Isoprene, copolymerization with

Table 4.11. Nitrogen balance on isoprene copolymerization with decomposition products of bis(nitrosoacetyl)benzidine (NAB)b 0...

Trcns-1,3-pentadiene and isoprene copolymerize with Ti(On-Bu)4/ AlEta [293]. Isoprene units in the copolymer chain are predominantly of 3,4 structure while the pentadiene units are mixed of cis and trans 1,4, 1,2 and 3,4 structure composition was independent of Al/Ti ratio. Kinetic measurements were not reported but reactivity ratios (Table 26) show somewhat greater reactivity of the pentadiene and a tendency for alternation. 

Butyl rubber and other isobutylene polymers of technological importance iaclude various homopolymers and isobutylene copolymers containing unsaturation achieved by copolymerization with isoprene. Bromination or chlorination of the unsaturated site is practiced commercially, and other modifications are beiag iavestigated. 

Cationic polymerizations work better when the monomers possess an electron-donating group that stabilizes the intermediate carbocation. For example, isobutylene produces a stable carbocation, and usually copolymerizes with a small amount of isoprene using cationic initiators. The product polymer is a synthetic rubber widely used for tire inner tubes ... 

The distinction between the rates of homo- and copolymerization apparently is misapprehended by some workers. For example, a recent review 141) discusses the results of McGrath 142) who reported butadiene to be more reactive in polymerization in hexane than isoprene, whether with respect to lithium polybutadiene or polyisoprene, although the homopropagation of lithium polyisoprene in hexane was found to be faster than of polybutadiene. The miscomprehension led to the erroneous statement14l) McGrath 142) results regarding the rate constants for butadiene and isoprene place in clear perspective the bizarre assertion 140) that butadiene will be twice as reactive as isoprene (in anionic co-polymerization). 

Supported Lewis acids are an interesting class of catalysts because of their operational simplicity, filterability and reusability. The polymer-bound iron Lewis-acid 53 (Figure 3.8) has been found [52] to be active in the cycloadditions of a, S-unsaturated aldehydes with several dienes. It has been prepared from (ri -vinylcyclopentadienyl)dicarbonylmethyliron which was copolymerized with divinylbenzene and then treated with trimethylsilyltriflate followed by THF. Some results of the Diels-Alder reactions of acrolein and crotonaldehyde with isoprene (2) and 2,3-dimethylbutadiene (4) are summarized in Equation 3.13. 

Monomer reactivity ratios and copolymer compositions in many anionic copolymerizations are altered by changes in the solvent or counterion. Table 6-12 shows data for styrene-isoprene copolymerization at 25°C by n-butyl lithium [Kelley and Tobolsky, 1959]. As in the case of cationic copolymerization, the effects of solvent and counterion cannot be considered independently of each other. For the tightly bound lithium counterion, there are large effects due to the solvent. In poor solvents the copolymer is rich in the less reactive (based on relative rates of homopolymerization) isoprene because isoprene is preferentially complexed by lithium ion. (The complexing of 1,3-dienes with lithium ion is discussed further in Sec. 8-6b). In good solvents preferential solvation by monomer is much less important and the inherent greater reactivity of styrene exerts itself. The quantitative effect of solvent on copolymer composition is less for the more loosely bound sodium counterion. 

Figures 8-10 is almost parallel to Curve 5, and shows no sign of the inversion phenomenon. Accordingly, during the copolymerization with a butadiene-to-isoprene molar ratio of 75 to 25, there is a considerable amount of isoprene incorporated in a rather random fashion with the butadiene. The overall rate is nevertheless controlled by the slower rate-determining step of butadiene polymerization. The copolymer composition at low conversion has been determined to be rich in butadiene, for example 83.5 to...

Butyl rubber is produced by a process in which isobutylene is copolymerized with a small amount of isoprene using aluminum chloride catalyst at temperatures around — 150° F. (20). The isoprene is used to provide some unsaturation, yielding a product that can be vulcanized (43). Vulcanized Butyl rubber is characterized by high tensile strength and excellent flex resistance furthermore, as a result of its low residual unsaturation (only 1 to 2% of that of natural rubber) it has outstanding resistance to oxidative aging and low air permeability. These properties combine to make it an ideal material for automobile inner tubes (3), and Butyl rubber has continued to be preferred over natural rubber for this application, even when the latter has been available in adequate supply. 

Butyl rubber is one product formed when isobutylene is copolymerized with a few percents of isoprene. In the Exxon process an isobutylene-methyl chloride mixture containing a small amount of isoprene is mixed at — 100°C with a solution of AICI3 in methyl chloride. An almost instantaneous reaction yields the product, which is insoluble in methyl chloride and forms a fine slurry. Molecular weight can be controlled by adding diisobutylene as a chain-transfer agent. Increased catalyst concentration and temperature also result in lowering molecular weight. The product can be vulcanized and is superior to natural rubber. A solution process carried out in C5-C7 hydrocarbons was developed in the former Soviet Union.471,472... 

In contrast to QM, three monomers of TCK, HCX and CTCX have been found to undergo spontaneous copolymerization with various vinyl monomers such as styrene (St), isoprene, vinyl acetate, acrylonitrile, and methyl methacrylate 58 59,60). For the copolymerization systems of TCX-St, HCX-St, CTCX-St, (Fig. 5) CTCX-... 

Korotkov and Rakova (53) found that butadiene was more active in copolymerization with isoprene with lithium catalyst, although in homopolymerization isoprene is three times faster. Korotkov and Chesnokova (33) studied the copolymerization of butadiene and styrene with n-butyl lithium in benzene. Butadiene polymerized before much of the styrene was consumed. They claimed the formation of block polymers consisting initially of polybutadiene and the polystyrene chain attached. 

Similar to our earlier study (2) some of the materials had a dark band at the phase interface which is believed to be rich in the isoprene component This phenomenon arises from the slow rate of copolymerization of the isoprene monomer with the n-butyl acrylate. The major experimental results of this and succeeding sections are summarized in Table II which shows phase domain dimensions for the several samples. 

Considerable efforts have been directed, primarily in Kennedy s group [3], to synthesize a series of block copolymers of isobutene with isoprene [90,91], styrene derivatives [92-104], and vinyl ethers [105-107]. Figure 7 lists the monomers that have been used for the block copolymerizations with isobutene. The reported examples include not only AB- but also ABA- and triarmed block copolymers, depending on the functionality of the initiators (see Chapter 4, Section V.B, Table 3). Obviously, the copolymers with styrene derivatives, particularly ABA versions, are mostly intended to combine the rubbery polyisobutene-centered segments with glassy styrenic side segments in attempts to prepare novel thermoplastic elastomers. These styrene monomers are styrene, p-methylstyrene, p-chlorostyrene, a-methylstyrene, and indene. 

Some heterocyclic monomers may undergo random copolymerization with vinyl monomers. This is a case of cyclic acetals (e.g., 1,3-dioxolane) which forms the random copolymers with styrene [308,309] or isoprene [310], Apparently, the oxycarbenium ions, being in equilibrium with tertiary oxonium ions (cf., Section II.B.6.b), are reactive enough to add styrene ... 

As previously noticed, butyl rubber (HR), poly(methylpropene-co-2-methyl-1,3-butadiene), is a random copolymer of isobutene and 0.7-2.2 mol% of isoprene. The industrial slurry process used all over the world consists in a low-temperature copolymerization initiated by A1C13 in meth-ylchloride. In contrast to 1,3-butadiene, isoprene copolymerizes readily with the more reactive isobutene. Reactivity ratios of the pair isobutene-isoprene, ri = 2.5 0.5 and r2 = 0.4 0.1, measured at the conditions of industrial process [10], show that the copolymerization behaves ideally (ri-r2 = 1), and, at the used low concentration of isoprene, isolated units of this latter comonomer are randomly distributed along the chain with 90% M-p-aiw-enchainment [52,53] ... 

Dienes were copolymerized with vinyl monomers such as p-chlorostyrene 17), acrylic esters18,19), vinyl carborane, isoprenylborane 20,2)), and ferrocenyl derivatives 21). The reaction conditions were similar (65 °C, dioxane, 72 h, 3 mole % of initiator). Liquid low-molecular-weight (Mn < 7000) copolymers were obtained. High concentration of p-chlorostyrene 17), acrylates, or methacrylates18,19) in the initial solution leads to higher copolymerization yields compared with systems rich in diene. Molecular weight and polydispersity vary in the same manner 15). The reactivity of dienes decreases in the series chloroprene (85-98%) > butadiene (64-83%) > isoprene (43-73%). 

Cationic polymerization is initiated by acids. Isobutylene, for example, undergoes cationic polymerization to a tacky material used in adhesives. Copolymerization with a little isoprene gives butyl rubber, used to make automobile innertubes and tire liners. A variety of acids can be used sulfuric acid AICI3 or BF3 plus a trace of water. We recognize this process as an extension of the dimerization discussed in Sec. 6.15. 

An elastomer that is entirely or mostly polydiene is, of course, highly unsaturated. All that is required of an elastomer, however, is enough unsaturation to permit cross-linking. In making butyl rubber (Sec. 32.5), for example, only 5% of isoprene is copolymerized with isobutylene. 

Isobutylene was copolymerized with methylbenzenes by cationic methods to give products containing 1-2% of the comonomer. Attachment of polyplvalolactone grafts was accomplished using the same chemistry as that for Isoprene block-graft copolymers. 

Butadiene-styrene copolymerization was attempted using the L3Ln-RX-AlR3 system [78]. Especially, (CF3COO)3Nd/C5HlxBr/AhBu3 (1 3 15) was found to be active for this type of copolymerization, with the ris-content of butadiene unit amounting 97.8% and the styrene content to ca. 32%. However, for the system of isoprene/styrene, the traws-1,4-polyisoprene copolymer was produced exclusively. 

Isobutylene CH3 CHt=C - 1 CH3 Polyisobutylene (PIB) CH3 —f-cHj-c— CH, Lubricating oils, sealants, copolymerized with 0.5-2.5 mol% isoprene to produce Butyl rubber for tire inner tubes and inner liners of tubeless tires. 

Isobutene is used in the field of elastomers, mainly to manufacture a special rubber, butyl rubber, by copolymerization with small amounts of isoprene. It serves essentially for the manufacture of inner tubes, but its production remains modest and accounts for... 

A plot of the mole fraction of isoprene in the SFR prepared copolymers as a function of the mole fraction of isoprene in the feed is shown in Figure 2. The data points are the results for the SFRP process initiated with BPO at 125 C in the presence of TEMPO the curve represents data reported by Wiley and Davis (6) for a conventional styrene/isoprene copolymerization initiated with peroxide at 100 C. The... 

Cationic Polymerization. A small amount of isoprene is cationicaUy copolymerized with isobutjdene in the commercial process for making butyl mbber, wherein the isoprene provides the unsaturation required for sulfur vulcanization. Homopolymerization of isoprene by cationic catalysts can lead to cyclized products and loss of unsaturation (70,71), as for example, during polymerizations initiated by boron trifluoride (72). Cationic polymerization of isoprene with BF, SnCl or AlCl catalysts in pentane, chloroform, or ethjibenzene from —78 to 30°C at around 50% conversion gives about 90% /n7 -l,4-polyisoprene stmcture the balance is 1,2 and 3,4 microstmcture (no microstmcture) (73). An insoluble powder was formed by... 

Copolymerization initiated by A proceeds readily at low temperatures and gives isobutene—isoprene copolymers structurally identical to those prepared commercially utilizing a conventional Lewis-acid initiator. That is to say, there is no incorporation of isoprene in a 1,2- or 3,4-fashion, as would be anticipated at least in part for a Ziegler—Natta process. As with polyisobutene, lower temperatures result in higher molecular weights (polydispersities 2) while materials with high M values and a low polydisper-sity index could be obtained only at very low contents of isoprene. consistent with observations that chain-transfer processes are extremely facile following isoprene incorporation. - ... 

The surprising result is that the fastest rate constant is associated with the crossover reaction of the poly(styryl)lithium chain ends with butadiene monomer (/isb) conversely, the slowest reaction rate is associated with the crossover reaction of the poly(butadienyl)lithium chain ends with styrene monomer ( gg). Similar kinetic results have been obtained for styrene-isoprene copolymerization [204]. 

Electron-withdrawing substituents in anionic polymerizations enhance electron density at the double bonds or stabilize the carbanions by resonance. Anionic copolymerizations in many respects behave similarly to the cationic ones. For some comonomer pairs steric effects give rise to a tendency to altemate. The reactivities of the monomers in copolymerizations and the compositions of the resultant copolymers are subject to solvent polarity and to the effects of the counterions. The two, just as in cationic polymerizations, cannot be considered independently from each other. This, again, is due to the tightness of the ion pairs and to the amount of solvation. Furthermore, only monomers that possess similar polarity can be copolymerized by an anionic mechanism. Thus, for instance, styrene derivatives copolymerize with each other. Styrene, however, is unable to add to a methyl methacrylate anion, though it copolymerizes with butadiene and isoprene. In copolymerizations initiated by w-butyllithium in toluene and in tetrahydrofuran at-78 °C, the following order of reactivity with methyl methacrylate anions was observed. In toluene the order is diphenylmethyl methacrylate > benzyl methacrylate > methyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > t-butyl methacrylate > trityl methacrylate > a,a -dimethyl-benzyl methacrylate. In tetrahydrofuran the order changes to trityl methacrylate > benzyl methacrylate > methyl methacrylate > diphenylmethyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > a,a -dimethylbenzyl methacrylate > t-butyl methacrylate. 

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