Molecular weight dependence of helical conformation of amino acid-based polyphenylacetylenes

Article abstract of DOI:10.1002/pola.24941

A well-defined living polymerization catalyst, [(nbd)Rh{C(Ph)=CPh ₂}(PPh₃)]/PPh₃, produced a series of amino acid based polyphenylacetylene derivatives having a wide range of molecular weights (M_n: 1500-128,800) with relatively narrow polydispersity. On the basis of the CD and UV-vis spectra of the obtained polymers measured in DMF, plotting the [I] and Kuhn dissymmetry factor, g, against the molecular weight revealed that their predominantly one-handed helical conformation strongly depended on their molecular weights.

Full text of DOI:10.1002/pola.24941

WWW.POLYMERCHEMISTRY.ORG
NOTE
Molecular Weight Dependence of Helical Conformation of Amino
Acid-Based Polyphenylacetylenes
Masashi Shiotsuki, Shohei Kumazawa, Naoya Onishi, Fumio Sanda
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura Campus, Nishikyo-ku,
Kyoto 615-8510, Japan
Correspondence to: M. Shiotsuki (E-mail: shiotsuki@adv.polym.kyoto-u.ac.jp) or F. Sanda (E-mail: sanda@adv.polym.kyoto-u.ac.jp)
Received 24 May 2011; accepted 11 August 2011; published online 9 September 2011
DOI: 10.1002/pola.24941
KEYWORDS: living polymerization; molecular dynamics; organometallic catalysts; polyacetylenes
(Table 1). The polymers were isolated by preparative high-per-
formance liquid chromatography (HPLC) (eluent: CHCl3) for the
complete removal of the catalyst residue and undesired
oligomers. The ranges of number-average molecular weights
(Mns) of poly(1a) and poly(1b) were 1500–128,800 and 2700–
70,300, respectively, which correspond to ‘‘N’’s of 5–407 for
poly(1a) and 8–213 for poly(1b).
INTRODUCTION Dynamic helical polymers, such as polyiso-
cyanates,1 polysilanes,2 and polyacetylenes,3–7 readily invert
the helical sense owing to relatively low energy barriers for the
helix reversal.8 Thus, they can transform their higher order
structures, for example, from helix to coil and/or from right- to
left-handed helices, offering their potential application to chiral
sensors, molecular memory, and so on. Lifson, Green, Tera-
moto, Sato and coworkers proposed the statistical mechanical
theory for helix inversion of polyisocyanates, in which a poly-
mer chain of optically active polyisocyanates is composed of
biased right- and left-handed long helical sequences separated
by the reversal points.1,9–13 This theory expects that the chirop-
tical properties of the polymers considerably depend on their
molecular weights, because the molecular weight and the aver-
age length of the one-handed helical sequence determine the
number of helix reversal per molecule. It was actually proven
by the experiments using poly[(R)-2-deuterio-1-hexyl isocya-
nate]; for example, the specific rotation [a] at a wavelength of
300 nm rapidly increases with the degree of polymerization (N)
below about 1000 but tends to level off to be limited to an
apparent constant value in a higher molecular weight region.11
Helical polyacetylenes are amenable to the same theory,8 as
experimentally demonstrated by circular dichroism (CD) spec-
troscopy along with atomic force microscopy,14 variable temper-
ature NMR spectroscopy,15 thermodynamic calculations,16 and
so on. However, there has been no systematic study directly
showing the dependence of optical properties of the polymers
on their molecular weights, to the best of our knowledge. This
study deals with the chiroptical behavior of predominantly one-
handed helical amino acid based polyphenylacetylene deriva-
tives depending on the molecular weights.
The CD and UV–vis spectra of the obtained polymers were
measured in dimethylformamide (DMF) at room temperature
(Fig. 1). Poly(1a)s with Mns of 5100–128,800 showed split-type
intense Cotton effects in the conjugated polyene chromophore
region, whereas poly(1a)s with the lowest class of molecular
weight (Mn = 2600) did not show any Cotton effect [Fig. 1(a)]. It
suggests that poly(1a)s with the mid-range Mn formed predomi-
nantly one-handed helical structures in DMF. The Cotton effects
tend to become larger with the increase of Mn of poly(1a) in the
mid-range Mn. However, its trend obviously decelerated in the
higher Mn region more than 10,000 of Mn. Finally, the intensity
increase of the Cotton effect seems to achieve saturation at the
highest Mn region. Analogous poly(1b), having a Me group on
the carbamate nitrogen, showed almost identical tendencies to
poly(1a), although the lowest molecular weight poly(1b) (Mn
¼
2700) still show slight Cotton effect in its CD spectrum [Fig. 1(b)].
A similar tendencies of this molecular weight dependence for
both poly(1a) and (1b) were also seen in the CD spectra with
their CHCl3 solutions.
The optical properties of poly(1a) and poly(1b) were further
investigated with the Kuhn dissymmetry factor, g (g ¼ De/e, in
which De ¼ [y]/3298), which gives quantitative information of the
degree of preferential screw sense.21 Plotting Mn (and degree of
polymerization N calculated based on Mn and the formula
weight of the monomer units) versus g values at the wave-
lengths of about 290, 340, and 390 nm clearly shows the molec-
ular weight dependence of the optical properties of poly(1a) and
poly(1b) [Fig. 1(c) and (d), respectively]. The g values of poly(1a)
greatly extended with increase of Mn, but the development
abruptly turned dull at Mn of about 10,000 and the g value
appeared to become saturated. Poly(1b) showed the almost
same tendency as displayed in Figure 1(d), although the turning
point was slightly shifted to a lower molecular weight region.
RESULTS AND DISCUSSION
We chose the binary Rh catalyst system consisting of
[(nbd)Rh{C(Ph)¼CPh2}(PPh3)]17 and PPh3 to the polymerization
of monomers 1a and 1b (Scheme 1). This kind of catalyst pro-
motes cis-stereoregulated living polymerization of phenylacety-
lene-type monomers and N-propargylamides as reported in the
literature.18–20 The corresponding polymers with relatively
small molecular weight distribution were successfully obtained
Additional Supporting Information may be found in the online version of this article.
C
V
2011 Wiley Periodicals, Inc.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2011, 49, 4921–4925
4921

NOTE
WWW.POLYMERCHEMISTRY.ORG
weight of the polymer, corresponding to the constant forma-
tion of helices and relatively low frequency of helix reversals.
This tendency considerably changed at the molecular weight
region with Mn higher than 10,000. The growth of g value
tends to be slower compared with that in the second region
and also appears to be almost saturated. These behaviors
appear fitting to the statistical mechanical theory proposed by
Lifson, Green, Teramoto, Sato and coworkers,1,9–13 as previ-
ously displayed by the variable temperature CD spectra and
the thermodynamic calculations.16 In this theory, the numbers
of helix reversal in one polymer chain is the key importance; a
short polymer chain (in the mid-range molecular weight
region) does not tend to have a number of reversal points and
thus the De rise directly reflects the molecular weight of the
polymer. On the contrary, a longer polymer chain can have
more helix reversal points, which results in the apparent satu-
ration on helix bias and eventually slow growth of the g value
against molecular weight increase.
EXPERIMENTAL
Instruments
Polymers were purified by preparative HPLC on JAIGEL-1H
and JAIGEL-2H. 1H (400 MHz) and 13C NMR (100 MHz) spectra
TABLE 1 Polymerization of 1a and 1ba
SCHEME 1 Polymerization of amino acid based phenylacety-
lene derivatives 1a and 1b with the well-defined catalyst,
[(nbd)Rh{C(Ph)¼CPh2}(PPh3)]/PPh3.
[M]0
[M]0/ Yieldb
Mw/
Nd
c
c
Entry Monomer (M)
[Rh]
(%)
Mn
Mn
1
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1b
1b
1b
1b
1b
1b
1b
1b
1b
1b
0.20
0.20
0.20
0.20
0.20
0.20
0.10
0.05
0.01
0.001
0.20
0.20
0.20
0.20
0.20
0.20
0.20
0.20
0.10
0.05
0.01
75
60
50
25
20
15
10
5
66
128,800 1.11 407
44,100e 1.32 139
2
74
95
The helical conformation of substituted polyacetylenes is
stabilized by the intramolecular hydrogen bonding between
the side chains having appropriate substituents such as am-
ide23 and carbamate24 moieties. In CHCl3, monomer 1a (c ¼ 20
mM) and poly(1a) (Mn ¼ 12,800, c of the monomer unit ¼ 20
mM) displayed C¼O stretching absorption peaks at 1692 and
1,669 cm–1, respectively, both of which rationally consist of the
overlapped two carbonyl absorption peaks of the carbamate
and amide. The observed shift by 23 cm–1 between 1a and
poly(1a) suggests the formation of intramolecular hydrogen
bonding in poly(1a).25 It is noted that the poly(1a)s with lower
Mns exhibited smaller shifts of C¼O absorption in the IR spec-
tra (Table 2), corresponding with the relationship between the
molecular weight of poly(1a) and its CD intensity. Methylated
monomer 1b displayed separated two C¼O stretching peaks at
1,696 and 1,665 cm–1 assignable to amide and carbamate moi-
eties. Poly(1b)s showed a single broad peak with an apparent
peak top at 1,684–1,685 cm–1 (Table 2). Although the peak shift
for poly(1b) was not as large as that for poly(1a), the result
supports the presence of intramolecular hydrogen bonding.
Each result displayed in Figure 1(c) and (d) consists of three
different regions along their abscissa axes. In the lowest mo-
lecular weight region, there is no significant excess formation
of one-handed helices because the Cotton effect was not
clearly observed in this region and the IR spectra did not show
any significant shift of C¼O stretching peaks. It suggests that
3
19,500 1.24
12,800 1.21
10,000 1.24
8900 1.21
7300 1.25
5100 1.24
2600 1.28
1500 1.64
62
40
32
28
23
16
8
4
92
5
59
6
>99
>99
48
7
8
9
1
49
10
11
12
13
14
15
16
17
18
19
20
21
a
0.5 >99
5
150
82
79
58
65
37
64
67
66
57
42
85
70,300 1.14 213
50,300f 1.13 152
100
60
50
70
25
20
15
10
5
18,700 1.19
16,000 1.22
15,300 1.23
7100 1.41
6100 1.45
5800 1.34
5700 1.18
3800 1.17
2700 1.14
57
48
46
21
18
18
17
12
8
1
In DMF/CH2Cl2 (3/1 v/v), 30 ꢀC, for 24 h.
Isolated yield.
b
c
Estimated by GPC (PSt Standard).
d
Degree of polymerization calculated by dividing the Mn by the formula
weight of the monomer unit [316.39 for poly(1a), 330.42 for poly(1b)].
helix formation requires
a certain level of polymer chain
length.26 The mid-range molecular weight region showed the
e
Mw (SEC) ¼ 58,200, Mw (MALLS) ¼ 67,200 (dn/dc ¼ 0.147 mL/g).
f
strongly rising up of the g value with increasing molecular
Mw (SEC) ¼ 56,800, Mw (MALLS) ¼ 93,100 (dn/dc ¼ 0.142 mL/g).
4922
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2011, 49, 4921–4925

WWW.POLYMERCHEMISTRY.ORG
NOTE
FIGURE 1 CD and UV–vis spectra of series of poly(1a)s and poly(1b)s [(a) and (b)] and Mn-g value plots for poly(1a)s and poly(1b)s
[(c) and (d)]. The solid curves in (c) and (d) were the theoretical lines calculated with the equations based on ref. 22 (also see Sup-
porting Information Table S1).
were recorded on JEOL EX-400 and JEOL AL-400 spectrome-
ters. Elemental analyses were performed at the Microanalytical
Center of Kyoto University. IR spectra were measured on a
JASCO FT/IR-4100 spectrophotometer. Melting points (mp)
were measured on a Yanaco micro melting point apparatus.
Number- and weight-average molecular weights (Mn and Mw)
of polymers were determined by size exclusion chromatogra-
phy (SEC) on TSK gel a-M and TSK gel GMHXL, using a solu-
tion of LiBr (10 mM) in DMF as an eluent at a flow rate of 1.0
mL/min, calibrated by polystyrene standards at 40 ꢀC. CD and
UV–vis spectra were measured in a quartz cell (optical distance:
1 cm) using a JASCO J-820 spectropolarimeter. Specific rota-
tions ([a]D) were measured on a JASCO DIP-100 digital polarim-
eter with a sodium lamp as a light source. Absolute Mws27 of
polymers were determined by multiangle laser light scattering
(MALLS) equipped with SEC on a Dawn E instrument (Wyatt
Technology; Ga–As laser, k ¼ 690 nm). The SEC was performed
on three linear-type polystyrene gel columns (Shodex KF-805L),
using a solution of LiBr (10 mM) in DMF as an eluent at a flow
rate of 1.0 mL/min at 40 ꢀC. The refractive index increment (dn/
dc) was measured in DMF at 40 ꢀC on an Optilab DSP refrac-
tometer (Wyatt Technology; k ¼ 690 nm, c < 2.5 mg/mL).
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2011, 49, 4921–4925
4923

NOTE
WWW.POLYMERCHEMISTRY.ORG
TABLE 2 Solution-State IR Spectroscopic Data of Monomers 1a
and 1b, Poly(1a), and Poly(1b)a
N-tert-Butoxycarbonyl-N-methyl-L-valine
4-ethynylanilide (1b)
The title compound was synthesized from Boc-N-Me-Val-OH
and 4-ethynylaniline in a similar manner to N-tert-butoxycar-
bonyl-L-valine 4-ethynylanilide. Yield: 36% (white powder).
[a]2D3: ꢁ86.5ꢀ (in DMF, c ¼ 0.10 g/dL), ꢁ161.4ꢀ (in CHCl3, c ¼
0.10 g/dL). 1H NMR (CDCl3): d ¼ 8.47 (s, 1H, ArNHCO), 7.49 (d,
J ¼ 8.4 Hz, 2H, HAr), 7.44 (d, J ¼ 8.8 Hz, 2H, HAr), 4.10 (d, J ¼
11.0 Hz, 1H, CHCH(CH3)2), 3.03 (s, 1H, CBCH), 2.83 (s, 3H,
NCH3), 2.38 (br m, 1H, CH(CH3)2), 1.49 (s, 9H, C(CH3)3), 1.03 (d,
J ¼ 6.4 Hz, 3H, CH3), 0.93 (d, J ¼ 6.4 Hz, 3H, CH3). 13C NMR
(CDCl3): d ¼ 171.1 (ArNHCO), 157.6 (N(CH3)COO), 138.5 (CAr),
132.9 (CAr), 119.2 (CAr), 117.4 (CAr), 83.4 (CBCH), 80.9 (CBCH),
76.6 (CH(CH3)3), 66.5 (CHNCH3), 30.8 (CH(CH3)2), 28.3
Wavenumber (cm–1
)
Compound
C¼O
NAH
1508
1508
1505
1511
1511
1515
1a
1ab
1692c
1694c
1669c
1672c
1689c
poly(1a) (Mn ¼ 12,800)
poly(1a) (Mn ¼ 7300)
poly(1a) (Mn ¼ 1500)
1b
1696, 1665 (amide,
carbamate)
1bb
1697, 1668 (amide,
carbamate)
1684c
1684c
1685c
1517
(CH(CH3)3), 26.0 (NCH3), 19.9, and 18.7 (CH3s of iPr). IR (cmꢁ1
,
KBr): 3322, 2974, 2109 (CBC), 1689, 1656, 1597, 1399, 1361,
1310, 1244, 1156, 838. Mp: 103 ꢀC. Anal. Calcd for C19H26N2O3:
C 69.06, H 7.93. Found: C 69.06, H 7.90.
poly(1b) (Mn ¼ 50,300)
poly(1b) (Mn ¼ 5,800)
poly(1b) (Mn ¼ 2700)
1518
1518
1519
Polymerization
The polymerization of 1a and 1b was performed under the
conditions described in each entry of Table 1. A solution of
[(nbd)Rh{C(Ph)¼CPh2}(PPh3)] and PPh3 in DMF/CH2Cl2 (3/1 v/v)
was added to a solution of a monomer in DMF/CH2Cl2 (3/1 v/v)
in a glass tube equipped with a three-way stopcock under ar-
gon. The resulting mixture was stirred at 30 ꢀC for 24 h. After
confirming the formation of a polymer with a target molecular
weight by GPC, the polymer was isolated by preparative HPLC
(see the details in Section Instruments). The polymer samples
were finally freeze-dried from their benzene solution for com-
plete removal of solvent.
a
Measured in CHCl3 (c ¼ 20 mM).
b
c ¼ 10 mM.
c
Two carbonyl peaks of the carbamate and amide were overlapped.
Materials
TRIAZIMOCH [4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmor-
pholinium chloride] was offered by Tokuyama and used without
further purification. DMAP [4-(N,N-dimethylamino)pyridine]
(Wako), p-ethynylaniline (Wako), Boc-Val-OH [N-(tert-butoxycar-
bonyl)-L-valine] (Kokusan Chemical), Boc-N-Me-Val-OH [N-(tert-
butoxycarbonyl)-N-methyl-L-valine] (Watanabe Chemical Indus-
tries), triphenylphosphine (Aldrich) were purchased and used as
received. [(nbd)Rh{C(Ph)¼CPh2(PPh3)}] (nbd ¼ 2,5-norbornadiene)
was prepared by the reported method.17 DMF and CH2Cl2 for
polymerization were purified by distillation over CaH2.
CONCLUSIONS
This work revealed that the chiroptical properties of the amino
acid based polyacetylenes strongly depend on their molecular
weight, which is applicable to the thermodynamic calculation
estimating the free energy difference between the right- and
left-handed helical states, the energy of the helix reversal state.
Further investigation including these calculations is now under
progress.
Monomer Synthesis
N-tert-Butoxycarbonyl-L-valine 4-ethynylanilide (1a)
TRIAZIMOCH (water content: 14.7%, 7.81 g, 24.1 mmol) and
DMAP (293 mg, 2.40 mmol) were added to a stirred solution of
4-ethynylaniline (2.34 g, 20.0 mmol) and Boc-Val-OH (5.21 g,
24.0 mmol) in THF (50 mL) at 0 ꢀC, and the reaction mixture was
stirred overnight at room temperature. Ethyl acetate (50 mL) was
added to the mixture, and the organic phase was sequentially
washed with 0.5 M HCl aq (50 mL), saturated NaHCO3 aq.
(50 mL), and saturated NaCl aq. (50 mL). The organic phase was
dried over anhydrous MgSO4, filtered, and concentrated under
reduced pressure. The residue was purified by column chroma-
tography (SiO2, hexane/ethyl acetate ¼ 4/1 v/v). Yield: 67% (yellow
powder). [a]2D3: þ8.1ꢀ (in DMF, c ¼ 0.10 g/dL), ꢁ55.1ꢀ (in CHCl3, c
¼ 0.10 g/dL). 1H NMR (CDCl3): d ¼ 8.60 (br s, 1H, ArNHCO), 7.43
(d, J ¼ 8.4 Hz, 2H, HAr), 7.38 (d, J ¼ 8.4 Hz, 2H, HAr), 5.32 (br m,
1H, NHCOO), 4.07 (br m, 1H, CHCH(CH3)2), 3.03 (s, 1H, CBCH),
2.21 (br m, 1H, CH(CH3)2), 1.44 (s, 9H, C(CH3)3), 1.02 (d, J ¼ 6.8
Hz, 3H, CH3), 1.00 (d, J ¼ 6.8 Hz, 3H, CH3). 13C NMR (CDCl3): d ¼
170.5 (ArNHCO), 156.5 (NHCOO), 138.1 (CAr), 132.8 (CAr), 119.4
(CAr), 117.7 (CAr), 83.4 (CBCH), 80.5 (CBCH), 76.7 (C(CH3)3), 61.0
(CHNH), 30.6 (CH(CH3)2), 28.3 (C(CH3)3), 19.4 and 18.2 (CH3s of
iPr). IR (cm–1, KBr): 3306, 2967, 2109 (CBC), 1671, 1599, 1527,
1406, 1368, 1287, 1249, 1172, 840. Mp: 133 ꢀC. Anal. Calcd for
ACKNOWLEDGMENT
The authors acknowledge the supports from a Grant-in-Aid for
Scientific Research from the Japan Society for Promotion of Sci-
ence and a Grant-in-Aid from the Kyoto University Global COE
Program, International Center for Integrated Research and
Advanced Education in Materials Science. The authors are
grateful to Prof. Mitsuo Sawamoto, Prof. Takaya Terashima and
Mr. Yuta Koda at Kyoto University for measurement of SEC-
MALLS, and Prof. Yo Nakamura at Kyoto University for helpful
discussion. In this research work the authors used the super-
computer of ACCMS, Kyoto University.
REFERENCES AND NOTES
1 Green, M. M.; Peterson, N. C.; Sato, T.; Teramoto, A.; Cook,
R.; Lifson, S. Science 1995, 268, 1860–1866.
2 Fujiki, M.; Koe, J. R.; Terao, K.; Sato, T.; Teramoto, A.; Wata-
nabe, J. Polym J 2003, 35, 297–344.
3 Tabata, M.; Sone, T.; Sadahiro, Y. Macromol Chem Phys
C18H24N2O3: C 68.33, H 7.65. Found: C 68.40, H 7.57.
1999, 200, 265–282.
4924
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2011, 49, 4921–4925

WWW.POLYMERCHEMISTRY.ORG
NOTE
4 Aoki, T.; Kaneko, T.; Teraguchi, M. Polymer 2006, 47, 4867–4892.
5 Akagi, K. Chem Rev 2009, 109, 5354–5401.
20 Zhang, W.; Shiotsuki, M.; Masuda, T. Polymer 2007, 48,
2548–2553.
6 Liu, J.; Lam, J. W. Y.; Tang, B. Z. Chem Rev 2009, 109,
5799–5867.
21 Dekkers, H. P. J. M. In: Circular Dichroism: Principles and
Applications, 2nd ed.; Berova, N.; Nakanishi, K.; Woody, R. W.,
Eds.; Wiley-VCH: Weinheim, 2000; Chapter 7 Circularly Polar-
ized Luminescence: A Probe for Chirality in the Excited State.
7 Shiotsuki, M.; Sanda, F.; Masuda, T. Polym Chem 2011,
2, 1044–1058.
8 Yashima, E.; Maeda, K.; Iida, H.; Furusho, Y.; Nagai, K. Chem
Rev 2009, 109, 6102–6211.
22 Green, M. M. In: Circular Dichroism: Principles and Applica-
tions, 2nd ed.; Berova, N.; Nakanishi, K.; Woody, R. W., Eds.;
Wiley-VCH: Weinheim, 2000; Chapter 17 A Model for How Poly-
mers Amplify Chirality.
9 Lifson, S.; Green, M. M.; Andreola, C.; Peterson, N. C. J Am
Chem Soc 1989, 111, 8850–8858.
10 Lifson, S.; Felder, C. E.; Green, M. M. Macromolecules 1992,
23 Nomura, R.; Tabei, J.; Masuda, T. J Am Chem Soc 2001,
25, 4142–4148.
123, 8430–8431.
11 Gu, H.; Nakamura, Y.; Sato, T.; Teramoto, A.; Green, M. M.;
Andreola, C.; Peterson, N. C.; Lifson, S. Macromolecules 1995,
28, 1016–1024.
24 Sanda, F.; Nishiura, S.; Shiotsuki, M.; Masuda, T.; Macromo-
lecules 2005, 38, 3075–3078.
25 The conformational analysis by the molecular orbital (MO)
calculation supported the formation of a helical structure stabi-
lized by intramolecular hydrogen bonding between the amide
and carbamate groups at the side chains of poly(1a) (See Sup-
porting Information Figs. S1 and S2).
12 Gu, H.; Sato, T.; Teramoto, A.; Varichon, L.; Green, M. M.
Polym J 1997, 29, 77–84.
13 Yoshiba, K.; Hama, R.; Teramoto, A.; Nakamura, N.; Maeda,
K.; Okamoto, Y.; Sato, T. Macromolecules 2006, 39, 3435–3440.
14 Sakurai, S.; Ohsawa, S.; Nagai, K.; Okoshi, K.; Kumaki, J.;
Yashima, E. Angew Chem Int Ed 2007, 46, 7605–7608.
26 The MO calculation of poly(1a) (2–20 mers) supported that a
certain degree of polymerization is necessary for the polymer
to adopt a regulated helical conformation. Otherwise, the tor-
sional angles of the single bonds at the main chain deviate
from the most stable value (ca. 145ꢀ) (See Supporting Informa-
tion Fig. S3).
15 Nomura, R.; Fukushima, Y.; Nakako, H.; Masuda, T. J Am
Chem Soc 2000, 122, 8830–8836.
16 Morino, K.; Maeda, K.; Okamoto, Y.; Yashima, E.; Sato, T.
Chem Eur J 2002, 8, 5112–5120.
27 As shown in Table 1, SEC-MALLS revealed the absolute
molecular weights of poly(1a) and poly(1b) somewhat larger
than those estimated by SEC [115% for poly(1a) (Entry 2), 163%
for poly(1b) (Entry 12)]. This is probably because both of the
polymers adopt a helical conformation that is more packed than
randomly coiled polystyrenes used for SEC calibration. The result
also suggests that poly(1b) is more ‘‘packed’’ than poly(1a).
17 Onitsuka, K.; Yamamoto, M.; Mori, T.; Takei, F.; Takahashi,
S. Organometallics 2006, 25, 1270–1278.
18 Miyake, M.; Misumi, Y.; Masuda, T. Macromolecules 2000,
33, 6636–6639.
19 Nakazato, A.; Saeed, I.; Shiotsuki, M.; Sanda, F.; Masuda, T.
Macromolecules 2004, 37, 4044–4047.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2011, 49, 4921–4925
4925