POLYMERIZATION OF ETHYLENE AT ATMOSPHERIC PRESSURE A Demonstration Using a "Ziegler" Type Catalyst ALBERT ZILKHA, NISSIM CALDERON, JOSEPH RABANI, and MAX FRANKEL The Hebrew University, Jerusalem, Israel
IN TBE following, a simple experiment on the polymerization of ethylene at atmospheric pressure is described using a "Ziegler" type catalyst prepared from amyl lithium and titanium tetrachloride. The experiment can be used for discussion on the new type of heterogeneous, stereospecific catalysts and on the polymerization of ethylene or a-olefins to isotactic polymers at low pressure; and as a part of an advanced laboratory course in polymerization reactions. Ziegler' discovered that ethylene can be polymerized at low pressure using mixed organometallic catalysts. In his publications, most of them patents, he pointed out that such catalysts could be prepared by the interaction of metal alkyls of elements of Groups I, 11, I11 of the periodic table with halides of the sub groups of the fourth to the sixth group. He suggested that these catalysts owe their special activity to the fact that they contain transition metal, e.g., titanium in a lower valency state, while the second component of the catalyst, the metal alkyl present, acts as reducing agent. The fact that these catalysts ought to be bimetallic has been confirmed by the work of Natta2 who showed that a catalyst containing one metal derivative alone did not cause polymerization of ethylene. The mixed organometallic types of catalysts have been shown to polymerize also other a-olefins3 and to be stereospecific. The polyethylene produced in this t,ype of polymerization is different in many important properties from that produced a t high pressures and temperatures. Contrary to the latter, it is essentially linear in structure as determined by I.R. spectra, its melting point is about 20'-30' higher, and it possesses a higher degree of crystallinity, as seen from X-ray diffraction patterns. These properties have made low pressure polyethylene a special type of plastic material having many important differences in technological properties as compared with high pressure polyethylene. One of the earliest publicatious of Ziegleri on the preparation and use of such organometallic catalysts I ZIEGLER, K., E. HOLZKAMP, H. BREIL,AND H. MARTIN, Angem C h a . , 67, 426 (1955); Angw. Chem., 67, 541 (1955). ' NATTA,G., P. PINO,G. MAZZANTI, AND U.GIAANINI, J . Am. C h . Soe., 79, 2975 (1957). a NATTA, G., Angew. Chem., 68, 393 (1956). 'ZZIEGLER, K., AND H. MARTIN,Makromol. Chm., 18, 186 (1956).
for the polymerization of ethylene a t atmospheric pressure was devoted to the suggestion of a lecture experiment in which it was intended to demonstrate the polymerization of ethylene using the special catalyst prepared from aluminum diethyl chloride and titanium tetrachloride. However, the preparation of such aluminum alkyls is experimentally difficult and hazardous owing to its tendency to self ignition. The experiment described here is safe and can easily be carried out without special apparatus requirements and there is no danger of spont,aneous ignition. EXPERIMENTAL
Preparation of Amy1 Lithium. In a 250-ml. threenecked flask fitted with an adapter (for passing through nitrogen), a dropping funnel, and a mechanical stirrer, are placed 3 g. lithium wire and 50 ml. petroleum ether c.P., b.p. 60'-SO0, previously boiled under nitrogen for several minutes to expel dissolved air. Two milliliters of a solution of 20.7-ml. amyl chloride dissolved in 25 ml. petroleum ether is added, and the reaction mixture is stirred at high speed until the reaction starts as indicated by the formation of turbidity owing to the lithium chloride formed; the rest of the amyl chloride solution is then added during 20 minutes under outside coolmg in an ice-salt mixture. The reaction mixture assumes a brownish-blue color. It is stirred for another 2l/%hours and then fdtered under nitrogen through a funnel containing a plug of glass wool into a graduated flask previously flushed with nitrogen. The filtration removes the unreacted lithium, but lithium chloride passes through; it does not interfere with the polymerization. The solution is allowed to settle and an aliquot portion is taken from the supernatant liquid, hydrolyzed in water, and titrated against 0.1 N HC1. The yield is about 70%. The amyl lithium can be. kept for several days in an icebox and can, therefore, be prepared in advance. Excess lithium or amyl lithium can be destroyed by the addition of ethanol. NOTE: I n a11 operations the nitrogen should be oxygen free. It may be purified by passing it through a quartz tube containing copper wire heated at 600°C.; or through pyrogallol solution (15%) in sodium hydroxide (20%); or through an alkaline sodium hydrosulfite solution to which sodium p-anthraquinone JOURNAL O F CHEMICAL EDUCATION
sulfonate is added as catalyst (so called Fieser's sol u t i ~ n ) .Subsequently ~ it is dried over concentrated sulfuric acid. Preparatimz of the Polymerization Catalyst, and the Polymerization of Ethylene. To the polymerization vessel consisting of a l-liter ground joint threenecked flask fitted with a mechanical stirrer, a condenser with calcium chloride tube, two gas adapters, one for flushing the apparatus with nitrogen, the other for passing ethylene, and a thermometer, 0.05 mole amyl lithium in petroleum ether solution is transferred, and diluted to 400 ml. with the same solvent. Two milliliters of anhydrous titanium tetrachloride is added in one portion by means of a pipet fitted with a rubber suction bulb, and stirring is started. A brownish-hlack precipitate is formed and the temnerature rides after a few minutes bv about 10°C. FIESEE,L. F., "Experiments in Organic Chemistry," 3rd ed., D. C. Heath & Co., Boston, 1955, p. 299; FIESER,L. F., J. Am. Chem. Soe., 46, 2630 (1924).
VOLUME 35, NO. 7, JULY, 1958
Stirring is contmued for 20 minutes to complete the formation of the catalyst and then ethylene C.P. grade is passed. Immediately small particles of polyethylene start to form and the temperature rises. After 10 to 15 minutes the rise in temperature is about 20°C. Stirring is more difficult as the solution becomes filled with solid polyethylene. The polymerization is stopped after 30 minutes. The catalyst is decomposed by addition of 40 ml. butanol. The polyethylene is filtered and washed with concentrated hydrochloric acid-methanol 1: 1, until it becomes white, then it is washed with vater and dried at 80°C. Yield is about 18 g. The polyethylene has a melting point of 125"-130°C. It yields tough semitransparent waxy films r b e n pressed in a Carver press at l4O0C. It is insoluble in organic solvents, concentrated acids, and alkalies in the cold. I t dissolves to a small degree in boiling tetralin, decalin, and similar solvents from which it crystallizes out on cooling.
