Caprolactam ring formation

The overall rate of conversion of e-caprolactam to polymer is higher than the polymerization rate of e-aminocaproic acid by more than an order of magnitude [Hermans et al., 1958, I960]. Step polymerization of e-aminocaproic acid with itself (Eq. 7-57) accounts for only a few percent of the total polymerization of e-caprolactam. Ring-opening polymerization (Eq. 7-58) is the overwhelming route for polymer formation. Polymerization is acid-catalyzed as indicated by the observations that amines and sodium e-aminocaproate are poor initiators in the absence of water and the polymerization rate in the presence of water is first-order in lactam and second-order in COOH end groups [Majury, 1958]. 

Ring-opening polymerization is another reaction that can be used to form polyamides and it can be contrasted with the ring-formation reactions described in Section 2.1.3. An important example is the polymerization of caprolactam to give nylon 6 which takes place at 520 K and is catalysed by water. 

Step growth polymerization can also take place without splitting out a small molecule. Ring-opening polymerization, such as caprolactam polymerization to nylon 6, is an example. Polyurethane formation from a diol and a diisocyanate is another step growth polymerization in which no small molecule is eliminated. 

Dutton reported on the synthesis of an e-caprolactam analog of an anthelmintic cyclic peptide. The a-hydroxy-e-caprolactam 44 was generated in an ex chiral pool synthesis staring from malic acid. The a-hydroxy carboxylic acid unit was protected as a dioxolanone in 43. The protective group served simultaneously as the reactive function during cyclization lactam 44 formation succeeded by ring opening of the dioxolanone 43 by the nucleophilic attack of the amino function, Eq. (8) [14]. 

Generation of free-radicals by Kolbe s reaction is well-known [Eq. (10)]. Formation of a radical-cation of monomer [Eq. (11)] has never been been proved and is only a possible conjecture from the right reverse consideration of the radical-anion formation at the cathode [Eq. (6)], although the perchlorate anion has actually been found to yield an unstable perchlorate free-radical by discharge at the anode. Nor is it certain that the monomer radical-cation is formed by direct discharge from the anode [Eq. (12)]. The ring-opening polymerization of oxides, caprolactam and isocyanides is also initiated on the electrode. A few examples of condensation polymerization have developed recently, like Eq. (7) and (12). Details of this work are described in the appropriate section. 

Comparing the structure of the monomer with that of the polymer as shown in Table I, we see that the polymerization of the / -carboxy-methyl caprolactam must involve isomerization of the monomer ring system. This isomerization may be described by several possible processes, all of which are characterized by reaction between the amide and acid group of the / -carboxymethyl caprolactam. Based upon the results of our studies on the structure of this polymer (5) we may eliminate confidently those processes according to which the formation of the glutarimide moiety results either by intrachain cyclization or by trans-cyclization of certain intermediate polymer structures. The former would involve a polymer formed by a conventional ring opening polymerization ... 

HCl-HOAc-AcjO, POCls, BiCla, neat with FeCls, and polyphosphoric acid. The reaction has been done in supercritical water and in ionic liquids. A polymer-bound Beckman rearrangement has been reported. Sim-ply heating the oxime of benzophenone neat leads to A-phenyl benzamide. The oximes of cyclic ketones give ring enlargement and form the lactam,as in the formation of caprolactam (83) from the oxime of cyclohexanone. Heating an... 

A similar isomerization preceding or following the polymerization has been observed with methylene-bis-caprolactam. Due to the preferential formation of stable five- and six-membered rings, the polymerization of... 

Although these earlier definitions were based on the chain structure of the polymers, they were closely related, as just described, to the mode of formation as well. It soon became apparent that such a classification has serious shortcomings, as so-called polycondensates could result from addition polymerization reactions. For example, although Nylon 6 can be prepared by the polycondensation reaction of e-aminocaproic acid (Braun et al., 1984), it is now synthesized by the ring-opening addition polymerization of e-caprolactam (Sandler and Karo, 1992), and this process has a profound effect on the... 

A wide variety of difunctional molecules have been converted into polymers by the step-growth process. It is also possible to prepare polymers from a single difunctional molecule if the two functional groups are reactive toward one another. The preeminent example of such a case is the formation of Nylon 6 from caprolactam by ring-opening and subsequent linear polymerization of the resulting amino acid ... 

Strain-free, small rings, such as caprolactam or caprolactone, have a high rate constant, kj, for their formation. On equilibration with the chain reaction, they lead to a fixed ratio of rings to chains, R, as is shown at the bottom of Fig. 3.13 ... 

Fradet and co-workers reported on the thermal ROP of y-carboxyethyl- s-caprolactam and y-aminoethyl- s-caprolac-tam (compare Scheme 7). Both monomers were polymerized in bulk at 250 °C. In both cases, the authors observed that monomer conversion was limited and did not exceed a plateau value of 0.53 (after a reaction time of 3 h) or 0.57 (after 30 min) for y-carboxyethyl- s-caprolactam and y-aminoethyl- s-caprolactam, respectively. The limiting monomer conversion was ascribed to ring-chain equilibria in both cases. The polymerizations could be accelerated by the addition of polyamidation catalysts, such as phosphorous and hypo-phosphoric acids, but no change of the maximum monomer conversion was observed. In a control experiment, 4-aminoethyl-1,7-heptanedioic acid was polymerized via thermal polymerization however, this only resulted in a low molecular mass compound. This was attributed to the much faster rate of the intra- versus the intermolecular amidation reaction. Cross-linked material was obtained, when both monomers were heated for a prolonged time, and loss of NH3 was observed, which was ascribed to amidine formation and deamination. 

A third route to polymers includes ring opening reactions as in the formation of nylon-6 from -caprolactam (Scheme 3). 

In situ polymerization was very useful as well for the fabrication of polyamide/CNT composites. For example, polyamide-6/MWNT composites have been successfully prepared by in situ hydrolytic polymerization of e-caprolactam in the presence of pristine and carboxylated MWNTs. e-Caprolactam monomer was found to form an electron transfer complex with MWNTs, yielding a uniform, polymerizable master solution, which was very beneficial for the formation of composites with well-dispersed MWNTs. In 2005, Gao and co-workers reported a new and improved chemical-processing technology that enables the continuous spinning of SWNT/ polyamide-6 fibers via the in situ ring-opening polymerization of caprolactam in the presence of SWNTs. In addition, this process resulted in an interesting hybrid material with characteristics of both the fiber and the matrix, and with excellent compatibility between the SWNTs and nylon 6. ... 

---