Two Mechanisms of Polymerisation

Introduction

Long chains can be assembled by two fundamentally different growth patterns. In step growth, small and large molecules react with one another through functional groups. In chain growth, an active end adds monomer repeatedly. The distinction predicts when high molar mass first appears.

Core explanation

In step-growth polymerisation, a monomer can react with another monomer, a dimer with a trimer, or two long oligomers with each other, provided compatible reactive end groups remain. Early conversion therefore produces a broad population of short species; very high degree of polymerisation emerges only when functional-group conversion approaches completion and stoichiometry is well balanced. Polyester formation from a diol and a diacid is a common example. In chain-growth polymerisation, initiation creates an active site, often a radical, ion or catalyst-bound chain end. Propagation adds monomer at that active end repeatedly; chains can become long even while much monomer remains unconsumed. Polyethylene made by addition to ethene is an example. Termination or transfer may stop a chain and create another. These descriptions concern kinetic pathways, not merely whether a small molecule is eliminated. Some step-growth reactions are addition reactions without a small by-product, and some ring-opening systems have mechanisms that complicate simple labels. The key practical comparison is the time course: step growth consumes functional groups throughout the mixture while chain growth consumes monomer at a minority of active ends. Monomer conversion and molar-mass evolution therefore look different in the two classes.

Step-by-step reasoning

Identify which species can react at each stage. If any molecules with complementary end groups may combine, classify step growth. If monomer adds mainly to a special active chain end, classify chain growth. Then predict whether long chains appear only late or can appear early.

Visual explanation

Draw three snapshots at low, intermediate and high conversion. In step growth, many short rods merge into longer rods over time. In chain growth, a few active rods lengthen rapidly while many single monomer circles remain.

Real-world analogy

Building a wall by allowing every partly built brick stack to join another resembles step growth. Adding bricks only to designated growing ends resembles chain growth. Both yield large structures, but the distribution of sizes develops differently.

Real-world example

Polyesters used in fibres are commonly made through step-growth reactions between bifunctional reactants. Polyethylene from ethene uses a chain-growth route. Manufacturers track conversion differently because high molar mass appears at different points.

Why?

Step growth is limited by the probability that many functional-group connections have occurred in sequence. Chain growth concentrates repeated additions on relatively few active centres. This difference, rather than the final chain appearance, explains the contrasting molar-mass profiles.

Common misconception

Condensation and step growth are not exact synonyms. Water or alcohol loss describes a reaction product; step growth describes who can react with whom. A step-growth polyurethane can form without elimination of a small molecule.

Worked example

Question: At 40% monomer conversion, one synthesis already contains very long chains alongside abundant monomer. Which mechanism is more likely? Reasoning: Long chains arising before nearly all reactive groups are consumed are characteristic of propagation at active ends. Answer: Chain growth is more likely than an ideal bifunctional step-growth process.

Quick check

1. In which mechanism can two long oligomers directly join? Answer: Step growth, provided their terminal functional groups are compatible and reactive.

Exam focus

Explain the reacting species and the timing of high molar mass. Do not classify only by observing water loss; specify whether growth occurs by general functional-group coupling or monomer addition at active ends.

Advanced insight

Real polymerisations can combine stages: a chain-grown polymer bearing reactive ends may later be cross-linked by step-growth chemistry. Mechanism labels describe the bond-forming process under discussion rather than a permanent identity of every product.

Summary

Step growth allows reactions among monomers, oligomers and polymer chains with suitable functional groups; high molar mass usually requires very high conversion. Chain growth lengthens active chains by repeated monomer addition and can produce long chains early. The mechanism is not determined solely by whether a small molecule is lost.

Practice questions

1. Why does step growth typically need near-complete functional-group conversion for long chains? Answer: Each long chain requires many successive couplings among molecules, so unreacted end groups must become rare.

2. What species adds to an active end during ordinary chain growth? Answer: Monomer molecules, which repeatedly extend the same growing chain.

3. Does every step-growth reaction release water? Answer: No. Step growth can use addition reactions such as formation of some polyurethanes without small-molecule elimination.

4. Which route better explains long polymer appearing while much monomer remains? Answer: Chain growth, because individual active chains can propagate rapidly at modest overall conversion.