Silica polymerization mechanism

In connection with the aforementioned study on polymerization mechanism of MMA77,78), Miyamoto et al. developed a preparatory method of separating blends of isotactic and syndiotactic PMMA82 The principle was based on a competitive adsorption of these different stereoisomeric polymers from a nonpolar solution (chloroform) onto an adsorbent surface (silica gel). The procedure was quite simple, as described below A given polymer blend was dissolved in chloroform, in which no stereocomplex formation usually occurs, and silica gel was then dispersed in this solution for adsorptive equilibration with the polymer species. The isotactic species could be isolated as the adsorbed component. In practice, its purity was ca. 80—90%, which depended on the added amount of silica gel. By repeating the same procedure, the purity could be enhanced. 

From this brief discussion it is clear that, even if the CO-reduced catalyst is simpler than the industrial catalyst, the high degree of heterogeneity of the Cr(II) species present on the surface of the amorphous silica makes the comprehension of the polymerization mechanism on the Phillips catalyst a complex and difficult task that still requires work (5). The heterogeneity of the Cr(II) structure is reflected in a... 

A serious problem in the control of silica polymerization is that, in general, initial particle formation is a result of random bond formation between a range of polysilicate species in solution. Moreover, the aggregation process is uncontrolled under conventional preparation conditions. It is important to understand the basic mechanisms of particle formation and aggregation in aqueous solution, as the physical properties of such silicas are determined primarily by the coordination number and nature and strength of interactions between essentially spherical primary particles. 

This paper examines some factors which affect not only the overall activity, but also the rate of termination of polyethylene chains growing on the Phillips Cr/silica polymerization catalyst. Although the theme of this symposium is not the termination but the initiation of polymer chains, the two aims are not inconsistent because on the Phillips catalyst the initiation and termination reactions probably occur together. They are both part of a continuous mechanism of polymerization. One possibility, proposed by Hogan, is shown below. The shift of a beta hydride simultaneously terminates one live chain while initiating another ... 

Strelko and co-workers (71-73) investigated the properties of the siloxane bond and the polymerization mechanism for silicic acid on the basis of the difference in the electronic structure of the Si-O bonds in silanol and the siloxane groups of silica. A kinetic equation was proposed to describe the polymerization of silicic acid throughout the entire pH range. The authors believe that the molecular mechanisms for the formation of globular skeletons in silica gels are based on the polymerization and depolymerization of silicic acids. 

Keywords Ethylene polymerization mechanisms Heterogeneous model catalysts Homogeneous model catalysts Molecular modeling Phillips Cr/silica catalyst Polyethylene Polymerization kinetics... 

Parallel to the progress in the basic understanding on the nature of active sites and polymerization mechanisms, several modified Phillips catalysts with better performance and improvements in the structures and properties of PE products through surface modification of the silica support and catalyst with Ti, F, Al, or B compounds have been successfully developed and commercially applied during... 

Malani, A. Auerbach, S. M. Monson, P. A., Probing tthe Mechanism of Silica Polymerization at Ambient Temperatures Using Monte Carlo Simulations. 

Silica encapsulation has been widely studied both in the open literature and in the patent literature. The first studies were reported by Hergeth et ah, who described the elaboration of composite particles made from quartz powders and polyvinyl acetate through seeded emulsion polymerization [85]. These authors showed that the number of seed particles must exceed a minimal value to prevent formation of new particles and, thus promote seed particles growth. The polymerization was proved to take place in the vicinity of the surface according to the admiceUar polymerization mechanism described previously, and the so-produced interfacial polymer was shown to display physical properties different than those of the bulk polymer. 

In summary, iep and pzc of silica have been variously reported to be from pH 0.5 to 3.7 according to a review of the literature on this point by Parks (55), who cited 12 references. However, a pH of around 2 0.5 appeared to be an average for various types of silica ranging from purified ground quartz to colloidal silica. Some variation may be expected, depending on whether the surface is crystalline or amorphous, possibly on particle size, and especially on the presence of impurities. The question remains how the iep determined from maximum gel time or minimum rate of disappearance of monomer relates to the polymerization mechanisms involved. 

Marsh, Klein, and Mermeulen (32) prepared a comprehensive review of the literature on polymerization kinetics and equilibria and conducted a detailed study to establish the polymerization mechanism in the range of silica concentrations of 200-1800 ppm and pH 4-10 at 25 C. Starting with Si(OH) solution prepared from NajHjSiO -xHjO by ion exchange at about pH 2.5, mixtures buffered with acetate were made at various concentrations and pH values and the rates of polymerization... 

The polymerization of alloocimene [121] is reported in the patent literature I39-I41). Silica gel, synthetic silicates, and various Lewis acids were used as the catalysts. However, no details concerning the structure of the polymeric products were given. More recently, the cationic polymerization of alloocimene was investigated by Jones (71). An intra-intermolecular polymerization mechanism was postulated (11-57). The iodine number of the polymer indicated the loss of two double bonds per monomer unit during polymerization. Interestingly, the use of Ziegler-type catalysts did not yield cyclic polymeric structures (95, 96). 

This proposed mechanism is based largely upon the fact that the maximum reaction rate is attained at pH 8 to 9, at which concentration of both species is reasonably high. On the other hand, monosilicic acid polymerizes at room temperature when the concentration exceeds approximately 120 ppm (McKeague and Cline [1963]). The reaction is catalyzed by OH", F , and silica gel, and a polymerization mechanism in which a six-coordination silicate ion H2O Si(OH)j combines with Si(0H)4 is envisaged by Iler [1955]. The hypothesis that condensation involves a temporary expansion in the coordination number of silica from four to six is also claimed by Okblerse [1961] from kinetic studies on submicroporous and macroporous silica. In a study of the polymerization and condensation process, Baumann [1958] showed... 

The silanization reaction has been used for some time to alter the wetting characteristics of glass, metal oxides, and metals [44]. While it is known that trichlorosilanes polymerize in solution, only very recent work has elucidated the mechanism for surface reaction. A novel FTIR approach allowed Tripp and Hair to prove that octadecyl trichlorosilane (OTS) does not react with dry silica. 

Zorbax PSM particles are made from small (80-2000 A), extremely uniform colloidal silica sol beads. In a patented polymerization process, these beads are agglutinated to form spherical particles. The size of the Zorbax PSM particles is controlled by the polymerization process, and the pore size is determined by the size of the silica sol beads. After polymerization, the silica is heated to remove the organic polymer and sinter the particles. The result is a spherical, porous, mechanically stable, pure silica particle that provides excellent chromatographic performance (Pig. 3.1). 

The polymerization filling was effected by the ion-coordination mechanism [17-19]. The monomers were ethylene, propylene, allene, os-butylene, butadiene. The fillers were mineral materials such as ash, graphite, silica gel, glass fibers. The ultimate aim of filler conditioning prior to polymerization is to secure, on its surface, metal complex or organometallic catalysts by either physical or chemical methods [17-19],... 

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