Stereochemical control mechanisms in propylene polymerization mediated by C1-symmetric CGC titanium catalyst centers

Alessandro Motta, Ignazio L. Fragalà, Tobin J Marks

Research output: Contribution to journalArticle

26 Citations (Scopus)

Abstract

This work analyzes stereochemical aspects of olefin polymerization processes mediated by the C1-symmetric constrained geometry catalyst H2Si(ind)(tBuN)TiCH3 + (ind = indenyl), including the role of the cocatalyst/counteranion. The energetics of catalyst activation are first analyzed and shown to compare favorably with experiment. The energetics of heterolytic ion pair separation are next scrutinized, and the effects of solvation environment are assessed. Computed thermodynamic profiles for ethylene insertion at H2Si(ind)( tBuN)TiCH3 + indicate that the kinetics of insertion processes at the H2Si(ind)(tBuN)TiR+ cation can be analyzed in terms of SCF potential energies. We next compare the energetic profile for ethylene insertion at the naked H2Si(ind)( tBuN)TiCH3 + cation with that at the related H2Si(ind)(tBuN)TiCH3 +H 3CB(C6F5)3 - ion pair to understand counterion effects. It is seen that the counterion, although affecting overall catalytic activity, does not significantly influence enchainment stereochemistry or polymer microtacticity. Next, the second ethylene insertion at H2Si(ind)(tBuN)Ti(nC 3H7)+H3CB(C6F 5)3 - is analyzed to evaluate counteranion influence on the propagation barrier. It is found that the ethylene uptake transition state is energetically comparable to the first insertion transition state and that solvation has negligible effects on the energetic profile. These findings justify analysis of the propylene insertion process within the less computationally demanding "naked cation" model. Thus, monomer enchainment at H2Si(ind)(tBuN)TiR+ is analyzed for H2Si(ind)(tBuN)TiCH3 + + propylene (first insertion) and for H2Si(ind)(tBuN) Ti(iC4H6)+ + propylene (second insertion). Data describing the first insertion highlight the sterically dominated regioselection properties of the system with activation energies indicating that olefin insertion regiochemistry is predominantly 1,2 (primary), while the second insertion similarly reflects the catalyst stereoinduction properties, with steric effects introduced by the growing chain (mimicked by an isobutyl group) preferentially favoring insertion pathways that afford isotactic enrichment, in agreement with experiment.

Original languageEnglish
Pages (from-to)7327-7338
Number of pages12
JournalJournal of the American Chemical Society
Volume129
Issue number23
DOIs
Publication statusPublished - Jun 13 2007

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Titanium
Polymerization
Propylene
Ethylene
Cations
Catalysts
Positive ions
Solvation
Alkenes
Olefins
Ions
Stereochemistry
Potential energy
Thermodynamics
Catalyst activity
Polymers
Activation energy
Monomers
Experiments
Chemical activation

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

Stereochemical control mechanisms in propylene polymerization mediated by C1-symmetric CGC titanium catalyst centers. / Motta, Alessandro; Fragalà, Ignazio L.; Marks, Tobin J.

In: Journal of the American Chemical Society, Vol. 129, No. 23, 13.06.2007, p. 7327-7338.

Research output: Contribution to journalArticle

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