Biocompatible polyester macroligands: New subunits for the assembly of star-shaped polymers with luminescent and cleavable metal cores

The synthesis of a series of star-shaped, biocompatible polyesters-polylact ides (PLAs), polycaprolactones (PCLs), and various copolymer analogues-with either labile iron(II)tris-bipyridyl or luminescent ruthenium(II) tris-bip yridyl cores is described.(1) These polymers were readily assembled by a co nvergent, metal-template-assisted approach that entailed the synthesis of b ipyridine (bpy) ligands incorporating PLA- and PCL-containing arms and subs equent chelation of the "macroligands" to iron(II) or ruthenium(II). Specif ically, the polyester macroligands bpyPLA(2) and bpyPCL(2) were prepared by a stannous octoate catalyzed ring-opening polymerization of DL- or L-lacti de and epsilon -caprolactone, using bis(hydroxymethyl)-2,2'-bipyridine as t he initiator. Copolymers bpy(PCL-PLA)(2) and bpy(PLA-PCL)(2) were generated in an analogous manner using bpyPLA(2) and bpyPCL(2) as macroinitiators. P olymers with narrow molecular weight distributions and with molecular weigh ts close to values expected based upon monomer/initiator loading were produ ced. The macroligands were subsequently chelated to iron(II) to afford six- armed, iron-core star polymers, which were characterized by UV-vis and H-1 NMR spectroscopy. Estimated chelation efficiencies for formation of the sta r polymers (M-n calcd: 20--240 kDa) were high, as determined by UV-vis spec tral analysis. Within the molecular weight range investigated, differential scanning calorimetry and thermogravimetric analysis revealed that the smal l amounts of metal in the polyester stars and differences in polymer archit ecture had little effect on the thermal properties of the PLA/PCL materials . However, thin films of the red-violet colored iron-core stars exhibited r eversible, thermochromic bleaching. Solutions and films of the polymers als o responded (with colorless) to a variety of chemical stimuli (e.g., acid, base, peroxides, ammonia), thus revealing potential for use in diverse sens ing applications. Likewise, the polyester macroligands were chelated to rut henium(II) to produce both linear and star-shaped polymers, which were char acterized by UV-vis and H-1 NMR spectral analysis. Molecular weights of the polymers were determined by gel permeation chromatography (M-n(MALLS): 6-3 0 kDa) with in-line, UV-vis diode-array detection, confirming the presence of the [Ru(bpy)(3)](2+) core in the eluting polymer fractions. As was the c ase with the corresponding iron-core polyesters, estimated chelation effici encies were high.