Photoinduced rac/meso interconversions of bridged bis(indenyl) zirconium dichlorides

Kaminsky, Werner; Schauwienold, Anne-Meike; Freidanck, Frank

doi:10.1016/1381-1169(96)00240-3

Cited by 49 publications

(26 citation statements)

References 24 publications

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“…Indeed it is known that daylight irradiation can produce photoinduced rac/meso interconversion of the metallocene catalysts. [5] This is in fact the explanation suggested by van Reenen to account for our data.…”

supporting

confidence: 52%

“…Moreover, chain termination occurs more frequently on the meso than on the more sterically hindered racemic form. [5,6] This is in contrast with the fact that all copolymers prepared with MBI, as well as those prepared with EI, featured narrow polydispersities and an almost random distribution of the comonomer (r P Á r Cm % 1) which are typical of single site catalysts. This holds both for copolymers with 1-pentene and with other higher alpha-olefins.…”

contrasting

confidence: 42%

“…Indeed under the polymerization conditions followed by van Reenen, MBI seems to contain a percentage of the meso form that should be remarkable. In fact, although propylene is known to have a very low reactivity on the meso form of MBI, [5,6] at least 40% (we guess this datum from inspection of the spectrum, as van Reenen's note does not contain any experimental details) of the sample is constituted by atactic poly(propylene).…”

Section: Runmentioning

confidence: 99%

“…Moreover, if the catalyst contains a mixture of the meso and racemic isomers, in principle, the ensuing copolymer should be a mixture of isotactic and atactic copolymers, the latter being likely characterized by higher comonomer content and lower molecular weight than the former. In fact it has been shown by Kaminsky [5,6] that, differently from propylene, the higher a-olefins have remarkably greater polymerization activity with the meso form of dimethylsilyl bridged metallocenes than with the racemic one. Moreover, chain termination occurs more frequently on the meso than on the more sterically hindered racemic form.…”

mentioning

confidence: 99%

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Unexpected Formation of Atactic Blocks in Propylene/1‐Pentene Copolymers from rac‐Me₂Si(2‐MeBenz[e]Ind)₂ZrCl₂

Forlini

Tritto

Stagnaro

2004

Macro Chemistry & Physics

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In a previous paper, [1] that has inspired the Note by van Reenen, [2] we reported on a study of propylene/1-pentene copolymerisation in the presence of two isospecific methylaluminoxane (MAO) activated zirconocenes: rac-Et(Ind) 2 -ZrCl 2 (EI) and rac-Me 2 Si(2-MeBenz[e]Ind) 2 ZrCl 2 (MBI).Two series of propylene/1-pentene copolymers with a wide range of compositions were prepared with both catalysts. We observed a surprising result concerning the influence of the ligand structure on the copolymer stereoregularity. In fact, with the moderately isospecific catalyst EI, the isotacticity value of the poly(propylene) homopolymer was maintained for the entire series of copolymers, while with the highly stereospecific catalyst MBI the isotacticity of the copolymers differed from that of the reference homopolymer and gradually decreased at increasing comonomer content and also at increasing the steric hindrance of the comonomer. [1,3] For the sake of clarity, here we present again the methyl region of 13 C NMR spectra of the highly isotactic homopolymer from MBI (Figure 1a, run 11 [1] ) and of two copolymers containing 6.90 and 10.31 mol-% of 1-pentene, respectively (Figure 1b and 1c, run 15-16 [1] ).In Table 1, the distribution of the methyl centered steric pentads of the samples of Figure 1 and of the corresponding Summary: Supplementary experimental findings are reported in addition to those presented in a previous paper dealing with the microstructure of propylene/1-pentene copolymers prepared with rac-Me 2 Si(2-MeBenz[e]Ind) 2 ZrCl 2 . The fractionation results which demonstrate that the atactic part of the copolymer can not be separated from the isotactic one confute the hypothesis suggested by other authors according to which the catalyst used could contain a percent of impurities of the meso (aspecific) form. This evidence is in agreement with the observed narrow polydispersities and the almost random distribution of the comonomers (r P Á r Cm % 1) which are typical of single site catalysts. 13 C NMR methyl region of pentane-soluble (a) and pentaneinsoluble (b) fractions of copolymer prepared with MBI.

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supporting

confidence: 52%

contrasting

confidence: 42%

Section: Runmentioning

confidence: 99%

mentioning

confidence: 99%

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Unexpected Formation of Atactic Blocks in Propylene/1‐Pentene Copolymers from rac‐Me₂Si(2‐MeBenz[e]Ind)₂ZrCl₂

Forlini

Tritto

Stagnaro

2004

Macro Chemistry & Physics

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“…8. As such, the Franck-Condon driven progressions appear formally similar to those common in electronic excitation between states with different geometries (36)(37)(38)(39)(40)(41) Witkowski, and coworkers (42,43). This feature of the surface is included in an approximate interpolation of the behavior along each of the three OHO directions displayed in SI Appendix, Fig.…”

Section: Strong Anharmonic Coupling Inmentioning

confidence: 99%

Hidden role of intermolecular proton transfer in the anomalously diffuse vibrational spectrum of a trapped hydronium ion

Craig

Menges

Duong

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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We report the vibrational spectra of the hydronium and methylammonium ions captured in the C 3v binding pocket of the 18-crown-6 ether ionophore. Although the NH stretching bands of the CH 3 NH 3 + ion are consistent with harmonic expectations, the OH stretching bands of H 3 O + are surprisingly broad, appearing as a diffuse background absorption with little intensity modulation over 800 cm −1 with an onset ∼400 cm −1 below the harmonic prediction. This structure persists even when only a single OH group is present in the HD 2 O + isotopologue, while the OD stretching region displays a regular progression involving a soft mode at about 85 cm −1. These results are rationalized in a vibrationally adiabatic (VA) model in which the motion of the H 3 O + ion in the crown pocket is strongly coupled with its OH stretches. In this picture, H 3 O + resides in the center of the crown in the vibrational zeropoint level, while the minima in the VA potentials associated with the excited OH vibrational states are shifted away from the symmetrical configuration displayed by the ground state. Infrared excitation between these strongly H/D isotope-dependent VA potentials then accounts for most of the broadening in the OH stretching manifold. Specifically, low-frequency motions involving concerted motions of the crown scaffold and the H 3 O + ion are driven by a Franck-Condon-like mechanism. In essence, vibrational spectroscopy of these systems can be viewed from the perspective of photochemical interconversion between transient, isomeric forms of the complexes corresponding to the initial stage of intermolecular proton transfer.vibrational spectroscopy | hydrogen bonding | vibrationally adiabatic | proton transfer

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Metallocenes

Wasserman¹

2002

Encyclopedia of Catalysis

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Ever since their discovery by Sinn and Kaminsky in 1979, the family of high activity homogeneous olefin polymerization catalysts known as metallocenes has been the subject of intense research and commercial development. While a structurally diverse group, metallocene catalysts are generally considered to have a few common features: a highly electrophilic, early transition metal or rare‐earth atom as the point of polymer chain attachment; a π‐bound cyclopentadienyl ring or isoelectronic analogue attached to this atom; and characteristically narrow distributions of polymer molecular weight and short‐chain branching. While polymerization of ethylene and propylene is reviewed in some detail, the nonpolymerization catalysis and (co)polymerization of functional monomers is also surveyed. The current commercial status of metallocene technology is also briefly discussed.

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Photoinduced rac/meso interconversions of bridged bis(indenyl) zirconium dichlorides

Cited by 49 publications

References 24 publications

Unexpected Formation of Atactic Blocks in Propylene/1‐Pentene Copolymers from rac‐Me₂Si(2‐MeBenz[e]Ind)₂ZrCl₂

Unexpected Formation of Atactic Blocks in Propylene/1‐Pentene Copolymers from rac‐Me₂Si(2‐MeBenz[e]Ind)₂ZrCl₂

Hidden role of intermolecular proton transfer in the anomalously diffuse vibrational spectrum of a trapped hydronium ion

Metallocenes

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Photoinduced rac/meso interconversions of bridged bis(indenyl) zirconium dichlorides

Cited by 49 publications

References 24 publications

Unexpected Formation of Atactic Blocks in Propylene/1‐Pentene Copolymers from rac‐Me2Si(2‐MeBenz[e]Ind)2ZrCl2

Unexpected Formation of Atactic Blocks in Propylene/1‐Pentene Copolymers from rac‐Me2Si(2‐MeBenz[e]Ind)2ZrCl2

Hidden role of intermolecular proton transfer in the anomalously diffuse vibrational spectrum of a trapped hydronium ion

Metallocenes

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Unexpected Formation of Atactic Blocks in Propylene/1‐Pentene Copolymers from rac‐Me₂Si(2‐MeBenz[e]Ind)₂ZrCl₂

Unexpected Formation of Atactic Blocks in Propylene/1‐Pentene Copolymers from rac‐Me₂Si(2‐MeBenz[e]Ind)₂ZrCl₂