1 polymerization with silica

Table 72.6 Ethylene polymerization with silica-supported rare-earth metal silylamide complexes.

In a tour-de-force with respect to the arcane patent literature, Hlatky reviews heterogeneous single-site catalysts. This is complemented by an article by Fink, Steinmetz, Zechlin, Przybyla, and Tesche on the specific subject of propylene polymerization with silica-supported metallocene/MAO catalysts. The mechanistic role of MAO and other activators is then analyzed by Chen and Marks. They define structure-activity relationships that are certain to promote future research and advances. 

Propene Polymerization with Silica-Supported Metallocene/MAO Catalysts... 

Joachim Zechlin was born in Essen, Germany, in 1970. Fie finished his Ph.D. work in 1999 at the Max-Planck-lnstitut ftir Kohlenforschung in Mulheim a.d. Ruhr under the direction of Professor Gerhard Fink. Flis work was focused on kinetic investigations of propylene polymerization with silica-supported metallocene catalysts. Since 1999 he has been working for the Bayer AG in Dormagen, Germany. 

Monoi T, Ikeda H, Sasaki Y, Matsumoto Y Ethylene polymerization with silica-supported CrrCH(SiMe3)2l3 catalyst. Effect of sihca calcination temperature and Cr content, Polvni J35(7) 608-611, 2003. 

The analysis of cigarette smoke for 16 different polyaromatic hydrocarbons is described in this experiment. Separations are carried out using a polymeric bonded silica column with a mobile phase of 50% v/v water, 40% v/v acetonitrile, and 10% v/v tetrahydrofuran. A notable feature of this experiment is the evaluation of two means of detection. The ability to improve sensitivity by selecting the optimum excitation and emission wavelengths when using a fluorescence detector is demonstrated. A comparison of fluorescence detection with absorbance detection shows that better detection limits are obtained when using fluorescence. 

Silica fouling is the accumulation of insoluble silica on anion resins. It is caused by improper regeneration which allows the silicate (ionic form) to hydrolyze to soluble silicic acid which in turn polymerizes to form colloidal silicic acid with the beads. Silica fouling occurs in weak-base anion resins when they are regenerated with silica-laden waste caustic from the strongbase anion resin unless intermediate partial dumping is done. 

The use of a polymeric support also affords a unique opportunity to control independently the variables that may affect the chiral recognition process, which is hard to achieve with silica. For example, the type and number of reactive sites can be easily adjusted with a polymer support. We recently reported an extensive study of the... 

Figure 15 High-surface area silica treated with aqueous solution of 1 wt% vinyltrimethoxy silane. A silica was polymerized with styrene and washed with CS2 three times. Polystyrene produced in experiment A was deposited with B silica and the silica washed with CS2 three times. (From Ref. 77.)...

However, when using supports with weak linkage between the primary particles of the catalyst, its splitting occurs quickly and it is unlikely to influence the shape of the kinetic curve. For example, in the case of chromium oxide catalyst reduced by CO supported on aerosil-type silica, steady-state polymerization with a very short period of increasing rate is possible (see curve 1, Fig. 1). 

Figure 4.3 Synthesis of onoaeric and polymeric siloxane bonded phases by reaction of organochlorosilanes with silica gel under different conditions.

Also, here, the effect of the adsorption layer of HPC on encapsulation of silica particles in polymerization of styrene in the presence of silica particles has been investigated. Encapsulation is promoted greatly by the existence of the adsorption layer on the silica particles, and the dense adsorption layer formed at the LCST makes composite polystyrene latices with silica particles in the core (7.). This type of examination is entirely new in polymer adsorption studies and we believe that this work will contribute not only to new colloid and interface science, but also to industrial technology. 

It was apparent that the dense adsorption layer of HPC which was formed on the silica particles at the LCST plays a part in the preparation of new composite polymer latices, i.e. polystyrene latices with silica particles in the core. Figures 10 and 11 show the electron micrographs of the final silica-polystyrene composite which resulted from seeded emulsion polymerization using as seed bare silica particles, and HPC-coated silica particles,respectively. As may be seen from Fig.10, when the bare particles of silica were used in the seeded emulsion polymerization, there was no tendency for encapsulation of silica particles, and indeed new polymer particles were formed in the aqueous phase. On the other hand, encapsulation of the seed particles proceeded preferentially when the HPC-coated silica particles were used as the seed and fairly monodisperse composite latices including silica particles were generated. This indicated that the dense adsorption layer of HPC formed at the LCST plays a role as a binder between the silica surface and the styrene molecules. 

The precipitated silica (J. Crosfield Sons) was heated in vacuo at 120° for 24h. before use. Two grades of surface areas 186 and 227 m g l (BET,N2), were used during this project. Random copolymers, poly(methyl methacrylates) and polystyrene PS I were prepared by radical polymerization block polymers and the other polystyrenes were made by anionic polymerization with either sodium naphthalene or sodium a methylstyrene tetramer as initiator. The polymer compositions and molecular weights are given in Table I. 

Various novel imprinting techniques have also been presented recently. For instance, latex particles surfaces were imprinted with a cholesterol derivative in a core-shell emulsion polymerization. This was performed in a two-step procedure starting with polymerizing DVB over a polystyrene core followed by a second polymerization with a vinyl surfactant and a surfactant/cholesterol-hybrid molecule as monomer and template, respectively. The submicrometer particles did bind cholesterol in a mixture of 2-propanol (60%) and water [134]. Also new is a technique for the orientated immobilization of templates on silica surfaces [ 135]. Molecular imprinting was performed in this case by generating a polymer covering the silica as well as templates. This step was followed by the dissolution of the silica support with hydrofluoric acid. Theophylline selective MIP were obtained. 

Despite the problems with silica, it has remained dominant as a stationary phase for the analysis of bases for the same reasons as it has for the separation of other classes of solute. Polymeric phases still give lower efficiency than silica phases, and at low pH seem to suffer the same overloading effect as silica-based phases. lonogenic groups seem to be introduced into polystyrene-divinyl-based phases during their manufacture, and these can lead to tailing of bases at intermediate pH where these groups become ionized. Other phases, such as those made from zirconia, show some promise for the analysis of bases but have not been fully evaluated as yet. 

Membranes with extremely small pores ( < 2.5 nm diameter) can be made by pyrolysis of polymeric precursors or by modification methods listed above. Molecular sieve carbon or silica membranes with pore diameters of 1 nm have been made by controlled pyrolysis of certain thermoset polymers (e.g. Koresh, Jacob and Soffer 1983) or silicone rubbers (Lee and Khang 1986), respectively. There is, however, very little information in the published literature. Molecular sieve dimensions can also be obtained by modifying the pore system of an already formed membrane structure. It has been claimed that zeolitic membranes can be prepared by reaction of alumina membranes with silica and alkali followed by hydrothermal treatment (Suzuki 1987). Very small pores are also obtained by hydrolysis of organometallic silicium compounds in alumina membranes followed by heat treatment (Uhlhom, Keizer and Burggraaf 1989). Finally, oxides or metals can be precipitated or adsorbed from solutions or by gas phase deposition within the pores of an already formed membrane to modify the chemical nature of the membrane or to decrease the effective pore size. In the last case a high concentration of the precipitated material in the pore system is necessary. The above-mentioned methods have been reported very recently (1987-1989) and the results are not yet substantiated very well. 

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