Neodymium tris-diarylphosphates: Systematic study of the structure–reactivity relationship in butadiene and isoprene polymerisation
Graphical abstract
Introduction
The discovery of catalytic systems based on lipophilic organic neodymium salts (phosphates and carboxylates) for diene polymerisation that occurred more than 30 years ago [1] triggered vigorous development of neodymium-induced polymerisation of dienes [2]. According to the commonly accepted mechanism of catalysis proposed by Kwag [3] and developed by Anwander [2b] (see Scheme 1), the catalytic species A and B contain in their structures one anion of the initial neodymium salt. Thus, the structures of neodymium salt anions can affect their catalytic properties.
Although there are many publications on lanthanide-catalysed diene polymerisation, and detailed reviews are available on this topic [2], little is known about the relationship between the anion structure of lanthanide salts and the catalytic properties of the corresponding catalysts. For example, the dependence of the performance of neodymium catalysts on the structure of neodymium organophosphate and carboxylate anions has not been systematically studied. Anwander prepared some neodymium derivatives based on orthosubstituted benzoic acids (see Fig. 1b). Unfortunately, due to the difficulty of obtaining the benzoic acids used and the moderate performance of the resulting neodymium complex catalysts, no further studies of the Anwander-type catalysts derived from substituted benzoic acids were performed. Some comparison data have been presented in the patent of Asahi [5], see Fig. 1a, but the Japanese salts are not individual compounds and presumably represent gels [6]. Additionally, no information has been presented by Asahi about neodymium derivatives with bulky phosphates. Therefore, the range of ligands studied is insufficient to draw conclusions about the relationship between the anion structure of neodymium salts and the catalytic properties of the corresponding catalysts. The influence of the electronic properties of ligands on the catalyst properties has been barely studied so far. Kobayashi [7] investigated the activity of catalysts based on neodymium and gadolinium halogenated acetates as a function of the acidity of the acid involved (RCOOH, where R = CH3, CH2Cl, CHCl2, CCl3, CF3), but no unambiguous dependence was derived for neodymium. In a paper [8] dealing with samarocenes, the authors demonstrated that the complex with medium-size ligands was most active in butadiene polymerisation. However, because of the very low activity and poor accessibility of these complexes, they are of little value for industrial applications. In addition, the nature of these compounds (metallocene compounds with divalent metals) differs strongly from the nature of neodymium salts used in industry. Correspondingly, the mechanism of catalysis and the structural dependence can differ appreciably from those of catalysis by neodymium salts.
Generally speaking, the influence of the ligands structure on the catalyst properties of the corresponding catalysts may be caused by several factors. First, the steric and electronic properties of the anion can affect the properties of a catalytic species because the anion is part of this species (see Scheme 1). Second, the lipophilic properties of the anion affect the solubility of both the initial neodymium salt and the final catalyst and intermediates. Researchers have focused primarily on this aspect; most often, salts with highly lipophilic anions are chosen (see Fig. 1). Third, the anion structure may affect the degree of association of the neodymium salts. Depending on the bulk of the ligand and the type of solvation, these salts can be monomeric [9], dimeric [9], [10], or polymeric [3], [4], [11]. For neodymium polymerisation, oligomeric salts are used most often [2]; meanwhile, oligomerisation prevents complete transfer of neodymium into catalytic species, which decreases the activity and deteriorates other catalytic properties [3].
Previously, we demonstrated [10a] that lanthanide tris-arylphosphates with structurally rigid bulky ligands based on 2,6-di-tert-butyl-4-methylphenol (ionol) are specific compounds. In contrast to all other known organic salts of lanthanides, they are individual non-oligomeric anhydrous compounds (dimers) that form low-viscosity solutions in hexane. The structures of some lanthanide tris-arylphosphates were characterised by X-ray crystallography, and it was shown that neodymium derivative 1 has promising properties in diene polymerisation (see Fig. 2).
The purpose of this work was to systematically study the catalytic properties of neodymium tris-phosphates with structurally rigid diarylphosphate ligands in stereoregular butadiene and isoprene 1,4-cis-polymerisation. We demonstrated that diverse neodymium tris-diaryl phosphates can be obtained in two steps from most readily available ortho- and para-substituted phenols through diarylphosphoric acids.
Section snippets
Experimental procedure
Esters 2–15 were prepared by the procedures published for 2 and 3 [10], [12]. Neodymium tris-phosphates were obtained from Nd(NO3)3 × 6H2O (Reakhim) and NdCl3 × 6H2O (Aldrich). Hexane used in the catalyst synthesis and polymerisation was dried by refluxing over sodium wire. Isoprene, butadiene and piperylene were dried twice over granulated alumina and were recondensed into a tube with a metallic tap containing granulated alumina. NMR spectra were recorded on a Bruker Avance 400 instrument in CDCl3
Synthesis of diarylphosphoric acids
The diarylphosphoric acids were prepared by reactions of the most readily available phenols with phosphorus oxychloride followed by hydrolysis of the intermediate chloroanhydrides [12]. Hydrolysis was conducted either in aqueous pyridine or in aqueous acetone (see Scheme 2). The first step of our study resulted in a set of diarylphosphoric acids containing various substituents and suitable for the synthesis of neodymium derivatives.
Synthesis of neodymium tris-diarylphosphates
Neodymium tris-diarylphosphates were prepared by reactions of
Effect of the spatial properties of ligands on the catalyst activity
The data presented in Table 2 (entries 1–28) can be used to study the dependence of the catalyst activity on both the bulk of the ligands and the electronic properties of the substituents. Fig. 3 shows the dependence of the catalyst activity on the bulk of the ortho-substituents in diaryl phosphates.
The curves for the activity versus bulk of the substituents in the ligands (Fig. 3) show that a change in the ligand size has no significant effect on the activity. However, phosphates with more
Conclusion
We prepared a series of neodymium tris-diarylphosphates with a variety of substituents in aryl phosphate ligands and used them to investigate the dependence of the catalytic properties of neodymium salts on the anion structure. The obtained data unambiguously indicated that the structure of the ligand of the initial salt has a pronounced effect on the properties of the neodymium catalyst. This effect is caused by the following three factors:
- 1.
Electronic properties of substituents.
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