Methacrylic stereoblock copolymers via the combination of catalytic chain transfer and anionic polymerization

Article abstract of DOI:10.1021/ma2020535

A novel synthetic pathway toward stereoblock copolymers by combining catalytic chain transfer polymerization with anionic polymerization is described. Catalytic chain transfer polymerization (CCTP) has been used to synthesize vinyl-terminated polymers, wh

Full text of DOI:10.1021/ma2020535

ARTICLE
pubs.acs.org/Macromolecules
Methacrylic Stereoblock Copolymers via the Combination of Catalytic
Chain Transfer and Anionic Polymerization
Gemma C. Sanders,†,‡ Timo J. J. Sciarone,†,‡ Hanneke M. L. Lambermont-Thijs,† Robbert Duchateau,† and
Johan P. A. Heuts*,†
†Laboratory of Polymer Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
‡Dutch Polymer Institute (DPI), John F. Kennedylaan 2, 5612 AB Eindhoven, The Netherlands
S Supporting Information
b
ABSTRACT: A novel synthetic pathway toward stereoblock
copolymers by combining catalytic chain transfer polymeriza-
tion with anionic polymerization is described. Catalytic chain
transfer polymerization (CCTP) has been used to synthesize
vinyl-terminated polymers, which, after the Michael addition of
α-lithioisopropyl isobutyrate, were used as macroinitiators for
the anionic polymerization of methacrylate-type monomers.
The resultant polymer consists of a predominantly atactic block
(originating from the free radical polymerization) and a more
isotactoid or syndiotactoid block (originating from the anionic polymerization).
’ INTRODUCTION
poly(methyl methacrylate) (pMMA) were made in the same
polymer. This method, which involved the sequential anionic
polymerization of diphenylmethyl methacrylate and trityl metha-
crylate followed byhydrolysis and methylation, israthertedious. In
1994, a more direct route was developed involving the use of
trialkylaluminum species to convert the selectivity of an isotactic
pMMA macroinitiator toward syndiotacticity.39 Group IV metal-
locenes have also been used in combination with boron or
aluminum species to create stereoblock copolymers of methacrylates.40
By exchanging a site-controlled polymerization with a chain-end
controlled synthesis, the selectivity of the growing polymer
chain could be effectively converted from being isotactic-rich
to syndiotactic-rich pMMA. Group IV metallocene/Lewis acid
hybrid catalysts have also been employed to synthesize isotacticÀ
syndiotactic block copolymers via a diastereotopic ion-pairing poly-
merization method.41 Although literature surrounding isotacticÀ
syndiotactic pMMA block copolymers is prevalent, we are not
aware of any information regarding stereoblock copolymers of
pMMA in the literature, in which one block is atactic. AtacticÀ
isotactic stereoblock copolymers of polyacrylamides, on the other
hand, have been reported.42À44 The first block, made via a con-
ventional RAFT technique, is atactic, the selectivity of which
switches to isotacticity upon addition of a Lewis acid. Carpentier and
co-workers45 reported, what they claimed to be, the first example of
a block copolymer of polystyrene with syndiotactic and atactic
blocks by combining ZieglerÀNatta polymerization with ATRP.
The combination of CCTP with other polymerization mech-
anisms has, hitherto, been underexploited as a technique.
Block copolymers have sparked the interest of academic and
industrial chemists alike for many decades because they provide a
facile and versatile route to combining the properties of two or
more homopolymers while obtaining a controlled architecture.1À6
They have been synthesized via sequentially fed living polymer-
ization techniques such as anionic7 or cationic8 polymerization,
atom transfer radical polymerization (ATRP),9 reversible additionÀ
fragmentation chain transfer (RAFT) polymerization,10 nitroxide-
mediated radical polymerization (NMRP),11 and more recently
using sequentially fed coordinative chain-transfer olefin poly-
merization12À14 and the so-called Dow shuttling chemistry.15
The resultant polymers often show improved properties com-
pared to blends of their homopolymer counterparts, contributing
their greatest assets to the block copolymer. This opens the door
to a broad range of applications areas, such as compatiblisers,16À19
self-assembly,20,21 drug delivery,22,23 and surfactants,24,25 which
can benefit from the combined properties of the copolymer.
Unfortunately, the use of a sole polymerization technique to
synthesize the block copolymer frequently restricts the mono-
mers that can be used and the properties that can be obtained. By
combining one or more techniques, a vast array of block co-
polymers can be synthesized. Recently, living radical polymeri-
zation techniques (e.g., ATRP, RAFT, and NMRP) have been
used in conjunction with catalytic olefin polymerization,1 ring-
opening polymerizations,2 ring-opening metathesis polymerization,3
and extensively with click chemistry.6 Catalytic chain transfer poly-
merization (CCTP),26À33 on the other hand, has rarely been used in
combination with alternative polymerization mechanisms2,24,34À37
and has never been combined with a non-radical technique.
Received: September 8, 2011
Stereoblock copolymers of methacrylates were first synthesized
Revised:
Published: November 23, 2011
November 8, 2011
by Doherty et al.38 where blocks of syndiotactic and isotactic
r
2011 American Chemical Society
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Scheme 1. Synthetic Approach to Block Copolymers: (a) Catalytic Chain Transfer Polymerization (CCTP); (b) Michael
Addition of α-Lithioisopropyl Isobutyrate; (c) Anionic Polymerization
Herein, we demonstrate a novel route to synthesizing block copoly-
mers via the combination of catalytic chain transfer polymerization
and anionic polymerization, wherein the blocks have differing tacti-
cities (Scheme 1). The inclusion of an anionically derived element
allows the possibility of introducing “non-radical” properties to the
block copolymer, such as tacticity and potentially crystallinity, via
either side- or chain-end control producing iso- or syndiotactic
polymers. This is impossible using CCTP alone. Block copolymers
with differing tacticities are of interest in fields such as coatings and
surface modifiers.46 The atactic component can provide good mis-
cibility with the substrate, while the isotactic or syndiotactic blocks
tend to form crystalline domains capable of enhancing the scratch and
chemical resistance of the coating, protecting the substrate.47,48
In CCTP, certain low-spin CoII species such as bis[(difluoro-
boryl)dimethylglyoxymate]cobalt(II) (COBF) can be used in
ppm quantities under free-radical conditions to catalyze the chain
transfer to monomer reaction and thus efficiently control the
molecular weight.26À33 In addition, the polymers formed during
CCTP contain a terminal double bond which gives rise to the
name “macromonomers”. CCTP is particularly efficient when
methacrylate (as opposed to acrylate and styrenic) monomers are
used; however, as with all free radical polymerization techniques,
the resulting polymers are basically atactic. The polydispersity
indices of polymers made via CCTP are consistent with that of
chain transfer dominated polymerization reactions and are usually
around 2. This can, however, be significantly reduced when CCTP
is used in combination with ATRP49 or RAFT.50
The anionic polymerization of methacrylates enables a degree
of stereoregularity to be introduced, which is almost always
impossible using radical methods.51À54 The anionic polymeriza-
tion of methacrylate monomers with lithium ester enolate-based
initiators, such as α-lithioisopropyl isobutyrate (1, Scheme 1),
proceeds via a Michael addition mechanism. Unfortunately, the
use of this type of initiator means that the polymerization
reactions are prone to two major problems.55 First, lithium ester
enolates, such as 1, are known to have a tendency to cluster,
resulting in inhomogeneous initiation and low initiator efficiency.
In addition, due to the mechanism of this polymerization
technique, a carbanion is formed, which can attack the ester
carbonyl functionality in the methacrylate-based polymer, result-
ing in the formation of dead cyclic species. Luckily, there is
extensive literature available with a huge selection of preventative
methods for both aggregation and backbiting termination.51,55
Care must be taken, however, as the choice of additives can
greatly influence the tacticity of the growing polymer chain.
In this present study we investigate the use of CCTP-derived
macromonomers, modified with 1, as macroinitiators for the
anionic polymerization of methacrylic monomers. By combining
these two different polymerization mechanisms, stereoblock
copolymers comprising of an atactic block and a syndio/isotactic
block were synthesized. The versatility of this reaction toward
macromonomers based on alternative methacrylic monomers
and different chain lengths was also investigated.
’ EXPERIMENTAL SECTION
General Considerations. All syntheses and manipulations of air-
and moisture-sensitive materials were carried out in oven-dried Schlenk-
type glassware on a dual manifold Schlenk line, a vacuum line (typically
1À100 mbar), or in a nitrogen-filled glovebox (typically <1.0 ppm of
oxygen and moisture).
Materials. Methyl methacrylate (MMA, 99%) and benzyl metha-
crylate (BzMA, 96%) were purchased from Aldrich and passed over a
column of activated basic alumina to remove the inhibitor. For anionic
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Table 1. Properties of the Synthesised pBzMA and pMMA
Synthesis of pBzMA2. A 250 mL round-bottom flask was charged
with AIBN (100 mg, 0.61 mmol) and 2 (50 mg, 0.126 μmol) inside a
glovebox. The flask was closed with a rubber septum and removed from
the glovebox. BzMA (50 mL) was injected via the septum, and the
mixture was stirred at 60 °C for 16 h under a nitrogen atmosphere. The
oligomerization was quenched by addition of hydroquinone and cooling
in ice. Residual monomer was removed under reduced pressure at
elevated temperatures. The residual yellow oil was vacuum distilled
using a Kugelrohr setup (10À3 mTorr, 200 °C) to afford the pure dimer
in 60% yield. 1H NMR (400 MHz, chloroform-d1, 298 K): δ 7.38 (m,
10H, Ph), 6.29 (s, 1H, dCH2), 5.53 (s, 1H, dCH2), 5.20 (s, 3H,
OCH2), 5.10 (s, 3H, OCH2), 2.74 (s, 2H, CH2), 1.25 (s, 6H, Me2) ppm.
13C{1H} NMR (100 MHz, chloroform-d1, 298 K): δ 176.7 (CdOester),
167.2 (CdOacryl), 137.2 (CdCH2), 136.0 (dCH2), 28.5 (Caromatic),
66.5 (OCH2), 43.0 (CH2), 40.9 (CMe2), 24.9 (CMe2) ppm.
Synthesis of Model “Macro”initiator. Compound 1 (10 mg,
72 μmol) was dissolved in 0.4 mL of C6D6 and the purity checked via
1H NMR. pMMA2 (14 mg, 72 μmol) was added and allowed to stir for
1 h. 1H NMR was used to determine that all of the starting enolate 1 was
consumed. The reaction mixture was then quenched with 10% HCl
solution, and the product again was analyzed using 1H NMR. 1H NMR
(400 MHz, THF-d8/toluene-d8, 298 K): δ 4.89 (septet, 1H, OCH-
(Me)2), 3.40 (s, 6H, OMe), 3.39 (s, 6H, OMe), 2.55 (m, 1H, CH),
1.99À2.10 (m, 2H, CH2), 1.50À1.59 (m, 2H, CH2), 1.06À1.12 (m,
18H, 3 Â Me2) ppm.
SECc
1H NMRd
2 (ppm)a
xb
Mn/g molÀ1 PDI
Mn/g molÀ1
pBzMA2
pBzMA6
pBzMA76
pMMA2
pMMA15
pMMA40
pMMA120
392
94
352
1050
14000
200
0.88
0.78
1150
1.6
2.2
5.5
191
4.3
3.3
2.1
13500
0.68
0.40
0.84
1700
3800
1.6
1.8
2.6
1500
4000
12000
13000
a The amount of 2 is defined as moles of 2 per 106 moles of monomer.
b Monomer conversions determined gravimetrically. c Values reported against
polystyrene standards. d Calculated based on the ratio of vinylic protons
(5.4 and 6.2 ppm) to ester protons (BzMA: 5.0 ppm; MMA: 3.6 ppm).
polymerizations, MMA and BzMA were purified further by being dried
over CaH2 and distilled, followed by titration against neat trioctylalu-
minum and a second distillation. Triisobutylaluminum (TIBA) was
purchased from Aldrich and used as received. Lithium chloride (98%)
was purchased from Aldrich and dried in a vacuum oven at 100 °C
overnight before introduction to the glovebox. Azobis(isobutryonitrile)
(AIBN) was recrystallized twice from methanol. The bis(methanol)
Anionic Polymerization from Model “Macro”initiator.
Compound 1 (2.5 mg, 18 μmol) was dissolved in 0.5 mL of dry THF,
pBzMA2 (12.5 mg, 36 μmol) was then added, and the mixture allowed
to stir for 15 min. MMA (90 mg, 900 μmol) was then added and stirred
at 20 °C for 2 h. The reaction product was quenched with acidified
ethanol and dried overnight in a vacuum oven. Mn = 31 000 g/mol,
PDI = 1.7; conversion = 90%.
Reaction of 1 with 2 equiv of pMMA2. Compound 1 (340 mg,
2.5 mmol) was suspended in toluene (20 mL), and then pMMA2 (1.0 g,
5.0 mmol) was added dropwise. The clear solution was stirred at room
temperature for 18 h. Toluene was evaporated under reduced pressure,
and the residue was taken up in methanol (20 mL). All volatiles were
evaporated under reduced pressure, and the resulting foam was taken up
in Et2O (50 mL). The organic layer was washed with hydrochloric acid
(10%, 30 mL) and then separated, and the aqueous layer was extracted
with Et2O (30 mL). The combined organic fractions were dried
over Na2SO4, and volatiles were removed under reduced pressure to
give an oil. A small amount of the monoaddition product Me2C-
(CO2Me)CH2CH(CO2Me)CH2CMe2CO2iPr was removed of the oil
by vacuum distillation (80 °C, 400 mTorr).
Typical Procedure for the Isotactic Anionic Polymeriza-
tion of MMA. In a glovebox, 1 (14 mg, 100 μmol) and LiCl (4 mg,
100 μmol) were dissolved in 0.99 g of dry toluene. Outside the glovebox,
0.01 mL of THF was added. The vials were cooled to À78 °C. MMA
(0.80 g, 8 mmol) was then added under an argon atmosphere, and the
mixture was stirred for 30 min at À78 °C. The reaction was quenched
with acidified ethanol and precipitated in pentane. The residual solvent
was removed using a vacuum oven at 60 °C overnight. In some
experiments, as indicated in text, LiCl was omitted and only toluene
used as solvent.
Typical Procedure for the Isotactic Anionic Polymeriza-
tion of MMA from a BzMA Macroinitiator. In a glovebox,
1 (14 mg, 100 μmol) and LiCl (4 mg, 100 μmol) were dissolved in 1 g
of dry toluene. In a separate vial, the macromonomer (100 μmol) was
dissolved in 0.99 g of dry toluene. After dissolution, the two crimp cap
vials were removed from the glovebox. To the vial containing 1 and LiCl,
0.01 mL of THF was added. Both vials were cooled to the appropriate
temperature. The two solutions where then combined under an argon
atmosphere and allowed to mix for 5 min. Cold MMA (0.45 mL, 4.2 mmol)
complex of COBF, COBF (MeOH)2 (2), was prepared as described
3
previously.56,57 The chain transfer activity of the complex was deter-
mined in methyl methacrylate (MMA) bulk polymerization at 60 °C and
found to be equal to 30 Â 103. For all experiments, a single batch of
catalyst was used. Toluene and tetrahydrofuran (THF) were purchased
from Biosolve and used as received for the CCTP polymerizations. For
use in the macroinitiation experiments, the solvents were dried over
molecular sieves prior to use. α-Lithioisopropyl isobutyrate, 1, and
MeAl(BHT)2 (3, BHT = 2,6-(t-Bu)2-4-Me-C6H2O) were synthesized
according to literature methods.58,59
Synthesis of Macromonomers via CCTP. Complex 2 (see
Table 1) and AIBN (42 mg, 0.25 mmol) were placed in a flask equipped
with a stirrer bar and underwent three vacuumÀargon cycles. BzMA or
MMA (50 mL) and toluene (50 mL) were deoxygenated and added
using a syringe. The mixture was heated to 60 °C and allowed to react for
16 h, after which it was quenched by cooling in ice and addition of
hydroquinone. For macromonomers with an Mn < 2000 g/mol, the
residual monomer and solvent were removed via vacuum evaporation
immediately after stopping the reaction. The resulting product was
redissolved in THF, passed over a column of basic alumina, and dried at
80 °C in a vacuum oven for at least 48 h to remove all traces of solvent
and water. For macromonomers with an Mn > 8000 g/mol, the product
was further diluted with toluene, precipitated in a large excess of pentane,
and then dried in a vacuum oven at 80 °C for at least 48 h to remove all
traces of solvent and water.
Synthesis of pMMA2. A 250 mL round-bottom flask was charged
with AIBN (213 mg, 1.3 mmol) and 2 (100 mg, 0.26 mmol) inside a
glovebox. The flask was closed with a rubber septum and removed from
the glovebox. MMA (100 mL, 936 mmol) was injected via the septum,
and the mixture was stirred at 80 °C for 6 h under a nitrogen atmosphere.
The oligomerization was quenched by addition of hydroquinone and
cooling in ice. Residual monomer was removed under reduced pressure
at room temperature. The residual yellow oil was vacuum distilled
1
(325 mTorr, 38 °C) to afford the pure dimer in 42% yield. H NMR
(400 MHz, chloroform-d1, 298 K): δ 6.12 (s, 1H, dCH2), 5.43 (s, 1H,
dCH2), 3.64 (s, 3H, OMe), 3.55 (s, 3H, OMe), 2.52 (s, 2H, CH2), 1.07
(s, 6H, Me2) ppm. 13C{1H} NMR (100 MHz, chloroform-d1, 298 K):
δ 177.3 (CdOester), 176.8 (CdOacryl), 137.3 (CdCH2), 127.8 (dCH2),
51.8 (OMe), 51.6 (OMe), 42.8 (CMe2), 41.0 (CH2), 24.8 (CMe2) ppm.
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Scheme 2. Reaction Scheme for the Synthesis of a Macroinitiator and Its Hydrolysis
was then added, and the mixture was stirred for 4 h at the desired
temperature. The reaction was quenched with water. The residual
solvent was removed using a vacuum oven at 60 °C overnight.
out using a Zorbax Eclipse XDB-C8 column, 4.6 Â 150 mm, 5 μm on an
Agilent 1100 series setup. Initially, the eluent was a 4:1 mix of methanol/
THF for 2 min and then gradually changed to pure THF over 25 min.
The flow rate was 1 mL/min. The temperature of the column was 50 °C.
Detection was carried out using a Polymer Laboratories evaporative light
scattering detector (ELSD). Differential scanning calorimetry (DSC) was
performed on a TA Q100 DSC. Approximately 5 mg of dried polymer
was weighed in aluminum hermetic pans. Temperature profiles from 0 to
200 °C with a heating and cooling rate of 1, 10, and 20 °C/min were
applied. TA Universal Analysis software was used for data acquisition. Glass
transition temperatures were determined from the second heating run.
Typical Procedure for the Syndiotactic Anionic Polymer-
ization of MMA from a BzMA Macroinitiator. Complex 3
(45 mg, 93.4 μmol) and macromonomer (47 μmol) were dissolved in
toluene for 45 min. A solution of 1 (47 μmol) in toluene was added and
allowed to stir for 60 min. MMA (0.22 g, 2.2 mmol) was added, and the
reaction mixture was stirred for 1 h at room temperature. For longer
macromonomers, the polymerization time was increased to 2 h and a
solution of TIBA (typically 0.5 wt % of the macromonomer) was added
to the macromonomer 2 h prior to the addition of 3 in order to remove
any trace of water. Reaction mixtures were quenched with acidified
ethanol, and the solvent was removed under vacuum. Polymers with an
Mn > 8000 g/mol were precipitated twice from pentane.
Syndiotactic Anionic Polymerization of pBzMA. Complex 3
(12 mg, 24 μmol) and a solution of 1 in toluene (1.6 mg, 12 μmol) were
dissolved in 1.5 g of toluene to cleanly generate an initiator/catalyst pair.
BzMA (0.21 g, 1.2 mmol) was then added, and the mixture was
stirred for 2 h at 20 °C. The reaction mixture was quenched with acidified
ethanol and precipitated in pentane. 81% conversion; Mn = 15 200 g/mol,
PDI = 1.5 (determined against PS standards in THF using SEC).
Syndiotactic Anionic Polymerization of pMMA. Complex 3
(12 mg, 24 μmol) and a solution of 1 in toluene (1.6 mg, 12 μmol) were
dissolved in 1.5 g of toluene to cleanly generate an initiator/catalyst pair.
MMA (0.12 g, 1.2 mmol) was then added, and the mixture was stirred for
2 h at 20 °C. The reaction mixture was quenched with acidified ethanol
and precipitated in pentane. 98% conversion; Mn = 9900 g/mol, PDI =
1.4 (determined against PS standards in THF using SEC).
Measurements. Gel permeation chromatography was carried out
using a Waters 2695 separations module, a Model 2487 UV detector
(254 nm), and a Model 2414 differential refractive index detector (40 °C).
Injection volume used was 50 μL. Tetrahydrofuran (Biosolve, stabilized
with BHT) was the eluent, flow rate 1.0 mL/min. The column set used was
a PLgel guard (5 μm particles) 50 Â 7.5 mm precolumn, followed by two
PLgel columns in series of 500 Å (5 μm particles) and 100 Å (5 μm
particles), respectively. Calibration was performed using polystyrene
standards (Polymer Laboratories, Mn = 370 up to Mn = 40 000 g/mol).
Data acquisition and processing were performed using Waters Empower 2
software. 1H NMR spectra were recorded on a Varian Mercury
Vx (400 MHz) spectrometer at 400 MHz. Chloroform-d1, THF-d8,
benzene-d6, toluene-d8, and tetramethylsilane were used as solvent and
internal standard, respectively. MALDI-ToF-MS was carried out using a
voyager DE-STRspectrometer from AppliedBiosystemsin reflector mode.
trans-2-(3-(4-tert-Butylphenyl)-methyl-2-propenylidene)malononitrile
doped with potassium trifluoroacetate was used as the matrix. It was
deposited from THF solution onto a stainless steel sample substrate, and the
solvent was allowed to evaporate. The polymer was then deposited as a
dilute (∼1 mg/mL) solution in THF. This resulted in each polymeric
species being observed as its K+ adduct with molecular mass M + 31. The
spectrometer was calibrated using poly(ethylene oxide) standards for
the lower mass range and polystyrene standards for the higher mass
range. Gradient polymer elution chromatography (GPEC) was carried
’ RESULTS AND DISCUSSION
Model studies, based on dimers prepared by CCTP, were
carried out to establish the reactivity of these species toward 1 as
well as to probe their ability as initiators to polymerize
methacrylic monomers anionically. Once established, macro-
monomers were used as macroinitiators. BzMA macromonomers
were initially selected as the starting blocks for the block
copolymer due to its UV activity; pBzMA is UV-active at
254 nm, whereas MMA shows no absorption at this wavelength,
allowing the simple detection of pBzMA-containing (block)
copolymers via SEC with UV detection.
Synthesis of Macromonomers. Poly(benzyl methacrylate)
(pBzMA) and poly(methyl methacrylate) (pMMA) macromono-
mers were synthesized by CCTP in toluene at 60 °C using AIBN
as the initiator. The final properties of these macromonomers are
collected in Table 1. The macromonomers were analyzed using
1H NMR to confirm that all had a terminal vinylic group. For
the lower molecular weight macromonomers, this was also
confirmed using MALDI-ToF-MS.
Synthesis of Macroinitiator (Model Studies). To investigate
the conversion of the macromonomer into a macroinitiator,
model studies were carried out. As a model for the macromono-
mer, an MMA dimer, pMMA2, was synthesized using CCTP,
followed by purification via vacuum distillation. Equimolar
quantities of the stable enolate Me2CdC(OiPr)OLi (1) and
pMMA2 were reacted in benzene-d6 (Scheme 2).
The 1H NMR spectrum of the initially formed product enolate
features one broad signal for all methylene protons, while all other
groups give their individual resonances with resolved coupling
patterns. Hydrolysis using10% aqueous HCl converts the spectrum
into that of the product ester, displaying diastereotopic methylene
protons as well as diastereotopic CMe2 groups and a quintet at 2.55
ppm which corresponds to the newly formed proton adjacent to
the ester functionality, confirming the formation of a “macro”-
initiator (Figure S1, Supporting Information).
In a separate experiment to confirm that the resulting coupling
product of the methacrylic dimer with 1 is indeed capable of
inducing initiation, the UV-active BzMA dimer, pBzMA2, was reacted
with lithium ester enolate, 1, followed by the addition of MMA.
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Polymerization ensued, and the resultant product was analyzed
using SEC equipped with dual UV/DRI detectors. The molecular
weight distribution (Figure 1) shows that a polymeric material
has been synthesized with an Mn of 31 000 g/mol and that the
UV and DRI signals overlay. Since pBzMA2 is UV-active at
254 nm, whereas MMA is inactive, this result is indicative that
pBzMA2 has indeed acted as a macroinitiator and has been
incorporated into the polymer.
was confirmed via MALDI-ToF-MS (Figure S2, Supporting
Information). Backbiting of 4 does not occur as this would result
in the formation of an unstable 4-membered ring. The observed
predominance of backbiting in this reaction, which is in fact a
model reaction for propagation of pMMA2, precludes the lithium
enolate initiated anionic (homo) polymerization of CCT-derived
macromonomers.
We can, nevertheless, conclude that (the models for) macro-
monomers can act as macroinitiators for the anionic polymeri-
zation of MMA, allowing this synthesis of block copolymers.
Block Copolymer Synthesis. AtacticÀIsotactic Block Copol-
ymers. One of the most common methods to synthesize isotactic
pMMA from lithium ester enolates, such as 1, is by carrying out
the polymerization in toluene.55 Under these conditions and at
room temperature, isotactic pMMA with an [mm] = 74% can
be synthesized.60 However, as illustrated by the model reactions,
the polymerization of MMA from a macroinitiator suffers
significantly from backbiting reactions, more so than the simple
polymerization of pMMA from 1; in other words, the (macro)-
initiator efficiency is lower than that of 1. The addition of 1%
THF and the lowering of the temperature to À78 °C assist
in minimizing backbiting termination reactions,55 although
the addition of THF also affects the tacticity, decreasing the
degree of isotacticity (Figure S3, Supporting Information).
Further addition of THF (i.e., >1%) results in a significant loss
in stereocontrol,61 justifying the choice of a 99:1 ratio of
toluene to THF as a suitable solvent mixture to perform the
polymerizations.
The addition of 2 equiv of pMMA2 to 1, however, resulted in a
backbitten species (6, Scheme 3). The presence of this species
The in situ formation of a macroinitiator was carried out by
reaction of pBzMA6 macromonomer and 1, followed by the
addition of MMA at a range of temperatures and in the absence
and presence of LiCl (Table 2). Temperature choice is para-
mount in anionic polymerizations, especially when there is an
Figure 1. Molecular weight distribution (relative to PS, measured in
THF) illustrating that pBzMA2 is capable of initiating the anionic
polymerization of MMA: (- - -) DRI signal, (—) UV signal. Experimental
conditions: solvent = THF, temperature = 20 °C, reaction time = 2 h.
Scheme 3. Mechanistic Reaction Scheme of the Backbiting Reaction of Macroinitiators
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Table 2. Conditions and Results for the Anionic Polymeri-
zation of MMA Using pBzMA6 as the Macroinitiator
(pBzMA6/1/MMA = 1:1:10)
temp/°C
equiv of LiCla
Mnb/g molÀ1
PDIb
MMA convc
1
2
3
4
5
6
20
0
0
0
0
1
1
1
2100
2400
1920
2020
2500
1800
15d
33e
1.9
1.7
1.9
1.6
0.94
1.00
0.26
0.82
1.00
0.50
À78
20
0
À78
a Molar equivelents of LiCl with respect to 1. b Determined using SEC,
measured against PS standards in THF using DRI detector. c Conversion
of monomer determined gravimetrically after 4 h reaction time.
d Bimodal Mp = 2600 and 83 900 g molÀ1. Bimodal Mp = 2700 and
e
122 500 g molÀ1
.
opportunity for backbiting, as well as other side reactions, to
occur. In general, anionic polymerizations of this type are carried
out at À78 °C.55 The reactions shown in Table 2 also show a
strong dependence on temperature. As can be seen in entries
1À3 and 4À6, decreasing the temperature decreases the PDI of
the resulting polymer. In comparing entries 1 and 3, this
difference is most obvious as the molecular weight distribution
of resultant polymer transforms from bimodal to monomodal.
This suggests a significant reduction in the amount of side
reactions that have occurred. The use of lower temperatures also
suppresses the rate of polymerization, decreasing the degree of
conversion obtainable after 4 h.
The use of salts, such as lithium chloride, has been reported to
assist in the deaggregation of lithium ester enolate clusters,
allowing homogeneous initiation to occur.62,63 From Table 2 it
is evident that the presence of lithium chloride does have a
profound effect on the polydispersity of the polymers synthe-
sized. At all temperatures, a decrease in PDI is observed when the
reaction is carried out in the presence of lithium chloride. The
most noticeable being the decrease from a bimodal distribution
with PDI of >15 to a monomodal distribution with PDI of
<1.9 when the reaction is carried out in the presence of LiCl at
0 °C or even at room temperature. Although the PDI of the
polymer decreases when the temperature is decreased from 0 °C
to À78 °C when lithium chloride is used, the difference is not
great. It should also be noted here that the observed PDIs are
larger than one would normally expect from an anionic poly-
merization, but one must recall that the starting macroinitiator
already has a PDI ≈ 1.6. In addition, the rate of reaction is
compromised. Lithium chloride does not have a pronounced
effect on the tacticity of the polymer formed (Figure S3,
Supporting Information).
Figure 2. Molecular weight distribution (relative to PS, measured in
THF) of pBzMA6 and p(BzMA6-b-MMAx) (entry 5, Table 2) normal-
ized to the amount of polymer.
and UV signals observed in Figure 1 is not observed here in
Figure 2 as a balance exists between the conversion of macro-
monomer (n > 1) to macroinitiator and backbiting, which does
not exist for the dimers (n = 1), as detailed in Scheme 3.
Gradient polymer elution chromatography (GPEC) was car-
ried out to further confirm the presence of a block copolymer
(Figure 3). By using an analytical technique which separates
based on chemical composition as opposed to hydrodynamic
volume, it was possible to distinguish between the two homo-
polymers (pBzMA6 and pMMA) and the block copolymer
p(BzMA6-b-MMAx) (Figure 3). The observed oligomeric dis-
tribution of the macromonomer has been reported before in the
literature.64 It is obvious that the resultant product contains both
unreacted pBzMA6 macromonomer and pMMA homopolymer
(13À28 and 37À38 min, respectively), but a broad third peak
corresponding to a copolymer can also be seen, spanning
between the two homopolymers. The broadness is as expected
because the polymer consists of a variety of block sizes originat-
ing mainly from the macroinitiator pBzMA6 and also somewhat
from the second block.
The existence of a copolymer was further confirmed using
MALDI-ToF-MS (Figure 4a) in which multiple distributions
were observed. As depicted in the spectrum, a distribution with a
repeat unit of 176 Da, corresponding to pBzMA homopolymer, is
seen. The end group of this pBzMA has been identified as the
result of backbiting (i.e., after reaction of the macromonomer
with 1, the chain end has bitten back onto itself to form a six-
membered ring, as shown in Scheme 3, preventing the initiation
of MMA polymerization from this polymer chain). Minimal
unreacted macromonomer is observed in the spectrum, indicat-
ing that the first step of the reaction with the lithium enolate gives
almost 100% conversion. A distribution with a repeat unit of 100
Da is also seen establishing the presence of pMMA, but the end
group could not as yet be determined. Enolates are known to
have limited thermal stability and thus decompose to a wide
variety of species. It is assumed that one of these species is
responsible for initiating the polymerization of MMA or that this
decomposition reaction has occurred after completion of the
reaction. Underlying these two distributions, which show a much
A more in-depth analysis was carried out on the polymer
formed in entry 5 (Table 2). A UV detector was used in tandem
with a DRI detector for SEC measurements. The DRI signal
produced using SEC for the polymer made in entry 5 clearly
shows a shift toward a higher molecular weight after the reaction
has taken place. A shift in the UV signal is also observed, although
the UV and DRI chromatograms of the block copolymer do not
overlap perfectly. This illustrates that while the UV-active
macromonomer does form part of the block copolymer, it may
not have been distributed evenly, suggesting that some pMMA
homopolymer has also been produced; pMMA can be formed as
a result of initiation from unreacted 1. The perfect overlap of DRI
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Figure 4. MALDI-ToF-MS spectrum of p(BzMA6-b-MMAx) (entry 5,
Table 2): (a) unfractionated and (b) fractionated, showing multiple
distributions, where 1a = pBzMA, 2 = pMMA, and 3 = p(BzMA6-
b-MMAx).
Figure 3. Gel permeation elution chromatogram showing (a) pBzMA6,
(b) pMMA, and (c) the presence of a p(BzMA6-b-MMAx) (entry 5,
Table 2) as well as some residual macromonomer and pMMA.
higher intensity, is a third distribution with a difference of 24 Da
between the peaks. This is consistent with the difference between
two MMA units and one BzMA unit, verifying the presence of a
copolymer of MMA and BzMA. Again, the end group could not
be elucidated, but it appears to be different than the end group
of pMMA.
To unequivocally prove the existence of the block copolymer,
isolation of the copolymer was attempted using GPEC fractiona-
tion. Several fractions were taken between 25 and 35 min. This
procedure was repeated 15 times in order to obtain sufficient
material to acquire a MALDI-ToF-MS spectrum; the fractions
were run a second time to check the purity. In each case, some
residual homopolymer remained in the spectra. Fortunately, one
fraction eliminated all traces of pBzMA giving a much clearer
spectrum containing only pMMA and p(BzMA6-b-MMAx)
(Figure 5). The MALDI-ToF-MS spectrum of the fractionated
species shows two distributions (Figure 4b). One corresponds to
pMMA and the other to p(BzMA-b-MMA), the end groups of
which are consistent with those in the unfractionated spectrum
(Figure 4a). Note that the intensities observed in MALDI-ToF-
MS are not representive of the actual amounts of polymer
present as the measurement is not quantitative.
Figure 5. GPEC chromatogram showing the results of fractionation of
p(BzMA6-b-MMAx) (entry 5, Table 2).
formed, although the degree of [mm] triads is lower than if only
toluene is used as the solvent (Figure S3, Supporting In-
1
Free-radical polymerizations are generally highly random and
as such produce atactic chains. However, from literature60 and
model studies we know that 1 is capable of producing isotactic
pMMA, characterized by a high degree of [mm] triads. Under the
conditions used here, pMMA with an isotactoidal tendency is
formation). H NMR was used to determine the extent of the
tacticity of p(BzMA6-b-MMAx) (entry 5, Table 2) as shown
in Figure 6. The CH3 protons of the pMMA, between 0.6 and
1.2 ppm, can be used to indicate the tacticity of the pMMA
component of the polymer. At 1.2 ppm a quintessential [mm]
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triad can be seen; however, both [mr] and [rr] triads, at 1.0 and
0.8 ppm, respectively, are also present. This suggests that the
polymer is partially isotactic in nature, although there is an
element of atacticity derived from the macromonomer (acting
either as the first block or as unreacted material). It was also
difficult to determine the exact degree of isotacticity due to the
underlying atacticity of the macromonomer.
It is clear from the evidence presented above that block
copolymers of atactic benzyl methacrylate and isotactic-rich
methyl methacrylate can be made via the combination of CCTP
and anionic polymerization. However, this route proved to be
rather inefficient due to backbiting termination of the macro-
initiator as well as the formation of pMMA from unreacted 1.
AtacticÀSyndiotactic Block Copolymers. Aluminum compounds
have been used previously to control the tacticity of methacrylic
monomers during anionic polymerizations.60,65 Rodriguez-
Delgado et al.60 reported the use of MeAl(BHT)2 (3) in com-
bination with 1 as an effective route to preventing both clustering
of lithium ester enolates and backbiting termination. Furthermore,
the polymerization of MMA under these conditions can be carried
out in toluene at room temperature to produce syndiotactic
pMMA (71.3% [rr], 24.7% [mr], 4.0% [mm]). Further reduction
in the temperature to 0 and À40 °C increases the stereoselectiv-
ity toward higher syndiotacticity, but only by around 8%.
To first determine whether this route is feasible, a short BzMA-
based macromonomer (pBzMA6) was used as a macroinitiator.
After mixing with 2 equiv of 3, 1 equiv of 1 was added to form a
macroinitiator in situ, followed by 47 or 100 equiv of MMA
(entries 1 and 2 in Table 3, respectively). Figure 7 shows the
molecular weight distributions of the starting macromonomer
and the final polymer synthesized in entry 2 (Table 3). A clear
increase in the molecular weight from the macromonomer to the
block copolymer can be observed from 1150 to 24 000 g/mol. In
addition, the signals recorded from the UV and DRI detectors of
the SEC instrument correlate well with each other, indicating the
inclusion of the pBzMA6 macromonomer in the final polymer.
A small amount of unreacted macromonomer remains, which is
more obvious in the UV trace of the product mixture.
The molecular weight of the block copolymer obtained in
entry 2 is too high for analysis via MALDI-ToF-MS; however,
the block copolymer made in entry 1 (Table 3) has a molec-
ular weight that can be easily analyzed using this technique
(Figure 8). Three distinct distributions can be seen. Distribution
3 indicates that a copolymer has been synthesized; the difference
Figure 6. 1H NMR (400 MHz, CDCl3) of (a) pBzMA6 and (b) block
copolymer p(BzMA6-b-MMAx) (entry 5, Table 2).
Figure 7. Molecular weight distribution of p(BzMA6-b-MMA95) nor-
malized based on amount of polymer.
Table 3. Conditions and Results for the Anionic Polymerization of MMA or BzMA Using a Methacrylic Macromonomer as the
Macroinitiator in the Presence of 3
block copolymera
theor final Mnb/g molÀ1
Mn /g molÀ1
c
PDIc
block 1:block 2d
monomer conve
1
2
3
4
5
6
7
8
9
p(BzMA6-b-MMA47)
p(BzMA6-b-MMA95)
4500
10600
22000
18000
9500
5300
17400
19500
16200
6000
2.6
1.6
1.9
2.2
2.2
1.7
1.7
1.7
1.5
6:30
0.75
0.94
0.73
0.24 f
0.97
0.57
1.00
0.50
0.83
6:142
76:114
p(BzMA76-b-MMA77
)
p(BzMA76-b-BzMA18
)
p(MMA15-b-BzMA43
p(MMA40-b-BzMA29
)
15:29
40:12
)
9900
10100
8000
p(MMA40-b-MMA50)
p(MMA120-b-BzMA26
8800
)
22000
21300
17600
24000
120:31
p(MMA120-b-MMA110
)
a Ratio of monomers determined via SEC. b Corrected for conversion. c Determined using SEC, measured against PS standards in THF. d Determined
using 1H NMR based on CO2CH2Ph (BzMA, 4.8 ppm) and CO2CH3 (MMA, 3.6 ppm) integrals. e Determined gravimetrically after 2 h reaction time.
f 4.5 h reaction time.
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Figure 8. MALDI-ToF-MS of p(BzMA6-b-MMA47), where 1 = unreacted pBzMA6, 1b = hydrolyzed pBzMA macroinitiator, and 3 = p(BzMA-
b-MMA).
between the peaks is 24 Da (consistent with the difference
between two MMA units and one BzMA unit) and an end group
corresponding to two protons and a connecting group originat-
ing from the lithium ester enolate (see overall scheme, Scheme 1).
The most intense distribution (1) corresponds to unreacted
macromonomer, but quantitative conclusions are difficult to
draw from the intensity of the peaks due to the intrinsic nature
of the MALDI-ToF-MS technique and the fact that at lower
masses the block copolymer is less prevalent. In addition, a third
distribution is seen (1b), which corresponds to the hydrolyzed
macroinitiator. This suggests that 3 is highly efficient at prevent-
ing backbiting as the backbitten product is not observed in this
spectra (contrary to the case where 1% THF in toluene is used to
prevent backbiting, vide supra). The most important result,
however, is that no pMMA is observed, which considering the
high propensity and ease of pMMA to “fly” in MALDI-ToF-MS
strongly suggests that reaction of 1 with the macromonomer is
more efficient in the presence of 3.
On the basis of the SEC and MALDI-ToF-MS data presented
above, it can be concluded that the use of 3 in the polymerization
of MMA from a pBzMA macromonomer is a successful route to
synthesizing (stereo) block copolymers.
Figure 9. Molecular weight distribution of pMMA15 and p(MMA15-
b-BzMA43), normalized based on amount of polymer.
monomers for the second block, an increase in molecular weight is
observed, accompanied by a slight decrease in polydispersity index.
To investigate the versatility of this reaction toward macro-
monomers based on different methacrylic monomers, a short
MMA-based macromonomer (pMMA15), which has a similar
molecular weight to pBzMA6 of around 1000 g/mol, was used as
the macroinitiator. A second block of BzMA was polymerized
anionically from pMMA15 as shown in entry 5 (Table 3).
Again, an increase in molecular weight was seen (from 1700 to
6000 g/mol, Figure 9), and the DRI and UV detector signals of
the block copolymer overlap well. MALDI-ToF-MS (Figure S4,
Supporting Information) also confirms the presence of a block
copolymer. Longer pMMA macromonomers (pMMA40 and
pMMA120) can also be used to produce macroinitiators for both
MMA and BzMA polymerization (entries 6À9, Table 3).
The use of 3 in the anionic polymerization of methacrylates
using CCTP-derived macromonomers as macroinitiators proves
to be a highly effective route to synthesizing block copolymers
and has proven its versatility toward different methacrylic
monomers as well as varying chain lengths.
The limits of the polymerization technique have also been
investigated by using longer macromonomers as macroinitiators.
In entries 3 and 4 (Table 3), a benzyl methacrylate macromono-
mer with an Mn = 13 500 g/mol was used to prepare the
macroinitiator for the polymerization of both MMA and BzMA.
Note that here the use of a 1:1:2 molar ratio of macromonomer/
1/3 (as per entries 1, 2, and 5, Table 3) resulted in the incomplete
conversion of the macromonomer into the macroinitiator (as
evidenced by the residual vinylic protons attributed to the
1
macromonomer seen by H NMR). It was thought that this
may arise from residual water trapped within the macromonomer
due to the increased viscosity of higher molecular weight
macromonomers. Indeed, the addition of a small amount of
TIBA (typically 0.5 wt % compared to the macromonomer) to
the macromonomer solution prior to the reactions alleviated this
issue to a certain extent as TIBA acts as a scavenger for water.
Although 3 can also act as a scavenger, the reactivity is much
lower than that of TIBA. Further, TIBA is known to give no
adverse effect in terms of the polymerization mechanism or
tacticity.60 In both cases, where MMA and BzMA were used as
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Figure 10. 1H NMR (400 MHz, chloroform-d6) of (a) p(BzMA)6 macromonomer and (b) p(aBzMA)6-b-(sMMA)95.
As reported by Rodriguez-Delgado et al.,60 the use of 3 in
toluene for the polymerization of methacrylic monomers results
in syndiotactic polymers. The tacticity of syndiotactic metha-
crylic polymers is characterized by an [rr] triad, which can be
observed easily in 1H NMR. At room temperature, pMMA with
a tacticity of 71.3% [rr], 24.7% [mr], and 4.0% [mm] was
obtained.60 The polymerization of BzMA under these same
conditions has not been investigated in the aforementioned
article. However for completeness, pBzMA has been synthesized
under the same conditions for the purposes of this article and
using 1H NMR the tacticity determined to be 77.5% [rr], 20.9%
[mr], and 1.6% [mm]. Compared to the tacticity of pBzMA made
via CCTP (60.6% [rr], 34.5% [mr], and 4.8% [mm], Figure S5),
it is clear that a more syndiotactic pBzMA can be made under the
anionic polymerization conditions described by Rodriguez-
Delgado et al.
due to overlapping peaks. In fact, only the CdO region
(175À179 ppm) in 13C NMR allowed the adequate separation
of the peaks and thus accurate determination of the tacticity of
the copolymers. For the longer macroinitiators, the syndiotacti-
city calculated for the second block, regardless of the nature of
the starting macromonomer or the monomer added for the
second block, was 79À81% (Figure 11).66 Although the starting
macromonomers synthesized via CCTP (Table 1) generally have
a slight tendency toward syndiotacticity ([rr] = 60À65%),67 the
second blocks show an increase in syndiotactoid character. The
lower molecular weight macroinitiators show a slightly broader
degree of syndiotacticity from [rr] = 74% to [rr] = 82%
(Figure 12).68 It is assumed that even higher syndiotacticities
can be achieved when polymerizing at lower temperatures;60
however, this is beyond the scope of the current paper.
Further examination of the tacticity of the two blocks was
attempted using DSC. Unfortunately, only a marginal difference
in Tg between the syndiotactic and atactic homopolymers was
found, spanning the range in which an accurate Tg can be
determined (Figure S6, Supporting Information). The relatively
short chain lengths as well as the respective degrees of syndio-
and atacticity are thought to play a role. Applying the FoxÀFlory
equation, a Tg of 103 and 117 °C for atactic and syndiotactic
pMMA was found for the homopolymers made via CCTP and
anionic polymerization, respectively (with similar molecular
weights to that of the block copolymers). In addition, a
Tm could not be identified even using a range of heating rates
(1À20 °C/min), due to the difficulties in achieving thermal
crystallization of syndiotactic pMMA.69,70 Regardless, given that
the homopolymers of pMMA and pBzMA made under these
anionic conditions give rise to syndiotactic polymers and the
NMR evidence presented in Figures 10À12, it is feasible to
assume that the second blocks made via this route do indeed
consist of methacrylic polymers with a syndiotactoid character.
Figure 10 shows both the 1H NMR spectra of p(aBzMA)6 (a)
and p(a-BzMA)6-b-(s-MMA)95 (b). The block copolymer made
in entry 2 (Table 3) was chosen due to its high degree of
polymerization of MMA compared to BzMA. In this way, the
tacticity of the pMMA block made via anionic polymerization
can be probed more accurately, with minimal interference of
the atactic pBzMA macromonomer. As anticipated, pBzMA6
is clearly atactic demonstrated by the methyl protons around
1 ppm (e) in Figure 10a. On the other hand, the methyl protons
around 1 ppm of p(a-BzMA)6-b-(s-MMA)95 (Figure 10b) show
a high concentration of racemic [rr] triads which is indicative
of a highly syndiotactic material (71% [rr], 27% [mr], and
2% [mm]), which is comparable to the values obtained for the
homopolymer of pMMA under these conditions.60 The deter-
mination of the tacticity of the remaining polymers synthesized
in Table 3 was investigated using 13C NMR. This was necessary
particularly for the BzMAÀMMA copolymers, where the char-
acteristic triads for pMMA and pBzMA were indistinguishable
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of catalytic chain transfer polymerization (CCTP) and anionic
polymerization techniques. This provides a novel route to
synthesizing block copolymers containing blocks of different
tacticities. CCTP has been used to synthesize an atactic block;
the second block was then made via anionic polymerization. The
anionic polymerization was carried out either in the presence of
lithium chloride and a toluene/THF solvent mixture to give an
isotactoid block or in the presence of MeAl(BHT)2 (3) to give
rise to a second block syndiotactoid in nature. Although block
copolymers could be made using both methods, the use of
MeAl(BHT)2 was much more efficient, particularly with respect
to the conversion of the macromonomer into a macroinitiator as
no evidence of homopolymerization of the second monomer was
observed. The method also proved to be versatile toward
macromonomers of different methacrylates and chain lengths.
Although the synthetic procedures described in this paper
require further optimization and/or purification steps (depend-
ing on the monomers and method used), we believe this to be the
first example of atacticÀsyndiotactic and atacticÀisotactic
methacrylic block copolymers.
’ ASSOCIATED CONTENT
Supporting Information. 1H NMR spectrum of the
S
b
“macro”initiator derived from pMMA2 and 1, MALDI-ToF-
MS of cyclic backbitten product of the macroinitiator, 1H
NMR of pMMA made anionically from 1 under a range of
conditions, MALDI-ToF-MS spectrum of p(MMA15-b-BzMA43),
and 1H NMR of pBzMA made via CCTP and anionic polymer-
ization and DSC of pMMA. This material is available free of
charge via the Internet at http://pubs.acs.org.
Figure 11. 13C NMR (100 MHz, chloroform-d6) showing the tacticity
of (a) pMMA15 and (b) p(MMA15-b-BzMA43).
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: j.p.a.heuts@tue.nl.
’ ACKNOWLEDGMENT
This work is part of the research programme of the Dutch
Polymer Institute DPI, Eindhoven, The Netherlands, Project
No. #651.
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