Inclusion of Polymers within the Crystal Structure of Tris(o

Table 2 records the unit cell dimensions of various inclusion adducts formed with ..... use of the Philips analytical X-ray X'PERT MPD (NSF grant DMR-...
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Chem. Mater. 1999, 11, 1243-1252

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Inclusion of Polymers within the Crystal Structure of Tris(o-phenylenedioxy)cyclotriphosphazene Harry R. Allcock,* A. Paul Primrose, and Nicolas J. Sunderland Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802

Arnold L. Rheingold and Ilia A. Guzei Department of Chemistry, University of Delaware, Newark, Delaware 19716

Masood Parvez Department of Chemistry, University of Calgary, Alberta, Canada T2N 1N4 Received September 3, 1998. Revised Manuscript Received March 15, 1999

Tris(o-phenylenedioxy)cyclotriphosphazene was found to form hexagonal host-guest inclusion adducts (clathrates) with the polymers: cis-1,4-polybutadiene, trans-1,4-polyisoprene, polyethylene, poly(ethylene oxide), and polytetrahydrofuran. Single-crystal X-ray diffraction studies of both the polyethylene and poly(ethylene oxide) inclusion adducts revealed the presence of individual polymer chains extended along tunnel-like voids within the host lattice. Both polyethylene and poly(ethylene oxide) were incommensurate within the tunnel-like voids preventing the exact elucidation of the polymer conformation. Average repeat unit lengths calculated for these polymers suggest that both are in an extended conformation. Both the polyethylene and poly(ethylene oxide) adducts crystallized from benzene in the space group P63/m. Inclusion adducts of the five different polymers were examined by differential scanning calorimetry (DSC) and powder X-ray diffraction. In each case, the melting point of the inclusion adduct was higher than either the melting point of the pure polymer or the pure host. The melting points of oligomeric polyethylene and poly(ethylene oxide) inclusion adducts increased as the molecular weight of the included oligomers increased. Removal of cis-1,4-polybutadiene and poly(ethylene oxide) from the host lattice was achieved through chemical decomposition of tris(o-phenylenedioxy)cyclotriphosphazene. Linear polybutadiene was separated from a mixture with highly branched polybutadiene through selective adduct formation.

Introduction Much attention is being directed toward the utilization of noncovalent bonding interactions such as hydrogen bonding,1 metal-ligand bonding,2 and π-π stacking forces3 in the design of novel nano- and microscale suprastructures. Such structures mimic many biological systems in the sense that molecules easily and cleanly self-assemble to form large-scale architectures, having (1) (a) Russel, V. A.; Evans, C. C.; Li, W.; Ward, M. D. Science 1997, 276, 575. (b) Wang, X.; Simard, M.; Wuest, J. D. J. Am. Chem. Soc. 1994, 116, 12119. (c) Desiraju, G. R. Angew. Chem., Int. Ed. Engl. 1995, 34, 2311. (d) Verkataraman, D.; Lee, S.; Zhang, J.; Moore, J. S. Nature 1994, 371, 591. (e) Mathias, J. P.; Simanck, E. E.; Whitesides, G. M. J. Am. Chem. Soc. 1994, 116, 4326. (e) Chang, Y. L.; West, M. A.; Fowler, F. W.; Lauher, J. W. J. Am. Chem. Soc. 1993, 115, 5991. (2) (a) Lehn, J.-M. Chem. Eur. J. 1997, 3, 99. (b) Munakata, M.; Kuroda-Sowa, T.; Maekawa, M.; Nakamura, M.; Akiyama, S.; Kitagawa, S. Inorg. Chem. 1994, 33, 1284. (c) Fujita, M.; Kwon, Y. J.; Miyazawa, M.; Ogura, K. J. Chem. Soc., Chem. Commun. 1994, 1977. (d) Stang, P. J.; Chen, K. J. Am. Chem. Soc. 1995, 117, 1667. (e) Goodgame, D. M. L.; Menzer, S.; Smith, A. M.; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 574. (3) (a) Ashton, P. R.; Goodnow, T. T.; Kaifer, A. E.; Reddington, M. V.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. F.; Vicent, C.; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1989, 28, 1396. (b) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 2, 5525. (c) Hunter, C. A. Chem. Soc. Rev. 1994, 101.

a certain form or function. Related to this research is the investigation of inclusion adducts. Various organic and organometallic compounds have the ability to form inclusion adducts in which guest molecules are trapped within cage, tunnel, or layered voids present within the host crystal structure.4 Unlike zeolites and zeolite-type solids,5 these host compounds often crystallize in a certain manner, depending on the structure of the guest. The structures shown in Scheme 1 represent a unique class of inorganic-organic, highly tailorable host molecules. These compounds are synthesized by the reaction of hexachlorocyclotriphosphazene with various difunctional aryl compounds (Scheme 1). Tris(o-phenylenedioxy)cyclotriphosphazene (1)6-9 has proved to be (4) Barrer, R. M. In Inclusion Compounds; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Eds.; Academic Press: London, 1984; Vol. 1, p 191. (5) (a) Iwamoto, T. In Inclusion Compounds; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Eds.; Academic Press: London, 1984; Vol. 1, p 29. (b) Vo¨gtle, F. In Supramolecular Chemistry; Wiley: Chichester, 1991; p 171. (6) Allcock, H. R.; Siegel, L. A. J. Am. Chem. Soc. 1964, 86, 5140. (7) Allcock, H. R.; Levin, M. L.; Whittle, R. R. Inorg. Chem. 1986, 25, 41. (8) Siegel, L. A.; van der Hende, J. H. J. Chem. Soc. A. 1967, 817.

10.1021/cm980611e CCC: $18.00 © 1999 American Chemical Society Published on Web 04/28/1999

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Allcock et al. Scheme 1

the most versatile of these species, since it forms inclusion adducts with a wide range of small molecules. The paddle-wheel shape of 1 allows for the formation of a hexagonal-type host packing arrangement that retains guest molecules within the tunnel-like voids that penetrate the crystal lattice. (Figure 1) These tunnels are ∼5 Å in diameter at their narrowest points and run perpendicular to the plane of the phosphazene rings. Tris(2,3-naphthalenedioxy)cyclotriphosphazene (2) forms a similar inclusion adduct with benzene, but with a tunnel diameter of approximately 10 Å.10 Recent work has shown that the crystal structures of the inclusion adducts formed from 2 are greatly dependent on the structure of the guest.11 To date, structures 3 and 4 are known to form inclusion adducts with only a few small molecules.12,13 In addition to forming clathrates during crystallization from a guest solvent, crystals of pure 1 also imbibe small molecules directly from the liquid or vapor state to form inclusion adducts. If the guest is of sufficient size, the inclusion adduct can be stable at room temperature and atmospheric pressure for long periods of time.6 The ability of 1 to form adducts through direct (9) Allcock, H. R. J. Am. Chem. Soc. 1964, 86, 2591. (10) Allcock, H. R.; Stein, M. T. J. Am. Chem. Soc. 1974, 96, 49. (11) Kubono, K.; Kurata, H.; Isoda, S.; Kobayashi, T. J. Mater. Chem. 1993, 3, 615. (12) Allcock, H. R.; Teeter-Stein, M.; Bissell, E. C. J. Am. Chem. Soc. 1974, 96, 4795. (13) Allcock, H. R.; Kugel, R. L. Inorg. Chem. 1966, 5, 1016.

imbibition has proved useful in several different ways. Previous studies have shown that direct imbibition discriminates against larger guest molecules.6,14 For example when an equimolar mixture of cyclohexane and heptane is exposed to 1, heptane is preferentially absorbed with almost a 100% exclusion of cyclohexane.6 Applications have been developed from this for separations technology.15 Other research has shown that γ-irradiation of various dienes, acrylates, and vinyl monomers included within 116-19 through direct imbibition results in the formation of linear, and, in some cases, stereoregular polymers. Most recently, we reported in a communication that 1 interacts with polyethylene or poly(ethylene oxide) in solution to form inclusion adducts that contain the polymers as guests.20 These adducts are stable at temperatures above 300 °C. Before this discovery, only (14) Allcock, H. R.; Allen, R. W.; Bissel, E. C.; Smeltz, L. A.; Teeter, M. J. Am. Chem. Soc. 1976, 98, 5120. (15) (a) Goldup, A.; Westway, M. T. (British Petroleum Co.) U.S. Patent 3,472,762, 1969. (b) Goldup, A.; Westway, M. T. (British Petroleum Co.) U.S. Patent 3,499,944, 1970. (c) Haresnape, J. N. (British Petroleum Co.) U.S. Patent 3,504,47, 1969. (16) Allcock, H. R.; Ferrar, W. T.; Levin, M. L. Macromolecules 1982, 15, 697. (17) Allcock, H. R.; Levin, M. L. Macromolecules 1985, 18, 1324. (18) Allcock, H. R.; Dudley, G. K.; Silverberg, E. N. Macromolecules 1994, 27, 1039. (19) Allcock, H. R.; Silverberg, E. N.; Dudley, G. K. Macromolecules 1994, 27, 1033. (20) Primrose, A. P.; Parvez, M.; Allcock, H. R. Macromolecules 1997, 30, 670.

Structure of Tris(o-phenylenedioxy)cyclotriphosphazene

Figure 1. Representation of the molecular and crystal structures of 1. The spirocyclic side groups are oriented at right angles to the plane of the phosphazene ring. The right hand structure shows the hexagonal crystal arrangement of 1 in which the 5-Å diameter tunnel voids are visible.

perhydrotriphenylene (PHTP)21,22 and urea23-25 were known to readily form crystalline inclusion adducts with preformed polymers. Potential uses for this phenomenon include the separation of polymers on the basis of polymer-host interactions. For example, it was recently reported that polytetrahydrofuran (M h w ) 2 000) extracted from an adduct with urea had a lower polydispersity than before clathration.25 The formation of polymer-host inclusion adducts may also facilitate the study of polymer conformations and chain mobility with the macromolecules isolated from the influence of chain-chain or chain-solvent interactions. Solid-state 2H and 13C NMR spectroscopy has recently proved useful for determining polymer mobility within PHTP.22,26-28 In this paper we report that a variety of polymers interact with 1 to form inclusion adducts. The structure and properties of these adducts were examined by X-ray powder diffraction, scanning electron microscopy (SEM), gel permeation chromatography (GPC), and differential scanning calorimetry (DSC). This study includes singlecrystal X-ray diffraction analyses of both the polyethylene (PE) and poly(ethylene oxide) (PEO) inclusion adducts with 1. Experimental Section Materials. Hexachlorocyclotriphosphazene (Ethyl Corp./ Nippon Fine Chemical) was purified by recrystallization from heptane, followed by vacuum sublimation (0.050 mmHg, 40 °C). Catechol (Aldrich) was recrystallized from toluene and sublimed (0.050 mmHg, 60 °C). Triethylamine (Aldrich) was dried and distilled from sodium benzophenone. Polyethylene (M h w ) 1 000) and poly(ethylene glycol) (M h w ) 35 000) were used as received from Polysciences. Poly(ethylene oxide) (M hw ) 300 000), trans-1,4-polyisoprene (M h w ) 410 000), cis-1,4(21) Farina, M.; Natta, G. J.; Allegra, G.; Loffelhotz, M. Polym. Sci., Part C 1967, 16, 2517. (22) Sozzani, P.; Bovey, F. A.; Schilling, F. C. Macromolecules 1991, 24, 6764. (23) Monobe, K.; Yokoyama, F. T. J. Macromol. Sci., Phys. 1973, B8, 277. (24) Chenite, A.; Brisse, F. Macromolecules 1991, 24, 2221. (25) Chenite, A.; Brisse, F. Macromolecules 1992, 25, 776. (26) Sozzani, P.; Bovey, F. A.; Schilling, F. C. Macromolecules 1989, 22, 4225. (27) Sozzani, P.; Behling, R. W.; Schilling, F. C.; Bruckner, S.; Helfand, E.; Bovey, F. A.; Jelinski, L. W. Macromolecules 1989, 22, 3318. (28) Schilling F. C.; Amundson, K. R.; Sozzani, P. Macromolecules 1994, 27, 6498.

Chem. Mater., Vol. 11, No. 5, 1999 1245 polybutadiene (M h w ) 2 000 000-3 000 000), 60% unsaturated polybutadiene (M h w ) 1,800), 99% unsaturated polybutadiene h w ) 250), poly(ethylene (M h w ) 1 500), polytetrahydrofuran (M glycol) (M h w ) 1 500 and 400), poly(ethylene glycol) methyl ether (M h w ) 750 and 350), tetraethylene glycol dimethyl ether, and ethylene glycol dimethyl ether were used as received from Aldrich. The linear oligomers, CH3(CH2)nCH3, where n ) 5, 7, 9, 11, 13, 15, and 28, were also used as received from Aldrich. Synthesis of 1. Tris(o-phenylenedioxy)cyclotriphosphazene was synthesized by a modification of a previously described method.9 Hexachlorocyclotriphosphazene was allowed to react with catechol in the presence of triethylamine in tetrahydrofuran (THF). The resultant crystals were washed with water and 1 was recovered by Soxhlet extraction using benzene. This solid was then filtered from benzene, and was sublimed twice under vacuum (0.050 mmHg, 200 °C). Preparation of Inclusion Adducts. The polyethylene-1 and trans-1,4-polyisoprene-1 inclusion adducts were prepared by the interaction of powdered, crystalline tris(o-phenylenedioxy)cyclotriphosphazene with an equivalent weight amount of each polymer in refluxing benzene. These solutions were refluxed for 24 h to ensure complete recrystallization of 1. Refluxing benzene or heptane was used as the solvent for the formation of the cis-1,4-polybutadiene-1 adduct. After cooling, the solids were collected by filtration, washed with either water or hexane to remove unclathrated polymer, and evacuated at 0.050 mmHg pressure for 48 h to remove residual small molecules. The poly(ethylene oxide)-1 inclusion adduct was made by mixing poly(ethylene oxide) with an equal weight amount of 1 in water at room temperature for 24 h. Polytetrahydrofuran (M h w ) 250) was directly imbibed as a liquid within 1. Both PEO and polytetrahydrofuran form inclusion adducts with 1 even though this host is completely insoluble in these systems. X-ray-quality crystals of the PE (M hw ) 1 000)-1 and PEO (M h w ) 1 500)-1 adducts were grown through slow evaporation of benzene from solutions containing PE or PEO and saturated with 1. Polymer Recovery. Host 1 in the cis-1,4-polybutadiene-1 adduct was decomposed by hydrolysis with 10 molar equiv of Et3N and water in THF. After drying, the cis-1,4-polybutadiene was extracted with hexane and precipitated into methanol. This polymer extraction method was also used to retrieve the 99% unsaturated polybutadiene from 1. The removal of PEO from 1 required a reaction of 1 with 6 molar equiv of catechol and 12 molar equiv of Et3N in dry THF. After drying, the PEO was then extracted from the resultant solids using CH2Cl2. Equipment. The melting temperatures of the polymer-1 inclusion adducts were estimated by differential scanning calorimetry (DSC) using a Perkin-Elmer 7 series thermal analysis system. A heating rate of 20 °C/min was employed, and an indium standard was used for calibration. Thermal weight loss measurements of host 1 and cis-1,4-polybutadiene adduct were made using a SSC 5200 Haak-Buchner thermogravimetric analyzer equipped with a HP model 712/60 Power Risk Station under an atmosphere of compressed air at a flow rate of 30 cm3/min, using a heating rate of 10 °C/min. A Rigaku Gigerflex X-ray diffractometer with a copper source was used to examine the powdered polymer-1 adducts. X-ray diffractograms were obtained at a scan rate of 2° 2θ/min between 2θ ) 3 and 60°. 31P, 1H, and 13C solution NMR spectra of 1, and the initial and recovered polymers were recorded with a Bruker WM-360 spectrometer operated at 146, 360, and 90.5 MHz, respectively. 31P NMR chemical shifts are relative to 85% H3PO4 at 0 ppm, with positive shift values downfield from the reference. The molecular weight of cis-1,4-poly(butadiene) was estimated using a Hewlett-Packard HP 1090 gel permeation chromatograph equipped with an HP-1047A refractive index detector, American Polymer Standards AM gel 10 µm guard, AM gel 10 µm linear, and AM gel 10 µm 104 Å columns. The chromatograph was calibrated versus polystyrene standards (Polysciences). The samples were eluted with a 0.1 wt % solution of tetra-n-butylammonium nitrate (Aldrich) in THF (OmniSolv). The molecular weight analysis of poly(ethylene oxide) was carried out using a Waters Associates liquid/gel permeation chromatographic system consisting of a Model 501

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Table 1. Crystallographic Data formula formula weight space group a, Å c, Å V, Å3 Z cryst color, habit D(calc), g cm3 µ(Mo KR), cm-1 T, K diffractometer radiation R(F), %a R(wF), %b R(wF2), %a

(PE-1)

(PEO-1)

C18H12O6P3N3‚C4 507.27 P63/m 11.5192(8) 10.068(1) 1156.9(1) 2 colorless block 1.456 3.01 200(2) Rigaku AFC6S Mo KR (λ ) 0.71073 Å 4.1 3.6 ---

C18H12O6P3N3‚CxOx 519.27 P63/m 11.4833(4) 10.0799(5) 1152.12(8) 2 colorless block 1.497 3.05 213(2) Siemens P4/CCD 4.53 --12.02

∑[w(Fo2

Quantity minimized ) ) R ) ∑∆/∑(Fo), ∆ ) |(Fo - Fc)|. b Quantity ∑∆/∑(Fo); R(w) ) ∑∆w1/2/∑(Fow1/2), ∆ ) |(Fo - Fc)|. a

R(wF2)

- Fc2)2]/∑[(wFo2)2]1/2; minimized ) ∑∆2; R )

HPLC pump, Model U6K injector, and a Model 410 differential refractometer controlled by a Waters Millenium hardware/ software computer system. Separations were achieved with a column bank consisting of Waters Associates Ultrahydrogel Linear and Ultrahydrogel 120 GPC columns eluted at a rate of 1.0 mL/min with a mobile phase consisting of HPLC grade chloroform. The chromatograph was calibrated using poly(ethylene oxide) standards from Aldrich and Polysciences, Inc. Scanning Electron Microscopy. Crystals of the benzene adduct of 1 were grown by slow evaporation of benzene saturated with 1. Crystals of the PEO (M h w ) 300 000)-1 adduct were obtained through slow evaporation of a benzene solution containing both PEO and 1. The resultant crystals were washed extensively with water to remove unclathrated PEO and were dried by evacuation. The crystals were then coated with a 10-nm layer of gold/palladium and examined using a JEOL JSM-5400 and JSM-6300F electron microscope at accelerating voltages of 20-30 kV. Crystallographic Structural Determination. Singlecrystal X-ray diffraction of the PE-1 crystalline adduct was performed on a Rigaku AFC6S diffractometer (Table 1). The PEO-1 crystalline adduct was examined using a Siemens P4/ CCD diffractometer. The systematic absences in the diffraction data were consistent with the reported space groups. In both cases either of the hexagonal space groups P63 or P63/m was indicated; in both cases the latter centrosymmetric space group was preferred on the basis of the chemically reasonable and computationally stable results of refinement. The structures were solved using direct methods, completed by subsequent difference Fourier synthesis and refined by full-matrix least-squares procedures. The data were corrected for Lorenz and polarization for both structures, and also for absorption for the PE-1 adduct. In both cases all non-hydrogen atoms were refined with anisotropic displacement coefficients except for the carbon atoms comprising the polymeric chain. In the case of the PEO-1 adduct, the hydrogen atoms on the polymer were ignored. All other hydrogen atoms were treated as idealized contributions. All software and sources of the scattering factors are contained in the SHELXTL (version 5.03) program library (G. Sheldrick, Siemens XRD, Madison, WI).

Results and Discussion Crystal Structures. The PE and PEO inclusion adducts of 1 formed through crystallization in benzene are very similar. Both have hexagonal packing patterns in which individual polymer chains are extended along tunnel-like voids. (Figures 2 and 3) Motion of the PEO and PE chains within 1 was observed at temperatures

Figure 2. View down the c axis of the polyethylene-1 inclusion adduct unit cell.

Figure 3. View along the ab plane of the poly(ethylene oxide)-1 inclusion adduct unit cell. (Some guest and host species have been omitted for clarity.) Table 2. Unit Cell Dimensions of Hexagonal Inclusion Adducts of 1 guest

a, Å

c, Å

ref

poly(ethylene) poly(ethylene oxide) mesitylene o-xylene p-xylene bromobenzene benzene

11.53 11.504 11.65 11.915 11.68 11.66 11.80

10.07 10.08 10.20 10.046 10.12 10.09 10.054

this work this work Allcock et al.4 Allcock et al.4 Allcock et al.4 Siegel8 Allcock et al.4

well below 0 °C. (Table 1) This indicates that weak interactions are present between the polymer guests and 1. The hexagonal crystal structure of 1 in both complexes closely resembles that found for inclusion adducts of this host with small molecules. Table 2 records the unit cell dimensions of various inclusion adducts formed with 1. The a dimension of the adduct unit cell is related to the tunnel diameter. This dimension in both the PEO-1 and PE-1 adduct unit cell is decreased by 0.30 and 0.27 Å, respectively, compared to that for the benzene-1 adduct. This decrease may reflect an increased stability of the overall adduct. Despite the reduction in the tunnel diameter, the unit cell c axis dimension remains similar to the others noted in Table 2. The tris(o-phenylenedioxy)cyclotriphosphazene molecules have a 3-fold symmetry within both the PE and PEO inclusion adducts. As seen in Figures 4 and 5, the

Structure of Tris(o-phenylenedioxy)cyclotriphosphazene

Figure 4. Molecular structure and atomic numbering scheme for 1 and PE in the PE-1 inclusion adduct.

Figure 5. Molecular structure and atomic numbering scheme for 1 in the PEO-1 inclusion adduct.

paddle-wheel-like structure of 1 consists of pendent phenyl groups linked perpendicularly to the phosphazene ring through etheric oxygen linkages. The molecular structure of 1 in Figures 4 and 5 is very similar to that found in other adducts where 1 is the host compound. This indicates that the packing of 1 to form an adduct is not significantly altered by differences in the guest structure. The O-P-O bond angles of 1 in both Figures 4 and 5 are somewhat deformed due to strain within the five-membered heterocyclic rings. As seen in Tables 3 and 4, the ring strain within 1 results in O-P-O bond angles of ∼97°. Similar ring strain is seen in cyclic phosphates such as (o-phenylenedioxy)chlorophosphine (99°)29 and methylethylene phosphate (99°).30 In other cyclotriphosphazene structures that lack a strained side-unit ring, the X-P-X bond angles (29) Arbuzov, B. A.; Naumov, V. A.; Shaidulin, S. A.; Mukmenev, E. T. Dolk. Akad. (30) Steitz, T. A.; Lipscomb, W. N. J. Am. Chem. Soc. 1965, 87, 2488.

Chem. Mater., Vol. 11, No. 5, 1999 1247

are near 102°. For example, hexachlorocyclotriphosphazene, which also has a planar ring structure, has Cl-P-Cl angles near 101.4°.31 It also has N-P-N and P-N-P bond angles near 120°. However, with the formation of the five-membered rings in 1, these two bond angles diverge resulting in N-P-N bond angles near 117.4° and P-N-P bond angles near 122.6°. As shown in Figures 4 and 5, both PEO and PE are disordered within the adduct structure, resulting in a chainlike guest electron density distribution. This electron distribution is very different from that of benzene in 1.8 This indicates that adduct formation is highly selective toward PE and PEO over benzene. Discontinuity of the electron density for PE and PEO suggests that the repeating components of both polymers have favored positions along the tunnel axis. Movement or static disorder of both PE and PEO within 1 limited the determination of the exact polymer conformation by X-ray crystallography. It is unlikely that there is significant movement of the polymer chains out of the host lattice due to the thermodynamic stability they impart to the overall structure. Similar X-ray crystallographic disorder of included polyethylene in PHTP has been attributed to polymer rotation.22,28 The mobility of PE and PEO within 1 is highly probable since, like perhydrotriphenylene, host 1 interacts with the guests through weak van der Waals forces only. The thermal parameters of all atoms in both PE and PEO are rather large which also suggests high thermal activity. The inability to resolve the carbon and oxygen atoms of PEO in the PEO-1 adduct suggests that atom sites of neighboring PEO chains are disordered in their placement along the tunnel axis. Attempts to “freeze” both the PE and PEO chains within the host lattice at temperatures near -70 °C proved unsuccessful. Despite the disorder in both polyethylene and poly(ethylene oxide), the average repeat unit length for these polymers included within 1 could be estimated. This was accomplished through consideration of the average number of PE and PEO atoms per adduct unit cell. As seen in Table 3, the average distance between the centers of electron density for PE was 1.25-1.26 Å. This distance may be attributed to the average distance between carbon atoms in polyethylene. This corresponds to a PE repeat unit length of 2.52 Å. This length is comparable to those found for PE within both urea23 and PHTP.21 In all three of these adducts PE assumes an extended trans-planar chain conformation, as is found in the pure crystalline polymer. The average repeat unit length for PEO in 1 was estimated to be 3.02 Å. PEO in the pure crystalline state has a repeat unit length of 2.76 Å. It is likely that, following inclusion within 1, PEO is extended to a near-trans-planar conformation. This elongation of PEO to better fit the adduct voids has also been detected for PEO included within PHTP.21 Scanning electron micrographs of both PEO-1 and benzene-1 inclusion adduct crystals are shown in Figure 6. The crystal morphology of the benzene-1 adduct was very smooth, showing hexagonal plateletlike growth along the tunnel axis direction of the adduct (Figure 6a). Fracture of this crystal occurred during imaging, and this could be due to the release of benzene (31) Bullen, G. J. J. Chem. Soc., A 1971, 1450.

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Table 3. Bond Lengths and Angles for the PE-1 Adducta P(1)-O(1) P(1)-O(1a) P(1)-N(1) P(1)-N(1c) O(1)-C(1) O(1)-P(1)-O(1a) O(1)-P(1)-N(1) O(1)-P(1)-N(1c) N(1)-P(1)-N(1c) P(1)-O(1)-C(1) P(1)-N(1)-P(1d) a-d

Bond Lengths, Å C(1)-C(1a) 1.386(6) C(1)-C(2) 1.363(4) C(2)-C(3) 1.389(4) C(3)-C(3a) 1.395(6) C(4)e-C(5)e 1.25(2)

1.608(2) 1.608(2) 1.579(4) 1.580(4) 1.398(3) 97.0(2) 110.1(1) 110.2(1) 117.4(3) 109.9(2) 122.6(3)

Bond Angles, deg O(1)-C(1)-C(1a) O(1)-C(1)-C(2) C(1a)-C(1)-C(2) C(1)-C(2)-C(3) C(2)-C(3)-C(3a) C(5)e-C(4)e-C(5b)e

111.5(1) 126.5(3) 122.0(2) 116.9(3) 121.1(2) 180.0

C(4)e-C(5b)e C(5)e-C(6)e C(2)-H(1) C(3)-H(2)

C(4)e-C(5)e-C(6)e C(5)e-C(6)e-C(5a)e C(1)-C(2)-H(1) C(3)-C(2)-H(1) C(2)-C(3)-H(2) C(3)-C(3)-H(2)

1.25(2) 1.26(2) 1.03 1.10

180.0 180.0 118.3 124.7 113.5 125.1

Symmetry operations: (a) x, y, 1/2 -z; (b) x, y, -z; (c) 1 - y, x - y, z; (d) 1 - x + y, -x, z. e Polyethylene atoms.

Table 4. Bond Lengths and Angles for the PEO-1 Adduct P(1)-O(1)a P(1)-O(1) P(1)-N(1)a P(1)-N(1) O(1)-C(3) O(1)b-P(1)-O(1) O(1)-P(1)-N(1) O(1)b-P(1)-N(1) N(1)a-P(1)-N(1) P(1)-O(1)-C(3) P(1)c-N(1)-P(1)

Bond Lengths, Å 1.6086(13) C(1)-C(1)b 1.6086(13) C(1)-C(2) 1.580(2) C(2)-C(3) 1.583(2) C(3)-C(3)b 1.402(2) Bond Angles, deg 97.12(9) O(1)-C(3)-C(3)b 110.30(7) O(1)-C(3)-C(2) 110.29(7) C(1)b-C(1)-C(2) 117.38(13) C(3)-C(2)-C(1) 109.64(11) C(3)b-C(3)-C(2) 122.63(13)

1.391(5) 1.399(3) 1.379(2) 1.375(4)

111.69(9) 122.9(2) 121.48(11) 116.1(2) 122.38(11)

a-c Symmetry operations: (a ) -y + 1, x - y, z; (b) x, y, -z + 1/ ; 2 (c) -x + y + 1, -x + 1, z.

from the adduct (Figure 6b). The PEO (M h w ) 300 000)1 inclusion adduct crystal, as shown in Figure 6c, did not undergo fracturing during observation. Although the overall shape of the crystalline solid was hexagonal, the surface was very rough, and rodlike crystals were trapped within the greater structure. High-quality crystalline PEO-1 adducts, such as those used for single-crystal X-ray diffraction, were formed using low molecular weight PEO (M h w ) 1 000). These crystals were precise and had very smooth surfaces. A reduction in adduct crystal quality with increased guest polymer molecular weight has been reported previously for urea crystallized with poly(tetrahydrofuran).25 Figure 6d suggests that growth of the PEO-1 inclusion adduct occurs in a rodlike manner in which PEO appears to seed the crystallization of 1 along the length of the tunnel axis. A lateral view of these rodlike crystals, as shown in Figure 6e, indicates that the severing of a crystal may result in slippage of individual polymer chains to form coalesced polymer fibers that span the crystal fracture. Over-exposure of these fracture sites to the electron beam of the SEM resulted in heating and subsequent melting of the bridging fibers. Previous research on urea-polyethylene inclusion adducts has produced similar SEM images which suggest bridging of fractured urea with polyethylene fibers.23 Nanocrystalline Inclusion Adducts. Interaction of cis-1,4-polybutadiene (M h w ) 2 000 000-3 000 000) with microcrystals of 1, suspended in refluxing heptane, yielded crystalline inclusion adducts. 1H NMR of this adduct indicated that less than 5% heptane by weight is co-included with cis-1,4-polybutadiene. A DSC thermogram of this adduct showed a sharp crystal melting transition at 328 °C, as indicated in Figure 7b. This

melting point is 78 °C higher than the melting point of pure 1 (250 °C) (Figure 7a). Formation of the cis-1,4polybutadiene adduct occurred in heptane despite the fact that 1 is not soluble in this solvent. This type of adduct formation has previously been observed when PEO is mixed with 1 in water.20 Water neither dissolves 1 nor initiates inclusion adduct formation on its own. PEO dissolved in water presumably penetrates the crystallites of 1 to form an inclusion adduct. Evidence supporting this was provided by the observation that a mixture of 1 with molten PEO (M h w ) 300 000) at 80 °C for 6 h resulted in adduct formation. It is likely that cis-1,4-polybutadiene in heptane behaves in a manner that is similar to PEO in water, by penetrating the lattice of 1 to form an inclusion adduct. However, in this case, cis-1,4-polybutadiene does not initiate adduct formation but simply replaces heptane which can readily escape from the adduct. The small DSC peak near 220 °C in Figure 7a has previously been attributed to a phase transition in pure 1 from a monoclinic (guest free) phase to a mixed monoclinic/hexagonal phase.6 X-ray powder diffraction of pure 1 at 220 °C was carried out to confirm this conclusion (Figure 8b). The resultant X-ray diffractogram indicated the presence of both the hexagonal (Figure 8a) and the monoclinic (Figure 8c) crystal phase structure of 1. The relative peak intensities indicated a predominance of the hexagonal packing pattern at 220 °C. The hexagonal crystal pattern for 1 began to appear as the crystals were heated to 180 °C. The discrepancy between this onset value and that indicated in Figure 7a may be explained by the fact that the heating rate for the X-ray diffraction analysis of 1 was extremely slow compared to the heating rate used for the DSC analysis of this host. Exposure of tris(o-phenylenedioxy)cyclotriphosphazene to solutions containing the polymers listed in Table 5 yielded inclusion adducts with melting points that were higher than those of the corresponding polymers or of pure 1. Polyethylene, trans-1,4-polyisoprene, and cis1,4-polybutadiene inclusion adducts with 1 were formed in refluxing benzene. Because these polymers and 1 are both soluble in benzene it is likely that adduct formation occurred through a crystallization process. This indicates that formation of the polymer adducts is thermodynamically favored over the formation of the benzene adduct. The melting points of the inclusion adducts listed in Table 5 fell within a range of 295 to 334 °C, with the polyethylene-1 adduct having the highest

Structure of Tris(o-phenylenedioxy)cyclotriphosphazene

Chem. Mater., Vol. 11, No. 5, 1999 1249

Figure 6. Scanning electron micrographs of single crystals of (a and b) the benzene-1 inclusion adduct and (c-e) the PEO-1 inclusion adduct.

melting point, and the polytetrahydrofuran-1 adduct the lowest. DSC thermograms of these adducts all indicated single endothermic melting transitions similar to that found for the cis-1,4-polybutadiene-1 adduct shown in Figure 7b. A residual endotherm due to possible melting of the unclathrated polymer or host was not detected. This suggests that the crystalline adducts isolated were fully complexed with polymer and that any unclathrated polymer was removed with the solvent wash. Thermogravimetric analysis indicated a 4% weight loss starting at 141 °C. This suggests that a small

amount of benzene is present with the polymer in the tunnel-like voids. The cis-1,4-polybutadiene-1 inclusion adduct formed from either benzene or heptane showed very similar thermal properties. Thus, polymer-1 adduct formation may be more dependent on the polymer structure than on the solvent used for the clathration procedure. X-ray diffraction patterns for the inclusion adducts listed in Table 5 retained their hexagonal pattern even after these solids had been evacuated at 0.050 mmHg and 220 °C. Under these conditions both heptane and benzene are easily removed from adducts

1250 Chem. Mater., Vol. 11, No. 5, 1999

Allcock et al. Table 6. Polymers That Did Not Form an Inclusion Adduct with 1 poly(propylene) poly(isobutylene) poly(dimethylsiloxane) poly(vinyl alcohol) cis-1,4-poly(isoprene)

poly(acrylonitrile) poly(styrene) poly(methyl methacrylate) poly[bis(2,2,2-trifluoroethoxy)phosphazene]

Figure 7. DSC thermograms (20 °C/min) of (a) pure 1, and (b) cis-1,4-poly(butadiene)-1 inclusion adduct.

Figure 9. Effect of average PE and PEO molecular weight on the melting point of PE-1 and PEO-1 inclusion adducts.

Figure 8. X-ray diffractograms of (a) the benzene-1 adduct (b) pure 1 at 220 °C, and (c) pure 1 at room temperature. Table 5. Melting Pointsa of Some Linear Polymers and of the Corresponding Inclusion Adducts (Tm of pure 1 ) 250 °C) Tm, °C polymers included within 1 poly(ethylene) (MW ) 1 000) cis-1,4-poly(butadiene) (MW ) 2 000 000-3 000 000) poly(ethylene oxide) (MW ) 300 000) trans-1,4-poly(isoprene) (M h w ) 410 000) poly(tetrahydrofuran) (M h n ) 250)

guest polymer

inclusion adduct

109 2b

334 329

65 60c -14c

308 302 295

a Determined by DSC. b In Polymer Handbook; Brandrup, J., Immergut, E. H., Eds.; Wiley: New York, 1989; Vol. 5, p 3. c From Aldrich.

of 1, a procedure that would normally induce a reversion of 1 to a monoclinic host crystal structure. This indicates that the host lattice packing of the adduct is stabilized by presence of polymers within the tunnel-like voids. 1H and 31P NMR spectra for these solids confirmed that no decomposition of the host or the polymer occurred during adduct formation.

Although polyethers and other less bulky macromolecules formed inclusion adducts with 1, polymers such as poly(dimethylsiloxane) and poly[bis(2,2,2-trifluoroethoxy)phosphazene] did not. (Table 6) This apparent selectivity may be a consequence of the relatively small cross-sectional diameter of the tunnels in 1. As shown in Table 2, only a limited expansion of the adduct unit cell for 1 occurs as the small-molecule guest size increases. It is likely that polymer chains, having radial diameters larger than the normal tunnel diameter of 1, are unable to form inclusion adducts with this host. cis-1,4-Polyisoprene did not form an inclusion adduct with 1, but trans-1,4-polyisoprene did. This suggests that separations based on very small changes in polymer microstructure may be possible by using inclusion host compounds such as 1. The melting points for various oligo- and poly-ethylene- and -ethylene oxide inclusion adducts of 1 were determined using a simple MEL-TEMP melting point device. As shown in Figure 9, an increase in adduct melting temperature occurs as the chain length of the included PE and PEO increases. The corresponding curves labeled PEO and PE show the relationship between polymer molecular weight and polymer melting point for PE and PEO before complexation with 1. Very little increase in the pure polymer melting points occur after the chain length of PEO and PE exceeds dp ) 56 (PEO), and dp ) 100 (PE). In a like manner, the melting temperatures of the adducts level off at fairly low guest molecular weights as shown by the curves labeled PEO-1 and PE-1. It seems likely that this logarithmic relationship between adduct melting temperature and

Structure of Tris(o-phenylenedioxy)cyclotriphosphazene

guest molecular weight can be extended to other polymer-1 inclusion adducts. A similar type of adduct melting point/guest molecular weight curve has been reported previously for the poly(ethylene glycol)-urea inclusion adduct.32 Polymer Recovery. A major aspect of this investigation included the characterization of the polymers, cis1,4-polybutadiene and poly(ethylene oxide) (PEO), removed from inclusion adducts of 1. Previous work in our program had shown that various polydienes and polyacrylates synthesized in the tunnels can be removed from inclusion adducts of 1 by simple heating of the adducts in boiling benzene.18,19 Polymer retrieval via this method is probably facilitated by the inability of these polymers to reform adducts with 1 in refluxing benzene. However, this method proved ineffective for the removal of either PEO or cis-1,4-polybutadiene from 1 due to the thermodynamic stability of the PEO and cis-1,4-polybutadiene adducts in refluxing benzene. Therefore, to isolate PEO and cis-1,4-polybutadiene, it was necessary to decompose 1 so that it could not reform an inclusion adduct. In the case of cis-1,4-polybutadiene, this was achieved by the hydrolysis of 1 with water in the presence of Et3N. The reaction of 1 with sodium hydroxide in dioxane results in both P-O and subsequent P-N bond cleavage.33 The initial cleavage of the P-O bonds proceeds quickly to relieve the strain present in the five-membered ring. Hydrolysis of 1 allowed a separation of cis-1,4-polybutadiene from the hydrolyzed products of 1 on the basis of solubility differences. We reported earlier that trans-1,4-polybutadiene formed within inclusion adducts of 1 is recovered using a very similar reaction to decompose 1.16 Poly(ethylene oxide) and the hydrolysis products of 1 are both soluble in water. Therefore, to isolate the PEO after clathration, it was necessary to use an alternative method to decompose the adduct. In this, catechol was employed as a nucleophile in place of water. The reaction of 1 with excess catechol in the presence of Et3N is known to yield the product 2-(o-hydroxyphenoxy)-2,2′spirobi[1,3,2-benzodioxaphosphole] triethylamine salt.9 This solid is insoluble in chlorinated solvents, and thus was separated from the mixture with PEO through simple filtration. The theoretical amount of PEO or cis-1,4-polybutadiene that can be included within 1 was estimated from the hexagonal unit cell structure of 1. Each unit cell of 1 contains one full tunnel segment that is roughly 10 Å long. The total length of tunnel void per unit cell (10 Å) was then multiplied by the number of adduct unit cells that can be generated given a certain weight amount of 1. The amount of linear polybutadiene or PEO needed to match this length was then determined, and was set as the maximum theoretical amount of polymer that can be retrieved. This approximation was made assuming a trans-planar conformation for both polymers. No consideration was given to the effects of end groups or adduct crystal size on this calculation. Approximately 1.21 g of cis-1,4-polybutadiene was recovered from 8.13 g of 1. This corresponds to an approximate 110% tunnel occupation. It is very likely that this value is an (32) Suehiro, K.; Nagano, Y. Makromol. Chem. 1983, 184, 669. (33) Allcock, H. R.; Walsh, E. J. J. Am. Chem. Soc. 1972, 94, 4538.

Chem. Mater., Vol. 11, No. 5, 1999 1251

overestimate due to the fact that polymer segments which extend out from the adduct surface are not taken into account. The percent tunnel occupation of PEO in the PEO-1 adduct was estimated after PEO recovery to be 53%. 1H NMR spectroscopy indicated that a significant amount of water is co-included with PEO within the host lattice voids of 1. This suggests that water competes strongly with PEO for inclusion within 1. Previous work in our group has shown that trace amounts of water present during crystallization of 1 in benzene results in the selective formation of a H2O-1 adduct.7 Reduced incorporation of PEO within 1 may also be attributed to the fact that while the cis-1,4polybutadiene adduct was formed in refluxing benzene, the PEO adduct of 1 was formed in water at room temperature. This may have allowed equilibrium forces to play a greater role in determining the relative amounts of PEO and water included within 1. GPC analysis indicated that adduct formation favored high molecular weight cis-1,4-polybutadiene within a narrow bell-shaped molecular weight distribution (PDI ) 3.00). Pure cis-1,4-polybutadiene not exposed to 1 had an average molecular weight (M h w) of 399 510.34 The average molecular weight (M h w) of the clathrated cis-1,4polybutadiene was 429 219. The molecular weight distribution curve for the clathrated cis-1,4-polybutadiene was shifted toward the high molecular weight range. The average molecular weight of clathrated PEO (M h w) 157 136) was somewhat lower than that of pure PEO (M h w ) 197 102) before clathration with 1. This may indicate that although rigorous measures were taken to prevent it, some degradation of PEO did occur during recovery. Separations. Linear polybutadiene was separated from branched polybutadiene through selective adduct formation with 1. The linear polybutadiene was 99% unsaturated and contained 25% 1,2-addition and 70% 1,4-addition. Minimal reaction of the remaining double bonds within the polymer chains (increased unsaturation) reduced the amount of side-chain growth. It was found through X-ray powder diffraction that the linear “99% unsaturated” polybutadiene (M h w ) 1 500) could be directly imbibed by 1 to form an inclusion adduct, whereas attempts to induce direct imbibition of branched “60% unsaturated” polybutadiene (M h w ) 1 800) into 1 were unsuccessful. Both the 99% and 60% unsaturated polybutadiene samples were phenyl terminated, indicating that end-groups were not a factor in adduct formation. It is likely that the 60% unsaturated polybutadiene, which consisted of 45% 1,2-addition and 15% 1,4-addition, is more branched than the 99% unsaturated polybutadiene. This is supported by the fact that the 60% unsaturated polybutadiene was highly viscous while the 99% unsaturated polybutadiene had a much lower viscosity. The branched polybutadiene did not form an adduct with 1 from a benzene medium. The melting point of the host recovered from the mixture with branched polybutadiene in benzene corresponded to that of pure 1 with benzene as a guest. Moreover, powder X-ray diffraction of this solid after evacuation (34) The determined molecular weight of cis-1,4-polybutadiene was much less than that reported from Aldrich. Regardless of this discrepancy a relative comparison between unclathrated and clathrated cis1,4-polybutadiene could be made.

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Allcock et al.

achieved through hydrolysis of 1 using Et3N and water as described earlier. Approximately 0.33 g of linear polybutadiene was recovered from 4.03 g of 1. The 1H NMR spectra of the recovered linear polybutadiene, shown in Figure 10c, matched that of the linear polybutadiene before clathration. This indicated that isolation of the linear polybutadiene had been achieved. Conclusions

Figure 10. 1H NMR of (a) solid from mixture of 60% unsaturated poly(butadiene) with 1, (b) solid from mixture of 60% and 99% unsaturated poly(butadiene) with 1, and (c) poly(butadiene) extracted from 1.

at 200 °C showed a guest-free monoclinic host packing pattern. As seen in Figure 10a, solution 1H NMR spectra of this material dissolved in CDCl3 showed only the resonances from 1 (at 7.04 and 7.10 ppm). Residual benzene was not detected (Figure 10a) due to the earlier release of this guest from 1 during heating. To determine the ability of 1 to separate linear from branched polybutadiene, 1 was exposed to a solution of both polymers in benzene. Examination of the crystals recovered by filtration showed selective absorption of the linear polybutadiene. Powder X-ray diffraction of the resultant microcrystals showed retention of the hexagonal packing pattern of 1 even after heating of the solid at 200 °C. The melting point of this material was elevated to near 280 °C. Solution 1H NMR spectroscopy of this product dissolved in CDCl3 indicated the presence of both 1 and the linear polybutadiene (Figure 10b). The absence of signals near 5.7 ppm, and the absence of a broad 1H NMR multiplet from 1.8 to 1.0 ppm, which are characteristic of the 1H NMR spectrum for the branched polybutadiene, indicated that no significant amount of the branched polybutadiene had been absorbed. From these results it was concluded that 1 in benzene can separate linear 99% unsaturated polybutadiene from the 60% saturated polybutadiene. Subsequent recovery of the linear polybutadiene from 1 was

We have shown that a range of linear polymers can be incorporated into the tunnels of host 1 to form polymer-1 inclusion adducts. Both poly(ethylene oxide) (PEO) and cis-1,4-polybutadiene form adducts with 1 from solvents in which 1 is insoluble. This may allow the use of 1 for the chromatographic separation of linear from branched oligomers and polymers. The melting points of polymer-1 adducts far exceed the melting points for urea and PHTP-based polymer inclusion adducts. Recovery of the included polymer from 1 was also shown to be feasible but required the hydrolysis of the host to prevent reformation of the polymer-containing inclusion adduct. Further work is currently being carried out to examine the possible uses of 1 for the separation of polymers based on both microstructure and chirality, and as a means to separate block copolymers from homopolymers. Acknowledgment. This work was supported by the Petroleum Research Fund of the American Chemical Society and the N. S. F. Polymers Program. We thank M. Angelone and R. Walsh for their assistance with the JEOL JSM class electron microscopes, and Dr. T. Mallouk (The Pennsylvania State University) for use of the Philips analytical X-ray X’PERT MPD (NSF grant DMR-9402860). We also thank Drs. Fritz Franzon and Fan Li of the Chemistry Department at Iowa State University for performing high temperature powder X-ray diffraction on tris(o-phenylenedioxy)cyclotriphosphazene (1). Supporting Information Available: Crystal data and structure refinement, atomic coordinates, anisotropic displacement coefficients, and observed and calculated structure factors for both structures. This material is available free of charge via the Internet at http://pubs.acs.org. CM980611E