Group IV diamine bis(phenolate) catalysts for 1-decene oligomerization
Graphical abstract
Introduction
Oligomerization of higher α-olefins by group IV single-site organometallic complexes has attracted huge attention during the past ten years. Oligomers of higher α-olefin monomers are known as polyalphaolefins (PAO)s which can be used as fragrances, surfactants, adhesives, corrosion protective coatings, and synthetic lubricants (engine oil). Higher α-olefin monomers are usually referred to long olefinic monomers such as 1-hexene, 1-octene, 1-decene, among others [[1], [2], [3], [4], [5]]. An extraordinary class of organometallic catalysts is group IV metal systems based on aryloxide ligands furnished with heteroatom donors (mostly ONNO-based bis(o-aminophenolato) ligands). The aforementioned ligands lead to the tuning of the electronic and steric features of the synthesized complexes [6,7]. Kol and coworkers reported that, for zirconium catalysts, the activity of the complexes and the nature of the synthesized polymers are mostly affected by the mixed information of steric and electronic factors of the substituents on the phenolate rings [8,9]. While bulky ortho substituents tend to isotactic polymerization, electron-withdrawing substituents cause to very high active catalysts, however, with less tacticity and low polymer molecular weight. Theoretical [[10], [11], [12], [13]] and experimental [[14], [15], [16], [17]] investigations on a variety of group IV catalysts with chelating alkoxide ligands have revealed that further electron density on the metal center decreases the insertion barrier energy for an inserting monomer. Furthermore, for a class of titanium bis(phenolate) complexes, it was shown that electron-donating groups in the ligand lead to the increase of catalytic activity [18].
Salan type ligands are another important class of ligands employed in olefin oligomerization and polymerization experiments. All Salan ligands reported to date have featured identical phenolate rings [19]. The feature of the phenolate substituents of the symmetric Salan ligands was discovered to affect strongly the efficiency of the resulting zirconium catalysts in α-olefin polymerization. Thus, electron-withdrawing groups cause to highly active complexes, while bulky substituents cause to slightly active, but highly isospecific complexes [[20], [21], [22], [23]].
Designing new polymerization/oligomerization catalysts is ordinarily performed along with traditional parameters such as catalyst activity, [[24], [25], [26]] average molecular weight (Mn), or molecular weight distribution (MWD); [[27], [28], [29], [30]] meanwhile, these data have limited values for estimation of the polymer’s molecular architecture, i.e. MWD, unsaturated end groups, etc., at various polymerization conditions. However, by using further experimental data, kinetic parameters for various reactions involved in oligomerization can be achieved.
Herein, we wish to describe for the first time the synthesis and structural investigation of various group IV catalysts including titanium, zirconium, and hafnium metals with [ONNO] ligand (Scheme 1), as well as their catalytic performance in 1-decene oligomerization. The effects of the different metal centers, and various oligomerization parameters on the catalytic productivity and selectivity in 1-decene oligomerization were evaluated experimentally. In addition, the predominant kinetic parameters for Cat1-3 were found and compared with the reported complexes in the literature.
Section snippets
Material
All experiments including catalyst preparation processes, and also oligomerization reactions were performed under inert atmosphere in glovebox. All used solvents were dried through a solvent purification system (MBraun SPS) and stored over activated molecular sieves in a glovebox prior to use. 3,5-di-tert-buthylcatechole and 1-decene of 94 % purity were provided from Aldrich Chemical Co. (Munich, Germany), then 1-decene was dried by distilling over sodium/benzophenone prior to use.
Characterization of complexes
A series of [ONNO]-based ligands was synthesized (Scheme 1). Reaction of tetra(isopropoxide) derivatives of Ti, Zr, and Hf with 1 eq of ligand (L1) in aforementioned solvent readily yields di(isopropoxy) complexes [49]. The NMR data of the resulted catalysts are strongly related to those previously reported of their Ti analogue [50]. The NMR data for compounds 1–3 confirmed their effective C2-symmetric structure (see supporting information, Fig S1-S6) [31]. It has been accepted that existence
Conclusions
Three group IV based diamine bis(phenolate) catalysts were synthesized and fully characterized by elemental analysis and NMR spectroscopy. We indicated that synthesized complexes cause to a remarkable correlation of molecular weight, molecular weight distribution, and tacticity in 1-decene oligomerization by using [Ph3C][B(C6F5)4] as cocatalyst. Then, we proposed a mechanism for each catalyst system by using kinetic parameters and rate constants which derived from the kinetic evaluation
CRediT authorship contribution statement
Ahad Hanifpour: Data curation, Investigation, Writing - original draft. Naeimeh Bahri-Laleh: Conceptualization, Methodology, Data curation, Writing - original draft, Supervision. Mehdi Nekoomanesh-Haghighi: Writing - review & editing, Investigation. Albert Poater: Conceptualization, Methodology, Writing - original draft.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors appreciate Iran Polymer and Petrochemical Institute (IPPI) for all of the supports provided under the grant number of 43794110. A. P. is a Serra Húnter Fellow and ICREA Academia Prize 2019, and thank the Spanish MINECO for project PGC2018-097722-B-I00.
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