Facile strategy to well-defined water-soluble boronic acid (Co)polymers

Article abstract of DOI:10.1021/ja074239s

A facile route to well-defined boronic acid (co)polymers from stable and easily manipulated boronic ester monomers is presented. The polymerization of 4-pinacolatoborylstyrene by reversible addition-fragmentation chain transfer (RAFT) yielded polymeric bo

Full text of DOI:10.1021/ja074239s

Published on Web 08/04/2007
Facile Strategy to Well-Defined Water-Soluble Boronic Acid (Co)polymers
Jennifer N. Cambre, Debashish Roy, Sudershan R. Gondi, and Brent S. Sumerlin*
Department of Chemistry, Southern Methodist UniVersity, Dallas, Texas 75275-0314
Received June 11, 2007; E-mail: bsumerlin@smu.edu
Scheme 1. Synthesis of Boronic Ester and Boronic Acid
Homopolymers and Block Copolymers by RAFT Polymerization
Organoboron polymers are important precursors to materials with
potential utility in catalysis, separations, and sensing applications.
Recent reports detail boron-containing polymers serving as elec-
trolyte materials for batteries, blue emissive polymers, self-healing
materials, and precursors for functional polyolefins.1 These polymers
have demonstrated promise in biological applications as well, which
is especially evidenced by water-soluble boronic acid-containing
polymers having garnered a great deal of attention for glucose or
RNA sensing, diabetes treatment therapies, and as supramolecular
materials.2 To more fully realize the potential of boronic acid
containing macromolecules in sensing and delivery applications, it
is vital to expand the capability to prepare such polymers with
precise control over topology, molecular weight, and composition.
Herein, we report the first, to our knowledge, successful synthesis
of well-defined water-soluble boronic acid copolymers from stable
and easily manipulated boron-containing monomers.
A variety of polymerization techniques have been considered
for the synthesis of polymers with pendant boron functionality, with
conventional radical polymerizations arguably being most successful
due to facile experimental setup and lack of significant side
reactions.3 Controlled radical polymerization techniques facilitate
the preparation of (co)polymers with predetermined molecular
weights, narrow molecular weight distributions, and high degrees
of chain-end functionalization, the latter of which facilitate the
preparation of complex macromolecular architectures that lead to
self-assembly and other advanced materials applications.4 Ja¨kle et
al. pioneered the efficient synthesis of organoboron vinyl (co)-
polymers via atom transfer radical polymerization (ATRP),5 either
from silylated precursors that were subsequently borylated6 or from
the polymerization of organoboron monomers.7 We are primarily
interested in the synthesis and aqueous solution behavior of
amphiphilic organoboron block copolymers, especially those with
acrylamido hydrophilic blocks. While the success of ATRP for the
polymerization of most acrylamido monomers has drastically
improved over the past few years, reversible addition-fragmentation
chain transfer (RAFT) polymerization8 has proven excellent for the
synthesis of a range of polyacrylamides.9 RAFT can be conducted
under relatively mild conditions, is applicable to nearly any
monomer susceptible to radical polymerization, and has been
employed to prepare a range of well-defined complex macro-
molecular topologies. The plethora of stimuli-responsive and water-
soluble acrylamido polymers, coupled with the potential biological
applications of controlled architecture boron-containing polymers,
suggests significant value in developing synthetic capabilities to
prepare such block copolymers.
challenging to characterize by gel permeation chromatography
(GPC). Therefore, we polymerized the pinacol ester of 4-vinylphen-
ylboronic acid and subsequently deprotected utilizing a mild and
convenient strategy to obtain the boronic acid (co)polymers. A
previous successful route to well-defined polymeric boronic acids
proceeded via ATRP and postpolymerization modification of
silylated precursors with toxic and air-sensitive BBr3.7
We polymerized 4-pinacolatoborylstyrene (pBSt) (1) with 2-dode-
cylsulfanylthiocarbonylsulfanyl-2-methylpropionic acid (2) as the
chain transfer agent (CTA) and 2,2′-azobisisobutyronitrile (AIBN)
as the initiator (Scheme 1). Polymerizations were conducted at 70
°C in anisole with a monomer concentration of 50 vol %. Reaction
stoichiometry ([monomer]/[CTA]/[initiator]) was varied, and the
agreement between theoretical and experimental molecular weights
was excellent over the course of the polymerizations (Figure 1).
Kinetics followed the expected dependence on the ratio of [CTA]/
[AIBN]. Increasing the amount of initiator relative to the CTA
resulted in somewhat faster rates, and with the exception of a slight
inhibition period being observed at low initiator concentrations,
pseudo-first-order kinetics were observed up to relatively high
conversions. As further evidence of polymerization control, poly-
(4-pinacolatoborylstyrene) (PpBSt) homopolymers were employed
as macro-chain transfer agents to synthesize block copolymers with
DMA. Successful chain extension confirmed end group retention
indicative of polymerization control, while simultaneously allowing
the preparation of amphiphilic diblock copolymers (Table 1).
Indeed, PpBSt145-b-PDMA273 (Mn ) 60800 g/mol, Mw/Mn ) 1.09)
formed micelles with an average hydrodynamic diameter of
approximately 98 nm in water, as determined by dynamic light
scattering (Figure 1).
We prepared homopolymers of selected molecular weights and
block copolymers with poly(N,N-dimethylacrylamide) (PDMA).
While there have been reports of uncontrolled random copolymers
of acrylamido and boron-containing monomers being prepared by
conventional radical polymerization,10 this is the first example of
block copolymers with organoboron and acrylamido segments.
Polymeric boronic acids tend to be extremely hygroscopic and are
Postpolymerization modification of silylated precursors prepared
by ATRP can successfully lead to polymeric boronic acids,7 but
deprotection of the stable and conveniently manipulated polymeric
pinacol esters has yet to be reported. Rather harsh conditions are
generally required to deprotect hindered boronic esters. Transes-
terification with another boronic acid followed by hydrolysis is an
attractive approach, but the requirement of an excess of this second
9
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10.1021/ja074239s CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
anionic polyelectrolyte boronates at high pH. On the other hand,
the polymeric ester precursors were insoluble under these condi-
tions, and thus the change in solubility serves as further evidence
of successful deprotection to the free boronic acid moieties. For
the block copolymers with PDMA, deprotection was sufficiently
mild to proceed without degradation of the acrylamido units.
This facile route leads to well-defined polymeric boronic acids
via RAFT polymerization of their stable pinacol esters. This is the
first example of boronic acid/ester block copolymers being prepared
by RAFT polymerization and the first time a block copolymer
containing organoboron and acrylamido segments has been prepared
by any polymerization method. The self-assembly potential of
organoboron amphiphilic copolymers was demonstrated by forma-
tion of polymeric micelles of PpBSt-b-PDMA in water. The
versatility of acrylamido polymers in applications requiring water-
solubility or stimuli-responsive behavior, combined with the
anticipated biological and catalytic utility of controlled architecture
boron-containing polymers, suggests significant promise in the
development of synthetic strategies toward water-soluble boronic
acid-containing macromolecules. Future studies will investigate the
responsive behavior of boronic acid block copolymers as a function
of specific small molecule organics.
Figure 1. (A) Pseudo-first-order kinetic plot and (B) Mn versus conversion
for RAFT polymerization of 4-pinacolatoborylstyrene (pBSt) with various
ratios of monomer (Mon), chain transfer agent (CTA), initiator (Init); (C)
GPC traces for a PpBSt homopolymer and block copolymer with poly-
(N,N-dimethylacrylamide) (PDMA); (D) hydrodynamic diameter distribution
for PpBSt145-b-PDMA273 in water.
Acknowledgment. Acknowledgment is made to SMU, the
Donors of the American Chemical Society Petroleum Research
Fund (Grant 45286-G7), Oak Ridge Associated Universities (Ralph
E. Powe Junior Faculty Enhancement Award), and the Defense
Advanced Research Projects Agency (Grant HR0011-06-1-0032)
for partial support of this research.
Table 1. Results from Synthesis of Poly(4-pinacolatoborylstyrene)
(PpBSt) and PpBSt-b-Poly(N,N-dimethylacrylamide) (PDMA)
c
time
min
conversionb
%
Mn
c
polymer
[M]/[CTA]/[I]a
g/mol
Mw/Mn
PpBSt
[100]/[1]/[0.15] 518
[100]/[1]/[0.20] 420
[200]/[1]/[0.30] 420
[100]/[1]/[0.40] 360
74
70
68
79
99
99
16 900
16 300
32 400
17 200
36 600
60 800
1.16
1.13
1.11
1.15
1.08
1.09
PpBSt
PpBSt
PpBSt
Supporting Information Available: Detailed experimental and
analytical details, and additional GPC traces. This material is available
free of charge via the Internet at http://pubs.acs.org.
PpBSt-b-PDMA [150]/[1]/[0.40] 360
PpBSt-b-PDMA [250]/[1]/[0.40] 300
a Stoichiometric ratio of monomer/chain transfer agent/initiator. b De-
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