Khalid A. M. Thakur

A Quantitative Method for Determination of Lactide Composition in Poly(lactide) Using 1H NMR

A method has been developed to quantitatively determine the composition of d-lactide and meso-lactide stereoisomer impurities in poly(lactide) containing predominantly l-lactide. In this method, the stereosequence information obtained from a few well-resolved resonances in the (1)H NMR spectrum representing RR and R stereogenic defects is used. The d-lactide and meso-lactide as minor components lead to RR and R stereogenic defects, respectively, which influence the isotactic chain length distribution and hence affect the polymer properties. Analytical equations relating the stereosequence probability to the lactide feed composition are not available due the complicated kinetics involved for the melt polymerization; viz. the preference for syndiotactic lactide addition decreases with reducing residual lactide concentration in the batch process. Hence, empirical correlations were determined by least-squares fit to the predictions for the specific stereosequence probabilities provided by Monte Carlo calculations of a number of lactide stereocopolymerizations. The Monte Carlo calculations simulate the kinetics observed for melt polymerization at 180 °C catalyzed by Sn(II) bis(2-ethylhexanoate) (Sn(II) octoate) in a 1:10 000 catalyst/lactide ratio.

Ultrafast studies on proton transfer in photostabilizers

Abstract Excited-state intramolecular proton transfer (ESIPT) is currently of considerable experimental theoretical interest and practical industrial interest, e.g. in polymer stabilizers. The vast majority of ESIPT systems involve transfer of a proton from an oxygen donor. 1,5-dihydroxyanthraquinone is typical, with ESIPT leading to dual fluorescence emission. The emission is conveniently discussed in terms of a four-state diagram, where N, N*, T and T* correspond to the normal 9,10-keto ground state, the normal 9,10-keto excited state, the 1,9-keto ground tautomeric state and the 1,9-keto excited tautomeric state respectively. The short-wavelength emission has been assigned to the normal excited state and the long-wavelength emission to the excited tautomer. ESIPT with nitrogen as the proton donor is rare and is often complicated by dimerization. Previous studies of 1-aminoanthraquinone and 1-(acetylamino)anthraquinone have found no evidence for ESIPT. We now report compelling evidence for ESIPT in 1-(acylamino)anthraquinones.

An alternative interpretation of the HETCOR NMR spectra of poly(lactide)

An alternative to the assignments proposed recently by Chisholm et al. for the 1H and 13C NMR resonances of poly(lactide) based on their HETCOR spectra is presented; we find that the HETCOR spectra are consistent with older assignments of the tetrad and hexad stereosequence resonances in the 1H and 13C NMR spectra and we believe that the influence of adjacent chiral units on the NMR chemical shift extends asymmetrically and is opposite in the case of 1H and 13C nuclei; this is in contrast to the assumption that the observed chiral repeat unit for any particular stereosequence resonance is the same for 1H and 13C NMR spectra as used by Chisholm et al. in their analysis.

Ultrafast experiments on intermolecular electron transfer in the benzene - bromine atom charge transfer complex

Abstract Ultrafast pump - probe measurements have been made on the benzene - bromine atom charge transfer (CT) complex in CCl4 and cyclohexane solutions. Ultrafast optical excitation of the CT band of the complex yields an ion pair, which is comprised of a benzene cation and a bromide anion. The rate of charge recombination between the bromide and the benzene cation in the ion pair has been observed to be much faster than the rate of diffusion apart. The charge recombination rate is accelerated at high benzene concentrations as a result of the formation of a benzene dimer cation - bromide ion pair which undergoes much faster charge recombination than the benzene cation - bromide ion pair.