Polymer substrate selection

Ohtaki, M., Komiyama, M., Hirai, H., and Toshima, N., Effects of polymer support on the substrate selectivity of covalently immobilized ultrafme rhodium particles as a catalyst for olefrn hydrogenation, Macromolecules, 24, 5567, 1991. 

Arshady R, Mosbach K. S3mthesis of substrate-selective polymers by host-guest polymerization. Maktomol Chem 1981 182 687-692. 

Sellergren B, Lespisto M, Mosbach K. Highly enantioselective and substrate-selective polymers obtained by molecular imprinting utilizing noncovalent interactions. NMR and chromatographic studies on the nature of recognition. J Am Chem Soc 1988 110 5853-5860. 

When the reactions of alkyl bromides (n-Q-Cg) with phenoxide were carried out in the presence of cosolvent catalyst 51 (n = 1 or 2,17 % RS) under triphase conditions without stirring, rates increased with decreased chain length of the alkyl halide 82). The substrate selectivity between 1-bromobutane and 1-bromooctane approached 60-fold. Lesser selectivity was observed for polymer-supported HMPA analogue 44 (5-fold), whereas the selectivity was only 1,4-fold for polymer-supported phosphonium ion catalyst 1. This large substrate selectivity was suggested to arise from differences in the effective concentration of the substrates at the active sites. In practice, absorption data showed that polymer-supported polyethylene glycol) 51 and HMPA analogues 44 absorbed 1-bromobutane in preference to 1-bromooctane (6-7 % excess), while polymer-supported phosphonium ion catalyst 1 absorbed both bromides to nearly the same extent. 

To clarify mechanisms of substrate selectivity, studies on elementary reaction steps with polymer-supported cosolvent catalysts must be carried out in detail. 

A first attempt to calculate the effective pore size within the polymer from the substrate selectivity has not been oversuccessful, since calculated pore sizes of about 7 A were obtained in polymers with measured pore sizes in the region of 150 to 200 A. Although, of course, the... 

A quantitative assessment of the effects of head group bulk on, S k2 and E2 reactions in cationic micelles has been made.148 The kinetics of the acid-catalysed hydrolysis of methyl acetate in the presence of cationic, anionic, and non-ionic surfactants has been reported on.149 The alkaline hydrolysis of -butyl acetate with cetyltrimethylammonium bromide has also been investigated.150 The alkaline hydrolysis of aromatic and aliphatic ethyl esters in anionic and non-ionic surfactants has been studied.151 Specific salting-in effects that lead to striking substrate selectivity were observed for the hydrolysis of /j-nitrophenyl alkanoates (185 n = 2-16) catalysed by the 4-(dialkylamino)pyridine-fimctionalized polymer (186) in aqueous Tris buffer solution at pH 8 and 30 °C. The formation of a reactive catalyst-substrate complex, (185)-(186), seems to be promoted by the presence of tris(hydroxymethyl)methylammonium ion.152... 

Compared to polymers, dendrimer architectures offer favourable conditions for fixation of catalytically active moieties thanks to their monodispersity, variability, structural regularity of the molecular scaffold, and numerous functionalisation possibilities. Catalytic units can be fixed - multiply if required - on the periphery, in the core of a dendrimer, or at the focal point of a dendron. If the dendrimers are suitably functionalised at the periphery, appropriate metal complexes can be directly attached to the surface of the molecule. In contrast, dendrimers functionalised in the core or at the focal point shield the catalytically active site through their shell structure in a targeted manner, for example to attain substrate selectivity in the case of reactants of different sizes [1]. The corresponding concepts of exodendral and endodendral fixation of catalysts were inttoduced in the context of functionalistion of carbosilane, polyether, and polyester dendrimers [2]. Exodendral fixation refers to attachment of the catalytic units to the... 

Molecular imprinting has recently attracted considerable attention as an approach to the preparation of polymers containing recognition sites with predetermined selectivity. The history and specifics of the imprinting technique pioneered by Wulff in the 1970s have been detailed in brilliant review article [40]. These materials, if successfully prepared, are expected to find applications in numerous areas such as the resolution of racemates, chromatography, substrate selective catalysis, and the production of "artificial antibodies". Imprinted monoliths have also recently received... 

Pyridine-bound resins were also prepared and successfully employed as polymer-supported phase transfer catalysts in bromide displacement from 1-bromoalkanes by salt phenoxide or naphthoxide, even though the controlling factor (diffusivity or preferential sorption) for the observed substrate selectivity effects was difficult to determined.[133]... 

Mackenzie W. M. and Sherrington, D. C. Substrate selectivity effects involving polymer-supported phase-transfer catalysts, Polymer, 1981, 22, 431—433. 

Fife and co-workers described a macromolecule, 5 containing 4-DAAP and a bis-(trimethylene) disiloxane backbone that exhibited enzyme-like substrate selectivity for the esterolysis of p-mtrophenyl alkanoates, 6 (Scheme 5.2). This synthetic polymer showed highest levels of activity toward substrate 6 (n = 14), when it was used as a nucleophilic catalyst for the solvolysis of a series of... 

The design of superior polymer catalysts is alw -s related to substrate selectivity and efficient turnover of catalysts. Some selectivity was observed among substrates that was due to hydrophobic and electrostatic factors. Especially the cooperative action of these two factors was effective for rate enhanconents. The efficiency of acid and alkali hydrolyses was increased iq> to 100 times in the presence of prdymers, but the simple polyelectrolyte effect would not yield further rate enhaiK menL More elaborate systems that include efficient general add and base catalyses must be developed. 

These properties are listed in order of usefulness for comparative review purposes. Liquid surface tension is the most fundamental property, because it pertains only to the material in question (provided the material is adequately pure) and the technique used for measurement. All the other properties listed are dependent also on solvents, contact-angle test liquids, and liquid or solid substrates selected. For solids, approaches such as the Owens-Wendt analysis (7) have supplanted the Zisman method (18) in recent years, but data from the Zisman method for organosilicon polymers are more available compared with data from the Owens-Wendt approach. Some useful data on aqueous surface tensions and Langmuir troughs are also available. Data for other listed properties are of less fundamental use and rather scanty. 

In competitive substrate binding experiments, the polymers showed selectivity for the templates with which they were made. On the other hand, non-templated control polymers showed statistical preference for both substrates. Some results on the substrate selectivities of these polymers are presented in Table 6.1. It is particularly notable from this study that these imprinted polymers could distinguish positional isomers (4 and 5), which differ in their imidazole spacing by only 4 A. 

Maximum binding capacities of these polymers towards different substrates and their apparent binding constant values were determined. These results indicate that the substrate selectivity exhibited by these imprinted polymers may involve a combination of cooperative two-site coordination of the Z> -imidazoles to the metal centres, as well as steric interaction with the cavity containing the binding sites. 

SUBSTRATE SELECTIVITIES OF MOLECULARLY IMPRINTED METALCOORDINATING POLYMERS TOWARDS VARIOUS R/S-IMIDAZOLE SUBSTRATES"... 

Hjerten and co-workers prepared substrate-specific polyacrylamide gels by carrying out cross-linking polymerisation in the presence of proteins [47]. Polymerisation of acrylamide and 7V,A -methylenebisacrylamide was carried out in the presence of cytochrome C and haemoglobin as the templates. After removal of the templates using surfactants and acetic acid, substrate selectivities of the polymer gels were tested by chromatography. The imprinted polymers reportedly showed preferential affinities for their templates. 

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