We report the synthesis of sterically-stabilized diblock
copolymer
particles at 20% w/w solids via reversible addition–fragmentation
chain transfer (RAFT) aqueous dispersion polymerization of
N
,
N
′-dimethylacrylamide (DMAC) in
highly salty media (2.0 M (NH
4
)
2
SO
4
). This is achieved by selecting a well-known zwitterionic water-soluble
polymer, poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC),
to act as the salt-tolerant soluble precursor block. A relatively
high degree of polymerization (DP) can be targeted for the salt-insoluble
PDMAC block, which leads to the formation of a turbid free-flowing
dispersion of PDMAC-core particles by a steric stabilization mechanism.
1
H NMR spectroscopy studies indicate that relatively high DMAC
conversions (>99%) can be achieved within a few hours at 30 °C.
Aqueous GPC analysis indicates high blocking efficiencies and unimodal
molecular weight distributions, although dispersities increase monotonically
as higher degrees of polymerization (DPs) are targeted for the PDMAC
block. Particle characterization techniques include dynamic light
scattering (DLS) and electrophoretic light scattering (ELS) using
a state-of-the-art instrument that enables accurate ζ potential
measurements in a concentrated salt solution.
1
H NMR spectroscopy
studies confirm that dilution of the as-synthesized dispersions using
deionized water lowers the background salt concentration and hence
causes in situ molecular dissolution of the salt-intolerant PDMAC
chains, which leads to a substantial thickening effect and the formation
of transparent gels. Thus, this new polymerization-induced self-assembly
(PISA) formulation enables high molecular weight water-soluble polymers
to be prepared in a highly convenient, low-viscosity form. In principle,
such aqueous PISA formulations are highly attractive: there are various
commercial applications for high molecular weight water-soluble polymers,
while the well-known negative aspects of using a RAFT agent (i.e.,
its cost, color, and malodor) are minimized when targeting such high
DPs.