Chapter
11
Polycyclopropanone Synthesis and Hydrogenolysis
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 16, 2016 | http://pubs.acs.org Publication Date: December 22, 1988 | doi: 10.1021/bk-1988-0364.ch011
Roderic P. Quirk and James H . Dunaway
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Institute of Polymer Science, University of Akron, Akron, OH 44325
Cyclopropanone was polymerized at -78°C to polymers with M =4,000-20,000 and M /M =1.9-2.2 using triethylamine as initiator. Infrared, H NMR, and C NMR spectral analyses of the polymers are consistent with a polyacetal structure for the polymer. Hydrogenolysis reactions of the cyclopropane rings in the polymer were carried out using Pt/carbon and Pd/carbon catalysts. Infrared, H NMR, and C NMR spectral analyses of the hydrogenolysis product are consistent with selective ring-opening of the least substituted cyclopropane carbon-carbon bond to form gem-dimethyl groups. Hydrogenolysis was accompanied by some degradation as shown by size exclusion chromatographic analysis. n
w
n
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Ketones are generally not polymerizable, despite claims that acetone can be polymerized at low temperatures (_1). A simple explanation for the lack of polymerizability of ketones compared to v i n y l monomers can be deduced from consideration of Pauling ( 2 ) average bond energies as shown i n Equations 1 and 2, where AHP (est) i s the estimated enthalpy of polymerization based upon the difference i n bond energies of the two single bonds formed i n the polymer compared to the double bond i n the monomer: ol
Pauling Bond Energies C=C 147 kcal/mol C=0 174 kcal/mol
AH^°^"(est)
> 2C-C 2x83.1 kcal/mol
-19.2 kcal/mol
(1)
+6 kcal/mol
(2)
> 2C-0 2x84 kcal/mol
Current address: P P G Industries, Glass Research and Development, Harmar Township, P A 15238 0097-6156/88/0364-0141 $06.00/0 © 1988 American Chemical Society
Benham and Kinstle; Chemical Reactions on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
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142
CHEMICAL REACTIONS ON POLYMERS
Thus, estimated bond energy changes for polymerization indicate that i n contrast to v i n y l monomers carbonyl groups have unfavorable enthalpies of polymerization, because of the high bond energy of the carbonyl group. The contribution from the enthalpy of polymerization would be expected to dominate the free energy of polymerization since entropies of polymerization are generally negative and unfavorable for a l l monomers (3). This suggests that ketones which have ring s t r a i n energy which i s decreased upon carbonyl addition might have more favorable thermodynamic parameters f o r polymerization. Cyclopropanone i s an interesting monomer i n t h i s respect since there i s much more bond angle s t r a i n (4) i n the monomer compared to the polymer as shown i n Equation 3. Therefore, i t would be predicted that cyclopropanone, unlike unstrained ketones, might have a favorable free energy of polymerization. Indeed, i t has been reported that
C-C(=0)-C Bond Angle Strain: 120-60°=60°
C-C(0 )-C Bond Angle Strain: 109-60°=49° 2
cyclopropanone undergoes polymerization, presumably catalyzed by adventitious moisture, when warmed to room temperature (5,6). Given t h i s encouraging information, i t was important to investigate t h i s interesting monomer and to determine not only i t s range of polymeriz a b i l i t y (e.g., anionic, c a t i o n i c , e t c . ) , but also the properties of the r e s u l t i n g polymer. Another i n t r i g u i n g aspect of the polymerization of cyclopropanone was the p o s s i b i l i t y that the resulting polymer could provide an i n d i r e c t route to the hithertofore unreported polymer of acetone. I t was envisioned that s e l e c t i v e hydrogenolysis of the cyclopropane ring i n polycyclopropanone could produce polyacetone as shown i n Equation 4. There i s precedent f o r the s e l e c t i v e cyclopropane r i n g opening of the least substituted cyclopropane carbon-carbon bonds
upon hydrogénation (]_ 8). Herein are reported r e s u l t s of i n v e s t i gations into the n u c l e o p h i l e - i n i t i a t e d polymerizations of cyclopropanone . 9
Experimental Materials. Ketene was synthesized as described by Andreades and Carlson (9) from diketene. Diazomethane was synthesized from N-methyl-N-nitroso-p_-toluenesulf onamide as outlined by Hudlicky (10). A t y p i c a l synthesis of cyclopropanone involved the slow addition of 400 mL of a 1.0 M d i e t h y l ether solution diazomethane to a 2-3-fold molar excess of ketene at -78°C. This synthesis was based on the
Benham and Kinstle; Chemical Reactions on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
11.
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Polycyclopropanone
QUIRK AND DUNAWAY
procedure described by van Tilborg et a l . (11). The reaction between diazomethane and ketene was e s s e n t i a l l y instantaneous as evidenced by the immediate disappearance of the c h a r a c t e r i s t i c yellow color of diazomethane and the evolution of nitrogen. After a l l of the diazo methane had been added, the excess ketene was removed by vacuum d i s t i l l a t i o n at -55 to -65°C. Infrared spectra of d i e t h y l ether solutions of cyclopropanone showed a strong absorption at 1820 cmc h a r a c t e r i s t i c of the cyclopropanone carbonyl (5,6). Yields for t h i s synthesis as determined by the a n a l y t i c a l method described by van Tilborg et a l . (11) ranged between 50-75%. Triethylamine was p u r i f i e d by d i s t i l l a t i o n from freshly crushed calcium hydride and stored under argon i n a r e f r i g e r a t o r .
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 16, 2016 | http://pubs.acs.org Publication Date: December 22, 1988 | doi: 10.1021/bk-1988-0364.ch011
1
Characterization. Infrared spectra were obtained with a Beckman Model FT-2100 FTIR spectrometer. H-NMR spectra were obtained on a Varian Model T-60 spectrometer (60 MHz) with deuterated chloro form as the solvent and (CH ) Si as the internal standard. C NMR spectra were obtained using a Varian XLA00 (100 MHz) spectrometer i n deuterated chloroform which also served as the i n t e r n a l standard. Glass t r a n s i t i o n temperatures and melting points were determined with a du Pont 1090 Thermal Analyzer using the DSC mode. Samples were sealed i n aluminum dishes and the analyses were carried out under an inert atmosphere (nitrogen) at heating rates of 5°C and 10°C per minute. Size exclusion chromatographic analyses were obtained with a Waters 150C GPC using a s i x μ-Styragel columns (ΙΟ , 5 x l 0 , ΙΟ , 10**, 10 , 10 Â) after c a l i b r a t i o n with standard polystyrene samples. The c a r r i e r solvent was THF at a flow rate of 1 mL per minute at 30°C. Vapor Pressure Osmometry (VPO) measurements were made on a Knauer Type 11.00 Vapor Pressure Osmometer using toluene as the solvent at AA°C. Sucrose octaacetate, recryst a l l i z e d twice from ethanol, was used as the standard f o r VPO. 1
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Polymerizations. Polymerization was t y p i c a l l y achieved by the addition of AO yL (28.8x10- mol) of triethylamine to 350 mL of a cold (-78°C) 0.8 M d i e t h y l ether solution of cyclopropanone. I n i t i ation was usually accompanied by a 10°C increase i n reaction temperature and the formation of a white p r e c i p i t a t e within 2-3 minutes. The polymerization solution was kept at -78°C for an a d d i t i o n a l two hours and then allowed to warm up slowly to room temperature overnight. The polymer was p u r i f i e d by p r e c i p i t a t i o n into methanol and dried under vacuum. The H NMR spectrum was characterized by a s i n g l e t at 61.0 ppm. The C NMR spectrum was characterized by singlets at 612.24 ppm and 689.26 ppm i n a roughly 2:1 area r a t i o . The molecular weights of the polymers were determined by a combination of vapor phase osmometry and size exclusion chromatography. Molecular weights ranged between 4,000 g/mol to >15,000 g/mol. A DSC trace of a 13,000 g/mol sample showed a sharp c r y s t a l l i n e melting point at 167°C and a discontinuity between 0-5°C which might have been due to a glass t r a n s i t i o n . The low molecular weight polymers (
