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New bridged zirconocenes for olefin polymerization: Binuclear and hybrid structures
Abstract
Four newly synthesized bridged bisindenyl zirconocenes which are derivatives of known structures have been tested in polymerizations of liquid propylene with methylaluminoxane as cocatalyst. They include two asymmetric and two binuclear systems which are suitable for isotactic polymerization. Surprising differences from the behavior of similar systems (regiospecificity, chain termination) are found and discussed.
... The interest in this area has primarily been driven by the potential to exploit intermetallic cooperativity/synergism between the two or more proximal metal centers to eventually enhance the performance of polymerization systems. For instance, several studies on dinuclear catalysts suggest that catalyst activity [3][4][5][6][7][8][9][10][11][12][13] and molecular weight, [3,7,10,14] tacticity, [13][14][15], or comonomer incorporation [8,[16][17][18][19][20] of/in the resulting polymers Catalysts 2018, 8, x FOR PEER REVIEW 3 of 19 Scheme 2. Synthesis of the para-and meta-phenylene-bridged bis(fulvenes) 1a-c. ...
... The interest in this area has primarily been driven by the potential to exploit intermetallic cooperativity/synergism between the two or more proximal metal centers to eventually enhance the performance of polymerization systems. For instance, several studies on dinuclear catalysts suggest that catalyst activity [3][4][5][6][7][8][9][10][11][12][13] and molecular weight, [3,7,10,14] tacticity, [13][14][15], or comonomer incorporation [8,[16][17][18][19][20] of/in the resulting polymers Catalysts 2018, 8, x FOR PEER REVIEW 3 of 19 Scheme 2. Synthesis of the para-and meta-phenylene-bridged bis(fulvenes) 1a-c. ...
New ligand platforms of the type p- or m-Ph{-CR(3,6-tBu2Flu)(Cp)}2 (para-, R = Me (2a), H (2b); meta-, R = Me (2c)) were synthesized via nucleophilic addition of the 3,6-tBu2-fluorenyl-anion onto the parent phenylene-bridged difulvenes (1a–c). The corresponding discrete homodinuclear zirconium and hafnium bis(dichloro ansa-metallocene) complexes, Ph[{-CR(3,6-tBu2Flu)(Cp)}MCl2]2 (p-, R = Me (3a-Zr2, 3a-Hf2), R = H (3b-Zr2); m-, R = Me (3c-Zr2), were prepared by salt metathesis reactions. An attempt to generate in situ a heterodinuclear complex 3a-Zr-Hf was also undertaken. For the first time, Atmospheric Pressure PhotoIonization (APPI) mass-spectrometric data were obtained for all dinuclear compounds and found to be in excellent agreement with the simulated ones. Preliminary studies on the catalytic performances of these dinuclear complexes, upon activation with MAO, in ethylene homopolymerization and ethylene/1-hexene copolymerization revealed a few differences as compared to those of the monometallic analogues. In particular, slightly lower molecular weights and a greater formation of short methyl and ethyl branches were obtained with the dinuclear systems.
... Although so far only one family of Group 4 bimetallic catalysts have been described, a wide range of covalently tethered alternatives have been investigated with varying successes. Phenylene [177][178][179]-, polymethylene [180][181][182]-, and silyl [183,184]-bridging moieties have all been shown to have some cooperative effects resulting in increased molecular weight or improved comonomer incorporation. Configurationally restrained ligands give an alternative to their flexible counterparts by maintaining proximate metal centres. ...
The discovery of transition metal-catalysed polymerisation of olefins in the 1950s by Ziegler and Natta was of huge importance. Over the last 30 years, the interest in homogenous polymerisation has not only grown but has changed focus from primarily studying the metallocene complexes of Group 4 to widespread exploration of post-metallocene systems. This is a consequence of extensive patenting of the metallocene catalyst systems, as well as for general interest in the improvement of polyolefin catalysis. The plastics industry produces more than 107 tonnes of polyethylene, polypropylene and polystyrene each year worldwide, and the study of well-defined homogenous catalysts is invaluable in uncovering the mechanism of polymerisation and the fundamental properties of the active species. This short Tutorial Review gives an overview of homogenous transition metal Ziegler–Natta catalysis in general as well as an overview of a selection of Group 4 post-metallocene catalysts. An overview of the synthesis and reactions of alkyl cations relevant to olefin polymerisation is also given. Finally, this review summarises recent advances in bimetallic catalysts for olefin polymerisation.
... Some researchers measured polymerization yields in liquid propylene, 17,18 but much more kinetic information can be extracted from polymerization rate profiles. The first article reporting rate profiles in catalytic liquid propylene polymerization was published by Samson et al., 4 and a number of other authors investigated the hydrogen effect on kinetics (see Pater et al. 2 and Shimizu et al. 3 ). ...
Liquid propylene batch experiments in the absence of a gas phase have been carried out using a highly-active MgCl2/TiCl4/phthalate/silane/AlR3 catalyst at varying temperatures (60-80°C) and molar hydrogen-monomer ratios of 0-10 mmol/mol. With increasing hydrogen concentration the
polymerization rate increases rapidly, reaching a constant value at concentrations above 1.4 mmol/mol;
pseudo-first-order catalyst deactivation constant increases;
molecular weight decreases;
polydispersity decreases slightly;
but average molecular weight and polydispersity increase with increasing temperature. Polymerization rate, deactivation constant, and average molecular weight can be modeled based on a consistent dormant site mechanism assuming an (averaged) quasi-single-site model. © 2006 American Institute of Chemical Engineers AIChE J,2006
Ion pairing is known to play a key role in stoichiometric and catalytic reactions mediated by charged transition metal complexes, especially when they are conducted in low-polar solvents. Molecular olefin polymerization catalysis by group IV metal complexes represents a textbook case in this respect, due to the high electrophilicity of the involved cationic active species and the very low relative permittivity (~ 2) of the commonly used hydrocarbon solvents. As a matter of fact, ion pairing has a huge impact on activity and selectivity of olefin polymerization catalysts. Ion pairing is strictly related to the activation process, since the cationic active species is usually generated by the reaction of a neutral precursor with a suitable cocatalyst (activator). The counterion generated from the latter reaction has to be weakly coordinating in order to disfavor its competition with the olefin for the occupancy of the coordination vacancy of the active metal center. Herein, after having recalled some generalities of olefin polymerization and nature of activators, using ion pairing as a reading key, the main findings concerning with the structure and self-aggregation tendency of ion pairs relevant to olefin polymerization, as deduced by means of advanced NMR techniques, are described. Representative examples of structure/activity relationships are then discussed, suggesting some rationale to interpret the different catalytic behavior of various ion pairs. Finally, cases in which the activation process, besides providing the expected active ion pair, leads to further layers of complexity (in situ ligand modification, dimer formation, etc.) are illustrated.
In a batch-wise operated autoclave reactor, liquid propylene was polymerized using a 4th generation, TiCl4/MgCl2/phthalate ester–AlEt3–R2Si(OMe)2, Ziegler–Natta catalyst system. By using a calorimetric principle it was possible to measure full reaction rate versus time curves for obtaining data on polymerization kinetics, under industrially relevant conditions. The influence of polymerization temperature, the hydrogen and monomer concentration, and the prepolymerization method on reaction kinetics were investigated.
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