Swaminathan Sivaram

Polymeric catalysts for chemo‐ and enantioselective epoxidation of olefins: New crosslinked chiral transition metal complexing polymers

Polymeric analogs of well-known chiral Mn(III)-salen complexes were synthesized and were used as recyclable catalysts for asymmetric epoxidation of olefins. For this purpose two different monomers, 2 and 3, bearing chiral Mn(III)-salen moieties were synthesized. The monomer 3 carries a bulky substituent closer to the Schiff base moiety, while monomer 2 lacks such a substituent. These metal complexed chiral monomers were subsequently copolymerized with ethylene glycol dimethacrylate producing insoluble crosslinked functional matrices that possess macroporous morphology. Chemo- and enantioselective catalytic activities of these two polymers were evaluated for epoxidation of olefins. Both polymers catalyzed the epoxidation of a variety of olefins at room temperature in the presence of iodosylbenzene (PhIO) as the terminal oxidant with yields comparable to the homogenous system. In terms of their enantioselective catalytic activity, polymer P-2 (obtained from 3) performed better than polymer P-1 (obtained from 2). Unfortunately, while the homogeneous systems are reported to offer over 80% enantioselectivity, with the present polymeric catalysts, enantioselectivity to a maximum of 30% were observed. Unlike the homogeneous system, use of an external nitrogenous donor played a very insignificant role in influencing enantioselectivity.

Polymeric metal complex catalyzed enantioselective epoxidation of olefins

Spectacular achievements in catalytic asymmetric epoxidation of olefins using chiral Mn(III)-salen complexes have stimulated a great deal of interest in designing polymeric analogs of these complexes and their use as recyclable chiral catalysts. Several strategies have been devised to anchor these chiral catalytic moieties to polymer supports. Techniques of copolymerization of appropriate functional monomers and chemical modification of preformed functional polymers have been utilized to prepare these polymers. Both organic and inorganic polymers have been used as the carriers to immobilize these metal complexes. Results of these investigations are reviewed in this article.

Studies on the Simultaneous Solid‐State Polymerization and Exchange Reactions of PET/PEN Oligomer Blends

Poly(ethylene terephthalate) (PET) and poly(ethylene-2,6-naphthalate) PEN oligomers were obtained by degrading high molecular weight PET and PEN. Crystalline PET/PEN blends of varying compositions were prepared by melt blending and then crystallizing at 175°C. Simultaneous solid-state polymerization (SSP) and exchange reactions were performed on these blends by heating just below the melting temperature under a nitrogen flow. 1 H NMR spectroscopy was used to study the progress of exchange reactions between terephthalic and naphthalate units. The inherent viscosity was measured to monitor the progress of SSP. WAXS studies indicated that the condensation and exchange reactions occurred in the amorphous phase, leaving the PET and PEN crystallites unaffected. Detailed thermal characterization showed that the crystallization behavior of the copolymers depended strongly on the extent of the exchange reaction.

Development of Structure and Morphology during Crystallization and Solid State Polymerization of Polyester Oligomers

Low molecular weight oligomers of poly- (ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) were synthesized and the crystallization behavior were examined. The crystallized oligomers underwent solid state polymerization (SSP) when subjected to temperatures close to melting temperature under nitrogen. The crystallites did not exhibit reorganization during SSP but the structure and morphology changed due to SSP. It was found that the catalyst used for the synthesis of the oligomers had an influence on both the crystallization and the SSP of the oligomer. Titanium isopropoxide (Ti(O i Pr) 4 ) reduces the crystallization rate of the oligomers but enhances the SSP rate, whereas, antimony trioxide (Sb 2 O 3 ) increases the crystallization rate but reduces the SSP rate.

Copolymerization of methyl methacrylate with lauryl methacrylate using group transfer polymerization

The syntheses of random and block copolymers (using sequential monomer addition) of methyl methacrylate (MMA) and lauryl methacrylate (LMA) have been investigated by group transfer polymerization (GTP) over a wide composition range using tetrabutylammonium bibenzoate (TBABB) as catalyst and 1-methoxy-1-(trimethylsiloxy)-2-methyl-1-propene (MTS) as initiator in tetrahydrofuran (THF) at room temperature. The absolute molecular weight of the copolymers were determined by SEC-MALLS. The observed molecular weights were generally higher than the calculated molecular weights. However, the molecular weight distributions were very narrow (1.02–1.1). Use of trimethylsilyl benzoate as a “livingness enhancer” improved the livingness of the first block (PLMA) and block copolymers with no detectable contamination of homopolymer. Statistical copolymers of MMA and LMA were prepared, and the reactivity ratios of the two monomers under the defined conditions were determined.

The Mukaiyama–Michael addition of a β,β-dimethyl substituted silyl ketene acetal to α,β-unsaturated ketones using tetra-n-butylammonium bibenzoate as a nucleophilic catalyst

The Michael addition of a β,β-dimethyl substituted silyl ketene acetal [Me2Cue605C(OMe)OSiMe3] to α,β-unsaturated ketones, namely, 2-cyclopentenone, 2-cyclohexenone, 3-methyl-2-cyclohexenone, isophorone, methyl vinyl ketone and mesityl oxide occurs smoothly in the presence of the nucleophilic catalyst, tetra-n-butyl ammonium bibenzoate (TBABB) in THF giving the corresponding 1,4-adducts in excellent yields.

Preparation of Polyurethane Microspheres via Dispersion Polycondensation Using Poly(1,4-isoprene)-block-poly(ethylene oxide) as Steric Stabilizer

A novel dispersion polymerization of a diisocyanate and a diol for the preparation of spherical polyurethane particles is reported. An amphiphilic block copolymer, namely, poly(1,4-isoprene)-block-poly(ethylene oxide) was used as a steric stabilizer. Monodisperse spherical particles were obtained in the size range from 0.2 to 2.0 μm. The polyurethane particle formation was dependent on the concentration of the steric stabilizer, the block segment molecular weight and the nature of dispersion medium. The polyurethane particles were stabilized by a mechanism involving physical adsorption of the steric stabilizer on the surface of the growing particle.

A magnesium chloride supported bis(cyclopentadienyl)-zirconium(IV) dichloride catalyst for the polymerization of ethylene

A magnesium chloride supported zirconocene catalyst was synthesized using a soluble tetrahydrofuran complex of magnesium chloride. The supported catalyst polymerizes ethylene with high activities in the presence of methylaluminoxane. The siginificant feature of the supported catalyst is that it polymerizes ethylene even at temperature as high as 100°C at low Al/Zr ratios. The polymer molecular weights obtained using supported catalyst are found to be higher compared to the unsupported catalyst. The molecular weight distribution index is in the range of 2.0-2.5. The active site concentration of the supported catalyst was found to be five times lower than that of the unsupported catalyst.

Conjugate Addition of a Silyl Ketene Acetal to α,β‐Unsaturated Lactones

Abstract Conjugate addition of a silyl ketene acetal [Me2C˭C (OMe)OSiMe3] to α,β‐unsaturated lactones (namely, 5,6‐dihydro‐2H‐pyran‐2‐one, 2(5H)‐furanone as Michael acceptor) occurs efficiently at room temperature in the presence of a nucleophilic catalyst, tetra‐n‐butyl ammonium bibenzoate (TBABB), in THF as well as Lewis acid catalysts such as Yb(OTf)3 and I2 in CH2Cl2, giving the corresponding 1,4‐adducts in excellent yields.

Synthesis of Hydroxy-Functional PMMA Macromonomers by Anionic Polymerization

Living anionic polymerization has been utilized to synthesize hydroxy end-functionalized PMMA macromonomers with styryl or allyl functionalities as the polymerizable end-groups. Protected hydroxy-functionalized alkyl lithium initiators have been used to initiate anionic polymerization of MMA. Subsequently the living chains with protected hydroxyl function have been terminated using 4-vinylbenzyl chloride (4-VBC) or allyl methacrylate (ALMA) to form α-hydroxy-ω-styryl and α-hydroxy-ω-allyl PMMA, respectively. These protected hydroxy-functionalized PMMA macromonomers have been characterized by GPC and 1H-NMR. Termination using 4-VBC led to 50% functionalization, whereas that using allyl methacrylate led to 100% functionalization of the hydroxy-PMMA.Living anionic polymerization has been utilized to synthesize hydroxy end-functionalized PMMA macromonomers with styryl or allyl functionalities as the polymerizable end-groups. Protected hydroxy-functionalized alkyl lithium initiators have been used to initiate anionic polymerization of MMA. Subsequently the living chains with protected hydroxyl function have been terminated using 4-vinylbenzyl chloride (4-VBC) or allyl methacrylate (ALMA) to form α-hydroxy-ω-styryl and α-hydroxy-ω-allyl PMMA, respectively. These protected hydroxy-functionalized PMMA macromonomers have been characterized by GPC and 1H-NMR. Termination using 4-VBC led to 50% functionalization, whereas that using allyl methacrylate led to 100% functionalization of the hydroxy-PMMA.