The anionic and the cationic partners of ionic liquids may act cooperatively and independently as nucleophilic and electrophilic catalysts. This ambiphilic propensity was demonstrated by kinetically discriminating the contributions of the anion (nucleophilic catalyst) and of the cation (electrophilic catalyst) to the solvent-free Baylis-Hillman dimerization of cyclohexenone catalysed by ionic liquids. 相似文献
Crosslinked polymer seed latexes of butyl acrylate, methyl methacrylate and methacrylic acid were synthesized with ethylene glycol dimethacrylate as a crosslinking agent in a first step. Three different processes of seeded emulsion polymerization were used to prepare an outlayer of polysiloxane on the above seed latex particles: (A) direct anionic polymerization of D4 (octamethyl tetracyclosiloxane) catalyzed by potassium hydroxide; (B) direct cationic polymerization of D4 onto the seed catalyzed by dodecylbenzene sulfonic acid; (C) a vinyl-containing polysiloxane prepared by copolymerization of D4 and vinyl septamethyl tetracyclosiloxane was added before the D4 cationic polymerization. Characterization by transmission electron microscopy showed that only process C provided satisfactory results. Film hardness was measured, and the latex film from process C demonstrated the lowest hardness of all the films. The mechanism of polymerization is discussed. 相似文献
Effects of emulsifying chemicals and emulsion aging on the sizing performance of alkenyl succinic anhydride (ASA) were investigated. The emulsifying chemicals were nonionic and anionic surfactants and cationic starches. Image analysis in emulsion quality control was applied. It was found that the use of surfactants may deteoriate remarkably the sizing efficiency if the surfactant and its dosage is not carefully chosen. The need for surfactants in emulsification was affected by the properties of ASA and the mixing energy available. The act of cationic starch both as an effective stabilizer and as a retention agent in ASA-sizing systems was established. 相似文献
The highly controlled and efficient polymerization of ethylene is a very attractive but challenging target. Herein we report on a Coordinative Chain Transfer Polymerization catalyst, which combines a high degree of control and very high activity in ethylene oligo- or polymerization with extremely high chain transfer agent (triethylaluminum) to catalyst ratios (catalyst economy). Our Zr catalyst is long living and temperature stable. The chain length of the polyethylene products increases over time under constant ethylene feed or until a certain volume of ethylene is completely consumed to reach the expected molecular weight. Very high activities are observed if the catalyst elongates 60 000 or more alkyl chains and the polydispersity of the strictly linear polyethylene materials obtained are very low. The key for the combination of high control and efficiency seems to be a catalyst stabilized by only one strongly bound monoanionic N-ligand. 相似文献
Desulphuration of trimethylthiirane (TMT), catalyzed by anionic and cationic catalysts for thiirane polymerization, has been studied. The possibility of obtaining polymers having mono-, di- and trisulphide bonds during TMT polymerization by typical cationic catalysts has been shown. Some peculiarities of the desulphuration reaction during the polymerization of thiiranes by ionic catalysts have been found. Differences in their catalytic action, shown by the formation of different quantities of both disulphide SSbonds in polymer chain and alkene, are discussed from the point of view of HSAB principle and symbiotic effect of ligands at the counterion. It is shown that desulphuration reactions during the cationic and anionic polymerization of thiiranes have a similar electronic nature, consisting of increase of coordinative state and the activation of the acceptor orbital of the thiirane sulphur atom. The nucleophilic attack by thiirane (during cationic polymerization) or growing thiolate-ion (during anionic polymerization) on the electrophilic sulphur atom of thiiranium intermediates respectively, is accompanied by concerted fragmentation of these intermediates, giving the alkene and disulphide fragment. Possible mechanisms have been suggested for thiirane desulphuration and SSbond formation in products of the ionic polymerization of thiiranes. HSAB principle proved to be very useful for discussion of experimental results and other aspects of thiirane polymerization. 相似文献
A novel surfactant has been synthesized for use in cationic and anionic ring-opening suspension polymerization of PEG-based macromonomers in silicon oil. A polymer of acrylate esters containing pentamethyldisiloxane and PEG was prepared by radical polymerization. The surfactant can stabilize an emulsion of PEG-based macromonomers, initiator, and solvent in silicon oil such that polymer beads are obtained by ring-opening polymerization, initiated either by a Lewis acid (cationic ring opening) or potassium tert-butoxide (anionic ring opening). The average bead size could be controlled by varying the stirring rate and the amount of surfactant and solvent. The surfactant does not interfere with the polymerization and can be removed together with residual silicon oil by a simple washing procedure. 相似文献