CHIRAL C-1-SYMMETRICAL GROUP-4 METALLOCENES AS CATALYSTS FOR STEREOREGULAR ALPHA-OLEFIN POLYMERIZATION - METAL, ANCILLARY LIGAND, AND COUNTERANION EFFECTS

This contribution reports the synthesis and activity as precatalysts f or stereoselective propylene polymerization of several chiral non-C-2 symmetric zirconocene and hafnocene complexes, (R)- and (S)-Me(2)Si(Me (4)C(5))-(C(5)H(3)R)MR(2) (R = Cl(2) or Me(3)) where R* = R,2S,5R)-tr ans-5-methyl-cis-2-(2-propyl)cyclohexyl ((-)-menthyl; M = Zr, (a) and M = Hf, (b)) and (1S,2S,5R)-trans-5 -methyl-cis-2-(2-propyl)cyclohexyl ((+)-neomenthyl; M = Zr (c)). Metallocene dichlorides were prepared f rom MCl(4) and Li(2)Me(2)Si(Me(4)C(5))(C(5)H(3)R) and converted to th e corresponding dimethyl complexes with MeLi . LiBr. All complexes wer e characterized by standard techniques, with absolute configuration es tablished by circular dichroism and X-ray diffraction. For the (R) - R = (-)-menthyldichloro complex (2a): space group = P2(1)2(1)2(1); a = 9.404(2), b = 9.817(3), c = 28.684(7) Angstrom(-120 degrees C), Z = 4 ; R(F) = 0.056, R(w)(F) = 0.061 for 2025 reflections having I > 3 sigm a(I). For the (R) - R = (-)-menthyldimethyl complex (3a): space group = P2(1); a = 9.501(3), b = 9.394(3), c = 15.565(3) Angstrom, beta = 1 03.76(2)degrees(-120 degrees C), Z = 2; R(F) = 0.037, R(w)(F) = 0.039 for 1528 reflections having I > 3 sigma(I). Reaction of either (R)-3a or (R)-3b with B(C6F5)3 in toluene yields two spectroscopically discer nible methyl cations. The temperature dependence of the ion-pair equil ibrium constant in toluene yields Delta H = -0.7(1) kcal/mol and Delta S = -3.1(1) eu for (R)-3a and Delta H = 0.14(3) kcal/mol and Delta S = -3.1(1) for(R)-3b. ''Cationic'' propylene polymerization catalysts w ere generated from 2 + methylalumoxane or 3 + methylalumoxane, B(C6F5) (3), Ph(3)C(+)B(C6F5)(4)(-), or HN((n)Bu(3))B-+(C6F5)(4)(-). Polymeriz ation activities, stereoregularities, and polymer molecular weights ar e strongly dependent on cocatalyst identities and concentrations, sugg esting strong, structure-sensitive ion-pairing effects, Polypropylene isotacticities as high as 95% mmmm pentad content are observed, with s tereoregularity increasing and polymerization activity falling with de creasing reaction temperature.