Catalytic Synthesis and Characterization of Biodegradable ...

Chapter 1
water and alcohols, which are either present in the lactide feed or can be added by demand.
Although high molecular weight PLA can be obtained through the ROP of lactide, side
reactions, such as intermolecular and intramolecular transesterification reactions exist and
perturb the chain propagation, broaden the molecular weight distribution, and yield cyclic
oligomers. Therefore, catalyst that has lower side reactions is desired. Indeed, aluminum
based single-site complexes have shown excellent controllability for the solution
52, 54, 61-67
polymerization of lactide.
Figure 1.3.5 ROP of lactide through a coordination-insertion mechanism.
DMAP 47 and lipase 49 can be used as the catalysts for the ROP of LacOCA. The
ring-opening reactions of LacOCA with alcohols are predicted to occur through the activation
of the alcohol and with both the traditional stepwise mechanisms, which involve tetrahedral
intermediates (Figure 1.3.6). Furthermore, DMAP is proposed to act as a bifunctional catalyst
through its basic nitrogen center and an acidic ortho-hydrogen atom. 48 The ROP of LacOCA
using DMAP as catalyst gives access to PLA of controlled molecular weights and low
polydispersities under mild conditions (typically within a few minutes at room temperature
with LacOCA). 47 Both lipase PS and Novozym 435 promote the ring-opening
polymerization of LacOCA (Figure 1.3.7). Accordingly, PLA of relatively high molecular
weights and low polydispersities are obtained in high yields within a few hours at 80 .
Slight preference for L-lacOCA over D-lacOCA is observed, and with Novozym 435, the
molecular weight of the obtained PLA can be controlled by varying the lipase loading.
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