Sectors, rotating

One of the early specialized techniques used for the study of radical reactions was the rotating sector method. The use of this technique (Fig. 6) for determining reaction kinetics was demonstrated by Melville for the gas phase polymerization of methyl methacrylate, and later by Bartlett and Swain for the liquid phase reaction, and by Carlsson and Ingold for tin hydride reductions.  

Other flow techniques were developed in subsequent years, but towards the end of the 1960s it was recognized that detectable concentrations of reactive radicals could be obtained in a static sample by ultraviolet photolysis of 

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The rate of initiation was measured directly and radical lifetimes were determined using the rotating sector method. The following results were obtained.f... 

Even though the rates of initiation span almost a 10-fold range, the values of k, show a standard deviation of only 4%, which is excellent in view of experimental errors. Note that the rotating sector method can be used in high-pressure experiments and other unusual situations, a characteristic it shares with many optical methods in chemistry. 

The lifetime of polystyrene radicals at 50 C was measuredt by the rotating sector method as a function of the extent of conversion to polymer. The following results were obtained ... 

The splitting and recombination of the beam is accomplished by means of two rotating sector mirrors which are geared to the same electric motor so that they work in unison (Fig. 17.12). The microprocessor which is used to operate such an instrument will automatically correct for the dark current of the photocell, i.e. the small current which passes even when the cell is not exposed to radiation. 

From this we can see that knowledge of k f and Rf in a conventional polymerization process readily yields a value of the ratio kp fkt. In order to obtain a value for kf wc require further information on kv. Analysis of / , data obtained under non-steady state conditions (when there is no continuous source of initiator radicals) yields the ratio kvlkx. Various non-stcady state methods have been developed including the rotating sector method, spatially intermittent polymerization and pulsed laser polymerization (PLP). The classical approach for deriving the individual values of kp and kt by combining values for kp kx. with kp/k, obtained in separate experiments can, however, be problematical because the values of kx are strongly dependent on the polymerization conditions (Section... 

The kinetics of many copolymerizations have now been examined with absolute (overall) propagation rate constants being determined by the rotating sector, PLP or FSR methods. A similar situation as pertains for the MMA-S... 

A superficially related dependence of on the medium has been observed by Norrish and Smith working with methyl methacrylate, and by Burnett and Melville with vinyl acetate. Rates in poor solvents are high, and determination of by the rotating sector method reveals what appears to be a decrease in kt in the poor solvents. This apparent decrease in kt accounts for the increased rate of polymerization. Actually, precipitation of the polymer seems to be responsible for the effect. The growing radicals become imbedded in precipitated droplets, presumably of very small size. The termination reaction is suppressed owing to isolation of the chain radical in one droplet from that in another. This gel effect is fairly common in systems yield-... 

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 second type of double-beam instrument is one where the light source is divided into two beams by a rotating sector mirror that alternately reflects and transmits the light. This results in a chopped beam of light that alternately passes through the reagent blank and the sample as shown in Fig. 5.18. 

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. 

The original value obtained from the rotating sector application was multiplied by a factor of 4 (see text). 

Few kinetic studies of reactions of alkyl radicals with tin hydrides other than Bu3SnH have been reported. Studies of the reactions of the tert-butyl radical with Me3SnH and Ph3SnH were performed by the rotating sector method,80 but an error in absolute values exists in that method as judged by differences in rate constants for reactions of Bu3SnH with alkyl radicals... 

Most detector systems require that the IR beam be modulated, where the source energy is adequately differentiated in the measured signal from the ambient background. One of the traditional approaches is to use some form of mechanical chopper , usually in the form of a rotating sector wheel, which modulates the beam by blocking the radiation in one or more sectors during a rotation. Note that this is not a requirement for FTIR systems where the interferometer naturally modulates the beam. 

Measurements of kp were performed by the rotating sector method and its variations until the late 1980s [Flory, 1953 Odian, 1991 Walling, 1957]. In the late 1980s advances in pulsed laser technology and size exclusion chromatography resulted in the development of a technique called pulsed laser polymerization-size exclusion chromatography (PLP-SEC) [Beuermann and Buback, 2002 Beuermann et al., 1997, 2000 Buback et al., 1986, 1992,... 

The rotating sector method requires the introduction of the parameter is, defined as the average lifetime of a growing radical under steady-state conditions. The radical lifetime is given by the steady-state radical concentration divided by its steady-state rate of disappearance ... 

The ratio kp/k, can be obtained from Eq. 3-157 from xs and the rate of polymerization under steady-state conditions. (The subscript s in Eq. 3-157 and subsequent equations in this section refers to steady-state values the absence of s denotes non-steady-state values.) The individual rate constants kp and k, can be determined by combining kpjk, with kpjit, 1 obtained from Eq. 3-25. Thus, the objective of the rotating sector method is the measurement of xs. 

The PLP-SEC method, like the rotating sector method, involves a non-steady-state photopolymerization [Beuermann, 2002 Beuermann and Buback, 2002 Komherr et al., 2003 Nikitin et al., 2002], Under pulsed laser irradiation, primary radicals are formed in very short times ( 10 ns pulse width) compared to the cycle time ( 1 s). The laser pulse width is also very short compared to both the lifetimes of propagating radicals and the times for conversion of primary radicals to propagating radicals. The PLP-SEC method for measuring kp requires that reaction conditions be chosen so that no significant chain transfer is present. The first laser pulse generates an almost instantaneous burst of primary radicals at high... 

The rate constants for propagation and termination have been determined for many monomers. Table 3-11 lists values for some of the common monomers. These data were based on rotating sector experiments. The monomers have been listed in order of decreasing kp values (which does not necessarily correspond to the exact order of decreasing kt values). The order of kp values is discussed in Sec. 6-3b, since kp is a function of both monomer reactivity and radical reactivity. 

A prime purpose of the IUPAC Working Party on Modeling of Kinetics and Processes of Polymerization has been standardization of the experimental conditions and calculation methods for obtaining rate constants and other parameters. Table 3-12 shows the PLP-SEC values of the propagation rate parameters for a number of monomers. For many monomers there is good agreement between the values obtained from the rotating sector and PLP-SEC methods. 

The rate constants kp, kt, and kp, kt can be obtained by rotating sector studies on the pure hydrocarbons. The cross-propagation and... 

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. 

Flow photolysis in benzene (B) and methanol (M) unless otherwise noted. c RH, rotating sector on hydrocarbon. ROOH, rotating sector on hydrocarbon in presence of hydroperoxide. 

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