Initiators commonly used

The activation energy Ed for the decomposition of the initiators commonly used is of the order of 30 kcal., while (Ep — Etl2) assumes values from 4 to 7 kcal. Hence, insofar as it is termination-controlled, the temperature coefBcient of the degree of polymerization is negative. Similarly for thermal polymerizations, the corresponding quantity... 

Initiators commonly used include dibenzoyl peroxide, lauryl peroxide, 2, 2 -azobis isobutyronitrile, and others that are suitable for use in the temperature range of approximately 60-90°C. 

From the slope and intercept of the linear plot of lni p versus l/T we can determine Erp and Ap Ad / AtY. For most monomers, Ep and Et are in the range 5-10 kcal/mol and 2-5 kcal/mol, respectively, while for most initiators commonly used in thermal homolysis Ed is in the range... 

MMA toward an alkyl radical [302]. Because of its low electron density (e=2.7) [302], this fluoromonomer readily undergoes anionic polymerization with amines and organic and inorganic salts in the presence of 18-crown-6 [301, 303]. However, anionic initiators commonly used for polymerization of MMA such as butyllithium and Grignard reagents fail to polymerize this monomer due to Sn2 addition-elimination [303,304]. All-acrylate polymers for 157 nm imaging have been prepared by radical terpolymerization of 2-trifluoromethyl-acrylate and methacrylate or acrylate [288] (Fig. 90). 

Vinyl acetate is polymerized in dispersion form using various initiators. Exanples of ionic initiators commonly used for free-radical emulsion polymerizations are ammonium, sodium or potassium persulfate. Topical nonionic hydrophobic initiators include 2,2 -azobis(isobutyronitrile) (AIBN) and benzoyl peroxide. Water-soluble nonionic initiators such as tertiary-butyl hydroperoxide are also employed. The initiator 4,4 -azobis(4-cyanovaleric acid) in its acid state is oil soluble, while neutralization causes it to become water soluble providing for further diversity in initiators. 

In contrast to solution, the higher viscosity of a lecithin liposome decreases the efficiency of free radical initiation, and retards their autoxidation. Artificial azo initiators have a very low efficiency (about 9%) when solubilized in the bilayer phase. For this reason, the oxidizability of lecithin dispersed in a liposome is much lower than in solution. Although the solvent cage effect may be unique to diazo initiators commonly used in kinetic studies, and is not necessarily relevant to food lipid systems, metal initiators which are relevant to foods and biological systems, may also be affected by solvent cage effects because of the hydrated layer in emulsions (Figure 10.4). 

The initiators used in aqueous solution polymerizations of the acrylic acids usually are the water-soluble initiators commonly used in emulsion polymerization, such as persulfate, percarbonate, and perphosphate salts. Monomer-soluble initiators have also been used, usually at sufficiently low concentrations or in the presence of water-soluble solvents to form a homogeneous system. 

Application of PMMA as fixative for bone was first demonstrated by Charnley [52]. The PMMA bone cement is composed of the liquid monomer MMA, a partially polymerized PMMA powder, an initiator (commonly used dibenzoyl peroxide), an activator (N, A-dimethyl-p-toluidine), a radiopacifier (visible to X-rays) such as barium sulfate or zirconium oxide, and a copolymer to influence the mixing and handling of the cement [53]. In some cases, an antibiotic (e.g., gentamicin) is included in the formulation to minimize infection during implantation. The polymerization is initiated by the interaction between the activator and the initiator, yielding a free radical... 

The water-soluble initiator commonly used is potassium or sodium persulfate, and the usual recipe for emulsion polymerization is 200 parts by weight of water, 100 parts by weight of the monomer, and 2-5 parts by weight of a suitable emulsifier [1,2]. The monomer should be neither totally soluble nor totally insoluble in the water medium and must form a separate phase. The emulsifier is necessary to ensme that the monomer is dispersed uniformly as in a true emulsion [3-8]. The polymer that is formed from emulsion polymerization is in the form of small particles having an average diameter around 5 pm. The particles form a stable emulsion in water. Their separation can be effected only through the... 

Z-matriccs arc commonly used as input to quantum mechanical ab initio and serai-empirical) calculations as they properly describe the spatial arrangement of the atoms of a molecule. Note that there is no explicit information on the connectivity present in the Z-matrix, as there is, c.g., in a connection table, but quantum mechanics derives the bonding and non-bonding intramolecular interactions from the molecular electronic wavefunction, starting from atomic wavefiinctions and a crude 3D structure. In contrast to that, most of the molecular mechanics packages require the initial molecular geometry as 3D Cartesian coordinates plus the connection table, as they have to assign appropriate force constants and potentials to each atom and each bond in order to relax and optimi-/e the molecular structure. Furthermore, Cartesian coordinates are preferable to internal coordinates if the spatial situations of ensembles of different molecules have to be compared. Of course, both representations are interconvertible. 

The nebulization concept has been known for many years and is commonly used in hair and paint spays and similar devices. Greater control is needed to introduce a sample to an ICP instrument. For example, if the highest sensitivities of detection are to be maintained, most of the sample solution should enter the flame and not be lost beforehand. The range of droplet sizes should be as small as possible, preferably on the order of a few micrometers in diameter. Large droplets contain a lot of solvent that, if evaporated inside the plasma itself, leads to instability in the flame, with concomitant variations in instrument sensitivity. Sometimes the flame can even be snuffed out by the amount of solvent present because of interference with the basic mechanism of flame propagation. For these reasons, nebulizers for use in ICP mass spectrometry usually combine a means of desolvating the initial spray of droplets so that they shrink to a smaller, more uniform size or sometimes even into small particles of solid matter (particulates). 

Only relatively few compounds can act as primary explosives and still meet the restrictive military and industrial requirements for reflabiUty, ease of manufacture, low cost, compatibiUty, and long-term storage stabiUty under adverse environmental conditions. Most initiator explosives are dense, metaHoorganic compounds. In the United States, the most commonly used explosives for detonators include lead azide, PETN, and HMX. 2,4,6-Triamino-l,3,5-triuitrobenzene (TATB) is also used in electric detonators specially designed for use where stabiUty at elevated temperatures is essential. 

Lead azide is not readily dead-pressed, ie, pressed to a point where it can no longer be initiated. However, this condition is somewhat dependent on the output of the mixture used to ignite the lead azide and the degree of confinement of the system. Because lead azide is a nonconductor, it may be mixed with flaked graphite to form a conductive mix for use in low energy electric detonators. A number of different types of lead azide have been prepared to improve its handling characteristics and performance and to decrease sensitivity. In addition to the dextrinated lead azide commonly used in the United States, service lead azide, which contains a minimum of 97% lead azide and no protective colloid, is used in the United Kingdom. Other varieties include colloidal lead azide (3—4 pm), poly(vinyl alcohol)-coated lead azide, and British RE) 1333 and RE) 1343 lead azide which is precipitated in the presence of carboxymethyl cellulose (88—92). 

Activation Parameters. Thermal processes are commonly used to break labile initiator bonds in order to form radicals. The amount of thermal energy necessary varies with the environment, but absolute temperature, T, is usually the dominant factor. The energy barrier, the minimum amount of energy that must be suppHed, is called the activation energy, E. A third important factor, known as the frequency factor, is a measure of bond motion freedom (translational, rotational, and vibrational) in the activated complex or transition state. The relationships of yi, E and T to the initiator decomposition rate (kJ) are expressed by the Arrhenius first-order rate equation (eq. 16) where R is the gas constant, and and E are known as the activation parameters. 

Initia.tors, The initiators most commonly used in emulsion polymerization are water soluble although partially soluble and oil-soluble initiators have also been used (57). Normally only one initiator type is used for a given polymerization. In some cases a finishing initiator is used (58). At high conversion the concentration of monomer in the aqueous phase is very low, leading to much radical—radical termination. An oil-soluble initiator makes its way more readily into the polymer particles, promoting conversion of monomer to polymer more effectively. 

For contact testing a couplant normally is used between the probe and the test piece. This material may be oil, water, or some gel or other Hquid or paste. Compatibihty with the test object is important, so that no unexpected chemical attack occurs, causing a crack to initiate. Whereas the frequency range for ultrasonic tests may extend from approximately 50 kH2 to 50 MH2, the range most commonly used for metallic test materials is 0.5—25 MH2. 

Some fabrication processes, such as continuous panel processes, are mn at elevated temperatures to improve productivity. Dual-catalyst systems are commonly used to initiate a controlled rapid gel and then a fast cure to complete the cross-linking reaction. Cumene hydroperoxide initiated at 50°C with benzyl trimethyl ammonium hydroxide and copper naphthenate in combination with tert-huty octoate are preferred for panel products. Other heat-initiated catalysts, such as lauroyl peroxide and tert-huty perbenzoate, are optional systems. Eor higher temperature mol ding processes such as pultmsion or matched metal die mol ding at temperatures of 150°C, dual-catalyst systems are usually employed based on /-butyl perbenzoate and 2,5-dimethyl-2,5-di-2-ethyIhexanoylperoxy-hexane (Table 6). 

Starters. Nearly any compound having an active hydrogen can be used as starter (initiator) for the polymerization of PO. The common types are alcohols, amines, and thiols. Thus in Figure 2 ROH could be RNH2 or RSH. The fiinctionahty is derived from the starter, thus glycerol results in a triol. Some common starters are shown in Table 4. The term starter is preferred over the commonly used term initiator because the latter has a slightly different connotation in polymer chemistry. Table 5 Hsts some homopolymer and copolymer products from various starters. 

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