Grade 12

Addition and condensation polymerization

How chain-growth addition and step-growth condensation build polymers, why their molecular-weight development differs, and how monomer structure determines repeat units and by-products.

1 Lower secondary 2 Upper secondary 3 Undergraduate 4 Advanced 5 Research

IntuitionIntuition: growing a chain

A polymer is a molecular chain assembled from many small building blocks. In chain-growth polymerization, reactive chain ends add monomers one at a time; in step-growth polymerization, any two molecules with suitable functional groups can join.

Compare radical chain growth with the stepwise formation of nylon-6,6 and water.

SchoolSchool level: monomers and repeat units

Definition: Monomer and polymer

A monomer is a small molecule that can form covalent bonds in polymerization. A repeat unit is the structural fragment repeated along the chain; it need not have the same formula as the monomer when atoms leave as a by-product.

\ce{n CH2=CH2 -> [-CH2-CH2-]_{n}}

Ethene loses its carbon–carbon double bond as new carbon–carbon single bonds form. No small molecule is expelled: the atoms of the monomer remain in the polyethylene repeat unit.

Examples of polymerization
Monomer(s)	Polymer / linkage	By-product
ethene	polyethylene (PE)	none
vinyl chloride	poly(vinyl chloride) (PVC)	none
adipic acid + hexamethylenediamine	nylon-6,6 (amide)	water
terephthalic acid + ethylene glycol	PET (ester)	water / methanol, route-dependent

Example: Counting repeat units

A polyethylene sample has $M_n = 28\,000$ g mol $^{-1}$ . Estimate its number-average degree of polymerization, using $M_0=28.05$ g mol $^{-1}$ .

Solution

$X_n pprox M_n/M_0 = 28\,000/28.05 pprox 998$ , soit environ 1 000 unités répétitives par chaîne. Les groupes terminaux sont négligés.

UndergraduateUniversity: mechanism and conversion

X_n=\frac{1}{1-p}\quad\text{(stoichiometric step growth)}

For an ideal balanced bifunctional step-growth reaction, $p$ is the fraction of functional groups that have reacted. Reaching $X_n=100$ requires $p=0.99$ ; high molar mass is obtained only near complete conversion. For chain-growth polymerization, long chains can exist at low overall monomer conversion because each active chain grows rapidly once initiated.

Mechanistic contrast
Feature	Chain growth	Step growth
Growing species	active chain end	monomer, oligomer, or polymer
High molar mass appears	early in conversion	near complete functional-group conversion
Examples	PE, PS, PMMA	polyesters, polyamides

Example: Near-complete conversion matters

For a balanced step-growth system, what $X_n$ is expected at $p=0.995$ ?

Solution

$X_n=1/(1-p)=1/0.005=200$ . Une petite baisse de conversion divise donc fortement la longueur moyenne des chaînes.

If the two functional groups are not present in equal amounts, stoichiometric imbalance also limits chain length. For a ratio $r\leq1$ of the less abundant to more abundant groups, $X_n=(1+r)/(1+r-2rp)$ ; exact balance is especially important at high conversion.

AdvancedAdvanced: equilibrium and reaction engineering

For condensation, removing water or an alcohol can shift a reversible equilibrium toward polymer, provided the reaction mixture stays close to the required functional-group stoichiometry. Chain-growth rates instead depend on initiation, propagation, termination, and transport; viscous radical systems can show autoacceleration when termination becomes diffusion-limited.

\Delta G_{\mathrm{pol}}=\Delta H_{\mathrm{pol}}-T\Delta S_{\mathrm{pol}}\qquad T_c\approx\frac{\Delta H_{\mathrm{pol}}}{\Delta S_{\mathrm{pol}}}\;(\Delta H,\Delta S<0)

ResearchResearch frontier: recyclable polymerization

References

Studies on Polymerization and Ring Formation. I. An Introduction to the General Theory of Condensation Polymers · W. H. Carothers, 1929
Principles of Polymerization, 4th ed. · G. Odian, 2004