Fluorous miniemulsions: A powerful tool to control morphology in metallocene-catalyzed propene polymerization

Authors


Abstract

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The nonaqueous miniemulsion polymerization of liquid propene using highly water sensitive metallocene catalysts is presented. In the emulsion perfluoromethylcyclohexane is applied as continuous phase in which the monomer is polymerized to high molecular weights. The droplets are stabilized by specially designed of hydrocarbon/perfluorinated block copolymers. This technique offers the possibility to generate polyolefin nanoparticles with a narrow size distribution and diameters below 100 nm. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

INTRODUCTION

Polyolefins play a vital role in modern life as indicated by the production of more than 100 million tons per year.1, 2 In addition to Ziegler–Natta catalysts,3, 4 metallocenes have been found to be suitable for olefin polymerization,5–9 and during the last decades, a vast number of new examples have been developed, making the synthesis of previously inaccessible polymers possible.10, 11 Most of them possess not only excellent activities in olefin polymerization but also yield high molecular weights and allow selective control over the tacticity.10, 12–14 Unfortunately, in homogeneous polymerization, the morphology, the size, and the shape of the obtained particles is seldom controlled. Furthermore, reactor fouling occurs due to local overheating but can be overcome by supporting the catalyst on magnesium chloride, silica, clay, or polymer supports.15 In these processes, the product is formed on micrometer- to millimeter-scale particles, whereas smaller ones are inaccessible. Another approach to avoid reactor fouling and maintain morphological control is through the use of aqueous emulsions. Although it is a rapidly developing research field, catalyst activities in water are lower because of the low hydrolytic stability of most metallocenes, and typically low molecular weights are obtained.16

Recently, various types of nonaqueous emulsions have been described, enabling the utilization of moisture and air-sensitive reactions.17–20 Mixtures of incompatible solvents of different polarities form stable emulsions of one solvent into the other with the use of an appropriate biphasic material. In these emulsion systems, nanometer scale spherical particles from numerous types of polymers have been generated, including polyurethanes, polyesters, and at the same time any side reactions with water was avoided.19, 20 This concept was further extended to a fluorous emulsion system, where the hygroscopic, active species, central to the polymerization, are immobilized inside the droplets and further protected by the inert medium. They have been employed in metallocene-catalyzed olefin polymerization as a continuous phase, utilizing high-molecular-weight emulsifiers and a dispersed hydrocarbon phase, hosting the metallocene catalyst.21, 22 Combined with excellent heat transfer, enabled by the high surface contact of the emulsion droplets, a decrease in the size of the polyolefin particles to the nanometer range was obtained. Only moderate activities were achieved, primarily, because of the slow diffusion of the monomer from the gas phase into the emulsion droplets. To overcome this limitation, a miniemulsion process,23 applicable to metallocenes, was developed. To overcome this limitation, a miniemulsion process, applicable to metallocenes is presented. Herein, we describe a procedure, where liquid propene is used directly as a dispersed phase in a perfluorinated miniemulsion. By design, a high monomer concentration is maintained inside the droplets throughout the entire polymerization. High activities are achieved, while lack of impurities is observed and morphology control preserved. A new type of emulsifier was developed, namely high-molecular-weight amphiphilic block copolymers based on a lipophilic and a fluorophilic block. In this way, emulsion stability over months was achieved, in comparison with the previously reported organic-in-fluorous emulsion, prepared with statistical copolymers.

EXPERIMENTAL

General, Materials, and Instrumentation

All reactions were carried out in an inert gas atmosphere, using standard Schlenk techniques. Unless otherwise stated, all compounds were purchased from Sigma Aldrich and Alfa Aesar and used without further purification. Styrene and pentafluorostyrene (2,3,4,5,6-pentafluorostyrene) were stirred over calcium hydride for 24 h, and then distilled under reduced pressure and stored under an argon atmosphere. Perfluoromethylcyclohexane (PFMCH) was degassed by bubbling with argon before use. Propene was purchased from BASF AG, Ludwigshafen. NMR spectra were collected on Bruker AMX 250, Bruker AC 300, or Bruker 500 model spectrometers. Chemical shifts δ are reported in parts per million (ppm), referenced to the residual proton of the deuterated solvent as an internal standard. Dynamic light scattering (DLS) or photon correlation spectroscopy (PCS) was measured using a frequency-doubled Nd:YVO4 laser (Verdi V2, Coherent) at a wavelength of 532 nm as coherent polarized source of light. The PCS experiment was performed with a multiple tau digital correlator (ALV 6010-160). Polymer melting points (Tm) were determined on a differential scanning calorimeter (DSC) using a heating rate of 10 °C min−1 from 20 to 200 °C. The heating cycle was performed twice, but only the results of the second scan are reported. Molecular weights and molecular weight distributions of polypropylene were measured at 135 °C by gel permeation chromatography (GPC) against polystyrene standards using PLgel Mixed-B column, a differential refractometer detector, and 1,2,4-trichlorobenzene as eluent.

Synthesis of the Emulsifiers

Polystyrene-TEMPO

In a Schlenk tube, styrene (0.24 mol, 25 g), AIBN (0.240 mmol, 39.4 mg), and TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl, 0.42 mmol, 65.6 mg) were combined, and three freeze-pump-thaw cycles were applied before the reaction mixture was heated to 125 °C for 12 h. Then, THF (20 cm3) was added to the mixture. The product was precipitated into 250 cm3 methanol. Filtering of the colorless solid and drying in vacuo yielded TEMPO end-capped polystyrene (60% conv., Mn = 19,000 g mol−1, PDI =1.27). The NMR data corresponded to that given in the literature for similar compounds.24

Polystyrene-block-Poly(2,3,4,5,6-pentafluorostyrene) PS-b-PFS

Polystyrene-TEMPO (PS-TEMPO; 0.2 mmol, 4.0 g, Mn = 19,000 g mol−1, PDI = 1.27) was combined with pentafluorostyrene (62 mmol, 12.0 g), and the reaction mixture was heated to 125 °C for 5 days. THF (20 cm3) was added to the mixture, and then it was precipitated into 200 cm3 methanol. Filtering of the colorless solid and drying in vacuo yielded the PS180-b-PFS150 block copolymer (70% conv., Mn = 49,000 g mol−1, PDI = 1.29).

1H NMR (300 MHz, CDCl3, 25 °C): δ = 1.19–2.91 (6H, [BOND]CH2CHAr[BOND]), 6.30–7.24 (5H, [BOND]C6H5).

Polystyrene-block-Poly[2,3,5,6-tetrafluoro-4-(1H,1H-perfluorooctyloxy)styrene-stat-poly(2,3,4,5,6-pentafluorostyrene)] PS-b-PerfluorooctylPFS

A solution of 1H,1H-perfluorooctan-1-ol (4.5 mmol, 1.8 g) in THF (5 cm3) was cooled to −78 °C, and then sec-butyllithium (4.2 mmol, 3 cm3, 1.3 M in hexane) was added and subsequently a solution of the PS180-b-PFS150 (0.01 mmol, 0.40 g) block copolymer in THF (4 cm3) was added dropwise. The reaction mixture was heated to 45 °C and stirred for 5 days, and then, after cooling, precipitated into methanol. The emulsifier was obtained by filtration and dried in vacuo to give a colorless solid (yield: 75–80%, degree of substitution: 50%).

1H NMR (300 MHz, CDCl3, 25 °C): δ = 0.93–2.79 (6H, [BOND]CH2CHAr[BOND]), 4.20–4.70 (2 H, [BOND]OCH2CF2[BOND]), 6.15–7.16 (5H, [BOND]C6H5), intensities ratio [BOND]OCH2CF2[BOND]:[BOND]C6H5 = 0.81:4.93, [BOND]C6H5:[BOND]CH2CHAr[BOND] = 4.93:7.98. 19F NMR (500 MHz, CDCl3, 25 °C): δ = −82.34 (4 F, Ar[BOND]F), −121.93 to −124.12 (12 F, [BOND]CF2), −127.23 (3 F, [BOND]CF3).

Polymerization of Propene

Propene polymerization was carried out in a Büchi Glas Uster polymerization autoclave (1 dm3) equipped with a mechanical stirrer, thermostat, and an upstream propene condensation column (2 dm3, −20 °C). Before use, the reactor was heated (80 °C) under a continuous nitrogen flow. PFMCH (225 cm3), emulsifier (250 mg), and 1 cm3 solution with scavenger (triisobutylaluminium, 1.0 M in hexane), and methylaluminoxane (MAO, 5 cm3, Crompton, EurecenAl510010T) were combined in a dried round-bottomed Schlenk flask and ultrasonificated for 10 min. The mixture was transferred via cannula into the reactor, before the latter was mechanically sealed. Liquid monomer (25 g, liquefied in a condensing column at 4 bar and −21 °C) was added by argon overpressure and the autoclave heated to 60 °C under stirring. The reaction was initiated by the addition of the catalyst solution ([dimethylsilanediylbis(3,3′-(2-methylbenz[e]indenyl))]-zirconium dichloride) (1 mgcm−3 toluene solution), using a pressure lock. After 60-min polymerization time, the reactor was cooled to room temperature. The reaction mixture was poured into methanol, and then the product was filtered and dried or, if necessary, isolated by centrifugation. 13C NMR spectra corresponded to an isotactic polypropylene (between 90 and 98% isotacticity for the different experiments). The isotacticity increases while increasing the polymerization pressure and the propene concentration inside the droplets. This was the reason for the increase in the pressure of the polymerization, which directly influenced the catalyst activity as well (see Table 2). On the other hand, while on the macrolevel the system is perfectly maintained, it may be well assumed that inside the droplets a higher temperature is achieved, even with the perfect heat transfer of the emulsion. This leads to an overheated place inside the droplets, and thus affect the stereoregularity. At lower pressures, this effect is better observed, as there also the diffusion still plays a role. That is why, at lower pressures, lower isotacticity is yielded. It can be said that inside the droplets, the process is very close to the homogeneous polymerization, and the only thing, preventing the whole system of overheating is the good heat transfer from the droplets toward the continuous phase.25 The melting points of the polymer were in the range 144–151 °C.

RESULTS AND DISCUSSION

The prerequisite for the polymerization of liquid propene in a miniemulsion are suitable emulsifiers, which stabilize the liquified monomer inside the fluorous phase even at elevated temperatures and high pressures. The statistical copolymer-based surfactants, reported in a previous work, do not provide sufficient emulsion stability, as these systems were stable for not more than a couple of hours, at most couple of days.21 Because of the more demanding polymerization conditions in comparison with the previously reported gas-phase process, the stabilization of the emulsions had to be improved.

Synthesis of the Emulsifiers and Emulsion Characterization

Emulsions formed with block copolymers are much more stable than in the presence of statistical copolymers.26 A new series of emulsifiers with a block structure was produced by nitroxide-mediated radical polymerization (Fig. 1).24, 27 They are based on block copolymers of styrene and pentafluorostyrene. Furthermore, a strategy was used to increase the fluorous content by substituting the fluorine atom in the p-position of the pentafluorostyrene repeat units with a fluorocarbon chain (Fig. 1).28 As the fluorous content was significantly increased, an improved stabilization effect on the perfluoroalkane/alkane interface was expected, which should provide a better control over the emulsion properties.29 Typically, no separation or any change in the emulsion characteristics was observed over months.30 The main reason is the limited dynamics of the block copolymers hampering destructive processes in the emulsion such as Ostwald ripening or coalescence.26

Figure 1.

Synthesis of the block copolymer emulsifiers.

Emulsions consisting of toluene and PFMCH with different solvent and emulsifier content were prepared, and their properties were evaluated by DLS. The detection system used consist of a combination of a single-mode fiber for mode selection and an avalanche photodetector setup. The autocorrelation functions were analyzed with the CONTIN algorithm and a single process was detected.31, 32 From the mean relaxation time, the hydrodynamic radius of the particles was obtained, via the Stokes-Einstein relation using the refractive index and viscosity of the solvent. An influence of the amount of emulsifier on the droplet size was observed—a decrease from 109 nm to 46 nm while increasing the amount of emulsifier from 0.5 to 10 wt %. Stable emulsions were observed up to 20 vol % dispersed phase, above this point phase separation occurred (Fig. 2, Table 1). Decrease in the droplet size was observed in the emulsions, containing emulsifier amount smaller than 5 wt % (Table 1) at elevated temperatures. This can be attributed to a change in the block copolymer dynamics. The energy of the whole system is increased, thus leading to reorganization of the stabilizer on the surface of the droplets and better stabilization. At higher emulsifier contents, this is not observed. There the stabilization is enough, and minimum of the droplet size is observed already at 25 °C.

Table 1. Mean Droplet Size of Oil-in-Oil Emulsions Consisting of Toluene and PFMCH: Hydrodynamic Radius at 25 and 40 °C
No.Emulsifier (wt %)Disp. Phase (vol %)Mean Radius at 25 °C nm−1Mean Radius at 40 °C nm−1
11510994
22.559763
32.5109048
410104645
55155745
Figure 2.

Dependence of the droplet size upon amount of the emulsifier quantity (left, dynamic light scattering of a fluorous/toluene emulsion, 10 vol % toluene) and temperature (right, 10 vol % toluene, 10 wt % emulsifier).

It is known that fluorinated solvents are immiscible with organic solvents below the UCST (upper critical solution temperature), whereas above this point, a single phase is observed.33, 34 It is very imperative to preserve the emulsion stability at the polymerization temperature, 60 °C.

To investigate the emulsion properties, DLS experiments were performed in a sealed, dust-free scattering cell, allowing measurements above the boiling point of the solvent, while preventing evaporation. Remarkably, the measurements showed no change in the mean droplet size in the range between 25 and 69 °C. A polydispersity of 1.1 was found in the analysis of the light-scattering results. These findings clearly indicated that a fluorous/hydrocarbon oil-in-oil emulsion system is not strongly influenced by the temperature, even above the boiling point (Fig. 2). Emulsions prepared by the previously reported emulsifiers21 break after a few hours, while the block copolymers provide much better overall stabilization and no change in the emulsion characteristics is observed, even after weeks.

Polymerization of Liquid Propene

After the formation of stable emulsions was established, these systems can be exploited for catalytic olefin polymerization. In contrast to the previously reported emulsion polymerization,21 here liquid propene was used directly as the dispersed phase, and miniemulsion conditions were applied (Fig. 3). As the process is performed at elevated pressure (up to 25 bar) and moreover, because of the reactivity of the monomer, much better heat transfer is required from the active polymerization centers toward the continuous phase.

Figure 3.

Schematic description of the polymerization procedure. Continuous phase—perfluoromethylcyclohexane; emulsion, prepared by adding the emulsifier and the cocatalyst (MAO) followed by ultrasonification; transfer of the system into the reactor, and saturation with liquid propene; polymerization after injection of the catalyst, followed by isolation of the product.

This was demonstrated by the polymerization of liquid propene, a highly exothermic process, and under homogeneous conditions the heat transfer was very poor, leading to excessive heat, melting of the polymer, and finally reactor fouling. Alternatively, performing the polymerization inside a miniemulsion, maximized heat transfer led to a product consisting of particles with spherical shape [Fig. 4(c,d)].

Figure 4.

Product morphology of PP produced inside an emulsion (a) and under homogeneous conditions (b), respectively, and SEM images of the polypropylene particles (c,d) produced by liquid propene miniemulsion polymerization.

Compared with the homogeneous liquid propene polymerization, the catalyst activity was much higher when performed in emulsion 3500 kgPP (mol Zr h)−1 compared with 300 kgPP (mol Zr h)−1 under homogeneous conditions (Table 2). The monomer conversion was higher than 50%, while in the case of homogeneous polymerization, it was below 1%. This is explained with local overheating and melting of the polymer, and subsequent decomposition of the catalyst. In the case of the miniemulsion, the reactor fouling was prevented because of the excellent heat transfer between the droplets and the continuous phase. This first led to high activities because of the constant monomer concentration inside the droplets and second to perfectly shaped spherical product particles. In a previous work,21 when gaseous propene was used as monomer inside the fluorous emulsion, lower activities were observed (Table 2, Run 8). As suggested there, the diffusion of the monomer through the continuous phase is the limiting factor.

Table 2. Liquid Propene Polymerization Results: Used Amount of Propene, Catalyst, and Al/Zr Ratio, as Well as Applied Pressure and Observed Activity and Yield
RunPropene (g)Pressure (bar)Catalyst (mg)Molecular Weight Mn (g mol−1) (PDI)Al/Zr RatioActivity [kgPP·(mol Zr·hr)−1]Yield (g) (%)
  • a

    Homogeneous polymerization.

  • b

    Gas-phase polymerization.21

1251028,000 (2.2)2,0001,0003.2 (13)
22518212,500 (7.5)2,0003,5009 (36)
325201.515,500 (3.5)3,0003,0008 (32)
42022238,000 (4.6)2,0003,00010 (40)
525251.5168,000 (3.6)3,0003,50014 (56)
62525241,000 (2.8)2,0003,00010 (40)
7a2501024,900 (2.1)2,0003001.5 (0.6)
8b2216,000 (3.5)2,0004007 (–)

This gas diffusion through the continuous phase was avoided by the direct use of liquified propene as the dispersed phase. This led to higher polymerization activities, as in the same time by performing the polymerization inside a miniemulsion, excellent control over the process, and the product morphology was afforded. The molecular weights reached more than 100,000 kg mol−1 (Mn), higher than those reported so far.21 The polydispersities, between 2 and 5, are typical for a metallocene-catalyzed polymerization. Moreover, the absence of additional solvent, used for the solubilization of the monomer, and acting as a polymerization “nanoreactor,” is another advantage. This minimizes the costs and the environmental impact of the process, as the continuous phase can be easily regenerated, with the unconsumed monomer captured and used in another polymerization.

CONCLUSIONS

A process for the polymerization of liquid propene is presented where the polymer is applied in a nonaqueous miniemulsion system consisting of a perfluorinated solvent as the continuous phase. The careful design of emulsifiers enabled the formation of emulsions, which showed no change in the droplet size with increase of the temperature even above the boiling point of the continuous phase solvent. The polymerization of liquid propene, which is otherwise difficult to control, is performed inside this system, and perfectly shaped spherical particles are formed and isolated after the process. The molecular weights are higher than the reported ones for propene polymerization in emulsion. At the end, the obtained polymer contains no impurities, and more importantly, the pelletization step after the polymerization to form a well-processable product is avoided. Furthermore, high monomer conversions are observed with catalyst activities above 3000 kgPP (mol Zr hr)−1. These values are much higher than in the case of gaseous propene, where diffusion is a key factor for the polymerization. Although inorganic and organic supports for metallocenes generate polyolefin particles in the range of several hundred micrometers to several millimeters, the miniemulsion process controls the product size and morphology down to the nanometer scale. This together with the absence of additional solvent impurities makes the polypropylene promising for many different applications such as cosmetics, polymeric fillers, adhesives, or precursors for coatings.

Acknowledgements

Basell AG, Frankfurt, is gratefully acknowledged for providing the catalyst. M. S. Hoffmann thanks the Stiftung Stipendien-Fonds des Verbandes der Chemischen Industrie e.V. for funding.

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