Catalytic Synthesis and Characterization of Biodegradable ...

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