Exploring the effect of alkyl end group on poly(L-lactide) oligo-esters. Synthesis and characterization

Poly(L-lactide) (PLLA) oligo-esters with α-hydroxyl-ω-alkyl (alkyl = −CH2−[CH2−CH2]n−CH3, where n = 1, 2, 4, 5, 6, 7, 8, 9, and 10) end groups were synthesized by ring-opening polymerization of L-lactide (L-LA) catalyzed by tin(II) 2ethylhexanoate Sn(Oct)2 in the presence of aliphatic alcohols as initiators (HO−CH2−[CH2−CH2]m−CH3, where m = 1, 2, 4, 5, 6, 7, 8, 9, and 10). High yields (∼ 62 to 71 %) and Mn(NMR) in the range of 2120-2450 Da (PLLA) were obtained. Effects of alkyl end groups on thermal properties of the oligo-esters were analyzed by DSC, TGA and SAXS. Glass transition temperature (Tg) gradually decreases with increase in the percent of −CH2−[CH2−CH2]m−CH3 end group, as results alkyl end group provides most flexibility to PLLA. An important effect of alkyl end group on a double cold crystallization (Tc1 and Tc2) was observed, and is directly related with the segregation phase between alkyl end group and PLLA. TGA analysis revealed that PLLA oligo-esters are more thermally stable with docosyl (−C22H45) respect to the butyl (−C4H9) end group, probably is due to steric hindrance of the end group (docosyl respect to butyl) toward intermolecular and intramolecular transesterification. SAXS analysis showed that alkyl end group as docosyl restricted the growth of lamellae thickness (D) due to steric hindrance. Characterization of hydroxyl and alkyl end groups in the PLLA oligo-esters was determined by MALDI-TOF, GPC, FT-IR and H and C NMR.