Factors affecting rate propagation

Fatigue damage is predominantly caused by crack growth under cyclic loading. Several factors affect the rate at which a crack propagates in a material ... 

Electron spin resonance detection of the propagating and mid-chain radicals involved in the polymerization of phenyl acrylate (PhA) and the determination of the rate constants for this monomer using the ESR method were reported by Azukizawa et al. [114]. Absolute values of kJiM were determined at infinitely low conversion, to minimize the factors affecting the radical concentration. 

Before any chemistry can take place the radical centers of the propagating species must conic into appropriate proximity and it is now generally accepted that the self-reaction of propagating radicals- is a diffusion-controlled process. For this reason there is no single rate constant for termination in radical polymerization. The average rate constant usually quoted is a composite term that depends on the nature of the medium and the chain lengths of the two propagating species. Diffusion mechanisms and other factors that affect the absolute rate constants for termination are discussed in Section 5.2.1.4. 

To complicate matters still more, propagation reactions are also affected by diffusion which means that the specific rate constant for propagation must be considered as a decreasing function of conversion. The reduction in initiator efficiency as polymerization proceeds is another factor to take into account for an accurate kinetic description (Russell et al., 1988b). 

Since the forward reaction in (29) is exothermic, the equilibrium is displaced to the left by increase in temperature this factor accounts in part for the anomalous temperature coefficient of reaction rate mentioned above. The apparent catalysis by propagating base is also explicable as acid catalysis since the carbamic acid is stoichiometrically derived from the base by reaction (29). That true base catalysis is not operative has been shown by the observation that addition of tertiary bases does not affect the reaction rate [17]. Further, the polymerization is catalysed by other weak acids such as hydrocinnamic [17] and a-picolinic acids [10, 17], which, if present in sufficient concentration under conditions of low CO2 pressure, reduce the order in initiating base to unity. Thus, under such conditions, with hydrocinnamic acid (HX) as catalyst the simple kinetic form (30) is achieved. 

As would be anticipated, the nature of substituent groups has a profound influence on the reactivity and polymerizability of an NCA. The rates of reaction between NCA and base molecules are affected by stereochemical and electronic factors further, in suitable circumstances, interactions between propagating species and side-chain groups may occur. 

The first two factors have been dealt with above. As far as is known, the rate constant is not affected by the colloidal nature of the emulsion process. In fact, recent determinations of the propagation rate constant (kp) of styrene in... 

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