Synthesis and characterization of multi-armed calixarene- and resorcinarene-core polylactide star polymers

Abstract

The renewable, biodegradable polymer polylactide (PLA) has been employed in biomedical applications, and its use is becoming more common within various commercial packaging materials. Architectural diversity within this class of polymers, however, is still somewhat limited; which, in turn, limits the properties of this important class of polymers. Herein, we describe the synthesis and characterization of a series of multi-armed resorcinarene- and calixarene-core PLA star polymers. These macrocyclic-core, four- and eight-armed star PLAs were prepared by tin(II)-catalyzed ring-opening polymerizations of L-lactide and racemic DL-lactide using hydroxyl-functionalized calixarene and resorcinarene initiators. The resulting polymers have narrow PDIs and molecular weights close to those targeted based upon monomer/initiator ratios, as determined by GPC, NMR spectroscopy, and MALDI-TOF mass spectrometry. Moreover, end-group analysis by ¹H NMR spectroscopy suggests that all alcohol groups within the calixarene and resorcinarene macrocycles initiate polymerization, which supports star polymer production. Hydrogenolysis studies with eight-armed resorcinarene-core star PLAs that incorporate a cleavable benzyl linker between the core and the arms have allowed the arms to be characterized independently from the intact star polymers. Finally, the thermal properties of the calixarene- and resorcinarene-core star PLAs have been analyzed by DSC. The lower molecular weight star polymers exhibited slower crystallization rates than linear analogs, which allowed the percent crystallinities of the samples to be readily controlled over a range from approximately 0–30%, depending upon the macrocyclic core and annealing time.