Catalytic Synthesis and Characterization of Biodegradable ...
Catalytic Synthesis and Characterization of Biodegradable ...
Catalytic Synthesis and Characterization of Biodegradable ...
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<strong>Synthesis</strong> <strong>and</strong> Electrochemistry <strong>of</strong> Schiff Base Cobalt(III) Complexes <strong>and</strong> Their<br />
<strong>Catalytic</strong> Activity for Copolymerization <strong>of</strong> Epoxide <strong>and</strong> Carbon Dioxide<br />
The Lewis basic monomer coordinated to the metal center in the axial site transferred to<br />
the propagating metal-polymer chain, thereby labilizing the metal alkoxide bond <strong>and</strong><br />
facilitating the insertion <strong>of</strong> CO2. 3, 15, 22-25 The rac-PO monomer might be first favorably<br />
inserted into the Co−OR bond <strong>of</strong> the Schiff base cobalt complexes, followed by the insertion<br />
<strong>of</strong> the CO2 monomer, as shown in Scheme 2.2. Subsequent alternating copolymerization <strong>of</strong><br />
rac-PO <strong>and</strong> CO2 afforded the alternating repeating unit structure to yield the polycarbonate.<br />
The side reaction to yield the cyclic carbonate could take place through the backbiting<br />
degradation <strong>of</strong> the growing polymer-catalyst complex, which may lead to the lower polymer<br />
yield. A similar mechanism has been proposed by Nguyen <strong>and</strong> co-workers using cobalt (salen)<br />
<strong>and</strong> N, N-dimethylaminoquinoline for the copolymerization <strong>of</strong> CO2 <strong>and</strong> PO. 15<br />
2.3.2 Complex Structure <strong>and</strong> Polymerization<br />
The copolymerizations <strong>of</strong> CO2 <strong>and</strong> rac-PO catalyzed by the series <strong>of</strong> L-Co III -dnp/Bu4NBr<br />
catalysts were studied, <strong>and</strong> the results are summarized in Table 2.3. The diimine-bridge (X)<br />
between the two nitrogen atoms in the Schiff bases significantly affected the catalytic activity.<br />
With X being (R,R)-1,2-cyclohexanediamine (L 1 -Co III -dnp in Scheme 2.1), the highest<br />
catalytic activity with a turn-over frequency (TOF) <strong>of</strong> 245 h -1 was accomplished (Table 2.3).<br />
Under the same conditions, when X was replaced by ethylenediimine or (R,<br />
R)-1,2-diphenylethylenediimine, the catalytic frequency was reduced to 210 <strong>and</strong> 190 h -1 for<br />
the L 2 -Co III -dnp/Bu4NBr <strong>and</strong> the L 3 -Co III -dnp/Bu4NBr catalysts, respectively. When X was<br />
2,2-dimethyl-1,3-propylenediimine, the L 4 -Co III -dnp/Bu4NBr catalyst showed the lowest<br />
activity. The catalytic activity <strong>of</strong> these complexes for the alternating copolymerization <strong>of</strong> CO2<br />
<strong>and</strong> rac-PO was in the order <strong>of</strong> L 1 -Co III -dnp > L 2 -Co III -dnp > L 3 -Co III -dnp >> L 4 -Co III -dnp,<br />
which revealed that the diimine-bridges with the three carbon atoms led to a lower activity.<br />
One could anticipate that the degree <strong>of</strong> polarization <strong>and</strong>/or the strength <strong>of</strong> the Co–O bond for<br />
the axial coordination should influence the rate <strong>of</strong> the monomer insertion <strong>and</strong> thus should<br />
affect the rate <strong>of</strong> the propagation during the polymerization. Although attempts to obtain all<br />
the molecular structures <strong>of</strong> the L-Co III -dnp complexes were not successful, the nature <strong>of</strong> the<br />
Co–O bond has been successfully determined by electrochemical methods (vide infra).<br />
Table 2.3 The catalytic activity for the copolymerization <strong>of</strong> CO2/rac-PO using the L-Co III -dnp/Bu4NBr<br />
catalyst systems. a)<br />
Run Complex<br />
TOF<br />
(h –1 ) b)<br />
Selectivity<br />
(%PPC) c)<br />
Head-to-tail<br />
Linkages (%) d)<br />
‐ 65 ‐<br />
Mn e)<br />
×10 4<br />
Mn f)<br />
×10 4<br />
1 L 1 -Co III -dnp 245 74 88 2.6 1.4 1.4<br />
PDI f)<br />
2 L 2 -Co III -dnp 210 60 71 1.4 1.1 1.3<br />
3 L 3 -Co III -dnp 190 75 82 4.0 2.6 1.2