Introduction: The "Rope-Trick" Synthesis of Nylon
In this instructable, I'll be walking through the classic demonstration synthesis of Nylon 6,10. The content of this instructable is also covered in the following video:
Step 1: Materials / Chemicals
Many of the chemicals required for this reaction are toxic, and so the synthesis should only be performed with all safety precautions taken into consideration, the MSDS for every chemical reviewed and on-hand, and within a functioning fume hood.
There's a kit here that has some of the required chemicals in very small quantities, but enough for a single demonstrations:
- 4g of 1,6 diamino hexane (hexamethyl diamine)
- 10ml 4% solution of sebacoyl chloride (sebacoyl dichloride)
- Na2CO3-10H20 (Sodium carbonate, hydrated)
- 100 ml of hexane
- 100 ml of water
- Functioning fume hood
- MSDS sheets for all chemicals
- latex/nitrile gloves
- 300ml + beaker (or a size appropriate for the quantity of liquids used)
- Glass rod or similar
- Waste container
Step 2: Step-Growth Polymerization
Nylon is formed by alternate bonding of our two reactants, sebacoyl chloride and diamino hexane, to form long chained molecules that we call polymers. For this specific reaction, we call the reaction a step-growth process, because the amines only want to react with the oxyls, and vice-versa. So the reaction proceeds in steps where molecule "A" reacts with molecule "B" to form a dimer A-B. Our dimer can then react with either an A or a B, to form a trimer A-B-A or B-A-B. These can then react with other reactants or other dimers/trimers/etc to form progressively longer chains.
This type of polymerization is very sensitive to the initial ratio of monomers A and B. Too much of either results in many small molecules rather than the more desirable long chains. For example, too much of A would favor molecules like A-B-A, A-B-A-B-A, A-B-A-B-A-B-A, etc. That is, A-type molecules are much more likely to bond at any given time than B-types, and therefore all the Bs get gobbled up quickly, and B-terminated chains never have the time to interact with other chains to form longer molecules, before finding a single A-type molecule instead (see animated gif).
Step 3: The "Rope-Trick" (controlling the Reaction)
Because step-growth polymerization is sensitive to the initial quantities of reactants, we can't simply mix A and B into a solution and expect long molecules. Instead, we need a way to control the reaction such that long chains are favored over short chains.
The way we do this is by dictating the location of the reaction. We can do this by creating an interface between two immiscible solvents, one containing A, and the other B. The chlorine groups at the ends of the sebacoyl chloride molecule make it non-polar, and therefore able to dissolve in an organic solvent like hexane. The amine groups (NH2) on the diamino hexane molecule like to form hydrogen bonds, and so they are somewhat willing to dissolve in water. This solubility decreases with increasing number of carbon atoms.
Prepping the solutions
We can create our two liquids by adding 10ml of sebacoyl chloride to 90 ml of hexane and 4g of diaminohexane into 100ml of water. To the diamino hexane solution, we then add Na2CO3, the quantity of which depends on the predicted amount of HCl produced. We'll get to that.
To our beaker, we first add the diaminohexane solution, because water has a higher density than hexane. Carefully, we then add the sebacoyl hexane solution, which floats atop the water and does not mix. Immediately, a thin white film can be seen at the interface of the two liquids. This is Nylon!
The Rope Trick
As soon as the polymer barrier forms, no further reaction takes place as the two reactants are physically separated. Now for the rope trick. Using tongs or a glass rod, or pretty much anything poke-y, we can grab the polymer at the interface and pull it out of the solution. As we do so, the two liquids are allowed once again to meet, and start adding to the chains we've already created within the layer we're extracting. If we wrap the polymer around a glass rod and start turning, we can pull nylon out of our solutions indefinitely, until one or both of the reactants are depleted.
The rope-trick is a clever way to get away with egregious differences in the initial quantities of our two monomers, A and B. That's because at all times, there's only one place for a reaction to occur, at a 2-D interface, and thus, opportunities for early-termination by an abundance of either molecule are eliminated.
Step 4: About That Na2CO3 We Added...
We added a base, Na2CO3, to our diaminohexane solution for a specific reason. The type of polymer reaction we are performing is a condensation reaction -- so-called because a small molecule is released as a byproduct. In many polymer reactions, this small molecule is water (H2O). However, because we're using a chlorinated reactant (sebacoyl chloride), this small molecule is instead hydrochloric acid (HCl).
To encourage forward progress in the reaction, this small molecule needs to be removed (Le Chatelier's principle). That's where the sodium carbonate comes into play. As a base, it neutralized the hydrochloric acid, allowing for further bonding to take place.
The exact quantity of Na2CO3 required depends on how much rope-tricking you'll be doing. If we assume that we'll take the reaction to completion, then we'll deplete all 4g of diamino hexane, and in turn, produce as many HCl molecules as molecules of diamino hexane (and therefore sebacoyl chloride) reacted.
To get a lower bound, we just need to solve the stochiometric equation:
4g diamino hexane x 6.023 x 10^23 (molecules / mol) / 116 (g/mol)
Xg Na2CO3 x 6.023 x 10^23 (molecules / mol) / 105 (g/mol)
X = 105 * (4/116) = 3.6g
If you're using a hydrated form of Na2CO3 like Na2CO3-10H20, make sure you take those ten water molecules into account:
X = 285 * (4/116) = 9.8g
And that's it! Be safe. Don't do this without expert supervision. Dispose of all waste products appropriately.