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3 results on '"multisite catalysis"'

1. C–H Activation and Olefin Insertion as Sources of Multiple Sites in Olefin Polymerization Catalyzed by CpAlkylHf(IV) Complexes

Author

Alceo Macchioni, Roger L. Kuhlman, Leonardo Tensi, Cristiano Zuccaccia, and Anthony P. Gies

Subjects
hafnocenes, Substituent, multisite catalysis, olefin polymerization, NMR, MALDI, ANSA-METALLOCENE CATALYSTS, HAFNIUM ALKYL COMPLEXES, ETHYLENE POLYMERIZATION, LIVING POLYMERIZATION, 1-HEXENE, ZIRCONIUM, LIGAND, COPOLYMERIZATION, PROPYLENE, MONOMER, 010402 general chemistry, Photochemistry, 01 natural sciences, Catalysis, chemistry.chemical_compound, Cyclopentadienyl complex, Polymer chemistry, MALDI, Olefin fiber, 010405 organic chemistry, Cationic polymerization, ANSA-METALLOCENE CATALYSTS, General Chemistry, Metallacycle, 0104 chemical sciences, chemistry, Intramolecular force, Molar mass distribution
Abstract

Intramolecular activation of hydrocarbyls to form metallacyclic complexes is a relatively fast process in cationic hafnocene catalysts bearing propyl-substituted Cp ligands. The resulting metallacycles are effective 1-hexene polymerization catalysts with activities comparable to that of the nonmetalated precursor. Ad hoc polymerizations of 1-hexene, using (CpPr)2HfMe2 as catalyst precursor, allow the isolation and characterization, via nuclear magnetic resonance (NMR) and matrix-assisted laser desorption ionization (MALDI) techniques, of polymers containing (CpCH2–CH2–CR3)2HfCl2 (R = H or polymeryl) units. The polymeryl substitutions arise from irreversible incorporation of polymer chains onto the cyclopentadienyl ligand substituent(s) via metallacycle intermediates. As a consequence of such “self-modification”, multiple active sites are generated by a nominally single-site catalyst; this may explain the broadening of the molecular weight distribution (MWD) and chemical composition distribution (CCD) obse...

Published
2016
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2. Selective distribution and contribution of nickel based pre-catalyst in the multisite catalyst for the synthesis of desirable bimodal polyethylene.

Author

Chen, Mei, Chen, Yuming, Li, Wei, Dong, Chuanding, Liang, Peng, Wang, Ning, Jiang, Binbo, Wang, Jingdai, and Yang, Yongrong

Subjects
*NICKEL, *CATALYST synthesis, *POLYETHYLENE, *YOUNG'S modulus, *MOLECULAR weights
Abstract

• POSS molecules selectively combine with the nickel pre-catalyst. • DFT calculations and DRIFTS have helped to understand the distribution of active sites. • The target dispersion of nickel sites enhances the branched UHMWPE fraction. • The overall mechanical properties of bimodal polyethylene are substantially improved. We report a selective immobilization strategy to control the catalytic performance of the target active species in hybrid multisite catalysts. The sterically bulky and electronically donated polyhedral oligomeric silsesquioxane (POSS) is introduced onto the support and offers extra immobilization sites for the target pre-catalyst. DFT and CO low-temperature adsorption facilitate to understand the distribution of active sites. The nickel pre-catalyst can interact with the POSS, endowing the scattered distribution of nickel sites. The large steric and electronic donated iron pre-catalyst exhibits the inertness to POSS, confining iron species on the POSS-free space. The selective dispersion of nickel species enhances its contribution in the synthesis of UHMWPE fraction with an exceptional activity, a much higher molecular weight and branching degree. This promoted contribution synchronously enhances the impact resistance (+27%), tensile strength (+53%), Young's modulus (+111%) and elongation ratio (+37%) of the synthesized bimodal PE compared to those of commercial UHMWPE. [ABSTRACT FROM AUTHOR]

Published
2020
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