Chain transfer

Lore & Background

Chain transfer was first proposed by Hugh Stott Taylor and William H. Jones in 1930 while studying polyethylene production from ethylene and hydrogen in the presence of ethyl radicals. The observed product mixture was best explained by postulating transfer of radical character from one reactant to another. Flory incorporated the radical transfer concept into his mathematical treatment of vinyl polymerization in 1937, coining the term 'chain transfer' to explain why average polymer chain lengths were usually lower than predicted by rate considerations alone.

The first widespread use of chain transfer agents came during World War II in the US Rubber Reserve Company's 'Mutual' recipe for styrene-butadiene rubber. Researchers at Standard Oil Development Company and the U.S. Rubber Company discovered that adding a mercaptan modifier produced a lower molecular weight, more tractable rubber and increased the polymerization rate. Although German scientists had become familiar with chain transfer agents in the 1930s, Germany continued to make unmodified rubber to the end of the war.

Throughout the 1940s and 1950s, progress was made in understanding chain transfer. Snyder et al. proved sulfur from a mercaptan modifier became incorporated into a polymer chain. Frank R. Mayo published a series of papers laying the foundation for determining chain transfer reaction rates. In the early 1950s, DuPont workers demonstrated that short and long branching in polyethylene was due to two different mechanisms of chain transfer to polymer.

Reader's Guide

Chain transfer is a fundamental concept in polymer chemistry that explains and controls molecular weight in addition polymerizations. By transferring the active center from a growing chain to another molecule, it limits chain length and can introduce branching. The reaction is categorized by the molecule that reacts with the growing chain: transfer to chain transfer agent (e.g., thiols like dodecyl mercaptan, halocarbons), transfer to monomer (which determines theoretical maximum molecular weight), transfer to polymer (leading to branched polymers, as in low-density polyethylene), and transfer to solvent (which can produce oligomers). Historically, chain transfer was first proposed in 1930 and named by Flory in 1937. Its practical importance grew during World War II with the development of synthetic rubber, where mercaptan modifiers improved processability. Since the 1980s, research has focused on living polymerizations such as catalytic chain transfer, RAFT, and iodine transfer polymerization, where the chain transfer reaction produces a polymer with similar activity, allowing controlled growth. The nature of chain transfer is now well understood and standard in textbooks.

Did You Know?

• Chain transfer was first proposed by Hugh Stott Taylor and William H. Jones in 1930.
• Flory coined the term 'chain transfer' in 1937.
• The first widespread use of chain transfer agents came during World War II in the US Rubber Reserve Company's 'Mutual' recipe for styrene-butadiene rubber.
• In the early 1950s, DuPont workers demonstrated that short and long branching in polyethylene was due to two different mechanisms of chain transfer to polymer.