C O MMU N I C A T I O N S
Scheme 1
Scheme 2
of reactivity provide an explanation for the presence of the sp3
carbons observed in the solid-state NMR experiment at 48 ppm.
The inability to obtain intact chains in the MALDI-TOF MS
experiment of poly(o-diethynylbenzene) is also suggestive of cross-
linking. However, there is clearly a substantial amount of nonaro-
matic unsaturation remaining as evidenced by rapid bromine uptake
2 2
(80 wt %) of suspensions of the polymer in CH Cl . (See Supporting
Information.)
spectra showing intact polymer chains, analysis of poly(o-diethynyl-
To conclude, the polymers prepared by a Kumada coupling and
the Bergman cyclization are structurally distinct. Spectroscopic and
spectrometric data indicate that the poly(1,4-dibromonaphthalene)
is a poly(1,4-naphthalene), whereas the poly(o-diethynylbenzene)
contains five-membered rings. In light of these dissimilarities, we
suggest that these latter polymers not be represented as a poly-
benzene) at the same laser power leads to numerous peaks of odd
mass (fragments) consistent with a polymer ablation process.11
Additional insight into the structural differences between the
polymers was obtained with 13C cross polarization magic angle
spinning (CP-MAS) NMR. The poly(1,4-dibromonaphthalene)
showed peaks in the NMR spectrum at 138 (1,4 positions), 133
(1,4-naphthalene).
(4a,8a positions), and 126 ppm (2,3,5,6,7,8 positions). Integration
of these resonances provides a qualitative ratio of 1:1:3. The
observed chemical shifts are comparable in position to those in
R-ternaphthyl: 139 (1,4 positions), 134 (4a,8a positions), and 126
ppm (2,3,5,6,7,8 positions). By contrast, the spectrum of the poly-
Acknowledgment. Financial support from the ACS PRF and
an Oak Ridge Associated Universities Ralph E. Powe Award. We
thank Flint Ink for providing the pyrolysis GC-MS data. J.P.J.
and K.D.L. are fellows of the NSF-sponsored IGERT program for
Molecularly Designed Electronic, Photonic, and Nanostructured
Materials at the University of Michigan.
(o-diethynylbenzene) contained a broad peak at 126 ppm and a
very broad peak at 48 ppm.
Consideration of the mechanism of the diradical polymerization
provides a likely explanation as to why the polymer generated
through this route differs so substantially from the poly(1,4-
dibromonaphthalene). The Bergman cyclization proceeds through
a p-didehydroarene intermediate which, for the case of o-diethynyl-
Supporting Information Available: IR spectral data, TGA traces,
1
3
C CP-MAS NMR spectra, MALDI-TOF MS data, reflectance UV-
vis data, powder X-ray diffraction patterns, and synthetic procedures
PDF). This material is available free of charge via the Internet at http://
(
benzene,12 is 17.8 kcal mol-1 higher in energy than the starting
pubs.acs.org.
diyne. This energy difference ensures that the concentration of the
References
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requires a highly regioselective attack at the 1 position, giving
diradical B, and subsequent attack at the 2′ position to yield naphthyl
(1) Jones, R. R.; Bergman, R. G. J. Am. Chem. Soc. 1972, 94, 660-661.
(
(
(
2) John, J. A.; Tour, J. M. J. Am. Chem. Soc. 1994, 116, 5011-5012.
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(
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residue C (Scheme 1). However, intermediate B can cyclize on
(6) For heterocylic examples, see: Prasanna, K.; Perera, U.; Krawiec, M.;
the 1′ carbon to yield a benzofulvenyl residue (F).1
3,14
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By contrast,
(
(
(
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2
354.
diradical adds a benzofulvenyl residue to give E. Intermediates F
and E are consistent with five-membered ring species observed in
the pyrolysis GC-MS experiment of the poly(o-diethynylbenzene).
Scheme 1 is reasonable, provided that cyclization of B and D to
C, F, and E occurs faster than the intermolecular addition of a
radical to a carbon-carbon triple bond. However, during the
reaction the monomer is present in high concentration and will be
able to react with a growing polymer chain before B and D can
cyclize, leaving an unreacted ethynyl group appended to the chain
9) NIST/EPA/NIH Mass Spectral Library with Search Program, 2000 Edition,
Standard Reference Database, NIST, Department of Commerce, Gaithers-
burg, MD.
(
(
(
10) Bromine termination had been assumed in the original report. This suggests
n
that the average x of 38, derived from residual halogen analysis, may
have been an overestimation.
11) Laser Fourier transform mass spectrometry for polymer characterization;
Brenna, J. T., Creasy, W. R., Zimmerman, J., Eds.; Advances in Chemistry
Series 236; American Chemical Society: Washington, DC, 1993.
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(13) Matzger, A. J.; Vollhardt, K. P. C. Chem. Commun. 1997, 1415-1416.
(
(
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15) The reported PDIs for other “Bergman polymers” (refs 2 and 3) tend to
be larger than the theoretical limiting value of 2 for a step-growth
mechanism. This is suggestive of cross-linking.
(Scheme 2). This accounts for the terminal triple bond observed in
the IR spectrum of this polymer. Moreover, cross-linking could
occur by addition to this unsaturation or to activated double bonds
in the benzofulvene units or polymer backbone.15 These latter modes
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J. AM. CHEM. SOC.
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