NOESY AND ROESY EXPERIMENTS AS VALUABLE TOOLS TO EXPLAIN THE LACK OF REACTIVITY OF LINEAR-DENDRITIC BLOCK COPOLYMERS

J. PÁEZ; P. FROIMOWICZ; L. GERBINO; M.N.BELGACEM; A. GANDINI; M. C. STRUMIA

Alta Gracia, Córdoba, Argentina

Conferencia; MAGNETIC RESONANCE IN A CORDUBENSIS PERSPECTIVE VI:NEW DEVELOPMENTS IN NMR; 2011

international society for magnetic resonance

In previous studies, our group reported the synthesis of a family of first-generation linear-dendritic block copolymers1 (Fig.1) bearing terminal amine groups, and the attempts to use them as substrates in Schiff´s base formation reactions and crosslinking polymerizations with diisocianates. To our surprise, in these reactions the aromatic copolymer 4 exhibited a much higher reactivity than its aliphatic analogues 1,2 and 3.2 This behavior is unusual for such functional groups and suggests that the molecular structure plays a key role in the reactivity of these compounds in the reaction conditions studied. The first evidence supporting this hypothesis was obtained from 1H NMR spectra. 1displayed signals with different peak resolutions in the same spectra. This effect was assumed to be caused by different degree of solvation of the different parts of the copolymers, which directly affected their mobility. The results suggest that the central oligoether segment was well solvated while the terminal arms were much less solvated. Similar results were obtained from compounds 2 and 3. On the contrary, copolymer 4 showed spectra where all signals were sharp and well resolved, indicating that the entire molecule was well solvated. These differences observed in 1H NMR spectra suggest that the aliphatic copolymers may aggregate, thus reducing their reactivity. Therefore, a series of 2D and 1D selective NOESY and ROESY experiments were performed in different solvents, choosing 1 as aliphatic model copolymer to explain the lack of reactivity. We specially analyzed the trough-space correlation between protons far away form each other in the same molecule. The results demonstrate that the amine-end groups are located spatially close to the central linear segment. A reason for this proximity is formation of hydrogen bonds that terminal –NH2 groups may form with the oxygen atoms from the ether central block, as well as with the tertiary amine nitrogen atoms. These bonds involve the terminal amine groups into hindered aggregates, thus reducing or limiting their reactivity. Conversely, in the case of 4 the results show no correlation between the dendritic part and the linear segment. Here, the aromatic moieties provide higher stiffness to the dendritic arms and decrease the basicity of the amine groups, rendering unfavorable the formation of hydrogen bonds and leaving the -NH2 groups accessible to react. In summary, we demonstrated the utility of NOESY and ROESY experiments to study the effect of the molecular structure on the reactivity of linear-dendritic block copolymers.