Question

Draw the mechanism for the propagation step on the free radical polymerization of styrene. Why does...

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?

Homework Answers

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.

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