Monomer formation

Chemical Properties. Vacuum thermal degradation of PTFE results in monomer formation. The degradation is a first-order reaction (82). Mass spectroscopic analysis shows that degradation begins at ca 440°C, peaks at 540°C, and continues until 590°C (83). 

Char formation and reduced monomer production are observed for all of these additives upon reaction with PMMA. Char formation increases as a function of temperature, for the hydrido cobalt compound, there is 5% char at 262°, 8.5% at 322°, 15% at 338°, and 19% at 375°C the cobalt(lll) cyanide produces 3% char at 338° and 11% at 375°C the cobalt(ll) cyanide yields 11% char at 375°C. At the highest temperature, 375°C, the amount of monomer formation is 22% for K3Co(CN)5, 11% for K3Co(CN)6, and 10% for HCo[P(OPh)3]4. Ideally one would hope to observe no monomer formation and complete char production. Such is not the case here, these materials probably have no utility as flame retardant additives for PMMA since monomer formation, even at a reduced level, will still permit a propagation of the burning process. While somewhat positive results for these three additives do not prove the validity of the hypothesis, we take this to be a starting point in our search for suitable additives, further work is underway to refine the hypothesis and to identify other potential hydrogenation catalysts and other additives that may prove useful as flame retardants for PMMA... 

Scheme 12. In the Ru-catalyzed conversion of disubstituted styrenyl ethers to chromenes the presence of ethylene is required for reaction efficiency as well as high yield of monomer formation...

In contrast to the reaction carried under an Ar atm, when (S)-49 was treated with 10 mol% la under an atmosphere of ethylene (22°C, CH2C12,24 h), (S)-41 was obtained in 81% isolated yield and >98% ee (Scheme 12). As expected, the use of ethylene atmosphere proved to be necessary for preferential monomer formation (10% of the derived dimer was also generated). These results indicate that the presence of ethylene is imperative for efficient metal-catalyzed chromene formation as well for processes involving disubstituted styrenyl ethers (25-30% yield of dimer 50 under argon). 

The second electronic transfer to the oxygen produces the diradical (C) which evolves into monomer formation. The latter possibility (IV) is a homolytlc cleavage giving another anion radical. If the process follows scheme III or IV, we must obtain monomer formation after the oxidation reaction in all cases. We have carried out the oxidation of carbanionic dimers derived from isoprene, crmethylstyrene, styrene, 1,1-diphenylethylene. 

With "direct oxidation" there Is a low amount of oxygen In the mixture and the coupling reaction and the alcoholate formation are favoured. This sequence prevents the monomer formation by the disappearance of the radical. 

The last observation concerns the oxidation of the lsoprene carbanionic dimers.— We do not observe the monomer formation, but the presence of monoalcohols with a high yield. In "Inverse oxidation we obtain 52% of monoalcohol and 80% In "direct oxidation . These monoalcohols have been Isolated and characterized (Fig. 1). They come from the transfer reaction from the primary radical to the solvent. The high yield of monoalcohol shows that the hydrogen transfer from the solvent Is a competitive reaction towards functionalization. This fact Is confirmed by the similar values of the two rate constants ( 108 M-i.S-1) . . ... 

T. Sawaguchi and M. Seno, Thermal degradation of polyisobutylene effect of rotational motion around C-C- bond on the /1-scission leading to monomer formation, Polym. Degrad. Stab., 54(l) 23-32, October 1996. 

PRPP amidotransferase is inactive as a dimer. Dimer formation is promoted by IMP, GMP, and ADP, monomer formation by PRPP. 

The activity of cinnamyl alcohol dehydrogenase (CAD EC 1.1.1.195) was already described in the 1970s and was mainly investigated with respect to lignin biosynthesis (see Petersen et al, 1999, for further information). It catalyses the reduction of cinnamaldehydes to cinnamyl alcohols with the help of NADPH the reaction is readily reversible (Fig. 4.7). From a functional point of view, CAD activity is involved in developmental lignification and in the formation of defence compounds. Several reviews have treated the involvement of this enzyme in lignin monomer formation (Boudet et al, 1998,... 

Experimental evidence for this disassociation and the resultant monomer formation was found both directly and indirectly by several researchers. " The PX monomer has a ground state tetraolephinic... 

This sequence of proton transfer to monomer (formation of activated monomer) followed by attack at the NH2 group from the chain end by the activated monomer and leading to the monomer addition with generation of the NH3 group thus represents the chain growth on the chain end B. 

Kudryashova EV, Visser AJWG, van Berkel WJH. Monomer formation and function of p-hydroxybenzoate hydroxylase in reverse micelles and in dimethylsulfoxide/water mixtures. Chem-BioChem. 2008 9 413-119. 

Paraffin dehydrogenation on Cr. Al203 Cracking of gas oil Vinyl chlonde monomer formation ... 

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