Initial rate constants

The thiol ( -dodecyl mercaptan) used ia this recipe played a prominent role ia the quaUty control of the product. Such thiols are known as chain-transfer agents and help control the molecular weight of the SBR by means of the foUowiag reaction where M = monomer, eg, butadiene or styrene R(M) = growing free-radical chain k = propagation-rate constant = transfer-rate constant and k = initiation-rate constant. 

Thus the thiol 0 2 25511 is capable of terminating a growiug chain and also initiating a new chain. If the initiation-rate constant, k is not much slower than the propagation-rate constant, the net result is the growth of a new chain without any effect on the overall polymerization rate (retardation). That represents a tme chain transfer, ie, no effect on the rate but a substantial decrease iu molecular weight (12). 

The initial rate constant values related to 10 mg of the metal film catalyst obtained from the kinetic plots (analogous to those represented in Fig. 15) diminishes by about one order of magnitude as a consequence of nickel film preexposure to atomic hydrogen (i.e. after transformation into... 

The following order of initiation rate constants was found by Grubbs et al. for 71a and some precatalysts containing one phosphine ligand 56d< 56k 71a<56h (cf. Scheme 15 for structures of 56d,h,k) [48b, 55]. Thus, 71a shows a rate of initiation comparable to that of 56k but three orders of magnitude higher than that of 56d. Nevertheless, 56d appears to be more reactive in RCM reactions than 71a [56]. Wakamatsu and Blechert were the first to report that the activity of precatalysts related to 71a can be dramatically enhanced by modification of the benzylidene unit [56]. For example, RCM of 75 using 1 mol% of BINOL-derived complex 71b yields the azacyclic product 76 in quantitative yield within 20 min (Eq. 10), whereas with 56d only 4% of 76 was obtained under these conditions [56]. 

Largest polymer chain length in polymer distribution Smallest polymer chain length in seed distribution and reactor effluent Initiation rate constant Propagation rate constant... 

Polysaccharide Schematic structure Initial rate constants min. Xl0 ... 

In thermal polymerization where the rate of initiation may also vary with composition, an abnormal cross initiation rate may introduce a further contribution to nonadditive behavior. The only system investigated quantitatively is styrene-methyl methacrylate, rates of thermal copolymerization of which were measured by Walling. The rate ratios appearing in Eq. (26) are known for this system from studies on the individual monomers, from copolymer composition studies, and from the copolymerization rate at fixed initiation rate. Hence a single measurement of the thermal copolymerization rate yields a value for Ri. Knowing hm and ki22 from the thermal initiation rates for either monomer alone (Chap. IV), the bimolecular cross initiation rate constant kii2 may be calculated. At 60°C it was found to be 2.8 times that... 

The rate constants in table 4 for Ru/AlaOs should be considered as initial rate constants since it was not possible to achieve a higher coverage of N— than 0.25. Furthennorc, it was not possible to detect TPA peaks for Ru/AlaOs within the experimental detection limit of about 20 ppm. Ru/MgO is a heterogeneous system with respect to the adsorption and desorption of Na due to the presence of promoted active sites which dominate under NH3 synthesis conditions. The rate constant of desorption given in table 4 for Ru/MgO refers to the unpromoted sites [19]. The Na TPD, Na TPA and lER results thus demonstrate the enhancing influence of the alkali promoter on the rate of N3 dissociation and recombination as expected based on the principle of microscopic reversibility. Adding alkali renders the Ru metal surfaces more uniform towards the interaction with Na. 

B, the initial rate constants are diminished, in contrast to those observed for the catalytic system (61). The reasons for the different effects of protein B on the two reactions with Hox are unknown. With Hox from M. capsulatus (Bath), activities of only —10% of the values observed under optimal catalytic conditions were found with the H202 shunt pathway, assuming specific activities to be greater than 200 mU/mg (59). As a consequence of the poor yields observed, the effect of protein B on the system was not investigated further. 

We have, therefore, taken a different approach (10). In a copolymer of acrylamide with acrylic acid, tEe initial rate constant for amide hydrolysis is given by... 

Figure 3. Initial rate constants for hydrolysis of acrylamide copolymers at 90, 108, and 120 °C.

The study of the detailed mechanism of free radical initiation (rate constant k ) and ozone decay (rate constant d) by the reaction with cyclohexane, cumene, and aldehydes gave the following results (7 = 298 K) ... 

Hence, the apparent initiation rate constant of POOH in PP depends on the following reactions bimolecular chain propagation (kp), intramolecular chain propagation (A pis),... 

Initiation, polymer autoxidation, 3 102,103 Initiation rate constants ( ), in VDC... 

Figure 4. Initiation Rate Constant (kj) for the Thermal Degradation of a Vinylidene Chloride/Methyl Acrylate (Five Mole Percent)/ 4-Vinylpyridine (0.1 Mole Percent) Terpolymer at 170 °C.

In all the above cases, degradation is clearly faster at higher intensities. It is possible to sho v [30] for all the above that the (initial) rate constant (k ) is a linear function of intensity (Fig. 5.27). 

To interpret these results the initial rate constants (relative to IBOA), for both the overlap band and the acrylate moiety were plotted against a molar equivalent ratio parameter, H, in the form of B = + w)- Figures 3 and 4 compare the changes in reactivity... 

Five different types of rate constants are of concern in radical chain polymerization—those for initiation, propagation, termination, chain transfer, and inhibition. The use of polymerization data under steady-state conditions allows the evaluation of only the initiation rate constant kd (or kt for thermal initiation). The ratio kp/k J2 or kp/kl can be obtained from Eq. 3-25, since Rp, Rj, and [M] are measurable. Similarly, the chain-transfer constant k /kp and the inhibition constant kz/kp can be obtained by any one of several methods discussed. However, the evaluation of the individual kp, k ktr, and kz values under steady-state conditions requires the accurate determination of the propagating radical concentration. This would allow the determination of kp from Eq. 3-22 followed by the calculation of kt, kIr, and kz from the ratios kp/ltj2, ktr/kp, and kz/kp. 

For any specific type of initiation (i.e., radical, cationic, or anionic) the monomer reactivity ratios and therefore the copolymer composition equation are independent of many reaction parameters. Since termination and initiation rate constants are not involved, the copolymer composition is independent of differences in the rates of initiation and termination or of the absence or presence of inhibitors or chain-transfer agents. Under a wide range of conditions the copolymer composition is independent of the degree of polymerization. The only limitation on this generalization is that the copolymer be a high polymer. Further, the particular initiation system used in a radical copolymerization has no effect on copolymer composition. The same copolymer composition is obtained irrespective of whether initiation occurs by the thermal homolysis of initiators such as AIBN or peroxides, redox, photolysis, or radiolysis. Solvent effects on copolymer composition are found in some radical copolymerizations (Sec. 6-3a). Ionic copolymerizations usually show significant effects of solvent as well as counterion on copolymer composition (Sec. 6-4). 

Table 27. 4a-Monodeuteration effect on initiation rate constants kj (44)/kg(44-d). Conditions as in Table 24...

Therefore the initiation rate constant in the monodeuterated molecule... 

The polymerization of lactones with the methyl substituent at different positions showed identical enantiopreferences in the polymerization reactions (Scheme 8b) as observed for the hydrolysis reaction (Fig. 10a) [26]. Also in this case, the alternating orientation of the methyl group from 3- to 5-MeCL in the faster reacting enantiomer suggested an odd-even effect. Comparison of the initial rate constants showed that the polymerization of 5-MeCL proceeded the fastest ki = 5h ), whereas 3-MeCL... 

Fig. 18. Variation of initial rate constant VJPa for decarboxylation of 6-nitrobenzisoxazole-3-carboxylate as a function of substrate concentration under conditions of excess substrafe. Initial velocities were corrected for the spontaneous hydrolysis of substrate in absence of polymer. Polymer is (C2H4N)m (C 2H25)0 2Sm(C2H5), 75m. The curve was drawn according to (30). with nk2 = 0.458 sec-1, and KM = 8.59 x 10-5 M.

Some semi-quantitative confirmation of these A factors comes from the consideration that the pyrolysis of C2H8 at 900°K. is a chain reaction in which the data on maximal inhibition indicate a chain length X of the order of 10. Since the only likely homogeneous, initiation process is the fission of C2H8 into 2CH3, the hypothetical first-order rate constant for the pyrolysis can be set equal to this initiation rate constant multiplied by X ... 

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