Active propagating

Since pp of the metal within the crevice is raised by the decrease in pH, the metal is active whereas the outer surface is passive, and this represents the active propagation stage of crevice corrosion there is a significant IR drop between the two zones. 

The influence of changes in these other variables on MWD in a homopolymerization has not yet been tested, but whatever perturbations are introduced to the feed in a radical polymerization in a laboratory-scale CSTR, they are unlikely to introduce dramatic changes in the MWD of the product because of the extremely short life-time of the active propagating chains in relation to the hold-up time of the reactor. This small change in MWD could be advantageous in a radically initiated copolymerization where perturbations in monomer feeds could give control over polymer compositions independent of the MWD. This postulate is being explored currently. 

The lanthanocene initiators also polymerize EtMA, PrMA and BuMA in a well-controlled manner, although syndiotacticity decreases as the bulk of alkyl substituent increases. Reactivity also decreases in the order MMA EtMA > PrMA > BuMA. Chain transfer to provide shorter polymer chains is accomplished by addition of ketones and thiols.460 The alkyl complexes (190) and (191) also rapidly polymerize acrylate monomers at 0°C.461,462 Both initiators deliver monodisperse poly(acrylic esters) (Mw/Mn 1.07). An enolate is again believed to be the active propagating species since the model complex (195) was also shown to initiate the polymerization of MA. 

In some cases they were able to quench deton completely by an applied magnetic field, and in one experiment they determined the temperature coefficient of voltage required to quench detonation in 20 mol % acetylene and 80% air to be positive. Malinovskii Lavrov stated that negative charges actively propagate the deton wave in 25/75 mol % acetylene... 

In an aftermath of large scale natural or man-made disasters there is likely to be lack of donor sites for SSG harvest to treat extensive bums. Use of isolated keratinocytes which can be actively propagated in vitro [24] and applied in vivo along with dermal substitutes may potentially solve the problem of epithelial deficiency and scarring complications, thus reducing the rates of morbidity and mortality in bums critical care. 

Complexation with PMDT. The study developed above has been repeated using the tertiary triamine PMDT. The results are analogous to those found in presence of TMEDA (figure 6). A first order reaction with respect to the complexe PILi/PMDT is evidenced so it seems that the active propagating species is the 1 1 chelate. 

The first viewpoint contradicts the autocatalytic character of the reaction, conductometric measurements in the polymerization system and some other facts (see below). Scheme (33) can be considered as completely experimentally substantiated. The following important proofs were obtained A direct experimental discovery of a quaternary ammonium alcoholate in the reaction system, 42) a full agreement of the nature of the active propagating site with all the existing kinetic and structural data l4,149 153 157 I58) establishment of the ionic behaviour of the propagating sites by comparison of the kinetic curves of the process with the character of the electric... 

Thus, in the presence of alcohols or other proton donors the polymerization of epoxy compounds under the action of TA proceeds according to the anionic mechanism to give quaternary ammonium alcoholate as the active propagating site [Scheme (33)]. 

This means that the active propagating sites such as free alkoxy ions and ion pairs are solvated with the hydroxyl groups. This must lead to an increase in the reactivity of the solvated ion pair as compared with that of the contact ion pair and decrease in the free ion reactivity, i.e. ultimately to the levelling off of the reactivity differences of these particles. 

Both reactions lead to the appearance of end hydroxyl and unsaturated groups in the polymer. The accumulation rate of these groups is expected to be controlled by the concentration of the active propagating sites (see Fig. 13) in accordance with Scheme (43). 

Based on these kinetic and microscopic observations, olefin polymerization by supported catalysts can be described by a shell by shell fragmentation, which progresses concentrically from the outside to the centre of the support particles, each of which can thus be considered as a discrete microreactor. A comprehensive mathematical model for this complex polymerization process, which includes rate constants for all relevant activation, propagation, transfer and termination steps, serves as the basis for an adequate control of large-scale industrial polymerizations with Si02-supported metallocene catalysts [A. Alex-iadis, C. Andes, D. Ferrari, F. Korber, K. Hauschild, M. Bochmann, G. Fink, Macromol. Mater. Eng. 2004, 289, 457]. 

The structure of the active propagation center in the cationic ring-opening polymerization (ROP) of cyclosiloxanes is still controversial. Trisilyloxonium ions generated from hexamethyl-cyclotrisiloxane, D3, and octamethylcyclotetrasiloxane, D4, were observed at low temperature by Olah et al. [I] and their participation as intermediates in cationic ROP of cyclosiloxanes was postulated. However, the main objection against this mechanistic concept is a relatively low rate of polymerization when an initiator able to form persistent tertiary oxonium ions is used [2]. 

Initiation involves the reversible reaction of water with TiCU to produce the active catalyst, which is then in a form capable of protonating the methylene of isobutylene (Eqs. 22.18 and 22.19). The active propagating species formed is a tightly bound ion pair. 

Stuart GJ, Sakmann B 1994 Active propagation of somatic action potentials into neocortical pyramidal cell dendrites. Nature 367 69—72... 

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