Mrinmay Mandal

Imino phenoxide complexes of group 4 metals: synthesis, structural characterization and polymerization studies

A complete library of new alkoxides containing group 4 metals and the imino phenol ligand were synthesized in high yields and purity. These compounds showed catalytic activity towards the polymerization of L-lactide (L-LA), rac-lactide (rac-LA), e-caprolactone (CL), δ-valerolactone (VL), rac-β-butyrolactone (rac-BL), ethylene and propylene. The zirconium catalysts were found to yield better polymerization results in comparison to the titanium and hafnium analogues. The number average molecular weight (Mn) values of the polymers are very high for LA and CL with controlled molecular weight distributions (MWDs). Analyses of MALDI-TOF and 1H NMR spectra of low Mn oligomers reveal that the ligand initiates the ring-opening polymerization (ROP). For ethylene and propylene polymerization, we have achieved moderate to good activity using MAO as a co-catalyst.

Zr(IV) complexes containing salan-type ligands: synthesis, structural characterization and role as catalysts towards the polymerization of ε-caprolactone, rac-lactide, ethylene, homopolymerization and copolymerization of epoxides with CO2

Zr(IV) complexes containing salan-type diamine bis(phenolato) ligands were synthesized and characterized by various spectroscopic techniques and X-ray crystallography. We observed the formation of dinuclear Zr complexes where each Zr centre adopts a distorted octahedral geometry. The metal centers are bridged with two isopropoxide groups and each metal center has two terminal isopropoxide moieties attached to it. These compounds show notable activities towards the ring-opening polymerization (ROP) of e-caprolactone (e-CL) and rac-lactide (rac-LA) in the presence and absence of benzyl alcohol as an external initiator, resulting in very high number average molecular weight (Mn) polymer with controlled molecular weight distributions (MWDs). There is a close proximity between observed molecular weight (Mobsn) and theoretical molecular weight (Mtheon). Analysis of MALDI-TOF and 1H NMR spectra of low Mn oligomers reveals that the isopropoxide groups as well as the OBn groups initiate the ROP. Kinetic studies reveal that the polymerization follows first-order kinetics and is faster in the presence of benzyl alcohol (BnOH). All the complexes were active precatalysts towards the polymerization of ethylene. In addition, all the complexes were found to copolymerize cyclohexene oxide (CHO) and propylene oxide (PO) with CO2, affording the formation of degradable polycarbonate with moderate Mn values and narrow MWDs. Their thermal properties were examined using DSC and TGA analysis. In the case of styrene oxide (SO), the coupled product of CO2 and SO was observed.

Imino phenoxide complexes of niobium and tantalum as catalysts for the polymerization of lactides, ε-caprolactone and ethylene

Imino phenoxide complexes of Nb and Ta show precise control towards the solvent free ring-opening polymerization of lactides (LA) and ε-caprolactone (CL). There is a close proximity between the observed number average molecular weight (M(n)) and theoretical molecular weight and the molecular weight distributions (MWDs) were found to be narrow. Analysis of low molecular weight oligomers of LA synthesized from these compounds revealed that the ligand is incorporated as one of the end terminal groups in the polymer chain. In addition, these complexes were realized to be precatalysts for the polymerization of ethylene.

Synthesis and structural characterization of titanium and zirconium complexes containing half-salen ligands as catalysts for polymerization reactions

Complexes 1–7 were synthesized from tridentate half-salen ligands in good yields and purity. They were characterized by various spectroscopic techniques, elemental analyses, and X-ray crystallography. With titanium, homoleptic and heteroleptic complexes could be synthesized independently whereas with zirconium only homoleptic complexes could be synthesized. All the complexes were efficient and good catalysts for the ring-opening polymerization (ROP) of lactides (LA) and e-caprolactone (e-CL). The molecular weights (Mn) of the resulting poly(lactic acid) (PLA) and poly(caprolactone) (PCL) are close to the theoretical values, and the molecular weight distributions (MWDs) are narrowly distributed. Kinetic study showed that the ROP had a first-order dependency on monomer concentration. Next, preliminary studies on copolymerization of cyclohexene oxide (CHO) with CO2 were performed in the presence of tetrabutylammonium bromide (TBAB) as a cocatalyst. The results from DSC and TGA were used to describe the thermal properties of the copolymer. In addition, all the complexes can effectively catalyze the cycloaddition reactions of CO2 with propylene oxide (PO) and styrene oxide (SO) using TBAB as a cocatalyst. The formation of the homopolymerization product from CHO is witnessed in the absence of any cocatalyst. All the complexes, when activated by methylaluminoxane (MAO), showed potent catalytic efficiency towards the polymerization of ethylene. The impact of polymerization parameters such as time, temperature and solvents on the polymerization of ethylene is discussed elaborately.

Kinetic investigation on the highly efficient and selective oxidation of sulfides to sulfoxides and sulfones with t-BuOOH catalyzed by La2O3

The selective oxidation of various sulfides to sulfoxides by a simple, efficient, and environmentally benign method is of prime focus. In this paper, we have explored a highly efficient protocol for the oxidation of alkyl aryl sulfides to sulfoxides with high selectivities catalyzed by La2O3 in the presence of 70% t-BuOOH solution (water). We obtained predominantly the monooxygenated product. The over oxidation of sulfides to sulfones was not observed under these conditions. The resulting products are obtained in good to excellent yields within a reasonable time without the use of ligands and other additives. The epoxidation of the double bond as well as allylic oxidation are not observed with allyl sulfides. Sulfones can be obtained quantitatively by altering the reaction conditions. The surface morphology and the catalyst reusability were verified by XRD, AFM and SEM techniques. The surface area of the La2O3 was measured using BET isotherms.