Cloud points in aqueous solutions of poly(n-isopropylacrylamide) synthesized by aqueous redox polymerization

ABSTRACT The mean-square radius of gyration, second virial coefficient and intrinsic viscosity were determined in methanol at 25.0 °C for poly(_N_-isopropylacrylamide) (PNIPA) samples in the

range of weight-average molecular weight _M_w from 1.04 × 105 to 2.65 × 106, which were synthesized by aqueous redox polymerization using a redox catalyst consisting of ammonium persulfate

and sodium metabisulfite. Analyses of the quantities showed that the samples have almost the same degree of branching as that of PNIPA samples previously synthesized by radical

polymerization using azobis(isobutyronitrile) as an initiator in _tert_-butanol. The cloud point was also determined in aqueous solutions of two PNIPA samples with _M_w=1.04 × 105 and 1.86 ×

105 and compared with the results of the previous samples. It was found that the cloud point is definitely higher for the present samples than for the previous ones, and, moreover, the _M_w

dependences of the cloud point for the two kinds of samples are opposite to each other. As the two kinds of samples have almost the same stereochemical composition and degree of branching,

such difference in the behavior of the cloud point is considered to arise from the difference in their chain-end groups. SIMILAR CONTENT BEING VIEWED BY OTHERS CONFORMATIONS OF HYDROGENATED

RING-OPENED POLY(NORBORNENE)S IN DILUTE SOLUTION Article 19 December 2023 THERMAL HYSTERESIS OF AGGREGATION STATES OF THERMORESPONSIVE BLOCK COPOLYMERS FORMING INTERMOLECULAR HYDROGEN BONDS

Article 23 June 2021 PHASE BEHAVIOR OF Π-CONJUGATED POLYMER AND NON-FULLERENE ACCEPTOR (PTB7-TH:ITIC) SOLUTIONS AND BLENDS Article Open access 02 December 2022 INTRODUCTION In a series of

experimental studies recently made of aqueous poly(_N_-isopropylacrylamide) (PNIPA) solutions,1, 2, 3, 4 it was shown that the behavior of their cloud-point curves is considerably affected

by the kind of end group of the PNIPA sample used1, 3 and also by primary structure, that is, the degree of branching of the sample.2 Furthermore, it was pointed out that the cloud-point

curve does not seem to correspond directly to a liquid–liquid binodal.4 The PNIPA samples used previously1, 2, 3, 4 were synthesized by radical polymerization using azobis(isobutyronitrile)

(AIBN) as an initiator and by living anionic polymerization using diphenylmethylpotassium as an initiator, so that they had hydrophobic groups at their chain ends, that is, the

isobutyronitrile group for the former and the diphenylmethyl group for the latter. The above-mentioned aqueous solution behavior of PNIPA may arise from the effect of the hydrophobic

chain-end groups, which is enhanced with decreasing molecular weight. To verify more clearly the effect of the chain-end groups on the behavior of the cloud-point curve, it is desirable to

pursue further investigations of aqueous solution behavior of PNIPA samples with a hydrophilic chain-end groups. In this study, therefore, we prepared such PNIPA samples by aqueous redox

polymerization using a redox catalyst consisting of ammonium persulfate and sodium metabisulfite, whose chain-end groups are hydrophilic and/or ionic ones. We first characterized the PNIPA

samples so prepared by analyzing dilute solution properties such as the mean-square radius of gyration 〈_S_2〉, second virial coefficient _A_2 and intrinsic viscosity [_η_], in methanol at

25.0 °C, then determined the cloud point in the aqueous solutions, and finally made a comparison of present results with the previous ones for the PNIPA samples having the hydrophobic

chain-end groups.1 EXPERIMENTAL PROCEDURE MATERIALS Original PNIPA samples were synthesized by aqueous redox polymerization using a redox catalyst consisting of ammonium persulfate

(NH4)2S2O8 (Wako Pure Chemical Industries, Ltd, Osaka, Japan) as an oxidative reagent and sodium metabisulfite Na2S2O5 (Wako Pure Chemical Industries, Ltd) as a reductive reagent, following

the procedure reported by Wooten _et al._5 The monomer _N_-isopropylacrylamide (Tokyo Chemical Industry Co., Ltd, Tokyo, Japan; ∼20 g), which had been recrystallized three times from a 9:1

mixture of _n_-hexane and benzene and then dried in a vacuum for 12 h, was dissolved in 250 ml of phosphate-buffered saline (PBS) with pH=7.3–7.5 (Lonza Walkersville, Inc., Walkersville, MD,

USA). Subsequently, 0.09 g of Na2S2O5 in 1 ml of PBS was added to the monomer solution, followed by 0.03 g of (NH4)2S2O8 in 1 ml of PBS. The reaction mixture was then stirred under dry

nitrogen at 25 °C for 24 h. Then, the polymerization mixture was poured into 250 ml of methanol, to precipitate an original polymer sample.6 The resulting polymer was then dissolved in pure

water and dialyzed seven times against pure water for 24 h using a cellulose tube. The original samples so prepared were separated into fractions of narrow molecular weight distribution by

fractional precipitation using acetone as a solvent and _n_-hexane as a precipitant, or by a column elution method with a 6:4 mixture of _n_-hexane and acetone as an eluent. We obtained nine

test samples named R_x_ (_x_=10, 19, 21, 48, 66, 96, 130, 220 and 270), which are generally called R samples. Each of the samples was dissolved in 1,4-dioxane, then filtered through a

Teflon membrane Fluoropore (Sumitomo Electric Industries, Ltd, Osaka, Japan) with a pore size of 1.0 μm, and finally freeze-dried from their 1,4-dioxane solutions. They were dried in a

vacuum at ∼80 °C for 12 h just before use. The ratio _M_w/_M_n of the weight-average molecular weight _M_w to the number-average molecular weight _M_n was determined for the 9 samples from

analytical gel permeation chromatography in the same manner as before1, 3 using tetrahydrofuran as an eluent and 12 standard polystyrene (PS) samples as reference standards. To specify the

chain-end group of the present PNIPA samples, we synthesized an extra PNIPA sample with a very small _M_w by mixing 10 g of _N_-isopropylacrylamide in 250 ml of PBS with 1.50 g of Na2S2O5 in

25 ml of PBS and 0.50 g of (NH4)2S2O8 in 25 ml of PBS, in the same manner as above. The reaction mixture was stirred under dry nitrogen at 25 °C for 30 min. The resulting polymer was

dialyzed and then dried in the above-mentioned manner. The extra sample was named R0.5, whose values of _M_w, _M_n and _M_w/_M_n were determined to be 4.77 × 103, 3.34 × 103 and 1.43,

respectively, from analytical gel permeation chromatography with a column Shodex SB-804 HQ (Showa Denko KK, Tokyo, Japan) connected to a solvent delivery pump PU-980 (JASCO Corporation,

Tokyo, Japan) and a refractive index detector RI-8000 (Tosoh Corporation, Tokyo, Japan); a 0.2 M aqueous NaNO3 solution was used as an eluent and nine standard poly(oxyethylene) samples as

reference standards. Tetrahydrofuran used in analytical gel permeation chromatography was of reagent grade with no stabilizer. Deuterated dimethyl sulfoxide used for 1H- and 13C-nuclear

magnetic resonance (NMR) spectroscopy was of reagent grade. Methanol used for static light scattering (LS) and viscosity measurements was purified by distillation after refluxing over

calcium hydride for ∼6 h. Water used for the determinations of the cloud point was highly purified through a water purification system Simpli Lab (Millipore Corporate, Billerica, MA, USA);

its resistivity was 18.2 MΩ cm. 1H- AND 13C-NMR To determine the values of the fraction _f_r of racemo diads for all the nine samples, R10 through R270, 1H NMR spectra for the samples in

deuterated dimethyl sulfoxide at 170 °C at ∼1 wt% were recorded on a spectrometer EX-400 (JEOL Ltd, Tokyo, Japan) at 399.8 MHz using an rf pulse angle of 90° and a pulse repetition time of 8

 s. To specify the chain-end group of the present PNIPA samples, the 13C NMR spectrum of the extra sample R0.5 in deuterated dimethyl sulfoxide at 150 °C at ∼20 wt% was recorded on the same

spectrometer, operated at 100.5 MHz with a pulse repetition time of 3 s. Tetramethylsilane was added to each solution as an internal standard. LIGHT SCATTERING LS measurements were carried

out to determine _M_w and _A_2 for all nine samples, R10 through R270, and 〈_S_2〉 for the eight samples, R19 through R270, in methanol at 25.0 °C. A light-scattering photometer Fica 50

(Fica, Saint Denis, France) was used for all measurements with vertically polarized incident light of wavelength _λ_0=436 nm. To calibrate the apparatus, the intensity of the light scattered

from pure benzene was measured at 25.0 °C at a scattering angle of 90°, where the Rayleigh ratio _R_Uu(90°) of pure benzene was taken as 46.5 × 10−6 cm−1.7 The depolarization ratio _ρ_u of

pure benzene at 25.0 °C was determined to be 0.41±0.01. The scattered intensity was measured at eight different concentrations and at scattering angles ranging from 22.5 to 142.5°, and then

converted to the excess unpolarized components Δ_R_Uv of the reduced scattered intensity using the scattered intensity from the solvent methanol. The data obtained were treated by using the

Berry square-root plot.8 For all samples, corrections for the optical anisotropy were unnecessary as the degree of depolarization was negligibly small. The most concentrated solution of each

sample was prepared gravimetrically and made homogeneous by continuous stirring at room temperature for 1 or 2 days. It was optically purified by filtration through a Teflon membrane

Fluoropore with a pore size of 0.45 or 0.10 μm. The solutions of lower concentration were obtained by successive dilution. The weight concentrations of the test solutions were converted to

polymer mass concentrations _c_ using the densities of the respective solutions calculated from the partial specific volumes _v_2 of the samples and the density _ρ_0 of the solvent methanol.

The quantity _v_2 was measured using an oscillating U-tube density meter DMA5000 (Anton-Paar, Graz, Austria). The values of _v_2 so determined in methanol at 25.0 °C were 0.869, 0.876,

0.878 and 0.883 cm3 g−1 for the samples R10, R19, R21 and R48, respectively, and 0.884 cm3 g−1 for the samples with _M_w≳6 × 105, independent of _M_w. For _ρ_0 of methanol at 25.0 °C, we

used the literature value 0.7866 g cm−3.9 The refractive index increment ∂_n_/∂_c_ was measured at a wavelength of 436 nm using a differential refractometer DR-1 (Shimadzu Corporation,

Kyoto, Japan). The values of ∂_n_/∂_c_ in methanol at 25.0 °C were determined to be 0.1865, 0.1864, 0.1855 and 0.1846 cm3 g−1 for samples R10, R19, R21 and R48, respectively, and 0.1851 cm3 

g−1 for samples with _M_w≳6 × 105, independent of _M_w. For the refractive index _n_0 of methanol at 25.0 °C at a wavelength of 436 nm, we used the literature value 1.3337.9 VISCOSITY

Viscosity measurements were carried out for all nine samples, R10 through R270, in methanol at 25.0 °C using a conventional capillary viscometer of the Ubbelohde type. The flow time was

measured to a precision of 0.1 s, keeping the difference in the flow time between the solvent and solution longer than 20 s. The test solutions were maintained at 25.0 °C within ±0.005 °C,

during the measurements. The most concentrated solution of each sample was prepared in the same manner as in the case of the LS measurements. The solutions of lower concentration were

obtained by successive dilution. The polymer mass concentrations _c_ were calculated from the weight fractions and the densities of the solutions. Density corrections were also made in the

calculations of the relative viscosity _η_r from the flow times of the solution and solvent. The data obtained for the specific viscosity _η_sp and _η_r in the range of _η_r<1.8 were

treated, as usual, by the Huggins (_η_sp/_c_ vs _c_) and Fuoss–Mead (ln _η_r/_c_ vs _c_) plots, respectively, to determine [_η_] and the Huggins coefficient _k_′. (Note that the two plots

have the same intercept.) TRANSMITTANCE OF LIGHT The intensity of light passing through aqueous solutions of the samples R10 and R19 at various weight fractions _w_ was measured using a

home-made apparatus with incident light of wavelength 650 nm from a laser diode module. A given test solution contained in a cylindrical cell of outer diameter 10 mm was immersed in a water

bath and stirred continuously. The temperature of the water bath was made to increase at a rate of ∼1.5 °C h−1. During the continuous increase in temperature from 30 to 35 °C, the intensity

of the light passing through the cell was monitored by a photodiode. The output of the photodiode and the solution temperature, measured simultaneously using a platinum resistance

thermometer combined with a programmable digital multimeter 7555 (Yokogawa Electric Corporation, Tokyo, Japan), were recorded on a personal computer at intervals of 10 s. Then, the

(relative) transmittance, as defined as the ratio of the intensity of light through the test solution at a given temperature to the intensity at a lower temperature at which the test

solution is apparently transparent, was determined as a function of temperature. The measurements were carried out for each sample at eight concentrations in the range of 0.5%≲_w_≲10%. The

most concentrated solution of each sample was prepared gravimetrically and made homogeneous by continuous stirring for 2 days at ∼5 °C. The solutions of lower concentration were obtained by

successive dilution. RESULTS AND DISCUSSION CHARACTERIZATION STEREOCHEMICAL COMPOSITIONS AND CHAIN-END GROUPS As shown in the second column of Table 1, the values of the fraction _f_r of the

racemo diads determined for all nine samples, R10 through R270, from 1H NMR spectra using the assignments of the 1H signals as proposed by Isobe _et al._,10 are almost the same (0.53–0.54)

and rather in good agreement with the _f_r values (0.51–0.52) determined previously for the samples synthesized by radical polymerization using AIBN as an initiator in _tert_-butanol and

benzene,1 which are hereafter called T and B samples, respectively. Figure 1 shows the 13C NMR spectrum of the extra sample R0.5 in the range of the chemical shift _δ_ from 10 to 80 p.p.m.,

where signals from the methylene carbon Ca (34≲_δ_/p.p.m.≲36)11 and the methine carbon Cb (41≲_δ_/p.p.m.≲42)11 in the main chain, the methine carbon Cc (38≲δ/p.p.m.≲40)11 and the methyl

carbon Cd (20≲δ/p.p.m.≲22)11 in the side group, and the carbons in the solvent dimethyl sulfoxide (38≲_δ_/p.p.m.≲40) are observed. We note that a signal from the carbonyl carbon

(172≲_δ_/p.p.m.≲174)11 is out of the range. The small signal with the asterisk observed at _δ_≃54 p.p.m. may arise from methylene carbons adjacent to the initiating chain-end groups.

Analogous to polymers synthesized by aqueous redox polymerization using (NH4)2S2O8 and Na2S2O5,12, 13 the initiating chain-end of sample R0.5 may probably be sulfonate, sulfate or hydroxy

groups, as shown in Figure 2. According to the report by Ebdon _et al._,12 which stated that the signals from the methylene carbons adjacent to the sulfonate, sulfate and hydroxy groups at

the initiating end of a vinyl polymer appear at 51≲_δ_/p.p.m.≲52, 61≲_δ_/p.p.m.≲62 and 64≲_δ_/p.p.m.≲65, respectively, the signal with the asterisk in Figure 1 may be assigned to the

methylene carbon adjacent to the sulfonate end-group. As signals from the methylene carbons adjacent to the sulfate and hydroxy end groups are inappreciable in Figure 1, it may be concluded

that almost all of the initiating chain-ends in the sample R0.5 are sulfonate groups. Although we could not directly specify the type of the initiating chain-end group for the nine samples,

R10 through R270, because of the weakness of the signal from the methylene carbon adjacent to the initiating chain-end group, the sulfonate group may also be predominant in the samples

synthesized in the same manner as was sample R0.5. As for the terminating end group of the present PNIPA samples, we do not have any detailed information. MEAN-SQUARE RADIUS OF GYRATION IN

METHANOL AT 25 °C The values of _M_w and _M_w/_M_n determined from LS measurements in methanol at 25.0 °C and analytical gel permeation chromatography, respectively, for all nine samples,

R10 through R270, are given in the third and fourth columns, respectively, of Table 1. The fifth column of the table gives the values of 〈_S_2〉 for the eight samples R19 through R270

determined simultaneously from the LS measurements in methanol at 25.0 °C. We note that the value of 〈_S_2〉 for sample R10 has been omitted because the slope of the Berry square-root plot

against the square of the magnitude of the scattering vector was not sufficiently large for R10 to evaluate 〈_S_2〉 accurately. Figure 3 shows double-logarithmic plots of 〈_S_2〉 (in Å2)

against _M_w; the unfilled circles represent the present values for the R samples, and the solid curve smoothly connects the data points. For comparison, plots for the T and B samples have

been reproduced from Figure 1 of ref. 2. The filled triangles and squares represent the values for the T and B samples, respectively, and dashed curves smoothly connect the respective data

points. The slope of the plot of the R samples in the range of _M_w≳106 is 0.94 and intermediate between 1.12 and 0.90 for the T and B samples, respectively. The slope 0.94 is less than 1.2,

which corresponds to linear flexible polymers with very large _M_w in good solvents, and smaller than unity, which corresponds to unperturbed linear polymers. This implies that the primary

structure of the R samples is not linear but branched, as in the cases of the T and B samples.2 The values of 〈_S_2〉 for the R samples are in rather good agreement with the values for the T

samples, but definitely larger than the values for the B samples. For a given polymer chain, 〈_S_2〉 generally decreases with the increasing degree of branching.14 Considering the fact that

the R, T and B samples have almost the same stereochemical composition, therefore, the degrees of branching of the R and T samples agree with each other and are smaller than that of the B

samples. Strictly, however, the data points for the R samples deviate downward from the point for the T samples for _M_w≳2 × 106. This implies that the degree of branching is somewhat larger

for the R samples than for the T samples in the range of very large _M_w. SECOND VIRIAL COEFFICIENT IN METHANOL AT 25.0 °C The values of _A_2 determined from LS measurements in methanol at

25.0 °C for all nine samples, R10 through R270, are given in the sixth column of Table 1. The _A_2 values are of order 10−4 cm3 mol g−2, indicating that methanol at 25.0 °C is a good solvent

for the present R samples as well as for the previous T and B samples. Figure 4 shows double-logarithmic plots of _A_2 (in cm3mol g−2) against _M_w for PNIPA in methanol at 25.0 °C. For

comparison, plots for the T and B samples have been reproduced from Figure 2 of ref. 2. The symbols and curves have the same meaning as those in Figure 3. The values of _A_2 for the R

samples are in rather good agreement with the values for the T samples over the whole range of _M_w examined and are definitely larger than the values for the B samples. The quantity _A_2 is

proportional to an effective volume, excluded to a polymer chain by the presence of another, and decreases with an increasing degree of branching. Therefore, the above result indicates that

the effective volume of a given R sample is almost the same as that of a T sample with the same _M_w. The seventh column of Table 1 gives the values of the interpenetration function Ψ for

the eight samples R19 through R270 in methanol at 25.0 °C calculated from with the values of _M_w, 〈_S_2〉 and _A_2 given in the third, fifth and sixth columns, respectively, of the table,

where _N_A is the Avogadro constant. The quantity Ψ, which is nearly equal to 0.24 for linear flexible polymers with very large _M_w in good solvents, is sensitive to the density profile of

repeat units constituting a given polymer14, 15 and becomes larger than 0.24 if the segment density of the polymer is larger than that of the linear flexible polymers. Figure 5 shows plots

of Ψ against log _M_w for the R samples in methanol at 25.0 °C along with plots for the T and B samples, which have been reproduced from Figure 4 of ref. 2. The unfilled circles, filled

triangles, filled squares and solid and dashed curves have the same meaning as those in Figures 3 and 4. For comparison, literature data for linear PS (♦),16, 17 regular 4-arm star PS ()18

and regular 6-arm star PS (),19 all measured in benzene at 25.0 °C, are also shown in Figure 5; the dotted curves smoothly connect the data points for each kind of PS. The value of Ψ for PS

increases with the increasing number of arms because of the above-mentioned nature of Ψ. Note that the linear PS corresponds to two-arm star PS. As a natural consequence of the behavior of

〈_S_2〉 and _A_2, shown in Figures 3 and 4, respectively, Ψ for the R samples is close to that for the T samples in the _M_w range of ≲2 × 106, but deviates upward as _M_w increases from ∼2 ×

106. As for the B samples, Ψ increases with increasing _M_w and becomes larger than Ψ for the regular four-arm star PS, which indicates that the degree of branching in the B samples is

definitely larger than that in the R and T samples. INTRINSIC VISCOSITY IN METHANOL AT 25 °C The values of [_η_] and _k_′ determined from viscosity measurements in methanol at 25.0 °C for

all nine samples, R10 through R270, are given in the eighth and ninth columns of Table 1. In accordance with the fact that methanol at 25.0 °C is also a good solvent for the present R

samples as in the cases of the previous T and B samples, deduced from the above-mentioned results of _A_2, the _k_′ values lie between 0.3 and 0.4. Figure 6 shows double-logarithmic plots of

[_η_] (in 100 ml per g) against _M_w for PNIPA in methanol at 25.0 °C. For comparison, plots for the T and B samples have been reproduced from Figure 3 of ref. 2. The symbols and curves

have the same meaning as those in Figures 3 and 4. As in the cases of 〈_S_2〉 and _A_2, shown in Figures 3 and 4, respectively, the values of [_η_] for the R samples are in rather good

agreement with the values for the T samples and definitely larger than the values for the B samples. The quantity [_η_] is proportional to the hydrodynamic (molar) volume and decreases with

the increasing degree of branching. Therefore, the above result indicates that the hydrodynamic volume of a given R sample is almost the same as that of a T sample with the same _M_w. The

slope of the plot for the R samples in the range of _M_w≳106 is 0.58 and is smaller than 0.8, which corresponds to linear flexible polymers with very large _M_w in good solvents, as in the

cases of the T and B samples.2 As a minor point, we note that the values of [_η_] for the R samples with _M_w≲105 are somewhat larger than the values for the T samples. This subtle

discrepancy may arise from the difference in the effective hydrodynamic volume of the chain-end group between the R and T samples. The last column of Table 1 gives the values of the

Flory–Fox factor Φ for the eight samples, R19 through R270, in methanol at 25.0 °C calculated from with the values of _M_w, 〈_S_2〉 and [_η_] given in the third, fifth and eighth columns,

respectively, of the table. The quantity Φ, which is nearly equal to 2 × 1023 mol−1 for linear flexible polymers with very large _M_w in good solvents, is sensitive to the degree of

branching of a given polymer and takes a larger value if the polymer has branches. Figure 7 shows plots of Φ against log_M_w for the R samples in methanol at 25.0 °C along with plots for the

T and B samples, which have been reproduced from Figure 5 of ref. 2 and plots for the linear, regular 4-arm and regular 6-arm PS.16, 17, 18, 19 All the symbols and curves have the same

meaning as those in Figure 5. The difference in the behavior of Φ among the R, T and B samples is almost the same as in the case of Ψ shown in Figure 5. From the results of the 1H- and

13C-NMR spectra and the dilute solution properties given in this subsection, it may be concluded that the R samples have almost the same stereochemical compositions as those of the T and B

samples, the same degree of branching as that of the T samples for _M_w≲2 × 106, and the hydrophilic sulfonate group at the initiating end in place of the hydrophobic isobutyronitrile group

for the T and B samples. CLOUD POINT IN AQUEOUS SOLUTIONS Figure 8 shows plots of the (relative) transmittance against temperature for the aqueous solutions of the PNIPA samples, R10 and

R19, both at _w_=5.05%. It should be noted here that the shape of the transmittance curve for each solution is almost independent of the rate of increase in temperature if the rate is slower

than 1.5 °C h−1. For comparison, the transmittance curves (dashed) previously determined for the aqueous solutions of the T samples, T5 (_M_w=5.17 × 104) and T13 (_M_w=1.31 × 105), and the

B samples, B5 (_M_w=4.65 × 104) and B14 (_M_w=1.44 × 105), all at _w_≃5%, have been reproduced from Figure 2 of ref. 1. The curves for the R samples are located to the right (high

temperature side) of the curves for the T and B samples. It is interesting to see that the curve moves to the right (high temperature side) with increasing _M_w for the T and B samples, but

moves to the left (low-temperature side) with increasing _M_w for the R samples. The cloud point in a given test solution is, in principle, determined to be the temperature at which the

solution just starts to become turbid. As seen from Figure 8, however, it is somewhat difficult to determine such a temperature unambiguously because the transmittance for each solution

starts to decrease from 100% rather gently. Following the previous studies,1, 3 therefore, the cloud point in each solution was determined as the temperature at which the transmittance

became the threshold value 90%, for convenience. In Figure 8, the symbols and represent the cloud points determined for samples R10 and R19, respectively, and the symbols ▾, ▴, and represent

the cloud points previously1 determined for samples T5, T13, B5 and B14, respectively. As a natural consequence of the behavior of the transmittance curves, the cloud points for the R

samples are the highest among the three kinds of samples. Finally, Figure 9 shows the cloud-point curves in aqueous solutions of the R samples R10 () and R19 () in the range of 0.5%≲_w_≲10%.

For comparison, the cloud-point curves previously determined in aqueous solutions of the T samples T5 (▾) and T13 (▴) and of the B samples B5 () and B14 () have been reproduced from Figure

3 of ref. 1. The solid and dashed curves smoothly connect the cloud points for the R samples and the T and B samples, respectively. In the figure, the literature data reported by de Azevedo

_et al._20 are also shown by the symbols . Although their PNIPA sample with _M_w=6.15 × 105 was prepared in the same manner as in the case of the present R samples, their experimental

procedure to determine the cloud point was different from ours: the temperature at which the slope of the plot of the reciprocal of the intensity of light scattered from an aqueous PNIPA

solution against temperature changes suddenly during the heating process was adopted as the cloud point. Despite the difference in _M_w and the procedure to determine the cloud point, the

present data for R10 and R19 qualitatively agree with the data by de Azevedo _et al._20 As expected from the results shown in Figure 8, the cloud-point curves for the R samples are located

on the high temperature side among the three kinds of samples over the whole range of _w_ examined. It is more important to note that the cloud-point curve for the R samples shifts to the

low-temperature side with increasing _M_w, while the curves for the T and B samples in the range of _w_≳2% shift to the opposite side. Otake _et al._21 and Schild and Tirrell6 pointed out

that the cloud point in an aqueous solution of PNIPA synthesized by aqueous redox polymerization was somewhat higher than that of PNIPA synthesized by radical polymerization in organic

solvents using AIBN as an initiator, although the reductive reagent _N,N,N′,N′_,-tetramethylethylenediamine of a redox catalyst used by them was different from ours. Furthermore, Xia _et

al._22, 23 reported that the cloud point in an aqueous solution of PNIPA synthesized by atom transfer radical polymerization with a hydrophilic nonionic group at the initiating chain-end

decreased with increasing molecular weight. These experimental results are consistent with ours, as shown in Figure 9. The dependence of the cloud point on _M_w for the R samples is rather

consistent with the prediction of conventional polymer solution thermodynamics.24 We note that such dependence could also result from the hydrophilicity of the chain-end group: the

hydrophilic effect increases with decreasing _M_w. It was previously3 shown that the cloud-point curve in an aqueous solution of a PNIPA sample with a strongly hydrophobic chain-end group,

which was synthesized by living anionic polymerization and had the diphenylmethyl group at the initiating chain-end, shifted remarkably to the low-temperature side with decreasing _M_w.

Furthermore, it was recently reported that amphiphilic A-B block copolymers with a PNIPA block as the hydrophilic part formed aggregates in their aqueous solutions, and their cloud points

were lower than those in an aqueous PNIPA solution.25, 26, 27, 28 These facts imply that the above-mentioned noteworthy difference in the behavior of the cloud-point curve between the R

sample and the T and B samples arises from the hydrophobicity of the chain-end groups, whose effect relatively increases with decreasing _M_w. We close our discussion with reference to the

previous opinion about the difference in the behavior of the cloud points between the T and B samples:1, 2 the increase in the number of branch points results in the increase in the number

of hydrophobic chain-end groups and, therefore, in the decrease in the cloud point. CONCLUSION The series of PNIPA samples having sulfonate groups at the initiating chain-ends were prepared

by aqueous redox polymerization using a redox catalyst consisting of (NH4)2S2O8 and Na2S2O5, which were referred to as R samples. The R samples were shown to have the same stereochemical

compositions as those of the PNIPA samples, previously1 synthesized by radical polymerization using AIBN as an initiator in _tert_-butanol and benzene, which were called T and B samples,

respectively. From the analyses of 〈_S_2〉, _A_2 and [_η_] in methanol at 25.0 °C, the R samples were shown to have the same degree of branching as that of the T samples in the range of

_M_w≲2 × 106, but have a somewhat larger degree of branching for _M_w≳2 × 106 (the degree of branching of the B samples is the largest of all.2). For the aqueous solutions of the two R

samples, R10 (_M_w=1.04 × 105) and R19 (_M_w=1.86 × 105), the cloud-point curves were determined in the range of 0.5%≲_w_≲10%. The curves for R10 and R19 were appreciably higher than the

curves previously1 determined for the two T samples (_M_w=5.17 × 104, _M_w=1.31 × 105). (The curves for the B samples were lower than those for the T samples.1) It was found that the

cloud-point curve for the R samples shifts to the low-temperature side with increasing _M_w, while the curves for the T and B samples in the range of _w_≳2% shift to the opposite side. The

difference in the behavior of the cloud-point curve between the R and T samples, both with almost the same degree of branching and stereochemical composition, arises from the difference in

the chain-end group: the R samples have the hydrophilic sulfonate group and the T samples have the hydrophobic isobutyronitrile group. REFERENCES * Kawaguchi, T., Kojima, Y., Osa, M. &

Yoshizaki, T. Cloud points in aqueous poly(_N_-isopropylacrylamide) solutions. _Polym. J._ 40, 455–459 (2008). Article  CAS  Google Scholar  * Kawaguchi, T., Kojima, Y., Osa, M. &

Yoshizaki, T. Primary structure of poly(_N_-isopropylacrylamide) synthesized by radical polymerization. Effects of polymerization solvents. _Polym. J._ 40, 528–533 (2008). Article  CAS 

Google Scholar  * Kobayashi, K., Yamada, S., Nagaoka, K., Kawaguchi, T., Osa, M. & Yoshizaki, T. Characterization of linear poly(_N_-isopropylacrylamide) and cloud points in its aqueous

solutions. _Polym. J._ 41, 416–424 (2009). Article  CAS  Google Scholar  * Kawaguchi, T., Kobayashi, K., Osa, M. & Yoshizaki, T. Is a ‘cloud-point curve’ in aqueous

poly(_N_-isopropylacrylamide) solution binodal? _J. Phys. Chem. B_ 113, 5440–5447 (2009). Article  CAS  Google Scholar  * Wooten, W. C., Blanton, R. B. & Coover, H. W. Effect of pH on

homopolymerization of _N_-isopropylacrylamide. _J. Polym. Sci._ 25, 403–412 (1957). Article  CAS  Google Scholar  * Schild, H. G. & Tirrell, D. A. Microcalorimetric detection of lower

critical solution temperatures in aqueous polymer solutions. _J. Phys. Chem._ 94, 4352–4356 (1990). Article  CAS  Google Scholar  * Deželić, Gj. & Vavra, J. Angular dependence of the

light scattering in pure liquids. _Croat. Chem. Acta_ 38, 35–47 (1966). Google Scholar  * Berry, G. C. Thermodynamic and conformational properties of polystyrene. I. Light-scattering studies

on dilute solutions of linear polystyrenes. _J. Chem. Phys._ 44, 4550–4564 (1966). Article  CAS  Google Scholar  * Johnson, B. L. & Smith, J. in _Light Scattering from Polymer

Solutions_ (ed. Huglin, M. B.) Ch 2 (Academic Press, London, 1972). Google Scholar  * Isobe, Y., Fujioka, D., Habaue, S. & Okamoto, Y. Efficient lewis acid-catalyzed stereocontrolled

radical polymerization of acrylamides. _J. Am. Chem. Soc._ 123, 7180–7181 (2001)and its supporting information. Article  CAS  Google Scholar  * Zeng, F., Tong, Z. & Feng, H. N.m.r.

investigation of phase separation in poly(_N_-isopropylacrylamide)/water solutions. _Polymer_ 38, 5539–5544 (1997). Article  CAS  Google Scholar  * Ebdon, J. R., Huckerby, T. N. &

Hunter, T. C. Free-radical aqueous slurry polymerizations of acrylonitrile: 1. End-groups and other minor structures in polyacrylonitriles initiated by ammonium persulfate/sodium

metabisulfite. _Polymer_ 35, 250–256 (1994). Article  CAS  Google Scholar  * Omidian, H., Zohuriaan-Mehr, M. J. & Bouhendi, H. Aqueous solution polymerization of neutralized acrylic acid

using Na2S2O5/(NH4)2S2O8 redox pair system under atmospheric conditions. _Int. J. Polym. Mater._ 52, 307–321 (2003). Article  CAS  Google Scholar  * Yamakawa, H. _Modern Theory of Polymer

Solutions_ (Harper & Row, New York, 1971) Its electronic edition is available on-line at the URL:http://www.molsci.polym.kyoto-u.ac.jp/archives/redbook.pdf. Google Scholar  * Yamakawa,

H. _Helical Wormlike Chains in Polymer Solutions_ (Springer, Berlin, 1997). Book  Google Scholar  * Miyaki, Y., Einaga, Y. & Fujita, H. Excluded-volume effects in dilute polymer

solutions. 7. Very high molecular weight polystyrene in benzene and cyclohexane. _Macromolecules_ 11, 1180–1186 (1978). Article  CAS  Google Scholar  * Miyaki, Y. _Dilute Solutions of

Polystyrene over a Very Wide Range of Molecular Weight_ (Ph. D. Thesis, Osaka University, Osaka, 1981). Google Scholar  * Okumoto, M., Nakamura, Y., Norisuye, T. & Teramoto, A.

Excluded-volume effects in star polymer solutions: four-arm star polystyrene in benzene. _Macromolecules_ 31, 1615–1620 (1998). Article  CAS  Google Scholar  * Okumoto, M., Iwamoto, Y.,

Nakamura, Y. & Norisuye, T. Excluded-volume effects in star polymer solutions. Six-arm star polystyrene in benzene. _Polym. J._ 32, 422–427 (2000). Article  CAS  Google Scholar  * de

Azevedo, R. G., Rebelo, L. P. N., Ramos, A. M., Szydlowski, J., de Sousa, H. C. & Klein, J. Phase behavior of (polyacrylamides + water) solutions: concentration, pressure and isotope

effects. _Fluid Phase Equilib._ 185, 189–198 (2001). Article  Google Scholar  * Otake, K., Inomata, H., Konno, M. & Saito, S. Thermal analysis of the volume phase transition with

_N_-isopropylacrylamide gels. _Macromolecules_ 23, 283–289 (1990). Article  CAS  Google Scholar  * Xia, Y., Yin, X., Burke, N. A. D. & Stöver, H. D. H. Thermal response of

narrow-disperse poly(_N_-isopropylacrylamide) prepared by atom transfer radical polymerization. _Macromolecules_ 38, 5937–5943 (2005). Article  CAS  Google Scholar  * Xia, Y., Burke, N. A.

D. & Stöver, H. D. H. End group effect on the thermal response of narrow-disperse poly(_N_-isopropylacrylamide) prepared by atom transfer radical polymerization. _Macromolecules_ 39,

2275–2283 (2006). Article  CAS  Google Scholar  * Flory, P. J. _Principles of Polymer Chemistry_ (Cornell University Press, Ithaca, NY, 1953). Google Scholar  * Chung, J. E., Yokoyama, M.,

Suzuki, K., Aoyagi, T., Sakurai, Y. & Okano, T. Reversibly thermo-responsive alkyl-terminated poly(_N_-isopropylacrylamide) core-shell micellar structures. _Colloids Surf. B_ 9, 37–48

(1997). Article  CAS  Google Scholar  * Chung, J. E., Yokoyama, M., Aoyagi, T., Sakurai, Y. & Okano, T. Effect of molecular architecture of hydrophobically modified

poly(_N_-isopropylacrylamide) on the formation of thermoresponsive core-shell micellar drug carriers. _J. Control. Release_ 53, 119–130 (1998). Article  CAS  Google Scholar  * Kujawa, P.,

Segui, F., Shaban, S., Diab, C., Okada, Y., Tanaka, F. & Winnik, F. M. Impact of end-group association and main-chain hydration on the thermosensitive properties of hydrophobically

modified telechelic poly(_N_-isopropylacrylamides) in water. _Macromolecules_ 39, 341–348 (2006). Article  CAS  Google Scholar  * Duan, Q., Narumi, A., Miura, Y., Shen, X., Sato, S., Satoh,

T. & Kakuchi, T. Thermoresponsive property controlled by end-functionalization of poly(_N_-isopropylacrylamide) with phenyl, biphenyl, and triphenyl groups. _Polym. J._ 38, 306–310

(2006). Article  CAS  Google Scholar  Download references ACKNOWLEDGEMENTS This research was supported by a Grant-in-Aid (22350050) and in part by the global COE program International Center

for Integrated Research and Advanced Education in Materials Science, from the Ministry of Education, Culture, Sports, Science and Technology, Japan. AUTHOR INFORMATION AUTHORS AND

AFFILIATIONS * Department of Polymer Chemistry, Kyoto University, Kyoto, Japan Tomoyo Ise, Kouta Nagaoka, Masashi Osa & Takenao Yoshizaki Authors * Tomoyo Ise View author publications

You can also search for this author inPubMed Google Scholar * Kouta Nagaoka View author publications You can also search for this author inPubMed Google Scholar * Masashi Osa View author

publications You can also search for this author inPubMed Google Scholar * Takenao Yoshizaki View author publications You can also search for this author inPubMed Google Scholar

CORRESPONDING AUTHOR Correspondence to Masashi Osa. RIGHTS AND PERMISSIONS Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Ise, T., Nagaoka, K., Osa, M. _et al._ Cloud points

in aqueous solutions of poly(_N_-isopropylacrylamide) synthesized by aqueous redox polymerization. _Polym J_ 43, 164–170 (2011). https://doi.org/10.1038/pj.2010.121 Download citation *

Received: 23 August 2010 * Revised: 01 October 2010 * Accepted: 13 October 2010 * Published: 15 December 2010 * Issue Date: February 2011 * DOI: https://doi.org/10.1038/pj.2010.121 SHARE

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clipboard Provided by the Springer Nature SharedIt content-sharing initiative KEYWORDS * aqueous solution * cloud point * intrinsic viscosity * mean-square radius of gyration *

poly(_N_-isopropylacrylamide) * second virial coefficient