Functionalized bio-polyesters by nitroxide radical coupling

The preparation of functionalized bio-polyesters by radical grafting of functional molecules is a controversial issue. The well known approach of functionalizing polyolefins, which is the peroxide-initiated melt grafting of maleic anhydride, has been described as a method to improve miscibility and thereby thermal and mechanical properties of polyester blends and composites. Nonetheless, the covalent grafting of functional moieties on polyester chains has not been fully characterized. The major difficulties encountered are due to the inability of IR spectroscopy to differentiate the C=O stretching signals of covalent grafted moieties with those of the polyester chains. In addition, data are not available about the control of chain degradation and crosslinking side reactions during the functionalization process. In this work, a post-reactor modification method based on the reaction between nitroxide functional derivatives and macroradicals of poly(butylene succinate) (PBS) and poly(lactid acid) (PLA) was investigated. This approach, which was recently fine tuned by our group for polyolefins [1], allows to achieve a good grafting efficiency and a minor extent of side reactions. In particular, 4-benzoyloxy-TEMPO and 4-(1-naphtoate)-TEMPO, a pro-fluorescent nitroxide, were successfully grafted onto PBS and PLA, as evidenced by UV-Vis and fluorescence spectroscopies. The grafting was also confirmed and estimated by 1HNMR, whereas the preservation of molecular weight and distribution was evidenced by SEC analysis.