Polyxylylenes are thermoplastics used as encapsulants for electronic devices. Five polyxylylenes were prepared by pyrolysis of [2.2]paracyclophanes and characterized by solid state {sup 13}C NMR spectroscopy. The chemical shift data, in combination with interrupted decoupling experiments, allowed assignment of resonances to their carbon sources in the polymers. This confirmed the integrity of the xylylene building block in the polymers and is consistent with linear polymers. No crosslinking could be detected within the NMR sensitivity limits. Residual paracyclophane was detected by {sup 13}C CP MAS NMR spectroscopy in the polyxylylene samples prepared at room temperature; however discrete {sup 13}C resonances due to amorphous and crystalline phases in the polymers were not resolved

Assink, R. A.

Jamison, G. M.

Loy, D. A.

McNamara, W. F.

Prabakar, S.

Schneider, D. A.

UNT Digital Library

Characterization of Polyxylylenes with Solid State 13C Nuclear Magnetic Resonance Spectdoscopy. Douglas A. Loyt*, Roger A. Assink?, Gregory M. Jamisont, W. Frere McNamarat, S. PrabakarS, and Duane A. Schneidert. t Sandia National Laboratories, Albuquerque, NM 871 851407 $ Advanced Materials Laboratory, University of New Mexico, Albuquerque, NM 8 Introduction. Polyxylylenes are an important class of thermoplastics used as encapsulants for electronic devices.1 They are most conveniently prepared from [2.2]paracyclophanes by a vapor deposition polymerization technique known as the Gorham Process (Scheme 1).2 In this process, [2.2]paracyclophane is sublimed into a furnace where it is thermally cleaved into two equivalents of xylylene m ~ n o m e r . ~  The end result of the Gorham process is a solvent-free film composed of linear, highly crystalline, high molecular weight p01ymer.~ Polyxylylene and chloro-substituted polyxylylenes are crystalline enough to result in insolubility up to the melt temperatures (300 "C). Solvent and chemical resistance, in combination with the ability of vapor deposition polymerization to evenly coat irregular surfaces with polymer, makes pollyxylylenes a valuable class of encapsulants. - m 2 @- 650 "C \ /  R R Scheme 1. Formation and polymerization of xylylene monomers 1-5 from paracyclophanes. These same characteristics also make polyxylylene and poly-haloxylylenes difficult to characterize by analytical techniques, such as solution nuclear magnetic resonance (NMR) spectroscopy. Solid state Cross Polarization Magic Angle Spinning (CP MAS) NMR spectroscopy is a convenient and routine alternative to x-ray diffraction techniques used to characterize molecular structure of both types of materials.5 We became interested in structurally characterizing polyxylylene materials as part of our study of fullerene-xylylene copolymers.6 It was noted during this investigation that l3C NMR spectra had not been previously reported for any of the polyxylylenes. Here we describe the preparation of polyxylylene P1, poly-2- ethylxylylene P2, poly-2-chloroxylylene P3, polydichlorox ylylene P4, and poly(a, a, a', a ' - te t ra f lu~roxyly lene)~  P5 (Figure 1) by the pyrolysis of their respective [2.2]paracyclophanes and their characterization using solid state 13C CP MAS NMR spectroscopy. DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor my agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracj, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark @4 6: i, or otherwise does not necessarily constitute or imply its endorsement, recom- or favoring by the United States Government or any agency thereof. The views of authors expressed herein do not nmssady state or reflect those of the Government or any agency thereof. D B T R I m  m s  DOCUMENT 1s I - ~-~ DISCLAIMER Portions of this document may be illegible in electronic inrage products. fmnbes are produced from the best a-le oFiginal dolvment Et Q P1 Q P4 P2 P5 P3 Figure 1. Polyxylylenes (Pl-P5) prepared by pyrolysis of [2.2]paracyclophanes and characterized by solid state l3C NMR spectroscopy. Experimental General. [2.2]paracyclophane, dichloro[2.2]paracyclophqne, tetrachloro~2.2]paracyclo- phane, and diethyl[2.2]paracyclophane, were obtained from Union Carbide and used as received. a, a, a', a'-Octafluoro[2.2]paracyclophane was obtained from Professor William R. Dolbier, Jr. at the University of Florida. Infrared analyses of the films were performed in a Perkin-Elmer 1750 Fourier Transform Ilpfrared spectrometer. Solution l3C NMR spectra of [2.2]paracyclophane precursors were obtained in deuterochloroform with a Bruker AM 200 MHz spectrometer. Solid state 13C NMR spectra were obtained at 50.19 MHz with a Chemagnetics console interfaced to a Nicolet 1280 data station. Samples were spun between 4-6 kHz. Chemical shifts are relative to a hexamethylbenzene standard set to 17.2 ppm for the methyl carbons. Signal precision in the solid state NMR spectra was k 0.1 ppm for most resonances. Spinning side bands in the spectral data are abbreviated as ssb. Interrupted decoupling experiments were performed by inserting a 60-90 ps delay between cross polarization and acquisition. Polymer Synthesis. Paracyclophanes were sublimed (or distilled) into an evacuated quartz tube heated to 650 "C (0.5-1 mm Hg) and the xylylene monomers generated were condensed/polymerized on glass at either room temperature, cooled to 0 "C with an ice bath or cooled to -78 "C with a dry ice-acetone bath. The films could be peeled from the glass and were dried under dynamic vacuum for 18 hours at room temperature. Thick films were opaque and generally white, though polyxylylene films were occasionally tinted grey-green. Thin films prepared at room temperature were transparent. Results. Thermolysis of paracyclophanes in a flow-through pyrolysis apparatus afforded thin films of polyxylylenes. In all cases, some of the monomer was converted into a film of carbon lining the inside of the quartz tube; the majority passed through the hot zone of the tube to condense and polymerize on glass surfaces. Yields of the polymers based on monomer ranged between 20-50% and were based only on material that condensed on the inner surface of a receiving flask placed in a controlled temperature bath. Flexible polyxylylene films were peeled from the glass surface on which they had formed and were characterized by infrared and 13C CP MAS NMR spectroscopies. The infrared spectra of the polyxylylenes Pl-P4 are similar to reported spectra-lc The solution 13C NMR spectrum of [2.2]paracyclophane (Figure 2A) was obtained to provide a reference for the solid state l3C CP MAS NMR spectra of the insoluble polyxylylenes. The solution l3C NMR spectrum was composed of three signals representing the substituted (139.6 ppm) and unsubstituted (133.0 ppm) aromatic and benzylic methylene (35.7 ppm) carbons.* The solid state 1’3C CP MAS NMR spectrum of polyxylylene P1 (Figure 2B) also displayed three analogous, but somewhat broad peaks for the substituted (140 ppm) and unsubstituted (133 ppm) aromatic and benzylic methylene (40 ppm) carbons. No end-group methyls (23 ppm) were observed in any of the polyxylylenes, indicating that the polymers were not oligomeric. Interrupted decoupling experiments with P1 were used to confirm the peak assignments of the aromatic carbons (Figure 3). With the insertion of a 90 ps delay between cross polarization and acquisition, the carbon nuclei are exposed to the dipole interactions of the surrounding protons. Resonances of canbon nuclei directly bonded to a proton are broadened and attenuated more than the resonances of carbon nuclei not directly bonded to a proton. The benzylic methylene at 40 ppm disappeared entirely and the peak (CH aromatic) at 128.8 ppm was sharply attenuated. The resonance at 140 ppm (C aromatic) was attenuated far less than the other two peaks and became the dominant peak in the spectrum. In some samples of polyxylylene, shoulders on the aromatic peaks were observed that were determined to be due to entrapped paracyclophane. Soxhlet extraction of polyxylylene (P 1) with benzene recovered considerable [2.2]paracyclophane (14.2% by weight). In addition, when a, a’-13C enriched polyxylylene was washed with benzene and re-examined by 13C CP MAS NMR, the subsequent spectrum of the polymer revealed that the shoulder on the benzylic carbon resonance had disappeared. The solid state spectra for the functionalized polyxylylenes P2-P4 were similar to that of the unsubstituted polyxylylene P 1 with broad overlapping resonances in the aromatic region and the benzylic methylene appearisg near 40 ppm- The ethyl- substituted P2 also displayed additional carbon peaks at 24 and 16 ppm. Interrupted decoupling generally eliminated the benzylic methylene completely while the attenuation of peaks in the aromatic region resulted in either simplification of the spectra or the appearance of previously obscured peaks (P3 & P4). Two aromatic carbon resonances were observed in the solid state 13C NMR spectrum of polytetrafluoroxylylene (P5) at 133.9 and 127.3 ppm- The lack of a 13C NMR peak for the benzylic methylene carbons is not unexpected as the two fluorine substitiuents are not decoupled from the benzylic carbon, therefore dipole-dipole coupling eliminates the resonance for the fluorinated methylenes.10 Interrupted decoupling strongly attenuated the 127.3 pprn peak (CH aromatic) leaving the peak at 134.2 ppm (C aromatic) relatively unchanged. Conclusions Polyxylylenes Pl-P5 were successfully characterized using solid state 13C CP MAS NMR spectroscopy. The 13C NMR chemical shift data, in combination with the results of interrupted decoupling experiments, allowed assignment of resonances to their respective carbon sources in the polymers. These NMR data confirmed the integrity of the xylylene building block in the polymers, and are consistent with linear polymers. No evidence of crosslinking could be detected within the sensitivity limits of the NMR. Residual paracyclophane was detected by 13C CP MAS NMR spectroscopy in the polyxylylene samples prepared at room temperature. However, discrete 13C resonances due to amorphous and crystalline phases in the polymers were not resolved. Table. l3C NMR and Interrupted Decoupling Chemical Shifts for polyxylylenes. Polymer 13C NMR Resonances (ppm) Interrupted Decoupling (ppm) Aromatic Aliphatic Aromatic Aliphatic P1 141.4, 139.7, 39.8 141.4, 140.2, ---- 128.9 129.4 P2 141.4, 139.7, 39.8,24.0, 16.0 ---- ---- 128.9 P3 142.3, 137.4, 36.9 142.3, 137.1, ---- 134.3, 130.5 134.8 P4 139.0,132.5 33.7 138.8, 135.0 ---- P5 133.9, 127.3 ---- 134.2 ---- Acknowledgments We would like to thank Professor William R. Dolbier, Jr. at the University of Florida for the sample of octafluoroparacyclophane. We would also like to thank David R. Wheeler at Sandia for his valuable comments and discussions. This work was supported by the United States Department of Ehergy under contract #DE- AC04AL8 5 OOO. References 1. For reviews of polyxylylene and xylylene chemistry see: a) Ereede, L. A.; Swarc, M. Q. Rev. Chem. SOC. 1958,12,301. b) Lee, S. M. Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed., Vol 24, John Wiley & Sons, New York, 1983, 744-771. c) Mulvaney, J. E. Pulym. Sci. Technul. 1984,25. d) Beach, W. F.; Lee, C.; Bassett, D. R.; Austin, T. M.; Olson, R. Encyclopedia of Polymer Science and Engineering, 2nd ed., Vol 17, John Wiley & Sons, New York, 1989,990-1025. 2. Gorham, W. F., U. S. Pat. 3,342,754 (Sept 19, 1967). 3. The driving froce for ring opening is the strain energy of the [2.2]paracyclophane which was determined to be 138 kJ/mol. See Hopf, H.; Marquard, C. in Strain and Is Implications in Organic Chemistry (A. de Meijere and S. Blechert, Eds.) Kluwer, Dordrecht, 1989, p. 297. 4. Gorham, W. F. Am. Chem. Soc. Polym. Chem. Preprints 1965,6,73. 5 a) Koenig, J. L. Spectroscopy of Polymers, American Chemical Society, Washington, D.C., 1992, Chapters 8-9. b) High Resolution NMR Spectroscopy of Synthetic Polymers in Bulk, R. A. Komoroski, Ed., VCH Publishers, Deerfield Beach, Florida, 1986. 6. Loy, D. A.; Assink, R. A. J. Am. Chem. SOC. 1992,114,3977-78. 7. Dolbier, W. R., Jr.; Asghar, M. A.; Pan, H. Q.; Celewicz, L. J. Org. Chem. 1993,58, 1827. 8. 13C NMR: 6 139.4 (C aromatic), 132.8 (CH aromatic), and 35.7 (methylene) ppm from Takemura, T.; Sato, T. Can J .  Chern. 1976,54,3412. 9. Review: Vogtle, F.; Newmann, P. Top. Curr. Chem. 1974,48,67. 10. Slichter, C. P. Principles of Magnetic Resonance, 3rd ed., Springer-Verlag, Berlin, 1990, p 65. a 4 b A C Y C a I I I I I I I I 1 160 140 120 100 80 60 40 20 0 B b' 1 1 a' C' Figure 2. Solution 13C NMR spectrum of [2.2]paracyclophane (A) and solid state 13C CP MAS NMR spectrum of polyxylylene (B) prepared by pyrolyzing [2.2]paracyclophane at 650 "C. The three peaks at 77.00 ppm in A are due to solvent and the asterisks mark spinning side bands in B. * a Interrupted Decoupling a b * * v \  I I I I I I 1 I I I I I I I I 0 PPm I 200 100 Figure 3. The l3C CP MAS NMR spectrum of polyxylylene P1-(-78) obtained with interrupted decoupling. Peaks due to carbon nuclei with hydrogens attached (b) are attenuated more than those without hydrogens (a). The resonance due to the methylene carbons (c) at 38 ppm has disappeared entirely (the asterisks mark spinning side bands). 

Characterization of polyxylylenes with solid state {sup 13}C nuclear magnetic resonance spectroscopy

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Characterization of polyxylylenes with solid state {sup 13}C nuclear magnetic resonance spectroscopy

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