Pentafluorophenyl derivatives transition metals

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],... 

The syndiotacticity of SPS results from the homogeneous coordination polymerization mechanism. Many metal complexes were examined, but only group 4 transition metal complexes or rare earth metal complexes with cocatalysts such as MAO, pentafluorophenyl borate, or borane derivatives allow the synthesis of SPS with high activities [6-10]. Some results are summarized in Thble 2.1. 

Pentafluorophenyllithium in ether reacts slowly with mercury at 15° C to afford bis(pentafluorophenyl)mercury, a compound recently prepared by another route (Section VII, B). The lithium reagent has very recently been used to prepare pentafluorophenyl derivatives of boron (79a), tin 19b), lead 19b), and several transition metals 19c). 

It has recently been observed (153a) that the shifts of the ortho fluorine atoms in pentafluorophenyl transition metal complexes are less shielded than similar fluorine atoms in pentafluorophenyl derivatives of main group metals. For possible reasons for the relative shifts in —CFz—M and CsFs—M compounds, see refs. (153) and (153a). 

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