Molecular dynamics of star-shaped poly(L-lactide)s in tetrahydrofuran as solvent monitored by fluorescence spectroscopy

Linear telechelic, alpha,omega-ditelechelic, and star-shaped tri-, tetra-, penta-, and hexa-arm poly(L-lactide)s (PLAs) fitted at every arm with pyrene end group have been prepared. Internal dynamics and mobility of the PLA chains in tetrahydrofuran solution at 25 degrees C, with regard to the number of PLA arms in one macromolecule and the individual arm average degree of polymerization, was followed by fluorescence spectroscopy. Analysis of both static and time-resolved spectra of the star-shaped polymers revealed dynamic segmental motion resulting in end-to-end cyclization, accompanied by an excimer formation. Probability and rate of the latter reaction increased with increasing number of arms and with decreasing their polymerization degree. Moreover, time-resolved measurements revealed that for macromolecules containing few arms (2 or 3) the pyrene moieties are located in the interior of the star-shaped PLAs, whereas in the instance of the higher number of arms (4-6) they are located at the periphery of the star-shaped PLAs. Thus, increasing the number of arms leads to their stretching away from the center of the star-shaped PLA macromolecule. (c) 2005 Wiley Periodicals, Inc.