Ring Opening Polymerization and the Formation of Nylon 6

Ring Opening Polymerization and the Formation of Nylon 6

Ring Opening Polymerization And the Formation of Nylon 6

THEORY:

The ring-opening polymerization of caprolactam is a commercial process. The nylon produced, Nylon 6, is used extensively in automobile tire cords and for gears and bearings in small mechanical devices.

The catalyst used in this reaction is sodium hydride; therefore, this is referred to as an anionic polymerization. The sodium hydride initiates the reaction by removing the weakly acidic lactam proton from caprolactam to form a N anion (a good nucleophile). The anionic N subsequently bonds to the weakly electrophilic carbonyl-C on another monomer molecule. In order to accommodate the electron pair donated by the N anion, the second caprolactam molecule undergoes ring scission. Its carbonyl-C to N bond breaks heterolytically producing an NH anion, which propagates the chain.

Ordinarily anionic polymerizations must be run in the absence of oxygen, but the addition of polyethylene glycol serves to complex with the sodium ion (just as 18-crown-6 ether does) and enhances the catalytic activity of sodium hydride.

PROCEDURE:
Do the heating in a fume hood. Wear safety glasses and avoid contact with the reagents.

Caprolactam is toxic, a sensitizer, an irritant and a neurologic hazard.

Sodium hydride is strongly basic, i.e., corrosive, and is explosive in water.

  1. Clamp a test tube above a Bunsen burner in a fume hood. Angle the test tube toward the back of the fume hood.
  2. In the test tube place 4 g of caprolactam and 0.25 g polyethylene glycol Heat the mixture with a small, cool flame and as soon as it has melted remove the flame and add 50 mg of gray (not white) sodium hydride (50% dispersion in mineral oil).
    Close the container of NaH immediately after dispensing since it is moisture sensitive.
  3. Mix the catalyst with the reactants by stirring with a Pasteur pipette (or swirling the test tube) and heat the mixture rapidly to boiling (200° to 230°C) over a 2 minute period. Polymerization takes place rapidly, as indicated by an increase in viscosity.
  4. If polymerization has not occurred within 5 minutes remove the tube from the heat, cool it somewhat, add another 50 mg of sodium hydride, mix and reheat.
  5. When the solution is so viscous that it will barely flow, insert a wood stick or Pasteur pipet and draw fibers from the melt

QUESTIONS:

  1. Identify the polymerization type as one of the following: chain growth, step growth (condensation or rearrangement) or ring scission.
  2. Identify the polymerization technique as one of the following: bulk, solution, suspension or emulsion.
  3. Identify the polymer type as either thermoplastic or thermoset.
  4. Identify the end use of the polymer as one of the following: rubber (elastomer), plastic (resin), fiber, coating, adhesive (sealant), film, composite, cellular or biopolymer.

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BB/CH605/caprolactamexp.doc