Polystyrene PS Resins

These are, without doubt, the most widely used solid supports. PS systems used for the synthesis of peptides and small molecules consist of 1% cross-linked hydrophobic resins obtained by suspension polymerization from styrene and divi-nylbenzene. For other uses, PS with 2% cross-linking, which is mechanically more stable than those with less cross-linking, is also employed. This 2% PS was used 

Based on the early work of Mutter [37], the first commercially available PEG-PS resins were developed independently in the mid-1980s by Zalipsky, Albericio, and Barany (PEG-PS) [38-39] and Bayer and Rapp (TentaGel) [40-41]. These systems can be synthesized either by reaction of preformed oligooxyethylenes with amino-methylated polystyrene beads (PEG-PS) or by graft polymerization on polystyrene beads (TentaGel). 

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As far as polymer supports for microwave-assisted SPOS are concerned, the use of cross-linked macroporous or microporous polystyrene (PS) resins has been most prevalent. In contrast to common belief, which states that the use of polystyrene resins limits reaction conditions to temperatures below 130 °C [14], it has been shown that these resins can withstand microwave irradiation for short periods of time, such as 20-30 min, even at 200 °C in solvents such as l-methyl-2-pyrrolidone or 1,2-dichlorobenzene [15]. Standard polystyrene Merrifield resin shows thermal stability up to 220 °C without any degradation of the macromolecular structure of the polymer backbone, which allows reactions to be performed even at significantly elevated temperatures. 

The most common solid supports in SPS are hydrophobic polystyrene (PS) resin beads (9), which are representatives of the class of so-called gelatinous solid supports. They consist of PS cross-linked with 1-2% divinylbenzene (DVB) and are described schematically in Fig. 1.1, which shows the appearance of a hydroxymethyl-grafted PS resin. 

Synthesis of the chiral catalysts to introduce enantioselectivity in carbene transfer reactions is a subject of great interest. Often copper and rhodium chiral catalysts are of choice for the carbene transfer reactions. In some reports, immobilized chiral dirhodium (II) catalyst were employed successfully in asymmetric cyclopropanation reactions. Ubeda and coworkers reported the immobilization of chiral Rh2(02CR)2(PC)2 (PC = ort/io-metalated phosphine) compounds on cross-linked polystyrene (PS) resin by an... 

Any standard resin used for solid-phase peptide synthesis can be used with microwave irradiation including, but not limited to, polystyrene (PS) resin, polyethylene glycol-PS (PEG-PS) resin, PEG, and cross-linked ethoxylate acrylate (CLEAR) resin. 

The earliest published example of microwave-assisted SPOS involved diisopropyl-carbodiimide (DlC)-mediated solid-phase peptide couplings [24], Numerous Fmoc-protected amino acids and peptide fragments were coupled with glycine-preloaded polystyrene Wang resin (PS-Wang) in DMF, using either the symmetric anhydride or preformed N-hydroxybenzotriazole active esters (HOBt) as precursors (Scheme 12.1). 

This ABA block copolymer consists of stiff polystyrene (PS) and resilient polybutadiene blocks. The domains of these TPEs have characteristic Tt values of 100 and —80 C, respectively. The polybutadiene blocks retain their flexibility at low temperatures, and the polystyrene blocks lose their stiffness when the polymer is heated above 110 C. A related thermoplastic is a transparent AB block copolymer of styrene and butadiene (K-resin). 

PBDEs are used in different resins, polymers, and substrates at levels ranging from 5 to 30% by weight (EU 2001). Plastic materials that utilize PBDEs as flame retardants include ABS polyacrylonitrile (PAN) polyamide(PA) polybutylene terephthalate (PBT) polyethylene (PE) cross-linked polyethylene (XPE) polyethylene terephthalate (PET) polypropylene (PP) polystyrene (PS) high-impact polystyrene (HIPS) polyvinyl chloride (PVC) polyurethane (PUR) and unsaturated polyester (UPE). These polymers and examples of their final products are summarized inTable 5-2 (Hardy 2002 WHO 1994a). 

Typical solid supports used in organic syntheses are resin beads formed from cross-linked polystyrene (PS 40-150 pm diameter), polystyrene-... 

Figure 7.4. Single-bead IR spectra of the product for the reaction 3 at various times on PS = polystyrene, AP = ArgoPore, Tube = Microtube reactor, and PS-PEG = polystyrene-polyethyleneglycol resin.

Abbreviations DBU, l,8-diazabicyclo[5.4.0]undec-7-ene DMF, A/fV-dimethylformamide MCPBA, 3-chloroperbenzoic acid NMP, A-methyl-2-pyrrolidinone Pol, undefined polymeric support PS, cross-linked polystyrene Wang resin, 1-2% cross-linked polystyrene with p-benzyloxybenzyl alcohol linker. 

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