Chain polymerization butyl rubber

Butyl mbber, a copolymer of isobutjiene with 0.5—2.5% isoprene to make vulcanization possible, is the most important commercial polymer made by cationic polymerization (see Elastomers, synthetic-butyl rubber). The polymerization is initiated by water in conjunction with AlCl and carried out at low temperature (—90 to —100° C) to prevent chain transfer that limits the molecular weight (1). Another important commercial appHcation of cationic polymerization is the manufacture of polybutenes, low molecular weight copolymers of isobutylene and a smaller amount of other butenes (1) used in adhesives, sealants, lubricants, viscosity improvers, etc. 

The most industrially significant polymerizations involving the cationic chain growth mechanism are the various polymerizations and copolymerizations of isobutylene. In fact, about 500 million pounds of butyl rubber, a copolymer of isobutylene with small amounts of isoprene, are produced annually in the United States via cationic polymerization [126]. The necessity of using toxic chlorinated hydrocarbon solvents such as dichloromethane or methyl chloride as well as the need to conduct these polymerizations at very low temperatures constitute two major drawbacks to the current industrial method for polymerizing isobutylene which may be solved through the use of C02 as the continuous phase. 

Butyl rubber (HR) is widely used for inner tubes and as a sealant. It is produced using the cationic polymerization with the copolymerization of isobutylene in the presence of a small amount (10%) of isoprene. Thus, the random copolymer chain contains a low concentration of widely spaced isolated double bonds, from the isoprene, that are later cross-linked when the butyl rubber is cured. A representation is shown in structure 5.20 where the number of units derived from isobutylene units greatly outnumbers the number of units derived from the isoprene monomer. The steric requirements of the isobutylene-derived units cause the chains to remain apart giving it a low stress to strain value and a low Tg. 

When the polymeric initiator contains many halogens, there will be many grafted side chains, and the product is called a comb or brush polymer. A variety of polymers can be used as the polymeric initiator, including polymers containing vinyl chloride and 4-chloromethylstyrene units, and halogenated natural and butyl rubbers. Graft copolymers are discussed further in Chaps. 5, 6, and 9. 

Polymerization of 2-methylpropene in the presence of small amounts of 2-methyl-1,3-butadiene (isoprene) gives a copolymer with enough double bonds to permit cross-linking of the polymer chains through vulcanization. The product is a hard-wearing, chemically resistant rubber called butyl rubber. It is highly impermeable to air and is used widely for inner tubes for tires. 

Alkenes that easily form carbocations are good candidates for cationic polymerization, which is just another example of electrophilic addition to an alkene. Consider what happens when pure isobutylene is treated with a trace of concentrated sulfuric acid. Protonation of the alkene forms a carbocation. If a large concentration of isobutylene is available, another molecule of the alkene may act as the nucleophile and attack the carbocation to form the dimer (two monomers joined together) and give another carbocation. If the conditions are right, the growing cationic end of the chain will keep adding across more molecules of the monomer. The polymer of isobutylene is polyisobutylene, one of the constituents of butyl rubber used in inner tubes and other synthetic rubber products. 

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... 

This method of grafting requires the addition of a living cationic polymer to a preformed polymer having several nucleophilic centers in the chain. An example is epoxidized butyl rubber (see above). It was observed that the model of the reactive unit, namely the trisubstituted epoxide 2,3-epoxy-2,4,4-trimethylpentane, terminates THF polymerization 153) ... 

Isoprene is used as the comonomer in butyl rubber (0.5-2.5 mol%) (Kresge et al., 1987) because the isobutene-isoprene reactivity ratios are more favorable for inclusion of the diene than those of the isobutene-butadiene pair (Kennedy, 1968). Thus, for the former pair, the r (isobutene) = 2.5, and r (isoprene) = 0.4 (Kennedy, 1968). It should be noted, however, that as discussed previously, such r values can be markedly influenced by the nature of the initiator and solvent used in the polymerization. The values just quoted are applicable to the commercial butyl rubber process, as described earlier. It has been shown that the isoprene unit enters the chain predominantly in a 1,4-conflguration (Chen and Field, 1967). 

Although, there are many more cationicaUy polymerizable monomers than anionically polymerizable ones, relatively few cationic polymerizations (e.g., isobutene polymerization to produce polyisobutene and butyl rubber - copolymer of isobutene with small fractions of isoprene [18]) are performed industrially because macrocations are highly reactive and prone to suffer termination and chain transfer reactions. 

The SIN of PNA and BR was prepared as follows. Butyl rubber was dissolved in nonyl acrylate. Dinitrobenzene or trinitrobenzene was employed as a vulcanizing agent for the BR, which contained about 1% double bonds. The polymerization of the NA was initiated by benzoyl peroxide. A crosslinked network of PNA is formed by chain transfer to the a-hydrogen. 

The use of liquid and supercritical CO2 as an alternative, environmentally friendly or benign solvent should be mentioned here. De Simone reported the first examples of carbocationic polymerization in hquid and supercritical CO2. Vinyl ethers and oxetane containing fluorinated side chains yielded low molecular weight products (77). Kennedy reported the carbocationic polymerization of isobutylene in supercritical and liquid CO2 (78), also yielding low molecular weight polymer. Butyl rubber was reportedly produced in liquid C02/hexane mixture (79). However, it seems that the initial excitement surroimding liquid and supercritical CO2 as alternative solvents for polymerization has disappeared. 

The principal feature that both anionic and radical polymerization have in common is that a highly reactive species reacts with a monomer containing an alkene to generate a chain-extended species that itself is a highly reactive species (e.g., an anion or a radical). The two other principal chain polymerization mechanisms that are widely used are cationic and metal-catalyzed polymerizations. Cationic polymerization is used for the preparation of butyl rubber and metal-catalyzed... 

For example, as shown in Scheme 6.39 the sulfuric acid-catalyzed polymerization of 2-methylpropene (fxobutylene [(CH3)2C=CH2]) discussed above, (Scheme 6.32) when carried out in the presence of a few percent of 2-methyl-l,3-butadiene (isoprene, [CH2=C(CH3)-CH=CH2]) results in a material that retains enough double bonds to allow chains of the hydrocarbons to be cross-linked. The vulcanized material is called butyl rubber. ... 

Polyisobutene (PIB) and butyl rubber are unusual in that they are polymerized commercially by the process of cationic polymerization. As in the case of free radical and other polymerizations involving a double bond the process involves a chain reaction but in this case the active species is a carbonium ion. The chemistry of cationic polymerization is complex and outside the scope of this book but has been the subject of a number of reviews (Tsukamoto and Vogl, 1971 Plesch, 1963). There are however a number of salient features which should be noted. 

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