Propagation process

The benefit of such a model is that better understanding of the wave propagation process may be gained. Also, it is possible to make controlled parameter studies in order to understand the influence of e.g. defect orientation, probe angle and frequency on the test results. Results may be presented as A-, B- or C-scans. 

A chain reaction polymerization of vinyl monomer, which is usually carried out by a photoinitiator to produce a primary radical (R ), which can interact with a monomer molecule (M) in a propagating process to form a polymer chain composed of a large number of monomer units (see Eq. [2] and reaction Scheme [3]. 

Such functionality can also be of great practical importance since functional initiators, transfer agents, etc. are applied to prepare end-functional polymers (see Section 7.5) or block or graft copolymers (Section 7.6). In these cases the need to maximize the fraction of chains that contain the reactive or other desired functionality is obvious. However, there are also well-documented cases where weak links formed by initiation, termination, or abnormal propagation processes impair the thermal or photochemical stability of polymers. 

Combining control over architecture with control over the stereochemistry of the propagation process remains a holy grail in the field of radical polymerization. Approaches to this end based on conventional polymerization were described in Chapter 8. The development of living polymerization processes has yet to substantially advance this cause. 

Images of the flame propagation process in an HCSI engine. Color scale qualitatively represents burning intensity. The presence of the spark is highlighted in the first image. 

This addition reaction can be repeated over and over in the propagation process, and each step adds two CH2 groups to the growing polymer chain. The propagation can be written in the following general form ... 

For the high resolution case, the phase-contrast effects are automatically introduced owing to the combined effect of defocus and spherical aberration, which gives rise to an image of a structure complicated by the fact that also the amplitude term, resulting from the propagation process, interacts in a non-linear way with the phase term [16,89,90,96]. 

In the propagation process of Ziegler-Natta polymerization, the insertion of olefin into a metal-carbon bond is the most important basic step, but many questions concerning to this process remained unanswered for a long time. 

Abstract. Auto-accelerated polymerization is known to occur in viscous reaction media ("gel-effect") and also when the polymer precipitates as it forms. It is generally assumed that the cause of auto-acceleration is the arising of non-steady-state kinetics created by a diffusion controlled termination step. Recent work has shown that the polymerization of acrylic acid in bulk and in solution proceeds under steady or auto-accelered conditions irrespective of the precipitation of the polymer. On the other hand, a close correlation is established between auto-acceleration and the type of H-bonded molecular association involving acrylic acid in the system. On the basis of numerous data it is concluded that auto-acceleration is determined by the formation of an oriented monomer-polymer association complex which favors an ultra-fast propagation process. Similar conclusions are derived for the polymerization of methacrylic acid and acrylonitrile based on studies of polymerization kinetics in bulk and in solution and on evidence of molecular associations. In the case of acrylonitrile a dipole-dipole complex involving the nitrile groups is assumed to be responsible for the observed auto-acceleration. 

Ideally, the Pd-OCH3+ or Pd-H+ species that initiate the catalytic cycle regenerate at the termination step of the chain propagation process. Chain transfer occurs via protonolysis or methanolysis. 

A careful analysis based on these experimental results excluded a chain-propagation process [33a]. On account of the 3-position of the methylthio or methoxy substituent in the thiophene or pyrrole rings, three isomeric dimers may be formed. The main reaction path can be deduced from the mesomeric forms of the radical cations (2)". The two most important mesomeric structures are those with the unpaired electron in... 

A radical mechanism sequence requires three distinct types of process initiation, propagation, and termination. Initiation is the formation of two radical species by bond fission, whereas propagation involves reaction of a radical with a neutral molecule, a process that leads to generation of a new radical. Because radicals are so reactive, the propagation process may continue as long as reagent molecules are available. Finally, the reaction is brought to a... 

However, the resonance forms in which the double bonds are conjugated are inherently more stable than that with the unconjugated double bonds (see Section 9.2). Accordingly, the hydroperoxide subsequently formed upon reaction with oxygen will have conjugated double bonds. Abstraction of a hydrogen atom to form the hydroperoxide is part of the chain propagation process. 

Consider propagation by polystyryl sodium ion pairs in a 1 M styrene solution in tetrahydrofuran. For an ion pair concentration of 2.0 x 10-3 M, calculate the relative contributions of contact and solvent-separated ion pairs to the propagation process. Use appropriate data from Table 5-12. 

Only active species (e.g., macro radicals or macro ions) can add further monomer molecules in the propagation process. 

Fig. 18 Schematic of the conformational changes from mushroom to brush state that occur in graft-polymerized PDMAm during the chain propagation process...

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