2-Methylpropene polymerization

The initiator for isobutene (2-methylpropene) polymerization is usually a Lewis acid with a proton source. We shall illustrate initiation... 

In 1946, Evans et al. [236] observed that 2-methylpropene polymerization with BF3 only starts in the presence of a small amount of water. By analogy to enzyme-co-enzyme terminology, they called water the cocatalyst. The term... 

A specific, greatly simplified form of the general scheme (79) is the initiation of 2-methylpropene polymerization [302]. It occurs in the presence of VC14 (Ti halogenides, etc.) and photons. The donor-acceptor complex of 2-methylpropene with VC14 forms an intermediate... 

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. 

The exact enthalpy of polymerization for a particular monomer will depend on the steric and electronic effects imposed by the substituents attached to the E=E double bond. For olefins, resonance stabihzation of the double bond and increased strain in the polymer due to substituent interactions are the most important factors governing AHp For example, propylene has a calculated AH of -94.0 kJ moT, whereas the polymerization of the bulkier 2-methylpropene is less exothermic (-78.2 kJ moT ) [63]. Due to resonance effects, the experimentally determined AH of styrene (-72.8 kJ mol ) is less exothermic than that for propylene, while that for bulkier a-methylstyrene is even less favorable (-33.5 kJ moT ) [63]. In general, bulky 1,2-disubstituted olefins (i.e., PhHC= CHPh) are either very difficult or impossible to polymerize. 

Copper(II) triflate has also been used for the carbenoid cyclopropanation reaction of simple olefins like cyclohexene, 2-methylpropene, cis- or rran.y-2-butene and norbomene with vinyldiazomethane 2 26,27). Although the yields were low (20-38 %), this catalyst is far superior to other copper salts and chelates except for copper(II) hexafluoroacetylaeetonate [Cu(hfacac)2], which exhibits similar efficiency. However, highly nucleophilic vinyl ethers, such as dihydropyran and dihydrofuran cannot be cyclopropanated as they rapidly polymerize on contact with Cu(OTf)2. With these substrates, copper(II) trifluoroacetate or copper(II) hexafluoroacetylaeetonate have to be used. The vinylcyclopropanation is stereospecific with cis- and rra s-2-butene. The 7-vinylbicyclo[4.1.0]heptanes formed from cyclohexene are obtained with the same exo/endo ratio in both the Cu(OTf)2 and Cu(hfacac)2 catalyzed reaction. The... 

The hydrofluorination of alkenes with anhydrous hydrogen fluoride has been already described extensively in Houben-Weyl, Vol. 5/3, pp 100-101. In the case of ethene, the yield of fluoroethane increases on raising the temperature (90°C, 20-25 atm), however, the procedure should be carried out at lower temperatures with higher alkenes because of their tendency to polymerize thus, 2-fluoropropane is formed in 60-75% yield at 0-45 C. Similar procedures have been described for 2-fluorobutane, 2-fluoro-2-methylpropane and 2-fluoro-2-methyl-butane from but-l-ene, 2-methylpropene and 2-methylbut-2-ene, respectively.63 Cyclohexene reacts at — 78 C with hydrogen fluoride to give fluorocyclohexane (70%) at 100 C polymerization is observed.59,60 Two equivalents of hydrogen fluoride to allene are taken up at — 70 C, to form 2,2-difluoropropane (50%).64... 

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 selective orientation may indicate a steric hindrance from the branch. Internal olefins like 2-butene tend to be poisons. They adsorb strongly but do not copolymerize to any significant extent. 2-Methylpropene is not very reactive either. In the absence of ethylene, a-olefins can be polymerized over Cr/silica but their reactivity is much lower than that of ethylene. Sometimes adding a-olefins to the reactor will improve activity, not because they are more reactive monomers, but because they are better reducing agents. 

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... 

In the presence of 60% sulfuric acid, 2-methylpropene is not converted to a long-chain polymer, but to a mixture of eight-carbon alkenes. The mechanism is like that of the polymerization of 2-methylpropene under nearly anhydrous conditions, except that chain termination occurs after only one 2-methylpropene molecule has been added ... 

The polyethene produced in this way has from 100 to 1000 ethene units in the hydrocarbon chain. The polymer possesses a number of desirable properties as a plastic and is used widely for electrical insulation, packaging films, piping, and a variety of molded articles. Propene and 2-methylpropene do not polymerize satisfactorily by radical mechanisms. 

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. 

Methylpropene can be removed from the reaction mixture by distillation and easily is made the principal product by appropriate adjustment of the reaction conditions. If the 2-methylpropene is not removed as it is formed, polymer and oxidation products become important. Sulfuric acid often is an unduly strenuous reagent for dehydration of tertiary alcohols. Potassium hydrogen sulfate, copper sulfate, iodine, phosphoric acid, or phosphorus pentoxide may give better results by causing less polymerization and less oxidative degradation which, with sulfuric acid, results in the formation of sulfur dioxide. 

The rates of incorporation of various monomers into growing radical chains have been studied in considerable detail. The rates depend markedly on the nature of the monomer being added and on the character of the radical at the end of the chain. Thus a 1 -phenylethyl-type radical on the growing chain reacts about twice as readily with methyl 2-methylpropenoate as it does with ethenylbenzene a methyl 2-methylpropenoate end shows the reverse behavior, being twice as reactive toward ethenylbenzene as toward methyl 2-methylpropenoate. This kind of behavior favors alternation of the monomers in the chain and reaches an extreme in the case of 2-methylpropene and bu-tenedioic anhydride. Neither of these monomers separately will polymerize... 

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. 

Situations (a) and (b) used to be considered easily understandable but case (c), on the other hand, was difficult to explain. It now appears that even the behaviour in cases (a) and (b) is not simple. Biddulph et al. [238] and especially Cheradame and Sigwalt [239] have observed only partial polymerization of 2-methylpropene with TiCl4 at low temperatures. After prepolymerization, the reaction could not be revived by further addition of a TiCl4 solution. However, polymerization proceeded to completion when... 

The resonance energy of the allylic radical is 48.8 kJ mol-1 [57, 58]. It is easily formed and its reactivity is low. It reacts reluctantly with monomers. This is why propene, 2-methylpropene, 1-butene (and higher members of the homologous series) do not polymerize by the radical mechanism... 

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. 

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

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 comparable polymerization of 2-methylpropene is always more rapid and leads to a higher molecular mass when initiated by y-irradiation [60] than when carried out in MeCl2 with the initiators BF3, A1C13 or EtAlCl2... 

With 2-methylpropene as M, both linear and star macromers have been prepared [92-94]. Many kinds of inifers may, of course, be used. For example Kress and Heitz prepared macromers from poly(oxytetramethylene) chains with acrylate or methacrylate end groups, by THF polymerization initiated by superacids with anhydrides as co-initiators - transfer agents [95]. 

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. 

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 ... 

However, in most cases ionic polymerization of the alkene is observed. Only a few examples of the formation of fluoroalkanes have been described. Thus, l-fluoro-3,3-dimethylbutane (2) is obtained in 26 /o yield from the reaction of ethene and 2-methylpropene in hydrogen fluoride. 

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