Bimodal reactivity

There are three possible active sites in a,(3-unsaturated acylzirconocene chlorides with respect to nucleophiles, namely the (3-unsaturated carbon, the acyl carbon, and the Zr—chlorine bond. The reactions of a, (3-unsaturated acylzirconocene chlorides with nucleophiles indicate bimodal reactivity (nucleophilic or electrophilic) at both the acyl and P-carbons (Scheme 5.37) [40],... 

The bimodal profile observed at low catalyst concentration has been explained by a combination of two light generating reactive intermediates in equihbrium with a third dark reaction intermediate which serves as a way station or delay in the chemiexcitation processes. Possible candidates for the three intermediates include those shown as "pooled intermediates". At high catalyst concentration or in imidazole-buffered aqueous-based solvent, the series of intermediates rapidly attain equihbrium and behave kineticaHy as a single kinetic entity, ie, as pooled intermediates (71). Under these latter conditions, the time—intensity profile (Fig. 2) displays the single maximum as a biexponential rise and fall of the intensity which is readily modeled as a typical irreversible, consecutive, unimolecular process ... 

J.A. Nicholls, Two Phase Detonations with Bimodal Drop Distribution , 3rd Intern Colloq Gas Dynamics of Expins and Reactive Systems (1971) 14) N.M. Laurendean I. Glassmar,... 

The kinetics of alkaline hydrolysis of a series of eleven vinylsulphone reactive dyes fixed on cellulose have been investigated at 50 °C and pH 11. Bimodal hydrolytic behaviour was observed under these conditions, the reaction rates being rapid at first but becoming slower as the concentration of fixed dye remaining gradually decreased. These results were attributed to differences in the degree of accessibility of the sites of reaction of the dyes within the fibre structure [87]. 

Under intermediate , i.e., not extreme, conditions of monomer reactivity and solvent polarity the polymers formed from styrenes and some other monomers by HA have a bimodal DPD. A change to a less reactive M or to a solvent of lower polarity leads to an increase in the fraction of the product of lower DP, and vice versa [44]. 

Possibility of unfired and/or once-fired end groups Broad or bimodal and molecular weight distribution Mjj control by nature of inifer (Cj), very reactive or un-reactive inifer unsuitable 100% conversion of both I and M only in specific cases Reactor control difficult, danger of heat jump on rapid BClj introduction... 

It is not an absolute necessity for LCP to have no free ions. If free ions are present, LCP is possible only if there are fast equilibria between free ions, ion pairs, and covalent species. If the equilibrium between free ions and ion pairs is slow, the result is a bimodal distribution. Further, to have any possibility of LCP with free ions present, the concentration and reactivity of the free ions should not be such that the reaction is too fast. 

The relative stability of the delocalized, non-vertical radical cation relative to a localized, vertical isomer was demonstrated also in gas phase experiments [404]. The molecular ions of m/e 132 obtained by gas phase ionization of the [4 + 2] dimer exhibited a bimodal decay, a result which was interpreted as evidence for the presence of two isomeric ions with different structures. The possibility that the reactive ion is a species with excess internal energy was discounted, when equivalent decay curves were observed in experiments using 10 eV and 70 eV electron impact ionization energy. In dramatic contrast, the molecular ions derived from the [2 + 2] dimer fail to react apparently the ion population resulting in this experiment is homogeneous [404],... 

The binary systems have two possible qualitatively different forms of the composition distribution of the products of the complete conversion (p = 1) shown in Fig. 4. The bimodal distribution is realized when the initial system composition x° is inside the basin of the stable SP corresponding to the homopolymer of the monomer Ms and the reactivity ratio of the corresponding radical rs < 0.5. In the opposite case when 0.5 < rs < 1 the composition distribution is unimodal. These conclusions derived at first [169-170] from the expressions (5.4) can also be obtained as a particular care from the general results [6, 134] of the analysis of the trajectory behavior near SP for the copolymerization of any number m of monomers. 

Although very broad molecular weight distributions will result from slow exchange between one dormant and one reactive propagating species, the distribution will always be monomodal. However, the molecular weight distributions of many carbocationic polymerizations are bimodal. Bimodal molecular distributions are produced in systems with two propagating species with either different reactivities, or with identical reactivity but different lifetimes in their active form [268]. Unfortunately, there is not enough experimental detail on the evolution of M and polydispersity as a function of conversion to interpret and explain all of the literature data reported. 

The microporosity of a new tubular silicatelayered silicate nanocomposite formed by the intercalation of imogolite in Na -montmorillonite has been characterized by nitrogen and m-xylene adsorption. The nitrogen adsorption data yielded liquid micropore volume of -0.20 cm g as determined by both the t-plot and the Dubinin-Radusikevich methods. The t-plot provided evidence for a bimodal pore structure which we attributed to intratube and intertube adsorption environments. The m-xylene adsorption data indicated a much smaller liquid pore volume (-0.11 cm g ), most likely due to incomplete filling of intratubular pores by the planar adsorbate. The FTIR spectrum of pyridine adsorbed on the TSLS complex established the presence of both Bronsted and Lewis acid sites. The TSLS complex was shown to be active for the acid-catalyzed dealkylation of cumene at 350 C, but the complex was less reactive than a conventional alumina pillared montmorillonite. 

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