Direct Methyl Group Exchange between Cationic Zirconium Ziegler−Natta Initiators and Their Living Polymers:  Ramifications for the Production of Stereoblock Polyolefins

Ethylene polymerization using novel titanium catalytic precursors bearing N,N‐dialkylcarbamato ligands

“…In particular, bis(phenoxyimine) complexes of group 4 metals, which were discovered independently by Mitsui Chemical and by Coates et al,19, 20 represent the most outstanding precursors for ethylene polymerization. During the last decade, precursors containing chelate di(amido),21–24 alkoxo,25, 26 and amidinate27 ligands gained great interest for the synthesis of potential postmetallocene catalysts for the polymerization of olefins.…”

Investigation of Dynamic Intra‐ and Intermolecular Processes within a Tether‐Length Dependent Series of Group 4 Bimetallic Initiators for Stereomodulated Degenerative Transfer Living Ziegler–Natta Propene Polymerization

“…[11,12] All of these new crystalline compounds were fully characterized, including a determination of their solid-state structure by single-crystal X-ray analysis and, as presented in Figure 1, none of these static structures appear to display any significant deviations within the geometrical parameters associated with each of the metal centers when compared to the corresponding values obtained for the solid-state structure of 1. [13] This observation suggests that no substantial nonbonded steric interactions between the two metal centers have been introduced as the result of the presence of the n-alkyl tethering link, and indeed, variable 298 K) spectroscopy of 3-5 revealed that, for each bimetallic complex, rapid interconversion between the meso and racemic (rac) diastereomeric configurations of the two metal centers is rapidly occurring on the NMR time scale as the result of facile metal-centered epimerizations that have associated barriers of DG c°< 10 kcal mol À1 at the coalescence temperature, T c = 203 K. When compared to a similar barrier determined for metal-centered racemization in 1,…”

“…Charts presented in Figure 3 serve to provide a more detailed quantitative look at these microstructural differences which become fairly apparent and informative through side-by-side comparisons of the levels of incorporation of a selected subset (i.e., mmmm, rmrm, and rrrm) of the ten possible pentad stereoerrors. [13,15] Most significantly, at 100% activation, all three bimetallic initiators, 3-5, are highly isoselective, with the nature and integrated intensities of the pentad stereoerrors observed being consistent with exclusive enantiomorphic site control during propagation (i.e., mmmr:mmrr:mrrm % 2:2:1, see Figure 2). [15] Additionally, as Figure 3 reveals, the degree of stereocontrol that was achieved during propagation at 100% activation revealed no appreciable quantitative difference in the % mmmm pentad content for PP prepared using mononuclear 1 and C 8 -linked bimetallic 3, whereas, a slight decrease in this high degree of stereoselectivity is observed as the tether length decreases (cf.…”

“…[14,15] Figure 2 presents a subset of the 13 C NMR spectra for the methyl resonances of these PP samples along with those obtained by employing mononuclear 1 under identical conditions, and in particular, [Zr] tot / [2] o . At first glance, these data clearly reveal that significant differences in PP stereochemical microstructures have indeed arisen as the result of both the level of activation, which is expected on the basis of previous results obtained for 1 under degenerative transfer conditions, [3] and, perhaps more gratifyingly, as a result of the length of the tether within the bimetallic initiators.…”

“…1 H NMR spectra were recorded at 400 MHz at ambient temperature while 13 C { 1 H} NMR spectra were recorded at 125 MHz, using 1,1,2,2-tetrachloroethane-d 2 as the solvent at 70 8C. …”