21.10:
Radical Chain-Growth Polymerization: Chain Branching
Radical chain-growth polymerization always leads to branched polymers. For example, free-radical polymerization of ethylene yields low-density polyethylene with heavy branching.
During free-radical polymerization, the highly reactive radical may abstract hydrogen from a non-radical carbon. This abstraction can be either intramolecular—that is, from the same polymer chain—or intermolecular—that is, from another growing or terminated chain.
If the hydrogen abstraction occurs anywhere on a terminated chain or on a growing chain away from the α carbon of the radical site, the propagation site transfers from the end of the abstracting chain to that point within the polymer chain. This leads to chain branching.
The distance between the abstraction site and the nearest end of the chain determines the size of the branch in the polymer chains.
In intramolecular chain branching, 1,5-hydrogen abstraction is favored because of a relatively stable six-membered transition state resembling the chair conformation of cyclohexane. So, the resulting polymer has butyl branches.
21.10:
Radical Chain-Growth Polymerization: Chain Branching
The skeletal structure of polymers synthesized via radical polymerization is always branched. For example, the polymerization of ethylene by radical polymerization results in a low-density grade of polyethylene with a heavily branched skeletal structure. Here, the radical site abstracts hydrogen from the growing chain, and the radical site shifts from the end (a primary carbon center) to anywhere within the growing chain (a secondary carbon center). Consequently, the part of the chain from the abstraction site to the end of the chain becomes a branch.
In intermolecular hydrogen abstraction, the radical site abstracts hydrogen from another growing polymer chain and terminates its chain growth. A new branch grows in the other chain where the hydrogen was removed. Figure 1 demonstrates the intermolecular hydrogen abstraction and the consequent chain growth from the middle of the polymer chain.
Figure 1: The intermolecular hydrogen abstraction (top) and the subsequent addition of the newly formed radical site to a monomer (bottom).
The radical site abstracts hydrogen from the same growing chain in intramolecular hydrogen abstraction. The length of the branch depends on the distance between the radical site and the abstraction site. Intramolecular 1,5-hydrogen abstraction frequently occurs due to the relatively stable six-membered transition state, resembling the chair conformation of cyclohexane. The resultant butyl branches are commonly found in branched polymers. Figure 2 shows the intramolecular hydrogen abstraction and the formation of the butyl branch.
Figure 2: The intramolecular hydrogen abstraction (top) and subsequent addition of the newly formed radical site on a monomer (bottom).
In general, intramolecular hydrogen abstraction is more probable than intermolecular hydrogen abstraction. Also, short-chain branching is more common than long-chain branching, as the ends of the chains are more accessible.
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.