Enantioselective synthesis of silanol

An enantioselective nucleophilic substitution reaction of achiral dialkoxysilane has been developed. The reaction proceeds with efficient stereocontrol on the silicon chirality center to give the enantioenriched silyl ether, which can be converted to the silanol without loss of enantiopurity. We have analyzed the steric course of the reaction by using DFT calculations and propose a transition state model to explain the observed enantioselectivity.     

Enantioselective Synthesis of Silacyclopentanes

A variety of functionalized silacyclopentanes were synthesized by highly enantioselective β‐eliminations of silacyclopentene oxides followed by stereospecific transformations. The reaction mechanism of the β‐elimination was elucidated by DFT calculations. An in vitro biological assay with an oxy‐functionalized silacyclopentane showed substantial binding to a serotonin receptor protein.     

Lipase-catalyzed synthesis and curing of high-molecular-weight polyricinoleate

High-molecular-weight polyricinoleate, with an M(w) of 100,600, was enzymatically prepared by the polycondensation of methylricinoleate using immobilized lipase from Pseudomonas cepacia (IM-PC) in bulk in the presence of 4 A molecular sieves at 80 degrees C for 7 d. Polyricinoleate was a viscous liquid at room temperature with a glass transition temperature (T(g)) of -74.8 degrees C, showed no crystallinity and was biodegraded by activated sludge. Polyricinoleate was readily cured using a dicumyl peroxide at 170 degrees C for 30 min to produce a chloroform insoluble crosslinked polyricinoleate with a hardness of 50A using durometer A.     

Lipase‐Catalyzed Ring‐Opening Polymerization of β‐Butyrolactone: End‐Group Analysis of Poly(3‐hydroxybutanoate) Using Supercritical Fluid Chromatography

The ring‐opening polymerization of (R,S)‐β‐butyrolactone (BL) in bulk was analyzed with respect to the polymer structure of the resulting poly[(R,S)‐3‐hydroxybutanoate)] [P(3HB)] by isolation of the pure form using preparative supercritical CO₂ fluid chromatography. It was confirmed that the four‐membered BL was polymerized in bulk by lipase to yield the corresponding cyclic, hydroxy‐ and crotonate‐terminated P(3HB)s. The relative ratios of the three types of polymers depended on the lipase concentration as well as on the monomer conversion. It was also confirmed that both cyclic and linear P(3HB) polymer species were subject to hydrolysis, and inter‐ and intramolecular transesterification by lipase to produce two series of polymers having linear and cyclic structures with higher and lower molecular weight. The formation of the cyclic P(3HB) iss regarded as the characteristic feature of the lipase‐catalyzed polymerization of BL.     

Isospecific copolymerization of styrene and a styrene‐terminated polyisoprene macromonomer with the Ni(acac)2/MAO catalyst

The copolymerization of styrene (St) with a styrene‐terminated polyisoprene macromonomer (SIPM) by a nickel(II) acetylacetonate [Ni(acac)₂] catalyst in combination with methylaluminoxane (MAO) was investigated. A SIPM with a high terminal degree of functionalization and a narrow molecular weight distribution was used for the copolymerization of St. The copolymerization proceeded easily to give a high molecular weight graft copolymer. After fractionation of the resulting copolymer with methyl ethyl ketone, the insoluble part had highly isotactic polystyrene in the main chain and polyisoprene in the side chain. Lowering the MAO/Ni molar ratio and the polymerization temperature were favorable to producing isospecific active sites. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1241–1246, 2000     

A new strategy for sustainable polymer recycling using an enzyme: poly(ε‐caprolactone)

Enzymatic degradation and polymerization using an enzyme were analyzed with respect to the establishment of a sustainable chemical recycling system for poly(ε‐caprolactone) (PCL) which is a typical biodegradable synthetic plastic. As the typical example, the enzymatic degradation of PCL having an M_n of 110 000 using lipase CA in toluene containing water at 70°C for 6 h afforded a unimodal oligomer having an M_n of about 1 000 quantitatively consisting of linear and cyclic oligomers. This was again polymerized by lipase CA in toluene under restricted water concentration to produce PCL having an M_n of greater than 70 000.     

Enzyme‐Catalyzed Preparation of Aliphatic Polyesters Containing Thioester Linkages

Aliphatic polyesters containing thioester linkages were enzymatically prepared by both the copolymerization of lactone with mercaptoalkanoic acid and by the transesterification of polyesters with mercaptoalkanoic acids. The enzymatic copolymerization of ε‐caprolactone with 11‐mercaptoundecanoic acid (11MU) and 3‐mercaptopropionic acid (3MP) was performed under reduced pressure using an immobilized lipase from Candida antarctica (CA). The transesterification of poly(ε‐caprolactone) and poly[(R)‐3‐hydroxybutyrate] was carried out with 11MU and 3MP using lipase CA under reduced pressure.

     

Well‐Controlled Biodegradable Nanocomposite Foams: From Microcellular to Nanocellular

Via a batch process in an autoclave, the foam processing of neat polylactide (PLA) and two different types of PLA/layered silicate nanocomposites has been conducted using supercritical carbon dioxide as a foaming agent. The morphological correlation between the dispersed silicate particles with nanometer dimensions in the bulk and the obtained closed‐cell structure of the foam is discussed. This is the first report that deals with the possibility of preparing biodegradable nanocellular polymeric foams via nanocomposite technology.

SEM image of the freeze‐fracture surface of a PLA/layered silicate foam, exhibiting closed‐cell structure.