Formation of Short Chain Oligomers

PET contains about 2-3 % of short chain oligomers, which cause problems in the processing of the polymer. Oligomers can occur as linear or cyclic molecules and can be extracted by suitable solvents. Different compounds have been identified depending on the solvent and the analysis technique used [49-52], After their extraction from the polymer, oligomers will reform by thermal treatment of the extracted sample [49], and a dynamic equilibrium between polymer and oligomers has been proposed. 

Among the oligomers, the cyclic trimer has been postulated to be uniquely stable [53, 54], This could be due to either a mechanism favouring the formation of trimer (kinetic control) or to the trimer having a lower energy than other oligomers (thermodynamic control), thus decreasing its rate of further reaction. 

Lower reactivity of short chain oligomers had already been found by Challa [41], who determined that the reaction rate for the transesterification of BHET was half that for longer molecules. Challa assumed that the reactivities of the dimer and the trimer may also be lower but was not able to verify this hypothesis. 

Ha and Choun [51] confirmed these findings from the investigation of cyclic oligomer formation at 270 °C. They derived a rate equation for cyclic oligomer formation taking thermal degradation of the polymer into account. 

Freire el al. [56] investigated the formation of oligomers in PET samples heated to temperatures between 150 and 270 °C. In this temperature range, the concentration of cyclic oligomers remained nearly constant with time, whereas the concentration of linear oligomers increased markedly, probably due to hydrolytic or glycolytic degradation reactions. 

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Attempts to polymerize pyrrole dimers and trimers were not initially very successful, with evidence for the formation of short-chain oligomers rather than continuous films [119]. However, good quality films have been obtained from the dimer, particularly in the presence of moderately nucleophilic anions [107,120]. 

Milieu conditions in gastrointestinal tract can influence the pectin structure and properties. Under the acid conditions of the stomach (pH 2-4) extraction of pectin from plant cell walls and hydrolysis of side chains can occur. In small intestine (pH 5-6) -elimination of main chains or de-esterification seems to be possible. In caecum and colon (pH 6-8) a strong fermentation of pectin takes place causing depolymerization to oligomers and leading to formation of short chain fatty acids and gases. The presence of OligoGalA is not yet clarified. 

Although the oligomerization of mononucleotides under the above conditions provides plausible models for the prebiotic synthesis of short-chain oligomers, it is doubtful that this type of condensation is applicable to the formation of long polymers. 

Chemical vapor polymerization of polyimides follows a different route from that of parylenes described above, in that, it is usually a two step process. First the monomers are adsorbed on the surface of the substrate resulting in the formation of a short-chained oligomer intermediate, and then the films are cured at a higher temperature ( 300°C) to form the desired... 

Telomerization reactions, the formation of short oligomers from dienes, represent a very efficient organic transformation with an overall atom economy of 100%, and they have been the subject of intensive research in both academic and industrial laboratories. Complexes of palladium are known to catalyze the reaction of dienes with a variety of nucleophiles. Mechanistically, the reactions are thought to proceed by allyl coordination of two butadiene molecules to a palladium(O) center followed by the formation of a C-C bond. The eight-carbon chain is then attacked by a nucleophile at the terminal or at the 3 position. The reaction usnally leads to a mixture of cis/trans isomers and n- and iio-prodncts. When the nncleophile is methanol, l-methoxyocta-2,7-diene 1 (n-product) is generally the major prodnct, which is a nseful precnrsor for plasticizer alcohols (octanols), solvents, corrosion inhibitors, and monomers for polymerization. ... 

Oxidation causes the formation of hydroperoxides and conjugated compounds, which by cleavage give aldehydes, alcohols, ketones, lactones, acids, esters, and hydrocarbons. Radical mechanisms lead to the formation of dimers, other oligomers, and oxidized TAG. The latter have one or more acyl group with an extra oxygen (hydroxy, keto, epoxy derivatives). Other oxidation products are TAG with short-chain fatty acyl and n-oxo fatty acyl groups. 

The progression of an ideal emulsion polymerization is considered in three different intervals after forming primary radicals and low-molecular weight oligomers within the water phase. In the first stage (Interval I), the polymerization progresses within the micelle structure. The oligomeric radicals react with the individual monomer molecules within the micelles to form short polymer chains with an ion radical on one end. This leads to the formation of a new phase (i.e., polymer latex particles swollen with the monomer) in the polymerization medium. 

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