Wayne R. Sorenson
Continentol Oil Compony Ponco City, Oklahoma
Polymer Synthesis in the Undergraduate Organic Laboratory
Among the objectives of the undergraduate elementary organic laboratory course are these: to give the student first hand contact with the organic reactions he has seen in his text; in so doing, to let him encounter the principles of organic chemistry as a functioning reality; to teach the student some of the typical operating techniques of an essentially experimental discipline; and to acquaint the student with the physical character and behavior of organic compounds. If these points are agreed upon, the place of experiments which demonstrate polymer-forming reactions in the undergraduate laboratory must be conceded. Polymer-forming reactions may involve only a multiple repetition of a classical organic reaction, such as esterification; or, in brilliant contrast, they may be represented by something as exotic as carbodiiinide formation.' Chain growth may involve an important category of reactions not readily demonstrable by other than polymerization^.^ Free radical reactions are probably an illustration. Polymerization can readily demonstrate various organic reactions involving different operating methods and skills, which will be discussed later. Macromolecular substances constitute a large proportion of commercially manufactured organic materials. They are substantially different in most physical characteristics from simple organic compounds; this important fact can be demonstrated easily by preparing viscous solutions, by observing the range of melting temperatures, and by casting or melt pressing tough films. Contact with some examples of this distinctive, scientifically and commercially important, class of organic materials would seem to be a most valuable experience a t the undergraduate level. The experiments that follow require equipment and facilities believed to be within the scope of the undergraduate organic laboratory. The chemicals used can he purchased or else they are the subjects of preparative procedures that are, in themselves, good examples of synthetic reactions. The polymers can, in most cases, be cast from solution or melted in a simple laboratory press to a tough film; success or failure here indicates whether satisfactorily high molecular weight has been obtained, and will serve as a rough indication of the relation of mechanical properties to molecular weight. As in most phases of polymer synthesis, the success of the reaction will depend on the purity of the chemicals used and the skill with which they are combined. Improper functional group balance
' CAMPBELL, T. W., AND MONAGLE, J. J., J. Am. C h .Soc.,
84, 1493 (1962).
MAYO,F. R., J. CHEM. EDUC., 36,157 (1959).
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through weighing and handling errors and introduction of small amounts of moisture or oxygen to a system can badly impair the successful operation of a polymerization. Above all, purity of the chemicals used (solvents as well as the monomers themselves) is of very great importance, and many failures will be traceable to this factor. I n practice, the most certain analytical test for the purity of a material is successful formation of high-molecular-weight polymer. Methods for melt pressing of solid polymer or casting films from solution are described in chapter 2 of Sorenson and CampbelL3 The first method requires a small laboratory press; the second needs only a lahoratory vacuum oven, as is used for drying organic compounds. I n cases where a polymer is soluble in a low-boiling solvent such as methylene chloride or tetrahydrofuran, air-drying in a hood is often practical. Considerable satisfaction can be derived from the fabrication of mechanically strong films by these two relatively easy methods; this constitutes one of the rewards of working with macromolecules. Polymerization of Trioxane to Polyformaldehyde
Polyformaldehyde can be obtained by the polymerization of pure formaldehyde gas or by the polymerization of trioxane, the cyclic trimer of formaldehyde. Trioxane is more convenient for laboratory experiments of the type described in this paper. I t is easier to purify trioxane and certainly the manipulations of a solid are easier than handling a gas.
I n a clean, dry test tube about 30 X 175 mm, place 10 g of trioxane recrystallized from methylene chloride (about 300 ml/kg of trioxane) and 12 ml of cyclohexane which has been dried over calcium hydride. Lightly stopper the tube (with a rubber stopper wrapped in aluminum foil) and warm to 65-70' in a water bath. Prepare a catalyst solution in advance by adding 2.0 ml BFa etherate to 40 ml benzene dried over calcium hydride. By means of a syringe or, if necessary, a calibrated medicine dropper, add 0.15 ml of catalyst solution to the test tube in the bath and again lightly stopper the tube. Within about five minutes the trioxane-cyclohexane solution will become immobilized by the growth of the polymer throughout the tube. Then drop the bath temperature to 4045', and maintain this temperature about 30 min. 8 S o ~ ~ ~W.s oR.,~ AND , CAMPBELL, T. W., "Preparative Methods of Polvmer Chemistrv." Interscience Publishers. Ino.. New York, 196;. p. 111.
Next, pour the contents of the tube onto a suction filter and wash the polymer with 125 ml isopropanol containing 10% aqueous ammonia. Make the solid polymer into a slurry with 125 rnl of isopropanolaqueous ammonia in a beaker, using a stirring rod to crush any agglomerated pieces, and again pour it onto a suction filter and wash with isopropanol. The polymer can be dried in a vacuum oven a t 50-60' overnight. The yield should be about 6.5-7.0 g (6570%). The polyn~ershould have an inherent viscosity around 2.0, as measured on a solution of 0.5 g in 100 ml dimethylformamide a t 60'. Tough films can be pressed a t around 190" but they are somewhat bubbly because of evolution of formaldehyde from the polymer. The crystalline melting point of the polymer is approximately 180°. The polymer can be stabilized to thermal degradation to a large extent by esterifying the terminal hydroxyl groups with acetic anhydride. Most, if not all, of the end groups are -OH as a result of termination of the growing chains with protonic impurities, primarily water. The latter can act as chain transfer agent. This may occur as:
-
and chain growth
Depolymerization occurs on heating by an "unzippering" of formaldehyde from free -OH end groups. End capping is accomplished by placing 5 g of finely powdered polymer, 200 ml acetic anhydride (caution! lachrymatory), and 0.5 g anhydrous sodium acetate in a three-neck flask equipped with stirrer and reflux condenser. Stir and reflux the mixture for one hour and separate the polymer by filtration with suction. Wash the polymer on the filter with hexane, make into a slurry twice in a beaker with hexane, and filter. Dry the polymer as in the preceding section; it now shows considerably greater stability to heat when pressed to a film, as is evidenced by the lower extent of bubble formation in the film. About 80-90% of the charged polymer is recovered. The difference in behavior of the two polymers can be qualitatively assessed by placing a part of the original and the capped polymer on the surface of a hot plate a t around 220-230". Both polymers should give viscous melts which can be pressed out wit,h a spatula. The uncapped polymer should readily decompose with the evolution of formaldehyde; the capped polymer will be more stable. Poly [ethylene methylene bi~(4-~hen~lmrbamofe)] from the Polyadditian of a Diol to a DiisocyanateS
The reaction of an alcohol with an isocyanate is commonly illustrated in the organic chemistry laboratory course by the synthesis of urethanes, either for themselves or as derivatives of alcohols. Polyurethanes are readily prepared by this fundamental reaction:
' SORENSON AND CAMPBELL, op. d., p. 82;
OVERBERGER, C. G., editor, "Macromolecular Syntheses," Vol. 1, John Wiley & Sons, Inc., New York, 1963, p. 73.
Purify methylene bis(4-phenylisocyanate) by distilling through a Vigreux column, bp 148-150°C/0.12 mm. As an alternative, the diisocyanate can be recrystallized by dissolving it in an equal volume of boilmg hexane, treating with decolorizing charcoal, and filtering into an equal volume of ice-cold hexane. I n this way, oiling out of the diisocyanate is prevented. A white crystalline solid results, mp 42'. It should be stored under nitrogen in the cold. Subsequent weighi n g ~should be conducted rapidly. (Avoid contact of the diisocyanate with the skin.) Purify the ethylene glycol by distillation (bp 7g°C/4.4 mm, n,25'C = 1.4300, and % H 2 0 = 0.05 or less). Purify the solvents by distillation: dimethylsulfoxide, bp 66'C/5 mm; 4methylpentanone-2, bp 115°C. For many undergraduate laboratories, it may be desirable to prepurify a large of these solvents and reagents for - quantity . student use. Place forty ml of 4-methylpentanone-2 and 25.02 g of methylene bis(4-phenylisocyanate) in a three-neck round-bottom flask e a u i ~ ~ with e d stirrer and condenser and protected from moisture. Stirring the suspension rapidly, add 6.20 g of ethylene glycol in 40 ml of dimethylsulfoxide. Heat the reaction a t 1 1 5 T for 11/% hr, then pour the clear, viscous solution into water to precipitate the polyurethane. The tough, white polymer should then be chopped up in a home blender, washed with water, and dried in a vacuum oven at 90°C. Inherent viscosity is 1.0 in N,N-dimethylformamide a t room temperature (cone. 0.5%). Films may be dry cast from dimethylformamide or directly from the originally prepared polymerization solution. The polymer-melt temperature is 255'C and the glass transition temperature is 90°C. A
A.
The method of interfacial polycondensation involving the reaction of a diacid chloride, dissolved in an organic solvent, with a diamine (or bisphenol) in water with enough base to react with the released HCI has been described in this J o ~ r n a l . ~ For highest molecular weight, the polymerization in most cases should he run with rapid stirring, as in a high-speed kitchen blender. If such a device is available, this approach is recommended. However, the polymerization can be dramatically executed by adding the reactants in such a way as to avoid mixing. I n either case, the reaction is: 0
0
'MOR~AN P., W., AND KWOLEK, 8. L., J. CHEM.EDUC.,36, 182 (1959). This paper, entitled "The Nylon Rope Trick," has received as much circulation as any ever published in these pageges. Volume 42, Number I , January 1965
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The following example uses excess diamine to neutralize the HCI formed: Place a solution of 3.0 ml sebacoyl chloride in 100 ml dist,illed tetrachloroethylene into a 200-ml tall-form beaker. Over the acid chloride solution, carefully pour a solution of 4.4 g hexamethylenediamine in 50 ml water. (The diamine is handled most conveniently as a standardized stock solution of about 20% in water. The commercially available solid diamine may also be used without further ~nrification.) Grasp the polymeric film which forms a t the interface of the two solutions with tweezers and raise from the beaker w a continuously forming rope. If a mechanical wind-up device is placed above the beaker, the polymer may he wound up continuously until one of the reactants is exhausted. Wash the polymer several times with 50% aqueous ethanol or acetone and dry in a vacuum oven a t 60°C. The product has an inherent viscosity of from 0.4 to 1.8 in m-cresol, 0.5% conc a t 25'C, depending on the reaction conditions. The polymer melt temperature is 215OC. Fibers and films can be obtained from the melt or from formic acid solutions. The following procedurea uses a blender for rapid stirring; a higher molecular weight, granular polymer is produced. Into a l-qt home blender, place a solution of 2.32 g (0.02 mole) of hexamethylenediamine and 1.60 g (0.04 mole) of sodium hydroxide in 330 1111 of water. Turn the blender to high speed with a rheostat, and add, over a period of about 15 seconds, a solution of 4.78 g (0.02 mole) of sebacoyl chloride in 250 ml of tetrachloroethylene. A smooth addition without loss of product may be made by adding the solution through a powder funnel inserted in a hole made in the plastic cover of the blender. I t is also helpful to place an aluminum foil on top of the jar but under the cover. Diamines and alkalies may be made up as concentrated aqueous solutions (0.2 g/ml) and dispensed from burets. After the mixture has been stirred for 2 minutes, collect the polymer on a fritted glass filter and wash with water until it is free of alkali and salt. Paper filters contaminate the product with cellulose fiber, which degrades and discolors if the polyamide is later melted. The washing may be hastened by the use of such organic solvents as acetone or alcohol or mixtures of these with water. They help to remove the waterimmiscible solvent. Stirring in the blender will also speed the washing. A final rinse with ethyl ether yields a fluffierform of dried ~roduct. The granular polymer should be dried in air a t a temperature below 100°C or in a vacuum oven. The yield is about 4.8 g. (85%). The inherent viscosity is 1.&1.8 determined in m-cresol a t 30°C and 0.5 g of polymer per 100 ml of solution. Poly(2,6-Dimethyl-p-phenylene etherP
This interesting and unusual polymer can be obtained by oxidative bromine displacement from 4-bromo-2,6dimethylphenol.
' OVERBERGER, op. cit., p. 13.
7 SORENSON AND C A M P B E L op. ~ cil., p. 281; OVERBERGER, op. eil., p. 75.
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An alternative route is the oxidative coupling of 2,6dimethylphenol; this reaction is described below:
Into a 500-ml wide-mouthed Erlenmeyer flask equipped with a paddle stirrer, air inlet tube, and thermometer, place a mixture of 50 ml pyridine, 170 ml nitrobenzene and 0.22 g copper(1) chloride. (The solvents should be puriiied by distillation. The copper(1) chloride can be purified by dissolving it in conc hydrochloric acid, filtering, and pouring into water to precipitate; the solid is then washed with alcohol and ether and dried under vacuum.) Air,freed of oily and mechanical contaminants by passing it through a n Erlenmeyer flask trap and a tube packed with glass wool, should be passed through the solution while it is stirred vigorously. Copper(1) is converted to copper(I1) by oxygen. After 10 min, add 10 g of 2,6-dimethylphenol (make sure that it is pure by recrystallizing a t least twice from heptane). If no temperature rise is noted in about 30 min, heat the mixture to 30-32' with a warm water bath. The viscosity of the solution increases as the polymerization proceeds. After about an hour, or when no further viscosity increase occurs, precipitate the polymer by pouring the solution into ly0conc aqueous hydrochloric acid in methanol (150 ml). Filter the solid, make into a slurry with 5% cone hydrochloric acid in methanol, filter again, dissolve it in chloroform, filter, and reprecipitate into methanol. The yield is about 8 g of a polymer with an inherent viscosity of about 1.0, aa measured in chloroform (0.5 g/100 ml), and an osmotic n~olecularweight of 28,000. Suspension Polymerization o f Styrenes
The polymerization of styrene has probably been studied in greater detail and by more methods than any other monomer; and no other monomer has, as a result, contributed quite so much to our knowledge of the detailed mechanisms of polyaddition, especially those followings free radical or anionic course.
The following sequence of reactions involves free radical initiation. The styrene should be broken up into droplets by stirring in water and kept separated by stirring and by the surface action of the added chemicals. The polymerization occurs within the styrene droplets, each one acting essentially as a site of hulk polymerization. The latter term simply refers to
polymerization without benefit of diluent. The considerable amount of heat evolved can prove difficult to remove in bulk polymerizations, hut this condition is mitigated by the relatively high surface-to-volume ratio and the heat-dissipating effect of the water. Bulk polymerization of styrene is commercially feasible and widely done; suspension methods are also used, but to a lesser extent. Suspension polymerization should not be confused with emulsion polymerization where the particle size of the dispersed polymerization sites is far smaller and where the kinetics, and hence the course of the reaction, are quite different. I n emulsion systems, a quite stable latex results, and must be coagulated in order to collect the polymer. A 1-1round-bottom three-neck flask, equipped with a good mechanical stirrer and a condenser, is used in this reaction. It is desirable to have a nitrogen inlet to the flask, if this gas is available, since oxygen should he excluded to preclude its inhibiting action. Add to the flask 250 ml distilled water, boil the water for five minutes to expel dissolved oxygen, then cool with a stream of nitrogen passing through the flask as a purge. The nitrogen will blanket the water and the subsequently formed mixture. (In lieu of this, chips of dry ice can he carefully added to the cooling water and a carbon dioxide blanket maintained.) When the water is a t room temperature, the flask is further charged with 0.02 g sodium laurylsulfate (Duponol C),8 0.75 g sodium polyacrylate, 2.5 g sodium sulfate, 75 g styrene, and 0.5 g benzoyl peroxide. The styrene should he inhibitor-free; either distill the monomer in vacuum (bp 50°/14 min, discarding the first 5-10y0 and retaining a middle fraction), or wash it twice with 5% aqueous sodium hydroxide and three times with distilled water. The sodium polyacrylate suitable for suspension polymerization purposes can be obtained from Rohm & Haas Co.; Cyanamerlo 370 from American Cyanamid is a similar product. Stir the mixture vigorously and heat on a steam bath for 4 hrs. There is little tendency for the monomer droplets to coalesce until they become viscous as polymer forms. Eventually, hard beads will form. If stirring is stopped when the beads are still tacky, agglomeration may occur, although the suspension aids tend to prevent this. The particle size of the final bead is a function of, among other things, the efficiency of stirring, and it is not easy to reproduce a specific size in the laboratory. (If a t anytime the reaction mixture should begin to reflux with great vigor, or in any other way appear to be out of control, turn the heat off and, if necessary, add portions of cold water through the condenser.) If no polymerization appears to occur in 2 hrs, another 0.5 g of peroxide should be added. At the end of the four hours, filter the heads of polystyrene, wash well with water, and dry in a vacuum oven a t 60°. The conversion depends on the duration of polynlerization and inhibitors present that may produce an induction period before polymerization begins. Conversion should be nearly complete. The polymer can be cast to clear, stiff, brittle films from a 10-20%
* Trademark of E. I. du Pont de Nemoura & Co., Inc. Trademark of American Cyanamid.
lo
solution in benzene, or pressed to films a t 150°. Emulsion Copolymerization of isoprene-Sfyrene"
The use of the emulsion polymerization technique and the copolymerization technique (i.e., the polymerization of a mixture of two monomers) have probably been of greatest importance in the production of synthetic rubber. Actually, the copolymer of butadiene and styrene was used as the most important synthetic rubber. However, because of its low boilingpoint ( - 5 T ) , bntadiene presents some difficulties in a simple laboratory experiment, so that its analog, isoprene (2-methylbutadiene-l,3), bp 34"C, can be used in its place. As stated in the previous experiment, emulsion polymerization differs from suspension polymerization in leading to a much finer polymer dispersion, called a "latex." Unlike suspension polymerization, where the coarse droplets of monomer polymerize into little beads, emulsion polymerization actually involves the monomer that is dissolved in the water. Since the polymer chain formed is not soluble in water, it "precipitates" in the form of a small colloidal particle, which then absorbs more monomer and continues to polymerize and grow in size. In this way a latex of colloidal size is produced, where the particles are stabilized by a protective layer of soap or detergent. The polymer can then be obtained from the latex by a coagulation by means either of acid or of alcohol. The reaction can be pictured thus:
Actually, about three units of isoprene are polymerized for every unit of styrene. Also, some (about 10%) of the isoprene does not add in a 1,4 manner as shown, but as 1,2 and 3,4 units. This reaction, like that in the previous experiment, proceeds by a free radical mechanism, so that a peroxide is generally used to initiate the polymerization. A "recipe" which is suitable for use a t room temperature follows: Distilled water Isoprene Styrene Sodium Oleate t-Dodecanethiol Diisopropylbenzene monohydroperoxide Tetraethylene pentamine
Wt. in orams 180:0 75 25 5.0 0.1 0.2 0.2
The polymerization is best carried out in a 4-02. round-form bottle having a screw cap lined with a Neoprene or nitrile rubber gasket. The bottle should be agitated either by tumbling slowly (every 2 seconds), end-over-end, or in a laboratory shaker. A 30% portion of the above recipe makes a suitable charge for such a bottle.
" This procedure waa kindly supplied by Professor M. Morton of the University of Akron, Akron, Ohio.
Volume 42, Number
I , Januory 1965
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The water should be freed from dissolved oxygen, as described in the previous experiment, and the styrene should be distilled, as before, to free it from inhibitor. The isoprene (Phillips Petroleum Co., Polymerization Grade) should also be distilled (bp 34OC) to free it from inhibitor. Make up the aqueous sodium oleate solution first and charge it into the bottle. The thiol ("Sulfole BR," Phillips Petroleum Co.) acts as a chain-transfer agent and is used in order to prevent the molecular weight of the polymer from becoming too high (leading to a crosslinked, insoluble, tough rubber). Dissolve this and the hydroperoxide initiator (Hercules Powder Co.), in a weighed amount of styrene and charge into the bottle, followed by the isoprene. Add the cocatalyst, tetraethylene pentamine (Mathesou, Coleman & Bell), last, as a solution in a small quantity (0.5 ml) of water. Before capping the bottle, flush the contents for about 5 minutes with nitrogen, using a long hypodermic needle (or fine capillary tube) which reaches to the bottom of the bottle. If a source of nitrogen is not available, the polymerization procedure should be modified as follows: After charging the isoprene, add a slight excess of isoprene (0.1 @) and warm the whole bottle gently in a water bath to a temperature of 35'C until the isoprene begins to boil. Place the cap loosely on the bottle, and allow the isoprene to evaporate, to flush out any air. This procedure helps to reduce the induction period caused by presence of oxygen, but may still result in a 30-min induction period.
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Place the bottle in the shaker (or on the rotor) and allow to polymerize for 2-3 hours. A bluish fluorescence, which marks the beginning of the polymerization (colloidal particles), should he visible after 15-30 min. After the reaction has run its allotted time, open the bottle and coagulate the contents by pouring it slowly into 500 ml of rapidly stirred isopropanol. Filter the crumb-like polymer under suction and dry in an oven a t 100°C. for one hour. A 2&30% conversion of monomer should be obtained within 2-3 hours. The copolymer can be converted to an elastic film either by casting from a 10-20% solution in benzene or by pressing in a heated press at about 150°C. A relatively extensive reference literature now exists for the preparation of polymers,3s4and it would he safe to venture that examples of polymer syntheses will begin to appear routinely in undergraduate organic laboratory manuals. Since organic polymer chemistry is increasingly recognized as a logical segment of organic chemistry, it will he increasingly taught and practiced accordingly. And, just as the physical chemistry aspects of conventional areas of organic chemistry are essential to their proper understanding and practice, so is it with the physical chemistry of polymer systems. Vinyl polymerization, for example, is not properly viewed except in the light of its kinetic as well as its synthetic aspects. The arrival of organic polymer chemistry in the undergraduate laboratory can surely be considered a distinct step toward a broader chemical outlook for the undergraduate.
