Rate constants for initiator decomposition

The rate of radical generation or production (Rpr) is related to the rate constant for initiator decomposition, c, by Equation 10.3 where [I] is the instantaneous initiator concentration. The factor of 2 is included because two initiator or primary radicals are... 

A fourth and probably the most useful method of determination of initiator eflSciency is based on the dead-end effect in polymerization technique which is treated in a later section. This technique allows treatment of kinetic data obtained under dead-end conditions to evaluate both the rate constant for initiator decomposition (kj) and the initiator efficiency (/) under experimental conditions. 

In the above scheme M stands for the monomer concentration, I is the concentration of the initiator, and [R ] and [P ] mean the concentration of primary and polymer radicals, respectively S stands for the chain transferring agent, Ri denotes the decomposition rate of the initiator, and Rp the rate of polymerization. The rate constant for initiator decomposition is kd, for initiation is ki, for propagation Ap, and for termination is hr. The above is based on an assumption that kp and ki are independent of the sizes of the radicals. This is supported by experimental evidence, which shows that radical reactivity is not affected by the size, when the chain length exceeds dimer or trimer dimensions. ... 

Note I In a radical polymerization, the rate of radical production from an initiator that provides two similar radicals is where 1 is the rate constant for initiator decomposition. 

Rp is the polymerization rate, kp is the rate constant for the propagation step, is the rate constant for initiator decomposition, k is the rate constant for the termination step, [M] Is monomer concentration, f Is an efficiency factor for the initiators and [I], [oxidant], and [reductant] are concentrations of the initiators. The general first-order dependence of rate on monomer concentration is altered when the macroradicals are terminated by radicals directly formed from the initiator, and when the monomer is involved in the initiation step. 

Following earlier studies of the oxidation of formic and oxalic acids by pyridinium fluoro-, chloro-, and bromo-chromates, Banerji and co-workers have smdied the kinetics of oxidation of these acids by 2, 2Tbipyridinium chlorochromate (BPCC) to C02. The formation constant of the initially formed BPCC-formic acid complex shows little dependence on the solvent, whilst a more variable rate constant for its decomposition to products correlates well with the cation-solvating power. This indicates the formation of an electron-deficient carbon centre in the transition state, possibly due to hydride transfer in an anhydride intermediate HCOO—Cr(=0)(0H)(Cl)—O—bpyH. A cyclic intermediate complex, in which oxalic acid acts as a bidentate ligand, is proposed to account for the unfavourable entropy term observed in the oxidation of this acid. 

Table 11.3 lists the pseudo first-order rate constants for the decomposition of hydroxybenzoic acids. It shows that the ratio of the mean initial decomposition rate of HBAs under argon, R(HBA)Ar, to that under air, R(HBA)air, is equal to (3.0 + 4.9 + 5.1)/(2.7 + 3.4 + 3.1) = 1.4 (see Table 11.3). The ratio of the rate of OH radical formation under argon to that under air was estimated to be R( OH)Ar/.R( OH)air = 20/15 = 1.3 and is close to the value of R(HBA)Ar/R(HBA)air, which suggests that the decomposition of HBAs is mainly caused by OH radicals, and oxygen molecules have little effect on the decomposition.

A free-radical polymerization is carried out in a CSTR with an average residence time of 240 min. The reaction is first order to monomer M and half order to initiator I, and the pseudo-first-order rate constant for the monomer reaction is 2.3 x 10 min when the initiator concentration is 0.01 M. The decomposition rate constant for initiator is 4.1 x 10 min. ... 

The constants kj and k, are the rate constants for initiator dissociation and monomer addition, respectively. Since initiator dissociation (ecp. 7.10) is much slower than monomer addition (ecp. 7.11), the first step of the initiation step (initiator dissociation) is the rate-limiting step. Some of the initiator radicals may undergo side (secondary) reactions, such as combination with another radical, that preclude monomer addition. Therefore only a fraction, f (an efficiency factor), of the initial initiator concentration is effective in the polymerization process. Also, decomposition of each initiator molecule produces a pair of free radicals, either or both of which can initiate polymerization. Based on these observations, the rate expression for initiation may be written as ... 

Consider the free-radical polymerization of methyl methacrylate in toluene solution at 77°C, initiated by AIBN. When the initial monomer concentration was 2.07 M and the initial AIBN concentration was 2 X 10 M, the initial rate of polymerization was determined to be Vp = 2.49 X 10" M min. a. Determine the initial rate of initiation, Vj, and kj(k by considering that the rate constant for the decomposition of AIBN at 77°C is k = 5.7 X 10 min, and that virtually all radicals are capable of initiating chains. 

As described in Exercise 14.43, the decomposition of suUuryl chloride (SO2CI2) is a first-order process. The rate constant for the decomposition at 660 K is 4.5 X 10 s (a) If we begin with an initial SO2CI2 pressure of 450 torr, what is the pressure of this substance after 60 s (b) At what time will the pressure of SO2CI2 decline to one-tenth its initial value ... 

Reaction constants for initiator decomposition, initiation reaction and propagation reaction Overall reaction rate constant (m /mol s)... 

Using this equation, you can calculate the concentration of NO2 at any time during its decomposition if you know the rate constant and the initial concentration. At 330°C, the rate constant for the decomposition of NO2 is 0.775 L/(mol s). Suppose the initial concentration is 0.0030 mol/L. What is the concentration of NO2 after 645 s By substituting into the previous equation, you get... 

If the rate constant for the decomposition of N2O5 is 6.2 X 10 /min, what is the half-life (The rate law is first order in N2O5.) How long would it take for the concentration of N2O5 to decrease to 25% of its initial value to 12.5% of its initial value ... 

Hydrogen peroxide in aqueous solution decomposes by a first-order reaction to water and oxygen. The rate constant for this decomposition is 7.40 X 10 /s. What quantity of heat energy is initially liberated per second from 2.00 L of solution that is 1.50 M H2O2 See Appendix C for data. 

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