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During one of the methods of polymerization:

Nylon 6,6 is prepared from polycondensation of hexamethylene diamine (HMD) and adipic acid. The repeat unit of the polymer is $\ce{–[NH(CH2)6 NHCO(CH2)4CO]}$. The diamine, which melts at 40.87 °C, is normally used in the form of a concentrated aqueous solution. The dibasic acid is used in its pure solid form (m.p.= 152.1 °C). In the preparation of nylon 6,6 polymer, the first step is a preparation of salt formed from a mixture of precise stoichiometric quantities of the two monomers. The salt can be prepared by mixing the alcoholic solutions of the two components, the pure salt precipitates and is dissolved in water to give a solution. Alternatively, the salt can be prepared by mixing a dispersion of the diacid in water with a solution of the diamine. The aqueous salt solution is prepared so that it has a concentration of about 50-60%. For the preparation of nylon 6,6 polymer, the salt solution is subjected to evaporation at the boil, possibly at elevated pressures, until concentration ≥ 60% are achieved. The concentrated salt solution together with a small amount (0.5%) of a molecular weight stabilizer is then heated in a reactor under a blanket of nitrogen so that temperature increases gradually and pressure reaches, typically, 1.73 MPa (250 psi). As water evaporates and temperature increases from about 212°C to 275°C, the molecular weight of the polymer reaches about 4000. The further reaction is achieved by a gradual decrease in reactor pressure to atmospheric pressure and then holding the polymer under these conditions for about one hour. At this point, the polymer is not quite equilibrated but molecular weights are in the range of 12000 to 17000. The process is devised to remove all the liquid water present in the salt solution as well as almost all the potential water, which may get liberated on the reaction of carboxyl (-COOH) and amine end groups present in the polymer. High molecular weight nylon for industrial applications is obtained by conducting the final stages of melt polymerization under reduced pressure (to remove water) or by addition of chain coupling agents (to help improve stoichiometric balance). The finished polymer is then extruded in the form of a ribbon or strand, quenched with water and cut to form chips. After drying, the chips are sent for subsequent spinning operation. Alternatively, the polymer may be sent directly to a spinning machine without prior solidification. In nylon 6,6 the polymerization reaction goes almost to completion.

Now my question is in the above method of polymerization involves formation of aqueous nylon salt from pure salt. I want to know why the precipitated pure salt isn't converted to polymer but is made aqueous and then converted to polymer? Isn't there anyway of forming the polymer from pure nylon salt (fully crystal dehydrated salt) instead of the aqueous one from the reactants?

Note: I'm ok with reacting the pure salt with any chemical substance to form the polymer.

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  • $\begingroup$ From the preparation above: "The salt can be prepared by mixing the alcoholic solutions of the two components, the pure salt precipitates " Collect this precipitate and dry under vacuum without heating. That will give you the pure salt. $\endgroup$ – Waylander Mar 30 at 7:53
  • $\begingroup$ @Waylander oops! I had made a mistake in my question. I've corrected it now. My question was actually why the salt is made aqueous? $\endgroup$ – Somanna Mar 30 at 8:01
  • $\begingroup$ I presune it is so it can be pumped through the reactor to control the polymerisation and give the desired range of chain lengths. If you just heat the salt you'll have less mixing $\endgroup$ – Waylander Mar 30 at 8:11
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The problems with reacting pure nylon salt are as follows:

  1. The molecules of hexamethylene diamine (HMD) and adipic acid will have a very hard time accessing each other's reacting groups, because of limited mobility within the solid structures.
  2. The reaction is a condensation, so it will produce a substantial amount of water anyway, so pretty much any reason for keeping the reactants dry is irrelevant, because the final polymer will end up wet anyway.

It might be possible, but because of (1), your product might be impure, containing a lot of small unreacted adipic acid and HMD particles.

Also, If you don't want to deal with the tedious procedures above, considering that they were probably written for industrial applications, consider using this procedure. It's a lot more lab friendly.

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