Nonlinear Radical Polymerization

Thus far, we have primarily considered free radical linear polymerizations of monomers containing a functionality of 2. With the exception of branching that can occur from chain transfer to polymer, a monomer with a functionality of 2 can only lead to linear polymers. But what if we use a monomer that has a functionality greater than 2? Much like what we discussed for step polymerization, we will get branching and possible formation of crosslinks leading to a network structure. In fact, many monomers for free radical polymerization that have a functionality higher than 2 are often called crosslinkers. Examples of common crosslinkers are shown in Figure 5.7 and Figure 4.2 in the textbook.

Figure 5.7: Examples of crosslinkers for free radical polymerization

Source: Lauren Zarzar

Polymerization of a monomer and crosslinker would create a network polymer, such as diagramed in Figure 5.8. The monomers that have a functionality of 2 form linear polymers, that are linked by the crosslinker.

Figure 5.8: Schematic diagram of a network polymer formed from monomer 'X' and crosslinker 'Z'.

Source: Replicated from Figure 4.19 from Young, Robert J., and Peter A. Lovell.
Introduction to Polymers, Third Edition, CRC Press, 2011