonded - molecule, oligomer, polymer, particle - the participant of chemical modification. Modifier reacts chemically with the surface functional groups. The basic types of surface reactions are given below (S support X, Y - modifiers, Xs, Ys-surface bonded groups inherent surface functional groups are not shown) ... 

Calix[n]arenes are cyclic condensation products of para-substituted phenol derivatives and formaldehyde [29], They are highly interesting for the development of sensitive coatings due to their conformational flexibility and the ease by which they may be modified chemically. Chemical modification can be done either in the meta position, or by reactions at the hydroxy group. In this way, bulky substituents [30], chelating substituents [31], aromatic residues [32], crown ethers [33,34], peptides [35,36], etc. can be introduced. A first approach to combinatorial synthesis of calix[4]arene receptors has been published by Reinhoudt and co-workers [37,38], who prepared calixarenes with different substituents. In solution, these calixarenes lead to formation of hetero-oligomers with barbiturates, and these hetero-oligomers were detected by MALDI-TOF mass spectrometry and H-NMR spectroscopy. 

The use of thiol compounds as transfer agents exclusively leads to monofunctional oligomers. To achieve bifunctionality requires a chain-end modification. Fock et al. [20-23] developed a new method based on the chemical chain-end modification of polymethacrylate telomers. These were previously obtained by telomerization reaction in the presence of mercaptoethanol or thioglycolic acid as transfer agents. The chemical modification can be summarized in two strategies ... 

Getting the bifunctionality requires a chemical modification of the terminal halogen from the oligomer obtained by ATRP. Two different concepts are possible to obtain the bifunctionality (Scheme 14) ... 

Scheme 14 Synthesis of telechelic oligomers by chemical modification of prepolymers previously obtained by ATRP...

This section is devoted to the synthesis of oligomers, which are not real telechelics, but are able to give telechelics by chemical modification. 

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