Metallocene precatalysts, activation

Unsymmetrical Metallocene Precatalysts Activated with MAO in Propylene Polymerization... 

Figure 3.9 Activation of a Group 4 metallocene precatalyst with a main-group metal-alkyl cocatalyst.

With discoveries of boron-based cocatalysts such as triphenyl-boron, ammonium tetraphenylborate salts, and finally pentafiuorophenyl derivatives of borate [B(C6H5)4] , olefin polymerization catalysis was developed without a reliance on alkylaluminum species. Although the activity with nonfiuori-nated boron-based cocatalysts was invariably low, the fiuorinated analogs exhibited olefin polymerization behavior similar to that of metallocene/MAO catalyst systems. The boron and borate compoimds are typically used in a 1 1 molar ratio with transition metal (stoichiometric or near stoichiometric). Because these activators do not alkylate the transition metal, the metallocene precatalyst employed must already bear alkyl groups. Thus, zirconocene dimethyl species combine with boron or borate activators to nerate active cationic polymerization catalysts. Figure 8 shows typical activation reactions with borate (a, b) and boron (c) activators. 

Although metallocene silyl and hydride complexes are the active species, many researchers have sought to develop more convenient precatalysts. The original dimethyltitanocene system reported by Harrod is reasonably easy to prepare, but Corey140 and others have shown that in situ catalyst generation from metallocene dichlorides (Ti, Zr, and Hf) and "BuLi is both simpler and equally effective. However, Harrod and Dioumaev have shown... 

FIGURE 1.13 C2-Symmetric bis(cyclopentadienyl) ansa-metaUocenes 14-17 for isoselective propylene polymerization. Only one enantiomer of the racemic pair is shown. Note that 17 is a gronp 3 metallocene that does not require activation by a Lewis acidic species (R = hydride actual precatalyst structure is a dimer). 

This chapter will discuss all known group 3 and 4 doubly bridged ansa-metallocenes made to date. When polymerization data is unavailable, comments will be made on the perceived viability of the compounds as precatalysts for a-olefin polymerization based on their structure and symmetry. For the a-olefin polymerization precatalysts described herein, the correlation between catalyst structure and polymer tacticity will be discussed. Further, the correlation between catalyst structure and regiocontrol, polymerization activity, and polymer molecular weight will be addressed when there is pertinent data present for a given precatalyst. 

When enantiomorphic site control is operating, C2-symmetric precatalysts with two interannular linkers are predicted to yield isotactic polypropylene. In most cases, this has been borne out by experiment, but the steric environment of the metallocene wedge (as illustrated in Figure 4.3) has important implications for both activity and regiocontrol. In particular, substituents in the 4-position of the cyclopentadienyl ring can be influential. 

Doubly bridged flni fl-metallocenes are a relatively small yet significant category of group 3 and 4 metallocenes. More experimentation is needed to supplement what has already been learned about how precatalyst stmcture and symmetry affects a-olefin polymerization activity, catalyst... 

As mentioned in Sect. 2.4, complex 8 is prepared by selective hydrogenation of the fluorenyl moiety of the parent highly syndiotactic-specific metallocene molecule 6. Its crystal structure is presented in Fig. 23. Complex 8 fulfills all symmetry requirements that one would expect a priori from a would-be syndiotactic-specific precatalyst molecule, nevertheless, after its activation with MAO and its exposure to propylene, complex 8 produces polypropylene chains with perfectly atactic microstructure [28, 30]. 

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