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 T 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 x 10(3) to 7.4 x 10(4), prepared by ring-opening polymerization of LA initiated with Sn(OC4H9)(2) (THF. 80 degreesC), 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) ( = 13.4) and LA monomer. As coinitiator/catalyst tin(II) octoate (Sn(Oct)(2)) has been used (bulk polymerization, 120 degreesC). H-1 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. (C) 2003 Elsevier Science Ltd. All rights reserved.