Orski et al.
Article
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This hydrophilicity, along with the cross-links that occur between
the polymer chains and the surface (and often between the polymer
chains themselves), create a hydrogel matrix that is sensitive to
(ROMP),
and reversible addition-fragmentation chain
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transfer (RAFT) polymerization.
As controlled polymer architectures of active ester polymers
have advanced in solution, there are few reports of these polymers
grafted from solid interfaces. The R u€ he group has polymerized
N-methacryloyl-β-alanine succinimide ester via a free-radical poly-
merization from a surface bound 2,2 -azobis(2-methylpropio-
nitrile) (AIBN) initiator and demonstrated the functionalization
of the activated ester with small molecules and oligomers on poly-
mer thicknesses up to 80 nm. More recently, Cullen and co-
workers used ATRP to grow polymer brushes of 2-vinyl-4,
-dimethyl azlactone from a surface to immobilize RNase A
17
pH and ionic strength of its solution environment. This can create
an environmental background response that can complicate or
even mask binding events that occur at the substrate interface.
N-Hydroxysuccinimide (NHS) esters are the most common
reactive group of activated esters and are widely used to conjugate
biological analytes to solid supports in their native form, where
the N-terminus of peptides, or lysine side chains, covalently bind
0
45
1
8
at the active ester site. While NHS esters have a half-life on the
order of hours in aqueous systems, their rate of hydrolysis
4
1
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and showed the enzyme maintained activity while covalently
increases in basic environments.
Also, side reactions invol-
4
6
attached to the polymer matrix. In order to fully develop active
ester brushes as a versatile and practical template for sensors and
microarrays, the stability of the activated ester brushes and the
ability to make controlled block copolymer architectures must be
evaluated. By polymerizing activated ester monomers using a
controlled polymerization technique, the sensing platform has
two facets of control. The NHS ester provides the desired
attachment chemistry, while the ability to polymerize blocks of
different monomers can control the surrounding microenviron-
ment and density of functional groups in the brush. This allows
for systematic tuning of properties that are important in array
development such as nonspecific binding, polymer solubility, and
accessibility of the functional moiety to solution analytes. Herein,
we report the surface initiated ATRP of N-hydroxysuccinimide
ving ring-opening or glutarimide formation between two active
2
1
esters are possible. Functionalization strategies can minimize
side reactions in active ester functionalization, such as using a
high concentration of the desired amine moiety to be attached or
functionalizing in organic solvents in the presence of a proton
1
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acceptor such as triethylamine or 4-(dimethylamino)pyridine.
More recently, the synthesis of polymers containing active ester
groups has emerged as a popular strategy for coupling synthetic
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and biomacromolecules.
By polymerizing a monomer with
an active ester functionality, not only is the extra functionaliza-
tion step avoided, but the polymer becomes a template for many
types of pendant functional groups and molecules. Pendant group
functionalization is straightforward and shows first order reac-
3
2
tionkineticswith very fast reaction times. It has beenshown that
synthetic polymers coupled to proteins can improve the overall
protein stability, solubility, and biocompatibility and are being
4-vinyl benzoate (NHS4VB). We demonstrate controlled poly-
merization through a kinetic study and formation of block co-
polymers with 2-hydroxyethyl acrylate, tert-butyl acrylate, and
styrene in which the NHS block can be either buried or exposed in
the brush layer, depending on monomer sequence in the poly-
merization. To test and quantify the functionalization efficiency,
the poly(NHS4VB) brushes are functionalized with primary
amine containing chromophores to quantify the amount of
activated ester present in the brush architecture and evaluate
the conversion efficiency of amide formation along the chain
backbone. The NHS active ester polymer brushes have the
potential to serve as a universal scaffold for the attachment of
both large and small analytes for sensor and microarray surfaces.
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utilized in the fields of biotechnology and medicinal research.
Controlled polymerizations containing active esters, including the
development of block copolymers, have been achieved in solution
by controlled polymerization techniques such as atom transfer
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5-37
radical polymerization (ATRP),
nitroxide mediated poly-
3
8
merization (NMP), ring-opening metathesis polymerization
(
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DOI: 10.1021/la902553f 2137