Porous silsesquioxane polymers and their functionality

High surface area, porous adsorbents have a wide variety of applications including gas storage, catalysis, as selectively permeable membranes, as adsorbents for solid phase extraction, as well as in liquid chromatographic separations in a wealth of modi. The rapidly developing field of engineering in micrometer-sized dimension, in particular of "lab-on-a-chip" technologies, demands an increase in surface-to-volume ratios of design and structure elements in order to allow functionality such as loading and selectivity in accordance with suitable flow through properties. These should be characterized by good mass transfer properties that ensure good efficiency of such entities. Porous, monolithic materials for such flow-through applications demand facile, repeatable and relative ease of preparation to ensure desired function. For flow through applications, hierarchically-structured silica monoliths offer a number of advantages including the high permeability to flow (due to existence of convectively accessible pores in micrometer-sized dimensions) together with a mesoporous pore space in the skeleton of the silica gel structure, that provides surface area and a high density of interacting functionalities. However, the inherent preparation in multiple steps via sol-gel synthetic routes tends to be highly sensitive to adjustable operational variables and preparation of such materials in microfluidic components is known to be a technological challenge. This led to a limited accessibility by a broad application-oriented audience. In contrast, polymer monoliths derived from a simple and robust free-radical polymerization have lower rigidity, lower surface areas and possess swelling propensity in usually employed hydro-organic solvents. Their simple preparation and chemistry convinces by a wealth of monomeric precursors that can be polymerized photochemically- or thermally- initiated. Furthermore, they are easily functionalizable and possess good scaling capability to fill molds of a wide variety of shapes from a centimeter down to a single micrometer range. In this proposal we want to pursue and systematically study routes for the preparation of hierarchically-structured adsorbents based on new organic/inorganic precursors, in particular polyhedral (vinyl)silsesquioxanes leading to porous monolithic three-dimensionally adhered entities. We want to understand the basics of directly influencing pore and scaffold formation and the following functionalization enabling potential technological application such as chromatography, solid phase extraction, catalysis, etc. Preparation through radically-initiated polymerization as well as "click" chemistry is proposed.

The theme treated in the project is located at the interface between synthetic (organic) chemistry and the miniaturized engineering sciences, particularly the development of new engineering-related materials. Applications of such materials are found in the life sciences and in an increasingly miniaturized scientific world. In the project a premise has been pursued, that allowed tiny nano-building blocks (polyhedral oligomeric silsesquioxanes (POSS)) being molecularly linked together in such a way that nano-, micro-, and macro-structured networks of certain mechanical, porous, and chemical properties resulted. In particular, two conceptually significant synthetic routes were followed, an uncontrolled and easiest way of a free-radical coupling chemistry, as well as a newly developed, radical-mediated linking chemistry based on the so-called thiol-ene click concept. Both conceptual approaches were systematically studied with respect to resulting materials properties such as mechanics and porous properties. Furthermore, studies on the interaction of target molecules of a variety of properties with such materials on a technically best possible level were performed The simplest and uncontrolled linking of such precursors led to monolithically-structured materials which exhibited particularly very large internal surface areas. In addition to these inherently existing large internal surface areas, the developed single-step and scalable synthetic approach allowed for the simultaneous adjustment of pore sizes on the order of nanometers up to several micrometers. This was a previously unknown experimental opportunity. Developed implications are particularly touching sustainable applications of green catalysis using aqueous solvents and nanoparticle conjugates used in miniaturized reactors, which could be operated continuously with very high robustness. In the second approach, a radical-mediated step-growth was developed, which promised an inherently better-controlled network formation reaction as a basic concept. It resulted in fundamentally homogeneous, soft and gel-like materials, as well as solid materials with adjustable porosity, all based on a chemically identical network formation reaction. Globally, in the project a new class of materials was developed successfully and their use demonstrated in high-end applications such as miniaturized separation processes and catalytic applications under consideration of sustainability issues.