In situ polymerization of starch with lactic acid in aqueous solution and the microstructure characterization

Copolymers of starch grafted with poly (lactic acid) were directly prepared by in situ reaction of cornstarch with lactic acid in aqueous media in the presence of stannous 2-ethyl hexanoate [Sn(Oct)2]. The procedure of the graft reaction was elucidated, based on the HPLC analysis, as the ring-opening polymerization (ROP) from starch surface of the small amounts of lactide produced in situ in the reaction system. The detailed chemical microstructure of the resultant graft copolymer was clarified by one- and two-dimensional NMR spectroscopy. Analysis of 13C NMR spectra of the carbons in starch ring as well as the carbonyl carbons in LA moiety demonstrates that the reactivity of hydroxyl groups at glucopyranan unit of starch decreases in the order of C-6>C-3>C-2. The average length of PLA grafts at individual starch ring carbons was also investigated.