Water as a reaction medium for nickel(II)-catalyzed ethylene polymerization

WATER AS A REACTION MEDIUM FOR NICKEL(II)- CATALYZED ETHYLENE POLYMERIZATION

Stefan Mecking* and Florian M. Bauers Institut für Makromolekulare Chemie und Freiburger Materialforschungszentrum der Albert-Ludwigs-Universität Freiburg, Stefan-Meier-Str. 31, D-79104 Freiburg, Germany

Introduction

As the most environmentally friendly common solvent conceivable, water also possesses other unique properties as a reaction medium: it is highly polar and immiscible with most organic compounds, has a high heat capacity and also features a strong propensity for micelle formation in the presence of surfactants.

These features are exploited in radical emulsion and suspension polymerization of olefinic monomers, such as butadiene, styrene and acrylates on a large scale.

Also, transition metal catalysis in aqueous media involving the synthesis of low molecular weight compounds (commercialized in the Ruhrchemie/Rhone- Poulenc hydroformylation process) is currently attracting strong interest.¹ By contrast transition metal catalyzed coordination polymerization reactions in water have received less attention, as the common early transition metal catalysts² are extremely sensitive to moisture.

Due to their less oxophilic nature, late transition metal complexes are generally less sensitive to polar media. However, the C-C linkage of ethylene in general (i.e. in organic reaction media) usually yields dimers and oligomers, due to the propensity of late transition metal alkyl complexes for β-hydride elimination.³ In comparison to early transition metal catalysts, only a relatively limited number of late metal catalysts applicable to polymerization yielding high molecular weight products are known. Most of them are based either on neutral nickel(II) complexes⁴ or on cationic iron, cobalt, nickel or palladium complexes.⁵ The neutral nickel(II) complexes contain a formally monoanionic bidentate ligand, usually binding via an oxygen donor and another donor atom such as P or N, whereas the cationic complexes contain neutral multidentate ligands with bulky substituted nitrogen donor atoms. The aforementioned stability of late transition metal complexes with respect to polar media is demonstrated by the tolerance of such catalysts towards polar functionalized comonomers^4c,5b,c and polar organic solvents^4b,c,5c Concerning water as a medium for coordination polymerization, a very slow (1 turnover/day) reaction of ethylene to higher molecular weight linear product, catalyzed by a rhodium complex, has been reported.⁶

Experimental

Materials. Deionized water was degassed prior to use. Toluene, THF, pentane and acetone were distilled from Na resp. P2O5 under argon. Ethylene (99.8 %) was used as received. [Rh(H2C=CH2)2(acac)] was prepared according to literature procedures.⁷

Instrumentation. Polymer molecular weights were determined by high temperature gel permeation chromatography (GPC) vs. linear polyethylene standards. ¹H and ¹³C NMR spectra of polyethylenes were acquired in C2D2Cl4

at 130 °C (Bruker AMX-300, 300 MHz for ¹H and 75 MHz for ¹³C).

Complex preparation.⁸ The ligand 4-MeC6H4C(=O)C(SO3-Na⁺)=PPh3

was prepared by sulfonation of 4-MeC6H4C(=O)C(H)=PPh3 using SO3•py or SO3•dioxane in methylene chloride, followed by neutralization of the resulting zwitterion with NaOH.⁹ Complex 1 was prepared by stirring a solution of equal amounts of 4-MeC6H4C(=O)C(SO3-Na⁺)=PPh3, [Ni(cod)2] and PPh3 in THF at 50 °C for several hours, followed by cooling to room temperature while stirring overnight. Removal of solvent followed by toluene extraction of the residue, filtering and precipitation with pentane yielded 1 in 88% yield. ³¹P{¹H} (202 MHz, C6D6, ext 85% H3PO4): δ 35.4 (d, ²J(P,P) = 276 Hz; =C(SO3-)P), 21.0 (d,

2J(P,P) = 276 Hz; PPh3).

Polymerization Experiments were performed in a mechanically stirred 200 mL steel reactor (Büchi AG, Ulster/Switzerland). Typically, a solution of the nickel complex in water was transferred to the reactor under an argon atmosphere followed by a solution of the phosphine scavenger in a small amount of organic solvent. In comparative experiments in organic solvents, solutions of the two components were subsequently transferred to the reactor (total volume of water and organic solvents: 100 mL).

After the given reaction time, the reactor was rapidly cooled and the ethylene pressure was released. The solid polymer was filtered from the aqueous phase and dried in vacuo at 50 °C.

Results and Discussion

The sulfonate complex 1 was prepared similar to procedures briefly reported in the literature.⁸ The sulfonate group renders the complex water- soluble. Also, in polymerization in aprotic solvents introduction of the sulfonate group has been reported to result in an increase of molecular weight of the polyethylene formed.^4b,c

O Ni

P Ph

PPh₃ Na^{+ -}O3S

Ph Ph

1

In order to produce higher molecular weight polymer using 1 as a catalyst precursor, absence of the strongly coordinating ligand PPh3 during polymerization is required.^4c As a phosphine scavenger not sensitive to water, [Rh(H2C=CH2)2(acac)] was utilized.

The stability of C-C linkage catalysts based on nickel(II) complexes with formally monoanionic ligands towards polar media including water had been noted early on by the original inventors.¹⁰ It has been a basis for the first industrial application of two-phase transition metal catalysis, utilizing diol solvents. However, successful polymerization affording higher molecular weight products in water has not been reported to the best of our knowledge. Complexes 2 and 3 were found to be completely inactive for ethylene polymerization in organic media in the presence of 1000 eq water (in combination with a phosphine scavenger).^4c,8b

2: R¹ = H, R² = Ph, R³ = Ph 3: R¹ = Ph, R² = OEt, R³ = Et

O Ni

P Ph

PR³3 R¹

Ph Ph

R²

For this reason, we were somewhat surprised to observe formation of linear polyethylenes employing complex 1 in water, utilizing only a small amount of water miscible (acetone) or immiscible (toluene) organic solvent to enable injection of the phosphine scavenger (cf. table). By comparison to polymerization in neat toluene or acetone (entries 5 and 6), polymer molecular weight is significantly reduced and productivity is lowered in aqueous media. By their molecular weights, the polymers obtained in runs 3 and 4 compare to typical polyethylene waxes.

In the context of possible side reactions in transition metal catalysis in aqueous media¹ (such as hydrolysis of metal-alkyl species, attack of water on coordinated substrates or coordination of water to the metal center as a ligand), regarding polymerization reactions a conceivable effect of water on chain transfer is of specific interest as small (absolute) changes in the overall chain transfer rate will strongly influence product molecular weight. For instance, the presence of additional ligands is known to promote chain transfer reactions in ethylene oligomerization to low molecular weight compounds.¹¹ Despite of the observed lowering of polymer molecular weight, no dramatic effect of water on chain transfer reactions is clearly evident from the data presented: the lower molecular weights can also be attributed to a slower chain growth as reflected by the lower productivity.

The question arises whether the observed lowering of productivity and polymer molecular weight in aqueous media are due (at least partially) to an unfavorable interaction of water with the nickel(II) catalyst or primarily to the low solubility of ethylene in water directly slowing down chain growth (it must be noted, that clearly distinguishing between these possibilities is not trivial, since e.g. a conceivable competition of water and ethylene for binding to the metal center would be influenced by ligand concentrations). To this end, utilization of larger amounts of acetone as a water miscible cosolvent to increase ethylene solubility has no significant effect on polymer yield (entries 1 and 2 vs.

First publ. in: Polymer Preprints 41 (2000), 1, pp. 209-210

Konstanzer Online-Publikations-System (KOPS) URL: http://www.ub.uni-konstanz.de/kops/volltexte/2008/6199/

URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-61996

3), which may be interpreted as an indication for interaction of water with the catalyst lowering the productivity. However, a conceivable rapid initial polymerization accompanied by complete irreversible deactivation of the catalyst in the early stages of the reaction can be ruled out, as comparison of runs with different reaction times (entry 1 vs. 2) shows that the catalysts are still active for polymerization after several hours in water.

Interestingly, performing the nickel(II)-catalyzed ethylene polymerization in the presence of ionic or non-ionic surfactants (SDS, Triton X-100), stable polyethylene emulsions are obtained. Typically, emulsions with particle sizes in the range of ∅ 80 to 300 nm are obtained at a catalyst productivity of e.g. 1300 mol(ethylene)/mol(Ni) in a 2 h experiment.

In conclusion, linear polyethylene can result from the coordination polymerizaton of ethylene in water as a reaction medium at high catalyst activities. In the presence of surfactants, polyethylene emulsions can be obtained.

Acknowledgments. The authors thank R. Mülhaupt for his interest in our work. Financial support by BASF AG is gratefully acknowledged, and we thank B. Manders and M. O. Kristen for valuable discussions. A generous loan of palladium chloride was provided by Degussa-Huels AG. GPC analyses were carried out by D. Lilge (BASF) and ³¹P NMR analyses were provided by D.

Hunkler (Freiburg). We thank G. Moerber for excellent technical assistance.

Table 1. Polymerization Results.

reaction conditions results

entry No.

n(cat.) / µmol

reaction medium (solvent ratios v/v)

reaction time

polymer yield / g

producti- vity^a)

Mw / (g·mol^-1)

Mn / (g·mol^-1) (Mw/Mn)

1 130 acetone/H2O 50:50 1.5 h 2.5 680 n.d. n.d.

2 121 acetone/H2O 50:50 3 h 3.2 940 n.d. n.d.

3 108 acetone/H2O 5:95 2 h 2.2 710 2230 970 (2.3)

4 89 toluene/ H2O 5:95 2 h 5.9 2360 3030 960 (3.1)

5 12 toluene 2 h 9.0 26680 580000 13900 (42)

6 26 acetone 2 h 22.2 30440^b) 94000 3766 (25)

Reaction conditions: 70 °C, 50 atm ethylene, total volume of water and/or organic solvent: 100 mL. Catalyst: 1 / [Rh(H2C=CH2)2(acac)] (Ni / Rh 2:1).

n.d. = not determined. a): mol(ethylene)/mol(Ni). b): probably mass transfer limited.

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