Radical polymerization of ethene

The diacyl peroxide dissociates and releases carbon dioxide gas. Alkyl radicals are produced, which in turn initiate chains. 

Chains propagate by adding successive ethylene units, until their growth is stopped by combination or disproportionation. 

The radical at the end of the growing polymer chain can also abstract a hydrogen atom from itself by what is called back biting. This leads to chain branching. 

The polyethylene produced by radical polymerization is not generally useful unless it has a molecular weight of nearly 1,000,000. Very high molecular weight polyethylene can be obtained by using a low concentration of the initiator. This initiates the growth of only 

Polyethylene has been produced commercially since 1943. It is used in manufacturing flexible bottles, films, sheets, and insulation for electric wires. Polyethylene produced by radical polymerization has a softening point of about 110°C. 

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High-temperature and -pressure free radical polymerization of ethene, to produce low-density polyethene (LDPE). 

Figure 6.2-24 Near-infrared absorbance spectra, recorded during free-radical polymerization of ethene at 190 °C and 2630 bar initial pressure (the arrows indicate the direction of the absorbance change with the reaction time).

A MECHANISM FOR THE REACTION ] Radical Polymerization of Ethene (Ethylene) 484... 

The free-radical polymerization of ethene (ethylene, CH2=CH2) to polyethylene is a simple example of the addition process (Figure 15.18). The monomer reacts to form a free radical, a species that has an unpaired electron, which then forms a covalent bond with an electron of another monomer ... 

A great number of reaction types have been investigated, the majority involve radical initiated organic chain reactions. A limited number of radiation induced synthesis has, however, been developed to the pilot plant or industrial scale. Examples of rqxrrted industrial synthesis are sulfoxidation and sulfochlorination of hydrocarbons for deterg t production and polymerization of ethene. 

High-temperature SEC finds wide application in polymerization studies, as the molecular mass distribution is an artefact of the various reactions involved in polymerization, initiation, termination, and transfer. It is diagnostic of living systems and random polymerization reactions, such as condensation and radical initiated polymerizations, for which the distributions are Poisson and normal respectively. In the polymerization of ethene and propene by Ziegler-Natta catalysts, the determination of the concentration of active centres as a function of conversion defines catalyst type. Similar studies have been made in the study of chain scission by thermal degradation or by irradiation, in defining the number of molecules produced from the inverse of the number average molecular mass and random chain scission eventually leads to a normal molecular mass distribution, with polydispersities close to 2.0. This has, of course, been widely used to produce narrow from broad molecular mass distribution samples prior to fractionation. 

Free radical polymerization, another example of a chain reaction (this chapter), is quite common and, for many alkenes and dienes, is the preferred method of polymer formation. Typically, as shown in Table 6.3 and Scheme 6.45, the initiator of the free radical process is a peroxide (such as di-fm-butylperoxide [(CH3)3C-0-0-C(CH3)3]). In Scheme 6.45a, the radical polymerization of ethylene (ethene, CH2=CH2) normally carried out at high pressure (>1(F atm) is shown, while as shown in Scheme 6.45b, the radical polymerization of a diene, 2-chloro-l, 3-butadiene [chloroprene, CH2=C(C1)CH=CH2], produces the all tram or (Z)-polymer called neoprene. ... 

Since the polymerization of ethene develops excess heat, radical polymerization on a laboratory scale is best carried out in a discontinuous, stirred batch reactor. On a technical scale, however, column reactors are widely used. The necessary pressure is generally kept around 180 to 350 MPa and the temperature ranges from 180 to 350 °C [24-29]. Solvent polymerization can be performed at substantial lower pressures and at temperatures below 100 °C. The high-pressure polymerization of ethene proceeds via a radical chain mechanism. In this case chain propagation is regulated by disproportionation or recombination. 

This discussion of chain polymerization has centered on free-radical polymerization of an ethenic monomer. Conjugated dienes such as 1,3-butadiene often polymerize as bifunctional monomers with 1,4-addition rather than as tetrafunctional monomers. 

A widely used synthetic procedure is radical polymerization, polymerization by a radical chain reaction (Section 13.9). In a typical procedure, a monomer (such as ethene) is compressed to about 1000 atm and heated to 100°C in the presence of a small amount of an organic peroxide (a compound of formula R—O—O -R,... 

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 ... 

Examples include control of molecular weight in step-growth polymerization, number-average degree of polymerization in step-growth polymerization of nonstoichio-metric monomer mixtures, free-radical and anionic polymerizations of styrene, and ethene oligomerization to linear 1-olefins in the Shell Higher Olefins Process. 

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... 

Dithienothiophenes give cation polymeric radicals capable of further copolymer addition" while polystryene with a narrow polydispersity has been prepared through the use of an end-capped photoactive anthryl group. ° Large differences in radical termination rates have been found to be responsible for the marked variations in molecular weights of polymer from the UV flash polymerisation of 1,3-butadiene. tra 5-l,2-bis(5-Phenyl-2-oxazolyl)ethene has been found to exhibit low laser conversion efficiency due to preferential dimerisation while thermally activated patterns can be formed on the surface of poly(methyl methacrylate) by coating with photodimerisable 9-anthraldehyde. " ... 

Metallocene-catalyzed Z-N polymerization is finding increased use on an industrial scale. One application is the production of linear low-density polyethylene (mLLDPE),99 which is a linear polymer with short branches incorporated deliberately at various points along the chain. Short branches are produced by Z-N copolymerization of ethene with 1-butene, 1-pentene, and 1-hexene rather than through radical mechanisms of chain transfer and backbiting. Thus, the process is... 

Chain-growth polymerization involves the sequential step-wise addition of monomer to a growing chain. Usually, the monomer is unsaturated, almost always a derivative of ethene, and most commonly vinylic, that is, a monosubstituted ethane, 1 particularly where the growing chain is a free radical. For such monomers, the polymerization process is classified by the way in which polymerization is initiated and thus the nature of the propagating chain, namely anionic, cationic, or free radical polymerization by coordination catalyst is generally considered separately as the nature of the growing chain-end may be less clear and coordination may bring about a substantial level of control not possible with other methods. ... 

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