Synthesis and assembly behavior of heteronucleobase-functionalized poly(ε-caprolactone)

Article abstract of DOI:10.1021/ma9026614

The heteronucleobase (adenine and uracil)-functionalized poly(ε-caprolactone) (A-PCL-U) possessing supramolecular structure has been successfully synthesized through the combination of ringopening polymerization and Michael addition reaction. Attachment of multiple hydrogen-bonding units to chain ends of PCL results in phase separation and substantial increase in the viscosity. The association constant (K^a) between adenine and uracil groups of A-PCL-U calculated from NMR measurement is 18.9 M^-1. XRD and DSC analyses indicate that the crystalline structure of the A-PCL-U is changed to some extent due to phase segregation as compared to the PCL homopolymer. The AFM micrograph demonstrates that phase segregation is formed from the hard nucleobase chain ends and the soft poly(ε-caprolactone) chains of the supramolecular complex.

Full text of DOI:10.1021/ma9026614

Macromolecules 2010, 43, 1245–1252 1245
DOI: 10.1021/ma9026614
Synthesis and Assembly Behavior of Heteronucleobase-Functionalized
Poly(ε-caprolactone)
I-Hong Lin, Chih-Chia Cheng, Ying-Chieh Yen, and Feng-Chih Chang*
Institute of Applied Chemistry, National Chiao Tung University, HsinChu, Taiwan
Received December 1, 2009; Revised Manuscript Received January 6, 2010
ABSTRACT: The heteronucleobase (adenine and uracil)-functionalized poly(ε-caprolactone) (A-PCL-U)
possessing supramolecular structure has been successfully synthesized through the combination of ring-
opening polymerization and Michael addition reaction. Attachment of multiple hydrogen-bonding units to
chain ends of PCL results in phase separation and substantial increase in the viscosity. The association
constant (K
18.9 M . XRD and DSC analyses indicate that the crystalline structure of the A-PCL-U is changed to some
a
) between adenine and uracil groups of A-PCL-U calculated from NMR measurement is
-1
extent due to phase segregation as compared to the PCL homopolymer. The AFM micrograph demonstrates
that phase segregation is formed from the hard nucleobase chain ends and the soft poly(ε-caprolactone)
chains of the supramolecular complex.
Introduction
component of polymer science and are expected to expand further
in the future.
In both DNA and RNA, hydrogen-bonding interaction has
drawn a great deal of interest because of novel structure
organizations.
and highly directional molecular recognitions exhibit unique
physical properties, such as high specificity, controlled affinity,
and reversibility.
containing multiple hydrogen bondings have received significant
interest in applications such as surgical fixation devices, con-
trolled drug delivery, and tissue engineering scaffolds.
In biological systems, complementary multiple-hydrogen-
bonding interactions are paramount and the basic concept of
molecular recognition which occurs in adenine-thymine (A-T)
1
-3
Biomimetic polymers with moderately strong
3
5
and guanine-cytosine (G-C) base pairs in DNA. A wide
variety of synthetic polymers were functionalized with the self-
complementary nucleobases, such as adenine, thymine, and
4-16
Biocompatible and biodegradable polymers
1
5,26,29,36
uracil;
however, reports describing the incorporation of
3
7-39
17,18
heteronucleobase unit in biocompatible materials are rare.
How-
We have studied the biocomplementary interactions of a DNA-
like side-chain homopolymer with alkylated nucleobases
mediated by hydrogen-bonded thymine-adenine (T-A) base
ever, reports on synthetic biocompatible and biodegradable
polymers containing nucleobase molecular recognition sites are
19-22
still rare.
It is still a challenge to control the supramolecular
4
0
polymers with secondary (and higher) bonding through specific
pairs. Recently, we also reported that the interassociation
equilibrium constant (K ) between adenine (A) and uracil (U)
is of ca. 671 M , which is stronger than that between adenine (A)
2
3
interactions from different hydrogen-bonding motifs.
a
-
1
In order to utilize the complementary nature of nucleobase pairs,
numerous reports regarding the effect of the placement of chain
ends on polymers or small molecules using supramolecular mo-
-
1 41
and thymine (T) (ca. 534 M ). Taking the above discussion
into account, poly(ε-caprolactone) with molecular recognition
units of adenine-uracil (A-U) base pairs was prepared, and
the bioactivity was introduced into poly(ε-caprolactone) through
the combination of ring-opening-polymerization and quantita-
tive Michael addition strategies. The supramolecular polymeri-
zation and self-assembly behaviors of the new poly(ε-capro-
lactone) derivative through complementary A-U pairs were
24-27
28
tifs on material properties have been reported.
Meijer et al.
recently reported the synthesis of oligomeric poly(ε-capro-
lactone)s containing self-complementary quadruple-hydrogen-
bonding 2-ureido-4[1H]-pyrimidinone (UPy) units. Mixing of the
UPy-functionalized poly(ε-caprolactone) with UPy-functionalized
peptides results in bioactive supramolecular structure that exhibits
29,30
1
strong and specific cell binding properties. Sivakova et al.
analyzed using H NMR titration, wide-angle X-ray diffraction
developed low molecular weight poly(tetrahydrofuran) consisting
of modified nucleobase moieties and possesses free-standing
film behavior. Besides functionalizing chain ends of polymers, it
(
WAXD), Fourier transform infrared (FTIR), differential scan-
ningcalorimetry (DSC), Ubbelohde viscometer, andatomic force
microscopy (AFM).
31-33
is also possible to place nucleobases on polymer side chains.
For side-chain architectures, these proton donors and acceptors
are appended to polymers that have main chains in forms of homo-
or block copolymers and which the self-assembly occurs via
hydrogen bonding interaction, usually in multiple fashion. For
instance, Rotello and co-workers have studied low molecular
weight polystyrene derivatized with diamidopyridine units which
can be noncovalently cross-linked with bis-thymine units. Such
nucleobase end-functionalized strategies have become a critical
Experimental Section
Materials. ε-Caprolactone (ε-CL, 99.5%, ACROS), dimethyl
sulfoxide (DMSO, 99%, ACROS), and dimethylformamide
(DMF, 99%, Fisher Chemical) were dried over calcium hydride
34
(
CaH
pressure prior to use. Uracil (U, 99%, ACROS), adenine (A,
9%, Sigma), triethylaluminum (AlEt , 0.9 M in hexane,
2
, 95%, ACROS) for 24 h and then distilled under reduced
9
3
Fluka), glacial acetic acid (HPLC grade, TEDIA), ethylene
carbonate (99%, Aldrich), sodium hydroxide (NaOH, 99%,
Acros), acryloyl chloride (97%, Alfa Aesar), tetrahydrofuran
*Corresponding author. E-mail: changfc@mail.nctu.edu.tw.
r 2010 American Chemical Society
Published on Web 01/14/2010
pubs.acs.org/Macromolecules

1
246 Macromolecules, Vol. 43, No. 3, 2010
Lin et al.
Scheme 1. Synthetic Route to Nucleobase-Terminated PCL
basis of the concurrent appearance of resonances l, m, and o at
.38, 6.12, and 5.80 ppm, respectively, which are assigned to the
6
olefinic protons of the acrylate-terminated PCL prepolymer.
Finally, Micheal addition of the acrylated A-PCL with hetero-
cyclic uracil results in adenine-uracil nucleobase-terminated
PCL [A-PCL-U] with complementary multiple-hydrogen-bond-
ing end groups.
0
Synthesis of 9-(2 -Hydroxyethyl)adenine [HEA]. A solution of
adenine (4.96 g, 36.7 mmol), ethylene carbonate (3.30 g, 37.5
mmol), and a trace of NaOH in DMF (150 mL) was heated to
reflux for 2 h. After filtration and removal of the solvent under
reduced pressure, the crude product was recrystallized from
0
ethanol to give 4.68 g (71%) of 9-(2 -hydroxyethyl)adenine
[
HEA] as colorless powder; mp 232-234 ꢀC. Quantitative
1
functionalization was confirmed using H NMR spectroscopy.
1
H NMR (500 MHz, DMSO, δ): 8.14 (1 H, s, ArH), 8.08 (1 H, s,
ArH), 7.18 (2 H, s, NH
t, J=5.5 Hz, CH ), 3.75 (2 H, q, J=5.4 Hz, CH ).
2
), 5.00 (1 H, t, J=5.4 Hz, OH), 4.19 (2 H,
2
2
Preparation of the Prepolymer of Adenine-End-Capped PCL
A-PCL). 0.2 mL of a toluene solution of triethyaluminum
(
(0.1 mol/L) was added into the DMSO (5 mL) solution containing
1
Figure 1. Stacked H NMR spectraof(a) hydroxyl-terminated A-PCL,
(
0
-4
b) acrylated-terminated A-PCL, and (c) uracil nucleobase-terminated
A-PCL-U (M = 4023 g/mol, M /M = 1.03).
9-(2 -hydroxyethyl)adenine [HEA] (0.1375 g, 8.3 ꢀ 10 mol)
under an argon atmosphere. The mixture was stirred at 27 ꢀC
for 30 min and then evaporated to dryness, and the byproduct
n
w
n
(
THF, HPLC grade, TEDIA), potassium tert-butoxide (t-Bu-
2-propanol was removed. After repeating this procedure three
OK, 97%, Fluka), and methanol (MeOH, HPLC grade, TED-
IA) were all used as received. Control PCL was obtained by ring-
opening polymerization as previously described, and it has a
times, 5 mL of caprolactone (0.044 mol) dissolved in 25 mL of
dry DMSO was added to the reaction mixture maintained at 0 ꢀC.
The mixture was maintained at 27 ꢀC for 4 h under an argon
atmosphere, and the reaction was terminated by adding excess
acetic acid (0.2 mL of acetic acid/0.8 mL of DMSO). Two-thirds of
the initial solvent was evaporated, and the residue was precipitated
into methanol. The product was dried until constant weight under
vacuum and gave yield of 3.6 g (74%). Quantitative functionaliza-
4
2
n
quoted molecular weight of M =5524.
Synthetic Routes. Adenine-uracil nucleobase-terminated
PCL, A-PCL-U, was synthesized via a four-step synthesis as
shown in Scheme 1. In the first step, adenine is condensed with
0
ethylene carbonate in hot DMF to afford the intermediate 9-(2 -
hydroxyethyl)adenine [HEA] with 70% yield after recrystalliza-
1
1
4
tion from ethanol as reported earlier. Through the second step,
3
tion was confirmed using H NMR spectroscopy. H NMR (500
MHz, CDCl , δ): 8.3(1H, -NdCHN-), 7.8(1H, =NCHdN-),
the ring-opening polymerization of ε-caprolactone (ε-CL) initi-
3
6
.0 (2H, -NH
2
), 3.9-4.2 (polycaprolactone, 2H per repeating
ated by 9-(2-hydroxyethyl)adenine [HEA] using Al(Et) as a
catalyst produces PCL containing adenine-functionalized
chain end with narrow molar mass distributions (M /M =
3
unit, -COOCH -), 4.4 (2H, -NCH CH -), 3.6 (2H,
2
2
2
-NCH
unit, -COdCH
repeating unit, -COCH
CH CH -), 1.2-1.4 (polycaprolactone, 2H per repeating unit,
-COCH CH CH -).
2
CH
2
-), 2.1-2.4 (polycaprolactone, 2H per repeating
CH -), 1.5-1.8 (polycaprolactone, 2H per
CH -, 2H per repeating unit, -COO-
w
n
1
hydroxyl groups of A-PCL in THF solution to introduce the
.03-1.08). Then, acryloyl chloride was reacted with terminal
2
2
2
2
1
acrylate end group. H NMR spectroscopy further confirms
quantitatively end-group functionalization (Figure 1) on the
2
2
2
2
2

Article
Macromolecules, Vol. 43, No. 3, 2010 1247
-4
Synthesis of Acrylated A-PCL. A-PCL (3.6 g, 6.8 ꢀ 10 mol)
was added under nitrogen into a 100 mL flame-dried, septum-
sealed, two-neck, round-bottomed flask with a magnetic stir
bar. The flask was equipped with an addition funnel and placed
in ice bath. Tetrahydrofuran (THF) (50 mL) was added under
nitrogen to prepare an 8 wt % polymer solution. Triethylamine
Table 1. Results of Polymerization for Adenine-Uracil Nucleobase-
Terminated PCL with Different Molecular Weights
a
b
c
d
entry
[M]/[I]
M
n,NMR
M
n,theo
w n
M /M
control PCL 5K
A-PCL-U 10K
A-PCL-U 5K
A-PCL-U 4K
a
60/1
100/1
64/1
5524
9242
5113
4023
5782
9970
5880
4430
1.38
1.07
1.08
1.03
(TEA) (3-fold excess, 0.25 mL) in 5 mL of THF was added by
syringe into the reaction mixture under nitrogen, and the
45/1
b
M = CL, I = initiator. Mn,NMR was determined on the basis of the
integral ratio of the methylene signal on the PCL polymer backbone
mixture was cooled to 0 ꢀC. Acryloyl chloride (3-fold excess,
0
.2 mL) dissolved in 10 mL of THF and syringed into the
(
-OCdOCH
group (NH -, 6.0 ppm). ) ꢀ yield;
Mn,theo denotes the theoretical number-average molecular weight of
2
-, 3.9-4.2 ppm) and signal on the primary amine end
addition funnel dropwisely to the reaction mixture under nitro-
gen. The reaction was performed for 24 h at 27 ꢀC, and the
mixture was filtered to remove triethylamine hydrochloride salt.
The THF was removed using rotary evaporation, and the
product was redissolved in chloroform, precipitated repeatedly
into methanol, and dried under reduced pressure at 25 ꢀC for
c
2
M
n,theo = ([M]/[I] ꢀ MCL þ M
I
d
PCL. Weight-average molecular weight (M ) and number-average
w
n
molecular weight (M ) are determined by GPC.
used to compute the d-spacing corresponding to the comple-
mentary behavior. Real-time small-angle X-ray scattering
(SAXS) measurement was performed at BL01B SWLS beamline
in the National Synchrotron Radiation Research Center
NSRRC), Taiwan. The incident X-ray beam was focused
vertically by a mirror and monochromated to the energy of
0.5 keV by a germanium (111) double-crystal monochromator.
The wavelength (λ) of the X-ray beam was 1.180 95 A. AFM
micrographs were recorded at 37 ꢀC in air using a Digital
Instrument Multimode Nanoscope IV operating in the tapping
regime mode using silicon cantilever tips (PPP-NCH-50,
2
4 h, giving yield of 3.2 g (88%). Quantitative functionali-
1
1
zation was confirmed using H NMR spectroscopy. H NMR
(
500 MHz, CDCl , δ): 5.8-6.5 (3H, -OCCHdCH ,), 8.3 (1H,
3
2
(
-
NdCHN-), 7.8 (1H, =NCHdN-), 6.0 (2H, -NH ), 3.9-4.2
2
(
polycaprolactone, 2H per repeating unit, -COOCH -), 4.4 (2H,
2
1
-
NCH
lactone, 2H per repeating unit, -COdCH
polycaprolactone, 2H per repeating unit, -COCH
per repeating unit, -COOCH CH -), 1.2-1.4 (polycaprolac-
tone, 2H per repeating unit, -COCH CH CH -).
2
CH
2
-), 3.6 (2H, -NCH
2
CH
2
-), 2.1-2.4 (polycapro-
CH -), 1.5-1.8
CH -, 2H
˚
2
2
(
2
2
2
2
2
2
2
2
04-497 kHz, 10-130 N/m).
Synthesis of Heteronucleobase-Functionalized A-PCL-U. The
acrylated A-PCL (3.2 g) was added under nitrogen to a flame-
dried, 50 mL round-bottomed flask containing a magnetic stir
bar. Potassium tert-butoxide (10 mg) was added as a catalyst
under nitrogen, and the flask was sealed with a rubber septum.
Results and Discussion
Characterizations of A-PCL-U Polymers. Successful in-
corporation of heterocyclic uracil motif is confirmed using
H NMR spectroscopy. The olefinic proton at 5.8-6.4 ppm
disappears, confirming the completion of Micheal addi-
tion between the heterocyclic uracil unit and the acrylated
A-PCL. Four new resonances characteristic of uracil are
1
3
6
.6 g (22-fold molar excess) of uracil was dissolved separately in
0 mL of DMSO maintained at 120 ꢀC, and the solution was
cooled to 60 ꢀC after uracil was completely dissolved; then the
solution was added by syringe into the A-PCL reaction mixture
under nitrogen maintained at 60 ꢀC. The reaction was allowed to
proceed for 3 days, and during this time, the uracil was pre-
cipitated partially. The mixture was filtered, and the DMSO was
removed at 60 ꢀC and 100 mTorr. The product was redissolved
in chloroform, precipitated into methanol, and dried under
reduced pressure at 50 ꢀC for 24 h, resulting product yield
1
observed in the H NMR spectrum of A-PCL-U at 10.2 ppm
(
(
s, -CONHCO-), 7.35 ppm (q, -N-CH-), 5.60 ppm
r, -CONH-), and 2.72 ppm (o, -OCOCH CH N-).
Methylene resonances (p, -OCOCH CH N-) associated
with ester are overlapped with the methylene protons in the
PCL repeat units at 3.9-4.2 ppm. The formation of comple-
mentary multiple hydrogen bonding was characterized via the
analysis of A-PCL-U as a 2.5 wt % (0.75 mM) solution in
2
2
2
2
2
.5 g (78%). Quantitative functionalization was confirmed using
1
1
H NMR spectroscopy. H NMR (500 MHz, CDCl
9
5
3
, δ):
.7-10.2 (1H, -CONHCO-), 7.2-7.5 (1H, -NCH-),
.5-5.7 (1H, -COCH-), 2.6-2.8 (2H, -OCOCH CH N-).
1
chloroform-d. The H NMR resonances of the A-PCL-U are
2
2
1
compared with the A-PCL precursor. Except for the NH
protons (changed from 6.00 to 6.44 ppm), all these resonances
of the A-PCL-U remain unshifted from their original chemical
shifts in the corresponding A-PCL precursor. The change in
the chemical shifts of the NH resonance indicates the forma-
tion of hydrogen-bonding interaction between adenine and
uracil attached to chain ends of the A-PCL-U. Molecular
weights and molecular weight distributions of PCL and
A-PCL-U are listed in Table 1. In addition, number-average
M was calculated from the relative H NMR integration ratio
n
of PCL methylene protons (-OCdOCH
2
-) at 3.8-4.2 ppm
compared to the primary amino protons of the adenine unit (2H,
NH ) at 6.0 ppm.
-
2
Characterizations. FTIRspectra were recorded usinga Nicolet
Avatar 320 FTIR spectrometer; 32 scans were collected at a
-
1
spectral resolution of 1 cm . The conventional KBr disk
method was employed: the sample was dissolved in THF, then
1
cast onto a KBr disk, and dried under vacuum at 80 ꢀC. H
NMR spectra were recorded on a Varian Inova 500 MHz
spectrometer equipped with a 9.395 T Bruker magnet and
operated at 500 MHz. The weight-average molecular weight
1
molecular weights are also calculated from the H NMR
integration ratios of the methylene signal on the PCL polymer
backbone (-OCdOCH -, 3.9-4.2 ppm) to signal on the
2
(
M
M
w
), number-average molecular weight (M
) were measured using a Waters 410 GPC system equipped
with a refractive index detector and three Ultrastyragel columns
n w
), and PDI (M /
primary amine end group (NH -, 6.0 ppm).
2
n
Reversibility of Supermolecular Polymer. The reversibility
of supramolecular polymer formation was investigated by
˚
(
100, 500, and 1000 A) connected in series. The system was
1
using variable-temperature H NMR spectroscopy in
calibrated using polystyrene (PS) standards. Thermal analysis
was carried out using a DSC instrument (TA Instruments Q-20)
1
,1,2,2-tetrachloroethane-d because the ability to form or
2
break the hydrogen bonding by external stimuli is a key
reason for the interest in these materials. The temperature
dependence of the NH proton chemical shift of the A-PCL-U
under an atmosphere of dry N . Samples were weighed (3-5 mg)
2
and sealed in an aluminum pan, which was scanned from 30 to
1
60 ꢀC at a scan rate of 20 ꢀC/min. WAXD spectra of powders
4
K complex at 10 wt % in 1,1,2,2-tetrachloroethane-d is
2
were obtained using a Rigaku D/max-2500 X-ray diffracto-
meter. The radiation source was Ni-filtered Cu KR radiation at a
wavelength of 0.154 nm. The voltage and current were set at
shown in Figure 2. The NH resonance shifts to upfield region
systematically from 10.0 to 8.6 ppm as the temperature is
raised from 25 to 100 ꢀC. The gradual decrease in the
3
0 kV and 20 MA, respectively. Bragg’s law (λ=2d sin θ) was

1
248 Macromolecules, Vol. 43, No. 3, 2010
Lin et al.
1
Figure 2. H NMR NH chemical shift of the A-PCL-U 4K as a
function of temperature (10 wt %). Sample was allowed to equilibrate
for 10 min at each temperature.
Figure 4. Benesi-Hildebrand plots adenine/uracil association in
CDCl (M = 4023, 27 ꢀC).
3 n
1
via H NMR titration experiment. The association constants
K ) for hydrogen-bonded complexes characterized by 500
(
a
1
MHz H NMR at 25 ꢀC in chloroform-d and the A-PCL-U
concentration systematically increased from 0.83 to 13.3 wt %.
The position of the uracil N-H resonance in A-PCL-U
shifts with increasing the concentration of the adenine. In
addition, the resonances of the amido NHs are relatively
broader, indicating a fast exchange rate between associ-
ated and dissociated A-U complex on the NMR time
4
5
scale. The Benesi-Hildebrand model, a mathematical
method of determining the association constant (K ) from
a
NMR titration experiment, is employed to fit nonlinear
chemical shift data for a dimeric hydrogen bond association
with the assumption that the complex is formed in 1:1
4
6,47
stoichiometry using the following equation:
1
1
1
¼
þ
ð1Þ
Δδ
KaΔδmax½APCLUꢁ δmax
Figure 3. Plots of specific viscosity of A-PCL-U and control PCL in
toluene solution vs concentration.
where Δδmax is the maximum change of the chemical shift of
the uracil NH protons. The slope of the double-reciprocal
intensity of the NH resonance with increasing temperature
can be attributed to the gradual dissociation of complemen-
plot is 1/K Δδmax, and the intercept is 1/Δδmax. A double-
a
4
4
tary hydrogen bonds. However, as soon as the sample is
cooled from 100 to 25 ꢀC, the NH resonance returns to its
original position at 10.0 ppm, suggesting that the nucleobase
end-functionalized hydrogen-bonded supramolecular com-
plexes are thermoreversible in 1,1,2,2-tetrachloroethane-d2.
Solution Viscosity. The effect of hydrogen-bonding asso-
ciations on solution viscosity of A-PCL-U in toluene at
ambient temperature was measured using an Ubbelohde
reciprocal plot based on the association of A-U complex
possesses a linear relationship as shown in Figure 4. The K
a
-
1
obtained from the slope of the plot is 18.9 M , which is
consistent with K values reported previously for adenine-
a
thymine (or adenine-uracil) base pair recognition (ca.
-1
25,48
10-100 M in CDCl ).
3
FTIR Analyses. After discovering the complementary be-
havior of A-PCL-U in solution, the assembly behavior of these
complexes in the bulk state was investigated using FTIR
spectroscopy at various temperatures (from 25 to 160 ꢀC).
Figure 5a illustrates FTIR spectra in the N-H stretching
viscometer. Plots of specific viscosities (η ) of control PCL
sp
5
2
K and PCL 52K with respect to concentration ranging from
0 to 200 mg/dL are linear, indicating that there are no
-
1
significant physical entanglements or noncovalent interac-
tions between these control polymers. However, the forma-
tion of hydrogen-bonding interaction between adenine and
uracil of A-PCL-U results in substantial increase in ηsp
relative to that the PCL as shown in Figure 3. The connection
of heteronucleobase units in solution driven by hydrogen-
bonding interaction leads to an entangled reversible supra-
molecular heteropolymerization. The substantial increase in
the solution viscosity of A-PCL-U relative to control PCL
can be attributed to the formation of the supramolecular
polymerization of A-PCL-U in solution.
region of the A-PCL-U where the band at 3500 cm corre-
sponds to free amide NH group and peaks at 3320 and
-
1
3206 cm are attributed to the A-U interaction. At 25 ꢀC,
the N-H stretching region (Figure 5a) shows a number of
-
1
broad absorption bands in the 3200-3400 cm
Vibrations at 3206 and 3320 cm correspond to medium
region.
-
1
37
strength hydrogen-bonded N-Hs, implying that A-PCL-U
indeed forms hydrogen-bonded interaction in the solid state.
Upon heating from 25 to 160 ꢀC, the signals at 3320 and
3206 cm , corresponding to N-H stretching, shift toward
higher wavenumber, and the free amide N-H stretch-
ing vibration at 3500 cm increases gradually, indicating
-
1
1
-1
H NMR Titration. Quantitative analysis for binding
affinity of intermolecular heterocomplexation was obtained
the change in the nature of the hydrogen bonding at high

Article
Macromolecules, Vol. 43, No. 3, 2010 1249
-
1
temperature. In the 1640-1800 cm
region at 40 ꢀC
Figure 5b), two carbonyl stretching peaks at 1734 and 1724
crystallization effect is more sensitive to temperature than the
hydrogen-bonding interaction within A-PCL-U.
(
cm correspond to the amorphous and crystalline carbonyl
stretching, respectively. Upon heating from 40 to 60 ꢀC, the
amorphous CdO band at 1734 cm appears as the weakly
-1
X-ray Diffraction Measurements. The wide-angle X-ray
diffraction (WXRD) patterns between 3ꢀ and 40ꢀ reflecting
the crystalline structure of the A-PCL-U sample are illu-
strated in Figure 6A. The diffraction peaks of the control
-
1
hydrogen-bonded amide indicated by the existence of bands at
3
-
1
320 and 3206 cm (Figure 5a), implying that the melt-
49
PCL 5K agree well with those reported previously. The
intense diffraction peaks located at 2θ=21.39ꢀ, 22.00ꢀ, and
23.70ꢀ are assigned to the (110), (111), and (200) reflections
ing of A-PCL-U starts around 60 ꢀC where the hydrogen-
bonded amide appears. Therefore, we can conclude that the
5
0
of PCL. However, the diffraction peaks of the A-PCL-U
shift to lower values of 2θ, implying that the crystalline
structure of the A-PCL-U is different. During crystalliza-
tion, the chain ends cannot be rejected from the early stage of
the crystal growth and thus alter the final structure of PCL
5
1
crystals. To further investigate the effect of nucleobase
on the A-PCL-U, the quenching treatment was carried out.
These samples need to be quickly cooled down below -70 ꢀC
in order to avoid or reduce the crystallization of A-PCL-U
from the disordered melt state. At -70 ꢀC, a reflection at
˚
lower angle corresponding to d-spacing larger than 30 A
appears, implying that a significant reduction in the fraction
3
0
of crystallization occurrs at the temperature. However,
careful comparison does show that both (110) and (200)
peaks are reduced in intensity, and new diffraction peaks are
apparently present. The new diffraction peak can be related
to the hard segment because of the immisciblility between
PCL and the terminal nucleobase hard segment units tends
to self-assembled into microdomains. This observation does
suggest that the crystal structure of the nucleobase-termi-
nated PCL did change to a certain extent. Furthermore, the
reflection peak at 2θ = 19.4ꢀ (d = 0.44 nm) suggests that
within these “hard” domains there are stacks of the nuclo-
base formed through π-π interactions. Figure 6B illustrates
the presence of a broader primary peaks and higher-order
reflections at q/q* ratios of 2 and 3 of A-PCL-U 5K,
indicating that A-PCL-U is arranged as lamellae. The first
-
1
peak located at the position of q=0.051 nm indicates the
presence of a lamellar structure due to the occurrence of the
long period 12.3 nm for A-PCL-U, which is also in good
agreement with wide-angle X-ray diffraction analyses.
Thermal Analyses. For A-PCL-U and control PCL, DSC
experiments were heated to 160 ꢀC to fuse the crystals and
then quenched to -90 ꢀC for the second DSC scans. The
second heating DSC thermograms of all A-PCL-U and PCL
are shown in Figure 7. Essentially all A-PCL-U complexes
Figure 5. Variable temperature FT-IR spectra in the (a) 3000-3600
-1
-1
cm region and (b) 1640-1800 cm region of A-PCL-U 4K.
Figure 6. (A) XRD curves: (a) control PCL 5K, (b) A-PCL-U 4K, (c) A-PCL-U 5K, (d) A-PCL-U 10K, and (e) sample (a) after -70 ꢀC. (B) Small-
angle X-ray diffraction of A-PCL-U 5K recorded with Cu KR X-ray radiation.

1
250 Macromolecules, Vol. 43, No. 3, 2010
Lin et al.
Figure 8. Plot of t1/2 vs crystallization temperature for samples of
control PCL and different molecular weight of nucleobase-termi-
nated PCL.
Figure 7. Heating DSC thermograms of pure and nucleobase-termi-
nated PCL with variable molecular weights (second scans): (a) control
PCL 5K, (b) A-PCL-U 4K, (c) A-PCL-U 5K, (d) A-PCL-U 10K.
Table 2. Values of n, k, and t1/2 at Various T
c
display melting transitions at about 50 ꢀC, which are lower
than the control PCL. In addition, the melting temperature
decreases with increasing the fraction of nucleobase unit in
the complex, and the dual melting transitions are observed
for those higher fraction nucleobase A-PCL-U as shown in
Figure 7. The dual melting transitions of A-PCL-U can be
attributed to melting of the initial crystals followed by
recrystallization and final melting of the crystals grown
T
c
(K)
n
k
t
1/2 (min)
control PCL 5K
A-PCL-U 4K
A-PCL-U 5K
A-PCL-U 10K
303
07
311
315
303
2.7
3.2
2.8
2.6
2.7
2.6
2.8
2.7
2.7
2.6
2.8
2.8
2.8
2.8
2.8
2.8
6.7
3.37
0.56
0.096
0.41
0.023
0.00084
0.00025
0.41
0.021
0.00013
0.0002
3.48
0.67
1.1
2.65
5.44
1.605
5.18
25.16
59.225
2.365
3.67
10.01
40.73
0.945
2.235
3.31
3
3
07
311
15
5
2
during the heating scan. To further investigate the thermal
behaviors of A-PCL-U, DSC measurements were carried out
3
303
307
311
315
303
(see the Supporting Information). All samples were first
heated up to 80 ꢀC at various heating rates and cooled at a
rate of 10 ꢀC/min. If the process is followed, when the heating
rate is increased, the lower melting peak will be magnified,
and the peak with higher melting temperature will decrease
as shown in Figure S1. The higher melting peak was
decreased with increasing heating rate. Therefore, the dual
melting behavior is indeed originated from the recrystalliza-
tion during the heating process. Since the A-PCL-U 10K
does not show dual melting transition behavior, rearrange-
ment through melting and recrystallization are retarded or
even inhibited with low fraction of nucleobase units in the
polymer chain. As a result, the content of the supermolecular
binding motif in the bulk plays a critical role in affecting the
crystallization behavior because of the phase segregation
between the nucleobase pairs and polycaprolactone chains.
Isothermal Crystallization Kinetics. In order to study the
isothermal crystallization kinetics, the weight fraction of
crystallinity, X(t), was calculated according to the follow
3
3
3
07
11
15
0.79
0.077
0.001
10.86
The crystallization kinetics is analyzed using the Avrami
treatment:
5
5
log½ -lnð1 -XðtÞÞꢁ ¼ log k þ n logðtÞ
ð3Þ
where X(t) is the weight fraction of material crystallized after
time t, n is the Avrami value which depends on both the
nature of the primary nucleation and the growth geometry of
the crystalline entities, and k is the overall kinetic rate
constant which depends on rates of nucleation and growth.
The values of k and n can be calculated from the intercept and
slope of eq 3. The half-time of crystallization, t1/2, is defined
as the time required for half of the final crystallinity to be
5
3,54
equation:
developed. The plots of t1/2 as functions of T for various
c
R dH
samples are shown in Figure 8, and the values of k, n, and t1/2
t
dt
dt
0
are summarized in Table 2. The obtained noninteger n values
dt
XðtÞ ¼
ð2Þ
R dH
in all cases are from mixed growth or surface nucleation
modes. The value of k decreases with increasing the nucleo-
base concentration and the crystallization temperature.
Results from the overall crystallization rate decrease
with increasing the concentration of the nucleobase chain
¥
0
dt
where the integral in the numerator is the heat generated at
time t and that in the demominator is the total heat generated
up to the end of the crystallization process. The typical
isotherms data (see the Supporting Information) of these
A-PCL-U and control PCL shows increasing in the end-
functional concentration results in significant shift of the
characteristic curves along the time axis, suggesting that
the crystallization rate becomes progressively decreased.
5
6
ends.
Surface Morphology. Supramolecular binding motifs show
well-defined fiberlike morphology due to the microphase sepa-
ration of the hard block segments (such as adenine and uracil
29,57
nucleobase pairs). Similar structure is also expected to be
present in the supramolecular polymers; the microphase
58

Article
Macromolecules, Vol. 43, No. 3, 2010 1251
amide hydrogen bonding and the soft polycaprolactone
chains results in wormlike hard domains.
Summary
Ring-opening polymerization and Micheal addition reaction
were employed to synthesize heteronucleobase (A and U)-func-
tionalized PCL. Incorporation of multiple hydrogen-bonding
units into PCL chain ends results in phase separation and
dramatical increased viscosity as a result of the formation of
supramolecular complexes chain. The K value of the A-PCL-U
a
-
1
4
K is calculated as 18.9 M using NMR measurements. In the
bulk state, FTIR spectroscopies provide positive evidence for
hydrogen-bonding interactions within these hard nucleobase
chain ends. In addition, WXRD and DSC measurements further
provide evidence that the crystalline structure of the A-PCL-U is
changed to some extent due to phase segregation between the
hard chain ends and the polycaprolactone chains. The SAXS
technique has been employed to investigate in detail the presence
of a broader primary peaks and higher-order reflection at q/q*
ratios of 2 and 3 of A-PCL-U 5K, indicating that A-PCL-U is
arranged as lamellae. Furthermore, AFM surface morphology
shows phase separation of end-group aggregation through
hydrogen-bonding interaction in the form of wormlike structure.
Acknowledgment. This study was supported financially by
the National Science Council, Taiwan (Contract NSC-98-2221-
E-009-006).
Supporting Information Available: Results about DSC of
A-PCL-U analyses. This material is available free of charge via
the Internet at http://pubs.acs.org.
References and Notes
Figure 9. (a) AFM phase images at 500 nm scan sizes of A-PCL-U. Z
ranges are 10 nm; Δj is 15ꢀ and 10ꢀ. Data obtained in tapping mode in
air at 37 ꢀC. In the phase images the hard phase of the polymer appears
brighter than the soft phase. (b) Idealized phase-segregated morphology
in the solid state.
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