Soluble polystyrene

Kragl and Wandrey made a comparison for the asymmetric reduction of acetophenone between oxazaborolidine and alcohol dehydrogenase.[59] The oxazaborolidine catalyst was bound to a soluble polystyrene [58] and used borane as the hydrogen donor. The carbonyl reductase was combined with formate dehydrogenase to recycle the cofactor NADH which acts as the hydrogen donor. Both systems were run for a number of residence times in a continuously operated membrane reactor and were directly comparable. With the chemical system, a space-time yield of 1400 g L"1 d"1 and an ee of 94% were reached whereas for the enzymatic system the space-time yield was 88 g L 1 d"1 with an ee of >99%. The catalyst half-life times were... 

Plenio et al. tested an adamantyl phosphine ligand bound to soluble polystyrene (Figure 4.43) in various palladium-catalyzed C-C coupling reactions.[62] The retention of metal complexes of the polymer-bound phosphine ligand were determined to be higher than 99.95%. 

Polymer-supported tin hydrides are being increasingly used to avoid the separation problem,407 12 and the reagent Bu2SnHLi, described above, is useful for attaching a stannyl hydride group to a polymer such as poly(4-chlorobutyl-styrene).397 The use of a non-cross-linked, soluble polystyrene has also been recommended.288... 

In contrast to some theoretical predictions (23, 46, 52) aggregation or phase separation in block copolymers occurs at a slightly higher total concentration than in polymer mixtures. Covalent bonding of the two kinds of blocks thus slightly increases the mutual solubility. Polystyrene blocks with M = 2000 dissolve in polybutadiene (Af = 75000) up to concentrations of about 20% (33). Therefore, special mechanical properties are, in general, only to be expected in sequence copolymers above a certain block length (in most cases M8 > 103). 

It was our intention when this new work began to outline, more fully, the effect of the polymeric structure on the effectiveness of rose bengal as a photosensitizer and to compare the behavior of the polymer-bound dye to the behavior of the dye - free - in solution. In the present paper, therefore, we report on the photochemical and spectral properties of new singlet oxygen sensitizers based on soluble polystyrenes. These new derivatives are referred to as P -RB. 

The values of the optical purity are in many cases different from those reported in the original papers since they have been recalculated assuming in all cases ott,5 = 238° (1 = 1, neat) as the maximum value for the rotatory power of hydratropaldehyde381 b (R,R) enantiomer 0 (S,S) enantiomer d The absolute configuration at the P atom is unknown Chirality not specified f Soluble polystyrene containing (4R, 5R)-2-[p-phenyl-4,5-bis(diphenylphosphinomethyl)]-l,3-dioxolane groups ... 

The desorganization by dilution of the periodic structures of block copolymers has been studied by electron microscopy after polymerization of the solvent66,70,71. Two types of solvents have been used styrene, which is the monomer for the soluble polystyrene block and which prevents any incompatibility between polymeric chains during polymerization of the monomer, and methyl methacrylate which allows the study of the effect of incompatibility between polymeric chains during polymerization of the solvent. 

From their chemical architectures the gel-type resins can be classified as belonging to one of two different types of solid supports. For hghtly crosshnked polystyrene and polyacrylamide resins the reactive sites are located along the polymer chains in a statistical manner. Reaction kinetics can be expected that are sinnilar to those associated with a soluble polystyrene that was proposed by Shemiakin et al.f l for peptide synthesis. However, reaction rates were found to depend on the location of the reactive sites on such linear noncrosslinked polystyrenes. In fact, reactive sites located on a flexible polymer loop should exhibit a different kinetic behavior to those close to a more rigid, crosshnked section. 

Linear soluble polystyrene-supported triphenylphosphine/carbon tetrachloride has been used as a condensing agent for the peptide ntiiesis in homogeneous solution After the desired condensation, the polymer is precipitated quantitatively and removed by filtration. The efficiency of this technique has been demonstrated by the preparation of several dipeptide derivatives in 84-95% yields. 

Copolymer E, of the type PS/DVBA B-CHa—0(CH2)3SnBu2Cl, appears to be sensitive to temperature and therefore induces some tin contamination in the product. However, soluble polystyrene having the same spacer seems to be better at avoiding tin contamination. 

Structural analysis of linear polymers molecularly dissolved in a suitable solvent using and solution phase NMR spectroscopy is long established [87-89]. Not surprisingly therefore when a linear soluble polymer is used as a support in solid phase synthesis and solution phase NMR spectroscopy can be a powerful tool in following the chemical synthesis on the support [90]. Figure 15.3.58, for example, shows a series of H NMR spectra of dissolved linear polymer samples taken at various stages in the solid phase synthesis of oligoethers on soluble polystyrene [91]. The various chemical steps Fig. 15.3.59 are clearly demonstrated. 

A new type of soluble polystyrene-supported palladium complex was synthesised (Figure 6.1) as an excellent and recyclable palladacycle catalyst for carbon-carbon bond formation in Heck, Suzuki and Sonogashira reactions to give high yields of the desired products. 

Fig. 20 Base-soluble polystyrene with a pendant hexafluoroisopropanol...

Soluble polystyrene supports differ from the terminally functionalized PEGs and polyethylene oligomers discussed above in that the catalyst moieties are attached to polystyrene via pendant groups, the loading of which can affect both the catalyst activity and separability. One example of a simple polystyrene-supported catalyst is the polystyrene copolymer-supported quaternary ammonium salts 66 and 67 [ 103]. These copolymers can be prepared with varying ratios of the styrene unit in the copolymer - the most active catalysts had 20-40 mol% of the vinylbenzylammonium groups in the copolymers. The utility of these catalysts was studied in a variety of solvents in the addition reaction of glycidyl methacrylate and carbon dioxide (Eq. 23). Polar solvents were most useful. The necessary polymer supports for preparation of catalysts 66 and 67 were prepared from chloromethylstyrene-styrene or chloromethyl-styrene-iV,JV-dimethylacrylamide copolymers that were in turn prepared by radical polymerization of the styrene or acrylamide monomers. The catalysts were recycled up to four times with small (ca. 6%) decreases in activity - de-... 

Immobilization of chiral ligands to effect asymmetric induction in alkylation of aromatic aldehydes by diorganozinc reagents promoted by PEG-im-mobilized ligands 54-57 can also be promoted by soluble polystyrene-bound species. A recent example of this is work where a polystyrene-bound BINOL was prepared [ 105]. This polymer 69 was used to form titanium-BINOLate and AlLibis(binaphthoxide) catalysts for Et2Zn reaction with benzaldehyde and for asymmetric Michael additions of stabilized carbanions to cyclohexenone. While good stereoselectivities were obtained with these catalysts, the synthetic yields were modest. 

While precipitation / crystallisation is easily automated, dialysis and gel permeation require more laborious workup procedures. In addition, precipitation / crystallisation seems to be less predictive and reproducible. Notably, it has been shown with soluble polystyrenes in the synthesis of e.g. oligosaccharides that the yields of purified polymer-bound products decreased considerably as the oligosaccharide chain grew due to increased solubility of the sugar-polymer conjugate in polar solvents.13... 

Pentene was hydroformylated to Cg aldehydes at 22 °C under 0.1 MPa H2/CO. The reaction solution was membrane-filtered and the products (77% n-hexanal and 23% methylpentanal) were analyzed by GC. The retained catalyst could be recycled twice [3], Along with Ohkubo et al, Bayer and Schurig also reported on soluble polymers as supports for asymmetric catalytic systems. The soluble polystyrene-bound analogue of DlOP (4,5-bis(diphenylphosphinomethyl)-2,2-dimethyl-l,3-dioxalane) was used for the asymmetric Rh-catalyzed hydroformylation of styrene but the ee of the predominantly obtained branched product was only 2% [Eq. (2)]. 

The applications reported for polymer-supported, soluble oxidation catalysts are the use of poly(vinylbenzyl)trimethylammonium chloride for the autooxidation of 2,6-di-tert-butylphenol [8], of copper polyaniline nanocomposites for the Wacker oxidation reaction [9], of cationic polymers containing cobalt(II) phthalocyanate for the autooxidation of 2-mercaptoethanol [10] and oxidation of olefins [11], of polymer-bound phthalocyanines for oxidative decomposition of polychlorophenols [12], and of a norbornene-based polymer with polymer-fixed manganese(IV) complexes for the catalytic oxidation of alkanes [13], Noncatalytic processes can also be found, such as the use of soluble polystyrene-based sulfoxide reagents for Swern oxidation [14], The reactions listed above will be described in more detail in the following paragraphs. 

In qualitative kinetic studies, Hodge was also able to show that the linear soluble phosphinated polystyrene reagent was only slightly less reactive than a similar DVB cross-linked polystyrene reagent with the same alcohol substrate. Recycling of the soluble polystyrene reagent was not explicitly described. 

Self quenching of excited state dye by ground state dye with rose bengals immobilized to soluble polystyrenes occurs, but it is not observed with the beads. 

Rotational diffusion rates of 2,2,6,6-tetramethyl-4-piperidinol-l-oxyl, bound to 2% cross-linked polystyrene with DF 0.02, arc slower than those of the soluble nit-roxyl (331. The rates (as the inverse rotational correlation time v ) increase with increased swelling of the polymer, from 3 x 10 s with no solvent or with the nonsolvents ethanol and 2-propanol, to 3 x 10 s i with benzene, to >10 s for a benzene solution of the corresponding soluble polystyrene. Increased cross-linking (4% and 12% DVB) gives decreased swelling a decreased rotational diffusion rates. 

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