Polymer Reference Materials: Round-Robin Tests for the Determination of Molar Masses

Determination of molar masses of macromolecules by size exclusion chromatography-light scattering not requiring knowledge of refractive index increments

A new approach for the calibration of SEC-light scattering (SEC-LS) setups is proposed, which requires solely the molar mass of a reference polymer. Neither the specific refractive index increment of the calibrant nor of the analyte is required. Comparison of the molar masses derived in different solvents for a large number of chemically different polymers shows that the new approach yields the same molar masses as if molar masses were derived using dn/dc to calibrate the light scattering setup. The approach therefore allows easier determination of molar masses by SEC-LS.

Ring-Opening Polymerisation ofrac-Lactide Using a Calix[4]arene-Based Titanium (IV) Complex

cone-25,27-Dipropyloxy-26,28-dioxo-calix[4]arene titanium (IV) dichloride(1)has been assessed in the ring-opening polymerisation ofrac-lactide (L,D-LA). The polymers formed (PLDA) turned out to display an isotactic stereoblock microstructure (determined by NMR) despite the fact that the catalyst hasC2vsymmetry. Two techniques were applied for initiating the polymerisation reaction, microwave irradiation, and conventional thermal treatment. The polymers obtained were all characterised by NMR, IR, HPLC-SEC, DSC, and MALDI-TOF analysis. The use of microwave irradiation, applied for the first time to calixarene-based catalysts in the presence of therac-lactide monomer, increased the polymerisation rate compared with that obtained by the other method. On the other hand, standard thermal treatment enabled a slightly better control than microwave irradiation over the molecular weight and molecular weight distribution of the polylactides formed.

How much does band broadening adulterate results deduced from chromatograms measured by size-exclusion chromatography really?

A number of polystyrene samples and standards were measured with several column combinations which differed in their extent of band broadening, sigma(BB). Comparison of the derived chain length distributions showed in some cases good agreement even despite strong distinctions in the determined sigma(BB) values. The calculated number and weight averages of the samples were almost identical in most cases for the column combinations used. Furthermore, the points of inflection of the standards and of multimodale distributions composed of narrow (Poisson like) peaks in microemulsion were examined. When conferred with the theoretical values which were calculated for assumed Poisson distributions the respective deviations from the "true" ones were as high as 10% (almost reaching 20% in unfavorable cases). Simple (correction) procedures were tested in order to obtain the actual average values as well as the effective points of inflection of narrow distributions.