ABSTRACT

Abstract A well-defined amine functionalized silica surface displaying evidence of isolated amine sites is prepared using a molecular patterning technique. These aminosilicas serve as a support for immobilized organometallic catalysts, specifically constrained geometry catalysts for olefin polymerizations. The patterned catalysts show an activity of 27-32 kg polymer/mol Ti hr for the polymerization of ethylene with methylaluminoxane as cocatalyst. This activity is significantly higher than catalysts made via traditional supporting techniques on a densely loaded amine-functionalized silica, which show an activity of 4-10 kg polymer/mol Ti hr. This activity increase, when combined with reactivity and probe molecule studies, provides further evidence of the site-isolated behavior of the patterned amine sites and the unique nature of active sites that can be assembled on these aminosilica materials. Introduction Organometallic complexes are extremely versatile catalysts because of the chemical properties which can be incorporated into them – the symmetry, sterics and electronics of the active center can all be controlled using molecular design principles. However, since homogeneous catalyst recovery and recyclability is a costly industrial problem, the implementation of these catalysts is not straightforward. For this reason, transition metal complex catalysts have been immobilized on solid supports for decades (1). Supporting a catalyst has the potential to result in a material with all the attributes of both homogeneous (high rates and selectivities) and heterogeneous catalysts (ease of separation). However, in practice this is rarely achieved as the performance of the supported catalysts is usually significantly inferior compared to their homogeneous analogues. In reality, the supported catalysts typically display both the drawbacks of heterogeneous (low rates and selectivities) and homogeneous catalysis (difficult separation due to metal leaching) instead of the attributes of both systems. While this phenomenon is due to many factors, a primary cause

is the formation of multiple types of ill-defined potential active sites on the solid support (1). As an example, consider single-site transition metal complexes such as Group 4 metallocenes and related complexes, which are a new class of olefin polymerization catalysts that have been recently developed (2). Although the homogeneous systems are well-understood, the preparation of well-defined supported single-site polymerization catalysts has not progressed as quickly, as protocols which would lead to the synthesis of uniform, isolated organometallic species on surfaces are poorly developed (3). For example, Ti and Zr constrained geometry-inspired catalysts (CGCs) (4) have been supported on a variety of different solids using two different immobilization strategies. In the first approach, the organometallic precatalyst is contacted with a support (5). However multiple types of surface species are formed due to solid-metal atom interactions. In the alternate approach, the complex is assembled step-wise on the support (6). Steric crowding on the support and the heterogeneous nature of the support surface commonly also results in multiple types of sites.