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Chemistry 2131:
Organic Chemistry for the Life Sciences (3)

Radical Polymerization of Alkenes

• we have seen a number of reactions of alkenes that all shared a common feature, the attack of the electron pair that makes up the pi bond of the double bond on an electron deficient atom. In all of the reactions that we have looked at to date electrons have moved in pairs. Now we will look at a number of radical reactions, reactions that involve homolytic bond cleavage and homogenic bond formation.
• before starting to discuss these, we must remember that to push single electrons we use single headed or fishhook arrows.
• in the radical polymerization reaction, a number monomers will be joined covalently to form a polymer. So, a monomer is a single base unit, where a polymer is made up of many monomers strung together.
• there are three steps to the radical polymerization process:
  1. initiation
  2. propagation
  3. termination
• in the chain initiation step we must start the reaction, we must cause the pair of electrons of the pi bond to split. To do this we use an initiator. Some compounds decompose when heated to form radicals. One such compound is benzoyl peroxide. The electrons of the O-O bond split homolytically when head to generate to benzoyloxy radicals.
• when these radicals are in the presence of ethylene or other alkenes they can induce homolytic bond cleavage of the pi bond of the double bond to generate a new covalent bond between the initiator and the alkene, generating a new alkyl radical.
• we can now say that initiation has taken place. The next step is the propagation step. The alkyl radical forms a new bond with another alkene molecule, generating in turn a new alkyl radical. This continues until the concentration of free monomers drops enough that the propagation rate falls off.
• the reaction is completed with the termination step, where two alkyl radicals meet head to head and form a bond (homogenically) and no more radicals exist.
• lets look at the alkyl radicals in a little more detail. They are much like carbocations, they are planar and have bond angles of 120 degrees. Another important similarity to carbocations is the relative stabilities of the alkyl radicals. The more substituted the carbon the more stable the radical. So:

  methyl < primary < secondary < tertiary

• what implications does this have for the polymerization process. Let's look at the polymerization of propene (propylene) to polypropylene. Initiation will proceed similarly. The difference is that there will be preferential orientation of the alkenes such that the radical is on the more substituted carbon (the one with the methyl group).
• this leads to more regular spacing of the side chains.
• you can get fancy by including bifunctional compounds that have two alkene groups on them leading to branching.
• try the following problem: Use fish-hook arrows to describe the polymerization of styrene to polystyrene. How would the product differ if a few percent of 1,4-divinylbenzene were included in the reaction mixture. Show the mechanism of the latter.

2. Conjugated dienes:

• before leaving the alkenes, let's look at the conjugated dienes and larger alkenes. When a molecule contains alternating double bonds it is said to be conjugated. Why would we study these?
• let's look at two simple molecules with two double bonds. Make for yourself 1,3-butadiene and 1,4-pentadiene. What is different about them? The big difference is that for the 1,3 butadiene there are four carbon atoms in a row that are sp² hybridized. Each atom has a p orbital perpendicular to the plane of the molecule. We think of the pi bonds as being between the first and second and third and fourth carbon atoms, but these orbitals can interact with each other across the single bond as well.
• these interactions give rise to resonance structures that stabilize these molecules.
• take the example of the addition of HBr to 1,3-butadiene. The product is 71% 3-bromo-1-butene and 29% 1-bromo-2-butene. How can we account for this? Let's start with the simple case of the expected product, the 3-bromo-1-butene. If the carbocation is resonance stabilized a form can exist where the positive charge can reside on the terminal carbon. If this carbocation is attacked by the bromide ion then the 1-bromo-2-butene product is observed. The ratio of the products is expected because the secondary carbocation is more stable than the primary.
• this was an example of an allylic carbocation. Since the allylic carbocation has supporting resonance structures it is a very stable carbocation
• the allylic carbocation is a resonance hybrid, it is neither wholly like either resonance structure, but it is an average of the two. The pi electrons are delocalized over the area in question.