Iron-Binding Hydroxamic Acid Polymers.

The objective of this work was to determine stability constants of the iron(III) complexes of polyhydroxamic acids in which the hydroxamic acid (HA) groups were separated by more than 13 atoms. Two water-soluble polymers containing HA units in side arms branching from the polymer backbone, one with 15 atom spacing between HA groups (P-15) and one with 17 atom spacing (P-17) were prepared by the following reaction sequence: 1) Schotten-Bauman reaction of methacryloyl chloride with glycylgylcine or e-amino-n-caproic acid. 2) Condensation reaction between N-hydroxysuccinimide and the product of the Schotten-Bauman reaction using N,N’- dicyclohexylcarbodiimide as a condensing agent. 3) Solution phase radical polymerization of the active ester using a,a\u27-azobis(isobutyronitrile) as initiator. 4) Reaction of the resulting polymer with N-methylhydroxylamine. The iron complex with P-15 (glycylglycine spacer) was water soluble and was found to have a ratio of three HA units per one iron(III) atom. The logarithm of the overall stability constant of the 3:1 complex (29.0) was determined spectrophotometrically (^max at 434 nm) by competition for iron between the polymer and another chelating ligand. When iron was added to aqueous solutions of P-17 (e-amino-n-caproic acid spacer), the solutions became turbid and precipitates formed. No meaningful absorbance readings could be obtained hence the mole ratio Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. xiv and stability constant for the iron complex was pot determined. The poor solubility of the complex could be caused by the more hydrophobic character of the polymer due to the methylene groups in the spacer or the possibility of the iron atoms serving as crosslinks which joined P-17 molecules together. Crosslinked polymers are generally insoluble in most solvents. Evidence that crosslinking was possibly occurring in the formation of the P-17 complex was obtained by observing the viscosities of solutions of copolymers containing HA units upon the introduction of iron. Copolymers consisting mainly of acrylamide and varying amounts of hydroxamic acids were prepared by essentially the same method as the homopolymers. Active ester monomers (p-alanine spacer) were copolymerized with acrylamide and the resulting copolymers were treated with Nmethylhydroxylamine. Aqueous solutions of the copolymers were prepared in viscometers and small aliquots of iron solution were added. Crosslinking of copolymers occurred intermolecularly (viscosity increase upon introduction iron) in concentrated solutions of the copolymers but occurred intramolecularly (viscosity decrease) in dilute solutions of the copolymers. 3+ Maximum viscosity changes occurred when the HA/Fe ratio was approximately three. The viscosity of the iron complexes returned to the same level a.s with no iron added when each hydroxamic acid unit bound a separate iron atom. The magnitude of the viscosity changes was dependent upon the loading of HA units in the copolymer and dependent upon the molecular weight of the copolymer. Copolymers with higher HA content showed greater viscosity changes, and higher molecular weight copolymers also Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. X V showed magnified viscosity changes. Rearrangement of the kinetically-favored intermolecular 3:1 complex to the thermodynamically-favored intramolecular 3:1 complex in concentrated solutions of the copolymers was evidenced by the slow decrease in intrinsic viscosity with respect to time. The rearrangement was shown to follow first order kinetics and the rate constant for the rearrangement was determined to be 5.2 x 10- 6 sec- 1 at 30° and 5.0 x 10 5 sec-x at 60°C. When iron(III) was added to highly concentrated solutions of the copolymer, gels were formed. The average distances between HA units on the same copolymer chain and between HA units on different copolymer chains in solutions of the copolymers at the gel point were calculated. Gels were formed when the distance between HA units on different chains was less than the distance between HA units on the same chain. This allowed one to estimate the gel point of copolymers based upon the HA content in the copolymer and indicates that Flory\u27s theory for the critical value of a gel point applies to systems in which metal ions serve as crosslinking agents