Half-sandwich metallocene

Syndiotactic polystyrene was first obtained only recently by Ishihara et al. [5] in polymerisation with a homogeneous catalyst derived from a transition metal compound such as monocyclopentadienyltitanium trichloride and methylalu-minoxane in toluene. Since then, several authors have reported on the synthesis of syndiotactic polystyrene promoted by different catalysts based on metal hydrocarbyls such as benzyl compounds, half-sandwich metallocenes (e.g. monocyclopentadienyl, monopentamethylcyclopentadienyl and monoindenyl metal derivatives), metal alkoxides, metallocenes and some other compounds. These catalysts are commonly derived from titanium or zirconium compounds, either activated with methylaluminoxane or aluminium-free, such as those activated with tris(pentafluorophenyl)boron, and promote the syndiospecific polymerisation of styrene and substituted styrenes [5-10,21,48-70], Representative examples of the syndiospecific polymerisation of styrene using catalysts based on various titanium compounds and methylaluminoxane are shown in Table 4.2 [6,52,53,56,58],... 

When supported on AI2O3 as a carrier, half-sandwich metallocenes such as CpTiCl3 and Cp TiCl3 also gave rise to suitable stereospecific catalysts that could even be activated by Al(z -Bu)3 [69]. However, in contrast to the respective homogeneous catalysts (yielding syndiotactic polystyrene), polymerisation with these heterogeneous catalysts afforded isotactic and syndiotactic polystyrenes. 

What are the advantages of half-sandwich metallocene-based catalysts as compared with heterogeneous Ziegler-Natta catalysts in styrene polymerisation What are the possible consequences of this for developing industrial processes ... 

Let us recall that half-sandwich metallocene-based catalysts, either activated with methylaluminoxane or aluminium free, such as those activated with tris(per-fluorophenyl)boron, also promote the syndiospecific polymerisation of styrene. 

Conjugated dienes have been polymerised using supported half-sandwich metallocene catalysts. For instance, catalysts derived by supporting CpTiCl3 on alumina-silica gels, containing—0-Ti(Cp)Cl2 species, displayed activity in isoprene polymerisation without the addition of any other activator. Depending on the alumina-silica gel composition, the kind of polymerisation medium and the temperature, these catalysts exhibited various activities and selectivities polyisoprenes with a predominant 3,4 structure and mixed 1,2/ trans-1,4 structure were obtained [118,119],... 

In contrast to heterogeneous Ziegler-Natta catalysts, homogeneous catalysts based on biscyclopentadienyl derivatives of group 4 transition metals, which contain cationic metallocene species of formally d° 14-electronic structure, hardly promote the polymerisation of conjugated dienes, since the diene can act as a donor of four electrons rather than of two electrons as in monoolefin polymerisation (let us recall that the polymerisation of conjugated dienes is catalysed by half-sandwich metallocene-based catalysts). However, it has been reported [162] that statistical copolymers of ethylene and butadiene were obtained with the Cp2ZrCl2— [Al(Me)0]x catalyst. 

Some half-sandwich titanium compounds with cyclopentadienyl ligands have been shown to be the most active catalysts for synthesis of these polymers. Fluorinated half-sandwich metallocenes, synthesized by Roesky et al. (263), have activities of up to a factor of 30 greater than those of chlorinated compounds. Polymerization has been carried out within a temperature range of 10-70°C (264). 

Half-sandwich metallocenes (120) (see Half-sandwich Complexes) play important role in the copolymerization of ethene and various a-olefins. They are distinguished by a sterically accessible active site. These catalysts are remarkably stable up to polymerization temperatures of 160 °C. 

Figure 1 Possible structures of half-sandwich metallocenes. (R is either an electron lone pair or an auxiliary substituent.)...

Due to the inert-pair effect. Group 13 elements sometimes produce low-valent monomeric metallocenes as well as high-valent oligomeric metallocene clusters and polymeric metallocene chains. Half-sandwich metallocenes of Group 13 elements can be found in the gas phase, but exist as clusters and polymeric forms in the solid state. Schleyer calculated the CpE, E = Be, B, C, and N, molecules at the SCF level and found the shortest C-C ring bond length in CpB (18). The... 

The Group 14 metallocenes are the most widely known series of isolated main group sandwich compounds. Schleyer calculated the model compounds T -CpC and T -CpSi and found that the pentahapto carbon compound is less stable than the single-bonded fulvenyl cation or the phenyl cation (36). On the other hand, Tj -CpSi has been predicted to be the most stable one among possible isomers. A mass spectrometry study of Group 14 half-sandwich metallocenes has been made and their atomization energies were calculated (57). It was concluded that nido-cluster is the most stable stmcture and that the stability of these metallocenes increases from CpPb to CpC+. 

The aminolysis of the homoleptic group 4 metal dimethylamides with amines, alkohols, and acidic hydrocarbons such as cyclo-pentadiene was first described by Chandra, G. Lappert, M. F. J. Chew. Soc. A 1968, 1940. This method was first applied to the synthesis of half-sandwich metallocene analogues by Hughes, A. K. Meetsma, A. Teuben, J. H. Organowetallics 1993, 12, 1936. 

The first example of reaction of a half sandwich metallocene with a protein was reported a few years later. Tetramethylcyclobutadiene nickel(II) dichloride 2 was shown to react with the enzyme horse hver alcohol dehydrogenase to afford a modified enzyme devoid of zinc and enzymatic activity and containing 8 mol nickel per mol protein. The mode of binding of this organometallic complex had not been fully elucidated at that time but was thought to occur by substitution of the chloride ligands by nucleophilic residues [3],... 

It is in many other aspects different from all the catalyst systems t discussed in previous sections. It is based on a half-sandwich metallocene molecule that cmitains an amido-type N-Ti bond and is a 12-electron system (14-electron system at the most if one considers the contribution of the nitrogen s lone pair of electrons to the overall N-Ti bonding) leaving a cationic 10-electron (maximum 12 including the lone pair electrons of N) electron configuration active site. ... 

Since metallocene catalysts have coordination power for chain staeoregularity, efforts are made to produce tactic polymers from nonolefin sources such as styrene, MMA, and vinyl chloride, which are normally polymerized using free-radical processes. Unfortunately, metallocenes based on early transition metals are too sensitive to polarity. Only styrene can be polymerized to high molecular weight. Syndiotactic PS was produced using half-sandwich metallocene in 1986 by Ishihara et The materials have high melting tempaa-... 

Schellenberg, J. The syndiospecific polymerization of styrene in the presence of fluorine-containing half-sandwich metallocenes. J. Polym. Sci. Part A Polym. C/zew.,38,2428-2439 (2000). 

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