Intramolecular Hydrogen Bonding in Isotactic Poly(methacrylamide)s and Its Implications for Control of Side-Chain Orientation

Abstract

KEY WORDSPoly(methacrylamide) / Intramolecular Hydrogen Bonding / Secondary Structure / Circular Dichroism / The photoprocesses of polymers differ from those of isolated low-molecular-weight analogues 1: excitation energy can be transported in the polymer through the pendant chromophores as in crystals or concentrated solutions of low-molecular-weight analogues 2 • Excitation energy, however, can be trapped as excimers (excited dimers) which are formed with pairs of chromophores suitably situated for their formation along the polymer chain. The chromophore interactions in the excited state, i.e., excitation energy transport and trapping, are affected by the stereoregularity and conformation of the main chain 3 as well as by the steric hindrance introduced in the side chain. 4 While it is not fully understood how these structural differences actually affect energy migration and excimer formation, one may be able to enhance the former and suppress the latter if one properly controls the orientation of the chromophores along the polymer chain. If this is possible, "a molecular wire" will be obtained which allows efficient one-dimensional transport of excitation energy along the polymer chain. Studies along this line have been carried out using polypeptides where side-chain chromophores are arranged at regular intervals along the helical main chain. 5 Previously, we have prepared poly(L-glutamines) where naphthalene chromophores are introduced in the side chain via amide linkage and examined how the sidechain structure affects the hydrogen bonding interactions among the side-chain amide groups and subsequent ordering of the side-chain chromophores. 6 In the present study we prepared isotactic and atactic polymethacrylamides, i.e., poly(l-naphthylmethyl methacrylamide) (1) and poly[(S)-1-(1-naphthyl)ethyl methacrylamide] (2), where naphthalene chromophores are likewise introduced in the side chains via amide linkage and examined how the stereoregularity and the side-chain structure affect the hydrogen bonding interactions among the side-chain amide groups and the subsequent orientation of the side-chain chromophores.

Effects of Side-Chain Structure and Solvent on Intramolecular Hydrogen Bonding in Isotactic Poly(methacrylamide)s

“…As reported previously, 3 theIR spectra do not change in the concentration range of 0.01-0.5 M (in terms of repeating unit), indicating that the side-chain amide groups are intramolecularly hydrogen-bonded. The relative intensities of amide N-H stretching vibration (amide A band) at ca.…”

“…In accord with the above observation, the CD spectrum of isotactic 1 in dioxane is very much similar to that in EDC (Figure 4 spectrum very much similar to that in chloroform-d reported previously. 3 The CD spectrum of isotactic 2 in dioxane, however, is markedly different from that in Wavelength (nm) Figure 5. UV and CD spectra of isotactic 2 in EDC (-).…”

“…Another question is: which section of the "partially hydrogen-bonded structure", formed in isotactic 2 (in dioxane) and in atactic 2 (both in EDC and in dioxane), 3 gives an exciton coupling interaction whose chirality in the 1 Bb band of the naphthyl chromophore is apparently opposite to that of the "fully hydrogen-bonded structure"? We speculate that the observed exciton coupling is due to two monomer units hydrogen-bonded only to each other, i.e., "dimers", and that these "dimers" are formed between the same i and i + 2 monomer units along the polymer chain as in the "fully hydrogen-bonded structure" with the intervening i + 1 monomer unit not hydrogen-bonded to nearby monomer units.…”

Synthesis and chiral recognition ability of optically active poly{N‐[(R)‐α‐methoxycarbonylbenzyl]methacrylamide} with various tacticities by radical polymerization using Lewis acids

“…The absorption around 3435 cm −1 is based on the free amide proton. In the IR spectra of the isotactic polymers, a shoulder peak around 3300 cm −1 , based on the intramolecularly hydrogen‐bonded amide proton,37–39 can be observed. The relative intensity of the hydrogen‐bonded peak increased with the increasing isotacticity.…”

Synthesis, Chiroptical Properties, and Chiral Recognition Ability of Optically Active Polymethacrylamides Having Various Tacticities

“…For instance, poly[(meth)acrylamide]s have different solubilities, 1,2,10 thermo-responsibilities, 4 hydrogen bonding formations, and conformations depending on the tacticity. 11 We also demonstrated that the chiral recognition ability of poly[(R) -1] was affected by the stereoregularity. 5 In this study, the radical polymerizations of four methacrylamides bearing optically -5), in the presence and absence of Lewis acids were carried out to synthesize the polymethacrylamides having different tacticities, and the relationship between the stereoregularity and the chiroptical properties of the polymers was discussed.…”