Polyamides acidic proton reaction

Proposed mechanisms for polycondensations are essentially the same as those proposed in the organic chemistry of smaller molecules. Here, we will briefly consider several examples to illustrate this similarity between reaction mechanisms for small molecules and those forming polymers. For instance, the synthesis of polyamides (nylons) is envisioned as a simple Sn2 type Lewis acid-base reaction, with the Lewis base nucleophilic amine attacking the electron-poor, electrophilic carbonyl site followed by loss of a proton. 

Compounds for which the Bronsted acidity is high enough to protonate oxazolines and for which the corresponding anion is a suitable nucleophile to attack the C5 carbon can couple via a ring-opening reaction. Carboxylic acids fulfill these requirements and thus polyester or polyamide acid end groups will react with bisoxazolines or bisoxazines (Scheme 6). 

Nylon is the common name for polyamides. Polyamides are generally made from reactions of diacids with diamines. The most common polyamide is called nylon 6,6 because it is made by reaction of a six-carbon diacid (adipic acid) with a six-carbon diamine. The six-carbon diamine, systematically named hexane-1,6-diamine, is commonly called hexamethylene diamine. When adipic acid is mixed with hexamethylene diamine, a proton-transfer reaction gives a white solid called nylon salt. When nylon salt is heated to 250 °C, water is driven off as a gas, and molten nylon results. Molten nylon is cast into a solid shape or extruded through a spinneret to produce a fiber. 

Lactams may be converted to the corresponding nylons by water-initiated polymerization. The reaction may also be initiated by a base that requires anhydrous conditions and proceeds at high reaction rates [9,10]. Initiators used include carbonates, hydrides, alcoholates, and hydroxides of alkali and alkaline earth metals. The conversion of lactams to polyamides is also possible by a cationic process, entailing initiation in an anhydrous medium by either strong protonic acids or their dissociable salts [11]. 

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