Step polymerisation with Polyfunctional monomers

If monomers which have functionalities greater than 2 are used for step polymerisation the product that forms consists of an infinitely large three-dimensional network and the polymerisation is characterised by sudden gelation at some point before the reaction is complete. The gel point is observed readily as the time when the mixture suddenly loses fluidity as viscosity rises sharply 

Gelation occurs at relatively low conversions of monomer to polymer hence the number-average molar mass at the gel point is low. By contrast, however, the weight-average molar mass becomes infinite at the gel point. 

In considering step polymerisation with polyfunctional molecules a number of assumptions are made. They are (i) that all functional groups are equally reactive, (ii) that reactivity is independent of molar mass or solution viscosity, and (iii) that all reactions occur between functional groups on different molecules, i.e. there are no intramolecular reactions. It is found experimentally that these assumptions are not completely valid and tend to lead to an underestimate of the extent of reaction required to bring about gelation. 

---