Polymethylene-Based Eight-Shaped Cyclic Block Copolymers

Article abstract of DOI:10.1021/acs.macromol.9b02571

A new strategy was developed for the synthesis of well-defined eight-shaped polymethylene (PM, equivalent to polyethylene)-based cyclic block copolymers, [c-(PM-b-PCL)]₂, according to the following four steps: (a) synthesis of a B-thexyl-boracyclic initiator by cyclic hydroboration of 1,4-pentadiene-3-yl 2,2,5-trimethyl-1,3-dioxane-5-carboxylate with thexylborane; (b) polyhomologation of dimethylsulfoxonium methylide using this boracyclic initiator to afford α,ω-dihydroxyl linear PM (PM-OH)₂; (c) synthesis of 4-miktoarm star block copolymers, (PM-alkyne)₂-(PCL-N₃)₂, by esterification, deprotection, and ring-opening polymerization (ROP); and (d) intramolecular cyclization of (PM-alkyne)₂-(PCL-N₃)₂, under high-dilution condition by copper-catalyzed "click" chemistry. All intermediates and final products were characterized by high-temperature gel permeation chromatography, proton nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, and differential scanning calorimetry. Initial studies on the thermal and self-assembly behavior of the cyclic block copolymers and their corresponding precursors revealed a significant influence of the cyclic structure on the properties.

Full text of DOI:10.1021/acs.macromol.9b02571

Article
pubs.acs.org/Macromolecules
Cite This: Macromolecules XXXX, XXX, XXX−XXX
Polymethylene-Based Eight-Shaped Cyclic Block Copolymers
†
Yu Jiang and Nikos Hadjichristidis*
Physical Sciences and Engineering Division, KAUST Catalysis Center, Polymer Synthesis Laboratory, King Abdullah University of
Science and Technology (KAUST), Thuwal 23955, Saudi Arabia
*
S Supporting Information
ABSTRACT: A new strategy was developed for the synthesis
of well-defined eight-shaped polymethylene (PM, equivalent
to polyethylene)-based cyclic block copolymers, [c-(PM-b-
PCL)] , according to the following four steps: (a) synthesis of
2
a B-thexyl-boracyclic initiator by cyclic hydroboration of 1,4-
pentadiene-3-yl 2,2,5-trimethyl-1,3-dioxane-5-carboxylate with
thexylborane; (b) polyhomologation of dimethylsulfoxonium
methylide using this boracyclic initiator to afford α,ω-
dihydroxyl linear PM (PM-OH) ; (c) synthesis of 4-miktoarm
2
star block copolymers, (PM-alkyne) -(PCL-N ) , by ester-
2
3 2
ification, deprotection, and ring-opening polymerization
(
(
ROP); and (d) intramolecular cyclization of (PM-alkyne)2-
PCL-N ) , under high-dilution condition by copper-catalyzed
3
2
“
click” chemistry. All intermediates and final products were characterized by high-temperature gel permeation chromatography,
proton nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, and differential scanning calorimetry.
Initial studies on the thermal and self-assembly behavior of the cyclic block copolymers and their corresponding precursors
revealed a significant influence of the cyclic structure on the properties.
INTRODUCTION
this living C1 polymerization, boron compounds were used as
initiators to polymerize dimethylsufoxonium methylide,
leading to well-defined hydroxyl-functionalized polymethylene.
Different PM-based (co)polymers were constructed by
combining this efficient strategy with other polymerization
■
The relationship between polymer architecture and various
properties is an important research subject in both academia
1
and industry. Among different architectures, cyclic polymers,
which contain a unique non-chain-end structure and different
32
2
,3
methods, such as ring-opening polymerization (ROP), atom
physical properties, have attracted considerable attention.
Compared to their linear analogues, many differences on
properties such as crystallinity, glass-transition temperature,
and melt-flow dynamics, are discovered because of the absence
3
3−36
transfer radical polymerization (ATRP),
reversible
addition-fragmentation chain-transfer polymerization
45
46
(
RAFT), and iodine transfer polymerization. Recently,
4
our group has also used polyhomologation to build well-
of chain ends. Several novel model cyclic polymers were
defined PM-based architectures by designing/synthesizing
synthesized and investigated, including single cyclic poly-
4
7−55
5
−9
10−12
novel borane initiators.
mers
shaped,
and complex cyclic polymers
such as tadpole-
23
1
3−15
16−18
19,20
In our previous paper, the synthesis of PE-based single
cyclic block and tadpole copolymers was achieved by
combining polyhomologation, Diels−Alder reaction (anthra-
cene-terminated PE and an azido-maleimide derivative), ROP,
chain-end transformation to introduce the alkynyl groups, and
cyclization (Scheme 1a). In the present work, the synthesis of
eight-shaped copolymers is based on a dihydroxy-protected
diene (1,4-pentadien-3-yl 2,2,5-trimethyl-1,3-dioxane-5-carbox-
ylate) precursor and thexylborane chemistry to afford a
difunctional boron initiator, followed by polyhomologation,
first chain-end transformation to introduce alkynyl groups,
ROP, second chain-end transformation to introduce azido
groups, and cyclization (Scheme 1b). The development of this
successful procedure was achieved after testing many different
sun-shaped,
and eight-shaped.
However,
the polymers involved in the above examples are mainly
polydienes, polycaprolactone (PCL), polystyrene, poly(2-
vinylpyridine), and poly(ethylene oxide). With respect to
polymethylene (PM), there are only a few reports on the
synthesis of homocyclic PM.
Polymethylene (equivalent to polyethylene, PE) is the most
widely used polymer because of its low cost, excellent physical
properties, and recyclability. However, there are still some
limitations in its applications in many areas. For example, poor
adhesion and incompatibility have been found between PM
and polar polymers; to overcome these disadvantages, it is very
important to develop synthetic strategies toward novel PM-
21−23
24
25
26−30
based polymeric materials. Shea et al.
have recently
reported a new living method, namely, polyhomologation (or
C1 polymerization), which is a powerful tool for the synthesis
Received: December 5, 2019
31−45
of well-defined functionalized linear polymethylenes.
In
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.macromol.9b02571
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Scheme 1. (a) Synthesis of Single Cyclic and Tadpole-Shaped Copolymers Starting from 9-anthracene-methyl allyl ether
(previous work); (b) Synthesis of Eight-Shaped Cyclic Copolymer Starting from 1,4-pentadien-3-ol (This Work)
synthetic strategies, one of which is given in the Supporting
Information.
combined organic extracts were washed with brine, dried over
MgSO , filtered, and concentrated to give the crude 1,4-pentadien-3-
4
ol.
DCC (5.8 g, 28 mmol), DMAP (0.34 g, 2.8 mmol), and 2,2,5-
trimethyl-1,3-dioxane-5-carboxylic acid (4.9 g, 28 mmol) were added
to a solution of crude 1,4-pentadien-3-ol (2.1 g, ∼25 mmol) in THF
EXPERIMENTAL SECTION
Materials. Ethyl formate (97%), vinylmagnesium chloride solution
1.6 M in tetrahydrofuran (THF)), copper(I) bromide (CuBr,
■
9
2
(
(
(
50 mL) at 0 °C under Ar. The mixture was stirred for 14 h at room
9.999%), borane dimethylsulfide complex solution (1.0 M in THF),
temperature, followed by washing with brine. The organic extracts
t
,3-dimethylbut-2-ene (≥99%), Bu-P (2.0 M in THF), sodium azide
2
were dried over MgSO , filtered, and concentrated. The residue was
4
NaN , 99.5%), 2,2,5-trimethyl-1,3-dioxane-5-carboxylic acid (97%),
3
purified by chromatography with silica gel (ethyl acetate/petroleum
amberlyst-15(wet) ion-exchange resin, 4-(dimethylamino)pyridine
DMAP, ≥99%), trimethylamine N-oxide dihydrate (TAO·2H O,
ether = 1:4) to give a colorless oil as the product (6.2 g, 52% yield).
(
≥
1
2
H NMR (CDCl , 500 MHz, tetramethylsilane (TMS)): δ 1.24 (3H,
3
99%), and N,N′-dicyclohexylcarbodiimide (DCC, 99.8%) were
s), 1.41 (3H, s), 1.45 (3H, s), 3.68 (2H, d, J = 10.0 Hz), 4.24 (2H, d, J
purchased from Aldrich and used as received. N,N,N′,N″,N″-
=
10.0 Hz), 5.25 (2H, d, J = 10.0 Hz), 5.36 (2H, d, J = 10.0 Hz), 5.78
Pentamethyldiethylenetriamine (PMDETA, 99%, Aldrich) and ε-
(
(
2
dd, J = 10.0 Hz, 10.0 Hz), 5.83-5.90 (2H, m); mass spectrometry
MS) (electrospray ionization (ESI)) calcd. for C H O : [M + H]
41.1434; found: 241.1437.
caprolactone (CL, ≥99%) were distilled over calcium hydride (CaH )
2
+
1
3
21
4
under reduced pressure before use. Toluene was refluxed over
sodium/benzophenone and distilled under nitrogen atmosphere just
before use. Dimethylsulfoxonium methylide was prepared according
to Corey’s method followed by switching the solvent from THF to
Synthesis of (PM-OH) (Scheme 1). A typical polyhomologation
2
procedure is as follows. To a solution of 1,4-pentadien-3-yl 2,2,5-
trimethyl-1,3-dioxane-5-carboxylate (1.08 g, 4.5 mmol) in THF (4.0
41,56
toluene.
53
mL), freshly prepared thexylborane (4.7 mL, ∼0.89 M) was added
dropwise at 0 °C under Ar, and the mixture was kept under stirring
for 4 h at room temperature to give the organoborane solution.
To a 100 mL Schlenk flask charged with methylide solution (30
mL, 1.0 M in toluene), 2.0 mL of the above-prepared organoborane
solution was added at 65 °C under Ar. After stirring at 65 °C for 10
min, 0.10 mL of the reaction solution was taken out and added to
water containing phenolphthalein. The neutral solution (pH
indicator) indicates the consumption of the ylide. Then, TAO·
Measurements. The high-temperature gel permeation chroma-
tography (HT-GPC) measurements were carried out with Viscoteck
50 instrument equipped with two PLgel 10 μm MIXED-B columns
3
and 1,2,4-trichlorobenzene (TCB) as the eluent at a flow rate of 0.8
1
mL/min at 150 °C. The system was calibrated by PS standards. H
and 13C NMR spectra were recorded with a Bruker AVANCE III-400,
5
00, or 600 spectrometer, and Fourier transform infrared (FT-IR)
spectra with a NICOLET iS10 FT-IR instrument. Differential
scanning calorimetry (DSC) measurements were performed using a
Mettler Toledo DSC1/TC100 system under an inert atmosphere
2H O (0.20 g) was added to the reaction mixture. After stirring at 90
2
°
C for 4 h, the mixtures were poured into cold methanol (300 mL).
(
°
nitrogen). The sample was heated from room temperature to 150
The white solids were filtered, dried under vacuum, and characterized
C, cooled to −40 °C, and finally heated again to 150 °C with a
1
by H NMR and HT-GPC [(PM -OH) , M = 3400, PDI =
n, NMR
heating/cooling rate of 10 °C/min. The second heating curve was
110
2
1
.36].
used to determine the melting temperature (T ). Dynamic light
m
Synthesis of (PM-alkyne)2 (Scheme 2). A typical synthesis
scattering (DLS) measurements were carried out with a Malvern
Zetasizer Nano ZS equipped with a TurboCorr correlator. The light
source was a 30 mW He−Ne laser emitting a vertically polarized light
of 632.8 nm wavelength.
Synthesis of 1,4-Pentadien-3-yl 2,2,5-trimethyl-1,3-diox-
ane-5-carboxylate (Scheme 1). Ethyl formate (4.0 mL, 50
mmol) was added dropwise to a solution of vinylmagnesium chloride
solution (63 mL, 100 mmol, 1.6 M in THF) in dry THF (30 mL) at 0
procedure is as follows. A solution of (PM110-OH) (1.36 g, 0.40
2
mmol), 4-pentynoic acid (400 mg, 4.0 mmol), N,N″-dicyclohex-
ylcarbodiimide (DCC, 824 mg, 4.0 mmol), and N,N-dimethylpyridin-
4
-amine (DMAP, 49 mg, 0.40 mmol) in toluene (20 mL) was stirred
at 90 °C overnight. The solution was cooled down, and the mixture
was concentrated by a rotory evaporator and precipitated in methanol.
The resulting white solid was filtered, dried under vacuum, and
1
°
C under argon (Ar). After completion of the addition, the mixture
was kept under stirring for further 1 h at room temperature. Then, the
characterized by H NMR and GPC [(PM110-alkyne)
2
, Mn, NMR =
3600, PDI = 1.36].
reaction was quenched with a saturated NH Cl solution (30 mL) and
the mixture was extracted with ethyl acetate (20 mL × 3). The
Synthesis of (PM-alkyne) -(PCL-OH) (Scheme 2). A typical
2 2
4
synthesis procedure is as follows. A solution of (PM110-alkyne) (1.08
2
B
DOI: 10.1021/acs.macromol.9b02571
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g, 0.30 mmol) and Amberlyst-15 (wet) ion-exchange resin (300 mg)
in toluene (20 mL) was stirred at 90 °C overnight. The solution was
cooled down, and the mixture was concentrated by a rotory
evaporator and precipitated in methanol. The white solid was filtered
and dried under vacuum to give (PM -alkyne) -(OH) .
Đ(M /M ) = 1.36/1.27). All HT-GPC traces were mono-
modal with narrow molecular weight distributions (Figure 1).
w
n
The proof of the successful synthesis of (PM-OH) came
2
1
from H NMR spectra. As shown in Figure 2a,b, the
1
10
2
2
t
characteristic chemical shift due to vinyl protons in 1,4-
pentadiene-3-yl 2,2,5-trimethyl-1,3-dioxane-5-carboxylate at δ
Bu−P (0.625 mL, 2 M, 1.25 mmol) was added to the solution of
10 2 2
2
(
PM1 -alkyne) -(OH) (900 mg, 0.25 mmol) in toluene (10 mL),
=
5.9-5.8 ppm (m, H ), δ = 5.3, 5.4 ppm (d, H ) disappeared
then caprolactone (5.70 g, 50 mmol) was added to the mixture. The
b
a
resulting solution was stirred in a preheated oil bath at 90 °C for 14 h.
after the hydroboration/polyhomologation/oxidation/hydrol-
Then, the reaction was quenched by adding CH COOH/MeOH (10
ysis reaction, and a new peak at δ = 3.65 ppm of the −CH −
3
2
vol %) and cooled to room temperature. The mixture was
concentrated by a rotory evaporator and precipitated in methanol.
The white solid was filtered, dried under vacuum, and characterized
connected to the hydroxyl group (H ) appeared, an indication
a′
of polymerization initiation from the two vinyl groups
initiating sites. The signals (H ) at δ = 3.65 and 4.21 ppm,
1
e
by H NMR and GPC [(PM -alkyne) -(PCL -OH) , 2.2 g, M
=
1
10
2
15
2
n, NMR
assigned to protons of the −CH − from the dioxane group,
3
2
7.0 × 10 , PDI = 1.39].
Synthesis of (PM-alkyne) -(PCL-N ) (Scheme 2). A typical
indicate that the 2,2,5-trimethyl-1,3-dioxane-5-carboxylate
group remained intact during the polyhomologation. The
observed integration area ratio of aliphatic protons at δ = 1.1−
1.5 ppm (Hb&g, two protons for each methylene unit, two PM
2
3 2
synthesis procedure is as follows. A solution of (PM 10-alkyne)2-
1
57
(
PCL -OH) (1.16 g, 0.10 mmol), 2-azidoacetic acid (200 mg, 2.0
15 2
mmol), DCC (412 mg, 2.0 mmol), and DMAP (25 mg, 0.20 mmol)
in toluene (10 mL) was stirred at 90 °C overnight. The solution was
cooled down, and the mixture was concentrated by a rotory
evaporator and precipitated in methanol. The resulting white solid
chains) [Sb&g/4H] to that of protons assigned to −CH −
2
connected to the hydroxyl group and −CH − from the
2
dioxane group at δ = 3.65 ppm (H , four protons from two
a′&e
PM chains and two protons from the dioxane group) [Sa′&e
1
was filtered, dried under vacuum, and characterized by H NMR and
/
3
GPC [(PM110-alkyne) -(PCL -N ) , M
= 7.1 × 10 , PDI =
2
15
3
2
n, NMR
6
H] was derived by the following formula: m = (S /4H):
b,g
1.39].
(
Sa′,e/6H), which is the degree of polymerization of each PM
Synthesis of [c-(PM-b-PCL)] (Scheme 2). A typical synthesis
2
chain. This integrated ratio was used to determine the number-
average molecular weight of the macroinitiator (PM-OH)2.
procedure is as follows. Copper(I) bromide (CuBr, 29 mg, 0.2 mmol)
and PMDETA (35 mg, 0.20 mmol) were dissolved in toluene (250
mL). The mixture was degassed by three freeze−pump−thaw (FTP)
Synthesis and Characterization of 4-Miktoarm Star
cycles and then put in a preheated oil bath at 90 °C; then, (PM
-
Copolymers, (PM-alkyne) -(PCL-N ) . The 4-miktoarm star
1
10
2
3 2
alkyne) -(PCL -N ) (116 mg, 0.01 mmol) in toluene (150 mL) was
2
15
3
2
copolymer, (PM-alkyne) -(PCL-N ) , precursor of eight-
2 3 2
added to the CuBr/PMDETA solution via a peristaltic pump at a rate
of 1.4 mL/h. After completion of the addition, the reaction solution
was concentrated, purified by passing through a neutral alumina
column using toluene as eluent to remove the copper, and
precipitated in methanol three times. The white solid was filtered,
shaped cyclic, was synthesized according to the following
general steps (Scheme 2): (a) introduction of an alkynyl group
on both PM chain ends to get (PM-alkyne) ; (b) removal of
2
the OH-protective group to afford (PM-alkyne) -(OH) ; (c)
2
2
1
ROP of ε-caprolactone with the difunctional initiator to obtain
the 4-arm star copolymers (PM-alkyne) -(PCL-OH) ; and (d)
introduction of azido groups on both PCL chain ends to afford
dried under vacuum, and characterized by H NMR and GPC. {[(c-
3
(
PM110-b-PCL ))] , M
= 7.1 × 10 , PDI = 1.39}.
2
2
15
2
n, NMR
(
PM-alkyne) -(PCL-N ) .
2 3 2
RESULTS AND DISCUSSION
■
1
And more specifically, the −OH groups of the synthesized
As shown in Scheme 1B, 1,4-pentadien-3-ol was converted to
,4-pentadien-3-yl 2,2,5-trimethyl-1,3-dioxane-5-carboxylate by
(
PM-OH) was reacted with 4-pentynoic acid, in the presence
2
of DMAP and DCC, to give (PM-alkyne) . To ensure the
2
esterification with 2,2,5-trimethyl-1,3-dioxane-5-carboxylic
acid. The cyclic hydroboration of 1,4-pentadiene-3-yl 2,2,5-
trimethyl-1,3-dioxane-5-carboxylate with thexylborane afforded
the B-thexyl-boracyclic compound, which was used as initiator
for polyhomologation of dimethylsulfoxonium methylide in the
complete esterification of −OH groups, a 10-fold excess of 4-
1
pentynoic acid was used. The H NMR spectrum of polymer
(
(
PM-alkyne) is shown in Figure 3a. A comparison to that of
2
PM-OH) (Figure 2b) shows that the signal of the methylene
2
protons adjacent to the −OH groups shifted from δ = 3.67
ppm (a′) to 4.14 ppm (a″), and new signals (g), (h), and (i)
appeared at δ = 2.02 and 2.56 ppm, attributed to the terminal
proton of the alkyne functional group and to the methylene
protons adjacent to the triple bond, respectively. The
esterification was further verified by FT-IR analysis. A
comparison of the spectrum of (PM-alkyne)2 to that of
next step. After oxidation-hydrolysis with TAO·2H O, 2,2,5-
2
trimethyl-1,3-dioxane-functionalized α,ω-dihydroxyl poly-
methylene (PM-OH)2 was obtained. Two samples with
different molecular weights were synthesized, and their
molecular characteristics are listed in Table 1. The molecular
1
weights were determined by the H NMR end-group analysis
3
(
Mn, NMR = 3.4/11.3 × 10 ), and the polydispersity index by
(PM-OH) , shows that a characteristic signal of alkyne groups
2
HT-GPC with polystyrene standards (polydispersity index:
−1
appeared at 3314 cm (Figure 4b).
For polymer (PM-alkyne) , the protective group was cleaved
2
Table 1. Molecular Characteristics of (PM-OH)2
under mildly acidic conditions to give two free −OH groups in
the middle of the polymethylene chain. The two −OH groups
a
b
a
b
entry
Sample
Mn, NMR
Mn, GPC.
DP
Đ
were utilized to initiate the ROP of ε-caprolactone with the
1
2
(PM110-OH)2
(PM396-OH)2
3400
11 300
4500
12 000
110
396
1.36
1.27
t
phosphazene superbase Bu-P as a catalyst in toluene at 90 °C.
2
1
The successful ROP was confirmed by H NMR, FT-IR, and
HT-GPC techniques. Comparing the H NMR spectra of
1
a
1
Mn, NMR and DP were calculated from H NMR spectra (600 MHz,
C D Cl , 90 °C) using the area ratio of protons of terminal CH OH
(
PM-alkyne) -(PCL-OH) with the corresponding (PM-
2 2
2
2
4
2
b
alkyne) (Figure 3b,a), the fingerprints of PCL block appeared
at δ = ∼3.7 ppm to the backbone. Đ(M /M ) and Mn, GPC,
2
w
n
determined by HT-GPC (1,2,4-trichlorobenzene, 150 °C, PS
at δ = 1.40, 1.71, 2.34, and 4.12 ppm. Also, the signal of the
methylene protons adjacent to the −OH groups from the end
standards).
C
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Figure 1. GPC traces of linear PMs and the corresponding 4-miktoarm star copolymers (TCB at 150 °C).
1
Figure 2. H NMR spectra of (a) 1,4-pentadien-3-yl 2,2,5-trimethyl-1,3-dioxane-5-carboxylate in chloroform-d at 25 °C and (b) (PM -OH) in
1
10
2
C D Cl at 90 °C.
2
2
4
of the PCL chain appeared at δ = 3.67 ppm (j). In the FT-IR
Finally, the 4-miktoarm star copolymer (PM-alkyne) -(PCL-
2
spectrum of (PM-alkyne) -(PCL-OH) (Figure 4c), a strong
N ) was obtained by esterification of the −OH groups of the
2
2
3 2
−
1
band at 1723 cm , attributed to the polycaprolactone,
appeared after ROP. The GPC curves in Figure 1a,b show
the variation of molecular weights and molecular weight
distributions of the 4-miktoarm star copolymers prepared from
the linear PM precursors. A slight shift of the elution peak of
PCL chain ends with 2-azidoacetic acid. This signal (j) of the
methylene protons adjacent to the −OH groups shifted to δ =
4.12 ppm (j′) after esterification. At the same time, a new
signal (k) appeared at δ = 3.89 ppm, attributed to the
methylene protons adjacent to the azido group (Figure 3c).
From the FT-IR spectra (Figure 4d), the characteristic
absorption of the azido group around 2110 cm− can be
observed.
(
PM110-alkyne) -(PCL -OH) to the lower-molecular-weight
2 15 2
1
side, compared to the linear PM precursor (PM110-OH) , was
2
observed. In the case of (PM396-alkyne) -(PCL -OH) , a shift
2
8
2
to a slightly higher-molecular-weight side occurred, compared
to the corresponding linear one. There was no significant shift
between the GPC traces of 4-miktoarm copolymers and the
linear precursors, and this should be attributed to the branched
Synthesis and Characterization of Eight-Shaped
Copolymers [c-(PM-b-PCL)] . The intramolecular cyclization
2
by “click” reaction was carried out by a slow and continuous
addition of the star copolymer (PM-alkyne) -(PCL-N )
2
3 2
58
structure of the star polymer. The hydrodynamic volume
increases with molecular weight and decrease when the
architecture is changed from the linear (2-arm star) to 4-arm
star (same-arm molecular weight). Therefore, it does not
change much in this case. Similar results were also observed in
solution into the catalyst solution. To avoid the formation of
a byproduct from intermolecular condensation, cyclization was
carried out under high-dilution conditions. The GPC traces
and molecular characteristics of the precursors and the final
products are shown in Figure 5 and Table 2. M
determined by the H NMR end-group analysis; M
was
and
n, NMR
53
1
our previous work.
n, GPC
D
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Scheme 2. Synthesis of [c-(PM-b-PCL)] from (PM-OH)2
2
Đ(M /M ) were determined by HT-GPC with polystyrene
As shown in Figure 3d, a comparison with the corresponding
(PM-alkyne) -(PCL-N ) (Figure 3c) shows that the signal (i)
w
n
standards (Table 2). The hydrodynamic volume of cyclic
polymer is smaller than that of their linear precursors, and this
feature could be described by using R, the ratio of the peak
value of molecular weight (Mp, GPC) of the cyclic polymer to
2
3 2
belonging to the terminal proton of the alkyne disappeared,
and a new signal at 7.42 ppm (i′) appeared, assigned to proton
of triazole ring formed from the alkyne and azido groups. A
new signal (g′+h′) between 2.90 and 3.10 ppm also appeared,
due to the transformation of alkyne to triazole, similar to our
59
that of the linear precursor. The R values are 0.90 and 0.91,
respectively. Comparing the GPC traces in Figure 5, it is
obvious that GPC trace of [c-(PM110-b-PCL )] shifted to
23
previous report. And there is still a weak signal at 2.56 ppm
assigned to the methylene protons adjacent to the triple bond,
meaning that not all of the alkyne group was transferred to
triazole. This might be from the trace amount of unreacted 4-
arm precursor or the intermolecular condensation byproduct.
It is very difficult to separate the unreacted or bimolecular
byproduct from the eight-shaped product because of its poor
solubility, but the high yield of the eight-shaped cyclic product
is still confirmed by comparing the integration area of these
two peaks (g+h, g′+h′) in Figure 3d (see details in Figure S6,
ESI†). From the FT-IR spectra (Figure 4e), it could be
observed that the characteristic absorption of the azido group
15
2
longer elution time, meaning smaller hydrodynamic volume
compared to the star precursor. For the GPC trace of [c-
(
PM396-b-PCL )] , there is a rather big shoulder observed,
8 2
indicating the existing of byproduct. This byproduct is possibly
due to the intermolecular click reaction (linear polyconden-
sates or bigger cyclics). The increasing distance of the terminal
groups of polymer chain favors rather intermolecular than
intramolecular reaction. The Đ(M /M ) values of the cyclic
w
n
products are 1.35 and 1.39 (Table 2). All cyclic polymers were
characterized by 1H NMR and FT-IR spectroscopy after
precipitation in methanol and drying.
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1
Figure 3. H NMR spectra of (a) (PM -alkyne) , (b) (PM -alkyne) -(PCL -OH) , (c) (PM -alkyne) -(PCL -N ) , and (d) [c-(PM396-b-
3
96
2
396
2
8
2
396
2
8
3 2
PCL )] in C D Cl at 90 °C.
8
2
2
2
4
the corresponding cyclic polymers. Unfortunately, due to the
weak signals of the light scattering and viscometer responses
(
Figure S3, Supporting Information), it was impossible to
extract conclusions.
Actually, we first tried to synthesize this PM-based eight-
shaped copolymer according to another strategy (Scheme S1;
see details in the Supporting Information) from a linear
precursor with two alkyne groups at the chain ends and two
azido groups in the middle of the chain, followed by
1
intramolecular click chemistry. The H NMR results showed
the absence of cyclization due to the steric hindrance of azido
groups in the middle of the chain, so we improve the strategy
by using 4-miktoarm star-type precursor instead of the linear
one.
The thermal behavior of the cyclic copolymers and their
corresponding precursors, studied by differential scanning
calorimetry (DSC), are shown in Figure 6. The 4-miktoarm
star copolymer (PM110-alkyne) -(PCL -N ) with M = 7100
2
15
3 2
n
Figure 4. FT-IR spectra of (a) (PM110-OH) , (b) (PM110-alkyne)2,
2
exhibited two relatively melting peaks at 106.5 and 44.6 °C,
attributed to the PM and PCL blocks, respectively (Figure 6a).
For the corresponding cyclic [c-(PM110-b-PCL )] , the
(
c) (PM110-alkyne) -(PCL -OH) , (d) (PM -alkyne) -(PCL -
N ) , and (e) [c-(PM -b-PCL )] .
2 15 2 110 2 15
3
2
110
15
2
15
2
melting point (T ) of PM slightly decreased (100.8 °C)
m
around 2110 cm−1 and the alkyne group around 3314 cm−1
disappeared after click cyclization. From all of the above, it can
be concluded that the intramolecular cyclization was
successful.
while the T of PCL slightly increased to 45.3 °C (Figure 6b).
m
A similar trend was observed in the case of [c-(PM396-b-
PCL )] (Figure 6d) compared to the corresponding
8
2
precursors (Figure 6c), at significantly lower melting temper-
atures of the PM block. The melting temperatures of the PCL
block were present with a very small peak at around 37 °C,
Triple-detection GPC (refractometry, light scattering at λ =
6
70 nm and viscometry) was also used to compare the linear to
F
DOI: 10.1021/acs.macromol.9b02571
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Figure 5. GPC traces of eight-shaped cyclic copolymers and the corresponding 4-miktoarm star precursors (TCB at 150 °C).
Table 2. Molecular Characteristics of 4-Miktoarm Star
Copolymers (PM-alkyne) -(PCL-N ) and the
Corresponding Eight-Shaped Cyclic Products [c-(PM-b-
this case, DMF was chosen as a selective solvent for PCL, and
therefore, the DMF-phobic PM block could form the core.
Solutions of [c-(PM110-b-PCL )] and their corresponding
2
3 2
15
2
PCL)]2
precursors (PM110-alkyne) -(PCL -N ) in DMF were pre-
2 15 3 2
pared with the same concentration. The size distribution of the
formed nanoparticles in all DMF solutions was measured by
dynamic light scattering (DLS) at room temperature (Figure
7). The size of the nanoparticles formed by [c-(PM110-b-
a
b
b
c
entry
1
sample
Mn, NMR Mp, GPC
Đ
R
(PM110-alkyne) -(PCL -N )
7100
7100
5400
1.39
1.39
1.26
1.35
0.91
0.90
2
15
3
2
[
c-(PM110-b-PCL )]
4900
13 100
11 900
15
2
2
(PM396-alkyne) -(PCL -N )
13 400
13 400
2
8
3
2
[
c-(PM396-b-PCL )]
8
2
a
1
Mn, NMR was calculated from H NMR spectra (600 MHz, C D Cl )
2
2
4
b
at 90 °C. Đ(M /M ) and M
, determined by HT-GPC (1,2,4-
w
n
p, GPC
c
trichlorobenzene, 150 °C, PS standards). R is the ratio of the M
p, GPC
of cyclic polymers to their linear precursors.
Figure 7. Size distribution of the formed micelles in DMF: (a)
(
PM110-alkyne) -(PCL -N ) and (b) [c-(PM -b-PCL )] .
2 15 3 2 110 15 2
PCL )] was smaller than that of (PM -alkyne) -(PCL -
1
5
2
110
2
15
N ) (Figure 7: 414 nm for a; 251 nm for b). The smaller peak
3
2
Figure 6. DSC traces for copolymers: (a) (PM110-alkyne) -(PCL -
in Figure 7b around 25 nm must be contributed to the
unimolecular chain which does not self-assemble. The decrease
of the micelle size may be attributed to the formed cyclic
topology, which will restrict the aggregation of polymethylene
2
15
N ) ; (b) [c-(PM -b-PCL )] ; (c) (PM -alkyne) -(PCL -N ) ;
3
2
110
15
2
396
2
8
3 2
and (d) [c-(PM396-b-PCL )] .
8
2
probably due to its low composition. The more complex the
block. The PDI of the (PM110-alkyne) -(PCL15-N ) solution
is 0.048 according to the DLS, indicating a monodisperse
particle system. A higher value of 0.706 was observed for [c-
2 3 2
structure, the higher the influence on T and X , due to the
m
c
reduced mobility of the chains.
(
PM110-b-PCL )] solutions because of the byproducts and
Finally, the self-assembly behaviors of (PM110-alkyne)2-
15 2
the unimolecular chains.
(
PCL -N ) and [c-(PM110-b-PCL )]2 copolymers, as an
1
5
3 2
15
example, were investigated and compared. PM-based block
copolymers in selective solvents for one block self-assemble to
form micellar structures. The morphology and size of the self-
assembled micelles are affected by different architectures. In
CONCLUSIONS
A novel synthetic strategy toward well-defined eight-shaped
cyclic block copolymers consisting of PM and PCL blocks was
■
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DOI: 10.1021/acs.macromol.9b02571
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developed by combining the novel thexylborane chemistry
with polyhomologation, ROP, and click reaction. 1,4-
Pentadien-3-yl 2,2,5-trimethyl-1,3-dioxane-5-carboxylate
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which was used for the polyhomologation of dimethylsulfoxo-
nium methylide to give α,ω-dihydroxyl polymethylene.
Subsequent esterification, deprotection, and ROP resulted in
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-miktoarm star copolymers, followed by intramolecular click
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1
(
1
by HT-GPC, H NMR, and FT-IR spectroscopy. The
reversible linear-cyclic topological conversion of telechelic polymers
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difference in the thermal properties and the self-assembly
behavior was observed by DSC and DLS, also indicating the
successful synthesis of eight-shaped cyclic block copolymers.
This general strategy expands the applicability of the
difunctional borane initiator and opens new horizons toward
PM/cyclic-based complex polymers.
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Construction of double-eight and double-trefoil polymer topologies
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.macromol.9b02571.
■
4
(
7, 8214−8223.
*
S
12) Satoh, Y.; Matsuno, H.; Yamamoto, T.; Tajima, K.; Isono, T.;
Satoh, T. Synthesis of well-defined three-and four-armed cage-shaped
polymers via “topological conversion” from trefoil-and quatrefoil-
shaped polymers. Macromolecules 2017, 50, 97−106.
1H NMR spectra (Figures S1 and S2); triple-detection
GPC traces (Figure S3); FT-IR spectra (Figure S4);
NMR spectra (Figures S5 and S6); and synthetic
(13) Zhang, F.; Quirk, R. P.; Gerislioglu, S.; Wesdemiotis, C.;
Bekele, S.; Tsige, M.; Koh, Y. P.; Simon, S. L.; Foster, M. D. Synthesis
and Characterization of Well-Defined, Tadpole-Shaped Polystyrene
with a Single Atom Junction Point. Macromolecules 2018, 51, 9509−
strategy of [c-(PM-b-PCL)] (Scheme S1) (PDF)
2
9
518.
AUTHOR INFORMATION
Corresponding Author
E-mail: nikolaos.hadjichristidis@kaust.edu.sa. Tel: +966-(0)
2-8080789.
■
(14) Dong, Y.-Q.; Tong, Y.-Y.; Dong, B.-T.; Du, F.-S.; Li, Z.-C.
Preparation of tadpole-shaped amphiphilic cyclic PS-b-linear PEO via
ATRP and click chemistry. Macromolecules 2009, 42, 2940−2948.
*
(
15) Li, H.; Riva, R.; Jerome, R.; Lecomte, P. Combination of ring-
́ ̂
1
opening polymerization and “click” chemistry for the synthesis of an
amphiphilic tadpole-shaped poly (ε-Caprolactone) grafted by PEO.
Macromolecules 2007, 40, 824−831.
ORCID
Nikos Hadjichristidis: 0000-0003-1442-1714
Present Address
Key Laboratory of Catalysis and Materials Science of the State
Ethnic Affairs Commission & Ministry of Education, South-
Central University for Nationalities, Wuhan, Hubei Province
30074, China
Notes
(16) Fan, X.; Wang, G.; Huang, J. Synthesis of macrocyclic
molecular brushes with amphiphilic block copolymers as side chains.
J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 1361−1367.
†
(17) Xia, Y.; Boydston, A. J.; Grubbs, R. H. Synthesis and direct
imaging of ultrahigh molecular weight cyclic brush polymers. Angew.
Chem., Int. Ed. 2011, 50, 5882−5885.
4
(18) Boydston, A. J.; Holcombe, T. W.; Unruh, D. A.; Frechet, J. M.;
́
The authors declare no competing financial interest.
Grubbs, R. H. A direct route to cyclic organic nanostructures via ring-
expansion metathesis polymerization of a dendronized macro-
monomer. J. Am. Chem. Soc. 2009, 131, 5388−5389.
ACKNOWLEDGMENTS
Research reported in this publication was supported by King
Abdullah University of Science and Technology (KAUST).
■
(19) Isono, T.; Satoh, Y.; Miyachi, K.; Chen, Y.; Sato, S.-i.; Tajima,
K.; Satoh, T.; Kakuchi, T. Synthesis of linear, cyclic, figure-eight-
shaped, and tadpole-shaped amphiphilic block copolyethers via t-Bu-
P4-catalyzed ring-opening polymerization of hydrophilic and hydro-
phobic glycidyl ethers. Macromolecules 2014, 47, 2853−2863.
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