Catalytic Synthesis and Characterization of Biodegradable ...

Chapter 7
7.1. Conclusion
In this thesis, the author described the catalytic synthesis and characterization of
biodegradable polyesters and their radical copolymers. In this chapter, the author has
described the synthesis of biodegradable polyesters by using a series of newly synthesized
cobalt-Schiff-base complexes catalysts. The catalyst activity, polymerization mechanism,
functionality of PLA by incorporating with stable TEMPO radical-substituted polymers and
important conclusions derived from this study are summarized.
In chapter 1, the author summarized “copolymer of epoxide and carbon dioxide”,
“polylactide”, “others biodegradable polyesters”, “electroactive polymers” and “radical
polymers”, respectively.
In chapter 2, a series of Cobalt Schiff-base complexes were investigated as the catalyst for
the alternating copolymerization of CO2 and rac-PO in the presence of Bu4NBr. The
poly(propylene carbonate) (PPC) and cyclic propylene carbonate (PC) selectivity of the
resultant copolymers were determined by modification of the length of the diimine bridges
between the two nitrogen atoms in the ligands. The L 1 -Co III -dnp/Bu4NBr catalyst exhibited
the highest activity, PPC/PC selectivity, and degree of head-to-tail linkages. The
L 2 -Co III -dnp/Bu4NBr catalyst showed slightly lower head-to-tail linkages. For the dimine
bridges containing three-carbon chains between the two nitrogen atoms in the Schiff bases,
the corresponding L 4 -Co III -dnp complex displayed the lowest catalytic activity. Based on the
electrochemical measurements to determine the half-wave potentials of the complexes, the
catalytic activity of these complexes was compared. The higher stability for the axial group’s
metal–O (phenolate) bond reflected the more negative E1/2 of the Co III + e -
‐ 130 ‐
Co II redox
couple for the L-Co III -dnp complexes with the diimine-bridge composed of two carbon atoms,
which led to the higher catalytic activity for the copolymerization of PO and CO2. In the case
of the cobalt complexes with more positive Co(II/III) potentials and thus with a lower electron
density on the cobalt center, the coordination of a labile propagating chain end to the cobalt
center is considered to determine the overall polymerization rate.
In chapter 3, Co(III) complexes with Schiff base ligands and Tin(II) alphatates, were used
as catalysts for the ROP of LacOCA to produce poly(lactic acid). Intra- and/or
inter-transesterification were suggested to coincide with the polymerization, which resulted in
a relatively large polydispersity in molecular weights. The carbonyldioxy group was