Polymerization of ϵ‐Caprolactam to Nylon‐6 Catalyzed by Barium σ‐Borane Complex under Mild Condition

Abstract

A mild and low-temperature synthesis of nylon-6 is achieved through the ring-opening polymerization of ɛ-caprolactam using barium σ-borane complex as the catalyst. Due to the mild nature of the catalyst, this protocol provides an easy means of synthesizing nylon-6, for the first time with a precise and predictable molecular weight. From various model experiments, it is evident that the σ-coordinated boron hydride acts as a mild hydride donor in the first step of the reaction to generate lactamate anions, which then undergo ring-opening polymerization in-situ with the remaining caprolactam molecules under the influence of the dehydrogenated boron metal complex.Polyamides are among the most versatile synthetic polymers and are indispensable to modern life. The widespread utility of these durable high-strength materials is manifested from the various applications in the form of fibers, industrial yarns, floor covering and engineering plastics or films. [1] Among all the polyamides available today, poly (ɛ-caprolactam) or nylon-6 alone represents approximately 45 % of worldwide production in terms of volume. [2] With the increase in environmental regulation toward reduction of industrial energy consumption and carbon dioxide (CO 2 ) emission, significant efforts are being made to develop a greener and more efficient technology for making nylon. [3][4][5][6][7][8][9][10] The industrial production of nylon-6 is commonly carried out by means of anionic polymerization, which is performed hydrolytically at 250°C or at a relatively lower temperature of 150°C in the presence of an initiator. [11][12][13][14][15][16][17][18][19][20] Buchmeiser et al. reported a series of N-heterocyclic carbenes (NHCs) as initiators at temperatures of 180-200°C for anionic ring-opening homo as well as copolymerization of lactams to give corresponding polyamide PA 12 and PA 6/12. [21][22][23][24][25][26][27][28][29] In addition to intense energy requirements, these high-temperature reactions have several other drawbacks, including the formation of undesired side products. The properties of nylon-6 synthesized through anionic polymerization are often shaped by irregularities in the polymer chain because of side reactions such as Claisen-type condensations that occur during the polymerization of ɛ-caprolactam. These side reactions are strongly marked by weak heat exchange with the surrounding medium during typical bulk polymerization. [30] Ricco et al. have studied the influence of fast activators on the chain regularity and polymorphism of synthesized nylon-6. [31] At the same time, cationic initiation is not as useful as its anionic counterparts because of poor conversion and low molecular weight of the resultant polymer.Furthermore, while the use of an alkali or alkaline metal or alkali metal hydride enables the synthesis of nylon-6 in good proportion, the side reactions adulterate the product with amines and water, which destroy reactive initiator and propagate a species that often promotes polymer branching and chain irregularities. Therefo...

Highly Chemoselective Hydroboration of Alkynes and Nitriles Catalyzed by Group 4 Metal Amidophosphine–Borane Complexes

“…The preparation of catalysts was carried out using a borane derivative of the aminophosphine ligand ( L1-H2 ), which was synthesized by the reaction between N , N ′-bis­(diphenylphosphino)-benzene-1,2-diamine [Ph 2 PNHC 6 H 4 NH-PPh 2 ] and dimethyl sulfide borane complex in a 1:2 molar ratio in toluene (Tol) at room temperature (rt). , The titanium complex, with the molecular formula [{Ph 2 P­(BH 3 )­N} 2 C 6 H 4 Ti­(NMe 2 ) 2 ] ( 1 ) was synthesized utilizing a convenient method, in which the ligand L1-H2 was treated with tetrakis­(dimethylamido) titanium­(IV) in a 1:1 molar ratio in toluene at room temperature (Scheme ). In contrast, the reaction of anhydrous zirconocene dichloride (Cp 2 ZrCl 2 ) with dianionic lithium salt [{Ph 2 P­(BH 3 )­N} 2 C 6 H 4 ]­Li 2 (THF) 4 afforded the corresponding bis-cyclopentadienyl zirconium complex [Cp 2 Zr­{Ph 2 P­(BH 3 )­N} 2 C 6 H 4 ] ( 2 ) in good yield with the elimination of 2 equiv of lithium chloride (Scheme ).…”

Changes in Number and Antibacterial Activity of Silver Nanoparticles on the Surface of Suture Materials during Cyclic Freezing

“…To assess the dynamics of sorption of AgNPs, materials of metric size 2-0 (or #2/0) were used, including two materials of natural origin: silk, which is a protein fiber consisting of fibroin and sericin [ 39 ], and catgut, consisting of collagen filaments [ 40 ]. For comparison, we used an SM of artificial origin “Perlon” (nylon-6), which is a polymerization product of ε-caprolactam [ 41 ]. Silk was a sterile non-absorbable polyfilament braided thread of natural origin produced by Volot TM (Moscow, Russia).…”

Alkali and Alkaline Earth Metal Complexes as Versatile Catalysts for Ring‐Opening Polymerization of Cyclic Esters

“…Next, the barium borane complex ( 87 ) was also tested as a catalyst for the synthesis of nylon‐6 through the ROP of ϵ‐caprolactam [73] . The current protocol using barium borane complex as a catalyst for ROP of ϵ‐caprolactam provides an efficient and economical way to synthesize nylon‐6, since the reaction occurs at a low temperature and without the need for high pressure.…”