Free-Radical Polymerization of Ethylene

Step 1 Homolytic dissociation of a peroxide produces alkoxy radicals that serve as free-radical initiators  

Step 2 An alkoxy radical adds to the carbon-carbon double bond  

Step 3 The radical produced in step 2 adds to a second molecule of ethylene  

The radical formed in step 3 then adds to a third molecule of ethylene, and the process continues, forming a long chain of methylene groups. 

In the free-radical polymerization of ethylene, ethylene is heated at high pressure in the presence of oxygen or a peroxide. 

Mechanism 6.9 shows the steps in the free-radical polymerization of ethylene. Dissociation of a peroxide initiates the process in step 1. The resulting peroxy radical adds to the carbon-carbon double bond in step 2, giving a new radical, which then adds to a second molecule of ethylene in step 3. The carbon-carbon bond-forming process in step 3 can be repeated thousands of times to give long carbon chains. In spite of the -ene ending to its 

Free-Radical Polymerization of Ethylene THE OVERALL REACTION  

In Chapter 1, it was mentioned that highly branched low density polyethylene and copolymers made with polar comonomers are produced only by free radical polymerization at very high pressure and temperature. (All other forms of commercially available polyethylene are produced with transition metal catalysts under much milder conditions see Chapters 3, 5 and 6.) In this chapter we will review how initiators achieve free radical polymerization of ethylene. Low density polyethylene and copolymers made with polar comonomers are produced in autoclave and tubular processes, to be discussed in Chapter 7, 

When linear low density polyethylene from the Unipol gas-phase process became commercially available in 1975 (1), predictions of the demise of LDPE were widespread. 

Though linear low density polyethylene of the period had better mechanical properties and, at that time, could be produced at lower cost, it could not match LDPE s ease of processing and optical properties (especially clarity). Linear low density polyethylene did indeed displace LDPE in many applications. However, LDPE not only survived, it actually grew (2, 3), albeit at a slower pace than other forms of polyethylene. Though more than 75 years have elapsed since its 

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FIGURE 6.17 Mechanism of peroxide-initiated free-radical polymerization of ethylene. 

Reaction conditions for the free radical polymerization of ethylene are 100-200°C and 100-135 atmospheres. Ethylene conversion is kept to a low level (10-25%) to control the heat and the viscosity. However, overall conversion with recycle is over 95%. 

As mentioned in Section 9.3, Jackson (141) has obtained estimates of the chain-transfer coefficient of the growing radical with polymer in the free-radical polymerization of ethylene, C,p, by choosing the value so as to fit the MWD. As the polymerization conditions for the polymers mentioned in Table 10.1 are not disclosed, it is necessary to choose typical conditions 220° C and 2000 atm will be selected. Under these conditions Ctp, the ratio of the rate constant for attack on polymer (per monomer unit) to that for propagation, in a homogeneous phase, was found to be about 4.0 x 10 3. This is in good agreement with the known transfer coefficients for the lower alkanes (160), when allowance is made for the differences in pressure and temperature (100). The relation between Ctp and k is ... 

Eastham, A. M. Some Aspects of the Polymerization of Cyclic Ethers. Vol. 2, pp. 18-50. Ehrlich, P. and Mortimer, G. A. Fundamentals of the Free-Radical Polymerization of Ethylene. Vol. 7, pp. 386r-448,... 

Transfer to polymer, causing reactivation of a polymer molecule al some point along its length, leads to the growth of branches. The process can occur intermolecularly and also intramolecularly the latter process is particularly important in the free radical polymerization of ethylene at high pressure where it leads to the production of numerous short branches which considerably affect the properties of the polymer. 

Besides these laboratory experiments, the analysis of industrial reactors may also reveal segregation effects, as for instance in reactors for free radical polymerization of ethylene where the initiator feedstream is likely to be mixed by an erosive process (175). Polymerization and polycondensation reactors offers an especially interesting field for future applications of micromixing. 

Ehrlich, P. and Mortimer, G. A. Fundamentals of the Free-Radical Polymerization of Ethylene. 

To begin, let s consider the anionic polymerization of styrene. For an initiator, we will choose an organometallic compound an organic compound bonded to a metal atom) such as butyllithium, C4H9 Li+. Although the details differ, you should recognize the overall similarity of the mechanism for this anionic polymerization to that for the free radical polymerization of ethylene, above (initiation, propagation, and termination). 

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