Living Polymerization of 1-Hexene by Cationic Zirconium and Hafnium Complexes that Contain a Diamido/Donor Ligand of the Type [H3CC(2-C5H4N)(CH2NMesityl)2]2-. A Comparison of Methyl and Isobutyl Initiators

Addition of [Ph 3 C][B(C 6 F 5 ) 4 ] to [MesNpy]Hf(i-Bu) 2 ( 1a; [MesNpy] 2 - = [H 3 CC(2-C 5 H 4 N)(CH 2 N-2,4,6-Me 3 C 6 H 2 ) 2 ] 2 - ) yields {[MesNpy]Hf(i-Bu)}[B(C 6 F 5 ) 4 ] ( 2a ) quantitatively. Compound 2a decomposes in a first-order manner at 0 °C in C 6 D 5 Br with k d = 9.2 × 10 - 6 s - 1 . The polymerization of 1-hexene by 2a at 0 °C in C 6 D 5 Br was found to follow first-order kinetics with a k p = 0.10(1) M - 1 s - 1 . The polymerization is well-behaved up to 10 °C with Δ H ⧧ = 10.82 kcal mol - 1 and Δ S ⧧ = −23.01 cal mol - 1 K - 1 (at [Hf] = 1 M). Up to 600 equiv of 1-hexene were polymerized by 2a in chlorobenzene to give atactic poly[1-hexene] with the expected M n values and polydispersities between 1.02 and 1.05. Compound 1a could be activated with B(C 6 F 5 ) 3 to yield {[MesNpy]Hf(CH 2 CHMe 2 )}[HB(C 6 F 5 ) 3 ] ( 2a ‘), which will polymerize 1-hexene in C 6 D 5 Br at 0 °C with k p = 0.048(1) M - 1 s - 1 and in toluene with k p = 0.0111(3) M - 1 s - 1 . Well-behaved inhibitors for 1-hexene polymerization include diisopropyl ether, hexyltrimethylsilyl ether, triethylamine, and tributylamine, but not diphenyl ether or dimethylaniline, both of which undergo CH activation. Activation of [MesNpy]Zr(i-Bu) 2 ( 1b ) with [Ph 3 C][B(C 6 F 5 ) 4 ] in C 6 D 5 Br gave {[MesNpy]Zr(i-Bu)}[B(C 6 F 5 ) 4 ] ( 2b ), which decomposed at 0 °C by β-hydride elimination in a first-order manner with k d = 8.3 × 10 - 6 s - 1 . Polymerization of 100 equiv of 1-hexene by 2b appeared to be well-behaved at temperatures up to 10 °C (Δ H ⧧ = 8.08 kcal mol - 1 and Δ S ⧧ = −33.34 cal mol - 1 K - 1 for [Zr] = 1 M), although sensitivity of 1b to light limited reproducibility in general. Activation of [MesNpy]ZrMe 2 ( 4 ) with [Ph 3 C][B(C 6 F 5 ) 4 ] resulted in formation of primarily catalytically inactive {[MesNpy] 2 Zr 2 Me 3 }[B(C 6 F 5 ) 4 ]. However, ∼10% of the theoretical amount of {[MesNpy]ZrMe}[B(C 6 F 5 ) 4 ] is not trapped by [MesNpy]ZrMe 2 and therefore is available to initiate polymerization of 1-hexene slowly at room temperature. Bulk polymerization results obtained for this system support the assertion that only ∼10% of the added metal centers are active for polymerization. Activation of [MesNpy]HfMe 2 with [Ph 3 C][B(C 6 F 5 ) 4 ] also results in formation of inactive {[MesNpy] 2 Hf 2 Me 3 }[B(C 6 F 5 ) 4 ].