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Langmuir 2009, 25, 48-50
Syndiotactic Poly(N-n-propylacrylamide) Shows Highly Cooperative Phase Transition Takeshi Mori,*,†,‡ Tomohiro Hirano,*,| Atsushi Maruyama,§ Yoshiki Katayama,†,‡ Takuro Niidome,†,‡ Yoichi Bando,| Koichi Ute,| Shinji Takaku,⊥ and Yasushi Maeda*,⊥ Department of Applied Chemistry, Center for Future Chemistry, and Institute for Material Chemistry and Engineering, Kyushu UniVersity, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan, DiVision of Life System, Institute of Technology and Science, The UniVersity of Tokushima, 2-1 Minamijosanjima, Tokushima 770-8506, Japan, and Department of Applied Chemistry and Biotechnology, UniVersity of Fukui, 3-9-1 Bunkyo, Fukui 910-8507, Japan ReceiVed October 20, 2008. ReVised Manuscript ReceiVed NoVember 22, 2008 Syndiotactic poly(N-n-propylacrylamide)s (PNNPAM) with various racemo (r) diad contents were synthesized, and their phase transition behaviors were studied by high-sensitivity differential scanning calorimetry and FT-IR spectroscopy. The cooperativity of the phase transition increased with the increase in the r diad content. Compared with the atactic PNNPAM, the number of cooperative units, which shows the monomeric units length undergoing cooperative phase transition, increased 2.5- to 4-fold in the syndiotactic polymers. From the results of FT-IR spectroscopy and quantum chemical calculations, it was determined that the high cooperativity of the syndiotactic polymers resulted from local formation of an ordered structure in a dehydrated state.
Water-soluble synthetic polymers such as poly(N-alkylacrylamide)s and poly(N-isopropylmethacrylamide) show coil-globule transitions in response to temperature change.1 This transition has been recognized as a similar phenomenon to the denaturation of small globular proteins.1 However, there are several distinctive differences between the two transitions. One of the differences is the high cooperativity of the synthetic polymers. The synthetic polymers with relatively high molecular weight exhibit a sharp phase transition with a maximum half-width (∆T1/2) of 1-2 °C,1c,d,2 while proteins commonly exhibit a broad phase transition (∆T1/2 ∼ 10 °C).3 Such high cooperativity of the synthetic polymers has resulted in their recent application to a wide variety of fields because the cooperativity is a critical characteristic. Considering that proteins are composed of 20 kinds of amino acids, it can be surprising that the phase transition of proteins still occurs in such cooperative manner. Thus, the cooperativity of the phase transition of synthetic polymers, which is usually homopolymeric, may be increased by learning the molecular design of the proteins. One of the causes that reduce the cooperativity of the synthetic polymers’ phase transition would * To whom correspondence should be addressed. E-mail moritcm@ mbox.nc.kyushu-u.ac.jp (T.M.),
[email protected] (T.H.),
[email protected] (Y.M.). † Department of Applied Chemistry, Kyushu University. ‡ Center for Future Chemistry, Kyushu University. § Institute for Material Chemistry and Engineering, Kyushu University. | The University of Tokushima. ⊥ Fukui University. (1) (a) Ptitsyn, O. B.; Kron, A. K.; Eizner, Y. Y. J. Polym. Sci., Polym. Symp. 1968, 16, 3509–3517. (b) Kubota, K.; Fujishige, S.; Ando, I. J. Phys. Chem. 1990, 94, 5154–5158. (c) Tiktopulo, E. I.; Bychkova, V. E.; Ricka, J.; Ptitsyn, O. B. Macromolecules 1994, 27, 2879–2882. (d) Tiktopulo, E. I.; Uversky, V. N.; Lushchik, V. B.; Klenin, S. I.; Bychkova, V. E.; Ptitsyn, O. B. Macromolecules 1995, 28, 7519–7524. (e) Wu, C.; Zhou, S. Macromolecules 1995, 28, 8381– 8387. (f) Ito, D.; Kubota, K. Macromolecules 1997, 30, 7828–7834. (2) (a) Schild, H. G.; Tirrell, D. A. J. Phys. Chem. 1990, 94, 4532–4356. (b) Ding, Y.; Ye, X.; Zhang, G. Macromolecules 2005, 38, 904–908. (c) Tang, Y.; Ding, Y.; Zhang, G. J. Phys. Chem. B 2008, 112, 8447–8451. (d) Yamazaki, Y.; Tada, T.; Kunugi, S. Colloid Polym. Sci. 2000, 278, 80–83. (3) (a) Privalov, P. L. FEBS Lett. 1974, 40, S140-S153. (b) Privalov, P. L. AdV. Protein Chem. 1979, 33, 167–241. (c) Giancola, C. J. Therm. Anal. Calorimetry 2008, 91, 79–85. (d) Stathopulos, P. B.; Rumbeldt, J. A. O.; Karbassi, F.; Siddall, C. A.; Lepock, J. R.; Meiering, E. M. J. Biol. Chem. 2006, 281, 6184–6193.
be the molecular weight distribution. To our knowledge, there is no report that proves that the molecular weight distribution affects the cooperativity of synthetic polymers. However, several groups have reported that the phase transition temperature of synthetic polymers depends on their molecular weight, especially when the molecular weight is relatively small.2c,4 Thus, it seems reasonable that the molecular weight distribution reduces the cooperativity. The other explicit difference between synthetic polymers and proteins is stereoregularity. The proteins are completely stereoregular polymers exclusively composed of L-amino acids, whereas synthetic polymers are generally obtained as atactic polymers if special polymerization techniques are not adopted. It is clear that the stereoregularity of proteins is responsible for their cooperative phase transition. Thus, we hypothesized that synthetic polymers show higher cooperativity if the stereoregularity is provided. Several groups including ours have reported that the phase transition temperature of synthetic polymers clearly depends on their tacticity.5 However, to date, the effect of the tacticity on the cooperativity has not been examined. In a present report, we synthesized syndiotactic poly(N-n-propylacrylamide) (PNNPAM) with various racemo (r) diad contents and examined their phase transition behaviors with high-sensitivity differential scanning calorimetry (DSC) and FT-IR spectroscopy. Syndiotactic PNNPAMs with various r diad contents (52-71) were synthesized according to our original method (Table 1).5f Figure 1 shows the high-sensitivity DSC traces of the diluted solutions (1.0 mg/mL) of each polymer with a heating rate of 1.0 °C/min. With an increase of r content, the endothermic peaks (4) Tong, Z.; Zeng, F.; Zheng, X.; Sato, T. Macromolecules 1999, 32, 4488– 4490. (5) (a) Hirano, T.; Okumura, Y.; Kitajima, H.; Seno, M.; Sato, T. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 4450–44460. (b) Ray, B.; Okamoto, Y.; Kamigaito, M.; Sawamoto, M.; Seno, K.; Kanaoka, S.; Aoshima, S. Polym. J. 2005, 37, 234–237. (c) Katsumoto, Y.; Kubosaki, N. Macromolecules 2008, 41, 5955–5956. (d) Ito, M.; Ishizone, T. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 4832–4845. (e) Kobayashi, M.; Ishizone, T.; Nakahama, S. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 4677–4685. (f) Hirano, T.; Nakamura, K.; Kamikubo, T.; Ishii, S.; Tani, K.; Mori, T.; Sato, T. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 4575–4583.
10.1021/la8034837 CCC: $40.75 2009 American Chemical Society Published on Web 12/10/2008
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Langmuir, Vol. 25, No. 1, 2009 49 Table 1. Parameters of Syndiotactic PNNPAM
sample
ra
mm/mr/rra
nmb
nrb
r52 r57 r66 r68 r71
51.8 56.8 65.6 68.2 70.9
21.1/51.2/22.7 18.9/50.3/30.8 12.4/43.6/44.0 11.1/38.4/50.5 8.3/37.7/54.0
1.82 1.75 1.57 1.58 1.44
2.08 2.22 3.02 3.63 3.86
Tcd, °C
Mn, ×10-4 c Mw/Mnc 3.44 2.71 3.86 5.57 5.50
2.8 2.7 2.5 2.6 3.0
22.3 (23.2) 23.9 (24.3) 24.7 (25.3) 25.1 (25.7) 25.2 (25.9)
Tpe, °C ∆T1/2f, °C ∆Hg ∆Cph ∆Hvi 22.6 23.9 24.9 25.2 25.4
1.73 0.63 0.56 0.50 0.41
1.64 1.50 1.52 1.81 1.68
0.7 1.2 1.7 2.2 2.8
0.353 0.982 1.11 1.25 1.51
nj
∆Tk, °C
fl
220 650 730 690 900
7 6.5 5 4.5 4.5
0.225 0.247 0.256 0.270 0.320
a Determined by 1H and 13C NMR. b Average sequence length of m and r calculated from the triad values according to the litearature.9 c Determined by SEC. d Cloud point determined by turbidimetry (conc. ) 1.0 mg/mL, rate ) 1.0 °C/min) in H2O and D2O (bracketed value is Tc in D2O). e Temperature of endothermic peak. f Maximum half-width of endothermic peak. g Calorimetric enthalpy (kcal/mol-monomer). h Endothermic peak height (kcal/°C/mol-monomer). i van’t Hoff enthalpy (Mcal/mol-monomer) calculated from the equation,10 ∆Hv ) 6.9Tp2/∆T1/2. j Number of cooperative units, n ) ∆Hv/ ∆H. k Width of phase transition determined from temperature dependence of ∆∆A. l Fraction of hydrogen-bonded CdO at 37 °C determined from amide I band.
Figure 1. High-sensitivity DSC traces of syndiotactic PNNPAM solutions (1.0 mg/mL). Heating rate ) 1.0 °C/min.
resulting from dehydration of the polymer chains became significantly sharper and shifted to higher temperature. The same tendency was observed when the polymer concentration was more diluted (0.05 mg/mL) or the heating rate was decreased (0.2 °C/min). Parameters determined by the DSC measurements are complied in Table 1. The sharper endothermic peak with an increase in
r content can be confirmed by ∆T1/2 values. It is obvious that the enhanced cooperativity of the syndiotactic PNNPAMs does not result from the narrowness of the molecular weight distribution, based on the fact that r71 with the largest Mw/Mn value showed the highest cooperativity among the polymers examined. Because the Mn value tends to increase with increasing r content, there was a concern that the cooperative phase transition results from increasing molecular weight, not from the increasing r content. Thus, we prepared two other polymers with different molecular weights but having almost constant r content (r ∼ 69), and found no effect of the Mn values on the cooperativity (Supporting Information Table S2). The number of cooperative units n, which shows the monomeric units length undergoing cooperative phase transition, was calculated from the ratio of van’t Hoff enthalpy (∆Hv) and calorimetric enthalpy (∆H).3a The n values roughly increased with an increase in r contents. r71 has 4 times as large an n value as the atactic polymer (r52). Next, we evaluated the cooperativity of PNNPAM by FT-IR spectroscopy. For high-fidelity measurements, the high concentration (1.0 × 102 mg/mL) was adopted and D2O was used as a solvent instead of H2O to avoid the overlapping between O-H bending and the amide I band.6 The FT-IR spectra of r52 and r71 below and above the phase transition temperatures and their differential IR spectra are shown in Figure 2a,b, respectively. Red shifts of antisymmetric C-H stretching (νas(C-H), ∼2970 cm-1) and amide II′ (amide II mode for COND species, ∼1480 cm-1) and blue shift of amide I (∼1630 cm-1) were observed
Figure 2. (a) FT-IR spectra of r52 and r71 solutions below (blue, 7 °C) and above (red, 39 °C) the phase transition temperature. (b) Differential IR spectra between the above two temperatures. (c) Temperature dependence of ∆∆A for three bands. Shaded areas show temperature ranges of phase transition. These ranges were determined from the intersection temperatures between two baselines and experimental curves. Polymer concentration ) 1.0 × 102 mg/mL. Heating rate ) ca. 1 °C/min.
50 Langmuir, Vol. 25, No. 1, 2009
during the phase transition.7 To raise the visibility of temperature dependence of the peak shifts, ∆∆A, which is the difference between ∆A values at positive and negative peaks, was adopted (Figure 2c). The three bands of each polymer shifted in the same temperature range; 18 to 25 °C (∆T ) 7 °C) for r52 and 21 to 25.5 °C (∆T ) 4.5 °C) for r71, respectively. The amide II and νas(C-H) bands exhibited the same shifts in H2O (Supporting Information Figure S1). The ∆T value decreased with an increase in r content (Table 1). Thus, the cooperative phase transition of a syndiotactic polymer is also confirmed by the shift of vibration bands during phase transition. We speculated on the mechanism of the cooperativity increase with an increase in r contents as follows. When the stereoregularity increases, the polymer chain may form locally ordered structures at a dehydrated state. The local formation of ordered structure of syndiotactic PNNPA would induce the cooperative phase transition in such long segments. The hypothesis of the local formation of ordered structure seems realistic because several stereoregular synthetic polymers form locally ordered helical structures in solutions even when the stereoregularity is not so high.8 The result of quantum chemical calculations of syndiotactic PNNPAM supports our hypothesis (Supporting Information Figure S2). We found that syndiotactic PNNPAM (octamer) forms a helical structure with intramolecular hydrogen bonding among amide groups of adjacent monomeric units. When the amide I band, which is derived mainly from the CdO stretching vibration, was separated into the two fractions by Gaussian fitting (Figure 3), a fraction (f) resulting from the intra/intermolecular hydrogen-bonded CdO (1628 cm-1) becomes higher in the syndiotactic polymer (r71) than the atactic one (r52). The f value increased with the increase in r content (Table 1). This efficient intra/intermolecular hydrogen bonding would be derived from the ordered structure of syndiotactic PNNPAMs in a dehydrated state. We calculated the average sequence length of r (nr) from triad values (Table 1). The nr values increased with the r content and reached ca. 4 in r71, which is a value twice as large as that of the atactic polymer (r52). Such relatively long sequence lengths in the r-rich polymers would enable the local formation of an ordered structure in a dehydrated state. In conclusion, we have shown the enhancement of cooperativity in the phase transition of the syndiotactic PNNPAM. The (6) As shown in Table 1, the Tcs in D2O were slightly higher than those in H2O because of a difference in the strength of their hydrogen bonding. The same positive shift of phase transition temperature in D2O was also reported for poly(Nisopropylacrylamide); Wang, X.; Wu, C. Macromolecules 1999, 32, 4299–4301.
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Figure 3. Baseline-subtracted amide I band of (a) r52 and (b) r71 below (left, 7 °C) and above (right, 39 °C) phase transition temperature, and best fitted Gaussian components. Two fractions centered at 1628 cm-1 (red) and 1651 cm-1 (blue) result from hydrated and intra/intermolecular hydrogen-bonded CdO, respectively.7 Polymer concentration ) 1.0 × 102 mg/mL.
enhanced cooperativity probably results from the local formation of an ordered structure in the syndiotactic PNNPAM in a dehydrated state. The ordered structure formation through the coil-globule transition will be intriguing because the phenomenon is reminiscent of the unfold-fold transition of proteins. To the best of our knowledge, this is the first report revealing that stereoregularity affects the cooperativity of phase transition of temperature-responsive polymers. The present finding will be important especially for the application of temperature-responsive polymers, because the cooperativity is a critical characteristic. The study of the effect of stereoregularity on the other temperatureresponsive polymers is now in progress in our group. Acknowledgment. We thank Dr. Motonori Kudou, Dr. Akihito Hashidzume, and Dr. Shuichi Goda for valuable discussion and suggestion. T.H. thanks the financial support of a Grant-in-Aid for Young Scientists (B) (18750102) from the Ministry of Education, Culture, Sports, Science and Technology. Supporting Information Available: Experimental procedures and characterization data. This material is available free of charge via the Internet at http://pubs.acs.org. LA8034837 (7) IR bands were assigned according to our previous report. Maeda, Y.; Nakamura, T.; Ikeda, I. Macromolecules 2001, 34, 1391–1399. (8) (a) Nagasawa, M.; Murase, T.; Kondo, K. J. Phys. Chem. 1966, 69, 4005– 4012. (b) Reiss, C.; Benoit, H. J. Polym. Sci., Polym. Symp. 1968, 16, 3079–3088. (c) Dybal, J.; Speˆva´cˆek, J. S.; Schneider, B. J. Polym. Sci., Part B: Polym. Phys. 1986, 24, 657–674. (9) Randall, J. C. Macromolecules 1978, 11, 592–597. (10) Mabrey, S.; Sturtevant, J. M. Methods Membr. Biol. 1978, 9, 237–274.
