Thermoreversible cis-cisoidal to cis-transoidal isomerization of helical dendronized polyphenylacetylenes

Article abstract of DOI:10.1021/ja055406w

High cis content (81-99%) cis-transoidal polyphenylacetylene (PPA) jacketed with amphiphilic self-assembling dendrons, poly[(3,4-3,5)mG2-4EBn] with m = 8, 10, 12, 14, 16, and (S)-3,7-dimethyloctyl, were synthesized by Rh(C≡CPh)(nbd)(PPh₃)₂

Full text of DOI:10.1021/ja055406w

Published on Web 10/11/2005
Thermoreversible Cis-Cisoidal to Cis-Transoidal
Isomerization of Helical Dendronized Polyphenylacetylenes
Virgil Percec,*,† Jonathan G. Rudick,† Mihai Peterca,‡ Martin Wagner,†
Makoto Obata,† Catherine M. Mitchell,† Wook-Dong Cho,†
Venkatachalapathy S. K. Balagurusamy,†,‡ and Paul A. Heiney‡
Contribution from the Roy & Diana Vagelos Laboratories, Department of Chemistry, UniVersity
of PennsylVania, Philadelphia, PennsylVania 19104-6323, and Department of Physics and
Astronomy, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104-6396
Received August 8, 2005; E-mail: percec@sas.upenn.edu
Abstract: High cis content (81-99%) cis-transoidal polyphenylacetylene (PPA) jacketed with amphiphilic
self-assembling dendrons, poly[(3,4-3,5)mG2-4EBn] with m ) 8, 10, 12, 14, 16, and (S)-3,7-dimethyloctyl,
were synthesized by Rh(CtCPh)(nbd)(PPh3)2 (nbd ) 2,5-norbornadiene)/N,N-(dimethylamino)pyridine
(DMAP) catalyzed polymerization of macromonomers. The resulting cylindrical PPAs self-organize into
hexagonal columnar lattices with intracolumnar order (Φhio) and without (Φh). The polymers with m ) 12,
14, and 16 exhibit also a hexagonal columnar crystal phase (Φh,k). The reversible Φh,k-to-Φhio-to- Φh phase
transition in these dendronized PPAs was analyzed by a combination of differential scanning calorimetry
and small and wide-angle X-ray diffraction experiments performed on powder and oriented fibers. In the
io
Φh,k and Φh phases, the dendronized PPAs form helical porous columns. The helical pore disappears in
the Φh phase. This change is accompanied by a decrease of the external column diameter that is induced
by stretching of the polymer backbone along the axis of the cylinder. The helix sense of the porous PPA
is selected by homochiral alkyl dendritic tails. This transition is generated by an unprecedented conversion
of the PPA backbone from the cis-cisoidal conformation in the Φh,k and Φhio phases to the cis-transoidal
conformation in the Φh phase. Under the same conditions, the pristine cis-PPA undergoes cis-trans
isomerization and irreversible intramolecular 6π electrocyclization of 1,3-cis,5-hexatriene sequences followed
by chain cleavage. These processes are eliminated in the dendronized cis-PPA below its decomposition
temperature.
interactions have been employed in an attach-to strategy7 and
to self-assemble dendronized supramolecular polymers.8 Po-
Introduction
Dendronized polymers approach the architectural complexity
of biomacromolecules and the dimensions required for molecular
nanoscale construction.1 Composed of a linear polymer back-
bone jacketed by dendritic appendages on every repeat unit,
dendronized polymers are prepared via divergent,2 attach-to,3
or macromonomer strategies.3b,4,5,6 More recently, noncovalent
lymerization of convergently synthesized dendritic macromono-
mers renders the highest degree of structural perfection,
particularly when combined with living polymerizations.4
Regardless of the synthetic method employed, two classes of
dendrons have been used in the construction of dendronized
polymers: self-assembling4a-f,5c,j,l-o,6,7a,8 and non-self-assem-
bling.2,3b,c,4g,h,5a,b,d-i,k In solution, dendronized polymers adopt
a cylindrical structure whose stiffness and microenvironment
are determined by the dendritic side chain.2a,c,3b,5k,6c Less well-
defined structures are obtained in bulk.1,5b,c,h,k We have elabo-
rated a strategy to overcome this limitation by jacketing
polymers with self-assembling dendrons that produce periodic
order in the bulk state.3a,4a-f,6,7a The resulting polymers display
a diversity of shapes that self-organize into one-dimensional
(1-D) smectic, two-dimensional (2-D) p6mm hexagonal, p2mm
simple rectangular, and c2mm centered rectangular columnar,
† Department of Chemistry, University of Pennsylvania.
‡ Department of Physics and Astronomy, University of Pennsylvania.
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10.1021/ja055406w CCC: $30.25 © 2005 American Chemical Society
J. AM. CHEM. SOC. 2005, 127, 15257-15264
15257

A R T I C L E S
Scheme 1
Percec et al.
and three-dimensional (3-D) Pm3hn cubic, {Im}3hn cubic, P42/mnm
tetragonal, and 12-fold symmetry lattices.4a-f,6,7,9
Efforts are under way to understand the evolution of homo-
chirality at interfaces.8,10,11 Polymers adopting a preferred helical
screw sense are of particular interest because individual
molecules might be resolved by scanning tunneling (STM) and
atomic force (AFM) microscopy.11 Cis-transoidal polyphenyl-
acetylenes (PPAs) adopt a helical conformation12a,13 whose
screw sense is selected by incorporation of chiral substituents
or by complexation of chiral substrates.14 Visualization of
predominantly single-handed helical superstructures on mica or
highly ordered pyrolytic graphite (HOPG) corroborates solution
spectroscopy but is limited by irreversible structural changes
undergone by the polymer during visualization.11d,e Intramo-
lecular cyclization via 6π electrocyclization of 1,3-cis,5-
hexatriene sequences in the PPA backbone generate cyclohexa-
diene repeat units at temperatures as low as 20 °C in solution
(Scheme 1).12 Re-aromatization of the cyclohexadiene moiety
results in extrusion of triarylbenzene and concomitant chain
cleavage at higher temperatures.12 A second chain cleavage
event occurs in solutions exposed to O2 and ambient light, which
likely proceeds via direct oxidation of the polyene backbone12d
(not shown in Scheme 1).
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Direct visualization of dendron-jacketed polymers offers the
opportunity to quantitatively probe fundamental questions about
molecular conformation and the formation of 1-D, 2-D, and 3-D
periodic states.1f Retrostructural analysis of the bulk self-
organized lattices confirms that single molecules are investigated
and permits rational design of polymers with controlled shape,
stiffness, and size that can reverse traditional trends encountered
in their pristine macromolecules.6b Previous studies of flexible
polymer backbones jacketed with self-assembling dendrons have
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shaped oligomers (i.e., L , a, where L is the length of the chain
and a is the lattice parameter, which is the same as the diameter
of a cylindrical macromolecule). Longer oligomers (L < a)
behave as short, flexible stacks. Polymers of a given degree of
polymerization (DP) exhibit rodlike character when L > a and
become stiff when L . a. Polymerization of macromonomers
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spherical object). In the ideal case where DP ) µ′, the spherical
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15258 J. AM. CHEM. SOC. VOL. 127, NO. 43, 2005

Isomerization of Helical Polyphenylacetylenes
A R T I C L E S
Scheme 2 a
a DPTS ) N,N-dimethylaminopyridinium tosylate; nbd ) 2,5-norbornadiene; DMAP ) N,N-(dimethylamino)pyridine.
Table 1. Structural Analysis of Poly[(3,4-3,5)mG2-4EBn]
We have prepared a library of dendronized cis-transoidal
PPAs to examine helix selection within the cylindrical object.
Herein, we report cis-transoidal PPAs jacketed with a particular
dendron substitution pattern that matches the helical pitch of
the pristine PPA. Experimental observations in the solid state
and solution demonstrate helical porous order within the
cylindrical object. At elevated temperatures, the polymer
undergoes an unprecedented cis-cisoidal-to-cis-transoidal con-
formational isomerization that closes the pore and circumvents
the intramolecular 6π electrocyclization occurring in pristine
cis-PPA. This isomerization is reversible. Structural analysis
confirms that molecular weight and cis content of dendronized
PPAs remain unchanged, thus providing a gating mechanism
for the porous structure.
a
b
b
m
DPth
Mn
Mw/Mn
cis contentc
8
dm8
10
12
14
48
53
56
52
49
52
45 400
49 000
101 000
61 300
50 000
1.28
1.38
1.35
1.22
99%
95%
81%
87%
91%
99%
1.24
d
d
16
-
-
a Theoretical degree of polymerization ) [4EBn]0/[Rh]0. b Determined
by GPC (THF, 1 mL/min) calibrated with polystyrene standards. c Deter-
mined as prescribed in ref 12a. d Insoluble in THF.
substituted acetylene by sublimation was employed to avoid side
reactions related to trace metal impurities. Sequential reduction
of the methyl ester with LiAlH4 followed by desilylation in
basicmethanolprovidedthedesired4-ethynylbenzylalcohol(4EBnOH),
which was also purified by sublimation. The synthesis and
structural analysis of (3,4-3,5)12G2-CO2H was reported
previously.6c Preparation of dendrons with the same substitution
pattern and different alkyl tails was accomplished using the same
Results and Discussion
Synthesis. The self-assembling macromonomers, (3,4-3,5)-
mG2-4EBn with m ) 8, 10, 12, 14, 16, and (S)-3,7-
dimethyloctyl were selected from a library designed by analogy
to our previously reported dendronized polystyrenes.6 Synthesis
and polymerization of the dendritic macromonomers are il-
lustrated in Scheme 2. The polymerizable apex was synthesized
according to modified literature procedures.15 Methyl 4-bro-
mobenzoate was subjected to Pd-catalyzed Sonogashira cross-
coupling with trimethylsilylacetylene. Purification of the di-
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9
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A R T I C L E S
Percec et al.
Table 2. Thermal Analysis of Poly[(3,4-3,5)mG2-4EBn] by Differential Scanning Calorimetry
polymer
thermal transitions (°
C) and corresponding enthalpy changes (kcal/mol)a
poly[(3,4-3,5)8G2-4EBn]
poly[(3,4-3,5)dm8G2-4EBn]
poly[(3,4-3,5)10G2-4EBn]
poly[(3,4-3,5)12G2-4EBn]
poly[(3,4-3,5)14G2-4EBn]
poly[(3,4-3,5)16G2-4EBn]
Φh,gio,b 46 (0.56); Φhio,c 96 (1.26); Φh,d 130 dece
Φ
Φ
Φ
Φ
Φ
h,g,f 50; Φh, 120 dec
h,g,f 37 (0.38); Φh, 60 (-0.35); Φhio, 91 (1.25); Φh, 130 dec
h,k,g 0 (3.63); Φhio, 88 (1.11); Φh, 140 dec
h,k, 24 (5.63); Φhio, 87 (0.50); Φh, 120 dec
h,k, 49 (12.71); Φhio, 89 (1.66); Φh, 140 dec
b
a Data from second heating DSC scans at 10 °C/min. Φh,gio, glassy state of hexagonal columnar lattice with intracolumnar order. c Φhio, hexagonal
columnar lattice with intracolumnar order. d Φh, hexagonal columnar lattice. e dec, onset of decomposition. f Φh,g, glassy state of hexagonal columnar lattice.
g Φh,k columnar hexagonal crystal.
convergent synthetic strategy. The macromonomers were pre-
pared by condensation of each of the dendritic carboxylic acids
with 4EBnOH following the method of Moore and Stupp.16
Synthesis and structural analysis of the four possible PPA
isomers (i.e., cis-cisoidal, cis-transoidal, trans-cisoidal and
trans-transoidal) was demonstrated using Ziegler-Natta-type
catalysts.12 Catalysts based on Rh(I) have emerged as a
convenient method for stereoselective preparation of cis-
transoidal PPA.17 We are particularly interested in liv-
ing polymerization methods for preparation of dendronized
polymers.4a-f Only a few Rh-based catalysts have been reported
to exhibit characteristics of a living polymerization of arylacety-
lene monomers.17a Noyori and co-workers have shown that Rh-
(CtCPh)(nbd)(PPh3)2 (nbd ) 2,5-norbornadiene), in the pres-
ence of 10 equiv of N,N-(dimethylamino)pyridine (DMAP),
induces stereospecific polymerization of arylacetylenes with
controlled molecular weight, narrow molecular weight distribu-
tion (Mw/Mn), and chain ends active for reinitiation.18 We have
employed this catalytic system to prepare the dendronized PPAs
(Scheme 2).
Two methods for the analysis the cis and trans contents of
PPA have been advanced.12a,b The more detailed method12b
accounts for the fractions of cis-polyene, trans-polyene, and
cyclohexadiene repeat units formed during polymerization and
purification. The resonance for the methine proton of cyclo-
hexadiene repeat units is broad and found in the range 4.5-3
ppm. Due to overlap with the alkyl ether proton resonances
associated with the alkyl tails (3.8 ppm in CDCl3), the broad,
weak resonance due to cyclohexadiene repeat units cannot be
integrated reliably. A simplified analysis determines cis content12a
by the ratio of the integrated area for the resonance at 5.8 ppm
to all resonances in the range 7.2-5.5 ppm (i.e., all aromatic
and vinylic resonances). This approach overlooks the overlap
of cis-polyene resonances with vinylic resonances from cyclo-
hexadiene repeat units but offers a reasonable index for the
structural composition of PPAs. Table 1 summarizes the
molecular weight and structural composition of the polymer.
Relative molecular weight (Mn) and molecular weight dis-
tribution (Mw/Mn) determined by gel permeation chromatography
(GPC) relative to polystyrene standards are reported in Table
1. Absolute molecular weight determination by light scattering
on polystyrenes and polymethacrylates jacketed with self-
assembling dendrons confirmed that GPC results obtained with
polystyrene standards underestimate the true molecular weight.6
We and others have noted this behavior with a range of
dendron-polymer backbone combinations.2c,d,4b,d,g,5k Addition-
ally, we have shown that extremely high molecular weight
fractions exhibit anomalous elution behavior.6c With regard to
the present polymers, we do not anticipate such behavior.
Considering the theoretical DP based on the ratio of monomer
to Rh and the previously noted low initiator efficiency, the
dendronized PPAs considered here are significantly shorter than
the corresponding dendronized polystyrene.
Structural and Retrostructural Analysis. Differential scan-
ning calorimetry (DSC) and thermal optical polarized micros-
copy (TOPM) were used to confirm the presence of hexagonal
columnar phases. Table 2 summarizes the phase behavior
determined by DSC. Increasing number of methylene units (m)
in the alkyl tails raises the melting temperature, such that
crystalline phases are observed for m ) 12, 14, and 16. In all
cases, decomposition occurs before isotropization. Nonetheless,
reversible heating and cooling cycles can be obtained if the
maximum temperature is kept 20 °C below the observed
decomposition temperature. While all of the dendronized PPAs
exhibit minor inflections during the first heating cycle that are
due to various thermal history events, only the hexagonal
io
columnar crystal (Φh,k) to Φhio (m ) 12, 14, and 16) and Φh
-
Φh phase transitions are reversible and reproducible during all
subsequent scans. In addition, exothermic transitions on heating
(m ) 10) due to kinetic effects induced by close proximity to
glass transition and chain stiffness are sometimes observed.17b
Hexagonal columnar phases identified by the combination
of DSC and thermal optical polarized microscopy (TOPM) were
assigned by X-ray diffraction (XRD). Table 3 enumerates the
lattice symmetry, observed d-spacings, lattice parameter (a),
experimental density at 20 °C (F20), and number of dendrons
per column stratum. For all peripheral alkyl chain variations, a
hexagonal columnar (Φh) phase is observed. While several
thermal transitions are observed during the first heating cycle,
the lattice parameter changes very little except through the
highest temperature transition. The corresponding change in
lattice parameter cannot be explained exclusively by the
contraction of the peripheral alkyl chains.
An intracolumnar microphase-segregated model consisting
of a disordered liquidlike aliphatic coat jacketing the aromatic
part of the dendron has been demonstrated for both macro-
molecular4d,5l and supramolecular19 columns generated from self-
assembling dendritic building blocks. In the Φh phase, the
aromatic part of the dendron is also disordered liquidlike.
(16) Moore, J. S.; Stupp, S. I. Macromolecules 1990, 23, 65-70.
(17) (a) Sedla´cek, J.; Vohl´ıdal, J. Collect. Czech. Chem. Commun. 2003, 63,
1745-1790. (b) Percec, V.; Keller, A. Macromolecules 1990, 23, 4347-
4350.
(18) (a) Kishimoto, Y.; Eckerle, P.; Miyatake, T.; Ikariya, T.; Noyori, R. J.
Am. Chem. Soc. 1994, 116, 12131-12132. (b) Kishimoto, Y.; Eckerle, P.;
Miyatake, T.; Kainosho, M.; Ono, A.; Ikariya, T.; Noyori, R. J. Am. Chem.
Soc. 1999, 121, 12035-12044.
io
However, in the Φh phase, the aromatic part displays intra-
(19) Ungar, G.; Abramic, D.; Percec, V.; Heck, J. A. Liq. Cryst. 1996, 21, 73-
86.
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15260 J. AM. CHEM. SOC. VOL. 127, NO. 43, 2005

Isomerization of Helical Polyphenylacetylenes
A R T I C L E S
Table 3. Measured d Spacings and Structural Analysis of the Hexagonal Columnar Lattices Generated from Poly[(3,4-3,5)mG2-4EBn]
Cylindrical Macromolecules
d spacings (Å)
polymer
T (°
C)
lattice
d100
d110
d200
d210
aa (Å)
F20b (g/cm3)
µc
poly[(3,4-3,5)8G2-4EBn]
-20
60
105
23
50
100
25
70
105
30
70
105
30
p6mm
p6mm
p6mm
p6mm
p6mm
p6mm
p6mm
p6mm
p6mm
p6mm
p6mm
p6mm
p6mm
p6mm
p6mm
p6mm
p6mm
p6mm
38.2
38.6
35.4
37.0
36.3
35.0
40.6
40.5
38.3
43.7
43.2
38.9
46.0
45.1
40.7
47.2
47.3
42.2
22.1
22.3
20.4
21.4
21.0
20.2
23.4
23.4
22.1
25.2
24.9
22.4
26.5
25.8
23.2
27.6
27.2
24.4
19.3
19.4
17.7
18.8
18.4
17.6
20.4
20.3
19.2
22.0
21.7
19.5
23.3
22.7
20.5
24.0
23.7
21.1
44.3
44.7
40.8
43.0
42.1
40.5
47.0
46.9
44.0
50.6
49.6
44.9
53.2
52.0
46.9
55.1
54.6
48.7
1.019
4.94
poly[(3,4-3,5)dm8G2-4EBn]
poly[(3,4-3,5)10G2-4EBn]
poly[(3,4-3,5)12G2-4EBn]
poly[(3,4-3,5)14G2-4EBn]
poly[(3,4-3,5)16G2-4EBn]
-
-
1.009
1.014
5.0
5.25
70
100
30
70
105
18.0
1.005
5.25
a Lattice parameter, a ) 2 d100 3, where d100 ) (d100
the column (Dcol) are equal. F20, experimental density measured at 20 °C. Number of dendrons in a column stratum ) ( 3NAa tF20)/2M, where
+
3d110 + 2d200
+
7d210)/4; for columnar phases, the lattice parameter (a) and diameter of
x
x
x
b
c
2
x
Avogadro’s number NA ) 6.0220455 × 1023, layer thickness t ) 4.7 Å, and M is the molecular weight of the monomer.
order helical feature at 13.5 ( 0.2 Å (Figure 2). This
demonstrates that the first-order feature observed in the other
samples at 6.5 ( 0.2 Å is due to intracolumnar helical order.
CD spectra of poly[(3,4-3,5)dm8G2-4EBn] in methyl cyclo-
hexane at 2 °C and 22 °C further support the presence of a
helical conformation in the polymer backbone in solution (Figure
3). The decrease of the Cotton effect intensity at elevated
temperatures is consistent with the dynamic helical behavior
observed in cis-transoidal PPAs.13 Since at the transition from
solution to solid-state XRD data show that the dendronized
polymer maintains its helical conformation, the helix sense
Figure 1. Square of the lattice parameter (a2) as a function of the number
of methylenic units (m). The steepest increase is observed for the low-
temperature range (determined between 60 and 70 °C) when the aliphatic
selected by the stereocenter in solution is preserved in solid
state.
XRD patterns obtained during heating and cooling cycles
indicate that the lattice parameter undergoes a discontinuous
and sudden change that is both reversible and reproducible.
Figure 4a,b presents plots of Dcol over a range of temperatures
with the corresponding DSC traces showing the high-temper-
ature transition on heating and cooling cycles. Furthermore, the
angle at which the dendron tilts relative to the column axis is
tails are more rigid. In the high-temperature phase (determined from 100
and 105 °C, Table 3) the alkyl tails are more flexible, allowing a repacking
of the dendrons.
columnar order that involves helicity, well-defined dendron tilt,
and other features that give rise to diffuse XRD peaks. Due to
the liquidlike aliphatic coat, the intracolumnar order is not
correlated from one column to another in the Φhio phase. Figure
1 compares the square of the experimental lattice parameter (a2),
which is analogous to the square of the column diameter (Dcol2),
with the number of methylenic units (m) in the n-alkane chains.
In agreement with the microphase-segregated model, the linear
fit of the experimental data indicates that the density of the
aliphatic region of the column is constant for all measured chain
lengths. Since, as will be discussed later, the lattice dimensions
are temperature dependent, the trend from Figure 1 is only
qualitative. Nevertheless, it demonstrates a marked difference
io
∼40° for all alkyl chain lengths in the low-temperature Φh
phase. The change in Dcol at the DSC phase transition (Figure
4a,b) is responsible for the difference observed in the trend from
Figure 1.
The reconstructed electron density maps for poly[(3,4-3,5)-
12G2-4EBn] in the Φh,k phase at 20 °C, in the Φhio at 50 °C,
and in the Φh phase at 110 °C were calculated from the XRD
data with the following phase choice for the four diffraction
peaks of the hexagonal columnar phase: (10) +, (11) -, (20)
io
io
between column diameters in the Φh and Φh phases.
-, and (21) -. The electron density maps of the Φh,k and Φh
XRD Patterns of oriented fibers of all dendronized PPAs
phases are almost identical. Figure 5 shows the electron density
maps in the Φh,k phase at 20 °C and in the Φh phase at 110 °C.
The electron density maps from Figure 5 support the intracolumn
phase-segregated model. The electron density map of the lower-
temperature Φh,k and Φh phases exhibits a drop in electron
density at the column core. The significance of such an
exhibit a feature at 6.5 ( 0.2 Å that indicates intracolumnar
io
helical order in the Φh phase. This feature persists through
any phase transition observed by DSC, except the highest
temperature transition (Figure 2). Furthermore, this feature is
recovered on cooling. In the case of (3,4-3,5)16G2-4EBn, the
diffraction pattern of the oriented fiber in the Φhio phase shows,
besides the first-order feature at 6.6 Å, a diffuse weak second-
io
observation has recently been discussed for a series of self-
assembling dendritic dipeptides.8 In the Φh,k and Φh phases,
io
9
J. AM. CHEM. SOC. VOL. 127, NO. 43, 2005 15261

A R T I C L E S
Percec et al.
Figure 2. Wide-angle diffraction patterns of aligned fibers (horizontal fiber axis is indicated by arrow) collected in the temperature sequence indicated by
the dotted arrows for poly[(3,4-3,5)mG2-4EBn] with m ) 12, 14, 16. q ) (4π/λ)sin θ is the momentum transfer. i, four diffuse X-shape short-range helical
features (4.4 ( 0.2 Å); j, 6.5 ( 0.2 Å first-order helical features (corresponding to the dendron registry along the column axis); k, four X-shape diffuse spots
corresponding to the 13.5 ( 0.2 Å second-order helical feature (slightly enhanced for poly[(3,4-3,5)16G2-4EBn] at -20 °C) (see Figure 6); l, average
stratum thickness in the Φh,k phase (4.1 ( 0.2 Å); t, four diffuse X-shape features due to dendron tilt (57 ( 9°); (hk0), higher-order reflection of the
hexagonal columnar phase. Diffractionlike spots near the beam stop on the equators are from the beam stop.
previously.8 In the highest-temperature Φh phase, this feature
is absent from the electron density map, indicating that the
helical conformation is compressed (cis-cisoisal) in the low-
temperature Φh,k and Φhio phases and stretched (cis-transoidal)
in the high-temperature Φh phase. This difference is in agree-
ment with the difference between the cis-cisoidal and cis-
transoidal conformations of pristine PPA, as well as with the
difference between Dcol in the two phases (Figure 1).
Structural Model. In combination with the observations
above, molecular modeling studies constrained by the XRD
results clarify the significance of the Φhio-to-Φh phase transition.
Figure 3. Circular dichroism (top, left axis) and UV-vis spectra (bottom,
right axis) of poly[(3,4-3,5)dm8G2-4EBn] in methyl cyclohexane ([4EBn]
Given a helical conformation of the polymer backbone, the
dihedral angle (â) associated with the cisoidal and transoidal
conformations in cis-PPA is the only available mechanism to
explain the observed column pore and its disappearance at
elevated temperature. Two such angles are illustrated in Figure
) 9.1 × 10-4 M) at 2 (blue curves) and 22 °C (red curves).
the columns have a small pore of 3.8 ( 1.2 Å. The size of the
pore was calculated from the intensity of the X-ray diffraction
peak data collected at 20 °C by using the procedure reported
9
15262 J. AM. CHEM. SOC. VOL. 127, NO. 43, 2005

Isomerization of Helical Polyphenylacetylenes
A R T I C L E S
Figure 4. (a) Temperature dependence of the column diameter and the corresponding DSC traces (red, heating; blue, cooling) for poly[(3,4-3,5)mG2-
4EBn] (m ) 12); (b) same data for m ) 16. (c) Representative small-angle XRD plots for poly[(3,4-3,5)16G2-4EBn], indicating an enhancement of the
io
(11) and (21) peaks in the Φh,k and Φh phases at 20 and 60 °C compared to the Φh phase at 110 °C.
Figure 5. Electron density maps reconstructed from the XRD data of poly-
[(3,4-3,5)12G2-4EBn] (a) in the Φh,k phase at 20 °C and in the Φhio at 50
°C and (b) in the Φh phase at 110 °C.
Figure 7. 1H NMR Spectra and GPC data for (a) pristine poly[(3,4-3,5)-
12G2-4EBn] and samples annealed (b) under argon at 100 °C for 30 min,
(c) in air at 100 °C for 30 min, and (d) in air at 85 °C for 3 h. Cis content
Figure 6. Models illustrating (a) the dihedral angle (â) about the single
bond in the polyene backbone, (b) side and top views of PPA oligomers
with â ≈ 65° (cis-cisoidal) and 100° (cis-transoidal) and the corresponding
change in length along the helix axis, (c) side views of space-filling models
of PPA and poly[(3,4-3,5)mG2-4EBn] showing the helical registry of
aromatic rings that explains the 6.6 (i-j) and 13.5 Å (i-k) helical features
in the Φhio phase (see Figure 2), and (d) a top view of a space filling model
of poly[(3,4-3,5)mG2-4EBn] with low-electron-density core.
determined as described in ref 12a.
Thermal Stability. In bulk or solution, thermally induced
6π electrocyclization of 1,3-cis,5-hexatriene sequences in cis-
PPA to form cyclohexadiene repeat units occur via the cisoidal
conformation.12 While very little cyclization is observed at room
temperature in bulk, the rate of cyclization increases with
temperature and becomes significant above 60 °C. Furthermore,
at temperatures above 120 °C, extrusion of triphenylbenzene
with concomitant chain cleavage is observed due to re-
aromatization of cyclohexadiene repeat units.12 The temperature-
triggered relaxation (unwinding) of the PPA backbone upon
heating is expected to eliminate the intramolecular cyclization
reaction.
The stability of these polymers toward thermal cycling was
investigated. A fiber sample of poly[(3,4-3,5)10G2-4EBn]
extruded at 95 °C and cycled during XRD experiments between
50 and 100 °C over 3.5 h was dissolved in THF, and its
molecular weight (Mn) and molecular weight distribution (Mw/
6 from both top and side views. The side views indicate a
corresponding stretch of the helix axis. Furthermore, when â
≈ 65° (i.e., cis-cisoidal PPA), the aromatic rings of the PPA
backbone are in registry consistent with the observed 6.6 Å
features in the fiber XRD patterns. When â ≈ 100° (i.e., cis-
transoidal PPA), the registry is not apparent. Appending the
dendrons to the polymer backbone in a conformation that both
creates a 40° tilt relative to the column axis and adequately
fills space creates an aromatic core whose diameter matches
that calculated above. Increasing â results in a larger tilt angle
and, together with contraction of the alkyl tails, a smaller column
diameter.
9
J. AM. CHEM. SOC. VOL. 127, NO. 43, 2005 15263

A R T I C L E S
Percec et al.
Mn) were determined by GPC. The initial values are reported
in Table 1, and for the thermally treated sample, Mn ) 101 000
and Mw/Mn ) 1.30. Films of poly[(3,4-3,5)12G2-4EBn] cast
from CH2Cl2 were thermally treated under several conditions.
Figure 7 illustrates 1H NMR spectra of the pristine polymer, as
well as a film annealed at 100 °C under anaerobic conditions
for 30 min and films annealed under ambient atmosphere at
100 °C for 30 min and at 85 °C for 3 h. The measured cis
content and GPC results show nominal differences. The steric
demand of the dendron provides a driving force against
intramolecular 6π electrocyclization in bulk samples.
homochiral tails. In bulk, a Φhio-t-Φh phase transition was
identified as a temperature-triggered relaxation of the polymer
backbone, which leads to a transition from a compressed helical
porous structure to a stretched helical nonporous structure. A
combination of XRD, electron density calculations, and mo-
lecular modeling studies was used to explain this process by
an unprecedented cis-cisoidal-to-cis-transoidal conformational
isomerization of the dendronized cis-PPA. The thermoreversible
stretching of the polymer backbone impedes the intramolecular
6π electrocyclization encountered in annealed pristine cis-PPA
samples and provides a gating mechanism for the helical porous
structure.
Conclusions
From a library of dendronized cis-transoidal PPAs, we have
selected a dendritic architecture whose match to the helical pitch
of the polymer backbone allowed a detailed structural and
retrostructural analysis of the dendronized PPA. Variation of
the peripheral alkyl tails has led to a quantitative analysis of
the structure, the results of which are in agreement with a
microphase-segregated model for the cylindrical PPA. XRD
experiments have clarified the internal structure. In the Φh,k and
Φhio phases, the dendrons are titled at ∼40° to a helical porous
column axis. Solution CD experiments demonstrate the selection
of a single-handed helix sense by incorporation of dendritic
Acknowledgment. Financial support by the National Science
Foundation (DMR-99-96288 and DMR-01-02459) is gratefully
acknowledged.
Supporting Information Available: Experimental part con-
taining materials, methods, synthesis, and structural analysis and
additional XRD patterns from oriented fiber samples. This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA055406W
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15264 J. AM. CHEM. SOC. VOL. 127, NO. 43, 2005