Chain termination occurs when two radical species (each odd-electron) react to form one or two new molecules without radical (even-electron). Chain transfer occurs when a radical species reacts with a nonradical species. The result must be at least one radical species. In the most common occurence, the chain end radical attacks a weak bond. An atom gets transfered to the chain end.
After this happens, the current chain is terminated. A new chain may start or not, depending on reactivity of new radical.
In many cases, a chain transfer agent is added deliberately to the reaction mixture. Many compounds work well for this purpose, but mercaptans (also known as thiols) are the most general. Example for styrene + butyl mercaptan:
The sulfur-centered radical reinitiates very efficiently. The result is a dimunition of the molecular weight without changing the overall rate of conversion of monomer to polymer. (Using more initiator is another way to decrease MW, but the reaction rate would increase proportionally, a possibly dangerous situation.)
Naturally, there are many even-electron species present in the reaction mixture (i.e., monomer, initiator, solvents, polymer chains, etc.), and all of therse may participate in transfer reactions, depending on the relative reactivities of the structures involved. Here is an example of transfer to initiator featuring acrylonitrile and benzoyl peroxide (BPO):
One chain is terminated, but another one initiates. This particular reaction reduces MW and wastes initiator (i.e., an initiator molecule is consumed, but no new chains are begun). Sometimes this precess is called induced decomposition of the initiator. It is a common side reaction for the peroxy initiators, but happens less often with the azo initiators.
If the chain end radical attacks an atom on the backbone of the same or another chain, the result is a new radical that can reinitiate to form a branch. This reaction happens very commonly during the free radical polymerization of polyethylene.
The chemistry is driven by the greater stability of the secondary radical that forms, compared to the primary one at the chain end. Eventually, even the branches get branches. The most convenient site of attack is a hydrogen atom that is a short distance back on the same chain, so short branches are very common. Typical: 3-6% short (<=6 carbons) branches, 0.2% long branches in the polyethylene chain. Branches disrupt chain packing, and decrease degree of crystallinity, creating low density polyethylene (LDPE, #4 in the recycling codes). LDPE is much softer than the version that is linear (made by coordination polymerization).
Chain transfer to polymer occurs to a lesser extent in many other systems. Truly linear polymers are actually rare.
Consider what happens if chain transfer occurs, but the new radical is incapable of reinitiation. If so, this is a kind of termination reaction. The agent responsible is called an inhibitor. There are many kinds of inhibitors, but the most common are phenols with bulky groups ortho to the OH group, as in this example:
(BHT stands for "butylated hydroxy toluene," a slang name for 2,6-di-tert-butyl-para-cresol.)
Most commercial monomers are packaged with traces of inhibitor to prevent premature polymerization. The inhibitor can be removed prior to polymerization by distillation, chromatography, or extraction. In many cases, it is simply left alone, and additional initiator is used to overwhelm the inhibitor.
Inhibitors are added in minute quantities to many other chemicals (e.g., ether, THF) to interupt radical chain reactions that lead to decomposition. They are also used in foods to slow oxidation that leads to spoilage.
Molecular oxygen presents peculiar behavior toward free radical polymerization.
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