Catalytic propene polymerization: determination of propagation, termination, and epimerization kinetics by direct NMR observation of the (EBI)Zr(MeB(C6F5)3)propenyl catalyst species.
Treatment of (EBI)ZrMe2 with B(C6F5)3 and 100 equiv of 1-hexene in toluene-d8 at -40 degrees C for 1 h, followed by addition of >5 equiv of propene, generates a living polypropene catalyst (3) with a polyhexene "tail". Upon further addition of propene, speciation of the catalyst and propagation kinetics are measured directly by NMR, yielding a propagation rate law that is first order in [propene] and [living catalyst] with a rate constant of 2.6 M-1 s-1 at -40 degrees C and the activation parameters DeltaH = 3.4(0.2) kcal/mol and DeltaS = -42(2) cal/(mol K). In the absence of propene, elimination of vinylidene-terminated polymer is observed with a first-order rate constant of 3.3 (0.5) x 10-5 s-1 at -40 degrees C and DeltaH = 12.9(1.4) kcal/mol and DeltaS = -23(5) cal/(mol K). Use of 1-13C-propene and 1,1-2H-2,3-13C-propene and 1H, 19F, and 13C NMR spectroscopy provide rigorous characterization of the living catalyst and enable the first direct observation of both 1,3-isomerization and chain-end epimerization processes at a metallocene catalyst. Isomerization and epimerization occur with rates that are similar to that of vinylidene termination and are consistent with previous mechanistic hypotheses featuring isomerization via a tertiary alkyl intermediate and switching between enantiofaces of the coordinated alkene.
Sillars
Douglass R
DR
Department of Chemistry, University of Wisconsin, 1101 University Avenue, Madison, WI 53706, USA.
Landis
Clark R
CR
D000475:Alkenes / Q000737:chemistry*; D003080:Cold Temperature; D007536:Isomerism; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D009942:Organometallic Compounds / Q000737:chemistry; D011126:Polypropylenes / Q000138:chemical synthesis* / Q000737:chemistry; D015040:Zirconium / Q000737:chemistry
D000475:Alkenes; D009942:Organometallic Compounds; D011126:Polypropylenes; C013658:propylene; D015040:Zirconium