ω-Dimethyl ammonium tetrakis-pentafluorophenyl borate polyisoprene as an organic template for alkylated metallocenes toward the synthesis of polyethylene beads

Abstract

In this work, we report the self-assembly in heptane of ω-dimethyl ammonium tetrakis-pentafluorophenyl borate polyisoprenes into micellar aggregates and the use of these nano-objects to support alkylated metallocenes toward ethylene polymerization. Dynamic light scattering was used to demonstrate the self-assembly and the formation of the micellar structures having a core of polar dimethyl ammonium tetrakis-pentafluorophenyl borate moieties, and a corona of polyisoprene (PI). The latter were then used as organic supports for alkylated metallocenes to produce unprecedented millimetric polyethylene beads without loss of catalytic activity.

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Introduction

In order to fit with industrial processes and thus to control the polyolefin (PE, PP) morphology, i.e. formation of beads, it is of prime importance to anchor the catalytic system onto a support. Obviously, the most investigated supports actually used in industry are inorganic.¹ Nevertheless, the use of versatile organic carriers may be in some cases of potential interest, notably when the catalytic systems are completely poisoned by the inorganic supports. Organic supports have already been reported such as functionalized polystyrene microgels^2–4 or systems in which the catalyst is self-immobilized.⁵ Recent book chapters^5–7 and a review⁸ give a good overview of the domain. Moreover, we have shown that micellar aggregates of functional polymers and copolymers respectively in toluene and heptane can be effectively used as supports for post-metallocene and metallocene catalysts activated with aluminium derivatives (MAO, AlR₃).^9–11 This methodology of supporting single-site catalysts allowed us to produce PE beads, most often, of micrometric size. Another important route to activate alkylated metallocenes is the use of borane or borate reagents.^12,13 Contrarily to aluminium co-catalysts which are required in a large amount (Al/Met > 100 to 1000 eq.) for a full activation,¹² alkylated metallocenes only need 1 equivalent of borate salt with respect to the catalyst to generate the active species.^12,14 To support such activators, organic supports have already been reported such as slightly crosslinked poly(4-vinylpyridine) enabling the fixation of the active species.¹⁵ Direct fixation of borate derivatives by radical copolymerization of styrene and 4-styryltris(pentafluorophenyl)borate has been also described, enabling the direct incorporation of the co-catalyst within the support.¹⁶ In the continuation of our previous investigations that dealt with the use of micellar aggregates as organic supports for halogenated metallocenes and post-metallocenes, we report in this manuscript the first use of ω-dimethyl ammonium tetrakis-pentafluorophenyl borate polyisoprene ([PI–NMe₂H]⁺[B(C₆F₅)₄]⁻) self-assemblies in heptane as both organic support and activator of bis(indenyl) dimethyl zirconium, [Ind₂ZrMe₂].

Results and discussion

Prior to the ethylene polymerization, two [PI–NMe₂H]⁺[B(C₆F₅)₄]⁻ were synthesized and respectively dissolved in heptane. The self-assembly of these functional polyisoprenes was analyzed by dynamic light scattering (DLS). For that purpose, solutions of [PI–NMe₂H]⁺[B(C₆F₅)₄]⁻ were prepared at concentrations required for ethylene polymerization. DLS analyses (Table 1) revealed the formation of [PI–NMe₂H]⁺[B(C₆F₅)₄]⁻ self-assemblies in heptane.

Table 1 DLS results of [PI–NMe₂H]⁺[B(C₆F₅)4]⁻ dispersion at different concentrations in heptane

Run^a	Support	Conc. of support/mg mL^−1	R H/nm
a V (heptane) = 10 mL. b Relation Γ = f(q^2) non-linear.
1	[PI15–NMe2H]^+[B(C6F5)4]^−	0.07	451
2		0.1	519
3	[PI29–NMe2H]^+[B(C6F5)4]^−	0.1	n.d ^b
4		0.15	251

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The relaxation times were determined by applying the CONTIN analysis¹⁷ to the autocorrelation functions. All the hydrodynamic radii (R_H) values of the so-formed nano-objects were calculated from the diffusion coefficient using the Stokes–Einstein equation. For all the investigated systems, a linear variation of the frequency (inverse of the relaxation time) with q², whereby the line passes through the origin, is the hallmark of a translational diffusive process typical of spherical objects (Fig. 1, inset).

Fig. 1 Relaxation curve and corresponding Contin fit obtained by DLS analysis (θ = 90°) of [PI₁₅–NMe₂H]⁺[B(C₆F₅)₄]⁻ in heptane (c = 0.07 mg mL⁻¹, T = 30 °C). The inset shows the decay rate Γ dependency to the square scattering vector q² of the biggest population (run 1).

For both [PI–NMe₂H]⁺[B(C₆F₅)₄]⁻, dispersions were clear without any sedimentation. In the case of [PI₁₅–NMe₂H]⁺[B(C₆F₅)₄]⁻, at the concentration of 0.07 mg mL⁻¹, one peak is detected on the relaxation curve (Fig. 1).

A first population, less intense and not visible at all angles (not present at θ = 90°), can be considered as temporary and unstable self-assemblies. The second main population has a hydrodynamic radius of 451 nm (Table 1) that is more representative of larger aggregates. At the concentration of 0.1 mg mL⁻¹, spherical aggregates with a diameter size of 519 nm (Table 1) are detected. Due to the very good solubility of PI and the nonsolubility of N,N-dimethylanilinium tetra(pentafluorophenyl)borate ([PhMe₂H]⁺[B(C₆F₅)₄]⁻) in heptane, it is believed that [NMe₂H]⁺[B(C₆F₅)₄]⁻ moieties are logically embedded within the core of the micellar aggregates.

[PI₂₉–NMe₂H]⁺[B(C₆F₅)₄]⁻ also demonstrated self-assembly properties. Nevertheless, at the concentration of 0.15 mg mL⁻¹, the nano-objects formed were found not to have a translational diffusive process which can be due to the higher PI chain length that decreases the stability of the aggregates (Table 1). At the concentration of 0.15 mg mL⁻¹, the maximum angle of measurement was 70° because of the too much decreasing scattered intensity, suggesting a small quantity of scattering objects belonging to two populations of nano-objects with the main one at 251 nm, the second being not visible at all the angles.

The nano-objects obtained through the self-assembly of [PI–NMe₂H]⁺[B(C₆F₅)₄]⁻ in heptane were subsequently used as supports of alkylated metallocene, e.g. Ind₂ZrMe₂. Polymerizations were then performed at 30 °C under 1 bar of ethylene pressure during 35 min. The influence of the support and its concentration on the productivity and morphologies of the PE formed were investigated; data are collected in Table 2.

Table 2 Polymerization of ethylene in the presence of the catalytic system composed of [PI–NMe₂H]⁺[B(C₆F₅)₄]⁻ and Ind₂ZrMe₂

Run^a	N(Zr)/μmol	B/Zr	Support	Conc. of support/mg mL^−1	Activity kg/(mol Zr h bar)	Productivity g/(g sup.)	T f/°C^b	χ c ^b	M w ^c/kg mol^−1	D ^c	Morphology
a Experimental conditions: V (heptane) = 30 mL; reaction time = 35 min; pethylene = 1 bar; n (TIBA) = 0.1 mmol. b Determined by DSC. c Determined by GPC (1,2,4-trichlorobenzene at 150 °C).
Blank	1	1.5	/	0	1700	/	137	66	360	2.8	Beads agg.
1	1	1.5	[PI15–NMe2H]^+[B(C6F5)4]^−	0.1	1400	266	138	69	310	2.7	Beads (0.6 mm)
2	1	1		0.07	1400	403	n.d	n.d	320	3.5	Beads (1 mm)
3	0.5	1.5		0.05	1700	335	n.d	n.d	n.d	n.d	Beads agg.
4	0.5	1		0.03	1200	340	n.d	n.d	n.d	n.d	Beads agg.
5	1	1.5	[PI29–NMe2H]^+[B(C6F5)4]^−	0.15	500	116	139	71	440	3.7	Beads (1.5 mm)
6	1	1		0.1	300	94	n.d	n.d	n.d	n.d	None
7	0.5	1		0.05	370	101	n.d	n.d	n.d	n.d	None

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First, it is worth noting that the presence of polyisoprene support does not poison the catalyst. In the presence of [PI₁₅–NMe₂H]⁺[B(C₆F₅)₄]⁻ used at any concentration, very good activities of the catalytic system were obtained (between 1200 kg/(mol Zr h bar) and 1700 kg/(mol Zr h bar)), in the same range as those measured in the absence of support. As the required quantity of support/activator for a complete activation of the catalyst is low, the productivity observed with [PI₁₅–NMe₂H]⁺[B(C₆F₅)₄]⁻ is higher than the one obtained with [PI₂₉–NMe₂H]⁺[B(C₆F₅)₄]⁻, over 400 g/(g support).

The polyethylene synthesized under homogeneous conditions exhibits a morphology of linked and ill-defined aggregated beads that can be explained by the presence of [PhMe₂H]⁺[B(C₆F₅)₄]⁻ aggregates in heptane (Fig. S4 in the ESI†). Interestingly, in the presence of [PI₁₅–NMe₂H]⁺[B(C₆F₅)₄]⁻, at the highest concentrations (0.1 mg mL⁻¹ and 0.07 mg mL⁻¹), well-defined PE beads with a size diameter of 0.5 mm and 1 mm were obtained.

From eqn (1), used to estimate the size of the polyethylene particles, with D_(polymer) the expected diameter of the polyolefin particles and D_{(catalyst particles)} the diameter of the support (self-assembled nano-objects in our methodology), polyethylene beads with a diameter around 7 μm would have been expected, for instance in the case of run 2. As the PE particles obtained have a diameter around 1 mm, it suggests that the latter beads are an agglomerate of smaller particulates, as can be seen through microscopy (Fig. 2(a)). This feature proves that the fragmentation process does not occur most probably because of the flexibility of the nano-objects in opposition to what is generally observed with inorganic supports:

Fig. 2 Photography of PE beads prepared with Ind₂ZrMe₂ as a catalyst and [PI₁₅–NMe₂H]⁺[B(C₆F₅)₄]⁻ as a support/activator (run 1) (a) and [PI₂₉–NMe₂H]⁺[B(C₆F₅)4]⁻ as a support/activator (run 5) (b).

In the cases of polymerizations performed in the presence of [PI₂₉–NMe₂H]⁺[B(C₆F₅)₄]⁻, the catalytic activities (between 300 kg/(mol Zr h bar) and 500 kg/(mol Zr h bar)) are lower than those obtained under homogeneous conditions. One can explain this activity decrease and also the loss of morphology by the good solubilization of the activator moiety attached to longer PI chain, thus suppressing any local concentration effect. However, at the concentration of 0.15 mg mL⁻¹, millimetric beads of polyethylene are obtained (Fig. 2(b)). This behavior demonstrates the need to have activator species trapped in the core of the self-assemblies to generate well-defined PE beads. It is also an indirect proof of the role of such organic supports in the formation of these beads.

A kinetics study of the ethylene pressure decrease was undertaken to evaluate the influence of the support on the polymerization (Fig. 3).

Fig. 3 Decrease of the ethylene pressure during the polymerizations in the presence and in the absence of [PI–NMe₂H]⁺[B(C₆F₅)₄]⁻.

Data corresponding to the blank were not measured after 25 min to the cessation of stirring. When the polymerization is performed in the presence of [PI–NMe₂H]⁺[B(C₆F₅)₄]⁻, the reaction can continue longer and the agitation is not stopped anymore. The kinetic of ethylene polymerization is also slower in the presence of [PI–NMe₂H]⁺[B(C₆F₅)₄]⁻ as suggested by the slope of the curves corresponding to run 2 and run 5 compared to blank. The polyethylenes were then analyzed to prove the role of the support in the process.

First, elemental analysis revealed 80 ppm and 55 ppm of fluorine in the polyethylene respectively from run 1 and run 5, demonstrating an encapsulation of the borate anion. To check the presence of polyisoprene in the produced PE beads, dynamic mechanical analyses were performed. The tangent δ curve as a function of the temperature shows several thermo-mechanical transitions (Fig. 4).

Fig. 4 DMA analysis of polyethylenes produced in the presence of [PI–NMe₂H]⁺[B(C₆F₅)₄]⁻ and Ind₂ZrMe₂—comparison with a blank.

For both samples, a transition is visible at −120 °C which corresponds to the T_g of the PE. DMA analyses of PE samples synthesized in the presence of the support (run 5, [PI₂₉–NMe₂H]⁺[B(C₆F₅)₄]⁻ at the concentration of 0.15 mg mL⁻¹, theoretically 0.9 wt% of PI in the PE bead) and under homogeneous conditions, reveal a difference above −50 °C which can be attributed to the presence of polyisoprene, blended with polyethylene chains. The last transition, beginning at 0 °C, is linked to the reorientations and vibrational movements in the crystals of polyethylene.¹⁸

Conclusion

In summary, we report the use of ω-dimethyl ammonium tetrakis-pentafluorophenyl borate polyisoprene self-assemblies in heptane as support for alkylated zirconocene toward ethylene polymerization. This support enables, thanks to its self-assembly properties, the control of the morphology of PE beads. These self-assemblies act as nano-reactors and yield unprecedented millimetric beads of polyethylene without loss of catalytic activity. This result demonstrates the need to have a high concentration of activator embedded in the self-assemblies to generate millimetric polyethylene beads. This paper shows the easy use of ω-functionalized polymers as supports for very sensitive alkylated metallocene catalysts in aliphatic solvent toward ethylene polymerization.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Polymer synthesis and characterisation. See DOI: 10.1039/c2py00588c

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