Catalytic Synthesis and Characterization of Biodegradable ...
Catalytic Synthesis and Characterization of Biodegradable ...
Catalytic Synthesis and Characterization of Biodegradable ...
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
Chapter 2<br />
useful for designing a highly efficient catalytic system.<br />
Table 2.4 Cyclic voltammogram data for the transition between Co III + e -<br />
Complex Epc (mV) E1/2 (mV) ΔE ipa/ipc<br />
L 1 -Co III -dnp 32 107 150 0.64<br />
L 2 -Co III -dnp 84 134 99 0.83<br />
L 3 -Co III -dnp 52 163 221 0.82<br />
L 4 -Co III -dnp -71 32 206 0.78<br />
‐ 68 ‐<br />
Co II <strong>of</strong> L-Co III -dnp. a)<br />
a) Determined using 1 mM solution <strong>of</strong> L-Co III -dnp in DMF in the presence <strong>of</strong> 0.1 M NBu4ClO4 at carbon<br />
electrode vs. Ag/Ag + at the scan rate <strong>of</strong> 100 mV s -1 .<br />
2.5 Conclusions<br />
A series <strong>of</strong> Cobalt Schiff-base complexes were investigated as the catalyst for the<br />
alternating copolymerization <strong>of</strong> CO2 <strong>and</strong> rac-PO in the presence <strong>of</strong> Bu4NBr. The<br />
poly(propylene carbonate) (PPC) <strong>and</strong> cyclic propylene carbonate (PC) selectivity <strong>of</strong> the<br />
resultant copolymers were determined by modification <strong>of</strong> the length <strong>of</strong> the diimine bridges<br />
between the two nitrogen atoms in the lig<strong>and</strong>s. The L 1 -Co III -dnp/Bu4NBr catalyst exhibited<br />
the highest activity, PPC/PC selectivity, <strong>and</strong> degree <strong>of</strong> head-to-tail linkages. The<br />
L 2 -Co III -dnp/Bu4NBr catalyst showed slightly lower head-to-tail linkages. For the dimine<br />
bridges containing three-carbon chains between the two nitrogen atoms in the Schiff bases,<br />
the corresponding L 4 -Co III -dnp complex displayed the lowest catalytic activity. Based on the<br />
electrochemical measurements, the half-wave potentials <strong>of</strong> the complexes were obtained <strong>and</strong><br />
the catalytic activity <strong>of</strong> these complexes was compared. The higher stability for the axial<br />
group’s metal–O (phenolate) bond indicated the more negative E1/2 <strong>of</strong> the Co III + e -<br />
redox couple in the L-Co III -dnp complexes, <strong>and</strong> the higher catalytic activity for the<br />
copolymerization <strong>of</strong> PO <strong>and</strong> CO2. In the case <strong>of</strong> the cobalt complexes with more positive<br />
Co(II/III) potentials <strong>and</strong> lower electron density on the cobalt center, the end <strong>of</strong> propagating<br />
chain to the cobalt center was considered to determine the overall polymerization rate.<br />
However, the L 4 -Co III -dnp with the most negative E1/2 possessed the lowest catalytic activity,<br />
which may be due to the unfavorable trigonal bipyramidal geometry during the transition state,<br />
preventing the Lewis bases from bonding to the metal center.<br />
2.6 Experimental Part<br />
Materials<br />
All experiments involving air- <strong>and</strong>/or water-sensitive compounds were carried out using<br />
st<strong>and</strong>ard Schlenk techniques under a dry argon atmosphere. All solvents were purified by<br />
Co II