Lactam anion

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. 

Polyamide macromonomers can be made by reaction of the terminal acyllactam function with an unsaturated nucleophile such as the anion derived fromp. vinylbenzyl amine 8I). The nucleophilicity of the latter is higher than that of the lactam anion, and the reaction is straightforward. 

The details of the anionic polymerization of nylon 6 have been extensively reviewed (1-8) and will only be discussed briefly as they affect the star-polymerization of nylon 6. Nylon 6 is polymerized anionically in a two-step process (Figure 1). The first step, creation of the activated species 3, is the slow step. The e-caprolactam monomer reacts in the presence of a strong base (such as sodium hydride) to form the caprolactam anion 2. This anion reacts with more caprolactam monomer to form 3. The reaction of this activated species with lactam anions occurs rapidly to form the nylon 6 polymer 4. 

The polymerization of lactams (cyclic amides) can be initiated by bases, acids, and water [Reimschuessel, 1977 Sebenda, 1976, 1978 Sekiguchi, 1984]. Initiation by water, referred to as hydrolytic polymerization, is the most often used method for industrial polymerization of lactams. Anionic initiation is also practiced, especially polymerization in molds to directly produce objects from monomer. Cationic initiation is not useful because the conversions and polymer molecular weights are considerably lower. 

Strong bases such as alkali metals, metal hydrides, metal amides, metal alkoxides, and organometallic compounds initiate the polymerization of a lactam by forming the lactam anion XXXIV [Hashimoto, 2000 Sebenda, 1989 Sekiguchi, 1984], for example, for e-caprolactam with a metal... 

The lactam anion reacts with monomer in the second step of the initiation process by a ring-opening transamidation to form the primary amine anion XXXV. Species XXXV, unlike... 

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. 

The MWD is usually broader than the most probable distribution as a result of branching, which occurs in the later stages of reaction. As the monomer and lactam anion concentrations decrease, there is an increasing tendency for the polymeric amide anion XLII to attack the... 

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. 

Nucleophilic attack by the amide anion can occur at either the exocyclic or endocyclic carbonyl. The former regenerates the lactamate anion, whereas the latter results in polymerization. Although the locus of nucleophilic attack has no major effects in a homopolymerization, it can exert considerable control over the copolymerizations and on copolymer structure. 

The anionic catalysts listed earlier react with lactam monomer to first form the salt, which in turn will dissociate to the active species, namely, the lactam anion. A strongly dissociating catalyst in low concentrations, therefore, is always preferable to weakly dissociating catalysts in higher concentrations. The catalytic activity of the various alkali metal and quaternary salts of a lactam generally follows the extent of their ionic dissociation that is controlled by the cation. Activity of a salt decreases with increasing size of the cation due to restricted mobility and decreased ionization potential. 

Being very strong bases, organometallic compounds are capable to start the polymerization (7, 12, 30, 51, 62). If organomagnesium compounds are used, the interaction of magnesium cations with lactam anions can probably reduce the dissociation of the salt analogous the basic character is certainly weakened in lactam complexes formed by addition of hydrated oxides of Ti, Zr, Hf, Th or Ce (48). 

The anionic polymerizability of 67 is much lower than that of 58. This difference is due not only to the lower ring-strain of 67 and but also to a greater steric hindrance that the lactam anion of 67 receives when it comes closer to the carbonyl... 

This adduct is presumably formed by the mechanism illustrated in Scheme 15 [74]. As in the conventional anionic polymerization of other lactams, 69 reacts with potassium pyrrolidonate to give a lactam anion 71. However, as soon as 71 is... 

Under substantially anhydrous conditions, the anionic polymerization of lactams possessing an unsubstituted amide group may be initiated by any strong base capable of forming the free lactam anion... 

The polymer amide anion is in equilibrium with monomeric lactam and lactam anions are regenerated... 

The net result of reactions (24) and (25) is the incorporation of one linear monomer unit into the polymer chain with regeneration of both the active end group and the lactam anion. 

The high speed of anionic polymerization is due to the fact that both reacting species are chemically activated and hence highly reactive the lactam anion represents an activated monomer with increased nucleo-philicity and the terminal N-acylated lactam unit represents an activated end group (growth centre) with increased acylating ability. 

Acylation of a lactam anion with diacylamine, i.e. the reverse of reaction (29),... 

Acylation of the monomer activates the latter towards the nucleophilic attack by a lactam anion. Hence, it is rational to use the term activator for lactam derivatives capable of growing (e.g., A-acyllactams) or such compounds which produce the latter much faster than the initiator alone (e.g., acid chloride). In most cases, the activator or its fragments become part of the polymer molecule. Anionic polymerizations in which an activator has been added are designated as activated polymerizations, whereas in non-activated polymerizations the growth centres are produced by the initiator, reaction (23). 

A great variety of substances is capable of acting as activators. The first group comprises iV-substituted lactams with polar substituents at the nitrogen. These iV-substituted lactams must be able to acylate lactam anions with opening of the lactam ring of the activator at a rate comparable to the rate of polymerization. The main representatives of this group are acyllactams [84, 89, 107—110] (V), iV-substituted [111] (VI) or iV,Af-disubstituted [112] carbamoyllactams (VII), JV-carboxylic acid esters of lactams [112] (VIII) and salts of lactam-N-carboxylie acids [107] (IX), viz. 

In the presence of a strong base, e.g., lactam anions, the imide groups containing at least one ct-hydrogen undergo a Claisen type condensation analogous to the condensation of esters or ketones [121, 125], viz. 

Substituted keto amides (X)—(XIV) having one a-hydrogen are much more acidic than polymer or lactam amide groups and they lower the concentration of lactam anions through the equilibria... 

However, it follows from the processes discussed so far that in some reactions growth centres are also regenerated. For example, decomposition of the anion of a substituted keto amide yields lactam anions as well as isocyanate which is an effective activator, reaction (35). Acyl-lactam growth centres are also generated from keto amide and isocyanate, scheme (45). At high temperatures, lactam anions are generated also from carbonate [130,145,146]... 

The pattern of side reactions in the polymerization of a-substituted lactams is slightly different. The keto timide structures (XV) formed in the Claisen type condensation have no acidic a-hydrogen so that the concentration of lactam anions cannot decrease so dramatically as with the acidic keto amides (X)—(XIV). Consequently, the position of the condensation equilibrium is shifted in favour of the initial components. Also the main products formed from af,a -disubstituted keto amides [136], scheme (52), are different from those of the monosubstituted keto amides, scheme (45). 

Since lactam anions are acylated by diacylamine groups much faster than with cyclic acyl groups of acyllactams [94], the extent of such branching should be unimportant as long as a few percent of monomer are present. It has been confirmed that the majority of imide groups in anionic polycaprolactam are present as acyllactam [123,150]. 

In the activated polymerization, the number of polymer molecules is primarily determined by the concentration of added activator molecules but additional growth centres are formed in the slow disproportionation reactions (23) and (26)—(28) as long as lactam anions are present. Simultaneously, side reactions of the growth centres start to interfere (Section 4.3), so that the number of polymer molecules is not equal to the number of added (or formed) growth centres. It has been established [134] that in the anionic polymerization of caprolactam, both activator (A) and initiator (I) are consumed in a ratio A[A]/A[I] = 2.5 already within the first minutes (at temperatures above 190°C). At lower ratios of initial concentrations, the surviving strong base increases the number of... 

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