Network formation reaction scheme

Recently the polymeric network (gel) has become a very attractive research area combining at the same time fundamental and applied topics of great interest. Since the physical properties of polymeric networks strongly depend on the polymerization kinetics, an understanding of the kinetics of network formation is indispensable for designing network structure. Various models have been proposed for the kinetics of network formation since the pioneering work of Flory (1 ) and Stockmayer (2), but their predictions are, quite often unsatisfactory, especially for a free radical polymerization system. These systems are of significant conmercial interest. In order to account for the specific reaction scheme of free radical polymerization, it will be necessary to consider all of the important elementary reactions. 

One subtle comparison between the thermograms was that in both systems the reaction scheme seemed to be identical. The DSC thermograms showed similar behavior during network formation. The thermoplastic did not seem to alter the process. The reaction mechanism was not dependent on the linear polymer incorporated. This reinforced the notion that a true SIPN was attained in this experiment. 

A formal reaction scheme, given by Germain and Laugier to describe the formation of all observed partial oxidation products, is shown below. This scheme comprises three reaction paths corresponding to side chain oxidation, oxidative coupling and direct oxidation of the nucleus as initiating reactions. In reality, an even more complex reaction network is to be expected. 

Reaction Schemes and Networks. Within the last few years a series of review articles have appeared concerning the oxidation of propylene to acrolein (10-16). It is generally assumed that the first reaction step, the formation of an adsorbed allylic species, is rate-determining for the formation of acro-... 

A similar diagram was presented in a previous study for the impromoted PC oxidation of phenol (Salaices et al., 2004). The reaction scheme introduced in this chapter incorporates all carboxylic acids detected in the oxidations of the various aromatic species, as well as the existing relationships among the intermediate species. A very important fact is that this newly developed reaction network describes the Fe-assisted PC oxidation of phenol as well as the unpromoted PC reaction. One important difference between the reaction scheme for the impromoted PC reaction and that of the Fe-assisted PC reaction is the step relating the formation of 1,2,4-THB from... 

Although the major interest in experimental and theoretical studies of network formation has been devoted to elastomer networks, the epoxy resins keep apparently first place among typical thermosets. Almost exclusively, the statistical theory based on the tree-like model has been used. The problem of curing was first attacked by Japanese authors (Yamabe and Fukui, Kakurai and Noguchi, Tanaka and Kakiuchi) who used the combinatorial approach of Flory and Stockmayer. Their work has been reviewed in Chapter IV of May s and Tanaka s monograph Their experimental studies included molecular weights and gel points. However, their conclusions were somewhat invalidated by the fact that the assumed reaction schemes were too simplified or even incorrect. It is to be stressed, however, that Yamabe and Fukui were the first who took into account the initiated mechanism of polymerization of epoxy groups (polyetherification). They used, however, the statistical treatment which is incorrect as was shown in Section 3.3. 

Carbyne complexes containing the Mn=CR moiety undergo an interesting variety of reactions. " Scheme 13 provides a sampler of these reactions. The electrophilic nature of the carbyne in the cationic Cp(CO)2Mn=CR" has been exploited in cycloadditions, metatheses, and ketene formation methodologies. Extended chains containing unsaturated carbon networks that are truncated by metal... 

Fig. 14. Reaction scheme for the network formation processes in the free-radical monovinyl-divinyl co-polymerization...

Scheme 3.2. Molecular probes used for UV absorption and fluorescence monitoring of tbe reactions for network formation.

Kinetic approaches represent realistic and comprehensive description of the mechanism of network formation. Under this approach, reaction rates are proportional to the concentration of unreacted functional groups involved in a specific reaction times an associated proportionality constant (the kinetic rate constant). This method can be applied to the examination of different reactor types. It is based on population balances derived from a reaction scheme. An infinite set of mass balance equations will result, one for each polymer chain length present in the reaction system. This leads to ordinary differential or algebraic equations, depending on the reactor type under consideration. This set of equations must be solved to obtain the desired information on polymer distribution, and thus instantaneous and accumulated chain polymer properties can be calculated. In the introductory paragraphs of Section... 

Fig. 13 (a) Reaction scheme for the preparation of the CDTE based tetra-linker 1. (b) Network formation of 3 via DA/rDA chemistry during the deprotection reaction and the cycling process, (c) Pressed pellet of 3. (d) Broken specimen after rheology experiments, (e) Healed specimen with identical mechanical properties as the original one. Reproduced with permission from [60]... 

Under hydro(solvo)thermal conditions, Cd and Zn coordination networks ean be obtained by reactions of metal salts with cyanopyridine or pyiidinecarboxaldehyde. Cyano-, carboxal-dehyde-, and ester-substituents slowly hydrolyze to form corresponding earboxylie acid facilitating network formation. For instance, bis[4- 2-(4-pyridyl)ethenyl benzoato]-Zn° and Cd with eightfold diamondoid network structures were obtained by slow hydrolysis of (E)-4-(4-cyanos-tyryl)pyridine under hydro(solvo)thermal conditions (3-D diamondoid net Scheme 6a). ... 

This substantiates the positive role of O2 in promoting benzaldehye and benzoic acid formation. From the above-mentioned two examples, it is apparent that there are many parallel reactions in the aerobic oxidation of benzyl alcohol, besides the direct dehydrogenation of benzyl alcohol to benzaldehyde, which result in the undesired by-products. On the basis of these studies and other literature evidence, a network of reactions occurring during the aerobic oxidation of benzyl alcohol over Pd/AljOj catalyst is presented in Scheme 12.1. After elucidating the mechanisms of reactions that lead to by-products, the next step in catalyst development is the complete elimination or at least suppression of these byproducts. It is logical to assume that all these reactions may not have the same active sites, and it is important to identify the different active sites for different reactions. 

The curing process is the set of chemical reactions that leads to the formation of a highly crosslinked 3D network For epoxy/amine the chemical process that leads to network formation can be described according to the scheme ... 

Photochemical reactions occur where and when irradiation is carried out. The spatial and temporal control thus makes them the favorite choice for many polymer transformations. The well known photolabile ort/zo-nitrobenzyl group has been applied to the generation of bases. This method has been found convenient for causing patterning and two stage polymer network formation (Scheme 18). ... 

Figure 5.4. A reaction scheme differentiating an IPN from an IPS. The IPS is a variation on the semi-SIN idea, where the two monomers are mixed, followed by a sequential polymerization. The first polymer is crosslinked, and the second linear.Mi, M2—Addition of monomers Ci, C2—addition of crosslinking agents Xi, X2—addition of catalysts Pi, P2— formation of polymer network L2—formation of linear polymer —triisocyanurate ring.

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