Biomedical Engineering Reference
In-Depth Information
Neutral Methacrylates
O
H
O
O
O
O
O
N
N
O
n
OH
O
OEOMA
O
HEMA
O
HPMA
O
DMAEMA
O
DEAEMA
O
O
n
O
OH
O
OEOA
O
HEA
Neutral Acrylamides
Neutral Vinylpyridines
Anionic Monomers
N
NH
2
N
ONa
ONa
N
SO
3
Na
O
O
O
O
O
N
NaMA
NaA
NaSS
4
-
VP
2
-
VP
A
m
DMA
DMAEMA
Scheme 3
Chemical structures of common hydrophilic monomers
To overcome these issues, conditions were intentionally developed that allowed
ATRP in water to be performed: (a) creating a high ratio of Cu(II)/Cu(I), (b) using
a high overall concentration of copper catalyst, (c) using excess of halide salts [
95
,
97
] (d) selecting a ligand, such as TPMA with significant Cu/TPMA stability in
water [
110
] and (e) using chlorine as the chain end rather than bromine [
42
,
45
] to
reduce the concentration of radicals, and to minimize the loss of deactivator and
polymer chain ends. Very recently, a few reports have demonstrated the preserved
chain-end halide functionality for successful chain extension with another polymer
block in a second batch reaction [
97
,
108
].
The first well controlled ATRP of oligo(ethylene oxide) methacrylate (OEOMA)
in water was reported in which excess amount of halide salts were intentionally
added to suppress the dissociation of Cu(II) deactivators [
11
,
95
]. In another report,
a Cu(0) powder and large amount (10,000 ppm) of Cu(II) was generated by pre-dis-
proportionation of CuBr/Me
6
TREN in water before addition of monomer and ini-
tiator for the ATRP of
N
-isopropyl acrylamide (NIPAM),
N
,
N
-dimethyl acrylamide
(DMA), oligo(ethylene oxide) acrylate (OEOA) and 2-hydroxyethyl acrylate (HEA)
monomers [
108
]. The polymerization showed well controlled polymer chain struc-
ture with preserved bromine chain-end functionalities.
Alternatively, ATRP has been successfully conducted in organic solvents that
dramatically decrease the K
ATRP
equilibrium constants as compared to those in
pure aqueous systems. The organic solvents suitable for the ATRP of hydrophilic
monomers are largely determined by the solubility of the monomers and the pro-
duced polymers in the solvent. Several organic solvents, including dichloroben-
zene [
111
], anisole [
112
], toluene [
113
], DMF, DMSO, alcohols [
114
-
117
] and
their mixtures [
118
] have been successfully applied to the synthesis of neutral
hydrophilic polymers of PHEMA, PDMAEMA and POEOMA (Scheme
3
). When
charged monomers are used in the ATRP for direct synthesis of charged polye-
lectrolytes, the choice of organic solvents becomes more limited and sometimes,
water becomes an essential co-solvent, together with organic solvents including
alcohol [
119
-
121
] DMF [
122
] DMSO and pyridine, to dissolve the polyelectro-
lytes and achieve a homogenous system.
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