Metallocene systems, spectroscopic

Cationic alkyl metallocene complexes are now considered the catalytically active species in metallocene/MAO systems. Spectroscopic observation has confirmed the presence of cationic catalytic centers. X-ray photoelectron spectroscopy (XPS) on the binding energy of Zr(3d5/2) has suggested the presence of cationic species, and cationic hydride species such as ZrHCp2 that are generated by /1-hydride elimination of the propagating chain end... 

Most of the spectroscopic investigations discussed above were carried out on well-defined metallocene systems, either isolated species or those generated from a well-defined metallocene alkyl precursor activated with one equivalent of a borane or borate activator. Most practical polymerisation catalysts, on the other hand, include a scavenger, usually an aluminum alkyl, and may contain ill-defined activators such as methylaluminoxane (MAO), usually at high MAO/Zr ratios. Such systems are less amenable to quantitative studies nevertheless, the identifications of species such as those depicted in Schemes 8.5-8.8 has enabled similar compounds to be identified in more complex mixtures. An idea of the possible mode of action... 

It is now well recognised that the active species is a cationic complex, or more precisely a solvent-separated or tight ion pair, the structure of which depends on the mode of catalyst activation. Early spectroscopic and synthetic studies on metallocene dimethyl precursors helped to outline the principal reaction pathways, these have been reviewed [16, 21, 23]. Some of this chemistry is briefly summarised here since it presents the background for the understanding of later studies on methyl-aluminoxane (MAO) systems. 

The cocatalyst has various functions. The primary role of MAO as a cocatalyst for olefin polymerization with metallocenes is alkylation of the transition metal and the production of cation-like alkyl complexes of the type Cp2MR+ as catalytically active species (91). Indirect evidence that MAO generates metallocene cations has been furnished by the described perfluorophenyl-borates and by model systems (92, 93). Only a few direct spectroscopic studies of the reactions in the system CP2MCI2/MAO have been reported (94). The direct elucidation of the structure and of the function of MAO is hindered by the presence of multiple equilibria such as disproportionation reactions between oligomeric MAO chains. Moreover, some unreacted trimethylaluminum always remains bound to the MAO and markedly influences the catalyst performance (77, 95, 96). The reactions between MAO and zirconocenes are summarized in Fig. 8. 

Isomeric (s-cis- and (i-fra/w-V-conjugated diene)zirconocene and -haf-nocene complexes exhibit pronounced differences in their characteristic structural data as well as their spectroscopic features. These differences exceed by far the consequences expected to arise simply from the presence of conformational isomers of the 1,3-diene unit. While (f-rra/u-butadiene)-zirconocene (3a) shows a behavior similar to a transition metal olefin TT-complex, the (.r-cu-diene)ZrCp2 isomer 5a exhibits a pronounced alkylmetal character (23, 45). Typical features are best represented by a tr, 7T-type structure for 5 (55). However, the distinctly different bonding situation of the butadiene Tr-system/bent-metallocene linkage is not only reflected in differences in physical data between the dienemetallocene isomers 3 and 5, but also gives rise to markedly different chemical behavior. Three examples of this are discussed in this section the reactions of the 3/5 isomeric mbcture with carbon monoxide, ethylene, and organic carbonyl compounds. 

In connection with the elucidation of the structures of the products obtained upon reaction of the tropone (496) with perchloric acid, the 6-methoxy-substituted derivatives have been prepared by reaction of (496) with methyl iodide and silver fluoroborate. -Comparison of u.v. spectra indicated that the salts between the tropones (496) and perchloric acid were 6-hydroxy-derivatives of the perchlorate of (497). U.v. and n.m.r. spectra of the systems (496)—(499) are discussed and their properties and reactivities compared with the corresponding JV-phenyl-pyrrole analogue. A preliminary report of the polarographic, i.r., and u.v. spectroscopic properties, as well as the dipole moments, of both the C-annelated systems (496) and (497) without methyl groups in the thiophen ring and also the ion (500) with alkyl, hydroxy, and methoxy-groups in the seven-membered ring has been published. Values of piirR+ between 6 and 7 have been obtained for this system. The spectroscopic properties of metallocene derivatives such as (501)—(503) have also... 

The two-state propagation mechanism cannot be tested by physical and spectroscopic properties of the ani-PP or kinetic results. We have reasoned that if the metallocene complex have dissimilar n -systems, and that their stereochemical controls of the states associated with them are different, then very different kind of PP may be produced. In particular, if one state is Isospeclfic and the other state is nonspecific, and if a chain propagates alternately via these two states, then the macromolecule would contain alternate crystallizable and noncrystallizable segments, such material would be a thermoplastic elastomer of homopolypropylene. 

---