Constant of initiation

A kinetic study for the polymerization of styrene, initiated with n BuLi, was designed to explore the Trommsdorff effect on rate constants of initiation and propagation and polystyryl anion association. Initiator association, initiation rate and propagation rates are essentially independent of solution viscosity, Polystyryl anion association is dependent on media viscosity. Temperature dependency correlates as an Arrhenius relationship. Observations were restricted to viscosities less than 200 centipoise. Population density distribution analysis indicates that rate constants are also independent of degree of polymerization, which is consistent with Flory s principle of equal reactivity. 

The formation of radicals from hydrogen peroxide in cyclohexanol was measured by the free radical acceptor method [60] the effective rate constant of initiation was found to be equal to ki = 9.0 x 106 exp(—90.3/RT) s 1. For the first-order decomposition of H2O2 in an alcohol medium, the following reactions were discussed. 

The autoxidation of ethers occurs with self-acceleration as autoxidation of hydrocarbons. The kinetics of such reactions was discussed earlier (see Chapter 2). The autoacceleration of ether oxidation occurs by the initiating activity of the formed hydroperoxide. The rate constants of initiation formed by hydroperoxides were estimated from the parabolic kinetic... 

For olefins, cyclic, or better hi- or tricyclic ring structures with large ring strain (norborn-2-enes or norbornadienes for instance) are required. Alternatively, 1-alkynes can be used. In this case, the term 1-alkyne polymerization applies. This reaction proceeds via a- or j6-insertion of the alkyne into the metal-carbon double bond (Scheme 1). Both insertion mechanisms lead to a conjugated polymer. With a few exceptions [1-3], polymerizations based on a-insertion are the preferred ones, since they offer better control over molecular weights due to favorable values of kj/kp (ki, kp = rate constants of initiation and propagation, respectively). 

In the preceding explanation, it has been assumed that the degree of dissociation was increased by the presence of an electric field no field effect on the rate constant of initiation was assumed. Equation (13) can be rewritten as... 

The thermochemical characteristics of l,3,5-trinitro-l,3,5-triazepane, such as energies of dissociation of N-NOz bonds, enthalpies of formation, vaporization, and combustion, as well as enthalpy of formation of amine radicals, have been determined <2004MI92>. The rate constants of initial monomolecular stages of thermal decomposition in the solid phase were measured for its furazano-fused analog 20 <1999RCB1250> and the ratio of the rate constants of decomposition in the melt and solid states, characterizing the reaction retardation in the crystal lattice, was determined. The kinetics of the thermal decomposition of 20 has also been studied <1995MI885>. 

XY, F2C = CRR, k , kpl, kTe and ktr represent the telogen, the monomer, the rate-constant of initiation, the rate-constants of propagation, of termination and of transfer, respectively. 

A constant rate of absorption was observed (Figure 2). The efficiency of amine for terminating chain reactive oxidation is calculated according to the following reaction proposed by Hammond et al. (9), in which the rate constant of initiation kt was taken to be 8.16 X 10"4 mole/min. from the rate of evolution of nitrogen in a simple decomposition of azobisiso-butyronitrile in chlorobenzene at 62.5°C., and the efficiency e of initiator was taken to be 0.58 (10). 

Although this formally requires that k, - °°, controllable molecular weight is obtained if the rate constant of initiation is comparable to that of propagation (kf 2 kp) (see Chapter 4). As demonstrated subsequently, if initiation is slow, the number average molecular weight is initially too high but becomes equal to the theoretical value once initiator is consumed. 

The rate constants of initiation of styrene by triflic acid are estimated to be roughly k,- 10 M sec-1 at 0° C in CH2CI2 this is 10,000 times smaller than the corresponding propagation rate constant under similar conditions [17]. However, it does not take into account the higher order kinetics in acid [134]. Initiation of more nucleophilic monomers is faster, with ki = 103 M l-sec-1 for a-methylstyrene [21] and kj = 5104 M -sec l for p-methoxystyrene [23], as determined by stopped-flow methods at ambient temperature in CH2CI2 and C2H4CI2. 

Table 12 lists the rate constants of initiation in numerous systems calculated from spectrometric measurements. These rate constants are 103-106 times smaller than the rate constants of propagation (kp+ = 10s mol" L-sec- ) under similar conditions, with the largest differences occurring with less reactive monomers such as styrenes and with sterically hindered monomers such as a-methylstyrene. 

The rate constants of nearly all of the elementary reactions in trityl-initi-ated polymerizations of cyclopentadiene [216], p-methoxystyrene [186], vinyl ethers [217], and a-methylstyrene [218] were determined by kinetic measurements, sometimes combined with conductometric measurements. Monomer conversion was followed by either dilatometry, spectroscopy, or calorimetry. Initiation was followed by the decrease in the 410-nm absorption of the trityl carbenium ions (e = 36,000 mol- L em-1), caused by their reaction with monomer by either direct addition or hydride abstraction. The initiator was assumed not to be consumed in any other reactions. The reaction orders (usually first order in each reagent) and rate constants of initiation were then determined by plotting the rate of initiation versus the initial monomer and initiator concentrations according to Eq. (52). 

The intercept of a plot of [N] vs. [M]0 yields the number of chains formed by initiation ([AT] ), or more precisely, the sum of chains formed by initiation and intramolecular chain transfer to counteranion. The rate constant of initiation kt is calculated from the intercept, and k, and ktrm are obtained from the slope after solving the integral value graphically. 

Kinetics of initiated radical polymerization of styrene is well studied at the initial stages. At the high-conversion of polymerization the growth of viscosity of media- influences the mechanism and kinetics of process greatly (12),In our research we have attempted to obtain tne empirical dependence of some rate constants of initiated polymerization of styrene on conversion with the aim of using them at mathematical simulation of the process. 

Having clearly established the nature of the individual chemical reactions which make up the polymerization, the authors use a spectroscopic technique to determine rate constants of initiation and propagation. 

Thus, measuring the concentration of initiator as a function of time, the rate constant of initiation can be determined. In a few systems, when the equilibrium monomer concentration is relatively high (THF, DXL), initiation has been studied under conditions at which propagation is excluded ([M]o < lM]e). 

H-NMR spectra of this system, recorded after different times from the beginning of the reaction, ate shown in Fig. 3. Reaction (59) can also be followed by -NMR, recording gnals A (fi = 88.0 ppm from CeF ) and B (6 = 85.2 ppm). The reaction is treated as a second-order reaction and the plot of [THFlt 8 h constant of initiation... 

In Table 6 some of the values of the rate constants of initiation determined to date and their corresponding activation parameters are listed. 

The polymers possess one sulfonyl group per chain, which can be utilized as end-functional polymers as discussed later (section III.B.l). Narrower MWDs (MJMn = 1.2—1.4) were obtained in MMA polymerization with 1-32 as well as 1-33 and 1-34 in conjunction with CuCl/L-1 in />xylene at 90 °C.175 In a homogeneous system with CuCl/L-4,1-32 can afford narrow MWDs MJMn = 1.1—1.3) for styrene, MMA, and nBA.176 The fast addition of the sulfonyl radical to these monomers was evidenced by H NMR analysis of the reactions, where the apparent rate constants of initiation are 4 (for styrene and MMA), 3 (nBA), and 2 (MA) orders of magnitude higher than those of propagation. A similar controlled and homogeneous polymerization of MMA with 1-32 (X = CH3)/ CuBr/L-4 was reported in diphenyl ether at 90 °C.178 A better control of molecular weights and MWDs with 1-32 (X = CH3)/CuBr/L-9 in diphenyl ether was also... 

In these equations, ki, kp, and kjp are the specific rate constants of initiation, propagation, and depropagation reactions, respectively. 

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