Isobutene, polymerization

Addition of metallic oxides to isobutene polymerized by high energy radiation leads to a spectacular increase in the yield.313. It seems that some ions are stabilized by complexing with the surface of the oxide and such an interaction prevents their recombination with the gegen-ions. These observations confirm therefore the suggested cause of inefficient ionic polymerization in systems exposed to ionizing radiation. 

Isobutene polymerized, 157 lsoelectronic system, 246 Isomerization energies, for butane, 75 for pentane, 75 Isoprene, 169 Isotactic placement, 172... 

Extraction of sec-butanol from isobutene Polymerization to form Teflon... 

For isobutene polymerized in ethyl chloride by SnCl4 and H20 a transfer mechanism involving the catalytic complex has been suggested on kinetic grounds [4] ... 

Strong Lewis acids and suitable reaction conditions (e.g., high dielectric constant, low temperature) unbalance the electron structure of olefins. The effect of nucleophilicity is demonstrated in the simplest olefinic series ethylene polymerization is practically impossible with aluminum chloride in methyl chloride diluent at —100° C. propene yields a low molecular weight oil under the same reaction conditions and isobutene polymerizes with extreme rapidity to high molecular weight rubbery products. 

This situation is somewhat reminiscent to that encountered in enzyme chemistry where the active biocatalyst is a combination of an apo-enzyme and a coenzyme, the components alone being complete inactive. Substrate specificity, which is so characteristic for enzymatic processes is also high in carbonium ion chemistry. For example styrene is polymerized by titanium tetrachloride—water, but not by titanium tetrachloride— alkyl chlorides 37) however, with stannic chloride catalyst alkyl chlorides are effective cocatalysts 88). In the same vein Plesch (93) showed that water is a better cocatalyst than acetic or chloroacetic acid in conjunction with titanium tetrachloride in isobutene polymerization, but Russel (94) found just the opposite with stannic chloride. 

The tertiary propagating carbenium ions in isobutene polymerizations are stabilized by eight /3-H atoms through hyperconjugation. Therefore, 1,1-disubstituted alkenes polymerize cationically much more readily than monosubstituted a-olefins. 

Although carboxylic acids generally form 1 1 adducts with alkenes, the resulting esters are easily ionized in the presence of either Lewis or protonic acids. The higher efficiency of chlorinated acetic acids relative to hydrogen halides is ascribed to the ability of their 1 1 adducts to coordinate with excess acid. Alkyl halides are eventually formed when carboxylic acids are used to initiate polymerization in the presence of a Lewis acid due to migration of the carboxylate moiety to the Lewis acid [Eq. (25)]. Similarly, styrene and isobutene polymerizations initiated by preformed alkyl acetate adducts in the presence of BC13 always produce Cl-terminated chains [104,105]. 

The 1 1 adducts of various carboxylic acids and styrene, vinyl ethers, and isobutene have been isolated and used as initiators in the presence of Lewis acid activators. The polymerization rates correlate with the basicity of the leaving groups. However, isobutene polymerizes =103 times slower when initiated by pivalates and isobutyrates in the presence of BC13 than when initiated by acetates, even though they have similar pKa values [106]. Coordination of the covalent adducts with BC13 is evidently hindered when the alkyl substituents are bulkier. More detailed studies on vinyl ether polymerizations using a series of substituted benzoates demonstrate that the pKa values of the parent acid affects both the initiation rate and dynamics of ionization, and therefore the ability to prepare well-defined polymers [107]. 

If initiation is faster or comparable to propagation and termination is negligible, kinetic plots are straight in semilogarithmic coordinates. Initiation is faster than propagation and not kinetically detectable in polymerizations of isobutene and styrene initiated by cumyl derivatives because the initiator is more easily ionized than the propagating species. However, if the initiator is less easily ionized than the propagating species as in a-methyl-styrene polymerizations initiated by cumyl derivatives, and in isobutene polymerizations initiated by /-butyl derivatives (cf., also Section III. A.5), then initiation may be incomplete and the overall polymerization rate will increase continuously. 

Polymerizations which are second order in Lewis acid have also been observed, including self-initiated polymerizations with AlBrj [181], and isobutene polymerizations initiated by alkyl esters and halides activated by TiCU [175,182],... 

Kinetic analysis of isobutene polymerizations initiated by BCh was recently used to distinguish between haloboration and self-ionization of the Lewis acid [Eq. (38)]. 

The rates of initiation and propagation are comparable when the covalent initiator and dormant chain ends have similar structures. Therefore, 1-phenylethyl precursors are useful initiators for styrene polymerizations, but are poor initiators for a-methylstyrene and vinyl ether polymerizations. Similarly, cumyl derivatives are good initiators for isobutene and styrene, but are poor initiators for vinyl ethers their initiation of a -methylstyrene is apparently slow [165]. 1-Alkoxyethyl derivatives are successful initiators for vinyl ethers, styrenes, and presumably isobutene polymerizations [165,192]. /-Butyl derivatives initiate polymerization of isobutene slowly [105]. This is mirrored in model studies that show that /-butyl chloride undergoes solvolysis approximately 30 times slower than 2-chloro-2,4,4-trimethylpentane [193]. This may be due to insufficient B-strain in monomeric tertiary precursors [194]. In contrast, monomeric and dimeric or polymeric structures of secondary esters and halides apparently have similar reactivity. 

However, the equilibrium monomer concentrations of disubstituted alkenes is measurable. The equilibrium constants for dimerization, tri-merization, and polymerization of a-methylstyrene have been determined as a function of temperature under anionic conditions [12] similar values should be obtained under cationic conditions. Unfortunately, the equilibrium position can t be determined directly under cationic conditions due to the irreversible side reactions of isomerization and indan and spirobiindan formation (Section II. A). The equilibrium monomer concentrations of isobutene and isopropenyl vinyl ethers should also be relatively high, albeit lower than those of a-methylstyrenes. However, the true equilibrium can t be reached with these monomers due to irreversible side reactions, and reliable data are therefore not available. Nevertheless, the ceiling temperature of isobutene polymerization is apparently between 50 and 150° C. 

Transfer in a-methylstyrene and isobutene polymerizations occurs with /3-proton elimination from either the endo- or exoposition [Eq. (47)] [14,132]. The latter is not only favored kinetically, but produces a more reactive alkene which may copolymerize. In contrast, the thermodynamically more stable internal alkene should be too sterically hindered to copolymerize. 

The equilibrium constants with nucleophiles such as tertiary amines are so large, that carbenium ions practically do not exist. Thus, tertiary amines and pyridine apparently react with carbenium ions irreversibly and therefore terminate carbocationic polymerizations. Somewhat weaker nucleophiles such as 2,6-dimethylpyridine (lutidine), sulfides, and tris(p-chlorophenyl)phosphine are good deactivators in vinyl ether polymerizations because they react reversibly with monomer, thus maintaining a low concentration of carbenium ions without causing elimination. However, the equilibrium constants in styrene and isobutene polymerizations with amines, sulfides, and phosphines are too large to generate a sufficient stationary concentration of carbenium ions to complete polymerization in a reasonable amount of time. 

Much less information is available on transfer constants in polymerizations of other alkenes. It appears that the transfer constant to anisole in isobutene polymerizations is smaller than in styrene polymerizations, and much closer to values predicted by model studies Cx 5 x 10 3 [323], Calculated Cx values are relatively independent of temperature. 

A later study by the same group showed that, as the initiator, not only PhEtCl but also its bromide counterpart (1-phenylethyl bromide) can be used that, as the added salt, not only the chloride but also the bromide and the iodide (nBi NY Y = Br, I) can be used (see Figure 23) [25], As in isobutene polymerization, the polymer s w-end (tail group) is a chloride... 

Sigwalt proposed, based on calculated transfer constants, that for indene and related isobutene polymerizations, the use of nucleophilic additives does not stabilize the growing carbocation but adjusts the relative rates of initiation and propagation to achieve efficient initiation and narrow MWDs of the polymers [163,164,232]. 

Sometimes, an apparently ideal model of the growing species such as f-butyl halide for isobutene polymerization and cumyl halide for a-methylstyrene polymerization may not be sufficiently reactive. In both cases the ionization ability and initiation efficiency for the monomeric species is much lower than that for the dimeric/macromolecular species ... 

To assure a sufficient initiation rate, it is recommended to use a compound that ionizes more readily than the macromolecular covalent species. This is the case for 1-alkoxyhaloethanes used as initiators for a-methylstyrene and cumyl halides for isobutene polymerizations [5,40], The latter system may, however, lead to intramolecular cyclization. Blocking either the ortho or meta position in the aromatic ring improves the efficiency of initiation with cumyl derivatives [5]. 

Similarly, the microgel-mediated linking of living poly(isobutene) chains with divinylbenzene gives star-shaped rubbery polymers [213]. The living isobutene polymerization is initiated with a tertiary chloride/TiCl4... 

Nonetheless, the evidence for a carbocationic mechanism to this point is circumstantial and based largely on the presumption that isobutene polymerization must involve carbocationic initiation. However, it must be noted that, in principle, the polymeric products could also be formed via a Ziegler—Natta process, as in Scheme 2. This sequence of steps would... 

Already before reporting this combined inifer and living polymerization approach, Kennedy and coworkers developed a controlled isobutene polymerization method based on cumyl ester initiators (Scheme 8.6) with boron trichloride as activator and incremental monomer addition [28], The livingness of the polymerization was demonstrated by the linear increase of number-average molar mass and the constant number of polymer chains (A) with the amount of PIB obtained (wp, as measure for conversion) as well as the narrowing of the molar mass distribution with conversion (Fig. 8.1) [28]. 

Moreover, the absence of unsaturations in the resulting polymers clearly indicated the absence of chain transfer reactions. Similar living polymerization characteristics were reported for cationic isobutene polymerizations initiated with cumyl methyl ethers (Scheme 8.6) with BCI3 as activator [29, 30] as well as with cumyl ethers and cumyl esters as initiators together with titanium tetrachloride as activator [31],... 

In a recent study on isobutene polymerization, Bochmann and co-workers examined the reaction of dinuclear, zirconocene hydride initiators with isobutene (and related olefins) at low The final product shown in Scheme 23 results from insertion and C-H activation of 3equiv. of monomer per molecule of dinuclear initiator one of the monomers is hydrogenated in this process. 

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