Tailoring the Metallocene Structure To Obtain LLDPE by Ethene Homopolymerization: An Experimental and Theoretical Study
摘要:
The mechanism of formation of branched polyethylene in the polymerization promoted by meso-metallocene/MAO systems has been investigated by combining DFT calculations and experimental results on differently featured meso-zirconocenes. A possible explanation for the required meso structure in the formation of branches considers a competition of ethene insertion and beta-hydrogen transfer to the monomer. General mechanistic considerations in order to design catalytic systems able to synthesize LLDPE by ethene homopolymerization have been outlined.
Regiochemistry of the Styrene Insertion with CH2-Bridged ansa-Zirconocene-Based Catalysts
摘要:
eMethylenebis(indenyl)zirconium dichloride substituted in C(3), activated by methylalumoxane, is able to give polystyrene and ethylene-styrene copolymers. In this study hydrooligomers, whose structure, determined by C-13 NMR and GC-MS techniques, gives information about the regiochemistry and the stereochemistry of styrene insertion, have been purposefully prepared. The regiochemistry of the styrene insertion is related to the encumbrance of substituents in C(3). rac-[Methylene-(3-R-1-indenyl)(2)]ZrCl2 with R = H, CH3, or CH2CH3 induces a prevailingly secondary styrene insertion into the zirconium-carbon bond. With increasing the substituent's steric hindrance (R = CH(CH3)(2)), regiochemistry inversion occurs and the primary insertion becomes prevailing. The analysis of ethylene-styrene copolymers obtained in the presence of the different catalysts allows confirming the correlation between regiochemistry and comonomers' reactivity. Besides, also the stereospecificity can be evaluated from the structure of the hydrotrimers, when the insertion is primary. Whereas the isospecificity in the absence of substituents (secondary insertion) and in the presence of the tert-butyl substituent (primary insertion) is well-known, a surprising syndiospecificity is observed when the indenyl ligand bears the isopropyl substituent in C(3).