No abstract

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

The MWD function and moments are derived for a "living" polymerization process which proceeds via active and "dormant" species and where the activity is directly exchanged between these chain ends in a bimolecular reaction ("degenerative transfer"). Such a mechanism is believed to be applicable to many "living" polymerizations (e.g., anionic, group transfer, cationic, and radical). For constant monomer concentration (slow addition of monomer or low conversion), the polydispersity index, PJPn, depends on the ratio of molar concentrations of monomer and initiator, = M/Io, the degree of polymerization, Pn, and the ratio of rate constants of exchange and propagation, ß = keJkv. In a limiting case (ß > 1 and Pn » 1), Pw/Pn ~1 + 2 /(ß ). The molecular weight distributions are always narrower than those obtained for a batch process, where monomer concentration decreases during polymerization.

We have observed dual transitions in various physical properties while investigating the surfactant-like behavior of the ionic liquid (IL) 1-butyl-3-methylimidazolium octylsulfate [C(4)mim][C(8)SO(4)] in aqueous media. Interestingly, in conductivity measurements, it was found that the second transition, which is comparatively weak in pure water, can be modulated and tuned to the desired concentration without affecting the first transition by the addition of a suitable electrolyte. Using an array of techniques, such as conductivity, ultrasonics, TEM, cryo-TEM, DLS, (1)H NMR and 2D (1)H-(1)H NOESY, it has been revealed that the first transition corresponds to the anionic aggregation with imidazolium cations adsorbed as counterions, and the second transition corresponds to the restructuring of the initially formed aggregates in a way that the alkyl chain of the imidazolium cation is incorporated towards the micellar core, leading to a unique mixed micelle-type structure in a single IL system.

For various mechanism of group-transfer polymerization (GTP), the concentration of the active chain ends is calculated. The calculated reaction orders with respect to catalyst and initiator concentrations, respectively, are compared to experimental data for the GTP of methyl methacrylate in THF catalyzed by both bffluoride and benzoate and of n-butyl acrylate in toluene catalyzed by mercury iodide.The comparison reveals that for catalysis by benzoate and mercury iodide, the experimental readion orders are only consistent with the associative mechanism of GTP, i.e, with an activated silyl ketene acehl aa the active species. For the catalysis with the lesa nucleophilic benzoate anion, the existence of a dissociative mechanism (with enolate ione or ion pain aa the active species) can be neither excluded nor proven. The effect of silyl esters which have been used in ylivingnesa enhancers" is discuseed and quantified in terms of all mechanisms discussed.