Characterization of star-shaped poly(l-lactide)s by liquid chromatography at critical conditions

A series of linear and star-shaped poly(l-lactide)s (PLA's) have been prepared by living polymerization of l,l-dilactide (LA) and analyzed by liquid chromatography at critical conditions (LC-CC). For the analysis of the PLA's LC-CC conditions have been used corresponding to silica gel as the stationary phase and a mixture of 1,4-dioxane/n-hexane (56.25/43.75 by vol%) at 50 °C as the mobile phase. At the critical point of adsorption, a series of linear C₄H₉-PLA-OH's having molar masses (M_n) in the range from 2.3×10³ to 7.4×10⁴, prepared by ring-opening polymerization of LA initiated with Sn(OC₄H₉)₂ (THF, 80 °C), showed no dependence of the elution volumes on molar mass. In subsequent experiments, star-shaped PLA's bearing various numbers of PLA-OH arms (R-(PLA-OH)_x) have been prepared in a controlled synthesis starting from various polyols (R-(OH)_x) containing exclusively primary hydroxyl groups: diethyleneglycol (x=2), trimethylolpropane (x=3), di(trimethylolpropane) (x=4), dipentaerithritol (x=6), and poly(3-ethyl-3-hydroxymethyloxetane) (〈x〉=13.4) and LA monomer. As coinitiator/catalyst tin(II) octoate (Sn(Oct)₂) has been used (bulk polymerization, 120 °C). ¹H NMR analysis of the resulting star-shaped polymers revealed that all OH-groups in the polyols started growth of the PLA chains. The series of star-shaped PLA's have been analyzed by LC-CC as well as by two-dimensional (2D) chromatography (i.e. LC-CC versus size exclusion chromatography (SEC)) with regard to possible structural imperfections. It has been shown, that the LC-CC elution volumes of the resulting R-(PLA-OH)_x increase with the number of PLA-OH arms, allowing discrimination of the individual R-(PLA-OH)_x's in their mixture. An exponential increase of the retention volume as a function of the number of arms has been found. Eventually, LC-CC measurements of the elution volumes carried out for acetylated star-shaped PLA's (R(PLA-OOCCH₃)_x) have shown that for the interactions of the R-(PLA-OH)_x macromolecules with the column packing the hydroxyl end-groups are mostly responsible.