Synthesis and Properties of Novel Poly(Hexyl-Substituted Lactides) for Pharmaceutical Applications

Abstract

Monohexyl-substituted lactide (mHLA) was synthesized by reaction of 2-hydroxyoctanoic acid with 2-bromopropionyl bromide, and polymerized with tin(II) 2-ethylhexanoate (Sn(Oct)2) or 4-dimethylaminopyridine (DMAP) in the presence of benzyl alcohol by ring-opening polymerization (ROP). Poly(monohexyl-substituted lactide) (PmHLA) of predictable molecular weights and narrow polydispersities were obtained in convenient bulk conditions at 100 °C within short polymerization times. The polymerizations were well controlled, showing a 'living' character for targeted degrees of polymerization up to DP = 60 as evidenced by molecular weight versus conversion studies and 1H NMR end group analysis. The hexyl groups have a strong impact on the glass transition temperature (Tg), which is low for PmHLA compared to standard poly(D,L-lactide) (PLA). Tg and zero shear viscosities at 25 °C can be controlled by the polymer molecular weight, ranging from ?22 °C for Mn = 2800 g/mol to ?10 °C for Mn = 9100 g/mol and 140 to 4850 Pa.s, respectively. These data are in correspondence with the Fox and Flory equations. The degradation mechanism of the PmHLA polymer in phosphate buffer pH 7.4 at 37 °C was shown to be similar to that of the standard PLA ('bulk erosion' type), with a slightly higher degradation rate, leading to the non-toxic degradation products lactic acid and 2-hydroxyoctanoic acid. PmHLA has the great potential as an alternative to conventional PLA/PLGA for drug delivery systems. By the hexyl-substitution the biodegradable PLA-ester backbone is conserved but the hydrophobicity is increased in comparison to standard PLA, while a viscous polymer is obtained. This leads to advantageous injectable solvent-free drug delivery systems, in which drugs can easily be incorporated by simple mixing.