Synthesis of Well-Defined Polyethylene-Based 3-Miktoarm Star Copolymers and Terpolymers

Article abstract of DOI:10.1021/acs.macromol.6b00291

Novel polyethylene (PE)-based 3-miktoarm star copolymers A₂B, (AB)₂B and terpolymers (AC)₂(BC) [A: PE; B, C: polystyrene (PS) or poly(methyl methacrylate) (PMMA)] were synthesized by combining boron chemistry, polyhomologation, and atom transfer radical polymerization (ATRP). 1,4-Pentadiene-3-yl 2-bromo-2-methylpropanoate was first synthesized followed by hydroboration with thexylborane to afford B-thexylboracyclanes, a multi-heterofunctional initiator with two initiating sites for polyhomologation and one for ATRP. After polyhomologation of dimethylsulfoxonium methylide the α,ω-dihydroxyl polyethylene (PE-OH)₂-Br produced served as macroinitiator for the ATRP of styrene to afford (PE-OH)₂-(PS-Br). Both (PE-OH)₂-Br and (PE-OH)₂-(PS-Br) were transformed to two new trifunctional macroinitiators (PE-Br)₂-Br and (PE-Br)₂-(PS-Br) through esterification reactions and used for the synthesis of (AB)₂B and (AC)₂(BC) 3-miktoarm star co/terpolymers. All intermediates and final products were characterized by ¹H NMR, high temperature gel permeation chromatography (HT-GPC), and differential scanning calorimetry (DSC). The synthetic method is a general one and can be used for the synthesis of complex PE-based architectures by combination with other living/living-controlled polymerization techniques.

Full text of DOI:10.1021/acs.macromol.6b00291

Article
pubs.acs.org/Macromolecules
Synthesis of Well-Defined Polyethylene-Based 3‑Miktoarm Star
Copolymers and Terpolymers
Zhen Zhang, Maryam Altaher, Hefeng Zhang, De Wang, and Nikos Hadjichristidis*
Physical Sciences and Engineering Division, KAUST Catalysis Center, Polymer Synthesis Laboratory, King Abdullah University of
Science and Technology (KAUST), Thuwal 23955, Kingdom of Saudi Arabia
S
* Supporting Information
ABSTRACT: Novel polyethylene (PE)-based 3-miktoarm star copolymers A2B, (AB)2B and terpolymers (AC)2(BC) [A: PE; B,
C: polystyrene (PS) or poly(methyl methacrylate) (PMMA)] were synthesized by combining boron chemistry,
polyhomologation, and atom transfer radical polymerization (ATRP). 1,4-Pentadiene-3-yl 2-bromo-2-methylpropanoate was
first synthesized followed by hydroboration with thexylborane to afford B-thexylboracyclanes, a multi-heterofunctional initiator
with two initiating sites for polyhomologation and one for ATRP. After polyhomologation of dimethylsulfoxonium methylide the
α,ω-dihydroxyl polyethylene (PE-OH)2-Br produced served as macroinitiator for the ATRP of styrene to afford (PE-OH)2-(PS-
Br). Both (PE-OH)2-Br and (PE-OH)2-(PS-Br) were transformed to two new trifunctional macroinitiators (PE-Br)2-Br and (PE-
Br)2-(PS-Br) through esterification reactions and used for the synthesis of (AB)2B and (AC)2(BC) 3-miktoarm star co/
terpolymers. All intermediates and final products were characterized by 1H NMR, high temperature gel permeation
chromatography (HT-GPC), and differential scanning calorimetry (DSC). The synthetic method is a general one and can be
used for the synthesis of complex PE-based architectures by combination with other living/living-controlled polymerization
techniques.
A very few examples dealing with the synthesis of PE-based
star polymers directly from ethylene catalytic polymerization,
theoretically leading to 100% PE, can be found in the literature.
For example, a trinuclear Pd−diimine complex was used for the
“living” polymerization of ethylene to afford well-defined 3-arm
star PE.7 Another example is the synthesis of hydroxyl-
terminated PE followed by “click” chemistry or ATRP to
produce the miktoarm star.8 The difficulty associated with the
catalytic ethylene polymerization is the lack of efficiency to
produce end-functionalized PEs, which are intermediates for the
synthesis of miktoarm stars in combination with other
polymerization techniques.9 Therefore, it is clear that more
powerful methods are needed toward the synthesis of PE-based
polymeric materials.
INTRODUCTION
■
1
́
Miktoarm (mikto from the greek word μικτος, meaning mixed)
star polymers, which contain chemically different arms, have
attracted considerable attention in past decades. Because of their
unique chemical structure, they have been used in many studies
including drug delivery,2 micellization,3 and bulk self-assembly.4
One of the most established synthetic methods for miktoarm
stars is based on anionic polymerization high vacuum techniques
and the use of appropriate linking agents such as divinylbenzenes,
double 1,1-diphenylethylenes, or chlorosilanes.5 Through the
chlorosilane protocol a plethora of miktoarm stars were
synthesized including AB2, ABCD, etc., where A, B, C, and D
are PS, polyisoprene, polydimethylsiloxane, poly(2-vinyl-
pyridine), etc.5a−d The synthesis of PE-based miktoarm stars is
very important since PE is one of the most common industrial
polymers thanks to its excellent physical properties and easy
processability. A way toward these materials is the synthesis of
polybutadiene (PBd) stars followed by hydrogenation to afford
the PE-stars. Unfortunately, the maximum 1,4 microstructure of
PBd by anionic polymerization is 93%, and consequently the PE
synthesized by this method is contaminated by 7% butene units.6
Polyhomologation, an alkylborane-initiated polymerization of
dimethylsulfoxonium methylide developed by Shea et al.,10 is a
powerful tool for the synthesis of well-defined linear poly-
methylenes (polymethylene, PM, equivalent to PE) without side
Received: February 6, 2016
Revised: March 14, 2016
© XXXX American Chemical Society
A
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(2.1 g, ∼ 25 mmol) in THF (50 mL) at 0 °C under Ar. The mixture was
stirred for 10 h at room temperature followed by dilution with hexane
(20 mL) and washing with brine. The organic extracts were dried over
MgSO4, filtered, and concentrated. The crude product was purified by
distillation under vacuum to give a colorless oil as product (3.4 g, 58%
yield). 1H NMR (CDCl3, 500 MHz, TMS): δ 1.94 (6H, s), 5.27 (2H, d, J
= 10.0 Hz), 5.38 (2H, d, J = 15.0 Hz), 5.70−5.73 (1H, m), 5.82−5.89
(2H, m). 13C NMR (CDCl3, 125 MHz, TMS): 30.7, 55.8, 76.4, 117.8,
134.3, 170.5. MS (ESI) calcd for C9H13BrO2Na: [M + Na]+ 255.00;
found: 254.85.
Synthesis of Macroinitiator (PE-OH)2-Br. A typical polyhomolo-
gation procedure is as follows. To a solution of 1,4-pentadiene-3-yl 2-
bromo-2-methylpropanoate (0.23 g, 1.0 mmol) in THF (4.0 mL) a
freshly prepared thexylborane (0.50 mL, ∼0.73 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.
A 100 mL Schlenk flask, charged with methylide solution (10 mL, 1.7
M in toluene) and toluene (20 mL), was heated to 60 °C with stirring
under Ar. Then 2.0 mL of the above prepared organoborane solution
was added. After stirring at 60 °C for 10 min, 0.10 mL of the reaction
solution was taken out and added to water containing phenolphthalein.
A neutral solution (pH indicator) indicated the complete consumption
of the ylide. Then TAO·2H2O (0.40 g) was added, and the mixture was
kept under stirring at 80 °C for 4 h. The mixture was poured into cold
methanol (300 mL), and the white solids were filtered, dried under
vacuum, and characterized by 1H NMR and HT-GPC [(PE546-OH)2-Br,
Mn,NMR = 15.5 × 103, PDI = 1.29].
Synthesis of Functionalized 3-Miktoarm Star Copolymer (PE-
OH)2-(PS-Br). A typical ATRP procedure is as follows. CuBr (2.6 mg,
0.018 mmol), styrene (0.45 mL, 3.9 mmol), and toluene (5.0 mL) were
placed into a 100 mL Schlenk flask. The mixture was subjected to two
freeze−pump−thaw cycles, and then PMDETA (8.0 μL, 0.036 mmol)
was added and the mixture was kept stirring at room temperature for 20
min under Ar. Then the macroinitiator (PE546-OH)2-Br (0.20 g, 0.013
mmol) was added, and the mixture was subjected to another two
freeze−pump−thaw cycles. The solution was immediately immersed
into an oil bath set at 100 °C to start the polymerization under stirring.
After 24 h, the polymerization was stopped by cooling in a liquid
nitrogen bath. The reaction mixture was poured into cold methanol
(300 mL) with stirring. The polymer was filtered, washed with
methanol, dried under vacuum, and characterized by 1H NMR and HT-
GPC (Mn,NMR = 19.3 × 103, PDI = 1.30).
alkyl groups. Well-defined PE-based linear, cyclic, and brush
polymers have been synthesized by polyhomologation or by
combination with other living or controlled/living polymer-
ization techniques.11,12 For example, 3-arm PE homostars were
synthesized by polyhomologation followed by a “stitching
reaction” or polyhomologation initiated by 1-boraadamantane.13
In the case of miktoarm stars, the only examples given in the
literature are the PE-PS2 and PE-PMMA2 3-miktoarm stars
synthesized by combining polyhomologation and ATRP.14
Recently, we reported a strategy based on polyhomologation
initiated by B-thexylsilaboracyclic derivatives, having two silicon-
connected initiating sites and one blocked, for the synthesis of
PE-based 3-miktoarm star PE2-PS, 4-arm PE homostars, and PE-
branched double graft copolymers.15 In this paper, a novel multi-
heterofunctional initiator is presented for the synthesis of A2B,
(AB)2B, and (AC)2(BC) (A, PE chain; B, C: PS or PMMA
chain) 3-miktoarm star co/terpolymers using the “core-first”
strategy16 and a combination of polyhomologation with ATRP.17
EXPERIMENTAL SECTION
■
Materials. Ethyl formate (97%), vinylmagnesium chloride solution
(1.6 M in THF), copper(I) bromide (99.999%), pentamethyl-
diethylenetriamine (PMDETA, 99%, Aldrich), 2,3-dimethylbut-2-ene
(⩾99%), trimethylamine N-oxide dihydrate (TAO·2H2O) (⩾99%,
Aldrich), borane dimethyl sulfide complex solution (5.0 M in diethyl
ether), 2-bromoisobutyryl bromide (BIBB, 98%), and pyridine (99.8%)
were purchased from Aldrich and used as received. Methyl methacrylate
(99%) and styrene (⩾99%) were distilled over calcium hydride under
reduced pressure. Tetrahydrofuran and toluene were refluxed over
sodium/benzophenone and distilled under a nitrogen atmosphere just
before use. Dimethylsulfoxonium methylide was prepared according to
Corey’s method followed by switching the solvent from THF to
toluene.18 Thexylborane was prepared by adding 2,3-dimethylbut-2-ene
to borane dimethyl sulfide complex solution (5.0 M in diethyl ether) in
THF at 0 °C.19
Measurements. The high temperature gel permeation chromatog-
raphy (HT-GPC) measurements were carried out at 150 °C with the
Agilent PL-GPC 220 instrument equipped with one PLgel 10 μm
MIXED-B column and a differential refractive index (DRI) detector.
1,2,4-Trichlorobenzene (TCB) was the eluent at a flow rate of 1.0 mL/
min. The system was calibrated with PS standards. Triple-detection HT-
GPC measurements were carried out on a Viscoteck HT-GPC module
350 instrument with two PLgel 10 μm MIXED-B columns equipped
with a refractive index, a light scattering (λ = 670 nm), and a viscometry
detector. TCB was used as an eluent (0.8 mL/min) at 150 °C. The
tridetector HT-GPC system was calibrated with PS standard (Mw = 115
× 103 g/mol, PDI = 1.05). The 1H and 13C NMR spectra were recorded
with a Bruker AVANCE III-500 or 600 spectrometer. DSC measure-
ments were performed using a Mettler Toledo DSC1/TC100 system
under an inert atmosphere (nitrogen). The sample was heated from
room temperature to 150 °C, cooled to −10 °C, and finally heated again
to 150 °C with a heating/cooling rate of 10 °C/min. The second heating
curve was used to determine the glass transition temperature (Tg),
melting temperature (Tm), and degree of crystallinity. The crystallinity
Xc of the samples was derived from Xc = ΔHm/ΔHm+, where ΔHm (288
kJ kg−1) is the specific enthalpy of melting for 100% crystalline PE.20
Synthesis of 1,4-Pentadiene-3-yl 2-Bromo-2-methylpropa-
noate. Ethyl formate (4.0 mL, 0.050 mol) was added dropwise to a
solution of vinylmagnesium chloride solution (63 mL, 0.10 mol, 1.6 M in
THF) in dry THF (30 mL) at 0 °C under argon (Ar). After completion
of the addition, the mixture was kept under stirring for further 1 h at
room temperature. Then the reaction was quenched with saturated
NH4Cl solution (30 mL), and the mixture was extracted with ethyl
acetate (20 mL × 3). The combined organic extracts were washed with
brine, dried over MgSO4, filtered, and concentrated to give the crude
1,4-pentadien-3-ol as a yellow oil.
Synthesis of 3-Miktoarm Star Copolymer (PE-b-PS-Br)2-(PS-
Br). A typical esterification and ATRP procedure are given below for the
synthesis of (PE568-b-PS57-Br)2-(PS57-Br), as an example. (PE568-OH)2-
Br (0.20 g, 0.024 mmol of OH) and toluene (20 mL) were placed into a
100 mL Schlenk flask, and the mixture was kept under stirring vigorously
at 100 °C under Ar. Pyridine (0.10 mL, 1.2 mmol) was added followed
by 2-bromo-2-methylpropionyl bromide (0.12 mL, 1.0 mmol), and
stirring for 10 h. The reaction mixture was cooled to room temperature
and poured into 300 mL of acidic methanol (containing 30 mL of 1 M
aqueous HCl). The polymer was filtered, washed successively with
methanol (2 × 20 mL), 1 M aqueous HCl (2 × 10 mL), and methanol (2
× 20 mL), and dried at 50 °C for 3 h in vacuum to give an off-white solid
(PE568-Br)2-Br (0.19 g, 90% yield).
ATRP of styrene (0.63 mL, 5.4 mmol) catalyzed by CuBr (5.5 mg,
0.038 mmol)/PMDETA (16 μL, 0.076 mmol) and initiated by (PE568
-
Br)2-Br (0.15 g, 0.018 mmol of Br) in toluene (5.0 mL) at 100 °C gave
the polymer (PE568-b-PS57-Br)2-(PS57-Br) (Mn,NMR = 34.2 × 103, PDI =
1.35). More details on the ATRP procedure are given above.
Synthesis of 3-Miktoarm Star Terpolymer (PE-b-PMMA-Br)2-
(PS-b-PMMA-Br). As an example the synthesis of (PE360-b-PMMA83-
Br)2-(PS60-b-PMMA83-Br) is given. (PE360-OH)2-(PS60-Br) (80 mg,
0.010 mmol of OH) was converted to (PE360-Br)2-(PS60-Br) (47 mg,
58% yield) by reaction with 2-bromo-2-methylpropionyl bromide (0.12
mL, 1.0 mmol) in the presence of pyridine (0.10 mL, 1.2 mmol) in
toluene (5 mL) at 100 °C.
ATRP of MMA (0.55 mL, 5.2 mmol) catalyzed by CuBr (1.7 mg,
Pyridine (2.2 mL, 28 mmol) and 2-bromoisobutyryl bromide (3.4
mL, 28 mmol) were added to a solution of crude penta-1,4-dien-3-ol
0.012 mmol)/PMDETA (5.0 uL, 0.024 mmol) and initiated by (PE360-
Br)2-(PS60-Br) (47 mg, 0.0085 mmol of Br) in toluene (5.0 mL) at 100
B
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°C gave the polymer (PE360-b-PMMA83-Br)2-(PS60-b-PMMA83-Br)
(Mn,NMR = 41.4 × 103, PDI = 1.41). More details on the ATRP
procedure are given above.
heterofunctional initiator (58% yield, two steps) by esterification
with 2-bromoisobutyryl bromide (Scheme 1C). Hydroboration
of 1,4-pentadiene moiety with thexylborane gave a mixture of
five- and six-membered boracyclanes (2a and 2b, Scheme 1B)19
exactly as in the case of methylphenyldivinylsilane.15
RESULTS AND DISCUSSION
■
Synthesis of PE-Based Macroinitiator (PE-OH)2-Br. In
our previous work,15 the heterofunctional initiator 3-(methyl-
divinylsilyl)propyl 2-bromo-2-methylpropanoate, synthesized
from 3-chloro-1-propanol in five steps, was used followed by
cyclic hydroboration, ATRP, and polyhomologation to afford the
PE2-PS 3-miktoarm star copolymers (Scheme 1A).
The 1,4-pentadiene group of the new initiator 1,4-pentadiene-
3-yl 2-bromo-2-methylpropanoate, after transformation to B-
thexylboracycles, was used to initiate the polyhomologation and
the 2-bromo-2-methylpropanoate group the ATRP. In our
previous work when first ATRP and then polyhomologation
were performed, the molecular weight of the obtained miktoarm
star copolymers was much higher than expected.15 This
discrepancy was attributed on one hand to the poor efficiency
of the PS-macroinitiator for polyhomologation and on the other
hand to the unknown concentration of boron macroinitiator, as a
consequence of accidental oxidation before polyhomologation.
Hence, we decided to start with the polyhomologation followed
by ATRP (Scheme 2). To make sure the whole thexylborane will
be consumed, since unreacted (initiator of polyhomologation)
will contaminate the miktoarm star with linear PE, excess of 1,4-
pentadiene-3-yl 2-bromo-2-methylpropanoate was used. The
resultant organoborane solution was added to the preheated
ylide solution (60 °C); the mixture became neutral in 10 min,
indicating that the ylide was rapidly and quantitatively consumed.
After the oxidation/hydrolysis with trimethylamine N-oxide
dihydrate (TAO·2H2O), a bromo-functionalized α,ω-dihydroxyl
polyethylene (PE−OH)2-Br was obtained. Seven samples with
different molecular weights were synthesized (Table 1). The
Scheme 1. (A) Previous Work: Laborious Synthesis Route for
A2B Miktoarm Star; (B) Cyclic Hydroboration of 1,4-
Pentadiene with Thexylborane; (C) This Work: Facile
Synthesis of A2B, (AB)2B, and (AC)2(BC) 3-Miktoarm Stars
Starting from Ethyl Formate
1
molecular weights were determined by H NMR end group
analysis (Mn,NMR = (3.3−32.9) × 103, Figure 1) and the
polydispersities by high temperature gel permeation chromatog-
raphy (HT-GPC) with polystyrene standards (Mw/Mn = 1.28−
1.59). Increasing the reaction temperature from 60 to 80 °C did
not improve the polydispersity (Table 1, entries 1 and 2). The
high polydispersity (1.59) but still monomodal (Figure 2A,
green) of the highest molecular weight PE (32.9 × 103, Table 1,
entry 7) is probably due to its insolubility (heterogeneous
polymerization).
Since boranes are very sensitive to air, the amount of the
initiator is less than that calculated from thexylborane;
consequently, the molecular weight of the PE produced is
higher than expected. To exactly control the molecular weight of
Considering the laborious and time-consuming synthesis of
the previous heterofunctional initiator, the following facile
synthetic route was developed. 1,4-Pentadien-3-ol was first
prepared by reaction of vinylmagnesium chloride with ethyl
formate, and without purification it was converted to the
Scheme 2. Synthesis Routes toward A2B 3-Miktoarm Star Copolymers (PE-OH)2-(PS-Br) and (AC)2(BC) Star Terpolymers (PE-
b-PMMA-Br)2-(PS-b-PMMA-Br)
C
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the PE, the real concentration of the organoborane solution
should be determined. For this purpose, (PE546-OH)2-Br
(Mn,NMR = 15.5 × 103, PDI = 1.29) was synthesized first
(Table 1, entry 6), and the number-average degree of
polymerization of PM (one chain) was calculated to be
(Mn,NMR − 233)/14/2 = 546 (233 is the molecular weight of
the diene initiator, 14 of the −CH2− repeating unit, two chains).
Consequently, the actual concentration of organoborane is 17
mmol/(546 × 2 × 2.0 mL) = 0.0078 mmol/mL (17 is the
amount of dimethylsulfoxonium methylide in mmol, 10 mL
corresponds to 1.7 mmol/mL, see Experimental Section; 2.0 mL
is the volume of organoborane solution; two chains). Using this
actual concentration, a new (PE1168-OH)2-Br, with controlled
molecular weight as shown by comparison of theoretical and
Table 1. Molecular Characteristics of (PE-OH)2-Br
a
b
Mn,theor Mn,NMR
DP
Mn,GPC
× 10−3 PDI
a
b
entry
1
sample
× 10−3
× 10−3 (PM)
c
(PE110-OH)2-Br
(PE113-OH)2-Br
(PE360-OH)2-Br
(PE185-OH)2-Br
(PE568-OH)2-Br
(PE546-OH)2-Br
(PE1168-OH)2-Br
−
−
−
−
3.3
3.4
110
4.1
3.4
1.34
d
c
2
113
360
1.37
1.38
1.28
1.36
1.29
1.59
c
3
4
5
6
10.3
5.4
17.4
7.5
e
185
14.7
e
16.4
15.5
32.9
568
28.7
18.4
44.0
−
546
f
7
29.9
1168
a
Mn,NMR and DP [degree of polymerization of one polymethylene
1
(PM) chain] were calculated from H NMR spectrum (toluene-d8)
using the area ratio of protons in terminal CH2OH at δ = ∼3.5 ppm to
b
the ones on the backbone. Mn,GPC, PDI = Mw/Mn, determined by
c
HT-GPC (1,2,4-trichlorobenzene, 150 °C, PS standards). Not
determined molecular weight (Mn,theor = 29.9 × 103, Mn,NMR
=
d
32.9 × 103, PDI = 1.59) (Table 1, entry 7) was synthesized.
measured. Polyhomologation reaction was performed at 80 °C.
e
Sample synthesized for determining the real concentration of
The proof of the successful synthesis of (PE-OH)2-Br was
derived from 1H NMR and GPC results. As shown in Figures 1A
and 1B, the characteristic chemical shifts of vinyl protons in 1,4-
pentadiene-3-yl 2-bromo-2-methylpropanoate at δ = 5.9−5.8
f
organoborane initiator. Mw,GPC‑LS = 47.2 × 103, PDIGPC‑LS = 1.34,
determined by triple-detection HT-GPC; see Supporting Information.
Figure 1. NMR spectra of (A) 1,4-pentadiene-3-yl 2-bromo-2-methylpropanoate in chloroform-d. (B) (PE546-OH)2-Br in toluene-d8 at 90 °C. (C)
(PE546-OH)2-(PS37-Br) in 1,1,2,2-tetrachloroethane-d2 at 90 °C.
D
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Figure 2. GPC chromatograms of linear PEs and the corresponding miktoarm star polymers.
Table 2. ATRP Reaction Conditions and Molecular Characteristics of A2B, (AB)2B, and (AC)2(BC) Stars
a
b
c
d
d
c
star co/terpolymer
(PE113-OH)2-(PS22-Br)
macroinitiator
M
[M]/[Br]
Mn,NMR × 10−3 Mn,GPC × 10−3
PDI
PE (wt %)
(PE113-OH)2-Br
(PE360-OH)2-Br
(PE546-OH)2-Br
(PE1168-OH)2-Br
(PE110-Br)2-Br
St
300/1
260/1
600/1
600/1
200/1
300/1
600/1
5.7
16.5
19.3
57.8
10.5
34.2
41.4
2.7
16.8
17.5
45.0
5.4
1.29
1.30
1.30
1.49
1.40
1.35
1.41
60
62
80
57
30
48
24
(PE360-OH)2-(PS60-Br)
(PE546-OH)2-(PS37-Br)
(PE1168-OH)2-(PS239-Br)
St
St
St
e
(PE110-b-PMMA24-Br)2-(PMMA24-Br)
(PE568-b-PS57-Br)2-(PS57-Br)
MMA
St
(PE568-Br)2-Br
31.3
(PE360-b-PMMA83-Br)2-(PS60-b-PMMA83-Br)
(PE360-Br)2-(PS60-Br)
MMA
25.5
a
b
c
M: monomer. The reaction was performed in toluene at 100 °C for 24 h and [Br]/[CuBr]/[PMDETA] = 1:1.4:2.8. Mn,NMR and PE content were
d
calculated from 1H NMR spectra (toluene-d8 or 1,1,2,2-tetrachloroethane-d2, 80 or 90 °C). Mn,GPC, PDI = Mw/Mn, determined by HT-GPC (1,2,4-
e
trichlorobenzene, 150 °C, PS standards). Mw,GPC‑LS = 67.2 × 103, PDIGPC‑LS = 1.30, determined by triple-detection HT-GPC; see Supporting
Information.
ppm (m, Hc), δ = 5.4 ppm (d, He), and δ = 5.3 ppm (d, Hd)
disappeared after hydroboration/polyhomologation/oxidation/
hydrolysis reaction, and a new peak at δ = 3.4 ppm for the
−CH2− connected to hydroxy group (He′) appeared, indicating
that the polymerization initiated from both initiating sites. The
observed integration area ratio of the aliphatic protons at δ =
1.1−1.5 ppm (Hc′&d′, two protons for each methylene unit, two
PE chains) [Sc′&d′/4H] to that of protons assigned to −CH2−
connected to hydroxyl group at δ = 3.4 ppm (He′, two protons for
each PE arm, two end groups) [Se′/4H] was derived from the
following formula: n = (Sc′&d′/4H):(Se′/4H), which is the degree
of polymerization of each PE chain. This integrated ratio was
used to determine the number-average molecular weight of the
macroinitiator (PE-OH)2-Br, Mn,NMR = (Sc′&d′/Se′) × MWCH2
(14) × 2 + MWinitiator (233). The B-thexyl site was inactive to
polyhomologation since only one peak appeared in the GPC
traces (Figure 2A, red and green).
ppm (He′, two protons for each PE arm, total two PE arms) [Se′/
4H] was derived from the following formula: m = (Sk&l/5H):
(Se′/4H) = 37, which is the degree of polymerization of PS chain.
The integration area ratio of aliphatic protons at δ = 1.0−1.4 ppm
(Hc′&d′, two protons for each methylene unit, two PE arms)
[Sc′&d′/4H] to that of end group’s protons [Se′/4H] was derived
from the same formula as above: n = (Sc′&d′/4H):(Se′/4H) = 660,
which is the degree of polymerization of one PE arm.
Considering that the peaks of Hg,i&H O were close to the peaks
2
of Hc′&d′, the integration of protons [Sc′&d′] was not so accurate
giving n = 546 instead of the above calculated value 660. This
indicates that ATRP initiated by (PE546-OH)2-Br although the
protons of Ha&b were not identifiable in 1H NMR spectra. The
number-average molecular weight of the A2B star (PE546-OH)2-
(PS37-Br) is Mn,NMR = m (37) × MWSt(104) + n (546) × MWCH2
(14) × 2 + MWinitiator (233) = 19.3 × 103.
Four A2B star samples were synthesized, and the correspond-
ing molecular characteristics are shown in Table 2. The GPC
curves in Figure 2A show the variation of molecular weights and
molecular weight distributions of the star polymers prepared
from the linear PE macroinitiators. A slight shift of the elution
peak of star polymer (PE546-OH)2-(PS37-Br) to the lower
molecular weight side, as compared to the linear PE macro-
Synthesis of A2B Star Copolymers. To synthesize A2B
miktoarm star copolymers, ATRP was carried out using (PE-
OH)2-Br as macroinitiator. For example, the ATRP of styrene
was performed with (PE546-OH)2-Br as macroinitiator and
copper(I) bromide (CuBr)/N,N,N′,N′,N″,N″-pentamethyl-
diethylenetriamine (PMDETA) as catalyst/ligand in toluene.
High temperature (100 °C) was needed since linear PE possesses
poor solubility at lower temperatures. The ratio [St]/[Br] was
600; the reaction was terminated after 24 h, followed by
precipitation in methanol to give the polymer. The fingerprints
initiator (PE546-OH)2-Br, was observed. In the case of (PE1168
-
OH)2-(PS239-Br), a shift to slightly higher molecular weight side
occurred, compared with the corresponding linear one. This can
be attributed to the branched structure of the star polymer
compared to the linear one.21 Separation of macromolecules by
GPC is based on the size or more specifically the hydrodynamic
radius (Rh). To compare the size of the linear and star polymers
(PE11686-OH)2-Br and (PE1168-OH)2-(PS239-Br) were charac-
terized by HT-GPC equipped with refractive index (RI), light
1
for PS and PE blocks are evident in the H NMR spectrum
(1,1,2,2-tetrachloroethane-d2, 90 °C) (Figure 1C). The observed
integration area ratio of the aromatic protons at δ = 6.4−7.3 ppm
(Hk&l, five protons for each St unit) [Sk&l/5H] to that of protons
assigned to the −CH2− connected to hydroxyl group at δ = 3.7
E
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Scheme 3. Synthesis Routes toward (AB)2B Star Copolymers (PE-b-PS-Br)2-(PS-Br) and (PE-b-PMMA-Br)2-(PMMA-Br)
scattering (LS), and viscometry detectors. The calculated Rh of
the linear PE was 6.9 nm, and the branched polymer was 7.6 nm.
Considering the molecular weight of the star copolymer
(Mw,GPC‑LS = 67.2 × 103, PDIGPC‑LS = 1.30) increased largely
compared to that of the linear one (Mw,GPC‑LS = 47.2 × 103,
PDIGPC‑LS = 1.34), the Rh of the star copolymer increased slightly,
which is in accordance with the tiny shift of the elution peak from
13.85 to 13.81 mL (RI peak, see Figure S1, Supporting
Information).
Synthesis of (AB)2B Star Copolymers. To synthesize
(AB)2B star copolymers, a macroinitiator with three ATRP
initiating sites was prepared from (PE-OH)2-Br by esterification
of the two hydroxyl groups with 2-bromoisobutyryl bromide at
100 °C in toluene (Scheme 3). As shown in Figure 3A, the
characteristic chemical shift due to the protons connected with
the hydroxyl groups in (PE568-OH)2-Br at δ = 3.4 ppm (He)
disappeared after esterification, and a new peak at δ = 4.0 ppm of
the −CH2− connected to ester group (He′) appeared, indicating
the complete formation of (PE568-Br)2-Br. Then ATRP of
styrene initiated by the resultant (PE568-Br)2-Br with CuBr/
PMDETA as catalyst/ligand at 100 °C in toluene was performed.
After 24 h, the polymer was precipitated in methanol and dried
under vacuum. By the same method, (PE110-Br)2-Br was
synthesized and used as macroinitiator for the ATRP of MMA.
The ATRP reaction conditions and the characteristics of the two
(AB)2B star copolymers are shown in Table 2.
As shown in Figure 3B and Figure S2, the fingerprints of
PMMA and PS segments are clearly found in the synthesized
(AB)2B star copolymers. By comparison of the integration of the
relevant peaks, the degrees of polymerization of the PS and
PMMA segments were calculated [(PE568-b-PS57-Br)2-(PS57-Br)
and (PE110-b-PMMA24-Br)2-(PMMA24-Br), see Supporting
Information]. The evolution of molecular weight for (AB)2B
star copolymers was investigated by HT-GPC. As shown in
Figure 2B, the elution peak of (PE110-b-PMMA24-Br)2-
(PMMA24-Br) shifted to a lower molecular weight side, and
the elution peak of (PE568-b-PS57-Br)2-(PS57-Br) shifted to a
slightly higher molecular weight side, when compared with the
peak of the corresponding linear PE. The polydispersity index
was relatively low (Mw/Mn ≤ 1.4) and the trace monomodal,
suggesting that the arm growing step was well-controlled,
indicating the high efficiency of the ATRP macroinitiator.
Synthesis of (AC)2(BC) Star Terpolymers. To synthesize
(AC)2(BC) star terpolymers, the two terminal hydroxyl groups
of A2B star copolymer (PE360-OH)2-(PS60-Br) were reacted with
2-bromoisobutyryl bromide at 100 °C in toluene to give (PE360
-
Br)2-(PS60-Br) containing three initiating sites for ATRP
1
(Scheme 2). As shown in the H NMR spectrum (Figure S3),
the characteristic chemical shift due to protons connected with
hydroxyl group in (PE360-OH)2-(PS60-Br) at δ = 3.7 ppm
disappeared after esterification, and a new peak at δ = 4.0 ppm of
the −CH2− connected to ester group appeared, indicating the
completely conversion of the −OH to −Br. Then the ATRP of
MMA initiated by the synthesized (PE360-Br)2-(PS60-Br) with
CuBr/PMDETA as catalyst/ligand at 100 °C in toluene was
1
performed, and the obtained polymer was characterized by H
NMR and HT-GPC. The fingerprints of the PMMA segment
were evident in the 1H NMR spectrum (Figure S3): the signals of
CH3−CO protons at 3.7 ppm, −CH2−C− at 1.9 ppm, and
CH3−C− at 1.1 and 0.9 ppm. The degree of polymerization of
the PMMA segment was calculated by comparison of the
integration of the relevant peaks [(PE360-b-PMMA83-Br)2-(PS60-
b-PMMA83-Br), see Supporting Information]. The esterification
reaction and the PMMA chains were also confirmed by FT-IR
spectroscopy (Figure S4). The characteristic ester carbonyl
absorbance peak at ∼1729 cm−1 increased, in accordance with
the increase amount of ester carbonyl groups in the polymer. The
Figure 3. NMR spectra of (A) (PE568-OH)2-Br and (PE568-Br)2-Br in
toluene-d8 at 90 °C and (B) (PE110-b-PMMA24-Br)2-(PMMA24-Br) in
1,1,2,2-tetrachloroethane-d2 at 90 °C.
F
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GPC curves in Figure 3C show that the elution peak of (PE360-b-
PMMA83-Br)2-(PS60-b-PMMA83-Br) shifted to higher molecular
weight than that of (PE360-OH)2-(PS60-Br). All peaks are
monomodal/symmetric and the polydispersity of final star
terpolymer low (Mw/Mn = 1.41, Table 2), indicating the chain
extension polymerization from A2B to (AC)2(BC) star polymer
was highly efficient.
DSC Analysis of Linear PEs and PE-Based Star Co/
Terpolymers. The DSC traces for the linear PEs and the
corresponding 3-miktoarm star co/terpolymers are shown in
Figure 4 and Figure S5. The linear PEs with Mn 3300−32.9 × 103
new horizons toward PE-based complex macromolecular
architectures when combining with other living and living/
controlled polymerization techniques, such as ring-opening
polymerization (ROP), nitroxide-mediated polymerization
(NMP), and reversible addition−fragmentation chain transfer
(RAFT).
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.6b00291.
Figures S1−S5 (PDF)
AUTHOR INFORMATION
Corresponding Author
*(N.H.) Tel + 966-(0)12-8080789; e-mail nikolaos.
hadjichristidis@kaust.edu.sa.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Research reported in this publication was supported by the King
Abdullah University of Science and Technology.
■
REFERENCES
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CONCLUSION
■
A “core-first” synthetic strategy toward well-defined PE-based
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DOI: 10.1021/acs.macromol.6b00291
Macromolecules XXXX, XXX, XXX−XXX