Metal-alkyl-free catalysts

The primary initiation reaction remains unclear. A similar situation exists with other metal-alkyl free catalysts (66. 67). Polymer chain growth may be pictured to occur by a coordinated anionic mechanism (Reaction 19). 

The strongest evidence in favor of propagation at the transition metal-alkyl bond is the existence of one-component, that is, metal-alkyl-free polymerization catalysts. Of these systems the Phillips catalyst was studied most thoroughly because of its commercial importance. Originally it was believed that Cr(VI) ions stabilized in the form of surface chromate and perhaps dichromate resulting from the interaction of Cr03 with surface hydroxyl groups above 400°C are the active species in polymerization 286,294... 

Many transition metals and their compounds with organic ligands initiate the polymerization of alkenes and/or dienes. Some of them do not need any special treatment to this end while others require the presence of some organic or mineral compound or a special physical modification. In contrast to ZN catalysts, they are active without an organometal of Groups I—III. They are commonly known as metal alkyl free (MAF) catalysts. Many of their features are, of course, in common with ZN catalysts. MAF catalysts initiate stereoselectively controlled polymerization. Even less is known of their operating mechanism than that of ZN catalysts. It is assumed that propagation also occurs on the transition metal-carbon bond. 

Jang, Y Choi, D. S. Han, S. Effects of tris(pentafluorophenyl)borane on the activation of a metal alkyl-free Ni-based catalyst in the polymerization of 1,3-butadiene. J. Polym. ScL, Part A Polym. Chem. 2004,42,1164-1173. 

The formation of high polymers of olefins in the presence of titanium halogenides with no specially added organometallic co-catalysts was discovered long ago [see (147), and the references therein], A complete description of various alkyl-free polymerization catalysts based on the use of transition metal chlorides may be found in the review by Boor (17), where a comparison of these catalysts with traditional two-component systems is given. 

The use of weakly coordinating and fluorinated anions such as B(C6H4F-4)4, B(C6F5)4, and MeB(C6F5)3 further enhanced the activities of Group 4 cationic complexes for the polymerization of olefins and thereby their activity reached a level comparable to those of MAO-activated metallocene catalysts. Base-free cationic metal alkyl complexes and catalytic studies on them had mainly been concerned with cationic methyl complexes, [Cp2M-Me] +. However, their thermal instability restricts the use of such systems at technically useful temperatures. The corresponding thermally more stable benzyl complexes,... 

It was discovered by Ziegler in Germany and Natta in Italy in the 1950s that metal alkyls were very efficient catalysts to promote ethylene polymerization at low pressures and low temperatures, where free-radical polymerization is very slow. They further found that the polymer they produced had fewer side chairrs because there were fewer growth mistakes caused by chain transfer and radical recombination. Therefore, this polymer was more crystalline and had a higher density than polymer prepared by free-radical processes. Thus were discovered linear and high-density polymers. 

Table 10 as well as the Tables 11 and 12 show the differences in energy of the transition states responsible for enantiomeric and regioisomeric excess. They have been calculated on the basis of the enantiomeric and isomeric ratios. They correspond to the difference in free activation energies (AAG ) for a single-step formation of the metal-alkyl complex intermediate from the substrate and the catalyst complex. 

Otsuka et al. (110, 112) studied the polymerization of butadiene in the presence of an aged Co2(CO)8/2 MoC15 catalyst. The product obtained was predominantly an atactic poly(l,2-butadiene), the 1,2-structure being favored by low reaction temperature (e.g., at 40° C, 97% 1,2 at 30° C, > 99% 1,2). Similar experiments with a Ni(CO)4/MoCl5 catalyst yielded a polymer with 85% cis- 1,4-structure. The results of Otsuka et al. have been confirmed by Babitski and co-workers (8), who studied the polymerization of butadiene by a large number of binary catalysts, based on transition metal halide, transition metal carbonyl combinations. These systems are of interest as further examples of alkyl-free coordination polymerization catalysts for dienes (9, 15a, 109). Little is known of the origins of stereospecificity of these reactions. 

Sigman and Jacobsen reported the first example of a metal-catalyzed enantioselective Strecker-type reaction using a chiral Alnl-salen complex (salen = N,N -bis(salicyhdene)-ethylenediamine dianion) [4]. A variety of N-allylimines 4 were evaluated in the reaction catalyzed by complex 5 to give products 6, which were isolated as trifluoroacetamides in good yields and moderate-to-excellent enantioselectivities (Scheme 3). Substituted arylimines 4 were the best substrates, while alkyl-substituted imines afforded products with considerably lower ee values. Jacobsen and co-workers also reported that non-metal Schiff base catalysts 8 and 9 proved to be effective in the Strecker reaction of imines 7 with hydrogen cyanide to afford trifluoroacetamides 10 after reaction with trifluoroacetic anhydride, since the free amines were not stable to chromatography (Scheme 4) [5]. 

A new approach for the synthesis of functionalized 4-alkylquinolines was developed utilizing electrogenerated carbanions <07SL1031>. The desired 4-alkylquinolines 83 were synthesized through a sequential alkylation/heterocyclization of p-(2-aminophenyl)-a,p-ynones 84 and the electrogenerated carbanions of nitroalkanes 85. This novel approach avoided metal and base catalysts and is performed under solvent free conditions. 

Chapter 1 is used to review the history of polyethylene, to survey quintessential features and nomenclatures for this versatile polymer and to introduce transition metal catalysts (the most important catalysts for industrial polyethylene). Free radical polymerization of ethylene and organic peroxide initiators are discussed in Chapter 2. Also in Chapter 2, hazards of organic peroxides and high pressure processes are briefly addressed. Transition metal catalysts are essential to production of nearly three quarters of all polyethylene manufactured and are described in Chapters 3, 5 and 6. Metal alkyl cocatalysts used with transition metal catalysts and their potentially hazardous reactivity with air and water are reviewed in Chapter 4. Chapter 7 gives an overview of processes used in manufacture of polyethylene and contrasts the wide range of operating conditions characteristic of each process. Chapter 8 surveys downstream aspects of polyethylene (additives, rheology, environmental issues, etc.). However, topics in Chapter 8 are complex and extensive subjects unto themselves and detailed discussions are beyond the scope of an introductory text. 

The first example of a fully recyclable fluorous chiral metal-free catalyst was reported by Maruoka and coworkers, who described the enantioselective alkylation of a protected glycine derivative (Scheme 5.17) with various benzyl- and alkyl bromides, in the presence of the quaternary ammonium bromide 62 as a phase-transfer catalyst [77]. Reactions were performed in a 50% aqueous KOH/toluene biphasic system in which 62 was poorly soluble. Nevertheless, the alkylated products were obtained in good yields (from 81 to 93%), with enantioselectivity ranging from 87 to 93% ee. Catalyst 62 was recovered by extraction with FC-72, followed by evaporation of the solvent, and could be used at least three times without any loss of activity and selectivity. 

Certain soluble catalysts polymerize propylene to highly syndiotactic polymers that are free of the isotactic form. Natta, Pasquon, and Zambelli (7 8) showed that VCl (or vanadium triacetylacetonate) in combination with AlRnX-type metal alkyls and anisole, polymerize propylene to highly syndiotactic polypropylene. These apparently homogeneous catalysts were used at low temperatures (-40 to -78 °C). Stereochemistry was initially explained by a mechanism involving repulsion between the methyl groups on the last added and new monomer unit to achieve orientation (83. 84). 

One and two electron oxidative addition processes that involve electron transfer between alkyl radicals and transition metal species have been exploited in organic synthesis for many years. These reactions can ultimately result in the formation of stable metal-alkyl complexes. The formation of such organometallic species during ATRP would have several implications on the role of the catalyst. The relative bond dissociation energies of the the Mt-R, Mt-X, and R-X bonds would ultimately dictate whether polymerization would be inhibited by the formation of a Mt-R bond, whether initiation efficiency might just be reduced, or whether the entire polymerization could be mediated through the reversible formation of such a Mt-R bond (as in stable free radical polymerization, or SFRP).[ ]... 

ATRP generally initiated with the formation of an alkyl free radicle by the hemolytic cleavage of an alkyl halide (R-X) by a catalyst (transition metal such as Cu with suitable ligands, M[ /L) (Fig. 2.22). The generated free radicle can either propagate with a suitable monomer, resulting in the formation of a polymer (rate constant kp), or can terminate (k,) or it can be reversibly deactivated with the metal-halide ligand complex kdeact)- The overall rate of polymerization depends on... 

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