Proton lactams

The ring torsional angles of the amide and O-protonated amide moieties in these structures are close to zero and the C(O)—N bond length is shorter in the protonated lactam (445) (1.298 A) than it is in the lactam itself (444) (1.337 A), as expected from the dominant resonance structures in these two compounds. The room temperature 15N and 13C NMR spectra of the lactam (444) and its O-trimethylsilyl derivative have been determined but do not give much information about the solution conformation of these compounds (76JA5082, 82JMR(46)163>. 

One of the few systems for which the initiation with protonic acid has been comprehensively studied is the cationic polymerization of lactams Initiation of the cationic polymerization of lactams with HCl gives predominantly a nwnomer molecule protonated at the oxygen atom but a small amount of N-protonated lactam is assumed to be present in the tautomeric equilibrium ... 

The protonated primary amino group is in equilibrium with the protonated lactam ... 

These reactions are similar to those describing the initiation and polymerization reactions in the anionic polymerization. It follows from this scheme, that only reactions involving the protonized lactam molecule, (90) and (99), can contribute to chain growth. Reaction (98) results in the incorporation of one monomer unit, only when followed by aminolysis (93b) at the cyclic carbonyl of the acyllactam. 

In polymerizations initiated with salts of tertiary amines, the growth centres (i.e. primary amine groups and acyllactam structures) must be formed first, before propagation can start. Due to the low concentration of protonated lactam, the rate of polymerization is much lower than in polymerizations initiated with salts of lactams or primary and secondary amines (Fig. 21). 

Hydrolysis of the enantholactam in 0.5 5% H2SO4 at 59.5-94°C indicated that two mechanisms are operating (i) via complexation of the lactam with HsOz" ", and/or (ii) via a complex of the protonated lactam with a water molecule <83izv777>. 

Acylation of these amine groups by molecules of other protonated lactams results in the... 

The above reaction results in the destruction of the equilibrium and a regeneration of the strongly acidic amide salt. Total lactam consumption results from repetitions of the above sequences and formations of new aminoacyllactam molecules. Initiations of polymerizations with acid salts of primary and secondary amines result in chain growths that proceed predominantly through additions of protonated lactams to the amines ... 

The rate at which the initiating amines are incorporated is proportional to the basicity. As the conversion progresses, the concentration of protonated lactams in the reaction mixture decreases while that of the protonated polymer amide groups increases. The latter takes part in the initiation reactions with lactam molecules and in exchange reactions with polymer molecules ... 

Acylation of these amine groups by molecules of other protonated lactams results in the monomers becoming incorporated into the polymers [117]. The growth centers are preserved and a molecule of lactam is protonated. This occurs in two steps [117] ... 

For example, the 10-membered 6-pelargonolactam in solution at 25 °C consists mainly of the tram isomer with only 5-10% of the cis isomer, while in crystals, it exists as a mixture of cis and tram isomers. Conversely, in solutions of the 11-membered lactam, only the tram isomer is present. Interestingly, in protonated lactams, the amide configuration is sometimes different from that of the neutral lactams, as shown by the analysis of the hydrochlorides. Drastic is the effect of protonation on ri-caprylolactam, which changes its configuration from the nonplanar transoid form to a nearly planar cisoid form. 

Two potential sites of protonation exist in unsubstituted amides the nitrogen atom and the oxygen atom. Protonation occurs preferentially at the oxygen (structure 11), but a small amount of N-protonated lactam (structure 10) is present. 

Due to the lack of resonance stabilization, the nitrogen-protonated lactam has a high acylating capability and is the most active species capable of reaaing with a ladam molecule at the nitrogen atom. 

N-protonated amide. Fast acylation of the monomers with the protonated lactam results in the formation of aminoatyl laaam cations as ammoniirm salts [13] ... 

These reactions destroy amine groups, which are regenerated by the initiation equilibrium of disproportionation between lactam and protonated amide [13]. The sequence of the disproportionation reaction [13] and the bimolecular aminolysis [21] contributes to the total lactam consumption with the formation [25] of new aminoacyl lactam molecules (20). ° Hence, the incorporation of lactam units into the polymer chain proceeds by the reaction of a protonated lactam with amine groups [18] as well as by the sequence of reactions [13] and [21]. 

The free lactam cannot be acylated to give the (aminoacyl) lactam owing to the N-substituent. On the other hand, the anion of the initiating add (e.g.. Cl") can undergo acylation by the O-protonated lactam giving rise, for example, as described in reaction [31], to the acyl chloride of the amino acid ... 

Additionally, the O-protonated lactam may undergo a nucleophilic attack [34] by the terminal nitrogen of the amino acid (the donor), this reaction nonetheless being an order of magnitude slower than the condensation of the aminoacyl chlorides [34] ... 

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