Biocompatible Polyester Macroligands: New Subunits for the Assembly of Star-Shaped Polymers with Luminescent and Cleavable Metal Cores

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Abstract
The synthesis of a series of star-shaped, biocompatible polyesterspolylactides (PLAs), polycaprolactones (PCLs), and various copolymer analogueswith either labile iron(II) tris-bipyridyl or luminescent ruthenium(II) tris-bipyridyl cores is described.1 These polymers were readily assembled by a convergent, metal-template-assisted approach that entailed the synthesis of bipyridine (bpy) ligands incorporating PLA- and PCL-containing arms and subsequent chelation of the “macroligands” to iron(II) or ruthenium(II). Specifically, the polyester macroligands bpyPLA2 and bpyPCL2 were prepared by a stannous octoate catalyzed ring-opening polymerization of dl- or l-lactide and ε-caprolactone, using bis(hydroxymethyl)-2,2‘-bipyridine as the initiator. Copolymers bpy(PCL−PLA)2 and bpy(PLA−PCL)2 were generated in an analogous manner using bpyPLA2 and bpyPCL2 as macroinitiators. Polymers with narrow molecular weight distributions and with molecular weights close to values expected based upon monomer/initiator loading were produced. The macroligands were subsequently chelated to iron(II) to afford six-armed, iron-core star polymers, which were characterized by UV−vis and 1H NMR spectroscopy. Estimated chelation efficiencies for formation of the star polymers (Mn calcd: 20−240 kDa) were high, as determined by UV−vis spectral analysis. Within the molecular weight range investigated, differential scanning calorimetry and thermogravimetric analysis revealed that the small amounts of metal in the polyester stars and differences in polymer architecture had little effect on the thermal properties of the PLA/PCL materials. However, thin films of the red-violet colored iron-core stars exhibited reversible, thermochromic bleaching. Solutions and films of the polymers also responded (with color loss) to a variety of chemical stimuli (e.g., acid, base, peroxides, ammonia), thus revealing potential for use in diverse sensing applications. Likewise, the polyester macroligands were chelated to ruthenium(II) to produce both linear and star-shaped polymers, which were characterized by UV−vis and 1H NMR spectral analysis. Molecular weights of the polymers were determined by gel permeation chromatography (Mn(MALLS): 6−30 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 case with the corresponding iron-core polyesters, estimated chelation efficiencies were high.

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