Activated aluminum alkyl, initiator

Aluminum alkyl initiators usually need water to be converted into their active form (alumoxanes). This has clearly been shown in the polymerization of 3,3-bis-(chloromethyl)-oxetane initiated with (i-C4H9)3Al and When the overall... 

This review article is concerned with chemical behavior of organo-lithium, -aluminum and -zinc compounds in initiation reactions of diolefins, polar vinyls and oxirane compounds. Discussions are given with respect to the following five topics 1) lithium alkylamide as initiator for polymerizations of isoprene and 1,4-divinylbenzene 2) initiation of N-carboxy-a-aminoacid anhydride(NCA) by a primary amino group 3) activated aluminum alkyl and zinc alkyl 4) initiation of stereospecific polymerization of methyloxirane and 5) comparison of stereospecific polymerization of methyloxirane with Ziegler-Natta polymerization. A comprehensive interpretation is proposed for chemistry of reactivity and/or stereospecificity of organometallic compounds in ionic polymerizations. 

Activated Aluminum Alkyl and Zinc Alkyl Initiators... 

In the past, several aluminum-alkyl, halide, and alkoxide complexes supported by multidentate ligands were examined for their catalytic lactide polymerization activities. To this end, monomeric aluminum complexes 148a, b (Fig. 21) were synthesized in our laboratory for producing polyesters with thiolate end groups [137]. These complexes initiated polymerizations under reflux condition in toluene and xylene forming PLAs with narrow molecular weight distributions (PDIs 1.15-1.25). 

Numerous binary and ternary diene polymerization initiator systems with neodymium as the rare-earth metal component have been designed empirically and investigated since the early discoveries in the 1960s. Commercially used neodymium-based catalysts mostly comprise Nd(III) carboxylates, aluminum alkyl halides, and aluminum alkyls or aluminum alkyl hydrides [43, 48,50-52]. Typically, the carboxylic acids, which are provided as mixtures of isomers from petrochemical plants carry solubilizing aliphatic substituents R. They are treated with the alkylaluminum reagents to generate the active catalysts in situ (Scheme 11). 

Accordingly, a broadening of MWD should be possible only by creating inhomogeneity in the active centre valences so as to promote different capabilities of monomer coordination and insertion and, thus, different propagation constants. The correspondence between narrow MWD and a unique oxidation state of the transition metal has been also pointed out by Christman for the ethylene polymerization with vanadium compounds-aluminum alkyls homogeneous systems. In this case, addition of a promoter causes re-oxidation of the deactivated sites (V") to the same identical initial ones (V "). 

Bier et al. found that in propylene polymerization, at 50 °C with aluminum alkyl reduced TiCl3—A1(C H5)2CI, the polymer polydispersity increased (Q from 6 to 10 in the first 30 minutes) and then decreased with time (Q approximately 4). Such a phenomenon could be justified by a decrease in the number of active centres with time after an initial period of rapid increase, together with the living nature of the polymeric chain which is only terminated by monomolecular disproportionation... 

Finally, there is also the development for commercial use of aluminum alkyls as polymer catalysts 298, 300). This arose initially from the observation that organo compounds of active metals, such as Li or K would add across olefinic double bonds repeatedly ... 

One first assumed that polymerization with Ziegler-Natta catalysts, such as aluminum-alkyls plus halides, works by a simple ionic mechanism. Since single aluminum alkyls normally cause anionic and titanium halides a cationic chain reaction (Chapter 8), the two components of the initiator should neutralize each other and only the excess one over the other should be active. If this were true, then either one of the components alone should be able to initiate the polymerization of ethylene or propylene, but this is not the case. A simple anionic or cationic mechanism can therefore not explain the polymerization with Ziegler-Natta catalysts. 

The nature of the Ziegler-Natta catalyst systems is not precisely known. In the case of insoluble catalyst systems, it is, however, certain that the true catalysts are not simple coordination adducts of aluminum alkyl and the metal halide, but complex reactions occur during the initial period of aging , often needed to achieve high activity. The reactions probably involve exchange of substituent groups (Allcock and Lampe, 1990 Odian, 1991) such as those shown in Eqs. (9.1)-(9.3) ... 

Ziegler-Natta polymerization is well known to involve a two-stage process [148, 149]. In the first stage, an aluminum alkyl (such as trialkyl aluminum) is reacted with TiCU in order to produce active jS-TiCls. The alkyl radicals, which are also produced in this reaction, are terminated by coupling and create inert products. Subsequent alkylation of -TiCb then occurs to generate the titanium species that is capable of initiating the polymerization of olefins such as ethylene (Scheme 11.37). 

Haszeldine et al. [478] studied the effect of vanadium compounds together with zinc and aluminum alkyls in tetrahydrofuran (THF). The most satisfactory modified Ziegler-Natta catalyst system is obtained when vanadium oxytrichloride and triisobutyl aluminum react together in the presence of an excess THF. The initiation mechanism and the structure of the active complexes is studied in a further publication of Haszeldine. The results demonstrate that the active centers in the catalyst system VOCl3/Al(iBu)3/THF are formed by interaction of vanadium oxydichloride tetrahydrofuranate and triisobutylalu-minum in the presence of THF ... 

The role of methylalumoxane (MAO) as a cocatalyst to activate zirconocene compounds such as Cp ZrCl to create a single-site ethylene polymerization catalyst is similar, in some respects, to the role simple aluminum alkyls (AlRj) play in activating Ziegler catalysts. For example, MAO acts as an alkylating agent to form the initial Zr-carbon bond (Zr-CH ) necessary to initiate the polymerization process. However, experimental evidence obtained by a variety of methods clearly has shown that the MAO reacts with the zirconium center to form a zirconium cation of the type [CpjZr-CHS] in which the zirconium is not reduced to a lower oxidation state, but remains as a d° metal and Zr(IV) oxidation state. The MAO, therefore, forms an anion moiety to complete the ion pair necessary to create the active species, as illustrated in Equation 4.1. 

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