Polymerization silicon-mediated

The controlled polymerization of (meth)acrylates was achieved by anionic polymerization. However, special bulky initiators and very low temperatures (- 78 °C) must be employed in order to avoid side reactions. An alternative procedure for achieving the same results by conducting the polymerization at room temperature was proposed by Webster and Sogah [84], The technique, called group transfer polymerization, involves a catalyzed silicon-mediated sequential Michael addition of a, /f-unsaluralcd esters using silyl ketene acetals as initiators. Nucleophilic (anionic) or Lewis acid catalysts are necessary for the polymerization. Nucleophilic catalysts activate the initiator and are usually employed for the polymerization of methacrylates, whereas Lewis acids activate the monomer and are more suitable for the polymerization of acrylates [85,86]. 

Group-transfer polymerizations make use of a silicon-mediated Michael addition reaction. They allow the synthesis of isolatable, well-characterized living polymers whose reactive end groups can be converted into other functional groups. It allows the polymerization of alpha, beta-unsaturated esters, ketones, amides, or nitriles through the use of silyl ketenes in the presence of suitable nucleophilic catalysts such as soluble Lewis acids, fluorides, cyanides, azides, and bifluorides, HF. ... 

Previous mechanistic studies of another silicon-mediated reaction, "group transfer polymerization" of methacrylatesl - ] indicated the involvement of pentacoordinate silicon species. Our working hypothesis for the subject reaction of silyethers is that formation of pentacoordinate silicon species (Scheme 5) should increase reactivity and promote nucleophilic character in the alkoxy ligands. 

These TMS-carbamate-mediated NCA polymerizations resemble to some extent the group-transfer polymerization (GTP) of acrylic monomers initiated by organo-silicon compounds [40]. Unlike GTPs that typically require Lewis acid activators or nucelophilic catalysts to facilitate the polymerization [41], TMS-carbamate-mediated NCA polymerizations do not appear to require any additional catalysts or activators. However, it is still unclear whether the TMS transfer proceeds through an anionic process as in GTP [41] or through a concerted process as illustrated in Scheme 14. 

During the last 5 years, there have been several reports of multiblock copolymer brushes by the grafting-from method. The most common substrates are gold and silicon oxide layers but there have been reports of diblock brush formation on clay surfaces [37] and silicon-hydride surfaces [38]. Most of the newer reports have utilized ATRP [34,38-43] but there have been a couple of reports that utilized anionic polymerization [44, 45]. Zhao and co-workers [21,22] have used a combination of ATRP and nitroxide-mediated polymerization to prepare mixed poly(methyl methacrylate) (PMMA)Zpolystyrene (PS) brushes from a difunctional initiator. These Y-shaped brushes could be considered block copolymers that are surface immobilized at the block junction. 

Hybrid graft copolymers having silicon-based polymer backbones were also prepared by the metal-mediated radical polymerizations of styrene. The phenyl groups of poly[(methylphenyl)silylene] were bromomethylated and then employed as the grafting points of polystyrene (G-16).294,441 Polysiloxane can be employed also as a backbone (G-17) by introduction of benzyl chloride units into the pendant vinyl-functionalized poly(dimethylsiloxane).409... 

In the acidic route (with pH < 2), both kinetic and thermodynamic controlling factors need to be considered. First, the acid catalysis speeds up the hydrolysis of silicon alkoxides. Second, the silica species in solution are positively charged as =SiOH2 (denoted as I+). Finally, the siloxane bond condensation rate is kinetically promoted near the micelle surface. The surfactant (S+)-silica interaction in S+X 11 is mediated by the counterion X-. The micelle-counterion interaction is in thermodynamic equilibrium. Thus the factors involved in determining the total rate of nanostructure formation are the counterion adsorption equilibrium of X on the micellar surface, surface-enhanced concentration of I+, and proton-catalysed silica condensation near the micellar surface. From consideration of the surfactant, the surfactants first form micelles as a combination of the S+X assemblies, which then form a liquid crystal with molecular silicate species, and finally the mesoporous material is formed through inorganic polymerization and condensation of the silicate species. In the S+X I+ model, the surfactant-to-counteranion... 

One other type of chemically modified electrode has been used in the study of cytochrome c, as described by Lewis and Wrighton. In this work platinum, gold, and p-type silicon substrates were modified with a polymeric overlayer which contained bipyridinium-type one-electron redox centers. These immobilized mediators transfer electrons between the electrode surface and diffusing ferricytochrome c at the potential of the mediators which is ca. 0.59 V more negative than the formal potential of cytochrome c. Oxidation of ferrocytochrome c is precluded because of the difference in the formal potentials of cytochrome c and the immobilized mediator. This same group has recently immobilized 2,3,4,5-tetramethyl-l-(dichlorosilylmethyl)-[2]-ferrocenophane on platinum. The juxtaposition of the formal potentials... 

To achieve enhanced cell adhesion and proliferation in vitro which probably leads to a better integration of the prosthesis into the surrounding tissue the cell adhesion mediator fibronectin was coupled to the surface of silicone. Figure 40 shows the two principal steps of the appHed surface modification procedure. Argon plasma treatment and subsequent exposure to air of the silicone samples leads to the formation of hydroperoxide groups on its surface which are used to initiate polymerization of acrylic acid (AAc), methacrylic acid (MAAc) and gly-cidyl methacrylate (GMA) generating carboxylic and epoxy groups, respectively. 

ABSTRACT. Polysilanes, (-SiRR -)n, represent a class of inorganic polymers that have unusual chemical properties and a number of potential applications. Currently the most practical synthesis is the Wurtz-type coupling of a dihalosilane with an alkali metal, which suffers from a number of limitations that discourage commercial development. A coordination polymerization route to polysilanes based on a transition metal catalyst offers a number of potential advantages. Both late and early metal dehydrogenative coupling catalysts have been reported, but the best to date appear to be based on titanocene and zirconocene derivatives. Our studies with transition metal silicon complexes have uncovered a number of observations that are relevant to this reaction chemistry, and hopefully important with respect to development of better catalysts. We have determined that many early transition metal silyl complexes are active catalysts for polysilane synthesis from monosilanes. A number of structure-reactivity correlations have been established, and reactivity studies have implicated a new metal-mediated polymerization mechanism. This mechanism, based on step growth of the polymer, has been tested in a number of ways. All proposed intermediates have now been observed in model reactions. 

Temple K, JaMe F, Sheridan JB, Manners 1 (2001) The nature of the active catalyst in late transition metal-mediated ring-opening polymerization (ROP) reactions mechanistic studies of the platinum-catalysed ROP of silicon-bridged [l]ferrocenophanes. J Am Chem Soc 123 1355-1364... 

Scheme 5 Silicone rubber from siloxane-styrene copol)uners via nitroxide-mediated radical polymerization.

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