Ethene radical polymerization

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,... 

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).

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. ... 

Section 4.6.2 illustrates the experimental procedures that have recently been applied toward the study of high-pressure free-radical polymerization processes. Section 4.6.3 presents results of propagation, termination, chain-transfer (to monomer and to polymer), and P-scission rate coefficients for ethene homopolymerization. Recent results from experiments and modeling investigations into high-pressure copolymerizations (with ethene being one of the monomers) are reported in Section 4.6.4, together with information on homopolymerization rate coefficients of the comonomer species. 

Alternatively, polymerization can be carried out by radical polymerization in supercritical ethene under extremely harsh reaction conditions such as 2000bar and 300 C. Clearly, when handling such... 

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

Ethene (ethylene) polymerizes by a radical mechanism when it is heated at a pressure of 1000 atm with a small amount of an org ic peroxide (called a diacyl peroxide). 

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. ... 

Scheme 6.45. Free radical polymerization catalyzed by di-ferf-butyl-peroxide [(CH3)3 C-0-0-C(CH3)3]) of (a) ethene (ethylene [CH2=CH2]) at high pressure to yield polyethylene. Step 1 is termed initiation as the initial species needed for polymerization have been formed in this step. Steps 2 and 3 are called propagation since a new radical forms for each that is used. The chain continues. Steps 4 and 5 are examples of tenmnation. All steps that destroy radicals and do not create new ones terminate the reaction. In Part b 2-chloro-l,3-butadiene [chloroprene, CH2=C(C1)CH=CH2] undergoes a similar polymerization to yield neoprene rubber.

Synthetic polyolefins were first synthetisized by decomposition of diazomethane [2]. With the exception of polyisobutylene, these polymers were essentially laboratory curiosities. They could not be produced economically. The situation changed with the discovery of the high pressure process by Fawcett and Gibson (ICI) in 1930 ethene was polymerized by radical compounds [3]. To achieve a sufficient polymerization rate, a pressure of more than 100 MPa is necessary. First produced in 1931, the low density polyethene (LDPE) was used as isolation material in cables. 

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. 

Oxygen or peroxides are used as the initiators. Initiation is very similar to that in many other free-radical polymerizations at different temperatures according to their half-live times (Table 1). The pressure dependence is low. Ethene polymerization can also be started by ion radiation [48-51]. The desired molecular weight is best adjusted by the use of chain transfer reagents. In this case hydrocarbons, alcohols, aldehydes, ketones, and esters are suitable [52,53]. 

At elevated temperatures, ethene can be copolymerized with a number of unsaturated compounds by radical polymerization [174-180] (Table 7). The commercially most important comonomers are vinyl acetate [181], acrylic acid, and methacrylic acid as well as their esters. Next to these carbon monoxide is employed as a comonomer, as it promotes the polymer s degradability in the presence of light [182]. 

While it is difficult to copolymerize ethene and polar monomers by Ziegler- or singlesite catalysts because of the great reactivity of the active sites to polar groups, it is commercialized to use free radical polymerization by high ethene pressure. 

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 ... 

Two pieces of kinetic information are essential with respect to applying chemical initiators in radical polymerization the decomposition rate coefficient fed defined as the loss in initiator concentration with time and the efficiency of initiation/, which refers to the fraction of primary radicals that actually start chain growth. As initiator decomposition rates are mostly deduced from independent experiments, by monitoring initiator decay in (inert) solution, it is recommended to select the solvent as close as possible to the monomer system of interest. The solvent material in which the fed measurements have been performed needs to be stated when efficiencies/are presented. For use with ethene polymerization, fed from experiments in solution of heptane should be well suited. 

An example of radical polymerization is that of ethene in the presence of an organic peroxide at high pressures and temperatures. The reaction proceeds by a mechanism that, in its initial stages, resembles that of the radical addition to alkenes (Section 12-13). The peroxide initiators cleave into alkoxy radicals, which begin polymerization by addition to the double bond of ethene. The alkyl radical thus created attacks the double bond of another... 

---