Precipitation approach

Cortes et al. [18] have quantitatively determined polymer additives in a polycarbonate homopolymer and an ABS terpolymer. In that case, a multidimensional system consisting of a microcolumn SEC was coupled on-line to either capillary GC or a conventional LC system. The results show losses of certain additives when using the conventional precipitation approach. An at-column GC procedure has been developed for rapid determination (27 min) of high-MW additives (ca. 1200Da) at low concentrations (lOOppm) in 500- xL SEC fractions in DCM for on-line quality control (RSD of 2-7%) [36], Also, SEC-NPLC has been used for the analysis of additives in dissolution of polymeric... 

Table 12. Thermally regenerable capacity of resins" prepared by the precipitation approach...

Triallylamine hydrochloride Methacrylic acid in formic acid Precipitation approach 0.60 ... 

Curve A in Figure 13-6, which is the titration curve for the chloride/iodide mixture just considered, is a composite of the individual curves for the two anionic species. Two equivalence points are evident. Curve B is the titration curve for a mixture of bromide and chloride ions. Clearly, the change associated with the first equivalence point becomes less distinct as the solubilities of the two precipitates approach one another. In the bromide/chloride titration, the initial pAg values are lower than they are in the iodide/chloride titration because the solubility of silver bromide exceeds that of silver iodide. Beyond the first equivalence point, however, where chloride ion is being titrated, the two titration curves are identical. 

The surface area and pore volume of the support both increased with the first increments of phosphate added. They went through an optimum at a P/Al atomic ratio of about 0.2. Pore volume was high in this series because all the samples were washed in n-propanol before the final drying step. The surface area dropped considerably as P/Al in the precipitate approached the stoichiometric 1.0, because of the formation of crystalline AIPO4. This transition caused a large increase in the pore diameter as P/A1 increased. 

Precipitative methods for metal sulphides were initially used by Brus and co-workers, in which they prepared CdS by reaction between cadmium sulphate and ammonium sulphide. Meanwhile, co-precipitation approach has been successfully applied to the synthesis of various metal chalcogenide particles involving metathesis reactions between metal and chalcogenide ions that instantaneously produce precipitates of metal (Zn, Cd, Fig, Pb, Sb, In, Ga, etc.) chalcogenides (Equation (]))... 

Hu and coworkers [126, 127] firstly developed an in situ precipitation approach to fabrication of 3D ordered chitosan rods with a structure of concentric circles through the formation process illustrated in Fig. 11. Briefly, the chitosan/acetic acid solution is filled into a bag made of chitosan membrane, and then immersed into 5% NaOH aqueous solution. When OH ions from the outside solution permeate into... 

Early examples of the precipitation approach include the aqueous solution polymerizations reported by Chaimberg et al. [53] for the graft polymerization of polyvinylpyrrolidone onto silica. The nonporous silica particles were modified with vinyltriethoxysilane in xylene, isolated and dispersed in an aqueous solution of vinylpyrrolidone. The reaction was performed at 70°C and initiated by hydrogen peroxide, after which precipitation on the surface occurred, leading to encapsulation. Nagai et al. [54] in 1989 reported on the aqueous polymerization of the quaternary salt of dimethylaminoethyl methacrylate with lauryl bromide, a surface-active monomer, on silica gel. Although the aim was to polymerize only on the surface, separate latex particles were also formed. 

Wilcox et al. have demonstrated an application of the precipitation approach for MBH adduct isolation. By using a diaryl alkene alcohol (14) as a precipitating auxiliary (based on the solubility switch of structural isomerization), the corresponding MBH acids 17 were obtained from the reaction of acrylate 15 and aldehydes in the presence of DABCO, followed by isomerization and cleavage, in moderate to good yield (Scheme 2.11). 

Yue and Cohen prepared (ZnCd)S2 colloids by multiple loading experiments [164]. The carboxylic acid coordination sites are regenerated and can be re-used to make onion-type binary clusters. This technique allows for the possibility of cluster-size control and the synthesis of core-shell clusters through multiple metal-loading and reduction cycles. Recharging leads to ZnCdS colloids. In a similar fashion, bimetallic Au/Pd nanocolloids have been synthesized as catalysts for hydrogenation of dienes [244]. Eisenberg and coworkers used this method to increase the size of CdS colloids [267]. A one step precipitation approach was used to prepare the bicomponent particles Cd and Se in the presence of polyvinyl alcohol (PVA) [268]. 

---