An unprecedented cation radical chain Diels-Alder polymerization leading to novel carbazole polymers

Article abstract of DOI:10.1016/S0040-4039(00)02301-7

The polymerization of 3,6-bis(trans-1′-propenyl)-N-phenylcarbazole in the presence of tris(4-bromophenyl)aminium hexachloroantimonate (1^+·) leads to soluble, high molecular weight, thermally stable cycloaddition polymers containing carbazole units in the main polymer chain. The reaction appears to proceed via a highly efficient cation radical chain mechanism which circumvents the usual hole transfer step of the propagation cycle. This polymerization represents the first observation of direct cation radical Diels-Alder cycloaddition polymerization.

Full text of DOI:10.1016/S0040-4039(00)02301-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 1437–1439
An unprecedented cation radical chain Diels–Alder
polymerization leading to novel carbazole polymers
Nathan L. Bauld* and Yeonsuk Roh
Department of Chemistry and Biochemistry, The University of Texas, Austin, TX 78712, USA
Received 25 September 2000; accepted 11 December 2000
Abstract—The polymerization of 3,6-bis(trans-1%-propenyl)-N-phenylcarbazole in the presence of tris(4-bromophenyl)aminium
+
’
hexachloroantimonate (1 ) leads to soluble, high molecular weight, thermally stable cycloaddition polymers containing carbazole
units in the main polymer chain. The reaction appears to proceed via a highly efficient cation radical chain mechanism which
circumvents the usual hole transfer step of the propagation cycle. This polymerization represents the first observation of direct
cation radical Diels–Alder cycloaddition polymerization. © 2001 Elsevier Science Ltd. All rights reserved.
Cation radical chain cycloaddition polymerizations
have only recently been observed.1 The examples thus
far reported exclusively involve cyclobutanation as the
initial cycloaddition mode for monomer attachment.
We now wish to report the first example of a highly
efficient and direct cation radical chain Diels–Alder
polymerization. This polymerization is of special inter-
est in that one of the double bonds of the conjugated
diene system is part of an aromatic ring, and the
resulting polymers contain carbazole residues in the
main polymer chain, a type of polymer which is of
particular interest for applications in optoelectronic
devices.2
The chemistry of a monofunctional analogue, 3-(trans-
1%-propenyl)-N-phenylcarbazole (2),3 provides the nec-
essary foundation for understanding the polymerization
mechanism. When 2 was treated in dichloromethane
solution at 0°C with 1+ (15 mol%) for 1 minute,
’
followed by quenching with excess saturated methano-
lic potassium carbonate, aqueous workup and column
chromatography, a 34% yield of the mixture of
diastereoisomeric dimers (3) was obtained (Scheme 1).
The structure of these dimers was established by 1H and
13C NMR and H–H correlation spectra, as well as by
LRMS and HRMS.4 The same dimer mixture was
obtained in 43% yield when the reaction was carried
Scheme 1.
Keywords: cation radical; polymerization; carbazole; chain mechanism; Diels–Alder.
* Corresponding author.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)02301-7

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N. L. Bauld, Y. Roh / Tetrahedron Letters 42 (2001) 1437–1439
out in the presence of 2,6-di-tert-butylpyridine, thus
precluding the possibility of an acid catalyzed or other
carbocation-mediated process.5 Several close precedents
already exist for analogous reactions in which, in an
overall sense, cation radical Diels–Alder reactions
involving addition to a double bond of an aromatic
ring are observed.6 These reactions are generally con-
sidered to be stepwise in nature, proceeding via distonic
cation radical intermediates which subsequently cyclize
to the ortho position of the aromatic ring (Scheme 2).
The aromaticity of the ring is then restored via pro-
cesses which are not yet well defined.
with those of 3.8 When the polymerization of 4 was
carried out in the presence of 10 mol% of 2 (to cap the
polymer at a lower molecular weight), the resulting
polymer was found to have a weight average molecular
weight of 62 100, and the NMR spectrum could then
be conveniently observed at ambient temperatures.
Finally, even when the polymerization process was
carried out with a 2:1 ratio of 2:4, an 85% yield of
oligomers (average nonamers) was formed, providing
further evidence that the polymerization is indeed
occurring via a chain growth process. The proton
NMR spectrum of these oligomers is virtually identical
to that of the high molecular weight polymer, except
for line width. In addition to these oligomers, a small
amount (10% yield) of the 2:1 adduct (2:4) was isolated
from this mixture, and the NMR spectrum of this
‘trimer’ proved to be virtually identical in the aliphatic
region to that of the dimer 3.
We then synthesized and polymerized 3,6-bis(trans-1%-
propenyl)-N-phenylcarbazole (4; Scheme 3).7 When this
monomer was treated in dichloromethane solution at
0°C with 1+ (15 mol%) polymerization occurred
’
rapidly. Quenching with methanolic potassium carbon-
ate followed by
a
typical two phase water/
dichloromethane workup gave a glassy polymer which
was highly soluble in dichloromethane and other
organic solvents. This polymer was found to have MW
It is significant that, notwithstanding the rather modest
yields of dimers which are obtained in the reaction of
the monofunctional analogue 2, the polymerization of 4
is extremely rapid and highly efficient in generating
Diels–Alder type linkages. It is considered to be a key
to the efficiency of cation radical cycloaddition poly-
merizations that the hole transfer which is required for
the neutralization of the adduct cation radicals and
which, in the case of monofunctional substrates, is
required to be intermolecular, can be intramolecular
and therefore potentially much faster in the case of
difunctional molecules. The special efficiency observed
in the present polymerizations appears to derive from
the circumstance that even intramolecular hole transfer
is circumvented, because the adduct cation radical, as it
1
106 200 (PDI 3.1). The H NMR absorptions of the
polymer in CDCl3 solution were unusually broad, so
that the NMR spectrum was examined at 50°C. The
polymer has no detectable traces of absorptions (methyl
and olefinic) corresponding to unreacted propenyl end
groups, indicating that both propenyl groups of the
monomer have reacted essentially completely. Cycload-
dition polymerization is thus indicated, since linear
polymerization of a difunctional monomer would nec-
essarily generate an insoluble, network polymer. In the
aliphatic region, the chemical shifts of the NMR
absorptions of the polymer correspond very closely
Scheme 2.
Scheme 3.

N. L. Bauld, Y. Roh / Tetrahedron Letters 42 (2001) 1437–1439
1439
3. Prepared from N-phenylcarbazole by (1) monoacylation
with propionyl chloride/AlCl3 (59%), (2) reduction with
sodium borohydride in ethanol/THF (99%), and (3) dehy-
dration by phosphorus oxytrichloride/pyridine (91%).
4. The chemical shift assignments for the two major dimers
were derived from H–H COSY spectra. Only the crucial
assignments for the protons attached to saturated carbon
are given here: Isomer A: l 4.63 (Ha, d, J=6.2), 2.94 (He,
ddd, J=6.8, 5.0, 1.8), 2.34 (Hb, dq, J=6.6, 6.8), 1.92 (Hd,
m), 1.80 (Hc, tq, J=7.4, 6.6), 1.43 (Hf, d, J=6.8), 1.03 (Hg,
d, J=6.0); Isomer B: l 4.78 (Ha, d, J=4.2), 3.4 (He, ddd,
J=6.5, 4.8, 1.8), 2.79 (Hb, dq, J=4.2, 2.6), 1.92 (Hd, m),
1.70 (Hc, tq, J=7.4, 5.0), 1.21 (Hf, d, J=7.0), 1.03 (Hg, d,
J=6.0). The aromatic protons of both isomers absorbed in
the range of l 7.1–8.12 and were very similar to those of
N-phenylcarbazole except that the C4 proton of the aryl
substituent (Ar in Scheme 1) is a singlet, and the C4 proton
of the other carbazole ring is missing.
Scheme 4.
is initially formed, is presumably already delocalized, in
significant part, upon the reactive propenyl moiety
(Scheme 4). Propagation of the chain by reaction at this
propenyl moiety is therefore feasible.
5. Gassman, P. G.; Singleton, D. A. J. Am. Chem. Soc. 1984,
106, 7993–7994.
Acknowledgements
6. (a) Asanuma, T.; Yamamoto, M.; Nishijima, Y. J. Chem.
Soc., Chem. Commun. 1975, 608–609; (b) Kadota, S.;
Tsubono, K.; Makino, K.; Takeshita, M.; Kikuchi, T.
Tetrahedron Lett. 1987, 28, 2857–2860; (c) Wilson, R. M.;
Dietz, J. G.; Shepherd, T. A.; Ho, D. M.; Schnapp, K. A.;
Elder, R. C.; Watkins, J. W.; Geraci, L. S.; Campana, C.
F. J. Am. Chem. Soc. 1989, 111, 1749–1754.
The authors wish to acknowledge the Robert A. Welch
Foundation (F-149) for support of this work.
References
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the same manner as the monopropenyl derivative, i.e.
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8. 1H NMR (500 MHz, CDCl3, 50°C) l 4.8–4.6 (br), 3.5–3.3
(br), 2.9–2.6 (br), 2.3–2.2 (br), 1.9–1.6 (br), 1.4–0.9 (br).
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.