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 p…
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.
View the full transcript and gain access to JoVE Core videos
Q1: Why does radical chain-growth polymerization always produce branched polymers?
During radical chain-growth polymerization, the highly reactive radical abstracts hydrogen from non-radical carbons in polymer chains. This hydrogen abstraction transfers the radical site from the chain end to an internal position, creating a branch point. The part of the chain between the abstraction site and the original chain end becomes a branch. This branching mechanism is inherent to radical polymerization and unavoidable.
Q2: What is the difference between intramolecular and intermolecular hydrogen abstraction in polymerization?
Intramolecular hydrogen abstraction occurs when the radical abstracts hydrogen from the same growing polymer chain, creating branches within that chain. Intermolecular hydrogen abstraction happens when the radical abstracts hydrogen from a different growing or terminated chain, terminating one chain's growth while initiating branching in another. Intramolecular abstraction is more probable than intermolecular abstraction.
Q3: How does low-density polyethylene form during radical polymerization of ethylene?
Free-radical polymerization of ethylene produces low-density polyethylene through extensive chain branching. The reactive radical abstracts hydrogen from growing or terminated polyethylene chains, shifting the radical site to internal positions. This creates numerous branch points throughout the polymer structure, resulting in the characteristic low density and heavily branched skeletal structure of this polyethylene grade.
Q4: Why is 1,5-hydrogen abstraction favored in intramolecular chain branching?
1,5-hydrogen abstraction is favored because it proceeds through a relatively stable six-membered transition state that resembles the chair conformation of cyclohexane. This favorable transition state geometry makes the reaction kinetically preferred over other abstraction distances. The result of 1,5-hydrogen abstraction is the formation of butyl branches, which are commonly found in branched polymers produced by radical polymerization.
Q5: What determines the size of branches formed during radical polymerization?
The distance between the hydrogen abstraction site and the nearest end of the polymer chain determines branch size. When abstraction occurs close to the chain end, small branches form. When abstraction occurs far from the end, larger branches result. Short-chain branching is more common than long-chain branching because the ends of chains are more accessible to the reactive radical site.
Q6: How does the radical site shift during hydrogen abstraction in polymer chains?
When the radical abstracts hydrogen from a non-radical carbon, the radical site transfers from its original position at the chain end (a primary carbon center) to the abstraction point within the chain (a secondary carbon center). This shift in radical position allows polymerization to continue from the new site, with the segment between the new radical site and the original chain end becoming a branch.
Q7: What happens to chain growth when intermolecular hydrogen abstraction occurs?
During intermolecular hydrogen abstraction, the radical abstracts hydrogen from another polymer chain, terminating that chain's growth at the abstraction point. Simultaneously, a new radical site forms at the abstraction location in the other chain. Polymerization then continues from this new radical site, creating a branch that grows from the middle of the previously growing chain.