Essentially, the same result was obtained by analyzing
a high conversion (96.0%) poly-(L)-lactide sample
(PLA3, see Figure 3). This sample was prepared from
pure L enantiomer, at low [M]/[I] ratio (Table 1) and
with addition of pyridine in order to decrease the
transesterification reactions. Starting from a pure
enantiomer, it is not possible to determine by NMR the
amount of transesterification, since the “forbidden”
modes do not exist.7,8 The MALDI-TOF mass spectrum
of the PLA3 sample reported in Figure 3 was recorded
in reflectron mode (allowing a higher resolution with
respect to the linear mode) and shows well-resolved
signals approximately in the 1000-4000 Da region. The
inset in Figure 3 shows an expanded portion of the
spectrum with two distinct mass series, due to openchain
oligomers clustered to Na+ ions and terminated
by methoxy ester and carboxylic end groups, respectively.
The carboxylic end group may be formed by
hydrolysis of ester groups, owing to the presence of a
trace of water in the synthesis mixture.
Since the unfractionated poly-(D,L)-lactide samples
(PLA1 and PLA2) failed to show the presence of cyclic
oligomers, as well as the high conversion (96.0%) poly-
(L)-lactide sample (PLA3) prepared in the presence of
pyridine, a high conversion (98%) poly-(D,L)-lactide
sample obtained after a long period of polymerization
(18 weeks) was examined and more particularly the
oligomer part obtained by treating the raw unfractionated
polymer with methanol. This low MW fraction
(PLA4) was then analyzed by MALDI-TOF.
The spectrum in Figure 4, obtained in reflectron
mode, shows well-resolved signals approximately in the
500-5000 Da region. The most intense peak is at 599
Da, and it corresponds to the sodium adduct of the cyclic
octamer of lactic acid. The inset in Figure 4 shows an
expanded portion of the spectrum with four distinct
mass series, due to open-chain and cyclic oligomers
clustered to Na+ and K+ ions, respectively. Therefore,
the MALDI spectrum in Figure 4 shows without ambiguity
the presence of open-chain and cyclic oligomers
(approximately in 60/40 molar ratio), and this result
indicates the occurrence of intramolecular exchange
reactions (Scheme 2b).
These preliminary results obtained on polylactides
prepared using the SALENAlOCH3 initiator system
indicate that polymerization and transesterification
reactions are occurring in parallel from the beginning
(as indicated by the results on the low conversion PLA1
sample).
A more careful examination of different polymers
obtained with this particular initiator and with other
initiators is necessary to substantiate a general scheme
of transesterification reactions occurring in the course
of ring-opening lactide polymerization.