Self-assembly and secondary structures of linear polypeptides tethered to polyhedral oligomeric silsesquioxane nanoparticles through click chemistry

Article abstract of DOI:10.1002/pola.24640

In this study, we used click chemistry to synthesize a new macromolecular self-assembling building blocks, linear polypeptide-b-polyhedral oligomeric silsesquioxane (POSS) copolymers, from a mono-azido-functionalized POSS (N ₃-POSS) and several alkyne-poly(γ-benzyl-L-glutamate) (alkyne-PBLG) systems. The incorporation of the POSS unit at the chain end of the PBLG moiety allowed intramolecular hydrogen bonding to occur between the POSS and PBLG units, thereby enhancing the α-helical conformation in the solid state, as determined through Fourier transform infrared spectroscopy and wide-angle X-ray diffraction analyses. POSS-b-PBLG underwent hierarchical self-assembly, characterized using small-angle X-ray scattering, to form a bilayer-like nanostructure featuring α-helical or β-sheet conformations and POSS aggregates. Thermogravimetric analysis indicated that the thermal degradation temperature increased significantly after incorporation of the POSS moiety, which presumably formed an inorganic protection layer on the nanocomposite's surface.

Full text of DOI:10.1002/pola.24640

Self-Assembly and Secondary Structures of Linear Polypeptides
Tethered to Polyhedral Oligomeric Silsesquioxane Nanoparticles
Through Click Chemistry
YUNG-CHIH LIN, SHIAO-WEI KUO
Department of Materials and Optoelectronic Science, Center for Nanoscience and Nanotechnology,
National Sun Yat-Sen University, Kaohsiung, Taiwan
Received 7 January 2011; accepted 21 February 2011
DOI: 10.1002/pola.24640
Published online 17 March 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: In this study, we used click chemistry to synthesize a
new macromolecular self-assembling building blocks, linear
polypeptide-b-polyhedral oligomeric silsesquioxane (POSS)
copolymers, from a mono-azido–functionalized POSS (N3-POSS)
and several alkyne-poly(c-benzyl-L-glutamate) (alkyne-PBLG) sys-
tems. The incorporation of the POSS unit at the chain end of
the PBLG moiety allowed intramolecular hydrogen bonding to
occur between the POSS and PBLG units, thereby enhancing
the a-helical conformation in the solid state, as determined
through Fourier transform infrared spectroscopy and wide-angle
X-ray diffraction analyses. POSS-b-PBLG underwent hierarchical
self-assembly, characterized using small-angle X-ray scattering,
to form a bilayer-like nanostructure featuring a-helical or b-sheet
conformations and POSS aggregates. Thermogravimetric analy-
sis indicated that the thermal degradation temperature
increased significantly after incorporation of the POSS moiety,
which presumably formed an inorganic protection layer on the
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nanocomposite’s surface. 2011 Wiley Periodicals, Inc. J Polym
Sci Part A: Polym Chem 49: 2127–2137, 2011
KEYWORDS: block copolymers; nanocomposites; polypeptides;
POSS; self-assembly
INTRODUCTION Polypeptides have received much attention
recently because of their potential applications in various
scientific fields, their close relationship to proteins, their flex-
ibility in terms of functionality, and their molecular recogni-
tion properties.1–4 The secondary structures of peptide
chains play crucial roles in the formation of the well-defined
tertiary structure of proteins.5 Because of its a-helical sec-
ondary structure, poly(c-benzyl-L-glutamate) (PBLG) has a
significant effect on the formation of hierarchically ordered
structures. a-Helical PBLG units serve as rigid rod-like struc-
tures in the solid state and in solution,6 resulting in unique
behavior such as thermotropic liquid crystalline ordering7,8
and thermoreversible gelation,9,10 respectively.
dimensional (3D) architectures for the self-assembly of
supramolecular structures through the ROP of c-benzyl-L-glu-
tamate N-carboxyanhydride (c-Bn-Glu NCA) using amino-
propyl isobutyl-POSS as a macroinitiator. Because of POSS’s
unique structure, it is a useful building block for the prepa-
ration of a variety of nanostructured materials.24–32 The
presence of a POSS moiety at the chain end allowed intra-
molecular hydrogen bonding to occur between the POSS and
PBLG units, thereby enhancing the latter’s a-helical confor-
mation in the solid state, as characterized using infrared,
Raman, and solid state NMR spectroscopy.23 Nevertheless,
because we had prepared this POSS-b-PBLG system through
the so-called divergence or ‘‘graft from’’ method, it was diffi-
cult to distinguish how the POSS nanoparticles influenced
the secondary structure of PBLG.33
Performing conformational studies of model polypeptides is
an important step on the road toward mimicking the biologi-
cal activity of more complex proteins.11 Indeed, there are
many reported methods available for synthesizing polypep-
tide-b-nonpeptide (rod–coil) block copolymers based on rigid
PBLG helices.12–22 The polypeptide domains in these systems
are typically obtained from an amino-terminated polymer
through ring-opening polymerization (ROP) of a-amino acid
N-carboxyanhydride (NCA) monomers. In a previous study,23
we combined the well-defined macromolecular architectures
of polyhedral oligomeric silsesquioxane (POSS) and PBLG to
generate polymeric building blocks having distinct three-
As a result, in this study we used the convergence or cou-
pling method to synthesize linear PBLG-b-POSS copolymers
through click chemistry. First, we synthesized alkyne-PBLG
species with different degrees of polymerization through
ROP of c-Bn-Glu NCA using propargylamine as a macroinitia-
tor (Scheme 1). Next, we synthesized linear PBLG-b-POSS
copolymers from a mono-azido–functionalized POSS (N3-
POSS) and the alkyne-PBLG species using a click reaction
(Scheme 2). Finally, we used differential scanning calorime-
try (DSC), thermogravimetric analysis (TGA), Fourier
Correspondence to: S.-W. Kuo (E-mail: kuosw@faculty.nsysu.edu.tw)
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Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 2127–2137 (2011) 2011 Wiley Periodicals, Inc.
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SCHEME 1 Synthesis of alkyne-
PBLG species.
transform infrared (FTIR) spectroscopy, wide-angle X-ray dif-
fraction (WAXD), and small-angle X-ray scattering (SAXS) to
characterize the thermal properties, self-assembly, and sec-
ondary structures of the PBLG units.
the corner-capping reaction had reached completion. The
substitution of the chloride atom with the azide group led to
the appearance [Fig. 1(a–c)] of a stretching vibration band
for the azido group at 2102 cmꢀ1, confirming that N3-POSS
had been obtained. The complete substitution of chloride
atoms by azido groups was also confirmed in the 1H NMR
spectrum in Figure 1(b). After the substitution reaction, the
signal of the benzyl CH2 group connected to the chloride
atom (4.60 ppm) shifted to higher field (4.36 ppm) for the
benzyl azide. The lack of any remnant signal at 4.60 ppm
suggested that the substitution reaction had reached comple-
tion. Figure 2 presents the 29Si NMR spectrum of the mono-
benzyl chloride POSS; we assign the resonances at ꢀ55.44,
ꢀ67.61, ꢀ68.27, and ꢀ81.27 ppm to the silicon nuclei of the
silsesquioxane.34 The ratio of the integration intensities of
the signals for these silicon atoms confirmed that the
RESULTS AND DISCUSSION
Synthesis of N3-POSS
N3-POSS was prepared through the corner-capping reaction
of trichloro[4-(chloromethyl)phenyl]silane and isobutyltrisila-
nol-POSS and subsequent substitution with NaN3. Figure
1(a) presents FTIR spectra of the isobutyltrisilanol-POSS,
mono-benzyl chloride POSS, and N3-POSS, recorded at room
temperature. The spectrum of mono-benzyl chloride POSS
[Fig. 1(a,b)] reveals that the signal for the OH group of iso-
butyltrisilanol-POSS had totally disappeared, indicating that
SCHEME 2 Synthesis of POSS-b-PBLG species.
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plets at 7.31–7.38 ppm. We determined the molar masses of
alkyne-PBLG using the 1H NMR spectra and the following
equation:
IbMBLG
Mn;PBLG
¼
þ Mpropargylamine
If
where Ib and If represent the intensities of the CH2
protons b (alkyne-PBLG) from the PBLG main chain and the
CH2 protons f of the propargylamine initiator, respectively,
and Mpropargylamine is the molar mass of the propargylamine
initiator.33 Table 1 lists the molecular weights of the various
alkyne-PBLG systems, as determined through 1H NMR spec-
troscopic and GPC analyses.
Synthesis of Linear POSS-b-PBLG Copolymers
1
Figure 4 presents H NMR spectra of N3-POSS, alkyne-PBLG,
and linear POSS-b-PBLG. Figures 1(b) and 3 provide the
peak assignments for the spectra of alkyne-PBLG and N3-
POSS; Scheme 2(d) provides them for the linear PBLG-POSS.
The resonance of benzylic CH2 group of N3-POSS connected
to the azide unit (4.36 ppm) shifted downfield significantly
to 5.56 ppm for POSS-b-PBLG. In addition, the signals for the
isobutyl groups attached to silicon atoms of the POSS moiety
appeared at about 0.63, 0.93, and 1.85 ppm and the proton
on the nitrogen atom of PBLG resonated as a singlet at 7.9
ppm, confirming the synthesis of POSS-b-PBLG. Figure 5(b)
reveals that the C¼¼O and amide carbon atom signals appear
in the 13C NMR spectrum of alkyne-PBLG at 172.3 and 171.4
ppm, respectively, while those of the phenyl ring appear at
136.4 ppm, with the other phenyl ring carbon atoms appear-
ing as a signal at about 128 ppm. The signals for the ben-
zylic carbon atom and the amino acid a-carbon atoms
(NHCOC) appear at 66.0 and 52.0 ppm (b-sheet conforma-
tion), respectively. A small peak at 55.1 ppm arises from a-
carbon atoms in the a-helix conformation. The signals of the
alkyne carbon atoms of alkyne-PBLG appear at 81.0 and
73.4 ppm; Figure 5(a) provides assignments of the remain-
ing signals for the carbon atoms of alkyne-PBLG. Figure 5(b)
FIGURE 1 (A) FTIR spectra of (a) isobutyltrisilanol-POSS, (b) Cl-
POSS, and (c) N3-POSS. (B) 1H NMR spectra of (d) Cl-POSS
and (e) N3-POSS in CDCl3.
octahedral silsesquioxane was obtained. In addition, the
chemical shifts of the signals in the 29Si NMR spectrum of
N3-POSS were identical to those of its precursor, implying
that the cage structure of N3-POSS was the same as that of
the mono-benzyl chloride POSS. Thus, the complete cubic
structure of the siloxane cage remained unchanged during
the synthesis.
Synthesis of Alkyne-PBLG
1
Figure 3(a) presents the H NMR spectrum of the BLG mono-
mer in CDCl3. The benzyloxycarbonyl ring protons appeared
as signals between 7.31 and 7.38 ppm, whereas the benzylic
protons appeared as a doublet at 5.11 ppm. We assign the
singlet at 6.4 ppm to the proton on the ring nitrogen atom;
the other alkyl CH2 protons appeared upfield as multiplets
between 2.0 and 2.6 ppm. Figure 3(b) displays the 1H NMR
spectrum of alkyne-PBLG in a mixture of CDCl3 and 15%
TFA. The signal for the proton on the nitrogen atom of
alkyne-PBLG appeared as a singlet at 7.9 ppm; the singlets
at 2.6 and 4.0 ppm correspond to the CBCAH and CBCCH2
protons, respectively; the aromatic protons appear as multi-
FIGURE 2 29Si NMR spectra of (a) Cl-POSS and (b) N3-POSS.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 3 1H NMR spectra of (a)
BLG monomer (in CDCl3) and
(b) alkyne-PBLG (in CDCl3 and
15 wt % TFA).
displays the 13C NMR spectrum of POSS-b-PBLG. The signals
for the alkyne carbon atoms are absent, but those of the iso-
butyl carbon atoms of the POSS moiety were present at 0.2,
25.0, and 29.0 ppm; Figure 5(b) provides assignments of all
of the other signals for the carbon atoms of POSS-b-PBLG.
FTIR spectroscopic analysis (Fig. 6) confirmed the complete
disappearance of the characteristic signals for the azido and
acetylene groups. The signal at 2105 cmꢀ1, corresponding to
the absorbance of the azido group of N3-POSS, was absent in
the spectra of the linear POSS-b-PBLG; the absorption band
of the SiAOASi (siloxane) groups of the POSS moiety
appears at 1100 cmꢀ1 in the spectra of the POSS-b-PBLG
copolymers, indicating that the azido and acetylene function-
alities had participated in the click reactions. Taken together,
the 1H NMR, 13C NMR, and FTIR spectra all confirmed the
successful synthesis of POSS-b-PBLG. Table 1 also lists the
molecular weight of the various POSS-b-PBLG species, as
determined through 1H NMR spectroscopic and GPC
analyses.
Thermal Analyses of POSS-b-PBLG Copolymers
Figure 7 presents DSC thermograms of the alkyne-PBLG and
linear PBLG-b-POSS systems. During the second heating run
of each of these systems, we observed only one glass
TABLE 1 Molecular Characteristics of the Alkyne-PBLG and
POSS-b-PBLG Systems
a
b
b
Compound
Mn
Mn
Mw
PDIb
DPa
PBLG5
1,020
2,340
4,470
7,370
11,580
1,960
3,300
5,400
8,300
12,500
NDc
ND
ND
5
10
20
33
53
5
PBLG10
3,220
4,080
5,290
7,340
ND
3,720
4,300
6,700
9,500
1.11
1.05
1.26
1.29
ND
PBLG20
PBLG33
PBLG53
POSS-b-PBLG5
POSS-b-PBLG10
POSS-b-PBLG20
POSS-b-PBLG33
POSS-b-PBLG53
a
ND
4,050
5,000
5,800
8,760
4,520
5,500
1.12
1.10
1.26
1.24
10
20
33
53
7,300
10,800
Determined from 1H NMR spectra.
Determined from GPC analysis.
Not detectable.
FIGURE
4
1H NMR spectra of (a) N3-POSS (in CDCl3), (b)
b
c
alkyne-PBLG5 (in CDCl3), and (c) POSS-b-PBLG5 (in d6-DMSO).
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FIGURE 5 13C NMR spectra of
(a) alkyne-PBLG5 (in d6-DMSO)
and (b) POSS-b-PBLG5 (in TFA).
transition temperature (Tg, at ca. 20 ꢁC), which increased
upon increasing the degree of polymerization.35 The glass
transition behavior of the alkyne-PBLG and linear POSS-b-
PBLG systems was almost identical; theꢁvalue of Tg of the
linear POSS-b-PBLG was higher (by ca. 4 C) than that of the
linear alkyne-PBLG only for the lowest molecular weight
PBLG5, presumably because the b-sheet conformation of
alkyne-PBLG5 transformed into a rigid rod-like a-helix struc-
ture after incorporation of the POSS unit into the linear
POSS-b-PBLG5 system (i.e., due to the larger POSS content in
this block copolymer than in the other, higher molecular
weight POSS-b-PBLG species). In the following section, we
characterize the transformations of the secondary structures
of the alkyne-PBLG and linear POSS-b-PBLG systems, as
determined through FTIR spectroscopy and WAXD analyses.
Conformational Study of the Peptide Segment
We recorded FTIR spectra at room temperature to obtain in-
formation regarding the conformations of the peptide seg-
ments in the alkyne-PBLG and linear POSS-b-PBLG systems
(Fig. 8). Analysis using the second derivative technique11
indicated that the amide I band at 1655 cmꢀ1 was character-
istic of the a-helical secondary structure. For polypeptides
possessing a b-sheet conformation, the position of the amide
I band is shifted to 1627 cmꢀ1, while that of a random coil
or turn population is located at 1693 cmꢀ1; the free C¼¼O
FIGURE 6 FTIR spectra of (a) N3-POSS, (b) POSS-b-PBLG5, and
FIGURE 7 DSC traces (second heating runs) of (a) alkyne-PBLG
and (b) POSS-b-PBLG.
(c) alkyne-PBLG5.
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of curve fitting of the signals for the amide I groups for the
b-sheet, a-helical, and random coil structures; Figure 10
summarizes the results. The fraction of a-helical secondary
structures increased on increasing the degree of polymeriza-
tion in both the alkyne-PBLG and POSS-b-PBLG systems, sim-
ilar to the findings reported by Papadopoulos et al.35 At a
low degree of polymerization (DP < 20), both secondary
structures were present, but as the degree of polymerization
increased, the a-helical secondary structure was favored. In
addition, each of the linear POSS-b-PBLG systems obtained
after performing the click reactions had a higher fraction of
the a-helical secondary structure and lower fractions of b-
sheet and random coil secondary structures than did its
corresponding alkyne-PBLG system at the same degree of
polymerization of the polypeptide.
We observed similar phenomena in the WAXD analyses at
393 K of the secondary structural changes of the alkyne-
PBLG and POSS-b-PBLG systems (Fig. 11). Here, we discuss
the effects of chain length and the incorporation of POSS on
the secondary structure of PBLG. For the alkyne-PBLG spe-
cies [Fig. 11(a)], the diffraction pattern of PBLG at a DP of
20 revealed the presence of two secondary structures. The
first peak, at a value of 2h of 4.57ꢁ, reflects the distance
(d ¼ 1.67 nm) between the backbones in the antiparallel b-
pleated sheet structure; the second, at 16.2ꢁ (d ¼ 0.47 nm)
represents the intermolecular distance between adjacent
peptide chains with one lamella. The three reflections at
higher angles, with relative positions 1:31/2:41/2, relative to
FIGURE 8 FTIR spectra of (A) alkyne-PBLG species incor-
porating for (a) PBLG5, (b) PBLG10, (c) PBLG20, (d) PBLG33, (e)
PBLG53 and (B) POSS-b-PBLG species incorporating (f) PLBG5,
(g) PLBG10, (h) PLBG20, (i) PLBG33, and (j) PLBG53
.
unit of the side chain group of PBLG provided a signal at
1734 cmꢀ1, while the signals at 1607 and 1594 cmꢀ1 repre-
sented stretching of the benzene units of the side chain
groups of PBLG. Next, we used the deconvolution technique
in a series of Gaussian distributions to quantify the fraction
of each of the peaks (Fig. 9). Table 2 summarizes the results
FIGURE 9 Curve fitting data from the analysis of the FTIR spectra of the alkyne-PBLG species incorporating (a) PBLG5, (b) PBLG10
,
(c) PBLG20, and (d) PBLG53, and their corresponding POSS-b-PBLG systems.
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TABLE 2 Curve-Fitting of Secondary Structure of Alkyne-PBLG and Linear PBLG-b-POSS
(t: Wavenumber (cm21), W1/2: Half-Width (cm21), Af: Area Fraction of Each Peak)
Random Coil
a-Helix
b-Sheet
Sample
t
W1/2
Af
t
W1/2
Af
41.3
t
W1/2
Af
PBLG5
1693
1693
1692
1691
1693
–
27
27
26
27
25
–
18.9
18.7
12.6
8.8
5.9
0
1653
1654
1655
1654
1654
1654
1653
1652
1652
1652
32
32
30
28
26
26
28
30
38
36
1626
1627
1625
1626
1627
–
19
19
20
18
20
–
39.8
29.8
33.4
17.0
18.0
0
PBLG5-POSS
PBLG10
51.5
54.0
74.2
76.1
PBLG10-POSS
PBLG20
PBLG20-POSS
PBLG33
100
–
–
0
100
100
100
100
–
–
0
PBLG33-POSS
PBLG53
–
–
0
–
–
0
–
–
0
–
–
0
PBLG53-POSS
–
–
0
–
–
0
the primary peak at q*, are indexed according to the (10),
(11), and (20) reflections of a 2D hexagonal packing of cylin-
ders composed of 18/5 a-helices with a cylinder distance of
1.36 nm.35 The structure of PBLG has been as a nematic-like
paracrystal with a periodic packing of a-helices in the direc-
tion lateral to the chain axis.4 The broad amorphous signal
at about 16ꢁ originates mainly from the long amorphous
side chain. Further increases in the DP of PBLG to 33 and 53
resulted in disappearance of the diffraction peak at 4.57ꢁ
associated with the b-sheet secondary structure, suggesting
the absence of that particular conformation for the longer
peptides. In addition, the a-helical conformations were better
packed for the longer peptides. Decreasing the length of the
PBLG chain (DP ¼ 5 and 10) led to destabilization of the a-
helical secondary structures, causing the b-sheet secondary
structures to be observed as signals at values of 2h of 4.57ꢁ
and 16ꢁ, respectively. These results are consistent with those
from our FTIR spectroscopic analyses: For low DPs (<20),
the alkyne-PBLG systems featured both secondary structures;
as the DP increased, however, the a-helical secondary struc-
ture was favored.
Figure 11(b) presents the WAXD patterns of the linear POSS-
b-PBLG recorded at 393 K. For the linear POSS-b-PBLG spe-
cies having longer PBLG segments (DP > 20), we observe
FIGURE 11 WAXD patterns (recorded at 393 K) of (a) alkyne-
PBLG and (b) POSS-b-PBLG species featuring with different
degrees of polymerization.
FIGURE 10 Secondary structures of alkyne-PBLG and POSS-b-
PBLG systems featuring different degrees of polymerization.
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POSS-b-PBLG20. Structurally speaking, the PBLG tethers the
POSS units from its crystalline domains; meanwhile, the scat-
tered POSS moieties gather and lock into the copolymers to
form crystalline entities. The cleaner system can now phase-
separate into two different types of crystalline domains,
which further pack into a lamellar-like sequence having a
bilayer structure. The characteristic spacing (50 Å) observed
from the SAXS data is consistent with the bilayer structure
of the POSS-b-PBLG5 copolymer [Scheme 3(a)]. The observed
spacing is close to the thickness (50 Å) expected for the
repeated packing of a ‘‘{POSS}-{PBLG5 b-sheet}-{PBLG5 b-
sheet}-{POSS}’’ bilayer structure, as estimated from twice the
sum of the POSS length (2 ꢂ 11 Å2) and peptide length (2 ꢂ
5 ꢂ 3 Å3). For this calculation, we estimated the length of
polypeptide segment by considering the b-sheet conforma-
tion to have a length of about 3 Å for each repeat unit,
rather than the length of 3.8 Å expected for fully extended
peptide bonds38; we estimated the length of isobutyl-POSS
to be about 11 Å from first diffraction peak at a value of 2h
of 7.1ꢁ. In addition, we would expect the observed spacing
of the POSS-b-PBLG20 copolymer to be 82 Å, for the repeated
FIGURE 12 WAXD analyses of (a) alkyne-PBLG20 and (b) POSS-
b-PBLG20 species.
only one set of Bragg peaks at diffraction angles having a
ratio of 1:31/2:41/2, indicating a columnar hexagonal arrange-
ment of the molecules. On decreasing the length of the pep-
tide segment (DP < 20) of the linear POSS-b-PBLG systems,
a considerable fraction of the peptide segments featured a b-
sheet secondary structure, similar to behavior of the corre-
sponding alkyne-PBLG systems. Figure 12 summarizes the
WAXD patterns of alkyne-PBLG20 and POSS-b-PBLG20. After
performing the click reaction, POSS-b-PBLG20 featured a
higher fraction of a-helical secondary structures than did its
precursor alkyne-PBLG20. Therefore, it appears that even if
peptides only partially adopt the a-helical and b-sheet con-
formations, when anchored to POSS in the form of a block
polymer they undergo conformational stabilization such that
most of the peptide segments are constrained in the a-helical
secondary structure. In addition, Figure 11(b) reveals that
the WAXD pattern of POSS-b-PBLG5 features more two addi-
tional diffraction angles (at ca. 1.6 and 7.1ꢁ) relative to that
of pure alkyne-PBLG5, suggesting microphase separation into
a lamellar-like sequence featuring a bilayer structure and
(101) diffraction planes for hexagonal POSS crystals,
respectively.
packing of
a ‘‘{POSS}-{PBLG20 a-helix}-{PBLG20 a-helix}-
{POSS}’’ bilayer structure, estimated from twice the sum of
the POSS length (2 ꢂ 11 Å2) and the peptide length (2 ꢂ
20/3.6 ꢂ 5.4 Å3), as indicated in Scheme 3(b). The length of
the polypeptide segment was estimated by considering the
18/5 a-helical conformations to have a length of 5.4 Å for
each pitch of the helix.
Figure 14 provides the thermal stabilities under N2 of the
POSS-b-PBLG5 and alkyne-PBLG5 nanocomposites. To com-
pare their thermal stabilities, we used the 20 wt % weight
loss temperature as a standard. We observe a significantly
increased (ca. 80 ꢁC) decomposition temperature (Td) for
POSS-b-PBLG5 relative to that of alkyne-PBLG5, presumably
because of the nano-reinforcement effect of incorporating
POSS moieties into polymeric matrixes. A recent study dem-
onstrated that POSS units can significantly retard thermal
Gallot et al.36,37 used SAXS to investigate the morphologies
of linear polyvinyl/polypeptide coil/rod block copolymers in
the solid state. They found a large-scale hexagonal-in-lamel-
lar morphology of alternating polyvinyl and polypeptide
sheets with a-helical polypeptide chains arranged in a
hexagonal array. To observe the correlation between the self-
assembly of the hexagonally packed polypeptides and the
crystallization of the POSS units, we conducted SAXS analy-
ses of the POSS-b-PBLG systems (Fig. 13). The broad peak,
assumed to be a Bragg reflection, at a value of q of 0.125
corresponds to an ordering spacing of about 50 Å for POSS-
b-PBLG5; this phenomenon is consistent with the WAXD data
at a value of 2h of 1.6ꢁ. Increasing the length of the PBLG
chain resulted in a Bragg reflection at a value of q of 0.076,
corresponding to an ordering spacing of about 82 Å for
FIGURE 13 SAXS patterns (recorded at room temperature) of
POSS-b-PBLG species incorporating different degrees of
polymerization.
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cial AcOH overnight, followed by washing with absolute
EtOH and ethyl ether, and then drying under vacuum. N,N-
Dimethylformamide (DMF), sodium azide (NaN3), triethyl-
amine (TEA), trichloro[4-(chloromethyl)phenyl]silane, and
N,N,N,N,N-pentamethyldiethylenetriamine (PMDETA, 99%)
were purchased from Aldrich. All solvents were distilled
before use. c-Benzyl-L-glutamate N-carboxyanhydride was
prepared according to a literature procedure43 and stored at
ꢁ
ꢀ30 C before use.
Mono-Benzyl Chloride POSS (Cl-POSS)
A solution of trichloro[4-(chloromethyl)phenyl]silane (1.00
mL, 5.61 mmol), isobutyltrisilanol-POSS (4.05 g, 5.11 mmol),
and TEA (2.20 mL, 15 mmol) in dry tetrahydrofuran (THF,
30.0 mL) was stirred at room temperature for 7.5 h under
Ar and then filtered to remove the precipitated NEt3ꢃHCl.
The clear THF solution was added dropwise into a beaker of
MeCN and rapidly stirred. The resulting product was filtered
off and dried under vacuum. (4.61 g, 80%).
1H NMR (CDCl3): d 7.65 (d, 2H), 7.39 (d, 2H), 4.60 (s, 2H),
1.92–1.82 (m, 7H), 0.98–0.94 (m, 42H), 0.65–0.62 (m, 14H)
ppm. 29Si NMR (CDCl3): d ꢀ55.44, ꢀ67.61, ꢀ68.27, ꢀ81.27
ppm. FTIR (KBr, cmꢀ1): 2956 m, 2871 m, 1465 m, 1400 w,
1366 w, 1332 w, 1230 m, 1110 vs, 1039 m, 838 m, 741 m,
694 w.
Mono-Benzyl Azide POSS (MBA-POSS, N3-POSS)
NaN3 (1.10 g, 16.9 mmol) was added to a solution of mono-
benzyl chloride POSS (1.62 g, 1.72 mmol) in DMF (40 mL)
and then the mixture was heated at 80 ꢁC overnight. After
cooling to room temperature, the mixture was diluted with
CHCl3 (150 mL) and washed with 1 M NaHCO3 (2 ꢂ 100
mL) and water (2 ꢂ 100 mL). The organic layer was dried
(Na2SO4) and the solvent evaporated under vacuum to yield
a white solid (1.14 g, 70%).
SCHEME 3 Proposed model for the self-assembly of the (a)
POSS-b-PBLG5 and (b) POSS-b-PBLG20 copolymers.
1H NMR (CDCl3): d 7.67 (d, 2H), 7.32 (d, 2H), 4.36 (s, 2H),
1.92–1.82 (m, 7H), 0.98–0.94 (m, 42H), 0.65–0.62 (m, 14H)
ppm. 29Si NMR (CDCl3): d ꢀ55.44, ꢀ67.61, ꢀ68.26, ꢀ81.19.
motion; at the same time, they can act as flow aids at ele-
vated temperatures. In addition, the increased values of Td
for the nanocomposites may also result from increased chain
spacing, which gives rise to lower thermal conductivity.39 In
materials featuring POSS nanoparticles, thermal motion is
restricted, thereby reducing the number of pathways for
organic decomposition; the inorganic POSS component pro-
vides additional heat capacity, thereby stabilizing the materi-
als against thermal decomposition. Char yield, another indi-
cator of thermal stability, also increased after tethering the
POSS nanoparticles. Thus, the thermal stability of PBLG
improved as a result of the formation of network structures
and because of the presence of the inorganic silsesquiox-
ane.40–42
EXPERIMENTAL
Materials
Isobutyltrisilanol-POSS was obtained from Hybrid Plastics.
Propargylamine was purchased from Tokyo Kasei Kogyo.
Copper(I) bromide (CuBr) was purified by washing with gla-
FIGURE 14 TGA analyses of alkyne-PBLG5 and POSS-b-PBLG5
species.
SYNTHESIZE OF MACROMOLECULAR SELF-ASSEMBLING BUILDING BLOCKS, LIN AND KUO
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FTIR (KBr, cmꢀ1): 2956 s, 2871 m, 2102 m, 1465 m, 1399
w, 1383 w, 1366 w, 1334 w, 1230s, 1106 vs, 1038 m, 838 m,
803 w, 744 m. MALDI-TOF mass: 951 (g/mol).
(NSRRC), Taiwan. A triangular bent Si (111) single crystal
was employed to obtain a monochromated beam having a
wavelength (k) of 1.3344 Å. The XRD patterns were collected
using a imaging plate (IP; Fuji BAS III; area ¼ 20 ꢂ 40 cm2)
curved with a radius equivalent to the sample-to-detector
distance (280 mm). The two-dimensional (2D) XRD patterns
observed for the sample (typical diameter: 10 mm; thick-
ness: 1 mm) were circularly averaged to obtain a one-dimen-
sional (1D) diffraction profile I(Q), with the value of Q cali-
brated using standard samples of Ag-Behenate and Si
powder (NBS 640b). SAXS experiments were performed
using the SWAXS instrument at the BL17B3 beamline of the
NSRRC, Taiwan; the X-ray beam had a diameter of 0.5 mm
and a wavelength (k) of 1.24 Å. The blend samples (thick-
ness: 1 mm) were sealed between two thin Kapton windows
(thickness: 80 lm) and analyzed at room temperature.
Propargyl-Terminated PBLG33
Bz-L-GluNCA (4.00 g, 15.2 mmol) was weighed in a dry-box
under pure Ar, placed in a flame-dried Schlenk tube, and dis-
solved in anhydrous DMF (40 mL). The solution was stirred
for 10 min and then propargylamine (12 lL, 175.2 lmol)
was added using a N2-purged syringe. After stirring for 40 h
at room temperature, the polymer was recovered through
precipitation in diethyl ether and drying under high vacuum.
1H NMR (CDCl3): d 7.84 (s, 1H, NH), 7.35 (m, 5H, ArH), 5.1
(d, 2H, CH2Ar), 4.59 (s, 1H), 3.99 (d, 2H), 2.45 (d, 2H), 2.18
(s, 1H), 2.10–1.90 (m, 2H).
PBLG-b-POSS Diblock Copolymers
N3-POSS (0.15 g, 0.158 mmol), alkyne-PBLG (0.09 g, 0.028
mmol), and CuBr (3.59 mg, 0.025 mmol) were dissolved in
DMF (15 mL) in a flask equipped with a magnetic stirrer
bar. After one brief freeze/thaw/pump cycle, PMDETA (5.27
lL, 0.025 mmol) was added; the reaction mixture was then
carefully degassed through three freeze/thaw/pump cycles,
placed in an oil bath thermostated at 60 ꢁC, and stirred for
24 h. After evaporating all of the solvent under reduced
pressure, the residue was dissolved in CH2Cl2 and passed
through a neutral alumina column to remove copper cata-
lysts. Evaporating of the solvent yielded the linear PBLG-b-
POSS as a dark powder.
CONCLUSIONS
We have prepared well-defined linear POSS-b-PBLG copoly-
mers through ROP of Glu-NCA followed by click reactions
with a monofunctional azide-POSS. After attaching the PBLG
blocks to the POSS nanoparticles, the fraction of a-helical
secondary structures increased as a result of intramolecular
hydrogen bonding between the POSS and PBLG moieties.
The POSS-b-PBLG systems exhibited greater conformation
stability and superior thermal properties relative to those of
pure PBLG. The self-assembly of POSS-b-PBLG formed a
bilayer-like nanostructure featuring a-helical or b-sheets and
POSS aggregates, as evidenced using SAXS analysis.
Characterization
1H NMR spectra were recorded at room temperature using a
Bruker AM 500 (500 MHz) spectrometer, with the residual
proton resonance of the deuterated solvent acting as the
internal standard. Molecular weights and molecular weight
distributions were determined through gel permeation chro-
matography (GPC) using a Waters 510 high-performance
liquid chromatograph (HPLC) equipped with a 410 differen-
tial refractometer and three Ultrastyragel columns (100, 500,
and 103 Å) connected in series, with dimethylacetamide
(DMAc) as the eluent (flow rate: 0.4 mL/min). Thermal anal-
ysis through differential scanning calorimetry (DSC) was per-
This study was supported financially by the National Science
Council, Taiwan, Republic of China, under contracts NSC
97-2221-E-110-013-MY3 and NSC 97-2120-M-009-003. The
WAXD experiments were conducted at the 17A1 beamline at
the NSRRC, Taiwan.
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