Chapter 4: Study Guide and Self-Assessment
A study of polymerization kinetics is not only crucial to an understanding of the fundamentals of polymerization, but also to its practical implementation by process engineers. First, it provides information on how long a reaction takes. As you work through this chapter you will see that step-growth polymerizations generally require long times, but some addition polymerizations occur so rapidly that they could be considered explosive. The relative rates at which different reactions occur (e.g., the rate at which chains are terminated relative to the rate at which they propagate), can have a profound effect on molecular weight and microstructure (e.g., in copolymerization, where the composition and sequence distributions of the copolymer will depend upon the relative rates at which the monomers add to the growing chain end). In this chapter we will discuss the kinetics of step-growth (condensation) and addition polymerizations. Copolymerization will be treated in a separate chapter.
Objectives
Upon successfully completing this chapter you should be able to:
- Derive equations describing the kinetics of step-growth polymerizations using polyesterifications as an example.
- Understand that step-growth polymerizations are generally slow and high molecular weight polymer is only obtained at high degrees of conversion (i.e., near the end of the polymerization when nearly all the monomers have been reacted).
- Describe the kinetics of the main features of free radical chain polymerization (initiation, propagation, termination and chain transfer).
- Show how the use of the steady-state assumption allows the derivation of equations describing the rate of polymerization, conversion, the kinetic chain length and the effect of chain transfer.
- Describe how the equation for the rate of polymerization provides an understanding of things like the Tromsdorff effect and why ethylene can only be polymerized free radically at high pressure and elevated temperature.
- Show how these equations are modified to describe aspects of ionic polymerizations, such as chain transfer to solvent and living polymerizations.
Self-Assessment Questions