Schiff bases polymeric

A linear correlation was also found between the a-factors and the isotropic e.p.r. A values of the complexes. Changes in A are ascribed primarily to changes in the covalency of the metal-ligand bond. A computer analysis of the electronic spectra indicated xy < xz, yz < x — y < z as the most probable order of metal d-orbital energies. Difficulties in correlating spectroscopic and structural data on oxovanadium chelates of even closely-related ligands are illustrated by the observation that within the series of Schiff bases polymeric, dimeric, and monomeric structures are encountered (see also p. 52). 

The Schiff base polymeric ligand, P-dhtenH2 (0.5g) and manganese(III) acetate dihydrate (Ig) in ethanol (100ml) were refluxed with stirring for 12h and 60h respectively for complexes A and B. The complexes were filtered, washed with hot ethanol, ether and dried under vacuum. 

Lipid oxidation products can interact with proteins and amino acids, and can affect the flavor deterioration and nutritive value of food proteins. Peroxyl radicals are very reactive with labile amino acids (tryptophane, histidine, cysteine, cystine, methionine, lysine and tyrosine), undergoing decarboxylation, decarbonylation and deamination. Methionine is oxidized to a sulfoxide combined cysteine is converted to cystine to form combined thiosulfinate (Figure 11.4). Aldehydes, dialdehydes and epoxides derived from the decomposition of hydroperoxides react with amines to produce imino Schiff bases (R-CH=N-R ). Schiff bases polymerize by aldol condensation producing dimers... 

Primary aromatic amines react with aldehydes to form Schiff bases. Schiff bases formed from the reaction of lower aUphatic aldehydes, such as formaldehyde and acetaldehyde, with primary aromatic amines are often unstable and polymerize readily. Aniline reacts with formaldehyde in aqueous acid solutions to yield mixtures of a crystalline trimer of the Schiff base, methylenedianilines, and polymers. Reaction of aniline hydrochloride and formaldehyde also yields polymeric products and under certain conditions, the predominant product is 4,4 -methylenedianiline [101 -77-9] (26), an important intermediate for 4,4 -methylenebis(phenyhsocyanate) [101-68-8], or MDI (see Amines, aromatic amines, l thylenedianiline). 

X-ray diffraction analysis of crystalline poly(schiff base)s and their low molecular models shows that the formation of molecular complexes is accompanied by an increase in interplanar distances and, in a number of cases, by complete amor-phization. Molecular complexes of poly(schiff base)s with Br2 decompose with time, because of the bromination of the donor components, forming C—Br bonds. Substitution of hydrogen by bromine in phenyl groups occurs only in cases in which these groups are not included into the main polymeric chain. 

To reduce the chance of side reactions, such as the dimerization of the diamine to tUjtw -diaminodihexylamine in (3.17) and the degradation of adipic acid to Schiff bases in (3.16), the precondensation can be carried out either for 30-60 min below 250°C or very rapidly (seconds) at 250-290°C. In the (pre)polymerization step, a concentrated PA salt solution is pumped into a set of heating tubes. These tubes have several heating zones as their diameter gradually increases in size.5,6,28... 

One method of solving the kinetics dilemma is well known in coordination chemistry that is, start with a labile metal ion and render it inert during the course of the synthetic reaction. We have accomplished this in the case of zirconium(IV) by starting with tetrakis(salicylaldehydo)zirconium(IV), which is quite labile, and polymerization with 1,2,4,5-tetraaminobenzene in a Schiff-base condensation reaction in situ (6). The polymeric product contains a "double-headed" quadridentate ligand, which is much more inert to substitution. However, 1,2,4,5-tetraaminobenzene has become very expensive. Therefore, the synthesis of a zirconium polymer with 3,3, 4,4 -tetraaminobiphenyl (commercially 3,3 -diami nobenzidine) with zirconium salicylaldehyde, Zr(sal)4 (7) has been undertaken as shown below ... 

A range of tetradentate Schiff-base ligands have also been employed to prepare discrete aluminum alkoxides. The most widely studied system is the unsubstituted parent system (256), which initiates the controlled ROP of rac-LA at 70 °C in toluene. The polymerization displays certain features characteristic of a living process (e.g., narrow Mw/M ), but is only well behaved to approximately 60-70% conversion thereafter transesterification causes the polydispersity to broaden.788 MALDI-TOF mass spectroscopy has been used to show that even at low conversions the polymer chains contain both even and odd numbers of lactic acid repeat units, implying that transesterification occurs in parallel with polymerization in this system.789... 

Another example where PEG played the role of polymeric support, solvent, and PTC was presented by the group of Lamaty [72]. In this study, a Schiff base-proteded glycine was reacted with various electrophiles (RX) under microwave irradiation. No additional solvent was necessary to perform these reactions and the best results were obtained using cesium carbonate as an inorganic base (Scheme 7.64). After alkylation, the corresponding aminoesters were released from the polymer support by transesterification employing methanol in the presence of triethylamine. 

Five-coordinate aluminum alkyls are useful as oxirane-polymerization catalysts. Controlled polymerization of lactones102 and lactides103 has been achieved with Schiff base aluminum alkyl complexes. Ketiminate-based five-coordinate aluminum alkyl (OCMeCHCMeNAr)AlEt2 were found to be active catalyst for the ring-opening polymerization of -caprolactone.88 Salen aluminum alkyls have also been found to be active catalysts for the preparation of ethylene carbonate from sc C02 and ethylene oxide.1 4 Their catalytic activity is markedly enhanced in the presence of a Lewis base or a quaternary salt. 

The reaction of Zr(OPr ) with the Schiff bases acetylacetone-2-hydroxy-alkylimine, N-[(3-hydroxy-2-naphthyl)methylene]-2-hydroxyalkylamine, and o-hydroxyacetophenone-2-hydroxyalkylimine (HL) affords the polymeric Zr(OPr )2L and ZrL2 compounds. N.m.r. and i.r. spectral studies indicated that the N atom of the Schiff base is co-ordinated in these compounds. 

Heat treatment of carbon black impregnated with a salt of [Fe(phen)3] gives an oj gen reduction catalyst. i The compounds [Fc2(88)Cl4], [Fe(89)Cl2] and [Fe(90)Cl2] are potential ethene polymerization catalysts they all contain five-coordinated iron(II). The chiral terpy ligands (91) (R R = all three combinations of H, Pr ) and the Schiff base analogue (92), also... 

Schiff base ligands are a particularly suitable alternative in ROP of lactide because of their ease of preparation and the possibility to easily tunable steric and electronic properties. Chisholm and coworkers reported Schiff base-supported zinc amide and phenoxide complexes 51a, b (Fig. 10) [75], which catalyze the polymerization of L-lactide in benzene at room temperature yielding 90% conversion in 3 h for 51a and 72 h for 51b. The significant difference in reactivity between the catalysts can be taken from the rate of initiation, which is slower for 51a owing to presence of bulky 2,6-ferf-butylphenoxide leaving group. 

Recently, N,N,0-tridentate Schiff-based zinc alkoxide complexes 52a-53e have been developed by our group [76]. All complexes efficiently initiate the polymerization of L-lactide at 25 °C with >90% conversion within 30-240 min, with only one exception, 52c which is inactive. The polymerization was well-controlled (PDI = 1.04—1.09) and showed that the reactivity decreases with an electron-withdrawing... 

Darensbourg and coworkers reported a systematic investigation of ROP catalyst performance along with kinetic and mechanistic studies for the polymerization of L- and rac-lactide using calcium complexes derived from tridentate Schiff base ligands (Fig. 13) [90, 91]. With calcium catalysts 77a-d, used in melt and solution polymerization of L-lactide, it was found that calcium salen catalyst 77d with bis (phosphoranylidene)ammonium azide as a co-catalyst is much less active than the calcium complexes with tridentate Schiff base ligands, as reflected in the monomer L-lactide conversions of 59-80% for 77a-c but only 35% for 77d as initiator. 

Fig. 13 Calcium complexes supported by N,0-donor Schiff base ligands Table 4 Ca complexes in rac-lactide and L-lactide polymerization...

---