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Solid state and solution characterization of caprolactam cyclic dimer, a by-product in the synthesis of nylon 6
Abstract
The dimer of ϵ-caprolactam (1,8-diazacyclotetradecan-2,9-dione), an impurity always present in industrial nylon 6, was characterized by mass spectrometry and solution 13C NMR analysis. DSC and X-ray diffraction lead to the complete description of the behavior of two crystalline phases: the one stable at room temperature and showing a transition to the other form at 244°C; the second one, more symmetrical, melting at 347°C and slowly converting to the first one at temperatures below 200°C. Both structures are stabilized by intermolecular hydrogen bonds, and the transition is due to conformational changes (from G+TTG− to G+G+TG+ on the aliphatic sequence and from eclipsed to ≈90° on the NCH2 bonds) which can be taken into account in the discussion about the tight chain folding of nylon 6.
... The WXRD patterns of the a-form and b-form cyclic dimer crystals are shown in figure 4 and their X-ray diffraction spectra of the cyclic dimer are displayed in figure 3b. The characteristic peaks of the a-form appear at 2u as follow: 10 [19], who disclosed that this packing rearrangement is connected to an important conformational change along the aliphatic sequence of the dimer. In the centre of the b-form crystal, hydrogen bonds build up an almost square two-dimensional lattice with triclinic cell dimensions for axes b and c (8.91 Å with an angle of 908), each cyclic dimer molecule is connected by four hydrogen bonds to four other molecules, which make the crystal more stable at a lower temperature. ...
The characteristics of pure α- and β-form of the cyclic dimer (1,8-diazacyclotetradecane-2,9-dione) were systematically and integrally investigated during this study. The results showed that the α-form could dissolve and rapidly transform into the β-form in methanol, and in caprolactam solution at a lower temperature, an interesting transition occurred and formed co-precipitates, which refract colourful light under PLM. However, these dimers can aggregate in water, and they are then transformed into multi-slice layers and compact structures. The detailed transition behaviours between the two forms were further measured by FT-IR, XRD and DSC by varying the temperature from 25°C to 360°C, respectively, which showed that there are two endothermic transitions over the course of the heating programme. At a temperature of approximately 242°C, the β crystals were initially converted into α crystals, and then they melted when the temperature reached over 345°C. A video recorded under a light microscope also showed that the sublimation of the β cyclic dimer occurred after the transition. However, the α-form might sublimate at temperatures lower than 150°C when mixed with volatile matter.
In this study, a modified torque rheometer is used to investigate the volatilization behavior of monomer and oligomers in polyamide-6 (PA6) melt under dynamic film–forming conditions with negative pressure. The surface renewal model is employed to simulate the volatilization behavior. The effects of actual processing factors and simulation results on the volatilization behavior of monomer and oligomer are analyzed comparatively. It is found that the monomer and oligomers removal rate increase continuously with increasing temperature, residence time, and rotational speed. And, the cyclic dimer, which is extremely harmful to spinning, can be removed. It is found that the removal of monomer and oligomers continued to increase with increasing temperature, residence time, and spinning speed. Moreover, cyclic dimers, which are extremely harmful to spinning, are also removed. Additionally, it is discovered that the polycondensation reaction of PA6 results in an increase in the molecular weight and viscosity at lower temperatures (250 °C), while a higher temperature (270 °C) and shear rate (150 r/min) cause a reduction in viscosity and molecular weight.
Nylon‐6 is selectively depolymerized to the parent monomer ε‑caprolactam by the readily accessible and commercially available lanthanide trisamido catalysts Ln(N(TMS)2)3 (Ln = lanthanide). The depolymerization process is solvent‐free, near quantitative, highly selective, and operates at the lowest Nylon‐6 to ε‑caprolactam depolymerization temperature reported to date. The catalytic activity of the different lanthanide trisamides scales with the Ln3+ ionic radius, and this process is effective with post‐consumer Nylon‐6 as well as with Nylon‐6 + polyethylene, polypropylene or polyethylene terephthalate mixtures. Experimental kinetic data and theoretical (DFT) mechanistic analyses suggest initial deprotonation of a Nylon terminal amido N‐H bond, which covalently binds the catalyst to the polymer, followed by a chain‐end back‐biting process in which ε‐caprolactam units are sequentially extruded from the chain end.
Nylon‐6 is selectively depolymerized to the parent monomer ϵ‐caprolactam by the readily accessible and commercially available lanthanide trisamido catalysts Ln(N(TMS)2)3 (Ln=lanthanide). The depolymerization process is solvent‐free, near quantitative, highly selective, and operates at the lowest Nylon‐6 to ϵ‐caprolactam depolymerization temperature reported to date. The catalytic activity of the different lanthanide trisamides scales with the Ln³⁺ ionic radius, and this process is effective with post‐consumer Nylon‐6 as well as with Nylon‐6+polyethylene, polypropylene or polyethylene terephthalate mixtures. Experimental kinetic data and theoretical (DFT) mechanistic analyses suggest initial deprotonation of a Nylon terminal amido N−H bond, which covalently binds the catalyst to the polymer, followed by a chain‐end back‐biting process in which ϵ‐caprolactam units are sequentially extruded from the chain end.
The cyclic dimer 1,8-diazacyclotetradecane-2,9-dione is one of the most important components of caprolactam solution concentrates and triggers significant agglomeration during the recovery process in industrial polyamide-6 plants. For this reason, the agglomeration behaviour and morphology changes of cyclic dimers in solution concentrates were investigated. Precipitates separated from an ∼80 wt.% solution concentrate were explored. Cyclic dimers aggregated in the solution concentrates at lower temperatures to form larger, thicker, and compacted structures or multi-layers structures. An appropriate increase in the solution temperature and addition of fresh caprolactam aid in avoiding agglomeration, but high temperatures (over 140 ℃) initiate the hydrolytic polymerisation of caprolactam in the solution concentrate, which aggravates blockages. Based on these results, an optimised process model is introduced to avoid aggregation.
The ring-opening polymerization (ROP) of lactams is extensively reviewed, covering all aspects related to their polymerizability, in terms of both thermodynamic feasibility and kinetic likeliness, with greater attention to the many different aspects of the anionic polymerization, also in terms of its industrial applications. Other polymerization mechanisms (e.g., cationic) or routes (e.g., enzymatic) are synthetically introduced and updated. Specific lactams (ε-caprolactam (CL), ω-laurolactam, 2-pyrrolidone, and C-substituted β-lactams) and their anionic polymerization are described in detail, with a mention of the living polymerization of the latter family of lactams. Anionic copolymers of the most relevant lactams are listed, with a description of the synthetic routes adopted for each pair.
Formation of polymers through ring-opening reactions of cyclic compounds is an important process in polymer chemistry. In such polymerizations, chain-growth takes place through successive additions of the opened structures to the polymer chain:
Natural abundance 15N NMR spectra of various aliphatic lactams and of the corresponding polyamides were measured in trifluoroacetic acid. Cis and trans amide bonds were found to have nearly identical shifts and identical 1JNH coupling constants. Increasing chain length of the monomeric units and increasing ring size result in a downfield shift of the 15N signals. The 13C NMR spectra display, on the other hand, remarkable shift differences for lactams and Nylons. The polyamides show a monotonic downfield shift of the carbonyl signal with increasing chain length of the monomeric unit in contrast to the lactams. The 13C NMR spectra of the lactams were measured in various solvents; they show a downfield shift of the carbonyl signals with increasing acidity of the solvent. Protic solvents also favor strongly the cis-form of 1-aza-2-cyclononanone over the trans-form.
Es wird eine reproduzierbare Methode zur quantitativen Trennung der cyclischen Oligomeren des ϵ-Caprolactams aus Heißwasserextrakten von Caprolactam-Polymerisat angegeben. Molekulargewichtsbestimmungen auf kryoskopischem Wege in verschiedenen Lösungsmitteln ergaben abweichend von Literaturbefunden, daβ darin je ein ringförmiges Di-, Tri- und Tetrapeptid der ϵ-Aminocapronsäure vorliegt.
(S)-Poly(2-aminobutyric acid) (1) [(S)-poly(imino-1-methyl-3-oxotrimethylene)] was transformed into its N-trifluoroacetyl derivative 4. The IR spectra and the circular dichroism of both substances were compared. In addition, (S)-poly[N-(1-phenylethyl)acrylamide] (2) ((S)-poly-[1-(1-phenylethylaminocarbonyl)ethylene]) was trifluoroacetylated leading to polymer 5. 4 was found to react as trifluoroacetylating agent with L-1-phenylethylamine (L-3) twice as fast as it reacts with D-3. Accordingly, the reaction of 4 with D,L-3 leads preferentially to the L-form of N-(1-phenylethyl)trifluoroacetamide with optical yields up to 6%. The reaction of 5 with D,L-3 affords the D-form of N-(1-phenylethyl)trifluorcacetamide with optical yields up to 13%.
The methanolic extract of a copolymer of 6-caprolactam with 8-octanelactam was analyzed; cyclic oligomers were identified by mass spectroscopy. The cyclic homodimers and codimer were separated by thin-layer chromatography. The quantitative analysis of methanolic extract involved direct evaporation into the ion source of mass spectrometer.
The nylon oligomer (6-aminohexanoic acid cyclic dimer) degradation genes on plasmid pOAD2 of Flavobacterium sp. KI72 were cloned into Escherichia coli vector pBR322. The locus of one of the genes, the structural gene of 6-aminohexanoic acid linear oligomer hydrolase, was determined by constructing various deletion plasmids and inserting the lacUV5 promoter fragment of E. coli into the deletion plasmid. Two kinds of repeated sequences (RS-I and RS-II) were detected on pOAD2 by DNA-DNA hybridization experiments. These repeated sequences appeared five times (RS-I) or twice (RS-II) on pOAD2. One of the RS-II regions and the structural gene of the hydrolase overlapped.
A System of Computer Programs for the Automatic Solution of Crystal Struc-tures from X-ray Diffraction Data
- Woolfson
Woolfson, A System of Computer Programs for the Automatic Solution of Crystal Struc-tures from X-ray Diffraction Data, Univs. of York, England, and Louvain-la-Neuve, Belgium, 1980