The two dominant polymerization mechanisms build molecular weight very differently:
Chain-growth: few chains grow FAST to huge Xn immediately,
monomer pool depletes while chain count stays low.
Step-growth: ALL monomers link a little at a time —
Xn = 1 / (1 − p) (Carothers equation, p = conversion)
In chain-growth (addition) polymerization — e.g. polyethylene, polystyrene — an initiator creates a small number of reactive centers that each add monomers one at a time very rapidly, so high molecular weight appears almost immediately in a few chains while most monomer is still unreacted. In step-growth (condensation) polymerization — e.g. nylon, polyester — every monomer can react with every other, so chain length grows slowly and uniformly across the whole pool; very high molecular weight only appears once conversion is extremely high (>99%).
- Mechanism toggle — switches between the fast-few-chains and slow-all-together growth patterns.
- Monomer concentration — more monomer available to react raises achievable chain length.
- Initiator level (chain-growth) — more initiator means more chains start, but each ends up shorter since they compete for the same monomer pool.
- Temperature — speeds up both propagation and (for chain-growth) termination kinetics.
This distinction is why manufacturers pick step-growth for tough engineering plastics like nylon and chain-growth for cheap high-volume commodity plastics like polyethylene.