Teacher's Notes:

There has been much concern in the past over the reproducibility and the cost of this experiment. The key to reproducibility is to keep the conditions as nearly identical as possible from plate to plate. If care is taken, this experiment gives reliable results. Some of the lack of reproducibility can be attributed to the difficulty in assigning the middle of what appears to be a rather long, blurry spot for a polydispersed sample. This difficulty can be reduced by opting to record the more easily assigned bottom of the spot. The GPC results confirm that the bottom of the spot if a meaningful measurement, in that it correlates with the weight average molecular weight. Another point in which this procedure differs from those previously reported involves the resulting calibration curve. A correlation coefficient of 1 was obtained for a second order polynominal fit, whereas correlation coefficients of .96-.99 were obtained for a linear fit of the same data.

Recycling of the reverse phase TLC plates results in a significant cost reduction. Recycling can be easily accomplished with only minor modifications in the procedure. These modifications are time costly, as the student is required to spend more time using the UV lamp. The laboratory directors will determine which procedure best serves their needs. Reverse phase TLC plates can be recycled by a method discussed in a separate procedure.

If this experiment has been paired with the microscale bulk polymerization of styrene, the distribution of molecular weight for this synthesis may be determined.

This method of determination of molecular weight of polystyrene is applicable to all polystyrene samples whose molecular weight falls between 2K and 100K. Samples whose molecular weight falls outside this window give significantly poorer correlations with their corresponding GPC data.

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