Adjusting conformational switching behavior of helical polycarbodiimides through substituent induced polarity effects

Article abstract of DOI:10.1002/pola.24484

Recent discoveries on the improved versatility of helical polycarbodiimides capable of undergoing low energy reversible conformational changes from realignment of their restricted polyarene pendant groups has led to seven new polycarbodiimides that each present unique information in regards to how the electronics and connectivity of the arene π-system play a crucial role in the behavior of these polymers. In addition to their individual anomalous behavior, this series of functional polymers unlock new answers toward the global understanding of the governing forces behind this complex switching process. Through the incorporation of functional groups covalently attached to the naphthalene pendant, dramatic changes and new application of these systems are realized. Variable temperature polarimetry is used to observe the reversible conformational changes of these chiral polymers.

Full text of DOI:10.1002/pola.24484

Adjusting Conformational Switching Behavior of Helical
Polycarbodiimides Through Substituent Induced Polarity Effects
JUSTIN G. KENNEMUR, CHRIS A. KILGORE, BRUCE M. NOVAK
Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh, North Carolina 27695
Received 13 October 2010; accepted 3 November 2010
DOI: 10.1002/pola.24484
Published online 3 December 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Recent discoveries on the improved versatility of heli-
cal polycarbodiimides capable of undergoing low energy reversi-
ble conformational changes from realignment of their restricted
polyarene pendant groups has led to seven new polycarbodii-
mides that each present unique information in regards to how
the electronics and connectivity of the arene p-system play a cru-
cial role in the behavior of these polymers. In addition to their
individual anomalous behavior, this series of functional polymers
unlock new answers toward the global understanding of the gov-
erning forces behind this complex switching process. Through
the incorporation of functional groups covalently attached to the
naphthalene pendant, dramatic changes and new application of
these systems are realized. Variable temperature polarimetry is
used to observe the reversible conformational changes of these
C
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chiral polymers.
2010 Wiley Periodicals, Inc. J Polym Sci Part
A: Polym Chem 49: 719–728, 2011
KEYWORDS: aromatic interactions; carbodiimide; chiral; confor-
mational change; functionalization of polymers; helical; heter-
oatom containing polymers; molecular machine; optical switching;
polarization; polycarbodiimide; polyguanidine; reversible; stimuli-
sensitive polymers; tunable
INTRODUCTION Substituent effects on aromatic systems
attached to, or along, a polymer backbone present a unique
and interesting way of inducing electronic perturbations on
a localized region of the polymer repeat unit. Even more
interesting is observing how these perturbations translate to
the global properties of the polymer system. The substitution
of electron withdrawing groups on arene systems, in com-
parison with electron donating substituents, can give impli-
cations toward how noncovalent dipolar p-interactions,
conjugation, and/or partial delocalization of these regions
play a role on the observed properties of the polymer. We
recently published the discovery of a hallmark, versatile,
polycarbodiimide (Poly-1, Fig. 1) capable of undergoing
dynamic changes in conformation through a low-energy con-
certed realignment of naphthalene pendant groups influ-
enced by solvent and temperature.1 It has been determined
that these observations are not due to polymer aggregation
and that this reversible shutter-like reorientation of the aro-
matic pendant groups occur without inversion of the static
helical backbone. Instead, it is a secondary layer of chirality
created during the polymerization by these constricted arene
pendant groups that gives rise to such dramatic chiro-optical
changes. To our knowledge, these polymers remain the only
reported examples of synthetic helical macromolecular sys-
tems that undergo reversible changes in chiral polarization
without inversion of backbone helicity and without the influ-
ence of chiral pendant groups, chiral solvent, and/or other
chiral perturbations such as chiral guest molecules.2
In addition to the potential application of these polymer
systems toward tunable polarizers, chirality based chemo-
sensors, biomimetic materials, optical display materials, and
data storage devices; we are also invested in the novelty of
such a concerted macromolecular expression of chirality
transfer. Aromatic p-based interactions are found throughout
nature and, in biochemistry, these complex noncovalent
interactions are found to play important roles in structural
stability, molecular recognition, and the overall properties of
biomolecules.3–6 It is therefore with great interest that we
continue our fundamental understanding of the governing
forces behind the unique switching behavior of these poly-
carbodiimide systems. This understanding may prove useful
for unlocking new answers toward how noncovalent p-inter-
actions can affect the conformational behavior of complex
functional macromolecules. The last few decades have seen
prominent advancements toward the understanding of
aromatic-aromatic interactions which consist of a complex
ballet of solvophobic, Van der Walls, and electrostatic stack-
ing forces.7–11 The aforementioned Poly-1 is unique in the
sense that it holds a large density of arene substituents
affixed to a static chiral scaffolding. This allows the ordered
registry of the helix to amplify aromatic interactions and
arene conformational transitions to be easily observed
through chiro-optical techniques, such as polarimetry. In
addition, the long aliphatic alternate polycarbodiimide
pendant group allows good polymer solubility in a variety of
solvents and dampens aggregation and aromatic interaction
Correspondence to: B. M. Novak (E-mail: bruce_novak@ncsu.edu)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 49, 719–728 (2011) 2010 Wiley Periodicals, Inc.
SWITCHING BEHAVIOR OF HELICAL POLYCARBODIIMIDES, KENNEMUR, KILGORE, AND NOVAK
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 1 Library of Naphthalene Pendant Group Based Helical Polycarbodiimides.
between chains. Due to the buffering of aggregation from
this large aliphatic corona, the behavior of this polymer
translates directly to how these arene substituents interact
along the chain, albeit through aromatic-backbone or
aromatic-aromatic interactions, and not due to inter-chain
interactions. (i.e., this is a fully intra-molecular phenomenon)
This manuscript will take into account the effects that
substituents have on the arene pendant groups and how
they translate to changes in the conformational switching
process of the polycarbodiimide. A secondary study will take
into account how disruption of such behavior comes from
alteration of the pendant group aromaticity by partial satura-
tion and by changing the proximity of these arene groups to
the backbone. A new library of seven select polycarbodii-
mides (Poly 2–8) have been chosen and synthesized to
study these effects. Since all of the polymers in Figure 1 are
obtained from nonsymmetric carbodiimide monomers having
two different pendant groups, we acknowledge the chance
for regioirregularity by indicating both regioisomers possible
on the resulting polymers.
carbon, acetic anhydride, nitric acid (fuming), and N,N-di-
methylaminoethylamine (DMAEA) were purchased from
Aldrich (Sigma-Aldrich, Milwaukee, WI) and used without
further purification unless otherwise noted. Triethylamine and
common laboratory solvents were purchased from Fisher
Scientific, Fair Lawn, NJ. Dibromotriphenylphosphorane salt
was purchased from VWR (Suwanee, GA) and used as
received. Solvents used for polymerization or catalyst synthesis
were dried, distilled, and degassed and stored over molecular
sieves prior to use. UV-Vis experiments were performed in
spectroscopy grade THF also purchased from Aldrich.
Instrumentation Used for Characterization
of Monomers and Polymers
Infrared spectroscopy was performed on a Jasco FT-IR 140
Fourier transform infrared spectrometer using potassium
bromide crystal windows purchased from Aldrich. UV-Vis
spectroscopy was performed on a Jasco V-550 UV-Vis spec-
trometer using high clarity quartz cells. Polarimetry was per-
formed on a Jasco P-1010 polarimeter with interchangeable
wavelength filters and using a jacketed 0.5 dm cell. Cell tem-
perature was adjusted by a Neslab RTE-140 circulation bath
attached to the jacketed cell and the solution temperature
within the cell was monitored by an Omega K-Type thermo-
couple attached to a Barnant digital thermocouple thermo-
meter. 1H and 13C NMR analyses were performed on a
Mercury 300 or 400 spectrometer using deuterated solvents
(Cambridge Isotope Laboratories) with tetramethylsilane
internal standard. Mass spectra were obtained at the NCSU
EXPERIMENTAL
Materials
1-aminonaphthalene, 1-methylnaphthalene, 4-methoxy-1-nitro-
naphthalene, 4-bromo-1-aminonaphthalene, 4-chloro-1-amino-
naphthalene, 5-aminoquinoline, 4-aminoindan, 1-naphthyl-
methylamine, n-dodecyl isocyanate, titanium tetraisopropoxide,
(R)-(þ)-1,10-bi-2,20-naphthol (BINOL), palladium (10%) on
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ARTICLE
SCHEME 1 General synthesis of carbodiimide monomers.
Department of Chemistry Mass Spectrometry Facility using
electrospray ionization (ESI) on an Agilent Technologies
6210 LC-TOF mass spectrometer.
7.75 (m, 2H), 8.09–8.17 (m, 2H), 8.61 (d, 1H). 13C NMR (75
MHz, CDCl3) d (ppm): 20.5, 123.9, 124.1, 124.9, 125.2, 125.4,
127.4, 129.2, 133.4, 142.6, (ipso-NO2 carbon was not
resolved due to extremely slow relaxation time). IR (cmꢁ1):
3084 (m, CAH aryl), 3057 (m, CAH aryl), 3016 (m, CAH
aryl), 2964 (m, CAH methyl), 1599 (m, C¼¼C), 1576 (m,
C¼¼C), 1510 (vs., NO2), 1334 (vs., NO2 bend).
General Preparation of Monomers
Each monomer was synthesized by reacting the appropriate
aryl amine with dodecyl isocyanate to form the stable N,N0-
disubstituted urea precursor which was then dehydrated
using dibromotriphenylphosphorane salt and triethylamine
(Scheme 1) and purified by column chromatography. Specif-
ics to the procedure have been previously reported.1 Any
additions or changes to the general procedure are listed with
each monomer.
4-Methyl-1-aminonaphthalene was synthesized from 4-nitro-
1-methylnaphthalene through hydrogenation described below:
15.46 g (82.6 mmol) of 4-nitro-1-methylnaphthalene was
added to a jar and dissolved in 40 mL of toluene with a stir
bar. 277.5 mg of Pd/C (10%) was added to the solution. Jar
was lowered into a Parr vessel with a moat of toluene added
around the jar to limit toluene from evaporating out of the
jar. The Parr reactor was sealed and pressurized with 60 psi
of H2 gas. The Parr reactor was partially submerged in an oil
bath on a stir plate and the reaction solution was heated at
100 ꢀC for 24 h with continual H2 pressure. After cooling,
the jar was removed from the vessel and the purple-red so-
lution was filtered through a plug of celite to remove cata-
lyst. After drying over anhydrous sodium sulfate for 15 min,
the filtrate was dried to a red oil by vacuum.
N-(1-naphthyl)-N0-(n-octadecyl)carbodiimide (Mono-1)
This has been previously reported and characterized.1
N-(4-methoxy-1-naphthyl)-N0-(n-dodecyl)
carbodiimide (Mono-2)
5.46 g (27 mmol) of purchased 4-methoxy-1-nitronaphthalene
was dissolved in 50 mL toluene and hydrogenated in a Parr
reactor using a catalytic amount of 10% Pd/C. After stirring
under H2 pressure at 50 psi and a temperature of 80 ꢀC for
10 h, the resulting pale yellow solution was removed from the
reactor and filtered through Celite. (Note: Discoloration of this
amine occurs very rapidly and protection of the amine from
oxidation by reacting with isocyanate needs to be done
directly following filtration) No yield or characterization done
due to instability of the amine. Amine was protected by addi-
tion of 1.25 molar equivalents of dodecyl isocyanate to form
white fluffy urea after recrystallization from 50:50 EtOH:
n-BuOH. (85% Yield overall from starting 4-methoxy-1-nitro-
naphthalene.) Subsequent dehydration and purification
resulted in carbodiimide monomer as a pale yellow oil.
Yield: 10.049 g (77%). 1H NMR (300 MHz, CDCl3) d (ppm):
2.60 (s, 3H), 4.03 (s, br, 2H), 7.71 (d, 1H), 7.13 (d, 1H),
7.48–7.55 (m, 2H), 7.87 (d, 1H), 7.97 (d, 1H). 13C NMR (75
MHz, CDCl3) d (ppm): 19.2, 109.9, 121.6, 124.4, 124.9, 125.1,
125.2, 125.9, 127.0, 133.4, 140.7. IR (cmꢁ1): 3348, 3375 (br,
NAH). HRMS-ESI: Mtheoretical
157.0963, DM ¼ 0.13 mmass units (0.84 ppm), C11H11N.
¼
157.0964, Msample
¼
Reaction of 4-methyl-1-aminonaphthalene with dodecyl iso-
cyanate in DCM refluxed overnight yielded white solid urea
which was recrystallized from MeOH. Subsequent dehydra-
tion resulted in Mono-3 as yellow oil after purification.
1
Yield: 87%. H NMR (300 MHz, CDCl3) d (ppm): 0.88 (t, 3H),
1.20–1.50 (br, 18H), 1.71 (m, 2H), 3.44 (t, 2H), 3.99 (s, 3H),
6.74 (d, 1H), 7.20 (d, 1H), 7.48–7.55 (m, 2H), 8.22 (d, 2H).
13C NMR (75 MHz, CDCl3) d (ppm): 14.4, 22.9, 27.0, 29.4,
29.6, 29.8 (overlapped), 29.9 (overlapped), 31.7, 32.2, 47.3,
55.9, 103.9, 119.5, 122.2, 123.5, 126.0, 126.2, 126.7, 129.6,
129.7, 137.0, 152.9. IR(cmꢁ1): 3070 and 3043 (w, CAH aryl),
3001 (w, CAH MeO), 2924 (vs., CAH alkyl), 2852 (s, CAH
alkyl), 2129 (vs., N¼¼C¼¼N), 1589 (s, C¼¼C), 1267 (s, arylAO
1
Yield: 73% H NMR (300 MHz, CDCl3) d (ppm): 0.88 (t, 3H),
1.20–1.50 (br, 18H), 1.70 (m, 2H), 2.65 (s, 3H), 3.46 (t, 2H),
7.18–7.25 (m, 2H), 7.51–7.56 (m, 2H), 7.94 (d, 1H), 8.31 (d,
1H). 13C NMR (75 MHz, CDCl3) d (ppm):14.4, 19.5, 22.9,
27.1, 29.4, 29.6, 29.8 (overlapped), 29.9 (overlapped), 31.7,
32.2, 47.2, 119.4, 124.3, 124.4, 125.7, 126.4, 126.7, 129.0,
131.0, 133.5, 135.6, 136.4. IR (cmꢁ1): 3066 (w, CAH aryl),
3033 (w, CAH aryl), 2925 (vs., CAH alkyl), 2854 (s, CAH
alkyl) 2135 (vs., N¼¼C¼¼N), 1581 (m, C¼¼C). HRMS-ESI:
Mtheoretical ¼ 351.2795, Msample ¼ 351.2796, DM ¼ ꢁ0.09
mmass units (ꢁ0.24 ppm), C24H34N2.
stretch), 1020 (m, H3CAO stretch) . HRMS-ESI: Mtheoretical
¼
367.2744, Msample ¼ 367.2745, DM ¼ ꢁ0.13 mmass units
(ꢁ0.37 ppm), C24H34N2O.
N-(4-methyl-1-naphthyl)-N0-(n-dodecyl)
carbodiimide (Mono-3)
4-nitro-1-methylnaphthalene was synthesized from pur-
chased 1-methylnaphthalene according to literature.12
N-(4-bromo-1-naphthyl)-N0-(n-dodecyl)
carbodiimide (Mono-4)
Monomer was prepared from commercially available 4-
bromo-1-aminonaphthalene and n-dodecyl isocyanate as ini-
tially described.
Yield: 15.46 g (27%). m.p. 61–64 ꢀC (lit. 68 ꢀC) 1H NMR
(300 MHz, CDCl3) d (ppm): 2.79 (s, 3H), 7.39 (d, 1H), 7.60–
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
1
Yield: 77% H NMR (300 MHz, CDCl3) d (ppm): 0.88 (t, 3H),
recrystallized from MeOH and subsequently dehydrated as
previously described to yield a light yellow oil following
purification.
1.20–1.50 (br, 18H), 1.70 (m, 2H), 3.48 (t, 2H), 7.14 (d, 1H),
7.55 (t, 1H), 7.61 (t, 1H), 7.68 (d, 1H), 8.18 (d, 1H), 8.30 (d,
1H). 13C NMR (75 MHz, CDCl3) d (ppm):14.4, 22.9, 27.0, 29.3,
29.6, 29.7, 29.8, 29.9 (overlapped), 31.6, 32.2, 47.1, 118.1,
120.0, 124.4, 126.8, 127.3, 128.0, 129.9, 130.2, 132.8, 134.9,
137.9. IR (cmꢁ1): 3068 and 3043 (w, CAH aryl), 2924 (vs.,
CAH alkyl), 2852 (s, CAH alkyl) 2139 (vs., N¼¼C¼¼N), 1583 (s,
C¼¼C). HRMS-ESI: Mtheoretical ¼ 415.1743, Msample ¼ 415.1742,
DM ¼ ꢁ0.07 mmass units (ꢁ0.17 ppm), C23H31BrN2.
N-(4-chloro-1-naphthyl)-N0-(n-dodecyl)
carbodiimide (Mono-5)
Monomer was prepared from commercially available 4-
chloro-1-aminonaphthalene and n-dodecyl isocyanate as ini-
tially described.
1
Yield: 82%. H NMR (300 MHz, CDCl3) d (ppm): 0.89 (t, 3H),
1.05–1.40 (br, 20H), 2.95 (t, 2H), 4.78 (s, 2H), 7.42–7.59 (m,
4H), 7.81 (d, 1H), 7.88 (d, 1H), 8.06 (d, 1H). IR (cmꢁ1): 3047
(m, CAH aryl), 2924 (vs., CAH alkyl), 2853 (s, CAH alkyl)
2127 (vs., N¼¼C¼¼N), 1693 (m, NCN-CH2-aryl). HRMS-ESI:
Mtheoretical ¼ 350.2722, Msample ¼ 350.2725, DM ¼ ꢁ0.3 mmass
units (ꢁ0.85 ppm), C24H34N2.
Preparation of (R)-BINOL-titanium(IV)-
diisopropoxide Catalyst
All polymerizations were performed using (R)-1,10-binaphth-
2,20-oxy (BINOL) titanium (IV) diisopropoxide catalyst which
was synthesized according to previous literature.1,13
1
Yield: 82% H NMR (300 MHz, CDCl3) d (ppm): 0.88 (t, 3H),
1H NMR (300 MHz, CDCl3 stored over molecular sieves) d
(ppm): 1.06 (d, 12H), 4.49 (m, 2H), 6.75 (d, 2H), 7.15 (d,
2H), 7.34 (m, 4H), 7.46 (d, 2H), 7.86 (d, 2H).
1.20–1.50 (br, 18H), 1.72 (m, 2H), 3.48 (t, 2H), 7.20 (d, 1H),
7.48 (d, 1H), 7.53–7.64 (m, 2H), 8.21 (d, 1H), 8.31 (d, 1H).
13C NMR (75 MHz, CDCl3) d (ppm):14.4, 22.9, 27.0, 29.3,
29.6, 29.7, 29.8, 29.9 (overlapped), 31.6, 32.2, 47.1, 119.5,
124.3, 124.6, 126.2, 126.7, 127.7, 130.0, 131.6, 135.0, 137.1.
IR (cmꢁ1): 3070 and 3047 (w, CAH aryl), 2925 (vs., CAH
alkyl), 2854 (s, CAH alkyl), 2137 (vs., N¼¼C¼¼N), 1585 (s,
C¼¼C). HRMS-ESI: Mtheoretical ¼ 371.2249, Msample ¼ 371.2248,
DM ¼ 0.06 mmass units (0.17 ppm), C23H31ClN2.
General Preparation of Polymers
Polymerization and work-up procedures are fully described in
previous literature.1 Unless otherwise noted, polymerizations
were performed at room temperature using 1 g portions of
monomer and approximately 200:1 molar equivalents of mono-
mer to catalyst predispersed in less than 2 mL of dry CHCl3. Po-
lymerization times ranged from 5–7 days unless otherwise
noted. Yields of polymers are reported below. IR and 1H NMR
spectra for each polymer are also reported. For IR analysis, thin
films of polymer were cast onto a potassium bromide crystal
window using CHCl3 as a solvent at room temperature. The loss
of the strong carbodiimide (N¼¼C¼¼N) absorption and the arrival
of a strong imine (C¼¼N) absorption is indicative of successful
polymerization. Due to substantial broadening, little information
other than further confirmation of successful polymerization is
derived from 1H NMR spectra at this time and this is deter-
mined by extreme broadening of signals in addition to aniso-
tropic shielding of some methylene hydrogens into negative
ppm values. Due to long delay times, long analysis times, and
poor resolution of 13C NMR analyses, this was not performed on
these polymers. GPC also results in substantial broadening of
eluted peaks. This has been a continual observation of these
polymer systems1 and early experiments performed by Goodwin
and Novak14 using a low-angle light scattering detector proved
that polymers of similar molecular weight were eluting over a
very long range of elution times. Previously reported data has
shown that using >100:1 molar ratio of monomer:catalyst
results in a sufficient degree of polymerization for these poly-
mers1 and, as such, this ratio was kept similar for each polymer-
ization to reduce differences that may result from large varia-
tions in molecular weight from Poly-1 to Poly-8.
N-(5-quinolyl)-N0-(n-dodecyl)carbodiimide (Mono-6)
Monomer was prepared from commercially available 5-ami-
noquinoline and n-dodecyl isocyanate as initially described.
1
Yield: 79% H NMR (300 MHz, CDCl3) d (ppm): 0.88 (t, 3H),
1.20–1.60 (br, 18H), 1.73 (m, 2H), 3.49 (t, 2H), 7.33 (d, 1H),
7.40 (m, 1H), 7.63 (t, 1H), 7.86 (d, 1H), 8.60 (d, 1H), 8.91
(m, 1H). 13C NMR (75 MHz, CDCl3) d (ppm): 14.4, 22.9, 27.0,
29.3, 29.6, 29.7, 29.7–29.8 (overlapped), 29.9, 31.6, 32.1,
47.1, 120.0, 121.0, 124.4, 125.8, 129.4, 132.4, 135.0, 138.0,
149.2, 151.1. IR (cmꢁ1): 3062 and 3030 (w, CAH aryl), 2925
(vs., CAH alkyl), 2854 (s, CAH alkyl) 2133 (vs., N¼¼C¼¼N),
1589 (m, C¼¼C), 1610 and 1570 (s, C¼¼C and C¼¼N ring
stretch). HRMS-ESI: Mtheoretical ¼ 338.2591, Msample ¼ 338.2593,
DM ¼ ꢁ0.23 mmass units (ꢁ0.69 ppm), C22H31N3.
N-(4-indanyl)-N0-(n-dodecyl)carbodiimide (Mono-7)
Monomer was prepared from commercially available 4-ami-
noindan and n-dodecyl isocyanate as initially described.
1
Yield: 73%. H NMR (300 MHz, CDCl3) d (ppm): 0.88 (t, 3H),
1.25–1.43 (br, 28H), 1.67 (m, 2H), 2.08 (m, 2H), 2.93 (dd,
4H) 3.38 (t, 2H), 6.87 (d, 1H), 6.98 (d, 1H), 7.08 (t, 1H). 13C
NMR (75 MHz, CDCl3) d (ppm): 14.4, 23.0, 25.0, 27.1, 29.4,
29.6, 29.8 (overlapped), 29.9 (br, overlapped), 30.9, 31.5,
32.2, 33.4, 47.3, 120.9, 121.4, 127.5, 136.2 136.7, 138.7,
146.2. IR (cmꢁ1): 3059 (w, CAH aryl), 2925 (vs., CAH alkyl),
2852 (s, CAH alkyl) 2135 (vs., N¼¼C¼¼N), 1587 (s, C¼¼C
aryl). HRMS-ESI: Mtheoretical ¼ 327.2795, Msample ¼ 327.2797,
DM ¼ ꢁ0.25 mmass units (ꢁ0.77 ppm), C22H34N2.
Poly-N-(1-naphthyl)-N0-(n-octadecyl)carbodiimide (Poly-1)
This polymer has been previously reported and characterized.1
Poly-N-(4-methoxy-1-naphthyl)-N0-(n-dodecyl)
carbodiimide (Poly-2)
N-(1-methylnaphthyl)-N0-(n-dodecyl)carbodiimide (Mono-8)
Dodecyl isocyanate was added to a solution 1-methylnaph-
thylamine (1:1 molar ratio) in DCM at 0 ꢀC. The urea was
Monomer:catalyst (molar ratio) 200:1. Yield: 77%. 1H NMR
(300 MHz, CDCl3) d (ppm): ꢁ0.61 (br, overlapping) 0.66
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ARTICLE
TABLE 1 Experimental Polymerization Parameters for Molecular Weight Study of Poly-1
Mono-1
Cat.
Mono:Cat.
Ratio
Vol. CHCl3
(mL)
Rxn Time
(days)
Theo.
Yield
Wt.a Mono-1 (g)
(mmol)
(lmol)
% Yield
Mn
Corrected Mn
1.02
1.05
1.04
1.05
1.07
a
2.41
2.48
2.46
2.48
2.53
5.7
8.0
421:1
309:1
215:1
108:1
55:1
2
2
2
2
2
7
7
7
7
7
46
55
88
80
73
177K
130K
90K
81K
71K
80K
35K
17K
11.5
22.9
45.9
45K
23K
The formula weight for Mono-1 is (420.7 g/mol).
(br), 0.89 (br) 1.26 (br), 3.64 (br), 6.35 (br), 7.11–8.00 (br,
overlapping). IR (cmꢁ1): 3070 and 3047 (w, CAH aryl), 2995
(w, CAH MeO), 2925 (vs., CAH alkyl), 2852 (s, CAH alkyl),
1620 (vs., N¼¼C), 1589 (s, C¼¼C), 1273 (s, arylAO stretch),
1022 (m, H3CAO stretch).
2923 (vs., CAH alkyl), 2852 (s, CAH alkyl) 1645 (vs., N¼¼C),
1597 (w, C¼¼C aryl).
Poly-N-(1-naphthylmethyl)-N0-(n-dodecyl)
carbodiimide (Poly-8)
Monomer:catalyst (molar ratio) 200:1. Yield: 66%. 1H NMR
(300 MHz, CDCl3) d (ppm): ꢁ0.50–0.50 (br, overlapping),
0.53 (br), 0.69 (br), 0.88 (br), 3.11 (br), 4.53 (br), 5.09 (br),
6.80–8.00 (br, overlapping). IR (cmꢁ1): 3048 (m, CAH aryl),
2924 (vs., CAH alkyl), 2852 (s, CAH alkyl) 1645 (vs., N¼¼C),
1597 (w, C¼¼C aryl).
Poly-N-(4-methyl-1-naphthyl)-N0-(n-dodecyl)
carbodiimide (Poly-3)
Monomer:catalyst (molar ratio) 150:1. Yield: 52%. 1H NMR
(300 MHz, CDCl3) d (ppm): ꢁ1.27–0.82 (br, overlapping),
0.89 (br), 1.26 (br), 2.26 (br, overlapping), 3.36 (br), 6.70–
8.00 (br, overlapping). IR (cmꢁ1): 3056 (w, CAH aryl), 3008
(w, CAH aryl), 2924 (vs., CAH alkyl), 2852 (s, CAH alkyl)
1620 (vs., N¼¼C), 1585 (s, C¼¼C).
RESULTS AND DISCUSSION
After the discovery of Poly-1,1 derived from N-(1-naphthyl)-
N0-(n-octadecyl) carbodiimide monomer, (Mono-1, Table 1),
a new study was performed to probe the effects of molecular
weight on the observed conformational changes. We were
interested to see how molecular weight will affect the optical
switching amplitude (OSA), defined as the total difference in
specific optical rotation exhibited between the two confor-
mational positions of the naphthalene units. We also wanted
to observe any changes that molecular weight has on the
temperature at which these realignments occur within a
select solvent such as CHCl3. Due to the previously men-
tioned biased results of gel permeation chromatography,1,15
this study serves as a foundation for how we can expect mo-
lecular weight differences to effect the behavior of these sys-
tems. A relatively broad range of molecular weights can be
justifiably synthesized by altering the monomer to catalyst
ratio of these polymerizations followed by correction of the
theoretical number average molecular weight based on the
yield of polymer obtained. This series of polymerizations is
shown in Table 1. The previously reported (R)-1,10-binaphth-
2,20-oxy (BINOL) titanium (IV) diisopropoxide catalyst1,16,17
was used for each polymerization. When performing variable
temperature polarimetry on these polymers in CHCl3 (Fig.
2), it can be seen that the OSA is reduced with polymers of
substantially low molecular weight (<ꢂ20K). This is
believed to be caused by increased chain-end character
where helical unraveling is more prominent. Such dramatic
decreases in observed specific optical rotations were also
observed for helical polyisocyanates at lower molecular
weights.18 More specifically to our case, the lower molecular
weight chains are impacted more by this loss of steric con-
striction which can translate to less restricted realignment of
the naphthalene pendant groups and the loss of OSA
Poly-N-(4-bromo-1-naphthyl)-N0-(n-dodecyl)
carbodiimide (Poly-4)
Monomer:catalyst (molar ratio) 200:1. Yield: 79%. 1H NMR
(300 MHz, CDCl3) d (ppm): ꢁ1.20–0.80 (br, overlapping)
0.89 (br), 1.25 (br), 3.49 (br), 4.71 (br), 6.50–8.20 (br, over-
lapping). IR (cmꢁ1): 3070 and 3043 (w, CAH aryl), 2925
(vs., CAH alkyl), 2852 (s, CAH alkyl) 1633 (vs., N¼¼C), 1563
(m, C¼¼C).
Poly-N-(4-chloro-1-naphthyl)-N0-(n-dodecyl)
carbodiimide (Poly-5)
Monomer:catalyst (molar ratio) 200:1. Yield: 82%. 1H NMR
(300 MHz, CDCl3) d (ppm): ꢁ1.50–0.80 (br, overlapping),
0.89 (br), 1.26 (br), 3.51 (br), 4.77 (br), 6.50–8.30 (br, over-
lapping). IR (cmꢁ1): 3072 and 3045 (w, CAH aryl), 2925
(vs., CAH alkyl), 2854 (s, CAH alkyl), 1635 (vs., N¼¼C), 1567
(m, C¼¼C).
Poly-N-(5-quinolyl)-N0-(n-dodecyl)carbodiimide (Poly-6)
Monomer:catalyst (molar ratio) 200:1. Yield: 41% 1H NMR
(300 MHz, CDCl3) d (ppm): ꢁ1.93 (br) ꢁ1.30–0.65 (br, over-
lapping), 0.73 (br), 0.88 (br), 1.25 (br), 3.52 (br), 4.79 (br),
6.51 (br), 6.80–8.10 (br, overlapping), 8.68 (br). IR (cmꢁ1):
3062 and 3030 (w, CAH aryl), 2925 (vs., CAH alkyl), 2854
(s, CAH alkyl), 1637 (vs., N¼¼C), 1589 (m, C¼¼C), 1604 and
1570 (s, C¼¼C and C¼¼N ring stretch).
Poly-N-(4-indanyl)-N0-(n-dodecyl)carbodiimide (Poly-7)
Monomer:catalyst (molar ratio) 194:1. Yield: 79%. 1H NMR
(300 MHz, CDCl3) d (ppm): ꢁ0.50 (br), 0.24–0.80 (br, over-
lapping), 0.88 (br), 1.26 (br), 1.80–4.00 (br, overlapping),
6.40–7.60 (br, overlapping). IR (cmꢁ1): 3049 (w, CAH aryl),
SWITCHING BEHAVIOR OF HELICAL POLYCARBODIIMIDES, KENNEMUR, KILGORE, AND NOVAK
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
tion of a ring carbon such as the 5-quinolyl derivative (Poly-
6). Substituents with mild to strong electron withdrawing
character created complications in the synthetic process and
are currently under investigation.
Variable temperature polarimetry was performed on each
polymer in toluene, THF, and CHCl3, and it was discovered
that each polymer behaves anomalously in each solvent
(Fig. 3).
The N-(4-methoxy-1-naphthyl)-N0-(n-dodecyl) polycarbodii-
mide (Poly-2) shows profound differences when compared
with the previously reported (Poly-1).1 For convenience, a
summation of conformational switching temperatures for
each polymer with respect to solvent is shown in Table 2.
The energy requirements associated with each solvent have
been completely reversed for Poly-2, with toluene now
requiring the lowest energy (ꢂ5 ꢀC estimated midpoint
temp.) ꢀand CHCl3 and THF requiring higher temperatures of
24–27 C for the observed conformational changes. For Poly-
2, the OSA in THF reaches 1900ꢀ ranging from þ1500ꢀ to
ꢁ400ꢀ, the largest OSA reported thus far by these polymer
systems. These optical rotations are completely reversible
upon cooling and no significant helical racemization occurs
at these temperatures.
FIGURE 2 Variable temperature polarimetry study of various
molecular weight samples of Poly-1 in CHCl3 (c ¼ 0.20–0.22 g/
100 mL). Although the OSA is reduced for low molecular
weight polymers, the relative temperature of switching remains
the same.
observed. Another important observation is that Poly-1
undergoes these conformational changes at approximately
the same temperature regardless of molecular weight. There-
fore, it is concluded that altering the molecular weight of
these polymer systems can translate to a change in OSA but
not the energy required for switching to occur. It is hypothe-
sized that we must alter the chemical composition and elec-
tronic properties of the arene pendant groups in order to
observe changes in conformational switching temperatures
within a constant solvent.
For Poly-3, the energy required to observe conformational
changes has been greatly increased to the point that the
full process cannot be observed within the experimental
thermal limits in each solvent. Interestingly, the order of
energy required for each solvent (i.e., CHCl3 < THF < tolu-
ene) appears to remain the same when compared with Poly-
1 yet the addition of a pedestrian methyl substituent on
Poly-3 has caused a dramatic overall increase in required
energy.
To date, our working model to explain the reasoning behind
this dynamic behavior suggests that the difference in energy
associated with the relative positions of the arene pendant
groups is dictated by differences in additive polarity by hav-
ing their dipole moment aligned either with or against the
inherent dipole on the polycarbodiimide backbone.15,19 We
now know that solvent plays a crucial role on the relative
energy required for these transitions as well.1 It is therefore
plausible that modifying the electronics of the arene p-sys-
tem through substituent effects and altering the polarity of
the pendant groups will change the temperature necessary
to observe these realignments with respect to solvent. To
further test this hypothesis, the design and synthesis of a
series of substituted naphthalene pendant groups (Poly-2–6)
was performed. The substituents were selectively placed on
the ‘‘4’’ position of the 1-naphthyl pendant groups which lies
on the same axis of rotation as the aryl-N bond that connects
the naphthalene units to the backbone. This positioning
should minimize steric interference associated with the rota-
tion and repositioning of the naphthalene core. Additionally,
substituents on this position lie in direct resonance with the
aryl-N bond, further promoting the desired polarity differen-
The halogen substituted Poly-4 and Poly-5 are an interesting
case since halogen substituents on an arene system share
mixed effects by being r-withdrawing and p-donating. When
comparing Poly-4 and Poly-5 to each other, their switching
profiles look very similar; higher polarity solvents such as
CHCl3 and THF require much lower energy than the less
polar toluene. When comparing both of these polymers to
the parent Poly-1, the energy for toluene has been slightly
increased while the energy for THF has been decreased by
>15 ꢀC. Another interesting observation from these two
polymers is that the more electronegative chlorine substi-
tuent has decreased the energy of both the CꢀHCl3 and THF
switching profiles proportionally by about 10 C when com-
pared with bromine and a slight decrease in toluene is seen
as well. The full conformational change could not be
observed for Poly-5 in CHCl3 and THF because they are
below the thermal limits for the experiment.
Poly-6 presents even more anomalous behavior as a result
of the heterocyclic 5-quinolyl pendant group. This pendant
group is very similar in size to naphthalene but has very dis-
tinct electronics as a result of the nitrogen heteroatom. It is
found that CHCl3 is the only solvent that we can observe
ces. The substituents range from
a strongly donating
methoxy group (Poly-2), to a weakly donating methyl group
(Poly-3), to substituents with mixed effects such as halogens
(Poly-4,5). We also experimented with heteroatom substitu-
ꢀ
such conformational changes in and this happens at ꢂ20 C.
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ARTICLE
FIGURE 3 Variable temperature polarimetry of Poly-1–6 showing dramatic conformational switching energy differences between
each polymer with respect to solvent. For each plot, the polymer was tested in (a) CHCl3 green (b) THF blue and (c) toluene red.
Specific optical rotations were recorded within respective temperature ranges for each solvent using 435 nm light and a concentra-
tion range of 0.20–0.22 g/100 mL. Data results for Poly-1 have been previously reported1 but are included for clarity of
comparison.
Both THF and toluene exhibit negative optical rotations
throughout the thermal range, concluding that if conforma-
tional changes do occur ꢀwithin these solvents, it requires
temperatures less than 0 C. Poly-6 presents an exciting new
functionality to these reversible switching polycarbodiimides
due to the potential chemistry that can be performed on the
heterocyclic nitrogen. Recent research by our group has
shown evidence that coordination of borane to the nitrogen
lone pair of pyridinyl pendant group containing polycarbodii-
mides allow the use of these polymers as supports for local-
ized chemical transformations.20 This research can be further
extended to include Poly-6, potentially allowing a reversibly
chiral support for asymmetric catalysis. Such experiments
are currently underway.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 2 UV-Vis and Polarimetry Summation for New
low energy naphthyl chromophore absorbance region. The
donor substituents, methoxide (Poly-2) and methyl (Poly-3),
exhibit the largest shifts of 26 and 15 nm, respectively.
Weaker but identical shifts are observed for the halo-substi-
tuted Poly-4, 5 and a similar shift is also realized for Poly-6.
The resolution of peak maxima below 250 nm is limited by
THF; however, these absorptions are also attributed to the
naphthyl chromophores as seen by comparison to di-n-hexyl
polycarbodiimide (Poly-DnH), which contains no arene
groups. The extinction coefficient for the backbone imine
absorption, which is the only chromophore observed for
Poly-DnH, is significantly less than the higher energy naph-
thyl absorption and exhibits no significant absorption above
250 nm. This observation leads to the conclusion that the
imine absorption for Poly-(1–6) is buried underneath the
higher energy naphthyl chromophore absorption for these
arene based polymers.
Switching Polymers
Approx. Switching
Temp. (ꢀC)a
Polymer
ID
Conc.
ppm
kmax nm
(in THF)
CHCl3
THF
Toluene
Poly-1
Poly-2
Poly-3
Poly-4
Poly-5
Poly-6
a
4.3
4.0
4.2
4.0
4.0
4.2
308
334
323
315
315
314
1
20
36
23
>50
10
<0
23
26
5
>55
12
>70
55
<0
NDb
52
NDb
Temperatures taken at the midpoint of the OSA, slight extrapolations
were made if necessary.
b
Not determinable, no evidence of conformational changes are seen
within temperature limits.
In addition to substituent effects on naphthalene, Poly-7 was
synthesized to determine the importance of a fully aromat-
ized pendant group. The 4-indanyl derivative consists of two
fused rings (one aromatic and one saturated) and should
fulfill the constricted size and spatial requirements as
previously investigated.1 Variable temperature polarimetry of
Poly-7 revealed little to no change in specific optical rotation
in toluene, THF, and CHCl3 (Fig. 5). This observation
warrants conclusion that p-based interactions on a fully
aromatized pendant group play a vital role in the conforma-
tional changes observed. In other words, simply having a
UV-Vis spectroscopy was performed on each polymer to
compare auxochromic effects that each of these new arene
substituents have on the polymer absorption bands. Figure 4
shows the overlay of UV-Vis spectra for each polymer dis-
solved in THF while concentrations and peak absorption
maxima can be found in Table 2. The inset shows an enlarge-
ment of the naphthyl absorption region between 280 and
380 nm. When compared with the parent Poly-1, each of the
new substituted polymers exhibit a bathochromic shift at the
FIGURE 4 UV-Vis spectra overlay of Poly-(1–6) in THF at room temperature. Di-n-hexyl polycarbodiimide (Poly-DnH) is also shown
(4.3 ppm in THF) for comparison as a polycarbodiimide without arene chromophores.
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ARTICLE
changes with respect to solvent. The dramatic differences in
energy associated with each polymer and these conforma-
tional changes in CHCl3, THF, and toluene indicate that this
behavior is very complex and may be the result of multiple
forms of aromatic-aromatic and aromatic-backbone interac-
tions in addition to solvophobic and polarity effects. An
attempt to further probe some of these complexities has
revealed that complete aromaticity of the pendant group is
required for these rearrangements and that a constricted
direct attachment of the arene pendant group to the back-
bone of the polymer is also required. In addition, the incor-
poration of a heterocyclic nitrogen onto the arene pendant
groups, Poly-6, presents new functionality, upon which,
more utility of this chemistry can be built upon. These
results open the door to the possibility that an even larger
library of polyarene containing polycarbodiimides with con-
formational switching behavior is synthetically feasible.
Going forward, these substituents can be chemically modified
to perform even greater function on a reversibly chiral mo-
lecular switch.
FIGURE 5 Variable temperature polarimetry of Poly-7 in (a)
CHCl3, (b) THF, and (c) toluene. (c ¼ 0.20–0.22 g/100 mL) Partial
removal of aromaticity from a naphthyl group to an indanyl
group results in the complete loss of conformational switching
behavior.
The authors thank the NCSU Department of Chemistry Mass
Spectrometry Facility funded by North Carolina Biotechnology
Center and the NCSU Department of Chemistry for the HRMS
data. Research funding for this project is supported by the
Howard J. Schaeffer Award stipend.
constricted bulky pendant group is not enough to allow
these transitions.
Poly-8 was synthesized to experiment with the aforemen-
tioned second layer of embedded chirality that is sterically
affixed around the polymer helical backbone as a result of
the direct attachment of these arene systems. For Poly-8,
mobility and degrees of rotational freedom are introduced to
the arene pendant group through spacing from the poly-
carbodiimide backbone by a methylene unit. Our curiosity is
driven by how this increase in pendant group mobility will
affect the optical rotations of these polymers and if a second
corona of chiral order can still be retained by the 1-naph-
thylmethyl pendant group. Furthermore, this increase in
mobility may promote aromatic-aromatic based interactions
by allowing the pendant groups to obtain relative positions
that maximize them. Poly-8 exists as a very tacky polymer
after work-up unlike Poly-(1–7) which have a fluffy texture.
Polarimetry of Poly-8 reveals a large overall decrease in spe-
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