Polyethylene, macroporous

The majority of physisorption isotherms (Fig. 1.14 Type I-VI) and hysteresis loops (Fig. 1.14 H1-H4) are classified by lUPAC [21]. Reversible Type 1 isotherms are given by microporous (see below) solids having relatively small external surface areas (e.g. activated carbon or zeolites). The sharp and steep initial rise is associated with capillary condensation in micropores which follow a different mechanism compared with mesopores. Reversible Type II isotherms are typical for non-porous or macroporous (see below) materials and represent unrestricted monolayer-multilayer adsorption. Point B indicates the stage at which multilayer adsorption starts and lies at the beginning of the almost linear middle section. Reversible Type III isotherms are not very common. They have an indistinct point B, since the adsorbent-adsorbate interactions are weak. An example for such a system is nitrogen on polyethylene. Type IV isotherms are very common and show characteristic hysteresis loops which arise from different adsorption and desorption mechanisms in mesopores (see below). Type V and Type VI isotherms are uncommon, and their interpretation is difficult. A Type VI isotherm can arise with stepwise multilayer adsorption on a uniform nonporous surface. 

Custom-made macroporous polyacrylamide-poly(ethylene glycol) monolith (acrylate copolymer of acrylic acid, butyl acrylate, methylene-bisacrylamide with 3% (w/v) polyethylene glycol))... 

Besides the classical polymer introduced by Merrifield (1%-crosslinked chloromethylated polystyrene), a broad variety of polymeric supports is available for SPPS and some of the most popular resins are summarized in Table 1. The chemical structures of some selected resins are presented in Figure 1 and electron micrographs of several examples are displayed in Figure 2. In addition to the solid supports listed in Table 1, there are several other carriers used in peptide synthesis such as the gel-type and macroporous poly(meth-acrylates), coated surfaces like polystyrene films on polyethylene (PEt) sheets, polystyrene-coated polyethylene or polytetrafluoroethylene, and modified glass surfaces. (For recent reviews on polymeric carriers see refs . )... 

Fig. 1 Isocratic electrochromatography of peptides in a capillary filled with a macroporous polyacrylamide-polyethylene glycol matrix, derivatized with a C12 ligand (29%) and containing acrylic acid. Conditions mobile phase, 47% acetonitrile in a buffer voltage, 22.5kV (900 V/cm), 7 [jim sample concentration, 4-10 mg/mL detection, UV absorbance at 270 nm other conditions are described in Ref. 5. (From Ref. 5 reproduced with permission of the authors and the American Chemical Society.)...

Most polymeric redox reagents have been developed on microporous polystyrene, typically cross-linked with 1% divinylbenzene. Few examples have been reported for macroporous polystyrene, silica or other supports such as high-loaded cross-linked polyethylene imine (Ultraresins). Some problems specific for redox reactions can also arise from the reactivity of the polymer support itself. In cross-linked polystyrene, for example, benzylic positions can be oxidized at elevated temperatures and thus can account for a competing reaction pathway [8], Further reactivities are found for other solid supports as well. 

The activity of some polyethylene glycols bonded with macroporous copolymers of glycidylmethacrylate during the interfacial catalysis of the model reaction of sodium phenolate with n-BuBr, were investigated [79]. Rate constants of phenol alkylation in the aqueous system NaOPh-n-BuBr (in toluene)-PEG were measured at 60°C as a function of the molecular weight of PEG and its concentration. The obtained results were compared with the data on the kinetics of the reaction occurring in the presence of soluble PEG. Upon the insertion of immobilized and soluble PEG, alkylation rates increased 168 and 139 fold, respectively. The increase of the catalytic activity of immobilized PEG, which corresponded to a rise in the molecular weight of the polymer, was caused by an increase in the PEG ability to sorb alkali metal cations. 

A key enabler often employed in the synthesis of zeolites is the template, often called an organic directing agent. The template type is frequently different for microporous zeolites, mesoporous materials, and macroporous materials. The template can be an individual molecule (e.g., quaternary salts or linear amines), in-situ formed micelle clusters, or preformed structures (e.g., polyethylene spheres). 

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