Organic support for ethylene polymerization based on the self-assembly in heptane of end-functionalized polyisoprene

Abstract

In this work, we report how micellar structures obtained by the self-assembly in heptane of polyisoprene end-capped with a hindered tertiary alcohol moiety (PI-φ₂OH) can be efficiently used as organic supports for olefin polymerization single-site catalysts. PI-φ₂OH has been synthesized by end-capping living polyisoprenyllithium with an excess of benzophenone. Dynamic light scattering analysis indicates a self-assembly of PI-φ₂OH in heptane, a good solvent for polyisoprene and a poor one for the polar end-group. The so-formed micelle-like nanoparticles, composed of a di-phenyl alcohol group core and a polyisoprene corona were used as organic supports for catalytic system composed of aluminic activators, trimethylaluminium (TMA) or methylaluminoxane (MAO), and metallocene or post-metallocene catalysts, to produce micrometric polyethylene beads.

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Introduction

Metallocenes and late transition metal complexes are effective catalysts for the homogeneous polymerization of olefins, enabling the production of polyolefins with a tunable microstructure and molecular weight distribution. In order to control the polyolefin morphology (formation of beads) and also to prevent fouling of large scale reactors in industry, fixing the catalytic system on a support is required. The immobilization of catalytic systems is generally performed with a solid inorganic carrier^1,2 such as silica,³ alumina⁴ and magnesium chloride.^5,6 After polymerization, some residual inorganic support can remain in the polyolefin which affects the optical and mechanical properties of the final materials.^7,8 To avoid such a drawback, one solution is to replace the inorganic supports with organic templates. Most of the investigations carried out have been on the development of functional organic supports essentially based on polystyrene (PS).⁹ For instance, a metallocene catalyst can be attached to a PS backbone through its aromatic ligands.^10–13 Another strategy is to anchor the activator, generally methylaluminoxane (MAO) or trimethylaluminium (TMA), on a polar organic support thanks to weak oxygen-aluminium interactions. Typically, PS nanogel with a small number of peripheral ethylene oxide units was described as carrier for MAO/2,6-bis{1-2,6}-(diisopropylphenyl)imino]ethylpyridynyl iron dichloride, [MeDIP(2,6-iPrPh)₂FeCl₂], a catalytic system towards ethylene polymerization.¹⁴ However in all cases, a multistep synthesis of these organic templates is required that can be seen as a limitation for the industrialization.

A more straightforward strategy to design organic support is based on the self-assembly of end-functionalized polymers or block-copolymers.¹⁵ Bouilhac et al. have synthesized linear PS end-functionalized by a benzoic acid or a benzophenone moiety¹⁶ and investigated the self-assembly in toluene of these functional PS into micellar aggregates. The purpose was the encapsulation of Fe-based catalytic system to produce millimetric polyethylene (PE) beads with a high catalytic activity. Similarly, polystyrene-block-poly(4-vinyl benzoic acid) copolymers were synthesized and their ability to control the PE morphology was also proven.¹⁷ This new concept of support for the production of PE beads has been essentially applied on the self-assembly, in toluene, of PS-based supports with MeDIP(2,6-iPrPh)₂FeCl₂ as a catalyst.

Following this strategy, we describe in this paper the use of micellar structures based on the self-assembly in heptane of polyisoprene end-capped with a diphenyl-tertiary-alcohol moiety, as nanoreactors for the production of polyolefin beads. Diphenyl-tertiary-alcohol end-group has been chosen for its polarity which induces micellization in aliphatic solvent and also for the capacity of tertiary alcohol to entrap free TMA leading to an increase of the catalytic activity.¹⁸ MeDIP(2,6-iPrPh)₂FeCl₂, N,N′-bis(2,6-diethylphenyl)-2,3-(naphthalene-1,8-diyl)-1,4-diazabuta-1,3-diene) nickel(II) dibromide, [(α-diimine)nickel(II)] and bis(indenyl)zirconium dichloride [Ind₂ZrCl₂] (Scheme 1), were tested as catalysts in the presence of MAO or TMA as activators.

Scheme 1 Catalysts used with PI-φ₂OH as support for the polymerization of ethylene.

Results and discussion

Synthesis of PI-Φ₂OH

The anionic polymerization of styrene, ended by the addition of benzophenone to quench the active species has been reported by Quirk and colleagues.¹⁹ On this basis, we performed the anionic polymerization of isoprene in cyclohexane, initiated by s-butyllithium, and quenched the reaction by adding an excess of benzophenone in order to obtain low molar mass diphenylhydroxy-terminated polyisoprene (PI-ϕ₂OH, see Scheme 2).

Scheme 2 Synthesis of polyisoprene diphenyl alcohol, PI-ϕ₂OH, by anionic polymerization.

As illustrated in Table 1, anionic polymerization in apolar solvent leads to a very good control over the polymer dimensions together with the advantage of working above room temperature.

Table 1 Synthesis by anionic polymerization of PI-Φ₂OH^a

Run	Mn,SEC^b/g mol^−1	D	Mn,NMR^c/g mol^−1	f (%)
a Polymerization in cyclohexane at 50 °C. b Determined by SEC with THF as eluent (RI detector). c Determined by ^1H NMR.
1	2515	1.08	1900	100
2	5843	1.05	3900	100

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The chain-end funtionnalization has been checked by ¹H and ¹³C NMR. As it can be seen in the ¹H NMR spectrum (Fig. 1), the apparition of peaks at 7.2 and 7.5 ppm are representative of the aromatic protons and the one at 3.06 ppm of the proton of the alcohol function.

Fig. 1 ¹H NMR spectra (400 MHz, CDCl₃) of diphenyl alcohol terminated polyisoprene (run 1).

Properties of PI-Φ₂OH solubilized in heptane studied by dynamic light scattering

All the PI-Φ₂OH dispersions in heptane used for the dynamic light scattering (DLS) analysis were initially heated at 65 °C and then cooled down to 30 °C to favor the micellization process. The same methodology was followed in the presence of the aluminic activator (see later in the text).

First, DLS experiments were performed on a PI-Φ₂OH (M_n = 2515 g mol⁻¹) dispersion at 1 mg ml⁻¹ in heptane at 30 °C.

Collected data revealed the existence of a self assembly of PI-Φ₂OH in heptane into micelle-like structures that likely consist of an alcohol core and a polyisoprene corona (heptane being a good solvent for polyisoprene). The values of the hydrodynamic radii (R_H) of the different nanoparticles were calculated from the decay times using the Stokes–Einstein equation. These relaxation times were determined by applying the CONTIN analysis²⁰ to the autocorrelation functions; an example is given in Fig. 2 for PI-Φ₂OH of M_n = 2515 g mol⁻¹.

Fig. 2 Relaxation curve and corresponding Contin fit obtained by DLS analysis (θ = 50°) of PI-Φ₂OH (M_n = 2515 g mol⁻¹) in heptane (c = 1 mg mL⁻¹, T = 30 °C). The inset shows the dependency of the decay rate Γ on the square scattering vector q².

The DLS analysis of higher molar mass PI-Φ₂OH (M_n = 5843 g mol⁻¹) in heptane at 1 mg ml⁻¹ and 30 °C showed a very low diffusion intensity indicating the presence of a main population composed of free chains (2 nm size) together with a minor second population attributed to aggregates (125 nm). This result clearly proves that such PI is not capable to form micellar structures due to a too long size of the PI chain. For this reason, lower molar mass PI-Φ₂OH (M_n = 2515 g mol⁻¹) was only used as template for the synthesis of PE beads.

Effect of MAO addition onto PI-Φ₂OH self-assembly

The effect of MAO addition onto PI-Φ₂OH self-assembly process in heptane was investigated by DLS. For this purpose, MAO was added to PI-Φ₂OH dispersion and the whole system was maintained for 24 h at 65 °C then cooled down at 30 °C. The addition of MAO (Al/OH = 75) into PI-Φ₂OH dispersion (M_n = 2515 g mol⁻¹, c = 1 mg mL⁻¹) leads to an increase of the particle radius (from an average value of 290 nm to 530 nm), see Fig. 3. This phenomenon would indicate that some MAO is encapsulated within the aggregates.

Fig. 3 Autocorrelation function and relaxation times at θ = 90° of PI-Φ₂OH (M_n = 2515 g mol⁻¹) in heptane (c = 1 mg ml⁻¹, T = 30 °C) with MAO: Al/OH = 75.

The signal increase observed in the correlation function at high relaxation times also indicates the formation of aggregates which can be explained by the presence of non-encapsulated MAO. It is worth noting that in the absence of PI-Φ₂OH, the MAO is not dispersable in heptane at all. The encapsulation phenomenon is supported by the visual aspect of the dispersion which remains homogeneous and stable at least 2 h (no sedimentation observed). This means that MAO is fully dispersed in heptane thanks to the presence of PI-Φ₂OH.

Consequently, the decay rate versus squared scattering vector plot could not be fitted with a linear curve, explained by the presence of different populations of aggregates with a non-spherical shape (data not shown).

Effect of TMA addition onto PI-Φ₂OH self-assembly

TMA is soluble in heptane and purchased in solution (2 mol L⁻¹) in this solvent. The effect of TMA addition (Al/OH = 25) on PI-Φ₂OH self-assembly has also been checked by DLS (see Fig. 5). As for MAO, TMA was added to PI-Φ₂OH dispersions and the whole system was maintained for 24 h at 65 °C then cooled down at 30 °C. As expected, the addition of TMA onto PI-Φ₂OH dispersion (M_n = 2515 g mol⁻¹, c = 1 mg mL⁻¹) leads to an increase of the particle size with a R_H value of 551 nm without formation of larger aggregates as it was observed for MAO. In the case of TMA addition, the dispersion remains homogeneous and the decay rate versus squared scattering vector plot can be fitted with a linear curve, proof of the formation of spherical particles (inset in Fig. 4).

Fig. 4 Relaxation curve and corresponding Contin fit obtained by DLS analysis (θ = 50°) of PI-Φ₂OH (M_n = 2515 g mol⁻¹) in heptane (c = 1 mg mL⁻¹, T = 30 °C) with TMA: AL/OH = 25. The inset shows the decay rate Γ dependency to the square scattering vector q².

Fig. 5 SEC traces (viscosimeter) of PEs made with PI-Φ₂OH and MeDIP(2,6iPrPh)₂FeCl₂ as catalyst.

Polymerization of ethylene using MeDIP(2,6-iPrPh)₂FeCl₂

The micellar structures obtained from the self-assembly of PI-Φ₂OH in heptane were subsequently used to support the tridentate bis(imino) pyridinyl iron catalyst, MeDIP(2,6-iPrPh)₂FeCl₂. The polymerizations were then performed at 30 °C under 1 bar of ethylene pressure for 1 h.

The influence of the support on the production of PE using MeDIP(2,6-iPrPh)₂FeCl₂ was investigated; data are collected in Table 2.

Table 2 Polymerization of ethylene in the presence of the catalytic system composed of PI-Φ₂OH + MAO or TMA and MeDIP(2,6iPrPh)₂FeCl₂

Run	n(cata)/μmol	Activator	Al/Fe	Activity/kg mol(C)^−1 h^−1bar^−1	Al/OH	Tm/°C^c	χc^c	M w /kg mol^−1	D	Bead size/μm
a Experimental conditions: heptane, 30 mL; reaction time = 1 h; T = 30 °C; Pethylene = 1 bar. b Experimental conditions: heptane, 30 mL; reaction time = 1 h; T = 30 °C; Pethylene = 1 bar; reaction time before polymerization = 24 h at 65 °C under stirring; PI-Φ2OH: Mn = 2515 g mol^−1; [PI-Φ2OH] = 1 mg ml^−1. c Determined by DSC. d Determined by SEC (1,2,4-trichlorobenzene at 150 °C).
blank 1^a	0.6	MAO	1500	2413	—	131	62%	290	bimodal	—
1^b	0.6	MAO	1500	2987	75	134	65%	760	6.5	0.5
blank 2^a	0.6	TMA	500	1753	—	139	61%	530	bimodal	—
2^b	0.6	TMA	500	1965	25	133	28%	980	5	0.5–2
3^b	0.6	TMA	400	280	20	140	62%	1114	15.7	1–10
4^b	0.6	TMA	300	162	15	139	62%	831	9	1–4

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With MAO, an increase of the catalytic activity in the presence of the support compared to homogeneous conditions is observed (2987 kg PE/(mol Fe h bar, run 1, vs. 2413 kg PE/(mol Fe h bar), run blank 1). This increase can be explained by the entraping of the free TMA, present in the MAO solution, through the alcohol function of the support.

The use of TMA as a cocatalyst at a ratio Al/Fe = 500, leads to slightly higher values of catalytic activity in the presence of PI-Φ₂OH in comparison to homogeneous conditions (1753 kg PE/(mol Fe h bar), run blank 2, vs. 1965 kg PE/(mol Fe h bar), run 2). This can be reasonably explained by a local increase of the cocatalyst concentration thanks to the micellar support. Reduction of Al/Fe ratio below 500 leads to a sharp decrease of the catalytic activity. According to SEC analysis, oligomers were never formed (Fig. 5), which is in agreement with a suppression/reduction of transfer reaction to TMA, proving the efficiency of the support to trap the activator within the micelles. The reduction of the Al/Fe ratio also leads to a diminution of the molar masses (run 3 and run 4).

Additionally, the morphology of the PE beads was investigated by scanning electron microscopy.

In the absence of PI-Φ₂OH support, the so-formed PE does not exhibit any specific morphology. In contrast, both MAO and TMA activated catalytic systems supported with PI-Φ₂OH formed spherical PE particles (Fig. 6).

Fig. 6 SEM pictures of PE prepared with MeDIP(2,6iPrPh)₂FeCl₂ under homogeneous conditions (a) and prepared in the presence of PI-Φ₂OH (b) and (c).

MAO-based catalytic system enables the formation of rather well-defined PE spherical particles with a sizes of 0.5 μm range. In the case of TMA, the size of the PE particles remains in the micron range but varies upon the experimental conditions (Al/Fe ratio) as shown in Table 2. A better control of the particle size is observed with respect to the higher Al/Fe ratio which corresponds to the higher catalytic activity. The higher the catalytic activity, the smaller the PE beads.

Concerning the polymerization kinetic, the decrease of ethylene pressure was followed by connecting the schlenk to a guard bottle charged with 1 bar of monomer. The pressure drop in homogeneous conditions (blank 1) and in the presence of PI-Φ₂OH (run 1) are represented Fig. 7.

Fig. 7 Pressure decrease during ethylene polymerization with MeDIP(2,6iPrPh)₂FeCl₂/MAO under homogeneous conditions (Blank 1) and with PI-Φ₂OH (Run 1).

As can be observed, the decrease of ethylene pressure in both conditions follows the same profile, showing that PI-Φ₂OH does not modify the kinetic of ethylene polymerization. Further analyses dealing with kinetic studies in various experimental conditions are currently in progress.

Polymerization of ethylene using (α-diimine)nickel(II)

In order to extend this concept of supports to a large range of catalytic systems for olefin polymerization, micellar-like structures obtained from PI-Φ₂OH self-assembly have been tested as supports of nickel-based catalyst. (α-diimine)nickel(II) is known to produce branched PE (chain-walking mechanism) for which the number of branches can be tuned by catalyst structure, ethylene pressure and temperature.^21–23 Less bulky catalysts are more resistant to chain-walking process and produce less branched PE. Therefore, nickel catalyst with relatively small ethyl substituents in ortho positions of aryl rings was chosen. The influence of the support and the temperature on the efficiency of (α-diimine)nickel(II) have been studied; data are collected in Table 3.

Table 3 Polymerization of ethylene in the presence of the catalytic system composed of PI-Φ₂OH + MAO or TMA and (α-diimine)nickel(II)

Run	n(cata)/μmol	Activator	Al/Ni	T/°C	Activity kg/mol(C)^−1 h^−1 bar^−1	Al/OH	Tm/°C^c	χc^c	M w /kg mol^−1	D	Bead size/μm
a Experimental conditions: heptane, 30 mL; reaction time = 1 h; T = 30 °C; Pethylene = 1 bar. b Experimental conditions: heptane, 30 mL; reaction time = 1 h; T = 30 °C; Pethylene = 1 bar; reaction time before polymerization = 24 h at 65 °C under stirring; PI-Φ2OH: Mn = 2515 g mol^−1; [PI-Φ2OH] = 1 mg ml^−1. c Determined by DSC. d Determined by SEC (1,2,4-trichlorobenzene at 135 °C). e Bimodal distribution.
Blank 1^a	5	TMA	200	0	208	—	131	52%	413	7	—
1^b	5	TMA	200	0	11	84	133	34%	406	3.5^e	1–4
Blank 2^a	5	MAO	200	30	344	—	69	1%	156	2.6	—
2^b	5	MAO	200	30	317	84	63	1%	242	1.5	5–80
Blank 3^a	5	MAO	200	0	525	—	126	38%	433	4.8	—
3^b	5	MAO	200	0	61	84	122	31%	192	1.4	0.5
Blank 4^a	5	MAO	200	−10	324	—	128	44%	490	3.5	—
4^b	5	MAO	200	−10	90	84	126	40%	2 440	1.8	0.5
Blank 5^a	1	MAO	460	−10	1341	—	138	61%	773	5.8	—
5^b	1	MAO	460	−10	402	39	135	60%	1 720	1.4	0.5

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Catalytic activities of (α-diimine)nickel(II) in the presence of PI-Φ₂OH support vary from 11 to 402 kg PE/(mol Ni h bar) with respect to the activator and the temperature. Polymerizations activated by TMA showed generally lower activity compared to MAO activated ones.

Unlike polymerizations performed at 30 °C, for which the catalytic activities are comparable whatever the conditions, at low temperature (−10/0 °C), the catalytic activity was found to be much lower in the presence of PI-Φ₂OH support.

This behavior can be explained by the difficulty encountered by the catalyst to penetrate the micellar structure, thus impeding the effective formation of the active species.

As expected, the catalytic activity increased with the Al/Ni ratio, whatever the experimental conditions (activity = 402 kg PE/(mol Ni h bar) for MAO/Ni = 460, run 5, and activity = 90 kg PE/(mol Ni h bar) for MAO/Ni = 200, run 4). With MAO as an activator (runs 2–5), PEs with monomodal molar mass distributions and narrow dispersities, D, were obtained while PE with a bimodal distribution is obtained with TMA (run 1) (Fig. 8). As it could be expected, a decrease in the reaction temperature yields PEs with a higher crystallinity ratio (higher melting temperature) in agreement with a lower chain-walking rate and the formation of high molecular weight linear polyethylene (run 4).

Fig. 8 GPC chromatograms (refractive index) of PE produced with (α-diimine)nickel(II) and PI-Φ₂OH as support.

In all cases, scanning electron microscopy revealed the formation of spherical PE beads (Fig. 9). Interestingly, the size and size distribution of the PE particles depend on the experimental conditions.

Fig. 9 SEM pictures of PE prepared with (α-diimine)nickel(II) under homogeneous conditions (a) and prepared in the presence of PI-Φ₂OH (b) and (c).

While PE particles with a broad size distribution of between 5 μm and 80 μm are obtained at 30 °C (run 2), small and narrow distributed PE particles (0.5 μm) are formed at low temperature (−10 °C, run 4). These results clearly show that the PE particle size is strongly affected by the polyethylene microstructure (linear or branched).

The formation of highly linear polyethylene (high crystalline ratio) leads to the formation of a large number of particle nuclei yielding small PE particles with a narrow size distribution. Conversely, the formation of highly branched amorphous polyethylene favor the formation of large PE particles. This phenomenon can be reasonably supported by the aggregation of growing elementary particles explained by the amorphous state of the branched polyethylene.

Polymerization of ethylene using Ind₂ZrCl₂

This concept of supporting single-site catalysts was finally extended to classical metallocene. It is worth mentioning that previous attempts to trap metallocene/MAO catalytic system within PS-based micellar structures in toluene^16,17 lead to systems that revealed unsuccessful to produce PE beads with a controlled morphology, probably, because only few MAO was trapped by the support. In this study, Ind₂ZrCl₂ was tested with MAO as an activator.

The two PI-Φ₂OH supports (M_n = 2515 g mol⁻¹ and M_n = 5843 g mol⁻¹) were used for the polymerization of ethylene using the conditions mentionned in Table 4.

Table 4 Polymerization of ethylene in the presence of the catalytic system composed of PI-Φ₂OH + MAO and Ind₂ZrCl₂

Run	n(cata)/μmol	Al/Zr	M n (PI-Φ2OH)/g mol^−1	Activity/kg mol(C)^−1 h^−1 bar^−1	Al/OH	Tm/°C^b	χc^c	M w /kg mol^−1	D	Bead size/μm
a Experimental conditions: hetpane, 60 mL; reaction time = 1 h; Pethylene = 1 bar. b Experimental conditions: hetpane, 60 mL; reaction time = 1 h; Pethylene = 1 bar; Reaction time before polymerization = 24 h at 65 °C under stirring [PI-Φ2OH] = 1mg ml^−1. c Determined by DSC. d Determined by SEC (1,2,4-trichlorobenzene at 150 °C).
Blank 0^a	3	500	—	337	—	135	64%	n.d.	n.d.	—
1^b	3	500	2515	261	63	135	66%	306	4	1
2^b	3	500	5843	517	146	136	68%	306	3.3	1

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Surprisingly, the catalytic activities obtained in the presence of PI-Φ₂OH supports are comparable to the one obtained in homogeneous conditions. An activity increase is even noticed in the presence of the highest molar mass PI. The molar mass and molar mass distribution, D, of the so-formed PE remain the same, irrespective of the experimental conditions.

As it can be seen in Fig. 10, PE beads could be readily obtained in the presence of PI-Φ₂OH supports. The size of the PE beads stays around 1 μm irrespective of the support molar mass. This last result proves the validity of the concept that consist in using micellar structures as organic supports for metallocene catalysts.

Fig. 10 SEM pictures of PE prepared with Ind₂ZrCl₂ under homogeneous conditions (a) and prepared in the presence of PI-Φ₂OH (b).

Role of PI-Φ₂OH support in the production of PE beads

In order to check the presence of polyisoprene support in the so-formed PE beads, differential scanning calorimetry (DSC) was first carried out. This technique did not reveal the presence of a PI phase. Dynamic mechanical analysis (DMA) was therefore performed on PE samples prepared in both homogeneous and heterogenous conditions (Fig. 11).

Fig. 11 DMA analysis of PE produced with Ind₂ZrCl₂ on which the T_g of polyisoprene is visible.

Such an analysis was done on PEs produced with Ind₂ZrCl₂. The tan δ versus temperature plot shows a first peak at −114 °C, usually denoted γ, which corresponds to the T_g of the PE. A second peak at −55 °C is present only for the PE sample produced in presence of PI-Φ₂OH. This transition corresponds to the polyisoprene T_g proving that some PI is trapped within PE beads and thus that PI-Φ₂OH supports play a role in the formation of PE beads. The last transition, at 60 °C, denoted α, is related with vibrational motion and reorientation within the PE crystal.²⁴

Such an analysis was also performed on PE samples produced with MeDIP(2,6iPrPh)₂FeCl₂ but the presence of PI phase was not revealed in the PE beads because of the high catalytic activity (Table 2, runs 1 and 2).

Experimental section

Materials

Isoprene, sec-butyllithium, benzophenone, MAO, TMA were purchased from Aldrich. Cyclohexane and heptane were dried over polystyryl-lithium and distilled under vacuum. Isoprene was dried over di-n-butylmagnesium and distilled under vacuum and kept at 5 °C. Sec-butyllithium was filtered prior to use. Benzophenone was sublimated and kept under vacuum at room temperature. TMA (2 M solution in heptane), MAO (10%_w in toluene) were used as received. MeDIP(2,6-iPrPh)₂FeCl₂ was synthesized as reported in the literature²⁵ and kept in a glovebox under argon atmosphere. Ind₂ZrCl₂ was purchased from Strem and kept in a glovebox under argon atmosphere. (α-diimine)nickel(II) was synthesized as reported in the literature²⁶ and kept in a glovebox under argon atmosphere.

Synthesis of polyisoprene diphenyl alcohol terminated (PI-Φ₂OH)

A dry round bottom flask equipped with a magnetic stirrer was charged with 40 mL of cyclohexane, s-butyllithium (1.67 mL; 0.002 mol) and isoprene (5.88 mL). The flask was immersed in an oil bath at 50 °C for 2h. A solution (1M) of benzophenone in cyclohexane was then added (4 mL; 0.004 mol) and the mixture was stirred at 50 °C overnight. The polymer was precipitated into an excess of methanol, then an excess of ethanol at 30 °C to remove unreacted benzophenone and finally dried under vacuum at room temperature. M_n = 2515 g mol⁻¹, M_w/M_n = 1.07, ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 1.64 (CH3 of PI, b), 2.05 (CH2 of PI, m), 3.06 (H of OH, s), 5.1 (H of PI, b), 7.5–7.2 (aromatic, 10H, m).

Polymerization of ethylene

The desired quantity of PI-Φ₂OH support was introduced in a Schlenk tube and lyophilized during a night with dioxane. Dry heptane and activator (TMA or MAO) was added. The reactor mixture was heated to 65 °C for 24h. The reaction mixture was then connected to a 1 bar ethylene gas outlet using a rubber tube. Schlenk tube was purged by ethylene for 20 min to remove argon. Polymerization was initiated by injection of the required amount of catalyst in toluene via a syringe. After 1 h of polymerization, the vessel was disconnected from the ethylene outlet, and the polymer was precipitated by addition of ethanol containing 10% HCl. The precipitated polymer was filtered, washed, and dried to constant weight.

Analysis

¹H NMR spectra were recorded using Bruker AC-400 NMR spectrometer using CDCl₃ at room temperature. Size exclusion chromatography (SEC) was performed in THF at 40 °C at a flow rate of 1 mL min⁻¹ using a differential refractometer (Varian) and a UV-visible spectrophotometer (Varian) operating at 254 nm and 4 TSK columns (G5000HXL (9 μm), G4000HXL (6 μm), G3000HXL (6 μm), and G2000HXL (5 μm)). Calibration was performed using linear (1,4)-polyisoprene standards.

The PE weight average molar mass (M_w) and molar mass distribution of (α-diimine)nickel(II) produced PEs were determined using a Waters GPCV2000 SEC instrument, equipped with two PLgel (300 × 7.5 mm) columns, and an online viscometer and refractive index detector, at 135 °C in 1,2,4-trichlorobenzene as solvent at a flow rate of 0.9 mL min⁻¹.

The PE weight average molar mass (M_w) and molar mass distribution of MeDIP(2,6-iPrPh)2FeCl₂ and Ind₂ZrCl₂ produced PEs were determined using a Waters GPCV2000 SEC instrument, equipped with three columns PLgel Olexis Guard (300 × 7.5 mm), and an online viscometer and refractive index detector, at 150 °C in 1,2,4-trichlorobenzene as a solvent at a flow rate of 1 mL min⁻¹.

PE beads were observed under scanning electronic microscopy (SEM) analyses on a JEOL JSM 2500 apparatus.

Dynamic light scattering (DLS) measurements were performed using an ALV Laser goniometer, which consists of a 22 mW HeNe linear polarized laser with 632.8 nm wavelength and an ALV-5000/EPP multiple tau digital correlator with 125 ns initial sampling time. The samples were kept at constant temperature (30 °C) during all the experiments. The accessible scattering angle range is from 10° up to 150°. However, due to difficulties in removing dust, most of the dynamic measurements were done at diffusion angles higher than 40°. The dispersions were introduced into 10 mm diameter glass cells. Dynamic light scattering measurements were evaluated by fitting the measured normalized time autocorrelation function of the scattered light intensity with the help of the constrained regularization algorithm (CONTIN), which provides the distribution of relaxation times τ, A(τ), as the inverse Laplace transform of g⁽¹⁾(t) function:

The apparent diffusion coefficient D was obtained by plotting the relaxation frequency, Γ (Γ = τ⁻¹) versus q2 where q is the wavevector defined as

and λ is the wavelength of the incident laser beam (632.8 nm), θ is the scattering angle, and n the refractive index of the media. The diffusion coefficient was then determined by extrapolation to zero concentration, and hydrodynamic radius (R_H) was calculated from the Stokes–Einstein relationwhere k_B is the Boltzmann constant, Γ the relaxation frequency, T is the temperature, and η is the viscosity of the medium. Dispersions used for light scattering were prepared using the following method: heptane was preliminarily filtered through a 0.22 μm PTFE membrane and added to polymer chains and the activator when necessary. The dispersions were then left under stirring for 24 h at 65 °C for complete dissolution.

The DMA analysis measurements were performed with a Thermal Analysis RSA 3 instrument using a cylindrical geometry over a temperature range of −140 to 150 °C at a rate of 10 °C min⁻¹ under nitrogen flow. All samples were scanned at a frequency of 1Hz.

Conclusions

In this paper, we report the use of polyisoprene end-capped with a diphenyl alcohol, PI-Φ₂OH, as support for ethylene polymerization in an aliphatic solvent, i.e., heptane. The self-assembly of these end-functionalized PI oligomers in heptane allowed us to use the formed micellar structures as organic supports for both post-metallocene and metallocene catalytic systems. Ethylene polymerizations performed in the presence of such nanoreactors lead to the formation of PE beads with a micrometric size. Remarkably, this paper shows, for the first time, that the polyethylene microstructure (linear or branched) affects the size of the so-formed PE beads (in the case of Ni catalyst) and also that such concept of organic templates can be effectively extended to metallocene catalysts, which generally require a quite high concentration of aluminic activator.

Acknowledgements

The present research was financially supported by the French Ministry of Research and Education, the Centre National de la Recherche Scientifique, the Institut Universitaire de France and the Aquitaine Council; the Grant Agency of the Czech Republic (grant No. 104/07/P264) and Ministry of Education, Youth and Sports (program MSM 6046137302).

Notes and references

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