Chiroptical properties of graft copolymers containing chiral poly(n -hexyl isocyanate) as a side chain

Article abstract of DOI:10.1021/ma2019649

Chiral poly(n-hexyl isocyanate) (PHIC) macromonomers were synthesized by living anionic polymerization of n-hexyl isocyanate (HIC) using the functional initiator sodium N-(4-vinylphenyl)benzamide (Na-4VPBA) in tetrahydrofuran (THF) at -98 °C. Polymerizati

Full text of DOI:10.1021/ma2019649

ARTICLE
pubs.acs.org/Macromolecules
Chiroptical Properties of Graft Copolymers Containing Chiral
Poly(n-hexyl isocyanate) as a Side Chain
Priyank N. Shah, Joonkeun Min, Hyung-Jin Kim, Sang-Yoon Park, and Jae-Suk Lee*
School of Materials Science and Engineering and Department of Nanobio Materials and Electronics, Gwangju Institute of Science and
Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju 500-712, Republic of Korea.
ABSTRACT: Chiral poly(n-hexyl isocyanate) (PHIC) macro-
monomers were synthesized by living anionic polymerization of
n-hexyl isocyanate (HIC) using the functional initiator sodium
N-(4-vinylphenyl)benzamide (Naꢀ4VPBA) in tetrahydrofuran
(THF) at ꢀ98 °C. Polymerization was terminated by (S)-2-
acetoxypropionyl chloride using pyridine as a catalyst. Initiator
efficiency and aggregation behavior during polymerization were
also studied. Chiral PHIC macromonomers with varying mo-
lecular weight (MW) were synthesized with a narrow molecular
weight distribution (MWD). Graft copolymerization of the
chiral PHIC macromonomers was performed in toluene at 80 °C under vacuum using free radical polymerization with a 2,20-
azobis(2-methylpropionitrile) (AIBN) initiator. Several experiments were performed to determine the optimum polymerization
time for macromonomers and graft copolymers. Formation of the graft copolymer was confirmed using SECꢀMALLS, 1H NMR,
and circular dichroism (CD). CD spectra showed that graft copolymers adopted a conformation with opposite helical sense and a
lower CD intensity as compared with chiral PHIC macromonomers.
’ INTRODUCTION
Additionally, the NaꢀBA initiation system gives living PHIC
with a long enough lifetime to efficiently perform the termina-
tion reaction. Our group has reported helicity induction in
PHIC using a NaꢀBA initiation system and pyridine catalyzed
chiral termination. Helicity induction via introduction of a
terminal chiral moiety is due to the covalent chiral domino
(CCD) effect, in which a chiral moiety is attached to the α-end
of the polymeric chain.20 The terminal chiral moiety of PHIC
can induce helicity in up to ∼36 monomer units (i.e., ∼5.0 kDa)
through the CCD effect. The chiral initiator can induce helicity
in poly(aryl isocyanate) up to ∼12 monomer unit.10 Helicity
induction through the domino effect is also achieved in peptides
using covalent and noncovalent interactions21 and has great
biological importance. Several key physiological functions in
nature are controlled by stereochemical information transmis-
sion through a remote chiral moiety. Such signal transmission is
carried out by G-protein coupled receptors (GPCRs), which
effect conformational changes in the adjacent proteins and
transmit stereochemical information on the nanometer scale
through the domino effect.22
Macromolecular and supramolecular helical structures per-
form sophisticated functions in nature, including recognition,
replication, and catalytic activity. Inspired by these discrete
helical biopolymers, chemists have constructed various artificial
helical polymers and self-assembled supramolecules with con-
trolled helicity from small molecule precursors.1,2 The amplifica-
tion of chirality in various helical polymers such as poly(phenyl-
acetylene)s, polyisocyanides, and polysilanes have been studied
experimentally and theoretically.3,4 The transfer of chiral infor-
mation through noncovalent interactions to helical polymers by a
so-called memory effect is also well established.5,6
Among all synthetic helical polymers, polyisocyanate is one of
the most studied materials in helicity induction, due to its
dynamic right-handed (P) and left-handed (M) helical confor-
mations and its ability to induce optical activity.7,8 The groups of
Green,9 Okamoto,10 Zentel,11 and others12 have conducted a
number of noteworthy studies on helicity induction in polyiso-
cyanates. Isocyanate polymers with a single helical screw sense
can be obtained by using chiral initiators,13 monomers,14 or
solvents,15 which has led to the discovery of several interesting
phenomena such as the sergeants-and-soldiers principle9,16 and
the majority rule.17
At lower temperature, polyisocyanates undergo a noncova-
lently aggregated state, which is observable via a sudden rise in
CD intensity.23,24 This state was first observed by Bur et al. in
nonpolar solvents.25 Later, Green et al. observed polyisocyanates
in a pregelled state during the cooling process; a “pre-gel” is a
During polymerization poly(n-hexyl isocyanate) (PHIC)
undergoes rapid trimerization unless an additive is used.18
The polymerization of n-hexyl isocyanate (HIC) has been
successfully performed using sodium benzanilide (NaꢀBA)
and sodium benzhydroxide (NaꢀBH) as initiators; these mole-
cules act as initiators, and they also chain-end protection.19
Received: August 26, 2011
Revised:
September 18, 2011
Published: September 29, 2011
r
2011 American Chemical Society
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Scheme 1. (a) Synthesis of Initiator Sodium N-(4-Vinylphenyl)benzamide, (b) Synthesis of Chiral Macromonomer of
Poly(n-hexyl isocyanate) (PHIC), (c) Synthesis of Graft Copolymer of PHIC Macromonomer by Free Radical Polymeriza-
tion, and (d) Synthesis of Random Graft Copolymers of PHIC Macromonomer with Styrene by Free Radical Polymerization
noncovalently aggregated polymer state that forms before
precipitation.26 Aggregated biomacromolecules (peptides) cause
some neurodegenerative disorders like Alzheimer’s, Parkinson’s,
and Huntington’s diseases.27
Graft copolymers containing side chain PHIC are an example
of a covalently aggregated state. PHIC macromonomers have
been prepared by several groups for synthesis of such graft
copolymers, using different synthetic routes. Novak et al. have
synthesized PHIC macromonomers with acrylate ends using
living coordination polymerization.28 Se et al. also synthesized
acrylate-terminated PHIC but reported poor copolymerizability
with methyl methacrylate (MMA).29 Kikuchi et al. synthesized
styryl- or acrylate-terminated PHIC, which showed equivalent
reactivity to styryl-terminated PHIC with styrene; these research-
ers described poor copolymerizability of acrylate-terminated
PHIC with MMA due to steric hindrance around double
bonds.30,31 Moreover, in dimensional property analysis of the rod
brushes they observed that the main chain stiffness of the rod
brush is higher than that of brush consisting of flexible side
chains.32
In the present work, we examined the chiroptical properties of
the covalently aggregated chiral PHIC. For these purposes, chiral
graft copolymers containing poly(N-(4-vinylphenyl)benzamide as
a backbone and chiral PHIC as a side-chain have been synthesized
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using a “graft-through” technique. The chiral PHIC macromono-
mer was synthesized using a functional initiator and chiral
terminator through living anionic polymerization. This is the first
report so far on thesynthesisof PHICmacromonomer usingliving
anionic polymerization. During graft copolymerization, PHIC
macromonomers are converted from a discrete random state to
a covalently aggregated state. We use the phrase “covalently
aggregated chiral PHIC” interchangeably with “graft copolymer
of chiral PHIC”. Because of the diversity of results in existing
reports, the present work will be highly useful in understanding
changes in helical sense and the range of transmission of stereo-
chemical information in aggregated states of polymer.
the monomer HIC (0.790 g, 6.23 mmol) in 15.40 mL of THF and
addition to the initiator solution. The polymerization reaction was
terminated after 60 min by adding an excess of (S)-2-acetoxypropionyl
chloride (S-type of the chiral terminator, (S)-Ct), followed in quick
succession with pyridine, to the grown polymer chain. The polymer was
precipitated into methanol, filtered, and dried in vacuo. The polymer was
obtained with a yield of 99%. The dn/dc value of the PHIC macro-
monomer (CH-2) was obtained to be 0.0908 mL/g. The molecular
weight and molecular weight distribution of the polymer were obtained
to be 5.00 kDa and 1.10, respectively. Several polymerization reactions
were conducted to check the efficiency of the initiators; in those cases
polymerization reactions were terminated using acidified methanol.
Dissolution in THF and reprecipitation in methanol was repeated to
obtain highly pure polymers. The polymers were further dissolved in
benzene and freeze-dried for characterization. The solvent-soluble
components were analyzed by weighing the residue after evaporation
’ EXPERIMENTAL SECTION
Materials. n-Hexyl isocyanate (Aldrich, 97%), pyridine (Aldrich,
99.5%), and (S)-2-acetoxypropionyl chloride (Aldrich, [α]20D = ꢀ31°,
98%) were dried over CaH2 and vacuum distilled. Tetrahydrofuran
(THF, Fisher Scientific, GR grade) was distilled under N2 after refluxing
with sodium for 5 h and distilled again under vacuum from sodium
naphthalenide (NaꢀNaph) solution. Sodium (Aldrich, 99%), calcium
hydride (Junsei, 95%), naphthalene (Aldrich, 99%), 2,20-azobis-
(isobutyronitrile) (AIBN, TCI, 98%), 4-vinylaniline (Aldrich, 97%),
benzoyl chloride (Aldrich, 99%), triethylamine (TEA, Aldrich, 99%),
and dichloromethane (DCM, Aldrich, 99%) were used without further
purification.
1
of solvent, and HNMR was used to check whether any unreacted
monomers or trimers remained. PHIC macromonomer: 1H NMR
(CDCl3, 300 MHz)), δ (ppm): 0.9 (3H, CH3), 1.0ꢀ2.0 (8H, (CH2)4),
3.7 (2H, NꢀCH2-), 5.20 (dd, 1H, CH2dCHꢀ), 5.68 (dd, 1H,
CH2dCHꢀ), 6.71 (q, 1H, CH2dCHꢀ). 13C NMR (CDCl3, 75 MHz),
δ (ppm): 14.5 (CH3), 22.5 (CH2), 26.2 (CH2), 28.5 (CH2), 31.5 (CH2),
48.6 (NꢀCH2ꢀ), 156.8 (CdO). IR (KBr, cmꢀ1): 2959, 2932, 2860
(CH2), 1700 (CdO), 1349/1297 (disubstituted amide), 1227, 1175,
1092, 785, 728 (CH2).
Synthesis of the Graft Copolymer. The purified and freeze-
dried chiral PHIC (CH-1) macromonomer (0.40 g, 93.0 μmol) was
further dried under vacuum at 10ꢀ6 Torr. Polymerization of the
macromonomer was performed in 0.93 mL toluene using AIBN (1.53 mg,
11.3 μmol) as an initiator at 80 °C as shown in Scheme 1c. The mixture
of the macromonomer, solvent, and initiator was placed in glass ampules,
degassed three times using freezeꢀthaw cycles, sealed under high
vacuum (10ꢀ6 Torr) and placed in a thermostatted bath. After comple-
tion of the reaction, the reaction mixture was precipitated in methanol,
filtered and dried under vacuum. Unreacted macromonomer from the
product was completely removed by three rounds of fractional pre-
cipitation using THF and methanol. The graft copolymer was freeze-
dried with benzene. The chiral graft copolymer was obtained with a yield
of 86%. The dn/dc value of the graft copolymer was observed to be
0.0887 mL/g. The CG-1’s molecular weight and molecular weight
distribution was found to be 31.0 kDa and 1.43, respectively. PHIC
Initiator. N-(4-Vinylphenyl)benzamide (4VPBA) was synthesized
in one step as shown in Scheme 1a. A one-neck flask equipped with a
dropping funnel and nitrogen gas was charged with 4-vinylaniline (4.50 g,
37.8 mmol) and TEA (6.00 g, 59.3 mmol) in DCM. The stirred mixture
was maintained at 0 °C for 1 h, and benzoyl chloride (6.00 g, 42.7 mmol)
in DCM was then added dropwise to the solution and stirred in an ice
bath for 30 min. The reaction mixture was stirred at 0 °C for 3 h. After
completion of the reaction, the remaining salt was removed by filtration,
the remaining acidic filtrate was neutralized with sodium bicarbonate,
and the product was extracted with chloroform. The synthesized 4VPBA
was purified by recrystallization from ethanol three times and dried in
vacuo. Sodium N-(4-vinylphenyl)benzamide (Naꢀ4VPBA) in THF
(50 mL) was prepared at room temperature via the reaction of 4VPBA
(2.12 g, 9.50 mmol) with elemental sodium (0.180 g, 7.80 mmol), in a
1.2:1.0 ratio. When the color of the reaction mixture turned a light yellow
it was frozen in liquid nitrogen and connected to a high vacuum line
(10ꢀ6 Torr) for degassing. After complete degassing, the obtained
initiator was stored at ꢀ30 °C in glass ampules with break-seals. An
appropriate concentration of Naꢀ4VPBA was diluted prior to use.
4VPBA. 1H NMR (CDCl3, 300 MHz), δ (ppm): 6.71 (q, 1H,
CH2dCHꢀ), 5.68 (dd, 1H, CH2dCHꢀ), 5.20 (dd, 1H, CH2dCHꢀ),
7.88 (m, 1H, ꢀNH). 7.30ꢀ7.80 (8H, two aromatic rings). 13C NMR
(CDCl3, 75 MHz), δ (ppm): 113.18 (CH2dCHꢀ), 136.12
(CH2dCHꢀ), 165.64 (CdO), 120.13, 126.93, 128.81, 131.88,
134.02, 134.94, 136.12, 137.50 (8C, ꢀCꢀH of aromatic ring). IR
(KBr, cmꢀ1): 3340 (ꢀNH), 3081, 3054, 1600ꢀ1700 (CdO),
1600ꢀ1400 (CdC of aromatic ring), 840, 710. Anal. Calcd for
C15H13NO: C, 80.69; H, 5.87; N, 6.27; O, 7.17. Found: C, 80.71; H,
5.89; N, 6.25; O, 7.18.
1
graft copolymer: H NMR (CDCl3, 300 MHz)), δ (ppm): 0.9 (3H,
CH3), 1.0ꢀ2.0 (8H, (CH2)4), 3.7 (2H, NꢀCH2-). 13C NMR (CDCl3,
75 MHz), δ (ppm): 14.5 (CH3), 22.5 (CH2), 26.2 (CH2), 28.5 (CH2),
31.5 (CH2), 48.6 (N-CH2-), 156.8 (CdO). IR (KBr, cmꢀ1): 2958, 2931,
2860 (CH2), 1702 (CdO), 1350/1298 (disubstituted amide), 1228,
1172, 1093, 784, 729 (CH2).
Characterization. The 1H and 13C NMR spectra were measured
using a JEOL JNMLA300WB, using CDCl3 as the solvent. Chemical
shifts were referenced to tetramethylsilane (TMS) at 0 ppm. Molecular
weights were determined from the response of a multiangle laser light
scattering detector system (MALLS), SECꢀLS, (OPTI LAB-DSP
interferometric refractometry 478ꢀ009ꢀ690 and DAWN EOS laser
photometer 113-E, Wyatt Technology) with four columns (HR 0.5, HR
1, HR 3, and HR 4, Waters Styragel columns run in series with column
pore sizes 50, 100, 500, and 1000 Å, respectively). THF with triethyla-
mine (for the prevention of adsorption of hydrophilic polymer onto the
column) was used as the mobile phase at a flow rate of 1.0 mL/min. The
dn/dc value for the polymer in THF at 40 °C was measured with an LED
(Optilab DSP) source. After dn/dc was measured for five different
concentrations of each polymer sample, SECꢀLS data were gained with
refractive index detection at 40 °C. FTIR spectra were run with Perkin-Elmer
System 2000 using KBr pellets. Circular dichroism (CD) and ultraviolet
(UV) spectra were measured on a JASCO-J-720 spectropolarimeter using a
Living Anionic Polymerization of HIC and Chiral Termina-
tion. Chiral PHIC macromonomers were synthesized as shown in
Scheme 1b. The unidirectional initiator Naꢀ4VPBA was used to
synthesize chiral PHIC macromonomer under high vacuum (10ꢀ6
Torr) in a glass apparatus following a standard procedure.33 In a typical
polymerization procedure, a solution of Naꢀ4VPBA initiator (0.190 g,
0.860 mmol) in 7.30 mL of THF was transferred to the reaction flask
through the break-seal and the solution temperature was then equili-
brated to ꢀ98 °C. The polymerization was performed by dissolution of
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Table 1. Synthesis of Chiral Macromonomers of Poly(n-hexyl isocyanate) with and without Chiral Termination at ꢀ98 °C in
Tetrahydrofuran
amount of reagent (mmol)
Mn ꢁ 10ꢀ3
e
run
Naꢀ4VPBA
HIC
pyridine
(S)-Ct
time (min)
calcdd
obsde
MWD Mw/Mn
yield (%)
H-1a
H-2a
H-3a
H-4a
0.09
0.43
0.24
0.10
0.11
1.03
0.86
0.87
0.88
1.66
4.42
5.10
4.59
4.48
4.59
5.86
6.23
7.06
8.04
12.6
-
-
30
6.09
1.51
2.43
5.60
5.41
1.06
1.25
1.37
1.49
1.27
28.1
7.60
11.6
27.2
24.1
4.30
5.00
5.50
6.00
5.10
1.10
1.08
1.10
1.09
1.10
1.07
1.10
1.08
1.06
1.08
89(11)f
100
99
-
-
60
-
-
60
-
-
60
100
92(8)g
98
H-5a
-
-
80
CH-1b
CH-2b
CH-3b
CH-4b
CH-5b
3.56
2.97
3.45
2.75
3.35
2.98
2.46
2.78
2.59
2.86
60/20c
60/20c
60/20c
60/20c
60/20c
99
99
100
100
a Polymerization without chiral termination. b Polymerization with chiral termination. c Polymerization time is 60 min and termination reaction time is
20 min. d Mn = {[HIC]/[Naꢀ4VPBA] ꢁ molecular weight of HIC} + molecular weight of Naꢀ4VPBA + molecular weight of (S)-Ct. e Molecular
weight (Mn) and Molecular weight distribution (MWD) were measured by size exclusion chromatography multiangle laser light scattering
f
(SECꢀMALLS) in THF/Et3N at 40 °C. The unreacted monomer amounts are presented in parentheses. g The yield of trimer is presented in
parentheses. Naꢀ4VPBA: sodium N-(4-vinylphenyl)benzamide. (S)-Ct: (S)-type chiral terminator ((S)-2-acetoxypropionyl chloride). HIC: n-hexyl
isocyanate.
cell path length of 0.1 cm. Specific ellipticity values are expressed in molar
ellipticity [Θ]. All CD spectra were smoothed using a Savitzky-Golay filter
with OriginPro-8.1 software. All CD samples were prepared in concentration
of 1 mg/mL. Elemental analysis was carried out using an EA 1110 analyzer
(CE instrument, Italy).
chain-end by preventing the backbiting reaction. This dual
function for initiators can be attributed to the tendency of
Naꢀ4VPBA to form a cluster through charge interactions.
Synthesis of Chiral Macromonomers. HIC was introduced
into the reactor containing Naꢀ4VPBA; on addition, the color of
Naꢀ4VPBA changed from dark yellow to pale yellow, indicating
the formation of the amide anion of HIC. The growing polymer
chain has very low reactivity even with acid chloride, so pyridine
was used as a catalyst to ensure complete termination. Table 1
(CH-1 to CH-4) shows the result of living anionic polymeriza-
tion of HIC with Naꢀ4VPBA initiator in THF at ꢀ98 °C. The
PHIC macromonomers were obtained with quantitative yields of
∼99% in each case. The obtained chiral PHIC macromonomers
varied in molecular weight (MW) in the range of 4.3ꢀ6.0 kDa,
with a narrow molecular weight distribution (MWD), lower than
1.10, as determined by SECꢀMALLS. As shown in Figure 1a, the
1H NMR spectrum of CH-2 clearly indicates attributable to the
vinyl benzyl group of the initiator, acetyl group of terminator, and
hydrogen of aliphatic HIC chain. Moreover, no -NH peak was
evident, which clearly indicates that the introduction of the chiral
moiety to the N-terminus proceeded to completion. By relative
study of peak intensity of the functional initiator, chiral termi-
nator and monomer; the number of monomeric unit in the
polymeric chain was observed to be ∼37. SECꢀMALLS showed
that CH-2 had a MW of 5.0 kDa and DP ∼36, which was in well
accordance with the observed MW by 1H NMR. This suggests
that the present initiator was efficient enough to perform living
polymerization of HIC and afforded well-defined chiral PHIC
macromonomers with quantitative double-bond and chiral func-
tionality. The reactive vinyl group of the Naꢀ4VPBA initiation
system did not give any side reactions during polymerization.
Graft Copolymerization of Chiral PHIC. The purified macro-
monomer was polymerized using free radical polymerization,
which led to covalent aggregation of the chiral PHIC as shown in
Scheme 1c. Table 2 shows the results of free radical polymeri-
zation of the chiral PHIC macromonomer in toluene using AIBN
as an initiator at 80 °C. The PHIC macromonomers of different
MW (Mn = 4.30ꢀ11.6 kDa) were polymerized to establish the
’ RESULTS AND DISCUSSION
Role of Naꢀ4VPBA as an Initiator. 4VPBA was successfully
synthesized in one pot with 4-vinylaniline and benzoyl chloride
in the presence of triethylamine, as shown in Scheme 1a. Reac-
tion of 4VPBA with sodium metal in THF gave a dark yellow
solution of Naꢀ4VPBA. Even at room temperature, Naꢀ4VPBA
did not attack the vinyl group of other initiator molecules.
Naꢀ4VPBA has structural similarity to NaꢀBA, and it also
exhibited weak reactivity similar to NaꢀBA, due to efficient
charge delocalization by the phenyl rings and the carbonyl
group.19a To study the initiation and aggregation behavior of
Naꢀ4VPBA, several polymerizations was performed at different
time intervals as shown in Table 1. The optimum polymerization
time with 100% yield was determined to be 60 min. If polym-
erization was terminated before 60 min (H-1) a significant
amount of unreacted monomer remained in the traces of filtered
solution. If the polymerization time exceeded 60 min (H-5) the
polymer chain showed signs of degradation after complete
consumption of all monomer; as a result, trimers of HIC are
evident in found in the traces of filtered solution. The efficiency
of Naꢀ4VPBA initiator (efficiency of initiator = (calculated
MW/observed MW) ꢁ 100) was found to be between 19 and
21% in each polymerization (H-1 to H-5).
The anion content in Naꢀ4VPBA was estimated by reaction
of Naꢀ4VPBA with CH3COCl following a standard procedure
and was found to be quantitatively with at least 95% yield of the
N-acetyl-N-(4-vinylphenyl)benzamide.19a Despite of having
∼95% reactive anion, only ∼20% of the initiator molecules
initiated polymerization. Therefore, like NaꢀBA, as per our
expectation, Naꢀ4VPBA initiator also had a dual function;19a
the remaining ∼80% of initiator molecules protected the living
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optimum conditions. After 6 h, a unimodal curve of SEC for
the graft copolymer was observed with a very small peak
for unreacted macromonomer, which was removed from the
product mixture by fractional precipitation using THF and
methanol to give completely pure graft copolymer, as shown in
Figure 2. The degree of polymerization (DP) of PHIC macro-
monomer was approximately ∼8.0 in almost all cases, as shown
Chiroptical Properties of Chiral PHIC Macromonomers
and Their Graft Copolymers. CD spectra of chiral PHIC
macromonomers with varying MW in the range 4.3ꢀ6.0 kDa
are shown in Figure 3a. The chiral PHIC macromonomer
showed a positive Cotton effect at 255 nm due to a characteristic
nꢀπ* transition of the polyisocyanate backbone and had a
conformation with a P-helical sense. The CD intensity of chiral
PHIC macromonomer increased as the MW increased from
4.3 kDa to 5.0 kDa, and decreased gradually for 5.5 kDa and
6.0 kDa. Therefore, helicity can be induced by the covalent
domino effect in the chiral PHIC macromonomer by (S)-Ct in a
domain (MW) up to 5.0 kDa, i.e. up to a DP of ∼36. These data
are consistent in the context of domains (MW) influenced by
terminal chiral moiety for previously reports of chirally termi-
nated PHIC.20
1
in Table 2. Figure 1b shows the H NMR spectra of the graft
copolymer; the inset figure shows the disappearance of the vinyl
group peaks of the macromonomer after graft copolymerization.
Moreover, a clear shift in the SEC curve was observed for the
graft copolymers, as shown in Figure 2. The 1H NMR and SEC
data clearly show the formation of graft copolymer from PHIC
macromonomer.
Figure 3b shows the CD spectra of chiral PHIC macromono-
mers of different MW and their graft copolymers. Surprisingly,
the chiral graft copolymers adopted a conformation with M-he-
lical sense, which is opposite to the P-helical sense of chiral PHIC
macromonomers; moreover, the graft copolymers had lower CD
intensity as compared with the chiral PHIC macromonomer. In
other words, the PHIC adopted a helical conformation with
opposite sense at the covalently aggregated states. The intriguing
results convinced us to look carefully at the packing pattern of
PHIC chains in the graft copolymers.
Figure 1. (a) 1H NMR of chiral macromonomer of poly(n-hexyl
isocyanate) (CH-2) (b) 1H NMR of graft copolymer (CG-4) showing
disappearance vinyl group peaks of the chiral macromonomer.
Figure 2. SEC curve of chiral PHIC macromonomer (CH-1) and graft
copolymer (CG-1).
Table 2. Synthesis of Chiral Graft Copolymers at 80 °C in Toluene
amount of reagents (μmol)
Mn/MWDc
runa
AIBN
macromonomerb
styrene
macromonomer
graft copolymer
DPd
CG-1
CG-2
CG-3
CG-4
RG-5
RG-6
RG-7
9.3
9.0
93.0
90.4
95.4
93.2
40.1
40.4
39.5
-
-
(CH-1) 4.30 kDa/1.07
(CH-2) 5.00 kDa/1.06
(CH-3) 5.50 kDa/1.10
(CH-4) 6.00 kDa/1.08
(CH-5) 5.10 kDa/1.08
(CH-5) 5.10 kDa/1.08
(CH-5) 5.10 kDa/1.08
31.0 kDa/1.43
46.4 kDa/1.64
46.0 kDa/1.45
44.0 kDa/1.70
42.6 kDa/1.44
43.4 kDa/1.43
37.8 kDa/1.51
7.2
9.2
8.4
7.5
-
9.6
-
9.2
-
10.2
10.5
10.9
80
200
400
-
-
a All Polymerizations were conducted at 80 °C for 6 h. b Poly(n-hexyl isocyanate) macromonomer synthesized using Naꢀ4VPBA initiator and (S)-Ct
terminator. c Molecular weight (Mn) and Molecular weight distribution (MWD) were measured by size exclusion chromatography multiangle laser light
scattering (SECꢀMALLS) in THF/Et3N at 40 °C. d Degree of polymerization.
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Figure 3. (a) CD (upper) and UV (lower) spectra of chiral macromonomers of poly(n-hexyl isocyanate) with varying molecular weight, and (b) CD
(upper) and UV (lower) spectra of chiral macromonomers of poly(n-hexyl isocyanate) and the graft copolymers synthesized from them (spectra a and b
were measured in hexane at 1 mg/mL, with a cell path length of 0.1 cm at 25 °C).
Scheme 2. Representation of the Possible Aggregated States of the Chiral Poly(n-hexyl isocyanate) Macromonomers
During graft copolymerization, chiral PHIC macromonomers
are converted from a random state to a covalently aggregated
state, as shown in Scheme 2. As previously mentioned, noncova-
lently aggregated polyisocyanates show a sudden rise in CD
intensity.23ꢀ26 This can be explained in the context of the
packing of helical chains in the pregel in two different ways.7
Polyisocyanate chains align in close parallel packing in the
aggregates, and adopt an energetically favorable conformation
with reduced helix reversals that allows for longer helical
segments and enhanced cooperativity.26 In addition, high CD
intensity can occur due to the formation of a helical super-
structure of polyisocyanate chains in the aggregates, in which the
amide chromophores are in an additional chiral environment.
This kind of chiral arrangement of helical polymers in aggregates
has been reported in DNA samples.34
system differed from all previous systems in two ways: (a) here,
helicity was induced in the chiral PHIC macromonomer by the
covalent chiral domino effect20 and (b) the aggregation was
due to covalent bonding of the vinyl-terminated, optically active
PHIC macromonomers.
In the present system, the chiral PHIC macromonomer aggre-
gated to form a “head-to-head” (vinyl-to-vinyl group) covalently
aggregated state after graft copolymerization, as shown in Scheme2.
Due to steric hindrance of the bulky rod-polymer side chains, the
syndiotactic structure was the most favorable for the graft
copolymers.32 In the covalently aggregated state, two PHIC chains
are aligned opposite to each other, resulting in a longer chain that
contains the same chiral moiety ((S)-Ct) at both ends, as shown in
the dotted-line box of Scheme 2. This extended PHIC chain has a
covalent linkage (backbone of graft copolymer), in the exact
middle, as shown in the dark-lined box of Scheme 2. In the
noncovalently aggregated state, the polyisocyanates have high
CD intensity due to adoption of more energetically favored
The above-discussed chiroptical property was observed in the
noncovalently aggregated state of chiral polyisocyanates. In those
cases, helicity was induced by a cooperative effect. The present
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Scheme 3. Representation of the Modal Structures Chiral Random Graft Copolymers with Varying Amount of Styrene in
Backbone
between PHIC side-chains decreases and they come closer to
each other as shown in Scheme 3. The random graft copolymers
(RG-5 to 7) of PHIC macromonomer and styrene showed
positive Cotton effect at 255 nm and owned a P-helical
conformation like PHIC macromonomer. Therefore, both PHIC
macromonomer and its random graft copolymer with styrene
had the same helical conformation, and the random graft
copolymer did not adopt a conformation with an opposite helical
sense even in covalently aggregated state. This strongly sup-
ported our assumption that the graft copolymer adopted a
conformation with opposite helical sense compared with the
chiral PHIC macromonomer due to the double covalent chiral
domino effect of the extended PHIC chain containing chiral
moieties at both ends and a covalent linkage in the middle of
the chain.
Furthermore, in random graft copolymers, the CD intensity
increased from RG-7 > RG-6 > RG-5, as shown in Figure 4. It can
be interpreted like, as the amount of styrene increased in the
backbone of the random graft copolymer the CD intensity
increased, and as the amount of styrene decreased CD intensity
decreased. However, all the random graft copolymer had lower
CD intensity compared with chiral PHIC macromonomer. RG-5
Figure 4. CD (upper) and UV (lower) spectra of chiral macromono-
mers of poly(n-hexyl isocyanate) (CH-5), the chiral graft copolymers
(CG-4), and chiral random graft copolymers (RG-5 to 7) (all CD and
had the low space between consecutive PHIC side-chains as it
had 1:2 ratio of PHIC macromonomer:styrene. In such case,
PHIC side-chains were close to each other, which caused high
steric effect on the PHIC chain and reduced the free movement,
as a result the chiral influence by (S)-Ct on the PHIC domain
decreased significantly. In RG-7, more space was created be-
tween consecutive PHIC side-chains compared with RG-5 as it
had 1:10 ratio of PHIC macromonomer:styrene. Such a high
space increased the free movement of PHIC chains, and in-
creased the chiral influenced domain by (S)-Ct, which increased
the helicity induction. It can be interpreted that the reduced CD
intensity in the graft copolymers was due to steric hindrance at
the backbone of the graft copolymer chains, which restricted free
movement of the PHIC chain. This effect was termed as a “spacer
effect in chiral graft copolymers”. Higher the space between two
consecutive side-chain, higher the CD intensity and vice versa.
Adoption of opposite helical sense due to a covalent linkage
of the extended PHIC chain in graft copolymers can be
attributed to the adoption of opposite helical sense as observed
in different types of liked or kinked biomacromolecules. The
polypeptide chain containing a covalent linkage created by
UV spectra were measured in hexane at 1 mg/mL, with a cell path length
of 0.1 cm at 25 °C).
conformations.26 Therefore, it was assumed that the adoption of a
conformation with opposite helical sense (P to M) and a lower CD
intensity in the covalently aggregated state was due to the effect of
covalent linkage on the extended chain containing (S)-Ct at both
ends. The covalent linkage creates an obstacle against the driving
force applied by (S)-Ct toward helicity induction. Moreover, it
increases the steric hindrance and restricts the movement of PHIC
chains.
In order to confirm the extended chain effect, several random
graft copolymerizations of chiral PHIC macromonomers with
styrene were conducted in different ratios. In the random graft
copolymer, some space was created by styrene between two
chiral PHIC chains; therefore, there were no consecutively
aggregated chiral PHIC chains as were obtained by graft copo-
lymerization of chiral PHIC macromonomers as shown in the
dotted-line box in Scheme 2. Moreover, as the amount of styrene
increases the space between consecutively chiral PHIC chains
increases and as the amount of styrene decreases the space
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short linkers adopted opposite helical sense as an effect of
linkage.35 The simulation models of homochiral and hetero-
chiral helices with kinked structures exhibit a conformation
with an opposite helical sense when energy is minimized.36
The above explanation supported our assumption that cova-
lently aggregated chiral PHIC adopts a conformation with oppo-
site helical sense due to the linkage effect in the extended PHIC
chain of the graft copolymer. The detailed studies of the effect of
the side-chain length on CD intensity, as well as solvent and
temperature effects on the graft copolymers will be presented in
future publications.
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Living anionic polymerization of n-hexyl isocyanate has been
successfully performed using the functional initiator sodium
4-N-(4-vinylphenyl)benzamide (Naꢀ4VPBA), which serves both
as a chain initiator and an additive to prevent the backbiting
reaction. This dual function was confirmed by the clear difference
between initiation efficiency (∼20%) and an estimation of anion
content (∼95%) for Naꢀ4VPBA. Chiral poly(n-hexyl iso-
cyanate) (PHIC) macromonomers with varying and controlled
molecular weight (MW) were successfully synthesized using
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by 1HNMR, SECꢀMALLS and CD. Maximum optical rotation
of the macromonomer is obtained when Mn = 5.0 kDa, i.e.,
DP ∼ 36. Graft copolymers of optically active PHIC were also
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1
and the disappearance of the vinyl group peaks in H NMR
spectra. CD spectra showed that the graft copolymers had an
opposite helical sense compared with chiral macromonomers,
which was due to effect of the extended chain containing the
same ((S)-Ct) chiral moieties at the both ends and covalent
linkage in the middle. Moreover, we have established a “spacer
effect” in chiral random graft copolymers. Higher the amount of
spacer in backbone, higher is the CD intensity and vice versa. The
present work will be greatly useful in understanding helical
behavior and transmission of stereochemical information in
biomacromolecules and synthetic helical polymers through
covalently aggregated states of these polymeric chains.
’ AUTHOR INFORMATION
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Corresponding Author
*Telephone: +82 62 970 2306. Fax + 82 62 970 2304. E-mail:
jslee@gist.ac.kr.
’ ACKNOWLEDGMENT
This work was supported by the Program for Integrated
Molecular Systems (PIMS) and the World Class University
(WCU) program (Project No. R31-20008-000-10026-0). P.N.S
thanks the Institute for Information Technology Advancement
(IITA) of Korea for the scholarship.
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