Synthesis of polyisobutylene-based miktoarm star polymers from a dicationic monoradical dual initiator

Article abstract of DOI:10.1021/ma3007762

A dicationic monoradical dual initiator 3-[3,5-bis(1-chloro-1-methylethyl) phenyl]-3-methylbutyl 2-bromo-2-methylpropionate (DCCBMP) was designed for the preparation of A₂B type miktoarm star copolymers, where A is a polyisobutylene (PIB)-based homo or block copolymer that is produced by living carbocationic polymerization (LCP), and B is a polyacrylate or other polymer block produced by atom transfer radical polymerization (ATRP). DCCBMP was obtained by chlorination of 3-[3,5-bis(1-hydroxy-1-methylethyl)phenyl]-3- methylbutyl 2-bromo-2-methylpropionate (DCOHBMP), which was synthesized in a pure form via aerobic oxidation of 3-(3,5-diisopropylphenyl)-3-methylbutyl 2-bromo-2-methylpropionate using N-hydroxyphthalimide (NHPI)/Co(OAc) ₂?4H₂O catalyst system. Initiation efficiency of DCCBMP was 0.89-0.98 for LCP of isobutylene (IB) at -70 °C targeting molecular weights of 5K, 10K, and 20K g/mol, which was comparable to the standard dicationic initiator 5-tert-butyl-1,3-(1-chloro-1-methylethyl)benzene (t-Bu-m-DCC). Sequential monomer addition under LCP conditions produced narrow-polydispersity poly(styrene-b-isobutylene-b-styrene) (PS-PIB-PS) triblock copolymers. These polymers possessed a 2-bromo-2-methylpropionate (bromoester) ATRP initiating site in the approximate middle of PIB segment and two sec-benzylic chloride groups at the PS chain ends, which were also effective in initiating ATRP. With these PS-PIB-PS macroinitiators, poly(tert-butyl acrylate-b-styrene-b-isobutylene)₂-s-poly(tert-butyl acrylate) [(PtBA-PS-PIB)₂-s-PtBA] miktoarm star polymers were readily synthesized via ATRP of tert-butyl acrylate (tBA). The sec-benzylic chloride functionality was alternatively eliminated at high temperature (180-200 °C) under vacuum, yielding PS-PIB-PS macroinitiators containing only the bromoester site (thermolyzed PS-PIB-PS). Size exclusion chromatography (SEC) and nuclear magnetic resonance (NMR) spectroscopy showed that thermolysis removed all sec-benzylic chlorides without destruction of bromoester moieties. Thermolyzed PS-PIB-PS allowed synthesis of poly(styrene-b-isobutylene)₂-s- poly(tert-butyl acrylate) [(PS-PIB)₂-s-PtBA] star polymers with the potential hydrophilic segment attached only to the core of PS-PIB-PS. For both star terpolymer configurations, the PtBA blocks were converted to PAA via thermolysis at 150 °C to produce amphiphilic PIB-based miktoarm star polymers with narrow polydispersity indices (<1.1).

Full text of DOI:10.1021/ma3007762

Article
pubs.acs.org/Macromolecules
Synthesis of Polyisobutylene-Based Miktoarm Star Polymers from a
Dicationic Monoradical Dual Initiator
Yaling Zhu and Robson F. Storey*
School of Polymers and High Performance Materials, The University of Southern Mississippi, 118 College Drive # 5050, Hattiesburg,
Mississippi 39406, United States
S
* Supporting Information
ABSTRACT: A dicationic monoradical dual initiator 3-[3,5-
bis(1-chloro-1-methylethyl)phenyl]-3-methylbutyl 2-bromo-2-
methylpropionate (DCCBMP) was designed for the prepara-
tion of A2B type miktoarm star copolymers, where A is a
polyisobutylene (PIB)-based homo or block copolymer that is
produced by living carbocationic polymerization (LCP), and B
is a polyacrylate or other polymer block produced by atom
transfer radical polymerization (ATRP). DCCBMP was
obtained by chlorination of 3-[3,5-bis(1-hydroxy-1-
methylethyl)phenyl]-3-methylbutyl 2-bromo-2-methylpropio-
nate (DCOHBMP), which was synthesized in a pure form via aerobic oxidation of 3-(3,5-diisopropylphenyl)-3-methylbutyl 2-
bromo-2-methylpropionate using N-hydroxyphthalimide (NHPI)/Co(OAc)2·4H2O catalyst system. Initiation efficiency of
DCCBMP was 0.89−0.98 for LCP of isobutylene (IB) at −70 °C targeting molecular weights of 5K, 10K, and 20K g/mol, which
was comparable to the standard dicationic initiator 5-tert-butyl-1,3-(1-chloro-1-methylethyl)benzene (t-Bu-m-DCC). Sequential
monomer addition under LCP conditions produced narrow-polydispersity poly(styrene-b-isobutylene-b-styrene) (PS−PIB−PS)
triblock copolymers. These polymers possessed a 2-bromo-2-methylpropionate (bromoester) ATRP initiating site in the
approximate middle of PIB segment and two sec-benzylic chloride groups at the PS chain ends, which were also effective in
initiating ATRP. With these PS−PIB−PS macroinitiators, poly(tert-butyl acrylate-b-styrene-b-isobutylene)2-s-poly(tert-butyl
acrylate) [(PtBA−PS−PIB)2-s-PtBA] miktoarm star polymers were readily synthesized via ATRP of tert-butyl acrylate (tBA).
The sec-benzylic chloride functionality was alternatively eliminated at high temperature (180−200 °C) under vacuum, yielding
PS−PIB−PS macroinitiators containing only the bromoester site (thermolyzed PS−PIB−PS). Size exclusion chromatography
(SEC) and nuclear magnetic resonance (NMR) spectroscopy showed that thermolysis removed all sec-benzylic chlorides without
destruction of bromoester moieties. Thermolyzed PS−PIB−PS allowed synthesis of poly(styrene-b-isobutylene)2-s-poly(tert-
butyl acrylate) [(PS−PIB)2-s-PtBA] star polymers with the potential hydrophilic segment attached only to the core of PS−PIB−
PS. For both star terpolymer configurations, the PtBA blocks were converted to PAA via thermolysis at 150 °C to produce
amphiphilic PIB-based miktoarm star polymers with narrow polydispersity indices (<1.1).
sequester Paclitaxel on the highly successful Taxus drug-eluting
coronary stent.7,8
INTRODUCTION
■
Block copolymers containing polyisobutylene (PIB) elasto-
meric segments are of great interest due to the outstanding
environmental resistance, mechanical damping, impermeability,
and biocompatibility of PIB. Poly(styrene-b-isobutylene-b-
styrene) (PS−PIB−PS) triblock copolymers were first
synthesized in the early 1990s by Kennedy and co-workers
via living carbocationic polymerization (LCP) and the
technique of sequential monomer addition, involving bidirec-
tional polymerization of isobutylene (IB) followed by addition
of styrene.1,2 These copolymers exhibit strong phase separation
in the bulk and display properties typical of thermoplastic
elastomers (TPEs). They exhibit excellent low-temperature
flexibility and elongation properties imparted by the rubbery
PIB center block, and they possess elastic recovery and physical
strength properties due to the glassy PS segments.1−6 Because
of its general inertness, PIB also possesses excellent
biocompatibility, and in 2004, PS−PIB−PS was approved to
Over the years, efforts have been made to combine PIB with
other outer blocks to achieve different or improved properties.
For example, the upper use temperature range has been
extended by replacing the PS blocks with higher-Tg glassy
segments produced from p-chlorostyrene,9 p-methyl-
styrene,10−12 α-methylstyrene,13,14 indene,10,15 or p-tert-butyl-
styrene,16 or higher-Tg semicrystalline segments from p-chloro-
α-methylstyrene,17 and hydrophilic character has been imparted
by replacing styrene with p-hydroxystyrene18 or 4-(2-
hydroxyethyl)styrene.19 In all of these cases, sequential
monomer addition was used to form the block structure;
although in some cases a nonhomopolymerizable chain-capping
Received: April 16, 2012
Revised: June 14, 2012
© XXXX American Chemical Society
A
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Scheme 1. Schematic Synthesis of Amphiphilic Poly(acrylic acid-b-styrene-b-isobutylene)2-s-poly(acrylic acid) [(PAA−PS−
PIB)2-s-PAA] and Poly(styrene-b-isobutylene)2-s-poly(acrylic acid) [(PS−PIB)2-s-PAA] Miktoarm Star Terpolymers from
DCCBMP, Combining Living Carbocationic Polymerization (LCP) and Atom Transfer Radical Polymerization (ATRP)
monomer such as diphenylethylene and/or a change in Lewis
acid strength was used to facilitate the crossover reac-
tion.12,14,17,18
PAA, with water transmitting poly(acrylic acid) (PAA) block
segments. Transmission electron microscopy (TEM) revealed a
PIB continuous phase, and concentric PS (outer) and PAA
(inner) cylinders. At a composition of 50:25:25 PIB:PS:PAA
(wt %), the membranes were elastomeric and water permeable
(10−5−10−4 g−1 h−1 mmHg−1); below 25 wt % PAA, the PAA
domains were discontinuous within the PS cylinders and water
permeation decreased by an order of magnitude.31 However,
polymers prepared via site transformation from PS−PIB−PS
are limited to structures in which the third block (e.g., PAA) is
covalently attached to the PS block. This precludes
morphologies possessing an interface between PIB and the
third block, causing the PAA domains to be constrained within
the rigid PS cylinders and potentially limiting water swelling
and permeation. To address this issue, we have targeted a new
miktoarm star-branched configuration whereby the third block
is covalently attached to the PIB block (Scheme 1). This
structure retains the PS−PIB−PS configuration, providing
elastic recovery and strength by physically constraining the PIB
segments at their ends. In addition, phase-separated morphol-
ogies possessing an interface between PIB and the third block
are possible.
Miktoarm star polyisobutylene-based block copolymers have
been previously reported. Faust et al.33 reacted 2-(PIB)furan
with living PIB, thereby creating coupled PIB with an active
furanyl cation at the coupling junction. After adjustment of
Lewis acidity and solvent medium polarity, sequential addition
of methyl vinyl ether was carried out to produce the miktoarm
star block copolymer. This method was shown to provide
independent control over the degrees of polymerization of the
two PIB segments. In more recent work, Faust and Hirao34
used a postpolymerization, iterative divergent approach, using
functionalized diphenylethylene, to place 2n benzyl bromide
functions at the termini of either mono or difunctional PIB (B).
The bromide functions were then coupled with living anionic
poly(methyl methacrylate) (A) to make A2B, A4B, and A8B
miktoarm star polymers, and A2BA2, A4BA4, and A8BA8
“pompom” polymers. However, these methods, which con-
struct the PIB segments in a “toward the core” manner, do not
easily lend themselves to the synthesis of the star polymers of
Scheme 1, especially the (ABC)2A variety.
Since PIB can only be produced via cationic polymerization,
many monomer families, for example, methacrylates, acrylates,
(meth)acrylamides, etc., that do not undergo cationic polymer-
ization, cannot be combined with IB using sequential monomer
addition. Therefore, new strategies combining LCP with
different controlled/living polymerization mechanisms have
been developed to expand the number of potential PIB-based
block copolymers. Among the various controlled/living
polymerization mechanisms, atom transfer radical polymer-
ization (ATRP) is recognized to be versatile with regard to
monomer type and tolerant to a wide variety of functional
groups, such as allyl, amino, epoxy, hydroxyl, and vinyl; ATRP
is also easy to implement due to the availability and/or relative
ease of synthesis of ATRP initiators.20 It is therefore not
surprising that quite a number of reports have appeared
involving synthesis of PIB-based block copolymers using a
combination of LCP and ATRP. Site (mechanism) trans-
formations involving addition of styrene to living PIB to
produce sec-benzylic chloride functionality,21−23 functionalizing
hydroxyl-terminated PIB with bromoisobutyryl (or bromopro-
pionyl) groups,24,25 and addition of 1,3-butadiene to living PIB
to produce allyl halide end groups,26 have been used to
transform the PIB growing chain end into an ATRP initiating
site. Masar and co-workers also reported the synthesis of PIB−
PS−PMMA−PS−PIB pentablock copolymers, where PMMA is
poly(methyl methacrylate), by producing α,ω-dichloro−PS−
PMMA−PS triblock copolymer via ATRP followed by LCP to
add IB.27 Dual initiators containing both cationic and ATRP
initiating sites have also been successfully used to prepare PIB-
based block copolymers.28−30
In reference 23, we reported the synthesis and character-
ization of PtBA−PS−PIB−PS−PtBA pentablock terpolymers as
potential permselective barrier elastomers with enhanced
moisture transmission capabilities. The synthesis employed
LCP to first produce PS−PIB−PS, followed by site trans-
formation to ATRP to create the poly(tert-butyl acrylate)
(PtBA) outer blocks. Acid-catalyzed23,31 or thermal cleavage32
of the tert-butyl ester side groups yielded PAA−PS−PIB−PS−
B
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Scheme 2. Synthetic Routes to 3-[3,5-Bis(1-chloro-1-methylethyl)phenyl]-3-methylbutyl 2-Bromo-2-methylpropionate
(DCCBMP) via 3-(3,5-Diisopropylphenyl)-3-methylbutyl 2-Bromo-2-methylpropionate (DIPBMP): Radical Bromination (Left,
Previous33) and Aerobic Oxidation (Right, Current)
The architectures shown in Scheme 1 are readily prepared
using a dicationic initiator with an initiation site for another
polymerization process such as ATRP. We recently reported35
the synthesis of such an initiator, 3-[3,5-bis(1-chloro-1-
methylethyl)phenyl]-3-methylbutyl 2-bromo-2-methylpropio-
nate (DCCBMP) (Scheme 2, left). The key intermediate, 3-
(3,5-diisopropylphenyl)-3-methylbutyl 2-bromo-2-methylpropi-
onate (DIPBMP), was produced by the Friedel−Crafts
alkylation of diisopropylbenzene by 3-methyl-3-butenyl 2-
bromo-2-methylpropionate, 3. Selective radical bromination
of DIPBMP proved problematic, however, and after chromato-
graphic dehydrobromination, yielded a liquid diolefin inter-
mediate that was difficult to purify by vacuum distillation. We
therefore sought a different synthetic route, particularly one
that yielded a crystalline intermediate that could be purified by
recrystallization. It has been reported that benzylic CH of
alkylbenzenes can be oxidized with N-hydroxyphthalimide
(NHPI) and Co(OAc)2·4H2O catalyst system under oxygen
atmosphere at ambient pressure.36−38 Herein, we report a new
synthetic route (Scheme 2, right), in which DIPBMP is
oxidized using this procedure to yield the crystalline dihydroxy
intermediate, (DCOHBMP). This compound is a stable solid
and may be purified by recrystallization. It is convenient to
store the initiator in the form of DCOHBMP, and then to carry
out the final, high-yielding chlorination step immediately prior
to use.
remove these groups and leave only the desired bromoester
function. Ivan et al. found that poly(vinyl chloride) readily and
́
quantitatively dehydrochlorinates upon heating to 180−200
°C.40 Here the PS−PIB−PS macroinitiator is processed in the
same way to deactivate the PS chain ends and produce the
miktoarm star, poly(styrene-b-isobutylene)2-s-poly(tert-butyl
acrylate) [(PS−PIB)2-s-PtBA] after ATRP of tert-butyl acrylate
(tBA). The tert-butyl ester side groups are then cleaved using
the simple thermal treatment described by Kopchick et al.32 to
convert PtBA to PAA and yield the desired amphiphilic
poly(styrene-b-isobutylene)2-s-poly(acrylate acid) [(PS−PIB)2-
s-PAA] miktoarm star terpolymers (Figure 1, right).
The proper use of DCCBMP requires that the carbocationic
polymerization be done first, followed by ATRP. The LCP of
styrene naturally creates sec-benzylic chloride PS chain ends,
known ATRP initiators.39 Therefore, PS−PIB−PS polymers
synthesized from DCCBMP require a procedure that will
1
Figure 1. H and 13C NMR spectra of 3-(3,5-diisopropylphenyl)-3-
methylbutyl 2-bromo-2-methylpropionate (DIPBMP).
C
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Scheme 3. Synthesis of (PAA−PS−PIB)2-s-PAA and (PS−PIB)2-s-PAA miktoarm star polymers
The original PS−PIB−PS macroinitiator can also be used to
initiate tBA in three directions to produce poly(tert-butyl
acrylate-b-styrene-b-isobutylene)2-s-poly(tert-butyl acrylate)
[(PtBA−PS−PIB)2-s-PtBA] miktoarm star terpolymers, which
could be further converted into poly(acrylate acid-b-styrene-b-
isobutylene)2-s-poly(acrylate acid) [(PAA−PS−PIB)2-s-PAA]
in the same manner described above. Bates et al. reported that
architecturally asymmetric tetrablock terpolymer, poly-
[(ethylene oxide)-b-styrene-b-butadiene-b-(ethylene oxide)],
forms interesting bilayer vesicles at a high weight percentage
of the hydrophilic block when placed in a solvent selective for
the outer block.41 In addition, Balsara and co-workers
discovered platelet self-assembly of an amphiphilic tetrablock
copolymer, poly(styrenesulfonate-b-methylbutylene-b-ethyl-
ethylene-b-styrenesulfonate).42 We anticipate similar morpho-
logical behavior will be observed for our (ABC)2A star
polymers.
EXPERIMENTAL SECTION
■
Materials. 3-Methyl-3-buten-1-ol (≥97%), 2-bromo-2-methylpro-
pionyl bromide (98%), 1,3-diisopropylbenzene (96%), N-hydroxyph-
thalimide (NHPI) (97%), Co(OAc)2·4H2O (≥98.0%), acetonitrile
(anhydrous, 99.8%), methylcyclohexane (MCHex) (anhydrous, ≥
99%), triethylamine (TEA) (99.5%), 1,1,4,7,7-pentamethyldiethylene-
triamine (PMDETA) (99%), hexane (anhydrous, 99%), 2,6-lutidine
(99+%), TiCl4 (99.9%), CuIBr (99.999%), AlCl3 (99.99%), toluene
(anhydrous, 99.8%), CDCl3, and balloons with wall thickness of 15 mil
were used as received from Sigma-Aldrich, Inc. Compressed oxygen
was used as received from Praxair, Inc. Petroleum ether, CH2Cl2,
K2CO3, THF, MgSO4, and NaHCO3 were used as received from
Fisher Chemical Co. Dowex HCR-W2 ion-exchange resin (strong
cationic type) was used as received from Dow Chemical, Germany.
Isobutylene (IB) (99.5%, BOC Gases) and MeCl (99.5%, Alexander
Chemical Co.) were dried through columns packed with CaSO4 and
CaSO4/4 Å molecular sieves, respectively. tert-Butyl acrylate (tBA)
(99%) was passed through a K2CO3 and Al2O3 column to remove
inhibitor.
D
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Instrumentation. Absolute molecular weights and polydispersity
indices (PDIs) of polymers were determined using a size exclusion
chromatography (SEC) system (35 °C, THF, 1 mL/min) consisting of
a Waters Alliance 2695 Separations Module, online multiangle laser
light scattering (MALLS) detector (MiniDAWN, Wyatt Technology,
Inc.), interferometric refractometer (Optilab rEX, Wyatt Technology
Inc.), and two mixed D (5 μm beadsize) PL gel (Polymer Laboratories
Inc.) GPC columns connected in series. The dn/dc values used for PIB
homopolymers were calculated from the following equation:43 dn/dc =
DIPBMP (7.30 g, 18.4 mmol), NHPI (1.303 g, 8 mmol),
Co(OAc)2·4H2O (0.101 g, 0.41 mmol), and 25 mL MeCN. Because
Co(OAc)2·4H2O and NHPI are sparingly soluble in MeCN, an orange
heterogeneous mixture was obtained. The flask was then capped with a
balloon filled with pure oxygen. The reaction was stirred vigorously at
23 °C for 72 h. The solvent was vacuum stripped, and the solid was
washed with CH2Cl2 (3 × 25 mL). A clear, light-yellow liquid was
obtained after drying over MgSO4. The oxidized diol product,
DCOHBMP (2.61 g, 6.05 mmol, 33.1% yield) was obtained as a
white solid after removing minor quantities of several byproducts,
including dihydroperoxy, diolefin, and monohydroxy monohydroper-
oxy, as well as byproducts containing methyl ketone moieties, by
column chromatography (SiO2, hexane/THF (v/v) = 2:1 cosolvents
as the eluent). The preliminarily purified product was dissolved to
saturation in toluene at 60 °C. Upon cooling to −10 °C for several
hours, pure DCOHBMP was formed as white crystals, which were
0.116(1 − 108/M ) (M = number-average molecular weight); the
̅
̅
n
n
dn/dc values for block and miktoarm star copolymers were calculated
from the interferometric refractometer detector response and
1
assuming 100% mass recovery from the columns. Solution H and
13C nuclear magnetic resonance (NMR) spectra were obtained at 22
°C using CDCl3 as the solvent and tetramethylsilane as the internal
reference. Progress of the cationic polymerization of IB was monitored
using real-time, remote-probe (light conduit type) attenuated total
reflectance Fourier transform infrared spectroscopy (FTIR) (ReactIR
4000). Detailed descriptions of the SEC, NMR, and FTIR
instrumentation and corresponding procedures have been previously
published.44
The melting point of 3-[3,5-bis(1-hydroxy-1-methylethyl)phenyl]-
3-methylbutyl 2-bromo-2-methylpropionate (DCOHBMP) was meas-
ured using a Q200 (TA Instruments) differential scanning calorimeter.
The furnace atmosphere was purged with a 50 mL/min nitrogen
stream. Standard capped aluminum crucibles were loaded with ∼5 mg
of DCOHBMP solid, and the sample was subjected to a temperature
ramp of 1 °C/min from 35 to 100 °C.
High resolution mass spectrometry of DCOHBMP was performed
using a Waters LCT Premier XE benchtop orthogonal acceleration
time-of-flight (oa-TOF) mass spectrometer, and the sample was
analyzed by solids probe by electron-impact (EI) mode at 70 eV. A
crystal was placed into a borosilicate glass capillary, which was then
placed into the probe. Once the probe was inserted into the
instrument, the sample was heated quickly to 400 °C. The 218 mass of
heptacosa was used for a lockmass.
1
collected by filtration. Melting point =91.3 °C by DSC. H NMR
(CDCl3): δ = 1.40 (s, 6H, 4-H), 1.60 (s, 12H, PhCOH(CH3)2), 1.86
(s, 6H, (CH3)2CBr), 2.06 (t, 2H, 2-H), 2.31 (s, 2H, PhCOH(CH3)2),
4.01 (t, 2H, 1-H), 7.40 (m, 2H, 2,6-PhH), 7.43 (m, 1H, 4-PhH) ppm.
13C NMR: δ = 29.4 (C4), 30.7 ((CH3)2CBr), 31.9 (PhCOH(CH3)2),
37.0 (C3), 41.9 (C2), 55.9 ((CH3)2CBr), 63.8 (C1), 72.8 (PhCOH-
(CH3)2), 118.1 (4-PhC), 120.1 (2,6-PhC), 148.0 (1-PhC), 149.0 (3,5-
PhC), 171.6 (CO) ppm. HRMS (EI): C21H33O479Br [M]+, calcd
428.1562, found 428.1575; C21H33O481Br [M]+, calcd 430.1542, found
430.1553.
1
Figures A, B, and C, Supporting Information, show H and 13C
NMR spectra of diolefin, dihydroperoxy, and monohydroxy
monohydroperoxy byproducts, respectively.
Synthesis of 3-[3,5-Bis(1-chloro-1-methylethyl)phenyl]-3-
methylbutyl 2-Bromo-2-methylpropionate (DCCBMP). The
final product, DCCBMP, was obtained as a pale yellow liquid (2.63
g, 93.2% yield) by chlorination of DCOHBMP using anhydrous,
gaseous HCl, as previously reported.42 1H NMR (CDCl3): δ = 1.41 (s,
6H, 4-H), 1.86 (s, 6H, (CH3)2CBr), 2.01 (s, 12H, PhCCl(CH3)2),
2.06 (t, 2H, 2-H), 4.04 (t, 2H, 1-H), 7.50 (m, 2H, 2,6-PhH), 7.62 (m,
1H, 4-PhH) ppm. 13C NMR: δ = 29.2 (C4), 30.6 ((CH3)2CBr), 34.4
(PhCCl(CH3)2), 37.0 (C3), 41.8 (C2), 55.8 ((CH3)2CBr), 63.5 (C1),
69.9 (PhCCl(CH3)2), 120.2 (4-PhC), 122.1 (2,6-PhC), 146.0 (1-
PhC), 148.0 (3,5-PhC), 171.5 (CO) ppm.
Initiation Performance Test (Isobutylene Homopolymeriza-
tion). LCPs of IB were carried out within an inert atmosphere
glovebox equipped with a hexane/heptane cold bath, following the
previously described procedure.42 Polymerizations were performed at
−70 °C, using DCCBMP or 5-tert-butyl-1,3-(1-chloro-1-methylethyl)-
benzene (t-Bu-m-DCC) as the initiator. TiCl4 served as the catalyst,
and 2,6-lutidine as the Lewis base in 60/40 (v/v) MCHex/MeCl
cosolvents.
PS−PIB−PS Synthesis. PS−PIB−PS triblock copolymers were
produced via LCP and sequential monomer addition within a drybox
at −70 °C, using DCCBMP as the initiator, TiCl4 as the catalyst, and
2,6-lutidine as the Lewis base in 60/40 (v/v) MCHex/MeCl
cosolvents. FTIR (ReactIR 4000) was used to monitor isobutylene
and styrene conversions by observing the olefinic CH2 wag of IB
(887 cm−1) and syrene (907 cm−1).46 The DiComp probe was
inserted into a 250 mL 4-necked round-bottom flask equipped with a
temperature probe and a stirring shaft with a Teflon paddle. The
reactor was placed into the cold bath and allowed to equilibrate to −70
°C. Into the flask were charged 57.9 mL of prechilled MCHex, 38.6
mL of prechilled MeCl, 2,6-lutidine (0.0489 mL, 4.23 × 10−4 mol),
and DCCBMP (0.3005 g, 6.45 × 10−4 mol). The mixture was allowed
to stir for 10 min to reach thermal equilibrium before a background
spectrum was collected. Prechilled IB (8.50 mL, 0.106 mol) was added
to the flask, and then about 15 spectra were acquired to establish the
average intensity of the 887 cm−1 peak, A0, corresponding to the initial
monomer concentration. At this point, TiCl4 (0.707 mL, 6.44 × 10−3
mol) was injected into the flask. The initial molar concentrations of
reagents were [IB]0 = 1.0 M, [DCCBMP]0 = 6.1 mM, [2,6-lutidine]0 =
4.0 mM, and [TiCl4]0 = 61.0 mM, and the total volume was 105.7 mL.
Once the IB monomer was fully consumed (>99% conversion),
Synthesis of 3-(3,5-Diisopropylphenyl)-3-methylbutyl 2-
Bromo-2-methylpropionate (DIPBMP). First, 3-Methyl-3-butenyl
2-bromo-2-methylpropionate, 3, was produced by reacting 3-methyl-3-
buten-1-ol (2) with 2-bromo-2-methylpropionyl bromide (1) using a
variation of a previously reported procedure (Scheme 2).28 DIPBMP
was synthesized by the Friedel−Crafts alkylation of diisopropylben-
zene by 3, using a modification of the procedure described by Cheon
and Yamamoto,45 as follows: within an inert atmosphere glovebox
equipped with a hexane/heptane cold bath, a 250 mL two-necked,
round-bottom flask, equipped with mechanical stirrer, was charged
with 1,3-diisopropylbenzene (85.7 g, 0.528 mol) and AlCl3 (26.5 g,
0.200 mol). This mixture was chilled to −20 °C and stirred vigorously.
Then, 3 (40.6 g, 0.173 mol) was slowly added, and the slurry was
stirred vigorously for another 25 h. It was added to ice-cold water (500
mL), and this mixture stirred for 2 h. The organic phase was separated,
and the water layer was extracted with CH2Cl2. The combined organic
solutions were washed with brine and then DI water and dried over
anhydrous MgSO4. After filtration, CH2Cl2 was removed by vacuum
stripping, and the desired compound, 3-(3,5-diisopropylphenyl)-3-
methylbutyl 2-bromo-2-methylpropionate, was obtained as a light
1
yellow oil (34.0 g, 49.5% yield) upon vacuum distillation. H NMR
(CDCl3): δ = 1.25 (d, 12H, PhCH(CH3)2), 1.38 (s, 6H, 4-H), 1.86 (s,
6H, (CH3)2CBr), 2.02 (t, 2H, 2-H), 2.88 (m, 2H, PhCH(CH3)2), 4.03
(t, 2H, 1-H), 6.92 (m, 1H, 4-PhH), 7.02 (m, 2H, 2,6-PhH) ppm. 13C
NMR: δ = 24.1 (PhCH(CH3)2), 29.2 (C4), 30.7 ((CH3)2CBr), 34.3
(PhCH(CH3)2), 36.7 (C3), 42.1 (C2), 55.9 ((CH3)2CBr), 63.9 (C1),
121.3 (2,6-PhC), 121.8 (4-PhC), 147.8 (1-PhC), 148.5 (3,5-PhC),
171.5 (CO) ppm.
Synthesis of 3-[3,5-bis(1-hydroxy-1-methylethyl)phenyl]-3-
methylbutyl 2-bromo-2-methylpropionate (DCOHBMP). The
dihydroxy intermediate DCOHBMP was prepared via aerobic
oxidation of DIPBMP (Scheme 2, right) using the N-hydroxyph-
thalimide (NHPI)/Co(OAc)2·4H2O catalyst system.35 Into a 250 mL
Erlenmeyer flask equipped with a magnetic stirrer were added
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indicated by the 887 cm−1 absorbance approaching an asymptotic
value (Ar), a mixture of prechilled 25.7 mL of MCHex, 17.1 mL of
MeCl, and 7.1 g of styrene was added. These amounts were designed
to achieve [styrene]0 = 0.4 M, assuming no volume loss when IB
monomer was converted into polymer, while maintaining a 60/40 (v/
v) MCHex/MeCl cosolvents system. The FTIR instrument was reset
to monitor the disappearance of the styrene band at 907 cm−1. Once
the styrene conversion reached ∼50%, 20 mL of prechilled CH3OH
was added to quench the polymerization; styrene conversion was
limited in order to prevent chain transfer to polymer (EAS side
reaction). After warming to room temperature and loss of MeCl,
MCHex solvent and remaining styrene monomer were vacuum
stripped, and sufficient THF was added to completely dissolve the
polymer. The PIB−PS−PIB sample was then precipitated into a 5−
10× volume excess of MeOH and dried under vacuum to yield a white
solid product.
Star Polymer Synthesis. Miktoarm star terpolymers of the type
(PtBA−PS−PIB)2-s-PtBA were prepared by initiating tBA directly
from unmodified PS−PIB−PS, which inherently carries sec-benzylic
chloride PS chain ends as well as the bromoester functionality at the
initiator moiety. However, to produce (PS−PIB)2-s-PtBA, the sec-
benzylic chloride chain ends had to be deactivated; this was achieved
by heating PS−PIB−PS to 180−200 °C at 30 mmHg of vacuum
overnight.
ATRP of tBA was performed using CuBr as the catalyst, PMDETA
as the ligand, and PS−PIB−PS as the macroinitiator (MacroI)
following the same procedure as previously reported for ATRP of
methyl acrylate.42 Polymerizations were performed using a molar ratio
[MacroI]0:[CuBr]0:[PMDETA]0 = 1:1:1 in toluene at 70 °C, with
[MacroI]0 = 0.01 M and with [tBA]0 set at various levels to achieve
different molecular weights. Conversion of tBA was limited to ∼60%
to avoid coupling. After polymerization, the solution was passed
through a column packed with ion-change resin and neutral Al2O3 to
remove copper salt. Then THF was added to completely dissolve the
polymer, and the resulting solution was passed through a 0.2 μm filter
to remove any remaining Al2O3. The solution was then precipitated
into a 5−10× volume excess of MeOH, and white (PS−PIB)2-s-PtBA
was collected by filtration and dried under vacuum at room
temperature.
to 1,3-diisopropylbenzene, the olefinic carbon peaks disap-
peared, and corresponding new peaks a and d appeared at 29.2
and 36.7 ppm, respectively. Peak l (1-PhC) shifted downfield to
147.8 ppm. Peaks i and j shifted upfield to 121.8 and 121.3
ppm, respectively. Only four peaks were observed in the
aromatic carbon region, indicating that a symmetric structure
was obtained and that substitution occurred only at the 1-Ph
carbon.
In a previous report,33 we used selective bromination of
DIPBMP, followed by dehydrobromination, to produce the
diolefin, 3-(3,5-diisopropenylphenyl)-3-methylbutyl 2-bromo-
2-methylpropionate, an alternative intermediate for DCCBMP
(Scheme 2, left). This route was less than satisfactory, however,
because the purity of the liquid diolefin was only moderate, and
it was difficult to purify by vacuum distillation. We therefore
sought a different synthetic route, particularly one that yielded a
solid intermediate that could be purified by recrystallization.
We found that NHPI/Co(OAc)2·4H2O-catalyzed free radical
aerobic oxidation (Scheme.2, right) works satisfactorily to
convert DIPBMP into the di-tert-hydroxyl intermediate,
DCOHBMP, which is a crystalline solid at room temperature.
Minisci et al. showed that aerobic oxidation of cumene using
this catalyst system usually produces a methyl aryl ketone side
product (i.e., acetophenone in the case of cumene);35 however,
selectivity toward the desired cumyl alcohol could be increased
by using low reaction temperature and a nonpolar solvent
medium. Solvents recommended by these authors, such as
chlorobenzene and acetonitrile, as well as the nonpolar aliphatic
solvent hexane, were tested as reaction solvents for the room
temperature oxidation of DIPBMP. No reaction was observed
for hexane, and only very low conversion was obtained with
chlorobenzene. We next conducted a series of reactions in
acetonitrile at various temperatures (40 °C, 23 and 0 °C). We
found that the amount of methyl aryl ketone decreased from
about 35% to 21% when the temperature was reduced from 40
to 23 °C, but the time to reach full conversion of the DIPBMP
rose from <24 to about 72 h. At a reaction temperature of 0 °C,
conversion was still very low (<10%) after 24 h; this was
considered unreasonably slow, and the reaction was stopped.
We concluded that the most convenient conditions for the
reaction are the use of acetonitrile as the solvent at room
temperature, despite the relatively long reaction time required.
Isolation and purification of DCOHBMP from the crude
reaction mixture was carried out using a combination of column
chromatography and recrystallization. The crude product was a
sticky yellow liquid from which we were unable to obtain
crystals from toluene without preliminary purification. Column
chromatography (SiO2) effectively removed both the methyl
aryl ketone and hydroperoxy byproduct; however, it caused
some loss of the desired product to olefin via dehydration. The
product after column chromatography was a eutectic liquid that
could be recrystallized from toluene to yield pure DCOHBMP
as a stable, white solid. Since the olefin and hydroperoxy
byproduct also yield the desired final product, DCCBMP, upon
hydrochlorination, they could theoretically be further purified
and hydrochlorinated to increase the overall yield; however, we
did not attempt this optimization strategy in this initial study.
High-resolution mass spectrometry (HRMS) analysis of the
recrystallized product revealed two molecular ions, correspond-
ing to the presence of either 79Br and 81Br, thus confirming the
structure of DCOHBMP. Differential scanning calorimetry
(DSC) revealed the mp to be 91.3 °C.
To convert PtBA into PAA, the star polymers were exposed to 150
°C at 30 mmHg of vacuum overnight to obtain amphiphilic (PAA−
PS−PIB)2-s-PAA and (PS−PIB)2-s-PAA miktoarm star polymers.
RESULTS AND DISCUSSION
■
Synthesis of DCCBMP. The key intermediate for
DCCBMP, 3-(3,5-diisopropylphenyl)-3-methylbutyl 2-bromo-
2-methylpropionate (DIPBMP), was produced by Friedel−
Crafts alkylation of 1,3-diisopropylbenzene by 3-methyl-3-
butenyl 2-bromo-2-methylpropionate, 3, at −20 °C in a drybox.
AlCl3 was added at approximately an equal molar ratio to 3,
forming a thick, deep-red slurry. Low temperature was
employed to prevent unwanted substitution reactions at other
carbons on the 1,3-diisopropylbenzene.
1H and 13C NMR spectra of DIPBMP are shown in Figure 1.
Upon alkylation, the olefinic protons in 3 disappeared, and the
new methyl protons of the tether unit appeared at 1.38 ppm
(peak a). The triplet at 4.03 ppm (peak e) was assigned to the
methylene protons of the tether unit adjacent to the
bromoester (1-H). The methylene protons further from the
ester (2-H) were observed as a triplet at 2.02 ppm (peak d).
The methyl groups of the 2-bromo-2-methylpropionate moiety
(ATRP initiating site) exhibited a peak at 1.86 ppm (peak b).
The splitting pattern of the aromatic proton peaks changed
upon substitution at 1-Ph, yielding two main peaks at 6.92 ppm
(peak g) and 7.02 ppm (peak f). Peak integration values were
consistent with the proposed chemical structure. 13C NMR data
provided better resolved peaks for analysis. After 3 was attached
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Figure 2 shows the NMR characterization results for
DCOHBMP. In the proton spectrum, the multiplet at 2.88
adjacent methyl groups shifted to 69.9 and 34.4 ppm,
respectively. Moreover, the 13C NMR spectrum consisted of
exactly 13 signals, which were unambiguously assigned to the
13 carbons in DCCBMP, providing further evidence that the
targeted structure was obtained.
Initiation Performance Test. Three PIB samples, PIB-5K,
PIB-10K, and PIB-20K, were prepared in order to study the
cationic initiation performance of DCCBMP. Table 1 lists
NMR and SEC characterization data for these samples. Figure 4
shows the proton NMR spectrum of a representative sample,
PIB-5k. The PIB backbone methyl and methylene protons
appear at about 1.1 ppm (peak o) and 1.4 ppm (peak p),
respectively. Characteristic peaks for the DCCBMP initiator, b,
e, f, and g, are present, indicating that the radical initiating site
remains intact after cationic polymerization. Number-average
functionality with respect to the ATRP initiating site, Fn,Br, was
̅
quantified and found to be in the range 0.93−1 (Table 1). F
̅
n,Br
1
was calculated from H NMR data using eq 1, where ACH O is
2
Figure 2. 1H and 13C NMR spectra of 3-[3,5-bis(1-hydroxy-1-
methylethyl)phenyl]-3-methylbutyl 2-bromo-2-methylpropionate
(DCOHBMP) obtained by the aerobic oxidation of DIPBMP.
the integrated area of the methylene protons adjacent to the
ester group (peak e) and ACE is the sum of the integrated peak
areas of characteristic resonances representing the various
polymer chain ends, defined by eq 2.
ppm (peak j, Figure 1) associated with the benzylic protons of
DIPBMP was absent. The doublet representing the methyl
protons of the isopropyl groups, formerly at 1.25 ppm (peak c,
Figure 1), was converted into a singlet at 1.60 ppm (peak c)
upon aerobic oxidation. A new peak representing the hydroxyl
proton appeared at 2.31 ppm (peak h). Peaks f and g shifted
downfield to 7.40 and 7.43 ppm. Integrated areas of all peaks
were within 98.7% of the theoretical value. In the carbon
spectrum, the oxidized carbon and adjacent methyl carbons
shifted downfield to 72.8 ppm (peak h) and 31.9 ppm (peak c),
respectively, and peak i shifted upfield. The other three
aromatic carbon peaks, as well as all carbon signals for the
bromoester tail, were observed at essentially the same chemical
shift in reactant and product.
A
CH2O
F
=
̅
n,Br
ACE
ACE = Aexo + Aendo + Atert‐Cl/2 + 2Acoupled
(1)
(2)
In eq 2, Aexo is the area of the upfield exo-olefinic resonance at
4.64 ppm, Aendo is the area of the single endo-olefinic resonance
at 5.15 ppm, and Atert‑Cl is the area of the resonance at 1.96 ppm
due to the methylene protons of the tert-chloride end group.
Acoupled was calculated as follows:
(A4.75−5.0 − Aexo)
Acoupled
=
(3)
2
Figure 3 shows the 1H and 13C NMR spectra of pure
DCCBMP prepared by chlorination of DCOHBMP. The
methyl protons adjacent to chlorine were observed at 2.01 ppm
(peak c), which represents a downfield shift relative to their
position in the precursor. Peaks associated with the aromatic
protons shifted slightly downfield to 7.50 (peak f) and 7.62
(peak g) ppm. The carbon attached to chlorine and the
where A4.75−5.0 is the integrated area of the convoluted peaks
from 4.75 to 5.0 ppm associated with the downfield exo-olefinic
proton and the two identical protons of the coupled product.
Equation 2 represents our standard protocol for determination
of PIB end group composition and accounts for the possible
occurrence of all likely terminal chain end types, regardless of
whether they are actually present in the sample. In the present
case, the vast majority of the end groups are tert-Cl (>96% for
all samples).
In Table 1, initiation efficiency of DCCBMP, Ieff, was
calculated as X
SEC/MALLS data, respectively. X
/X
and M
/M
from NMR and
̅
̅
̅
̅
n,theo
n,PIB
n,theo
n,PIB
was calculated as the
̅
n,theo
molar ratio of monomer to initiator charged to the reactor;
= (X × MIB) + MI, where MIB and MI are the
M
̅
̅
n,theo
n,theo
molecular weights of isobutylene and the initiator, respectively.
1
X
̅
was calculated from H NMR data using eq 4,
n,PIB
AMe
3
X
̅
=
+ 2
n,PIB
ACE
(4)
where, AMe is the integrated peak area of the methyl protons in
the PIB repeat unit. Ieff of DCCBMP characterized by NMR
spectroscopy was 0.93−0.95. The SEC eulograms of all three
PIBs were symmetrical, and PDIs were all less than 1.02. Ieff
calculated from SEC data were in the range 0.89−0.98.
Figure 3. 1H and 13C NMR spectra of 3-[3,5-bis(1-chloro-1-
methylethyl)phenyl]-3-methylbutyl 2-bromo-2-methylpropionate
(DCCBMP).
Control experiments were conducted under the same
conditions using the standard difunctional cationic initiator,
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Table 1. Characterization of PIBs Prepared from Di-cationic Mono-radical Dual Initiator, DCCBMP, and Control Difunctional
a
Cationic Initiator, t-Bu-m-DCC
NMR
SEC
X
X
F
Ieff
M
̅
(g/mol)
M
(g/mol)
n,theo
Ieff
PDI
Ieff, t‑Bu‑m‑DCC
̅
̅
̅
̅
n,PIB
n,theo
n,Br
0.97
0.93
1.00
n,PIB
PIB-5K
PIB-10K
PIB-20K
89
82
0.92
0.94
0.95
5700
5070
0.89
0.95
0.98
1.02
1.01
1.01
0.97
1.00
0.99
175
358
164
340
10 170
20 020
9670
19 540
a
−70 °C; 60/40 MCHex/MeCl cosolvents (v/v); [IB]0 = 1.00 M; [2,6-lutidine]0 = 4.00 mM. [I]0 = 12.2 mM; [TiCl4]0 = 48.8 mM for 5K samples.
[I]0 = 6.10 mM; [TiCl4]0 = 48.8 mM for 10K samples. [I]0 = 3.05 mM; [TiCl4]0 = 91.5 mM for 20K samples.
these copolymers are given in Table 2 and Figure 6. As listed in
Table 2, PS−PIB−PS macroinitiators were prepared with very
Table 2. Molecular Weight Data for PS−PIB−PS
Macroinitiators Prepared from DCCBMP via LCP and
a
Sequential Monomer Addition
Mn,PIB (g/mol)
Mn,PS−PIB−PS (g/mol)
PDI
b
c
PS−PIB−PS-1
PS−PIB−PS-2
11 380
20 020
20 150
32 060
1.06
1.02
a
−70 °C; 60/40 MCHex/MeCl cosolvents (v/v); [IB]0 = 1.00 M;
b
[2,6-lutidine]0 = 4.00 mM; [St]0 = 0.4 M. [DCCBMP]0 = 6.1 mM;
[TiCl4]0 = 61.0 mM; [St]0/[DCCBMP]0 = 96; conv.(styrene) = 0.88
calculated based on SEC result. [DCCBMP]0 = 2.94 mM; [TiCl4]0 =
c
1
Figure 4. H NMR spectrum of PIB-5K initiated by DCCBMP.
88.2 mM; [St]0/[DCCBMP]0 = 160; conv.(styrene) = 0.72 calculated
based on SEC result.
5-tert-butyl-1,3-bis(1-chloro-1-methylethyl)benzene (t-Bu-m-
DCC). PDIs of the resulting PIBs were the same as those
obtained from DCCBMP, i.e., less than 1.02. Ieffs calculated
from SEC data were ∼1, comparable with those of DCCBMP.
In addition, real-time ATR-FTIR monitoring of IB polymer-
izations initiated from DCCBMP yielded linear first order
kinetic plots, similar to those obtained with t-Bu-m-DCC, as
illustrated in Figure 5. These results show that the presence of
Figure 6. SEC elution curves of PIB segment in PS−PIB−PS-1
(black), PS−PIB−PS-1 (red), PS−PIB−PS-1 after thermolysis
(green), which overlaps with PS−PIB−PS-1, and (PS−PIB)2-s-PtBA
star polymers, ATRP-1 (cyan) and ATRP-2 (blue).
narrow PDIs (1.06 for PS−PIB−PS-1, 1.02 for PS−PIB−PS-2).
The targeted molecular weight of the PIB block in PS−PIB−
PS-1 was 9670 g/mol; the experimental molecular weight was
11 380 g/mol, yielding Ieff = 0.85. For PS−PIB−PS-2, the
Figure 5. First-order kinetic plot for DCCBMP-initiated IB polymer-
ization at −70 °C. Conditions were as follows: 60/40 Hex/MeCl
cosolvents (v/v); [IB]0 = 1.00 M; [2,6-lutidine]0 = 4.00 mM;
[DCCBMP]0 = 12.2 mM; [TiCl4]0 = 48.8 mM targeting 5k.
experimental M
(20 020 g/mol) was almost the same as
̅
n,PIB
M
̅
(19 540 g/mol), yielding Ieff = 0.98. Conversion of
n,theo
styrene (0.88 for PS−PIB−PS-1, 0.72 for PS−PIB−PS-2) was
higher than targeted (0.50), because of the difficulty of
estimating the initial absorbance at 907 cm−1, when styrene is
added to the system already containing TiCl4. Thus, the PS−
PIB−PS polymers were composed of more PS volume than the
designed value.
Figure 6 shows the progression of SEC elution curves during
the synthesis of PS−PIB−PS-1, which is representative. PIB
initiated by DCCBMP (black) was characterized by a narrow
and symmetrical peak. Elution profile of the triblock copolymer
the ATRP initiating site on DCCBMP does not interfere with
or present any special problems in LCP and that pure
DCCBMP prepared via the aerobic oxidation route enables
precise control over molecular weight, polydispersity, and
functionality of the resulting polymers.
PS−PIB−PS Synthesis. PS−PIB−PS triblock copolymers
with bromoester functionality at the approximate center of PIB
block were prepared via sequential LCP of IB and then styrene,
using DCCBMP as the initiator. Results of SEC analysis of
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(red) was still symmetrical but shifted to lower elution volumes,
indicating that LCP of the second monomer, styrene, occurred
to form the targeted triblock copolymer.
Table 3. Molecular Weight Data for (PS−PIB)2-s-PtBA and
(PtBA−PS−PIB)2-s-PtBA Star Polymers Prepared via
a
ATRP
Figure 7 (upper) shows the 1H NMR spectrum of a
representative sample, PS−PIB−PS-2. The styrene backbone
[tBA]0/
[MacroI]0
Mn,star
(g/mol)
Mn,PtBA
(g/mol)
M
̅
PDI
n,theo
b
b
c
ATRP-1
ATRP-2
ATRP-3
ATRP-4
100
200
100
580
26 120
36 350
40 250
77 450
5970
16 200
8190
7690
15 380
7690
1.08
1.07
1.02
1.03
c
45 390
45 140
a
70 °C; toluene; [MacroI]0 = 0.01 M; [MacroI]0:[CuBr]0:
b
[PMDETA]0 = 1:1:1; quenched at 60% tBA conversion. using
thermolyzed PS−PIB−PS-1 as macroinitiator, producing (PS−PIB)2-s-
c
PtBA star polymers. using PS−PIB−PS-2 as macroinitiator, producing
(PtBA−PS−PIB)2-s-PtBA star polymers.
Figure 6 illustrates the SEC elution curves of (PS−PIB)2-s-
PtBA star polymers, ATRP-1 (cyan) and ATRP-2 (blue),
initiated by thermolyzed PS−PIB−PS-1. Both star polymers
possessed narrow PDIs (<1.1), and the elution profiles were
symmetrical with no apparent shoulders, indicating very high
blocking efficiency from the macroinitiator and negligible
radical−radical coupling during ATRP. High blocking efficiency
confirms that the ATRP initiating site of the DCCBMP initiator
survives LCP and thermolysis at 180−200 °C.
1
Figure 7. 1H NMR spectra of PS−PIB−PS-2 before (upper) and after
thermolysis (lower).
Figure 8 (upper) shows the H NMR spectrum of (PtBA−
PS−PIB)2-s-PtBA star polymer (ATRP-3). Addition of the
methylene and methine protons were observed at 1.4 ppm
(peak q) and 1.8 ppm (peak r). The broad peaks at 6.4−7.2
ppm (collectively denoted s) were assigned to the aromatic
protons both of the PS block and DCCBMP initiator. The
methylene protons next to the bromoester group were
observed as a well-defined triplet at 4.0 ppm (peak e). The
characteristic broad absorbance due to the ultimate CH of the
PS block (sec-benzyl chloride proton, peak t) was observed at
4.3−4.4 ppm.
Star Polymers Synthesis. As illustrated in Scheme 2,
quenching LCP of styrene with MeOH produces sec-benzylic
chloride end groups, which are known to be active ATRP
initiation centers.21−23 To selectively grow tBA only from the
designated bromoesters, PS−PIB−PS macroinitiators were
thermolyzed to eliminate HCl and thus deactivate the PS
chain ends. After thermolysis, the broad peak associated with
the sec-benzyl chloride CH at 4.3−4.4 ppm disappeared, as
shown in the lower spectrum in Figure 7. Small peaks appeared
at 3.1 and 6.1 ppm, which were attributed to the newly formed
olefinic chain ends. PS−PIB−PS macroinitiators were rean-
alyzed by SEC after thermolysis, and as expected, the molecular
weights of the treated and untreated PS−PIB−PS were about
the same. As shown in Figure 7, the elution curve of the
thermolyzed PS−PIB−PS-1 (green) overlaps with PS−PIB−
PS-1 (red), indicating that the treated and untreated macro-
initiators differ only at the PS chain ends.
1
Figure 8. H NMR spectra of (PtBA−PS−PIB)2-s-PtBA (ATRP-3)
star polymer (upper), and the corresponding (PAA−PS−PIB)2-s-PAA
obtained by thermolysis (lower).
ATRP of tBA was next used to produce (PtBA−PS−PIB)2-s-
PtBA and (PS−PIB)2-s-PtBA miktoarm terpolymers from PS−
PIB−PS and thermolyzed PS−PIB−PS macroinitiators, re-
spectively. SEC characterization results for these star polymers
are given in Table 3. PS−PIB−PS macroinitiators, either in
their original state or after thermal deactivation of the PS chain
ends, worked well in ATRP. The PtBA segments of the
resulting miktoarm stars possessed molecular weights very close
to the targeted values, and the overall PDIs were narrow (<1.1).
PtBA blocks introduced new peaks at 1.5 (peak u) and 2.2 ppm
(peak (v), corresponding to the methylene and methine
backbone protons, respectively, of PtBA. As expected, (PS−
PIB)2-s-PtBA prepared from thermolyzed PS−PIB−PS showed
the same characteristic proton signals.
Thermolysis has been reported to eliminate isobutylene
molecules from PtBA via β-type scission and thus produce PAA
hydrophilic blocks.32 This technique proved to also work very
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1
well for the present systems. Figure 8 (lower) shows the H
NMR spectrum of (PAA−PS−PIB)2-s-PAA polymer obtained
after thermolysis. The integrated area for the combined PIB
backbone methylene and PtBA tert-butyl protons (peak p and
t) decreased. 13C NMR gave better evidence for removal of the
tert-butyl groups. Figure 9 shows 13C NMR spectra of ATRP-3
ization. SEC results showed that high initiation efficiency (Ieff =
0.89−0.98) and near-monodisperse polymers (PDI ≤ 1.02)
were obtained when DCCBMP was used to polymerize IB,
targeting molecular weights of 5k, 10k and 20K g/mol. The
cationic initiation performance of DCCBMP was essentially
identical to the standard aromatic difunctional cationic initiator,
t-Bu-m-DCC, which was utilized as a control (Table 1). The
bromoester functionality, which was designed for the
subsequent ATRP, was observed in the proton NMR spectrum
of the resulting PIB. The number-average functionality, M
was calculated to be 0.93−1.00, indicating that radical initiating
group was intact during IB polymerization.
̅
n,Br
DCCBMP was utilized to prepare a series of amphiphilic
(PAA−PS−PIB)2-s-PAA and (PS−PIB)2-s-PAA miktoarm star
polymers using a combination of LCP, sequential monomer
addition, and ATRP techniques. PS−PIB−PS triblock copoly-
mers were produced by living carbocationic polymerization of
IB followed by sequential addition of styrene. After quenching
with MeOH, the polymers thus obtained carried sec-benzylic
chloride groups at the PS chain ends, which are able to initiate
radical polymerization. Thus, as obtained, these polymers
possessed three ATRP initiating sites and were used to produce
(PtBA−PS−PIB)2-s-PtBA miktoarm star polymers.
The PS terminal functionality could alternatively be
selectively deactivated by heating the polymers to 180−200
°C in a vacuum oven. Proton NMR spectra showed that only
sec-benzylic chloride groups were removed, with no change
observed for the bromoester group. SEC characterization of
PS−PIB−PS macroinitiators before and after thermolysis
generated identical polymer elution profiles, indicating that
there were no backbone structural changes. Thermolyzed PS−
PIB−PS macroinitiators enabled PtBA growth only from the
bromoester group, yielding (PS−PIB)2-s-PtBA star polymers
under the same ATRP conditions.
Both types of miktoarm stars were prepared with designed
composition and narrow PDIs (<1.1) as confirmed by NMR
and SEC analysis. Upon thermolyzing these star polymers,
PtBA was completely converted to PAA, yielding amphiphilic
PIB-based star polymers (PAA−PS−PIB)2-s-PAA and (PS−
PIB)2-s-PAA, in which the third block can share an interface
with the PIB block. Their intriguing self-assembly behavior in
aqueous solution as well as phase-separated morphology in the
solid state are currently being investigated.
Figure 9. 13C NMR spectra of (PtBA−PS−PIB)2-s-PtBA (ATRP-3)
star polymer (upper) and the corresponding (PAA−PS−PIB)2-s-PAA
obtained by thermolysis (lower).
before (upper) and after thermolysis (lower). The methyl and
quaternary carbons of the tert-butyl groups of PtBA, which
appear at 81 ppm (peak y) and 29 ppm (peak t) in the upper
spectrum, as reported,32,47 completely disappear after thermol-
ysis.
CONCLUSIONS
■
The dual initiator DCCBMP containing two tert-benzylic
chloride groups for LCP initiation and one α-bromoester for
ATRP initiation was synthesized in four steps. Instead of free
radical bromination of the DIPBMP intermediate,33 a new
aerobic oxidation using NHPI and Co(OAc)2·4H2O catalyst
system was employed to convert DIPBMP into DCOHBMP, a
new compound that may be purified by recrystallization. The
chemical structure of this crystalline solid was confirmed by
proton NMR spectroscopy and HRMS, and its melting point
was determined by DSC to be 91.3 °C. Chlorination of
DCOHBMP yielded the final product, DCCBMP, as a light
yellow liquid. The chemical structure of DCCBMP was
confirmed by NMR spectroscopy. Compared with free radical
bromination, this mild aerobic oxidation reaction can easily
convert benzylic CH(CH3)2 into COH(CH3)2 without
destroying other functional groups. Although side products
were observed, which reduced the yield, the technique provides
a better synthetic route and can be potentially applied to the
syntheses of other cumyl-type cationic polymerization
initiators.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 13C NMR spectra (CDCl3, 23 °C) of diolefin
byproduct, 3-(3,5-diisopropenylphenyl)-3-methylbutyl 2-
bromo-2-methylpropionate, dihydroperoxy byproduct, 3-[3,5-
bis(1-hydroperoxy-1-methylethyl)phenyl]-3-methylbutyl 2-
bromo-2-methylpropionate, and monohydroxy, monohydroper-
oxy byproduct, 3-[3-(1-hydroperoxy-1-methylethyl)-5-(1-hy-
droxy-1-methylethyl)phenyl]-3-methylbutyl 2-bromo-2-methyl-
propionate, of the NHPI/Co(OAc)2·4H2O catalyzed aerobic
oxidation of DIPBMP. This material is available free of charge
via the Internet at http://pubs.acs.org/.
AUTHOR INFORMATION
■
The initiation performance of DCCBMP was investigated by
conducting TiCl4-catalyzed IB polymerizations at −70 °C.
FTIR spectroscopy demonstrated a linear first-order kinetic
plot that passes through the origin, indicating fast initiation and
a constant concentration of active chain ends during polymer-
Corresponding Author
*E-mail: robson.storey@usm.edu.
Notes
The authors declare no competing financial interest.
J
dx.doi.org/10.1021/ma3007762 | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
(35) Zhu, Y.; Storey, R. F. Polym. Prepr. (Am. Chem. Soc., Div. Polym.
Chem.) 2010, 51 (2), 252−253.
ACKNOWLEDGMENTS
■
The authors would like to thank The University of Southern
Mississippi Composites Center, funded by The Office of Naval
Research and Northrop Grumman, Award No. N00014-07-1-
1057, for financial support of this research.
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dx.doi.org/10.1021/ma3007762 | Macromolecules XXXX, XXX, XXX−XXX