Lactams polymerization, activated monomer

Anionic polymerization of lactams was shown to proceed according to what is called the activated monomer mechanism. With bischloroformates of hydroxy-terminated poly(tetramethyleneglycol) and poly(styrene glycol) as precursors for a polymeric initiator containing N-acyl lactam ends, block copolymers with n-pyrrol-idone and e-caprolactam were obtained by bulk polymerizations in vacuum at 30 and 80 °C, respectively361. ... 

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... 

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. 

Polymerizations initiated by strong bases (R-, IIO, RO-) and tertiary amines (which are poor nucleophiles) proceed at much faster rates than do polymerizations initiated hy primary amines. Also, unlike the latter, where each polymer chain contains one initiator fragment (i.e., RNH—), these polymerizations do not result in incorporation of the initiator into the polymer chain. Polymerization proceeds by an activated monomer mechanism similar to that in the anionic polymerization of lactams. The reacting monomer is the NCA anion XLIV... 

What are the roles of an acylating agent and activated monomer in the anionic polymerization of lactams ... 

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... 

Lactam polymerization with anionically activated monomer has its counterpart in the cationic processes of lactam polymerization. This type of mechanism has also been observed recently in some polymerizations of oxygen-containing heterocycles (see Chap. 4, Sect. 2.3)... 

So far, not many systems are known to meet the conditions of the growth mechanism with activated monomer. Polymerizations of oxirane, chlo-romethyloxirane [145, 146] and of lactams [147] belong to this class. The existence of an electroneutral propagating macromolecule and of an activated monomer results in reduced probability of back-biting reactions and in easier preparation of macromers [145] by means of exchange reactions. 

Polymerization of oxirane (and of its derivatives) by the mechanism of activated monomer is so far exclusively cationic and can be represented by schemes (27) and (28) of Chap. 4. In contrast to the ring-opening polymerization of lactams, both the classical and the activated monomer mechanisms are operating in this case. Conditions can be found where one or the other mechanism predominates [339]. 

There are essentially two mechanisms of chain growth in the cationic polymerization of lactams. Either the cationically activated monomer reacts with neutral growth center or the neutral monomer molecule reacts with cationic active centers located at the end of the growing chain [214) ... 

In the polymerization of unsubstituted lactams, propagation proceeds mainly by the activated monomer mechanism ... 

The polymerization of any lactam starts with the ring opening of the monomer (or activated monomer) by the initiator... 

In homogenous media, most of the transacylation reactions are reversible and as soon as the first polymer amide groups are formed, the same kind of reactions can occur both at the monomer and at the polymer amide groups. Unless the active species are steadily formed or consumed by some side reaction, a set of thermodynamically controlled equilibria is established between monomer, cyclic as well as linear oligomers and polydisperse linear polymer. The existence of these equilibria is a characteristic feature of lactam polymerizations and has to be taken into account in any kinetic treatment of the polymerization and analysis of polymerization products. The equilibrium fraction of each component depends on the size of the lactam ring, substitution and dilution, as well as on temperature and catalyst concentration. 

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. 

This type of lactam polymerization is initiated under anhydrous conditions with acids or acid salts which do not split off water at the polymerization temperature (e.g., lactam or amine hydrochloride) as well as with some Lewis acids [176, 177]. The activated species is the monomer cation which takes part both in the initiation and propagation reactions. 

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 ... 

Anionic polymerization. For some heterocyclic monomers the unique chemical structure of the growing species follows unequivocally from the monomer structure. However, in many cases isomeric structures have to be taken into account. For instance, for symmetrical monomers, like thietane, the carbanion but not the thiolate anion was proposed (4). Unsymmetrically substituted monomers can provide active species by a- or B- ring scission. Unusual structure of activated monomer was proposed for NCA and lactams. These structures can not be distinguished by spectrophotometric methods, and application of H- or 13C-NMR looks more promising. 

The rate of assisted lactam polymerization is dependent on the concentration of base and JV-acyllactam, which determine the concentrations of activated monomer and propagating chains, respectively. The degree of polymerization increases with conversion and with increasing concentration of monomer or decreasing A -acyllactam concentration. These characteristics are qualitatively similar to those of living polymerizations, but lactam polymerizations seldom are living [15,16]. 

The mechanism of the cationic polymerization of lactams formally resembles the activated monomer mechanism, better known for the anionic process. In this media-... 

Polymerizations in the presence of an acylating agent are often referred to as assisted (or activated) polymerizations. The rate of lactam polymerization depends on the concentrations of the activated monomer and propagating chains, which, for an assisted lactam polymerization, are determined by the concentrations of base and iV-acyllactam (or acylatig agent), respectively. The degree of... 

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