Chain-growth chemistries

An important application of photochemical initiation is in the determination of the rate constants which appear in the overall analysis of the chain-growth mechanism. Although we shall take up the details of this method in Sec. 6.6, it is worthwhile to develop Eq. (6.7) somewhat further at this point. It is not possible to give a detailed treatment of light absorption here. Instead, we summarize some pertinent relationships and refer the reader who desires more information to textbooks of physical or analytical chemistry. The following results will be useful ... 

Environmental Considerations. Environmental problems in Ziegler chemistry alcohol processes are not severe. A small quantity of aluminum alkyl wastes is usually produced and represents the most significant disposal problem. It can be handled by controlled hydrolysis and separate disposal of the aqueous and organic streams. Organic by-products produced in chain growth and hydrolysis can be cleanly burned. Wastewater streams must be monitored for dissolved carbon, such as short-chain alcohols, and treated conventionally when necessary. 

Polylactides, 18 Poly lactones, 18, 43 Poly(L-lactic acid) (PLLA), 22, 41, 42 preparation of, 99-100 Polymer age, 1 Polymer architecture, 6-9 Polymer chains, nonmesogenic units in, 52 Polymer Chemistry (Stevens), 5 Polymeric chiral catalysts, 473-474 Polymeric materials, history of, 1-2 Polymeric MDI (PMDI), 201, 210, 238 Polymerizations. See also Copolymerization Depolymerization Polyesterification Polymers Prepolymerization Repolymerization Ring-opening polymerization Solid-state polymerization Solution polymerization Solvent-free polymerization Step-grown polymerization processes Vapor-phase deposition polymerization acid chloride, 155-157 ADMET, 4, 10, 431-461 anionic, 149, 174, 177-178 batch, 167 bulk, 166, 331 chain-growth, 4 continuous, 167, 548 coupling, 467 Friedel-Crafts, 332-334 Hoechst, 548 hydrolytic, 150-153 influence of water content on, 151-152, 154... 

Since COMC II (1995), published work in this area has included several studies of the influence of catalyst and monomer structure on the dehydropolymerization of silanes, as this likely affect the chemistry of polymer chain growth. 

Photochemically-generated radicals are encountered as reactive intermediates in many important systems, being a major driving force in the photochemistry of ozone in the upper atmosphere (stratosphere) and the polluted lower atmosphere (troposphere). The photochemistry of organic carbonyl compounds is dominated by radical chemistry (Chapter 9). Photoinitiators are used to form radicals used as intermediates in the chain growth and cross-linking of polymers involved in the production of electronic circuitry and in dental treatment. 

Nearly all synthetic polymers are synthesized by the polymerization or copolymerization of different "monomers." The chain growth process may involve the addition chain reactions of unsaturated small molecules, condensation reactions, or ringopening chain-coupling processes. In conventional polymer chemistry, the synthesis of a new polymer requires the use of a new monomer. This approach is often unsatisfactory for Inorganic systems, where relatively few monomers or cyclic oligomers can be Induced to polymerize, at least under conditions that have been studied to date. The main exception to this rule is the condensation-type growth that occurs with inorganic dl-hydroxy acids. 

A total of three equivalents of DIBAH and two equivalents of CO are consumed in each run with eventual precipitation of (C5H5)2ZrCl2. The sequence can be repeated with no loss of activity. This system may provide valuable insight into the notion of chain growth in CO reduction chemistry despite the fact that H2 is not employed directly as the reductant. 

The polymerization of some monomers does not fall neatly into either of the mechanisms discussed above. We will take up a few of them (e.g., anionic and coordination polymerizations) after we further develop step-growth and chain-growth polymerizations. Some polymerizations can proceed by either mechanism, depending upon the specific monomer or the reaction conditions. The most notable examples, ring-opening polymerization and some of the newer chemistries, are presented as separate categories toward the end of the chapter. 

Now it s time to learn more about the chemistry of chain-growth reactions. To begin, let s consider the most prevalent type of chain reaction, free radical polymerization. A free radical is defined as a species having one unpaired electron. For example, the methyl free radical would look like this ... 

It is important to appreciate that polymer produced by an anionic chain-growth mechanism can have drastically different properties from one made by a normal free radical reaction. Block copolymers can be synthesized in which each block has different properties. We mentioned in Chapter 4 that Michael Szwdrc of Syracuse University developed this chemistry in the 1950s. Since that time, block copolymers produced by anionic polymerization have been commercialized, such as styrene-isoprene-styrene and styrene-butadiene-styrene triblock copolymers (e.g., Kraton from Shell Chemical Company). They find use as thermoplastic elastomers (TPE), polymers that act as elastomers at normal temperatures but which can be molded like thermoplastics when heated. We will discuss TPEs further in Chapter 7. 

On Tuesday, introduce a discussion on the type of polymer—elastomer, chain-growth, step-growth, fiber, or copolymer—and introduce some of the actual chemistry of polymers. 

The fundamental chemistry of the structural adhesives described here can change very little. Vinyl and acrylic monomers polymerize by chain growth polymerization initiated by free radicals or ions. Isocyanate and epoxy compounds react with compounds containing active hydrogen in step growth polymeriza-... 

Bruice PY. (1995) Chain growth polymers. Organic Chemistry, pp. 1013-1019. Prentice Hall, New Jersey. 

In this review, we limit ourselves to the mechanisms of primary product formation, which are fimdamental to Fischer-Tropsch chemistry. Using new information mainly from computational studies, we focus on two coidlicting hypotheses regarding the key reaction steps that lead to chain growth. 

Iron is also an important Fischer-Tropsch catalyst, but in the active state, it is present as a carbide (70,71) which is characterized by a unique chemistry that we do not discuss. Computations concerning the Fischer-Tropsch reaction on iron were performed by Bromfield et al. (72) and by Lo and Ziegler (73), who investigated the chain-growth reaction. 

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