The “Needle in the Haystack” Makes the Difference: Linear and Hyperbranched Polyglycerols with a Single Catechol Moiety for Metal Oxide Nanoparticle Coating

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^–1 for CA-<i>lin</i>PG and from 3750 to 5750 g·mol^–1 for CA-<i>hb</i>PG with narrow and monomodal molecular weight distributions (<i>M</i>_w/<i>M</i>_n < 1.23 for <i>lin</i>PG and <i>M</i>_w/<i>M</i>_n = 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