Biomedical Engineering Reference
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Cl
Cl
O
Br
Br
O
O
NCS
O
O
Cl CN
ClAN (0.14)
Cl CN
ClPN (0.46)
O
Cl
EtCliB (0.022)
Br
EtBriB (2.7)
Br
MBriB (2.6)
BzCl (5.5x10
-3
)
PECl (0.010)
BzBr (0.10)
PEBr (0.17)
BzNCS (8.9x10
-6
)
k
act
(M
-1
s
-1
)
Extrapolated Values
10
4
10
-5
10
-3
10
-2
10
-1
10
0
10
1
O
N
Br CN
BrAN (7.1)
Br CN
BrPN (23)
Br
DEBrPA (0.040)
SCN
O
O
O
O
O
O
Br
O
O
Cl
O
O
Br
O
Cl
MClP (0.015)
O
O
O
MClAc (1.6x10
-3
)
I
MIP (0.53)
Br
t
BBrP (0.12)
Br
MBrP (0.33)
MBrAc (0.030)
BzSCN (1.2x10
-5
)
EBPA (5.3x10
3
)
Fig. 2
Effect of initiator structures on ATRP activation rate constants (
k
act
) with Cu
I
X/
N,N,N′,N′,N″-pentamethyldiethylenetriamine (
PMDETA
) (X
=
Br or Cl) in MeCN at 35 °C.
3°:
red
; 2°:
blue
; 1°:
black
; isothiocyanate/thiocyanate:
left half-filled;
; chloride:
open
; bromide:
illed
; iodide:
bottom half-filled;
; amide:
inverted filled triangle
; benzyl:
filled triangle
; ester:
open square
; nitrile:
open circle
; phenyl ester:
open diamond
. Reprinted with permission from
American Chemical Society [
42
]
selected alkyl (pseudo)halides. The range of k
act
spans over six orders of mag-
nitude and an examination of the structure of the ligands shows that the general
order of Cu complex activity for ligands is: tetradentate (cyclic-bridged) > tet-
radentate (branched) > tetradentate (cyclic) > tridentate > bidentate ligands.
Bridged cyclam (Cyclam-B), tris(2-dimethylaminoethyl)amine (Me
6
TREN)
and tris(2-pyridylmethyl)amine (TPMA) are among most the active while
2,2′-bipyridine (bpy) and pyridineimine are the least active. The nature of the
nitrogen atoms in the ligands also plays a role in the activity of the Cu com-
plexes and follows the order pyridine
≈
aliphatic amine > imine < aromatic
amines. Generally, alkyl amines complex to Cu(II) more strongly than pyri-
dines. A C2 bridge between N atoms generates complexes with higher activi-
ties than those with C3 or C4 bridges. Steric effects around the Cu center are
very important, with a Me
6
TREN catalyst complex being 10,000 times more
active than the Et
6
TREN complex [
41
]. Electronic effects are also very impor-
tant and copper complexes formed with bpy containing two p-dimethylamino
groups are 10
6
times more active than the unsubstituted ligand and a substi-
tuted TPMA, formed from three 3,5-dimethyl-4-methoxypuridine rings, is 10
3
times more reducing than TPMA [
43
,
44
].
The reactivity of different alkyl halides in ATRP depends on the structure of
the alkyl group and transferable (pseudo)halogen. It is important to select a suf-
ficiently reactive species for an efficient ATRP initiation of the polymerization
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