Ethylene, 30 Table free-radical polymerization

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 ... 

Table 20-6. Syndiotactic Diad Fractions and Transition Probabilities p for the Formation of Iso- and Syndiotactic Diads in the Free Radical Polymerization of Ethylene Derivatives, CHj CRR ...

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. 

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. ... 

Tactlclty data for the free radical polymerizations of a number of mono and dlsubstituted ethylenes are collected In Table 3 (3). As seen from this data, the predominant triad compositions in most cases are either the heterotactic or syndiotactic triads, but, in general, the r dyad placements are the predominant mode of addition. 

Table 1 is a survey of some of the studies which permit partial or nearly complete kinetic analysis and is not intended to be exhaustive. It furthermore cites only studies made under conditions where the phase compositions can be inferred with reasonable certainty. The paucity of entries does, however, reflect the authors opinion that only a very small fraction of the published literature merits detailed anal rtis. The entries indicate the breadth of the temperature and pressure range over which the free-radical polymerization of ethylene has been studied and the variety of phase conditions which can exist. 

Free-Radical Polymerization of Ethylene Table 7 (continued)... 

Monomer concentrations and pressures are not very high in Ziegler-Natta polymerization (about 2-3 MPa), contrary to the free radical polymerization of ethylene which requires very high pressures. For obvious reasons, industrial producers of polyolefins focus their attention on the productivity of a catalytic system which corresponds to the mass of polymer produced (and not its rate of formation) per mass unit of transition metal, without mention of time. In Table 8.18 the typical components of a currently used Ziegler-Natta system for the industrial production of polypropylene are given. 

Most addition polymers are formed from polymerizations exhibiting chain-growth kinetics. This includes the typical polymerizations, via free radical or some ionic mode, of the vast majority of vinyl monomers such as vinyl chloride, ethylene, styrene, propylene, methyl methacrylate, and vinyl acetate. By comparison, most condensation polymers are formed from systems exhibiting stepwise kinetics. Industrially this includes the formation of polyesters and polyamides (nylons). Thus, there exists a large overlap between the terms stepwise kinetics and condensation polymers, and chainwise kinetics and addition (or vinyl) polymers. A comparison of the two types of systems is given in Table 4.1. 

The original process for high pressure polyethylene was based on use of a high pressure autoclave and used air to introduce free radicals sufficient to initiate polymerization of ethylene. Principal features of the autoclave process are summarized in Table 7.2. 

Industrially, ethylene is polymerized by the high-, medium-, or low-pressure process in bulk, solution, or in the gas phase (Table 25-1). The high-pressure-process polymerizes by a free radical mechanism addition of about 0.05% oxygen to ethylene presumably produces CH2=CH(OOH), which decomposes to provide the start free radicals. Correspondingly, hydroxyl groups have been found in high-pressure poly (ethylene). Inter-molecular transfer by polymer or initiator free radicals produce main-chain free radicals that initiate the polymerization of ethylene ... 

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