Acetaldehyde-induced polymerization

Monitoring of acetaldehyde-induced polymerization of catechin and epicatechin by HPLC-MS demonstrated the formation of several methylmethine-linked flavanol dimers, trimers, and tetramers. Detection of the intermediate ethanol adducts confirmed the mechanism postulated by Timberlake and Bridle, which involves protonation of acetaldehyde in the acidic medium, followed by nucleophilic attack of the resulting carbocation by the flavan unit. The ethanol adduct then loses a water molecule and gives a new carbocation that undergoes nucleophilic attack by another flavanol molecule. Four dimers (C6-C6, C8-C8, and C6-C8, R and S) were formed from each monomeric flavanol. When both epicatechin and catechin units were present, additional isomers containing both types of units were... 

Es-Safi, N.E. et al.. Competition between (+)-catechin and (—)-epicatechin in acetaldehyde-induced polymerization of flavanols. J. Agric. Food Chem. 47, 2088, 1999. 

Figure 3b. Products issuedfrom acetaldehyde-induced polymerization...

Figure 3 General mechanism of the acetaldehyde-induced polymerization of flavan-3-ols and anthocyanins. (Adapted with permission from reference 31. Copyright 2002 American Chemical Society.)...

It has been known for a long time that decomposition and polymerization processes occur as a result of irradiation of acetaldehyde. The polymerization process, diminishing in importance towards shorter wavelengths, is induced by free radicals. Iodine traps the free radicals, therefore, no polymerization is observed in the iodine-inhibited reaction . Woolgar and Allmand suggested that the polymer formed was probably paraldehyde. On account of insufficient data, a detailed discussion of the polymerization processes is not justified. 

The dehydrogenation of ethanol over copper catalysts is not complete at 300° C. when moderate times of contact are used but if the temperature is raised to 350° C. or higher, secondary reactions become more and more evident. At temperatures above 350° C., copper catalysts begin to activate the decomposition of acetaldehyde to methane and carbon monoxide, to induce polymerization of the aldehyde, to cause dehydration processes to set in, to cause hydrogenation of the ethylene, and, in general, to promote secondary decompositions and condensations which complicate the product and destroy the activity of the catalyst. Hence, for the production of aldehydes and ketones it is desirable to use moderate temperatures of about 300° C. and to obtain maximum yields from the decomposition rather than maximum decomposition of alcohol per pass over the catalyst. 

By a technique which interposed a rotating sector between a source of UV radiation and a dilatometer bearing a monomer, variations in the rotational speed of the sector and size of the opening, controlled bursts of radiation strike the monomer and induce polymerization. From the frequency of exposure and the effect on the polymerization many of the kinetic constants were evaluated. In this connection it should be noted that vinyl acetate exhibits virtually no absorption of UV radiation at 290-300 nm. On the other hand, acetaldehyde has an extinction coefficient of 14 at 290 nm and an extinction coefficient of 15 at 300 nm. Therefore acetaldehyde can act as a photo-sensitizer for the polymerization of vinyl acetate at wavelengths above 299.8 nm [23]. At 366 nm, 2,2 -azo-bisisobutyronitrile has been used as a sensitizer [29,180]. Azobicyclohexane carbonitrile is a UV sensitizer suitable for use with a 124 watt mercury arc at 25°C which does not produce a dark reaction in rotating-sector experiments. Its absorption peak is at 350 mn with an extinction coefficient of 16 [181]. 

The polyacetaldehyde thus prepared has the same properties and structure as Letort s polyacetaldehyde which was obtained by crystallization polymerization. This indicates that crystallization polymerization, the only previously known method of acetaldehyde polymerization, is only a special case of acid-initiated or cationic acetaldehyde polymerization. We feel that Letort s proposal (29) that the peroxide-induced acetaldehyde polymerization (crystallization polymerization) as a radical polymerization can now be reinterpreted in the following way The initiating species, which is prepared by reacting acetaldehyde with a controlled amount of oxygen, is obtained via a... 

Because microoxygenation delays the beginning of MLF, this should be completed before inoculating the selected bacteria. Another reason to induce MLF after microoxygenation is because the lactic bacteria consume acetaldehyde, an essential intermediate in the polymerization reactions among phenolic compounds, as seen above. 

It is concluded that the incorporation of a small amount of alkali or alkaline earth oxide, V20g, amphoteric oxide, or oxide of heavy metal into silica gel induces a marked increase in the activity. This finding suggests that the proton-abstraction from a methyl group of acetaldehyde can be promoted by active sites with a relatively weak base, arising from V2O5 and amphoteric oxides. On the other hand, the formation of acrolein is accompanied by two sides reactions (1) formation of CO2 and methanol by Equations (4) and (5) which is promoted mainly by acid-base dual functions, and (2) polymerization of acrolein to unidentified polymers, which is promoted by strongly acidic sites. 

PET/PEN blends can be subjected to a solid state polymerization to increase the intrinsic viscosity or to reduce the acetaldehyde generation. In the course of the solid state polymerization, the extent of transesterification is increased. A low level of transesterification of the PEN/PET preform causes poor transparency, while a high level of transesterification prevents strain-induced crystallization and poor mechanical properties. 

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