Protonated lactams, polymerization

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 ... 

Taking into account the equOibriunuiature of lactam polymerization, there will be always enough monomer avaUable for protonation. However, the proportion of protonated monomer will decrease with conversion. 

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). 

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 ... 

Such compound differs in nucleophilicity from activated monomers. These salts are products of deprotonation of lactam monomers at the amides followed by reduction of the carbonyl functions. It is postulated that during lactam polymerizations, after each monomer addition, the active species form again in two steps [143]. In the first one proton exchanges take place ... 

Therefore, all initiators used for lactam polymerization so far may be divided into two types 1. strong bases which are capable of removing the eunlde proton to form a lactam anion and thus can start an anionic polymerization (J ), and 2. compounds with active hydrogen which can protonate the amide bond so that cationic polymerizations become possible (2). An excellent... 

Carbanions stabilized by phosphorus and acyl substituents have also been frequently used in sophisticated cyclization reactions under mild reaction conditions. Perhaps the most spectacular case is the formation of an ylide from the >S-lactam given below using polymeric Hflnig base (diisopropylaminomethylated polystyrene) for removal of protons. The phosphorus ylide in hot toluene then underwent an intramolecular Wlttig reaction with an acetyl-thio group to yield the extremely acid-sensitive penicillin analogue (a penem I. Ernest, 1979). 

An interesting bifunctional system with a combination of In(OTf)3 and benzoyl-quinine 65 was developed in p-lactam formation reaction from ketenes and an imino ester by Lectka [Eq. (13.40)]. High diastrereo- and enantioselectivity as well as high chemical yield were produced with the bifunctional catalysis. In the absence of the Lewis acid, polymerization of the acid chloride and imino ester occurred, and product yield was moderate. It was proposed that quinine activates ketenes (generated from acyl chloride in the presence of proton sponge) as a nucleophile to generate an enolate, while indium activates the imino ester, which favors the desired addition reaction (66) ... 

A variety of protonic and Lewis acids initiate the cationic polymerization of lactams [Bertalan et al., 1988a,b Kubisa, 1996 Kubisa and Penczek, 1999 Puffr and Sebenda, 1986 Sebenda, 1988]. The reaction follows the mechanism of acid-catalyzed nucleophilic substitution reactions of amides. More specibcally, polymerization follows an activated monomer mechanism. Initiation occurs by nucleophilic attack of monomer on protonated (activated) monomer (XXIV) to form an ammonium salt (XXV) that subsequently undergoes proton exchange with monomer to yield XXVI and protonated monomer. The conversion of XXIV to XXV involves several steps—attachment of nitrogen to C+, proton transfer from... 

Anionic polymerization of 58 activated by N-benzoyl lactam proceeds without side reactions. Since side reactions in the anionic polymerization of lactams are mainly caused by protonabstraction, the pKa value for the bridge-head methine proton adjacent to the lactam-carbonyl group in 58 must be higher than that for the a-methylene protons in 2-pyrrolidone. This is because the former monomer has a rigid bicyclic structure. 

In his review, Reimschuessel described [353] isomerization polymerizations of lactams containing a carboxyl group as a substituent or part of a substituent. Whereas normal lactam ring opening leads to polyamides, isomerization polymerization produces polyimides in a complicated way, with proton transfer and through bicyclic intermediates... 

In the cationic polymerization of lactams, initiated with protonic acids, macromolecules are formed having two ends (A and B in the structure below) potentially active (cf. Sect. 3.1) ... 

Termination involving intramolecular proton shift or intermolecular proton transfer, both followed by elimination of a molecule of water has been reported for the high temperature cationic polymerization of lactams a) intramolecular ... 

The catalyst used in this reaction is sodium hydride therefore, this is referred to as an anionic polymerization. The sodium hydride removes the acidic lactam proton to form an anion (1) that attacks the coinitiator, acetylcaprolactam (2), which has an electron-attracting acetyl group at-... 

At the very beginning of polymerization, the linear monomer units are incorporated into the polymer in reaction (24) which is followed by the fast proton exchange (25) regenerating lactam anions. The proton exchange between pyrrolidone and its anion... 

This reaction is similar to the disproportionation (23) occurring in the anionic polymerization except that the nucleophiles are activated in the latter case. The protonated primary amine group is in equilibrium with the lactam, viz. 

Due to its high speed, the cationic initiation reaction contributes to the over-all lactam consumption, in contrast to the anionic polymerization in which the disproportionation is usually very slow as compared with the fast propagation reactions involving acyllactam growth centres. Neutral and protonated polymer amide groups can also take part in the dispro-... 

In the polymerization initiated with strong acids under anhydrous conditions the major part of monomer is incorporated into the polymer via highly reactive intermediates [243]. In the presence of strong acids, protonation of -substituted lactams can occur both at the oxygen and nitrogen (see Section 3). Similarly as with the unsubstituted lactams (Section 5.1), the iV-protonated form (XXXI) resulting from the equilibrium... 

The polymerization occurs by the activated monomer mechanism, which supposes a two-step mechanism involving the acylation of the lactam anion (NaL) by the AT-acyllactam end-group followed by a fast proton-exchange with the monomer. In bulk polymerization, the preformed AT-acyllactams or their precursors (so-called CIs) are introduced into the system in order to avoid the slow initiation step due to the absence of AT-acyllactam groups at the beginning of polymerization ... 

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