---
title: "Chain Growth and Step Growth: Two Routes to a Polymer"
description: "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 m"
canonical: https://lightmysky.com/learn/science/chain-growth-and-step-growth-two-routes-to-a-polymer-mt_LADBgarlKs
source: https://lightmysky.com/learn/science/chain-growth-and-step-growth-two-routes-to-a-polymer-mt_LADBgarlKs.md
retrieved: 2026-09-12
---

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# 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.

Subject: Science · Area: Chemistry · Ages 22 to 24
Page: https://lightmysky.com/learn/science/chain-growth-and-step-growth-two-routes-to-a-polymer-mt_LADBgarlKs

## Ready when they can

- 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

## Lesson: Two ways to grow a chain

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.

**Example.** 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.

**Tip.** Sketch molar mass against conversion to tell the routes apart. High from the start means chain growth, while flat-then-steep means step growth.

**Recap.** Chain growth starts long, step growth finishes long, and conversion near one decides everything.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Nucleophilic Acyl Substitution and the Reactivity Ladder](https://lightmysky.com/learn/science/nucleophilic-acyl-substitution-and-the-reactivity-ladder-mt_b60fq2aQ7r)
- [Radical Selectivity, Stability and Allylic Positions](https://lightmysky.com/learn/science/radical-selectivity-stability-and-allylic-positions-mt_tgIlRLvc9x)

## Opens up

- [Controlled Radical Polymerisation and Block Architecture](https://lightmysky.com/learn/science/controlled-radical-polymerisation-and-block-architecture-mt_nQKYR0yEEs)