Macromolecules 1998, 31, 2889-2893
2889
“Living” Nature in Anionic Cyclopolymerization of 1,2:5,6-Dianhydro-3,4-di-O-methyl-D-mannitol Using the Potassium tert-Butoxide/18-Crown-6 Initiating System Takeshi Hatakeyama, Masatoshi Kamada, Toshifumi Satoh, and Kazuaki Yokota* Division of Molecular Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan
Toyoji Kakuchi* Division of Bioscience, Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan Received July 2, 1997; Revised Manuscript Received December 2, 1997
ABSTRACT: The effect of 18-crown-6 ether (18C6) in the anionic cyclopolymerization of 1,2:5,6-dianhydro3,4-di-O-methyl-D-mannitol (1) using potassium tert-butoxide (t-BuOK) was studied. The addition of 18C6 did not affect the regio- and stereoselectivity in the polymerization of 1, and the resultant polymer was (1f6)-2,5-anhydro-3,4-di-O-methyl-D-glucitol (2). The complexation of t-BuOK with 18C6 increased the initiator efficiency from 0.3 to 1.0, which was accompanied by enhancement of the apparent polymerization rate. The rate constant ratios of the transfer reaction to the propagation (ktr/kp) in the systems with and without 18C6 were 1 order of magnitude lower than the value in the polymerization of monoepoxides. The “living” nature of the t-BuOK/18C6 initiating system was evaluated by the two-step monomer resumption experiment.
Introduction
Scheme 1
Cyclizations of optically active epoxides are regarded as one of the useful methods for the synthesis of natural products consisting of cyclic ether structures, because stereocontrolled reactions can be realized. Thus, the stereospecificity in the cyclization of epoxide has received considerable attention,1-7 e.g., Nicolaou et al. reported a stereospecific ring-closure reaction of hydroxy epoxides to form five-, six-, and seven-membered rings1-3 based on the Baldwin rule8 utilizing the electronic effect of the substituent bonded to the epoxy group. We have reported the regio- and stereoselective cyclopolymerization of 1,2:5,6-dianhydrohexitols as a new method for producing a carbohydrate polymer.9-16 Using each of the cationic and anionic initiators, 1,2:5,6dianhydrohexitols produced polymers consisting of cyclic constitutional repeating units without cross-linking reactions. The polymer obtained by cationic polymerization was composed mainly of a five-membered cyclic unit along with the other cyclic ones.9-13 On the other hand, polymerization using the anionic initiator proceeded through a highly regio- and stereoselective mechanism to yield a polymer consisting only of the 2,5anhydrohexitol unit.12-16 For instance, 1,2:5,6-dianhydro-3,4-di-O-methyl-D-mannitol (1) was polymerized with potassium tert-butoxide (t-BuOK) to yield (1f6)2,5-anhydro-3,4-di-O-methyl-D-glucitol (2).14,15 Alkali metal and alkali metal alkoxide perform as the anionic initiators for the polymerization of oxiranes, lactones, and olefins. However, the initiators are insoluble or sparingly soluble in the polymerization solvents, resulting in low initiator efficiency. To remove this defect, complexing agents, such as crown ether and * To whom all correspondence should be addressed. Tel: +81-11-706-2255. Fax: +81-11-706-6602. E-mail: kakuchi@ eoas.hokudai.ac.jp.
cryptand, are utilized to form an initiating complex that is soluble in the polymerization solvents.17-23 In the anionic polymerization of glycidyl ethers, the addition of crown ethers promotes both initiation and propagation.20-22 Hence, it is of interest to investigate the effect of crown ether on the cyclopolymerization of monomer 1 in relation to more precise control of the polymerization system, i.e., to living polymerization. This paper deals with the effect of 18-crown-6 ether (18C6) for the anionic polymerization of 1 using t-BuOK in terms of the initiator efficiency, the apparent rate of polymerization, and the participation of side reactions. In addition, the living nature of the system with t-BuOK/18C6 initiator was evaluated by means of a twostep monomer resumption experiment. Experimental Section Measurements. 1H and 13C NMR spectra were recorded using a JEOL JNM-A400 II spectrometer in chloroform-d (CDCl3) with tetramethylsilane as the internal standard. The molecular weights of the resulting polymers were measured by gel permeation chromatography (GPC) in tetrahydrofuran on a JASCO HPLC system equipped with three polystyrene gel columns (Shodex KF-804L). The number-average molecular weight (Mn) and the molecular weight distribution (Mw/ Mn) were calculated on the basis of polystyrene calibration.
S0024-9297(97)00976-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/17/1998
2890 Hatakeyama et al.
Figure 1. Time-conversion plot for anionic cyclopolymerization of monomer 1: (open circles) with 18C6; (solid circles) without 18C6. (Data from ref 15). Conditions: [1] ) 1.0 mol‚L-1; [1]/[t-BuOK] ) 20; [18C6]/[t-BuOK] ) 2.0. Materials. Toluene was purified by the usual methods and distilled from sodium benzophenone ketyl. Potassium tertbutoxide (t-BuOK) was purified by sublimation under vacuum before use. 18-Crown-6 ether (18C6) was purified by recrystallization from acetonitrile. 1,2:5,6-Dianhydro-3,4-di-O-methyl-D-mannitol (1) prepared according to the reported procedures was freshly distilled from calcium hydride just before use.24 Cyclopolymerization. The polymerization using t-BuOK was carried out as described in a previous paper.12 A methanol solution of the crude product was neutralized with carbon dioxide. Potassium carbonate as a precipitate was eliminated by filtration, and then the solution was evaporated. Monomer conversion was estimated from the peak area ratio of methine protons in cyclized units (H-2 or H-5) to unreacted epoxy groups in the 1H NMR spectrum of the crude product. The product was purified by reprecipitation from chloroform-nhexane.
Results and Discussion Anionic cyclopolymerization of 1 was carried out using t-BuOK as an initiator in toluene at 60 °C. In the conventional polymerization without 18C6, the initiator gradually dissolved into the polymerization system after several hours and the solution turned brown. On the other hand, the reaction mixture was a brown solution throughout the polymerization in the presence of 18C6. In both systems, the resulting polymers were yellowbrown viscous liquids and soluble in methanol, chloroform, and THF and insoluble in n-hexane. Because the residual epoxy groups were not detected in the 1H NMR spectra of the polymers, the polymers were composed of only cyclized repeating units. There was no substantial difference in the 13C NMR spectra between the polymers obtained from both systems. Thus, the addition of 18C6 did not influence the regio- and stereoselectivity of the anionic cyclopolymerization of 1; i.e., the resultant polymer was (1f6)-2,5-anhydro-3,4-di-Omethyl-D-glucitol (2). Figure 1 shows the time-conversion plots for the polymerizations of 1 with and without 18C6. The monomer conversion came close to 100%; therefore, the polymerization was scarcely prevented by the addition of 18C6. The rate of monomer consumption for the system with 18C6 was faster than that for the system without 18C6. Assuming that the polymerization of 1 is a first-order reaction with respect to the monomer
Macromolecules, Vol. 31, No. 9, 1998
Figure 2. First-order time-conversion plot for anionic cyclopolymerization of monomer 1: (open circles) with 18C6; (solid circles) without 18C6. Conditions: [1] ) 1.0 mol‚L-1; [1]/[tBuOK] ) 20; [18C6]/[t-BuOK] ) 2.0.
concentration, the rate of the monomer consumption is derived as
-
d[M] ) kapp[M] dt
(1)
where [M] represents the instantaneous concentration of monomer and kapp denotes the apparent rate constant of the polymerization. Integration of eq 1 gives
ln
[M]0 [M]
) kappt
(2)
where [M]0 represents the concentration of monomer in the feed. The first-order time-conversion plots according to eq 2 are shown in Figure 2. For both systems, the plots were presented as upward-concave curves, implying the existence of the chain termination.25 The parameter kapp was estimated as the slope of the approximated line where conversion was less than 70%. For the system without 18C6, kapp was 1.42 × 10-1 h-1, which was determined from the plots in the range of 1-6 h. On the other hand, kapp was 4.26 × 10-1 h-1 for the system with 18C6 in the range 0.5-2 h. The addition of 18C6 increased the rate constant by 3 times. Figure 3 shows the plots of number-average degree of polymerization (DPn) versus conversion. For the polymerization system without 18C6, the DPn increased in proportion to conversion and went up to 36 at 100% conversion. The linearity in the plot was caused by little participation of side reactions, e.g., chain transfer, in the polymerization. The DPn of the polymer increased linearly with conversion also in the presence of 18C6. The ultimate DPn was about 17, which is close to the [1]/[t-BuOK] of 20 in the feed. In the polymerization of epoxide, the side reaction is generally known as a chain transfer reaction to a monomer. Taking this chain transfer reaction into consideration, the reciprocal of DPn is expressed by the following equation.
ktr f[I]0 1 ) + DPn kp [M]0 - [M]
(3)
where kp and ktr denote the rate constant of the propagation and the transfer reaction, respectively, f
Macromolecules, Vol. 31, No. 9, 1998
Figure 3. Plots of DPn vs conversion for monomer 1: (open circles) with 18C6; (solid circles) without 18C6. Conditions: [1] ) 1.0 mol‚L-1; [1]/[t-BuOK] ) 20; [18C6]/[t-BuOK] ) 2.0.
Dianhydro-3,4-di-O-methyl-D-mannitol Polymerization 2891
Figure 5. Variation of DPn as a function of [18C6]/[t-BuOK] ratio. Conditions: [1] ) 1.0 mol‚L-1; [1]/[t-BuOK] ) 20; [18C6]/ [t-BuOK] ) 2.0. Chart 1
Figure 4. Plots of 1/DPn vs 1/([M]0 - [M]) for monomer 1: (open circles) with 18C6; (solid circles) without 18C6. Conditions: [1] ) 1.0 mol•L-1; [1]/[t-BuOK] ) 20; [18C6]/[t-BuOK] ) 2.0.
denotes the initiator efficiency, and [I]0 represents the concentration of initiator in the feed. The plots of the reciprocal of DPn against the reciprocal of the polymer yield ([M]0 - [M]) are shown in Figure 4. The slope of this plot represents the concentration of active species, f[I]0 (mol‚L-1), and then f is estimated using [I]0 ) 0.05 mol‚L-1. In the plot for the system without 18C6, the slope was inclined to change with the progress of polymerization. The value of f was 0.30 in the conversion below 70% and then increased to 0.50 above this conversion. Therefore, the system without 18C6 would be considered to be polymerization with a slow initiation due to the sparing solubility of t-BuOK. The value of f is less than 1.0 also in the polymerization of epoxy compounds such as glycidyl ethers. Grobelny et al. proposed that the initiator efficiency of t-BuOK is lowered by aggregation of t-BuOK molecules with the monomers. On the other hand, the plot for the system with 18C6 was in good agreement with the linear approximation. The value of f was 1.03, which was independent of the variation of monomer conversion. Thus, in the system with 18C6, all of the initiators induced the propagation.
The addition of 18C6 caused an increase in the solubility of t-BuOK, resulting in the efficient formation of the initiating anion. Therefore, the increment in f brought about the enhancement of the apparent polymerization rate. According to eq 3, an intercept on the ordinate axis in Figure 4 corresponds to the ktr/kp ratio. The ktr/kp values were 5.9 × 10-3 and 6.5 × 10-3 with and without 18C6, respectively. In both systems, the values were 1 order of magnitude lower than the value of 2.0 × 10-2 in the polymerization of propylene oxide and glycidyl ethers. During the polymerization of epoxides, the side reaction is mainly deprotonation from the epoxide molecule, which proceeds through the E2 mechanism to yield the anionic species with an allyl ether end group. Such a chain transfer reaction, however, hardly occurs in the polymerization of 1, because the stereochemistry between the C-O bond at the R-position of the epoxy groups and the C-H bond at the adjacent 3or 4-position is cis form, as shown in Chart 1, which prevents the antiperiplanar conformation required for the progress of bimolecular elimination. The process of a side reaction in the polymerization of 1, thus, is obscure. Figure 5 shows graphically the variation in DPn as a function of [18C6]/[t-BuOK] ratio. The DPn decreased steeply with the addition of 18C6 and remained constant at 17 at a [18C6]/[t-BuOK] ratio of 0.5 or above. It is generally accepted that a macrocyclic polyether such as 18C6 first breaks up the association of ion pairs in nonpolar solvents and second complexes counterions so strongly as to increase the concentration of free-ion propagation species, resulting in a substantial increase in the polymerization rate. The experimental result in which the macrocyclic ligands are sufficient at a ratio
2892 Hatakeyama et al.
Macromolecules, Vol. 31, No. 9, 1998
Table 1. Cyclopolymerization of 1,2:5,6-Dianhydro-3,4-di-O-methyl-D-mannitol (1) Using t-BuOK/18C6a run 1 2 3 4d 5d 6d
additive none none none 18C6f 18C6f 18C6f
[1]/[t-BuOK]
yieldb (%)
Mnc
Mw/Mnc
DPn
5 10 20 5 10 20
96.5 97.0 98.5 86.8e 89.2e 92.5
1590 3030 6410 1130 1890 2980
1.27 1.41 1.53 1.35 1.51 1.38
9.1 17.4 36.8 6.5 10.9 17.1
a [1] ) 1.0 mol‚L-1; temp, 60 °C; solvent, toluene; time, 48 h. Yields of n-hexane-insoluble part. c Measured in THF by GPC using polystyrene as the standard. d Data from ref 15. e Some low molecular weight polymer was lost in reprecipitation. f [18C6]/[tBuOK] ) 2.0.
b
of 0.5 thus indicates that the intermolecular exchange of the ligands takes place more rapidly. The effect of the [1]/[t-BuOK] feed ratio is summarized in Table 1. In the systems without 18C6 (run 1-3), the DPn of the polymer obtained was twice the feed ratio. The polymer yield for the systems with 18C6 (run 4-6) decreased with decreasing feed ratio, which is due to the removal of n-hexane-soluble oligomers in reprecipitation. The DPns of the polymers were approximately equal to the feed ratio, suggesting that 18C6 completely broke up the association of t-BuOK in toluene. The addition of the complexing agent did not affect the molecular weight distribution (Mw/Mn) of the polymer in the range of 1.3-1.5. The molecular weight of the polymer can be precisely controlled in the system with 18C6. As discussed above, DPns of the resulting polymer were proportional to the conversion and f was about 1.0 throughout the polymerization in the system with 18C6. Therefore, the polymerization of 1 with the t-BuOK/ 18C6 initiating system would be taken as apparently living (called “living”) polymerization.26 As an evaluation of the “living” nature, the experiment for successive additions of two batches of monomer was carried out. A first batch of monomer 1 (2.87 mmol) was polymerized at [1]/[t-BuOK] ) 10 and [18C6]/[t-BuOK] ) 2.0. After 100% conversion was reached in 6 h, additional polymerization took place on adding a second monomer 1 (2.87 mmol) to the reaction system and was continued for 36 h. Figure 6 shows that the GPC traces of the polymers for both batches were unimodal. The DPn value for first and second batches were 11.1 and 18.9 relative to the calculated values of 10 and 20, respectively. The Mw/ Mn was slightly increased from 1.33 to 1.62 in the second polymerization. The broadening in Mw/Mn of the polymer through the two-step polymerization is owing to the occurrence of a chain termination. Although the second polymerization underwent a reaction with a slight lowering of the concentration of the propagating species, the “living” nature of the system was evident. Conclusion To establish a highly controlled polymerization system that produces the new carbohydrate polymer, (1f6)-2,5-anhydro-3,4-di-O-methyl-D-glucitol (2), the effect of a crown ether for the polymerization of 1,2:5,6dianhydro-3,4-di-O-methyl-D-mannitol (1) using t-BuOK was studied. The addition of 18-crown-6 ether (18C6) caused an increase in the initiator efficiency from 0.3 to 1.0 where the conversion was less than 70%. The apparent polymerization rate of 1 in the presence of 18C6 was faster than that with no additive by a factor
Figure 6. Gel permeation chromatograph traces of polymer 2 synthesized in toluene of 60 °C using t-BuOK/18C6: (a) after the first polymerization (Mn ) 1940, DPn ) 11.1, Mw/Mn ) 1.33) and (b) after the second polymerization (Mn ) 3290, DPn ) 18.9, Mw/Mn ) 1.62).
of 3.0. The complexation of t-BuOK with 18C6 produced the increment in the initiator efficiency, which was accompanied by enhancement of the apparent polymerization rate. Assuming the occurrence of a chain transfer reaction to a monomer, the rate constant ratios of the transfer reaction to the propagation (ktr/kp) were 5.9 × 10-3 and 6.5 × 10-3 with and without 18C6, respectively. Moreover, in the two-step monomer resumption experiment for the system with t-BuOK/18C6, the active chain end that was produced from the first batch of monomer continued to propagate on adding the second monomer. The DPn of the resultant polymer increased from 11.1 to 18.9, which corresponds to the calculated value of 20. Although the inactivation of the active species caused an increase in the molecular weight distribution, the polymerization of 1 with t-BuOK/ 18C6 proceeded in a “living” manner. References and Notes (1) Nicolaou, K. C.; Duggan, M. E.; Hwang, C.-K.; Somers, P. K. J. Chem. Soc., Chem. Commun. 1985, 1359. (2) Nicolaou, K. C.; Prasad, C. V. C.; Somers, P. K.; Hwang, C.K. J. Am. Chem. Soc. 1989, 111, 5330. (3) Nicolaou, K. C.; Prasad, C. V. C.; Somers, P. K.; Hwang, C.K. J. Am. Chem. Soc. 1989, 111, 5335. (4) Stork, G.; Maldonado, L. J. Am. Chem. Soc. 1974, 96, 5270. (5) Kuszmann, J. Carbohydr. Res. 1979, 73, 93. (6) Fujiwara, K.; Tokiwano, T.; Murai, A. Tetrahedron Lett. 1995, 36, 8063. (7) Hayashi, N.; Fujiwara, K.; Murai, A. Chem. Lett. 1996, 341. (8) Baldwin, J. E. J. Chem. Soc., Chem. Commun. 1976, 734. (9) Kakuchi, T.; Satoh, T.; Umeda, S.; Hashimoto, H.; Yokota, K. Macromolecules 1995, 28, 4062. (10) Kakuchi, T.; Satoh, T.; Umeda, S.; Hashimoto, H.; Yokota, K. Macromolecules 1995, 28, 5643. (11) Kakuchi, T.; Umeda, S.; Satoh, T.; Hashimoto, H.; Yokota, K. Macromol. Rep. 1995, A32 (7), 1007. (12) Kakuchi, T.; Satoh, T.; Mata, J.; Umeda, S.; Hashimoto, H.; Yokota, K. J. Macromol. Sci., Pure Appl. Chem. 1996, A33 (3), 325. (13) Satoh, T.; Hatakeyama, T.; Umeda, S.; Yokota, K.; Kakuchi, T. Polym. J. 1996, 28, 520. (14) Satoh, T.; Yokota, K.; Kakuchi, T. Macromolecules 1995, 28, 4762. (15) Satoh, T.; Hatakeyama, T.; Umeda, S.; Hashimoto, H.; Yokota, K.; Kakuchi, T. Macromolecules 1996, 29, 3447. (16) Satoh, T.; Hatakeyama, T.; Umeda, S.; Kamada, M.; Yokota, K.; Kakuchi, T. Macromolecules 1996, 29, 6681. (17) Orvik, J. A. J. Am. Chem. Soc. 1976, 98, 3322.
Macromolecules, Vol. 31, No. 9, 1998 (18) Koinuma, H.; Naito, K.; Hirai, H. Makromol. Chem. 1982, 183, 1383. (19) Stolarzewicz, A.; Grobelny, Z.; Arkhipovich, G. N.; Kazanskii, K. S.; Makromol. Chem. Rapid Commun. 1989, 10, 131. (20) Stolarzewicz, A.; Grobelny, Z. Makromol. Chem. 1992, 193, 531. (21) Stolarzewicz, A.; Neugebauer, D.; Grobelny, Z. Macromol. Chem. Phys. 1995, 196, 1295. (22) Stolarzewicz, A.; Neugebauer, D.; Grobelny, Z. Macromol. Chem. Phys. 1995, 196, 1301.
Dianhydro-3,4-di-O-methyl-D-mannitol Polymerization 2893 (23) Boileau, S.; Deffieux, A.; Lassale, D.; Menezes, F.; Vidal, B. Tetrahedron Lett. 1978, 20, 1767. (24) Kuszmann, J. Carbohydr. Res. 1979, 71, 123. (25) Penczek, S.; Kubisa, P.; Szymanski, R. Macromol. Chem. Rapid Commun. 1991, 12, 77. (26) Thomas, L.; Polton, A.; Tardi, M.; Sigwalt, P. Macromolecules 1992, 25, 5886.
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