Expt #2. Radical Polymerization of Styrene and Molecular Weight Determination by TLC

Relevant textbook readings - Mohrig Chapter 17. Smith chap 15.13-15.14

Literature Reference - Slough, G. A. J. Chem. Educ. 1995 72, 1031-1032

Overview - You will carry out the radical polymerization of styrene and measure the average molecular weight of the resulting polymer using thin layer chromatography (TLC).

Procedures for Styrene Polymerization

    1. Place a small piece of cotton in a Pasteur pipet and fill it with alumina to a height of approx 3-4 cm. Use this alumina column to filter 2 mL of styrene into a small disposable test tube (do not use one of the reaction tubes out of the micro kit!). 

    2. Add 50 mg of AIBN and and place a wood applicator stick into the tube to serve as a boiling aid and to allow monitoring of the reaction.

    3. Heat on a sand bath to around 135°C observing carefully for the onset of polymerization as indicated by yellowing , bubbling, and/or increasing viscosity of the solution. Once polymerization begins the reaction will generate its own heat so you will want to lower the applied heat by removing the tube somewhat from the hot sand.

    4. Remove the wood stick from the solution  periodically. When the fibers so formed are brittle (can be cleanly snapped in two) after cooling, the polymerization is done.

    5. Pour some of the hot polymer onto a warm watch glass or glass plate. Spread it out so that when it hardens you will get a thin film. Allow it to cool and use this thin film to obtain the IR spectrum. Alternatively, you can dissolve 5 mg of solid polymer in 5 drops of CH₂Cl₂ and the drip this solution onto a salt plat and obtain the IR after the solvent has evaporated off of the plate.

Procedures for Molecular Weight Measurement using TLC

    1. Obtain a plastic-backed, fluorescent TLC plate from the instructor. Handle it carefully with forceps to avoid getting fingerprints or other contaminants on it.

    2. Prepare standard solutions of polystyrene standards of four different molecular weights. Prepare the solutions in small vials taking care to label the vials appropriately. Use 25 mg of polystyrene standard and 3 mL CH₂Cl₂ for each solution.

    3. Prepare a solution of the polystyrene product you obtained in the same manner as step 2.

    4. Prepare and spot your TLC plate with each of the four standards and your product. Use the procedures detailed on pp 224-225 of Mohrig.

    5. Prepare a developing chamber as discussed in Section 17.3 of Mohrig. The solvent is a 1.15:1.00 (v/v) mixture of hexane and 1,4-dioxane.

    6. Develop the plate according to the procedures in Mohrig (pp 224-225). Make sure to remove the plate from the chamber before the solvent line reaches the top.

    7. Visualize the plate using a UV lamp as described in Mohrig (chap 17.4). Carefully outline all spots lightly with a pencil. Sketch the plate showing all distances in your lab notebook.

    8. Calculate and tabulate the R_f value for each substance.

    9. Use Excel to make a plot of the log of the average MW of the standards versus the log of (1/R_f - 1). The plot should be linear and can be used as a standard curve for determining the approximate MW of your product.

Assigned questions.

   1. Why was it necessary to filter the styrene through alumina prior to running this reaction? Hint: Look up styrene at Sigma-Aldrich's website and check the specifications of the product they sell.

   2. Why are polymer molecular weights limited? Identify some reactions that prevented our polystyrene molecular weights from reaching higher values.