Metallocene polymerization catalyst ion-pair aggregation by cryoscopy and pulsed field gradient spin-echo NMR diffusion measurements

Pulsed field gradient spin-echo (PGSE) NMR and cryoscopic measurements have been performed on a series of homogeneous metallocene polymerization catalyst ion-pairs to determine if aggregation is a significant phenomenon under typical polymerization conditions. Cryoscopic measurements on (Me5Cp)2ZrMe]+MeB(C6F5)3]- (1), rac-Et(Indenyl)2ZrMe]+MeB(C6F5)3]- (2), (1,2-Me2Cp)2ZrCHTMS2]+MeB(C6F5)3]- (3), Me2Si(Me4Cp)(t-BuN)TiMe]+MeB(C6F5)3]- (4), Me2Si(Me4Cp)(t-BuN)ZrMe]+MeB(C6F5)3]- (5), and Me2C(Fluorenyl)(Cp)ZrMe]+MeB(C6F5)3]- (6) were carried out in benzene in the 10-18 millimolal concentration range. PGSE measurements, using (p-tolyl)4Si as an internal standard, were also performed on catalyst ion-pairs 1, 4, 6, (Me5Cp)2ThMe]+B(C6F5)4]- (7), (Me2SiCp2)ZrMe]+MeB(C6F5)3]- (8), and Cp2ZrMe]+MeB(C6F5)3]- (9) in the 0.8-10.0 millimolar range. All results are consistent with a 1:1 ion-pair structural model and show little evidence for ion-quadruples or higher-order aggregates.