Efficient synthesis of macromolecular DO3A@Gn derivatives for potential application in MRI diagnostics: from polymer conjugates to polymer nanoparticles

Abstract

Herein, the synthesis of three different macromolecular DO3A@Gn conjugates based on poly(2-oxazoline)s is presented. Therefore, poly(2-methyl-2-oxazoline) is synthesized by a ring-opening, cationic polymerization and the polymerization is terminated with DO3A(tBu)3. The best results are obtained after 48 h at 120 °C with degree of termination of 86%. After deprotection of the DO3A ligand and complexation with Gn3+, relaxivity as measured with a magnetic field strength of 9.4 T (400 MHz) reveals values for r1 of up to 2.32 mm−1 s−1. The concept is extended to a block copolymer based on 2-heptyl-2-oxazoline and 2-methyl-2-oxazoline that is again terminated with DO3A(tBu)3 to form micelles with a size of 12.6 ± 0.7 nm after DO3A(tBu)3 termination and deprotection of the 1,4,7,10-tetraazacyclododecane-N,N,N,N-tetraacetic acid ligand. After complexation with Gn3+, relaxivity r1 is 10.1 mm−1 s−1 as determined from the slope of the plot of 1/T1 against the gadolinium(III) concentration at 9.4 T. Finally, crosslinked nanoparticles are prepared from amphiphilic macro-monomers that form micelles in water and are crosslinked throughout the core in the presence of azoisobutyronitrile (AIBN). The nanoparticle is 32.9 ± 7.8 nm in size after Gn3+ complexation and reveals a relaxivity r1 of 6.77 mm−1 s−1.