(200o) Liquid Phase Synthesis of Monodisperse PEGs By Nanostar Sieving

Polyethyleneglycol (PEG) is the go-to, FDA-approved polymer for modifying proteins, bio-molecules and nano-carriers. Monodisperse, or discrete, PEG is especially desirable because the resultant PEG-conjugates are unique molecular entities with narrowly defined physical, biological and pharmacokinetic properties. Monodisperse PEGs are produced by stepwise, or iterative, synthetic methods, and there is an active market in shorter monodisperse PEG derivatives. However, many applications still resort to polydisperse PEG because higher molecular weights (> ~1.5 kDa) of monodisperse PEG are difficult to synthesis and are not commercially available.

We have developed a scalable synthetic platform for the manufacture of sequence-defined heteropolymers and monodisperse homopolymers – Nanostar Sieving technology – that combines liquid phase coupling reactions with purification through a size-selective polymer membrane. Three strands of polymer are grown unidirectionally from a central hub giving a polymer nanostar. A rigid central hub was developed that both maximises membrane rejection of the growing polymer-star and provides a distinct chromatographic marker (lambda max = 290 nm).

This approach has been applied to oligonucleotides [Gaffney et al. Chem. Eur. J. 2015, 21, 9535-43] and peptides, but is even more appropriate to polyethers. Solid phase methods and supports are poorly compatible with Williamson etherification. By contrast, solution phase chain extension of PEG-stars is unaffected by mass transfer kinetics, the hub is chemically inert, and slow reactions (hours) can be continuously monitored to completion. PEG-star chain elongation was explored using Thp or Dmtr protected dodecagol (Eg12) toluene sulfonate building blocks, up to mono-methyl Eg60 (mPEG-2675) and Eg112 (PEG-4951), respectively. Synthesis of and purification of monodisperse PEG by nanostar sieving is ideal for efficient heterobifunctionalisation of the chain termini [Livingston et al. Chem. Eur. J. 2014, 20, 10038-51], and new building blocks are being developed to permit intra-chain functionalization at precise positions along the sequence.

This technology is being commercialised by EXACTMER, a new start-up from Imperial College London.