Molecular enlargement reactions

The changes in molecular size below 150 °C include not only a marked shift of the MWD toward lower MWs, but also a rapid decrease of the degree of LCB to zero. These facts demonstrate that molecular diminishing is heavily dominating initially. After some time when oxidation products have accumulated, molecular enlargement reactions start, however, and gradually become dominating. 

The difference in sensitivity to hydrolysis between the C-C-bonds formed by "peroxide curing" and the ester bonds made it possible to test the suggested scheme. n-C H was used as a model substance. The results from GPC-analysis are shown in Fig. 6. Hydrolysis by KOH in isopropanol at 50 °C for 1 week of the material heated at 123 °C show that ester formation is the only molecular enlargement reaction operating at this tenq>erature. At 180 C, on the other hand, it is evident that esterification is only partly responsible for the molecular enlargement, as there are significant amounts of enlarged material not affected by the KOH treatment. The loss of low MW material for the hydrolyzed samples is due to its solubility in the isopropanol. 

Thermo-oxidation increases the degradation rates considerably. During this type of degradation both molecular reduction and molecular enlargement reactions occur [9]. In starch-filled LDPE containing pro-oxidant in addition to starch it was shown that oxidation at 100°C initiates carbonyl formation after 5 days compared to pure LDPE which is unaffected under the same conditions at [10]. 

QEUC, see Quasi-molecular enlarged unit cell Quadricyclene, 20 323 valence isomerization of, 20 304 Quadricyclene, isomerization of, 24 146, 148 Quadrupolar interactions, 33 205-209 Quadrupole splitting, 26 126, 134, 140-142 Qualitative studies of simple reactions, 32 116 Quantitative treatment, structure effects, 29 155-162... 

Because of the accumulation of acid and alcoholic groups, reaction (I) becomes increasingly frequent. The formation of ester linkages provides another route to molecular enlargement besides the "peroxide curing". The ester bonds should be prone to hydrolysis, however. 

Even though the hydroformylation of alkenes is an important reaction for the functionalization of unsaturated compounds with enormous potential, the above mentioned examples already show the limitations of using molecularly enlarged catalysts in combination with membrane filtration the hydroformylation reaction is typically performed at elevated temperature (40-80 °C) as well as under a syngas... 

It was not until the synthesis was accomplished in two steps that it became possible to process these polymers from solution. In the first reaction step, two tetrafunctional monomers form a linear and soluble macromolecule by a polyreaction of two of the functional groups of each molecule. Subsequently, cyclocondensations take place in a second step, in which the heterocyclic rings are formed. This side reaction occurs intramolecularly. Therefore, it does not lead to a further molecular enlargement. The resulting polymers have heterocyclic units in their main chain they are mostly insoluble and infusible ... 

Compounds arising through molecular enlargement include most importantly SiH-containing methyl derivatives of 2,2-dichldro-l,3,5-trisilapentane. The proportion of these compounds will increase slightly with an increase in the mole equivalence of MeMgCl and with extended reaction time, but is suppressed by lowering the reaction temperature. 

Figure 2.11. (a) Novel wet-ETEM sample-holder for studies of catalyst-liquid reactions at the molecular level at operating temperatures. Ports for liquid injection (I) and heating (H) are shown, (b) Enlarged tip of the holder. L and T denote the tube for injecting the liquid on to the sample and the thermocouple, respectively (after Gai 2002). 

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