Poly(ferrocenylsilane-block-methacrylates) via Sequential Anionic and Atom Transfer Radical Polymerization

Article abstract of DOI:10.1021/ma0358172

A route to 2-bromoisobutyryl end-functionalized poly(ferrocenyldimethylsilane), allowing essentially quantitative end-capping was discussed. Following the formation of the organometallic block by anionic polymerization, a ruthenium-mediated controlled radical polymerization of methyl methacrylate was employed to grow the second block. Well-defined PFS-b-PMMA block copolymers with low polydispersities and controlled compositions were obtained. The use of atom transfer radical polymerization (ATRP) to form the second block implies that a wide variety of (meth)acrylic and other vinyl monomers can be used, which opens the way to novel hybrid organic-organometallic block copolymers and amphiphilic organometallic block copolymers.

Full text of DOI:10.1021/ma0358172

1686
Macromolecules 2004, 37, 1686-1690
trimethylsilyl or aminoalkyl methacrylate repeat units,
which are readily transformed into ionic blocks by
hydrolysis or quaternization, are employed. The only
reported example of a poly(ferrocenylsilane-block-meth-
acrylate) block copolymer, featuring a 2-(dimethylami-
noethyl) methacrylate block,18 was obtained in a route
applicable for this specific monomer. Thus, poly(ferro-
cenylsilane)-poly(methacrylate) block copolymers have
remained largely unexplored.
P oly(fer r ocen ylsila n e-block-m eth a cr yla tes)
via Sequ en tia l An ion ic a n d Atom Tr a n sfer
Ra d ica l P olym er iza tion
Igor Kor cza gin , Ma r k A. Hem p en iu s,* a n d
G. J u liu s Va n cso*
MESA+ Institute for Nanotechnology, University of Twente,
P.O. Box 217, 7500 AE Enschede, The Netherlands
Received December 2, 2003
Revised Manuscript Received J anuary 14, 2004
In this paper, we report the synthesis and character-
ization of poly(ferrocenylsilane-block-methyl methacry-
late) block copolymers, in which the organometallic
block is formed by anionic polymerization and the
organic block by atom transfer radical polymerization
(ATRP).19 The latter technique allows the controlled
polymerization of a much wider variety of vinyl mono-
mers than anionic polymerization schemes and imposes
less strict requirements on monomer purity.
In tr od u ction . The self-organization of block copoly-
mers constitutes a versatile means of producing ordered
periodic structures with phase-separated microdomain
sizes on the order of tens of nanometers.1 By varying
the relative molar masses of the blocks and the total
molar mass, control over morphology, microdomain size,
and spacing is easily achieved in the bulk state. In thin
films, asymmetric block copolymers can self-assemble
to ordered monolayers, provided that, next to block
copolymer composition, film thickness is controlled.
Block copolymer thin films2 are of significant interest
as templates for the fabrication of a variety of nanoscale
structures such as periodic dot arrays,3,4 nanopores,5
and nanowires.6,7
In a number of applications, the utility of the pro-
duced patterns, such as etch resistance or conductivity
relies on the introduction of inorganic elements in
separate loading steps. Poly(ferrocenylsilanes)8 owe a
number of useful characteristics such as redox activity
and a very high resistance to reactive ion etching9 to
the presence of alkylsilane and ferrocene units in the
main chain and combine these characteristics with the
properties and processability of polymers.
We demonstrated the use of asymmetric organic-
organometallic block copolymers featuring isoprene10 or
styrene11 units as organic and ferrocenyldimethylsilane
units as organometallic blocks in the formation of
monolayer thin films of densely packed organometallic
spheres in an organic matrix. Because of the very high
resistance of poly(ferrocenylsilanes) to reactive ion
etching,9 these thin films could be used as nanolitho-
graphic templates, enabling a direct transfer of the
nanoscale patterns into underlying silicon or silicon
nitride substrates with high aspect ratios.12,13 These
hybrid block copolymers could even be used to pattern
a variety of thin metal films into dot arrays for e.g. high-
density magnetic data storage.14
In block copolymer thin films, the presence of a
substrate and surface can induce orientation of the
microphase structure15 and can result in changes in
domain dimensions or phase transitions16,17 due to
substrate or surface preferences of one of the blocks.
After having established the thin film morphology of PI-
b-PFS and PS-b-PFS diblock copolymers, we were
interested to have access to PFS block copolymers with
more polar organic blocks, in particular poly(methyl
methacrylate) which has a higher affinity for silicon
substrates. In addition to organic-organometallic diblock
copolymers featuring polar blocks, amphiphilic organo-
metallic block copolymers are accessible when e.g.
Exp er im en ta l Section . a . Ma ter ia ls. Ferrocene
(98%), N,N,N′,N′-tetramethylethylenediamine (TMEDA,
99.5+%), n-butyllithium (1.6 M in hexanes), chlorodi-
methylsilane (98%), dichlorodimethylsilane (99%), al-
lyloxy-tert-butyldimethylsilane (97%), tetrabutylammo-
nium fluoride (1.0 M in THF), diisobutylaluminum
hydride (1.0 M in toluene), pyridine (anhydrous), methyl
methacrylate (MMA, 99%), 2-bromo-2-methylpropionic
acid (98%), 2-bromoisobutyryl bromide (98%), dichloro-
(p-cymene)ruthenium(II) dimer, and tricyclohexylphos-
phine were obtained from Aldrich. Allyloxytrimethylsi-
lane (>97%) and triethylamine (>99.5%) were obtained
from Fluka. The platinum cyclovinylmethylsiloxane
complex in cyclic methylvinylsiloxanes (3-3.5% Pt, CAS
68585-32-0) was obtained from ABCR, Karlsruhe. n-
Heptane (for synthesis) and 4-(dimethylamino)pyridine
(for synthesis) were purchased from Merck. Tetrahy-
drofuran (THF) for anionic polymerizations was distilled
from sodium-benzophenone under argon, degassed in
three freeze-pump-thaw cycles, and distilled by vacuum
condensation from n-butyllithium. Toluene for ATRP
was degassed on a high-vacuum line and distilled by
condensation. 2-Bromoisobutyric anhydride was pre-
pared according to a literature procedure.20 1H NMR
(CDCl3): δ 2.00 (CH3, s). The ATRP catalyst, (p-cy-
mene)ruthenium(II) chloride-tricyclohexylphosphine,
was synthesized as described earlier.21
1
b. Tech n iqu es. H NMR spectra were recorded in
CDCl3 on a Varian Unity Inova (300 MHz) instrument
at 300.3 MHz and on a Varian Unity 400 spectrometer
at 399.9 MHz. A solvent chemical shift of δ ) 7.26 ppm
was used as a reference. GPC measurements were
carried out in THF at 25 °C, using microstyragel
columns (bead size 10 µm) with pore sizes of 105, 104,
103, and 106 Å (Waters) and a dual detection system
consisting of a differential refractometer (Waters model
410) and a differential viscometer (Viscotek model
H502). Molar masses were determined relative to
polystyrene standards.
c. Mon om er P u r ifica t ion . [1]Dimethylsilaferro-
cenophane was prepared and purified as described
earlier.22,23 Methacrylate monomers were degassed on
a high-vacuum line in three freeze-pump-thaw cycles
and distilled under vacuum from calcium hydride im-
mediately before use.
* Corresponding authors. E-mail: m.a.hempenius@utwente.nl;
g.j.vancso@utwente.nl.
10.1021/ma0358172 CCC: $27.50 © 2004 American Chemical Society
Published on Web 02/13/2004

Macromolecules, Vol. 37, No. 5, 2004
Communications to the Editor 1687
Sch em e 1. Syn th esis of En d -Ca p p in g Rea gen ts w ith
P r otected Hyd r oxyl F u n ction a lity
d . An ion ic P olym er iza tion . [1]Dimethylsilaferro-
cenophane polymerizations were carried out in THF in
a MBraun Labmaster 130 glovebox under an atmo-
sphere of prepurified nitrogen (<0.1 ppm of H2O), using
n-butyllithium as initiator.
e. En d -Ca p p in g Rea gen ts. 3-(Tr im eth ylsiloxy)-
p r op yld im eth ylch lor osila n e (1). A two-necked 100
mL round-bottom flask fitted with a septum and con-
nected to a Schlenk line was evacuated and filled with
argon. Toluene (10 mL), allyloxytrimethylsilane (8.2 g,
63 mmol), chlorodimethylsilane (11.9 g, 126 mmol), and
platinum catalyst (2-3 droplets, ∼5 × 10-6 mol of Pt)
were added. After stirring the mixture under argon at
room temperature for 4 days, complete conversion was
achieved. The reaction mixture was degassed on a
vacuum line in three freeze-pump-thaw cycles, and
toluene and excess (CH3)2SiHCl were removed by vacuum
condensation. The product was purified by vacuum
distillation (bp 37-38 °C, 0.1 mm) and obtained as a
colorless oil (isolated yield 12.7 g, 90%). 1H NMR
(CDCl3): δ 3.56 (CH2O, t, 6.6 Hz, 2H); 1.62 (CH2, m,
2H); 0.81 (CH2Si, m, 2H); 0.42 (ClSi(CH3)2, s, 6H); 0.10
(OSi(CH3)3, s, 9H).
stirred in a thermostated oil bath (80 °C) for 14 h. After
cooling, the mixture was diluted with THF and dropwise
added to MeOH to precipitate the product. The block
copolymers were dried under vacuum.
Resu lts a n d Discu ssion . Our synthetic approach to
PFS-b-PMMA block copolymers combines the living
anionic ring-opening polymerization of [1]dimethylsila-
ferrocenophane24 with a living radical polymerization
of methyl methacrylate by means of ATRP.25 Anionic
polymerization of [1]dimethylsilaferrocenophanes allows
one to form well-defined organometallic blocks with
controlled block lengths and low polydispersities.
In ATRP, free radicals are generated through a
reversible redox process catalyzed by a transition metal
complex. Uniform growth of chains is accomplished
through fast initiation and a rapid reversible deactiva-
tion of free radicals.19 An ATRP system consists of an
initiator, a catalyst (transition metal complex), and
monomer. For the synthesis of the methacrylate blocks,
a ruthenium-based catalyst [RuCl2(p-cymene)(PR3)],
with R ) cyclohexyl, was chosen. Because of its high
catalytic activity and control over the polymerization
process, poly(methyl methacrylate) (PMMA) with poly-
dispersities as low as Mw/Mn < 1.1 can be obtained using
this catalyst complex.26
Various R-haloesters have been successfully employed
as ATRP initiators. Among these, 2-bromoisobutyryl
groups are particularly useful as initiator for the ATRP
of methyl methacrylate,27 as this initiator produces a
radical that is structurally nearly identical to the
propagating radical. Acrylates are polymerized success-
fully using 2-halopropionyl and 2-haloisobutyryl groups
as initiator.19b,25
Here, PFS homopolymers end-capped with a 2-bro-
moisobutyryl moiety serve as macroinitiators for meth-
acrylate polymerization. This group is easily introduced
by reacting hydroxyl-terminated PFS with e.g. 2-bro-
moisobutyryl bromide or 2-bromoisobutyric anhydride.
We attempted to prepare hydroxyalkyl-terminated PFS
by treating living PFS anions with styrene oxide, but
incomplete end-functionalizations were found. Similar
results were reported for the end-capping of living
polystyrene with this reagent.25
3-(Ter t-bu tyldim eth ylsiloxy)pr opyldim eth ylch lo-
r osila n e (2) was prepared by a Pt-catalyzed hydrosi-
lylation reaction of allyloxy-tert-butyldimethylsilane and
chlorodimethylsilane in toluene, similar as described for
1. The product was purified by vacuum distillation (bp
50-52 °C, 0.1 mm) and obtained as a colorless oil
1
(isolated yield 13.5 g, 86%). H NMR (CDCl3): δ 3.60
(CH2O, t, 6.6 Hz, 2H); 1.62 (CH2, m, 2H); 0.90 (SiC-
(CH3)3, s, 9H), 0.82 (CH2Si, m, 2H); 0.42 (ClSi(CH3)2, s,
6H); 0.06 (s, 6H, OSi(CH3)2).
f. Ma cr oin itia tor s. In a typical experiment, [1]-
dimethylsilaferrocenophane (600 mg, 2.48 mmol) in
THF (5 mL) was polymerized by adding n-BuLi (0.25
mL of a 0.2 M solution in n-heptane, 5 × 10-5 mol) at
room temperature. After 15 min, the solution was cooled
to -70 °C, and end-capper 1 (56 mg, 2.5 × 10-4 mol)
was added. After stirring for 2 h at -70 °C, the mixture
was allowed to come to room temperature and added to
MeOH (50 mL) to precipitate 3a , which was dried under
vacuum. The trimethylsilyl end group was cleaved in a
mixture of THF (5 mL), H2O (0.4 mL), and AcOH (2
droplets) by stirring at room temperature for 24 h,
yielding hydroxypropyl-terminated PFS 4, which was
precipitated in n-heptane and dried under vacuum.
Alternatively, living PFS was end-capped with 2 at
20 °C to produce 3b. Cleavage of the TBDMS ether, by
stirring with i-Bu2AlH (0.5 mL, 5 × 10-4 mol) in CH2-
Cl2 (10 mL) at 20 °C for 24 h under argon, followed by
precipitation in MeOH, stirring in CH2Cl2/H2O (pH )
5), and precipitation in n-heptane gave 4. Acylation of
4 (5 × 10-5 mol of OH) was carried out with 2-bro-
moisobutyric anhydride (0.16 g, 5 × 10-4 mol) in dry
pyridine (5 mL) in the presence of 4-(dimethylamino)-
pyridine (20 mg, 1.6 × 10-4 mol). The mixture was
stirred at room temperature under dry N2 for 48 h.
2-Bromoisobutyryl end-functionalized PFS 5b was pre-
We then explored the use of 3-(trimethylsiloxy)-
propyldimethylchlorosilane (1) and 3-(tert-butyldimeth-
ylsiloxy)propyldimethylchlorosilane (2) (Scheme 1) as
end-capping reagents for living anionic PFS. Chlorosi-
lanes in general are particularly successful end-capping
reagents in anionic polymerization due to their high
reactivity and lack of side reactions,28 and their utility
can be increased further by incorporating protected
functional groups. Nevertheless, only a few accounts
have appeared in the literature on the use of such
reagents in the end-functionalization of living polymer
anions.29,30 Chlorosilanes 1 and 2 were synthesized by
the hydrosilylation of allyloxyalkylsilanes with chlo-
rodimethylsilane in toluene (Scheme 1).
cipitated in MeOH and dried under vacuum. Mn
)
1.20 × 104 g/mol, Mw ) 1.26 × 104 g/mol, Mw/Mn ) 1.05.
g. Block Cop olym er Syn th esis. A glass tube con-
taining a magnetic stirring bar was charged with a PFS
macroinitiator (60-210 mg), (p-cymene)ruthenium(II)
chloride-tricyclohexylphosphine (5 mg), methyl meth-
acrylate (0.80 g, 8.0 mmol), and degassed toluene (0.8-
1.5 mL) in the glovebox and sealed. The mixture was

1688 Communications to the Editor
Macromolecules, Vol. 37, No. 5, 2004
Sch em e 2. Syn th esis of a P oly(fer r ocen ylsila n e) Ma cr oin itia tor for ATRP
Ta ble 1. Degr ees of P olym er iza tion DP n of Va r iou s P F S Ma cr oin itia tor s 5 Ba sed on GP C a n d 1H NMR An a lysis
c
d
e
DPn,NMR
DPn,NMR
DPn,NMR
b
Mn,GPCa (g/mol)
Mw/Mn
DPn,GPC
-CH3 (n-Bu)
-C(CH3)2Br
-Si(CH3)2
PFS-Br 5a
PFS-Br 5b
PFS-Br 5cf
8 000
12 000
12 000
1.02
1.05
1.07
33
50
50
34
44
46
49
64
60
46
72
52
a
b
Measured by GPC, relative to polystyrene standards. Calculated from Mn determined by GPC. c Calculated from ferrocenyl and
initiator integrals (CH3). Calculated from ferrocenyl and 2-bromoisobutyryl integrals. e Calculated from backbone Si(CH3)2 and end-
d
capper Si(CH3)2 integrals. f Obtained via 3b, deprotected using i-Bu2AlH.
The hydroxyl-protecting group should be stable in the
presence of the PFS carbanion and cleave quantitatively
under conditions that do not deteriorate the PFS chain.
Trimethylsilyl ethers are cleaved under very mild
conditions but are also prone to nucleophilic attack by
organolithium compounds.31 To prevent reaction of
living PFS anions with the trimethylsilyl ether moiety
of 1, end-capping reactions were carried out at -70 °C.
Hydrolysis of the trimethylsilyl ether in a mixture of
THF, water, and acetic acid produced the hydroxypro-
pyl-terminated PFS 4, which was then acylated with
2-bromoisobutyric anhydride in pyridine with 4-(dimeth-
ylamino)pyridine as a catalyst22,32 to macroinitiator 5
(Scheme 2).
tert-Butyldimethylsilyl (TBDMS) ethers are stable
toward organometallic reagents, allowing one to end-
cap living polymers with 2 at room temperature. TB-
DMS ethers are usually cleaved either under acidic
conditions or by treatment with tetra-n-butylammonium
fluoride (TBAF) in THF.33 Acidic hydrolysis is not an
option for cleaving a tert-butyldimethylsilyl protecting
group as acidic conditions (pH < 4) are not tolerated by
PFS chains. TBAF cleaves tert-butyldimethylsilyl ethers
quantitatively, but it also is a powerful desilylation
reagent.34 We observed a significant broadening and
tailing to lower molar mass values of PFS in GPC traces
upon deprotection of 3b by TBAF. Clearly, TBAF causes
chain scission in PFS. By treating 3b with diisobutyl-
aluminum hydride,35 however, a clean deprotection of
the TBDMS ether was achieved, yielding 4 without any
molar mass decline (Scheme 2).
mass decline of the PFS chains. A diagnostic end group
signal clearly observed in the H NMR spectra of 4 was
1
a triplet at δ ) 3.56 ppm, belonging to the methylene
protons adjacent to the hydroxyl group. This methylene
1
signal was no longer visible in the H NMR spectra of
5, nor was the signal of the acyloxymethylene group due
to overlap with the ferrocenyl signals of the PFS chain.
Clearly present, however, was the characteristic singlet
at δ ) 1.93 ppm due to the 2-bromoisobutyryl moiety.
Also, the signal due to the CH3 group of the n-butyl
initiator moiety was observed at δ ) 0.90 ppm. End-
group analysis of 5 was performed on the basis of the
ratio of the initiator and end-capper integrals to the
ferrocenyl integral at δ ) 4.01 and 4.22 ppm. Similarly,
the dimethylsilyl signal of the end-capper moiety was
compared with the dimethylsilyl signal of the polymer
backbone. In Table 1, the number-average degrees of
polymerization (DPn) obtained by GPC and by 1H NMR
are summarized. The values correlate reasonably well.
Comparison of the dimethylsilyl signals of the initia-
tor (s, δ ) 0.21 ppm) and end-capper (s, δ ) 0.24 ppm)
moieties of 4 and of the corresponding signals of 5 seems
to be the most accurate way to gauge the degree of end-
functionalization. End-capping with 2 followed by re-
ductive cleavage of the TBDMS ether by i-Bu2AlH and
acylation gave 90-95% end-functionalization. GPC
measurements following the formation of the second
block should provide further information on end-capping
efficiency.
The 2-bromoisobutyryl end-capped poly(ferrocenylsi-
lane) 5 was then used as initiator in the ruthenium-
mediated living polymerization of methyl methacrylate
(Scheme 3). In a typical experiment, macroinitiator 5a
(60 mg), ruthenium complex (5 mg), methyl methacry-
late (0.80 g, 8.0 mmol), and toluene (0.8 mL) were placed
in a glass tube under a nitrogen atmosphere. The tube
was sealed, and the reaction mixture was stirred at
Hydroxypropyl-terminated PFS 4 and corresponding
macroinitiators 5 of varying molar mass were obtained
and characterized using GPC and 1H NMR. GPC
showed 4 and 5 to be well-defined polymers, with
polydispersities Mw/Mn < 1.1, indicating that the end-
capping and deprotection steps did not cause any molar

Macromolecules, Vol. 37, No. 5, 2004
Communications to the Editor 1689
Sch em e 3. Syn th esis of P oly(fer r ocen yld im eth ylsila n e-block-m eth yl m eth a cr yla te) Block Cop olym er s
Ta ble 2. Mola r Ma ss Ch a r a cter istics of P F S-b-P MMA Block Cop olym er s 6
Mn,GPCa (g/mol)
Mn,NMRb (g/mol)
Mn,NMR (g/mol)
PFS-b-PMMA
Mn,GPCa (g/mol)
PFS-b-PMMA
Mw/Mn
PFS-b-PMMA
wt %
PFSc
PFS-Br
PMMA
PFS-b-PMMA 6a d
PFS-b-PMMA 6b1
PFS-b-PMMA 6b2
PFS-b-PMMA 6cd
8 000 (5a )
12 000 (5b)
12 000 (5b)
12 000 (5c)
114 000
116 300
53 200
33 600
122 000
128 300
65 200
45 600
95 700
101 700
66 900
40 500
1.06
1.10
1.06
1.18
7
9
18
26
d
d
a
b
Measured by GPC, relative to polystyrene standards. Calculated from ferrocenyl and CH3O integrals, based on Mn,GPC of the PFS
block. c Calculated from ferrocenyl and CH3O integrals. 6a : MMA ) 8.0 × 10-3 mol, PFS-Br ) 7.5 × 10-6 mol, toluene 0.8 mL, Ru
d
catalyst 5 mg, MMA conversion 45%. 6b1: MMA ) 7.3 × 10-3 mol, PFS-Br ) 8.5 × 10-6 mol, toluene 1.0 mL, Ru catalyst 5 mg, MMA
conversion 60%. 6b2: MMA ) 4.0 × 10-3 mol, PFS-Br ) 8.8 × 10-6 mol, toluene 1.0 mL, Ru catalyst 5 mg, MMA conversion 58%. 6c:
MMA ) 7.2 × 10-3 mol, PFS-Br ) 1.8 × 10-5 mol, toluene 1.5 mL, Ru catalyst 5 mg, MMA conversion 60%.
F igu r e 1. GPC traces of PFS macroinitiator 5a and its
F igu r e 2. GPC traces of representative PFS-b-PMMA block
corresponding PFS-b-PMMA block copolymer 6a .
copolymers.
80 °C for 14 h. After precipitation in methanol and
lowing the formation of the organometallic block by
drying under vacuum, PFS-b-PMMA block copolymer
6a was obtained. Figure 1 shows the GPC traces of
macroinitiator 5a and PFS-b-PMMA block copolymer
6a . Essentially all of the PFS macroinitiator was
converted into block copolymer, indicating that the end-
capping strategy (Scheme 2) enabled a near-quantitative
incorporation of 2-bromoisobutyryl groups.
Control over block copolymer composition and molar
anionic polymerization, a ruthenium-mediated con-
trolled radical polymerization of methyl methacrylate
was employed to grow the second block. Well-defined
PFS-b-PMMA block copolymers with low polydispersi-
ties (Mw/Mn < 1.1) and controlled compositions were
obtained. The use of ATRP to form the second block
implies that a wide variety of (meth)acrylic and other
vinyl monomers can be used, thus opening up the way
mass was achieved by varying the initiator concentra-
tion [I]0 with respect to monomer concentration [M]0.
Alternatively, the molar mass of the PFS macroinitiator
to novel hybrid organic-organometallic block copoly-
mers and amphiphilic organometallic block copolymers.
can be tuned, while keeping the monomer-to-initiator
ratio [M]0/[I]0 constant. Characteristic GPC traces of
obtained PFS-b-PMMA block copolymers are shown in
Figure 2. For block copolymers 6a , 6b1, 6b2, and 6c,
MMA conversions of 45%, 60%, 58%, and 60% were
reached, respectively, in 14 h. Block copolymer composi-
Ack n ow led gm en t. The authors gratefully acknowl-
edge the University of Twente and the Masif Program
of the MESA+ Institute for Nanotechnology for financial
support.
Refer en ces a n d Notes
1
tions were determined using H NMR by relating the
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integral of the CH3O signal at δ ) 3.59 ppm belonging
to the methacrylate repeat units to the ferrocenyl signal
integral at δ ) 4.01 and 4.22 ppm. Combined with Mn
values of the PFS block obtained by GPC, total block
copolymer molar masses Mn were calculated and com-
pared with values obtained by GPC. The results, sum-
marized in Table 2, are in good agreement. The block
copolymers described here are asymmetric, with PFS
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