Four polylactide samples, obtained by ring-opening polymerization with an aluminum alkoxide initiator derived from a Schiff's base, were characterized by MALDI-TOF mass spectrometry. The MALDI mass spectra of these polylactides show well-resolved signals that can be reliably assigned to polylactide oligomers. Remarkably, both even-membered and odd-membered oligomers are present in these MALDI spectra. The presence of odd-membered oligomers cannot be explained on the basis of the lactide ring-opening polymerization, and one must admit that ester-exchange reactions do occur parallel to the polymerization process, causing a random cleavage of the polylactide chain. Furthermore, evidence for the presence of cyclic lactides was found in the MALDI-TOF spectrum of a low molecular weight polylactide fraction, indicating that ester exchange occurs also in polylactides by intramolecular endbiting reactions (ring-chain equilibration), with formation of cyclic oligomers.
S(−) and R(+) enantiomers of α‐methyl‐α‐ethyl‐β‐propiolactone (MEEPL) were prepared in an eight‐step synthesis with respective optical purities of 99 and 97% determined by 1H‐NMR (250 MHz) spectroscopy. Polymers (PMEPL) of different enatiomeric compositions were prepared with an anionic‐type initiator. Substantial differences in physical properties were observed between the racemic and optically pure polymers; for example, the melting point of the latter is 42°C higher than that of the former. Chiroptical properties of PMEPLs are reported. The 13C‐NMR (100.62 MHz) spectra of the polymers indicated that the distribution of configurational units in the macromolecular chain is random.
The reaction of ketene with chloral and 2,2-dichloropropanal leads, respectively, to the synthesis of 0-(trichloromethyl)-0-propioIactone (CC13-PL) and ß-(1,1-dichloroethyl)-0-propiolactone (CH3CC12-PL). Both monomers were prepared in the optically active form with enantiomeric excesses of 100% and 95% for CCI3-PL and CH3CC12-PL, respectively, using quinidine as a catalyst, and an enantiomeric excess of 100% for CH3CC12-PL using brucine as a catalyst. Both monomers were also prepared in the racemic form. Enantiomeric excesses were determined from optical rotation (CC13-PL) or NMR spectroscopy (CH3CC12-PL and CC13-PL) after complexation of the lactone with a europium chiral shift reagent. Polymerization was carried out in bulk and toluene solution, under vacuum, using mainly triethylaluminum as initiator. Solution characterization of the polymers was conducted by viscometry, osmometry, and gel permeation chromatography. The polymers prepared from optically active monomers are less soluble in organic solvents than those prepared from racemic monomers.In addition, the former polymers are crystalline and exhibit a high-temperature melting peak (Tm = 275 °C for poly(CCl3-PL) and 235 °C for poly(CH3CC12-PL)) whereas the latter polymers are amorphous and decompose at about 200 °C.
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