Graft Density Dependence of Main Chain Stiffness in Molecular Rod

Aug 13, 2015 - Department of Polymer Science and Engineering, Graduate School of Science and Engineering, Yamagata University, 4-3-16, Jonan, Yonezawa...
0 downloads 8 Views 2MB Size
Download PDF
Article pubs.acs.org/Macromolecules

Graft Density Dependence of Main Chain Stiffness in Molecular Rod Brushes Moriya Kikuchi,† Ryo Nakano,‡ Yuji Jinbo,§ Yuta Saito,‡ Satoshi Ohno,‡ Daichi Togashi,‡ Kazushi Enomoto,‡ Atsushi Narumi,‡ Osamu Haba,‡ and Seigou Kawaguchi*,‡ †

Faculty of Engineering, Yamagata University, 4-3-16, Jonan, Yonezawa 992-8510, Japan Department of Polymer Science and Engineering, Graduate School of Science and Engineering, Yamagata University, 4-3-16, Jonan, Yonezawa 992-8510, Japan § Department of Biochemical Engineering, Graduate School of Science and Engineering, Yamagata University, 4-3-16, Jonan, Yonezawa 992-8510, Japan ‡

S Supporting Information *

ABSTRACT: The graft density dependence of main chain stiffness in rod brushes composed of a flexible linear polystyrene (PS) main chain and poly(n-hexyl isocyanate) (PHIC) rod side chains has thoroughly been investigated by static light (LS) and small-angle Xray scattering (SAXS) measurements in tetrahydrofuran (THF) at 25 °C. A series of statistical graft copolymers having different graft densities (σ) and graft chain lengths (PS-g-HIC-Ns-lg, where Ns is the weight-averaged degree of polymerization of HIC and lg is the average distance (spacing) of the main chain between the side chain joints) were prepared by the nearly azeotrope radical copolymerizations of styrene (M1) with PHIC macromonomers (M2) (r1 = 0.84 ± 0.1 and r2 = 1.1 ± 0.3) in n-hexane or the bulk at 60 °C. The compositional heterogeneities of the graft copolymers were carefully characterized by the ratio of the UV to RI signal in the SEC traces and the comparison with those predicted from the theory by Stejskal and Kratochvil [Macromolecules 1987, 20, 2624], implying that they were sufficiently small enough to allow one to study their dimensional characterizations. The graft density dependence of the root-mean-square cross-section radius of gyration (⟨Sc2⟩1/2) and the z-averaged root-mean-square radius of gyration (⟨S2⟩z1/2) of PS-g-HIC-Ns-lg was studied and rationalized as the function of σ and Ns. The σ-dependence of ⟨Sc2⟩1/2 could be quantitatively described by the theory for the semiflexible comb whose main and side chains have different chain stiffness. The weight-averaged degree of polymerization of the main chain (NM) dependence of ⟨S2⟩z1/2 was also quantitatively described by the cylindrical wormlike chain model. It was found that the change in the main chain stiffness (λb−1) resulting from the interactions among the side chains increases in proportion to the scaling law of λb−1 ∝ Ns1σ1.

■

INTRODUCTION

reports on their syntheses and applications. A reason is due to the fact that the heterogeneities in the branched architecture, composition, and molecular weight make the relevant characterization techniques, such as size exclusion chromatography (SEC) and light scattering (LS), quite inefficient in a strict sense.2 Among the others, the macromonomer technique provides a promising method to synthesize well-defined graft copolymers at least in the sense that the macromonomer, which forms the branches, is precharacterized.1,2 However, the difference in the reactivity in the copolymerization of a low molecular weight comonomer with a macromonomer also results in the graft copolymers having significant compositional heterogeneities to make the precise dimensional characterization difficult. The graft copolymers in which three characteristic length parameters as described above are equally

Graft (co)polymer is a representative synthetic branched polymer, composed of a linear backbone and pendant grafted side chains. It is often the counterpart of the star polymer having the different branching architecture. Unlike the star polymer, the graft polymer is specified by defining three characteristic length dimensions, that is, the length of the grafted chain, the length (spacing) between the grafted joints, and the contour length of the main chain. The graft (co)polymer is generally prepared by the techniques of grafting-from, grafting-(on)to, and grafting-through.1,2 A wide variety of graft (co)polymers have so far been prepared by these techniques, including the graft copolymers whose backbone is chemically different from the branched chain. Recent significant progress in synthetic polymer chemistry, including the controlled/living radical polymerization3−5 and the use of “click-chemistry”6 is extremely appreciated. However, very few fundamental studies on the dimensional properties of the graft copolymers have so far been reported compared with the many © XXXX American Chemical Society

Received: May 12, 2015 Revised: August 1, 2015

A

DOI: 10.1021/acs.macromol.5b01010 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Scheme 1. Schematic Representation of PS-graf t-PHIC Copolymer (PS-g-HIC-Ns-lg) Evaluated in This Study

well-defined have not yet been easily achieved. This is the reason why the fundamental properties and molecular characterizations for the graft copolymers have not yet been a subject of research as compared to the star polymers.2 The poly(macromonomer), a homopolymer of the macromonomer, which is prepared by homopolymerization of the macromonomer, is a well-defined graft (co)polymer having one of the highest grafting densities in which a tooth chain is grafted at every second carbon atom of the main chain.2 It is called a comb or molecular brush. Over the past two decades, the conformational properties of the molecular brush, cylindrical brush, or bottlebrush have attracted considerable attention,2,7−19 because the intrinsically flexible main and side chains were found to behave as stiff chains in a dilute solution and the solid state. Since such stiffening is considered to result from the interactions among the densely grafted side chains, to clarify the relation between the main chain stiffness parameter (λM−1) and the chemical structure of the polymer has been one of the important subjects in the polymer solution study. Chain conformation of brushes has rather been explained by considering them as a thick linear homopolymer possessing a large side chain at every repeating unit. One of the most reliable determination method for the λM−1 value can be achieved to investigate the molecular weight dependence of the chain dimension and to apply appropriately the Kratky−Porod (KP) wormlike cylinder model to the experimental data.7,8,15−19 The main chain stiffness parameter of the brush is generally given by the equation:17

λM −1 = λ 0−1 + λb−1 −1

This value of r1 means a nearly azeotrope copolymerization and significantly encouraged us to initiate the study. The present article reports many details of the experimental results of the syntheses, characterization, and dilute solution properties of the rod-grafted copolymers (PS-g-HIC-Ns-lg), where lg is the average length (spacing or pitch) of the main chain between the rod side joints. A schematic picture showing the lengths characteristic of the graft copolymers, lM, lg, and the

(1) −1

where λ0 is the intrinsic backbone stiffness and λb relates to the excess free energy against the bending, which is caused from the collisions among the dense side chains, that is, the excluded-volume effects. Whereas the side chain molecular weight dependence of λb−1 in eq 1 for the flexible brushes has been studied experimentally15,18,19 and theoretically,16,20−22 a consensus has not always been obtained to date. In previous papers, we reported the synthesis and characterization of rod-like styryl-ended or α-methacryloyl-ω-acetyl ended poly(n-hexyl isocyanate) (PHIC) macromonomers (VBHIC-Ns23−25 or MA-HIC-Ns,26−28 where Ns is the weightaveraged degree of polymerization of HIC), their radical copolymerizabilities,23,26 and conformational properties of the resulting rod brushes.24,25,28 The value of λM−1 of the rod brushes linearly increased with the increasing of Ns that followed the scaling law of λM−1 ∝ Ns1. The observed power law exponent value could not be explained by the theories for the rod brushes.29 The scope of the present study is focused to elucidate experimentally the relationship between the main chain stiffness and the graft density, because the linear main chain without any grafting chain is originally very flexible polystyrene (PS) with λ0−1 of ca. 1.84 nm,30 as shown in Scheme 1. Whereas the main chain stiffness of the graft copolymer would be thought to decrease with increasing the grafted-chain spacing, few study has been reported yet,19 due to the difficulties of the preparation of a series of well-defined graft copolymers having different grafting densities. We reported that the apparent reactivity ratio of the macromonomer relative to styrene (St), 1/r1 in the radical copolymerizations of St (M1) and VB-HICNs (M2) under the condition of [M1]/[M2] ≪ 1 is close to one, implying that the compositional heterogeneities accompanied by an increase in the conversion are remarkably suppressed.23,26

Figure 1. Schematic picture showing three parameters, lM, lg, and Ls, characteristic of the PS-graf t-PHIC copolymer (PS-g-HIC-Ns-lg).

side chain contour length (Ls) is shown in Figure 1, in which the lg value was calculated from the following equation: lg = lM(x + 1)

(2)

where lM is the contour length per backbone monomer and 0.22 nm is used in this study,25 and x is the mole ratio of St (M1) to macromonomer (M2) in the graft copolymer, x = d[M1]/d[M2]. The statistical graft copolymers having various graft densities were synthesized by the “grafting through” technique.1,2 The values of the reactivity ratio, r1 and r2, for the radical copolymerizations were newly determined and the compositional heterogeneities of the graft copolymers which play a dominant role in the reliabilities of the studies were carefully characterized and discussed. The molecular and dimensional characterizations of the graft copolymers were performed in tetrahydrofuran (THF) at 25 °C by small-angle X-ray scattering (SAXS) and LS measurements. The rootmean-square cross-section radius of gyration (⟨Sc2⟩1/2) and zaveraged root-mean-square radius of gyration (⟨S2⟩z1/2) for PSg-HIC-Ns-lg were studied and compared to the current theories, and the graft density dependence of λM−1 was rationalized as the functions of Ns and grafting density. B

DOI: 10.1021/acs.macromol.5b01010 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

■

precipitated into methanol, filtered and dried under reduced pressure. The styrene conversion was then determined by the weight method using the following equation:

EXPERIMENTAL SECTION

Materials. Benzene (Kanto Chemical Co., Tokyo, Japan) was purified by washing with concentrated H2SO4, water, and a sodium hydroxide solution, then drying with calcium chloride, followed by distilling from a sodium/benzophenone ketyl solution. Dichloromethane (Kanto Chemical Co., Japan) and n-hexane (Kanto Chemical Co., Japan) were distilled over calcium hydride (CaH2) prior to use. nHexyl isocyanate (HIC) (Tokyo Kasei Kogyo Co., Ltd., Tokyo, Japan) was distilled over CaH2 under reduced pressure just before use. St (Wako Pure Chemical Industries, Osaka, Japan) was purified by washing with a sodium hydroxide solution and saturated NaCl solution, then drying with calcium chloride, followed by distilling over CaH2 under reduced pressure. Trichlorocyclopentadienyl titanium(IV) (CpTiCl3) (Kanto Chemical Co., Japan), methanol (Wako Pure Chemical Industries, Japan), and THF (Kanto Chemical Co., Japan) were used as received. The radical initiator, dimethyl-2,2′-azobis(2methyl propionate) (V-601) (Wako Pure Chemical Industries, Japan) was used after freeze-drying from benzene. Spectroscopic grade THF (Kanto Chemical Co., Japan) was used for the UV, specific refractive index increment (dn/dc) and small-angle X-ray scattering (SAXS) measurements. Unless otherwise specified, all other reagents were purchased from commercially available sources and used as received. Synthesis of PHIC Macromonomer (VB-HIC-Ns). p-Vinylbenzyl alcohol (VBA) was synthesized from p-vinylbenzyl chloride (AGC Seimi Chemical Co., Ltd., Kanagawa, Japan) by the reaction with sodium acetate, followed by alkaline hydrolysis. The titanium alkoxide complex was prepared according to Novak and co-workers.31 In a drybox, a solution of 9.1 mmol sodium p-vinylbenzyl alkoxide in 20 mL of dry benzene, prepared by the reaction of VBA with a 10-fold excess molar amount of NaH, was slowly added to the solution of 9.1 mmol CpTiCl3 in 50 mL of dry benzene. The reaction was carried out at room temperature for 1 h, then the reaction mixture was filtered through a G4 filter to remove the sodium chloride. The solution was freeze-dried to afford a yellow solid. The macromonomer, VB-HIC-Ns was synthesized as follows. In a drybox, to a 20 mL flask with a magnetic stir bar, measured amounts of the titanium complex and dichloromethane were added, then the flask was capped with a rubber septum. After the catalyst was completely dissolved, HIC was added through a syringe, the flask was fitted with a ground-glass stopper and removed from the drybox. The polymerization was carried out at room temperature for 24 h. After the polymerization was complete, to the solid orange mass was added methanol to terminate the active end-group. The precipitates were poured into methanol and filtered. The polymer was redissolved in THF containing 5% methanol and reprecipitated into methanol to afford a white solid material. The reprecipitation procedure was done three times. Finally, the polymer was dissolved in dry benzene and freeze-dried. Homopolymerization of VB-HIC-Ns. The polymacromonomer sample was prepared in a sealable ampule by the radical homopolymerization of VB-HIC-Ns using V-601 as the initiator in n-hexane at 60 °C for 24 h. After polymerization, the reaction mixture was dissolved in THF and precipitated into methanol, followed by five washings with n-hexane to remove any unreacted (unpolymerized) macromonomer. Finally, the polymer was dissolved in dry benzene, filtered, and freeze-dried. Synthesis of Graft Copolymer PS-g-HIC-Ns-lg. The graft copolymers, PS-g-HIC-Ns-lg samples, were prepared by the conventional radical copolymerization of St (M1) with VB-HIC-Ns (M2). Depending on the weight ratio of M1 to M2 in the feed, two different experimental procedures were used. For the weight ratio of M1/M2 > 1, the copolymerization was conducted in the bulk. A typical protocol for the radical copolymerization of styrene with VB-HIC-32 in the bulk was as follows: styrene (290 mg, 2.79 mmol), VB-HIC-32 (232 mg, 0.049 mmol), and V-601 (4.6 mg, 0.020 mmol) were loaded into a 5 mL glass ampule with a stopcock, degassed three times by a freeze− thaw process and then subsequently sealed off under reduced pressure. The copolymerization reaction was carried out at 60 °C for 24 h. The reaction was stopped by exposure to air. The reaction mixture was

convn(M1) =

W − W0,VB − HIC − Ns W0,St

× 100 (3)

where W is the weight of the reaction mixture of the graft copolymer and unreacted macromonomer and W0,i is the weight of species i in the feed. Any unreacted VB-HIC-Ns was eliminated from the reaction mixture by five washings and decantations with n-hexane. The complete removal of the unreacted VB-HIC-Ns without losing the resulting graft copolymers was confirmed by 1H NMR and SEC measurements of the n-hexane-soluble and -insoluble parts. On the other hand, for the weight ratio of M1/M2 < 1, the copolymerizations were conducted in n-hexane. A typical protocol for the radical copolymerization was as follows: styrene (16.3 mg, 0.16 mmol), VB-HIC-32 (372 mg, 0.078 mmol), V-601 (3.0 mg, 0.013 mmol), and n-hexane (0.381 g) were loaded into a 5 mL glass ampule with a stopcock, degassed three times by the freeze−thaw process and then subsequently sealed off under reduced pressure. The copolymerization was carried out at 60 °C for 48 h. The reaction was stopped by exposure to air. The reaction mixture was precipitated into methanol, filtered and dried under reduced pressure. The conversion of VB-HIC-Ns was determined by the UV signal area ratio of the graft copolymer (PS-g-HIC-Ns-lg) to the unpolymerized VB-HIC-Ns in the SEC trace using the following equation:

convn(M 2) =

UVPS ‐ g ‐ VB ‐ HIC ‐ Ns UVPS ‐ g ‐ VB ‐ HIC ‐ Ns + UVunreacted VB ‐ HIC ‐ Ns

× 100

(4) where UVi is the UV signal area of species i at the wavelength of 252 nm. The molar extinction coefficient at 252 nm was determined to be 1.27 × 102 L monomer-mol−1 cm−1 for the linear PS (Mw = 4.15 × 105 g mol−1, Mw/Mn = 2.34) and 9.66 × 104 L macromonomer-mol−1 cm−1 for the VB-HIC-32. Thus, for the weight ratio of M1/M2 < 1, the UV signal of the graft copolymer is mainly responsible for the grafted PHIC chain, but not for PS. The unreacted VB-HIC-Ns was removed from the reaction mixtures by a recycling preparative size-exclusion chromatography system of LC-9201 (Japan Analytical Industry (JAI) Co., Ltd., Tokyo, Japan) equipped with a UV-3740 (JAI Co., Ltd., Japan) using two columns (Shodex HF-2002 and HF2003; Showa Denko K.K., Tokyo, Japan) in THF as the eluent at the flow rate of 3.5 mL min−1 and room temperature. Determination of Reactivity Ratio, r1, and r2. The radical copolymerizations of St (M1) with VB-HIC-47 (M2) at different mol ratios were carried out in n-hexane with V-601 as the initiator at 60 °C for 60 or 90 min. The experimental procedures are the same as already mentioned. The conversion of VB-HIC-47 was controlled to less than 10%. The reaction mixture was poured into methanol, filtered and dried under reduced pressure. The unreacted VB-HIC-47 was removed from the mixtures by recycling preparative size-exclusion chromatography. The 1H NMR spectrum of polystyrene-graft-PHIC (run 4 in Table S1) is shown in Figure S1. The mole ratio in the graft copolymer, x = d[M1]/d[M2], was determined by the following equation: x=

⎡ (a) ⎤ 3NNMR ⎣ (c) ⎦ − 4 d[M1] = d[M 2] 5

(5)

1

where (a) is the H NMR peak intensities due to the phenyl and phenylene protons, (c) is those due to the methyl protons in the VBHIC-47, and NNMR is the number-averaged degree of polymerization of PHIC determined by the 1H NMR. The reactivity ratios, r1 and r2, were determined by a curve fitting method based on the well-known copolymerization equation (Mayo−Lewis equation):

F1 = 1 − F2 = C

r1f12 + f1 f2 r1f12

+ 2f1 f2 + r2f2 2

(6) DOI: 10.1021/acs.macromol.5b01010 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

we first describe many details of the characterizations of the synthesized graft copolymers, because this is one of the most important points to guarantee the reliability of the present studies. The Reactivity Ratios, r1 and r2, in the Radical Copolymerization of St with VB-HIC-47. Table S1 shows the radical copolymerization results of St(M1) with VB-HIC47(M2) at different mole ratios, [M1]/[M2], in n-hexane at 60 °C. The 1H NMR spectrum of polystyrene-graf t-PHIC (run 4 in Table S1) is shown in Figure S1. The mole ratio of the M1 and M2 composition in the graft copolymers, d[M1]/d[M2], determined from eq 5 is listed in Table S1. Figure S2 shows the compositional curve for the copolymerization together with the best-fit curve (solid line) calculated from eq 6 with the values being r1 = 0.84 ± 0.1 and r2 = 1.1 ± 0.3. The value of r1 determined in this study is in good agreement with that reported in a previous paper23 in which the r1 values (0.76− 0.92) were determined under the conditions of [M1]/[M2] ≫ 1. Whereas a relatively large experimental error is unavoidable in the determination of the r2 value, the determined values of r1 and r2 clearly imply that the copolymerization proceeds in a manner of nearly azeotrope and at least the compositional heterogeneities accompanied by the conversion are remarkably suppressed during the copolymerization. Preparation and Characterization of PS-g-HIC-Ns-lg. The copolymerization conditions, conversion, copolymer composition, and characteristics of the resulting graft copolymers (PS-g-HIC-Ns-lg), in which Ns = 26, 32, 58, and 115 and lg ranged from 0.22 to 368, are listed in Tables S2 and S3. Their composition data are also plotted in Figure S2, showing that the compositions of the graft copolymer are close to those in the feed within the experimental errors. A typical example of the absolute calibration curve, RI, and UV signals in the SEC chromatogram of the PS-g-HIC-32−0.79 sample is presented in Figure 2 along with the data of ⟨S2⟩z1/2

where F1 and F2 are the mole fraction of M1 and M2 in the graft copolymer, respectively, and f1 and f 2 are the mole fraction of M1 and M2 in the feed, respectively, r1 = k11/k12 and r2 = k22/k21 are the monomer reactivity ratios of M1 and M2, respectively, and kij is the propagating rate constant of a propagating chain ending in Mi reacting with a monomer Mj, with i and j = 1 or 2. Measurements. The weight-averaged molecular weight (Mw), polydispersity index (Mw/Mn), and z-averaged mean-square radius of gyration (⟨S2⟩z) of the PS-g-HIC-Ns-lg samples were determined in THF at 25 °C by using an SEC equipped with a multiangle light scattering (MALS) detector (DAWN-DSP; Wyatt Technology Co., Santa Barbara, CA; wavelength, λ = 632.8 nm) (SEC-MALS). The SEC measurements were carried out using an RI detector (Shodex RI101; Showa Denko K.K., Japan) and a UV detector (UV-8020; Tosoh Co., Tokyo, Japan) with three columns (Shodex KF802, KF806, and KF806L; Showa Denko K. K., Japan) in THF as the eluent with the flow rate of 1.0 cm3 min−1 and a 40 °C column temperature. The Rayleigh ratio at the scattered angle of 90° was determined using pure toluene.32 The angular dependence of the excess reduced scattering intensity was analyzed by the Berry method.33 All the polymer solutions were prepared by a gravimetric method, and the polymer mass concentrations were calculated by the same method employed in our previous study.24 The value of dn/dc for the PS-g-HIC-Ns-lg samples was measured in THF at 25 °C using a differential refractometer (DRM-1021; Otsuka Electronics, Osaka, Japan, wavelength: λ = 632.8 nm). The molar extinction coefficients of the polymers at the wavelength of 252 nm were determined in THF at 25 °C using a UV spectrophotometer (V-530, JASCO Co., Tokyo, Japan). The SAXS experiments were performed at 25 °C using NANO-Viewer (Rigaku Co., Tokyo, Japan) at an incident X-ray wavelength λ of 0.1542 nm with a sample-to-detector distance of 652 mm. The scattering vector (q) defined as 4πsin(θ)/λ, where 2θ is the scattering angle, was calibrated from the peak of the Bragg reflection of powdery lead stearate. The scattering intensity (I(q)) was detected using a high-speed 2D X-ray detector (PILATUS 100K; DECTRIS Ltd., Baden, Switzerland) with 487 × 195 pixels to cover the range of the scattering vectors from 0.1 to 1.0 nm−1. The excess scattering intensities (ΔI(q)) were obtained as the difference between the value of I(q) for the solvent and solution at the same q by taking into account the X-ray transmittance. To obtain the values of [Cp/ ΔI(q)]Cp=0 at an infinite dilution, the scattering intensities at each q were extrapolated to Cp = 0. The details were described in a previous study.34 The scanning probe microscopy (SPM) measurements were carried out to observe the chain conformations of the graft copolymers in the solid state. The sample preparation and the measurement conditions were the same as those in our previous study.28

■

RESULTS AND DISCUSSION Table 1 lists the characteristics of the VB-HIC-Ns macromonomers with Mw ranging from 0.342 × 104 to 1.48 × 104 g mol−1 and Mw/Mn ranging from 1.09 to 1.24 determined by the absolute calibration curve of PHIC in the SEC. Before going into the dimensional characterization of the graft copolymers,

Figure 2. RI and UV signals in SEC trace of PS-g-HIC-32−0.79 in THF, together with SEC absolute calibration curve (○) and ⟨S2⟩z1/2 (□) determined by SEC-MALS. Filled circles represent the ratio of UV to RI signals at each retention volume of PS-g-HIC-32−0.79 sample.

and the UV/RI signal ratio. The Mw and Mw/Mn values for PSg-HIC-Ns-lg, which were determined from the corresponding absolute calibration curve, are listed in Table S3. The values of log Mw and log ⟨S2⟩z1/2 linearly decrease with the increase in the retention volume showing that the graft copolymers are appropriately separated in the order of their hydrodynamic volume. To investigate the size dependence of the compositional heterogeneity, we used the signal ratio of UV to RI from the SEC trace. As described in the Experimental Section, the molar extinction coefficient of the linear PS and VB-HIC-32 at the wavelength of 252 nm in THF is 1.27 × 102 L monomermol−1 cm−1 and 9.66 × 104 L macromonomer-mol−1 cm−1, respectively. The latter is 760 times higher than the former. In

Table 1. Characteristics of VB-HIC-Ns Macromonomers VB-HIC-Ns

Mn × 10−4 (g mol−1)a

Mw × 10−4 (g mol−1)b

Nsc

Mw/Mna

Lsd (nm)

VB-HIC-26 VB-HIC-32 VB-HIC-47 VB-HIC-58 VB-HIC-115

0.409 0.476 0.660 0.782 1.53

0.342 0.429 0.611 0.755 1.48

26 32 47 58 115

1.11 1.09 1.19 1.24 1.24

4.5 5.7 8.2 10.2 20.2

a Determined by 1H NMR. bDetermined by SEC calibrated with a series of PHICs as the standard.24 cWeight-averaged degree of polymerization of HIC. dCalculated by the equation of Ls = (NsM0)/ ML with M0 = 127.2 g mol−1 and ML = 725 g mol−1nm−1.

D

DOI: 10.1021/acs.macromol.5b01010 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules addition, the dn/dc values of the linear PS and PHIC at the wavelength of 632.8 nm in THF at 25 °C are also determined to be 0.189 g−1 cm3 and 0.090 g−1 cm3, respectively. Therefore, the UV to RI signal intensity ratio provides a measure of the compositional heterogeneities of the graft copolymers. It should be noted in Figure 2 that the RI/UV ratio for PS-g-HIC-32− 0.79 is almost independent over the entire retention volume region. This clearly demonstrates that the composition of styrene and macromonomer in the PS-g-HIC-32−0.79 sample is almost independent of the molecular size and molecular weight. Differential Weight Distribution of Chemical Composition in the Statistical Graft Copolymers. Stejskal and Kratochvil35 reported the differential weight distribution of the chemical composition [W(w1)] for the radical copolymerization of a low molecular weight monomer, M1, and a macromonomer, M2, which is given by the equation:

(

W (w1) =

3 2

) 1 dz 2 a + 3/2 1 dw1 Γ(a + 1)Γ( 2 ) (1 + z ) Γ a+

In conclusion, based on the obtained reactivity ratio, UV/RI signal ratio in the SEC trace, together with the theoretical calculation of the differential weight distribution is that the compositional heterogeneities of the graft copolymers are low enough to study their dimensional properties. Graft Density Dependence of Cross-Section Radius of Gyration, ⟨Sc2⟩1/2. The characteristics of the PS-g-HIC-Ns-lg samples are listed in Table S3. The cross-section mean-square radius of gyration (⟨Sc2⟩) for the PS-g-HIC-Ns-lg samples was determined from the initial slope in the cross-section Guinier plot, ln[qΔI(q)/Cp]Cp=0 versus q2, within the cross-section

(7)

where Γ(w1) is the gamma function, a is related to the numberand weight-averaged degrees of polymerization, Nn and Nw, and defined as a−1 = Nw/Nn − 1, and w1 is the weight fraction of M1 in the graft copolymer. The parameters in eq 7 are given by Nwt (w1 − w1̅ )2 z2 = 2apw1̅ (1 − w1̅ ) [(1 − t )w1 + t ]2 ⎤1/2 ⎡ 4tw1̅ (1 − w1̅ ) p = ⎢1 + (r1r2 − 1)⎥ [(1 − t )w1̅ + t ]2 ⎦ ⎣

Figure 3. Cross-section Guinier plots of ln[qΔI(q)/Cp]Cp=0 as a function q2 for PS-g-HIC-58-lg samples in THF at 25 °C.

(8)

Guinier region (a typical example is shown in Figure 3) by applying the following equation:36

(9)

⎡ ΔI(q) ⎤ k 2Nπ 1 ⎥ = ln − ⟨Sc 2⟩q2 + ··· ln⎢q L 2 ⎢⎣ Cp ⎥⎦ C =0

and ⎡ ⎤1/2 (1 − t )w + t Nwt dz 1̅ =⎢ ⎥ dw1 ⎣ 2apw1̅ (1 − w1̅ ) ⎦ [(1 − t )w1 + t ]2

p

where L is the contour length of the cylinder, k is the electron density contrast factor, and N is the number of cylinders. The values of ⟨Sc2⟩1/2 for the poly(macromonomer), PS-gHIC-Ns-0.22, at an infinite dilution are listed in Table 2. Figure 4 shows the Ns dependence of the ⟨Sc2⟩1/2 in which the ⟨Sc2⟩1/2 value linearly increases with the increasing Ns and is superimposed on that of the previous studies within the experimental errors. According to the worm-like comb model in which the main and side chains have different stiffness parameter, λM−1 and λs−1, respectively, ⟨Sc2⟩ is given by37

(10)

where t is the ratio of the molecular weights of M1 and M2, r1 and r2 are the reactivity ratio, and w̅ 1is the average composition of the weight fraction of M1 defined by the equation:

tF1 w1̅ = tF1 + F2

(12)

(11)

with F1 and F2 are the mole fraction of M1 and M2 in the graft copolymer, given by eq 6, respectively. An example of the Nw (denoted by NM in this study) dependence of the differential weight distribution, W(w1) of the chemical composition (w1) for the statistical graft copolymers, PS-g-HIC-32-lg, having different average compositions of the weight fraction of M1 (w̅ 1) is shown in Figure S3, where t = 0.024, r1 = 1.0, r2 = 1.0, and a = 2.0 for the recombination termination. It can be clearly seen that the distribution broadens with the increasing of w̅ 1, that is, the St composition, but steeply sharpens with the increasing degree of polymerization of the main chain (NM). Figure S4 shows the actual W(w1) calculated using the parameters, r1 = 0.84, r2 = 1.1, and a = 2.0 for the PS-g-HICNs-lg samples, which will be used later to study the dimensional properties. The values of NM were tuned to the lowest value in each graft copolymer used to study the solution properties. It is noted that the W(w1) for the graft copolymers with Ns = 26, 32, and 58 is narrow enough, but for the graft copolymers with Ns = 115, it has a considerably broader W(w1).

NM ⎧ 2⎡ Ls 1 1 1 ⎨Ls ⎢ − + − L T 2 ⎩ ⎣ 6λs 4λs 2 4λs 3Ls 8λs 4Ls 2 ⎡L ⎤ 1 (1 − exp(− 2λsLs))⎥ +Ls(L T − Ls)⎢ s − 2λs 2 ⎣ 2λs ⎦

⟨Sc 2⟩ =

+

⎪

⎪

⎤⎫ 1 (1 − exp(− 2λsLs))⎥⎬ 3 4λs Ls ⎦⎭

L T = NMLs + (NM − 1)lM

⎪

⎪

(13) (14)

where NM is the weight-averaged degree of polymerization of the main chain, LT is the total contour length of the brushes, and lM is the contour length per backbone monomer. When NM ≫ Ls and λsLs → 0 for the brushes with the rod side chains, eq 13 is approximated by eq 15: E

DOI: 10.1021/acs.macromol.5b01010 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Table 2. Cylindrical Wormlike Chain Parameters for PS-g-HIC-Ns-lg in THF at 25 °C

a

PS-g-HIC-Ns-lg‑

λM−1 (nm)

ML × 10−3 a (g mol−1 nm−1)

⟨Sc2⟩1/2 b (nm)

δc (nm)

B (nm)

PS-g-HIC-26−0.22 PS-g-HIC-26−0.29 PS-g-HIC-26−0.52 PS-g-HIC-26−0.80 PS-g-HIC-32−0.22 PS-g-HIC-32−0.79 PS-g-HIC-32−1.59 PS-g-HIC-32−2.67 PS-g-HIC-32−3.29 PS-g-HIC-32−14.2 PS-g-HIC-32−32.2 PS-g-HIC-58−0.22 PS-g-HIC-58−0.50 PS-g-HIC-58−1.14 PS-g-HIC-58−3.82 PS-g-HIC-58−9.76 PS-g-HIC-58−33.9 PS-g-HIC-58−58.9 PS-g-HIC-115−34.4 PS-g-HIC-115−150 PS-g-HIC-115−368

26 17 14 10 38 14 8.1 5.0 4.1 2.5 2.1 53 29 13 5.0 2.8 2.2 1.8 2.9 2.1 1.8

14.0 10.7 6.23 4.19 17.9 5.36 2.88 1.91 1.64 0.744 0.593 27.7 12.5 5.74 2.04 1.09 0.650 0.575 2.59 0.552 0.505

2.57 2.51 2.46 2.39 3.18 2.92 2.89 2.72 2.49 1.71 1.38 6.04 5.70 5.71 5.13 4.14 2.97 2.67 6.45 4.06 2.70

9.00 9.00 8.78 8.34 11.4 10.7 10.1 8.98 8.32 0 0 20.4 20.2 19.5 16.9 10.9 0 0 6.30 0 0

1.2 1.2 1.2 1.2 − 1.6 1.6 1.6 1.4 1.3 1.3 11 3.5 1.6 1.3 1.3 1.3 1.0 1.4 1.3

Calculated from eq 19. bDetermined by SAXS in THF at 25 °C. cCalculated from eq 20.

Figure 4. Ns dependence of the measured ⟨Sc2⟩1/2 for PS-g-HIC-Ns0.22 samples and rod brushes from previous studies24,25,28 in THF at 25 °C. The solid line is the theoretical value for semiflexible brush model calculated by eq 15 with lg = 0.22 nm.

⟨Sc 2⟩ ≈

Ls 3 3(Ls + lM)

Figure 5. Graft density (σ) dependence of ⟨Sc2⟩1/2 for PS-g-HIC-Ns-lg samples in THF at 25 °C, together with those of the theoretical values (solid and dotted lines) for the semiflexible brush model calculated by eq 15.

(15)

increasing σ, but in the high σ region, it gradually increases and approaches the value of the homopolymers. The solid and broken lines in Figure 5 are the theoretical curves calculated from eq 15 by replacing lM with lg. The experimental σ dependence of ⟨Sc2⟩1/2 is perfectly reproduced by the worm-like comb theory.37 Graft Density Dependence of Radius of Gyration, ⟨S2⟩1/2. The experimental data of ⟨S2⟩z1/2 and Mw for the graft copolymers are summarized in Tables S4, S5, S6, and S7. Figure 6 shows an example of the double-logarithmic plot of ⟨S2⟩z1/2 versus Mw for PS-g-HIC-32-lg samples in THF, together with that of the linear PS under the same condition (solid line).37 For the same Mw, the absolute value of ⟨S2⟩z1/2 for PS-gHIC-32-lg decreases with the increasing graft density due to the branching architecture and the increase in the number of grafted chains. To evaluate the effect of the grafted chain on the main chain stiffness, Mw is reduced to the main chain contour length, i.e., NM. Figure 7 shows the NM dependence of ⟨S2⟩z1/2 for the PS-g-HIC-Ns-lg samples [(a) Ns = 32, (b) Ns = 58, (c)

The Ls value is calculated by the equation of Ls = (NsM0)/ML with M0 being the molecular weight of HIC (127.2 g mol−1) and the molecular weight per unit contour length (ML = 725 g mol−1 nm−1) in THF at 25 °C.24 The solid line in Figure 4 is the calculated one from eq 15 with lM = 0.22 nm,24 perfectly describing the experimental Ns dependence of ⟨Sc2⟩1/2 for the present rod brushes. Figure 5 shows the double-logarithmic plots of ⟨Sc2⟩1/2 versus the graft density (σ) of the graft copolymers, PS-g-HIC-Ns-lg. If one assumes the PS main chain as a smooth cylinder with dPS in the diameter to that the side chains with a contour length (Ls) link, the graft density (σ), i.e., the inverse of the occupation area per side chain, may be given by eq 16:

σ=

1 πdPSlg

(16)

where dPS = 0.92 nm.30 It is seen from Figure 5 that in the low σ region, the ⟨Sc2⟩1/2 value relatively steeply increases with the F

DOI: 10.1021/acs.macromol.5b01010 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

To evaluate the main chain stiffness parameter (λM−1) of the graft copolymers and to rationalize it as a function of σ and Ns, the NM dependence of ⟨S2⟩z1/2 was analyzed by the cylindrical wormlike chain model with an end effect.25,38 The main chain contour length (LM) may be given by LM =

Mw +δ ML

(18)

where δ/2 denotes the contribution of the side chains near the ends to the main chain contour. The value of ML for PS-g-HICNs-lg with an average monomer molar mass is calculated by eq 19: Figure 6. Mw dependence of ⟨S2⟩z1/2 of PS-g-HIC-32-lg samples in THF at 25 °C, together with that of linear PS37 under the same condition. The solid and broken lines are the theoretical values for the perturbed cylindrical wormlike chain model having the parameters listed in Table 2.

ML =

MStF1 + M n,VB ‐ HIC ‐ Ns(1 − F1) lM

(19)

The contribution of the side chains near the ends to the main chain contour, δ, is statistically averaged averaged for the mole ratio, x, which is rounded to the natural number, using the following equation: n

δ=

2 ∑i = 1 (Ls − nlM) (20)

x+1

The parameters of ML and δ are listed in Table 2. The meansquare radius of gyration (⟨S2⟩) of a cylindrical wormlike chain may be expressed by30 ⟨S2⟩ = ⟨SM 2⟩ + ⟨Sc 2⟩

(21)

where ⟨SM2⟩ is the main chain mean-square radius of gyration of a polymer chain with a contour length LM and is given with the main chain stiffness parameter, λM−1, by39 ⟨SM 2⟩ =

LM 1 1 1 − + − 6λM 4λM 2 4λM 3L M 4λM 4L M 2

[1 − exp( −2λML M)]

(22)

−1

The value of λM was determined by fitting eq 22 to the experimental data in the region that the number of Kuhn’s segment, nK = λMLM, is less than 5. In this region, the intramolecular excluded-volume effects substantially disappear. In the region of the nK > 5, the influence of the intramolecular excluded-volume effects on the chain expansion in a good solvent is also considered using the quasi-two-parameter theories,40−43 which are given by eqs S1−S4. The comparison of the experimental data of the PS-g-HIC-Ns-lg samples with the theoretical curves (solid and broken lines) calculated from eqs 17−22 and S1−S4 using the model parameters listed in Table 2 is shown in Figures 6 and 7. The experimental NM dependences of ⟨S2⟩z1/2 for the graft copolymers of all the Ns and lg are quantitatively described in terms of the perturbed cylindrical wormlike chain model. Graft Density Dependence of Main Chain Stiffness. Figure 8 shows the double-logarithmic plots of the main chain stiffness parameter (λM−1) versus the graft density (σ) for the PS-g-HIC-Ns-lg samples, as well as that of the linear PS (λM−1 = 1.84 nm30) in the theta condition (solid line). It is noted that the main chain of the graft copolymers becomes stiff with the increase in σ. The main chain stiffness is relatively low in the low σ region, but gradually increases with σ, and linearly increases with σ in the high σ region. Also, for the same σ, the λM−1 increases with the increasing Ns.

Figure 7. NM dependence of ⟨S2⟩z1/2 for PS-g-HIC-Ns-lg samples [(a) Ns = 32, (b) Ns = 58, (c) Ns = 26, and (d) Ns = 115] in THF at 25 °C, together with that of linear PS (solid line)37 under the same condition. The solid and broken lines are the theoretical values for the perturbed cylindrical wormlike chain model having the parameters listed in Table 2.

Ns = 26, and (d) Ns = 115]. The NM values are calculated by the following equation: NM =

Mw MStF1 + M n,VB ‐ HIC ‐ Ns(1 − F1)

(17) −1

where MSt is the molecular mass of St (104 g mol ) and Mn,VB‑HIC‑Ns is the number-averaged molecular weight of VBHIC-Ns as listed in Table 1. An interesting but remarkable observation noted in this figure is that at the same NM, that is, the same main chain contour length, the value of ⟨S2⟩z1/2 increases with the increasing graft density in all the PS-g-HICNs-lg samples. This clearly indicates that the PS backbone becomes stiff with the increase in the grafting density, σ. G

DOI: 10.1021/acs.macromol.5b01010 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 8. Graft density (σ) dependence of the main chain stiffness parameter (λM−1) for PS-g-HIC-Ns-lg samples in THF at 25 °C, together with that of linear PS37 (thick solid line) under the same condition.

Figure 10. Plots of the main chain stiffness change, λb−1 versus the graft density (σ) of PS-g-HIC-Ns-lg.

Figure 9 shows the SPM phase images of (a) PS-g-HIC-29− 0.22,25 (b) PS-g-HIC-32−1.59, (c) PS-g-HIC-32−3.29, (d) PS-

Figure 11 shows the double-logarithmic plot of λb−1 versus Nsσ for the present graft copolymers, together with the

λ b −1 ∝ σ 1 ∝ lg −1

(23)

Figure 11. Universal plots of λb−1 with Nsσ for all the PS-g-HIC-Ns-lg samples.

previous studies for the polymacromonomers.24,25 All the experimental data are superimposed on each other to afford a generality, which follows a simple scaling law described by the following equation:

Figure 9. Phase images of SPM measurement for (a) PS-g-HIC-29− 0.22,25 (b) PS-g-HIC-32−1.59, (c) PS-g-HIC-32−3.29, (d) PS-g-HIC58−0.22, and (e) PS-g-HIC-58−3.82 samples on mica at room temperature.

λb

−1

⎛ N ⎞1 ∝ Nsσ ∝ ⎜⎜ s ⎟⎟ ⎝ lg ⎠ 1

(24)

This very simple, but meaningful power law exponent value for lg is superficially close to that in the theoretical equation for the rod brush:29

g-HIC-58−0.22, and (e) PS-g-HIC-58−3.82 samples on mica at room temperature. A single, cylindrical brush-like macromolecule is clearly observed. It is also seen that the brushes become flexible with the decreasing σ. The values of λM−1 in a two-dimensional state are estimated by the trajectory analyses28 to be 32 nm for PS-g-HIC-29−0.22, 8.2 nm for PS-g-HIC-32− 1.59, 6.2 nm for PS-g-HIC-32−3.29, 56 nm for PS-g-HIC-58− 0.22, and 7.0 nm for PS-g-HIC-58−3.82. These values are comparable to those in THF at 25 °C, which are listed in Table 2. As mentioned in the Introduction, the main chain stiffness parameter (λM−1) of the brush polymer is given by the sum of the intrinsic backbone stiffness, λ0−1 and the increment due to the side chains, λb−1. We describe the latter. Figure 10 shows the double-logarithmic plot of λb−1 versus σ for the PS-g-HICNs-lg samples in THF. An interesting observation to be noted is that the data of λb−1 for the PS-g-HIC-Ns-lg samples linearly increase with the increasing σ, implying the following simple scaling relation:

λ b −1 ∝

Ns 2 3 2ln(Ns/lg ) lg

(25)

Further experimental studies to understand the conformational properties of the rod brush polymers are currently in progress and will be reported soon.

■

CONCLUSIONS The graft density dependence of the main chain stiffness in graft copolymers, PS-g-HIC-Ns-lg, composed of a flexible linear polystyrene (PS) main chain and PHIC rod side chains has been thoroughly studied by LS and SAXS measurements in THF at 25 °C. A series of statistical graft copolymers having different graft densities (σ) and graft chain lengths were prepared by the nearly azeotrope radical copolymerizations of styrene (M1) with PHIC macromonomers (M2) (r1 = 0.84 ± H

DOI: 10.1021/acs.macromol.5b01010 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules 0.1 and r2 = 1.1 ± 0.3). The compositional heterogeneities of the graft copolymers were carefully characterized to demonstrate that they were sufficiently low enough to allow one to study their dimensional characterizations. The graft density dependence of ⟨Sc2⟩1/2 of PS-g-HIC-Ns-lg was quantitatively described by the theory for the semiflexible comb whose main and side chains have different chain stiffness. The NM dependence of ⟨S2⟩z1/2 was also quantitatively described by the perturbed cylindrical wormlike chain model. The change in the main chain stiffness (λb−1) resulting from the interactions among the side chains was found to follow the simple scaling law of λs−1 ∝ N1s σ1.

■

(13) Sahl, M.; Muth, S.; Branscheid, R.; Fischer, K.; Schmidt, M. Macromolecules 2012, 45, 5167−5175. (14) Shah, P. N.; Chae, C. − G.; Min, J.; Shimada, R.; Satoh, T.; Kakuchi, T.; Lee, J. − S. Macromolecules 2014, 47, 2796−2802. (15) Terao, K.; Hokajo, T.; Nakamura, Y.; Norisuye, T. Macromolecules 1999, 32, 3690−3694. (16) Nakamura, Y.; Norisuye, T. Polym. J. 2001, 33, 874−878. (17) Nakamura, Y.; Norisuye, T. Chapter 5 In Soft Matter Characterization, Borsali, R., Pecora, R., Eds.; Spring Science + Business Media, LLC: New York, 2008. (18) Inoue, K.; Yamamoto, S.; Nakamura, Y. Macromolecules 2013, 46, 8664−8670. (19) Hatanaka, Y.; Nakamura, Y. Polymer 2013, 54, 1538−1542. (20) Fredrickson, G. H. Macromolecules 1993, 26, 2825−2831. (21) Zhulina, E. B.; Vilgis, T. A. Macromolecules 1995, 28, 1008− 1015. (22) Subbotin, A.; Saariaho, M.; Ikkala, O.; ten Brinke, G. Macromolecules 2000, 33, 3447−3452. (23) Kikuchi, M.; Kawaguchi, S.; Nagai, K. Des. Monomers Polym. 2004, 7, 603−617. (24) Kikuchi, M.; Mihara, T.; Jinbo, Y.; Izumi, Y.; Nagai, K.; Kawaguchi, S. Polym. J. 2007, 39, 330−341. (25) Kikuchi, M.; Lien, L. T. N.; Narumi, A.; Jinbo, Y.; Izumi, Y.; Nagai, K.; Kawaguchi, S. Macromolecules 2008, 41, 6564−6572. (26) Kawaguchi, S.; Mihara, T.; Kikuchi, M.; Lien, L. T. N.; Nagai, K. Macromolecules 2007, 40, 950−958. (27) Lien, L. T. N.; Kikuchi, M.; Narumi, A.; Nagai, K.; Kawaguchi, S. Polym. J. 2008, 40, 1113−1120. (28) Saito, Y.; Lien, L. T. N.; Jinbo, Y.; Kumaki, J.; Narumi, A.; Kawaguchi, S. Polym. J. 2013, 45, 193−201. (29) Subbotin, A.; Saariaho, M.; Stepanyan, R.; Ikkala, O.; ten Brinke, G. Macromolecules 2000, 33, 6168−6173. (30) Konishi, T.; Yoshizaki, T.; Saito, T.; Einaga, Y.; Yamakawa, H. Macromolecules 1990, 23, 290−297. (31) Patten, T. E.; Novak, B. M. J. Am. Chem. Soc. 1996, 118, 1906− 1916. (32) Pike, E. R.; Pomeroy, W. R. M.; Vaughan, J. M. J. Chem. Phys. 1975, 62, 3188−3192. (33) Berry, G. C. J. Chem. Phys. 1966, 44, 4550−4564. (34) Ishikawa, T.; Kikuchi, M.; Kobayashi, M.; Ohta, N.; Takahara, A. Macromolecules 2013, 46, 4081−4088. (35) Stejskal, J.; Kratochvil, P. Macromolecules 1987, 20, 2624−2628. (36) Glatter, O.; Kratky, O. Small Angle X-ray Scattering; Academic Press: London, 1982. (37) Nakamura, Y.; Wan, Y.; Mays, J. W.; Iatrou, H.; Hadjichristidis, N. Macromolecules 2000, 33, 8323−8328 For corrections, see:. Nakamura, Y.; Wan, Y.; Mays, J. W.; Iatrou, H.; Hadjichristidis, N. Macromolecules 2001, 34, 2018. (38) Amitani, K.; Terao, K.; Nakamura, Y.; Norisuye, T. Polym. J. 2005, 37, 324−331. (39) Benoit, H.; Doty, P. J. Phys. Chem. 1953, 57, 958−963. (40) Domb, C.; Barrett, A. J. Polymer 1976, 17, 179−184. (41) Yamakawa, H.; Stockmayer, W. H. J. Chem. Phys. 1972, 57, 2843−2854. (42) Yamakawa, H.; Shimada, J. J. Chem. Phys. 1985, 83, 2607−2611. (43) Shimada, J.; Yamakawa, H. J. Chem. Phys. 1986, 85, 591−600.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01010. Copolymerization results and compositional curve in the radical copolymerization, 1H NMR spectrum of poly(styrene-graf t-VB-HIC-47), differential weight distribution, W(w1), characterization results for PS-g-HIC-Ns-lg samples in THF at 25 °C, and quasi-two-parameter theories (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*(S.K.) Telephone: +81-238-26-3182. Fax: +81-238-26-3182. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank Prof. J. Kumaki at Yamagata University for the SPM measurements. The AGC Seimi Chemical Co., Ltd., Japan, is acknowledged for the kind supplies of VBC. Support in part by Grants-in-Aid from the Ministry of Education, Science, Sports and Culture of Japan (16550105 and 19550117), by The Foundation for Japanese Chemical Research, and by the Saneyoshi Scholarship Foundation are gratefully acknowledged.

■

REFERENCES

(1) Ito, K. Prog. Polym. Sci. 1998, 23, 581−620. (2) Ito, K.; Kawaguchi, S. Adv. Polym. Sci. 1999, 142, 129−178. (3) Tsarevsky, N. V.; Matyjaszewski, K. Chem. Rev. 2007, 107, 2270− 2299. (4) Ouchi, M.; Terashima, T.; Sawamoto, M. Chem. Rev. 2009, 109, 4963−5050. (5) Satoh, K.; Kamigaito, M. Chem. Rev. 2009, 109, 5120−5156. (6) Iha, R. K.; Wooley, K. L.; Nyström, A. M.; Burke, D. J.; Kade, M. J.; Hawker, C. J. Chem. Rev. 2009, 109, 5620−5686. (7) Wintermantel, M.; Schmidt, M.; Tsukahara, Y.; Kajiwara, K.; Kohjiya, S. Macromol. Rapid Commun. 1994, 15, 279−284. (8) Kawaguchi, S.; Akaike, K.; Zhang, Z. − M.; Matsumoto, H.; Ito, K. Polym. J. 1998, 30, 1004−1007. (9) Subbotin, A. V.; Semenov, A. N. Polym. Sci., Ser. A 2007, 49, 1328−1357. (10) Sheiko, S. S.; Sumerlin, B. S.; Matyjaszewski, K. Prog. Polym. Sci. 2008, 33, 759−785. (11) Potemkin, I. I.; Palyulin, V. V. Polym. Sci., Ser. A 2009, 51, 123− 149. (12) Miyake, G. M.; Weitekamp, R. A.; Piunova, V. A.; Grubbs, R. H. J. Am. Chem. Soc. 2012, 134, 14249−14254. I

DOI: 10.1021/acs.macromol.5b01010 Macromolecules XXXX, XXX, XXX−XXX