Alkylaluminum

Thane et al. [46] reported that in pentane suspension, alkylaluminum compounds efficiently alkylate labile chlorines in PVC, and alternatively, PVC carbonium ions could alkylate aromatic compounds to give rise to polymers of increased stability. The values of 2-3% for labile chlorines estimated by them were considerably higher than now generally believed. 

Ziegler-Natta catalyst (Section 31.2) A catalyst of an alkylaluminum and a litanium compound used for preparing alkene polymers. 

It was theorized that cationic initiators containing Si-Cl functions in conjunction with alkylaluminum compounds would lead to polymers with Si-Cl head-groups which subsequently could be useful for the preparation of block copolymers by coupling. The following equations help to visualize this proposition ... 

Cationic Olefin Polymerization Using Alkyl Halide Alkylaluminum Initiator Systems... 

Further, while conventional Friedel-Crafts halides produce high molecular weight polyisobutylenes or polyisobutylene copolymers (e.g., butyl rubbers, HR) only at relatively low ( —100 °C) temperatures, alkylaluminum-based initiator systems produce high molecular weight materials at much higher ( —40 °C) temperatures. 

Since initiation with conventional Friedel-Crafts halides cannot be controlled, the fine-tuning of reactions becomes extremely cumbersome. In contrast, by the use of alkylaluminum compounds elementary events (initiation, termination, transfer) become controllable and thus molecular engineering becomes possible. Indeed, by elucidating the mechanism of initiation etc., a large variety of new materials, i.e., block3, graft4-6 bigraft7 copolymers, have been synthesized and some of their physical-chemical properties determined. 

Although the history of isobutylene polymerization chemistry can be traced back to the nineteenth century12, systematic research began barely forty years ago. The introduction of alkylaluminum coinitiators took place at about the same time13. Since early work has been reviewed by Kennedy and Gillham14, only a comprehensive review of recent study of isobutylene polymerization using alkylaluminum coinitiators will be presented. 

Alkylaluminum containing a /3-hydrogen relative to aluminum reacted preferentially by hydridation (reduction). 

Both chlorine and bromine in conjuction with alkylaluminums were active initiators in isobutylene polymerization. 

In a series of seven recent publications, these Italian authors21-27 reported isobutylene homo- and copolymerizations using alkylaluminum coinitiators in the presence of halogen, interhalogen compounds and alkyl halide initiators. The following conclusions21 are reported in the first paper. 

The gaseous reagents were distilled and collected inside the enclosure. The 10 vol.% alkylaluminum and 1 vol.% f-butyl halide solutions were freshly prepared. The reaction mixtures were stored manually or by a vortex stirrer. Since PIB tended to precipitate out of solution in highly swollen form, relative rates could thus be established by visual observation. 

Trimethyl-l-pentene (TMP) (2 ml) was mixed with MeX (5 ml) and desired amounts of alkylaluminum and f-BuX solutions were added. In a similar study, 2 ml TMP were mixed with 5 ml MeBr and 0.1 ml of a solution of AlBr3 MeBr complex in MeBr (10 vol.%) was added at -80 °C. The reaction was stopped after 20 mins, and the products separated and analyzed using Kennedy and Rengachary s method11J. 

In view of the chemical nature of alkylaluminums and methyl halides, complexation is most likely to be rapid and complete, i. e. K is large. Indeed Me3 Al and a variety of Lewis bases were found to complex rapidly2. Initiation, i.e., f-butyl cation attack on monomer, is also rapid since it is an ion molecule reaction which requires very little activation energy. Thus, it appears that Rj t. and hence initiator reactivity are determined by the rate of displacement Ri and ionization R2. 

Based on alkylaluminum families, initiator reactivity is found to decrease as EtAlQ2 > Et2 A1X > Me3Al and for Et2AlX systems as, Et2 All > Et2 AlBr > Et2AlCl. Based on initiator efficiencies at —60 °C, an overall initiator/coinitiator/ solvent reactivity sequence has been developed. 

In previous papers1,2 we described reactivity studies of cationic isobutylene polymerization using r-butyl halide initiators, alkylaluminum coinitiators and methyl halide solvents. The effects of these reagents as well as temperature on the overall rate of polymerization and polyisobutylene (PIB) yield were studied and reactivity orders were established. These results were explained by a modified initiation mechanism based on an earlier model proposed by Kennedy and co-workers3,4. This paper concerns the effects of f-butyl halide, alkylaluminums and methyl halide, as well as temperature and isobutylene concentration on PIB molecular weights. 

Kennedy and Rengachary4 similarly determined AEmv, Mv and M for PIB prepared by a host of alkylaluminum initiator systems. Recently, Cesca and coworkers20 have used Mv data to determine the mechanism of isobutylene polymerization using the Cl2/Et2AlCl/MeCl system. 

SO the Sgl mechanism and not the usual arenium ion mechanism is operating. Aromatic rings can also be deuterated by treatment with D2O and a rhodium(III) chloride or platinum catalyst or with CeDs and an alkylaluminum dichloride catalyst," though rearrangements may take place during the latter procedure. Tritium ( H, abbreviated T) can be introduced by treatment with T2O and an alkylaluminum dichloride catalyst. " Tritiation at specific sites (e.g., >90% para in... 

The addition of allcenes to alkenes can also be accomplished by bases as well as by the use of catalyst systems consisting of nickel complexes and alkylaluminum compounds (known as Ziegler catalysts), rhodium catalysts, and other transition metal catalysts, including iron. These and similar catalysts also catalyze the 1,4 addition of alkenes to conjugated dienes, for example. 

The metal catalyzed production of polyolefins such as high density polyethylene (HDPE), linear low density polyethylene (LLDPE) and polypropylene (PP) has grown into an enormous industry. Heterogeneous transition metal catalysts are used for the vast majority of PE and all of the PP production. These catalysts fall generally within two broad classes. Most commercial PP is isotactic and is produced with a catalyst based on a combination of titanium chloride and alkylaluminum chlorides. HDPE and LLDPE are produced with either a titanium catalyst or one based on chromium supported on silica. Most commercial titanium-based PE catalysts are supported on MgCl2. 

It should be noted that the selective reduction of phenylacetylene and diphenylacetylene to either the ds-alkene or the alkane was achieved using LiAlH4 in the presence of FeCk or NiCk as a catalyst [90, 91]. However, deuterolytic workup of the reaction mixtures gave deuterium incorporations <26%, indicating that these reagent systems are not well suited for the synthesis of vinyl- or alkylaluminum compounds from alkynes. 

Table VIII. Effect of reaction temperature on molecular weight of butadiene-propylene rubber (BPR). Polymerization conditions as in Table VII, with i-Bu3Al as alkylaluminum compound. Data from Ref. 19.

So far it has not been possible to determine the distribution of the third monomer units in molecular chains. Yet it is possible to follow the rate of third monomer incorporation in polymerization so as to estimate the heterogeneity of its distribution in the whole polymer. We have previously reported the marked difference in incorporation of DCFD in polymerization with V(acac)3 Et2AlCl and Vc q-Et3Al2Cl -ETGA (3). Figure 8 shows that V. in combination with various alkylaluminum halides and VOCL -Et.Al.Cl are not noticeably different in influencing the incor-pola. uluu of DCED during EEDM polymerization. Thus, difference in... 

Aluminum(III) complexes are amongst the most common Lewis acids. In particular, aluminum halide species (e.g., A1C13, AlBr3) are commercially available and are widely used for various reactions. Other types of Lewis acid such as aluminum alkoxides, alkylaluminum halides, and trialkylaluminum species are also used for many kinds of Lewis-acid-mediated reactions. 

AICI3 is a moisture-sensitive and strong Lewis acid. It is a first choice for Friedel-Crafts-type reactions, which provide numerous important transformations in laboratory and industry. It can also be applied to the transformation of alkenes to ketones via alkylaluminum halides.303 Hydrozirconation of an olefin and subsequent transmetalation from zirconium to aluminum gives the corresponding alkylaluminum dichloride, and the subsequent acetylation by acetyl chloride affords the corresponding ketone in high yield (Scheme 66). 

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