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I was reading about carbon polymers and noticed something interesting about the difference between the reaction used to form nylon and the reaction used to form aramids: in the former, the monomers are an amine and a carboxylic acid, while in the later the monomers are an aromatic amine and an aromatic acyl halide.

Aramid formation Formation of an aramid

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Adipic acid, used to create Nylon 66

This struck me as interesting because it seems like the reaction should be roughly the same for the creation of either polymer, with the only difference being the presence of the aromatic ring in the aramid in place of a simple carbon chain. Is the halide required for polymerization in the aramid, but not the nylon, or is there another, perhaps economic, reason for its use? Why are halides used in the creation of aramids, but not in simpler nylons?

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Aromatic amines are less nucleophilic than aliphatic amines because the aromatic amines have the non-bonding electron pair "distracted" in the resonance with the aromatic ring. Therefore, they need a more reactive counterpart (acyl halide instead of the acid).

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Good thinking - to make nylon, you would first need to convert the carboxyl groups on adipic acid into an acylating agent such as the acid chloride. The standard "nylon rope trick" demo uses adipoyl chloride. See for example: https://depts.washington.edu/chem/facilserv/lecturedemo/NylonRopeTrick-UWDept.ofChemistry.html

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    $\begingroup$ You don't need to convert nylon monomers to acid chlorides. The major manufacturing route works fine with acids and amines. $\endgroup$ – matt_black Jun 2 '16 at 14:58

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