SUMMARYStyrene was polymerized using different amounts of azoisobutyronitrile as initiator at temperatures of 70 "C, 75 "C and 80 "C in suspension. The course of reaction up to almost complete conversion was modeled within a classical kinetic framework. Optimal simultaneous descriptions of both conversion and average degree of polymerization data were possible using two sets of values for the variation of the overall termination rate coefficient k, with conversion. One explanation for this is that k, is chain length dependent. Evidence for this necessity was derived by considering all kinetic parameters, except the termination rate coefficient, as reliable absolute values.
Recent improvements in the physical understanding of ER fluids have led to the rational design of new ER materials with improved properties. This paper gives an overview of several recent developments in the formulation of ER fluids, concentrating on new particulate phases for ER dispersions. Examples of homogeneous ER fluids are also discussed. The trend leading to designed ER dispersions is demonstrated by a new class of electrorheological fluids based on non-aqueous polyurethane dispersions. The fluids exhibit an attractive combination of properties: low viscosity, high ER effect, and low conductivity. The dispersed phase consists of a specially developed polyurethane elastomer which solvates and stabilizes metal salts. The polymer network density influences the mobility of the dissolved ions, allowing a surprising degree of control over the ER effect. Properties such as the field strength dependence of the ER-effect, switching response, and conductivity of these fluids correlate directly with changes in the polymer structure. Electrorheological measurements in a couette viscometer (shear-mode) and in a model shock absorber (flow-mode) using a commercial polyurethane-based fluid show that the ER effect is also dependent upon the shearing geometry.
44Dialkylamino)pyridines have been used as supernucleophilic catalysts in a large number of reactions. In many systems, these catalysts provide a level of reactivity combined with selectivity that is unique. Several reviews of the synthesis and a p plications of these catalysts have appearedJ3 Facile commercial synthesis has made dimethylaminopyridine (1, DMAP) the most popular member of this class of supernucleophiles, although the pyrrolidine analog (2, PPY) is the most active. Both steric and electronic arguments have been put forth to explain its enhanced catalytic activity compared with DMAP.' 0 1 2Three main types of polymer-bound dialkylaminopyridines, have been reported. The first4s5 dealt with reaction of several carboxylic acid-containing dialkylaminopyridines with commercial polyethyleneimine (PEI) to give reagents of general structure 3. These compounds were examined for catalytic behavior in the hydrolysis of p-nitrophenyl esters and found to be highly active. 0 I 1 PE I N c R NR' 3 4The other two reports dealt with incorporation of the dialkylaminopyridine moiety into typical functionalized, crosslinked polystyrene (4) by both reaction on preformed polymel.6.' and synthesis of a monomer and subsequent copolymerization! Application of these catalysts to esterification reactions, under Michaelis-Menten conditions, yielded results comparable to those obtained with the corresponding small molecular analogs.The polymeric catalysts synthesized in these reports have several limitations arising from the nature of the substrate polymer. First, two of these p o l y m e w
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