06. Polymer Synthesis

Figure 1: Examples of natural and synthetic polymers

II. Nylon 6.6

Nylon is a synthetic polymer which is synthesized by joining lots of monomer units together of two distinct molecules: hexamethylenediamine and adipoyl chloride, as can be seen in Figure 2 below:

Figure 2: Synthesis of nylon 6.6

Nylon’s most important application is in the production of fibers for clothing, rope, and carpets. Nylon behaves as a thermoplastic and can be melted and molded to fit any desired shape. It’s used to make pipes, zippers, and wire insulation. In this lab, you will be synthesizing nylon 6.6 in the form of a nylon rope. Your synthesis would be the first step in many for a factory producing nylon commercially. The factory would continue to heat and stretch your nylon rope until they obtained a very fine thread of nylon that could be used to produce a multitude of different clothes.

In the synthesis of nylon 6.6, your two starting materials, hexamethylenediamine and adipoyl chloride, experience changes to specific bonds as they combine to create nylon 6.6. These reaction sites are commonly referred to as functional groups. Each functional group exhibits unique chemical properties. Hexamethylenediamine is composed of two primary amines (RNH₂) (R indicates a carbon-containing group) and six alkane groups (-CH₂-). When hexamethylenediamine reacts with adipoyl chloride (composed of two acid chlorides (RCOCl) and a similar 6 carbon alkane chain), the formation of a new functional group, an amide (R-NH-CO-), helps link these two molecules together. This new dimer can continue to react with other amines and acid chlorides to produce a longer and longer chain of amide linked molecules until the one or both reactants are completely used up. The amide linkage in nylon is exactly the same type of functional group created when amino acids link together to produce proteins.

Not only is the synthesis of nylon 6.6 a great demonstration of our ability to alter functional groups and obtain a new compound with new physical and chemical properties, but the molecules involved exhibit some unique chemical bonds that can be observed using infrared (IR) spectroscopy. Every functional group exhibits very distinctive IR absorptions. The IR absorptions can help us distinguish one type of bond from another. Experimentally, you will be obtaining an infrared spectrum for your product (nylon 6.6) to help confirm that you synthesized nylon 6.6 and as mentioned earlier you will be able to identify an unknown plastic with infrared spectroscopy.

Throughout this lab, we hope to investigate the bonding characteristics of our molecules in the synthesis of nylon 6.6 and other plastics used in the world around us. We will use IR spectroscopy to analyze our product and compare the various models we use to represent the bonding taking place in these molecules.

III. Formation and Structure of Polymers

The term polymer is derived from Greek words meaning “many parts.” The repeating units in a polymer are called monomers. Monomers can be single atoms but are usually groups of atoms such as the –CH₂— units in polyethylene or the amino acids in a protein. The total number of repeating units in a polymer chain is called the degree of polymerization or DP. DP can vary from a few thousand to several hundred thousand, with structural properties such as strength and flexibility correlating with higher DP and therefore higher molecular weight. Most synthetic polymers have a distribution of molecular weights.

Polymers can form from a single monomer or from two or more monomers. Homopolymers are polymers comprised of a single monomer. Polyethylene, polystyrene, and polylactic acid are homopolymers. Copolymers are formed from two or more monomers. Copolymers can be random, alternating, or block, depending on the distribution of the two monomers within the polymer chains. The polymer that you are synthesizing today, nylon 6.6, is an alternating copolymer of hexamethylene diamine and adipoyl chloride. Polyethylene terephalate is an alternating copolymer of ethylene and terephathic acid. Polyvinylalcohol-co-acetate is a random copolymer of vinyl alcohol and vinyl acetate. The structures of some example polymers with their monomer units are shown in Table 1, below.

In yet another way of classifying polymers, it is apparent that some polymers are formed from the opening of a double bond, and are called addition polymers, while in other polymers the monomers join to form a new bond with the elimination of a small molecule, and are called condensation polymers.

Note that only the vinyl acetate monomer is shown for polyvinyl alcohol-co-acetate. This polymer is made from polyvinyl acetate; then, the polymer undergoes a chemical reaction to turn some of the acetate groups to alcohol groups, creating a random copolymer.

Table 1: Structures of example polymers

Question for thought:

Which of the polymers in Table 1 are addition polymers? Which are condensation polymers?

IV. Infrared (IR) Spectroscopy

Infrared light is absorbed by most chemical compounds when there is a change in the dipole moment of the molecules making up the compound. Whereas absorption of visible light usually results in the excitation of electrons, absorption of infrared light usually results in vibrations of bonds. Infrared Spectroscopy is a chemical identification technique in which infrared (IR) light is passed through a sample and a spectrophotometer identifies which frequencies of light have been absorbed by the sample. Different types of bonds absorb different frequencies of IR light, so analysis of an IR spectrum can tell you what types of bonds are present in your sample.

Table 2 gives the absorption values for certain bonds in the functional groups that you will be seeing in this lab. Although a spectrum may contain extra peaks, a spectrum will always contain at least one peak for each type of IR-absorbing vibration present in a sample. NOTE: A range of absorption values is given for each bond because the environment surrounding each bond can influence the exact infrared absorption value in the IR spectrum.

Question for thought:

What IR absorptions do you expect to see in each of the polymers in Table 1? Examine the bonds in each of the polymer (not monomer) structures, and tell which absorptions from Table 2 you expect for each polymer. The polymers can contain one or more of the types of bonds and/or vibrations shown in Table 2.

V. Percent Yield

Reactions do not typically produce the predicted amount of product. Sometimes there are competing reactions. Sometimes the reaction is slow enough that the reaction does not complete within the time frame of an experiment. Percent yield is the percent ratio of the actual or experimental yield obtained in the experiment to the theoretical or expected yield. It is calculated to be the actual (experimental) yield divided by theoretical yield multiplied by 100%. Don't be surprised if you don't get 100% yield!

Synopsis of the Experiment

In Part 1 of the lab, you will synthesize nylon 6.6 and analyze your product by mass and by Infrared Spectroscopy.

In Part 2 of the lab, you will identify unknown polymers using Infrared Spectroscopy.

Experiment

Materials and Equipment

ATR-FTIR Spectrum One Spectrometer

For Part 1: Synthesis of Nylon 6.6

• • 2.5% (v/v) solution of adipoyl chloride [C₆H₈Cl₂O₂] in heptane [C7H16] (2.5% v/v means 2.5 mL in a total of 100 mL)

    • • • NOTE: pure adipoyl chloride has a density of 1.25 g/mL.

  • 2% (w/v) aqueous solution of hexamethylenediamine [C₆H₁₆N₂] also containing NaOH (2% w/v means 2g in a total of 100 mL)

  • 50% (v/v) ethanol in water solution

For Part 2: Identification of Unknown Polymers

• • Unknown polymers 1, 2, 3, and 4

Instructions

Students will work individually unless otherwise specified by your instructor.

Wear gloves when working with chemicals.

Part 1: Synthesis of Nylon 6.6 Rope

• 1. Put on gloves. Complete all work today inside the fume hood.

  2. Pour 10 mL of a 2% (w/v) aqueous solution of hexamethylenediamine into a 150 mL beaker inside the fume hood.

  3. Using a different graduated cylinder, measure 10 mL of a 2.5% (v/v) solution of adipoyl chloride in heptane inside the fume hood. Slowly pour it on top of the hexamethylenediamine solution.

  4. With the aid of tweezers pull up on the film that forms at the interface between the aqueous and organic layers slowly and coil it around a glass rod held horizontally about 5 inches above the beaker. You may have a lump of nylon to start with. That's ok. If the lump is large, you may want to transfer it to a new beaker that will then become your collection beaker. If you get liquid bubbles, do not squeeze them, as they may spray liquid.

  5. With your fingers spin the glass rod to collect the nylon that keeps continuously forming at the interface as can be seen in Figure 3 below.

Figure 3: Setup for creating nylon 6.6 rope

6. Slide the nylon rope off your glass rod and into your collection beaker. In the beaker, wash the nylon rope with ~25mL water. Use your glass stirrring rod to break any bubbles in your rope. Lift your nylon and drain the water into a waste beaker. Wash the nylon rope again, this time with ~25mL of a 50% solution of ethanol in water. (Why do we use ethanol here?) Drain.

7. Dry your polymer with ~4 paper towels. Press the nylon firmly between the paper towels until there are no remaining bubbles. Continue pressing until the paper comes away completely dry. Leave your nylon to dry and wait as long as possible, squeezing out the water constantly, before moving on to step 8.

8. Before you leave lab, weigh your nylon 6.6 and obtain an FTIR spectrum of it.

9. Before you clean up, look back at your reaction beaker and note observations. Has more nylon formed?

Part 2: Identification of Unknown Polymer using Infrared Spectroscopy

1. With your partner or group, record the IR spectra of the 4 unknown polymer samples which will be provided in lab. Take an IR spectrum of at least one unknown with your partner, and then exchange spectra with other students so that you have spectra for all four unknowns.

2. Identify the polymers based on the experimental absorbances of the peaks from 4000-1500 cm^-1 and the information in the IR table (Table 2).

   1. • To get some ideas as to what you polymer might be, start by looking for significant features in your spectrum, perhaps a strong peak (dips very low) around 1700 cm^-1 which indicates that a carbonyl group is present, or peaks just above and just below 3000 cm^-1, which are indicative of sp² or sp³C—H bonds.

      • Then, start eliminating polymers whose expected peaks are not present.

      • Once you've identified a polymer, create and complete a table similar to Table 3 below, listing the expected absorbance ranges from Table 2 in column 2, and the corresponding peaks you observed in column 1.

      • Make an IR table for each unknown.

3. Record the IR spectrum of your sample of nylon 6,6.

4. Make another IR table similar to Table 3 below for your nylon sample.
   Please note if you still have water in your Nylon sample. It will appear as an OH stretch and, if present, would indicate that your final mass of nylon is inflated.

References

¹Modified from Experimental Organic Chemistry; D.R. Palleros; 2000.