Macromolecular Systems I & II
Polymerization of Dimethyl 7-oxanorbornene-2,3-dicarboxylate
Professor David Spivak
Ring opening metathesis polymerization (AKA: ROMP) has generated a large amount of interest in the last decade. This method is especially important for the synthesis of conjugated polymers. Some good reading is a review by D. Breslow (Prog. Polym. Sci. 1993, 18, 1141), and the following papers: 1. Grubbs, R. H.; Miller, S. J.; Fu, G. C.Acc. Chem. Res. 1995, 28, 446. 2. Gilliom, L. R.; Grubbs, R. H. J. Am. Chem. Soc. 1986, 108, 733.
The experimental protocol you will use is the following, your wonderful TA has begun the operation already by accomplishing tasks 1 and 2.
1. Monomer (60 mg, 0.28 mmol) was weighed into a 1 dram vial, and (PCy3)2Cl2Ru=CHPh (2 mg, 0.0024 mmol) was added to the vial. To the vial was addied 1 ml CH2Cl2, and the vial was capped tightly. The mixture was shaken gently for 2 h, and the polymer mixture allowed to stand overnight.
2. The vial was opened to the air and 75ml of a solution consisting of CH2Cl2 (2 mL), ethyl vinyl ether (400 ml), and BHT (100 mg) was added. The reaction mixture was stirred for 2 h. (or so)
3. The viscous solution will be diluted with CH2Cl2 (1-2 mL) containing a trace of BHT. This solution will be passed through a short plug of silica gel (1.5-2.0 cm) in a disposable pipet. The polymer will be eluted using CH2Cl2 containing a trace of BHT. The combined eluants will be concentrated by rotary evaporation (or by passing a stream of N2 over the solution0 and redissolved in a minimum of CH2Cl2 (1 mL) with a trace of BHT.
4. A tared scintillation vial will be charged with 3 mL of methanol with a trace of BHT. The polymer solution will be added one drop at a time by pipet, with thorough shaking of the sample after each addition to induce precipitation.
5. The methanol will be decanted from the precipitated polymer, which will be dried under a stream of N2 to obtain a clear colorless polymer.
6. Once the polymer is dried, and the yielded weight determined, the polymer will be dissolved in deuterated chloroform (about 0.75 mL), put into an NMR tube, and labelled with your name.
Note: The 1H NMR of your isolated polymer product should show broad peaks typical of polymer spectra. The cis:trans ratio of the polymer can easily be deterined by integration (see Muhlebach, A.; Bernhard, P.; Buhler, N.; Karlen, T.; Ludi, A. J. Mol. Catal. 1994, 90, 143). The olefinic protons are observed at 5.88 ppm for the trans and 5.59 for the cis. You should check this cis:trans ratio with that obtained by integration of the protons that were originally at the bridgehead position of the monomer. These peaks appear at 4.69 for the trans and 5.05 fro the cis. By calculating these ratios, you will discover that there are additional stereochemical considerations due to the double bond geometry in the polymer that would not be present in the monomer.
1. Draw the structure of the polymer resulting from the free radical polymerization of norbornene. How does this structure differ from that resulting from the ring-opening metathesis polymerization of norbornene?
2. Write a mechanism for the ring-opening metathesis polymerization of the 7-oxanorbornene monomer.
3. Cyclohexene does not undergo ring-opening metathesis polymerization to any appreciable degree. Explain.
4. compare the NMR spectra of the monomer and the pure polymer. How and why do they differ?