ABSTRACT:Temperature dependences of conformations of poly(-phenethyl-L-aspartate) (PA) in block copolymer systems, in which polystyrene (PS) was attached to either N-terminus (PA n -N-PS m ) or C-terminus (PS n -C-PA m ) of the PA, respectively, were investigated in 1,1,2,2-tetrachloroethane (TCE) solutions by using 1 H NMR spectroscopy.The block copolymers were synthesized by combination of atom transfer radical polymerization and ring-opening polymerization of N-carboxy amino acid anhydride. Whereas PA homopolymer (H-PA n , n ¼ 68) with right-handed helical conformation started to transform to left-handed structure at 70 C, the helical-sense inversion of PA 68 -N-PS 38 occurred at a lower temperature, 40 C. Further transformation from left-handed conformation was not observed for both polymers in the temperature range of 70 C to 110 C. On the other hand, 1 H NMR spectra of PS 70 -C-PA 170 showed coexistence of right-and left-handed helices of PA at 8.2 and 8.8 ppm at 30 C, respectively. These peaks, however, disappeared above 90 C, and a new peak at 8.5 ppm was observed. Thus, the introduction of PS chain into PA termini affected conformational stability of PA in a different way, although the transformational behaviors were not influenced by the difference of the chain length of PS segments. [doi:10.1295/polymj.PJ2007020] KEY WORDS Poly(-phenethyl-L-aspartate) / Block Copolymer / Helical-Sense Inversion / Chain Terminus / NMR / Controlling the higher ordered structure of polymers is one of the most important factors for producing functional polymeric materials. In that context, polypeptides have been frequently investigated and utilized because they can assume various higher ordered structures. For example, poly(aspartate ester)s are known to form various conformations including right-handed helix (RH), left-handed helix (LH), -sheet, and random coil (RC) depending on structure of side-chain groups, temperature, and solvents. [1][2][3][4][5][6][7][8][9][10][11][12][13] In particular, RH and LH conformations are interesting because their formation can be reversibly controlled by thermal stimulus.5-8 The helical-sense inversion has been reported to be caused by difference in the conformational entropy of the side-chain between the two opposite helical structures. 9-13In addition to the side-chain groups, chain termini of poly(aspartate ester)s should have a significant effect on their conformations, thus, allowing us to control them. Although a number of reports addressing the effect has been limited so far, Ushiyama et al. have demonstrated that attaching helical poly(-benzyl-L-glutamate) (BG) segment at the chain terminus of poly(-phenethyl-L-aspartate) (PA) by block copolymerization have a significant influence on the helical-sense inversion of the PA segment.14 Thus, it would be intriguing if we can use other polymer segment in place of the BG, because then more detailed analysis will become possible without undesirable overlapping of the signals in instrumental measurements such as CD, IR, X-ray, and NMR.As...
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