Chiroptical switching polyguanidine synthesized by helix-sense-selective polymerization using [(R)-3,3′-dibromo-2,2′-binaphthoxy](di-tert- butoxy)titanium(IV) catalyst

Article abstract of DOI:10.1021/ja0453533

A series of chiral binaphthyl titanium alkoxide complexes were synthesized. Among them, chiral titanium complex [(R)-3,3′-dibromo-2,2′- binaphthoxy](di-tert-butoxy)titanium(IV) (R-3) exists as a crystallographic C₂ dimer in the solid state but a monomer in solution at room temperature. Application of R-3 in the helix-sense-selective polymerization of achiral carbodiimide, N-(1-anthryl)-N′-octadecylcarbodiimide (1), yielded a well-defined regioregular, stereoregular poly[N-(1-anthryl)-N′- octadecylguanidine] (poly-1b) with a relatively narrow polymer dispersity index of 2.7. Full racemization of poly-1b at +80 °C in toluene requires more than 100 h. Interestingly, poly-1b was found to undergo fast reversible chiroptical switching at +38.5 °C in toluene. Furthermore, at room temperature, poly-1b shows a positively signed Cotton effect in toluene, but negative ones in THF and chloroform, respectively. The chiroptical switching takes place around the toluene content of 90% (vol) in the mixed toluene/THF solvents. This is the first example of chiroptical switching phenomenon occurring in a helical polymer possessing no chiral moieties in the polymer chains. We believe this reversible chiroptical switching phenomenon occurs by reorientation of anthracene rings relative to the chain director.

Full text of DOI:10.1021/ja0453533

Published on Web 01/28/2005
Chiroptical Switching Polyguanidine Synthesized by
Helix-Sense-Selective Polymerization Using
[(R)-3,3′-Dibromo-2,2′-binaphthoxy](di-tert-butoxy)titanium(IV)
Catalyst
Hong-Zhi Tang, Paul D. Boyle, and Bruce M. Novak*
Contribution from the Department of Chemistry, North Carolina State UniVersity,
Raleigh, North Carolina 27695
Received August 2, 2004; Revised Manuscript Received December 13, 2004; E-mail: novak@chemdept.chem.ncsu.edu
Abstract: A series of chiral binaphthyl titanium alkoxide complexes were synthesized. Among them, chiral
titanium complex [(R)-3,3′-dibromo-2,2′-binaphthoxy](di-tert-butoxy)titanium(IV) (R-3) exists as a crystal-
lographic C2 dimer in the solid state but a monomer in solution at room temperature. Application of R-3 in
the helix-sense-selective polymerization of achiral carbodiimide, N-(1-anthryl)-N′-octadecylcarbodiimide (1),
yielded a well-defined regioregular, stereoregular poly[N-(1-anthryl)-N′-octadecylguanidine] (poly-1b) with
a relatively narrow polymer dispersity index of 2.7. Full racemization of poly-1b at +80 °C in toluene requires
more than 100 h. Interestingly, poly-1b was found to undergo fast reversible chiroptical switching at +38.5
°C in toluene. Furthermore, at room temperature, poly-1b shows a positively signed Cotton effect in toluene,
but negative ones in THF and chloroform, respectively. The chiroptical switching takes place around the
toluene content of 90% (vol) in the mixed toluene/THF solvents. This is the first example of chiroptical
switching phenomenon occurring in a helical polymer possessing no chiral moieties in the polymer chains.
We believe this reversible chiroptical switching phenomenon occurs by reorientation of anthracene rings
relative to the chain director.
acetylenes.2j-m Solvent-driven chiroptical switching has been
reported for poly(L-aspartate â-esters)2a,3b,c and poly(propiolic
esters).2l To date, however, all chiroptical switching polymers
are synthesized from chiral monomers, possessing stereo centers
in the main or side chains. Herein, we wish to report the first
chiroptical switching polymer (poly[N-(1-anthryl)-N′-octade-
cylguanidine], poly-1b, see Scheme 2), which possesses no
chiral moieties in polymer chains. Poly-1b is synthesized by a
highly regioregular, stereoregular, helix-sense-selective polym-
erization.
Introduction
Controlling and switching the chiroptical properties of
(macro)molecules is of continued interest because of potential
applications in sensor data storage, optical devices, and liquid
crystalline displays.1-6 Chiroptical switch can be controlled by
temperature,2 solvent,3 additives,4 irradiation,5 and electron
redox,6 with thermo-driven chiroptical switching polymers being
the most extensively studied. Examples include poly(L-aspartate
â-esters),2a,b polyisocyanates,2c-e polysilanes,1d,2f-i and poly-
(1) Typical reviews: (a) Hill, D. J.; Mio, M. J.; Prince, R. B.; Hughes, T. S.;
Moore, J. S. Chem. ReV. 2001, 101, 3893. (b) Okamoto, Y.; Nakano, T.
Chem. ReV. 1994, 94, 349. (c) Pu, L. Acta Polym. 1997, 48, 116. (d) Fujiki,
M. J. Organomet. Chem. 2003, 685, 15.
The helix-sense-selective polymerization of achiral monomers
using chiral catalysts or chiral solvents yields kinetically
controlled helical polymers, e.g., polyisocyanides,7a-c poly-
(quinoxaline-2,3-diyl)s,7d,e poly(trityl methacrylates),1b,7f-h poly-
(trityl methacylamides),7i polyacetylenes,7j and polyisocyanates.7k
(2) For poly(L-aspartate â-esters): (a) Bradbury, E. M.; Carpenter, B. G.;
Goldman, H. Biopolymers 1968, 6, 837. (b) Watanabe, J.; Okamoto, S.;
Satoh, K.; Sakajiri, K.; Furuya, H.; Abe, A. Macromolecules 1996, 29,
7084. For polyisocyanates: (c) Maeda, K.; Okamoto, Y. Macromolecules
1999, 32, 974. (d) Cheon, K. S.; Selinger, J. V.; Green, M. M. Angew.
Chem., Int. Ed. 2000, 39, 1482. (e) Tang, K.; Green, M. M.; Choen, K. S.;
Selinger, J. V.; Garetz, B. A. J. Am. Chem. Soc. 2003, 125, 7313. For
polysilylenes: (f) Fujiki, M. J. Am. Chem. Soc. 2000, 122, 3336. (g) Fujiki,
M.; Tang, H.-Z.; Motonaga, M.; Torimitsu, K.; Koe, J. R.; Watanabe, J.;
Sato, T.; Teramoto, A. Silicon Chem. 2002, 1, 67. (h) Fujiki, M.; Koe, J.
R.; Motonaga, M.; Nakashima, H.; Terao, K.; Teramoto, A. J. Am. Chem.
Soc. 2001, 123, 6253. (i) Teramoto, A.; Terao, K.; Terao, Y.; Nakashima,
H.; Sato, T.; Fujiki, M. J. Am. Chem. Soc. 2001, 123, 12303. For
polyacetylenes: (j) Tabei, J.; Nomura, R.; Sanda, F.; Masuda, T. Macro-
molecules 2004, 37, 1175. (k) Cheuk, K. K. L.; Lam, J. W. Y.; Lai, L. M.;
Dong, Y.; Tang, B. Z. Macromolecules 2003, 36, 9752. (l) Nakako, H.;
Nomura, R.; Masuda, T. Macromolecules 2001, 34, 1496. (m) Tabei, J.;
Nomura, R.; Masuda, T. Macromolecules 2003, 36, 573. (n) Yashima, E.;
Maeda, K.; Sato, O. J. Am. Chem. Soc. 2001, 123, 8159.
(4) (a) Novak, B. M.; Schlitzer, D. S. J. Am. Chem. Soc. 1998, 120, 2196. (b)
Yashima, E.; Maeda, K.; Okamoto, Y. Nature 1999, 399, 449. (c) Ishikawa,
M.; Maeda, K.; Mitsutsuji, Y.; Yashima, E. J. Am. Chem. Soc. 2004, 126,
732. (d) Miyake, H.; Yoshida, K.; Sugimoto, H.; Tsukube, H. J. Am. Chem.
Soc. 2004, 126, 6524. (e) Su, S.-J.; Takeishi, M.; Kuramoto, N. Macro-
molecules 2002, 35, 5752. (f) Berl, V.; Huc, I.; Khoury, R. G.; Krische,
M. J.; Lehn, J.-M. Nature 2000, 407, 720.
(5) (a) Koumura, N.; Zijlstra, R. W. J.; van Delden, R. A.; Harada, N.; Feringa,
B. L. Nature 1999, 401, 152. (b) Huck, N. P. M.; Jager, W. F.; de Lange,
B.; Feringa, B. L. Science 1996, 273, 1686. (c) Janicki, S. Z.; Schuster, G.
B. J. Am. Chem. Soc. 1995, 117, 8524. (d) Mayer, S.; Maxein, G.; Zental,
R. Macromolecules 1998, 31, 8522. (e) Muller, M.; Zentel, R. Macromol-
ecules 1994, 27, 4404. (f) Maxein, G.; Zentel, R. Macromolecules 1995,
28, 8438. (g) Muller, M.; R. Zentel Macromolecules 1996, 29, 1609. (h)
Mayer, S.; Zentel, R. Macromol. Chem. Phys. 1998, 199, 1675.
(6) (a) Zahn, S.; Canary, J. W. Science 2000, 288, 1404. (b) Zahn, S.; Canary,
J. W. Trends Biotechnol. 2001, 19, 251.
(3) (a) Khatri, C. A.; Pavlova, Y.; Green, M. M.; Morawetz, H. J. Am. Chem.
Soc. 1997, 119, 6991. (b) Bradbury, E. M.; Carpenter, B. G.; Robinson, C.
C.; Goldman, H. Macromolecules 1971, 4, 557. (c) Toniolo, C.; Falxa, M.
L.; Goodman, M. Biopolymers 1968, 6, 1579.
9
2136
J. AM. CHEM. SOC. 2005, 127, 2136-2142
10.1021/ja0453533 CCC: $30.25 © 2005 American Chemical Society

Chiroptical Switching Polyguanidine
A R T I C L E S
Scheme 1
meric titanium catalysts are required. However, to date, mon-
omeric titanium alkoxide complexes are few in number.
In the present study, we pursued two approaches to prevent
the d0 Ti(IV) aggregation and increase its reactivity by introduc-
ing bulky and electron-withdrawing groups onto the 3,3′-
positions of naphthalene rings, and tuning the bulkiness of
alkoxy groups. Among the titanium complexes synthesized, [(R)-
3,3′-dibromo-2,2′-binaphthoxy](di-tert-butoxy)titanium(IV), R-3
(Scheme 1), exists as a dimer in the solid state and a monomer
in the solution state at room temperature. Catalyzed by R-3,
helix-sense-selective polymerization of achiral carbodiimide of
N-(1-anthryl)-N′-octadecylcarbodiimide (1) yielded poly-1b with
high regioregularity, stereoselectivity and a relatively narrow
molecular-weight distribution of 2.7. Although poly-1b pos-
sesses no chiral moieties in the polymer chains, this material
exhibits thermo-driven and solvent-driven reversible chiroptical
switching phenomena.
Results and Discussion
Chiral titanium complexes were synthesized from (R)-2,2′-
binaphthoxy ligands (L1-L6) with an equivalent of the
corresponding Ti(IV) alkoxide in toluene or benzene (Scheme
1). Complexation rates of these ligands are retarded from L1
to L6, probably due to steric effects. For example, reacting bulky
bis(triphenylsilyl) substituted ligand, L6, with Ti(OEt)4 in
refluxing toluene for a day yielded a mixture of R-10 and the
starting material L6, as evidenced by the remaining resonance
peak at 4.64 ppm (-OH) in the 1H NMR spectrum. In contrast,
the reaction of the parent ligand, L1, with Ti(O-i-Pr)4 or Ti-
(O-t-Bu)4 at room temperature was complete within 1 h. R-10
is monomeric, as evidenced by its light yellow color in solution
and the single set of well-resolved quadruplet resonance peaks
at 3.25 ppm (-OCH2CH3) in the 1H NMR spectrum (see
Supporting Information). The less bulky bis(diphenylmethylsilyl)
substituted ligand, L5, gave monomeric R-9 having bulkier
isopropoxide groups, but aggregated, R-8, with the less bulky
ethoxide groups, indicated by the featureless alkyl-H resonance
Recently, we reported our preliminary results on the helix-sense-
selective polymerization of achiral carbodiimides using [(R)-
and/or (S)-binaphthoxy](diisopropoxy)titanium(IV), R-1 and/
or S-1, catalysts (Scheme 1).8 However, the helical polyguanidines
obtained possess regioirregular backbones. We concluded that
it is resulted from the multiple catalytically active species, such
as monomer, dimers, and trimers of titanium complexes.9 To
precisely control the regioselectivity in the polymerization of
unsymmetrical carbodiimides, structurally well-defined mono-
1
peaks in the H NMR spectrum and the red-orange color in
solutions. This reveals that the bulkiness of R2 also plays an
important role in determining the existing forms of the titanium
complexes. Complexes R-4 and R-6 possessing the isopropoxide
groups exist in aggregated forms in solution, but using the more
bulky tert-butoxide groups leads to monomeric R-5 and R-7
with ligands L3 and L4. The light yellow color of R-3 solutions
indicates that R-3 exists as a monomer. Parent ligand L1
produced aggregated R-1 and R-2 with red-orange colors.
Preliminary polymerization experiments were carried out to
test the activity of these monomeric titanium catalysts. The
bromo substituted catalyst R-3 shows the highest polymerization
activity as compared to other monomeric complexes. This rate
enhancement can be explained by both steric effects and the
electron-withdrawing character of the brominated binaphthol
ligand. In the following study, we therefore focused on the new
catalyst, R-3.
(7) For polyisocyanides: (a) Deming, T. J.; Novak, B. M. J. Am. Chem. Soc.
1992, 114, 7926. (b) Nolte, R. J. M.; van Beijnen, A. J. M.; Drenth, W. J.
Am. Chem. Soc. 1974, 96, 5932. (c) Kamer, P. C. J.; Nolte, R. J. M.; van
Beijnen, A. J. M.; Drenth, W. J. Am. Chem. Soc. 1988, 110, 6818. For
poly(quinoxaline-2,3-diyl)s: (d) Ito, Y.; Miyake, T.; Ohara, T.; Suginome,
M. Macromolecules 1998, 31, 1697. (e) Ito, Y.; Ihara, M.; Murakami, M.
Angew. Chem., Int. Ed. Engl. 1992, 31, 1509. For poly(trityl methacry-
lates): (f) Nakano, T.; Okamoto, Y. Macromolecules 1999, 32, 2391. (g)
Okamoto, Y.; Suzuki, K.; Ohta, K.; Hatada, K.; Yuki, H. J. Am. Chem.
Soc. 1979, 101, 4763. (h) Nakano, T.; Okamoto, Y.; Hatada, K. J. Am.
Chem. Soc. 1992, 114, 1318. For poly(trityl methacylamides): (i) Hoshika-
wa, N.; Hotta, Y.; Okamoto, Y. J. Am. Chem. Soc. 2003, 125, 12380. For
polyacetylenes: (j) Aoki, T.; Kaneko, T.; Maruyama, N.; Sumi, A.;
Takahashi, M.; Sato, T.; Teraguchi, M. J. Am. Chem. Soc. 2003, 125, 6346.
For polyisocyanates: (k) Okamoto, Y.; Matsuda, M.; Nakano, T.; Yashima,
E. Polym. J. 1993, 25, 391.
(8) (a) Tang, H.-Z.; Lu, Y.; Tian, G.; Capracotta, M. D.; Novak, B. M. J. Am.
Chem. Soc. 2004, 126, 3722. (b) Tian, G.; Lu, Y.; Novak, B. M. J. Am.
Chem. Soc. 2004, 126, 4082.
(9) (a) Boyle, T. J.; Barnes, D. L.; Heppert, J. A.; Morales, L.; Takusagawa,
F. Organometallics 1992, 11, 1112. (b) Balsells, J.; Davis, T. J.; Carroll,
P.; Walsh, P. J. J. Am. Chem. Soc. 2002, 124, 10336. (c) Davis, T. J.;
Balsells, J.; Carroll, P. J.; Walsh, P. J. Org. Lett. 2001, 3, 699. (d) Pescitelli,
G.; Di Bari, L.; Salvadori, P. Organomettallics 2004, 23, 4223.
The X-ray-quality single crystals were grown by extremely
slow diffusion of a nonsolvent, acetonitrile, into methylene
chloride solution of R-3. As shown in Figure 1, R-3 exists as
a dimer with a crystallographic C2 symmetry in solid, in which
the naphthylate oxygens are bridging the titanium centers, and
the t-BuO alkoxides are all terminal. The coordination environ-
9
J. AM. CHEM. SOC. VOL. 127, NO. 7, 2005 2137

A R T I C L E S
Tang et al.
Figure 1. X-ray crystal structure of R-3. Crystal data: 0.42 × 0.30 ×
0.28 mm, hexagonal, P64, a ) 21.190(5) Å, c ) 11.783(2) Å, V ) 4583.4-
(17) Å3, Z ) 6, dc ) 1.475 g cm-3, λ ) 0.71073 Å, 2θmax ) 55.8°, F(000)
) 2047.11, T ) 125 °C. Selected bond distances and angles are found in
Table 1.
1
Figure 2. Variable-temperature H NMR spectra of R-3 in CD2Cl2.
Table 1. Selected Intramolecular Bond Distances (A) and Angles
(deg) in R-3 and R-11a
R-3
R-11
O1-Ti1
2.175(3)
1.851(4)
1.750(4)
1.782(4)
2.128(7)
1.827(7)
1.741(9)
1.771(8)
1.953(7)
69.5(2)
O2-Ti1
O3-Ti1
O4-Ti1
O1a-Ti1
1.996(3)
Figure 3. Stereoregular structures of non-symmetrically substituted
polyguanidines prepared through the polymerization of an achiral carbo-
diimide with catalyst R-1.
O1a-Ti1-O1
O1a-Ti1-O2
O1a-Ti1-O3
O1a-Ti1-O4
O2-Ti1-O4
O2-Ti1-O3
O3-Ti1-O4
Ti1-O1a-Ti1a
Ti1-O1-Ti1a
C2-C3-C3′-C2′
69.03(13)
114.86(17)
95.66(16)
127.39(17)
108.87(18)
98.84(18)
105.25(19)
110.78(15)
110.78(15)
-56.8(7)
124.6(3)
94.7(3)
117.9(3)
110.2(4)
100.9(4)
102.0(5)
109.9(3)
110.6(3)
existing as a monomer above -50 °C but a monomer and dimer
mixture below -60 °C. This equilibrium is also supported by
the single methyl resonance peak at 1.02 ppm above -50 °C
and a split peak below -60 °C. This NMR study strongly
supports the previous conclusion of the monomeric nature of
R-3 that was based on the observation of the light yellow color
of R-3 in solution.
a The data of R-11 are taken from ref 9a.
We previously reported that R-1 and S-1 catalysts will
polymerize achiral, but non-symmetrically substituted carbodi-
imides (e.g., N-(1-isopropyl-6-methylphenyl)-N′-methylcarbo-
diimide (2)) to yield helix-sense-selective polymers that do not
fully racemize through helix inversions upon annealing.8b We
attribute this unusual behavior to a second level of embedded
chirality that results from the stereoselective orientation of both
the aromatic substituents and the imine groups (Figure 3). Full
racemization of these stereoregular structures requires not only
helix reversals but rotations around the N-aryl bonds and/or
inversion of the imine nitrogens. Because of steric interactions
between neighboring groups, these normally low energy pro-
cesses are strongly inhibited.
Catalyzed by R-3, helical poly-1b was obtained by polym-
erization of 1 in toluene at room temperature (Scheme 2). Both
poly-1a and poly-1b show high solubility in toluene, chloroform,
and tetrahydrofuran (THF). As shown in Figure 4, compared
to poly-1a (Mw ) 234 000, PDI ) 19.3), poly-1b (Mw ) 16 000)
has much narrower polymer dispersion index, PDI ) 2.7,
indicating that the single site catalyst R-3 offers superior control
over the polymerization. Furthermore, contrary to the regioir-
regular polymer structure of poly-1a, poly-1b has a well-defined
regioregular backbone as evidenced by the single CdN stretch-
ing at 1642 cm-1 in FT-IR spectrum (See Supporting Informa-
tion).
ment about each titanium center is best described as a highly
distorted trigonal bipyramid, with a bridging naphtholate (i.e.,
O1a) ligand and one t-BuO (i.e., O4) ligand occupying the axial
positions with respect to titanium, and one t-BuO (i.e., O3), a
terminal naphtholate (i.e., O2) and a bridging naphtholate (i.e.,
O1) ligand occupying the remaining equatorial sites. This
structure is quite similar to the dimer of R-11 [(R)-3,3′-dimethyl-
2,2′-binaphthoxy](diisopropoxy)titanium(IV)] (Scheme 1).9a As
listed in Table 1, Ti-O-t-Bu distances are nearly 0.1 Å shorter
than Ti-ONp distances, revealing that the bonds between Ti-
O-t-Bu are stronger due to the greater electron rich character
of the oxygen of the -O-t-Bu group. Meanwhile, compared to
the dimer of R-11, three significant differences are found: (1)
The two -O-t-Bu groups in R-3 are in different environments;
one is confined, but another has two orientations. (2) The dimer
of R-3 displays an exact crystallographic C2 symmetry, whereas
the dimer of R-11 has a slight puckering of 1,3-dioxadititana-
cycle and shows virtual C2 symmetry. (3) All the Ti-O
distances in R-3 are longer (0.01-0.05 Å) than those in R-11
(Table 1), revealing that R-3 occupies a larger space due to the
overall more crowded environment in R-3.
Figure 2 shows the variable-temperature 1H NMR spectra in
the aromatic regions of R-3 in CD2Cl2. When the tempera-
ture was lowered, the well-resolved resonance peaks observed
at -40 °C were broadened at -50 °C, and new resonance peaks
appeared below -60 °C. These results are interpreted as R-3
The C2 symmetric titanium catalyst possesses two different
Ti-O bonds of Ti-OR2 (a) and Ti-ONp (b). Based on the
9
2138 J. AM. CHEM. SOC. VOL. 127, NO. 7, 2005

Chiroptical Switching Polyguanidine
A R T I C L E S
Scheme 2
Figure 4. GPC chromatograms of poly-1a and poly-1b eluting with
chloroform at a rate of 1.0 mL/min.
Figure 5. Optical rotations, [R]D, of poly-1a and poly-1b versus annealing
time in toluene at 80 °C (c ) 0.1 g/100 mL).
previously proposed mechanism,10 bond a or b selectively inserts
into a carbodiimide and R2O- or NpO- becomes the end group
of the polymer chain. In this competition, the nucleophilicity
of -OR2 is greater than -ONp due to its greater electron-rich
character of its oxygen. Once completed, the polymerization is
quenched and the titanium alkoxide endgroup is protonolysis
removed from the amidinate chain end using methanol. This
mechanism predicts that the achiral R2O- not the chiral NpO-
is the end groups in the helical polyguanidines. To confirm this,
we carried out the polymerization of N,N′-dihexylcarbodiimide
(3) catalyzed by R-1 (the molar ratio of [3]/[R-1] is 5), and
found that Ar-H resonance peaks completely (i.e., the residual
chiral catalyst) disappeared after the purification by reprecipi-
tation of the polymer solution in THF or chloroform into
methanol. It demonstrates that no chiral binaphthyl groups
remain in the resulting polyguanidines.
Figure 5 shows the optical rotations, [R]80D, of poly-1a and
poly-1b in toluene at 80 °C versus annealing time. Compared
to poly-1a, the initial [R]80D, -560°, of poly-1b is much greater
in intensity indicative of greater diastereoselectivity but opposite
in sign. The racemization rate (t1/2) for poly-1b is 27 h, 6 times
longer than that of poly-1a. It is worth pointing out the
racemization rate of poly-1b is the slowest of all the
polyguanidines measured to date.
Figure 6. Variable-temperature [R]D of poly-1b in toluene (c ) 0.1 g/100
mL) at a heating rate of 1.5 °C/min.
toluene at +80 °C? To explore this, we measured the optical
rotations of poly-1b at different temperatures. The first observa-
tion is that these polymers show a drastic temperature depen-
dence in their optical rotations both in terms of magnitude and
sign. As shown in Figure 6, [R]D of poly-1b converts its sign
from positive (e.g., [R]31 ) +300°) at lower temperature to
D
negative ones (e.g., [R]44D ) -205°) at higher temperature. The
chiroptical switching temperature is 38.5 °C. As reported
previously,8a [R]20 of poly-1b in toluene is +130°. Both
D
This experiment, however, leads to a puzzle. Why is it that
poly-1b and poly-1a show optical rotations of opposite sign in
positively signed optical rotations of poly-1a and poly-1b at
lower temperatures indicate that the same M-conformations of
R-1 and R-3 give the same preferred screw-sense polymers at
the polymerization conditions.
(10) Shibayama, K.; Seidel, S. W.; Novak, B. M. Macromolecules 1997, 30,
3159.
9
J. AM. CHEM. SOC. VOL. 127, NO. 7, 2005 2139

A R T I C L E S
Tang et al.
Figure 7. Variable-temperature CD (top) and UV-visible (bottom) spectra of poly-1b in toluene (c ) 2.1 × 10-4 M, path length ) 10 mm).
To further understand the chiroptical switching phenomenon,
variable temperature CD and UV-visible spectra were recorded
(Figure 7). At 25 °C, poly-1b shows a positively signed Cotton
effect with the maximum ∆ꢀ ) +4.69 M-1 cm-1 at 380 nm,
corresponding to the UV-visible absorption maximum at 384
nm. The Kuhn’s dissymmetry ratio, gabs ( ) ∆ꢀ/ꢀ), is +8.2 ×
10-4, comparable to that (+14.2 × 10-4) of the stable helical
poly{N-(1-anthryl)-N′-[(R)-3,7-dimethyloctyl]guanidine} (poly-
4R, Scheme 2). When poly-1b was heated in a toluene solution,
it showed a weak Cotton effect at 40 °C, but gave an almost
mirror-image Cotton effect at higher temperature with that at
room temperature; and the UV-visible absorption decreased
slightly. For example, at 80 °C, poly-1b gave a negatively signed
Cotton effect with maximum ∆ꢀ ) -4.69 M-1 cm-1 at 372
nm, corresponding to the maximum UV-visible absorption at
382 nm. gabs, is -12.7 × 10-4, comparable to that (-11.0 ×
10-4) of the stable helical poly{N-(1-anthryl)-N′-[(S)-3,7-
dimethyloctyl]guanidine} (poly-4S, Scheme 2) in toluene at 80
°C. The most interesting observation is that when this toluene
solution was cooled to 25 °C, once again poly-1b gave a
positively signed Cotton effect albeit with lower intensity
compared to the original one at 25 °C. This reveals that the
chiroptical properties of helical poly-1b can be reversible
switched around 40 °C without racemizing the polymer. The
decrease in the intensity is due to slow racemization during the
entire thermal process.
absorption was observed. However, the gabs-values remained
constant. Figure 10 shows the variable temperature CD spectra
of poly-1b in tetrahydrofuran (THF). Slight increase in the
absolute gabs-values was observed.
Interestingly, poly-1b shows negative Cotton effects in chloro-
form and THF at all temperatures. These CD spectra resemble
that of poly-1b in toluene at 60 °C, but are of opposite in sign,
compared to that of poly-1b in toluene at 25 °C. This indicates
the solvent-driven chiroptical switching. Figure 11, which shows
the solvent-composition dependence of the gabs-values of poly-
1b, clearly demonstrates the chiroptical inversion driven by the
change in solvent composition between toluene and THF. The
chiroptical inversion occurs around 10% content of THF.
Scheme 3 shows the possible molecular motions leading to
the full racemization of poly-1b. They are the N-C bond
rotations (φ) in the backbone, N-CAr bond rotations (θ) in the
side chains, and the imine configuration interconversions (ω).
φ is related to the torsion angle of the main chains, which
determines the helical screw sense and the helical pitches. The
helical inversion barrier of poly-1b is unknown, though we
attempted to calculate it theoretically using polymer-consistent-
force-field (pcff). The barrier for a structural analogue of
polyguanidines, polyisocyanates, was estimated as 12.5 kcal/
mol by an empirical force field.11 Considering the stiffer
backbone of the polyguanidine, it is reasonable to assume that
the helical inversion barrier of poly-1b is greater than 12.5 kcal/
mol. Pcff calculations reveal a limited rotation between N-CAr
(0 < θ < 90°) because of the great bulkiness of anthracene
groups, indicating that the energy for the free rotations are
extremely high but a low energy for the limited reorientation
(wagging) of the anthracence rings. The barrier of imine
configuration interconversions in small molecules is in the range
of 20-26 kcal/mol (coalescence temperatures in the range of
50-180 °C).12 Thus, the energy barrier in poly-1b is probably
in the sequence of ∆E(ω) > ∆E(φ) > ∆E(θ). The full
Keeping in mind the fact of slow racemization in toluene at
80 °C, we further performed the heating-cooling cycles of poly-
1b in toluene in the temperature range of 25 °C and 60 °C. As
expected, no significant racemization was observed. Poly-1b
shows absolutely reversible chiroptical switching in the four
thermal cycles as we conducted. Figure 8 displays the CD and
UV-visible spectra of poly-1b in toluene in the first two
heating-cooling cycles.
Figure 9 shows the variable temperature CD and UV-visible
spectra of poly-1b in chloroform. When the temperature was
raised from 25 °C to 60 °C, slight decrease in the UV-visible
(11) Lifson, S.; Felder, C. E.; Green, M. M. Macromolecules 1992, 25, 4142.
(12) Jennings, W. B.; Boyd, D. R. J. Am. Chem. Soc. 1972, 94, 7187.
9
2140 J. AM. CHEM. SOC. VOL. 127, NO. 7, 2005

Chiroptical Switching Polyguanidine
A R T I C L E S
Figure 8. Variable-temperature CD (top) and UV-visible (bottom) spectra of poly-1b in toluene (c ) 2.1 × 10-4 M, path length ) 10 mm) in the
heating-cooling-heating thermal cycle. The sample is the same as that in Figure 7. The measurement was performed six month later compared to that in
Figure 7.
Figure 9. Variable-temperature gabs (top) and UV-visible (bottom) spectra of poly-1b in chloroform (c ) 2.1 × 10-4 M, path length ) 10 mm).
racemization of poly-1b occurring at +80 °C takes more than
100 h, and probably results from contributions by all three of
these processes.
the helix inversion in poly-1a are not involved in this reversible
chiroptical switching process. The blue-red shift in UV-visible
and CD absorptions above and below the chiroptical switching
temperature, however, suggest that the directions of the anthra-
cence rings cooperatively switch relative to the helix director
(i.e., wagging in θ around the N-CAr bond). The helical pitches
in these various states may also vary in this process. Hence,
although small contributions from helical reversals and imine
inversions cannot be ruled out, we believe that changes in the
helical pitches and the directions of the transition dipole
moments of anthracence may lead to this chiroptical switching
phenomenon.13 The clear reversible switching mechanisms are
under study.
Three mechanisms are possible to explain this interesting
chiroptical switching phenomenon: helix inversions, imine
inversions and/or rotations around the N-anthracene bonds. Of
the three, partial rotations (wagging) around the N-anthracene
bonds are the lowest energy process. Compared to the time-
consuming (100 h) and energy-demanding (+80 °C) full
racemization process, the reversible chiroptical inversion occurs
quickly (less than 1 min) by themo-driven at the lower
temperature of +38.5 °C in toluene and by changes in solvent
polarity, implying that the imine configuration inversion and
9
J. AM. CHEM. SOC. VOL. 127, NO. 7, 2005 2141

A R T I C L E S
Tang et al.
Figure 10. Variable-temperature gabs spectra of poly-1b in THF (c ) 2.1 × 10-4 M, path length ) 10 mm).
of 2.7. Poly-1b was found to undergo dramatic reversible
chiroptical switching that is extremely sensitive and can be
driven by heat and solvent polarity. Chiroptical switching occurs
at 38.5 °C in toluene and around 10% THF content in mixed
THF/toluene at 25 °C. This is the first example of chiroptical
switching occurring in a helical polymer possessing no chiral
moieties in the polymer chains, and may prove useful in low-
cost optical memory and switching applications. The reversible
chiroptical switching occurs at substantially lower energy than
racemization (>100 h, +80 °C). After evaluating the potential
motions that may be behind this switching phenomenon, we
believe the reversible chiroptical switching phenomenon occurs
by a cooperative switching of the orientation of the anthracene
rings. It remains an open question as to whether helical and/or
imine inversions play a role in this process. The clear mechanism
are under detailed study.
Figure 11. gabs-values at 380 nm of poly-1b in toluene/THF at 25 °C.
Scheme 3
Acknowledgment. We thank Professors A. Clay Clark, T.
Brent Gunnone, and Alan E. Tonelli at NCSU for use of their
CD and FT-IR instruments and the Silicon Graphics Indigo II
XZ computing system. We appreciate Professors Alan E.
Tonelli, Stefan Franzen, Dr. Xingwu Wang, and Xin Jia for
their help and discussion on the pcff calculations. We also thank
Jubo Zhang for his help in the preparation of the X-ray crystals.
Supporting Information Available: All experimental details
and crystallographic data for R-3 (PDF, CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
Concluding Remarks
We have synthesized a series of chiral binaphthyl titanium
complexes for use in helix-sense-selective polymerizations.
Among them, chiral titanium complex R-3 exists as a crystal-
lographic C2 dimer in solid but a monomer in solution at room
temperature. Application of R-3 in the helix-sense-selective
polymerization of achiral carbodiimide 1 yielded a well-defined
regioregular, stereoregular poly-1b with a relatively narrow PDI
JA0453533
(13) Tinoco, I. J. Chim. Phys. 1968, 65, 91.
(14) (a) Tsang, W. C. P.; Schrock, R. R.; Hoveyda, A. H. Organometallics 2001,
20, 5658. (b) Maruoka, K.; Itoh, T.; Araki, Y.; Shirasaka, T.; Yamamoto,
H. Bull. Chem. Soc. Jpn. 1988, 61, 2975. (c) Ooi, T.; Kameda, M.; Maruoka,
K. J. Am. Chem. Soc. 2003, 125, 5139. (d) van der Linden, A.; Schaverien,
C. J.; Meijboom, N.; Ganter, C.; Orpen, A. G. J. Am. Chem. Soc. 1995,
117, 3008.
9
2142 J. AM. CHEM. SOC. VOL. 127, NO. 7, 2005