Alok Sarkar

Highly Active and Iso-Selective Catalysts for the Ring-Opening Polymerization of Cyclic Esters using Group 2 Metal Initiators

A series of alkali and alkaline earth (Ae) metal complexes bearing 1,2-phenylene(bis-diphenylphosphinothioic/selenoic amine) [{Ph2 P(E)NH}2 C6 H4 ] (E=S (1-H2); Se (2-H2) ligands are reported. Alkali metal complexes [{Ph2 P(S)N}2 C6 H4 ]Na(THF)4 (3 a) [{Ph2 P(Se)N}2 C6 H4 ]Na(THF)4 (3 b), and [{Ph2 P(Se)N}2 C6 H4 ]K(THF)5 (4 b) were obtained in good yield by treating protic ligands 1-H2 or 2-H2 with metal hexamethyldisilazides [MN(SiMe3 )2 ] (M=Na or K) at ambient temperature. The Ae metal complexes formulated as [{Ph2 P(E)N}2 C6 H4 ]M(THF)3 [E=S, M=Ca (5 a), Sr (6 a), Ba (7 a); E=Se, M=Ca (5 b), Sr (6 b), Ba (7 b)] can be synthesized by using two routes. The molecular structures of the free ligand 1-H2 and metal complexes 5 a,b-7 a,b in their solid states were established. Complexes 3 a and 3 b are isostructural; however, in complex 4 b, an attachment different from ligand 2 was observed. The complexes 5 a,b-7 a,b are isostructural and each metal ion exhibits a distorted pentagonal bipyramidal geometry around it. All Ae metal complexes 5 a,b-7 a,b were tested for the ring-opening polymerization (ROP) of racemic lactide (rac-LA) and ϵ-caprolactone (ϵ-CL) at room temperature. Calcium complexes 5 a and 5 b show excellent iso-selectivity, with Pi values of 0.78-0.87 at 298 K and with a high degree of polymerization control, whereas the corresponding strontium complexes 6 a and 6 b exhibit moderate iso-selectivity, and barium complexes 7 a and 7 b yield only atactic polylactides (PLAs). In all cases, the catalyst initiates the ROP catalytic cycle in the absence of any external initiator. Kinetic studies of the polymerization reactions indicate the relative order of polymerization rate increases with increase in the size of the metal ion: Ba>Sr>Ca.

Ring Opening Polymerization and Copolymerization of Cyclic Esters Catalyzed by Group 2 Metal Complexes Supported by Functionalized P–N Ligands

We report the preparation of alkali and alkaline earth (Ae) metal complexes supported by 2-picolylamino-diphenylphosphane chalcogenide [(Ph2P(=E)NHCH2(C5H4N)] [E = S (1-H); Se (2-H)] ligands. The treatment of the protic ligand, 1-H or 2-H, with alkali metal hexamethyldisilazides at room temperature afforded the corresponding alkali metal salts [M(THF)2(Ph2P(=E)NCH2(C5H4N)] [M = Li, E = S (3a), Se (3b)] and [{M(THF) n(Ph2P(=E)NCH2(C5H4N)}2] [M = Na, E = S (4a), Se (4b); M = K, E = Se (5b)] in good yield. The homoleptic Ae metal complexes [κ2-(Ph2P(=Se)NCH2(C5H4N)Mg(THF)] (6b) and [κ3-{(Ph2P(=Se)NCH2(C5H4N)}2M(THF) n] (M = Ca (7b), Sr (8b), Ba (9b)] were synthesized by the one-pot reaction of 2-H with [KN(SiMe3)2] and MI2 in a 2:2:1 molar ratio at room temperature. The molecular structures of the protic-ligands 1-H and 2-H, as well as complexes 3a,b-5a,b and 6b-9b were established using single-crystal X-ray analysis. The Ae metal complexes 6b-9b were tested for ring-opening polymerization (ROP) of racemic lactide ( rac-LA) and copolymerization of rac-LA and ε-caprolactone (ε-CL) at room temperature. In the ROP of rac-LA, the calcium complex 7b exhibited high isoselectivity, with Pi = 0.89, whereas both the barium and strontium complexes showed lower isoselectivity with Pi = 0.78-0.62. In the copolymerization of rac-LA and ε-CL, both barium and strontium complexes proved to be efficient precatalysts for the formation of the block copolymer rac-LA-CL, but the reactivity of 9b was found to be better than that of 8b. All the polymers were fully characterized using differential scanning calorimetry, thermogravimetric analysis, and gel permeation chromatography analyses. Kinetic studies on the ROP reaction of LA confirmed that the rate of polymerization followed the order Ba ≫ Sr ≈ Ca.

Rubber nanocomposites with polyhedral oligomeric silsesquioxanes (POSS) as the nanofiller

Abstract This chapter provides a comprehensive review on the role of polyhedral oligomeric silsesquioxanes (POSS) as reinforcing fillers for polymer and rubber nanocomposites. POSS, having an Si O Si framework, provides the high thermal stability, while the peripheral organic functionality offers the better polymer–filler interface adhesion. The peripheral organic functionalities of POSS were reported to play a key role in controlling the POSS dispersion in fumed silica based addition cured liquid silicone rubber. The radical grafting of organo-functionalized POSS was reported to reinforce the ethylene propylene diene monomer (EPDM) based rubbers, whilst causing a detrimental effect on radically curable polysiloxane elastomers. On the other hand, the matrix–filler incompatibility and the preferential crystallization of POSS segments were effectively leveraged as tunable linkages to fabricate thermoplastic elastomeric materials. The use of POSS based molecular units were also explored for developing functional structures like radiation protective coatings, nanoporous organic–inorganic frameworks for gas storage, or in low dielectric composites.