Polyisobutene structure

Polyisobutene has a regular head-to-tail structure, and the crowding of the methyl groups is so great that the molecule can only be built from conventional atomic models with great... 

The proportions of the different kinds of initial groups depend on the nature and rate of the different transfer reactions by which they are formed and the proportion of initial groups derived from the catalyst will depend on the ratio of the rate of initiation to the sum of the rates of the transfer reactions. Finally, although the main chain of the polyisobutene molecule is very resistant to chemical attack [14], by virtue of its chemical structure and configuration, the end-groups are readily attacked by atmospheric oxidation. 

Tanaka, Chatani, and Tadokoro improved this model by refining the crystal structure of polyisobutene (182). The resulting structure is a 2/1 helix in which the structural unit contains four nonequivalent monomer units. In the crystal cell there are always eight monomer units arranged in three turns but the 8/3 helical symmetry is no longer retained. This example represents one of the most notable exceptions to the equivalence principle. Displacement from the exact helical conformation is small, however, and all the pairs of torsion angles fall inside the same energy well. 

Radiation-induced chlorination of polyisobutene in carbon tetrachloride was studied at various temperatures. The process is a chain reaction with a G value of about 10 to 105, depending on the reaction conditions. At very low dose rates (0.1 to 0.2 rad I sec), the chlorination rate is directly proportional to the dose rate. At higher dose rates, the rate approaches a square-root dependence on the dose rate. The termination reaction and the influence of oxygen are discussed. The reaction is first order with respect to chlorine concentration. An activitation energy of about 4 kcal/mole was obtained. In connection with the chlorination reaction, degradation of the polyisobutene takes place. This degradation was followed by osmometric measurements. The structure of the chlorinated product was briefly investigated by IR spectroscopy. 

To get a better insight into the chlorination reaction, we wanted to avoid a heterogeneous process. Instead of polyethylene or polypropylene, we used polyisobutene, which is soluble in carbon tetrachloride, as are its chlorination products. In addition, we were interested in the structure and properties of the chlorinated products, especially in comparison with polyvinyl chloride (PVC) and vinyl chloride/isobutene (VC/IB) copolymers. 

The structure of the chlorinated products obtained is not yet clear. McNeill and McGuchan concluded from NMR data that, in the thermal chlorination process, both methyl and methylene groups were chlorinated but that the methylene groups were more readily substituted and that even some disubstitution of the methylene groups occurred. We studied our products by infrared-spectroscopy Figure 6 shows the spectra of four chlorinated polyisobutenes with increasing chlorine content. 

However, practically no evidence was presented for such an unusual structure. In spite of the statement that this structure will be confirmed later, such a publication has not appeared, although the authors continued to discuss this problem 152). Very recently Bacskai and Lap-porte 153) repeated and extensively studied this problem and, on the basis of careful analytical work reported in detail, concluded that the 1,3 structure was erroneous. Thus, polyisobutenes obtainable with certain Ziegler-Natta type catalyst combinations also have the well known structure of alternating gem.-dimethyl-methylene groups. This conventional structure was established as early as 1940 by Thomas et al 154) in contrast to the contention of the Russian authors 155) ascribing this discovery to Rudenko in 1951. 

The reader should note that stereoisomerism does not exist if the substituents X and Y in the monomer 4-14 are identical. Thus there are no configurational isomers of polyethylene, polyisobutene, or polyfvinylidene chloride). It should also be clear that 1,2-poly-butadiene (reaction 4-3) and the 1,2- and 3,4-isomers of polyisoprene can exist as isotactic, syndiotactic. and atactic configurational isomers. The number of possible structures of polymers of conjugated dienes can be seen to be quite large when the possibility of head-to-head and head-to-tail isomerism is also taken into account. 

In some of the first studies connected with the discovery of the general phenomenon of cocatalysis, f-butanol was found to play such a role in the polymerisation of isobutene by boron fluoride There is an apparent contradiction between this observation and tire fact that polyisobutene chains bearing hydroxyl end groups , i.e. the same structure as f-butanol, do not exhibit a cocatalytic function (see all the evidence of limited yields due to water consumption by termination with counterion to give those end groups). The likely explanation of this dichotomy is that the OH groups on the polymer molecules are embedded in the macromolecular cofls and therefore much less reactive towards the Lewis acid-moncaner complex. 

These homopolymers have structure units as shown in Fig. 2.1, where the asterisk indicates asymmetric carbon atoms. Thus, polypropylene (PP), poly (butene-1) (PB1), and poly(4-methylpentene-l) (P4MP1) have different tactic forms. The most important commercial polyolefins are polyethylene, polyisobutene, and the isotactic forms, that is, iPP, iPBl, and iP4MPl. Polyisobutene was first polymerized by the IG Farbenindustries (BASF) in the late 1920s. Polyethylene was first polymerized by ICI in the late 1930s in a branched form (1). Linear polyethylene... 

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 linear structure is an essential condition for the drag reduction efficiency, a maximal stretching of the macromolecular coil, for a given molecular weight, being required. Thus, the best results are obtained in the case of completely linear poly(ethylene oxide) and polyisobutene, followed by poly(acrylamide) [1080]. 

The possibilities of atactic, isotactic and syndiotactic structures therefore do not arise. Polyisobutene appears to be completely linear no evidence for branching has been found. 

These polyisobutenes are rubbery but, because of their saturated structure, they can not be cross-linked by conventional systems such as those based on sulphur nor could they be cross-linked by peroxides. As a consequence the materials exhibit high cold flow and creep and are thus unsuitable in most conventional rubber applications. This deficiency was eventually overcome by Thomas and Sparks and their co-workers at Standard Oil by copolymerization of the isobutene with a small quantity of a diene monomer to give a polymer which may be vulcanized by conventional sulphur-based systems. Although... 

Polyisobutene and Butyl Rubber. Pol5dsobutene, referred to sometimes as pol5dsobutylene, is considered the precursor of butyl rubber and is processed by low temperature cationic polymerization of isobutylene. The chemical structure can be represented as P ... 

Other polymers are degraded - polypropylene, polystyrene, higher polyolefins, poly(vinyl-chloride) PVC, polyisobutene (PIB), poly(methyl methacrylate) (PMMA), etc. - according to mechanisms similar to thermal or UV radical degradation, depending on their polymer structure. 

Polyisobutene is non-crystalline when unstretched and is therefore soluble at room temperature in hydrocarbons and halogenated hydrocarbons. The material is resistant to most acids, alkalis and aqueous solutions, as would be expected from its saturated hydrocarbon structure and absence of tertiary hydrogen atoms. The lack of tertiary hydrogen atoms renders polyisobutene more resistant to oxidation than polypropylene also, the less numerous and partially shielded methylene groups in polyisobutene are less reactive than those in polyethylene. However, polyisobutene is rather susceptible to thermal degradation since chain scission is favoured by the greater stability of the resultant tertiary free radical ... 

Butyl rabber (polyisobutene Figure 10.1) has the relatively simple structure shown below ... 

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