Salen-based polymers

Another example of catalyst modification was achieved by synthesizing a Co(salen)-based catalyst with pendent ammonium salts (Figure 8.27) [54], This complex was able to catalyze the copolymerization of C02 and PO at TOF-values of up to 26000h 1, with high molecular weights and low polydispersities. Upon completion of the reaction the polymer/catalyst mixture was filtered through a pad of silica which yielded the purified polymer product. When the Co(salen) complex had been recovered from the silica it was recycled several times, with little to no loss of activity. 

A similar approach permitted to prepare a macroporous polystyrene-based polymer 306 via a radical copolymerization of a divinylsalen 304 or of a distyryl spaced salen 305 with styrene and DVB (ratio 10/75/15) (Scheme 125) [188]. 

Kingsborough, R.P., and T.M. Swager. 1998. Electroactivity enhancement by redox matching in cobalt salen-based conducting polymers. Adv Mater 10 1100-1103. 

In principle, rac-lactide, a racemic mixmre of d- and L-lactide, may be polymerized in a stereoselective fashion. Depending on the stereoselection as the ROP proceeds, the resulting polymer may thus exhibit different stereoregularities these directly influence the thermal and mechanical properties of the produced PLAs. In this regard, isotactic PLA stereoblocks and PLA stereocomplexes, which are of interest for their thermal and mechanical properties, may be produced via the ROP of rac-lactide initiated by an achiral derivative, provided the polymerization proceeds via a chain-end stereocontrolled mechanism i.e., the last inserted lactide unit stereo-controls the insertion of the incoming monomer. This strategy has been first validated using salen-based aluminum complexes such as 16 (Scheme 16, top) to produce PLLA-PDLA isotactic stereoblocks [95, 96]. Alternatively, the chiral racemic salen aluminum complex 17 was found to be suitable for the parallel stereoselective synthesis of isotactic poly(D-lactide) and poly(L-lactide) from rac-... 

Kureshy developed a polymer-based chiral Mn-salen complex (Figure 21). Copolymerization of styrene, divinylbenzene, and 4-vinylpyridine generated highly cross-linked (50%) porous beads loaded with pyridine ligands at 3.8 mmol g-1. Once the polymer was charged with the metal complex catalyst, enantioselective epoxidation of styrene derivatives was achieved with ee values in the range 16 46%. 79... 

Figure 21 Kureshy s polymer-based Mn-salen complex.

Ghromium complexes have been found to promote the co-polymerization of epoxides with G02- Recently, Darensbourg et al. have demonstrated that the chromium-salen complexes, remarkably more stable to the air and moisture than zinc-based co-polymerization catalysts, are effective catalysts for the co-polymerization of GHO with G02. " Under the condition of 5.9MPa GO2 pressure at 80 °G, complex 35a transforms GHO to the completely alternating co-polymer with a TON of 250 mol (mol of Zn) and a TOF of 10 mol (mol of Zn) h along with a small amount of eyelie earbonate production (Table 7). 

A one-sided binding of the complex to the support resulted in further improvements (92). This requires synthesis of nonsymmetric salen-type ligands, which is complicated by the tendency of such ligands to equilibrate to give mixtures containing symmetric Schiff bases. Excellent results were obtained with monomer 7d, diluted in a methacrylate polymer, by using a combination of meta-chloroperbenzoic acid (mCPBA) and A-methyl-morpholine-A-oxide (NMO) as the oxidants ... 

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