Polymerization reactions, chain reaction

Azobisnittiles are efficient sources of free radicals for vinyl polymerizations and chain reactions, eg, chlorinations (see Initiators). These compounds decompose in a variety of solvents at nearly first-order rates to give free radicals with no evidence of induced chain decomposition. They can be used in bulk, solution, and suspension polymerizations, and because no oxygenated residues are produced, they are suitable for use in pigmented or dyed systems that may be susceptible to oxidative degradation. 

Vinyl addition polymerizations, being chain reactions, are more complex than condensation polymerizations and they are generally more likely to include minor side reactions leading to branching or cross-linking. The structures of the pol3oneric products are consequently more difficult to establish with certainty. 

Porous organic polymers (Figure 10) are potential electrochromatographic stationary phases for the analysis of pharmaceuticals. The polymer network is generally formed inside the capillary by a stepwise chain polymerization reaction. Polymerization reaction mixtures usually consist of a combination of monomers and cross-linker, initiator, and a porogenic mixture of solvents. 

Note 2 The adjective chain in chain polymerization denotes chain reaction rather than polymer chain . 

O. Ito, in Z. B. Alfassi (ed.), Free Radical Polymerization and Chain Reactions in General Aspects of the Chemistry of Radicals, John Wiley and Sons, Inc., New York, 1999, p. 209. 

Polymer synthesis is a complicated process that can be carried out applying different methodologies starting from appropriate monomers [193-196], The most important methods of polymer synthesis are step-growth polymerization and chain reaction polymerization [194,196],... 

Chain. A linear or branched macromolecule is often called a chain because the repeating units are joined together like links in a chain. Many polymers are polymerized by chain reactions, which arc characterized by a scries of successive reactions initiated by a single primary event. Here the term chain is used to designate a kinetic sequence of reaction events which results in the production of a molecular chain composed of linked repeating units. 

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 ... 

Tetrafunctlonal monomers or polymers with vinyl side chains offer the unique possibility of quantum amplification by spatial polymerization. This reaction Is also shown In Figure 6. Multlcentered polymerization by chain reaction provides high speed and the mechanical-physical strength of a crossllnked polymer. However, because of the rapid Increase In viscosity, the amplification may not be as large as expected. 

Unlike many other polymerization reactions, chain transfer with monomer cannot occur. However, many compounds (proton donors in particular) function as chain-transfer agents when added to the system. In general, the mechanism of the chain-transfer reaction can be depicted this way ... 

In addition polymerization, a chain reaction appends new monomer units to the growing molecule one at a time. Each new unit creates an active site for the next attachment. The polymerization of ethylene gas (C2H4) is a typical example. The process begins with a monomer of ethylene gas in which the carbon atoms are joined by covalent bonds as below ... 

In a general sense, photoinitiated polymerizations are chain reaction mechanisms that include initiation, propagation, and termination steps during which a liquid solution is commonly converted into a solid polymer or highly cross-linked struaure when multifunctional monomers or macromers are used (Scheme 1). 

F and MF are typical precursors to furan monomers bearing a moiety which can be polymerized by chain-reaction mechanisms. Scheme 6.11 provides a non-exhaustive array of entries into such structures, which have all been synthesized, characterized and polymerized [4], A major exception to this general postulate is constituted by furfuryl alcohol (2-hydroxymethylfuran, FA), which is in fact still today the most important commercially available furan compound, obtained by the catalytic reduction of F involving more than 80 per cent of its world production. FA is widely used as a polycondensation monomer and does not therefore belong to the class of coirpounds shown in Scheme 6.11. 

The monomers appearing in Scheme 6.11 are all susceptible to polymerize via chain reactions but, according to their specific structure, their actual response to different types of initiation varies considerably. The organization of... 

Many authors divide polymerizations into chain reactions and stepwise reactions. Of course, all reactions proceed in steps, that is, one reaction step follows another. Termolecular reactions are rare but in the organic chemistry sense, the term stepwise reaction signifies that intermediate products can be isolated and subsequently again made to react. This means that, in the absence of impurities, certain reactions can be frozen. In actual fact, anionic addition polymerizations can be frozen at low temperatures and made to run again at higher temperatures. That this procedure is not possible in the presence of water or carbon dioxide is an experimental consideration and not a conceptual difficulty. If we lived in an isocyanate atmosphere, no steps could be isolated in the polyamide synthesis. Thus, such classifications are based on experimental expertise, which can never be the basis of a physical definition. 

In addition polymerization (sometimes called chain-growth polymerization), a chain reaction adds new monomer units to the growing polymer molecule one at a time through double or triple bonds in... 

Addition Polymerization Chemical chain-reaction process in which monomers bond to each other in the step-by-step growth of a polymer, without generating any by-products. 

A distinct characteristic of photoinitiated cationic polymerization is its lack of sensitivity toward atmospheric oxygen that does not scavenge cationic species, as well as its living character, because the propagating polymer cations are not reacting among themselves. In contrast to radical-initiated polymerization, the chain reaction will thus continue to proceed in the dark after the UV exposure, a feature that has been used to assemble nontransparent materials by UV-curable laminating adhesives. 

Metal catalyzed polymerizations are chain reactions necessitating a constant valence of the metal. In contrast to that metal catalyzed polycondensations are step growth reactions involving a change of the valence of the metal. Tuning of the reaction by structural variations of the metal catalyst are demonstrated with the Pd-catalyzed vinyl polymerization of norbomene, the alternating copolymerization of ethylene with carbon monoxide and the Heck reaction as examples. One and two electron processes can be involved in metal catalyzed polycondensations. The Heck reactions, the Ni-catalyzed synthesis of polyphenylenes, and Ru-catalyzed ArH-insertion reactions are discuss. ... 

The first stage, chain initiation, is the dissociation of chlorine molecules into atoms this is followed by two chain propagation reactions. Two molecules of hydrogen chloride are produced and the ejected chlorine atom is ready to react with more hydrogen. The final steps, chain termination, stop the reaction. Chain reactions are important in certain types of free-radical POLYMERIZATION reactions. 

By the kinetic scheme governing the polymerization reaction chain versus stepwise reactions... 

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