Biomedical Engineering Reference
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of the selected monomer. Reactivity of alkyl halides follows the order of
3° > 2° > 1°, in agreement with the bond dissociation energy needed for homo-
lytic bond cleavage. Also, radical stabilization is enhanced by the presence of a
ʱ
-cyano group which is more activating than either a
ʱ
-phenyl or ester group.
Ethyl
ʱ
-bromophenylacetate, which combines the activation effect of both benzyl
and ester species, is the most active initiator and is >10,000 times more reactive
than 1-phenylethyl bromide (PEBr) and >100,000 times more active than methyl
2-bromopropionate (MBrP). The reactivity of alkyl halides follow the order
I > Br > Cl and is higher than that of alkyl pseudohalides [
42
].
In ATRP, the dynamics of the exchange reactions may be even more impor-
tant than the overall values of the equilibrium constants. Radicals must be very
quickly deactivated, and the value of
k
deact
should be as large as possible. This
requires very rapid rearrangement of the catalyst complex from the L/Cu(II)-X
to L/Cu(I) species, resulting from minimal reorganization of the complex, as
accomplished with branched tetrapodal ligands. Figure
3
a, b [
31
,
45
] show the
correlation of
k
act
and
k
deact
with
K
ATRP
for various Cu
I
Br/ligand complexes with
a standard alkyl halide, ethyl 2-bromoisobutyrate (EtBriB), and for various ini-
tiators in the presence of a Cu
I
/TPMA at 22 °C in MeCN. The equilibrium con-
stants increase as a consequence of a large increase in
k
act
accompanied by a
small decrease in
k
deact
. The ideal catalyst for ATRP of less reactive monomers,
or when used at lower concentrations, should have a large value for
K
ATRP
(i.e.,
larger
k
act
) but also preserve a very large value for
k
deact
. The values of equilib-
rium and rate constants in ATRP scale very well with the electrochemical activ-
ity of the complexes, as shown in Fig.
4
[
31
,
46
]. A complex with a 300 mV
more negative redox potential is ca. 100,000 times more reactive than the less
reducing catalyst complex.
Fig. 3 a
Ligands:
1
Cyclam-B,
2
Me6TREN,
3
TPMA,
4
BPED,
5
TPEDA,
6
PMDETA,
7
BPMPA,
8
dNbpy,
9
HMTETA,
10
bpy,
11
N4[3,2,3],
12
N4[2,3,2].
b
Initiators:
1
MClAc,
2
BzCl,
3
PECl,
4
MClP,
5
EtCliB,
6
BzBr,
7
ClAN,
8
PEBr,
9
MBrP,
10
ClPN,
11
EtBriB,
12
BrPN,
13
EBPA. Reprinted with permission from American Chemical Society [
45
]
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