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Polymer End Group Analysis - The Determination of Average Molecular Weight: Procedure

Note: Even though its volatility is low, you should avoid breathing DMF vapor, and you should wash DMF off immediately if you get it on your skin.

  1. Weigh a sufficient quantity of PMDA (FW=218.12) to prepare 100 mL of 0.2 M solution, and dry at 160°C for 2 hours.

  2. Dissolve the solid in 100 mL of DMF. Your glassware must be dry before you start. Do not add any water to this solution or to any of the samples until specified in the procedure. If it is necessary to store the solution for a week, obtain a small bottle from the stockroom and line the cap with aluminum foil.

  3. Obtain a sample of PEG and accurately weigh 0.20-0.24 g samples into each of four dry 125 mL Erlenmeyer flasks. Note: With a viscous liquid sample it is not feasible to weigh by difference from a weighing bottle. Instead, weigh the dry flask, add the appropriate amount of polymer from a dropper (carefully, so as to deliver the sample to the bottom of the flask and not on the wall), and reweigh the flask. The top door of the analytical balance is very convenient for this weighing operation.

  4. Pipet 10 mL aliquots of the 0.2 M PMDA into each of the Erlenmeyers. Wash any PEG on the flask wall to the bottom as the pipet drains. Avoid adding extra solvent, because the mixture may become too dilute for the reaction to go to completion.

  5. Use a Mohr pipet to add one mL of 3 M IMDA (prepared ahead by the laboratory staff) to each flask. Mix the contents and allow one-half hour for the reaction. While waiting, proceed to the PMDA standardization described below.

  6. To standardize the PMDA solution pipet 10 mL aliquots into three Erlenmeyer flasks and add 1 mL of the IMDA solution to each.

  7. Add 30 mL deionized water and phenolphthalein to each (no waiting period required) and titrate with the standard 0.2 M NaOH.

  8. After waiting 30 minutes, add 30 mL deionized water to each flask. Add phenolphthalein indicator, and titrate with standard 0.2 M NaOH (prepared and standardized by laboratory staff).

***Place all organic waste in the designated container.