Polymerization, free-radical addition

The reaction engineering aspects of these polymerizations are similar. Excellent heat transfer makes them suitable for vinyl addition polymerizations. Free radical catalysis is mostly used, but cationic catalysis is used for non-aqueous dispersion polymerization (e.g., of isobutene). High conversions are generally possible, and the resulting polymer, either as a latex or as beads, is directly suitable for some applications (e.g., paints, gel-permeation chromatography beads, expanded polystyrene). Most of these polymerizations are run in the batch mode, but continuous emulsion polymerization is common. 

In addition polymerization, simple molecules or monomers are added to each other to form long-chain molecules (polymers) without by-products, thus yielding a polymeric product in which the molecular formula of the repeating unit is identical to that of the monomer. The molecular weight of the polymer so formed is thus the total of the molecular weights of all of the combined monomer units. There are three commonly used types of addition polymerization free-radical polymerization, cationic polymerization, and anionic polymerization, which are described below. An example of addition polymerization is given by ... 

Significant improvement in controlled polymerizations of a variety monomers, including styrene, acrylates, acrylamide, acrylonitrile, 1,3-dienes, and maleic anhydride has been achieved when alkoxyamines have been used as initiators for living, free radical polymerization.(696c, 697) Alkoxyamines can be easily synthesized in situ by the double addition of free radicals, generated by thermal decomposition of an azo-initiator, such as 2,2 -azo-h/.s-/.so-butyronitrile (AIBN), to nitrones (Scheme 2.206). 

Conjugated dienes such as 1,3-butadiene very readily polymerize free radically. The important thing to remember here is that there are double bonds still present in the polymer. This is especially important in the case of elastomers (synthetic rubbers) because some cross-linking with disulfide bridges (vulcanization) can occur in the finished polymer at the allylic sites still present to provide elastic properties to the overall polymers. Vulcanization will be discussed in detail in Chapter 18, Section 3. The mechanism shown in Fig. 14.3 demonstrates only the 1,4-addition of butadiene for simplicity. 1,2-Addition also occurs, and the double bonds may be cis or trans in their stereochemistry. Only with the metal complex... 

Free-Radical Addition. In free-radical addition polymerization, the propagating species is a free radical. The free radicals, R-, are most commonly generated by the thermal decomposition of a peroxide or azo initiator, I (see Initiators, free-radical) ... 

Addition Homo-Polymerization (Free-Radical Homo-Polymerization)... 

Radical Polymerization. Free radical polymerization consists of three steps, initiation, propagation, and termination or chain transfer. Initiation consists of two steps, decomposition of the initiator to form a radical species, followed by addition of the initiator to the first monomer unit. 

As a result of the advances in catalyst discovery for aqueous ethylene polymerization, silica-polyethylene nancomposites have been prepared with structures that vary with changing catalyst structure and silica composition." It is likely that many more advances in the area of high-tech composites with potential biological and nanotechnology applications will be made in the next few years through aqueous polymerization processes. In addition to free radical polymerizations and catalytic polymerizations, it should be noted that oxidative polymerizations can also be performed in aqueous media to yield conducting polymers. Recently, this has been used to prepare polypyrrole-coated latex particles that are expected to be interesting synthetic mimics for micrometeorites. 

As previously mentioned, in addition to free radical initiation, ethylene may be polymerized by use of transition metal catalysts. To place the importance of these catalysts in proper perspective, one must recognize that transition metal catalysts were used to produce about 73% of the global industrial output of polyethylenes in 2008 or about 56 million tons (124 billion pounds). 

Polymerization involves addition of free radicals to the double bond of the monomer addition, first, of the free radical generated from the initiator, and then of the growing polymer molecule. This is, of course, an example of chain-reaction polymerization. 

By the way, it should be noted that Barlett and Nozaki included sodium pyrophosphate in their formulation to counteract the decrease in pH as a persulfate-initiated polymerization proceeds. At a low pH, the monomer is said to be susceptible to hydrolysis [18]. Work by Rinby and co-workers has indicated that the nature of the addition of free radicals such as H2N and OH is profoundly influenced by changes in the pH of the medium fix>m 1.4 to approximately 7.8 [53-55]. The radicals formed from persulfate in emulsion systems may be expected to be influenced by pH variations also. 

The growing polymer in chain-reaction polymerization is a free radical, and polymerization proceeds via chain mechanism. Chain-reaction polymerization is induced by the addition of free-radical-forming reagents or by ionic initiators. Like all chain reactions, it involves three fundamental steps initiation, propagation, and termination. In addition, a fourth step called chain transfer may be involved. 

Addition (or free radical) polymerization Polymerization in which monomers are added to the growing chains, initiated by free radical agents. 

For isoprene rubber, the abstraction route predominates over radical addition. Two polymeric free radicals then unite to give a crosslink. 

Crosslinks could also form by a chain reaction, which involves the addition of polymeric free radicals to double bonds (Loan, 1967,1963). 

On the average, the chains formed as a result by addition to the new free radical site will be as long as the backbone chain itself. Similar branches may sometimes arise due to transfer of a growing polymeric free radical with a monomer molecule, and in termination by disproportionation. (See Appendix A.)... 

The hrst hve chapters (Part 1) present an overview of some methods that have been used in the recent hterature to calculate rate constants and the associated case studies. The main topics covered in this part include thermochemistry and kinetics, computational chemistry and kinetics, quantum instanton, kinetic calculations in liquid solutions, and new applications of density functional theory in kinetic calculations. The remaining hve chapters (Part II) are focused on apphcations even though methodologies are discussed. The topics in the second part include the kinetics of molecules relevant to combustion processes, intermolecular electron transfer reactivity of organic compounds, lignin model compounds, and coal model compounds in addition to free radical polymerization. 

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