First-order termination

The first term on the RHS of (III-8) represents mutual termination of radicals in the polymer particles (i.e. second order termination). The second term represents a first order termination of radicals in the polymer particles by monomer soluble impurities (MSI), which are present in the polymer particles due to their transfer in there with monomer during the monomer diffusion phase from monomer droplets. 

Howard and Ingold (7) proposed participation of a first-order termination reaction in the autoxidation of styrene. If such contributions from first-order terminations are real and widespread in autoxidation, more knowledge about them becomes essential. 

Non-Steady State Equations with Correction for Spontaneous Initiation and First-Order Termination. Thoroughly purified hydrocarbons should exhibit a square-root dependence of oxidation rate on initiation rate, R we found, however, that even if this behavior is obtained with Ri of the order of 10 8 mole per liter per sec., deviations may occur with the low rates of initiation used in the non-steady state measurements (R 10 n). Also, spontaneous initiation of the order of R — 10 12 may occur. If we assume that the deviations can be described as a constant first-order termination, we can derive corrected formulas for the nonsteady state behavior upon adding a small amount of inhibitor AH or initiator AR, as follows. 

Initial condition spontaneous initiation Rf, first-order termination constant X ... 

Moreover, it should be noticed that polymerization rates were determined from the maximum slope of the kinetic curves, namely at degrees of conversion between 20 and 40%. At that time, the large increase in viscosity of the photoresist may already have reduced the chain mobility, thus favoring radical isolation and first-order termination. It is therefore very likely that the intensity exponent of the photopolymerization rate equation will be less than 0.85 in the early stages and that it increases with conversion to reach almost unity in the solid network. Such a kinetic behavior was indeed observed for the photopolymerization of neat hexanedioldiacrylate (31). 

The thermal degradation of PMMA has been considered in detail13 33-36 and the case of first order termination is analyzed in Ref. 33. If the rate of termination is k(T), the PBEs for this case are... 

Repeated attempts in this laboratory to demonstrate the presence of unpaired electrons in polyacrylonitrile initiated by AIBN at 50°C., using a Varian spectrometer, have failed to detect any signal at all. Measurements have been made both at room temperature and at —196 °C., and we believe that we could have detected concentrations as low as 1014 radicals per ml. Hence, it is not possible under the conditions reported here to account for the kinetic results by the mechanism of first-order termination of growing chains by entrapment in precipitated polymer. This is not to say, however, that under other conditions, such as photo-initiated polymerization at low temperature, the trapped radicals are unimportant to the kinetics of polymerization. 

Equation (XIV.G.3) represents, of course, one special case among many possible examples of chain-branching reactions. Variations of this equation may be obtained by adding terms that represent homogeneous first-order termination ka C) to Eq. (XIV.6.3) or wall-initiation terms to the auxiliary boundary equation (XIV.6.4). The addition of terms which are second-order in radicals, such as second-order recombination in the gas phase, or second-order branching leads to equations which are nonlinear and which may only be solved cither numerically or by approximation. 

Kinetics of Polymerization Involving Unimolecular Unopposed (Irreversible) First-Order Termination... 

Chemical reactions that proceed through free radical intermediates can explode if there is a branching step in the reaction sequence. Consider the overall reaction of A = B with initiator / and first-order termination of free radical R ... 

Thermal degradation of polystyrene involves depropagation and first order termination [37]. Initial decomposition temperature is 300-330° [38]. The kinetics of the decomposition follows the general rule da/dt = k (1 - a)[1-(1 - where a is the conversion (and equal to the ratio of the volatilized mass to the initial mass), k is the overall rate constant, and b is a parameter depending on the pyrolysis temperature. 

The degradation behaviour in this case cannot be obtained from the preceding equations, which are restricted to first-order termination or second-order disproportionation termination. Another method [2] restricted to a most probable type initial distribution has shown that in this case, in the absence of transfer... 

Kinetics of the overall reaction proceeding in ethanol was investigated239 . A pseudo-first order termination was assumed to take place in this solvent, namely,... 

The detailed competition among propagation steps was worked out by Mayo and coworkers [20—24] however, the oxidation process is complicated still further by self-initiation [25,26] and by first-order termination [27]. 

A. Single Active Center with First-Order Termination Chain ... 

Example First-order termination steps are often associated with extraneous factors, for example, the adsorption of chain carriers on the walls of a reaction vessel. 

Here again, the propagation rate is virtually the polymerization rate, since the consumption of monomer by the initiation step is negligible. Unlike the free radical case, the rate here is first order in initiator concentration, obviously due to the first-order termination step. 

Note this route leads to first order termination)... 

For fractionated PMMA polymerised with AIBN as initiator and laurylmercaptan as transfer agent, the rate constants obtained indicated a change in thermal degradation mechanism at approximately 400 °C, where depropagation to the polymer chain end began to compete with the first-order termination process [a.l67]. This led to a kink in the Arrhenius plot of the rate constants, where below 400 °C the activation energy was approximately 130 kJ/mol and above 400 °C it was ca. 250 kj/mol. 

First order termination reaction, likely to be reaction with Oj. ) CI2 vapor pressure measurement in gas phase above solution. 

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