Ethylene polymerization in supercritical carbon dioxide with binuclear nickel(n) catalysts Damien Guironnet, Tobias Friedberger and Steran Mecking* Received 30th June 2009, Accepted 1st September 2009 First published as an Advance Article on the web 21st September 2009 DOl: IO.1039/b912883b A series of new, highly fluorinated neutral (KZ-N,O) chelated Ni(IJ) binuclear complexes based on salicylaldimines bridged in p-position of the N -aryl group were prepared. The complexes are single-component catalyst precursors for ethylene polymerization in supercritical carbon dioxide and toluene. Solubility of the catalyst precursors in supercritical carbon dioxide is effected by a large number of up to 18 trifluoromethyl groups per molecule. Semicrystalline polyethylene with a low degree of branching is formed (ca. 10 branches/WOO carbon atoms). Polymer microstructures are independent of the nature of the bridging moiety, while stability of the catalysts appears to differ. Introduction emulsion systems this can be crucial for the control of nanopartic\e 7d size and structure. ,e Concerning reactions in CO2 , tailoring of the 8 Polymerization of olefins catalyzed by complexes of d metals catalyst precursors to provide solubility in the reaction medium is (late transition metals) has been investigated intensively recently. I required. In comparison to their early transition metal counterparts, We now report binuclear salicylaldiminato Ni(u) methyl com­ they are much more tolerant towards functional groups in the plexes soluble in dense CO2 and their polymerization properties. 2a substrates ,b or reaction media.le Thus, ethylene and I-olefins can be copolymerized with electron-deficient polar vinyl monomers, such as e.g. acrylates3 or even acrylonitrile,. and multiple insertion Results and discussion of acrylates affords low molecular weight polyacrylate via an Synthesis of Iigands and complexes insertion mechanism.~·6 Polymerization of ethylene and I-olefins 7 can be carried out in oxygenated reaction media such as water or Most commonly, solubility of organic compounds and of catalysts, 8 dense carbon dioxide. in particular, in dense CO2 is brought about by longer perfluo­ Dense carbon dioxide, that is liquid or supercritical CO2 roalkyl groups, as exemplified by the commercially available tris[3- (scC02), possesses unique properties, such as the possibility of (perfluorooctyl)phenyl]phosphine. Introduction of such perfluoro 9 variation of its density and solvent properties over a wide range. alkyl moieties is associated with considerable synthetic effort and In polymerization processes and polymer processing, dense carbon specific work up procedures, A convenient alternative can be dioxide can be useful as a solvent or suspension medium.9I>,10 It can the introduction of a sufficiently large number of trifluoromethyl be removed conveniently by variation of the pressure, resulting groups. It is worth noting that such electron withdrawing groups in a dry polymer powder. While free-radical polymerization in will also modify the electrophilicity of the metal center and conse­ dense CO2 has been studied most intensely,1l various examples quently catalytic properties (vide infra). The binuclear complexes 1 14 of coordination polymerization have been reported. l- Ethylene reported here are based on 3 differently bridged salicyladimine polymerization has been studied in scC02 with cationic palladium ligands, 3a (benzidine bridge), 3b (methylene bridge) and 3c diimine complexesBa-C as well as with neutral (K2-N,O) nickel (hexafluoroisopropylidene bridge). complexes. 8<1 •• Dianilines la (benzidine scaffold) and lb (methylene bridge) Current developments of late transition metal catalyzed olefin were prepared according to known procedures. 18f lc (hexaflu­ polymerization have resulted in a renewedl~ interest in neutral oroisopropylidene bridge) was obtained analogously by selec­ Ni(u) complexes. 16 This class of catalysts appears to be particularly tive ortho-bromination of 4,4' -diamino diphenyl I, I, I ,3,3,3- 7 17 suited for polar (pro tic) reaction media. • Substantial effort is hexafluoropropane, and subsequent Suzuki coupling with 3,5- devoted to the finding of catalysts with increased polymerization bis(trifluoromethyi)phenyl boronic acid in good yield (overall productivity. One approach studied are binuclear complexes, yield: 63%). Condensation of the dianilines la-i: with 3,5- which often polymerize with higher activities than their mononu­ bis[3,5-bis(trifluoromethyl)phenyl] salicylaldehyde in toluene un­ 18 clear analogues. The origin of this increased reactivity is iIl­ der Dean-Starck conditions afforded the novel salicylaldimines understood, cooperative effects have been suggested to operate 2a-i: (Scheme I). The crude products obtained upon solvent in certain cases. ISh Another aim of catalyst development is an evaporation were washed with methanol. The identity and purity appropriate solubility in the specific reaction media. In aqueous of the dianilines and di(salicylaldimines) were established by 'H, BC NMR spectroscopy, mass spectrometry and elemental analysis (see Experimental). Chair of Chemical Materials Science, Dept. of Chemistry, University of Konstanz, 78464, Konstanz, Germany. E-mail: stefan.rnecking@uni­ Reaction of the respective di(salicylaldimines) with 3 equiv­ konstanz.de; Fax: +49 7531885152; Tel: +49 7531885151 alents of [(pyridine)2Ni(CH3)2] in cold diethylether suspension CF, CF, previously for other Ni(n)-Me salicylaldiminato complexes. '6g,'Sr,t9 Most characteristic are the high field nickel methyl resonances around -1.0 ppm in the IH NMR and -8 ppm in the llC NMR spectra. This unique resonance confirms that both metal centers H are magnetically identical in the binuclear structure, as expected. 01::':-b CF'--,,-,~::=:;;; Catalytic ethylene polymerization with binuclear (K'.N,O) " CF, salicylaldiminato Ni(n) methyl pyridine complexes F, CF J CF3 Polymerization studies were performed in toluene as a reaction 1a:X:O 1b:X"'CH! medium (Table I). In this case monitoring of the polymerization la: X: ClCF,), via mass-flow meters, not possible with the set-up for polymeriza­ CF, 20: X: 0 CF, tion in supercritical fluids, gives insights on catalyst stability. The 2b: X =CH;! 2.: X: C{CF,), mononuclear analogue 4Sd was studied as a catalyst precursor for Scheme 1 Preparation of binuclear salicylaldimine ligands. comparison (entries 1-7 and 1-8). was found to be a viable route for the preparation of complexes 3a-c (Scheme 2). The relatively poor solubility of the ligand in diethylether slows down the reaction. Consumption of the starting material could be followed by a colour change (from yellow to dark red), and by the evolution ofmcthanc. Decomposed excess Ni(u) starting material was removcd by filtration. The novel catalyst precursors are active for ethylcnc poly­ merization at 50 DC under 40 bar ethylene pressure in toluene. Indcpcndently of the bridge, activities obscrvcd wcrc on the same order as for the mononuclear analogue (3 x 104 TO h-' average in a 30 min experiment). At a polymerization temperature of 70 QC, -30 °C to RT + (pyrldine"NIMe,) . higher polymerization rates were observed (Tablc I). This is likely Et,o associated with an enhanced dissociation of the pyridine donor. At this temperature, the nature of the bridge does have a clear cffect on catalyst stability. Complexes 3a,b as well as 4 decomposed rapidly. By contrast, 3c consumed ethylene steadily over the 30 min F3C duration of the experiment. The polymers obtained are semicrystalline (x - 50%) with CF; CF, CF3 CF, a melt peak temperature in the range from 105 to 122°C. A 2a:X 3a: X = 0; R = 3,5-(CF 2b:X 3b: X = C~; R = 3, relatively low number of methyl branches was detected by llC 20: X 30: X = qCF,),; R = NMR spectroscopy (15 per 1000 carbon atoms) along with very Scheme 2 Preparation of ("-'-N.O) salicylaldiminato nickel-methyl pyri­ few ethyl branches (2 per 1000 carbon atoms). Note that these dine complexes. branches result from 'chain walking',20 commonly observed with late transition metal catalysts. In detail, the lowest branching All complexes are diamagnetic in solution as evidenced by sharp and highest polymer molecular weight was observed for 3c, resonances in the' Hand llC NMR spectra. This suggests a square which can be related to the electron-withdrawing character of planar coordination geometry of the nickel center, as reported the hexafluoropropylidine bridge. An increased electrophilicity is Table t Polymerization in toluene" Run Complex TI"C TOP IO J /h' M! IOJ /gmo]-' Mw lM! Tm'l"C X" (%) NM,IlOOOC' NE,/IOOOC' I-I 3a 50 18 23.0 2.4 115 50 14 < I 1-2 3a 70 64 3.8 3.1 104 45 19 2 1-3 3b 50 20 25.1 2.8 119 51 11 < I 1-4' 3b 70 25 2.6 3.9" 110 47 15 2 1-5 le 50 29 38.9 2.6 122 54 9 < I 1-6 le 70 91 12.7 2.1 115 54 14 < I 1-7' 4 50 32 14.3 2.4 110 48 17 2 1-8' 4 70 70 8.3 2.2 105 46 19 2 • Reaction conditions: 100 mL of toluene, p(ethylene) 40 bar. I 30 min, n(Ni) = 10 /-lIDO!. • n(Ni) 5 /-lIDO!' • TOF in mol (C,H4 ) mol (Ni)"' h-'. d Determined by GPC. ' Determined by DSC from second heat. f Determined by nC NMR. • Bimodal distribution. Table 2 Polymerization in scCO,' Run Cat. TrC TOP 10'/h-1 M/ 1O'/g mol- l Mw IM/ Tm'/oC X' (%) NM,/IOOOC' NHt/IOOOC' 2-1 h 3a 50 2.7 15.3 2.3 121 48 14 < I 2-21> 3b 50 4.3 29.2 2.5 119 47 n.d. n.d. 2-3 3b 70 10.4 1.8 3.8 98 36 17 2 2-4' 3b' 70 15.1 12.0 2.1 99 36 17 2 2-51> 3c 50 3.4 26.1 2.3 124 54 9 < I 2-6 3c 70 8.9 2.3 4.6 105 43 14 I 2·7 3cc 70 14.3 2.7 3.4 105 43 14 2 2·8 4d 50 5.7 33.3 2.1 123 49 5 < I 2·9 4d 70 10.9 2.9 3.4 97 40 19 2 • Reaction conditions: n(Ni) = 10 Ilmol, t 30 min, m(CO,) = 45.5 g, m(C,H.) = 6 g.