Propagation reaction, polyamides

The most popular approach for the synthesis of polyamides from amino acids [224] is the amino acid V-carboxyanhydride (NCA) method (Fig. 31) [225]. The initiation with a base such as a tertiary amine is followed by the reaction of another NCA molecule with the resulting activated NCA anirm to produce a dimer with an electrophilic N-acyl NCA end group and a nucleophihc carbamate group. Then, N-carbamic acid is detached as carbrai dioxide during the propagation reaction. It is known that deprotonation of NCA and attack of the NCA anion are relatively fast... 

In stepwise polymerization, there is no initiation, propagation, or termination as is the case in chain reaction polymerization. The polymerization depends entirely on the individual reactions of the functional groups of monomers. The four types of stepwise polymerization are the synthesis of polyester, polyamide, polyurethane, and polycarbonate. 

P-lactams. Monomers having a low rate constant of propagation are preferable for the synthesis of uniform polyamides. Drawbacks in the preparation of uniform polyamides from P-lactams are the very frequent occurrence of depolymerization and transamidation (at the acyl lactam chain end and on the polyamide chain), both causing broadening of molar mass distribution of the resultant polyamide. The latter reaction occurs more frequentiy at high conversion. Therefore, it is advisable to stop the polymerization at low to intermediate... 

Evidently, if R is polymeric the product (29) will be a block copolymer, of AB or ABA type according to the functionality of the activator. For the reaction to proceed satisfactorily the ring = chain equilibrium between lactam and polyamide must be favourable for polymerization in the conditions employed, and there must be an absence of side reactions causing loss of propagative anionicity or transfer leading to the formation of free lactam homopolymer. Successful polymerizations have been achieved giving products with polyether, " polystyrene SBR, polydiene or polyisobutene " blocks, usually in combination with nylon 6 as the polyamide segment(s) or, in some cases, with nylon 4 or nylon 8. 

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