Copolymerization of 1,6-Anhydro-P-~- Galactopyranose and 1,6-Anhydro- 6-D-Mannopyranose Derivatives

Transcription

1 Copolymerization of 1,6-Anhydro-P-~- Galactopyranose and 1,6-Anhydro- 6-D-Mannopyranose Derivatives HIROSHI ITO, VLADIMIR MAROUSEK,* and CONRAD SCHUERCH, Department of Chemistry, State University of New York, College of Environmental Science and Forestry, Syracuse, New York Synopsis 1,6-Anhydro-2,3,4-tri-O-(p-methylbenzyl)-~-D-galactopyranose (TXGal,M1) has been copolymerized with 1,6-anhydro-2,3,4-tri-O-henzyl-P-~-mannopyranose (TBMan,M*), the products characterized by NMR, specific rotation, and viscosity, and the reactivity ratios calculated. The reactivity ratios rl = 0.37 f 0.15 and r2 = 38 f 4 indicate that the anhydromannose derivative is about 100 times as reactive as that of anhydrogalactose. A comparison of glucose, mannose, and galactose copolymerizations suggests that the reactivity differences of the three propagating cations are comparatively small and the reactivity differences of the monomers large. This result is consistent with a mechanism proposed earlier. Methyl substitution on the aromatic rings of the p-xylyl groups inhibits the initiation process significantly relative to benzyl, but propagation only slightly. INTRODUCTION Stereoregular homo- and heteropolysaccharides have been synthesized by cationic ring-opening polymerization of 2,3,4-tri-O-substituted 1,6-anhydro sugars ,6-Anhydro-2,3,4- tri- 0 -(p-methylbenzyl)-p-~-gluco- pyranose (TXGlu) has been used as a comonomer in copolymerizations with 1,6-anhydro-2,3,4-tri-O-benzyl-~-~-galacto- (TBGa1)l' and -mannopyranose (TBMan)I3J4 and with 1,6-anhydro-2,3-di-O -benzyl-4-o-(2,3,4,6-tetra- 0 - benzyl-a-d-glucopyranosy1)-@-d-glucopyranose (HBMal).12 The p-methyl groups serve as analytical markers that permit the determination of reactivity ratios by NMR spectroscopy. In a critical analysis of these copo1ymerizationsl6 in which TXGlu was copolymerized with 1,6-anhydro-2,3,4-tri-O-benzyl- P-D-glucopyranose (TBGlu), it was shown that the p-methyl groups did not significantly influence the propagation of TXGlu. It was also confirmed by the method of Kelen and Tudos17J8 that true copolymerization occurred in all cases and that classical copolymerization theory adequately described the mechanism.16 In this work, 1,6-anhydro-2,3,4-tri-0-(p-methylbenzyl)-~-D-galactopyranose (TXGal) was synthesized and copolymerized with TBMan. A striking difference in reactivity of these very similar compounds is demonstrated, which appears to reflect a difference in reactivity in the monomers to cation attack rather than a difference in reactivity of the derived cations. * Present address: Department of Polymers, Institute of Chemical Technology, Praha 6--Dejvice, Suchbatarova 5, Czechoslovakia. Journal of Polymer Science: Polymer Chemistry Edition, Vol. 17, (1979) John Wiley & Sons, Inc /79/ $01.00

2 ~ 1300 ITO, MAROUSEK, AND SCHUERCH RESULTS AND DISCUSSION Copolymerizations of TXGal and TBGlu were first run at two concentrations (Table I) and the products compared with those obtained by Lin on copolymerizing TBGal and TXGlu.'l The data are close to those of'lin in spite of the fact that the p-xylyl and benzyl substituents were reversed between the two monomers. The values thus indicate that methyl substitution in the aromatic ring decreases propagation reactivity only slightly. This result is consistent with the data obtained on copolymerization of TXGlu and TBGlu, which showed a slightly but not significantly higher reactivity for TBGlu.16 Since the anhydrogalactose monomers were less reactive than the anhydroglucose monomersll and anhydroglucose less reactive than anhydromannose,13j4 it was expected that TXGal would be less reactive than TBMan. This was confirmed, and indeed TXGal did not homopolymerize at several catalyst concentrations, although TBGal homopolymerizes satisfactorily.6 Since TXGal copolymerizes satisfactorily, its failure to homopolymerize seems to be due to its inability to be activated by phosphorus pentafluoride in the initiation step. A similar effect of p-methyl substitution on the initiation step has been observed in the polymerization of TXGlu. The rate of homopolymerization of TXGlu was much less than that of TBGlu, and a higher catalyst concentration was required for reasonably rapid polymerization.16 Presumably the electron-donating effect of the p-methyl group gives a higher electron density on the xylyl ether oxygen atoms than on benzyl ether oxygen atoms. This should result in a stronger coordination of PF5 with the oxygens on C-2, C-3, and C-4 rather than on the anhydro ring oxygen. As a result less electron deficiency results on C-1, and, in the case of TXGlu, the rate of polymerization is slower than that of TBGlu. In the case of TXGal, no initiation took place. Nevertheless, as the data of Table I and ref. 6 indicate, cation attack during propagation is affected to only a minor degree. The anhydromannose derivative (TBMan) used in this study was synthesized by Sondheimer according to a recently developed method.lg In order to test its purity and polymerizability, it was treated with 1 mole % PF5 under our usual conditions-anhydrous methylene chloride as solvent, -60 C in uucuo. In 18 min, a highly stereoregular polymer was produced in 41.3% conversion with an intrinsic viscosity of 2.8. Addition of a small amount of TXGal(0.2 mole fraction) to TBMan, however, lowered conversion to only 0.5% in a copolymerization carried out for 2 hr. Therefore, 5 mole % of the catalyst was used thereafter to obtain moderate yields of copolymer over the entire range of monomer feed compositions. The results of the copolymerizations of TXGal with TBMan are summarized in Table 11. Time-conversion relationships and the mole fraction of TXGal in copolymer indicate that TBMan is much more reactive than TXGal, as expected. The reactivity ratios of TXGal (MI) and TBMan (M2) were evaluated by the integrated copolymer composition equation of Mayo and Lewis.20 A computer program similar to those designed for the copolymerization systems of TXGlu-TBGal,ll TXGlu-TBMan,l3 and TXGlu-TBGlu16 was used. An approximate p value 1 - rl p==

3 COPOLYMERIZATION OF TXGa1,Mi AND TBMan,M2 1301

4 Y TABLE I1 Copolymerization of 1,6-Anhydro-2,3,4-Tri-O-(p -Methylbenzyl)-P-D-Galactopyranose (TXGal) with 1,6-Anhydro-2,3,4-Tri-O-Benzyl-~-~-Mannopyranose (TBMan) at -6OoCa Mole fraction Mole Mole of TXGal in TXGal TBMan fraction Polymn. fraction unreacted feed Polymn. feed feed of TXGal time Copolymer yield of TXGal mixtureb b1ty Ivl= z No. (g) (9) in feed (min) (g) (%) in copolymerb Obs. Calc. (deg) (dl/g) $ 44d d <e 26.5<e " I%, i "0 0 s ::E $ a Total monomers, 1.0X 10-3 mole; p-chlorobenzenediazonium hexafluorophosphate, 14.2 mg, 5.0 X b Mole fractions of TXGal in the copolymers and in the unreacted feed mixtures were determined by 100-MHz NMR spectroscopy and corrected by multiplying mole; methylene chloride, 1 ml. All the polymers precipitated s as powdery or fibrous solids in petroleum ether. x by c Intrinsic viscosities and specific rotations were determined in chloroform at 25OC. d Polymerization temperature, -55 to -7OOC; [a]g calculated from known value for TBGal homopolymer. Some was lost due to an accident. T:

5 COPOLYMERIZATION OF TXGal,M1 AND TBMan,M was estimated from the reactivity ratios rl and r2 obtained by the differential method of Fineman and Ross.21 The Mayo-Lewis method gave the reactivity ratios for TXGal (MI) and TBMan (M2) as follows: rl = 0.37 f 0.15, r2 = 38 f 4 In Figure 1 is shown the instantaneous copolymer composition curve calculated from the reactivity ratios. The position of the horizontal lines corresponds to the copolymer composition obtained from IH-NMR analysis. The length of the horizontal line represents the range of the monomer composition from the feed to the final solution. The curve is shown as a broken line between 0.9 and 1.0 mole fraction TXGal in the feed because TXGal did not homopolymerize. Now three pairs of reactivity ratios have been obtained for 1,6-anhydroglycopyranoses as follows: kgal+gal kgai+giu , -- kgiu+eiu kglu+gd kglu+glu kman+man = 11.5 = 0.90, k Glu+Man k~al+~al k Gal+Man = 0.37, k Man+Glu kman+man - kman+gal - 38 The following approximate equality can be derived from the values above: k Glu+Glu/kGlu+Gal X k Man+Man/k Man+Glu k Man+Man/kMan+Gal k Gal+Gal/kGal+Glu k Glu+Glu/k Glu+Man kgal+galhgal+man The self-propagation rates cancel out in the equation above to give the following relationship: kgal+glu kglu+man - kgal+man X kglu+gal kman+glu kman+gal This equation can be modified as follows: kgal+glu kglu+man kman+glu X N kgal+man kglu+gal kman+gal If one assumes that ka+b = ka+c, i.e., the propagation rate is not dependent MOLE FRACTION OF TXGA IN MONOMER FEED Fig. 1. Instantaneous copolymer composition curve calculated from the reactivity ratios for TXGA-TBMN: rl = 0.37 f 0.15, r2 = 38 f 4.

6 1304 ITO, MAROUSEK, AND SCHUERCH on the monomer but mainly on the growing cation, one could interpret why the approximate equality above holds. The self-propagation rate, however, differs from the CI'OSS-prOpagatiOn rate, ka+a # ka+b, kb+b # kb+a, as has been shorn in the series of copolymerizations.11j3 This assumption, therefore, is not acceptable. kgal+glu kgiu+man kglu+gal X - kman+glu kgal+man kman+gal This form of the relationship is easily understood if one assumes that kb+a = kc+a; i.e., the propagation rate does not depend significantly on the growing cation but mainly on the monomer. In consequence, we assume that the propagation rate is mainly determined by the structure of the attacking monomer. It is of interest to relate this conclusion to the mechanism proposed previ- 0us1y.l~ It was pointed out that the only source of a different energy barrier for different monomers to copolymerize was a change in conformation of the entering monomer as it becomes the active propagating trialkyloxonium ion. Since this was the case, it was proposed that the trialkyloxonium ion could not retain the IC4 conformation of the monomer but must convert to a boatlike conformation. Our new conclusion that the relative reactivities depend on the monomer and not on the growing cation is consistent with this conclusion. If the cations derived from glucose, mannose, and galactose were in the lc4 conformation, the axial substituents on C-2 and C-4 on glucose cation might be expected to provide more steric hindrance to the approach of monomer than would the single axial substituent on C-2 or C-4 of galactose or mannose cation. However, if all three cations are in a boatlike conformation, no axial substituents lie in the direction from which monomer must approach to react stereospecifically. All substituents are either equatorial or axial on the opposite side of the cationic pyranose ring. It is, therefore, understandable that the copolymerization reactivities depend predominantly on differences in monomer reactivity. A number of physical properties of the polymers are of interest. The tribenzyl mannose homopolymer prepared at 5% catalyst concentration was formed in high conversion in a short polymerization time. These conditions are not optimal for stereoregularity or high molecular s eight.^ Nevertheless, the polymer has a high optical rotation and intrinsic viscosity (Table 11; cf. ref. 4). The viscosity of the copolymers decreases steadily and abruptly with increase in TXGal to 20% in the copolymer (Table 11). A similar effect is observed in the TXGal-TBGlu system (Table I) and was previously observed with TXGlu and TBMan copolymers.l3 However in the last case it was clear that the intrinsic viscosity decreased from a maximum for the TBMan homopolymer to a broad minimum for copolymers containing 15% or more of TXGlu. In contrast, the intrinsic viscosity of TXGlu-TBGlu copolymers depended almost linearly on copolymer composition.l6 Dependence of the optical rotation on the copolymer composition is also interesting. As shown in Table 11, the specific rotation first decreased with increasing content of TXGal, reached a broad minimum at about 0.15 mole fraction, and then increased toward the value of the TXGal homopolymer, which was calculated from the molecular rotation of the TBGal homopolymer.6j0j1 The specific rotation of the TXGal-TBGlu copolymer (Table I) decreased linearly with increase in the mole fraction of TXGal in copolymer to about 0.45 mole (2)

7 COPOLYMERIZATION OF TXGa1,Mi AND TBMan,M fraction, and after that it seemed to decrease very gradually toward the calculated value of the TXGal homopolymer. Although the specific rotation of the TXGlu-TBMan copolymer was linearly related to the copolymer composition,13 that of the TXGlu-TBGal copolymer had a broad minimum around 0.5 (ref. 11). As previously suggested, the change of the intrinsic viscosity and specific rotation with copolymer composition may in part or in some cases reflect differences in conformation.llj3 1%-NMR spectra of the polymers were measured in deuteriochloroform. The methyl carbon in the copolymers of TXGal with TBMan and with TBGlu gave a singlet resonance at 21.4 ppm. Carbon resonances of C-6 of both copolymers appeared as a single peak around 66 ppm. Assignments were not attempted for several peaks ranging from 72 to 81 ppm in the copolymer of TXGal with TBMan and those ranging from 72 to 83 ppm in the copolymer of TXGal with TBGlu, which are due to the C-2, C-3, C-4, and C-5 in the pyranose ring and the methylene carbons of the benzyl and xylyl groups. The a-anomeric carbon resonance of the TXGal-TBMan copolymers appeared at 98.9 ppm as a singlet, though that of the TXGal-TBGlu copolymers appeared at 98.9 and 97.8 ppm, corresponding to TXGal and TBGlu, respectively. The shape of the a-anomeric carbon resonance of the latter suggests that it may give information about the diad fraction, as that of the TXGlu-TBMan copolymer did.14 No P-anomeric carbon signals were detected. The proton resonances of the three p-methyl groups on the benzene rings of the monomer TXGal appear as a single peak at , as in the case of TXGlu (6 2.36).13J6 It is of interest to note that, though the change of the C-2 axial benzyl ether group in TBGlu to the equatorial position of TBMan does not affect the chemical shift of the C-1 proton (6 5.47), the change of the C-4 axial xylyl ether group in TXGlu to the equatorial position in TXGal causes an upfield shift of the C-1 proton about 0.1 ppm, though both groups are well separated from each other. The mole fraction of TXGal in the unreacted monomer mixture, therefore, can be determined from the relative intensities of the anomeric proton resonances in the case of the copolymerizations of TXGal with TBGlu and TBMan. The methyl proton signals in the copolymer of TXGal with TBMan and TBGlu appear at ,2.25, and The relative intensities of the three peaks are as follows: the center peak > the highest-field peak > the lowest-field peak. The reverse order of intensities of the highest-field and the lowest-field peaks was observed in the copolymer of TXGlu with TBMan.13 It would be interesting to know the relative intensities of the three methyl signals of the TXGal homopolymer, since the TXGlu homopolymer has three methyl peaks of equal intensity13j6 and, as TBMan units are introduced into the TXGlu sequences, the side peaks decrease in strength.13 Unfortunately, however, the TXGal homopolymer was not available for a similar comparison. EXPERIMENTAL NMR spectra were measured with a Varian XL-100 spectrometer in deuteriochloroform with tetramethylsilane as the internal standard. Chemical shifts are expressed in parts per million (pprn) downfield from the TMS resonance. Mole fractions of TXGal in copolymers and in recovered monomers were de-

8 1306 ITO, MAROUSEK, AND SCHUERCH termined from integration ratios of p-methylbromatic and p-methyllnonaromatic proton resonances. Optical rotations were determined in chloroform at 25OC in a Perkin-Elmer model 141 polarimeter, using a jacketed l-dm cell. Viscosities were measured in a Cannon-Ubbelohde semimicroviscometer at 25"C, using chloroform as solvent. The intrinsic viscosity [17] was obtained from the measurement at four different concentrations. Materials 1,6-Anhydro-2,3,4-tri-O-benzyl-~-~-rnannopyranose was prepared by cyclization of 2,3,4-tri-0-benzyl-6-O-p-toluenesulfonyl-~-mannopyranose with sodium ethoxide in ethanol. After isolation the product was refluxed with sodium hydroxide in ethanol, concentrated, washed in chloroform solution, recrystallized from ethanol and absolute ether-petroleum etherlg; mp 60-61OC (lit.4j360-6loc, OC); [a]e-31.4' (lit.4j3-31.2' to -31.4', 29.6O). 1,6-Anhydro-2,3,4-tri-O-acetyl-P-D-galactopyranose was synthesized according to the procedures previously described, starting from penta-0-acetyl-p-d-galactopyrano~e~~~~~~*~~ and xylylated give TXGal, using p -methylbenzyl chloride. By-product di-p-xylyl ether was removed by crystallizing it out. Crude TXGal was then purified by silica gel and gel permeation chromatography and finally recrystallized several times from absolute ether-petroleum ether (bp-4ooc); mp 47-48OC; [a]$ = -54.8'. Methylene chloride and p-chlorobenzenediazonium hexafluorophosphate were purified in the usual manner. Copolymerization Copolymerization was carried out under vacuum at -6OOC in anhydrous methylene chloride with 5 mole % PFS, as previously described. Total quantity of monomer was 1.0 X mole. Polymerization conditions are shown in Tables I and 11. Polymerization was terminated at -6OOC by adding a small amount of cold methanol. The polymer was precipitated twice by pouring the chloroform solution into petroleum ether and isolated by freeze-drying from benzene. The combined supernatant solutions were concentrated to dryness, further dried under vacuum, and used for NMR analysis. This research was completed with financial support from the Division of General Medical Sciences, National Institutes of Health, US. Public Health Service (Grant No. GM06168), which is gratefully acknowledged. References 1. E. R. Ruckel and C. Schuerch, J. Am. Chem. SOC., 88, (1966). 2. E. R. Ruckel and C. Schuerch, J. Org. Chem., 31, (1966). 3. E. R. Ruckel and C. Schuerch, Biopolymers, 5, (1967). 4. J. Frechet and C. Schuerch, J. Am. Chem. SOC., 91, (1969). 5. J. Zachoval and C. Schuerch, J. Am. Chem. SOC., 91, (1969). 6. T. Uryu, H. Libert, J. Zachoval, and C. Schuerch, Macromolecules, 3, (1970). 7. T. Uryu and C. Schuerch, Macromolecules, 4, (1971). 8. B. Veruovic and C. Schuerch, Carbohydr. Res., 14, (1970). 9. V. Masura and C. Schuerch, Carbohydr. Res., 15,6672 (1970). 10. J. W.-P. Lin and C. Schuerch, J. Polym. Sci. A-1, 10, (1972). 11. J. W.-P. Lin and C. Schuerch, Macromolecules, 6, (1973).

9 COPOLYMERIZATION OF TXGa1,Ml AND TBMan,M W. H. Lindenberger and C. Schuerch, J. Polym. Sci. Polym. Chem. Ed., 11, (1973). 13. K. Kobayashi and C. Schuerch, J. Polym. Sci. Polym. Chem. Ed., 15, (1977). 14. K. Kobayashi, R. Eby, and C. Schuerch, Biopolymers, 16, (1977). 15. T. Uryu, H. Tachikawa, K. Ohaku, K. Terui, and K. Matsuzaki, Makromol. Chem., 178, (1977). 16. H. It0 and C. Schuerch, J. Polym. Sci. Polym. Chem. Ed., 16, (1978). 17. T. Kelen and F. Tudos, Macromol. Sci. Chem., 9,l-27 (1975). 18. J. P. Kennedy, T. Kelen, and F. Tudos, J. Polym. Sci. Polym. Chem. Ed., 13, (1975). 19. S. J. Sondheimer, R. Eby, and C. Schuerch, Carbohydr. Res., 60, (1978). 20. F. R. Mayo and F. M. Lewis, J. Am. Chem. Soc., 66, (1944). 21. M. Fineman and S. D. Ross, J. Polym. Sci., 5, (1950). 22. E. M. Montgomery, N. K. Richtmyer, and C. S. Hudson, J. Am. Chem. SOC., 64, (1942). 23. J. A. Wolff, J. Am. Chem. Soc., 67, (1945). Received November 22,1977

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