Alkene undergoing radical polymerization

Radical hydrosilylation takes place according to a usual free-radical mechanism with silyl radicals as chain carriers. Products are formed predominantly through the most stable radical intermediate. Even highly hindered alkenes undergo radical hydrosilylation. This process, however, is not stereoselective, and alkenes that are prone to free-radical polymerization may form polymers. 

Table 28.2 Examples of Alkenes That Undergo Radical Polymerization ...

We have seen that the substituent on the alkene determines the best mechanism for chain-growth polymerization. Alkenes with substituents that can stabilize radicals readily undergo radical polymerization, alkenes with electron-donating substituents that can stabilize cations undergo cationic polymerization, and alkenes with electron-withdrawing substituents that can stabilize anions undergo anionic polymerizations. 

Many alkenes undergo addition polymerization under the right conditions. The chain-growth mechanism involves addition of the reactive end of the growing chain across the double bond of the alkene monomer. Depending on the structure of the monomer, the reactive intermediates may be carbocations, free radicals, or carbanions. 

A variety of reactants—including sulfur dioxide, carhon monoxide, and oxygen, which do not homopolymerize—undergo radical copolymerization with alkenes to form polymeric sul-fones [Bae et al., 1988 Cais and O Donnell, 1976 Dainton and Ivin, 1958 Floijanczyk et al., 1987 Soares, 1997], ketones [Sommazzi and Garhassi, 1997 Starkweather, 1987, and peroxides [Cais and Bovey, 1977 Mukundan and Kishore, 1987 Nukui et al., 1982] ... 

Problem 6.63 Describe (a) radical-induced, and (b) anion-induced, polymerization of alkenes. (c) What kind of alkenes undergo anion-induced polymerization M... 

Polymerization of isobutylene, in contrast, is the most characteristic example of all acid-catalyzed hydrocarbon polymerizations. Despite its hindered double bond, isobutylene is extremely reactive under any acidic conditions, which makes it an ideal monomer for cationic polymerization. While other alkenes usually can polymerize by several different propagation mechanisms (cationic, anionic, free radical, coordination), polyisobutylene can be prepared only via cationic polymerization. Acid-catalyzed polymerization of isobutylene is, therefore, the most thoroughly studied case. Other suitable monomers undergoing cationic polymerization are substituted styrene derivatives and conjugated dienes. Superacid-catalyzed alkane selfcondensation (see Section 5.1.2) and polymerization of strained cycloalkanes are also possible.118... 

Many alkenes undergo free-radical polymerization when they are heated with radical initiators. For example, styrene polymerizes to polystyrene when it is heated to 100 °C with a peroxide initiator. A radical adds to styrene to give a resonance-stabilized radical, which then attacks another molecule of styrene to give an elongated radical. 

An interesting stereo-electronic effect was found for the two isomeric ketones endo-6 and exo-6 (Chart 7). With the exo-isomer, the carbonyl and alkene moieties are isolated from each ofher, fhe compound undergoes ot-cleavage accompanied by CIDNP effects, and is a good initiator for radical polymerizations. In contrast, the... 

Any compound that readily undergoes homolytic cleavage to form radicals that are sufficiently energetic to convert an alkene into a radical can serve as an initiator for radical polymerization. Several radical initiators are shown in Table 28.3. 

Some alkenes undergo polymerization by more than one mechanism. For example, styrene can undergo polymerization by radical, cationic, and anionic mechanisms because the phenyl group can stabilize benzylic radicals, benzylic cations, and benzylic anions. The particular mechanism followed for the polymerization of styrene depends on the nature of the initiator chosen to start the reaction. 

Radical addition to a bonds (see above) generates a new alkyl radical species (52), which can undergo coupling, disproportionation, or other reactions. Since 52 is generated in the presence of the alkene in this particular reaction, a chain reaction is possible in which the newly formed radical adds to unreacted alkene to generate another radical. An example is the radical polymerization of an alkene, as shown in Figure 13.6 (cationic polymerization tends to be more efficient in many cases and may be accomplished under milder... 

Initiators of Alkene Polymerization Whether free-radical or coordination polymerization occurs depends primarily on the substance used to initiate the reaction. Free-radical polymerization occurs when a compound is present that undergoes homolytic bond cleavage when heated. Two examples include... 

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. 

The types of compounds that can be polymerized readily by the radical-chain mechanism are the same types that easily undergo free-radical addition reactions. Alkenes with aryl, ester, nitrile, or halide substituent groups that can stabilize the intermediate radical are most susceptible to radical polymerization. Terminal alkenes are generally more reactive toward radical-chain polymerization than more highly substituted isomers. The dominant mode of addition in radical-chain polymerization is head-to-tail. The reason for this orientation is that each successive addition of monomer takes place in such a way that the most stable possible radical intermediate is formed. For example, the addition to styrene occurs to give the phenyl-substituted radical to acrylonitrile, to give the cyano-substituted radical ... 

Like substituted ethylene, isoprene undergoes free radical polymerization. Polymerization of isoprene results in the polymer polyisoprene, which is a simple alkene having one double bond in each unit (Figure 9.3). Polymerization of isoprene may follow one of two pathways cii-polymerization or trans-polymerization. 

Under some conditions, alkenes can be polymerized by free radicals. In the examples we have discussed, radical addition to an alkene produced an alkyl radical, which then abstracted hydrogen (or halogen in the case of CX4) in a second propagation step to give the product of overall addition to the alkene. What if Step 4 is not easy If the concentration of alkene is very high, for example, or if the addition step (Step 3) is especially favorable, the abstraction reaction (Step 4) might not compete effectively with it. Under such conditions the alkyl radical can undergo addition to the alkene to form a new alkyl radical (the dimer). The dimer radical can react with alkene to form a trimer radical and so forth, until either a termination step intercedes or the supply of alkene is exhausted (Fig. 11.33). 

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