Chain Transfer Agents

Chain transfer agents, also called modifiers or regulators, have at least one weak chemical bond. These compounds react with the free-radical site of a growing polymer chain and interrupt chain growth. In the process of chain transfer, the radical is temporary transferred to the regulating agent which reinitiates growth by transfering the radical to another polymer or monomer:

Rr· + S → Pr + S·
S· + M → R1·
S· + Pr → Rr·

Chain-transfer agents are often added to control the chain length during synthesis to achieve certain mechanical and processing properties. Very effective chain transfer agents are halogen compounds, some aromatic hydrocarbons, and thiols (mercaptans). Some common chain transfer agents are shown below.

 
Compound Repeating Unit Boiling Point (°C)
Carbon tetrachloride CCl4 77
Carbon tetrabromide CBr3 190
Bromotrichloromethane BrCCl3 105
4-Methylbenzenethiol 195
Isooctyl 3-mercaptopropionate 111
Pentaphenylethane 457
tert-Nonyl mercaptan 188
4,4′-Thiobisbenzenethiol N/A
Notes
  1. Arkema Chain Transfer Agents

  • Summary

    • Chain Transfer Agents

  • are compounds that have at least one weak chemical bond. They are added to the monomer or monomer blend to regulate the molecular weight of the polymer during synthesis.

  • Very effective chain transfer agents are halogen compounds, certain aromatic hydrocarbons, and thiols (mercaptans).

  • An increase in temperature almost always increases the amount of chain transfer, thus lowering the molecular weight.