Anions lactams

The anionic polymerization of lactams proceeds by a mechanism analogous to the activated monomer mechanism for anionic polymerization of acrylamide (Sec. 5-7b) and some cationic polymerizations of epoxides (Sec. 7-2b-3-b). The propagating center is the cyclic amide linkage of the IV-acyllactam. Monomer does not add to the propagating chain it is the monomer anion (lactam anion), often referred to as activated monomer, which adds to the propagating chain [Szwarc, 1965, 1966]. The propagation rate depends on the concentrations of lactam anion and W-acy I lactam, both of which are determined by the concentrations of lactam and base. 

For copolymerizations proceeding by the activated monomer mechanism (e.g., cyclic ethers, lactams, /V-carboxy-a-amino acid anhydrides), the actual monomers are the activated monomers. The concentrations of the two activated monomers (e.g., the lactam anions in anionic lactam copolymerization) may be different from the comonomer feed. Calculations of monomer reactivity ratios using the feed composition will then be incorrect. 

The interpretation of the mechanism of anionic lactam polymerization based on the conventional scheme (ionic active centre with approaching monomer) could not exhaustively explain all the observed effects. Agreement could only be obtained when the acido-basic properties of lactams and polyamides had been respected. The equilibrium... 

The most important features of the anionic lactam polymerization are that ... 

As a matter of fact, structures (XVI) and (XVII) represent only a minor fraction of polymer molecules. Due to the great number of irregular structures which may be incorporated into the polymer molecules (Section 4.3), a great variety of types of macromolecules can be present in anionic polymers [95]. The nature of irregular structures formed during anionic lactam polymerization is primarily determined by the type and concentration of catalytic species and temperature, as well as by ring size and substitution of the lactam. Only polymers of o ,a-disubstituted lactams are free of irregular structures, and should be composed only of macromolecules of type (XVI) and (XVII). 

Due to the relatively fast side reactions consuming both initiator and growth centres, the evaluation of the kinetics of anionic polymerization becomes very difficult. We are dealing with a system of varying concentration of both active species which, according to schemes (45), (51) and (52), can be not only consumed but also regenerated in the complicated set of side reactions. Hence, the key problem of the anionic lactam polymerization consists in the determination of the instantaneous concentrations of lactam anions and growth centres. 

In the anionic lactam polymerization it is always the lactam anion which is incorporated into the polymer molecules either in reaction (24)... 

A polymerization equilibrium between growing polymer chain and monomer is set up in hydrolytic and anionic lactam polymerizations. The monomer remaining in the polymer act as a plasticizer, and so is removed by extraction with hot water, for example. 

Looking at these reactions, a specularity between the anionic and the cationic mechanism is observable. Different from conventional ionic polymerizations, the growth center in cationic and anionic lactam polymerizations is always neutral, and it is the lactam monomer that carries the ionic charge. Such polymerizations follow what is sometimes referred to as activated monomer mechanism, since the reaaive species are lactam cations/anions, thus activated monomers. However, in order to avoid misunderstandings, it must be well dear that this definition has nothing to do with the term activated... 

The use of strong bases alone is a limiting factor in the anionic lactam polymerization since high polymerization temperatures and relatively slow reaction rates are necessarily implied side reactions are, therefore, unavoidable. Moreover, only the more reactive lactams, such as CL and ((-enantholactam, readily polymerize in nonactivated reaction conditions. The less reactive lactams, such as 2-pyrrolidone and 2-piperidone, are much harder to polymerize because the formation of the imide dimer is more difficult. These limitations can be overcome if the imide is generated by reaction of... 

Carbon dioxide too has been used as activator in anionic lactam polymerization. In particular, it has been successfully employed in the polymerization of 2-pyrrolidone by Roda et The introduction of gaseous CO2 into the... 

Aliphatic and aromatic esters show an activation effect in the anionic lactam polymerization through a fast acylation of the lactam anion (already at 150 °C) with the formation of the corresponding N-acyl lactam growth center and of an alkoxy anion as by-product [92], The reaction is followed by neutralization of the alkoxy anion [93] ... 

The "zip-reaction (U. Kramer, 1978, 1979) leads to giant macrocycles. Potassium 3- ami-nopropyl)amide = KAPA ( superbase ) in 1,3-diaminopropane is used to deprotonate amines. The amide anions are highly nucleophilic and may, for example, be used to transam-idate carboxylic amides. If N- 39-atnino-4,8,12,16,20,24,28,32,36-nonaazanonatriacontyl)do-decanolactam is treated with KAPA, the amino groups may be deprotonated and react with the macrocyclic lactam. The most probable reaction is the intramolecular formation of the six-membered ring intermediate indicated below. This intermediate opens spontaneously to produce the azalactam with seventeen atoms in the cycle. This reaction is repeated nine times in the presence of excess KAPA, and the 53-membered macrocycle is formed in reasonable yield. 

In anionic polymerization the reaction is initiated by a strong base, eg, a metal hydride, alkah metal alkoxide, organometaHic compounds, or hydroxides, to form a lactamate ... 

P-Lactam antibiotics exert their antibacterial effects via acylation of a serine residue at the active site of the bacterial transpeptidases. Critical to this mechanism of action is a reactive P-lactam ring having a proximate anionic charge that is necessary for positioning the ring within the substrate binding cleft (24). 

O-Alkylation of A-unsubstituted /3-lactams to give the corresponding 2-alkoxy-l- etines can be achieved by reaction of the azetidin-2-ones with hard electrophiles (trialkyloxonium tetrafluoroborates) followed by treatment with base (cf. Section 5.09.4.3.1) (67JHC619, 69LA(725)124). In contrast, reaction of the A-unsubstituted azetidin-2-ones (73) or their derived anions with a variety of softer electrophiles results in A-substitution, and some representative reactions are illustrated in Scheme 7. 

In general, electron-releasing groups (e.g. —NH2, —OH) diminish or prevent covalent hydration by decreasing the electron deficiency in the nucleus. This diminution becomes ineffective if a new kind of stabilizing resonance is facilitated by the substituent, e.g. the urea-type resonance and the 4-aminopyridine-type resonance in 2- and 6-hydroxypteridine, respectively. The reluctance of the anions of these substances to form hydrates is attributed to the stable benzenoid system, e.g. 42, in the anhydrous anion compared with the predominantly lactam form of the neutral species, e.g. 43. 

Nevertheless, the adjacent position of the amide and acetylenic groups was used in another type of heterocyclization. The nitrogen atom in the amide group is a weak nucleophile. Therefore, the N anion should be generated by potassium ethoxide. There are two possible variants of nucleophilic addition to the triple bond. Only one takes place, i.e., the formation of y-lactam. After 7 h of heating in EtOH in the presence of KOH, amide 72 isomerized into the known isoindoline 73 in 80% yield (Scheme 128). 

A criterion for the position of the extent of the mesomerism of type 9 is given by the bond order of the CO bond, a first approximation to W hich can be obtained from the infrared spectrum (v C=0). Unfortunately, relatively little is known of the infrared spectra of amide anions. How-ever, it can be assumed that the mesomeric relationships in the anions 9 can also be deduced from the infrared spectra of the free amides (4), although, of course, the absolute participation of the canonical forms a and b in structures 4 and 9 is different. If Table I is considered from this point of view, the intimate relationship betw-een the position of the amide band 1 (v C=0) and the orientation (0 or N) of methylation of lactams by diazomethane is unmistakeable. Thus the behavior of a lactam tow ard diazomethane can be deduced from the acidity (velocity of reaction) and the C=0 stretching frequency (orientation of methylation). Three major regions can be differentiated (1) 1620-1680 cm h 0-methylation (2) 1680-1720 cm i, O- and A -methylation, w ith kinetic dependence and (3) 1730-1800 em , A -methylation, The factual material in Table I is... 

When diazomethane is slowly added to excess lactam, the anions formed can interact with unreacted lactam by means of hydrogen bonds to form ion pairs similar to those formed by acetic acid-tri-ethylamine mixtures in nonpolar solvents. The methyldiazonium ion is then involved in an ion association wdth the mono-anion of a dimeric lactam which is naturally less reactive than a free lactam anion. The velocity of the Sn2 reaction, Eq. (7), is thus decreased. However, the decomposition velocity of the methyldiazonium ion, Eq. (6a), is constant and, hence, the S l character of the reaction is increased which favors 0-methylation. It is possible that this effect is also involved in kinetic dependence investigations have shown that with higher saccharin concentrations more 0-methylsaccharin is formed. 

Nylon 4 is produced hy ring opening 2-pyrrolidone. Anionic polymerization is used to polymerize the lactam. Cocatalysts are used to increase the yield of the polymer. Carhon dioxide is reported to he an excellent polymerization activator. 

Indeno[l,2-rf]azepin-4-ol and its 10-bromo derivative appear to be in the lactam form 20. In base solution, however, there is UV and HNMR spectroscopic evidence for the 14k fully conjugated anions, e.g. 21.57... 

At higher water concentrations (at higher pressure) (Fig. 3.24), the reaction is considerably more rapid. The prepolymer obtained is further polymerized in another reactor, which has a working pressure of 1 bar or less. The total reaction time of the prepolymerization can be considerably shortened by processing it in an autoclave.812 28 In a laboratory, PA-6 can be synthesized in several ways from m-aminocaprolic acid, from lactam and from lactam and water, and anionically. 

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