Four Different Regioisomeric Polycarbonates Derived from One Natural Product, d‑Glucose

Strategies
for the preparation of polycarbonates, derived from
the natural product d-glucose, which have the potential to
degrade back into their bioresorbable starting material and CO2, were developed. By employing established carbohydrate protection/deprotection
chemistries, two d-glucose derivatives, methyl 4,6-O-benzylidene-α-d-glucopyranoside or methyl
α-d-glucopyranoside, were converted into four different
regioisomeric diol monomers, i.e., 1,4-, 1,6-, 2,6-, or 3,6-diols,
as confirmed by nuclear magnetic resonance (NMR) spectroscopy, infrared
(IR) spectroscopy, and mass spectrometry. Each type of regioisomeric
monomer was then employed in a condensation polymerization with phosgene,
generated in situ from triphosgene, as a comonomer,
in the presence of pyridine, to produce four types of polycarbonates
with different backbone regio-connectivity, as characterized by size
exclusion chromatography, NMR spectroscopy, and IR spectroscopy. Interestingly,
their thermal properties, i.e., glass transition temperature (Tg) and thermal degradation behavior, were tunable
by changing the topological composition of the monomeric unit. That
is, polycarbonates with 2,6- and 3,6-backbone connectivity resulted
in significantly higher Tg of ca. 85 and
83 °C, respectively, as compared to those with 1,4- and 1,6-backbone
connectivity, showing a Tg of ca. 33 °C,
as measured by differential scanning calorimetry. Furthermore, when
the thermal decomposition temperature was measured by thermogravimetric
analysis, the nonanomeric carbon backbone-based polycarbonates (2,6-
and 3,6-) exhibited higher thermal stability and a sharper decomposition
profile, with onset decomposition temperature (Td,onset) at 363 or 336 °C, as compared with those polymers
containing the anomeric carbon in the carbonate linkage (1,4- and
1,6-), having Td,onset at 171 and 163
°C.