Catalytic Synthesis and Characterization of Biodegradable ...

Chapter 2
useful for designing a highly efficient catalytic system.
Table 2.4 Cyclic voltammogram data for the transition between Co III + e -
Complex Epc (mV) E1/2 (mV) ΔE ipa/ipc
L 1 -Co III -dnp 32 107 150 0.64
L 2 -Co III -dnp 84 134 99 0.83
L 3 -Co III -dnp 52 163 221 0.82
L 4 -Co III -dnp -71 32 206 0.78
‐ 68 ‐
Co II of L-Co III -dnp. a)
a) Determined using 1 mM solution of L-Co III -dnp in DMF in the presence of 0.1 M NBu4ClO4 at carbon
electrode vs. Ag/Ag + at the scan rate of 100 mV s -1 .
2.5 Conclusions
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, the half-wave potentials of the complexes were obtained and
the catalytic activity of these complexes was compared. The higher stability for the axial
group’s metal–O (phenolate) bond indicated the more negative E1/2 of the Co III + e -
redox couple in the L-Co III -dnp complexes, and 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 lower electron density on the cobalt center, the end of propagating
chain to the cobalt center was considered to determine the overall polymerization rate.
However, the L 4 -Co III -dnp with the most negative E1/2 possessed the lowest catalytic activity,
which may be due to the unfavorable trigonal bipyramidal geometry during the transition state,
preventing the Lewis bases from bonding to the metal center.
2.6 Experimental Part
Materials
All experiments involving air- and/or water-sensitive compounds were carried out using
standard Schlenk techniques under a dry argon atmosphere. All solvents were purified by
Co II