Cationic Polymerization of 2-Methylpropene

Ethene does not polymerize by the cationic mechanism because it does not have sufficiently effective electron-donating groups to permit easy formation of the intermediate growing-chain cation. 2-Methylpropene has electron-donating alkyl groups and polymerizes much more easily than ethene by this type of mechanism. The usual catalysts for cationic polymerization of 2-methylpropene are sulfuric acid, hydrogen fluoride, or a complex of boron... 

Cationic polymerization of 2-methylpropene at temperatures about 170 K may be almost flash-like the transformation of tetrahydrofuran to an equilibrium polymer-monomer mixture may last tens to hundreds of hours at 260 K. Evidently the overall polymerization rate is a function of many factors which may be interconnected or may act separately. The aim of kinetic measurements is to describe the polymerization, and to find conditions under which it would proceed in the desired manner. This is usually only possible after the various factors and their consequences have been isolated and investigated. The rate of monomer consumption during polymerization mostly depends on the generation rate of active centres, and on their concentration and reactivity. 

FIGURE 9.43 The beginning stages of the cationic polymerization of 2-methylpropene. 

Polymerization of 2-methylpropene is not initiated by hydrofluoric acid alone. In the presence of TiCl4, polymerization is very rapid even at low temperatures [94], Termination by the F counter-ion is prevented by its complexation with TiCl4. The basicity of the TiCl4F anion is low, and this anion as such does not combine with the growing cation. 

The cationic polymerization of isobutylene (2-methylpropene) is shown in Section 8-16A. Isobutylene is often polymerized under free-radical conditions. Propose a mechanism for the free-radical polymerization of isobutylene. 

An interesting synthesis of block copolymers by cationic polymerization of vinyl compounds was described by Kennedy and Melby [277] who used 2-chloro-6-bromo-2,6-dimethylheptane as coinitiator. Br- is eliminated by triethylaluminium, and styrene can be polymerized, without transfer, on the generated carbocation. After all the styrene has reacted, diethylaluminium chloride is added to eliminate Cl- from the coinitiator and thus produce new carbocations on the polymer chain. In the presence of 2-methylpropene, the two-block copolymer poly(styrene)-6/ock-poly(2-methylpropene) is formed. 

Cationic polymerizations of styrene, 2-methylpropene, vinylnaphthalene, indene, etc. at temperatures about 273 K yield only low polymers. Polymerization of styrene with HC104 in chlorinated solvents at room tern-... 

The major industrial production of polymers obtained by cationic polymerization of alkenes is related to the 2-methylpropene (isobutene or isobutylene) homo- and copolymers which can be classified into three families ... 

When styrene (vinylbenzene) is commercially polymerized, about 1-3% of 1,4-divinylbenzene is often added to the styrene. The incorporation of some divinylbenzene gives a polymer with more strength and better resistance to organic solvents. Explain how a very small amount of divinylbenzene has a marked effect on the properties of the polymer. The cationic polymerization of isobutylene (2-methylpropene) is shown in Section 8-16A. Isobutylene is often polymerized under free-radical conditions. Propose a mechanism for the free-radical polymerization of isobutylene. 

On the basis of the mechanism of cationic polymerization predict the alkenes of molecu lar formula C12H24 that can most reasonably be formed when 2 methylpropene [(CH3)2C=CH2] IS treated with sulfunc acid... 

Monomers for manufacture of butyl mbber are 2-methylpropene [115-11-7] (isobutylene) and 2-methyl-l.3-butadiene [78-79-5] (isoprene) (see Olefins). Polybutenes are copolymers of isobutylene and / -butenes from mixed-C olefin-containing streams. For the production of high mol wt butyl mbber, isobutylene must be of >99.5 wt % purity, and isoprene of >98 wt % purity is used. Water and oxygenated organic compounds iaterfere with the cationic polymerization mechanism, and are minimized by feed purification systems. 

Synthetic polymers can be classified as either chain-growth polymen or step-growth polymers. Chain-growth polymers are prepared by chain-reaction polymerization of vinyl monomers in the presence of a radical, an anion, or a cation initiator. Radical polymerization is sometimes used, but alkenes such as 2-methylpropene that have electron-donating substituents on the double bond polymerize easily by a cationic route through carbocation intermediates. Similarly, monomers such as methyl -cyanoacrylate that have electron-withdrawing substituents on the double bond polymerize by an anionic, conjugate addition pathway. 

H. Cheradame, J. Habimana de la Croix, E. Rousset, and F.J. Chen, Synthesis of polymers containing pseudohalide groups by cationic polymerization. 9. Azido end-capped poly(2-methylpropene) by polymerization initiated by the system lewis acid-2-azido-2-phenylpropane, Macromolecules, 27(3) 631-637, January 1994. 

The formation of the unreactive allylic ion causes growth termination of cationically polymerizing 2-methylpropene chains [108]... 

Similarly, 2-methylpropene (isobutene) is an important monomer. It only polymerizes by a cationic mechanism, and its copolymers with dienes are known as butyl rubber. Higher 1-alkenes (1-butene, 1-hexene, 1-octene) are important copolymerization components [4, 5] they produce tailored branching of some polyethylene types prepared by a coordination mechanism. Longer-chain alkenes (Cjq, C,2, Cj ) are also sometimes used as comonomers... 

Some alkene monomers can be polymerized by a cationic initiator, as well as by a radical initiator. Cationic polymerization occurs by a chain-reaction pathway and requires the use of a strong protic or Lewis acid catalyst. The chain-carrying step is the electrophilic addition of a carbocution intermedi- I ate to the carbon-carbon double bond of another monomer unit. Not sur- I prisingly, cationic polymerization is most effective when a stable, tertiary carbocation intermediate is involved. Thus, the most common commercial use of cationic polymerization is for the preparation of polyisobutylene by treatment of isobutylene (2-methylpropene) with BF3 catalyst at -ttO C. The product is used in the manufacture of inner tubes for truck and bicycle tires. 

Analogous to the initiation of anionic polymerization by addition of nucleophiles to alkenes, cationic polymerization can be initiated by the addition of electrophiles. The alkenes that respond well to cationic polymerization are those that form relatively stable carbocations when protonated. Of these, the one used most often is 2-methylpropene, better known in polymer chemistry by its common name isobutylene. 

As the name implies, chain-reaction polymerization is a chain reaction in which the initiator may be a cation, anion, or free radical. An example of cationic polymerization is found in the polymerization of isobutylene (2-methylpropene) in the presence of protic or Lewis acid catalysts to give poly(isobutylene) (16), as depicted in Equation 22.6. The conversion of acrylonitrile to poly(acrylonitrile) (17) using sodium amide, a strong base, represents anionic polymerization (Eq. 22.7). 

Isobutylene (2-methylpropene) is a good example of a monomer that polymerizes rapidly under cationic conditions. The reaction is carried out commercially at - 80 °C, using BF3 and a small amount of water to generate BFaOH" H" " catalyst. The product is used in the manufacture of truck and bicycle inner tubes. 

Acid-catalyzed polymerizations, such as that described for poly(2-methylpropene), are carried out with H2SO4, HF, and BF3 as the initiators. Because they proceed through carbocation intermediates, they are also called cationic polymerizations. Other mechanisms of polymerizations are radical, anionic, and metal catalyzed. 

The most complex type of behavior is exhibited by 2-methylpropene which polymerizes, rearranges, cyclizes, and oxidizes. Although the dominant alkenyl cation has not been completely identified, it is known to be a cyclopentenyl cation with methyl groups at C-1, C-2, and C-3 (XII) (Deno et al., 1964). 

---