Chemistry Reference
In-Depth Information
be the limiting factor in catalysis by preventing the substrate from rearranging itself
within the active site of the enzyme. Unfavourable steric interactions may also be at
play. Even though CalB has a large acyl binding cleft which can accepted aromatic
rings and some branched aliphatic esters, the larger size of the
Me
2
SiO
- group may
be prohibitive. The effect of steric bulk on the hydrolysis of fatty acid esters by
CalB has been examined [
20
]. Ethyl-2-methylbutyrate was hydrolyzed with good
conversion, ~ 90 % by GC; increasing the steric bulk on the acid side of the fatty
acid ester, to ethyl benzoate and ethyl-2-phenylpropionate, elicited a decrease in
conversion to between 35-45 % [
20
].
6.5
Thermal Properties of Disiloxane Containing
Polyesters
Differential scanning calorimetry (DSC) was used to examine the thermal proper-
ties of the siloxane polyesters. Siloxane-polyester samples were transferred into
aluminium pans and cooled to − 150 °C at a rate of 10 °C/min. Each sample was
heated at 10 °C/min to 200 °C and subsequently cooled at 10 °C/min to − 150 °C.
Thermal transitions were taken from a second heating scan that was done at 10 °C/
min to 200 °C.
Figure
6.5
presents the DSC thermograms for siloxane monomers as well as rep-
resentative siloxane-containing polyesters. The glass transition temperature (
T
g
) for
polydimethylsiloxane (PDMS) is − 125 °C. By comparison, the observed values for
the disiloxane-derived diol and diester are higher, − 99 and − 109 °C respectively,
while the
T
g
for the silicone-diol was more comparable at − 118 °C. The
T
g
for the
polyester synthesized from only disiloxanes was found to be − 104 °C. The poly-
ester synthesized from the silicone diol had a
T
g
of − 115 °C, only slightly higher
than the free silicone-diol. As a result of the flexibility of the disiloxane linkage and
the amorphous nature of silicones, thermal transitions associated with melting and
crystallization were not determinable.
6.6
A Comparison of the Activation Energy
for N435-Mediated Polyesterification Reactions
The general dependence of the rate of a reaction on temperature can be quantified
using the Arrhenius Eq. (6.1).
−
E
RT
(6.1)
a
k e
=
.
The activation energy,
E
a
, and the Arrhenius factor can be interpreted directly from a
plot of ln
k
vs.
1/T
which yields a straight line with a slope of -
E
a
/R
and an intercept
