Chemistry Reference
In-Depth Information
Table 3.3 Properties of polyethylenimines with covalently-linked aminopyridines.
Second-order rate constants
b
Polymer
derivative
a
pK
a
pH 7.3
pH 9.2
k
2
(obs.)
k
2
(norm.)
k
2
(obs.)
k
2
(norm.)
PEI-Pyr-1
7.31
10 400
20 800
34 300
36 400
PEI-Pyr-2
6.64
14 700
25 000
53 300
55 500
PEI-Pyr-3
6.68
25 300
35 200
164 000
168 000
a)
Stoichiometric composition given: C
2
H
4
N
represents ethylenimine residue and (C
12
H
25
)
lauryl group. For PEI-600, m approximately
1400.
b)
Observed rate, (obs.); rate normalized to non-
protonated pyridine (norm.) adduct.
For these adducts, the polymer environment increases markedly the concentration
of the nucleophilic state of the pyridine nitrogen. Unsurprisingly, therefore, in every
case [30] the observed hydrolysis rates of p-nitrophenyl caproate (pH 7.3) are 50-2000-
fold greater than that for the isolated aminopyridine. The nucleophilic forms of the
small molecule aminopyridines are about 200 times more effective than imidazole.
The catalytic effectiveness of the small molecule or of the polymer adducts is in-
creased when the pH of the solution is raised from 7.3 to 9.2.
The influence of the polymer on the intrinsic effectiveness of the nonprotonated
pyridine moiety adduct can be assessed by comparison of the normalized rates
k
2
(norm.), i.e., those expressed in terms of the concentration of
>
N- species in
the solution. The intrinsic activity of the most effective nucleophiles (Table 3.3) is en-
hanced by a factor of 35, when it is attached to polyethylenimine. This increase is in
addition to that due to the ability of the polymer at any pH to increase the fraction of
aminopyridine in the uncharged
>
N- form. For a specific nucleophile, for which poly-
mer adducts have been prepared with different aminopyridine contents, the catalytic
effect, per pyridine, increases with decreasing residue concentration of the nucleo-
phile (Table 3.3).
The acceleration in cleavage rate of nitrophenyl caproate by the dialkylaminopyri-
dine polymers was shown to be a manifestation of true catalysis by complete hydrolysis
of a 50-fold excess of substrate over residue molar concentrations of pyridine moiety
attached to the polymer.
