Question

Draw the mechanism for the propagation step on the free radical polymerization of styrene. Why does polystyrene typically have a head to tail arrangement of its monomeric units?

Answer 1

Propagatio
During polymerization, a polymer spends most of its time in increasing its chain length, or propagating. After the radical initiator is formed, it attacks a monomer (Figure 1). In an ethene monomer, one electron pair is held securely between the two carbons in a sigma bond. The other is more loosely held in a pi bond. The free radical uses one electron from the pi bond to form a more stable bond with the carbon atom. The other electron returns to the second carbon atom, turning the whole molecule into another radical. This begins the polymer chain. Figure 2 shows how the orbitals of an ethylene monomer interact with a radical initiator.

Figure 1: Phenyl initiator from benzoyl peroxide (BPO) attacks a styrene molecule to start the polymer chain.

Figure 2: An orbital drawing of the initiator attack on ethylene molecule, producing the start of the polyethylene chain.
Once a chain has been initiated, the chain propagates (Figure 3) until there is no more monomer (living polymerization) or until termination occurs. There may be anywhere from a few to thousands of propagation steps depending on several factors such as radical and chain reactivity, the solvent, and temperature. The mechanism of chain propagation is as follows:

Figure 3: Propagation of polystyrene with a phenyl radical initiator.Polystyrene is entirely composed of the monomer Styrene.