Chemistry Reference
In-Depth Information
3.2.2
Enhanced Reactivity of Nucleophiles in Polyethylenimines (PEIs)
The numerous amine nitrogens of polyethylenimine can act as nucleophiles. In addi-
tion, they are locally concentrated (Figure 3.1). Furthermore, polymers with attached
aliphatic acyl groups provide apolar binding sites in proximity to the amine residues.
One might expect, therefore, to find progressively enhanced rates of aminolysis of
substrates with increasingly large apolar substituents. Quantitative measurements
of aminolysis rates indeed revealed enhancements of several orders of magnitude.
To assess the aminolytic effectiveness of the polymer amines, the cleavage reaction
of Eq. (2) has been followed. If an amine P-NH
2
is used in the aqueous solution, one
obtains RCONHP instead of RCOOH.
ð
2
Þ
Table 3.1 [15] lists the first-order rate constants, which are corrected for hydrolysis of
ester in buffer alone. Propylamine served as a reference amine; in its presence 10
2
k
(min
-1
) for aminolysis decreased progressively from 0.98 to 0.51 to 0.05 as the acryl
group increased from 2 to 12 carbons (Table 3.1). The sharp drop for nitrophenyl lau-
rate may be the result of micelle formation (even at 6
10
-6
M)
.
With nonacylated polyethylenimines (Table 3.1) the rate constant is increased ca. 4-
fold over that of propylamine. This small enhancement may merely be because a great-
er fraction of the polymer's primary amine groups are in the basic, NH
2
state. With
these polyethylenimines, as with propylamine, k drops with increasing length of the
hydrocarbon chain of the acyl nitrophenyl ester.
Markedly different trends in the rate constants appear for aminolysis by lauroyl-poly-
ethylenimine (containing 10 residue % lauroyl groups). For each nitrophenyl ester the
rate is substantially greater with lauroyl-polyethylenimine than with polymer contain-
10
2
min)
a
for amine acylations by p-nitrophenyl esters.
b
Table 3.1
First-order rate constants (k
Amine
c
p-Nitrophenyl acetate
p-Nitrophenyl caproate
p-Nitrophenyl laurate
Propyl
0.98
0.51
0.053
PEI-6
3.60
1.47
0.11
PEI-18
4.38
1.57
0.11
PEI-600
4.60
1.80
0.17
L(10%)-PEI-6
d
15.2
68.1
698
a)
Here k = k
a
- k
o
, where k
a
is the measured rate
constant in the presence of amine and k
o
is that
for the hydrolysis in tris buffer alone; k
o
is 0.94
substrate were made in acetonitrile; hence the
final buffer also contained 6.7% acetonitrile.
c)
Numeral following polyethylenimine multi-
plied by 100 is the molecular weight of the
polymer sample.
d)
This sample of PEI-6 has 10% of its nitrogens
acylated with lauroyl groups.
10
-2
min
-1
for the acetyl ester, 0.61
10
-2
min
-1
10
-2
min
-1
for the lauroyl ester. Measurements
made at pH 9.0 in 0.02
M
tris(hydroxymethyl)-
aminomethane buffer, 25
for the caproyl ester, and 0.023
C. Stock solutions of
8
