Editing Polymerization (section)

===Step-growth===
{{Main|Step-growth polymerization}}

In step-growth (or step) polymerization, pairs of reactants, of any lengths, combine at each step to form a longer polymer molecule. The [[Molar mass distribution#Number average molar mass|average molar mass]] increases slowly. Long chains form only late in the reaction.<ref name=":0">{{Cite book |last1=Allcock |first1=H. R. |url= |title=Contemporary polymer chemistry. |last2=Lampe |first2=Frederick Walter |last3=Mark |first3=James E. |publisher=Pearson/Prentice Hall |others=Frederick Walter Lampe, James E. Mark |year=2003 |isbn=0-13-065056-0 |edition=3rd |location=Upper Saddle River, N.J. |pages=29–30 |oclc=51096012}}</ref><ref name=":1">{{Cite book |last=Fried |first=Joel R. |url= |title=Polymer science and technology |publisher=Prentice Hall Professional Technical Reference |year=2003 |isbn=0-13-018168-4 |edition=2nd |location=Upper Saddle River, NJ |pages=23 |oclc=51769096}}</ref>

Step-growth polymers are formed by independent reaction steps between functional groups of monomer units, usually containing [[heteroatoms]] such as nitrogen or oxygen. Most step-growth polymers are also classified as [[condensation polymer]]s, since a small molecule such as water is lost when the polymer chain is lengthened. For example, [[polyester]] chains grow by reaction of [[Alcohol (chemistry)|alcohol]] and [[carboxylic acid]] groups to form [[ester]] links with loss of water. However, there are exceptions; for example [[polyurethane]]s are step-growth polymers formed from [[isocyanate]] and alcohol bifunctional monomers) without loss of water or other volatile molecules, and are classified as [[addition polymer]]s rather than condensation polymers.

Step-growth polymers increase in molecular weight at a very slow rate at lower conversions and reach moderately high molecular weights only at very high conversion (i.e., >95%).  '''Solid state polymerization''' to afford polyamides (e.g., nylons) is an example of step-growth polymerization.<ref name=ullmannC2>{{Ullmann |doi=10.1002/14356007.a21_487.pub3|title=Polyethylene|year=2014|last1=Jeremic|first1=Dusan|pages=1–42|isbn=9783527306732}}</ref>