21.9:
Radical Chain-Growth Polymerization: Mechanism
Radical chain-growth polymerization consists of three stages: initiation, propagation, and termination.
In the initiation step, a free radical generated by the homolytic fission of the radical initiator reacts with the monomer's double bond. It generates a sigma bond between the initiator and the monomer. As a result, the radical site shifts to the monomer.
The propagation step begins as this new radical species adds to another monomer. This step repeats to add more monomers to grow the polymer chain.
The monomers preferentially bond such that the radical site is always present on the more substituted carbon.
Finally, chain termination can occur by the coupling of radicals in two growing polymer chains.
Alternatively, one chain's radical abstracts hydrogen from the carbon adjacent to the radical site in the other polymer chain. So, the polymerization stops and yields a pair of alkyl-terminated and alkenyl-terminated polymers.
21.9:
Radical Chain-Growth Polymerization: Mechanism
The radical chain-growth polymerization mechanism consists of three steps: initiation, propagation, and termination of polymerization. The polymerization initiates when a free radical generated from the radical initiator adds to the unsaturated bond in the monomer. The unpaired electron of the free radical and one π electron in the unsaturated bond creates a σ bond between the free radical and the monomer. As a result, the other π electron in the unsaturated bond converts this species into the new free-radical. The mechanism of the initiation reaction between a phenyl free radical and a vinyl chloride monomer is shown in Figure 1.
Figure 1: The initiation step of the reaction mechanism of the free-radical polymerization of vinyl chloride using a phenyl free radical.
The propagation step is the addition of this new free radical to the unsaturated bond in another monomer. This propagation step repeats to add more monomers and grow the polymer chain. In each addition, the radical site shifts to the newly added monomer. The number of times the propagation step repeats determines the molecular weight of the polymer chain. The propagation step of vinyl chloride free-radical polymerization is shown in Figure 2.
Figure 2: The propagation step of the reaction mechanism of the free-radical polymerization of vinyl chloride.
The monomers add preferentially such that the unpaired electron is positioned on the more substituted carbon. For example, monosubstituted ethylene monomers such as propylene, vinyl chloride, and styrene add to the growing chain in a head-to-tail manner. Here, the radical site in the growing polymer chain is always on the substituted carbon (tail).
Due to the high reactivity of radical intermediates, the termination of polymer growth is inevitable. Different reaction pathways lead to the termination of polymerization. One is the coupling reaction of a radical site in a growing chain with a radical site in another. Another way of termination is the radical disproportionation reaction, where the radical site of one growing polymer chain abstracts hydrogen from the α carbon of the radical site in the other chain. So, the former chain is terminated with an alkyl group, and the latter is terminated with an alkenyl group. The reaction mechanisms of radical coupling and disproportionation in the termination of the growth of a poly(vinyl chloride) chain are depicted in Figure 3.
Figure 3: The termination of vinyl chloride polymerization via radical coupling (top) and disproportionation reaction (bottom).
Chain transfer reagents and chain inhibitors can also terminate a polymer chain's growth. However, in this case, the radical site is not eliminated but either removed from the growing chain or made less reactive.
Suggested Reading
- Bruice, P. Y. (2004). Organic Chemistry. Upper Saddle River: Pearson, 074–078.
- Oullette, R. J., & Rawn, J. D. (2014). Organic Chemistry: Structure, Mechanism, and Synthesis. Elsevier, 998–1001.
- Smith, J. G. (2008). Organic Chemistry. New York: McGraw-Hill Higher Education, 1146–1149.
- Wade Jr, L. G. (2013). Organic Chemistry. Upper Saddle River: Pearson, 1223–1226.