Induction period lactams

The acceleration at the start of the polymerization (Fig. 8, curve 1) represents an induction period in which certain active species are formed gradually. It has been established that the active chain growth centre is represented by an A/-acylated lactam [44, 83, 85—94] which is formed in the slow disproportionation reaction... 

The polymerization of the carboxy-lactam (XXXIII) proceeded with a distinct induction period [247] (tj) which was taken into account in the evaluation of the conversion data. The monomer is consumed in the isomerization reaction (152) with the formation of the amino anhydride (S) the rate being... 

Much faster reaction can be achieved with strong bases (/07). The chain is started by an N-acylimide which may be N-caproyl-caprolactam produced in a slow reaction from the monomer during an induction period or an N-acylamid produced by action of a cocatalyst like for example carboxylic acid chlorides or anhydrides on caprolactam. The cocatalyst action speeds up the reaction such, that fast polymerization below the melting point of the polymer becomes possible. The strong base, such as alkali metal, metal hydride, metal amid, or oiganometallic compound, activates the monomer by lactam anion formation ... 

The imide dimer (VIII) has been isolated and is the actual initiating species for the onset of polymerization [14]. The slowness of reaction in Eq. (10.72) accounts for the occurrence of an initial induction period of low reaction rate in lactam polymerization. The imide dimer is necessary for polymerization because the amide linkage in the lactam is not... 

The use of strong base alone for aninic polymerization of lactam is limiting. As noted previously, the polymerization is characterized by induction periods and, moreover, only the more reactive lactams, such as e-caprolactam and 7-heptanolactam ( -enantholactam), readily undergo polymerization, while the less reactive lactams, 2-pyrrolidinone, and 2-piperidinone, are... 

When weak carboxylic acids or acids of medium strength initiate lactam polymerizations at anhydrous conditions, there is an induction period.In addition, the rates of these reactions are proportional to the of the acids.It appears that different reaction mechanisms are involved, depending upon the acid strengths. The nucleophiles are present in equilibrium ... 

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 anionic polymerization of lactams is catalyzed by strong bases capable of forming lactam anions. In the presence of a suitable initiator such as an acyl lactam, a very fast reaction occurs at temperatures in the range of 100 C to 200°C. Without the initiator, polymerization temperatures appreciably above 200°C are necessary and a noticeable induction period is present. Anhydrous conditions are an essential pre-requisite for good yields and rates and reproductible results. The most relevant reactions in the activated polymerization of lactams are the following ones ... 

The nature and concentration of the initiator play a cmcial role in the nonactivated anionic polymerization, where the growing centers are formed in the slow reaction [46] between the monomer and the lactam anion bringing about the presence of some induction periods. On the other hand, the evaluation of the specific action of a given initiator in the activated lactam polymerization is more complex, since it cannot be taken in consideration apart from the activator used. It is necessary to consider here the dual system initiator/activator. It seems that the activation energy for the anionic ring-opening polymerization of CL is almost independent of the activator used, " whilst it is probably a fimction of the initiator nature. 

Induction periods are often observed in the polymerization of lactams whilst the concentration of (III) increases. These can be eliminated by inclusion of acylating agents (e.g. acid halides and anhydrides) that react rapidly with the lactam to form N-acyllactams which then propagate... 

System V was used in a CH3CN/THF mixture for the rapid and regioselective construction of benzolactones and lactams 21 from alkynyl alcohols or alkynylamides 19 and propiolates 20 (Scheme 1.6) [12]. An induction period of 1 h at room temperature is necessary to form the active species. The reaction mixture is then heated for 12 h at 80 °C. It is supposed that the [2 - - 2 - - 2] cycloaddition step precedes the transesterification. Remarkably, no pyridines form in the presence of CH3CN. 

During the nonactivated polymerization, the nonionic growth centers are formed continuously through a slow reaction between the lactam and its anion (see Equation 7.2), and their concentration is at least one order of magnitude lower than the initial concentration of the salt. The process is therefore characterized by a distinct induction period and a markedly lower rate than the activated polymerization. The key component here is the initiator, which has a crucial effect on the course of the polymerization process. On the other hand, it is an excellent kinetic tool for assessing the quality of the initiation, the purity of the system and the monomer, and the suitability of the experimental technique applied, and so on. 

In a continuation of his studies on asymmetric P-lactam synthesis, Evans [42] utilized a,P-epoxyaldehydes 49a and 49b, prepared in two steps from achiral allylic alcohols via Sharpless asymmetric epoxidation and Swern oxidation, as chiral glyoxal synthons for the ketene-imine cycloaddition. Diastereosel-ection was excellent, ranging from 90 10 to 97 3 with overall yield of 50 up to 84% (for Schiff base formation and cycloaddition) after recrystallization or chromatographic purification of the major diastereomer. The sense of asymmetric induction correlated with that obtained in the analogous glyceraldehyde reaction, as established by periodic acid cleavage to aldehydes 51. 

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