A new tricrystalline triblock terpolymer by combining polyhomologation and ring-opening polymerization. synthesis and thermal properties

Article abstract of DOI:10.1002/pola.29492

New tricrystalline triblock terpolymers, polyethylene-block-poly(ε-caprolactone)-block-poly(L-lactide) (PE-b-PCL-b-PLLA), were synthesized by ROP of ε-caprolactone (CL) and L-lactide (LLA) from linear ω-hydroxyl polyethylene (PE-OH) macroinitiators. The linear PE-OH macroinitiators were prepared by C1 polymerization of methylsulfoxonium methylide (polyhomologation). Tin(II) 2-ethylhexanoate was used as the catalyst for the sequential ROP of CL and LLA in one-pot polymerization at 85?°C in toluene (PE-OH macroinitiators are soluble in toluene at 80?°C). ¹H NMR spectra confirmed the formation of PE-b-PCL-b-PLLA triblock terpolymers through the appearance of the characteristic proton peaks of each block. GPC traces showed the increase in the number average molecular weight from PE-OH macroinitiator to PE-b-PCL, and PE-b-PCL-b-PLLA corroborating the successful synthesis. The existence of three crystalline blocks was proved by DSC and XRD spectroscopy.

Full text of DOI:10.1002/pola.29492

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A New Tricrystalline Triblock Terpolymer by Combining Polyhomologation
and Ring-Opening Polymerization. Synthesis and Thermal Properties
1
Viko Ladelta ,1 George Zapsas ,1 Yves Gnanou ,2 Nikos Hadjichristidis
1Polymer Synthesis Laboratory, Physical Sciences and Engineering Division, KAUST Catalysis Center, King Abdullah University of
Science and Technology (KAUST), Thuwal 23955, Saudi Arabia
2Physical Sciences and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabia
Correspondence to: N. Hadjichristidis (E-mail: nikolaos.hadjichristidis@kaust.edu.sa)
Received 28 July 2019; Revised 23 August 2019; accepted 28 August 2019
DOI: 10.1002/pola.29492
ABSTRACT: New tricrystalline triblock terpolymers, polyethylene-
block-poly(ε-caprolactone)-block-poly(L-lactide) (PE-b-PCL-b-PLLA),
were synthesized by ROP of ε-caprolactone (CL) and L-lactide (LLA)
from linear ω-hydroxyl polyethylene (PE-OH) macroinitiators. The
linear PE-OH macroinitiators were prepared by C1 polymerization
of methylsulfoxonium methylide (polyhomologation). Tin(II) 2-
ethylhexanoate was used as the catalyst for the sequential ROP of
CL and LLA in one-pot polymerization at 85 ꢀC in toluene (PE-OH
macroinitiators are soluble in toluene at 80 ꢀC). 1H NMR spectra
confirmed the formation of PE-b-PCL-b-PLLA triblock terpolymers
through the appearance of the characteristic proton peaks of each
block. GPC traces showed the increase in the number average
molecular weight from PE-OH macroinitiator to PE-b-PCL, and PE-
b-PCL-b-PLLA corroborating the successful synthesis. The existence
of three crystalline blocks was proved by DSC and XRD spectros-
copy. © 2019 The Authors. Journal of Polymer Science Part A: Poly-
mer Chemistry published by Wiley Periodicals, Inc. J. Polym. Sci.,
Part A: Polym. Chem. 2019
KEYWORDS:
crystalline
polymer;
metal
catalyst; poly-
homologation; ring-opening polymerization; triblock terpolymer
polyethylene-block-poly(L-lactide)
polyethylene-block-poly(ethylene oxide) (PE-b-PEO) are com-
monly used as compatibilizers in PE-based polymer blends.23–29
(PE-b-PLLA),
and
INTRODUCTION Crystalline block co/terpolymers composed of
two or three crystallizable blocks have attracted enormous
attention in polymer science.1 This is because of the combina-
tion of two or three crystalline blocks that have different
chemical nature in one polymeric chain giving rise to some
remarkable properties.2,3 Moreover, recent advances in syn-
thetic polymer chemistry unlock the opportunity to tune the
thermomechanical, morphologies, biocompatibility, biodegrad-
ability, and optoelectronic properties of the crystalline poly-
meric materials.4–8
In contrast to diblock copolymer, triblock terpolymers con-
sisting of three crystalline segments are less explored, owing
to the challenging synthetic procedure. Only two types of
tricrystalline triblock terpolymer are reported until now. The
first type is poly(ethylene oxide)-block-poly(ε-caprolactone)-
block-poly(L-lactide) (PEO-b-PCL-b-PLLA) triblock terpolymer,
which was prepared by ROP of ε-caprolactone (CL) and
L-lactide (LLA) from PEO macroinitiator in one-pot using
metal or organic catalysts.30–32
There are many reports on double-crystalline diblock copoly-
mers, for example, poly(ε-caprolactone)-block-poly(L-lactide)
(PCL-b-PLLA), poly(ethylene oxide)-block-poly(ε-caprolactone)
(PEO-b-PCL), poly(small lactones)-block-poly(macrolactones)
(PSLs-b-PMLs), poly(ethylene oxide)-block-poly(L-lactide) (PEO-
b-PLLA), and poly(p-dioxanone)-block-poly(ethylene oxide)
(PPDX-b-PCL).9–19 Those copolymers are intensively studied for
biomedical applications.20–22 On the other hand, double-
crystalline diblock copolymers composed of PE segments such
The second type is polyethylene-block-poly(ethylene oxide)-
block-poly(ε-caprolactone) (PE-b-PEO-b-PCL), which was
reported by Abetz and his coworker in 2006.33 To the best of
our knowledge, this is the only example of tricrystalline tri-
block terpolymer containing PE block in the chain due to the
challenging polymerization steps. The PE-b-PEO-b-PCL was
prepared in three consecutive steps. First, poly(butadiene)-
block-poly(ethylene oxide) (PB-b-PEO) precursor was
as
polyethylene-block-poly(ε-caprolactone)(PE-b-PCL),
Additional supporting information may be found in the online version of this article.
© 2019 The Authors. Journal of Polymer Science Part A: Polymer Chemistry published by Wiley Periodicals, Inc.
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and
reproduction in any medium, provided the original work is properly cited.
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synthesized in one-pot by anionic polymerization of
1,3-butadiene followed by ROP of EO in the presence of strong
phosphazene base t-BuP4. Second, the PB-b-PEO was purified
and dried by three freeze-drying cycles and used as the
macroinitiator for the ROP of CL at 180 ꢀC without any catalyst
(thermally activated ROP). Third, the PB segment in PB-b-PEO-b-
PCL was hydrogenated using Wilkinson catalyst to afford PE-b-
PEO-b-PCL. Due to the high temperature during the ROP of CL,
PEO block degraded through random chain scission that leads to
the formation of undesired PEO-b-PCL diblock copolymer. Fur-
thermore, the PE prepared by anionic polymerization of
1,3-butadiene and subsequent hydrogenation contains at least
4% ethyl side groups.
Sigma-Aldrich was distilled twice over CaH2 under dynamic
vacuum. LLA (Sigma-Aldrich, 99%) was recrystallized from
EtOAc three times and dissolved in anhydrous 1,4-dioxane,
cryo-evaporating the 1,4-dioxane, followed by drying under
vacuum overnight. Sn(Oct)2, (95%, Sigma-Aldrich) was dis-
tilled twice over anhydrous MgSO4 and activated 4 Å molecu-
lar sieves followed by azeotropic distillation with dry toluene.
All monomers, solvents, and catalysts for polymerizations
were stored under argon (Ar) in a glove box (LABmasterproSP,
MBraun, Germany).
Instrumentation
1H NMR measurements were performed using a Bruker AVANCE
III spectrometers operating at 400 or 600 MHz; toluene-d8 or
1
In this article, we report the synthesis of a new tricrystalline
triblock terpolymer polyethylene-block-poly(ε-caprolactone)-
block-poly(L-lactide) (PE-b-PCL-b-PLLA). Linear PE-OH with
100% functionality34 macroinitiator obtained from poly-
homologation of methylsulfoxonium methylide was used to
initiate the ROP of CL and LLA using tin(II) 2-ethylhexanoate
(Sn(Oct)2) as the catalyst. The molecular structure of the tri-
CDCl3 were used as the solvents. H NMR spectra were used to
calculate the number average molecular weight (Mn) of each block
by using the integrals of the characteristic signals from the end
groups of each block and methylene protons of PE, PCL, as well
as methine proton of PLLA. High-temperature GPC (HT-GPC) mea-
surements were performed with a Viscotek High-Temperature
Chromatograph (Malvern) with 2 x PLgel 10 μm MIXED-B, 300 x
7.5 mm columns. TCB was used as eluent at a flow rate of
1.0 mL min−1 and temperature 150 ꢀC. The instrument was cali-
brated with PS standards. DSC measurements were performed
with a Mettler Toledo DSC1/TC100 under nitrogen (N2), cali-
brated with indium (purity > 99.999%). The samples were first
heated from 25 ꢀC to 200 ꢀC at a heating rate of 5 ꢀC min−1 to
erase the thermal history and then cooled to −20 ꢀC at a cooling
rate of 1 ꢀC min−1. This cycle was repeated two times while melt-
ing and crystallization temperatures (Tm and Tc) were recorded.
X-ray diffractograms were obtained from XRD Bruker D2 Phaser
using Cu Kα radiation. The sample for XRD measurements was
deposited on a glass substrate with an approximate size
1.5 cm × 1 cm, preannealed at 180 ꢀC for 5 min, and cooled down
1
block terpolymer was characterized by H NMR spectroscopy
and GPC. Differential scanning calorimetry (DSC) and x-ray
diffraction (XRD) spectroscopy reveal the existence of three
crystalline blocks in the polymer.
EXPERIMENTAL
Materials
Methanol (MeOH), THF, dichloromethane (DCM), and ethyl
acetate (EtOAc) were purchased from VWR Chemicals
(HiPerSolv Chromanorm, United Kingdom). 1,2,4-trichloro-
benzene (TCB; ≥99%, anhydrous), 1,4-dioxane (anhydrous,
>99.9%), benzoic acid (99.5%), calcium hydride (CaH2, 95%),
trimethylsulfoxonium iodide (≥98%), benzyltributylammo-
nium chloride (98%), triethylborane (TEB) in hexane (1 M),
trimethylamine-N-oxide dihydrate (TAO.2H2O; ≥99%), and
sodium hydride (NaH) were purchased from Sigma-Aldrich
(United States) and used as received. Toluene was dried by
distillation over CaH2 and polystyrilithium. THF was refluxed
over sodium/benzophenone before used. CL (99%) from
to room temperature (rt) with a cooling rate ~2 ꢀC min−1
.
Typical Polymerization Procedure for the Synthesis of
PE-OH
PE-OH (entry 1, Table 1): PE-OH was prepared according to the
procedure published in the literature.34 In more details, 40.0 g
TABLE 1 Molecular Characterization and Thermal Properties of PE-b-PCL-b-PLLA and the Precursor Polymersa
f
f
Tm (ꢀC)
Tc (ꢀC)
Timeb (h)
Convc (%)
Mn, NMRd (kg mol−1
)
ĐM
PE
PCL
PLLA
PE
PCL
PLLA
e
Entry
sample
1
2
3
4
5
6
PE-OH-1
3
20
48
3
100
3.3
3.5
1.21
1.32
1.49
1.29
1.38
1.42
117.3
111.7
107.4
124.6
127.5
126.5
101.2
98.6
PE-b-PCL-1
87.3
94.3
100
55.8
47.7
40.1
34.5
PE-b-PCL-b-PLLA-1
PE-OH-2
19.5
7.1
155.1
154.2
80.4
107.8
119.1
110.5
116.4
100.6
PE-b-PCL-2
24
48
96.0
95.9
4.2
56.9
54.1
44.7
45.6
PE-b-PCL-b-PLLA-2
23
a PE-OH was prepared by polyhomologation. PE-b-PCL-b-PLLA was pre-
pared by sequential ROP of CL and LLA catalyzed by Sn(Oct)2 at 85 ꢀC
in toluene.
d Mn of the last block determined by 600 MHz 1H NMR spectroscopy (tol-
uene-d8, 80 ꢀC).
e Determined by HT-GPC in TCB at 150 ꢀC (PS standards).
f Measured by DSC under N2 atmosphere with heating and cooling rates
10 ꢀC min−1 (cooling rate 1 ꢀC min−1 for PLLA block).
b Polymerization time for the last block.
c Determined by 400-MHz 1H NMR spectroscopy (CDCl3, 55 ꢀC).
2
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of trimethylsulfoxonium iodide was added into a biphasic mix-
ture of DCM (280 mL) and water (400 mL), followed by the addi-
tion of 61.5 g of tributylbenzylamino chloride. The mixture was
kept in a dark place for 24 h. The aqueous phase was extracted
and washed with DCM (70 mL × 3). After removal of water on a
rotary evaporator, a white needle-like crystal was obtained. The
crystal was dissolved in methanol at 50ꢀC and then cooled to
room temperature slowly, followed by cooling to 0ꢀC inside a
between toluene and water three times to dry the PE-OH. The
white powder was stored inside the glove box under Ar.
Typical Polymerization Procedure for the Synthesis of PE-
b-PCL
In a glove box under Ar atmosphere, a dry polymerization flask
equipped with a stirrer bar was charged with 200 mg dry PE-OH-
1, 5.7 mg of Sn(Oct)2 (0.014 mmol), 0.31 mL CL (2.8 mmol), and
7 mL dry toluene. The flask was sealed and placed in an oil bath
at 85 ꢀC outside the glove box. The ROP was monitored by 1H
NMR spectroscopy. After 24 h, the conversion of CL reached
96.0%, 3.5 mL of the reaction mixture was withdrawn, quenched
with benzoic acid, and precipitated in cold MeOH. PE-b-PCL was
isolated by centrifugation and dried in vacuo for 24 h at 40 ꢀC. 1H
NMR (600 MHz, toluene-d8, 80ꢀC): δ/ppm = 3.96 (PE, [−CH2–
O–PCL]), 1.32 (PE, [−CH2−]), 3.35 (PCL, [OCO − CH2CH2
CH2CH2CH2 − OH]), 3.96 (PCL, [OCO − CH2CH2CH2CH2CH2−]),
2.11 (PCL, [OCO − CH2CH2CH2CH2CH2−]), 1.52 (PCL, [OCO − CH2
CH2CH2CH2CH2−]), 1.45 (PCL, [OCO − CH2CH2CH2CH2CH2−]),
fridge overnight.
A needle-like crystal product of tri-
methylsufoxonium chloride (19.5 g, yield: 84%) with high purity
was obtained.
Next, 10.8 g (0.45 mol) of NaH was inserted into a two-necked
flask connected with a reflux condenser and Ar/vacuum line.
After three subsequent vacuum-Ar circles, 180 mL of freshly
distilled THF was injected. It was followed with the addition of
32.0 g (0.38 mol) of dry trimethylsulfoxonium chloride. The
mixture was heated and refluxed at 80 ꢀC until the gas ceased
(5–6 h). The THF was removed under low pressure and
followed the transfer of 150 mL of dry toluene into the flask.
After 0.5 h of stirring, the resulting turbid solution was filtered
through a dry Celite 545 column. A clear and transparent solu-
tion with a light yellow color was obtained. The concentration
1.23 (PCL, [OCO − CH2CH2CH2CH2CH2−]); Mn,NMR PCL
=
4200 g mol−1
.
of ylide was determined by titration with
a standard
hydrochloric acid aqueous solution (1.05 mmol/mL; yield 72%).
The produced ylide solution was transferred into a two-neck
1000 mL flask and kept under stirring for 30 min. Then,
0.45 mL of TEB (0.40 mmol) was outright added into the flask
to initiate the polymerization, and the solution was left for
30 min under stirring. After consumption of methylide, a
10 times excess (4.0 mmol, 0.45 g) TAOÁ2H2O was added for
the oxidation/hydrolysis of tris-organoborane at 80 ꢀC to obtain
PE-OH quantitatively. After 16 h, the reaction was stopped by
cooling in an ice bath. The mixture was poured into cold metha-
nol (300 mL), and the white solids were filtered, dried under
vacuum, and characterized by HT-GPC and 1H NMR.
Typical Polymerization Procedure for the Synthesis of PE-
b-PCL-b-PLLA
The flask containing 3.5 mL (0.014 mmol OH) PE-b-PCL solu-
tion was transferred into the glove box. Then, 404 mg LLA
(2.8 mmol) was added to the flask. The flask was transferred
outside the glove box and heated in an oil bath at 85 ꢀC for
24 h. The conversion of LLA measured by 1H NMR was 95.9%.
The polymerization was quenched by the addition of benzoic
acid, and the polymerization mixture was precipitated in cold
MeOH. The polymer was recovered by centrifugation and
decantation of the solvents and dried in vacuo at rt for 24 h.
1H NMR (600 MHz, toluene-d8, 80 ꢀC): δ/ppm = 3.96 (PE,
[−CH2–O–PCL]), 1.32 (PE, [−CH2−]), 3.35 (PCL, [OCO − CH2
CH2CH2CH2CH2 − OH]), 3.96 (PCL, [OCO − CH2CH2CH2
CH2CH2−]), 2.11 (PCL, [OCO − CH2CH2CH2CH2CH2−]), 1.52
(PCL, [OCO − CH2CH2CH2CH2CH2−]), 1.45 (PCL, [OCO − CH2
CH2CH2CH2CH2−]), 1.23 (PCL, [OCO − CH2CH2CH2CH2CH2−]),
1.37 (PLLA, [–CHCH3]), 5.06 (PLLA, [OCO–CHCH3–]), 4.09
1H NMR (toluene-d8, 80 ꢀC): δ/ppm = 3.36 (2H, CH2 − OH), 1.32
(−CH2CH2−), 0.88 (3H, CH3); Mn,NMR PE-OH = 7100 g mol−1
.
A 100-mL Schlenk flask was charged with 1 g of the isolated
PE-OH and 20 mL of dry toluene. The flask was heated to dis-
solve PE-OH, and azeotropic distillation was performed
(PLLA, [OCO–CHCH3–COOH]); Mn,NMR PLLA = 23000 g mol−1
.
O
O
O
O
i) 3n
ii)
S
H
O
CH2
OH
O
x
y
x
B
Sn(Oct)2
toluene
85 oC
N
O
2H2O
PE-b-PCL
PE
O
O
O
O
O
O
H
O
O
O
O
z
y
x
O
PE-b-PCL-b-PLLA
SCHEME 1 Synthesis of tricrystalline triblock terpolymer PE-b-PCL-b-PLLA by ROP of CL and LLA from PE-OH macroinitiator
catalyzed by Sn(Oct)2.
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RESULTS AND DISCUSSION
5.01 ppm (Supporting Information Fig. S2). The conversion of
LLA for each block is 94.3% and 95.9% (entry 3 and
6, Table 1), respectively. It is worth noting that the remaining
CL was not polymerized during the ROP of LLA. Thus, the
resulting third block is a pure PLLA chain.
The polyhomologation reaction and subsequential ROPs of CL
and LLA from the PE macroinitiator with Sn(Oct)2 as the cata-
lyst are described in Scheme 1. Sn(Oct)2 was selected as the
catalyst instead of an organic catalyst such as phosphazene
superbase to avoid the formation (S,R) meso-lactide mono-
meric unit during the ROP of LLA at 80 ꢀC.35 The presence of
(S,R) meso-lactide in the PLA chain is unfavorable, because it
can worsen the thermal properties of PLLA block and lead to
an amorphous polymer.35
1H NMR spectra of the isolated PE-OH, PE-b-PCL, and PE-b-
PCL-b-PLLA (entry 1–6, Table 1) are presented in Figure 1
(from PE-OH-1 macroinitiator) and Supporting Information
Figure S3 (from PE-OH-2 macroinitiator). The CH2OH pro-
tons of PE (c) appear at 3.35 ppm, while the CH2 protons of
the main chain (b) were observed at 1.32 ppm. The integral
ratio of these protons was used to calculate the Mn of PE-OH.
After the copolymerization with CL, six new peaks appear in
the spectrum. The peak centered at 3.96 ppm (h) is assigned
as CH2OC O protons of PCL block and merged with the pro-
tons (i). The OCH2 triplet protons appear at 2.11 ppm
Two PE-OH macroinitiators having a different molecular
weight (PE-OH-1 and PE-OH-2) were prepared by poly-
homologation (entry 1 and entry 4, Table 1). The ROPs of CL
and LLA from the macroinitiators are monitored by 1H NMR
spectroscopy. The conversion of CL is determined by the inte-
gral ratio of CH2OCO protons (h) in the polymer at 4.08 ppm
and monomer at 4.15 ppm (Supporting Information Fig. S1).
The conversions of CL were 87.3% for PE-OH-1 (entry
2, Table 1) and 96.0% for PE-OH-2 (entry 5, Table 1). The
conversion of LLA is determined by comparing the integral
ratio of methine proton (k) in the polymer centered at
5.17 ppm with the methine proton in monomer centered at
(d) while the four protons of
CH2 CH2O (g) and
CH2 CH2 OC O (e) appear at 1.52 and 1.45 ppm, respec-
tively. The CH2 CH2 CH2 protons (f) arise at 1.23 ppm.
Two new protons emerged after the ROP of LLA from PE-b-
PCL macroinitiator; and methine proton (CH3 CHO CO) and
methyl ( CH3) protons of PLLA block (k and j) around 5.06
FIGURE 1 1H NMR spectra of PE-OH-1, PE-b-PCL-1, and PE-b-PCL-b-PLLA-1 prepared by ROP of CL and LLA from PE-OH-1
macroinitiator (entries 1–3, Table 1) with Sn(Oct)2 as the catalyst (toluene-d8, 80 ꢀC. The mark * denotes the methyl protons signal
from toluene).
4
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FIGURE
3
Endothermic DSC thermograms of triblock
terpolymers (entries 1–6, Table 1) prepared by ROPs of CL and
LLA from PE-OH macroinitiators; (a) PE-OH-1 and (b) PE-OH-2,
with Sn(Oct)2 as catalyst (heating rate is 10 ꢀC min−1, under N2).
broader elution traces in TCB at 150 ꢀC than in THF at
35 ꢀC.35 This explains why the apparent ĐM of triblock ter-
polymers is broader than the ĐM of PE-OH macroinitiators.
Nevertheless, all GPC traces are monomodal, indicating that
only the initiation from hydroxyl end-group takes place.
DSC analysis was performed in order to evaluate the thermal
properties (Fig. 3) of PE-OHs (black), PE-b-PCLs (blue), and
PE-b-PCL-b-PLLAs (green). The Tm of PE-OH-2 (124.6 ꢀC) is
higher than that of PE-OH-1 (117.3 ꢀC) due to the higher
molecular weight of PE-OH-2. The Tm of PE-OH-1 decreases to
111.7 ꢀC in PE-b-PCL-1 and 107.4 ꢀC on PE-b-PCL-b-PLLA-1,
which is attributed to the presence of PCL block that melted
earlier than PE block. On the contrary, the Tm of PE-OH-2
increases to 127.5 ꢀC in PE-b-PCL-2 and 126.5 ꢀC in PE-PCL-
PLLA-2. The reason behind this phenomenon is still under
investigation. The Tm of PCL is observed at 55.8 ꢀC for PE-
PCL-1 and 56.9 ꢀC for PE-b-PCL-2, which further decrease to
47.7 ꢀC (PE-b-PCL-b-PLLA-1) and 54.1 ꢀC (PE-b-PCL-b-PLLA-
2). The Tm of PLLA is observed at 155.1 ꢀC (PE-b-PCL-b-PLLA-
1) and 154.2 ꢀC (PE-b-CL-b-PLLA-2), indicating the formation
of crystalline PLLA block.
FIGURE 2 HT-GPC traces of PE-OH (black), PE-b-PCL (blue), and
PE-b-PCL-b-PLLA (green) in TCB (150 ꢀC, PS standards). (a) From
PE-OH-1 and (b) from PE-OH-2 macroinitiators
and 1.37 ppm, respectively. The end-group protons of PLLA
(m) are detected at 4.09 ppm, corroborating that PLLA block
is covalently linked to PCL block.
Figure 2 displays the HT-GPC traces of the terpolymers and
their corresponding precursors. The GPC traces show the
increase of molecular weight from PE-OH macroinitiators
(black) to PE-b-PCLs (blue) and PE-b-PCL-b-PLLAs (green), as
indicated by the shifting of the elusion traces to lower elution
volume. PE-OH-1 and PE-OH-2 prepared by polyhomologation
exhibit narrow molecular weight distribution (ĐM) and
monomodal traces (Fig. 2, black line). PE-b-PCL-1 and PE-b-
PCL-2 (blue) traces are slightly broadened (Table 1), implying
that transesterification side reactions occurred during the
ROP of CL at 80 ꢀC. Further broadening is observed on PE-b-
PCL-b-PLLA-1 and PE-b-PCL-b-PLLA-2 (green) traces. It
should be noted that PLLA homopolymer tends to exhibit
After melting and annealing of the polymers for 3 min, crystalli-
zation behavior of the triblock terpolymers and its precursors
was evaluated by applying a cooling scan 10 ꢀC min−1 (Fig. 4)
for PE-OHs (black), PE-b-PCLs (blue), and 1 ꢀC min−1 for PE-b-
PCL-b-PLLAs (green). A slow cooling scan (1ꢀC min−1) must be
applied to facilitate the crystallization of PLLA block.1
The Tc of PE-OH-2 decreases from 110.5 ꢀC to 100.6 ꢀC in PE-
b-PCL-b-PLLA-2. This could be due to the presence of PLLA
block that crystallizes earlier than the PE block, forcing the
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trace of PE in PE-b-PCL-b-PLLA-1 shows broad hump with
two peaks, probably related to the formation of two different
crystalline phases after cooling.
The crystal structures of PE-OHs (black), PE-b-PCLs (blue), and
PE-b-PCL-b-PLLAs (green) were analyzed using XRD (Fig. 5).
Samples were deposited on top of a glass substrate and heated
up to 180 ꢀC, annealed for 10 min, and cooled down to room tem-
perature at a roughly 2 ꢀC min−1. The diffractograms show the
appearance of the characteristic crystallographic planes on homo,
di-, and triblock terpolymers. PE-b-PCL-b-PLLAs exhibit peaks in
the range of 2θ = 15ꢀ − 25ꢀ. PE and PCL blocks are known to
form orthorhombic crystal structure, thus giving an identical peak
position on the diffractogram. The (200) crystallographic plane of
PE and PCL is observed around 23.7ꢀ < 2θ < 24.2ꢀ, while the
(110) crystallographic plane of PE and PCL appears around
21.5ꢀ < 2θ < 21.9ꢀ. Crystallographic planes of PLLA are identified
with the appearance of three peaks; 19.2ꢀ < 2θ(203) < 19.4ꢀ,
16.8ꢀ < 2θ
< 17.1ꢀ, and 15.0ꢀ < 2θ(010) < 15.2ꢀ, respec-
(200/110)
tively. Thus, the successful synthesis of tricrystalline terpolymer
is also confirmed by XRD. Slight deviations on the peak position
between the two triblock terpolymers are attributed to the differ-
ent molecular weights and compositions. Further details will be
discussed in a separate paper.
FIGURE 4 Exothermic DSC thermograms of triblock terpolymers
(entries 1–6, Table 1) prepared by ROPs of CL and LLA from PE-
OH macroinitiators; (a) PE-OH-1 and (b) PE-OH-2, with Sn(Oct)2
as catalyst (cooling rate is 10 ꢀC min−1, under N2).
crystallization of the PE block to occur at a lower temperature
than in the corresponding homopolymer. The Tc of PCL
decreases from 40.1 ꢀC (PE-b-PCL-1) to 34.5 ꢀC (PE-b-PCL-b-
PLLA-1). In contrast, the Tc slightly increases from 44.7 ꢀC
(PE-b-PCL-2) to 45.6 ꢀC (PE-b-PCL-b-PLLA-2). The Tcs of PLLA
blocks are observed at 107.8 ꢀC and 119.1 ꢀC for PE-b-PCL-b-
PLLA-1 and PE-b-PCL-b-PLLA-2, respectively. The exothermic
CONCLUSIONS
Tricrystalline triblock terpolymers, PE-b-PCL-b-PLLA, were
synthesized by combining polyhomologation with ROP. Poly-
homologation produced perfectly linear PE-OH and used as
the macroinitiator for the ROP of CL and LLA with Sn(Oct)2 as
the catalyst. 1H NMR spectroscopy and GPC confirmed the
molecular structure of triblock terpolymers, while DSC and
XRD revealed the existence of three crystalline blocks. The
synthetic approach presented in this work offers a straightfor-
ward strategy to synthesize PE-based tricrystalline triblock
terpolymer with well-defined microstructure. This new PE-b-
PCL-b-PLLA triblock terpolymer is essential for the advance-
ment of polymer crystallization field.
ACKNOWLEDGMENTS
The research reported in this publication was supported by
King Abdullah University of Science and Technology (KAUST).
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