Ethylene, chain growth polymerizations

Addition or chain-growth polymerization involves the opening of a double bond to form new bonds with adjacent monomers, as typified by the polymerization of ethylene to polyethylene ... 

In general, there are two distinctively different classes of polymerization (a) addition or chain growth polymerization and (b) condensation or step growth polymerization. In the former, the polymers are synthesized by the addition of one unsaturated unit to another, resulting in the loss of multiple bonds. Some examples of addition polymers are (a) poly(ethylene), (b) poly(vinyl chloride), (c) poly(methyl methacrylate), and (d) poly(butadiene). The polymerization is initiated by a free radical, which is generated from one of several easily decomposed compounds. Examples of free radical initiators include (a) benzoyl peroxide, (b) di-tert-butyl peroxide, and (c) azobiisobutyronitrile. 

Step-growth polymerization, 22, 24-25, 23, 84-86, 86,90-92,114-115, 261 compared with chain-growth polymerization, 88-89, 88-89 interfacial polymerization, 91-92 laboratory activities on synthesis of nylon, 228-230 synthesis of polyesters in the melt, 231-233 synthesis of polyurethane foam, 234-237 molar mass and, 86, 86 polycondensation of poly ethylene terephthalate), 90-91 polymers produced by, 86 types of monomers for, 90 Stereochemistry, 28, 37-39,41-42, 70 tacticity, 103-105 Stereoisomers, 41 Stereoregularity, 70 Stiffness, 142, 261 Strain, 142-143, 261 Strength... 

Ethylene is also polymerized by free-radical chain-growth polymerization. With ethylene, the free-radical intermediates are less stable, so stronger reaction conditions are required. Ethylene is commonly polymerized by free-radical initiators at pressures around 3000 atm and temperatures of about 200 °C. The product, called low-density polyethylene, is the material commonly used in polyethylene bags. 

In the previous chapter, the synthesis of polymers by step polymerization and the kinetics of the process were considered. We turn our attention now to chain-growth polymerizations. The reader should recall that the features.that distinguish step-growth and chain-growth polymerizations are summarized in Table 5.1. A large number of different class of unsaturated monomers, such as ethylene (CH2=CH2, the simplest olefin), a-olefins (CH2=CHR, where R is an alkyl group), vinyl compounds (CH2=CHX, where X = Cl, Br, I, alkoxy, CN, COOH, COOR, CeHs, etc., atoms or groups), and... 

Chain growth polymerizations very often contain a double bond however, cyclic ethers will polymerize in this manner [5], POM (polyoxymethylene) made by the Celanese method shown in Figure 3.6 is an example of a cyclic ether with this method. The Celanese route for the production of polyacetal yields a more stable copolymer product via the reaction of trioxane, a cyclic trimer of formaldehyde, and a cyclic ether (e.g., ethylene oxide or 1,3 dioxalane). 

The monomers used most commonly in chain-growth polymerization are ethylene (ethene) and substituted ethylenes. In the chemical industry, monosubstituted ethylenes are known as alpha olefins. Polymers formed from ethylene or substituted ethylenes are called vinyl polymers. Some of the many vinyl polymers synthesized by chain-growth polymerization are listed in Table 28.1. 

Chain-growth polymerization of monosubstituted ethylenes exhibits a marked preference for head-to-tail addition, where the head of one monomer is attached to the tail of another. 

Although ethylene and substituted ethylenes are the monomers most commonly used for chain-growth polymerization reactions, other compounds can polymerize as well. For example, epoxides undergo chain-growth polymerization reactions. If the initiator is a nucleophile such as HO or RO , polymerization occurs by an anionic mechanism. 

Throughout this chapter, the examples of polymerizations in compressed CO2 have been primarily for chain growth polymerization processes. However, step-growth methods represent an area of new interest for SCFs. Initial experiments in this area include the synthesis of aromatic polyesters such as poly(ethylene terephthalate) (PET) in SCCO2 as illustrated in Scheme 4.5-9 [144]. An advan-... 

In chain-growth polymerization, monomers can onlyjoin active chains. Monomers contain carbon-carbon double bonds (e.g., ethylene, propylene, styrene, vinyl chloride, butadiene, esters of (meth)acrylic acid). The activity of the chain is generated by either a catal) t or an initiator. Several classes of chain-growth polymerizations can be distinguished according to the type of active center ... 

To better understand the fundamental and practical differences between step-growth polymerization and chain-growth polymerization (see Table 1.4), consider the industrial chain-growth polymerization of ethylene (by either coordination polymerization or high-pressure free-radical polymerization) to produce polyethylene. 

From the perspective of the chemical industry, the single most important reaction of al-kenes is chain-growth polymerization, a type of polymerization in which monomer units are joined together without the loss of atoms. An example is the formation of polyethylene from ethylene ... 

The number of monomers that undergo chain-growth polymerization is large and includes such compounds as alkenes, alkynes, allenes, isocyanates, and cyclic compounds such as lactones, lactams, ethers, and epoxides. We concentrate on the chain-growth polymerizations of ethylene and substituted ethylenes and show how these compounds can be polymerized by radical and organometallic-mediated mechanisms. 

Radical chain-growth polymerization of ethylene and substituted ethylenes (Section 16.5A)... 

The practical value of the Fischer-H opsch reaction is limited by the unfavorable Schulz-Flory distribution of hydrocarbon products that is indicative of a chain growth polymerization mechanism. In attempts to increase the yields of lower hydrocarbons such as ethylene and propylene (potentially valuable as feedstocks to replace petrochemicals), researchers have used zeolites as supports for the metals in attempts to impose a shape selectivity on the catalysis [114] or to control the performance through particle size effects. [IIS] These attempts have been partially successful, giving unusual distributions of products, such as high yields of C3 [114] or C4 hydrocarbons. [116] However, the catalysts are often unstable because the metal is oxidized or because it migrates out of the zeolite cages to form crystallites, which then give the Schulz-Flory product distribution. 

Radical Chain-Growth Polymerization of Substituted Ethylenes (Section 29.6A)... 

The most synthetic polymers are prepared by a free radical chain-growth polymerization (active centers are radicals). Other active centers are carbocations (cationic chain-growth polymerization, e.g. isobutylene, vinylether, tetrahydrofuran) and carboanions (anioific chain growth polymerization, e.g. butadiene, ethylenoxide, caprolactam). Also organometallic complexes are used as chain carriers (coordination polymerization, e.g. ethylene, propylene, butadiene). [08IUP2]... 

At this temperature, the ethylene insertion was sufficiently slow, which allowed us to monitor the quasi-living behavior at the start of typical chain-growth polymerizations. The chain length increased linearly with time, but the increase in the amount of polymer was nonlinear. Assuming quasi-living behavior, the calculation of the number of active centers revealed that only 7% of the chromium centers were active after 10 min. This number increased to 14% after 30 min at -30 °C. 

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