Ethene addition polymerization

Addition polymerization is a reaction in which monomers with double bonds are joined together through multiple addition reactions to form a polymer. Figure 2.12 illustrates the addition polymerization reaction of ethene to form polyethene. Table 2.1, on the next page, gives the names, structures, and uses of some common polymers that are formed by addition polymerization. 

Ethene or ethylene is the most important organic chemical used in commercial applications. Annual production of ethylene in the United States was over twenty-five million tons in the year 2000. Propylene is also used in large quantities with an annual production of over thirteen million tons. Alkenes such as ethylene and propylene have the ability to undergo addition polymerization. In this process, multiple addition reactions take place and many molecules link together to form a polymer. A polymer is a long chain of repeating units called monomers. For example, the addition of two ethylene molecules can be represented as... 

Poly(cycloalkene)s obtained from the vinyl addition polymerization method exhibit extremely high melting points. The high melting points make the polymers unprocessable. For this reason, comonomers, such as ethene or propene are introduced to lower the melting points. Copolymers of this type are addressed as COCs. 

Addition polymerization In addition polymerization, all of the atoms present in the monomers are retained in the polymer product. When the monomer is ethene, an addition polymerization results in the polymer polyethylene, as shown in Table 23-3. Unsaturated bonds are broken in addition polymerization just as they are in addition reactions. The difference is that... 

In addition polymerization, monomers react to form a polymer chain without net loss of atoms. The most common type of addition polymerization involves the free-radical chain reaction of molecules that have C = C bonds. As in the chain reactions considered in Section 18.4, the overall process consists of three steps initiation, propagation (repeated many times to build up a long chain), and termination. As an example, consider the polymerization of vinyl chloride (chloro-ethene, CH2 = CHC1) to polyvinyl chloride (Fig. 23.1). This process can be initiated by a small concentration of molecules that have bonds weak enough to be broken by the action of light or heat, giving radicals. An example of such an initiator is a peroxide, which can be represented as R—O—O—R, where R and R represent alkyl groups. The weak 0—0 bonds break... 

Most V-based catalysts of the Ziegler-Natta type cause double-bond opening (addition polymerization) rather than ROMP of cycloalkenes. For example, V(acac)3/Et2AlCl causes only Ziegler-Natta-type addition polymerization of cyclobutene (Natta 1963) and its copolymerization with ethene (Natta 1962). 

Addition polymerization of ethene to form polyethene a homopolymer... 

An addition polymer ic a polymer formed by addition reactions between monomers that contain a double bond. For example, molecules of ethene can polymerize with each other to form polyethene, commonly called polyethylene. 

Carothers of DuPont company also invented a synthetic rubber, chloroprene. This polymer is produced by addition-polymerization unlike the polymers mentioned so far. Let us see how it is done with an example of polyethylene. Polyethylene is used in everyday life as a wrap, the thin plastic film. It is a polymer of ethylene, ethene in technical terms. Ethylene has the chemical formula of CHj=CHj. Addition polymerization can be conducted with a compound having one or two double bonds such a compound is often called olefin, Ethylene is the simplest olefin. One of the two bonds in a double bond is a free radical. A free radical has an unpaired (single) electron, which seeks another electron to pair up. The electrons in the n-bond provide such an electron. So what happens is this ... 

Norbomene polymerization is the most versatile of the cycloolefin addition polymerizations. Single-site catalysts such as metallocene compounds, constrained geometry catalysts (CGCs), and nickel or palladium diimine complexes, used in combination with MAO or borate cocatalysts, are active for the copolymerization of norbomene with ethene. The stmctme of the norbomene homo- and copolymers can be widely influenced by the symmetry and stmcture of the ligands on the transition metal complexes. 

Polyethylene (PE) is a thermoplastic commodity made by the chemical industry through the addition polymerization of ethene and which heavily used in a variety of consumer products. PE is classified into several different categories, based mostly on its density and branching. The mechanical properties of PE depend significantly on variables such as the extent and type of branching, the crystal structure and the molecular weight. The following are types of PE ... 

In addition polymerization, all the atoms present in the monomers are retained in the polymer product. Polyethylene is an example of an addition polymer. Its chain contains all the atoms present in the ethene monomers that bonded to form it. Condensation polymerization takes place when monomers each containing at least two functional groups combine with the loss of a small by-product, usually water. Nylon is an example of a condensation polymer. 

Ethenic polymerization is an economically important class of polymer-forming reactions whose kinetics typifies chain-growth polymerization [1]. The terms vinyl, olefin, or addition polymerization are often used, although they are more restrictive. Usually three stages are essential to the formation of a useful high polymer. 

Examine the sequence of structures corresponding to Ziegler-Natta polymerization of ethene, or more specifically, one addition step starting from a zirconocene-ethene complex where R=CH3. Plot energy (vertical axis) vs. frame number (horizontal axis). Sketch Lewis structures for the initial complex, the final adduct and the transition state. Indicate weak or partial bonding by using dotted lines. 

In the zirconocene-catalyzed polymerization of alkenes, Landis and coworkers [20] have reported in situ observation of a Zr-polymeryl species, 15, at 233 K (Figure 1.5). Complex 15 is formed by partial reaction of 14 with excess 1-hexene. Derivatives 16 and 17 are generated quantitatively from 15 by addition of ca. 10 equiv. of propene and ethene, respectively. No other intermediates, such as alkene complexes, secondary alkyls, diasteromers of 15 or 16, or termination products, accumulate to detectable levels. These NMR studies permit direct monitoring of the initiation, propagation and termination processes, and provide a definitive distinction between intermittent and continuous propagation behavior. 

Most technically important polymerizations of alkenes occur by chain mechanisms and may be classed as anion, cation, or radical reactions, depending upon the character of the chain-carrying species. In each case, the key steps involve successive additions to molecules of the alkene, the differences being in the number of electrons that are supplied by the attacking agent for formation of the new carbon-carbon bond. For simplicity, these steps will be illustrated by using ethene, even though it does not polymerize very easily by any of them ... 

Ethene can be polymerized with peroxide catalysts under high pressure (1000 atm or more, literally in a cannon barrel) at temperatures in excess of 100°. The initiation step involves formation of radicals, and chain propagation entails stepwise addition of radicals to ethene molecules. 

The addition of carbon-centered radicals to C-C double bonds (for a review see Giese 1983) is the key reaction in the free-radical-induced polymerization. In general, the rate constants of these reactions are only moderately high, but this process becomes fast and efficient, because in technical applications the polymerizing olefin is usually present at high concentrations. In aqueous solutions, the rate constant of the addition of the hydroxyethyl radical to ethene [reaction (29)], a non-activated C-C double bond, has been determined at 3 x 104 dm3 mol1 s1 (Soylemez and von Sonntag 1980). 

Chemists have been combining small, simple hydrocarbon molecules into long-chain complex molecules for many years. The process is called polymerization, and the resulting product is called a plastic. When a simple, small hydrocarbon molecule like ethene (C2H4) is polymerized, polyethylene results =CH2=CH2= — CH3(CH2)n CH3. (Comment The two horizontal bars denote a double bond between the carbon atoms.) This is the product that is used to make transparent wrap, flexible bottles, and thousands of other products. In addition, polyethylene makes an outstanding macrosculpture material. It can be molded into an endless variety of interesting shapes. It can be colored or left clear. 

C to form the dinuclear (butadienyl)zirconium system 120. Treatment with B(C6F5)3 leads to the formation of a mono-addition product (121), even in the presence of excess borane. Complex 121 (Scheme 40) shows only a marginal ethene polymerization activity. This led to the notion that such formation of dimeric zirconium complexes might represent desactivation pathways in homogeneous Ziegler-Natta catalyst chemistry.125... 

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