Consider this exercise: you have a bag of pop beads, and each bead represents a monomer. You reach into the bag, pick up two beads, and put them together (they “react”) to form a dimer. Then you put this dimer back into the “solution” of monomers (the bag). Reach in, and grab two more beads at random. Most likely, you pick two single beads - put them together, and throw the dimer back. After you do this a bunch of times, perhaps you reach in and grab a monomer and a dimer. Put them together, you’ve made a trimer. Throw it back! If you do this for long enough, pretty soon your bag will be full of short oligomers, and lots of them, without many monomers left. Only after a long time of doing this exercise will you actually start to piece together these short oligomers to create longer polymers.
This mechanism is characteristic of step growth polymerization. Step polymerization is characterized by:
- loss of monomer early on in the reaction (remember, you turned all those monomers into dimers, trimers, and other short oligomers);
- growth of polymers throughout the reaction;
- the average molar mass increases slowly (for the majority of the exercise, your bag had only very short oligomers in it — only after very long times did you start to get longer polymers);
- high extents of reactions are necessary to get long chain lengths (lots and lots of beads had to be combined into those dimers and trimers before you started to get longer polymers);
- no initiator is used (the bead monomers were inherently reactive with each other).