Polymer formation, competition

As seen from the above reaction steps, the DVB procedures for star-branched polymer formation consists of several competitive and consecutive reactions. It was the aim of our work to study various reaction variables which effect this process. As mentioned earlier, these include DVB/RLi molar ratio, reaction... 

Other systems Additions of perfluoro-2-butyne [143] and acetylene dicarboxylic ester [139] to perfluoro-aromatics will also occur (see Table 9.9 and Chapter 7, Section lllB). The extending anion may be trapped, and the more reactive the aromatic compound used, the more effective the competition with polymer formation (Figure 9.57) [144]. 

These two early findings lead to the concept of competitive ablation and polymerization (CAP) which emphasizes the importance of a balance between polymer formation and ablation [3]. The first finding demonstrated the control of ablation due to extremely reactive fluorine-related species (atomic fluorine, F , etc.) by chemical reactions, and the second demonstrated the role of discharge conditions that control the production of highly ablative species. 

Because polymer formation and ablation are competitive and opposing processes, polymer-forming plasma has the least ablative effect however, ablation in such plasmas cannot be completely ruled out. Sputtering of metals used as the internal electrodes for plasma polymerization has been recognized as a contamination of plasma polymers. Under certain conditions, the sputtering of the electrode materials becomes significant and plays an important role in the engineering of interface as described in Chapter 9. 

The presence of octamethylcyclotetrasilazane and bicyclic dimer in the pyrolysis products strongly suggests that at least two competitive reactions take place during plasma polymerization of HMCTSN namely, ring enlargement and dimerization. However, dimerization seems to be predominant and it is considered to be the first step of reaction leading to polymer formation. 

With the objective of promoting polymer formation at the surface of Ti02 pigments and prevent secondary nucleation, Haga et al. used a diazoic amidinium initiator previously anchored on the mineral surface [213], whereas Janssen used redox initiators [208]. Although real benefit was taken from the nature of the initiator, in particular in the case of hydrophilic monomers like MMA, there was still a competition between the formation of surface polymer and free latex particles in these systems. In both cases, better results were obtained when the monomer was introduced under starved-feed conditions, which enabled a significant decrease in the extent of secondary nucleation. 

Emulsion polymerization is the polymerization technique that starts with emulsified monomer in the continuous aqueous phase. Polymer formation takes place in the micelles and is initiated by water-soluble initiators. The monomers are insoluble or sparingly soluble in water. Emulsion polymerization is used very frequently in order to perform encapsulation of inorganic particles with polymers where water-based coatings are required. For the encapsulation of inorganic particles, seeded emulsion polymerization is performed hydrophobic inorganic particles are dispersed with normal surfactants or protective colloids in the aqueous phase. As polymerization on the surface of inorganic particles is always in competition with secondary particle formation, the concentration of the surfactants should be lower than their critical micelle concentration. However, homogeneous nucleation can also occur, which... 

Product mixtures from radical chain addition of hydrogen chloride to alkenes are much more complicated than is the case for hydrogen bromide. The problem is that the rate of abstraction of hydrogen from hydrogen chloride is not large relative to addition of the alkyl radical to the alkene. This results in the formation of low-molecular-weight polymers in competition with simple addition. 

As far as the electrical conditions are concerned, the polymers can be deposited under potentiostatic or galvanostatic conditions or by cyclic potential sweeps. When potentials or current densities beyond the optimum values are employed, the conductivity of the relevant polymers decreases this is due to the fact that the PHCs are overoxidized at the potentials used for their formation [47,48]. When the process is carried out at high anodic potentials or when the concentration of the monomer in the solution becomes too low and the rate of polymer deposition imposed by the applied electrical conditions cannot be sustained, degradation of the polymer may be competitive with polymer formation [49]. A decrease of the oxidation potential produces an increase of the conductivity and, in parallel, a bathochromic shift of the optical absorption maximum of the deposited polymer, indicative of an extension of conjugation. 

One of the most common outcomes when an RCM reaction fails is the competing CM of the substrate to form linear or cychc dimers, or potentially ohgomers or even polymers. The competition between RCM and CM in the metathesis of dienes will depend on a number of factors, predominantly related to the structure of the target cycloalkene. The strain introduced (AH composed of angle strain, transannular strain, and torsional strain) will depend on the substitution pattern of the diene, while the loss of entropy (AS) will depend on the number of rotors frozen in the product that were otherwise free to rotate in the substrate. The formation of macrocycles will predominantly be influenced by AS, as rings greater than c. 12 members in size typically suffer relatively little strain. Macrocycle formation is not considered in detail here, but has been reviewed recently this section will focus on rings of 5-10 members. 

There are numerous references in the literature to irreversible adsorption from solution. Irreversible adsorption is defined as the lack of desotption from an adsoibed layer equilibrated with pure solvent. Often there is no evidence of strong surface-adsorbate bond formation, either in terms of the chemistry of the system or from direct calorimetric measurements of the heat of adsorption. It is also typical that if a better solvent is used, or a strongly competitive adsorbate, then desorption is rapid and complete. Adsorption irreversibility occurs quite frequently in polymers [4] and proteins [121-123] but has also been observed in small molecules and surfactants [124-128]. Each of these cases has a different explanation and discussion. 

Materia] Selector Access to (1) GE Select, a comprehensive database in Microsoft Windows format of the family of GE polymers which allows users to sort for the GE product families and grades of materials that will best meet the specified property ranges, and (2) CAMPUS, a worldwide database for plastic materials with uniform global protocol for acquiring and comparing data on competitive plastic materials. 

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