The “Needle in the Haystack” Makes the
Difference: Linear and Hyperbranched Polyglycerols with a Single Catechol
Moiety for Metal Oxide Nanoparticle Coating
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Abstract
Multifunctional linear (CA-<i>lin</i>PG) and hyperbranched
polyglycerols (CA-<i>hb</i>PG) bearing a single catechol
unit were synthesized by use of an acetonide-protected catechol initiator
for the anionic polymerization of ethoxyethyl glycidyl ether (EEGE)
and glycidol, respectively. A key feature for the synthesis of the
hyperbranched structures was a selective, partial acetal deprotection
step. The single catechol unit among a large number of aliphatic 1,2-
and 1,3-diol moieties (i.e., the “needle in the haystack”)
in both linear and hyperbranched polyglycerols permits dispersion
of transition metal oxide nanoparticles in brine, as demonstrated
for manganese oxide (MnO) nanoparticles. Molecular weights of the
single catechol bearing PGs ranged from 950 to 2350 g·mol<sup>–1</sup> for CA-<i>lin</i>PG and from 3750 to 5750
g·mol<sup>–1</sup> for CA-<i>hb</i>PG with narrow
and monomodal molecular weight distributions (<i>M</i><sub>w</sub>/<i>M</i><sub>n</sub> < 1.23 for <i>lin</i>PG and <i>M</i><sub>w</sub>/<i>M</i><sub>n</sub> = 1.22–1.48 for <i>hb</i>PG). Both C-<i>lin</i>PGs and C-<i>hb</i>PGs are suitable hydrophilic capping
agents to generate highly hydroxyl-functional nanoparticles with hydrophilic
PG shell. The PG content of the polymer-coated MnO nanoparticles (diameter
17 nm) was in the range 21–54 wt %, as determined via TGA.
The MnO nanoparticles with a hydrophilic, multifunctional polyglycerol
shell may represent a promising alternative to iron oxide or gadolinium
contrast agents for MRI