Initiator efficiency measurement

The ability to conduct radical reactions without the use of tin reagents is important. Allylic triflones have been used to conduct allylation reactions on a range of substrates (39) as a replacement for allyltributylstannane (Scheme 28). The main limitation was that unactivated or trisubstituted triflones failed to undergo reactions. In other nontin radical methods, arenesulfonyl halides have been used as functional initiators in the CuCl/4,4 -dinonyl-2, 2 -bipyridine-catalysed living atom-transfer polymerization of styrenes, methacrylates, and acrylates.The kinetics of initiation and propagation were examined with a range of substituted arylsulfonyl halides with initiator efficiency measured at 100%. 

Initiator efficiency increases with reaction temperature (Table 3.4). It is also worth noting that apparent zero-conversion initiator efficiencies depend on the method of measurement. Better scavengers trap more radicals. The data in Table 3,4 suggest that monomers (MMA, S) are not as effective at scavenging radicals as the inhibitors used to measure initiator efficiencies. The finding suggests that in polymerization the initiator-derived radicals have a finite probability of... 

Furthermore, it was found that, although the catalytic efficiency measured in initial current was lower for 13, the latter was more stable. When carbon felt electrodes were coated with this polymer (approx. 1 x 10 mol on a 20x20x4 mm piece) the reduction of dibromostilbene was obtained on a mmol scale with only moderate da rease of mediator electroactivity The production of about 0.15 mmol of stilbene... 

The efficiency of the intitiator is a measure of the extent to which the number of radicals formed reflects the number of polymer chains formed. Typical initiator efficiencies for vinyl polymerisations lie between 0.6 and 1.0. Clearly the efficiency cannot exceed 1.0 but it may fall below this figure for a number of reasons, the most important being the tendency of the newly generated free radicals to recombine before they have time to move apart. This phenomenon is called the cage effect . 

A large-scale enzymic synthesis of the trisaccharide a-D-Neu5Ac-(2—>3)-j8-D-Gal-( 1 — 3)-d-G1cNAc (65), the tumor-associated carbohydrate antigen CA 50, has been achieved.113 This is a further illustration of the cross reactivity and efficiency of STB, This sialyltransferase, like STA, was partially purified from porcine liver, according to a modification of Conradt s procedure. 114 The initial rate measured for j8-d-Gal-( 1 - 3)-d-G1cNAc at saturating concentration was 18% of the one measured for the real substrate / -d-... 

The individual terms must be evaluated in order to obtain numerical values for N. In the simplest case the rate of coagulation, in the presence of adequate stabilizer, will be equal to zero. There is some question as to whether this can be accomplished in practice because of the extreme instability of primary particles which are 1 to 2 nm in size (16, 20). The rate of initiation may be obtained from literature values or from independent measurements. The initiator efficiency, f, must also be obtained. For example, in the thermal decomposition of persulfate ion the rate of initiation is given by... 

The above values are also based on an assumed initiator efficiency of 0.5 owing to the presence of solvent (9). Comparison of the calculated Mn with the measured value (18) (Mn = 23,830) indicates that a considerable amount of termination occurred by combination. 

Another milestone in the 1980s was the discovery that either water-soluble or hpid-soluble initiators with water-soluble or hpid-soluble phenolic antioxidants can be used for quantitative kinetic studies in micelles and lipid membranes . This made measurements in these systems less difficult than before when the initiators were included in high concentrations in lipid membranes due to low initiator efficiency. ... 

Molecular weight measurements confirmed that the degree of polymerization followed the relationship DP = 1 /(2 f ktkd)° 5][[M]/[I]]0-5, which is predicted by this rate law. The measured rate law is consistent with a very low initiation efficiency, where f =/[M]. 

Burnett et al.63,64 observed an anomalous rate increase in the polymerization of methyl methacrylate in halobenzene. Although the experimental data did not indicate any solvent effect on the rate of decomposition of azobisisobutyronitrile, the efficiency of initiation varied with solvents. Since an enhanced rate of incorporation of initiator fragments and incorporation of solvent fragments into the polymer were not observed, a mechanism describing the increase in the initiator efficiency through the participation of an initiator-solvent-monomer complex was postulated [Eq. (2.4)]. Henrici-Olive et al.65) reported, however, that the rate of the azobisisobutyronitrile decomposition at 50 °C, measured spectroscopically, is higher in halo-benzene than in benzene. Burnett et al.66) found a similar enhanced rate effect of halobenzenes with other initiators, supporting his mechanism. 

Initiator efficiency increases with reaction temperature (Table 3.4). It is also worth noting that apparent zero-conversion initiator efficiencies depend on the method of measurement. Better scavengers trap more radicals. The data in Table... 

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