In the development of PEG-ylated biopharmaceuticals a
first potential source of heterogeneity is low site-selectivity
and low conversion during attachment to, for example, the
protein or substrate.[6, 10, 11] The second potential source of
heterogeneity is that of the attached ligand itself; in polymeric
structures, such as PEG, this arises from a distribution of
different-length oligomers (polydispersity). Well-controlled
anionic polymerization can produce monofunctional PEG
with polydispersity indices (PDIs) approaching 1.04 (see
Supporting Information).[12, 13] Polydisperse oligomers give
rise to mixtures of PEG-ylated conjugates and regulatory
bodies are inclined to demand further investigations before
approving products of inexactly known composition.[1, 14]