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Molecules 2018, 23(5), 1007; https://doi.org/10.3390/molecules23051007

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

1
Laboratory of Inorganic Chemistry, Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis Zografou, 15771 Athens, Greece
2
Department of Materials Science, University of Patras, University Campus, 26504 Rio, Greece
3
Laboratory of Industrial Chemistry, Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis Zografou, 15771 Athens, Greece
*
Author to whom correspondence should be addressed.
Academic Editors: Albert Demonceau, Ileana Dragutan, Valerian Dragutan and Derek J. McPhee
Received: 2 March 2018 / Revised: 13 April 2018 / Accepted: 21 April 2018 / Published: 25 April 2018
(This article belongs to the Special Issue Organometallic Catalysis in Organic Synthesis)
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Abstract

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 °C, 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. View Full-Text
Keywords: aerogels; dendritic monomers; ring opening metathesis polymerization (ROMP); polymeric materials; ruthenium aerogels; dendritic monomers; ring opening metathesis polymerization (ROMP); polymeric materials; ruthenium
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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Kanellou, A.; Anyfantis, G.C.; Chriti, D.; Raptopoulos, G.; Pitsikalis, M.; Paraskevopoulou, P. 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. Molecules 2018, 23, 1007.

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