Deep-Dive Guides · Polymers

Organic Polymers — the complete deep dive

Two independent axes organise this entire topic. Axis 1 — mechanism: does the chain grow by chain-growth (a reactive centre adds monomers one at a time) or step-growth (any two functional groups condense, anytime)? This single distinction sets the kinetics, the molecular-weight evolution, and even which monomers work.

Free extract — the opening section of the guide, in full.

0. How to use this guide — the two axes

Fix the two axes before any single polymer. Almost every exam question is really asking "which mechanism?" and "what regularity/architecture?".

The three questions for any polymer/polymerisation: (1) Chain-growth or step-growth? Chain-growth needs an active centre (radical/ion/metal) and a C=C (or ring); step-growth needs bifunctional monomers with reactive groups (–OH, –COOH, –NH₂, –NCO) that condense. (2) How does molecular weight build with conversion? Chain-growth: high MW almost immediately, monomer consumed steadily; step-growth: MW rises only at very high conversion (Carothers). (3) What is the microstructure? tacticity (iso/syndio/atactic), geometric isomerism, branching/cross-linking — these decide crystallinity, T_g, T_m, and mechanical behaviour.

Feature	Chain-growth (addition)	Step-growth (condensation)
Monomer	unsaturated (C=C) or cyclic	bifunctional (di-ol/di-acid/di-amine/di-isocyanate)
Active species	radical / cation / anion / metal centre	none — any two ends react
Growth	monomer adds one at a time to a chain end	monomer+monomer, dimer+trimer, any+any
MW vs conversion	high MW at low conversion; monomer decreases steadily	MW low until >99% conversion; monomer vanishes early
By-product	usually none	often a small molecule (H₂O, HCl, MeOH)
Examples	PE, PP, PVC, PS, PMMA, PAN	nylon, PET, Bakelite, polyurethane, epoxy

Key nuance: "addition" and "chain-growth" are usually synonymous, as are "condensation" and "step-growth" — but not always. Polyurethanes form by a step-growth addition (diol + diisocyanate, no small-molecule by-product), and some ring-opening polymerisations are chain-growth yet lose no by-product. Classify by mechanism (how the chain builds), not by whether a by-product appears.

Read the rest of this guide

The remaining 11 sections — Fundamentals & classification, Chain-growth (addition) polymerisation, Step-growth polymerisation & the Carothers equation, Molecular weight — Mn, Mw, PDI and more — are part of ChemVidya Full Access, along with the other guides, the books, the study notes and 6,000+ practice questions with worked solutions.

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