in situ GRAFT COPOLYMERIZATION OF STARCH WITH LACTIC ACID

The biodegradable starch graft copolymer,starch-g-poly(lactic acid)(starch_g_PLA),was directly produced by in situ graft copolymerization of cornstarch with lactic acid using stannous 2-ethyl hexanoate[Sn(Oct)_2] as catalyst.The procedure of the grafting 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.Analysis of ()~(13)C_NMR spectra indicated that ROP of lactide was taken place through acyl_oxygen cleavage co_initiated by hydroxyl group at glucopyranan unit,yielding PLA grafts with a hydroxyl terminus.()~1H_and()~(13)C_NMR spectra were employed to assign the characteristics resonances of the copolymer.The positions of lactyl units at the glucopyranan ring as well as the corresponding distribution are determined.The DS values of the graft copolymer calculated based on()~(13)C_NMR spectra of starch ring carbons are indentical to those of carbonyl carbons in LA moieties.The reactivity of hydroxyl groups at glucopyranan rings decreases in the order of C_6C_3C_2,while the average length of PLA grafts increases in the order of C_3C_6C_2.The phenomena are proposed as a result of the difference of the steric hindrance of hydroxyl groups at glucopyranan units in starch.It was found that the grafted PLA length increased slightly with prolonging the reaction duration.The average length of PLA grafts was 1.33 for 5 h reaction,and that reached to 2.02 when the reaction duration increased to 13 h.