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Flashing polymerization conditions

The iodine-catalyzed reaction of aziridines with carbon dioxide leads to 2-oxazolidinones (251). Because carbon dioxide effectively polymerizes ethyleneimine, only low yields are obtained when unsubstituted ethyleneimine reacts with C02. However, direct insertion of carbon dioxide [124-58-9] into aziridines can be accomplished, with better yields, by ethoxycarbonylation of aziridines with subsequent elimination of ethylene under flash vacuum conditions (252). 1- Phenyl aziridine [696-18-4] can react with C02 under antimony [7440-56-0] catalysis to give AI-phenyl-2-oxazolidinone in good yields (253). At low temperatures and with the exclusion of atmospheric humidity, the reaction of ethyleneimine with carbon dioxide produces the unstable ethyleneiminium salt [51645-58-2] of IV-vinylcarbamic acid (254,255). [Pg.8]

As can be easily seen, the breadth of the PLP distribution is not only dependent on the termination mode, as this is the case for steady-state experiments. The polydispersity is additionally controlled by the polymerization conditions like initiator concentration and laser intensity, which both influence the value of the free radical concentration which is produced at each laser flash, p. The polydispersity is also different for different pulse periods, o-... [Pg.6966]

Polymerization. The polymerization runs are made in a 5 liter stainless steel reactor in a bulk homopolymerization. Cocatalyst (5.64 mmol), catalyst (0.2 to O.S mmol), hydrogen (0.2 to 2 bar) and liquid propylene (3. 5 liters) are succesively introduced under a nitrogen blanket and the heating to the polymerization temperature is started. Unless otherwise specified, the polymerization conditions are maintained for three hours. The polymerization is then stopped by simultaneously flashing the residual monomer and cooling down the reactor. The polymer is recovered and dried overnight. 500 to 800 gram of polymer are obtained. [Pg.406]

Solution Polymerization These processes may retain the polymer in solution or precipitate it. Polyethylene is made in a tubular flow reactor at supercritical conditions so the polymer stays in solution. In the Phillips process, however, after about 22 percent conversion when the desirable properties have been attained, the polymer is recovered and the monomer is flashed off and recyled (Fig. 23-23 ). In another process, a solution of ethylene in a saturated hydrocarbon is passed over a chromia-alumina catalyst, then the solvent is separated and recyled. Another example of precipitation polymerization is the copolymerization of styrene and acrylonitrile in methanol. Also, an aqueous solution of acrylonitrile makes a precipitate of polyacrylonitrile on heating to 80°C (176°F). [Pg.2102]

By special processes, like polymerization under shear conditions or flash-spinning of polymer solutions, it is furthermore possible to obtain materials which do not only have this new shish-kebab micro-morphology, but also a new macro-morphology, namely the polymer fibrids.35... [Pg.302]

Consider a situation in which a concentrated polymeric solution enters the extraction zone of, say, an extruder in circumstances when the pressure in the extraction zone. Pa, is less than the equilibrium partial pressure of the volatile component in the feed solution. Under these conditions the solution will be supersaturated at the extraction pressure, flashing of the volatile component will occur, gas bubbles of radius Rq will be formed, and the concentration will immediately fall from Wi to wq. If bubble formation occurs by homogeneous nucleation, the rate at which these bubbles will be formed per unit volume of solution should depend on the difference between the equilibrium partial pressure of the volatile component and the devolatilization pressure. Since this pressure difference is greatest when the solution first enters the extraction zone, the rate of formation of bubbles will at first be high but as devolatilization pro-... [Pg.88]

The decisive break came in 1991 when Scott and co-workers reported the formation of corannulene from 7,10-diethynylfluoranthene 19 under flash vacuum pyrolysis (FVP) conditions. The critical features of this approach appear to be the high temperature employed (900-1000 °C) and the gas-phase conditions. The high temperature provides energy for drastic fluctuations in 8 away from its equilibrium geometry, allowing the reactive centers to come close enough to form the new bonds while the gas-phase conditions prevent polymerization (although some polymeri-... [Pg.6]

Cationic polymerization of 2-methylpropene at temperatures about 170 K may be almost flash-like the transformation of tetrahydrofuran to an equilibrium polymer-monomer mixture may last tens to hundreds of hours at 260 K. Evidently the overall polymerization rate is a function of many factors which may be interconnected or may act separately. The aim of kinetic measurements is to describe the polymerization, and to find conditions under which it would proceed in the desired manner. This is usually only possible after the various factors and their consequences have been isolated and investigated. The rate of monomer consumption during polymerization mostly depends on the generation rate of active centres, and on their concentration and reactivity. [Pg.481]

Alkyl enol ethers polymerize under these conditions, and their reactivity, as that of other strong nucleophiles, has been determined with the LASER flash method (Table 7). Because this method of carbocation generation produces the nucleophilic counterions CL, eventual polymeriza-... [Pg.118]

Free and polymer-bound 2-benzyloxy-thioxanthone exhibit similar flash photolysis behaviour and the same photoreduction quantum yield in the presence of 2-(A, AT-diethylamino) ethanol. This clearly shows that the polymeric nature does not appear to affect photophysical properties of the thioxanthone moiety. The photoinitiated polymerization of MMA in benzene solution, using BOTX and poly(StX-co-St) in combination with 2-(MA -dieffiylamino) ethanol, indicates that the polymer-bound chromophore seems to operate in the same way and with similar efficiency as the free photoinitiator, at least in conditions of dilute chromophore concentration. [Pg.149]

Direct and non destructive analysis (HPLC, and HPLC coupled with MS) shows that ILC contains anhydro-oligosaccharides with degrees of polymerization ranging from 7 to 2 and levoglucosan. These species result from the depolymerization of the cellulose molecules, It is important to note that the chemical composition of ILC does not depend on the operating conditions (heat flux densities and flash times). [Pg.1036]

Poly(methyl methacrylate) is probably the most utilized polymeric methacrylate. The result for flash pyrolysis using Py-GC/MS analysis of a sample of poly(methyl methacrylate), CAS 9011-14-7, with = 15,000 is shown in Figure 6.7.27. The pyrolysis was done in similar conditions to other examples, namely at 600° C in He at a heating rate of 20° C/ms with the separation on a Carbowax column (see Table 4.2.2) and MS detection. The peak identification for the chromatogram shown in Figure 6.7.27 was done using MS spectral library searches and is given in Table 6.7.20. [Pg.385]

According to analysis and consideration detailed above for photoinitiated polymerization, it may now be concluded that Rp or [M ] can be varied by varying the frequei of blinking. Under otherwise comparable conditions, the average rate (Rp) at different flashing conditions with the same / is related to the steady-state Rp)s as... [Pg.484]

Under conditions of nanosecond laser-flash photolysis, a long-lived transient absorption assigned to VI, A 465 nm, analogous to V, the intermediate proposed in formation of iodobiphenyls from iodonium salts, is observed [83], The diphenylsulfinyl radical cation, /Lmai340, 750 nm, is similarly observed in acetone sensitized laser-flash photolysis. Photo-CIDNP observations, namely emissive polarization for benzene, suggest that the homolytic cleavage pathway also operates under conditions of direct photolysis, accounting, in part, for the diphenylsulfide product [83], as well as the photochemistry of polymeric triarylsulfonium salts [81] (see above). [Pg.330]


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See also in sourсe #XX -- [ Pg.107 ]




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