During polymerisation (the process in which 'monomers' crosslink chemical bonds to become 'polymers') most chemical substances physically contract in volume. This effect can cause problems in plastic moulding processes. Over the years, a variety of chemical monomers have been discovered which expand as they polymerise (typically 10-15%) - they are added to the plastic mix so as to stabilise the moulding process.
The expanding monomers are mainly based on 'cyclic' compounds (atomically structured as a ring) and it has been found that the larger the ring, the larger the expansion. The complex atomic mechanisms which drive the expansion are not known.