Chain Growth and Step Growth: Two Routes to a Polymer
Both routes end with long chains, but they get there differently. Chain growth builds full-length molecules from the start at active centres, while step growth couples everything with everything, so molar mass only climbs at very high conversion.
What a learner can do afterwards
- Sketches how molar mass changes with conversion for each mechanism
- Calculates degree of polymerisation from conversion for a step-growth reaction
- Explains why a small excess of one monomer caps the achievable chain length
- Reads a dispersity value and says what it implies about how the chains were made
1 · Read
Both routes end with long chains but get there differently. Chain growth builds full-length molecules from the start at a few active centres. Step growth couples everything with everything, so molar mass only climbs at very high conversion.
In step growth the average degree of polymerisation is one over one minus conversion. At 98 percent conversion that gives 50 units, and at 99 percent it gives 100. The last two percent matter more than all the rest, and a small excess of one monomer caps the length early.
Dispersity reports how uniform the chains are. A value near one means nearly equal chains from a controlled growth, while a value near two is the signature of classic step growth.
Sketch molar mass against conversion to tell the routes apart. High from the start means chain growth, while flat-then-steep means step growth.
Chain growth starts long, step growth finishes long, and conversion near one decides everything.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.