Pathways chain transfer

Propagation and Chain Transfer Pathways/Theoretical Studies. 128... 

Figure 9.8. Possible chain transfer pathways and propagation...

Beta hydrogen elimination is the sole chain transfer pathway and the source of the propagating species Cp2ZrH+. 

Direct (3-Mo elimination was observed when activating zirconocene methyl neopentyl complexes with B(C6F5)3.555 With sterically bulky Cp ligands, instantaneous isobutylene elimination is observed at — 75 °G however, for the bis-(Cp) compound, the zwitterionic neopentyl complex species is stable at 0 °C but undergoes clean and reversible f3-Me elimination at 25 °C (Scheme 177). This finding is consistent with (3-Me elimination as the major chain-transfer pathway in propylene polymerizations using a sterically encumbered metallocene catalyst. 

By means of C and H NMR, mainly propyl and 2-methyl-prop-1-enyl (vinylidene) end groups are detected in the polymers prepared by Cp2MtX2 (Mt = Zr, or Ti) catalyst. This indicates that the predominant monomer insertion is 1,2-insertion and that p-H elimination from the last inserted 1,2 unit is the major chain transfer pathway. " In PPs prepared with related catalyst systems (Cp )2ZrX2/MAO (Cp = ri -pentamethylcyclopentadienyl), wo-butyl and prop-1-enyl (vinyl) groups are found to be the major end groups, which suggests that chain transfer consists of predominant p-methyl elimination. ... 

Wawzonek et al. first investigated the mechanism of the cyclization of A-haloamines and correctly proposed the free radical chain reaction pathway that was substantiated by experimental data. "" Subsequently, Corey and Hertler examined the stereochemistry, hydrogen isotope effect, initiation, catalysis, intermediates, and selectivity of hydrogen transfer. Their results pointed conclusively to a free radical chain mechanism involving intramolecular hydrogen transfer as one of the propagation steps. Accordingly, the... 

It is tempting to attribute problems in reconciling data from model studies and actual polymerizations to difficulties associated with data interpretation. The polymerization experiments are often complicated by other termination pathways, in particular chain transfer, which must be allowed for when assessing the results. It is notable in this context that the discrepancies are most evident for reactions carried out at higher temperatures (Sections 5.2,2.1.1 and 5.2.2.1.2). 

The most important side reactions are disproportionation between the cobalt(ll) complex and the propagating species and/or -elimination of an alkcnc from the cobalt(III) intermediate. Both pathways appear unimportant in the case of acrylate ester polymerizations mediated by ConTMP but are of major importance with methacrylate esters and S. This chemistry, while precluding living polymerization, has led to the development of cobalt complexes for use in catalytic chain transfer (Section 6.2.5). 

Studies (see, e.g., (101)) indicate that photosynthesis originated after the development of respiratory electron transfer pathways (99, 143). The photosynthetic reaction center, in this scenario, would have been created in order to enhance the efficiency of the already existing electron transport chains, that is, by adding a light-driven cycle around the cytochrome be complex. The Rieske protein as the key subunit in cytochrome be complexes would in this picture have contributed the first iron-sulfur center involved in photosynthetic mechanisms (since on the basis of the present data, it seems likely to us that the first photosynthetic RC resembled RCII, i.e., was devoid of iron—sulfur clusters). 

Figure 9.3 k summary of the Physiological pathway of the Krebs cycle. The pathway starts with acetyl-CoA, since citrate synthesis is the flux-generating step. The physiological pathway includes the electron transfer chain, since there is no flux-generating step in this chain. The pathway is indicated by the broader lines. The pathway, therefore, starts with acetyl-CoA and finishes with CO2 and H2O, which are lost to the environment. Acetyl-CoA is formed from a variety of precursors glucose and fatty acids are presented in this figure. 

Electron-transfer chains in plants differ in several striking aspects from their mammalian counterparts. Plant mitochondria are well known to contain alternative oxidase that couples oxidation of hydroquinones (e.g., ubiquinol) directly to reduction of oxygen. Semiquinones (anion-radicals) and superoxide ions are formed in such reactions. The alternative oxidase thus provides a bypass to the conventional cytochrome electron-transfer pathway and allows plants to respire in the presence of compounds such as cyanides and carbon monoxide. There are a number of studies on this problem (e.g., see Affourtit et al. 2000, references therein). 

The US EPA characterizes As, Be, Sb, Cd, Cr, Cu, Pb, Hg, Ni, Se, Ag, Tl, and Zn as priority metals because of their potential hazardousness to human health. However, the environmental fate and effect of only a few metals (As, B, Cd, Cr, Cu, Mo, Ni, Pb, and Zn) have been studied extensively (Rechcigl 1995). For a given metal the potential to cause harm depends on the identifiable risk pathway, which is different for different metals. One pathway usually provides the highest probability of adverse affects to some receptor and is, therefore, the limiting pathway (Ryan Bryndzia 1997). The most toxic elements to humans are Hg, Pb, Cd, Ni, and Co. Some of the principal limiting pathways for various metals are the direct ingestion of Pb-contaminated soil by children plant phytotoxicity from Cu, Zn, Ni food-chain concentration and transfer of Cd and Hg to humans and food-chain transfer of Se and Mo to livestock (Ryan Bryndzia 1997). 

Fig. 8. Schematic representation of heteroatom-containing structural elements in polymers that are disposed for characterisation by 1H/X/Y triple resonance experiments where X = 13C and Y = 19F, 31P, 29Si, 119Sn, with possible coherence transfer pathways being indicated by single and double headed arrows.36 39 Selective observation of the correlations of the building blocks in (a)-(c) requires experiments involving out-and-back coherence transfer via Vc.h/ -A.x (a), Vc.h/ cx (b), or / . (c), whereas the simultaneous observation of all correlation signals originating from a chain of an isotope labelled sample (d) is feasible by means of a HCa(Y)-CC-TOCSY sequence.39...

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