Unperturbed dimensions of polystyrene derivatives. II. Poly-3,4-dichlorostyrene and poly-p-cyclohexylstyrene

The relationship of intrinsic viscosity to the number-average molecular weight has been obtained for poly-3,4-dichlorostyrene and poly-p-cyclohexylstyrene in a few solvents. Values of (〈L²〉₀/M)^1/2 have been estimated for poly-3,4-dichlorostyrene and poly-p-cyclohexylstyrene through the use of the treatment of Stockmayer and Fixman. The values of (〈L〉₀/M)^1/2 have been obtained as 2.18 ± 0.08 and 2.52 ± 0.07 for poly-3,4-dichlorostyrene and poly-p-cyclohexylstyrene, respectively. The σ value of a series of polystyrene derivatives has been found to increase with the bulk of side groups. This seems to indicate that the σ value is mainly determined by the steric repulsion between side groups.     

Effects of composition on the solution properties of styrene–acrylonitrile copolymers

Short-range interactions between chain units of random copolymers in solution may be influenced by the composition or precisely by the distribution of sequence lengths of the same monomer units. Steric factors were derived for random copolymers of styrene and acrylonitrile with different compositions from the relation between the limiting viscosity number and the molecular weight. Mark-Houwink relations were obtained in methyl ethyl ketone (MEK) or in N,N′-dimethylformamide (DMF) at 30°C. for random copolymers containing 0.383 (Co-1) and 0.626 (Co-2) mole fraction of acrylonitrile, the expressions are: [η] = 3.6 X 10^?4 M _w^0.62, for Co-1 in MEK; [η] = 5.3 X 10^?4 M _w^0.61, for Co-2 in MEK; [η] = 1.2 × 10^?4M _w^0.77 for Co-2 in DMF. With the Stockmayer-Fixman expression, these correlations become, respectively: [η]/M^1/2 = 1.24 × 10^?3 + 8.0 × 10^?7 M^1/2; and [η]/M^1/2 = 1.70 × 10^?3 + 6.3 × 10^?7 M^1/2; and [η]/M^1/2 = 1.68 × 10^?3 + 31.3 × 10^?7 M^1/2. From the unperturbed mean-square end-to-end distances, 〈L²〉₀, determined from the first terms of the latter expressions, together with 〈L²〉_0f calculated by assuming the completely free rotation, gives the steric factor σ = (〈L²〉₀/〈L²〉_0f)^1/2 as 2.25 ± 0.05 for Co-1, and 2.31 ± 0.10 for Co-2. These values of σ are close to those for polystyrene (σ = 2.22 ± 0.05) and for polyacrylonitrile (σ = 2.20 ± 0.05). Therefore, it is concluded that the dimensions of random copolymers of styrene and acrylonitrile in solution are not significantly influenced by the composition. In other words, the unperturbed dimensions are not affected by a change in the alternation tendency between styrene units with phenyl side groups having a large molar volume and acrylonitrile units with nitrile groups responsible for the electrostatic interactions. On the other hand, the long-range interactions reflect the effect of sequence length. The Huggins constant and the second virial coefficient obtained from the light-scattering measurements have optimum values at about 0.5 mole fraction of acrylonitrile, where the greatest tendency for alternation seems to exist.     

Unperturbed dimensions of isotactic poly(tert-butylethylene oxide)

The root-mean-square end-to-end distances of isotactic poly(tert-butylethylene oxide) fractions were determined in xylene at 80°C from intrinsic viscosity measurements and in o-dichlorobenzene at 80 and 100°C by light scattering. The characteristic dimension (〈L²〉₀/M)^1/2 was 1.04 × 10^?8 cm in xylene and 0.9 and 0.7 × 10^?8 cm in o-dichlorobenzene at 80 and 100°C, respectively. The value in xylene corresponds to a C_∞ of 15.9. This observation and the large negative temperature coefficient of (〈L²〉₀/M)^1/2 suggest that poly(tert-butylethylene oxide) exists in a helical block conformation under these experimental conditions. This conclusion is in agreement with earlier reported NMR measurements.     

Molecular dimensions of polydipropylsiloxamer

The molecular dimensions of polydipropylsiloxamer were studied by intrinsic viscosity measurements in toluene and in 2-pentanone. The relationships between the molecualr weight and the intrinsic viscosity were found to be: [η]_25°C., toluene = 4.35 × 10^?4 M^0.58; [η]_θ(10°C.), toluene = 1.09 × 10^?3 M^0.5; [η]_θ(76°C.), 2-pentanone = 8.71 × 10^?4 M^0.5. This held reasonably well for molecular weights from 25,000 to 3000,000. The root-mean-square end-to-end length ratio, (r₀² /M)^1/2 as calculated from the constant K, exceeds the free rotation value by approximately 100%. The disparity is greater than that found with polydimethylsiloxamer, indicating a lower degree of flexibility for the polydipropylsiloxamer. This is largely due to the short range steric interaction between near neighboring units of the chain. Gel permeation chromatography was also employed to demonstrate the lower degree of flexibility for polydipropylsiloxamer as compared with polydimethylsiloxamer.     

Solution properties of high molecular weight polystyrene

A series of polystyrenes with weight-average molecular weight M?_w up to 1.3 × 10⁷ was prepared by anionic polymerization in tetrahydrofuran (THF). Each sample was characterized by gel-permeation chromatography, light scattering, and viscometry. It was found that each sample had an almost symmetrical and very narrow molecular weight distribution (M?_w/M?_n < 1.07). The mean-square unperturbed radius of gyration 〈S²〉₀ was determined in trans-decalin at 20.4°C as 〈S²〉₀ = 7.8₆ × 10^?18M?_w (cm²). The particle scattering factor was well represented by the Debye equation irrespective of solvent in the range of M?_w < 4 × 10⁶, and only a small deviation was observed in benzene at higher molecular weights. The penetration function Ψ ≡ A₂M²/4π^3/2N_A〈S〉^23/2 was found to approach a relatively low asymptotic value of 0.21–0.23 at molecular weights above 2 × 10⁶ in benzene at 30°C, where A₂ is the second virial coefficient and N_A is Avogrado's number. It was also found that the theta temperature in trans-decalin was affected by the nature of polymer samples. A difference of about 3°C in the theta temperature was observed between two series of anionic polystyrenes, one prepared in THF and the other in benzene, but there was practically no difference in unperturbed chain dimension.     

Solution and thermal properties of poly(1,3-dioxocane)

Poly( 1,3-dioxocane) was synthesized by cationic ring-opening polymerization with triphenyl-methane hexafluoroantimoniate as the initiator and was studied with regard to its solubility, unperturbed chain dimensions, and thermal transitions. The intrinsic viscosity and Flory-Huggins interaction parameter were used to determine the solubility parameter, δ_p = 9.6 cal^1/2cm^?3/2, a value that agrees with that calculated empirically. Fractions were obtained from the solvent/non-solvent system benzene/methanol at 25°C. The number-average molecular weight M_n and intrinsic viscosity [η] were measured in toluene at 25°C. The relation [η] = 1.45₉. 10^?4 M_n^0.79 was found. A value of 5.3 was obtained for the characteristic ratio 〈r²〉₀/nl². Results are correlated with the main thermal transitions of this polyformal.     

Unperturbed chain dimensions of isotactic polypropylene in theta solvents

The unperturbed mean-square end-to-end distance 〈R₀²〉 and its temperature variation d In 〈R₀²〉/dT for isotactic polypropylene have been estimated from intrinsic viscosity data in three theta solvents, i.e., diphenyl, diphenyl ether, and dibenzyl ether, measured at their θ temperatures as determined by precipitation temperature measurements. The characteristic ratios, 〈R₀²〉/nl², where n is the number of bonds of length l in the main chain, evaluated by assuming Φ = 2.87 × 10²¹, are 5.80 in diphenyl (at θ = 125.1°C.), 5.41 in diphenyl ether (at θ = 142.8°C.), and 4.56 in dibenzyl ether (at θ = 183.2°C.). These values lead to the temperature coefficient d In 〈R₀²〉/dT = ?4.09 × 10^?3 deg.^?1 Results are compared with the data previously reported on polyethylene.     

Light-scattering studies on polystyrenes in isorefractive poly(methyl methacrylate)–toluene “solvents”

The rms radii of gyration 〈S²〉^1/2 and second virial coefficients Γ₂ of five monodisperse polystyrenes (M × 10^?5 = 1.6, 2.8, 4.2, 6.6) were measured in isorefractive toluene–poly(methyl methacrylate) (M?_v = 4.0 × 10⁴, 1.6 × 10⁵, and 6.3 × 10⁵) “solvents.” For a given PMMA, the concentration at which the θ condition (defined by Γ₂ = 0) was reached was independent of PS molecular weight, but varied inversely with PMMA molecular weight (0.10, 0.056, and 0.023 g/mL, respectively). When this θ condition is reached by adding PMMA to toluene, the radii of gyration are decreased by only about 15%, much less than when it is reached by going to a poor, low-molecular-weight solvent. This reflects the exclusion of PMMA from the PS coils, the internal environment of which is essentially pure toluene.     

Ultrasonic Solution Degradations of Poly(Alkyl Methacrylates)

Ultrasonic (70 W, 20 kHz) solution (2%) degradations of poly(alkyl methacrylates) have been carried out in toluene at 27°C and in tetrahydrofuran (THF) at -20°C. M_w and M_n of all polymers (before and after sonification) were computed from GPC. Irrespective of the alkyl substituent, M_w decreased rapidly at first and then slowly approached limiting values. All M_w/M_n ratios were in the vicinity of 1.5 at the limiting chain lengths. For identical M_n, the rate constants k were (4.2 ± 2.0) × 10^?6 min^?1 in toluene at 27°C and (5.4 ± 2.0) × 10^?6 min^?1 in THF at -20°C. For poly(isopropyl methacrylate) and poly(octadecyl methacrylate) with higher, but identical, M_n,0, k values were higher ((9.0 ± 1.0) × 10^?6 min^?1 at 27°C and (18.0 ± 1.5) × 10^?6 min^?1 at -20°C). This suggests that M_n,0 and not the bulk size of the alkyl substituents is the factor that determines the rate of degradation. Lowering of the temperature accelerates degradation due primarily to lower chain mobility of poly-(alkyl methacrylates) and enhanced cavitation. The average number of chain scissions ([(M_n)₀/(M_n)_t] - 1) calculated from component degradation data are much higher than those obtained with overall M_n,t values.     

Kinetic studies of the reactions of pentacyanoferrate(III) complexes with L‐ascorbic acid

The reduction of Fe(CN)₅L^2? (L = pyridine, isonicotinamide, 4,4′‐bipyridine) complexes by ascorbic acid has been subjected to a detailed kinetic study in the range of pH 1–7.5. The rate law of the reaction is interpreted as a rate determining reaction between Fe(III) complexes and the ascorbic acid in the form of H₂A(k₀), HA^?(k₁), and A^2? (k₂), depending on the pH of the solution, followed by a rapid scavenge of the ascorbic acid radicals by Fe(III) complex. With given Ka₁ and Ka₂, the rate constants are k₀ = 1.8, 7.0, and 4.4 M^?1 s^?1; k₁ = 2.4 × 10³, 5.8 × 10³, and 5.3 × 10³ M^?1 s^?1; k₂ = 6.5 × 10⁸, 8.8 × 10⁸, and 7.9 × 10⁸ M^?1 s^?1 for L = py, isn, and bpy, respectively, at μ = 0.10 M HClO₄/LiClO₄, T = 25°C. The kinetic results are compatible with the Marcus prediction. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 126–133, 2005     

Thermodynamic and conformational properties of partially butyralized poly(vinyl alcohol) in aqueous solution

Phase separation behavior has been studied in aqueous solutions of partially butyralized poly(vinyl alcohol) (BuPVA) with various degrees of butyralization x_Bu and various molecular weights. It is found that these systems exhibit both upper and lower critical solution temperatures. The theta temperatures θ_LCST, evaluated by means of Shultz–Flory plots, are found to be 25.1, 23.3, and 14.4°C for BuPVAs with x_Bu of 7.5, 9.9, and 12.7 mol %, respectively. The unperturbed dimension 〈R²〉₀/M is evaluated as ca. 1.2 × 10^?16 Ų in the above range of x_Bu from viscosity measurement at θ_LCST. Properties of the BuPVA solutions are compared with those of other PVA copolymers.     

Kinetic Study of the Bromine-Catalyzed Isomerization of Maleic Acid in Aqueous Ce(IV)-Br−-H2SO4 Medium

The presence of ceric and bromide ions catalyzes the isomerization of maleic acid (MA) to fumaric acid (FA) in aqueous sulfuric acid. A kinetic study of this bromine-catalyzed reaction was carried out. The reaction between ceric ion and maleic acid is first order with respect to Ce(IV). For [Ce(IV)]₀=5.0×10^?4 M, [H₂SO₄]₀=1.2 M, μ=2.0 M (adjusted by NaClO₄), and [MA]₀=(0.5–1.0)M, the observed pseudo-first-order rate constant (k₀₃) at 25° is k₀₃=7.622×10^?5 [MA]₀/(1+0.205[MA]₀). The reaction between ceric and bromide ions is first order with respect to Ce(IV). For [Ce(IV)]₀=5.0×10^?4 M, [H₂SO₄]₀=1.2 M, μ=2.0 M, and [Br^?]₀=(0.025–0.150)M, the pseudo-first-order rate constant (k₀₂) at 25° is k₀₂= (4.313±0.095)x10^?2[Br^?]²+(2.060±0.119)x10^?3[Br^?]. The reaction of Ce(IV) with maleic acid and bromide ion is also first order with respect to Ce(IV). For [Ce(IV)]₀=5.0×10^?4 M, [MA]₀=0.75 M, [H₂SO₄]₀=1.2 M, μ=2.0 M, and [Br^?]₀= (0.025–0.150)M, the pseudo-first-order rate constant (k₀₃) at 25° is k₀₃= (5.286±0.045)x10^?2[Br^?]²+(3.568±0.056)x10^?3[Br^?]. For [Ce(IV)]₀=5.0 × 10^?4 M, [Br^?]₀=0.050 M, [H₂SO₄]₀=1.2 M, μ=2.0 M, and [MA]₀=(0.15–1.0)M at 25°, k₀₃=(2.108×10^?4+2.127×10^?4[MA]₀)/(1+0.205[MA]₀). A mechanism is proposed to rationalize the results. The effect of temperature on the reaction rate was also studied. The energy barrier of Ce(IV)—Br^? reaction is much less than that of Ce(IV)—MA reaction. Maleic and fumaric acids have very different mass spectra. The mass spectrum of fumaric acid exhibits a strong metastable peak at m/e 66.5.     

Temperature dependence of limiting viscosity number and radius of gyration of cellulose diacetate in acetone

Acetone solutions of a cellulose diacetate fraction were studied by viscosity and light scattering methods over the range 12.6–50.3². The temperature dependences of the limiting viscosity number [η], the mean-square radius of gyration 〈s²〉, and the second virial coefficient A₂ were determined. The unperturbed mean-square radius of gyration 〈s²〉_o and the expansion factor α, were estimated by using theoretical relations to the interpenetration function. It was found that dln 〈s²〉_o/dT is ?6.4 × 10^?3 deg^?1, while α, is close to unity over the whole temperature range studied. The viscosity results are interpreted to show that the draining effect is not negligible and the Flory viscosity parameter Φ slightly increases with increasing temperature. It is finally concluded that the value of ?6.9 × 10^?3 deg^?1 for dln [η]/dT can be ascribable to the rapid decrease in 〈s²〉_o.     

Rheological studies of moderately concentrated polystyrene solutions. I. A new method for the extrapolation of the zero-shear viscosity

Measurements of the viscosity coefficient η of solutions of polystyrene (M_w = 6.0 × 10⁵ and 1.77 × 10⁶) in trans-decalin (TD, θ solvent) and toluene (TL, good solvent) as function of shear rate (11?10⁴ s^?1), concentration (4.24?11.21 wt %), and temperature (10–50°C) are reported. As a new theoretically grounded method for the determination of the zero-shear viscosity η₀ it is proposed to plot η as a function of $\left({\eta \dot \gamma} \right)^3$. The intercepts of the straight lines obtained by this procedure give η₀ in good agreement with directly measured values.     

Polar,functionalized diene‐based materials. V. Free‐radical polymerization of 2‐[(N‐benzyl‐n‐methylamino)methyl]‐1,3‐butadiene and copolymerization with styrene

2‐[(N‐Benzyl‐N‐methylamino)methyl]‐1,3‐butadiene (BMAMBD), the first asymmetric tertiary amino‐containing diene‐based monomer, was synthesized by sulfone chemistry and a nickel‐catalyzed Grignard coupling reaction in high purity and good yield. The bulk and solution free‐radical polymerizations of this monomer were studied. Traditional bulk free‐radical polymerization kinetics were observed, giving polymers with 〈M_n〉 values of 21 × 10³ to 48 × 10³ g/mol (where M_n is the number‐average molecular weight) and polydispersity indices near 1.5. In solution polymerization, polymers with higher molecular weights were obtained in cyclohexane than in tetrahydrofuran (THF) because of the higher chain transfer to the solvent. The chain‐transfer constants calculated for cyclohexane and THF were 1.97 × 10^?3 and 5.77 × 10^?3, respectively. To further tailor polymer properties, we also completed copolymerization studies with styrene. Kinetic studies showed that BMAMBD incorporated into the polymer chain at a faster rate than styrene. With the Mayo–Lewis equation, the monomer reactivity ratios of BMAMBD and styrene at 75 °C were determined to be 2.6 ± 0.3 and 0.28 ± 0.02, respectively. Altering the composition of BMAMBD in the copolymer from 17 to 93% caused the glass‐transition temperature of the resulting copolymer to decrease from 64 to ?7 °C. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3227–3238, 2001     

Bimolecular self-reactions of 2-arylindandione- 1,3-yl radicals studied by flash photolysis

Kinetic and thermodynamic data for reaction (1) of certain C-centered aromatic radicals (referred to in this paper by the numbers I to X) in chlorobenzene: have been obtained. The k₁ values of radicals varied between (1.1 ± 0.2) × 10⁶M^?1·sec^?1 (radical VIII) and (3.6 ± 0.7) × 10⁹M^?1 sec^?1 (radical VI) at 20°C. An investigation of the relationship between the recombination rates of radicals I–VIII and X and the solvent viscosity (mixture of toluene and dibutylphthalate, 0.6 < η < 18.4 cP) has shown that the recombination reactions involving radicals I–IV are limited by diffusion in solvents having a viscosity η> 10 cP and are activation reactions in solvents having a viscosity η < 10 cP. The recombination of radicals VIII and IX is an activation reaction, while that of radicals V–VII is diffusion-controlled in the entire viscosity range. The recombination of radical X is limited, in the viscosity range of 18.4 to 2 cP, by intrusion into the first coordination sphere of the partner, the effect of viscosity on the radical X recombination rate in the specified range being the same as its effect on diffusion-controlled reactions. The possible reasons of the discrepancies between the experimental fast recombination rate constants and the theoretical values calculated by the Debye–Smoluchowski theory are discussed. The equilibrium constant depends strongly on the nature of the substituent in the phenyl fragment: the substituents which increase unpaired electron delocalization in the radical intensify the dissociation of the respective dimer. Long-wave absorption bands have been recorded for radicals I–X and their extinction coefficients obtained. Dimers I–V are thermo- and photochromic compounds.     

Poly-N-vinylamides,complexation and conformational changes in aqueous solution

Interaction between small molecules (iodine and 1-anilinonaphthalene-8-sulphonate) and poly-N-methylacetamide (PVMA), poly-N-vinylcaprolactam (PVCL), poly-N-vinylpyrrolidone (PVP) and copolymers of N-vinyleaprolactam with N-vinylpyrrolidone (CP VCL-VP) and with N-vinyl-N-methylacetamide (VMA) were studied. All the polymers formed complexes with the small molecules. The stability constant for the complexes with the probe increases in the series PVMA < PVP < copolymer < PVCL indicating the essential role of hydrophobic interactions in the complex formation. With increasing temperature of aqueous solution from 18 to 33°, the constant decreased for PVP, PVMA and copolymers with low VCL content and increased for PVCL. A folding of PVC coils was observed over the temperature range, demonstrating a relationship between the conformational changes of macromolecules and the polymer complex formation. Addition of PVCL, PVMA and PVP to aqueous I₃^? solution brings about a shift of λ_max from 350 nm for free triiodide ions to 372, 367 and 364 nm respectively. It is explained by different dipole moments of the side substituents. The stability constant for the polymers and I₃^? was estimated as approx. 10⁵ M. The ability of the polymers to form complexes depends on chainlength. The greatest loss of the ability was observed for PVCL with $\text{M}{}_{\mathrm{n}}$ changing from 6 × 10³ to 2 × 10³. Interaction of the polymers with I₃^? is accompanied by a conformational shrinking of coils. In the case of the probe, forces of electrostatic repulsion between negative charges of sulphonate groups result in an unfolding of polimer coils.     

Crystal structure,DNA interaction,and antiproliferative activity of the cobalt(II) complex of demethylcantharate and imidazole

A new cobalt(II) complex, [Co(C₃H₄N₂)(C₈H₈O₅)(H₂O)₂]·2H₂O, of demethylcantharate(7-oxabicyclo[2,2,1]heptane-2,3-dicarboxylate, C₈H₈O₅) with imidazole has been synthesized from cobalt chloride, demethylcantharidin (NCTD) and imidazole. The complex was characterized by elemental analysis, IR, and X-ray single crystal diffraction. The complex crystallized in the monoclinic crystal system and P2₁/m space group with a?=?0.634790(10)?nm, b?=?0.963030(10)?nm, c?=?1.221770(10)?nm, α?=?90°, β?=?95.9700(10)°, γ?=?90°, V?=?0.742844(15)?nm³, M_r ?=?383.22, D_c ?=?1.713?g?cm^?3, Z?=?2, F(0?0?0)?=?398, μ?=?1.206?mm^?1, the final R?=?0.0291, and wR?=?0.0837 [I?>?2σ(I?)]. The interaction of the complex with deoxyribonucleic acid (DNA) was studied by electronic absorption spectra, fluorescence spectra, and viscosity measurements, which indicate that the complex binds to calf thymus DNA through a partially intercalative mode. The binding constant K _b for the complex was 2.62?×?10⁴?L?mol^?1. The antiproliferation activity test showed that the complex has high antiproliferative ability against human hepatoma cells SMMC7721 (with IC₅₀ being 42.8?±?0.9?µmol?L^?1) and human lung cancer cells A549 (with IC₅₀ being 65.1?±?3.2?µmol?L^?1). The inhibition rates of the complex are much higher than those of NCTD.     

THE INTERACTION OF 2-AMINO-2-HYDROXYMETHYL-1,3-PROPANEDIOL WITH COBALT(II) AND MANGANESE(II) IONS

Abstract

The stepwise complex formation between 2-amino-2-hydroxymethyl-1,3-propanediol (TRIS) with Co(II) and Mn(II) was studied by potentiometry at constant ionic strength 2.0 M (NaClO₄) and T = (25.0 ± 0.1)°C, from pH measurements. Data of average ligand number (Bjerrum's function) were obtained from such measurements followed by integration to obtain Leden's function, F ₀(L). Graphical treatment and matrix solution of simultaneous equations have shown two overall stability constants of mononuclear stepwise complexes for the Mn(II)/TRIS system (β₁ = (5.04 ± 0.02) M^?1 and β₂ = (5.4 ± 0.5) M^?2) and three for the Co(II)/TRIS system (β₁ = (1.67 ± 0.02) × 10² M^?1, β₂ = (7.01 ± 0.05) × 10³ M^?2 and β₃ = (2.4 ± 0.4) × 10⁴ M^?3). Slow spontaneous oxidation of Co(II) solutions by dissolved oxygen, accelerated by S(IV), occurs in a buffer solution TRIS/HTRIS⁺ 0.010/0.030 M, with a synergistic effect of Mn(II).     

Studies of micro-brownian motion of a polymer chain by the fluorescence polarization method. I. Fluorescent conjugates of polyacrylamide

Polyacrylamide having a fluorescent residue at the chain end was prepared by polymerization of acrylamide in the presence of a fluorescent dye. The segmental motion of the chain end in dilute solution was studied by the fluorescence polarization method on the fluorescent polyacrylamide conjugates thus obtained. The linear relation between 1/p and T/η₀ held for every sample studied in aqueous media, where p is the degree of polarization of the fluorescence, T is the absolute temperature, and η₀ is the viscosity of the medium. The mean relaxation time 〈ρ〉 of the conjugate was evaluated from these data as a function of the molecular weight of the conjugate. The value of 〈ρ〉 increased slightly with molecular weight, varying from 3.3 × 10^?9 to 7 × 10^?9 sec. The absolute values of 〈ρ〉 and its molecular weight dependence suggest that 〈ρ〉 represents the mean rotational relaxation time for the cooperative motion of about ten monomeric units at the chain end. The effect of the mean extension of polymer chain on the segmental motion was found to be negligible.