Chain dynamics in poly(amidoamine) dendrimers. A study of ^(13)C NMR relaxation parameters

Abstract

Two series of poly(amido amine) (PAMAM) dendrimers have been prepared. Both were built from an ammonia core via repetitive addition of methyl acrylate and ethylenediamine. End groups were either hydroxyl or amine, the former derived from 2-aminoethanol substituted for ethylenediamine in the final amidation step. The dynamic properties of these polymers have been examined with the aid of ^(13)C NMR relaxation measurements. Spin-lattice relaxation times (T_1) of the terminal carbons of the hydroxyl-terminated PAMAM in DMSO-d_6 were found to decrease as the number of chain termini increased from 3 (T_1 = 0.54 ± 0.01 s at 75 MHz) to 3072 (T_1 = 0.23 ± 0.01 s at 75 MHz). Decreases in the nuclear Overhauser enhancement (NOE) were also observed; NOE varied from 2.9 ± 0.2 to 2.4 ± 0.2 over this range of molecular size. T_1 was found to increase with temperature, sensitivity to temperature decreasing with increasing molecular weight. The T_1 values of the methylene carbons in the molecular interior were found to decrease initially with molecular weight but were independent of molecular weight above the second generation. The values of T_1 determined at lower field strength (4.70 T) were lower than those at higher field strength (7.05 T), but consistent correlation times (τ) were obtained at both frequencies upon the application of Schaefer's log χ^2 distribution function. The τ calculated for the terminal ^(13)C (2.6 x 10^(-11) to 6.3 x 10^(-11) s) varied only weakly with molecular weight. In contrast, τ was found to be a sensitive function of molecular size for the internal ^(13)C, ranging from 7.4 x 10^(-11) to 1.2 X 10^(-8) s. Similar results were obtained for hydroxyl-terminated PAMAM dissolved in D_2O and for the amine-terminated dendrimers in either DMSO-d_6 or D_2O. These results indicate that the chain dynamics are insensitive to any steric crowding that may occur at the molecular surface, but report instead a slowing of internal chain motions as molecular size increases.

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