Stabilizers precursor

The accessibility of ethyl-substituted stabilizer precursors suggests benzylic bromination with N-bromosuccinimide followed by base-catalyzed dehydrobromination. Tertiary amines were found to be particularly effective in the syntheses described in this paper. 

Diorganotins Catalysts for silicones, polyurethane foams polyvinyl chloride stabilizers precursor for forming Sn02 films on glass antihelminthics for poultry lubricating oil additives (Piver 1973 CEC 1978 WHO 1980 Chau etal. 1984 Blunden etal. 1985 Blunden and Chapman 1986 USPHS 1992). 

Ito, Kawakami and Sawamura recently described the borylation of al-lylic carbonates by B2pin2, catalyzed by bis(phosphine)copper(I) alkox-ides. It was proposed that bis(phosphine)copper(I) boryl species formed by alkoxide/boryl a-bond metathesis are key intermediates in the catalytic cycle [231]. Making use of related N-heterocyclic carbene stabilized precursors, Sadighi and co-workers have very recently isolated the thermally labile copper boryl complex (IPr)CuBpin (11.1) together with the products of oxygen atom, styrene and aldehyde insertion into the Cu-B bond (11.2-11.5 Scheme 24) [232,233,237]. The structure of 11.1 in the solid state reveals an approximately linear Cu(I) coordination geometry [ZB-Cu-C 168.1(2)°] and a Cu-B distance [2.002(3) A] which is somewhat shorter than the sum of the expected covalent radii [2.05 A] [106]. Yet further evidence for the... 

Although the hydrogen abstraction process is a useful technique for many practical purposes, it is not easy to control. Control is considerably improved when a diluent-soluble polymer carrying one or a few copolymerizable groups, often known as a stabilizer precursor, is used under conditions that minimize hydrogen abstraction grafting (e.g., use of initiators such as azobis-isobutyronitrile that produce moderately reactive free radicals). 

Stabilizer precursors can be made by copolymerization of a monomer that contains a condensable group. The copolymer is then reacted with a monomer contain-... 

Stabilizer precursors can be used in conventional dispersion polymerization only because the starting reaction mixture is homogeneous. Some nonaqueous dispersions, however, are made by processes in which one or more of the reactants is/are insoluble in the liquid diluent (e.g., polycondensation) [3.70]. In these cases, preformed graft copolymers must be used because they function not only as stabilizers for the final dispersion, but also as dispersants or emulsifiers for the starting materials. 

In dispersion polymerization [ 115 ], the monomers, the initiator and the stabilizer (or stabilizer precursor) are dissolved in a solvent that is not a solvent for the polymer. Polymerization starts in a homogeneous phase and the polymer precipitates forming unstable nuclei. The nuclei are stabilized by the stabilizer present in the system. This stabilizer may be included in the formulation or formed in the reactor by grafting onto the stabilizer precursor. Nucleation ends when the number of stable polymer particles increases to a point in which all new nuclei are captured by the existing stable particles. Because of the compartmentaliz-ation of the radicals among the polymer particles, the polymerization locus changes from... 

Deurbergue and Oberlin (1991) [218] categorized various stabilized precursors prior to carbonization by an aromaticity index (Al) ... 

Fig. 16. Pyridine-stabilized precursor polymer (a) [277] and al-koxy precursor polymer (b) [592] for the synthesis of poly(2,5-thienylene vinylene)...

In dispersion polymerization (Figure 4.2), the reaction mixture is initially a homogeneous solution of monomer and stabilizer (or stabilizer precursor) in the continuous medium. Upon addition of the initiator, polymerization starts in the homogeneous phase and the polymer is formed, which is not soluble in the continuous medium, precipitates, and it is stabilized by the stabilizer. This technique yields monodisperse particles with sizes in the range of 1-5 pm. 

FIGURE 4.2 Schematics of dispersion polymerization using a stabilizer precursor. 

For stabilization by grafting to stabilizer precursors, the relationship between the particle size and the operation variables is [129,130]... 

The reaction proceeds to completion only when an excess of the incoming thiol is used. Smaller amounts of thiol usually result in incomplete exchange. On the other hand, if too large an excess of thiol is used (more than 300 molar equivalents in most cases), the phosphine-stabilized nanoparticles rapidly decompose rather than undergoing ligand exchange [85]. Decomposition is also observed when the exchange reaction is carried out at elevated temperatures presumably due to the limited thermal stability of the phosphine-stabilized precursor particles in solution [86]. 

In the first step, an alkane-soluble methacrylate polymer or copolymer is prepared by classical free radical means using 2-ethylhexylmethacrylate EHMA or (EHMA + dye-Clfe-MA). This material serves as the stabilizer precursor. In the second step, the SP (10 g), vinyl acetate (90 g), AIBN (0.2 g), and isooctane (100 g) are mixed in an inert atmosphere and heated for 8 hours at 80 . Particles are purified as in recipe 1. Corelabelled particles are prepared by adding a trace (100 ppm) of dye-MA to the particle s3mthesis step. Stabilizer-labelled particles are prepared by introducing a labelled SP into the particle synthesis. Our particles had mean diameters of 200 nm to 300 nm and a monomer ratio of methacrylate/(vinyl acetate) of 1/9. 

The stabihzation process of mesophase pitch fibers is carried out between 250-3 50°C in air. During stabihzation, pitch molecules form crossUnks and the precursor fibers become non-meltable, ready for carbonization. The stabilized precursor fibers then are carefully pre-caibonized between 700-900°C to remove volatiles without the formation of voids. The pre-catbonized fibers are then carbonized at 1500-2000°C to form carbon fibers. To improve the fiber modulus, a graphitiza-tion process up to 3000°C can be applied to obtain graphite fibers. 

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