Structure-activity relationship in olefin polymerization catalysis: is entropy the key?

Activation parameters for propene polymerization mediated by a bis(phenoxyamine)Zr-dibenzyl catalyst in combination with MAO have been measured experimentally and calculated by DFT; experiment and calculation consistently indicate that the entropic term is the most important reason for the low chain propagation rate with this system. Based on this finding and a review of literature data on a variety of olefin polymerization catalysts, we propose a strong correlation between the propagation rate and how catalysts deal with the entropy loss of monomer capture.

Activation of a bis(phenoxy-amine) precatalyst for olefin polymerisation: first evidence for an outer sphere ion pair with the methylborate counterion.

A bis(phenoxy-amine)ZrMe(2) complex (1) was synthesized and activated with B(C(6)F(5))(3) obtaining an ion pair (2) having the MeB(C(6)F(5))(3)(-) counterion in the second coordination sphere as deduced by NMR studies. Both DFT calculations and NMR investigations suggest that the activation is coupled with a change of the binding modality of the tetradentate ligand from cis(N,N)-trans(O,O) in the octahedral dimethyl precursor to 1 cis(N, N)-cis(O,O) in the ion pair 2.

Binuclear complexes of bis-chelating ligands based on [1,4]dioxocino[6,5-b:7,8-b']dipyridine moieties.

Ligands based on a [1,4]dioxocino[6,5-b:7,8-b']dipyridine (doxpy) core were prepared and characterized. They all present two equal chelating moieties each one including one N, O, or S donor in addition to a pyridinic nitrogen. These ligands displayed high selectivity for the formation of binuclear complexes. At least one d(8) ion (Pd(II) or Pt(II)) complex was prepared for each type of ligand. The stereochemical behavior of the ligands is discussed on the basis of NMR spectra. Stable atropoisomers were obtained in the case of N-oxides or in case chiral centers were introduced in the ethereal bridge. As for the complexes, stable enantiomers appear to be in principle attainable for all the new compounds. A test on the cooperative ability of two Pd(II) centers has been grounded on the microstructure of the styrene/CO copolymer catalytically produced by a binuclear pyridine-imino complex. In fact, comparison with the microstructure of the copolymers produced by related single-site mono- and (open-chain) binuclear catalysts reveals significant difference, thus giving indication of possible synergic metal activity.