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Formation of Oligomers

Propylene-Based Routes. The strong acid-catalyzed carbonylation of propylene [115-07-1] to isobutyric acid (Koch reaction) followed by oxidative dehydration to methacrylic acid has been extensively studied since the 1960s. The principal side reaction in the Koch reaction is the formation of oligomers of propylene. Increasing yields of methacrylic acid in the oxydehydration step is the current focus of research. Isobutyric acid may also be obtained via the oxidation of isobutyraldehyde, which is available from the hydroformylation of propylene. The -butyraldehyde isomer that is formed in the hydroformylation must be separated. [Pg.252]

The study of plutonium hydrolysis is complicated by the formation of oligomers and polymers once the simple mononuclear hydrolysis species start forming. The relative mono-oligomer concentrations are dependent on the plutonium concentration - e.g. the ratio of Pu present as (Pu02)2(0H)22 to that as PuO2(0H)+ is 200 for [Pulx = 0.1 M, 5.6 for 10-1 M and 0.05 for 10 8 M. [Pg.220]

In a closer examination of the formation of oligomers, Ferris and co-workers found that the reaction is favoured by relatively high salt concentrations (e.g. 1M NaCl), while the presence of divalent cations is not necessary. The formation of RNA oligomers was found to be temperature sensitive the yields decrease when the temperature is raised from 4°C to 50° C. The addition of meteorite material (from 3 meteorites) does not catalyse the polymerisation reaction only galena (PbS) can do this. The authors thus assume that RNA oligomers could have been formed on the young Earth in solutions of alkali metal salts in the presence of montmorillonite and a pH value of 7-9 (Miyakawa et al., 2006). Ferris (2006) has provided a short but up-to-date survey of montmorillonite-catalysed RNA oligomerisation. [Pg.176]

Normally, only a small stoichiometric excess (2-30 mol%) of silane is necessary to obtain good preparative yields of hydrocarbon products. However, because the capture of carbocation intermediates by silanes is a bimolecular occurrence, in cases where the intermediate may rearrange or undergo other unwanted side reactions such as cationic polymerization, it is sometimes necessary to use a large excess of silane in order to force the reduction to be competitive with alternative reaction pathways. An extreme case that illustrates this is the need for eight equivalents of triethylsilane in the reduction of benzyl alcohol to produce only a 40% yield of toluene the mass of the remainder of the starting alcohol is found to be consumed in the formation of oligomers by bimolecular Friedel-Crafts-type side reactions that compete with the capture of the carbocations by the silane.129... [Pg.12]

There are several guidelines that should be followed in order to increase the chemoselectivity of the monoadduct. Firstly, radical concentration must be low in order to suppress radical termination reactions (rate constant of activation [fcal and fca2] < < rate constant of deactivation kd t andfcd2]). Secondly, further activation of the monoadduct should be avoided ( al> >kd2). Lastly, formation of oligomers should be suppressed, indicating that the rate of deactivation (kd 2[Cu"LmX]) should be much larger than the rate of propagation ( [alkene]). Alkyl halides for copper-catalyzed ATRA are typically chosen such that if addition occurs, then the newly... [Pg.223]

Here the radical 1 acts as a strong terminator to prevent the formation of oligomers and polymers. On the other hand, it is expected that the substituted diphenylmethyl radicals which are less stable than 1 serve as both initiators and primary radical terminators. In fact, it was reported [84] that the apparent polymerization reactivities decreased in the following order diphenylmethyl, phenylmethyl, and triphenylmethyl radicals, which were derived from the initiator systems consisting of arylmethyl halides and silver. [Pg.88]

Assume the formation of oligomers ( 4) is first-order with respect to phenol, and the formation of the other products (jti, k2, 3) is first-order with respect to each of phenol and 02. The following rate constants were obtained font noncatalytic studies and from experiments using a MnC>2/CeO catalyst ... [Pg.450]

Double-bonded species (i.e., dicoordinate ) , cr2 metal species) were reported for As, Sb, and Bi.2 The stability of these species toward the formation of oligomers with metal-carbon single bonds is increased by steric protection with... [Pg.905]

In addition to the tert-butyl enol ethers mentioned above (15% yield), the action of KOtBu on l-iodo-4-methylcyclohexene in DM SO furnished the dimers 85 and tri-mers of 81 in 30 and -25% yield (Scheme 6.24). As in the case of 6 (see Scheme 6.10), the formation of oligomers of 81 was completely suppressed on performance of this reaction in the presence of (tBu)2NO, whereas theenol ethers (86 and its 5-methyl isomer, with the former originating in part and the latter totally from 4-methylcydohex-yne) were observed as in the reaction in the absence of the stable radical. Instead of the dimers 85 and the trimers of 81, a mixture of the hydroxylamine derivatives 87 was isolated in 38% yield. These findings indicate that 81 has no diradical character, in contrast to its immediate dimer 84, which is hence trapped quantitatively by (tBu)2NO [61]. [Pg.262]

De A. 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. [Pg.54]

This reaction occurs not only in bicyclic lactams, but also in monobac-tams. Indeed, intramolecular nucleophilic amino attack has also been observed in an arylglycine-substituted monobactam (Fig. 5.8,b) [84b], However, ampicillin (see below, 5.43, Fig. 5.14), which also carries an a-amino side chain in the 6/3-position, does not exhibit such an enhanced rate. This difference in reactivity has been attributed to the steric hindrance of both the geminal Me groups and H-C(3), which impedes the attack by the a-amino group at the /3-face [84a], In contrast, penicillins with an amino substituent in the 6/3-acylamido side chain show an intermolecular reaction resulting in the formation of oligomers (see Sect. 5.2.5). [Pg.207]

The effident synthesis of macrocycles generally requires the use of high dilution conditions in order to depress the formation of oligomers and polymers with respect to the monomer ( High Dilution Principle ) (52, 53). [Pg.28]

The polymerization proceeds via a radical chain-reaction mechanism, judging from some features of the polymerization initiation by irradiation or upon heating, no formation of oligomers, and polymer formation irrespective of the medium or atmosphere. The propagating radicals are readily detected by ESR spectroscopy during polymerization in the crystalline state (Fig. 2), because termination between the propagating radicals occurs less frequently in the solid state [50]. [Pg.273]

A straightforward extension of DKR to polymer chemistry is the use of diols and diesters (AA-BB monomers) or ester-alcohols (AB monomers) as substrates (Scheme 10, routes A and B). Such reactions have been referred to as DKR polymerizations and lead to the formation of oligomers and/or polymers because of the bifunctional nature of the reagents. [Pg.104]


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Oligomer formation

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