ANIONIC POLYMERIZATION OF VINYL MONOMERS A Demonstration ALBERT ZILKHA, MICHAEL ALBECK, and MAX FRANKEL The Hebrew University, Jerusalem, Israel
INTHE following, experiments on the polymerization of styrene using hutyl lithium as catalyst, i.e., by anionic mechanism, are described. The experimental procedure described is suitable for flash polymerization of styrene and as an experiment within a laboratory course in polymerization reactions. It can be used also as a lecture demonstration for advanced students in organic chemistry. Two general catalytic methods, the one by free radical, and the other by ionic mechanisms, are available for the polyme~imtionof vinyl monomers. Of the ionic polymerizations, the anionic has been less investigated than the cationic. Beaman1 showed that some vinyl monomers, especially methacrylonitrile, polymerized rapidly and at low temperature on using sodium in liquid ammonia as catalyst. Higginson and Wooding2 studied the kinetics of the polymerization of styrene with potassium in liquid ammonia as catalyst. Comparatively the most extensively studied anionic polymerization is that of butadiene using metallic sodium.8
VOLUME 35, NO. 7, JULY, 1958
Anionic polymerization is not yet completely understood. I t is sensitive t o experimental conditions and difficult to control and to reproduce, owing to the velocity of the polymerization. Its behavior is quite different from free radical polymerization and makes the study of the reaction mechanism more difficult; traces of foreign substances may act either as promoters or as retarders. The polymers obtained are usually not homogeneous and often have low molecular weights, depending on the catalyst used and the experimental conditions. The activation energy for the anionic polymerizations is usually small, but often a critical catalyst concentration is necessary to start the polymerization. Likewise, control of the composition of copolymers polymerized by anionic mechanism is very difficult. Despite the above mentioned drawbacks, anionic BEAMAN, R., J . Am. Chem. Soe., 7 0 , 3115 (1948). HIGGINSON,W. C. E., AND N. S. WOODING,J. Chem. Soe., 1052. 7 - M ~ a ZIEGLER,K., A n g w . Chem., 49, 499 (1938)
----.
polymerization is a comparatively new field of research which may yet lead to important contributions in the field of plastics. The experiments described below seem to be useful to demonstrate within a short time the special properties of anionic polymerization, such as heat and speed of reaction, sensitivity t o conditions and other factors mentioned above. Butyl lithium polymerizes as well as styrene and other vinyl monomers, such as acrylonitrile, acrylamide, and methyl methacrylate having electron attracting groups attached t o the vinyl group, thus making the double bond pore acidic and therefore more responsive to anionic catalysts. Styrene, however, is a striking example, as it can be polymerized not only by anionic, but also by free radical and by cationic mechanisms; its varying behavior emphaeizes the contrast between the three kinds of polymerization. EXPERIMENTAL
Prewaration of Butvl Lithium. A ~roceduremodified from that of ~llman"' and of ~ o g e l ~ w used. as Three grams of lithinm in the form of wire and 70 ml. of ether dried over sodium were placed in a 250-ml. three-necked flask equipped with a mechanical stirrer, dropping funnel, reflux condenser with calcium chloride tube, and gas leading tube t o pass oxygen-free nitrogen through the apparatus. The flask was cooled in an ice-salt mixture and 24 g. butyl bromide in 25 ml. ether were added dropwise during 20 minutes. The reaction mixture was stirred a t high speed for another 60 minutes. The solution of the butyl lithium was filtered through a funnel containing a plug of glass wool into a glass stoppered bottle. This filtration removed the residual lithium. The concentration of the butyl lithium was measured by titration of an aliquot portion with standard acid after hydrolysis with water using phenolphthalein as indicator. The yield was 80Y0. Excess lithinm or butyl lithium catalyst was destroyed by the addition of ethanol. The butyl lithium can be kept for several days in an icebox and can therefore be prepared in advance. It was found unnecessary in the present polymerization experiments to free the styrene used (Hopkins and Williams, London), from inhibitors. Polymerizations of Styrene with Butyl Lithium. To 20 ml. styrene at room temperaure in a Pyrex, widemouthed bottle (the mouth of which is pointed away from the observers) is added rapidly from a pipet, with shaking, a solution of butyl lithium in ether. The solution used had a concentration of 0.00086 mole per ml. The color of the styrene changes to yellow and then to dark red-brown. This is due to the formation of carbanionic propagation centers.' After the addition of 3 ml. of the catalyst solution, the styrene attains maximum color intensity and almost "ILMAN, H., J. A. BEEL,C. G. BRANNEN, G. E. BULLOCK, G. E. DUNN,AND L. S. MILLER,J. Am. Chem. Soe., 71, 1499 (1949). VOREL,A. I., "Pr%cticalOrganic Chemistry," 3rd ed., Longmans, Green & Co., Inc., New York, 1956, p. 932. 6 F~onu, PAUL J., "Principles OF Polymer Chemistry," Cornell University Press, Ithaca, New Yark, 1953, p. 224.
346
immediately an extremely fast exothermic polymerization occurs, whereby most of the polystyrene formed, still hot, shoots out from the bottle, as a friable mass of white color, streaked with brown, which on standing in the air, or on washing with water, usually turns more or less entirely white. On slower addition of the catalyst, more of it is needed to obtain this "flash" polymerization, sometimes up to 15 ml., and the polymerization proceeds less energetically. The changes in color observed also take a longer time to appear. The color sometimes fades away, probably due t o destruction of catalyst, and reappears on addition of fresh catalyst. I n both cases, however, the polymerization is quantitative. The experiment may also be carried out in an open porcelain dish. When the butyl lithium is dropped on the styrene, the latter assumes a dark reddish-brown color and spreads mushroom-like to mans times its original volume on its sudden Some 8 ml. of butyl lithium are needed on rapid addition ta comolete the ~olvmerization in this case. Here also " the polymerization is quantitative. An experiment on copolymerization using this technique was carried out with styrene containing some divinyl benzene. The polymerization proceeded as usual and the polymer formed was, contrary t o polystyrene, insoluble in benzene in which it swelled to many times its previous size owing to its crosslinked character. The polymerization of styrene can be carried out also on a piece of filter paper. . This is dipped in styrene, then butyl lithium solution is dropped on it, whereupon the styrene polymerizes readily, with accompanying color changes on the paper. The filter paper becomes impregnated with polystyrene and thus is rendered mechanically strong and impermeable to water. In the following, experiments on the polymerization of styrene with aluminium chloride (cationic mechanism) and with benzoyl peroxide (free radical mechanism) are given in order to enable a comparison with anionic polymerization. Polymerization of Styrene with Aluminium Chloride. To 5 ml. styrene held in a beaker was added 0.3-0.4 g. pure anhydrous aluminium' chloride (the aluminium chloride should be pure as with impure catalyst the polymerization does not start immediately and more catalyst is needed). Immediately an energetic polymerization occurred; the styrene assumed a brownishblack color; the temperature rose considerably, and part of the styrene evaporated as white fumes. The styrene polymerized t o a black viscous pitch-like substance, which solidified and changed its color slowly to white on standing or boiling with methanol. Polymerization of Styrene with Benzoyl Peroxide. The 0.25 g. benzoyl peroxide (5% catalyst) was dissolved in 5 ml. commercial styrene and heated in a test tube in a beaker of boiling water. After 20 min. the styrene started to solidifv, the ~olvmerizationbeing completed after about 30 'minutes,-forming a trans: parent polymer. On using a smaller quantity of catalyst, the pol~merizationtook a longer time for conpletion.
.
~
~
~~
~~
~
JOURNAL OF CHEMICAL EDUCATION
