Disclosed are methods of preparing a macromolecular conjugated ligand and a metal complex thereof. The metal complex is targeted for use as a contrast agent, for example, in MRI. The method of preparing a macromolecular conjugated ligand comprises: (a) providing a compound of formula (I) ##STR00001## wherein R, A, and Pg are as defined herein, (b) reacting the compound of formula (I) with a macromolecular compound (e.g., dendrimer) in an organic solvent medium which is substantially free of water to obtain a macromolecular conjugated compound, and (c) removing the carboxyl-protecting groups to obtain a carboxyl-deprotected macromolecular conjugated compound. The metal complex can be prepared by reacting the carboxyl-deprotected macromolecular conjugated compound with an ion (e.g., Gd(III)). Also disclosed are two carboxyl-protected 1B4M-DTPA intermediate compounds.