Site-isolation model

The core first method starts from multifunctional initiators and simultaneously grows all the polymer arms from the central core. The method is not useful in the preparation of model star polymers by anionic polymerization. This is due to the difficulties in preparing pure multifunctional organometallic compounds and because of their limited solubility. Nevertheless, considerable effort has been expended in the preparation of controlled divinyl- and diisopropenylbenzene living cores for anionic initiation. The core first method has recently been used successfully in both cationic and living radical polymerization reactions. Also, multiple initiation sites can be easily created along linear and branched polymers, where site isolation avoids many problems. 

Carroll JB, Jordan BJ, Xu H, Erdogan B, Lee L, Cheng L, Tiernan C, Cooke G, Rotello VM. Model systems for flavoenzyme activity site-isolated redox behavior in flavin-functionalized random polystyrene copolymers. Org Lett 2005 7 2551-2554. 

Heferogeneous olefin epoxidation over solid titania-silica catalysts has been the subject of numerous publications in the open literature. The general picture that emerges is that isolated titanium (IV) species on a silica surface or in a zeolife mafrix are responsible for the high epoxidation activity [2]. This picture is supported by model catalyst work on titanium silasesquioxane complexes [3,4] that form active homogeneous epoxidation catalysts [5] and by various successful atfempfs fo prepare well-defined, site-isolated titanium complexes by grafting molecular precursors on mesoporous silica [6-9]. These site-isolated titanium complexes have been shown to possess catalytic activity in olefin epoxidation. 

The simplest analytic model for an isolated proton in a lattice assumes that it is situated in a potential well centred on an interstitial site. This model is particularly appropriate to protons in a transition metal lattice, where the electron from the hydrogen atom can be accommodated in the d-band of the metal, but is also applicable to many other cases as well - e.g. to molecular hydrogen trapped in ion-exchanged zeolites (see Section 6.8.2 below). The model assumes that there are no interactions between neighbouring hydrogen atoms and that there is little coupling with the lattice modes. This implies that M/wjp 1 where m is the mass of the proton and M is the mass of the lattice atom. In transition metals, with face-centred cubic (FCC), body-centred cubic (BCC) or hexagonal close packed (HCP) lattices, the proton normally sits on either octahedral or tetrahedral sites. In more complex intermetallic... 

The site isolation concept with simple weso-tetraaryl porphyrin hgands has found widespread apphcation in oxidation catalysis with immobihzed metallopor-phyrins, for example, catechol and lignin oxidation with Mn Cl(TPP) complexes 46 and H2O2 as oxidant [55, 56] or hydroquinone and thiol oxidation with Co (TPP) complexes 47 and O2 as oxidant [57, 58]. Although these reactions are of considerable synthetic utility [59], truly functional biomimetic models with a detailed knowledge of elementary reaction steps have not yet been developed. 

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