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 (eq 1) 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 Rf
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/Rf
- 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.
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