constitutional heterogeneity 

of polymers
https://doi.org/10.1351/goldbook.C01283
The variation in constitution from molecule to molecule in polymers with molecules uniform with respect to elemental composition. An example is a polymer composed of linear and branched molecules; another example is a statistical copolymer comprising two isomeric constitutional units.
Source:
Purple Book, 1st ed., p. 52 [Terms] [Book]