The Aqueous Ring-Opening Metathesis Polymerization of Furan-Maleic Anhydride Adduct Increased Catalytic Activity Using a Recyclable Transition Metal Catalyst Tito Vlswanathan and Jagdish Jethmaiani University of Arkansas at Little Rodn, Little Rock, AR 72204 Simple experiments that are visually dramatic and that can be conducted within a short period of time are attractive additions to chemistrv laboratorv curricula. If an experiment yields a pmduct"that can be-characterized at the molecular level, it can be a valuable teaching tool. The newly discovered aqueous ring-opening metathesis polymerization (ROMP) offers an opportunity for an experiment that should fit well within the context of a laboratory course in organic or polymer chemistry.
also more active in subseouent wlvmerizations. reaching a limiting value(after two brthrie i;o~yrne~zations~of'on~~ 10-12 s. This increase in catalvtic activitv is unique to the ROMP of the 7-oxanorborneie monomers, and the increased activity of the recycled catalyst solutions is attributed to the in situ formation of Ru2*-olefincomplexes. It is important to note that "classical" ROMP catalysts react with the 0x0 bridge in 7-oxanorbornene derivatives-and are deactivated by it.
New Ring-Opening Metathesis Polymerization Catalysis Areview has been published of new homogeneous olefm metathesis catalysis based on high oxidation state metal catalysts that have provided unprecedented control of the polymerization of cyclic alkenes ( I ) . Until recently, most of these catalysts were organotransition metal compounds that catalyzed the ROMP reaction in a nonaqueous environment. An example of such a reaction is the ROMP of norbornene using titanocyclobutanes (2).This reaction produces a living system with a narrow and controllable molecular weight distribution. The advantage of a living system is the possibility of preparing block wpolymers with well-defined structures and properties.
Experimental In this experiment the Diels-Alder adduct of furan with maleic anhydride is prepared, and the recrystallized adduct is polymerized in water with K2RuC15(JohnsonMattheyIAESAR) (Fig. 2). Purification of the polymer is followed by spectroswpic characterization.
ROMP of 7-Oxabornene Derivatives
Quite recently the ring-opening metathesis polymerization of 7-oxanorbornene derivatives by ruthenium catalysts in (emulsion) aqueous solution under an atmosphere of air has been shown to be a rapid process that provides quantitative yields of the desired ROMPpolymers (3).(See Fig. 1.)Even though a living system is not obtained, the molecular weights are as high as lo6or more with a polydispersity of 1.2.
Synthesis of the Adduct
The synthesis of exo-7-oxabicyclo[2.2.llhept-5-ene-2,3,dicarboxylate anhydride was accomplished by reacting furan with maleic anhydride. The product was purified by recrystallization according to a previously published procedure (4). (Preparation of this addud is often included in standard organic lab textbooks.) The exo confiration of the product has been confirmed by 'H NMR data, according to Lee and Herndon (5).The 13CNMR shift values obtained using CDC13as solvent (see Fig. 3) are
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Figure 1. ROMP in aqueous systems. Increased Catalytic Activity ofRecycled Solutions The catalytic activity observed has been attributed to the formation of Ru%-olefm complexes while water is believed to act as a wcatalyst by dramatically decreasing the initiation period required for the reaction. With RuC13 catalysts in aqueous solutions the initiation time is 3035 min. However, the used solutions of aqumua ruthenium ion after an initial polymerization are not only recyclable but
Figure 2. Synthetic path for the preparation of poly(exo-7-oxabicycio[2.2.1]hept-5-ene-2.3-dicarboxylate. Volume 70 Number 2 February 1993
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Figbre 3. ' 3 NMR ~ spectrum 01 furanmalec anhydr de polymer (CD,OD solvent) (top). "C NMR spectrm of fJrarcmaleicanhydr oe aad~ct(CDCI, solvent)(bottom). 169.9 ppm for carbonyl carbons 137 ppm for olefinic carbons 82.2 pprn for allylic earbons 48.7 ppm for the methine carbons
Polymer Synthesis A l-g sample of the recrystallized adduct (0.006-molland 0.014 g of KzRuC15(3.56 x 10" moll are mixed with 3.6 mL of deionized, degassed water. The resulting solution is heated in a water bath at 60 'C for 40 min. Aviscous liquid is obtained to which 50 mL of 1M HCl is added and stirred. The precipitate obtained is dissolved in 1.5% NaOH and the polymer is reprecipitated by the addition of 1M HCl. I t is then washed twice with 10-mLaliquots of water and vacuum dried. Near-quantitative yields of the polymer are obtained. Polymer Characterization Characterization of macromolecules can include IR,
NMR,DSC, and molecular weight measurements. Instruments for spectroscopic characterization, such as IR and NMR, are generally available in most chemistry departments. Molecular weight measurements may be made with GPCISEC using poIystyrene standards for both M, and M, and hence plydispersity (i.e., MJMd Number average molecular weight alone may be obtained if an osmometer is available. 166
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DSC gives information about thermal transitions upon heating the polymer. IR spectra of monomer and polymer can be run as a KBr pellet, or the polymer can be cast as a thin film on a NaCl plate. NMR can be run using CDC13 as solvent for the monomer and CDsOD for the polymer. Results and Discussion The cis-trans Ratio Figure 3, which shows the 13CNMR spectra of the monomer and polvmer. can be used to determine the cis-trans ratio in the polymer. Reaonances arising from the carbons alpha to n double bond (the allylic carbons) are typically the best indicators of the cis or trans configuration bf that bond. The cis double bond gives allylic peaks 5 ppm upfield from that of the trans double bond (6). The NMR and IR peaks of polymers are generally broad in comparison with peaks of smaller molecules.
Focal Points This experiment demonstrates the versatility of a transition metal catalvst to c a m out a h i ~ h hefficient wlvmerization in the ;heapest &ssible solv&< The shorttime reauired for the entire experiment makes it an ideal extension of the existing labor&ory organic synthesis of the adduct. We recommend including a discussion of polymers and the role of transition metal catalysts in the polymerization of alkenes as prelab lecture material.
Use of the Polymer
The polymer synthesized in this experiment i s a functionalized polymer (high carboxyl wntent) that is soluble in hot water, CH30H, and other polar solvents. It could replace some acrylic polymers that are currently used as blends in disposable packaging materials in Europe (7)because polycarboxylates can be extracted or solubilized in hot water or base. Thus, they can solve the problems of purification that are usually associated with recyclingpolymers. The polymer may also be of importance as a biologically active anionic polymer due to its structural similarity to the copolymerformed by the free radical polymerization of a mixture of maleic anhydride and divinyl ether (8,9).It may also find use as an ionophoric material or as a selective ion ~ermeable membrane (8). . Coincidentally, the polymer synthesis involves furan, which is a renewable resource material that is prepared by the decarbonylation of furfural. Furfural is prepared by (acidic) steam diwstion of rice hulls and oat hulls. Thus. this reaction has%wopotential benefits: ~
Creating a use for the enormous amount of hulls that are a source of pollution in several states Using this natural waste subPtance to manufacture polymers that are currently made fmm petmleum-based monomer8
Science curricula should include examples that stress the importance of research in our daily lives for finding intelligent solutions to old problems. Acknowledgment We would like to thank the Arkansas Science and Technology Authority and the National Science Foundation for helping us purchase the Bruker 200-MHz NMR used in this project. We would also like to thank Alan Toland for running the NMR of the monomer and the polymer. Literature Cited 1. Gmbhs,R. H.; Turnas,W. Science 1888,243,SW-915. 2. Grubbs, R. H.;Gilliom, L. R Roeeeding. d t h e 4th International Symposivm on Homogen~ovsCatabais, September 24-28, 1984, Leningrad, USSR (Gordon and Breach. NeuYork, 1985);L. R.Gfiliwn; R. H. Grubbs J A m C b m Soe. 1888,108. 733. 3. Nwak, B. M.; Grubbe, R. H. J.Amx C b m . Sm. lW, 110,7542. 4. Diela, 0.; Alder, K C h . B e . 1928.62.567. 5. Lee,M. W.; Hemdon, W. C. J. Om.C b m . 1818.43.518 6. Silwstein,R. M.;Basder,G. C.;MmrillT.C.SpeampeabieI&nti~tioioof07pnk Compounds,4th ed.: Wfiey: NY,1981. 7. Wielplulshi, L J. Pol-r &prints, Polymer Technology Crmfe=ence, Philadelphis, PA;Ametiean Chemical S d e t y 1991,32(2),136. 8. Grubhs, R. H.;Navah, B. M.US. Patent 4 883 861,1989. 9, Bredow, D. 'Biolagcally active qnthetie anionic p o l y m d In 'Cyelopolgmeriration and Polymera with Chain ring atructm", ACS SympoaivmSetie8 196; Butler 0. B.;Kresta, J.E.,Ed.: 1 9 8 2 , ~ 1.
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