Cluster-polystyrene solutions

As a check to confirm that no extraneous non-polymer-attached catalytic species were present, the following experiment was performed. Polystyrene without attached cyclopentadiene was exposed to Co2(C0)e, extracted using a Soxhlet extractor and dried in vacuo in exactly the same manner as was used to synthesize 5. When used under the above Fischer-Tropsch reaction conditions, these treated, white polystyrene beads did not discolor, release any detectable species into solution, cause a CO/H2 pressure drop, or result in the formation of any detectable amounts of methane. These observations argue against the presence of small amounts of occluded Co2(C0)e or C04 (CO) 12 which could conceivably have been active or precursors to active species. It should be noted that the above clusters were reported to be essentially inactive under Fischer-Tropsch conditions (140°C, toluene, 1.5 atm., 3/1 H2/CO, three days) leading to mere traces of methane (11). The lack of products under our conditions also indicates that, at least in the absence of resin-bound CpCo(C0)2 or its derivatives, the polystyrene support did not degrade. 

The preparation of solids with covalently attached POM complexes is a serious and worthwhile research target because these materials might be expected to be rather stable to POM leaching in solutions. Many new materials of this type have been reported in the literature [16,49,74,137-143] however, catalytic studies on covalently boimd POMs still remain a rare event. In 1992, Judeinstein reported the first POM-polymer hybrid where a lacunary Keggin POM cluster was covalently linked to polystyrene or polymethacrylate backbone through Si-0 bonds [137]. This approach has been further developed by several research groups. 

It should be acknowledged that Risen utilized the concept of the ionic domains in ionomers (Nafion sulfonates, sulfonated linear polystyrene) as microreactors within which transition metal partides can be grown and utilized as catalysts (23-25). Transition metal (e.g. Rh, Ru, Pt, Ag) cations were sorbed by these ionomers from aqueous solutions and preferentially aggregated within the pre-existing clusters of fixed anions. Then, the ionomers were dehydrated, heated and reduced to the metallic state with Hg. Risen discussed the idea of utilizing ionomeric heterophasic morphology to tailor the size and size distributions of the incorporated metal particles. The affected particle sizes in Nafion were observed, by electron microscopy, to be in the range of 25-40 A, which indeed is of the established order of cluster sizes in the pre-modified ionomer. 

A model for calculating viscosities of concentrated polymer solutions has been formulated and used successfully to predict viscosities of alkyd resin solutions in both pure aromatic solvents and in mixtures of hydrocarbons and oxygenated materials. It was also found to describe viscosity trends in polystyrene-diethylbenzene solutions accurately. The formulation explicitly accounts for the observation that concentrated solution viscosities increase markedly with decreasing compatibility between resin and diluent. The proposal of an empirical relationship which interprets the viscosity enhancement in poorer solvents in terms of increased chain-chain interactions is of interest. The model contains three constants which are fixed for a particular resin and are independent of diluent type. These are the Mark-HouuAnk constant, the parameter in the Martin viscosity equation, and the constant relating the postulated clustering to the solution thermodynamics of a particular solution. 

Polymers containing aryl groups also react efBdently with metal atoms to yield bis(arene)metal(0) complexes within the polymer chain . Thus, poly(methylphenyl-siloxanes) react at 0°C as a liquid with Ti, V, Cr, Mo and W atoms to give high yields of colored bis( / -arene)metal complexes. In contrast Co, Fe and Ni atoms )deld only metal slurries. Similarly, poly(oxyphenylene) and polystyrene (in a solvent) react with V and Cr atoms to yield colored solutions . By adjusting the metal-atom flux, small, polymer-supported Ti and Mo clusters can be prepared . In some cases the com-plexed metal atoms spontaneously migrate through the polymer fluid to form dimers . 

In this part of the chapter we discuss (a) the controlled thermolysis of thiolate solutions in polystyrene matrix at temperatures above the polymer glass transition temperature and (b) the reaction mechanism in the case of silver-polystyrene nanocomposite systems. However, the same reaction mechanism is probably involved in the thermolysis of other mercaptide-polystyrene systems. This technique has proven to be an excellent new preparative scheme for the generation of both metal and sulfide clusters in polymers. In particular, high-molecular-weight n-alkanethiolates have shown to be the most effective compound class since the low volatility of thermolysis by-products avoids film foaming during the annealing process. 

In this chapter, two new approaches for the synthesis of metal-polymer nanocomposite materials have been described. The first method allows the preparation of contact-free dispersions of passivated gold clusters in polystyrene, and it is based on a traditional technique for the colloidal gold synthesis—that is, the alcoholic reduction of tetrachloroauric acid in presence of poly(vinyl pyrrolidone) as polymeric stabilizer. The primary function of the stabilizer is to avoid cluster sintering, but it also allows us to isolate clusters by co-precipitation. It has been found that the obtained polymer-protected nanometric gold particles can be dissolved in alkane-thiol alcoholic solutions to yield thiol-derivatized gold clusters by thiol absorbtion on the metal surface. Differently from other approaches for thioaurite synthesis available in the literature, this method allows complete control over the passivated gold cluster structure since a number of thiol molecules can be equivalently used and the... 

Second Virial Coefficient below the 0 Temperature Though not specially mentioned, the discussion up to this point has been limited to the polymer in solvents in which A2 is non-negative, so that /3 and 2 are zero or positive in the binaiy cluster approximation. Available data on A2 for polymer solutions below 0 are still scant and fragmentary. This is mainly due to the technical difficulties explained in Section 2.3 of Chapter 4. Recently, Tong et al. [84] undertook fairly systematic measurements of A2 on polystyrene in cyclohexane, and Takano et al. [85] did similar work on poly(isoprene) in... 

Pailthorpe and Russel (1982) and Aninachalam et at. (1998) compared this approximation with the exact solution by Langbein for colloidal polystyrene spheres in an aqueous solution and for aerosol molecule clusters of tetrachloromethane (CCLi), respectively. In both cases the approximate approach underestimates the van-der-Waals interaction energy by approx. 10 to 20 %. Pailthorpe and Russel (1982) conclude that this deviation is comparable with the uncertainty due to inaccurate dielectric values. 

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