Chiral media, polymerization

The use of a convective macroporous polymer as an alternative support material instead of silica for the preparation of protein-based CSPs has successfully been demonstrated by Hofstetter et al. [221]. Enantioseparation was performed using a polymeric flow-through-type chromatographic support (POROS-EP, 20 pm polymer particles with epoxy functionalities) and covalently bound BSA as chiral SO. Using flow rates of up to 10 ml/min, rapid enantiomer separation of acidic compounds, including a variety of amino acid derivatives and drugs, could be achieved within a few minutes at medium efficiencies, typical for protein chiral stationary phases (Fig. 9.13). 

Radical polymerization of acrylates can be used to make low-molecular-weight oligomers under high dilution conditions with a relatively large concentration of chain transfer reagent. These conditions were extended to the intramolecular cycli-zation of two acrylate entities tethered by a chiral diol in the formation of a remote stereocenter and a medium-sized ring (Eq. (13.38)) f49J. 

The synthesis of chiral poly(depsipeptides), polymers with alternating amide and ester bonds, by lipase-catalyzed ring opening of 3-isopropyl morpholino-2,5-dione (19) was shown by Hocker and coworkers (Scheme 11.5) [26], Various lipases were tested for the bulk polymerization of these heterocyclic monomers at temperatures of 100 °C or above. PPL and lipase type III from a pseudomonas species were shown to be effective catalysts. The isolated polymers showed Mn values of 3.5-17.5 kgmol-1. The influence of reaction temperature, the amount of enzyme and the presence of water in the reaction medium were shown to be important factors on the high molecular weight fraction and were investigated in detail [26b]. Comparison of optical rotation values for polymers prepared by... 

Since dicyclohexyl-18Crown-6 (DC18C6) has the ability to take up ions and to transfer them across a lipophiUc medium, it has widely been used in or nic synthesis, in ion-extraction into nonpolar solvents, as chiral complexing agents, etc. Osada, Shinkai, et al. have found that membranes prepared under proper conditions retain the structure of the original crown compounds sufficiently to recognize metal ions and v,j+/Vl,+ and V,j+/vcs+ (ratios of ion-permeation rates) for plasma-polymerized membranes were 3.7 and 3.4, respectively [75],... 

The polymeric products had medium molecular weights, as evidenced by the values of their viscosity in the range 0.1-. 3 dl/g in various solvents at 25-45°C. Oligomers with values of M /Mn= 1.2-1.7 were obtained in the case of oxalates 2. Polymers 5 containing chiral centers in the spacer units were optically active and the sign of the optical rotation at 578 nm was positive as that of the starting glycolethers... 

Similar through-space asymmetric polymerization from achiral mono-, di-, or tri-thiophenes and pyrrole monomers was also achieved by the use of cholesteric liquid crystals as an asymmetric reaction solvent [19]. As no reaction occurred between the molecules of the liquid crystal and the monomers, the chiral morphology of the polymers (which have no chiral substituent) is considered to derive from the asymmetry produced by the chiral liquid crystal medium during polymerization. Heat treatment of the polymer causes disaggregation and a loss of chirality, and polymers prepared in this way exhibit an exiton splitting signal in the circular dichroism spectra in the absorbance region of the polymeric backbone they also display a circular polarized luminescence. A representative example is shown in Scheme 8.2 [19]. 

Two nonchiral compounds, acetyl CoA and its condensation product acetoacetyl CoA, are the most common metabolic intermediates serving as the precursors for the synthesis of poly(3HB) or other PHAs. Stereospecific reductases and hydratases convert acetoacetyl CoA into chiral (R)-(—)-3-hydroxyhutyric acid, which is polymerized into poly(3HB). From different starting substrates, different pathways are used for the synthesis of acetyl CoA, and different organisms may use distinct pathways to form (R)-(—)-3-hydroxybutyric acid (Steinbuchel 1996). Besides acetyl CoA, longer-chain acyl CoAs are synthesized via specialized reactions when appropriate substrates are available. The actual composition of the PHA formed thus depends on the types of monomers that are present in the cell, which in turn depends on the carbon substrates that are provided in the growth medium. This flexibility allows one to direct an organism to synthesize PHA copolymers with tailored monomer compositions that confer desirable properties. 

Apparently electrostatic association of cationic monomers 1 with polyanions does not provide strong enough complexation to control tacticity or other aspects of microstructure in the daughter polymer. This is likely due to the solvation of template, monomers and growing daughter chain by the reaction medium. Conditions that organize solution phase monomers at an ordered, nonsolvated template surface would be more likely to transfer tacticity, or chirality, or to control monomer sequence during polymerization. 

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