A tetragonal phase self-organized from unimolecular spheres assembled from a substituted poly(2-oxazoline)

Article abstract of DOI:10.1021/acs.macromol.6b02298

Synthesis and living cationic ring-opening polymerization of a 2-oxazoline containing self-assembling minidendrons with hexadecyl groups are reported. Structural analysis using X-ray diffraction, molecular modeling, and reconstructed electron density maps

Full text of DOI:10.1021/acs.macromol.6b02298

Article
pubs.acs.org/Macromolecules
A Tetragonal Phase Self-Organized from Unimolecular Spheres
Assembled from a Substituted Poly(2-oxazoline)
Marian N. Holerca,† Dipankar Sahoo,† Mihai Peterca,†,‡ Benjamin E. Partridge,† Paul A. Heiney,‡
and Virgil Percec*,†
‡
†Roy & Diana Vagelos Laboratories, Department of Chemistry, and Department of Physics and Astronomy, University of
Pennsylvania, Philadelphia, Pennsylvania 19104, United States
S
* Supporting Information
ABSTRACT: Synthesis and living cationic ring-opening
polymerization of a 2-oxazoline containing self-assembling
minidendrons with hexadecyl groups are reported. Structural
analysis using X-ray diffraction, molecular modeling, and
reconstructed electron density maps revealed multiple self-
organized periodic arrays, including columnar hexagonal
P6mm, cubic Pm3n, and tetragonal P4 /mnm phases, which
̅
2
have not been observed before in a single homopolymer. The
degree of polymerization (DP) of the poly(2-oxazoline)
programs the sequence of these periodic arrays. The cubic
Pm3n phase was observed for DP = 5 whereas the tetragonal
̅
P42/mnm for 10 ≤ DP ≤ 50. For DP ≥ 75, only a columnar hexagonal P6mm phase was accessible because the polymer chains
are too long to form a single supramolecular sphere. This sequence was rationalized by the columnar character of supramolecular
spheres to provide the richest diversity of structures assembled from a single polymer and an unprecedented example of a
tetragonal phase organized from macromolecular spheres comprising a single polymer chain.
the chain. Dendronized poly(2-oxazoline)s with n = 13
generated columnar hexagonal arrays at all values of DP,24
whereas spheres were generated from polymers with n = 14 and
15 at DP ≤ 75 and DP ≤ 50, respectively.25 These spheres self-
INTRODUCTION
■
Quasi-equivalent building blocks are critical to the self-assembly
of functional structures in biology.1−3 A classic example is the
self-assembly of icosahedral viruses, in which a nucleic acid is
coated with quasi-equivalent protein units.1,2 Polymers
functionalized with self-assembling dendrons have been
elaborated as mimics of biological quasi-equivalent assembly.3−5
Second-generation monodendrons with peripheral dodecyl
alkyl chains appended to a poly(methacrylate) or poly(styrene)
backbone adopted a conical conformation at low degrees of
polymerization (DP), forming spheres which organized into a
organized into cubic Pm3n lattices like those observed in
̅
dendronized poly(methacrylate)s and poly(styrene)s.3,5 Spher-
ical assemblies of a poly(2-oxazoline) with n = 12 have also
been reported to organize into a body-centered cubic (BCC)
Im3m structure.26,27
̅
Structural and retrostructural analysis28,29 has discovered
other 3D phases generated from spheres in self-assembling
dendrons,30 including tetragonal P42/mnm,31 and 12-fold liquid
quasicrystalline (LQC).32 These phases have also been
transplanted to other forms of organized soft matter33,34
including block copolymers35−40 and lyotropic liquid crystals
cubic Pm3n lattice.3,5 Polymers with higher DP adopted a
̅
hexagonal packing of columns with the polymer backbone at
their cores jacketed by monodendrons with a flat-tapered
conformation.3,5
self-organized from surfactants.41−43 Whereas cubic Pm3n
̅
Poly(2-oxazoline)s, also known as N-acyl-substituted poly-
(ethylenimine),6 were employed as models to generate 2D and
3D periodic arrays in the 1990s7−12 and have undergone a
recent revival in interest due to their potential biomedical
applications, as smart materials and for hierarchical self-
assembly.13−16 Vesicles assembled from AB and ABA
amphiphilic block copolymers containing the water-soluble
poly(2-methyloxazoline) in the hydrophilic segment(s) under-
go directed protein insertion and act as functional mimics of
biological membranes controlled by the composition of the
block copolymers.17−23 Supramolecular structures can also be
controlled via the length of the peripheral alkyl chain of the
monodendrons, defined by n, the number of methylene units in
lattices organized from spheres comprising a single polymer
chain have been observed in poly(methacrylate)s,3,5 poly-
(styrene)s,3,5 and poly(oxazoline)s,24,25 and tetragonal P42/
mnm phases from poly(oxazoline)s have been mentioned,31 no
examples of these spheres organizing into a tetragonal P42/
mnm phase have been reported.
Here we report a poly(oxazoline) functionalized with
monodendrons with n = 16 that self-organizes into a tetragonal
Received: October 22, 2016
Revised: November 30, 2016
© XXXX American Chemical Society
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Synthesis of (3,4)16G1-Oxz. 3,4-Bis(n-hexadecane-1-yloxy)-
benzoic acid (1) was synthesized by standard techniques used in our
laboratories.47
P42/mnm array. At DP < 50, multiple polymer chains assemble
into supramolecular spheres that organize into cubic Pm3n and
̅
tetragonal P42/mnm structures. Polymers with DP = 50
generate a tetragonal P42/mnm array formed from unimolecular
supramolecular spheres. At higher DP (DP ≥ 75), only
columnar structures are accessible.
3,4-Bis(n-hexadecane-1-yloxy)benzoyl Chloride (2). Compound 1
(21 g, 34 mmol) was suspended in CH2Cl2 (200 mL), and a catalytic
amount of DMF was added. SOCl2 (40 mL, excess) was added
dropwise, and the mixture was heated to reflux for 0.5 h. After cooling
to room temperature, the solvent and the excess SOCl2 were distilled
to yield 21.5 g (99.9%) of a white powder which was used without
further purification. 1H NMR (CDCl3, δ ppm, TMS): 0.89
(overlapped t, 6H, CH3), 1.25−1.65 (m, 52H, CH3(CH2)13), 1.79
(m, 4H, CH2CH2OAr), 4.04 (overlapped t, 4H, CH2OAr), 6.92 (d,
1H, J = 9.2 Hz, meta to COCl), 7.56 (d, 1H, J = 1.8 Hz, ortho to
COCl), 7.71 (dd, 1H, J = 9.2 Hz, J = 1.8 Hz, ortho to COCl).
N-[3,4-Bis(n-hexadecane-1-yloxy)benzoyl]-2-aminoethanol (3).
Compound 2 (20.8 g, 33 mmol) was dissolved in CH2Cl2 (125
mL) and slowly added to ice-cooled ethanolamine (25 mL, excess)
under vigorous stirring. The mixture was stirred at 0 °C for 1 h and
was then heated to 40 °C whereupon it was stirred for another 5 h.
The solution was poured into a separatory funnel and washed three
times with H2O, dried over MgSO4, and filtered. The solvent was
removed by rotary evaporation, and the crude product was
recrystallized twice from acetone at 0 °C to yield 17.8 g (82%) of a
white powder. Purity (HPLC): 99+%. TLC (1:1 hexanes:EtOAc): Rf =
0.21. 1H NMR (CDCl3, δ ppm, TMS): 0.89 (t, 6H, CH3, J = 6.3 Hz),
1.25−1.69 (overlapped peaks, 52H, CH3(CH2)13), 1.86 (m, 4H,
CH2CH2OAr), 3.2 (bs, 1H, OH), 3.62 (q, 2H, CH2OH, J = 5.0 Hz),
3.83 (t, 2H, NHCH2, J = 5.1 Hz), 4.02 (overlapped t, 4H, CH2OAr),
6.60 (bs, 1H, NH), 6.88 (d, 1H, meta to CONH, J = 8.2 Hz), 7.28 (dd,
1H, ortho to CONH, J = 8.2 Hz, J = 2.4 Hz), 7.40 (d, 1H, ortho to
CONH, J = 2.4 Hz). 13C NMR (CDCl3, δ ppm): 13.8 (CH3), 22.4
(CH3CH2), 26.0 (CH2CH2CH2O), 29.0, 29.1, 29.2, 29.3, 29.5
(CH3CH2CH2(CH2)10), 30.2 (CH2CH2O), 31.8 (CH3CH2CH2),
42.7 (NHCH2), 62.4 (CH2OH), 69.1, 69.5 (CH2OAr), 112.5, 112.7
(meta to CONH, ortho to CONH and O), 119.7 (ortho to CONH),
126.3 (ipso to CONH), 148.6, 152.0 (meta to CONH, ipso to O and
para to CONH), 168.2 (CONH).
EXPERIMENTAL SECTION
■
Materials. THF (Fisher, ACS reagent) was refluxed over sodium
ketyl and freshly distilled before use. CH2Cl2 (Fisher, ACS reagent)
was refluxed over CaH2 and freshly distilled before use. Benzene (both
Fisher, ACS reagents) was shaken with concentrated H2SO4, washed
twice with water, dried over MgSO4, and finally distilled over sodium
ketyl. H2SO4, dimethylformamide (DMF), KOH, and NaHCO3 (all
Fisher, ACS reagents) and SOCl2 (97%) and ethanolamine (99%) (all
Lancaster) were used as received. Methyl trifluoromethanesulfonate
(MeOTf) (Fluka, 97%) was vacuum-distilled.
Methods. The purity and the structural identity of the intermediary
and final products were assessed by a combination of techniques that
includes thin-layer chromatography (TLC), high pressure liquid
chromatography (HPLC), 1H and 13C NMR, and matrix-assisted
laser desorption/ionization time-of-flight (MALDI-TOF) mass spec-
trometry. TLC was carried out on precoated glass plates (silica gel
with F254 indicator; layer thickness, 200 μm; particle size, 2−25 μm;
pore size, 60 Å, from Sigma-Aldrich). HPLC was carried out using
PerkinElmer Series 10 high pressure liquid chromatography with a LC-
100 column oven, Nelson Analytical 900 series integrator data station,
and two PerkinElmer PL gel columns, 5 × 102 Å and 1 × 104 Å. THF
was used as solvent at an oven temperature of 40 °C. UV absorbance
at 254 nm was used as detector. Relative weight-average (Mw) and
number-average (Mn) molecular weights were determined with the
same instrument from a calibration plot constructed from polystyrene
1
standards. H NMR (500 MHz) and 13C NMR (126 MHz) spectra
were recorded on a Bruker DRX 500 instrument using the solvent
indicated.
Differential Scanning Calorimetry (DSC). Thermal transitions
were measured on PerkinElmer DSC-7 differential scanning
calorimeter (DSC). The heating and cooling rates were 10 °C/min.
The transition temperatures were measured as the maxima and
minima of their endothermic and exothermic peaks. Glass transition
temperatures (Tg) were read at the middle of the change in heat
capacity. Indium and zinc were used as calibration standards. An
Olympus BX-40 optical polarized microscope (100× magnification)
equipped with a Metler Toledo FP82HT hot stage and a Metler
Toledo FP80 central processor was used to verify thermal transitions
and to characterize anisotropic textures.
X-ray Diffraction (XRD). X-ray diffraction (XRD) measurements
were performed using Cu Kα1 radiation (λ = 1.542 Å) from a Bruker-
Nonius FR-591 rotating anode X-ray source equipped with a 0.2 × 0.2
mm2 filament and operated at 3.4 kW. Osmic Max-Flux optics and
triple pinhole collimation were used to obtain a highly collimated
beam with a 0.3 × 0.3 mm2 spot on a Bruker-AXS Hi-Star multiwire
area detector. To minimize attenuation and background scattering, an
integral vacuum was maintained along the length of the flight tube and
within the sample chamber. Samples were held in glass capillaries (1.0
mm in diameter), mounted in a temperature-controlled oven
(temperature precision: 0.1 °C, temperature range from −10 to
210 °C). Aligned samples for fiber XRD experiments were prepared
using a custom-made extrusion device.44 The powdered sample (∼10
mg) was heated inside the extrusion device. After slow cooling, the
fiber was extruded in the liquid crystal phase and cooled to 23 °C.
Typically, the aligned samples have a thickness of 0.3−0.7 mm and a
length of 3−7 mm. All XRD measurements were done with the aligned
sample axis perpendicular to the beam direction. Primary XRD analysis
was performed using Datasqueeze (version 3.0.7).45
2-[3,4-Bis(n-hexadecane-1-yloxy)phenyl]-2-oxazoline, (3,4)16G1-
Oxz. Compound 3 (20 g, 31 mmol) was dissolved in CH2Cl2 (750
mL), and SOCl2 (6.56 mL, 0.09 mol) was added dropwise at 23 °C.
The mixture was stirred for 15 min, after which the 1H NMR indicated
complete conversion of the starting material. The reaction was
neutralized by addition of NaHCO3 solution (sat. aq, 750 mL) and
vigorous stirring for 1 h. The aqueous layer was discarded, and the
organic layer was washed three times with water (300 mL), then dried
over MgSO4, and filtered. The solvent was distilled, and the product
was recrystallized twice from hexanes, purified on column chromatog-
raphy (SiO2, hexanes:EtOAc 5:1) and recrystallized again in acetone to
yield 15.54 g (80%) of a white solid. Purity (HPLC): 99+%. TLC
(CHCl3): Rf = 0.50; mp 65−67 °C. 1H NMR (CDCl3, δ ppm, TMS):
0.88 (t, 6H, CH3, J = 6.4 Hz), 1.25−1.70 (m, 44H, CH3(CH2)13), 1.78
(m, 4H, CH2CH2OAr), 4.02 (overlapped t, 6H, CH2OAr,
OCH2CH2N), 4.42 (t, 2H, OCH2CH2N, J = 9.2 Hz), 6.87 (d, 1H,
meta to CON, J = 9.2 Hz), 7.48 (d, 1H, ortho to CON, J = 2.6 Hz)
7.48 (dd, 1H, ortho to CON, J = 9.2 Hz, J = 2.6 Hz). 13C NMR
(CDCl3, δ ppm): 14.2 (CH3), 22.3 (CH3CH2), 26.2 (CH2CH2CH2O),
29.2, 29.4, 29.7 (CH3CH2CH2(CH2)10), 30.2 (CH2CH2O), 31.9
(CH3CH2CH2), 54.9 (NCH2), 67.8 (OCNCH2), 69.3, 69.5
(CH2OAr), 112.6, 113.1 (meta to OCN, ortho to OCN), 119.8 (ipso
to CON) 121.7 (ortho to OCN), 148.9 (meta and para to CON),
164.8 (OC = N). Anal. Calcd for C41H73O3N: C 78.41, H 11.72.
Found: C 78.55, H 11.61.
Polymerization of (3,4)16G1-Oxz. A Schlenk tube equipped
with a magnetic stirrer was dried at 200 °C overnight and charged with
monomer (3,4)16G1-Oxz (0.313 g, 0.5 mmol), which was dissolved in
a minimum amount of dry benzene and freeze-dried in the septum-
sealed Schlenk tube. After freeze-drying, the Schlenk tube was flushed
with Ar. The initiator, MeOTf, was added via syringe according to the
desired theoretical DP, and the tube was brought on a preheated oil
Molecular Modeling and Simulation. Molecular modeling and
simulation experiments were performed using Material Studio (version
5.0) software from Accelrys. Details were reported previously.46
1
bath at 160 °C. The melt was stirred until H NMR indicated the
B
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a
Scheme 1. Synthesis of (3,4)16G1-Oxz
a
Reagents and conditions: (i) SOCl2, cat. DMF, CH2Cl2, reflux, 0.5 h; (ii) HN(CH2)2OH, CH2Cl2, 0 °C, 1 h then 40 °C, 5 h; (iii) SOCl2, CH2Cl2,
23 °C, 15 min; (iv) sat. aq NaHCO3, 23 °C, 1 h.
complete conversion of the monomer. The melt was allowed to cool
to RT, whereupon it was dissolved in THF (2 mL) and end-capped by
the addition of KOH solution (25% aq w/w, 2 mL) followed by
stirring for 1 h. The solid polymer was isolated by filtration.
RESULTS AND DISCUSSION
■
Synthesis of Poly[(3,4)16G1-Oxz]. Poly[(3,4)16G1-Oxz]
was prepared via living cationic ring-opening polymerization of
a monodendritic 2-oxazoline monomer (Scheme 1) synthesized
according to reported procedures (Scheme 2).48 Polymer-
Scheme 2. Polymerization of Monodendritic 2-Oxazolines
a
(3,4)16G1-Oxz into DPm
a
m = 5, 10, 20, 30, 40, 50, 75, 100, and 200
Figure 1. DSC traces of poly[(3,4)16G1-Oxz] recorded upon (a) first
heating and (b) first cooling at a rate of 10 °C/min. Phases determined
ization in the bulk state at 160 °C and subsequent end-capping
of the polymer chain ends with potassium hydroxide afforded
nine polymers with degrees of polymerization (DP) ranging
from DP = 5 to DP = 200, denoted DPm, where m is the
number of monomer repeat units in the polymer chain.
by XRD are indicated. Phase notation: k = undetermined crystalline
phase; g = glassy phase; Φh = columnar hexagonal P6mm phase; Cub =
cubic Pm3n phase; Tet = tetragonal P4 /mnm phase; i = isotropic
̅
2
liquid.
Thermal Analysis by DSC and Structural Analysis by
XRD. The temperature of thermal phase transitions and their
associated enthalpies were analyzed by differential scanning
calorimetry (DSC) at 10 °C/min (Figure 1). Phases indicated
in Figure 1 were determined by X-ray diffraction (XRD) to be
discussed later. The DSC traces of all polymers (DPm, 5 ≤ m ≤
200) show a large endothermic transition on first heating
consistent with the melting of an undetermined crystalline
phase, k. Upon further heating, a glass transition, followed by a
2D columnar hexagonal phase, Φh, was observed. The
crystallization (Tk) and glass transition (Tg) temperatures are
mostly invariant to DP, and in all polymers, Tg > Tk. This
sequence of transitions involving crystallization at a lower
temperature than the glass transition (Figure 1b) was
elaborated by our laboratory49 to decouple the motion of the
side groups from that of the backbone in side chain liquid
crystalline polymers. Microphase segregation25,49,50 of the
polymer backbone from the side groups provides, for example,
two glass transition temperatures: one arising from independ-
ent motion of the polymer backbone and the other from the
cooperative but independent motion of the side groups. This
decoupled motion of the polymer backbone and aliphatic
periphery and microphase segregation within the organized
arrays have been reported also for poly(2-oxazoline)s.24,25
For DP75, DP100, and DP200, columnar hexagonal Φh is the
only ordered array observed above Tg. In contrast, DPm, 5 ≤ m
≤ 50, exhibit an additional 3D phase organized from spheres:
DP5 generates a cubic Pm3n (Cub) structure while DP10,
̅
DP20, DP30, DP40, and DP50 generate a tetragonal P42/mnm
(Tet) structure. XRD analysis to be discussed later demon-
strates that the Tet array in DP50 is generated from
supramolecular spheres comprising a single DP50 molecule.
Thermal analysis of poly[(3,4)16G1-Oxz] is summarized in
Table 1. The enthalpy changes associated with phase transitions
reflect the change of the degree of order of the assemblies of
molecules in between the two phases. Hence, large enthalpy
changes are observed at the transition from the 3D crystalline k
phase to an isotropic liquid or glassy state and also upon
melting the 2D Φh phase of DP = 75, 100, and 200 to an
isotropic liquid. In contrast, transitions between ordered
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Table 1. Theoretical and Experimental Molecular Weights Determined by GPC and Thermal Phase Behavior of
Poly[(3,4)16G1-Oxz]
ab
,
thermal transitions (°C) and corresponding enthalpy changes (kcal/mol)
Mn,th (g/mol)
Mn,GPC (g/mol)
Mw/Mn
heating
cooling
DP5
3229
6434
4452
7608
1.23
1.20
1.14
1.12
1.09
1.08
1.08
1.08
1.10
k 45 (4.23) g 63 Φh 80 (0.22) Cub 114 (0.06) i
k 32 (6.17) g 66 Φh 80−85 Tet 122 (0.10) i
i 80 (−0.21) Cub * Φh 54 g 33 (−4.40) k
i 102 (−0.04) Tet * Φh 26 (−5.44) k
i 95−105 Tet 65−75 Φh 33 (−5.95) k
i 95−105 Tet 65−75 Φh 25 (−5.99) k
i 95−105 Tet 65−75 Φh 25 (−5.23) k
i 95−105 Tet 74 (−0.03) Φh 26 (−5.34) k
i 81 (−0.12) Φh 32 (−5.27) k
b
DP10
DP20
DP30
DP40
DP50
DP75
DP100
DP200
12704
18794
25244
31514
47189
62864
136764
10624
14740
18577
19501
25087
23972
23810
k 40 (6.68) g 72 Φh 85−90 Tet 128 (0.11) i
k 33 (6.18) g 68−72 Φh 75−85 Tet 116 (0.02) i
k 33 (5.86) Φh 86 (0.05) Tet 105−115 i
k 33 (5.74) g 73 Φh 92 (0.14) Tet 100−110 i
k 43 (5.67) g 74 Φh 105 (0.15) i
k 44 (5.83) g 80 Φh 110 (0.17) i
k 52 (6.12) g 84 Φh 119 (0.26) i
i 88 (−0.15) Φh 32 (−5.41) k
i 97 (−0.25) Φh 39 (−5.84) k
a
b
Single values are obtained from DSC data from first heating and cooling scans at a rate of 10 °C/min. Ranges of values are estimated transition
temperatures obtained from XRD experiments. Exact transition temperature could not be determined by DSC. All phases confirmed by XRD. Phase
notation: k = undetermined crystalline phase; g = glassy phase; Φ = columnar hexagonal P6mm phase; Cub = cubic Pm3n phase; Tet = tetragonal
̅
h
P42/mnm phase; i = isotropic liquid.
Figure 2. (a, b) VWAXS XRD pattern of the Φh phase of (a) DP5 and (b) DP10 collected at 70 °C. The intensity of the top half of the pattern has
been reduced to demonstrate the absence of tilt correlation features. (c) Comparison of simulated and experimental WAXS XRD patterns of the Φh
phase of DP5. The simulated pattern has been calculated using the model shown in (e−i). Fiber axis and lattice parameters are indicated. (d)
Azimuthal plots of the 4.6 Å stacking feature in (a, b) illustrating the absence of tilt correlation features for DP5 and DP10. (e) Monomeric unit of
poly[(3,4)16G1-Oxz]. (f−h) Molecular models of DP5 showing (f) full molecules, (g) polymer backbone, and (h) polymer backbone and aromatic
rings of the dendrons. (i) Supramolecular column of DP5. Peripheral alkyl chains omitted for clarity. (j) Schematic representation of the Φh phase.
(k) Molecular model of DP10. (l−n) Supramolecular column of DP10 showing (l) side view with peripheral alkyl chains omitted for clarity, (m) side
view, and (n) top view. (o) Supramolecular columns packed into a hexagonal array. Yellow lines denote unit cells. Color code used in the model: O
atoms, red; H atoms, white; N atoms, blue; C atoms in the core, green; C atoms in the phenyl rings, orange; all other C atoms, gray.
phases, such as between the 2D Φh and 3D Tet phases in DP =
10, 20, and 30, may be accompanied by very small or even
negative enthalpies seen as exothermic transitions. In these
instances, transition temperatures could not be accurately
determined by DSC. Therefore, in these cases estimated
transition temperatures determined by XRD are provided as
ranges of values in Table 1.
Retrostructural Analysis of Columnar Hexagonal
Phases by XRD and Molecular Modeling. The structures
of the periodic arrays generated by poly[(3,4)16G1-Oxz] were
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Figure 3. (a, b) VWAXS XRD patterns of the Φh phase of (a) DP75 and (b) DP200 collected at 80 °C. The intensity of the top half of the pattern
has been reduced to demonstrate the presence of tilt correlation features. (c) Comparison of simulated and experimental WAXS XRD patterns of the
Φh phase of DP200. The simulated pattern has been calculated using the model shown in (e−i). Fiber axis and lattice parameters are indicated. The
simulation does not incorporate the distribution of fiber orientations which smears out the (hk1) reflections. (d) Azimuthal plots of the 4.6 Å
stacking feature in (a, b) illustrating tilt correlation features of ∼22° for DP75 and DP200. (e−i) Representative models of the supramolecular
columns generated from DPm, 20 ≤ m ≤ 200: (e) polymer backbone, side view; (f, g) polymer backbone and aromatic rings of the dendrons, (f) top
view and (g) side view; (h, i) full molecules including peripheral alkyl chains, (h) side view and (i) top view. Color code used in the model: O atoms,
red; H atoms, white; N atoms, blue; C atoms in the core, green; C atoms in the phenyl rings, orange; all other C atoms, gray.
determined via a combination of XRD measurements of
extruded oriented fibers, molecular modeling, and simulation of
XRD patterns. Three different sample-to-detector distances
were used to probe structural features on different length scales:
very wide-angle X-ray scattering (VWAXS) patterns were
recorded with a sample-to-detector distance of 7 cm, measuring
d-spacings in the range of 2.6 ≤ d ≤ 40 Å; wide-angle X-ray
scattering (WAXS) patterns were recorded at 11 cm (3.9 ≤ d ≤
60 Å); and intermediate angle X-ray scattering (IAXS) patterns
were recorded at 54 cm (17 ≤ d ≤ 300 Å).
As noted earlier, all samples of poly[(3,4)16G1-Oxz]
investigated here (DPm, 5 ≤ m ≤ 200) exhibit a columnar
hexagonal Φh phase. Figure 2 shows XRD patterns and
molecular models for the columns of the Φh phase of DP5 and
DP10. Equatorial features on the VWAXS patterns of DP5 and
DP10 (Figures 2a and 3b, respectively) are consistent with a
hexagonal packing of columns with diameters (Dcol = a) of 46.5
and 45.9 Å, respectively. There is also a strong meridional
feature on both patterns denoting a 4.6 Å stacking distance
between repeat units of the supramolecular column. Azimuthal
plots (Figure 2d) demonstrate that each of these 4.6 Å features
is a single peak without tilt correlation features,51 suggesting
that there is no tilt of the stacked unit relative to the column
axis.
columns thus formed (Figure 2m,n) self-organize into a
columnar hexagonal Φh array (Figure 2o).
XRD analysis of the Φh phase of DPm, 20 ≤ m ≤ 200,
indicates that all of these polymers adopt an identical structure
in their supramolecular columns. Representative XRD patterns
and a general molecular model for the columns of DPm, 20 ≤
m ≤ 200, are shown in Figure 3. The XRD patterns are very
similar to those of DP5 and DP10 (Figure 2a,b) and suggest
that there is a similar columnar hexagonal structure at all DPs.
The diameters of the columns in DPm, 20 ≤ m ≤ 200, range
from 42.7 to 45.8 Å, similar to those of DP5 and DP10 (46.5
and 45.9 Å). The longer polymer backbone in DPm, 20 ≤ m ≤
200, adopts a helical conformation at the center of the
supramolecular column (Figure 3e). A 4.6 Å stacking feature
observed by XRD (Figure 3a,b) represents the distance
between adjacent turns of the supramolecular helix.
An azimuthal plot of the VWAXS patterns (Figure 3d)
reveals that the 4.6 Å stacking feature is not a single peak, but
also includes overlapped tilt correlation features.51 These tilt
features indicate that the dendrons are tilted down from the
plane of the column stratum by approximately 22°. A model
consistent with XRD and experimental density is shown in
Figure 3e−i and is very similar to the structure adopted by DP5
and DP10 (Figure 2e−o), with the addition of the ∼22° tilt
revealed by XRD. Hence, the columns of the Φh phase of
poly[(3,4)16G1-Oxz] with all DPs adopt almost identical
structures in which a helical polymer backbone is jacketed by
monodendrons with a flat-tapered conformation.
A model consistent with these observations from XRD is
shown in Figure 2e−o. The flat-tapered conformation of the
dendrons assembles into disks which stack into supramolecular
columns (Figure 2e−j). A single molecule generates each
disklike column stratum for DP5 (Figure 2f), whereas a
molecule of DP10 spans two column strata (Figure 2k). The
Structural Analysis by XRD and Reconstructed
Electron Density Maps of Cub (Pm3n) and Tet (P4 /
̅
2
mnm) Phases. In addition to columnar hexagonal phases
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Figure 4. (a) Intermediate-angle X-ray scattering (IAXS) pattern of the Cub (Pm3n) phase of DP5 at 90 °C. Fiber axis and lattice parameters
̅
indicated. (b) Radial plot of IAXS pattern in (a). (c) Schematic unit cell of Cub. (d, e) Reconstructed electron density maps of the unit cell of the
Cub phase: (d) perspective view; (e) z = 0 plane. (f) IAXS pattern of the Tet (P42/mnm) phase of DP20 at 100 °C. Fiber axis and lattice parameters
indicated. (g) Radial plot of IAXS pattern in (f). (h) Schematic unit cell of Tet. (i, j) Reconstructed electron density maps of the unit cell of the Tet
phase: (i) perspective view; (j) z = 0 plane. In (c, h), spheres connected by continuous lines exhibit columnar character.
Table 2. Structural Analysis of Poly[(3,4)16G1-Oxz] by XRD
b
cell parameter
c
d
f
g
T
d10, d11, d20, d21 (Å) (Φh); d200, d210, d (Å) (Cub);
Dcol (Å) (Φh); Dsphere (Å) (Cub);
a
e211
h
sample
DP5
(°C) phase
a (Å) c (Å)
d
002, d410, d330, d411, d312 (Å) (Tet)
Dsphere (Å) (Tet)
70
90
Φh
Cub
45.8
87.8
−
−
−
82.4
40.2, 22.8, 19.4, 15.1
43.9, 39.3, 35.9
45.8
54.5
44.6
49.3
45.8
49.3
42.7
49.5
44.8
48.9
43.7
49.3
42.8
43.7
DP10
DP20
DP30
DP40
DP50
70
Φh
Tet
Φh
44.6
39.1, −, −, 14.2
41.2, 36.7, 35.7, 33.5, 31.2
115
80
151.3
45.8
−
81.3
−
39.6, −, −, −
40.6, 36.9, 35.9, 33.6, 31.1
37.0, −, −, −
120
80
Tet
152.2
42.7
Φh
Tet
Φh
100
80
151.2
44.8
82.3
41.1, 36.6, 35.6, 33.5, 31.2
−
81.1
−
38.8, −, −, −
40.6, 36.5, 35.7, 33.3, 30.9
37.8, −, −, −
41.0, 36.7, 35.7, 33.6, 31.2
37.6, 21.8, 18.5, 13.7
37.8, −, −, −
115
80
Tet
150.4
43.7
Φh
Tet
Φh
Φh
Φh
110
80
151.3
42.8
82.0
DP75
−
−
−
DP100
DP200
80
43.7
80
43.0
37.7, 21.7, 18.6, 13.8
43.0
a
b
Phase notation: Φ = columnar hexagonal P6mm phase; Cub = cubic Pm3n phase; Tet = tetragonal P4 /mnm phase. Cell parameters: a = (2/
̅
h
2
c
√3)(d100 + √3d110 + 2d200)/3 for Φh; a = (2d200 + √5d210 + √6d211)/3 for Cub; a = (√17d410 + 3√2d330)/2 and c = 2d002 for Tet. d-spacings for
d
e
Φh calculated using dhkl = 1/(4h2 + k2 + hk)/(3a2) + l2/c2). d-spacings for Cub calculated using dhkl = a/(h2 + k2 + l2)1/2. d-spacings for Tet
53
f
g
h
47
calculated using dhkl = 1/((h2 + k2)/a2 + l2/c2)1/2
.
For Φh, Dcol = a. For Cub, Dsphere = 2(3a3/32π)1/3
.
For Tet, Dsphere = 2(abc/40π)1/3
.
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Table 3. Structural Analysis of the Cub and Tet Phases of Poly[(3,4)16G1-Oxz]
a
b
c
d
e
f
phase
Dsphere (Å)
ρ
(g/cm3)
Mwt (g/mol)
μcell
μ
μ′
DP5
Cub
Tet
Tet
Tet
Tet
Tet
54.5
49.3
49.3
49.5
48.9
49.3
1.0
1.0
1.0
1.0
1.0
1.0
3229
6434
126.2
175.2
88.7
59.4
44.6
35.8
15.78 ∼ 16
5.84 ∼ 6
2.96 ∼ 3
1.98 ∼ 2
1.49 ∼ 1.5
1.19 ∼ 1
78.9
58.4
59.2
59.4
59.6
59.5
DP10
DP20
DP30
DP40
DP50
12704
18974
25244
31514
a
b
c
d
Dsphere calculated as in Table 2. Experimental density at 23 °C. Error 5%. Theoretical molecular weight. Number of molecules in the unit cell.
For Cub μcell = NAρa3/Mwt, and for Tet μcell = NAρabc/Mwt, where NA is Avogadro’s number (6.022 × 1023 mol−1), ρ is the experimental density
e
measured at 23 °C, a, b, c are lattice parameters, and Mwt is the theoretical molecular weight. Number of molecules in the supramolecular sphere.
For Cub μ = μcell/8, and for Tet μ = μcell/30. Number of monodendritic units in the supramolecular sphere, calculated using μ′ = μ × m for DPm.
f
exhibited by poly[(3,4)16G1-Oxz] with all DPs, two 3D phases
organized from spheres were also observed by XRD: cubic
Pm3n (Cub) for DP5 (Figure 4a,b) and tetragonal P4 /mnm
̅
2
(Tet) for DPm, m = 10, 20, 30, 40, and 50 (Figure 4f,g). The
XRD pattern for DP20 in the Tet phase presented in Figure 4f
is representative of the tetragonal phase observed in DPm, m =
10, 20, 30, 40, and 50.
The Cub and Tet lattices consist of eight and 30
supramolecular spheres, respectively (Figure 4c,h).31 Spheres
connected by continuous lines in Figure 4c,h exhibit columnar
character.30,31,52 Electron density maps reconstructed from
XRD support the formation of supramolecular spheres by DPm,
5 ≤ m ≤ 50 (Figure 4d,e,i,j). Lattice parameters derived from
XRD (Cub of DP5: a = b = c = 87.8 Å and representative Tet of
DP20: a = 151.3 Å, b = c = 82.4 Å) indicate that the
supramolecular spheres of DP5 in the Cub phase have a
diameter of 54.5 Å, whereas those of DP10, 20, 30, 40, and 50
in the Tet phase have diameters of 48.9−49.5 Å. Structural
parameters for poly[(3,4)16G1-Oxz] at all DPs are summarized
in Table 2.
Retrostructural Analysis and Molecular Modeling of
Spheres of Poly[(3,4)16G1-Oxz]. Taking the experimental
density of poly[(3,4)16G1-Oxz] at 23 °C (ρ = 1.0 g/cm3) with
the lattice parameters derived from XRD and molecular weight
of the polymer allows calculation of the number of polymer
chains in the unit cell, μcell. It is more convenient to consider
two related parameters: μ, the number of polymer chains per
supramolecular sphere, calculated by dividing μcell by the
number of spheres in the unit cell (8 for Cub and 30 for Tet),
and μ′, the number of dendrons per supramolecular sphere,
calculated by multiplying μ by the number of monomers per
polymer chain, m. All three values (μcell, μ, μ′) are tabulated in
Table 3. The number of polymer chains per supramolecular
sphere, μ, is plotted as a function of DP in Figure 5.
As expected from their XRD-derived diameters, the spheres
of the Cub and Tet phases contain significantly different
numbers of monodendritic units: for Cub, Dsphere = 54.5 Å and
μ′ ≈ 80, while for Tet, Dsphere ≈ 49 Å and μ′ ≈ 60. The spheres
of DP5, which organize into a Cub array, contain 80 dendrons
and hence contain, on average, 16 molecules of DP5 per sphere.
The agreement of the lattice parameters and of μ′ for the Tet
phases of DP10, DP20, DP30, DP40, and DP50 suggests that
the supramolecular spheres in all of these tetragonal phases are
essentially identical and contain, on average, 60 monodendrons.
This can also be seen from the plot in Figure 5, which shows
that a sphere is expected to contain only one molecule (μ = 1)
for DP = 60. These spheres can be generated from various sizes
of polymer chains provided that the number of dendrons per
spheres (μ = μ′ × m) is 60: for example, 6 molecules of DP10,
Figure 5. Plot of the number of polymer chains (μ) per
supramolecular sphere as a function of the degree of polymerization
(DP). The solid red line represents a reciprocal line of best fit (R2 =
0.99).
3 molecules of DP20, or 2 molecules of DP30. In line with
previous studies, the polymer backbones of these molecules are
expected to be segregated from the peripheral alkyl chains of
the dendrons, although the degree to which this segregation
occurs is not determined here.
For larger sphere-forming molecules (DP40 and DP50),
calculated values of μ would suggest a noninteger number of
polymer chains: μ = 1.5 for DP40 and 1.2 for DP50. However,
sharing a polymer chain between two (or more) spheres is
disfavored, as doing so would disrupt microphase segregation
between aliphatic and aromatic regions. Instead, the inherent
distribution of molecular weights represented by the poly-
dispersity25 of poly[(3,4)16G1-Oxz] (Table 2) provides a
mechanism for the formation of 60-dendron spheres from
samples of DP40 and DP50. Two (or more) imperfect
molecules within a sample, for example, a DP32 molecule
and a DP28 molecule, could self-assemble into a 60-dendron
sphere. In addition, μ′ represents an average number of
dendron units in a supramolecular sphere, and hence not all
spheres within a single sample will necessarily contain exactly
60 dendrons.
However, this polydispersity means that a sample of, for
example, DP = 50, will contain a substantial number of polymer
chains with DP > 60.24,25 Hence, there will be some molecules
too long to form a macromolecular sphere, leading to a small
proportion of columnar assemblies such as those observed by
XRD for DP ≥ 75. A plot of the dependence of transition
temperature determined by DSC (Figure 1) or estimated by
XRD (Table 1) against DP (Figure 6) exhibits a maximum of
the isotropization temperature (Ti) for DP = 20, with a
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biphasic mixture or no observable Tet phase at all. For DP ≥
75, the biphasic regime ends, the tetragonal phase is completely
eliminated, and only columnar hexagonal arrays are observed.
The stability of these columnar hexagonal arrays increases with
DP, as indicated by their increasing Ti values.
As observed in poly(methacrylate)s and poly(styrene)s,3,5
increasing DP beyond a certain threshold eliminates the
generation of 3D phases organized from spheres and instead
promotes the formation of columnar structures. This behavior
is observed too in poly[(3,4)16G1-Oxz]: for DP ≤ 50, phases
generated from spheres are observed, but for DP ≥ 75, only
columnar hexagonal arrays are formed. This behavior arises
from the geometric constraints of the polymer sphere. As the
number of dendrons in a molecule approaches 60, the number
of molecules forming the supramolecular sphere tends to one.
For molecules with more than 60 dendrons, the molecules
cannot pack into a single sphere without creating a hollow
sphere, which is disfavored. An alternative assembly, in which
the polymer chain forms a 60-dendron sphere and the excess
length of the polymer hangs from the sphere like a tail, is also
disfavored as it disrupts microphase segregation between
aromatic and aliphatic components. Therefore, there is no
suitable mechanism via which a molecule larger than DP60 can
form a sphere. Instead, the most favorable structure is an
elongated column in which the polymer backbone forms a
helical core. Hence, only columnar hexagonal phases are
observed for poly[(3,4)16G1-Oxz] molecules with DP > 60.
Molecular models of the supramolecular spheres formed by
DPm, m = 5, 10, 20, 30, 40, and 50, are shown in Figure 7. In all
molecules, the dendrons on the polymer backbone adopt a
Figure 6. Dependence of transition temperatures of poly[(3,4)16G1-
Oxz] as a function of DP. Transition temperatures that could not be
accurately determined by DSC and were estimated by XRD (Table 1)
are shown with error bars.
subsequent decrease in Ti with increasing DP to DP = 50. This
suggests the existence of a biphasic regime for DP > 20, in
which there is a small but increasing proportion of polymer
chains with DP > 60 which disrupt the formation of the Tet
phase. XRD data are consistent with the presence of a small
proportion of Φh in the Tet phase but cannot provide
conclusive evidence due to the overlap of the diffraction
features arising from the Φh and Tet phases (compare Figure
S6b with S6d). Even for a sample with DP = 60, the small
proportion of molecules with exactly 60 monomers in the
polymer chain (few % of all chains)24,25 would result in a
Figure 7. Molecular models of the supramolecular spheres of DPm with m = 5, 10, 20, 30, 40, and 50. Top row: single molecule, side view. Second
row: single molecule, viewed from the apex of the cone defined by the molecule or, for DP50, viewed from the top. Color code: O atoms, red; H
atoms, white; N atoms, blue; C atoms in the core, green; C atoms in the phenyl rings, orange; all other C atoms, gray. Third row: schematic
representation of single molecule. Bottom row: schematic representation of molecules packed into a supramolecular sphere. For clarity, the
schematic representations depict absolute segregation between the polymer backbone (green) and aliphatic regions (red), which is unrealistic. Color
code: green, polymer backbone; red, monodendrons; blue, surface of sphere.
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Figure 8. Summary of hierarchical self-organization of poly[(3,4)16G1-Oxz] into cubic Pm3n (Cub), tetragonal P4 /mnm (Tet), and columnar
̅
2
hexagonal P6mm (Φh) phases as a function of DP.
conical conformation rather than the flat-tapered conformation
observed in the columnar hexagonal Φh phase. DP5 forms a
molecular cone, 16 of which assemble into the supramolecular
spheres of the Cub phase. DP10 and DP20 also form cones
which generate spheres in the Tet phase. Larger polymers, such
as DP30 and DP40, occupy a hemisphere or larger portion of a
sphere, and hence only two molecules assemble into one
spherical object. Finally, a single molecule of DP50 generates a
sphere. This is the first example of a tetragonal phase self-
organized from spheres containing a single polymer chain.
Previous dendronized poly(oxazoline)s with n = 14 and 15
also exhibited 3D phases generated from spheres at low DPs
and columnar hexagonal arrays at higher DPs.25 However, in
spheres, such as tetragonal P42/mnm (20/30 or 67% close
contact spheres),55 known also as Frank−Kasper σ
phase,34,35,39,41 becomes more favored. Hence, increasing DP
disfavors a cubic Pm3n arrangement and favors tetragonal P4 /
̅
2
mnm. Increasing n at a given DP (e.g., DP50 for poly[(3,4)nG1-
Oxz]) also seems to exhibit a similar trend (Φh for n = 13;24
Pm3n for n = 14 and 15;25 and P42/mnm for n = 16), but the
̅
driving force for this dependence requires further experiments
in order to be elucidated. The phases self-organized from
poly[(3,4)16G1-Oxz] are schematically summarized in Figure
8.
both series of molecules, only the Cub (Pm3n) phase generated
from spheres was observed. Spheres generated from a single
̅
CONCLUSIONS
■
macromolecular dendron also organized into a cubic Pm3n
̅
A tetragonal P42/mnm (Tet) phase self-organized from spheres
containing a single polymer chain was discovered in a library of
poly(2-oxazoline)s functionalized with a self-assembling mini-
dendron. Structural analysis demonstrated the formation of
lattice.54 In contrast, poly[(3,4)16G1-Oxz] exhibits two
different 3D phases generated from spheres, which can be
programmed via the DP. Previous work has shown that there is
a high repulsion between spheres such as those generated by
poly[(3,4)16G1-Oxz].31 This repulsion is lowered if alkyl
chains can move or deform to accommodate a neighboring
sphere.31,55 Indeed, the high proportion of close contact
cubic Pm3n (Cub), tetragonal P4 /mnm (Tet), and columnar
̅
2
hexagonal P6mm (Φh) phases as a function of the degree of
polymerization (DP) (Figure 8). This represents the richest
diversity of periodic arrays self-organized from a single
component polymer known to us. We propose that the
transition from cubic to tetragonal phases is mediated by
changes in the deformability or quasi-equivalence of the
supramolecular spheres. It is anticipated that these results will
provide principles for the assembly of additional molecular
building blocks into previously unexpected 3D structures. The
results reported here will also facilitate the merger of self-
assembling dendrons and dendrimers28,29 with block copoly-
mers,35−40,57−64 other self-assembling building blocks,33,34 and
surfactants.41−43
spheres in a Pm3n lattice (= 6/8 or 75%) is favored by
̅
deformable quasi-equivalent spheres, and hence the most
commonly observed 3D phase generated from spheres is
Pm3n,28,29,55,56 known also as Frank−Kasper A15. Deformation
̅
of the sphere requires reorganization of the dense aliphatic
corona and coordinated movement of the polymer backbone at
the center of the sphere. In a sphere of DP5, there are 16
molecules with short polymer backbones which can move and
deform to allow close contact between spheres. However, at
higher DP, this becomes more difficult as the backbone
becomes longer and movement of the polymer backbones
becomes more highly coordinated. Deformation of the sphere
is therefore less favorable, and a lattice with fewer close contact
I
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(11) Fischer, H.; Ghosh, S. S.; Heiney, P. A.; Maliszewskyj, N. C.;
Plesnivy, T.; Ringsdorf, H.; Seitz, M. Formation of a Hexagonal
Columnar Mesophase by N-Acylated Poly(ethylenimine). Angew.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.6b02298.
■
S
(12) Kobayashi, S.; Uyama, H. Polymerization of Cyclic Imino
Ethers: From its Discovery to the Present State of the Art. J. Polym.
Sci., Part A: Polym. Chem. 2002, 40, 192−209.
X-ray diffraction data of DP5 (Φh and Cub phases),
DP10, DP20, DP30, DP40, and DP50 (Φh and Tet
phases), and DP75, DP100, and DP200 (Φh phase)
(PDF)
(13) Ohmae, M.; Fujikawa, S.-I.; Ochiai, H.; Kobayashi, S. Enzyme-
Catalyzed Synthesis of Natural and Unnatural Polysaccharides. J.
Polym. Sci., Part A: Polym. Chem. 2006, 44, 5014−5027.
(14) Hoogenboom, R. Poly(2-Oxazoline)s: A Polymer Class with
Numerous Potential Applications. Angew. Chem., Int. Ed. 2009, 48,
7978−7994.
AUTHOR INFORMATION
Corresponding Author
*E-mail percec@sas.upenn.edu; tel +1-215-573-5527; fax +1-
215-573-7888 (V.P.).
■
(15) Bloksma, M. M.; Rogers, S.; Schubert, U. S.; Hoogenboom, R.
Secondary Structure Formation of Main-Chain Chiral Poly(2-
Oxazoline)s in Solution. Soft Matter 2010, 6, 994−1003.
(16) Bloksma, M. M.; Rogers, S.; Schubert, U. S.; Hoogenboom, R.
Main-Chain Chiral Poly(2-Oxazoline)s: Influence of Alkyl Side-Chain
on Secondary Structure Formation in the Solid State. J. Polym. Sci.,
Part A: Polym. Chem. 2011, 49, 2790−2801.
ORCID
Mihai Peterca: 0000-0002-7247-4008
Benjamin E. Partridge: 0000-0003-2359-1280
Virgil Percec: 0000-0001-5926-0489
(17) Nardin, C.; Thoeni, S.; Widmer, J.; Winterhalter, M.; Meier, W.
Nanoreactors Based on (Polymerized) ABA-Triblock Copolymer
Vesicles. Chem. Commun. 2000, 1433−1434.
Notes
The authors declare no competing financial interest.
(18) Binder, W. H.; Einzmann, M.; Knapp, M.; Kohler, G. Domain
̈
Formation in Lipid Bilayer Membranes: Control of Membrane
Nanostructure by Molecular Architecture. Monatsh. Chem. 2004,
135, 13−21.
ACKNOWLEDGMENTS
■
Financial support by the National Science Foundation (DMR-
1066116 (V.P.) and DMR-1120901 (V.P. and P.A.H.)), the
Humboldt Foundation (V.P.), and the P. Roy Vagelos Chair at
Penn (V.P.) is gratefully acknowledged. B.E.P. thanks the
Howard Hughes Medical Institute for an International Student
Research Fellowship.
(19) Kumar, M.; Grzelakowski, M.; Zilles, J.; Clark, M.; Meier, W.
Highly Permeable Polymeric Membranes Based on the Incorporation
of the Functional Water Channel Protein Aquaporin Z. Proc. Natl.
Acad. Sci. U. S. A. 2007, 104, 20719−20724.
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Crossing of Cationic Ring-Opening Polymerization and Enzymatic
Ring-Opening Polyaddition. J. Polym. Sci., Part A: Polym. Chem. 2010,
48, 1251−1270.
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