Ring-Opening and Ring-Forming Polymerizations: An Organic and Polymer Laboratory Experiment Lon J. Mathias' Department of Polymer Science. University of Southern Mississippi. Hattiesburg, MS 39406 Tito Viswanathan University of Arkansas, Little Rock. AR 72204 Two experiments are described that are suitable for college polymer science and organic chemistry laboratories. Background information is given on ring-opening and ringforming polymerizations that includes industrial examples and Drooosed mechanisms. Polvmer svnthesis is combined . . u,it h characteri~.atiunby IR ipectruiaq)y and dilurr solution visa,sitv. 'l'he latter illustrates the . ~olvelt.ctrol\~te effect for . the charged ammonium polymer in water. Polyamines, polyamine salts and polyamides are used commercially in a wide range of applications. These include sizing for paper and textiles, recovery and recycle of trace metal contaminates from chemical plants, and flocculation of particulate matter for water clarification. These polymers are strong complexing and chelating agents for metal salts, and such complexes have been used for a variety of catalytic applications. In addition, many of the polyamine compounds are basic catalysts in their own right or in conjunction with organic comonomers and have been used to make a variety of synthetic chemicals. Several of the presently available commercial polymers containing amide and ammonium functionality are given in Figure 1.These include vinyl addition polymers from acrylamide ( 1 ) and N-vinylpyrrolidone (2) and step-growth polymers containing quaternary ammonium groups (polyionenes, 3) obtained from polycondensation of diamines and bishalides. In this experiment, we examine two major types of polymerization processes involving heterocyclic monomers or repeat units. Polymer synthesis involves ring-opening polymerization to yield a polyamide from an oxazoline, and cyclopolymerization of a diallylamine derivative to give a polymer containing pyrrolidine units. Ring-Opening Polymerization Mechanisms There are several commercially important polymers that are synthesized via ring-opening polymerization. Examples summarized in Figure 2 include such common polymers as polyoxyethylene (POE, 4), poly(butylene oxide) (PBO, 5 ) , nylon 6 (6), and poly(ethy1eneimine) (PEI, 7). This last polymer is obtained by a nonselective process that can involve attack on the ethyleneimine monomer by either chain ends or internal secondary amines of the growing polymer. These competing reactions lead to a highly branched polymer structure that contains primary, secondary, and tertiary amine units ( 1 ) . Several years ago, a novel synthesis of completely linear PEI was developed (2).The method utilized a ring-opening ~olvmerization also, but of a five-membered heterocvcle . . thnt resulted i n f o r m a t ~ m u i asut)stitutrdami~l~~rnrhrr rhan thc t'rw amin? obtained from ethsleue~mllw.Flrure :l iummarizes the initial and propagat& steps for this polymerization (3) as well as the hydrolysis reaction and the final polymer structure.
' Author to whom correspondence should be addressed
Figure 1. Commercial amide and ammonium polymers.
Figure 2. Common ring-opening polymerizations
R
I I
c=o
-GN-
H,O/
OH-
-
k
H
b + RCO; 8
Figure 3. Ring-opening polymerization mechanism for Bsubstituted line5 and subsequent polymer hydrolysis.
axaro-
This method gives linear P E I (8) by a two-step process. In addition, the intermediate polymers containing amide functionality have become important in their own right (4). Many of these polymers are being examined for various unique applications involving a combination of properties. Many are soluble both in water and in a wide range of Volume 64
Number 7 July 1987
639
In this experiment, the polymerizal~ilitgof diallgldimethslamrno~~ium chloride is rxnmined. The ~ o l s m e IrS Durified by precipitation from water and ~ h a r a c t e r i ~ eby d dilute solution viscosity.
Figure 4. Free radical cyclopolymerization
mechanism of a diallylammonium
monomer.
organic solvents. The amide functionality provides multiple sites for complexation and chelation of a variety of metal salts. The controlled spacing of the pendent amide derivatives alone the three-atom repeat unit in the backbone provides a novel alternative to the normal two-atom backbone obtained with vinyl polymerization (see.for example poly(N-vinylpyrrolidone), 2). Finally, partial hydrolysis can give polymers containing both amide and amine or ammonium groups that can interact with substrates, together or in a sequential fashion. The most common oxazoline derivative available todav is the 2-ethyl compound. In this experiment, the monomer is ~olvmerizedusine a cationic initiator to aive hiah-molecuiarlweight polym& that is characterizedby both IR and solubility behavior. Cyclopolymerlzatlon Cyclopolymerizations were first discovered by George Butler in the late '50's (5). Since then, a wide variety of monomers have been found to undergo cyclopolymerization. We concentrate here on a diallylamine derivative. The cyclopolymerization process involves formation of a heterocyclic ring during polymerization as illustrated in Figure 4. The monomer shown, diallyldimethylammonium chloride, is one of the most widely used commercial derivatives. The cvclo~olvmerization mechanism involves two seauen. . . rial prupagation s t e p ( 6 ) .Intvrmdrcll~srattack d a proparating radical is immediatels followed h\. an inrromolecular attack to form the heterocycie. surprisingly, this second step leads to the unstable primary radical through kinetic rather than thermodynamic control and is followed by immediate reaction with another monomer molecule. Two possible side reactions can occur in these polymerizations, involving crosslinking and chain transfer, but they are not observed. In general, cyclopolymerization of diallylammonium compounds proceeds cleanly to high-molecular-weight polymer with no cross-linkine. Onr of thv c;lrlirr drawhncks in such p~~lgmeri'zations involved the ute of ueroxide initiators. Exrensive sellowine of and inefficient initiation-lead to Tow the product yields and undesirable properties. A recently reported improvement on these polymerizations involves a new commercial initiator V-50 (2,2'-azohis(2-amidinopropane.2HCI, 9). This water-soluhle species cleanly forms carbon radicals that initiate diallylammonium cyclopolymerization to high yield (7).
Polymerization of 2-Ethyloxazoline A clean, dry test tube is fitted with a rubber septum fastened on with wire. Approximately 2 mL of 2-ethyloxazoline2is injected into the test tube, which is then suspended in an oil bath preheated to 120 "C. After a few minutes equilibration, the test tuhe is carefully removed and approximately 5 ,LL of dimethylsulfate2 is injected. The test tuhe is put back in the oil bath. The solution gradually becomes more viscous until it gels or solidifies (about 2 h). The test tuhe is removed from the oil bath and allowed to cool. After removing the septum, 5 mL of methylene chloride is added to dissolve the mixture. This solution is then poured slowly into 50 mL of rapidly stirring mixed hexanes. The solvent is carefully decanted from the solid polymer, which is washed again with more hexanes end finally isolated by filtration. Caution: dimethylsulfate is toxic and should he handled only in small quantities with good ventilation. Polymerization of Diallyldimethylammonium Chloride Commercial monomer is usually available as a 65 wt % solution in water.Vhis is suitable for direct polymerization. Approximately 5 mL of this solution is added to a test tuhe along with initiator 9 (V503, -0.05 g, -1 ma1 90). A septum cap is wired in place and the reaction mixture purged for 5-10 min with Nt through inlet and outlet needles in the septum. The test tuhe is then placed in a preheated water or oil bath at 60-65 "C. A small-diameter syringe needleisleft in the septum to relieve pressure framliberated Nz gas. Polymerization takes place rapidly to give a gelled or solid mass within 1-2 h. The polymer is isolated by precipitation into 100 mL ethanol stirring rapidly in a 250-mL beaker. Purification can be carried out by reprecipitation from water into ethanol. Discussion The two synthetic procedures are straightforward and can Ix! wrrird &I with minimum of special preparutions ~ n d precautions. Hmwvrr, dimethylsultate is toxic. Only enough material for immediate use should be nresent in the lab. and good ventilation and handling procedures should be used. The ~ o l v m e r sare imnortant commerciallv and reoresent less weliknown speciaky chemicals. In adzition, their synthesis introduces the student to heterocvclic c o m ~ o u n d sin the context of polymer formation. Two synthetic extensions of the experiment are possible. One involves synthesis of a poly(dially1amine) (71,a polymer that is more difficult to .purify . and characterize. (Diallvlamine is also toxic.^ Alternntiwly, the oxnzoline polyn~ercan be hyrlrds7ed in refluxing aqueous acid and nrutmli?ed to obtain the free amine polymer. These two polymers can then be compared with the other amide and amine polymers made in this experiment. Polymer characterization involves qualitative evaluation of solubility behavior, dilute solution viscosity, and IR spectroscopy. Solubility should be evaluated for common organic solvents, acetic acid, and aqueous acid and base solutions. The results can be compared with other available polymers. The students must be made aware of the importance of allowing sufficient time for dissolution and sweiling to take place (5-12 h). Unlike low-molecular-weight materials, which normally dissolve rapidly or not a t all, polymers take appreciable time to untangle and move away from the solid polymer mass. This effect becomes more pronounced the higher the molecular weight of the polymer. Dilute solution viscosity is one of the most common and useful initial characterization techniques for polymers. At
a
Aldrich Chemical Co.. Milwaukee. WI 53233 Poiysciences, Inc., Warrington, PA 18976. 640
Journal of Chemical Education
Figure 6. IR spectrum of poly(2emyloxazoline)asa thin film cast from CH2C12.
Figure 5. Reduced viscosity platsfor poly(N,Ndiallyldimethylammonlum chlorids) in water (upper curve) and in 0.5 M NaCl (lower curve).
the very least, a viscosity value of more than -0.1 dL/g tells you that you do have a polymer. More important, qualitative comparisons are possible for polymers of the same composition; that is, increasing viscosity values correlate directly with increasing molecular weight and polymer size. Detailed procedures have been puhlished previously for viscosity determinations (8.9). One very i&r&ting aspect of the viscosity behavior of oolv(diallvldimethvlammoniumchloride) is the oolyelectroiyte effect that it shows (10). While well-behaved polymers show a linear relationship with respect to concentration, polyelectrolytes usually show higher reduced viscosity with decreasing concentration. This is demonstrated in the upper plot of Figure 5 (viscosity with units of dL/g plotted against concentration in g/dL). Addition of electrolytes (NaCI) at relatively high concentrations (>0.5M) compensates for the polyelectrolyte effect by masking the electrostatic repulsion of cationic groups along the polymer backbone. This is shown in the lower portion of Figure 5 where plots of two
different tvnes . . . for the nolvmer .. of viscositv values (8.9) . . nlus rlectrdyte dhow h e m brh~vitlr. IR soectroscoov is the most routine soectral sharacrerizntion technique available for polymers. The formation and IR characterization of polymer thin films is facile (II), giving both qualitative (12)and quantitative (13) information. Figure 6 gives the spectrum of the oxazoline polymer. Functional group identification can he required of the students, although polymer spectra often display unexpected comhination bands and contaminant peaks from retained solvent and reactants. In summarv. this exoerimeut ~rovidesstudents in collese polymer science and brganic chemistry laboratories wiih exoerience in the svnthesis and characterization of two novel polymers. These polymers illustrate radical ring-forming and nucleo~hilicrina-ooeninr! reactions and are interestine - . because they contain or are made from heterocyclic units. Literature CHed 1. Cheng, C.; Muccia, D. M.: St. Pierro,T Mocromolrculss1985,18,2154. 2. Bsssiri.T.G.;Leuy,A.:Litt,M. J. Polym. Sei..Polym.Lett. L961,5.871. 3. Kobayarhi. s.; saeguss, T. I" Ringopening Polymeri*ofion; Ivin, K.; saegusa Eds.;Elsevier: London. 1935; Vol. 2. Chapter 11. 4. Kleskula, H.: Paul, 0. R. J. Appi. Polym. Sci. 1986.31.941. 5. But1er.G. B.:Crawshaw,A.; Miller, W. L. J. Am. Chem.Soc., 1958.80.3615. 6. Butler,G.B.Acct~.Chsm.Re& 1982.15.370. 7. Harada, S.;Harcgasa, S. Mokromoi. Chem., Rapid Commun. 1984.5. 27. 8. Perrin,J. E.; Ma.tin,G.C. J. Chem.Educ. 1983.60.516. 9. Msthia8.L. J. J. Chsm.Edue. 1983.60, 422. 10. Ander, P. J. Chem.Educ. 1919.56.481. 11. Man0.E. B.;Ourao,L.A.J. Chem Educ 1973.54 228 12. Webb, J.; Rssmussen. M.; Selinger, 9.J. Chem Edue. 1977.54. 303. 13. Allress, K. N.: Cowell, B. J.: Herd, A. C. J. Chsm. Educ. 1981,58,741.
Volume 64
Number 7
July 1987
641
