7206 Communications to the Editor
Macromolecules, Vol. 38, No. 17, 2005
Cooperative behavior is expected when ∆Gr is large or
the temperature low enough that it appears large.11 This
energy is about 4 kcal/mol for polyisocyanates11 and on
the order of 1 kcal/mol for bulky poly(propiolic ester)s.22
In summary, we have shown that the cis-transoidal
PPA helix undergoes irreversible structural changes
related to 6π electrocyclization of 1,3-cis-5-hexatriene
sequences in the polymer backbone. This process com-
plicates the investigation of chiroptical properties in
solution and in bulk. Since many of the anticipated
applications of PPAs hinge upon the helical conforma-
tion and its dynamics, it is important to elucidate this
process and it implications. We observe no electronic or
steric effects on intramolecular cyclization due to para-
substituents. As an alternative to elevated temperature
experiments, we show that chiroptical studies as a
function of copolymer composition provide the desired
qualitative picture. The results reported here demon-
strate that literature data on helical PPA are only
qualitative since no complete structural analysis was
reported in any previous publication.
Figure 2. (a) Plot of molar ellipticity for cis-poly[4-(menthO2C)-
PA] (CCl4, 373 nm, red) and cis-(-)-poly[4-(menthO2CN)PA]
(THF, 381 nm, blue) samples annealed at 60 °C. (b) Repre-
sentative CD spectra obtained at 0 and 8 h. Spectra were
recorded at 2 °C.
Acknowledgment. Financial support by the Na-
tional Science Foundation (DMR-01-02459) is gratefully
acknowledged.
Figure 3. Plots of (a) specific optical rotation ([R]D, 9) and
(b) molar ellipticity ([θ]) at 2 (O) and 22 °C (2) as a function of
the mole fraction of chiral monomer (F*) in cis-(-)-poly[4-
(menthO2C)PA-co-PA].
Supporting Information Available: Text giving synthe-
sis and characterization of monomers and polymers and details
of annealing experiments, a reaction scheme, and figures
showing CD and UV-vis spectra. This material is available
CD and UV-vis spectra confirm that electrocycliza-
tion results in an irreversible and diminished helical
character before impacting the conjugation length of the
polymer backbone. Figure 2 illustrates the decreased
intensity of the negative Cotton effect associated with
the polyene backbone. It is important to stress that this
outcome may be particular to these specific PPAs and/
or the conditions employed. Since 6π electrocyclization
is a stereospecific reaction,19 the newly formed stereo-
centers adjacent to the polyene backbone can either
reinforce or counteract the helical handedness selected
by the para-substituent. In addition to cyclization and
cis-trans isomerization,2,16 the amount of single-handed
helical character decreases at 60 °C. Both of these
events contribute to the stereochemical diversity of the
cyclization products.
References and Notes
(1) Simionescu, C. I.; Percec, V.; Dumitrescu, S. J. Polym. Sci.,
Polym. Chem. Ed. 1977, 15, 2497-2509.
(2) Simionescu, C.; Percec, V. Prog. Polym. Sci. 1982, 8, 133-214.
(3) Nakano, T.; Okamoto, Y. Chem. Rev. 2001, 101, 4013-4038.
(4) Yashima, E.; Maeda, K.; Nishimura, T. Chem.sEur. J. 2004,
10, 42-51.
(5) Morino, K.; Maeda, K.; Okamoto, Y.; Yashima, E.; Sato, T.
Chem.sEur. J. 2002, 8, 5112-5120.
(6) Cornelissen, J. J. L.; Rowan, A. E.; Nolte, R. J. M.;
Sommerdijk, N. A. J. M. Chem. Rev. 2001, 101, 4039-4070.
(7) Aoki, T.; Kokai, M.; Shinohara, K.-i.; Oikawa, E. Chem. Lett.
1993, 2009-2012.
(8) Yashima, E.; Matsushima, T.; Okamoto, Y. J. Am. Chem.
Soc. 1997, 118, 6345-6359.
(9) Yashima, E.; Maeda, K.; Okamoto, Y. Nature 1999, 399,
449-451.
As an alternative to the thermal analysis typically
used to examine helix dynamics, we show that sergeant-
and-soldiers20 experiments provide similar insight. The
advantage of this approach is that they minimize the
impact of cyclization because the polymers spend shorter
times in solution and lower temperature experiments
are most informative. Cis-transoidal copolymers of (-)-
4-(menthO2C)PA and PA have been prepared using a
[M1 + M2]/[Rh] ratio of 50:1. The mole fraction of chiral
(10) Schenning, A. P. H. J.; Fransen, M.; Meijer, E. W. Macromol.
Rapid Commun. 2002, 23, 265-270.
(11) Okamoto, N.; Mukaida, F.; Gu, H.; Nakamura, Y.; Green,
M. M.; Andreola, C.; Peterson, N. C.; Lifson, S. Macromol-
ecules 1996, 29, 2878-2884.
(12) Shinohara, K.-I.; Yasuda, S.; Kato, G.; Fujita, M.; Shigeka-
wa, H. J. Am. Chem. Soc. 2001, 123, 3619-3620.
(13) Shinohara, K.-i.; Kitami, T.; Nakamae, K. J. Polym. Sci.,
Part A: Polym. Chem. 2004, 42, 3930-3935.
(14) Moore, J. S.; Gorman, C. B.; Grubbs, R. H. J. Am. Chem.
Soc. 1991, 113, 1704-1712.
(15) Percec, V.; Rudick, J. G.; Nombel, P.; Buchowicz, W. J.
Polym. Sci., Part A: Polym. Chem. 2002, 40, 3212-3220.
(16) Percec, V.; Rudick, J. G. Macromolecules, 2005, 78, 7241.
(17) Langner, A.; Ehrlich, P. Macromolecules 1990, 23, 2203-2210.
(18) Percec, V. Polym. Bull. (Berlin) 1983, 10, 1-7.
(19) Woodward, R. B.; Hoffmann, R. J. Am. Chem. Soc. 1965,
87, 395-397.
(20) Green, M. M.; Reidy, M. P.; Johnson, R. J.; Darling, G.;
O’Leary, D. J.; Wilson, G. J. Am. Chem. Soc. 1989, 111,
6452-6454.
(21) Yashima, E.; Huang, S.; Matsushima, T.; Okamoto, Y.
Macromolecules 1995, 28, 4184-4193.
(22) Nomura, R.; Fukushima, Y.; Nakako, H.; Masuda, T. J. Am.
Chem. Soc. 2000, 122, 8830-8836.
1
monomer in the polymer (F*) was determined by H
NMR spectroscopy.
Figure 3 presents the Cotton effect intensity at 2 and
22 °C and optical rotation as a function of copolymer
composition. The linear correlation between response
and the amount of chiral comonomer indicates a dilution
effect upon increasing the amount of PA in the copoly-
mer. This is expected to occur when the excess energy
of converting a helical conformation to a helical reversal
(∆Gr) is small.11 Consistent with the above discussion,
the measured Cotton effect for all copolymers is more
intense at lower temperature. This effect is more
prominent with increasing F* because the steric bulk
of the majority monomer leads to an increase of ∆Gr.21
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