Rotating sector technique

Photoinitiation can be switched on and off extremely rapidly. For example, the time of laser flash can be as short as 1 psec (10-12 s) and shorter. The practical absence of time inertia of photoinitiation lies in the timescales of the experimental techniques for studying fast free radical reactions (flash photolysis, rotating sector technique, photo after-effect [109]). 

The most recent values for Arrhenius parameters are those of Dainton and Burns128, who performed a very careful study of the photochemical formation of phosgene, investigating the effects of light intensity, temperature and concentration and determined radical lifetimes by the rotating sector technique. 

Our application of the rotating sector technique to hydrocarbon oxidations has been described (14,15,18). Oxidation rates were measured at the longest convenient chain lengths and corrected for the absorption and evolution of gas in initiation and in peroxy radical-peroxy radical reactions. ,a -Azobiscyclohexylnitrile (ACHN) was used as the photoinitiator at 30°C. and a,a -azobis-l-propanol diacetate as the photoinitiator at 56°C. ,a -Azobisisobutyronitrile (AIBN) was used as a thermal initiator at 30° and 56°C. 

Ayscough56 also studied the recombination of trifluoromethyl radicals by the rotating sector technique and found a rate constant of 2.34 X 1013 (mol./cc.)-1sec.-1, which is close to that for methyl radical combination obtained in an analogous manner from acetone. 

Bodenstein, Brenschede, and Schumacher27 (BBS) evaluated all the data and published approximate values for K63> 63, kBS, and k66. More recently Burns and Dainton33 (BD) repeated the photochemical experiments between 25 and 55°C utilizing the rotating-sector technique to obtain accurate values of the constants. The values obtained are in Table XV. 

Equations (2) and (3) could permit a simple determination of fc 2/A-B, and the authors started their experimental work with this objective in mind. The values of Ez and A3 for the C2H2CI3 radicals were known from the work of Ayscough et al. (7) in which they applied the rotating sector technique to the photochlorination of cis-l,2-dichloroethylene and found logio A3 (1. mole"1 sec.-1) = 8.7 0.3 and i 3 = 2.7 0.6 kcal. mole-1. In view of these values, and Howlett s conclusion, based on thermal dehydrochlorination studies, that for C2H3CI2 radical A 2 = 1013 sec.-1 and f 2 = 22 kcal. mole-1, no detectable isomerization was expected below 200°C. However, in contrast to this, a concurrent isomerization was easily detectable at 30°C even at a chlorine pressure of 200 mm. A detailed investigation was therefore necessary to explain the reasons for the discrepancy. 

Termination rate constants for alkyl and benzyl radicals in solution range between 109 and 1010 M 1 sec-1.85 These rates correspond quite closely to that calculated for a diffusion-controlled reaction, about 8 x 109 M x sec-1 for the common solvents at room temperature.86 Gas-phase rotating sector results are similar a newer method, however, shows that in the gas phase the rotating sector technique overestimates termination rates. Recombination is fastest for methyl radicals (1010,5 M-1 sec-1) and slower for others (-CF3, 109-7 at 146°C ... 

Static systems, however, are not usually suitable for rate studies. Fessenden (145) was the first to realize this and modified the static system for "intermittent radical production" using pulsed radiolysis. With the advances in electronic digital equipment, Smaller and coworkers (146) have subsequently fully developed the pulse-radiolysis technique for ESR studies and have successfully detected radicals with lifetimes as short as two microseconds. Concurrent developments of such "intermittent radical production" concepts have also been accomplished in photolytic systems by using either a rotating sector technique (147,148) or the flash photolytic technique (6). At present the pulse-radiolysis technique enjoys the advantage of a short and intense pulse at a rapid repetition rate. Only flash photolytic systems using a pulsed laser can approach these desirable conditions. These techniques will no doubt be continuously improved, and their future in ESR study of... 

If one assumes the absolute propagation rate constant, kp (measured by means of the rotating-sector technique or other absolute methods in block or solution polymerization systems), to be independent of the physical properties of the system (temperature excepted), it is possible to use these values for emulsion polymerization systems too. 

Rate coefficients of chain propagation and termination reactions measured by the rotating sector technique... 

Dilatometry and rotating sector techniques were combined to follow the photopolymerization of acrylamide in AOT reverse micelles with AIBN as the initiator and toluene as the organic phase [51,52]. Very high polymerization rates were observed with total convCTsion to polym achieved in a few minutes and good heat transfer. A monoradical termination reaction was found (R oc [I] [M]° ), caused by a degradative chain transfer to toluene. The reaction occurs at the water-oil interface by transfer of the growing polymer radical to toluene, followed by the exo-diffusion of tiie new ben lic radical, the latter bdng too stable to reinitiate polymerization. 

Most studies have dealt either with the free radical polymerization of hydrophobic monomers—e.g., styrene [56-89], methyl methacrylate (MMA) [68,73,74,84,86,90-93] or derivatives [2,94,97], and butyl acrylate (BA) [98-100]—within the oily core of O/W microemulsions or with the polymerization of water-soluble monomers such as acrylamide (AM) within the aqueous core of W/O microemulsions [101-123]. In the latter case, the monomer is a powder that has to first be dissolved in water (1 1 mass ratio) so that the resulting polymer particles are swollen by water, in contrast with O/W latex particles, where the polymer is in the bulk state. The polymerization can be initiated thermally, photochemically, or under )>-radiolysis. The possibility of using a coulometric initiation for acrylamide polymerization in AOT systems was also reported [120]. Besides the conventional dilatometric and gravimetric techniques, the polymerization kinetics was monitored by Raman spectroscopy [73,74], pulsed UV laser source [72,78], the rotating sector technique [105,106], calorimetry, and internal reflectance spectroscopy [95]. 

To illustrate more clearly the nature of free radical polymerization, it is instructive to examine the values of the individual rate constants for the propagation and termination steps. A number of these rate constants have been deduced, generally using nonstationary-state measurements such as rotating sector techniques and emulsion polymerization [26]. Recently, the lUPAC Working Party on Modeling of kinetics and processes of polymerization has recommended the analysis of molecular weight distributions of polymers produced in pulsed-laser-initiated polymerization (PLP) to determine values of... 

Value probably too low due to complications in the application of rotating sector technique to hypochlorite systems. 

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