# Module 4: Molecular Weight

In this short module, we will learn all about molecular weight (MW), MW distributions, and polydispersity. Extent of reaction will be introduced. You will learn to complete MW calculations. Through the HW and quiz you will practice calculating MW, PDI, and p. The variables and equations can be confusing, but remember you have been calculating number averages since you were in elementary school! There is a 100% chance that you will be asked to calculate MW on the first exam and again on the final. Use the lecture notes and recorded lecture to review.

**Read Section 2.2**: *Molecular Weight – Definitions* in Chapter 2: *Microstructure and Molecular Weight* of the Polymer Science and Engineering eBook (or the equivalent in a print version).

Download the TE 200 Module 04 Study Guide and Checklist.

#### Study Guide for Module 04 (Can you do the following?):

- Given the equation, calculate the number-average molecular weight.
- Given the equation, calculate the weight-average molecular weight.
- Calculate the number average degree of polymerization.
- Calculate the weight average degree of polymerization.
- Identify key aspects of a MW distribution plot (
*e.g*., a narrow distribution versus a broad one, which curve has a higher [latex]\overline{M}_n[/latex], [latex]\overline{M}_w[/latex],*etc*.). - Explain the importance of the polydispersity index.
- Calculate polydispersity.
- Use your knowledge of polydispersity to convert from weight average to number average, and vice versa.
- Given the appropriate equations, calculate the polydispersity, [latex]\overline{X}_n[/latex] and [latex]\overline{X}_w[/latex] from the extent of reaction (p) for step growth polymers (and vice versa).
- Explain why the polydispersity of step growth polymers is
*ca*. 2.