Anionic polymerization of lactams

The anionic polymerizations of lactams are initiated by strong bases capable of forming lactam atiions  

Such bases can be alkali metals, metal hydrides, organometallic compounds, and metal amides. The initiation step of ring-opening amidation can be shown as follows  

In these reactions, the propagation centers are the cyclic amide linkages of the N-acylated terminal lactam rings. Acylation of the amide nitrogens have the effect of increasing the electron deficiencies of these groups. This in turn increases the reactivities of the cyclic amide carbonyls toward attacks by the nucleophilic lactam anions  

Very rapid proton exchange follows. This results in equilibrium between the lactam and the polymeric amide anions. 

The polymer amide anions can undergo acylation by acyllactam groups with accompanying ring opening or with formation of lactam anions. In the first instance, it is an alternate path of propagation with formation of imide groups  

The primary amine anions abstract protons very rapidly from other molecules of lactams to form 

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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. ... 

It is well known that the anionic polymerization of lactams in general is markedly facilitated by the addition of N-acetylated lactams60-62. The polymerization of 61 is also accelerated by the addition of its N-acetylated compound 65 (Table 8). 

Azolides used as promotors in the anionic polymerization of lactams,1231,1241 and as plasticizers for PVC 1251... 

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 ... 

Semicrystalline polyamide fine powders have been used as toughening agents for epoxy networks. The powders can be obtained by grinding granules, or directly by anionic polymerization of lactams, 6 or 12, in an organic solvent from which the formed semicrystalline polymer precipitates. Microporous powders with an average particle size in the range of 10 pm and a narrow particle-size distribution, are obtained. 

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. 

This statement has its limitations. Ionic polymerization in hydrocarbons is always kinetically complicated, it often starts only after the addition of a polar compound (co-initiator), and it is affected by the aggregation of initiating and propagating particles. In strongly polar media, activation of initiator by dissociation of acids and bases is easy. Such solvent is simultaneously a reactive transfer agent. Propagation usually does not occur, and only low molecular products are formed. Exceptions can, of course, be found. During anionic polymerization of lactams in DMF, the solvent only increases the amount of dissociated initiator [27]. 

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... 

Particularities of Anionic Polymerization of Lactams in Oi anic Media... 

The anionic polymerization of lactams has been explored in great depth by Wichterle and Sebenda and their co-workers at the Institute of Macromolecular Chemistry in Prague. (6-8)... 

The adaptation of the anionic polymerization of lactams to casting and reaction Injection molding of commercial products has undoubtedly proceeded at several companies. The progress presented here reports the activities at Monsanto Company since that Is the work with which the authors are most familiar. 

The initiation reaction yields an imide moiety, which constitutes a growth center for propagation reaction. Addition of certain imides such as acyl lactams as coinitiators essentially eliminates the initiation reaction and makes possible the polymerization at relatively low reaction temperatures. Mechanistic and kinetic aspects of the anionic polymerization of lactams have been treated quite extensively [16b]. The discussed subjects relate to the various equilibria governing the polymerization process. They comprise equilibria allied to monomer conversion, to the formation of cyclic oligomers, and to the effect of initiator concentrations. 

The acylation reactions shown above are much faster than the initiation reactions. As a result, there are induction periods in anionic polymerizations of lactams. In addition, steep increases in molecular weights take place at the beginning of the polymerizations. Bimolecular aminolyses may contribute to that, though their contributions to the total conversions are negligible. ... 

The Frunze etal. mechanism has much in common with the alkali lactamolytic mechanism of Champetier and Sekiguchi, except for the formation of the above-shown complex. Frunze et al. also believe that probably a single mechanism exists for the anionic polymerization of lactams that they describe as ion coordinative Tht contributions of various mechanisms via ion pairs or via free ions depend upon the nature of the alkali metal counterion and upon their capacity to coordinate with electron-donating compounds (activator and monomer). The growth of ion pairs may mainly be expected from a lithium counterion, while growth by free anions may be expected from potassium or cesium. 

A new group of catalysts, metal dialkoxyaluminum hydrides, for anionic polymerizations of lactams, were reported recently. A different anionic mechanism of polymerization apparently takes place. When -caprolactam is treated with sodium dialkoxyaluminum hydride, a sodium salt of 2 (dialkoxyaluminoxy)-l-azacycloheptane forms ... 

Discuss the anionic polymerization of lactams and compare that with the cationic one. 

Anionic polymerization of lactams offers the best approach to the preparation of polyamide containing block copolymers. Styrene-nylon 6 block copolymers were prepared by adding e-caprolactam to polystyrene macroanions terminated with bisphenol A bis(chlorofor-mate)(31). Yamashita prepared ABA block copolymers of styrene-a-pyrrolidone and styrene- -caprolactam by sequential addition to styrene macroanions( ). Similarly Stehlik and Sebenda prepared N-acrylamide containing block copolymers(33). Block copolymers of isoprene-pivalolactam have also been reported( . In these cases the lactam was added to "living" polyisoprene anions. 

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