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 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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