Polyimide aerogel monoliths are prepared by ring-opening metathesis polymerization (ROMP) of a norbornene end-capped diimide, bis-NAD, obtained as the condensation product of nadic anhydride with 4,4 0 -methylenedianiline. The density of the material was varied in the range of 0.13À0.66 g cm À3 by varying the concentration of bis-NAD in the sol. Wet gels experience significant shrinkage, relative to their molds (28%À39% in linear dimensions), but the final aerogels retain high porosities (50%À90% v/v), high surface areas (210À632 m 2 g À1 , of which up to 25% is traced to micropores), and pore size distributions in the mesoporous range (20À33 nm). The skeletal framework consists of primary particles 16À17 nm in diameter, assembling to form secondary aggregates (by SANS and SEM) 60À85 nm in diameter. At lower densities (e.g., 0.26 g cm À3 ), secondary particles are mass fractals (D m = 2.34 ( 0.03) turning to closed-packed surface fractal objects (D S = 3.0) as the bulk density increases (g0.34 g cm À3 ), suggesting a change in the network-forming mechanism from diffusion-limited aggregation of primary particles to a space-filling bond percolation model. The new materials combine facile one-step synthesis with heat resistance up to 200 °C, high mechanical compressive strength and specific energy absorption (168 MPa and 50 J g À1 , respectively, at 0.39 g cm À3 and 88% ultimate strain), low speed of sound (351 m s À1 at 0.39 g cm À3 ) and styrofoam-like thermal conductivity (0.031 W m À1 K À1 at 0.34 g cm À3 and 25 °C); hence, they are reasonable multifunctional candidate materials for further exploration as thermal/acoustic insulation at elevated temperatures.

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Monolithic hierarchical fractal assemblies of silica nanoparticles are referred to as aerogels, and despite an impressive collection of attractive macroscopic properties, fragility has been the primary drawback to applications. In that regard, polymer-cross-linked silica aerogels have emerged as strong lightweight nanostructured alternatives rendering new applications unrelated to aerogels before, as in ballistic protection, possible. In polymer-cross-linked aerogels skeletal nanoparticles are connected covalently with a polymer. However, the exact location of the polymer on the elementary structure of silica and, therefore, critical issues, such as how much is enough, have remained ambiguous. To address those issues, the internal nanoporous surfaces of silica wet-gels were modified with norbornene (NB) by cogelation of tetramethyl orthosilicate (TMOS) with a newly synthesized derivative of nadic acid (Si-NAD: N-(3-triethoxysilylpropyl)-5-norbornene-2,3-dicarboximide). As inferred by both rheological and liquid 29 Si NMR data, Si-NAD reacts more slowly than TMOS, yielding a TMOS-derived skeletal silica network surfacederivatized with NB via monomer-cluster aggregation. Then, ring-opening metathesis polymerization (ROMP) of free NB in the nanopores engages surface-bound NB moieties and bridges skeletal nanoparticles either through cross-metathesis or a newly described stitching mechanism. After solvent exchange and drying with supercritical fluid CO 2 into aerogels (bulk densities in the range 0.27−0.63 g cm −3 , versus 0.20 g cm −3 of the native network), the bridging nature of the polymer is inferred by a >10-fold increase in mechanical strength and a 4-fold increase in the energy absorption capability relative to the native samples. The crosslinking polymer was freed from silica by treatment with HF, and it was found by GPC that it consists of a long and a short component, with around 400 and 10 monomer units, respectively. No evidence (by SAXS) was found for the polymer coiling up into particles, consistent with the microscopic similarity (by SEM) of both native and cross-linked samples. Most importantly, the polymer does not need to spill over higher aggregates for greatly improved mechanical strength; mechanical properties begin improving after the polymer coats primary particles. Extremely robust materials are obtained when the polymer fills most of the fractal space within secondary particles.

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Polydicyclopentadiene (pDCPD) is a polymer of emerging technological significance from separations to armor. It is a paradigm of ring opening metathesis polymerization (ROMP) and some of its remarkable properties (e.g., strength) have been attributed to crosslinking of the pendant cyclopentenes. pDCPD should be an ideal candidate for strong nanoporous solids (aerogels), however, excessive swelling of the wet-gels precursors in toluene (up to 200% v/v), followed by de-swelling and severe deformation in acetone, renders the resulting aerogels unusable. Based on spectroscopic evidence (IR, solid state 13 C NMR and several liquid 1 H NMR controls), only 4-5% of the pendant cyclopentene double bonds of pDCPD are engaged in crosslinking, via Wagener-type olefin coupling. Deformation was rectified via free radical polymerization of methylmethacrylate (MMA) in the pores of pDCPD wet-gels. The uptake of PMMA was varied in the 13-28% w/w range by varying the concentration of MMA. Evidence (e.g., differential scanning calorimetry) though suggests that PMMA remains a linear polymer, hence the pDCPD/PMMA network resist deformation, not because of molecular-level crosslinking, but due to a synergism related to the nano-topology of the two components (see next paper of this issue). With cylindrical monoliths available, the nature of the interparticle chemical bonding in pDCPD/PMMA aerogels was probed top-down with thermal conductivity and compression testing, using linearpolynorbornene (pNB) aerogels as a control system. The latter, with no pendant cyclopentenes, has no chance for interpolymer chain crosslinking. The solid thermal conduction and stiffness of pDCPD/PMMA and pNB aerogels scale similarly, pointing to a common mechanism for interparticle bonding. That was assigned to cross-metathesis, effectively extending the polymer chains of one nanoparticle into another, and was reflected on very high polydispersities (8-13).