Polymerization condensation/step-reaction

Characteristic Condensation (step-reaction) polymerization Chain reaction (vinyl-type) polymerization... 

The process or chemical reaction in which the molecules of a monomer are linked together to form macromolecules whose molecular weight is a multiple of that of the original substance nM [M] , where n is a degree of polymerization, DP. It is said that the polymerization leads to oligomer if 10 < DP < 50 and to polymer if DP > 50. When two or more monomers are involved, the process is called copolymerization. Most polymerization processes are classified as condensation (step) reactions or addition (chain) reactions. 

There are two primary polymerization approaches step-reaction polymerization and chain-reaction polymerization. In step-reaction (also referred to as condensation polymerization), reaction occurs between two polyfunctional monomers, often liberating a small molecule such as water. As the reaction proceeds, higher-molecular-weight species... 

The most common form of step growth polymerization is condensation polymerization. Condensation polymers are generally formed from simple reactions involving two different monomers. The monomers are difunctional, having a chemically reactive group on each end of their molecules. Examples of condensation polymerization are the formation of nylon 66, a polyamide, and of poly(ethylene terephthalate), a polyester. Because condensation poly-... 

Failure to remove the alcohols generated in either of the equilibrium condensation steps will reduce the efficiency of the polymerization process. This effect can be explained by Le Chatelier s principle, which was discussed in Chapter 3. The volatile alcohols produced during polymerization act as a chemical stress on the product side of the reaction, which inhibits polymerization. Another implication of the equilibrium nature of this polymerization process is seen in the molecular weight distribution of the final polymer. All polyesters contain a few percent of low molecular weight oligomers, regardless of the polymerization process. 

Poly(3HB) is synthesized in bacteria from acetyl-CoA by a three-step reaction (Fig. 1). The first enzyme of the pathway, 3-ketothiolase, catalyzes the condensation of two molecules of acetyl-CoA to form acetoacetyl-CoA. Aceto-acetyl-CoA reductase subsequently reduces acetoacetyl-CoA to R-3-hydroxy-butyryl-CoA, which is then polymerized by the PHA synthase to produce poly(3HB). Since acetyl-CoA is present in plant cells in the cytosol, plastid, mitochondrion, and peroxisome, the synthesis of poly(3HB) in plants could, in... 

The urea-formaldehyde polymer is formed by a multi-step reaction process between urea and formaldehyde. The initial phase is a methylolation of the urea under slightly alkaline conditions with a formaldehyde-urea (F/U) molar ratio of 2.0 1 to 2.4 1. Condensation of the methylolureas from the methylolat ion reaction is at atmospheric reflux with a pH of 4 to 6. This condensation polymerization continues to a pre-determined viscosity, at which time the pH is adjusted with a suitable base to 7-3 to 8.0. The adhesive is then concentrated to a total solids content of 50 to 60 percent by vacuum distillation. Additional urea is then normally added to produce a final F/U molar ratio of 1.6 1 to 1.8 1. 

Domine and Gogos (88-90) considered a very long, very wide, and thin mold being fed by a constant temperature mixture of AA, BB molecules. Both types are bifunctional and the feed has a molecular weight Mq. The polymerization, assumed to be reversible, proceeds by the reaction of A-ends with B-ends, and follows idealized step polymerization (condensation) kinetics without the generation of a small molecule (91). Specifically, we have... 

In a polymerization process the chain length distribution or molar mass distribution (MMD) is influenced by a large number of factors and conditions the kinetics of the reaction plays a very important role. The calculation of the resulting MMD is thus very complicated. For one of the simplest cases, a step reaction with polycondensation, a first-order approach is given here. As an example we take a hydroxy acid HO-R-COOH, which, upon condensation, forms the chain -[-O-R-CO-]n. 

Step polymerization (condensation reactions) polyetheremide, polyesters, polyethylterephthalate (PET), polybutylterephtalate (PBT), polyamide 6.6, polyarylate, polyurethanes, polyamide 6... 

Carothers classification (condensation vs. addition) is primarily based on the composition or structure of polymers. The second classification (chainwise vs. stepwise) was proposed by P. J. Floiy, and is based on the kinetic scheme or mechanism governing the polymerization reactions. Step reactions are those in which the chain growth occurs in a slow, stepwise manner. Two monomer molecules react to form a dimer. The dimer can then react with another monomer to form a trimer, or with another dimer to form tetramer. Thus, the average molecular weight of the system increases slowly over a period of time. This is exemplified by the following polyesterification ... 

Polymerization reactions are divided into two groups known as step reactions (also called condensation reactions) and chain reactions, also known as addition reactions. Step reactions require bifunctional or polyfunctional monomers, while chain reactions require the presence of an initiator. 

There is an alternative, somewhat less meaningful system of classification addition polymerization, in which molecules of monomer are simply added together and con-iensation polymerization, in which monomer molecules combine with loss of some simple nolecules like water. As it happens, the two systems almost exactly coincide nearly all ases of chain-reaction polymerization involve addition polymerization nearly all cases pf step-reaction polymerization involve condensation polymerization. Indeed, some hemists use the term addition polymerization to mean polymerization via chain reactions. 

The reactions occurring for multivalent metal cations occur over many condensation steps and may not obey the kinetic rules outlined above. Silicate polymerization is known to depend on the presence of oligomeric ion species. - - Gibbsite (formally Al(OH)3) is known not to precipitate sufficiently rapidly from supersaturated solutions, but needs seeding for particle growth Moreover, depending on the temperature and the composition of the solution, different solid species may form. This is illustrated in Figure 8.21. ... 

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