Polymerization and condensation

One of the drawbacks of thermal cracking in an FCC is that a high percentage of the olefins formed during intermediate reactions polymerize and condense directly to coke. 

Transformation of the initially deposited remains of living organisms started in the water columns and in the upper layers of bottom sediments of ancient seas. Random polymerization and condensation reactions of degraded biopolymers are believed to have formed the initial geopolymers, which contain humin, fulvic and humic acids. Until now these compounds (very typical for soil humus as well) are... 

The refractory compounds in the HMW DOM pool seems to be generated through abiotic reactions that act to link degradation products into macromolecules. These new chemical bonds create molecular structures that enhance the overall refractory nature of the DOM. The chemical changes lead to increased crosslinking, aromaticity, cyclization, esterification, and nitrogen depletion. The general types of chemical reactions responsible are oxidations, polymerizations, and condensations. Considerable debate exists as to whether these reactions are wholly abiotic or whether they are, at least in part, microbially mediated. 

The catalytic coke produced by the activity of the catalyst and simultaneous reactions of cracking, isomerization, hydrogen transfer, polymerization, and condensation of complex aromatic structures of high molecular weight. This type of coke is more abundant and constitutes around 35-65% of the total deposited coke on the catalyst surface. This coke determines the shape of temperature programmed oxidation (TPO) spectra. The higher the catalyst activity the higher will be the production of such coke [1],... 

Boron trifluoride etherate is used as a catalyst in many organic reactions namely, alkylation, polymerization and condensation reactions. 

The grafting methods can in principle be divided into three categories, namely, radical polymerization, ionic polymerization, and condensation or addition polymerization. Only the first case is discussed in the following since the most common grafting methods belong to this category. 

Miscellaneous polymerization and condensation reactions have been reported in some patents as generating hydrocarbons from methane, fvir example, methane when heated at I. OO C on AljOs MgAljO (impregnated wjth Mg. Cr and Ft) givesethene and benzene [281. 

The structure of the polymer obtained was determined from its IR spectrum the extent of conversion by polymerization and condensation (phenolic curing reaction) were also calculated from IR spectra. 

In contrast to the selective preservation theory, the condensation pathway proposes that humic substances are derived from the polymerization and condensation of low-molecular-weight molecules that are products of the partial microbial degradation of organic residues (Kogel-Knabner, 1993). Under this scheme of increasing complexa-tion, fulvic acids would be the first humic substances synthesized, followed by humic acids and then humin (Stevenson, 1994). The two commonly accepted condensation models are the polyphenol theory and the sugar-amine or mela-noidin theory. 

This polymerization process is a polycondensation in which the molecular weight builds up slowly as the small molecules of water are eliminated. Most step polymerization processes are polycondensations thus the terms step polymerization and condensation polymerization are often used synonymously. The stepwise reaction leads successively from monomers to dimers, trimers, and so on, until finally polymer molecules are formed. The polymers obtained are classified by taking into account the functional group of the repeating unit, for example, polyesters (— CO —O—), polyamides ( — CO— NH —), polyurethanes (—O — CO — NH —), polyethers ( — O —), and polycarbonates ( — O — CO —O —). 

Explain the difference between addition polymerization and condensation polymerization. (23.5)... 

The formation of polymers from the monomers is known as polymerization reaction. When more than one basic unit forms the polymer, the process is also named copolymerization. The polymerization reactions can be classified into two main groups, addition polymerizations and condensation polymerizations (or polycondensations). For the addition polymerizations, the resulting polymer has the repeating unit with the same molecular formula as the monomer, and the molecular mass of the polymer is the sum of the molecular masses of all the monomer molecules. For the condensation polymerizations, the resulting polymer has the repeating unit with fewer atoms than that of the monomer or monomers, and the molecular mass of the polymer is less than the sum of molecular masses of the original monomer unit or units because small molecules are eliminated following this reaction. This classification is not adequate for the characterization of the polymer itself, because the same polymer can be formed by more than one type of reaction. For example, a polyamide can be formed by addition from a lactam or by condensation from an co-aminocarboxylic acid as shown below ... 

To construct a polymer, very many monomers must add to a growing polymer molecule, and the reaction must not falter after the first few molecules have reacted. This is achieved by having the polymer molecule retain highly reactive functional groups at all times during its synthesis. The two major types of polymer growth are addition polymerization and condensation polymerization. 

In the acidic route (with pH < 2), both kinetic and thermodynamic controlling factors need to be considered. First, the acid catalysis speeds up the hydrolysis of silicon alkoxides. Second, the silica species in solution are positively charged as =SiOH2 (denoted as I+). Finally, the siloxane bond condensation rate is kinetically promoted near the micelle surface. The surfactant (S+)-silica interaction in S+X 11 is mediated by the counterion X-. The micelle-counterion interaction is in thermodynamic equilibrium. Thus the factors involved in determining the total rate of nanostructure formation are the counterion adsorption equilibrium of X on the micellar surface, surface-enhanced concentration of I+, and proton-catalysed silica condensation near the micellar surface. From consideration of the surfactant, the surfactants first form micelles as a combination of the S+X assemblies, which then form a liquid crystal with molecular silicate species, and finally the mesoporous material is formed through inorganic polymerization and condensation of the silicate species. In the S+X I+ model, the surfactant-to-counteranion... 

A fourth theory postulates that stream humic substances are formed by a unique stream humification process, whereby simple reactive moieties are polymerized and condensed into humic substances unique to the stream environment. 

Figure 3.9. Hypothetical reaction process illustrating transfonnation of an initially precipitated Ni-Al LDH into a phyllosilicate-like phase during aging. The initial step involves the exchange of dissolved silica for nitrate within the LDH interlayer followed by polymerization and condensation of silica onto the octahedral Ni-Al layer. The resulting solid possesses structural features common to 1 1 and 2 1 phyllosilicates. (From Ford et al., 2001.)...

There are two fundamental polymerization mechanisms. Classically, they have been differentiated as addition polymerization and condensation polymerization. In the addition process, no by-product is evolved, as in the polymerization of vinyl chloride (see below) whereas in the condensation process, just as in various condensation reactions (e.g., esterification, etherification, amidation, etc.) of organic chemistry, a low-molecular-weight by-product (e.g., H2O, HCl, etc.) is evolved. Polymers formed by addition polymerization do so by the successive addition of unsaturated monomer units in a chain reaction promoted by the active center. Therefore, addition polymerization is called chain polymerization. Similarly, condensation polymerization is referred to as step polymerization since the polymers in this case are formed by stepwise, intermolecular condensation of reactive groups. (The terms condensation and step are commonly used synonymously, as we shall do in this book, and so are the terms addition and chain. However, as it will be shown later in this section, these terms cannot always be used synonymously. In fact, the condensation-addition classification is primarily applicable to the composition or structure of polymers, whereas the step-chain classification applies to the mechanism of polymerization reactions.)... 

Most (but not all) of the step polymerization processes involve polycondensation (repeated condensation) reactions. Consequently, the terms "step polymerization" and "condensation polymerization" are often used synonymously. In a condensation reaction between two molecules, each molecule loses one atom or a group of atoms at the reacting end, which leads to the formation of a covalent bond between the two, while the eliminated atoms bond with each other to form small molecules such as water—hence the term condensation reactions. Consider, for example, the synthesis of a polyamide, i.e., a polymer with amide -(-CONH-)- as the characteristic... 

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