Alkene undergoing cationic polymerization

Thus monomers such as isobutylene, styrene, methyl vinyl ether, and isoprene undergo polymerization by cationic initiators. The effect of alkyl groups in facilitating cationic polymerization is weak, and it is only the 1,1-dialkyl alkenes that undergo cationic polymerization. 

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... 

Since cationic polymerization involves carbocations, addition follows Markovnikov s rule to form the more stable, more substituted carbocation. Chain termination can occur by a variety of pathways, such as loss of a proton to form an alkene. Examples of alkene monomers that undergo cationic polymerization are shown in Figure 30.4. 

Table 28.4 Examples of Alkenes That Undergo Cationic 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. 

For monomers that undergo cationic polymerization, the process is terminated when the carbocation intermediate is deprotonated by a base or attacked by a nucleophile other than an alkene (such as water). 

Alkenes such as ethene, vinyl chloride, and acrylonitrile do not undergo cationic polymerization very readily. On the other hand, isobutylene undergoes cationic polymerization rapidly. Provide an explanation for this behavior. 

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... 

Cationic zirconocene species efficiently activate alkenes toward carbon—carbon bond formation via carbometalation, as has been demonstrated in studies of alkene polymerization. Today, some zirconocene catalysts are available that allow single additions of metal-alkyls (mainly aluminum-alkyls) to alkenes or alkynes, thereby forming stable alkyl- or alkenyl-metals that do not undergo any further oligomerization. On the other hand, carbozirconation with Cp2ZrRCl in the presence of stoichiometric or catalytic amounts of activators has also been realized. 

Af-Acyliminium ions are known to serve as electron-deficient 4n components and undergo [4+2] cycloaddition with alkenes and alkynes.15 The reaction has been utilized as a useftil method for the construction of heterocycles and acyclic amino alcohols. The reaction can be explained in terms of an inverse electron demand Diels-Alder type process that involves an electron-deficient hetero-diene with an electron-rich dienophile. Af-Acyliminium ions generated by the cation pool method were also found to undergo [4+2] cycloaddition reaction to give adduct 7 as shown in Scheme 7.16 The reaction with an aliphatic olefin seems to proceed by a concerted mechanism, whereas the reaction with styrene derivatives seems to proceed by a stepwise mechanism. In the latter case, significant amounts of polymeric products were obtained as byproducts. The formation of polymeric byproducts can be suppressed by micromixing. 

The number of monomers that undergo chain-growth polymerizations is large and includes such compounds as alkenes, alkynes, allenes, isocyanates, and cyclic compounds such as lactones, lactams, ethers, and epoxides. We concentrate on the chain-growth polymerizations of ethylene and substituted ethylenes and show how these compounds can be polymerized by radical, cation, anion, and organometallic-mediated mechanisms. 

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