Now try this variation of the bead exercise. In the bag, put a bunch of similarly colored beads (monomers) and a few differently colored beads (which will be our initiator molecules). The difference between this and the previous exercise is that monomer can only add to the initiator or a growing polymer containing the initiator. So reach in and grab two beads until you pull out an initiator and a monomer. Put them together, throw them back! Grab two more; if you grab two monomers, they won’t react with each other. Eventually, you will grab a monomer and a growing polymer fragment that has the initiator bead; keep adding monomers to that. What you find is that, in comparison to the step polymerization exercise, here you are forming relatively few numbers of polymers but each polymer chain that does form will grow longer, faster. You will also not use up your monomer as quickly; even when your polymers grow large, you’ll still have lots of monomer beads left. This exercise is similar to chain growth polymerization. Chain growth polymerization has the following characteristics (compare to step growth polymerization!):
- growth of polymer occurs by adding monomers to relatively few polymer chains;
- monomer remains even at long reaction times;
- average molar mass increases quickly;
- initiation is required.
We will be considering step and chain polymerization independently, and in great depth, in the coming lessons, but for now you should have a good idea of the similarities and differences between these two mechanisms.