Bulk Polymerization of Styrene Bulk polymerization of styrene will be conducted with different initial initiator concentrations. The polymers will be analyzed for their MW and PDI by size exclusion chromatography (SEC) using a calibration curve. The MW dependence on conversion will be investigated. The effect of the initiator concentration on polymer MW will be examined.

Materials.
Azobisisobutyronitrile (AIBN) and toluene (HPLC) were purchased from Sigma-Aldrich and used as received. Styrene (Sigma-Aldrich) was purified by passing through a silica column to remove inhibitors. CDCl3 was purchased from Cambridge Isotope and was used as received.

Methods.
Reaction procedure. To a vial charged with freshly purified styrene (10 g) was AIBN (0.7g, 0.35g or 0.17g) added at room temperature. The mixtures are then heated at 80˚C in an oil bath. Seven aliquots of the reaction mixture are removed at fixed time intervals (e.g., 0, 20, 40, 60, 80,100 min, 1 week) and analyzed by 1H NMR spectroscopy for conversion and SEC for polymer MW and PDI.
Sample preparation for SEC analysis. An aliquot of the reaction mixture (3mg) is dissolved in THF(1mL) and was filtered through a syringe membrane filter prior to injection into SEC column.
Sample preparation for 1H NMR analysis. An aliquot of the reaction mixture (0.1-0.5 mg) is dissolved in CDCl3 (1mL) and is transferred into a NMR tube for conversion analysis.

Lab report write-up.
Summarize and discuss your experimental results and describe the characterization techniques you used for this lab. The following questions should be addressed in the report.
How does SEC characterization of polymer MW and PDI work? What are the advantage and limitation of this technique?
How is a calibration curve constructed?
What is the mechanism for the bulk polymerization of styrene?
How did you obtain the conversion data?
How does the polymer MW depend on conversions? Why?
How does the polymer MW depend on the initiator concentration? why?
Consider the major source of errors for the polymer MW and PDI obtained by SEC measurement using a calibration curve.

Answers for references only.
Conventional SEC requires construction of calibration curve which correlates elution volume with MW. Every slice of SEC chromatograms can be analyzed by the calibration curve to get Mi. Intensity of the SEC chromatograms are typically an instrumental readout (e.g., DRI) that reflects the elute concentration (i.e., gi). So the Mn, Mw and PDI can be calculated from these (Mi, gi) values. The SEC analysis is a very time-efficient approach to get MW and PDI information for high MW polymers. If the calibrants and polymers of interests are of identical type, the measured MWs are in the most part reliable, although PDI is typically over-estimated. However, if they are different, the MW values are relative MW as the calibrant and polymer of interests have different hydrodynamic volume. This approach requires the system to be stable (e.g., stable flow rate) and only size-exclusion being the operative separation mechanism.
Calibration curve is typically built using a series of mono-disperse polymers with known MWs covering a wide range (several KD to MD). Their peak elution time and log MW are fit with a polynomial function. This function is the calibration curve.
We have been through this in class and exam. You should be able to write down equation depicting the initiation, propagation and termination steps.
Conversion is defined as the mole fraction of monomers that are converted into polymers. So it can be obtained from 1H NMR by integrating peaks due to polymer repeating unit and unreacted monomers and compare their relative intensity.
For all three runs (AIBN=0.7, 0.35, 0.17g), experimental results show that MW increase slightly with conversion at the beginning stage of the reaction (