Group IV Metallocene Catalysts

The presence of substituents on the rings and the bridge can modify the steric and electronic properties and the symmetry of the molecule. 

Metallocene complexes require activation to be transformed into active catalysts. This is done by organoaluminoxanes, usually by methylaluminoxane (MAO), which provide maximum activity.570 During activation first the metal is methylated followed by a carbanion abstraction to form a metallocene monomethyl cation with a free coordination site (65), which is the actual active catalytic species  

The bulky methylaluminoxane anion stabilizes the coordinatively unsaturated metal cation. Stabilization by noncoordinating anions such as carbosilane dendrimers is also viable.571 Aluminoxanes, however, are required to be used in large excess to be effective. Alternatively, the active catalyst can also be prepared by reacting a metal dialkyl with fluorinated boranes, borate salts or aluminate salts. 

Key steps of the polymerization mechanism are the coordination of the monomer to the coordinatively unsaturated cationic metal center followed by alkyl migration (insertion) into the metal-carbon bond to form the polymer chain and recreating the vacant coordination site, which allows the subsequent coordination of the next monomer  

One of the most active metallocene catalysts is the 66 ama-bis(fluorenylidene) dichloride complex 1 g of catalyst produces 28 500 kg of polyethylene in 1 h at 60°C.569 

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Scheme 6.5 First enantioselective hydrogenation by a Group IV metallocene catalysts.

In a quest to increase the efficiency of olefin polymerization catalysts and their selectivity in the orientation of the polymerization, the highly effective Group IV metallocene catalysts, M(Cp)2(L)2, have been studied, since they all display high fluxionality. Following methide abstraction, the metallocene catalysts of general formula M(Cp-derivatives)2(CH3)2 (M= Ti, Zr, Hf), were turned into highly reactive M+-CH3 cationic species. The activation parameters for the methide abstraction, derived from variable temperature NMR experiments, establish a correlation between the enthalpies of methide abstraction, the chemical shift in the resulting cation, and the ethylene polymerization activities [149]. 

Koga, N., Yoshida, T. and Morokuma, K, Theoretical Studies on Olefin Polymerisation using Group IV Metallocene Catalysts , in Ziegler Catalysts, Springer-Verlag, Berlin, 1995, pp. 275-289. 

Hydride complexes of Group IV metallocenes have been implicated as catalysts and as important intermediates in olefin hydrogenahon and polymerization reactions [232, 233]. 

In hydrogenation, early transition-metal catalysts are mainly based on metallocene complexes, and particularly the Group IV metallocenes. Nonetheless, Group III, lanthanide and even actinide complexes as well as later metals (Groups V-VII) have also been used. The active species can be stabilized by other bulky ligands such as those derived from 2,6-disubstituted phenols (aryl-oxy) or silica (siloxy) (vide infra). Moreover, the catalytic activity of these systems is not limited to the hydrogenation of alkenes, but can be used for the hydrogenation of aromatics, alkynes and imines. These systems have also been developed very successfully into their enantioselective versions. 

Collins and co-workers have performed studies in the area of catalytic enantioselective Diels—Alder reactions, in which ansa-metallocenes (107, Eq. 6.17) were utilized as chiral catalysts [100], The cycloadditions were typically efficient (-90% yield), but proceeded with modest stereoselectivities (26—52% ee). The group IV metal catalyst used in the asymmetric Diels—Alder reaction was the cationic zirconocene complex (ebthi)Zr(OtBu)-THF (106, Eq. 6.17). Treatment of the dimethylzirconocene [101] 106 with one equivalent of t-butanol, followed by protonation with one equivalent of HEt3N -BPh4, resulted in the formation of the requisite chiral cationic complex (107),... 

Soluble Ziegler-Natta catalysts can exhibit unique stereochemical properties. Group IV metallocenes in combination with methylaluminoxanes produce isotactic polypropylene with two different isotactic microstructures. The usual enantio-morphic site control is characteristic of enantiomeric racemic titano- and zirco-nocene complexes (e.g., ethylene-bridged indenyl derivatives279,349). In contrast, achiral titanocenes (e.g., [Cp2TiPh2]) yield isotactic polypropylene with microstructure 49, which is consistent with a chain end control mechanism 279,349-351... 

By Group IV metallocenes, lanthanides and other metal catalysts... 

Ziegler-Natta polymerization of alkenes is an important industrial process for the manufacture of polyolefins. Although it originally involved the use of the triethylaluminum-TiCft complex as the catalysts, many other transition metal complexes and /-block compounds (lanthanides) also catalyze the polymerization of alkenes. Group IV metallocenes exhibit particularly outstanding properties. 

Homogeneous catalyst systems without methylalumoxane, which consists of (CH3)3A1, (CH3)2A1F and group IV metallocenes, such... 

The discovery that group IV metallocenes can be activated by methylaluminox-ane (MAO) for olefin polymerization has stimulated a renaissance in Ziegler-Natta catalysis [63]. The subsequent synthesis of well-defined metallocene catalysts has provided the opportunity to study the mechanism of the initiation, propagation, and termination steps of Ziegler-Natta polymerization reactions. Along with the advent of cationic palladium catalysts for the copolymerization of olefins and carbon monoxide [64, 65], these well-defined systems have provided extraordinary opportunities in the field of enantioselective polymerization. 

Waymouth has polymerized silyl protected alcohols and amines, and non conjugated diene monomers, with cationic Group IV metallocene single site-catalysts. He has found that chiral [(EBTHI)ZrMc] X catalysts, where EBTHI = ethylene-1,2 bis(Ti -4,5,6,7-tetrahydro-l-indenyl), are more easily poisoned by silyl ethers than are [CP2 ZrMe] ] catalysts. Also [(EBTHI)ZrMe] X catalysts are inactive for the polymerization of 4-TMSO-l,6 heptadiene but readily polymerize with the more sterically hindered TBDMS protect monomer. 

In 1954, Natta s first experiments with propylene polymerization using heterogeneous catalysts yielded products that were mixtures of atactic and isotactic polymer chains. Shortly thereafter, Natta produced polymers that consisted primarily of isotactic chains by modifying the composition of the catalyst. The modification of group IV metallocenes to produce catalysts capable of isospecific polymerization has developed much more slowly but has recently seen dramatic success. These advances are outlined below. 

Indeed, Ishihara and co-workers succeeded in the first preparation of sPS through activation of a transition metal complex with MAO (36,37). Typically, Group IV metallocene complexes have been used as catalysts for the polymerization of sPS. Of these, the monocyclopentadienyl-type complexes of titanium have been found to give the highest pol5nnerization activity based on transition metal (38,39). Subsequent to the development of MAO as the sPS cocatalyst, it has been foimd that highly electrophilic activators, such as the tetrakis(pentafluorophenyl) borate type, can be used as cocatalysts for the production of sPS (40,41). 

The discoveries by Kaminsky and Sinn that Group 4 metallocenes in the presence of methylaluminoxane were extremely active for olefin polymerization35,36 caused a considerable resurgence of interest in the application of homogeneous catalysts for olefin polymerization. Surprisingly perhaps, the modification of group IV metallocenes to produce catalysts capable of isospecific polymerization developed more slowly than that of the heterogeneous catalysts by Natta, but has recently seen dramatic success. 

Pig. 16 DehydrocoupUng of primary and secondary silanes to polysilanes (typical catalysts are for example group IV metallocenes)... 

For the dehydropolymerization with catalysts other than group IV metallocenes, no mechanistic proposals have been made up to now. 

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