Unexpected stereomutation dependence on the chemical structure of helical vinyl glycopolymers

Article abstract of DOI:10.1039/c2cc00036a

A small change in chemical structure causes a remarkable influence on the stereostructure stability and mutarotational rate of helical vinyl polymers bearing laterally attached p-terphenyl pendants with an achiral butoxy terminal and a chiral galactosylox

Full text of DOI:10.1039/c2cc00036a

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COMMUNICATION
Unexpected stereomutation dependence on the chemical structure of
helical vinyl glycopolymerswz
Jiaxi Cui, Jie Zhang and Xinhua Wan*
Received 3rd January 2012, Accepted 14th March 2012
DOI: 10.1039/c2cc00036a
A small change in chemical structure causes a remarkable influence
on the stereostructure stability and mutarotational rate of helical
vinyl polymers bearing laterally attached p-terphenyl pendants with
an achiral butoxy terminal and a chiral galactosyloxy terminal.
To our surprise, such a small structure modification exerts a
remarkable influence on the stereostructure stability and
stereorotational rate of the polymer.
The two monomers, 2-(40-butoxyphenyl)-5-[40-(2,3,4,6-tetra-O-
acetyl-b-D-galactosyloxy)phenyl]styrene (1) and 2-[40-(2,3,4,6-tetra-
O-acetyl-b-D-galactosyloxy)phenyl]-5-(40-butoxyphenyl)styrene (3),
were prepared via multistep synthetic routes (Schemes S1 and S2,
ESIz). They were converted to P1 and P3 with radical polymeriza-
tion. Whereas, P2 and P4 were obtained by deacetylation of P1
and P3 in methanol, a poor solvent for the four polymers,
respectively (Scheme 1).
Switchable helical polymers, which adapt to the surrounding
environment via stereomutation, are stimuli-responsive materials
with a broad range of applications, such as in asymmetric
synthesis,1 chiral separation,2 biosensors,3 and electrooptical
devices.4 To fully explore their fascinating potential, delicate
molecular design and in-depth understanding of the mechanism
and dynamics of stereostructure change are essential. Glyco-
polymers, as a type of interesting functional materials, have been
attracting great attention due to the unique recognition abilities of
clustered saccharide groups.5 They are also of interest in inducing
a tunable helical polymer backbone and subsequently providing
asymmetric geometric patterns of saccharide groups.6
The monomers 1 and 3 show specific optical rotations ([a]23605) of
901 and 721 in THF, respectively. After radical polymerization, the
20
resultant polymers P1 and P3 display [a] values of 12031 and
365
11241, separately (Table 1). Such large increments in optical
rotation power indicate that the optical activities of the polymers
do not solely arise from the configurational chirality of the side
group, but also a chiral secondary structure, most likely helical
Recently, we reported a helical vinyl glycopolymer, poly{2,5-
bis[40-(b-D-galactosyloxy)phenyl]styrene} (PGPS), which undergoes
the transition from kinetically controlled conformation (KCC) to
thermodynamically controlled conformation (TCC) when annealed
in dimethylsulfoxide (DMSO).7 Each repeating unit of PGPS has
two glycosyl groups at the ends of the side p-terphenyl pendant. In
the present work, we designed and synthesized four novel helical
vinyl glycopolymers, poly{2-(40-butoxyphenyl)-5-[40-(2,3,4,6-tetra-
O-acetyl-b-D-galactosyloxy)phenyl]styrene} (P1), poly{2-(40-butox-
yphenyl)-5-[40-(b-D-galactosyloxy)phenyl]styrene} (P2), poly{2-[40-
(2,3,4,6-tetra-O-acetyl-b-D-galactosyloxy)phenyl]-5-(40-butoxy-
phenyl)styrene} (P3), and poly{2-[40-(b-D-galactosyloxy)phenyl]-5-
(40-butoxyphenyl)styrene} (P4). All these polymers consist of one
chiral glycosyl terminal and one achiral butoxyl terminal at the
end of the terphenyl group. P2 and P4 are deacetylated products
of P1 and P3. P1 and P2 differ from P3 and P4 only by the
position where the side-group is linked to the polymer main chain.
Scheme 1 Synthesis of P1, P2 and their deacetylated products P3, P4.
Table 1 Polymerization results and chiroptical properties of the
resultant polymersa
b
20
365
Polymer
Yield (%)
87
Mn ꢀ 10ꢁ4 b
7.8
Mw/Mn
[a] c/1
P1
P2
P3
P4
1.71
1203
1344
1124
23
84
6.9
1.65
Beijing National Laboratory for Molecular Sciences, Key Laboratory
of Polymer Chemistry and Physics of Ministry of Education, College
of Chemistry and Molecular Engineering, Peking University, Beijing
100871, China. E-mail: xhwan@pku.edu.cn; Fax: +86 10 6175 1708;
Tel: +86 10 6275 4187
a
Polymerization conditions: temperature, 60 1C; [M]/[AIBN] = 300;
reaction time, 24 h. P2 and P4 were obtained from P1 and P3 via
b
deacetylation in methanol, respectively. Number-average molecular
w This article is part of the ChemComm ‘Chirality’ web themed issue.
z Electronic supplementary information (ESI) available: Experimental
details, 1H NMR spectra of monomers and polymers, FT-IR spectra
of P1 and P2, conformation models of P2 and P4. See DOI: 10.1039/
c2cc00036a
weight (Mn) and weight-average molecular weight (Mw) were estimated
by GPC in THF calibrated against a series of standard polystyrene.
c Specific optical rotation in units of degrees was measured in a 1 dm cell
at a concentration of ca. 2.0 mg mLꢁ1 in DMSO at 20 1C.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 4341–4343 4341

24 h, the [a]63605 value reaches a constant value of about ꢁ15741. The
CD spectrum of P4 after annealing in DMSO also shows a negative
Cotton effect, opposite to the positive one before annealing,
implying the mutation of helical structure again (Fig. S1, ESIz).7,8
To shed light on the interplay between the stereostructure and
the glycosyl position, the conformational analyses of P2 and P4
were carried out by computer simulation,10 based on energy
minimization with the Compass forcefield followed by MD at
298 K. Both polymers adopt pine-like helices in which side chains
connected at given tilt angles occupy equably the space around the
backbone no matter they are isotactic, syndiotactic, or atactic
(Tables S1–S3, ESIz). Fig. 3 shows the computer simulated
conformations of atactic P2 and P4. The column of the rigid chain
can be divided into three parts: the twisting main-chain core, the
aromatic region with high electron density, and the periphery. The
glycosyl groups of P2 sit in the periphery of the polymer chain
while those of P4 in the aromatic region. The exterior groups have
more freedom to rotate, but interior ones are restricted by each
other within the chiral environment. These conformational models
can find evidence from CD and NMR experiments. P3 exhibits a
negative band at around 230 nm, corresponding to the absorption
of acetyl groups since its acetyl groups are close to the helical
polymer backbone, but P1 does not (Fig. 1). In P1, the protons of
glycosyl and attached acetyl groups exhibit considerable sharp
peaks and their chemical shifts are close to those of the corres-
ponding monomer while the peaks of the protons on the butoxy
group become relatively broad (Fig. S2, ESIz). Moreover, the peak
of the protons d shifts from 4 ppm to 3 ppm, implying a shield
effect of the high electron-intensity atmosphere. The protons on
butoxyl of P3 show comparatively sharp resonance peaks (as that
of e) while those on glycosyl exhibit broad signals. These signal
changes indicate that the glycosyl of P3 and the butoxyl of P1 are
restricted in the dense electron atmosphere of p-terphenyl, the
aromatic region of the chain column, while the glycosyl of P1
and the butoxyl of P3 are located in the periphery. After deacety-
lation, the peaks of the protons on acetyl disappear and those on
glycosyl are shifted to 3.4–3.8 ppm. Similar results are observed for
P4: the peaks of the protons on butoxyl of P2 are relatively weak
and broad while those of P4 are strong and sharp. Moreover, the
peaks of d can be observed in P4 but not in P2 because they shift to
the high field where they are covered by the signals of the protons
on glycosyl.
Fig. 1 CD and UV-vis absorption spectra of 1, 3, P1, and P3 in THF
at a concentration of 2 ꢀ 10ꢁ5 mol Lꢁ1
.
conformation, of the polymer main chain is generated.8 After
deacetylation of P1 in methanol, the resultant polymer P2 has
an [a]23065 value of 13441, slightly larger than that of P1, probably
indicating the reservation of the original stereostructure of P1.
The deacetylated product of P3, i.e., P4, shows a dramatically
decreased optical rotation (from 11241 to 231). This might be
attributed to the drastically increased mutability.9
Fig. 1 presents the UV-vis absorption and CD spectra of
the acetylated monomers and their polymers. The monomers
1 and 3 have two absorption peaks centered at 256 and 279 nm,
respectively, which are assigned as the electronic transitions of
vinyl groups and side p-terphenyl. The absorption of the vinyl
group disappears completely after polymerization. Compared to
the weak signals of monomers in CD spectra, the polymers display
intensive Cotton effects: P1 shows two intensive positive Cotton
bands centered at 250 and 297 nm, respectively; while P3 exhibits
two CD bands centered at 255 and 297 nm, as well as a negative
signal centered at 230 nm. These observations imply a twisting
arrangement of the side p-terphenyl groups along the polymer
backbone. This is consistent with the speculation on the formation
of the helical polymer main chain.
All the polymers were first annealed in DMSO at 90 1C to
investigate their optical stabilities. Both P1 and P3 display no
obvious change in optical rotation, indicating the stable
stereostructures. However, the removal of acetyl groups enables
P2 and P4 to adjust their stereostructures. It is interesting to note
that the stereostructure stability and mutarotational rate greatly
depend on the position of the glycosyl group. As shown in Fig. 2,
it takes as long as 300 days for P2 to change its optical rotation
from 13441 to almost zero at 90 1C. In sharp contrast, the
mutarotation of P4 at 90 1C is too fast to track its change in
optical rotation, and thus, instead the mutarotation was tracked at
a lower temperature, i.e., 60 1C. Its specific optical rotation
changes from 231 to ꢁ8001 within 68 min. After annealing for
Fig. 2 Annealing time dependence of specific optical rotation of
P2 at 90 1C (a) and P4 at 60 1C (b) in DMSO, c = 0.2.
Fig. 3 Computer simulated conformations of atactic P2 and P4.
c
4342 Chem. Commun., 2012, 48, 4341–4343
This journal is The Royal Society of Chemistry 2012

To further confirm the position of hydrophilic glycosyl groups,
the contact angle measurements of glycopolymers were carried
out. The acetylated polymers, P1 and P3, show contact angles of
751 and 791, respectively, while deacetylated compounds, P2 and
P4, reveal more hydrophilic values of 541 and 631, separately
(Fig. S6, ESIz). The similar contact angles of acetylated polymers
illuminate the nature of similar hydrophobicity. In comparison,
the contact angle of P4 is obviously larger than that of P2. Such a
result indicates that the hydrophilic glycosyls are indeed exposed
outside for P2 but shielded inside for P4.
an unexpected dependence on the position where the p-terphenyl
group is linked to the polymer backbone. The strong steric
repulsion between side-groups of P4, the glycosyl groups of which
are confined in the crowded inner part of the helix, accelerates the
mutarotation. Whereas the relatively weak steric repulsion between
side-groups of P2 with glycosyl groups in the exterior part of the
helix leads to a slower mutarotation. By taking advantage of
isolation and ready transition of KCCs, P2 and P4 may find
applications in chiral separation, asymmetric catalysis, and sensors.
The financial support of the National Natural Science
Foundation of China (No. 21074001) and the Research
Fund for Doctoral Program of Higher Education of MOE
(No. 20110001110084) are greatly appreciated.
On the basis of the above discussion, a possible explanation of
the stereomutation mechanism of P2 and P4 is proposed. P1 and
P3 have helical conformations with a dominant screw sense
stabilized by the steric repulsion of side groups. The freshly
prepared P2 and P4 in methanol inherit the conformations of
P1 and P3, respectively. Given the fact that the two polymers
obtained via direct radical polymerization of 2-(40-butoxyphenyl)-
5-[40-(b-D-galactosyloxy)phenyl]styrene (2) and 2-[40-(b-D-galacto-
syloxy)phenyl]-5-(40-butoxyphenyl)styrene (4) in DMF, which is a
good solvent for the monomer but not a solvent for the polymer
and may trap the polymer stereostructure once formed, display
almost the same optical rotation as that of P2 and P4, we
speculate that both P2 and P4 adopt KCCs due to the steric
interactions of side-groups (Table S4, ESIz).7 These KCCs are
different from their TCCs, unlike P1 and P3, and just stable at
low temperature. At the elevated temperature, KCC-to-TCC
transition takes place. In the case of P4, the bulky glycosyl
groups are confined in the close-packed aromatic region, which
gives rise to strong steric repulsion. As a result, KCC of the
chain is unstable and thus the mutarotation is accelerated. The
movement of glycosyl groups makes H-bonding between OH of
glycosyl groups possible, which may exert new induction power
to the helical conformation. On the other hand, the big glycosyl
residues of P2 extend to the periphery. They are less crowded
than P4 and of weaker stabilizability to the helical conformation.
Such a subtle balance finally leads to transition to TCC, i.e., the
slow racemization of the helix. Novak et al. have reported the
solvent- and temperature-driven dynamic conformational changes
of polycarbodiimides to the reversible shutter-like reorientation of
the aromatic pendant groups without inversion of the static helical
backbone.11 They thought that the secondary layer of chirality
created during the polymerization by the constricted arene
pendant groups gives rise to the dramatic chiro-optical changes.
Considering the severe restriction to the main chain rotation
caused by the bulky side-groups, it is reasonable to exclude the
helix–helix transition of polymer backbones for P2 and P4.
A low-energy concerted realignment of terphenyl pendant
groups influenced by solvent and temperature might induce
the observed stereomutation. More evidence is needed before an
adequate explanation can be made.
Notes and references
1 T. Yamamoto, T. Yamada, Y. Nagata and M. Suginome, J. Am.
Chem. Soc., 2010, 132, 7899; A. J. Boersma, R. P. Megens,
B. L. Feringa and G. Roelfes, Chem. Soc. Rev., 2010, 39, 2083;
T. Miyabe, Y. Hase, H. Iida, K. Maeda and E. Yashima, Chirality,
2009, 21, 44; T. Hasegawa, Y. Furusho, H. Katagiri and E. Yashima,
Angew. Chem., Int. Ed., 2007, 46, 5885; M. Reggelin, S. Doerr,
M. Kiussmann, M. Schultz and M. Holbach, Proc. Natl. Acad. Sci.
U. S. A., 2004, 101, 5461; L. Pu, Chem. Rev., 1998, 98, 2405.
2 Y. Okamoto, S. Honda, I. Okamoto, H. Yuki, S. Murata,
R. Noyori and H. Takaya, J. Am. Chem. Soc., 1981, 103, 6971;
Y. Okamoto and T. Ikai, Chem. Soc. Rev., 2008, 37, 2593.
3 E. Yashima, K. Maeda and Y. Okamoto, Nature, 1999, 399, 449; L. Pu,
Chem. Rev., 2004, 104, 1687; B. Chen, J. P. Deng and W. T. Yang,
Adv. Funct. Mater., 2011, 21, 2345; K. Maeda, H. Mochizuki, K. Osato
and E. Yashima, Macromolecules, 2011, 44, 3217; J. W. Y. Lam and
B. Z. Tang, Acc. Chem. Res., 2005, 38, 745.
4 E. Gomar-Nadal, J. Veciana, C. Rovira and D. B. Amabilino,
Adv. Mater., 2005, 17, 2095; Y. L. Zhu, N. Gergel, N. Majumdar,
L. R. Harriott, J. C. Bean and L. Pu, Org. Lett., 2006, 8, 355;
E. Gomar-Nadal, L. Mugica, J. Vidal-Gancedo, J. Casado, J. T. L.
Navarrete, J. Veciana, C. Rovira and D. B. Amabilino, Macro-
molecules, 2007, 40, 7521; R. Mruk and R. Zentel, Macromolecules,
2002, 35, 185.
5 A. Takasu, S. Horikoshi and T. Hirabayashi, Biomacromolecules,
2005, 6, 2334; J. J. Lundquist and E. J. Toone, Chem. Rev., 2002,
102, 555.
6 A. Tanatani, A. Yokoyama, I. Azumaya, Y. Takakura, C. Mitsui,
M. Shiro, M. Uchiyama, A. Muranaka, N. Kobayashi and
T. Yokozawa, J. Am. Chem. Soc., 2005, 127, 8553; M. Inouye,
M. Waki and H. Abe, J. Am. Chem. Soc., 2004, 126, 2022;
I. Wataoka, H. Urakawa, K. Kobayashi, T. Akaike, M. Schmidt
and K. Kajiwara, Macromolecules, 1999, 32, 1816.
7 J. X. Cui, A. H. Liu, J. G. Zhi, Z. G. Zhu, Y. Guan, X. H. Wan
and Q. F. Zhou, Macromolecules, 2008, 41, 5245.
8 Z. Yu, X. H. Wan, H. L. Zhang, X. F. Chen and Q. F. Zhou,
Chem. Commun., 2003, 974; J. X. Cui, A. H. Liu, J. G. Zhi,
Z. G. Zhu, Y. Guan, X. H. Wan and Q. F. Zhou, Macromolecules,
2008, 41, 5245; J. G. Zhi, Z. G. Zhu, A. H. Liu, J. X. Cui,
X. H. Wan and Q. F. Zhou, Macromolecules, 2008, 41, 1594;
J. X. Cui, X. C. Lu, A. H. Liu, X. H. Wan and Q. F. Zhou,
Macromolecules, 2009, 42, 7678; Z. G. Zhu, J. X. Cui, J. Zhang and
X. H. Wan, Polym. Chem., 2012, 3, 668.
9 It is difficult to estimate the original [a]
25
365
value of P4, which
25
changes too quickly. To get an [a] value close to the original one,
365
the optical rotation of P4 was measured as soon as it was dissolved.
10 A. Zhang, F. Rodrıguez-Ropero, D. Zanuy, C. Aleman, E. W. Meijer
´ ´
and A. D. Schluter, Chem.–Eur. J., 2008, 14, 6924.
In summary, we have demonstrated the synthesis and chiroptical
properties of four novel helical vinyl glycopolymers with an excess
of screw-sense. Although the KCCs and TCCs are identical for
both P1 and P3, they are different for P2 and P4. The KCCs of the
latter two polymers transform irreversibly to TCCs when annealed
in DMSO at elevated temperature. The mutarotational rate shows
¨
11 H. Z. Tang, P. D. Boyle and B. M. Novak, J. Am. Chem. Soc.,
2005, 127, 2136; H. Z. Tang, E. R. Garland, B. M. Novak, J. He,
P. L. Polavarapu, F. C. Sun and S. S. Sheiko, Macromolecules,
2007, 40, 3575; J. G. Kennemur, J. B. Clark, G. Tian and
B. M. Novak, Macromolecules, 2010, 43, 1867.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 4341–4343 4343