TY - JOUR
TI - Poly(urethane-norbornene) Aerogels via Ring Opening Metathesis Polymerization of Dendritic Urethane-Norbornene Monomers: Structure-Property Relationships as a Function of an Aliphatic Versus an Aromatic Core and the Number of Peripheral Norbornene Moieties
AU - Aspasia Kanellou
AU - George C. Anyfantis
AU - Despoina Chriti
AU - Grigorios Raptopoulos
AU - Marinos Pitsikalis
AU - Patrina Paraskevopoulou
JO - Molecules: A Journal of Synthetic Organic Chemistry and Natural Product Chemistry
PY - 2018
VL - 23
TODO - 5
SP - 1007
PB - MDPI
SN - 1420-3049
TODO - null
TODO - aerogels, dendritic monomers, ring opening metathesis polymerization (ROMP), polymeric materials, ruthenium
TODO - We report the synthesis and characterization of synthetic polymer aerogels based on
dendritic-type urethane-norbornene monomers. The core of those monomers is based either on
an aromatic/rigid (TIPM/Desmodur RE), or an aliphatic/flexible (Desmodur N3300) triisocyanate.
The terminal norbornene groups (three at the tip of each of the three branches) were polymerized via
ROMP using the inexpensive 1st generation Grubbs catalyst. The polymerization/gelation conditions
were optimized by varying the amount of the catalyst. The resulting wet-gels were dried either
from pentane under ambient pressure at 50 oC, or from t-butanol via freeze-drying, or by using
supercritical fluid (SCF) CO2. Monomers were characterized with high resolution mass spectrometry
(HRMS), 1H- and solid-state 13C-NMR. Aerogels were characterized with ATR-FTIR and solid-state
13C-NMR. The porous network was probed with N2-sorption and SEM. The thermal stability of
monomers and aerogels was studied with TGA, which also provides evidence for the number of
norbornene groups that reacted via ROMP. At low densities (<0.1 g cm-3) all aerogels were highly
porous (porosity > 90%), mostly macroporous materials; aerogels based on the aliphatic/flexible core
were fragile, whereas aerogels containing the aromatic/rigid core were plastic, and at even lower
densities (0.03 g cm-3) foamy. At higher densities (0.2–0.7 g cm-3) all materials were stiff, strong,
and hard. At low monomer concentrations all aerogels consisted of discrete primary particles that
formed spherical secondary aggregates. At higher monomer concentrations the structure consisted of
fused particles with the size of the previous secondary aggregates, due to the low solubility of the
developing polymer, which phase-separated and formed a primary particle network. Same-size fused
aggregates were observed for both aliphatic and aromatic triisocyanate-derived aerogels, leading to
the conclusion that it is not the aliphatic or aromatic core that determines phase separation, but rather
the solubility of the polymeric backbone (polynorbornene) that is in both cases the same. The material
properties were compared to those of analogous aerogels bearing only one norbornene moiety at the
tip of each branch deriving from the same cores.
ER -