Version 2 2024-02-23, 21:16Version 2 2024-02-23, 21:16
Version 1 2024-02-23, 17:16Version 1 2024-02-23, 17:16
journal contribution
posted on 2024-02-23, 21:16authored byRory J. McBride, Elisa Geneste, Andi Xie, Anthony J. Ryan, John F. Miller, Adam Blanazs, Christine Rösch, Steven P. Armes
We report a new one-pot
low-viscosity synthetic route to high molecular
weight non-ionic water-soluble polymers based on polymerization-induced
self-assembly (PISA). The RAFT aqueous dispersion polymerization of N-acryloylmorpholine (NAM) is conducted at 30 °C using
a suitable redox initiator and a poly(2-hydroxyethyl acrylamide) (PHEAC)
precursor in the presence of 0.60 M ammonium sulfate. This relatively
low level of added electrolyte is sufficient to salt out the PNAM
block, while steric stabilization is conferred by the relatively short
salt-tolerant PHEAC block. A mean degree of polymerization (DP) of
up to 6000 was targeted for the PNAM block, and high NAM conversions
(>96%) were obtained in all cases. On dilution with deionized water,
the as-synthesized sterically stabilized particles undergo dissociation
to afford molecularly dissolved chains, as judged by dynamic light
scattering and 1H NMR spectroscopy studies. DMF GPC analysis
confirmed a high chain extension efficiency for the PHEAC precursor,
but relatively broad molecular weight distributions were observed
for the PHEAC–PNAM diblock copolymer chains (Mw/Mn > 1.9). This has been
observed for many other PISA formulations when targeting high core-forming
block DPs and is tentatively attributed to chain transfer to polymer,
which is well known for polyacrylamide-based polymers. In fact, relatively
high dispersities are actually desirable if such copolymers are to
be used as viscosity modifiers because solution viscosity correlates
closely with Mw. Static light scattering
studies were also conducted, with a Zimm plot indicating an absolute Mw of approximately 2.5 × 106 g mol–1 when targeting a PNAM DP of 6000. Finally,
it is emphasized that targeting such high DPs leads to a sulfur content
for this latter formulation of just 23 ppm, which minimizes the cost,
color, and malodor associated with the organosulfur RAFT agent.