Preparation of organoboron block copolymers via ATRP of silicon and boron-functionalized monomers

Article abstract of DOI:10.1021/ma051615p

The organoboron block copolymers were fabricated by atom transfer radical polymerization (ATRP) of silicon and boron-functionalized monomer, MBpin. It was observed that the reactivity of the functionalized monomer was similar to that of styrene, though MB

Full text of DOI:10.1021/ma051615p

Macromolecules 2005, 38, 8987-8990
8987
Preparation of Organoboron Block
Copolymers via ATRP of Silicon and
Boron-Functionalized Monomers
Scheme 1. Synthesis of the Organoboron Block
Copolymer PSBpin-b-PS; 1-PEB ) 1-Phenylethyl
Bromide and PMDETA )
N,N′,N′,N′′,N′′-Pentamethyldiethylenetriamine
Yang Qin, Vishad Sukul, Dimitrios Pagakos,
Chengzhong Cui, and Frieder J a1 kle*
Department of Chemistry, Rutgers University Newark,
73 Warren Street, Newark, New Jersey 07102
Received July 22, 2005
Revised Manuscript Received September 12, 2005
Boron-containing polymers represent an important
class of materials that are extensively used, for example,
as intermediates in the synthesis of functionalized
1
polymers with polar side groups, supported reagents
2
and immobilized catalysts, scaffolds for the solid-phase
3
synthesis of carbohydrates, sensors and stimuli respon-
4,5
sive polymers including the sensing of sugars, separa-
6
tion media in biochemical applications, polymeric
7
8
electrolytes for batteries, flame retardants, ceramic
9,10
precursors,
and luminescent and other electronically
1
1
interesting materials. Their preparation generally
involves either direct polymerization of boron-function-
alized monomers or functionalization of polymers with
boron groups. Polymerization of boron monomers has
overall received more attention for the placement of
boron substituents in the side chains of non-cross-linked
polyolefins. The method of choice usually is standard
free radical polymerization due to the reasonably good
compatibility with boron-carbon bonds and the rela-
tively straightforward synthetic procedures involved.
Thus, heat-induced or AIBN (2,2′-azobis(2-methylpro-
pionitrile)) initiated free radical polymerization has been
applied to the homo- and copolymerization of olefins
5
,12
13
containing boronic acid and boronic ester,
borazine,
block copolymers are rare.22 Controlled free radical
polymerization methods such as atom transfer radical
polymerization (ATRP)23 have become an important tool
for the preparation of block copolymers and other
polymer architectures. Since ATRP tolerates various
functionalities,23 we studied the suitability of ATRP for
the controlled polymerization of boron-containing mono-
mers and the formation of block copolymers.
and polyborane and carborane14 moieties. Ring-opening
1
0,15
metathesis polymerization (ROMP)
and Ziegler-
have also commonly been em-
1
6,17
Natta techniques
ployed, and the latter are especially useful for the
polymerization of more highly reactive and more strongly
Lewis acidic organoboron species. A recent example is
the polymerization of a boron-functionalized styrene
monomer with a titanium catalyst, which yields syn-
17
diotactic boron-functionalized polymers. Postpolymer-
ization modification procedures represent an interesting
alternative that sometimes can circumvent commonly
encountered issues with compatibility of certain orga-
noboron monomers with given polymerization proce-
Two different routes for the preparation of boron-
containing block copolymers were investigated (Scheme
1). In the direct polymerization method, route A, we
attempted the first controlled polymerization of a boron-
containing monomer via ATRP. The resulting well-
defined organoboron polymer should in turn be able to
serve as a macroinitiator in the chain extension with
styrene or other monomers. We have previously shown
that trimethylsilyl-functionalized polystyrene (PSSi) can
be used as an efficient precursor to boron-modified
1
8
dures. Recent advances in this field have led to the
successful application of hydroboration chemistry to
vinyl group containing polyolefins,1 and even unfunc-
tionalized polyethylethylene (PEE) and polypropylene
have been successfully borylated through a novel transi-
9
2
0
21
tion-metal-catalyzed process. We have shown that
well-defined fully functionalized borylated polystyrene
is readily accessible through facile and high yield
polymer modification reactions using silylated polysty-
styrenic polymers. Thus, postpolymerization modifica-
tion (route B) represents an interesting alternative
method that entails controlled polymerization of a
trimethylsilyl-functionalized monomer and subsequent
chain extension with styrene, followed by highly selec-
tive boron-silicon exchange with BBr3.
2
1
rene as the precursor.
While all these methods are extensively used in the
preparation of boron-containing homo- and random
copolymers, despite potentially intriguing possibilities,
reports on the incorporation of boron into well-defined
The direct polymerization method (route A) starts
from the organoboron monomer MBpin, which was
obtained in moderate yield (∼50%) through treatment
of styreneboronic acid with pinacol followed by high-
vacuum distillation. The polymerization behavior of this
*
Corresponding author. E-mail: fjaekle@rutgers.edu.
1
0.1021/ma051615p CCC: $30.25 © 2005 American Chemical Society
Published on Web 09/30/2005

8988 Communications to the Editor
Macromolecules, Vol. 38, No. 22, 2005
Figure 2. Gel permeation chromatography (refractive index
detector) traces of PSBpin and PSBpin-b-PS (see Scheme 1)
during the chain extension reaction.
Chain extension of the resulting organoboron polymer
PSBPin was attempted by ATRP with styrene in bulk
at 110 °C (Scheme 1). To ensure full coverage of the
polymer with Br end groups, a sample that was obtained
at low conversion of MBPin was used as the macroini-
tiator (Mw ) 8540). The polymerization was monitored
by gel permeation chromatography (GPC) analysis of
samples withdrawn at predetermined time intervals.
The GPC traces in Figure 2 clearly show that the
molecular weight increases with time and the dispersity
remains narrow (PDI ) 1.04). This indicates that
thermally induced polymerization is insignificant. More-
over, in the traces for PSBpin-b-PS, no low molecular
weight shoulders are observed. The absence of any
detectable amount of the starting block, PSBpin, sug-
gests close to quantitative initiation and provides
further evidence for the quasi-living nature of the
polymerization of MBpin. The composition of the co-
Figure 1. (a) Kinetic data for atom transfer free radical
polymerization (ATRP) of 4-pinacolatoborylstyrene (MBpin) in
1
anisole (50 wt %) at 90 °C as determined by H NMR
spectroscopy. (b) Dependence of number-average molecular
n
weight (M ) and polydispersity index (PDI) on conversion based
on gel permeation chromatography with refractive index
detector (GPC-RI) relative to polystyrene standards. [M]
polymer after a reaction time of 100 min was studied
0
)
by a combination of GPC, 1H NMR, and elemental
4
0 0 0 0
.3 M and [M] :[1-PEB] :[CuBr] :[PMDETA] ) 100:1:2:4 (1-
analysis. All three methods gave the same ratio of PS
repeat units relative to PSBpin repeat units (ca. 5:1).
Thus, the chain extension of PSBpin with styrene can
at least during the early stages also be considered a
quasi-living process, and the molecular weight can be
controlled based on the reaction time (see Supporting
Information).
PEB ) 1-phenylethyl bromide; PMDETA ) N,N′,N′,N′′,N′′-
pentamethyldiethylenetriamine).
organoboron monomer was studied using a typical
protocol for ATRP24 with 1-phenylethyl bromide (PEB)
as the initiator and N,N′,N′,N′′,N′′-pentamethyldieth-
ylenetriamine (PMDETA)/CuBr as the catalyst system.
The polymerization conditions were optimized by vary-
ing monomer concentration, temperature, initiator-to-
catalyst ratio, and ligand-to-catalyst ratio. The best
results were obtained with a ratio of monomer:initiator:
catalyst:ligand of 100:1:2:4 at 90 °C. Under these
conditions the polymerization was well controlled, and
a pseudo first-order kinetic behavior was observed up
to ca. 60% conversion, as shown in Figure 1a. The
monomer reactivity was found to be similar to that of
styrene, though MBpin polymerized at a considerably
slower rate than 4-trimethylsilylstyrene (MSi).25 At
higher conversion deviations from ideal behavior were
found, which are likely due to increased significance of
As mentioned above, postpolymerization modification
of silylated polymers provides an alternative route to
boron-containing polymers (route B, Scheme 1). This
method is particularly attractive since we could previ-
ously show that very good control is possible in ATRP
even at relatively high conversions of 4-trimethylsilyl-
styrene and that the silyl groups can be quantitatively
replaced with boryl moieties.21 A trimethylsilyl-func-
tionalized polymer (PSSi) of low molecular weight
(
Mw ) 6243) was used as the macroinitiator for the
polymerization of styrene under ATRP conditions
Scheme 1). A kinetic analysis of the chain extension of
(
PSSi with styrene showed a linear dependence of
ln([M]0/[M]) vs time and indicated that the molecular
weight of the block copolymer can be controlled and the
dispersity remains narrow (see Supporting Information).
The block copolymer PSSi-b-PS was recovered by pre-
cipitation into hexanes/ethanol (1:2) and studied by gel
permeation chromatography with in-line light scattering
detector (GPC-LS) (Table 1). Treatment of PSSi-b-PS
with (i) BBr in CH Cl , (ii) Me SiOMe, and (iii) pinacol
2
6
chain transfer events.
Consistent with these observations is that the mo-
lecular weight increases linearly with conversion up to
about 60% conversion, while the polydispersity index
(
PDI) remains low in this regime (Figure 1b). Again,
deviations from linearity were observed at higher
conversions with values of the number-average molec-
ular weight (Mn) lower than expected, but with only
slightly higher dispersities. Thus, the polymerization of
MBpin can be considered a controlled process before it
reaches 60% conversion.
3
2
2
3
gave the organoboron block copolymer PSBpin-b-PS,
which was isolated by precipitation into methanol. We
have previously shown for the synthesis of the respec-
tive homopolymer PSBPin that this procedure leads to

Macromolecules, Vol. 38, No. 22, 2005
Communications to the Editor 8989
Table 1. GPC-LS (Gel Permeation Chromatography with
boron polymers through straightforward esterification
reactions. The results described herein thus open up
new possibilities in the various application fields that
involve organoboron polymers including organic/inor-
ganic hybrid materials, polymer electrolytes, polymer-
supported catalysts, polymeric sensors, and separation
media.
1
in-Line Light Scattering Detector) and H NMR Analysis
of Polymers from Route B (Scheme 1)
mole fraction of mole fraction of
Mw
(GPC-LS)
functional block functional block
1
polymer
PSSia
by GPC-LS
by H NMR
b
6243
19480
21530
1, DP ) 34
1
a
c
PSSi-b-PS
0.21
0.21
0.19
PSBpin-b-PSa
0.19
d
Acknowledgment is made to the National Science
Foundation (NSF CAREER Award CHE-0346828 to
F.J.), to the donors of the Petroleum Research Fund,
administered by the American Chemical Society, and
to the Rutgers Research Council for support of this
research. We also thank NSF for GPC and LS instru-
mentation provided through the NSF-MRI program
a
See Scheme 1 for polymer labeling. b Average degree of po-
lymerization based on weight-average molecular weight (Mw).
c
Derived from integration of corresponding signals of PSSi and
PS. d Derived from integration of corresponding signals of PSBpin
and PS.
highly selective polymer modification.21 The individual
(MRI 0116066).
reactions were followed by NMR spectroscopy and the
NMR spectra of the final product were found to be
nearly identical to those for the block copolymer ob-
tained through the direct polymerization route (see
Supporting Information). GPC-LS analysis was per-
formed before and after postpolymerization modifica-
tion, and the results are summarized in Table 1. The
Supporting Information Available: Experimental pro-
cedures and characterization details. This material is available
free of charge via the Internet at http://pubs.acs.org.
References and Notes
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MA051615P