Segmented block copolyesters using click chemistry

Article abstract of DOI:10.1002/pola.26168

Copper(I) catalyzed azide-alkyne 1,3-Huisgen cycloaddition reaction afforded the synthesis of triazole-containing polyesters and segmented block copolyesters at moderate temperatures. Triazole-containing homopolyesters exhibited significantly increased (～40 °C) glass transition temperatures (T_g) relative to high temperature, melt synthesis of polyesters with analogous structures. Quantitative synthesis of azido-terminated poly(propylene glycol) (PPG) allowed for the preparation of segmented polyesters, which exhibited increased solubility and mechanical ductility relative to triazole-containing homopolyesters. Differential scanning calorimetry demonstrated a soft segment (SS) T_g near -60 °C for the segmented polyesters, consistent with microphase separation. Tensile testing revealed Young's moduli ranging from 7 to 133 MPa as a function of hard segment (HS) content, and stress at break values approached 10 MPa for 50 wt % HS segmented click polyesters. Dynamic mechanical analysis demonstrated an increased rubbery plateau modulus with increased HS content, and the T_g's of both the SS and HS did not vary with composition, confirming microphase separation. Atomic force microscopy also indicated microphase separated and semicrystalline morphologies for the segmented click polyesters. This is the first report detailing the preparation of segmented copolyesters using click chemistry for the formation of ductile membranes with excellent thermomechanical response.

Full text of DOI:10.1002/pola.26168

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Segmented Block Copolyesters Using Click Chemistry
Stephen M. June, Philippe Bissel, Timothy E. Long
Department of Chemistry, Macromolecules and Interfaces Institute, Virginia Tech, Blacksburg, Virginia 24061-0344
Correspondence to: T. E. Long (E-mail: telong@vt.edu)
Received 15 March 2012; accepted 16 April 2012; published online
DOI: 10.1002/pola.26168
ABSTRACT: Copper(I) catalyzed azide-alkyne 1,3-Huisgen cyclo-
addition reaction afforded the synthesis of triazole-containing
polyesters and segmented block copolyesters at moderate tem-
peratures. Triazole-containing homopolyesters exhibited signifi-
cantly increased (ꢀ40 ꢁC) glass transition temperatures (Tg)
relative to high temperature, melt synthesis of polyesters with
analogous structures. Quantitative synthesis of azido-terminated
poly(propylene glycol) (PPG) allowed for the preparation of seg-
mented polyesters, which exhibited increased solubility and me-
chanical ductility relative to triazole-containing homopolyesters.
Differential scanning calorimetry demonstrated a soft segment
(SS) Tg near ꢂ60 ꢁC for the segmented polyesters, consistent
with microphase separation. Tensile testing revealed Young’s
moduli ranging from 7 to 133 MPa as a function of hard segment
(HS) content, and stress at break values approached 10 MPa for
50 wt % HS segmented click polyesters. Dynamic mechanical
analysis demonstrated an increased rubbery plateau modulus
with increased HS content, and the Tg’s of both the SS and HS
did not vary with composition, confirming microphase separa-
tion. Atomic force microscopy also indicated microphase sepa-
rated and semicrystalline morphologies for the segmented click
polyesters. This is the first report detailing the preparation of
segmented copolyesters using click chemistry for the formation
of ductile membranes with excellent thermomechanical
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2012 Wiley Periodicals, Inc. J Polym Sci Part A:
Polym Chem 000: 000–000, 2012
KEYWORDS: atomic force microscopy (AFM); block copolymers;
click chemistry; click polyesters; CuAAC; polyesters; segmented
block copolymers; step-growth polymerization; triazole
INTRODUCTION Polyesters, which Wallace Carothers pio-
neered in the 1930s,1 enable a wide range of technologies,
including fibers, beverage containers, liquid crystalline struc-
tural materials, and coatings. The primary synthetic strategy
for polyesters involves the melt polycondensation of a glycol
and either a dicarboxylic acid or a dimethyl ester. These
polymerizations, which are typically performed in the melt,
require extremely high temperatures (>250 ꢁC) and metal
catalysts to obtain high conversion and molecular weight. In
addition, most common polyesters exhibit glass transition
temperatures (Tg’s) below 110 ꢁC.2 Utilization of a lower
temperature polyaddition methodology may offer advantages
over traditional melt polyesterification. Segmented block
copolymers also offer many advantages over classical ther-
moplastic high performance random copolymers. Addition of
an alternating flexible soft segment (SS) often offers
increased solubility compared to the homopolymer of a hard
segment (HS). In addition, segmented block copolymers typi-
cally exhibit microphase separation, often leading to elasto-
meric properties. Classical examples of segmented block
copolymers include segmented polyurethanes, poly(siloxane
imides), and poly(urethane ureas). Our research group has
recently reported the many attributes of segmented block
copolymers.3–5 In addition, our group maintains a significant
interest in polyester design,6,7 and the synthesis of high per-
formance segmented block copolymers using step-growth
polymerization.8–10
Copper(I) catalyzed azide-alkyne 1,3-Huisgen cycloaddition
(CuAAC) offers many advantages in the synthesis of poly-
meric materials. Sharpless and coworkers first described
CuAAC as click chemistry in the early 2000s.11 This reaction
proceeds at relatively low reaction temperatures and in a
wide variety of solvents, and in particular, click reactions ex-
hibit remarkable tolerance to the presence of other func-
tional groups. In addition, the reaction proceeds quantita-
tively in the absence of reaction byproducts.12–14 For these
reasons, click chemistry has featured prominently in many
polymer publications in recent years as a convenient route
toward monomer synthesis,15 polymer coupling,16–22 post-
polymerization pendant functionalization,17,23–25 and the
synthesis of star and graft copolymers.20,26 Relatively few
examples exist where azido-alkyne coupling provides the
principal polymerization step. For example, due to quantita-
tive reactions, azido-alkyne coupling regularly finds utility in
the synthesis of dendrimers.14,26–32 Voit and coworkers first
reported step-growth polyaddition using click chemistry in
the synthesis of hyperbranched polytriazoles, although the
Additional Supporting Information may be found in the online version of this article.
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final product exhibited insignificant solubility.33 In 2008, Shi
and coworkers described the first soluble, hyperbranched tri-
azole-containing polymer.34,35
lane. 13C NMR spectra are included in the supplemental
information.
Thermogravimetric analysis (TGA) was conducted on a TA
Instruments Hi-Res TGA 2950 at a temperature ramp rate of
10 ꢁC /min under a N2 atmosphere. Tg’s were reported from
the second heat of a heat/cool/heat cycle (0–150 ꢁC) at a
The literature presents fewer examples of click chemistry as
the principal polymerization step toward linear polymers;
however, some key examples do exist.26,36,37 In 2005, Maty-
jaszewski and coworkers described the step-growth poly-
merization of telechelic azide- and alkyne-functionalized
macromonomers for the preparation of linear triazole-con-
taining polymers.37 Only two reports describe the synthesis
of ‘‘click polyesters’’, or polyesters where click chemistry acts
as the principal polymerization step. Takasu and coworkers
reported the synthesis of thermally stable linear polyesters
using click chemistry in 2009,38 and again in 2010.39 How-
ever, Takasu also reported limited solubility without thermo-
mechanical properties, and most polymer structures were
linear and aliphatic. The synthesis of aromatic or alicyclic
click polyesters and determination of their mechanical prop-
erties remains unprecedented.
ꢁ
ꢁ
heating rate of 10 C/min and a cooling rate of 50 C/min on
TA Instrument differential scanning calorimetry (DSC)
a
Q1000 under N2. Compression molding provided films for me-
ꢁ
chanical testing and all samples were melt pressed at 200 C
for approximately 2 min at ꢀ15,000 psi. All samples were
annealed at 150 ꢁC for a minimum of 18 h prior to use.
Dynamic mechanical analysis (DMA) was performed on a TA
Instruments DMA Q800 under an N2 atmosphere in tension
mode with strains of 15 um, frequencies of 1 Hz, and a ramp
ꢁ
rate of 3 C/min. Tensile testing was performed using rectan-
gular films on an Instron 5500R tensile frame with a cross-
head speed of 50 mm/min. AFM was performed in tapping
mode on a Veeco Multimode AFM using a low force constant
(5 N/m) tip.
In this article, we present the synthesis of polyesters and
segmented copolyesters using click chemistry. Comparison of
analogous polyesters, which did not contain triazole units in
the backbone, with triazole-containing polyesters revealed a
dramatic increase in glass transition temperature (Tg) upon
triazole incorporation; however, the click homopolyesters
polyesters exhibited poor mechanical ductility and solubility.
Utilization of azido-terminated poly(propylene glycol) tele-
chelic macromonomers allowed for the synthesis of seg-
mented click polyesters. The segmented click polyesters
exhibited solubility in a variety of solvents and formed
mechanically robust films, in sharp contrast to previous liter-
ature. Thermal and mechanical testing confirmed microphase
separation in ductile films, and atomic force microscopy
(AFM) further revealed microphase separated and semicrys-
talline morphologies.
Synthesis of Bis(2-bromoethyl)terephthalate (BrTP) (1)
The dropwise addition of 2-bromoethanol (14 mL, 200
mmol) in CH2Cl2 (50 mL) to a solution of terephthaloyl chlo-
ride (10 g, 49 mmol), and N(Et)3 (28 mL, 200 mmol) in
CH2Cl2 (100 mL) at 0 ꢁC followed by subsequent stirring at
ꢁ
23 C for 20 h afforded BrTP. Washing with 5% aqueous so-
lution of K2CO3 (3 ꢃ 50 mL), water (2 ꢃ 50 mL) and drying
over MgSO4 yielded 1 as a yellow solid. The crude product
was recrystallized from ethyl acetate to give pure 1 (13.05 g,
1
70%) as white crystals; H NMR (CDCl3, 500 MHz) d 3.26 (t,
4H), 4.26 (t, 4H), 7.45 (s, 4H).
Synthesis of Bis(2-azidoethyl)terephthalate (BATP) (2)
A mixture of 1 (10 g, 34 mmol), NaN3 (17.6 g, 270 mmol),
and TBAB (0.55 g, 1.7 mmol) in DMF (50 mL) was heated at
EXPERIMENTAL
ꢁ
80 C for 48 h. After cooling, a saturated aqueous solution of
Methods and Materials
LiBr (50 mL) was added and the mixture was extracted with
ethyl acetate (3 ꢃ 50 mL). The combined organic phases
were washed with water (3 ꢃ 50 mL), brine (50 mL), dried
over MgSO4 and the solvent removed under rotary evapora-
tion. The crude was recrystallized from ethyl acetate to give
2 (mp 81–83 ꢁC) (7.62 g, 59%) as white crystals; 1H NMR
(CDCl3, 500 MHz) d 3.62 (t, 4H), 4.52 (t, 4H), 8.14 (s, 4H).
Terephthaloyl chloride, dimethyl isophthalate, titanium(IV)
tetraisopropoxide, 2-bromoethanol, sodium azide, 1,4-butane-
diol, tetrabutylammonium bromide (TBAB), and diethylene
glycol (DEG) were purchased from Aldrich, isophthaloyl
chloride, 6-bromohexanoyl chloride, propiolic acid, and cyclo-
hexane dimethanol (70:30 trans:cis) from Alfa Aesar, and p-
toluenesulfonic acid (pTsA) from Fluka and used without fur-
ther purification. Eastman Chemical, Inc generously donated
cyclohexane dimethanol (99% trans) and Bayer MaterialS-
cience kindly provided poly(propylene glycol) (PPG) 4200,
2200 (Aclaim), and 1000 (Arcol). Chloroform, N, N-dimethyl-
formamide, N-methylpyrrolidone, dimethylsulfoxide, and tet-
rahydrofuran were purchased from Fisher Scientific and
used as received. Dichloromethane and triethylamine were
purchased from Fisher Scientific and distilled from calcium
hydride. 1H and 13C NMR spectra were recorded on a 400-
MHz Varian Inova spectrometer. Chemical shifts are reported
in ppm downfield from the internal standard tetramethylsi-
Synthesis of Bis(2-bromoethyl)isophthalate (BrIP) (3)
A mixture of isophthaloyl chloride (27.3 g, 0.13 mol), N(Et)3
(75 mL, 0.54 mol) in CH2Cl2 (400 mL), and 2-bromoethanol
(3ꢁ8 mL, 0.54 mol) in CH2Cl2 (50 mL) was added dropwise at
0 C. The mixture was stirred at room temperature for 20 h.
The mixture was washed with 5% aqueous solution of
K2CO3 (3 ꢃ 150 mL), water (2 ꢃ 150 mL) and dried over
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ꢁ
MgSO4. The crude was purified by silica chromatography
(CHCl3/hexanes 7:3) to give 3 (35.3 g, 72%) as white crys-
tals; 1H NMR (CDCl3, 500 MHz) d 3.23 (t, 4H), 4.23 (t, 4H),
7.59 (t, 1H), 8.28 (d, 2H), 8.75 (m, 1H).
were added at 23 C. The solution was heated under refluꢃ
for 18 h. After cooling, the solution was washed with an
aqueous saturated NaHCO3 solution (50 mL), water (3 ꢃ 50
mL), brine (50 mL) and dried over MgSO4. The solvent was
removed under rotary evaporation and the crude was recrys-
tallized from hexanes/ethyl acetate to give compound 7 as
pale yellow crystals. (mp 56–58 ꢁC) (6.02 g, 65%); 1H NMR
(CDCl3, 500 MHz) d 1.79 (m, 4H), 2.88 (s, 2H), 4.23 (m, 4H).
Synthesis of Bis(2-azidoethyl)isophthalate (BAIP) (4)
A mixture of 3 (19.5 g, 52 mmol), NaN3 (26.8 g, 410 mmol),
andꢁTBAB (0.84 g, 2.6 mmol) in DMF (100 mL), was heated at
80 C for 48 h. After cooling, a saturated aqueous solution of
LiBr (100 mL) was added and the mixture was extracted with
ethyl acetate (3 ꢃ 100 mL). The combined organic phases
were washed with water (3 ꢃ 100 mL), brine (100 mL), dried
over MgSO4 and the solvent removed under rotary evapora-
tion. The crude product was purified by silica chromatography
(CHCl3/hexanes 8:2) to give 4 (13.0 g, 66%) as white crystals;
Synthesis of DEG Dipropiolate (8)
To a solution of DEG (6.2 g, 58 mmol) in CHCl3 (100 mL),
pTsA (128 mmol) and propiolic acid (232 mmol) were added
ꢁ
at 23 C. The solution was heated under refluꢃ for 18 h. Af-
ter cooling, the solution was washed with an aqueous satu-
rated NaHCO3 solution (50 mL), water (3 ꢃ 50 mL), brine
(50 mL) and dried over MgSO4. The solvent was removed
under rotary evaporation and the crude was purified by
silica chromatography compound 8 as a yellow oil. (7.31 g,
1
ꢁ
(mp 73–75 C) H NMR (CDCl3, 500 MHz) d 3.63 (t, 4H), 4.53
(t, 4H), 7.59 (t, 1H), 8.28 (d, 2H), 8.75 (m, 1H).
1
60%); H NMR (CDCl3, 500 MHz) d 2.91 (s, 2H), 3.73 (t, 4H),
4.33 (t, 4H).
Synthesis of trans-1,4-Cyclohexyl Dimethylene
Dipropiolate (T-CHDMDP) (5)
A solution of trans-1,4-cyclohexyl dimethanol (5g, 35 mmol),
pTsA (14.6 g, 77 mmol), and propiolic acid (8.6 mL, 143 mmol)
in benzene (100 mL) was heated under refluꢃ for 20 h. The mix-
ture was washed with water (3 ꢃ 100 mL), brine (100 mL) and
dried over MgSO4. The solvent was removed under rotary evapo-
ration. The crude product was purified using silica chromatogra-
phy (CHCl3/hexanes 7:3) to give 5 (5.65 g, 65%) as white crys-
tals; (mp 119–121 ꢁC) 1HNMR (CDCl3, 500 MHz) d 1.04 (m, 4H),
1.68 (m, 2H), 1.85 (m, 4H), 2.89 (s, 2H), 4.03 (d, 4H);
Synthesis of Poly(propylene glycol)
Bis-6-bromohexanoate (9)
To a solution of PPG (1H NMR Mn ¼ 1932, 19.8 g 9.0 mmol)
in 100 mL of anhydrous dichloromethane, 2.76 mL (19.8
mmols–2.2 molar excess) of triethylamine was added by sy-
ringe and the reaction was cooled to 0 ꢁC. 3.02 mL (19.8
mmols–2.2 molar excess) of 6-bromohexanoyl chloride was
added dropwise, and the reaction was allowed to equilibrate
at room temperature and stirred overnight. The triethylam-
monium chloride was filtered from the solution, and the so-
lution was washed with saturated sodium bicarbonate (1 ꢃ
100 mL), deionized water (3 ꢃ 100 mL), and brine (1 ꢃ
100 mL). The solution was dried over MgSO4, and the sol-
vent was removed under rotary evaporation to yield 9 as a
clear oil (14.36 g, 72% yield). 10 was used without further
purification. The structure was confirmed using 1H NMR
spectroscopy (spectra included in supplemental information).
Synthesis of cis/trans-1,4-Cyclohexyl Dimethylene
Dipropiolate (70:30-CHDMDP) (6)
A solution of cis/trans-1,4-cyclohexyl dimethanol (70:30
trans:cis) (5.5 g, 38 mmol), pTsA (15.9 g, 84 mmol), and pro-
piolic acid (9.3 mL, 155 mmol) in benzene (100 mL) was
heated under refluꢃ for 20 h. The mixture was washed with
water (3 ꢃ 100 mL) and brine (100 mL) and the product
was dried over MgSO4. The solvent was removed under
rotary evaporation and the crude was recrystallized from hex-
anes/ethyl acetate to give 6 (4.43 g, 47%) as white crystals;
(mp 109–113 ꢁC) 1HNMR (CDCl3, 500 MHz) d 1.04 (m, 4H),
1.68 (m, 2H), 1.85 (m, 4H), 2.89 (s, 2H), 4.03 (d, 4H).
Synthesis of Poly(propylene glycol)
Bis-6-azidohexanoate (10)
To a solution of 10 (1H NMR Mn ¼ 2289, 14.36 g, 6.4 mmol)
in 100 mL DMF, 2.92 g (44.9 mmol) of sodium azide (seven
molar excess) was added. The mixture was allowed to react
for 48 h at 65 C. After cooling, a saturated aqueous solution
of NaCl (100 mL) was added and the mixture was extracted
Synthesis of 1,4-Butanedipropiolate (7)
To a solution of 1,4-butanediol (4.3 g, 48 mmol) in CHCl3
(100 mL), pTsA (106 mmol) and propiolic acid (192 mmol)
ꢁ
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SCHEME 1 Synthesis of bis-2-bromoethyl terephthalate and isophthalate and subsequent conversion into the corresponding bis-
azide for utility in Cu(I) catalyzed 1,3-Huisgen cycloaddition.
with ethyl acetate (3 ꢃ 100 mL). The combined organic
phases were washed with water (3 ꢃ 100 mL), brine (1 ꢃ
100 mL), dried over MgSO4, and the solvent removed under
rotary evaporation to yield 10 as a clear oil. The crude prod-
uct was purified by passing over silica with chloroform as an
was sparged for at least 10 min with Ar or N2 to remove any
dissolved oxygen. A catalytic amount (100 ppm) of both CuI
and Cu powder were aꢁdded to the reaction mixture. The solu-
tion was heated to 60 C and stirred overnight under an inert
atmosphere. The resulting mixture was precipitated into an
aqueous solution of N, N,N0, N0, N00-pentamethyldiethylene tria-
mine and vacuum filtration yielded the polymer in powder
form. The resulting polymer was dried overnight at reduced
pressure and 50 ꢁC to afford the dry product. Segmented click
polyesters were synthesized using a similar method.
1
eluent. The structure was confirmed using H NMR spectros-
copy (spectra included in supplemental information).
RESULTS AND DISCUSSION
Polymer Synthesis
In a representative synthesis, an equimolar amount of bisazide
and bispropiolate were dissolved in N,N-dimethylformamide
(DMF) or dimethyl sulfoxide (DMSO). The reaction solution
The synthesis of several bisazide and bispropiolate monomers
allowed for the utilization of click chemistry for the synthesis
of novel segmented polyesters. The reaction of 2-bromoethanol
SCHEME 2 Conversion of various diols into the corresponding propiolates for subsequent synthesis of polyesters using Cu(I) cata-
lyzed 1,3-Huisgen cycloaddition.
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TABLE 1 Comparison of Thermal Transitions of Triazole-
Containing Polyesters and Structurally Analogous Melt
Transesterification Polyesters
Synthesis of non-click polyester analogs using melt transes-
terification offered a control to elucidate the effects of the
triazole ring on the physical and chemical properties of the
polymers and to compare the glass transitions of the click
polyesters to structurally similar polyesters. The resulting
polyesters exhibited good solubility in chloroform and melt-
pressed into mechanically robust films. Scheme 4 compares
the chemical structures and synthetic conditions for both the
click polyesters and melt transesterification polyesters.
Triazole-
Containing
Polyesters
Melt
Polyesters
HO-R-OH
Tg (ꢁC) Tm (ꢁC) Tg (ꢁC) Tm (ꢁC)
cyclohexane dimethanol 121
99% trans
201
80
214
DSC allowed for a determination of Tg’s of the click polyest-
ers as well as the melt polyesters to compare the thermal
properties of the triazole-containing polyesters with struc-
turally analogous melt polyesters, and Table 1 illustrates
these results. Polyesters synthesized using click chemistry in
cyclohexane dimethanol 117
trans:cis
182
72
ND
butane diol
95
77
178
ND
25
25
142
ND
Diethylene glycol
ꢁ
all cases exhibited at least a 40 C higher Tg than their struc-
ND ¼ not detected in the DSC experiment.
tural analogues, and in most cases demonstrated a crystal-
line Tm, in contrast to the structural analogues, which only
exhibited crystalline Tm’s with a high degree of structural
regularity. In addition, some triazole-containing polyesters
with both terephthaloyl dichloride and isophthaloyl dichloride
afforded the corresponding dibromides for utilization in the syn-
thesis of bisazide monomers. Reaction of the dibromides with
sodium azide resulted in the formation of the corresponding
bisazides. 1H NMR spectroscopy confirmed the structures of the
synthesized bisazides. Scheme 1 details the synthesis of these
bisazides. Reaction of an excess of propiolic acid with 1,4-butane
diol, DEG, trans-1,4-cyclohexane dimethanol (CHDM), or a mix-
ture of trans- and cis-CHDM (the trans:cis ratio was 70:30) in
refluxing chloroform or benzene at 80 ꢁC for 18 h with an excess
of p-toluene sulfonic acid yielded monomers with bispropiolate
functionality, as depicted in Scheme 2. 1H NMR spectroscopy
confirmed the structures and purity of the bispropiolates.
ꢁ
exhibited Tg’s near 125 C. These results suggested that tria-
zole incorporation enhances chain stiffness and crystallinity,
likely due to intermolecular association.
Because of the high glass transition temperatures of the syn-
thesized triazole-containing polyesters, they were potentially
useful as a HS in segmented polyesters. Preparation of seg-
mented polyesters required the synthesis of an azide-func-
tionalized SS. PPG offered good solubility in many solvents, a
low Tg, and commercial availability with high (f ¼ 2) tele-
chelic functionality at a variety of molecular weights. Each of
these properties allowed for PPG to serve as the SS in the
synthesis of segmented polyesters. The esterification reaction
of 6-bromohexanoyl chloride and PPG (Mn ꢂ 1932) yielded
bromine-terminated PPG segments (PPG-dibromide) in
ꢀ70% isolated yield, in agreement with our previous
report.5 Subsequent reaction with an excess of sodium azide
yielded the azide-functionalized PPG oligomer (PPG-bisazide)
The utilization of click chemistry resulted in a diverse series
of triazole-containing polyesters. Preliminary syntheses with
bisazidoterephthalates yielded polymers that exhibited insig-
nificant solubility in most organic solvents. However, utiliza-
tion of isophthalate-derived bisazides afforded polymers that
demonstrated solubility in DMSO and N-methyl-2-pyrroli-
done (NMP) upon heating. The resulting polymers melt-
pressed into free-standing, albeit brittle films. The polymers
exhibited remarkably high glass transition temperatures (Tg)
and demonstrated good thermal stability (Table 1). Scheme
3 illustrates the synthesis of polyesters using 1,3 Huisgen
CuAAC click chemistry.
1
in ꢀ60% isolated yield. H NMR spectroscopy confirmed the
purity, functionality (f ¼ 2), and molecular weight of this
difunctional oligomer. Scheme 5 illustrates the synthesis of
PPG-dibromide and subsequent conversion into PPG-
bisazide.
SCHEME 3 1,3-Huisgen CuAAC as a synthetic route for polyesters containing 1,2,3-triazole units.
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SCHEME 4 Synthesis of triazole-containing polyesters and their structurally analogous melt transesterification polyesters for com-
parison of thermal properties.
The reaction of PPG-bisazide, cis/trans-1,4-cyclohexyl
dimethylene dipropiolate (70:30-CHDMDP), and bis(2-azidoe-
thyl)isophthalate (BAIP) in DMSO with catalytic Cu and CuI
afforded segmented polyesters with HS contents of 30, 40,
and 50 wt % HS, shown as Scheme 6. Equation 1 illustrates
the calculations utilized to determine HS content. The
obtained polyesters were tan, elastomeric, and readily dis-
persible in DMF, DMSO, and NMP. However, DLS demon-
strated aggregation of the polymers in all available SEC sol-
vents, so molecular weight data was unattainable. Melt
pressing yielded light tan, free-standing films that exhibited
flexibility and mechanical ductility. These results were in
contrast to the homopolyesters prepared using click chemis-
try, indicating the utility of synthesizing segmented block
copolyesters to enhance film ductility.
click polyesters. Figure 1 illustrates FTIR spectra at 2106
cmꢂ1 as a function of time. This wavenumber corresponded
to the azide stretching frequency. Initially, the absorbance at
2106 cmꢂ1 decreased linearly with time. After approximately
2 h, the peak reached a plateau, indicating a lowered reac-
tion rate due to a decreasing concentration of azide and
alkyne functional groups within the solution. After ꢀ8 h,
complete azide consumption occurred, indicating high con-
version and reaction completion.
DSC and TGA provided analysis of the thermal transitions of
the segmented polyesterꢁs, and Table 2 summarizes these
results. SS Tg’s near ꢂ60 C indicated microphase separation,
while 1st heat Tm’s illustrated the semicrystalline nature of
these polyesters, suggesting elastomeric properties. In addi-
tion, TGA demonstrated that ꢁthese polyesters exhibited Td(on-
In situ FTIR spectroscopy offered a method for evaluation of
temperatures above 300 C, confirming excellent thermal
set)
the reaction rates of the click reaction toward segmented
stability for melt processing.
SCHEME 5 Synthesis of azide-terminated PPG for preparation of segmented polyesters using Cu(I) catalyzed 1,3-Huisgen
cycloaddition.
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SCHEME 6 Synthesis of segmented polyesters using Cu(I) catalyzed 1,3-Huisgen cycloaddition.
FIGURE 2 Representative tensile traces for segmented click
polyesters with varying hard segment content.
FIGURE 1 In situ FTIR spectra at 2106 cmꢂ1 demonstrating
complete azide consumption at ꢀ8 h.
with values in all cases near 35%. These properties are con-
sistent with segment block copolymers. Table 3 illustrates
these results.
DMA provided an efficient means to further probe micro-
phase separation, as well as the mechanical properties as a
function of temperature. Figure 3 illustrates DMA traces of
storage moduli as a function of temperature for each HS con-
tent segmented click polyester. Table 4 shows the values of
the SS Tg, the HS Tg, and the HS Tm. Only minor variation
occurred among the SS Tg and HS Tg values, in good agree-
ment with the SS Tg values shown in Table 2. The slightly
increased values for Tg’s reported using DMA relative to
those obtained using DSC are consistent with the litera-
ture.40 This is due to the delayed effects of the onset of seg-
mental motion on mechanical properties. The endotherm
relating to the onset of segmental motion (DSC Tg) does not
significantly affect the mechanical properties until long range
CHDMBP þ BAIP
% hard segment ¼
ꢃ 100
CHDMBP þ BAIP þ PPG ꢂ bisazide
(1)
Tensile testing evaluated the ultimate mechanical properties
of these segmented click polyesters; Figure 2 shows repre-
sentative traces for each HS content. Tensile testing of melt
pressed films revealed Young’s moduli of 7, 22, and 133
MPa for the 30, 40, and 50 wt % HS segmented polyesters,
respectively. As expected, the moduli increased with increas-
ing wt % HS, consistent with microphase separated thermo-
plastic elastomers. Stress at break also increased as a func-
tion of HS content with values ranging from 1.4 to 9.2 MPa.
Strain at break did not exhibit a dependence on HS content
TABLE 2 Thermal Characterization of Segmented Click
TABLE 3 Tensile Data for Segmented Click Polyesters with
Polyesters
Varying Hard Segment Content
Hard Segment Content
Tg (ꢁC)
Tm (ꢁC)
Td (onset) (ꢁC)
Stress at
Strain at
E Modulus
(MPa)
HS Content
Break (MPa)
Break (%)
30 wt %
40 wt %
50 wt %
ꢂ61
ꢂ61
ND
114
168
171
313
329
321
30 wt %
40 wt %
50 wt %
1.4 6 0.1
3.5 6 0.3
9.2 6 0.6
32 6 3
38 6 2
35 6 4
7 6 1
22 6 4
133 6 7
ND ¼ not detected in the DSC experiment.
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FIGURE 3 Dynamic mechanical analysis of segmented click
polyesters with varying hard segment content.
segmental motion is achieved throughout the entire SS. The
Tm of the HS increased with increasing HS content, also con-
sistent with DSC results, perhaps suggesting a lower level of
crystallinity in lower HS content segmented click polyesters.
The observed glassy storage modulus did not substantially
change with HS content; however, the rubbery plateau modu-
lus exhibited a significant increase at room temperature as a
function of increasing HS content. These results show good
agreement with the tensile data and are consistent with lit-
erature precedence on semicrystalline segmented block
copolymers with varying HS content.41,42
AFM provided a method to further confirm the microphase
separated nature of the segmented clickꢁ polyesters. Melt
pressing and subsequent annealing at 140 C provided films
suitable for AFM. Figure 4(a) shows a 1 ꢃ 1 um image of
the 40% HS polyester. In this image, the lighter features rep-
resent the HS, whereas the darker features represent the SS.
Figure 4(b) shows a 5x5 um image of the same sample. At
this magnification, the microphase separated domains are
also visible; however, the brighter, ribbon-like features repre-
sent the semicrystalline domains. These results further con-
firm the microphase separated and semicrystalline nature of
these segmented click polyesters. Literature precedence
exists for AFM observation of short-range ribbon microstruc-
tures in semicrystalline segmented block copolymers, similar
to those in Figure 4(a), and transmission electron micros-
copy commonly depicts long-range ribbon microstructures in
semicrystalline segmented block copolymers similar to those
in Figure 4(b).42
FIGURE 4 AFM images of 40 wt % HS segmented click polyes-
ter demonstrated microphase separated morphology and crys-
talline domains.
CONCLUSIONS
This manuscript is the first reported synthesis of segmented
copolyesters using copper(I) catalyzed azide-alkyne 1,3-Huis-
gen cycloaddition. Click chemistry offered an efficient, low-
temperature method toward the synthesis of polyesters and
segmented block copolyesters. DSC confirmed a significant
increase (ꢀ40 ꢁC) in Tg of the triazole-containing homopo-
lyesters relative to structurally analogous polyesters synthe-
sized using melt transesterification. Utilization of azido-func-
tionalized PPG telechelic macromonomers for the synthesis
of segmented click polyesters with varying HS content
resulted in a marked increase in polymer solubility and film
ductility. DSC demonstrated microphase separation of the
segmented click polyesters, suggesting the potential for elas-
tomeric properties. Tensile testing revealed an increase in
Young’s moduli from 7 to 133 MPa and stress at break from
1.5 to 9 MPa as HS content increased from 30 to 50 wt %,
and DMA displayed microphase separation and an increase
in rubbery plateau modulus and crystalline Tm with
TABLE 4 Thermal Transitions of Segmented Click Polyesters
with Varying Hard Segment Content Obtained from Dynamic
Mechanical Analysis
Hard Segment
Content
Sample
Tg,1 (ꢁC)
Tg,2 (ꢁC)
Tm (ꢁC)
30 wt %
40 wt %
50 wt %
30 wt % HS
40 wt % HS
50 wt % HS
ꢂ52
ꢂ48
ꢂ52
57
55
62
87
154
175
8
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18 Johnson, J. A.; Baskin, J. M.; Bertozzi, C. R.; Koberstein, J.
T.; Turro, N. J. Chem. Commun. 2008, 3064–3066.
increasing HS content. AFM confirmed the existence of
microphase separated and semicrystalline morphological
domains. These results suggest versatility of click chemistry
toward the synthesis of high performance polyesters and
segmented block copolyesters.
19 Johnson, J. A.; Finn, M. G.; Koberstein, J. T.; Turro, N. J.
Macromolecules 2007, 40, 3589–3598.
20 Qin, A.; Lam, J. W. Y.; Tang, B. Z. Chem. Soc. Rev. 2010, 39,
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21 Pressly, E. D.; Amir, R. J.; Hawker, C. J. J. Polym. Sci. Part
A: Polym. Chem. 2011, 49, 814–819.
ACKNOWLEDGMENTS
The authors would like to thank Dr. Qin Lin and Dr. Rama Puli-
gadda for insightful discussions during the conception of this
research. This research was supported by Brewer Science, Inc.
under contract # N00014-07-C-0797.
22 Zhang, C.; Shen, X.; Sakai, R.; Gottschaldt, M.; Schubert, U. S.;
Hirohara, S.; Tanihara, M.; Yano, S.; Obata, M.; Xiao, N.; Satoh, T.;
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