Expt #4. Radical Polymerization of Styrene and Molecular Weight Determination

Relevant textbook readings - (1) Mohrig Chapter 7.1, 9.4. (2) Hart et. al. Chapter 14.1-14.2

Literature Reference - Wackerly, J. Wm.; Dunne, J. F  J. Chem. Educ. 2017 94, 1790-1793.

Overview - You will carry out the radical polymerization of styrene (eq 1) and obtain a 1H NMR spectrum of the resulting polystyrene. Integration of the NMR spectrum will allow calculation of the average molecular weight of the polymer.

Procedure

    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 dry 5-mL round-bottom flask (rbf) equipped with a magnetic stirbar.

    2. Add 2 mL toluene followed by 300 mg di-tert-butyl peroxide (DTBP).

    3. Attach a reflux condenser and reflux the mixture with stirring for 1 h. (See Chapter 7.1 in Mohrig).

    4. Cool to room temperature. Use a pipette to add the reaction solution to 30 mL methanol with rapid stirring. 

    5. Vacuum filter (Mohrig technique 9.4) and transfer the obtained solid to a pre-weighed watch glass. Place your labeled watch glass in the oven  (ca. 60 °C) for one week to dry. 

    6. Determine the yield of polystyrene and obtain the ¹H NMR spectrum in CDCl₃.

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. Use the integrations of the peaks at 6.2-6.7 (broad multiplet) and 1.2 ppm (sharp singlet) to determine the average molecular weight of your polymer. The instructor will explain during the lab.

    3. Write the mechanism of the reaction including initiation and termination steps.

    4. Use the mechanism to explain why polymer molecular weights are not infinite.