Macromolecules, Vol. 37, No. 21, 2004
Imidazole-Containing Polymers 7883
parameter is rather large when compared, for example,
to that of R-chymotrypsin (for the same substrate, Km
) 1.6 × 10-4 M).15
is similar to a stage of enzymatic catalysis. Indeed,
before a catalytic act takes place substrate is usually
sorbed by the active center of the enzyme. In some
enzymes, like acetylcholinesterase, pepsin, and R-chy-
motrypsin, the substrate binding site contains hydro-
phobic structures which can effectively interact with
hydrophobic moieties of the substrate, contributing to
the formation of a relatively stable complex. However,
in our system, that complex is somewhat less stable
than those formed by enzymes, judging by Michaelis-
Menten analysis of the kinetics of catalysis. Also,
catalytic activity in our case is much lower than for
enzymes: the catalytic activity of our most active
polymer catalyst (PVCL-Vim-20) toward NPA hydroly-
sis at 43 °C is over 200 times lower than that of
R-chymotrypsin at 25 °C. The latter was estimated from
the data of ref 15. Indeed, other factors inherent to
enzymes (e.g., geometric correspondence between active
site of an enzyme and substrate) which increase the
stability of the complex and catalytic activity of enzyme
are not involved in the substrate-catalyst interactions
in our system.
Vasilevskaya et al.16 explored the theoretical possibil-
ity of acceleration of the reaction if both catalyst and
substrate can adsorb at the oil/water interfaces in mini-
emulsions. It was shown that at certain optimum size
of miniemulsion droplets (normally around several
hundred nanometers) a significant acceleration of the
reaction can take place compared to the case where
the size of droplets is too small or too large as well as
to the case of complete phase separation. Furthermore,
it was shown that for the optimum size droplets a
Michaelis-Menten profile of the reaction rate vs sub-
strate concentration is typical when the surface of
miniemulsion droplets is saturated with substrate spe-
cies. Therefore, an increase of the reaction rate due to
concentrating of the substrate at the interface of ag-
gregates and the fact that the reaction at the interface
of aggregates obeys Michaelis-Menten-type kinetics are
theoretically justified.
There can be some additional factors that enhance
catalytic activity at the surfaces of globules or ag-
gregates. First, both catalyst and substrate are specif-
ically oriented due to polymer concentration gradient
in the interfacial layer. This factor can stimulate mutual
orientation of the interacting species which in some
cases can be beneficial for an elementary act of the
catalytic reaction. Second, the substrate species are
subjected to an additional high stress due to the polymer
concentration gradient. The second factor obviously can
decrease the activation energy of the reaction since the
stress increases the ground state energy of the sub-
strate, resulting in energy-rich species which can be
more readily converted into the product.
Ack n ow led gm en t. The financial support from the
Alexander von Humboldt Foundation, Program for
Investment in the Future, is highly appreciated.
Refer en ces a n d Notes
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Con clu sion
Temperature-sensitive poly(N-vinylcaprolactam-co-1-
vinylimidazole) and poly(N-isopropylacrylamide-co-1-
vinylimidazole) are catalytically active in the reaction
of hydrolysis of p-nitrophenyl acetate.
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The reaction rate does not follow Arrhenius-type
behavior upon temperature increase: in the tempera-
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exponential, while further increase of the temperature
decreases the reaction rate. This type of behavior is
observed when copolymers form aggregates in aqueous
solution. The temperature of aggregation precedes the
temperature of rapid growth of reaction rate. This
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as with the enhancement of concentration of catalyti-
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features with the catalytic action of enzymes. The active
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