Chemistry Reference
In-Depth Information
quickly become distorted. This leads to an increased amount of catalyst being used
which can lead to an erroneous interpretation of the resulting kinetic data.
The mass of the polysiloxane-containing diol is nearly an order of magnitude
larger than the mass of the disiloxane diol (2000 g/mol compared 250 g/mol). To
ensure that a direct comparison can be made between the two systems, the stoichi-
ometry between N435 and CPr-TMDS-DME was maintained. At lower tempera-
tures, 35-70 °C, polyester elongation rates were generally slow, and certainly slow-
er than the observed rates when the disiloxane diol was the acyl acceptor (Fig.
6.2
).
The reaction rates continued to increase as the temperature was increased reaching
1.40 h
−1
at 110 °C. Beyond 110 °C rates did not appear to increase. Because the cata-
lyst loading was controlled, the decrease in the elongation rates is a direct result of
the increase in the polysiloxane chain length of the diol monomer. This in essence
reduced the concentration of the reaction mixture reducing the number of successful
binding events between the enzyme and the substrates.
Similar to reactions involving disiloxane-derived monomers, monomer conver-
sion in the polysiloxane system increased with each successive increase in tem-
perature (Fig.
6.3
). The observed trend was similar between the two systems; that
is to say at lower temperatures monomer conversion was limited by the slow rate
of the reaction. However, even at 50 °C monomer conversion reached nearly 80 %
after 24 h and at temperatures higher than 60 °C monomer conversion consistently
reached 85-95 %. The other notable observation was that despite the slower re-
action rates, monomer conversion was typically more extensive when the polysi-
loxane diol was the acceptor suggesting that the increased hydrophobicity had a
positive effect on catalysis.
6.4.3
A Comparsion of Acyl-Donors
The increase in the molecular mass of polyesters is typically monitored by gel per-
meation chromatography (GPC). This method allows for the number average mo-
lecular weight (
M
n
), weight average molecular weight (
M
w
), and the polydispersity
index (PDI) to be determined. Alternatively NMR-based methods can be employed
to gauge the
M
n
of polymers. Because there are obvious differences between the
methodologies that have been presented in the literature, a direct comparison be-
tween experimental conditions is not practical. To put our results into context we
carried out a series of enzyme-mediated polymerizations at 100 °C using a series of
dimethyl esters (fumarate, maleate, phthalate, succinate, adipate, and sebacate), in
addition to CPrTMDS-DME, and a common diol, 1,8-octanediol.
The results indicated that when aliphatic diesters were the acyl donor, N435 did
not distinguish between the C4, C6 or C12 esters in the polymerization of poly-
octylene esters (Fig.
6.4
). The olefinic diesters, dimethyl fumarate and dimethyl
maleate, produced a significant drop in the reaction rate. The rate of polymeriza-
tion was reduced by approximately 65 % when dimethyl fumarate was the acyl do-
nor; with the
cis
isomer, dimethyl maleate, no polymerization was observed at all.
