Abstract
Propene homopolymers have been produced by employing three C1-symmetric metallocene molecules (1, 2 and 3), each having t-butyl substituent(s) on the Cp, on the fluorenyl or on both aromatic moieties activated with methylaluminoxane at different polymerization temperatures and monomer concentrations. Polymers’ microstructures determined by 13C NMR spectroscopy suggest that the otherwise dominant alternating mechanism governed by the chain migratory insertion is largely replaced by the competing site epimerization mechanism, as a direct result of the imposing steric bulk of the t-butyl substituent on one of the distal positions of the Cp moiety. This phenomenon is more pronounced with 3 when a second t-butyl is present in the same half-space of the molecule making the site epimerization mandatory. The lower activity of catalyst 3 with respect to catalyst 2 is also in line with the necessity for the polymer chain to back-skip (or the site to epimerize) to its original position before the subsequent monomer insertion. Chain end group analyses by 1H NMR spectroscopy have revealed that the formation of vinylidene end groups, either via β-H elimination or as a result of direct chain transfer to the monomer after an ordinary 1,2-insertion is the prevailing chain termination route. A correlation between the relative concentrations of vinylidene end groups of polypropene (PP) polymers produced with each catalyst and the corresponding polypropenes’ molar masses was found, indicating the lower the relative concentrations of vinylidene end groups, the higher the molar masses.
Highlights
The advent of homogeneous ansa metallocene catalysts for stereospecific olefin polymerization had been considered a groundbreaking event in this industrially very important process [1,2,3].The possibility of rationally modifying the ligand structure of these systems allow one to have a high degree of control over polymer tacticity, comonomer incorporation/distribution, molar masses and molar mass distributions, over their physical and mechanical properties
The three metallocene precatalysts (1, 2 and 3) shown in Scheme 1 were activated with MAO and evaluated in homogeneous polymerization of propene
They should be independent from monomer concentration, ; there is a clear relationship between propene concentration in polymerization and allyl end groups; the greater the propene concentration the lower the allyl end groups
Summary
The advent of homogeneous ansa metallocene catalysts for stereospecific olefin polymerization had been considered a groundbreaking event in this industrially very important process [1,2,3].The possibility of rationally modifying the ligand structure of these systems allow one to have a high degree of control over polymer tacticity, comonomer incorporation/distribution, molar masses and molar mass distributions, over their physical and mechanical properties. Since the discovery of these catalyst systems, their catalytic behavior and the origin of their syndiospecificity have been the subject of intensive debate by many authors in numerous publications [8,9,10,11,12,13,14,15]. These systems endowed with two enantiomorphic sites offer the possibility of testing the existing theories put forward on the origin of the catalyst’s stereocontrol [16,17].
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