Chain Transfer

Chain transfer is a process that occurs during polymerization, in which the active center is transferred from one species to another (Figure 4.12).

Figure 4.12: Chain transfer
TA can be initiator, monomer, polymer, the solvent, or something you add to purposefully transfer the radical (i.e., a transfer agent). Often, T is a hydrogen atom or halogen atom

Source: Lauren Zarzar

In Figure 4.12, we use T and A to just represent fragments of a molecule that are linked by a single bond. TA can be initiator, monomer, polymer, the solvent, or something you add to purposefully transfer the radical (i.e., a transfer agent). Often, T is a hydrogen atom or halogen atom. Notice that breakage of the single bond in TA happens by homolytic cleavage, as we have seen before, such that the radical has been transferred from one species to another. Chain transfer - the rates at which it happens, and the species between which the radical is transferred - can have dramatic effects on the structure of the resulting polymer. First, we will consider the impact of chain transfer to polymer.

Chain transfer to polymer

Chain transfer to polymer means that the radical is transferred to somewhere in or on a polymer - it can be the same polymer where the radical was initially (intramolecular chain transfer, or backbiting), or the active center can be transferred to a different polymer chain (intermolecular chain transfer). Both of these processes are shown in Figure 4.13.

Figure 4.13: Intramolecular and intermolecular chain transfer to polymer. Intramolecular chain transfer to polymer, sometimes called backbiting, most often leads to short branches. Intermolecular chain transfer to polymer tends to lead to longer branches.

Source: Lauren Zarzar

If we were to polymerize a monomer with a functionality of two (e.g., a monomer with a vinyl group) we expect to create a linear polymer, without branching. But if there is chain transfer to polymer, then we get a very different polymer skeletal structure that does have branching. If there is intramolecular chain transfer to polymer, sometimes called backbiting, the active center is transferred to somewhere else along the same polymer chain where the active center originated. Usually, it's transferred to a position pretty close to the polymer end resulting in the formation of a short branch. If there is intermolecular chain transfer to polymer, then the active center is transferred to another polymer chain. It could be transferred anywhere along the polymer; usually, this mechanism leads to branches that are longer.