Synthesis of High Molecular Weight Nylon 6 by Anionic Polymerization of ε-Caprolactam. Formation of Cyclic Oligomers

Abstract

ABSTRACT:The formation of cyclic oligomers in the process of the anionic polymerization of E-caprolactam was studied. Cyclic oligomer contents increased with chain initiator concentration and decrease in catalyst concentration. Cyclic dimer content was much higher than that of cyclic trimer to cyclic hexamer and sensitively influenced by the concentrations of catalyst and chain initiator in comparison with cyclic trimer to cyclic hexamer. A mechanism is proposed in which cyclic dimer is peculiarly formed at the acyl-lactam terminal group.KEY WORDS Cyclic Oligomer/ Cyclic Dimer/ Nylon 6/ Anionic Polymerization/ E-Caprolactam/ Catalyst/ Chain Initiator / The anionic polymerization of e-caprolactam has been extensively investigated because of the high yield and rate of polymerization. 1 -4 Fundamental features such as various catalysts, 2 • 5 • 6 propagation mechanism, 7 -10 and kinetics 11 -16 are well known.The authors carried out systematic studies directed on high molecular weight nylon 6 and obtained stable high-molecular weight polymer (the weight average molecular weight, Mw=9.5 x 10 5 ) by controlling the water content in monomer and the concentration of the catalyst and chain initiator. 1 7 -19 The kinetic equations of their anionic polymerization are in agreement with the ideal rate equation. 20 The formation of cyclic oligomers in anionic polymerization of e-caprolactam has not been studied in detail. Cyclic oligomer content (up to heptamer) has been measured only in a few studies. 21 -23 The proposed mechanism of cyclic oligomer formation still seems insufficient. 24 This study deals with kinetic data of the cyclic oligomer formation in the polymerization as a function of the concentrations of catalyst and chain initiator. The mechanism of the formation is discussed. EXPERIMENT AL MaterialsIndustrial fiber grade e-caprolactam as monomer, ethyl magnesium bromide (EtMgBr; Aldrich Chemical Co., Inc.) as catalyst in 1.0 mo11-1 tetrahydrofuran solution and N-acetyl-e-caprolactam (Ac-CL; Tokyo Chemical Industry Co., Ltd.) as monofunctional chain initiator were used for polymerization. PolymerizationWater in e-caprolactam was removed to less than 0.013 mol¾ by bubbling dry argon gas for 5 h in a three-neck flask at 120°C. Water content was controlled by adding water to this e-caprolactam and measured by a Karl Fischer auto titration equipment (Hiranuma industrial Co., AQV-5S). The chain initiator was injected into ecaprolactam with controlled water content. The mixture of monomer and initiator was poured into a glass 186 tube (inside diameter 20 mm, volume 20 ml) dried beforehand by heating above l00°C under 10Pa vacuum. The catalyst (EtMgBr) solution was injected into the tube via a syringe, and the tube was sealed after unnecessary solvent was evaporated at 10 Pa. The polymerization was carried out with shaking in an oil bath at 150°C (thermostated at± l .0°C).Polymerization was stopped by quenching to dry ice/methanol temperature ( -73°C). The product was shaved with a boring machine and analyzed. Analy...

Polymerization of lactams, 94. Formation of cyclic oligomers in anionic polymerization of 6-hexanelactam

“…On the other hand, within the period of time studied, the concentration of the cyclic dimer remained deeply below its equilibrium value. In contrast to the reported analysis 20) of the products of activated and non-activated 6-hexanelactam polymerizations initiated by 0.1 mol-% EtMgBr, the molar fraction of the cyclic trimer is, in the studied course of the polymerization, distinctly higher than that of the cyclic dimer. When compared with polymerizations initiated by 0.5 mol-% HLNa (Fig.…”

“…1). In both cases, the linear molecules bear the same types of endgroups 12,20) . For the two polymerization systems, the difference in the nature of counterions of the lactam anions and, particularly, of the anions located on the growing chains causes that different mechanisms of cyclic oligomers formation take place.…”

Identification of caprolactam oligomers and related compounds in aqueous extracts of nylon‐6

“…1 Its presence in solutions stored in poly(vinyl chloride) containers manufactured within a polyolefin overwrap 1 and in polyolefin containers, especially after gamma irradiation, 2,3 has been reported. Because the production of high molecular weight polymers (such as polyamides) is often accompanied by the formation of moderate molecular weight oligomers, 4,5 it not unexpected that such polymers would contain cyclic and open chained oligomers, 6 with oligomers up to the hexamer being reported and quantitated. 6,7 Such oligomers, along with the unreacted monomer, represent the extractable portions of polycondensate polymers.…”

“…Because the production of high molecular weight polymers (such as polyamides) is often accompanied by the formation of moderate molecular weight oligomers, 4,5 it not unexpected that such polymers would contain cyclic and open chained oligomers, 6 with oligomers up to the hexamer being reported and quantitated. 6,7 Such oligomers, along with the unreacted monomer, represent the extractable portions of polycondensate polymers. 8 In fact, cyclic oligomers of caprolactam are reportedly present in water extracts of polyamides (e.g., waste solutions associated with nylon-6 production).…”

Molecular Structure of the Complex of Hexano-6-lactam with Magnesium Bromide

“…It is noteworthy that during the above polymerizations initiated with magnesium compounds, the formation of cyclic oligomers is markedly suppressed compared with the polymerization initiated with the sodium salt of ecaprolactam [4] (CLNa). In the non-activated anionic polymerization, two types of end groups [5,6] of linear molecules, which can take part in cyclization reactions, are formed independently of the initiator type -primary amine groups and N-acyllactam groups which are regenerated in each propagation step. [5] The coordination of magnesium compounds with structurally activated endgroups of linear molecules or with amide groups within the linear chains may be one of the explanations of the difference in mechanisms of cyclic oligomer formation in the course of EtMgBr or CL 2 Mg initiated polymerization and CLNa initiated polymerization.…”