Changing the structural and physical properties of 3-arm star poly(δ-valerolactone)s by a branch-point design

Article abstract of DOI:10.1039/d1cc01092a

The chemical structure of a branch point of star-shaped polymers has been considered to have a small influence on the physical properties of the entire polymer. Contrary to this general notion, here we show that a 3-arm star polymer, composed of three poly(δ-valerolactone) arms extended from one side of a triptycene branch point, exhibits a remarkably high complex viscosity, compared to the analogous star-shaped polymers with a branch point of a triptycene isomer or triphenylethane.

Full text of DOI:10.1039/d1cc01092a

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Changing the structural and physical properties
of 3-arm star poly(d-valerolactone)s by a
branch-point design†
Cite this: Chem. Commun., 2021,
57, 3901
Received 27th February 2021,
Accepted 16th March 2021
Hibiki Ogiwara,a Fumitaka Ishiwari, ‡*ab Tadahiro Kimura,a Yukihiro Yamashita,a
d
Takashi Kajitani,bc Atsuki Sugimoto,d Masatoshi Tokita,
Masaki Takatabe and
a
DOI: 10.1039/d1cc01092a
Takanori Fukushima
*
rsc.li/chemcomm
The chemical structure of a branch point of star-shaped polymers perspective of branched polymers but also a conceptually new
has been considered to have a small influence on the physical strategy for tuning the physical properties of polymers by the
properties of the entire polymer. Contrary to this general notion, design of branch points.
here we show that a 3-arm star polymer, composed of three poly
We have recently reported that 1,8,13-trisubstituted tripty-
(d-valerolactone) arms extended from one side of a triptycene cenes and 1,8-disubstituted triptycenes, such as 1,8,13-trialkoxy
branch point, exhibits a remarkably high complex viscosity, com- and 1,8-dialkoxy triptycene, can self-assemble into a ‘‘2D + 1D’’
pared to the analogous star-shaped polymers with a branch point structure at large length scales, where the two-dimensional (2D)
of a triptycene isomer or triphenylethane.
sheets formed by nested hexagonal packing of the triptycene
units stack into a 1D multilayer structure.5 This peculiar self-
The physical properties of polymers are determined not only by assembling ability is remarkable and enables the formation of
the chemical structure of the main and side chains but also 2D + 1D structures even when the triptycene functionality is
by the topology (i.e., shape) of the polymer.1–3 For instance, attached to the termini of a liquid polymer, polydimethylsilox-
Hadjichristidis and coworkers2 revealed that star-shaped poly- ane with Mn = 18–24 kDa.5e These findings prompted us to
mers (Fig. 1) show lower viscosity, and smaller hydrodynamic further test the structuring capability through the construction
and gyration radii, compared with the corresponding linear- of a star-shaped polymer using 1,8,13-trisubstituted triptycene
shaped polymers.1 Although there have been many studies on for a branch point that can be regarded as the smallest
the chain-end effects of polymers,4 to the best of our knowledge, constituent in a polymer.
there is no report on the systematic investigation of the chemical
structure-dependence of the branch point of the star-shaped
polymers on their properties.1 Here we show that the design of
a branch point could change the properties of star-shaped poly-
mers and would lead to not only updating the general structural
a Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo
Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 226-8503, Japan.
E-mail: fukushima@res.titech.ac.jp
b RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan
c Materials Analysis Division, Open Facility Center, Tokyo Institute of Technology,
4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
d Department of Chemical Science and Engineering, Tokyo Institute of Technology,
2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8550, Japan
e International Center for Synchrotron Radiation Innovation Smart,
Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
† Electronic supplementary information (ESI) available: Materials and methods,
experimental details and analytical data, and a supporting movie. See DOI:
10.1039/d1cc01092a
Fig. 1 Schematic illustrations and the chemical structures of the three-
arm star-shaped polymers ((a) 1,8,13-Trip-PVLs, (b) 1,8,16-Trip-PVLs and
(c) TPE-PVLs), in which the branch-point structures are highlighted.
‡ Present address: Department of Applied Chemistry, Graduate School of Engi-
neering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan. E-mail:
ishiwari@chem.eng.osaka-u.ac.jp
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To investigate how the chemical structure of the branch TPE-PVL-S and 1,8,16-Trip-PVL-S underwent melting entirely and
point of the star-shaped polymers affects their properties, we partially, respectively (Fig. 2b). Upon subsequent heating at 80 1C,
designed three types of three-arm star-shaped polymers with a 1,8,16-Trip-PVL-S became liquid entirely (Fig. 2c). In sharp contrast,
different branch point as shown in Fig. 1. The branch point a sample of 1,8,13-Trip-PVL-S maintained the original shape even
of 1,8,13-Trip-PVLs (Fig. 1a), 1,8,16-Trip-PVLs (Fig. 1b) or after heating up to 80 1C, a much higher temperature than the Tm
TPE-PVLs (Fig. 1c) is composed of 1,8,13-trisubstituted tripty- of the PVL arms, and eventually became liquid at 90 1C (Fig. 2d).
cene, 1,8,16-trisubstituted triptycene or 1,1,1-triphenylethane, Although a clear branch-point-structure dependence on the melting
respectively. We chose poly(d-valerolactone) (PVL) as the arms of behaviors of the M- and L-series polymers was not observed, we
the star-shaped polymers, which can be readily grown by living thought from the observed behaviors of the S-series polymers that
ring-opening polymerization of d-valerolactone from the corres- the branch point could have an non-negligible influence on the
ponding hydroxyl group-terminated initiators using diphenyl properties of the polymer.
phosphate as a catalyst.6 Another merit of this arm design is that
The variable-temperature X-ray diffraction (VT-XRD) analysis
PVL has a cross-sectional area similar to that of alkanes and provided insight into the structural change of the star-shaped
would not impair the self-assembly ability of 1,8,13-trisubstituted polymers upon heating. At 30 1C, all polymers showed diffrac-
triptycene.
tions at around q = 15, 17 and 21 nmÀ1, which are characteristic
To evaluate the effect of molecular weight, we synthesized of diffractions from the (110), (200) and (210) planes of the
star-shaped PVLs with different molecular weights by changing crystalized PVL (marked with orange triangles in Fig. 3a–c, g–i
the feed ratio of d-valerolactone and initiator (Table S1, ESI†). and m–o).7,8 Notably, the XRD profile of 1,8,13-Trip-PVL-S
These polymers were obtained with a relatively small molecular (Fig. 3a) displayed clear diffraction peaks at q = 2.86, 5.76,
weight distribution (Mw/Mn) ranging from 1.04 to 1.20. Here- 9.10 and 17.9 nmÀ1 (marked with light blue triangles), along
after, based on the molecular weights, we distinguish the star- with diffractions from the crystallized PVL. This indicates the
shaped PVLs with ca. 5.5 kDa, 20 kDa and 41 kDa by adding presence of a periodic structure other than the crystallized PVL.
indexes S (small), M (medium) and L (large), respectively, to the For all of the star-shaped polymers, when heated at a tempera-
abbreviated name of each polymer. The chemical structures of ture (e.g., 60 1C) higher than the Tm of the PVL arms, the
a total of nine polymers thus obtained were unambiguously diffraction peaks due to the crystallized PVL disappeared
characterized by NMR and IR spectroscopy, and MALDI-TOF-MS (Fig. 3d–f, j–l and p–r). Surprisingly, 1,8,13-Trip-PVL-S, even
spectrometry. The molecular weights of the star-shaped PVLs, after melting of the PVL arms, still showed the diffraction peaks
determined by the SEC analysis, are shown in Table S2 (ESI†).
at a wide-angle region (light blue triangles in Fig. 3d). Further-
In differential scanning calorimetry (DSC), all of the star-shaped more, a diffraction peak at q = 15.3 nmÀ1, which had been
PVLs showed an endothermic peak due to melting of the PVL arms hidden by diffractions from the crystallized PVL, was newly
(Fig. S1, ESI†). Table S2 (ESI†) summarizes the melting points (Tm) detected. The XRD pattern of 1,8,13-Trip-PVL-S at 60 1C is very
and the associated enthalpy changes (DHm). These values depend similar to those typically observed for 1,8,13-substituted tripty-
on the molecular weight, but not on the chemical structure of the cene assemblies.2a Indeed, all diffraction peaks at 60 1C were
branch point. However, when taking a closer look at the melting successfully indexed by assuming a ‘‘2D + 1D’’ structure with a
behaviors of the polymers, we noticed an interesting difference in d-spacing of d001 = 2.15, d002 = 1.08, d100 = 0.69, d110 = 0.40,
the melting behavior for the S-series polymers. As shown in Fig. 2
(see also Movie S1, ESI†), when a solid sample of each S-series
polymer was heated at 55 1C (i.e., near Tm of the PVL arms),
Fig. 3 XRD patterns of the S-series (a–f), M-series (g–l) and L-series (m–r)
Fig. 2 Changes in the appearance of a solid sample of 1,8,13-Trip-PVL-S,
1,8,13-Trip-PVL-S or TPE-PVL-S upon heating on a hot plate ((a) 25 1C,
(b) 55 1C, (c) 80 1C and (d) 90 1C). See also Movie S1, ESI.†
polymers of 1,8,13-Trip-PVLs (red), 1,8,16-Trip-PVLs (blue) and TPE-PVLs
(green) measured at 30 1C (a–c, g–i and m–o) and 60 1C (d–f, j–l and p–r)
in a glass capillary with a diameter of 1.5 mm.
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d111 = 0.39, d200 = 0.35 and d201 = 0.34 nm, in which the 15.3 and 17.9 nmÀ1 (marked with light blue triangles), which
triptycene branch points assemble to form 2D hexagonal layers can be attributed to diffractions from the (100), (110) and (200)
(a = 0.80 nm), which are aligned one-dimensionally (c-axis) with planes of a 2D assembly of the triptycene branch point. The
a 2.16 nm separation (Table S3 ESI†).
weak assembly feature of the triptycene branch point in 1,8,
Fig. 4a illustrates a presumable assembly structure of 1,8, 16-Trip-PVL-S is consistent with the partial melting behavior of
13-Trip-PVL-S featuring molecular ordering of the triptycene this polymer (Fig. 2).
moiety. Although it is difficult to address how the PVL arms are
The rheological measurements allowed quantitative analysis
located in the polymer assembly structure, the polymer chains of the mechanical properties of the star-shaped PVLs (Fig. 5 and
might be extended out of the 2D triptycene assembly through a Fig. S3–S5, ESI†). Fig. 5a–c shows the frequency dependence of
gap between the triptycene units (Fig. 4a, right). Fig. 4b shows a complex viscosity (Z*) of the polymers measured using a
the detailed VT-XRD patterns of 1,8,13-Trip-PVL-S measured at rotational rheometer at 60 1C.9–11 Over a frequency range from
30–150 1C. The diffraction peaks originating from the 2D + 1D 0.01 to 10 Hz, TPE-PVLs (Fig. 5a–c, green) have almost constant
structure gradually decreased with increasing temperature and Z* values of 100, 101 and 102 Pa s for TPE-PVL-S, TPE-PVL-M and
almost disappeared at 130 1C. This indicates that the triptycene TPE-PVL-L, respectively (Table S2, ESI†), representing typical
assembly can be maintained even in the molten state of the PVL behavior of a Newtonian fluid.10 Notably, the Z* value of 1,8,
arms (Fig. 4c). Such a strong structuring ability of the triptycene 13-Trip-PVL-S at 10 Hz is ca. 102 Pa s; it gradually increases with
branch point in 1,8,13-Trip-PVL-S is considered responsible for decreasing frequency and reaches ca. 104 Pa s at 0.01 Hz. This
the shape-retaining property upon heating over Tm of the PVL pseudoplastic behavior10 may originate from the increase in
arms (Fig. 2).
apparent molecular weight, caused by the assembly of the
Meanwhile, diffractions due to the 2D + 1D structure of the triptycene branch point that results in the 2D + 1D periodic
triptycene branch points were absent in the XRD profiles of the order of 1,8,13-Trip-PVL-S.3 Likewise, 1,8,16-Trip-PVL-S was
M-series (Fig. 3g–l) and L-series polymers (Fig. 3m–r), as well as found to behave as a pseudoplastic material and its Z* values
TPE-PVL-S (Fig. 3c and f). However, on close inspection of the increase from ca. 100 to 2 Â 102 Pa s with decreasing frequency
XRD profiles of 1,8,16-Trip-PVL-S at 30 and 60 1C (Fig. 3b, e and from 10 to 0.01 Hz (Fig. 5a, blue). The fact that 1,8,16-Trip-PVL-S
see also Fig. S2, ESI†), very weak peaks were detected at q = 9.10, has much smaller Z* values than 1,8,13-Trip-PVL-S in the entire
measurement frequency range is consistent with the difference in
the melting and structuring behaviors between these polymers
(Fig. 2 and 3).
Interestingly, among the M-series polymers, only 1,8,
13-Trip-PVL-M showed pseudoplastic behavior to provide an
enhanced Z* value in the low frequency range (Fig. 5b, red),
although this polymer does not exhibit XRD diffractions asso-
ciated with a periodic 2D + 1D structural order (Fig. 3g and j). We
presume that the triptycene branch point of 1,8,13-Trip-PVL-M
Fig. 5 Frequency dependence of Z* of the S-series (a), M-series (b) and
Fig. 4 (a) Schematic illustration of the assembly structure of 1,8,13-Trip-
PVL-S at 25 1C. (b) The VT-XRD profiles of 1,8,13-Trip-PVL-S measured
upon heating in a glass capillary with a diameter of 1.5 mm. (c) Schematic
diagram showing the relationship between the two periodic structures
formed in the bulk phase of 1,8,13-Trip-PVL-S and their thermal stability.
L-series (c) polymers of 1,8,13-Trip-PVLs (red), 1,8,13-Trip-PVLs (blue) and
TPE-PVLs (green) measured at 60 1C. Temperature dependence of Z* of
the S-series (d), M-series (e) and L-series (f) polymers of 1,8,13-Trip-PVLs
(red), 1,8,13-Trip-PVLs (blue) and TPE-PVLs (green) measured at
frequency of 0.01 Hz.
a
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may assemble to form a 2D + 1D structure at least partially, but and ‘‘Dynamic Alliance for Open Innovation Bridging Human,
the content of the triptycene branch point (B1.2 wt%) in 1,8, Environment and Materials’’ from MEXT, Japan. The synchrotron
13-Trip-PVL-M (Mn = 20 kDa) was too small to be detected by XRD XRD experiments were performed at the BL45XU beamline in
and DSC experiments. Noteworthy is the fact that even in such a SPring-8 with the approval of the RIKEN SPring-8 Center
small content, the triptycene branch point has a significant (20140056, 20150068, 20160027 and 20170055).
impact on the mechanical properties of the star-shaped polymers.
Considering also the fact that the L-series polymers all behave as a
Newtonian fluid (Fig. 5c), when the triptycene content relative to
the PLV arms is further reduced, the effect would no longer be
Conflicts of interest
manifested.
As shown in Fig. 5d–f, the trends in the temperature
There are no conflicts to declare.
dependence of the Z* values are similar to those in the
frequency dependence (Fig. 5a–c). There is no apparent differ-
ence among the L-series polymers (Fig. 5f). The Z* values of
1,8,13-Trip-PVL-M (Fig. 5e, red) rapidly decrease as the tem-
perature increases and become almost identical to those of
1,8,16-Trip-PVL-M and TPE-PVL-M (Fig. 5e, blue and green) at
80 1C. A more notable difference was found in the behaviors of
the S-series (Fig. 5d). 1,8,16-Trip-PVL-S (Fig. 5d, blue) also
showed a rapid decrease in Z* with increasing temperature,
providing a value similar to that of TPE-PVL-S (Fig. 5d, green)
over 100 1C. While the Z* values of 1,8,13-Trip-PVL-S (Fig. 5d,
red) are scarcely changed up to 100 1C, upon further heating,
they rapidly decreased to reach a plateau over 120 1C. This
feature agrees well with that of VT-XRD of 1,8,13-Trip-PVL-S
(Fig. 4b and c) and clearly indicates a close relationship between
Z* values and structuring of the triptycene branch point.
In conclusion, we have shown the melting, structural, and
rheological behaviors of the three-arm star-shaped PVLs in
relation to the chemical structure of the branch point. Notably,
1,8,13-trisubstituted triptycene, when used as the branch point
of the small-(Mn = 5.5 kDa) and medium-size (Mn = 20 kDa)
polymers with a triptycene content of B4.5 wt% and B1.2 wt%,
respectively, resulted in the improvement of the complex
viscosity (0.01–10 Hz), thanks to its superb assembling
ability.5 To the best of our knowledge, this study demonstrates
for the first time that polymer properties can be modulated by
the design of branch points. Using the other types of multi-arm
branched polymers, further investigations to clarify the scope
and limitation of the triptycene branch point as a module to
change polymer properties are underway in our group.
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10 J. D. Ferry, Viscoelastic Properties of Polymers, Wiley, New York, 1980.
11 The Z* values of TPE-PVL-M and linear PVL with a similar Mn are
comparable to one another (Fig. S6, ESI†).
This work was supported by KAKENHI (16K14003, 19H04567,
17H01034 and 20H05868) from JSPS, JST CREST (JPMJCR18I4)
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