Free-radical polymerization of alkenes

The boxed essay Ethylene and Propene The Most Important Industrial Chemicals summarizes the main uses of these two alkenes, especially their polymerization to polyethylene and polypropylene, respectively. Of the methods used to prepare polyethylene, the oldest involves free radicals and is carried out by heating ethylene under pressure in the presence of oxygen or a peroxide initiator. 

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Free-radical polymerization of alkenes has been carried out in aqueous conditions.115 Aqueous emulsion and suspension polymerization is carried out today on a large scale by free-radical routes. Polymer latexes can be obtained as products (i.e., stable aqueous dispersions... 

Alternatively, refractory wools can be wrapped about the crucibles, however, in some cases pyrolysis on the large surface area can prevent reaction or cause free radical polymerizations of alkenes.(15) In larger than milligram quantities, Ti, V and the more refractory materials are best evaporated by electron beam. (13,14) In any case, for metals it is usual to degas the... 

The reaction kinetics in the presence of free radical reducing agent such as AIBN appear to be rather complex. The principle reason is the incorporation of additional reactions steps that involve AIBN. These steps are (a) decomposition of AIBN to generate free radicals, (b) reduction of copper(II) to copper(I) in the presence of AIBN and (c) free radical polymerization of alkene... 

Section 27.8 Free-radical polymerization of alkenes usually gives branched polymers of low crystallinity. The two main mechanisms by which branches form both involve hydrogen atom abstraction by the radical site. In one, a growing chain abstracts a hydrogen atom from a terminated polymer. 

Free-radical polymerization of alkenes (see Section 6.14) Many alkenes polymerize when treated with free-radical initiators. A free-radical chain mechanism is followed and was illustrated for the case of ethylene in Mechanism 6.9. 

Polymerization reactions. There are two broad types of polymerization reactions, those which involve a termination step and those which do not. An example that involves a termination step is free-radical polymerization of an alkene molecule. The polymerization requires a free radical from an initiator compound such as a peroxide. The initiator breaks down to form a free radical (e.g., CH3 or OH), which attaches to a molecule of alkene and in so doing generates another free radical. Consider the polymerization of vinyl chloride from a free-radical initiator R. An initiation step first occurs ... 

In Section 6.21 we listed three main methods for polymerizing alkenes cationic, free-radical, and coordination polymerization. In Section 7.15 we extended our knowledge of polymers to their stereochemical aspects by noting that although free-radical polymerization of propene gives atactic polypropylene, coordination polymerization produces a stereoregulai polymer with superior physical properties. Because the catalysts responsible for coordination polymerization ar e organometallic compounds, we aie now in a position to examine coordination polymerization in more detail, especially with respect to how the catalyst works. 

Scheme 8-20 Synthesis of initiator-terminator functionalized templates for controlling the free radical polymerization of activated alkenes.

Free-radical polymerization of dienes (Section 10.14) Conjugated dienes undergo free-radical polymerization under conditions similar to those of alkenes. The major product corresponds to 1,4-addition. 

Heated, the compound (III) leads to the formation of a di-teri-butyl rutroxide radical (II) and an alkyl radical, CeHsC (H)CH3, that exhibits a chemical structure similar to that of the styryl radical. The choice of (III) as the capping agent comes from its absence of reactivity toward alkenes and from the low strength of the bond formed with the active site (Catala et al., 1995). It allows free radical polymerization of styrene and substituted styrene monomers at 90°C with complete control of the molecular weight and monomer consumption (Joussel et al., 1997). 

The polymers, thus simply based on (CF2)n that subsequently come from that monomer are actually produced by the free radical polymerization of the alkene (tetrafluoroethene tetrafluoroethylene, CF2=CF2). They are known, generically, as... 

In some cases where a reaction involving a radical species occurred within cobalt porphyrin complexes, it has been possible to trap transient cobalt porphyrin hydride species. This was indeed observed during the synthesis of organocobalt porphyrin that resulted from the reaction of cobalt(n) porphyrin and dialkylcyanomethylradicals with alkenes, alkynes, alkyl halides, and epoxide. A transient hydride porphyrin complex was also involved in the cobalt porphyrin-catalyzed chain transfer in the free-radical polymerization of methacrylate. The catalytic chain transfer in free-radical polymerizations using cobalt porphyrin systems has been extensively investigated and will not be treated in this section. Gridnev and Ittel have published a comprehensive overview of the catalytic chain transfer in free-radical polymerizations. ... 

Scheme 2 Idealized stereoselection in free-radical polymerization of a monosubsti-tuted alkene, H2C=CHR. The polymer terminus is shown in a linear conformation.

Dimerization in concentrated sulfuric acid occurs mainly with those alkenes that form tertiary carbocations In some cases reaction conditions can be developed that favor the formation of higher molecular weight polymers Because these reactions proceed by way of carbocation intermediates the process is referred to as cationic polymerization We made special mention m Section 5 1 of the enormous volume of ethylene and propene production in the petrochemical industry The accompanying box summarizes the principal uses of these alkenes Most of the ethylene is converted to polyethylene, a high molecular weight polymer of ethylene Polyethylene cannot be prepared by cationic polymerization but is the simplest example of a polymer that is produced on a large scale by free radical polymerization... 

In their polymerization, many individual alkene molecules combine to give a high molecular weight product Among the methods for alkene polymerization cationic polymerization coordination polymerization and free radical polymerization are the most important An example of cationic polymerization is... 

It might be noted that most (not all) alkenes are polymerizable by the chain mechanism involving free-radical intermediates, whereas the carbonyl group is generally not polymerized by the free-radical mechanism. Carbonyl groups and some carbon-carbon double bonds are polymerized by ionic mechanisms. Monomers display far more specificity where the ionic mechanism is involved than with the free-radical mechanism. For example, acrylamide will polymerize through an anionic intermediate but not a cationic one, A -vinyl pyrrolidones by cationic but not anionic intermediates, and halogenated olefins by neither ionic species. In all of these cases free-radical polymerization is possible. 

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