Step-growth polymerization

Step-growth polymerization builds polymers through repeated reactions between functional groups on monomers, oligomers, and growing chains. High molecular weight develops mainly at high conversion, rather than by rapid growth at one active chain end.

Carothers equation: A relation connecting step-growth polymer number-average degree of polymerization with functional-group conversion and stoichiometric balance. It quantifies how conversion and reactant imbalance limit chain length.
Functional group: A defined group of atoms that gives a molecule characteristic chemical reactivity. Step-growth reactions join molecules through complementary functional groups.
Polyester: A polymer whose backbone contains ester linkages. Many commercial polyesters are produced by step-growth reactions between diols and diacids or their derivatives.
Chain-growth polymerization: A polymerization in which monomers add successively to an active chain end. Unlike step-growth, it can produce long chains early while much monomer remains unreacted.
Extent of reaction: The fraction of a reactant’s initial functional groups that have reacted. Step-growth chain length rises sharply as functional-group conversion approaches completion.
Monomer: A small molecule capable of joining other molecules to form a polymer. Step-growth monomers carry the reactive groups from which chains and networks are built.
Nylon: A family of synthetic polyamides characterized by amide linkages in the polymer backbone. Common nylon types form by step-growth reactions of diamines and diacids or lactams.
Free-radical polymerization: A chain-growth polymerization initiated and propagated by reactive radicals. Its active radical chain ends contrast with step-growth reactions among molecules of many sizes.
Stoichiometric imbalance: A mismatch in the numbers of complementary reactive groups supplied by polymer-forming reactants. Even slight imbalance leaves chain ends unpaired and caps attainable molecular weight.
Oligomer: A molecule containing a small number of repeating units. Oligomers react with both monomers and longer chains during step-growth.

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