Chain Transfer Reactions
in Cationic Polymerization

Chain transfer reactions are important termination reactions in cationic polymerization which affect the final molecular weight of the polymer and may also result in branching. These reactions occur readily because the β-hydrogen of the carbenium ion is strongly positively polarized due to hyperconjugation.1 Or in other words, not all of the positive charge resides on the positively charged carbon atom. Consequently, carbenium ions easily participate in many side reactions such as chain transfer, termination and/or isomerization.

There are several types of chain transfer reactions; three very common reactions are:

Many other chain transfer reactions are possible. For example, chain transfer to a solvent or to an impurity present in the mixture can occur, or a transfer agent might be intentionally added to the system. Typical examples of such transfer agents are water, alcohols and acids. These compounds can transfer a HO, RO, or RCOO anion to the chain carbocation which terminates the chain growth. For example:

Notes
  1. Hyperconjugation is the stabilising interaction of electrons of a sigma orbital (C-C, C-H) with an adjacent empty or partially filled non-bonding orbital which creates an extended molecular orbital of increased stability. This concept can also be applied to carbenium ions and radicals.

  • Summary

    Cationic Polymerization

    is a type of chain growth polymerization in which a cationic initiator transfers a proton to a monomer which then becomes reactive.

  • Chain Transfer Reactions

    are important termination reactions in cationic polymerization.  These reactions occur readily because the β-hydrogen of the carbenium ion is strongly positively polarized.

  • Chain transfer reactions reduce the average molecular weight of the polymer and may also result in polymer branching.

  • In some cases, transfer agents are intentionally added to the system. Typical examples of such transfer agents are water, alcohols and acids.