Initiating species characterization

Scheme 7 Initiation mechanisms proposed in literature for the CO-reduced Cr/Si02 catalyst. Vertical direction shows evolution of the initial species upon addition of one ethylene molecule. Horizontal direction shows all the possible isomeric structures characterized by an average C2H4/Cr ratio equal to 1, 2, and 3...

Unlike the case for MAO, one needs only an equimolar amount of the coinitiator to activate the initiator. In some reaction systems, higher polymerization activity is observed with an excess of initiator relative to coinitiator. Maximum activity is observed in some polymerization systems at a 2 1 initiator coinitiator ratio, which indicates that bimetallic species such as XXXVIII are the actual initiating species [Wang et al., 2003], A unique feature of the use of organoboranes and organoborates is that the complexes, both monometallic such as XL and XLI and the corresponding bimetallic complexes, have been isolated and characterized in a number of systems. 

A ternary Rh catalyst system, [(nbd)RhCl]2-LiCPh=CPh2-PPh3, " induces the living polymerization of phenylacetylenes. In the latter case, the initiating species is a vinylrhodium 10, which was isolated and well characterized by X-ray analysis.The details for the living polymerization are described in Section 11.16.4.1. 

The detailed description of all the proposed mechanisms is not the aim of this work (see Reference (i) for more details), but a few concepts are briefly discussed in the following (a) Scheme 11 may be read in two dimensions in the vertical direction, the evolution of the initial species upon addition of one ethene molecule is represented, whereas, in the horizontal direction, all the possible isomeric structures characterized by an average C fCv ratio equal to 2, 3, and 4 are reported, (b) In all the proposed reactions, the metal formally becomes Cr(IV) as it is converted into the active site. This hypothesis is supported by investigations of the interaction of molecular transition metal complexes with ethene (226,227). Furthermore, it has... 

Figures 13A,C, 15A-D, and 16A, B show the effect of [ED] (i.e., TEA concentration) on kc and kp at different temperatures. In general, kc decreases with increasing [ED] because the reactivity of the initiating cations is reduced. The effect of [ED] on kc must be very similar to those on the reactivity of growing species characterized by Eq. (32).

The usual tools for characterization of Cu(II) complexes are not applicable to the initiator species in pyridine. Thus, aside from the facts that active initiators cannot be isolated as solids and that their spectra consist of an intense, featureless charge transfer band which extends through the visible spectral region (Figure 1), the species are neutral (17), ESR-nondetectable (17), and cannot be reduced at a dropping mercury cathode in the range 0-(—1.65V) (vs. SCE, pyridine solvent, and tetraethylammonium perchlorate as electrolyte) (18). 

One of the most representative examples of venomous animals is constituted by the Conus marine snails [2]. Of the approximately 500 Conus species, about 40-100 prey primarily on Qsh (Qsh-hunting species), and these species use two parallel physiological mechanisms requiring multiple neurotoxins to immobilize Qsh rapidly [3] neuromuscular block and excitotoxic shock. Fishhunting Conus snails use a harpoon-like device to inject their venom in their preys. The venom contains a cocktail of neurotoxins that will cause a double-phase paralytic process (Table 5.1), with an initial phase characterized by a fast paralysis with tetanus and a second phase characterized by a Qaccid paralysis. Finally, the Qsh will be engulfed by the snail. 

Biosynthesis. The initiating species in C. biosynthesis in Hevea brasiliensis is a derivative of trans-tram-famesyl diphosphate (i.e. a C15 allylic diphosphate), which is presumably modified at the methyl carbon of the dimethylallyl group. [Y. Tanaka etal. Initiation of Rubber Biosjmthesis in Hevea brasiliensis Characterization of Initiating Species by Structural Analysis Phytochemistry 41 (19 ) 1501-1505]... 

X > 500 nm, meaning that the initial species absorbing at 383 nm are precursors of the metal particles. In view of the similar optical properties of these species and of transient Au clusters (6a), it seems reasonable to assume that the initial sp ies consist of Au particles containing only a few metal atoms. If this assumption is correct, it implies that these clusters are relatively stable toward oxidation by O2, which may allow for their isolation and characterization. 

The second way, plasma-induced polymerization, is characterized by the formation of initiating species under the influence of a plasma and subsequent polymerization in the condensed phase. One possibility for the initiation process is that it can take place by exposing liquid monomers to a plasma of different gases (helium, argon, nitrogen, NO, CO2, O2, CF4) [82] for several minutes. The presenee of radieal initiators, photo-initiators, and photosensitizers can influence the course of the polymerization reaction [83-86]. This technique is used to polymerize thin films for coating purposes. 

Tanaka, Y., Aik-Hwee, E., Ohya, N., Nishiyama, N., Tangpakdee, J., Kawahara, S., Wititsuwan-nakul, R., Initiation of rubber biosynthesis in Hevea brasiliensis characterization of initiating species by structural analysis. Phytochemistry 1996, 41 (6), 1501-1505. 

The photopolymerization threshold is determined by the production efficiency of initiating species from excited triplet states, which is characterized by the quantum yield of polymerization. The reactions that produce radicals should compete with monomer quenching, oxygen quenching and other pathways of deactivation of the excited states like phosphorescence emission. The threshold is also determined by the reactivity of radicals and monomers. 

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