the chiral conformation generated during its synthesis. When a
THF solution of the polymer was cooled to 5 uC, a slight increase
of the optical activity was observed, implying the kind of
behaviour exhibited by (R)-2. However, warming the solution up
to 55 uC resulted in a striking decrease in the intensity of the
Cotton effects, which was irrerversible when the solution was
cooled back to 25 uC.
When a THF solution of (R)-4 was maintained at 55 uC for 2.5 h
a gradual loss of optical activity was observed. The Cotton effects
at 283 and 252 nm after 30 min have molar absorptivities of 2.00
and 25.40, respectively, while at the end of the period they are 1.16
and 24.23, respectively. There were no detectable changes in the
UV-vis absorption spectra, and the 13C NMR spectra before and
after heating are identical, so there seems to be no chemical
change.
All this data implies that an irreversible conformational
change of the polymer backbone takes place in which the
kinetically determined chiral secondary structure adopted during
the course of the polymerisation of the precursor monomer is
lost to diastereomeric conformations that cannot revert back to the
original optically active one. This situation is not in accord with a
stable helical form, almost certainly determined by the conforma-
tion that the 2-octyl chain is forced to adopt in the presence of
the nitro group at the 2-position relative to it, and may even
be related with the dipole moment associated with this
conformation.9
Fig. 1 Variable temperature CD spectra of (R)-2 in THF.
and returned to 25 uC, the CD spectrum was absolutely identical.
This observation is consistent with a rigid secondary structure, that
largely accepted for the poly(isocyanide)s, which ‘‘breathes’’ upon
heating and cooling.
In stark contrast to polymer (R)-2, polymer (R)-4 does not show
the typical CD spectrum of a chiral aromatic poly(isocyanide).
Rather, a very weak negative Cotton effect is observed at around
360 nm, followed by a strong positive signal at 283 nm, and a
further strong negative Cotton effect at 252 nm (Fig. 2). The
monomer shows a negligible optical activity in these areas, and
therefore the chirality has been amplified during the polymerisa-
tion. However, the position of the Cotton effects strongly suggest
that the secondary structure of this polymer is not the usual one in
this family of macromolecules.
While the kind of irreversible conformational change presented
by the polymer is not unique,10 it is the first observation of this
kind of behaviour in aromatic poly(isocyanide)s, and is indicative
of a metastable secondary structure, and confirms that the as-
formed poly(isocyanide) is a kinetically-determined product.
The results presented show how a remote stereoelectronic effect,
the slight change introduced by the presence of the nitro group
˚
located about 10 A away from the isocyanide carbon atom in the
monomer, can dramatically change not only the secondary
structure of the polymer, but also its conformational stability.
This work was supported by grants from the DGI, Spain
(Project No. BQU 2003-00760), DGR, Catalonia (Project
2001SGR00362) and CICYT, Spain (Project No. MAT2003-
07806-CO2-01). We warmly thank Carmen Artal for the precursor
to (R)-5 and Teresa Sierra for helpful comments.
Furthermore, (R)-4 displays a dramatic and irrecoverable loss of
optical activity associated with a low thermodynamic stability of
David B. Amabilino,*a Jose´-Luis Serranob and Jaume Vecianaa
aInstitut de Cie`ncia de Materials de Barcelona (CSIC), Campus
Universitari, 08193-Bellaterra, Catalonia, Spain.
E-mail: amabilino@icmab.es; Fax: +34 93 580 5729; Tel: +34 93 580 1853
bInstituto de Ciencia de Materiales de Arago´n (CSIC), Universidad de
Zaragoza, 50009, Zaragoza, Spain. E-mail: joseluis@unizar.es;
Fax: +34 976 761209; Tel: +34 976 761209
Notes and references
1 F. Millich, J. Polym. Sci., Macromol. Rev., 1980, 15, 207; R. J. M. Nolte,
Chem. Soc. Rev., 1994, 23, 11; J. J. L. M. Cornelissen, A. E. Rowan,
R. J. M. Nolte and N. A. J. M. Sommerdijk, Chem. Rev., 2001, 101,
4039.
2 A. J. M. van Beijnen, R. J. M. Nolte, W. Drenth and A. M.
F. Hezemans, Tetrahedron, 1976, 32, 2017; A. J. M. van Beijnen, R. J.
M. Nolte, A. J. Naaktegeboren, J. W. Zwikker, W. Drenth and A. M.
F. Hezemans, Macromolecules, 1983, 16, 1679; P. C. J. Kamer, R. J.
M. Nolte and W. Drenth, J. Am. Chem. Soc., 1988, 110, 6818–6825;
P. C. J. Kamer, M. C. Cleij, R. J. M. Nolte, T. Harada, A. M.
F. Hezemans and W. Drenth, J. Am. Chem. Soc., 1988, 110, 1581–1587;
Fig. 2 Variable temperature CD spectra of (R)-4 in THF.
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 322–324 | 323