Radical Polymerization of Pendant (Meth)acrylates on a Rigid

Article pubs.acs.org/Macromolecules

Radical Polymerization of Pendant (Meth)acrylates on a Rigid Helical Polyisocyanate Backbone Producing Poly(meth)acrylates with “Ideally” Atactic Main Chain Configuration Atsushi Narumi, Hitomi Baba, Tetsuya Akabane, Yuta Saito, Satoshi Ohno, Daichi Togashi, Kazushi Enomoto, Moriya Kikuchi, Osamu Haba, and Seigou Kawaguchi* Department of Polymer Science and Engineering, Graduate School of Science and Engineering, Yamagata University, Jonan 4-3-16, Yonezawa 992-8510, Japan S Supporting Information *

ABSTRACT: This study includes a topic that poly(methyl methacrylate) (PMMA) with an “ideally” atactic main chain configuration, approximately triad tacticities of mm/mr/rr = 1/2/ 1, has been produced in solution by the “one-dimensionally regulated” radical polymerization using rigid helical polyisocyanate as a macromolecular template. Three types of isocyanate monomers with pendant (meth)acryloxy groups, such as 2(methacryloxy)ethyl isocyanate (MOI), 2-[2-(methacryloxy)ethoxy]ethyl isocyanate (MOI-EO), and 2-(acryloxy)ethyl isocyanate (AOI), were polymerized using a titanium alkoxide complex to produce the corresponding polyisocyanates, MOI-n, MOI-EO-n, and AOI-n, respectively, in which n represents the number-averaged degree of polymerization. From the SAXS measurements of MOI-26 in hexafluoro-2-propanol and MOI-EO38 in tetrahydrofuran at 25 °C, the polyisocyanates were found to be considered as rigid rod molecules. The radical polymerization of the pendant (meth)acryloxy groups was performed, and the polyisocyanate template was removed by alkali hydrolysis followed by methyl esterification, eventually producing the poly[methyl (meth)acrylate]s. The diad, triad, and pentad tacicities of the products were determined by the 1H and 13C NMR spectra, and these probabilities followed those calculated based on the Bernoulli statistics. The influences of the polymerization conditions, such as polymerization temperature and monomer concentration, on the tacticity of the final polymers were clarified. The effect of the flexible spacer between the polymerizable group and polyisocynate template was also evaluated. The radical polymerizations in which the polymerizable groups are molecularly regulated on a rod-like helical polyisocyanate backbone produced vinyl polymers with inimitable tacticities even in the solution state.

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INTRODUCTION Stereochemical control of the vinylic polymer main chain has been a continuously important issue in synthetic polymer chemistry because its physical properties are remarkably affected by the main chain microstructures. A typical example is the differences in the glass transition temperatures (Tg) for poly(methyl methacrylate) (PMAA) in which the Tg at the infinite molecular weight has been reported to be 49.6 ± 1.3 and 123.3 ± 1.7 °C for the uniform isotactic PMMA (it-PMMA) and syndiotactic PMMA (st-PMMA), respectively.1 The anionic polymerization has been earlier developed to prepare such highly stereoregular PMMA’s; as an example, the polymerizations of methyl methacrylate (MMA) using t-C4H9MgBr and t-C4H9Li/ AlEt3 in toluene at −78 °C were reported to produce st-PMMA and it-PMMA, respectively.2 Much effort has later been devoted to the stereospecific radical polymerization with the increasing attention to its versatility using a radical process.3 The stereospecific radical polymerization has been achieved through the design of the monomer structure. As an example, chiral oxazoline acrylamides were polymerized by 2,2′-azobis(isobutyronitrile) (AIBN) at 75 °C, followed by the removal of © 2015 American Chemical Society

the auxiliary to produce poly(acrylic acid) (PAA) and poly(methyl acrylate) (PMA) with a high degree of isotacticity.4 Furthermore, the radical polymerization of the methacrylate monomer with a bulky substituent, such as (1-phenyldibenzosuberyl)methacrylate, produced an almost perfect isotactic polymer.5 The use of additives and specific solvents was also reported to be effective to enhance the stereoregularity. The addition of Sc(OTf)3 to the MMA polymerization system produced PMMA with the meso-triad content of 22%.6 A fluoroalcohol has been used as the solvent to prepare syndiotactic-enriched polymers through the low-temperature radical polymerization7,8 and/or atom transfer radical polymerization (ATRP).9 Another interesting approach for the radical polymerizations has been demonstrated in the specific media in which the molecular mobility of the monomer (or polymerizable group) is regulated. For such a system, the rotation and diffusion of the Received: April 14, 2015 Revised: May 11, 2015 Published: May 22, 2015 3395

DOI: 10.1021/acs.macromol.5b00771 Macromolecules 2015, 48, 3395−3405

Article

Macromolecules

Scheme 1. Syntheses of PMMAMOI, PMMAMOI‑EO, and PMAAOI through (i) Living Coordination Polymerizations of MOI, MOIEO, and AOI with Titanium Alkoxide Complex, (ii) Radical Polymerizations of MOI-n, MOI-EO-n, and AOI-n, (iii) Hydrolyses of P(MOI-n), P(MOI-EO-n), and P(AOI-n), and (iv) Methyl Esterifications of PMAAMOI, PMAAMOI‑EO, and PMAAOI, Respectively

In previous studies, we have reported the preparation and characterization of styryl-ended or methacryloxy-ended poly(nhexyl isocyanate) (PHIC) macromonomers as well as their radical copolymerizabilities and conformational properties of the resulting rod brushes.23−25 Polyisocyanate is a polymer which is categorized as a semiflexible, stiff polymer. Especially, PHIC is the best characterized polymer which assumes a dynamic 83 helical conformation in the solution and solid states.26 We have become interested in the radical polymerization behavior of pendant (meth)acrylate groups regulated one-dimensionally on a rod-like helical polyisocyanate backbone. Accumulation of the double bonds on the main chain may increase the local concentration to enhance their polymerizabilities. A more interesting feature to be noted in this system is that the stereoselectivity of the propagating radical to the neighboring double bond would be greatly influenced by the perturbation due to the rigid helical conformation. In this study, we demonstrated the radical polymerizations of (meth)acryloxy groups in the solution states, which were covalently bonded on the polyisocyanate template. Scheme 1 shows the synthetic procedure. Three types of isocyanate monomers with pendant (meth)acryloxy groups, such as 2-(methacryloxy)ethyl isocyanate (MOI), 2[2-(methacryloxy)ethoxy]ethyl isocyanate (MOIEO), and 2-(acryloxy)ethyl isocyanate (AOI), were polymerized using a titanium alkoxide complex to produce the polyisocyanates, poly(MOI), poly(MOI-EO), and poly(AOI), called MOIn, MOI-EO-n, and AOI-n, respectively, in which n represents the number-averaged degree of polymerization. A flexible oxyethylene unit between the methacryloxy group and the backbone would affect the stereoregularity. Although the polymerization of MOI with the titanium alkoxide complex has already been reported by Novak et al. to afford MOI-n with a bimodal molecular weight distribution, no radical polymerization of the pendant methacryloxy groups has been reported.27 The radical polymerizations of MOI-n, MOI-EO-n, and AOI-n were demonstrated at various monomer concentrations and polymerization temperatures, followed by removal of the polyisocyanate template and methyl esterification to afford the respective products called PMMAMOI, PMMAMOI‑EO, and PMAAOI. We

monomer molecules and also the propagating radical species are restricted, which induces the stereospecificity in the radical polymerization process. To cite some representative studies, the topochemical polymerization of diethyl (Z,Z)-hexa-2,4-dienedioate in the crystalline state produced the meso-diisotactic stereospecific polymer.10,11 The radical polymerization of vinyl monomers in porous coordination polymers was shown to produce vinyl polymers with high contents of the meso-diad.12 Highly isotactic poly(acrylonitrile) with the triad tacticities of mm/mr/rr = 87/10/3 was also prepared by the inclusion polymerization using the urea crystal.13 Chitosan was used as a template for the radical polymerization of methacrylic acid (MAA) to produce meso-enriched poly(methacrylic acid) (PMAA) with triad tacticities of mm/mr/rr = 29/19/52.14 The extremely highly stereoregular PMAA was prepared by the radical polymerization of MAA within a molecular-scale stereoregular cavity prepared from the stereocomplex film composed of it-PMMA and st-PMAA.15,16 A methodology utilizing template molecules has also been developed, and this would be categorized as another regulation method in the sense that the polymerization is conducted in the solution states. The cyclopolymerization of the difunctional monomer with a template molecule has been demonstrated, eventually producing PMMA with the triad tacticities of mm/mr/ rr = 12/49/39, which was different from that of mm/mr/rr = 4/ 34/62 obtained by the conventional radical polymerization.17 The PMMA with the higher meso-triad tacticity of 60−65% was prepared by the radical polymerization of a difunctional monomer such as the calcium salt of MAA in the DMF/toluene mixed solvent.18 The template polymerizations using the linear polymer and/or cyclic oligomer have also been demonstrated, which include the radical polymerization of (meth)acrylate functional groups covalently connected to the polymeric or oligomeric backbones such as p-cresole−formaldehyde resin,19 poly(vinyl alcohol),20 poly(2-hydroxyethyl methacrylate),21 and cyclodextrin.22 However, no stereochemical control has been achieved for the newly constructed polymer main chain through the polymerization of the pendant polymerizable group. 3396


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reported by Novak et al.27 In a drybox, CpTiCl3 (0.76 g, 3.5 mmol) and dry benzene (2 mL) were slowly added to a three-necked 50 mL flask containing a magnetic stir bar. The flask was connected to a bubbler filled with triethylamine. The reaction mixture was stirred for 30 min to give a homogeneous solution. A solution of benzyl alcohol (0.37 g, 3.5 mmol) in dry benzene (2 mL) was slowly added to the mixture. The reaction mixture was stirred until the production of hydrochloride gas had stopped. The mixture was then transferred to a 50 mL roundbottom flask and freeze-dried to give a yellow powder (0.82 g, 81%). MOI-n. In a drybox, the titanium alkoxide complex (0.82 g, 2.82 mmol) and dry dichloromethane (2.62 g) were added to a 20 mL flask containing a magnetic stir bar. After the titanium alkoxide complex had completely dissolved, MOI (8.76 g, 56.5 mmol) was added to the mixture. The flask was sealed with septum cap, taken out of the drybox, and immersed in a cool bath at 5 °C for 24 h with stirring. After 3:7 methanol/chloroform (w/w) was added, the mixture was stirred at 5 °C for another 24 h to terminate the polymerization. The mixture was evaporated, and the residue was dropwise added to methanol. The formed precipitate was collected by filtration using a 10 μm membrane filter sheet to obtain the polymer, which was purified by reprecipitation using chloroform and methanol and dried in vacuo to give MOI-30 as a white solid (8.27 g, 91.0%). 1H NMR (400 MHz, CDCl3, ppm): δ 7.5− 7.3 (Ar), 6.4−6.0 (CH2), 5.7−5.5 (CH2), 5.3−5.1 (−CH2O−), 4.8−3.4 (⟩NCH2CH2O−), 2.0−1.8 (−CH3). MOI-EO-n. The same procedure as that for MOI-n was applied to the titanium alkoxide complex (0.52 g, 1.82 mmol), dry dichloromethane (1.69 g), and MOI-EO (8.35 g, 41.9 mmol), while the work-up procedure was performed as follows. After 4:6 ethanol/chloroform (w/ w) was added, the mixture was stirred at 5 °C for 24 h to terminate the polymerization. The mixture was evaporated, and the residue was redissolved in a small amount of ethanol and dropwise added to cold hexane with cooling. The supernatant was removed by decantation. The residue was evaporated, dissolved in a small amount of ethanol, and dropwise added to cold hexane with cooling. This precipitation/ decantation process was performed a total of three times. Finally, the residue was evaporated, dried in vacuo for 6 h, dissolved in benzene, and finally freeze-dried for 36 h to give MOI-EO-20 as a white solid (6.35 g, 76.1%). 1H NMR (400 MHz, CDCl3, ppm): δ 7.4−7.3 (Ar), 6.2−6.0 (CH 2 ), 5.6−5.4 (CH 2 ), 5.2−5.1 (−CH 2 O−), 4.5−3.3 (⟩NCH2CH2OCH2CH2−), 2.0−1.8 (−CH3). AOI-n. The same procedure as that for MOI-n was applied to the titanium alkoxide complex (0.36 g, 1.23 mmol), dry dichloromethane (1.14 g), and AOI (4.89 g, 34.7 mmol) to give AOI-20 as a white solid (3.49 g, 68.1%). 1H NMR (400 MHz, CDCl3, ppm): δ 7.4−7.3 (Ar), 6.5−6.3 (CH2), 6.2−6.0 (−CH), 6.0−5.8 (CH2), 5.3−5.2 (−CH2O−), 4.5−3.5 (⟩NCH2CH2O−). Determination of n for MOI-n, MOI-EO-n, and AOI-n. As an example, the determination of n is described for MOI-n. Figure S1a shows the 1H NMR spectrum of the product in CDCl3, which was obtained for the polymerization of MOI with the titanium alkoxide complex as the initiator in dichloromethane using the [M]0/[I]0 of 20.0 at 5 °C for 24 h (entry 1-2). The signals due to the protons for the MOI repeating units, such as the vinyl protons (e at 6.4−6.0 ppm and 5.7−5.5 ppm), methylene protons (c and d at 4.8−3.4 ppm), and methyl protons (f at 2.0−1.8 ppm), were observed. The signals of the protons due to the initiator-derived unit, such as the aromatic protons (a at 7.5−7.3 ppm), also appeared. The n value of 30 was determined by comparing the integration areas of the resonances due to the vinyl protons (e) and those due to the aromatic protons in the chain end (a) using the equation

report the tacticity of the resulting 10 poly(meth)acrylate samples and discuss how the chemical structure of the polyisocyanate template with a rigid helical structure influences the stereoregulation in the homogeneous radical polymerization.

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EXPERIMENTAL SECTION

Measurements. The 1H and 13C NMR spectra were recorded using a JEOL JNM-ECX400. The infrared (IR) spectra were recorded using a HORIBA FT-720 spectrometer. The differential scanning calorimetry (DSC) measurements were carried out using the EXTRA6000 DSC6200 thermal analyzer (Seiko Instruments Inc.) at a heating rate of 10 °C/min. The size exclusion chromatography (SEC) using tetrahydrofuran (THF) as the eluent was performed with the system consisting of a JASCO CO-2065 Plus oven, a JASCO PU-2080 Plus pump, a JASCO DG-2080-53 degasser, a Shodex KF-802 column (size: 8.0 mm × 300 mm; average beads size: 6 μm, exclusion limit: 5 kg mol−1), three Shodex KF-806L columns (size: 8.0 mm × 300 mm; average beads size: 10 μm; exclusion limit: 2 × 104 kg mol−1), and TOSOH RI-8020 detector at the flow rate of 1.0 mL min−1 at 40 °C. The SEC using chloroform as the eluent was performed with the system consisting of a JASCO CO-2065 Plus oven, a JASCO PU-2080 Plus pump, a JASCO DG-2080-53 degasser, a Shodex KF-804L column (size: 8.0 mm × 300 mm; average beads size: 7 μm; exclusion limit: 4 × 102 kg mol−1), three Shodex KF-806L columns (size: 7.8 mm × 300 mm; average beads size: 10 μm; exclusion limit: 2 × 104 kg mol−1), and a Shodex RI-71S detector at the flow rate of 1.0 mL min−1 at 40 °C. The number-averaged molecular weight (Mn) and the polydispersity index (Mw/Mn) were determined by the linear PMMA standards (Polymer Laboratories) with Mn(Mw/Mn)s = 1250 kg mol−1 (1.16), 300 kg mol−1 (1.12), 139 kg mol−1 (1.10), 60.2 kg mol−1 (1.06), 30.7 kg mol−1 (1.06), 10.3 kg mol−1 (1.07), 4.90 kg mol−1 (1.09), 2.68 kg mol−1 (1.09), 1.30 kg mol−1 (1.11), 0.625 kg mol−1 (1.30), and ethylbenzene. The small-angle X-ray scattering (SAXS) measurements of MOI-26 in hexafluoro-2-propanol (HFIP) and MOI-EO-38 in THF were carried out in at 25 °C using BL-10C with a synchrotron orbital radiation as the X-ray source set up in the Photon Factory of the High Energy Accelerator Organization at Tsukuba, Ibaraki, Japan. The wavelength of the X-rays was 1.488 Å. The scattered intensity was recorded by a position-sensitive proportional counter with 512 channels over a scattering vector range from 0.02 to 0.30 Å−1. The scattered vector was calibrated using the sixth peak of dry collagen. The details of the experimental procedure and data processing have been reported in a previous paper.25 Materials. Dry benzene and dry THF were prepared according to the literature.24 Dry dichloromethane was prepared by refluxing dichloromethane (Kanto Chemical Co., Japan, >99.5%) over calcium hydride (CaH2), followed by distillation. Dry chloroform was prepared by refluxing chloroform (Wako Pure Chemical Industries, Japan, >99.5%) over CaH2, followed by distillation. Benzyl alcohol (>99.0%), methyl methacrylate (MMA, >98.0%), and N,N-dimethylaniline (DMA, >99.0%) were purchased from Wako Pure Chemical Industries, Japan, and distilled over CaH2 under reduced pressure before use. 2(Methacryloxy)ethyl isocyanate (MOI, >97%), 2[2-(methacryloxy)ethoxy]ethyl isocyanate (MOI-EO, >95%), and 2-(acryloxy)ethyl isocyanate (AOI, >97%) were kindly supplied from Showa Denko K. K., Japan, and distilled over CaH2 under reduced pressure in the presence of 1,1-diphenyl-2-picrylhydrazyl (DPPH) just before use. 2,2'Azobis(isobutyronitrile), AIBN (Kanto Chemical Co., Japan, >97%), was purified by recrystallization three times from methanol. Trichlorocyclopentadienyltitanium (CpTiCl3) (Kanto Chemical Co., Japan, >99.0%), 2-methoxyethanol (Kanto Chemical Co., Japan, >99.0%), Nmethyl-N-nitroso-p-toluenesulfonamide (Wako Pure Chemical Industries, Japan, >95%), and tributylborane (1.0 M in THF solution, Aldrich Chemical Co., Inc.) were used as received. HFIP was purchased from the Central Glass Co., Japan, and distilled over CaH2 before use. All other chemicals were obtained from commercial sources and used as received unless otherwise stated. Benzyloxydichloro(cyclopentadienyl)titanium(IV). The titanium alkoxide complex was synthesized according to the method

n=

5e 2a

(1)

Radical Homopolymerization at 60 °C. MOI-30 (599 mg, 0.126 mmol), AIBN (0.006 g, 0.04 mmol), and chloroform (1.26 mL) were added to a 5 mL round-bottom flask to give a homogeneous solution. The mixture was subjected to three freeze−thaw cycles, sealed under nitrogen, and immersed in an oil bath at 60 °C with stirring. After 48 h, the formed gel-like polymeric product was transferred to a beaker. To the product, chloroform (100 mL) was added, and then the mixture was 3397


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Macromolecules

Table 1. Recipe and Result for Living Coordination Polymerizations of MOI, MOI-EO, and AOI and Characterizations of Productsa entry

monomer

[M]0/[I]0

product

yield (%)

Mn,NMRd (kg mol−1)

Mn,calcde (kg mol−1)

Mn,SECf (kg mol−1)

Mw/Mnf

1-1 1-2 1-3 1-4b 1-5c 1-6

MOI MOI MOI MOI-EO MOI-EO AOI

13.6 20.0 63.4 23.2 28.2 20.7

MOI-26 MOI-30 MOI-61 MOI-EO-20 MOI-EO-38 AOI-20

78.5 91.3 54.0 76.1 48.0 68.0

4.17 4.81 9.60 4.09 7.68 4.81

1.76 2.95 5.42 3.53

4.32 3.23g 8.29g 2.10h 2.68h 3.15g

1.13g 1.52g 2.45g 1.05h 1.06h 1.63g

2.75

Solvent, dichloromethane; temperature, 5 °C; polymerization time, 24 h. Precipitation solvent, hexane. cPrecipitation solvent, methanol. dMn,NMR = MBz + (Mmonomer × n), where MBz is the molecular weight of the end group and Mmonomer is the molecular weight of the corresponding monomers, such as MOI, MOI-EO, or AOI. eMn,calcd = MBz + (Mmonomer × [M]0/[I]0 × yield), where the conversion was assumed to equal the yield. fDetermined by SEC based on the calibrations using PMMA standards. gEluent, chloroform. hEluent, THF. a

b

μm membrane filter sheet. The filtrate was evaporated and purified by reprecipitation using benzene/methanol with a 1.0 μm membrane filter sheet. The resulting solid was dried in vacuo for 24 h to give a yellowishwhite solid (0.038 g, 32%).

stirred overnight at room temperature. The solid was collected by filtration using a 10 μm membrane filter sheet and dried in vacuo for 24 h to give P(MOI-30)/60 °C in quantitative yield. Radical Homopolymerization at −5 °C. MOI-30 (600 mg, 0.126 mmol), benzoyl peroxide (BPO, 0.037 g, 0.15 mmol), and chloroform (1.26 mL) were added to 5 mL round-bottom flask. The mixture was subjected to three freeze−thaw cycles, sealed under nitrogen, and immersed in a cool bath at −5 °C with stirring. N,N-Dimethylaniline (0.02 mL, 0.15 mmol) was added to the mixture to initiate the polymerization. After 48 h, the formed gel-like polymeric product was transferred to a beaker. To the product, chloroform (100 mL) was added, and then the mixture was stirred at room temperature overnight. The solid was collected by filtration using a 10 μm membrane filter sheet and dried in vacuo for 24 h to give P(MOI-30)/−5 °C in quantitative yield. Radical Homopolymerization at −50 °C. MOI-30 (600 mg, 0.126 mmol) was added to chloroform (1.26 mL) in a 5 mL roundbottom flask. The mixture was sealed with a silicon septum cap and immersed in a bath cooled at −50 °C with stirring. Nitrogen was bubbled through the mixture with stirring for 20 min to remove any oxygen. Tributylborane (0.01 mL, 1.0 M in THF solution) was added to the mixture using a disposable syringe with stirring. Oxygen was then injected into the mixture using a gastight syringe to initiate the polymerization. The mixture was stirred at −50 °C for 72 h to give a gellike polymeric product. The product was transformed to a beaker and chloroform (100 mL) was added; then the mixture was stirred at room temperature overnight. The solid was collected by filtration using a 10 μm membrane filter sheet and dried in vacuo for 24 h to give P(MOI30)/−50 °C in quantitative yield. Hydrolysis. P(MOI-30)/−50 °C (0.55 g) and saturated NaOH(aq) (10 mL) were added to a Teflon internal cylinder-type sealed vessel (Taiatsu Techno Corporation) containing a magnetic stir bar. The vessel was immersed in an oil bath at 110 °C with intense stirring for 60 h. During the reaction, ionic exchanged water was added to the mixture a total of three times (final concentration of NaOH = 40 wt %). The residue was transferred to a 100 mL beaker, and its pH was reduced to 6.0 by adding hydrochloric acid and then dialyzed against water using Spectra/Por dialysis tubing (MWCO 1000). A 20-fold excess amount of amphoteric ion-exchange resin (BIO-RAD AG 501-X8 (D) Resin, 20− 50 mesh) to the sodium polymethacrylate was added to the resulting aqueous solution and stirred for 1 h to obtain the poly(methacrylic acid) (PMAA) salt-free solution with a pH between 4.0 and 4.5. The resin was removed by filtration using a G4 glass filter, and the filtrate was freezedried and then dried in vacuo at 40 °C for 24 h to give PMAAMOI‑30/−50 °C as a white solid (0.17 g, 58%). Methyl Esterification. The hydrolyzed product (0.15 g, 0.52 mmol) and benzene (30 mL) were added to a 50 mL round-bottom flask containing a magnetic stir bar. A solution of diazomethane28 in ether, which was prepared by the reaction of N-methyl-N-nitroso-ptoluenesulfonamide with 2-methoxyethanol in ether and water containing potassium hydroxide, was added to the suspension, and the reaction mixture was vigorously stirred overnight. Acetic acid was added to quench the unreacted diazomethane, the ether was removed by evaporation, and the residue, benzene solution was filtered using a 10

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RESULTS AND DISCUSSION Isocyanate Polymerizations. Three types of isocyanate monomers with pendant (meth)acryloxy groups, such as 2(methacryloxy)ethyl isocyanate (MOI), 2-[2-(methacryloxy)ethoxy]ethyl isocyanate (MOI-EO), and 2-(acryloxy)ethyl isocyanate (AOI), were polymerized using a titanium alkoxide complex as an initiator in dichloromethane (Scheme 1). Table 1 lists the results of the polymerizations. The polymerizations were performed at 5 °C for 24 h using the monomer/initiator ratio in the feed ([M]0/[I]0) ranging from 13.6 to 63.4, producing products with yields ranging from 48.0 to 91.3%. The products were characterized by 1H NMR spectroscopy and SEC analysis (Figures S1 and S2). The results indicated that the respective products were assigned to the target poly(MOI), poly(MOIEO), and poly(AOI), called MOI-n, MOI-EO-n, and AOI-n, respectively, with the number-averaged molecular weight (Mn,SEC) ranging from 2.10 to 8.29 kg mol−1 and molecular weight distribution (Mw/Mn) ranging from 1.05 to 2.45. The n represents the number-averaged degree of polymerization determined by the 1 H NMR spectra (see Supporting Information). As summarized in Table 1, we prepared six products, such as MOI-26, MOI-30, MOI-61, MOI-EO-20, MOI-EO-38, and AOI-20, for this isocyanate polymerization stage. Table 1 also lists the number-averaged molecular weight determined by 1H NMR (Mn,NMR) and that calculated from the [M]0/[I]0 and polymer yield (Mn,calcd). The Mn,NMR values were from 4.17 to 9.60 kg mol−1, which were greater than the respective Mn,calcd values ranging from 1.76 to 5.42 kg mol−1. This implied that the efficiency for the initiation was not quantitative. We now describe the difference between MOI-n, MOI-EO-n, and AOI-n. A featured result was observed in their SEC profiles (Figure S2) which should be related to their solubility (Table S1). The SEC traces exhibited a main peak along with a shoulder in the lower molecular weight regions for MOI-30 and bimodal peaks for MOI-61. This would be attributed to the low solubility of both samples in the MOI monomer. The SEC trace showed multimodal peaks for AOI-20. This would also be due to the fact that AOI-20 was insoluble in both the AOI monomer and dichloromethane. The low solubility might eventually lead to the occurrences of the side reactions such as terminations and chain transfer reactions. Such phenomena were previously reported by Novak and co-worker.27 The polymerizations of isocyanate derivatives with the titanium alkoxide complex proceeded in a 3398


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Macromolecules living fashion; however, the occurrence of side reactions due to the low solubility of the products was suggested by the SEC measurements. In remarkable contrast, the SEC traces for the MOI-EO-n series displayed monomodal symmetrical peaks with low Mw/Mn values. MOI-EO-n was soluble in both the MOI-EO monomer and dichloromethane so that the system was homogeneous throughout the polymerizations. The coordination polymerizations of MOI-EO initiated by the titanium alkoxide complex proceeded in a living manner, producing the product with well-defined molecular weights. The low isolated yield of MOI-EO-38 after the reprecipitation using methanol as the precipitating solvent was 48.0% (entry 1-5). This was due to the partial solubilization of MOI-EO-38 in methanol. We changed the precipitating solvent from methanol to hexane for MOI-EO-20, in which the yield increased to 76.1% (entry 1-4). The featured result for the introduction of an oxyethylene unit was also observed in their 1H NMR spectra (Figure S1). As a representative example, the signals were broad for MOI-30, suggesting that the molecular mobility of the protons was significantly restricted due to their being covalently bonded to the rigid helical backbone of the polyisocyanate. On the other hand, the signals were relatively sharp for MOI-EO-38, except for those due to oxyethylene protons in the vicinity of the polyisocyanate main chain. Thus, the mobility of the methacryloxy groups should be strongly restricted by the mainchain for the MOI-n and AOI-n series as compared to those for the MOI-EO-n series. Conformation of MOI-26 in HFIP and MOI-EO-38 in THF. Molecular characterizations for MOI-n and MOI-EO-n were conducted by SAXS measurements. As seen in Table S1, MOI-n dissolves only in halogen solvents, such as chloroform, dichloromethane, and HFIP, at room temperature. Because the former two solvents have a very low X-ray transmittance, HFIP was used as the solvent in the SAXS measurements of MOI-n. On the other hand, THF was used for MOI-EO-n because of its high solubility in many organic solvents. A typical example of Berry’s plots of MOI-26 (Mw = Mn,NMR × Mw/Mn = 4710 g mol−1) with the polymer concentration Cp = 19.7 mg cm−3 in HFIP at 25 °C is shown in Figure 1 in which the inverse square root of the excess

q=

4π sin θ λ

(3)

where θ is the scattering angle and λ is the wavelength. Figure 2 shows the Cp dependence of ⟨S2⟩z1/2 to give the ⟨S2⟩z,01/2 value of

Figure 2. Plots of ⟨S2⟩z1/2 versus Cp for MOI-26 (○) in HFIP and MOIEO-38 (□) in THF at 25 °C.

1.80 nm for MOI-26 and 2.44 nm for MOI-EO-38 (Mw = 8140 g mol−1) by extrapolating to infinite dilution. These values seem slightly higher than that of PHIC (1.33 nm for n = 26 and 1.91 nm for n = 38) with the corresponding n of HIC in hexane, THF, and dichloromethane at 25 °C. Figure 3 shows the Kratky plots of

Figure 3. Kratky plots for HFIP solutions of MOI-26 and THF solutions of MOI-EO-38 with different polymer concentrations at 25 °C. The upper solid line represents the theoretical curve for the straight cylinder with L = 5.92 nm and d = 1.1 nm. The lower solid line represents the theoretical curve for the straight cylinder with L = 7.98 nm and d = 1.0 nm.

MOI-26 in HFIP and MOI-EO-38 in THF at 25 °C with different Cps. The both profiles clearly demonstrate that the polyisocyanates behave as a stiff rod conformation, as in the case of PHIC.23 The scattering profiles were compared to the theoretical scattering function calculated for the straight cylinder with the contour length, L and the diameter, d which was expressed by eq 429

Figure 1. Berry plots of ΔI(q)−1/2 as a function of q2 for HFIP solution of MOI-26 with Cp = 19.7 mg cm−3 at 25 °C.

P(q) = 2

scattered intensities ΔI(q)−1/2 was plotted versus the square of the scattering vector q. The value of z-averaged root-meansquare radius of gyration ⟨S2⟩z1/2 was determined to be 1.77 nm, from the initial slope and the intercept, according to eqs 2 and 3 1 ΔI(q)−1/2 = ΔI(0)−1/2 + ΔI(0)−1/2 ⟨S2⟩z q2 + ... (2) 6

∫0

π

2 ⎧ ⎡ qd ⎤⎫ 2 ⎧ ⎡⎛ qL ⎞ ⎤⎫ ⎪ J1⎣⎢ 2 sin θI ⎦⎥ ⎪ ⎬ sin θI dθI ⎨j0 ⎢⎜ ⎟cos θI ⎥⎬ ⎨ ⎦⎭ ⎪ qd sin θ ⎪ ⎩ ⎣⎝ 2 ⎠ I 2 ⎩ ⎭

( ) ( )

(4)

where j0(x) and J1(x) denote the zeroth-order spherical Bessel function and the first-order Bessel function, respectively. The solid lines calculated from eq 4 with L = 5.92 nm and d = 1.1 nm for MOI-26 in HFIP and with L = 7.98 nm and d = 1.0 nm for 3399


Article

Macromolecules

Table 2. Recipes and Results for Radical Polymerizationsa and Subsequent Removal of Polyisocyanate Template To Produce Hydrolyzed Products entry

starting material

[vinyl group]0 (mol L−1)

initiatorc

temp (°C)

polymerization product

form

yield (%)

hydrolyzed product

yield (%)

2-1 2-2 2-3b 2-4 2-5 2-6 2-7 2-8 2-9c 2-10

MOI-30 MOI-30 MOI-30 MOI-61 MOI-61 MOI-61 MOI-EO-38 MOI-EO-38 MOI-EO-38 AOI-20

1.0 1.0 1.0 1.0 0.10 0.010 1.0 1.0 1.0 1.0

AIBN BPO/DMA B(n-Bu)3/O2 AIBN AIBN AIBN AIBN BPO/DMA B(n-Bu)3/O2 AIBN

60 −5 −50 60 60 60 60 −5 −50 60

P(MOI-30)/60 °C P(MOI-30)/−5 °C P(MOI-30)/−50 °C P(MOI-61)/1.0 M P(MOI-61)/0.1 M P(MOI-61)/0.01 M P(MOI-EO-38)/60 °C P(MOI-EO-38)/−5 °C P(MOI-EO-38)/−50 °C P(AOI-20)/60 °C

gel gel gel gel gel solution gel gel gel gel

quant 92.6 83.7 quant quant quant quant quant 74.7 quant

PMAAMOI‑30/60 °C PMAAMOI‑30/−5 °C PMAAMOI‑30/−50 °C PMAAMOI‑61/1.0 M PMAAMOI‑61/0.1 M PMAAMOI‑61/0.01 M PMAAMOI‑EO‑38/60 °C PMAAMOI‑EO‑38/−5 °C PMAAMOI‑EO‑38/−50 °C PAAAOI‑20

58 43 15 40 67 47 65 57 68 52

Solvent, chloroform; polymerization time, 48 h. bPolymerization time, 72 h. c[AIBN]0, 0.01 mol L−1; [BPO] 0 = [DMA] 0, 0.04 mol L−1; [B(nBu)3]0, 0.02 mol L−1.

a

assigned to the target polymer consisting of the MOI-30 repeating unit, called P(MOI-30)/60 °C. The [vinyl group]0 of 1.0 mol L−1 would be only the “apparent” average value for this system. The local concentration of vinyl groups, [vinyl group]L was calculated by the equation

MOI-EO-38 in THF quantitatively describe the experimental scattering profiles, verifying that the polyisocyanates are considered as cylindrical rod molecules in solution. The L and d parameters used are discussed as follows. Because of the experimental limitation, it was difficult to determine reliably the cross-sectional radius of gyration. Thus, d value was determined to be 1.1 nm for MOI-26 and 1.0 nm for MOI-EO-38 as the fitting parameter, which significantly influenced the values in the q region higher than 1.0 nm−1. These values seem somewhat higher than that (0.79 nm) of PHIC previously determined in THF at 25 °C.23 The contour length, L is defined by L=

Mw ML

[vinyl group]L =

4n πd 2LNA

(6)

where NA is Avogadro’s number. The [vinyl group]L value was calculated to be 7.7 mol L−1 for MOI-n and 10.0 mol L−1 for MOI-EO-n. These values are very close to the molar concentration (9.4 mol L−1 at 20 °C) of the bulk MMA. It should be noted that the local concentration is independent of the [vinyl group]0 and n. This feature was supported by the experimental results for the polymerizations of MOI-61 at 60 °C at the lower [vinyl group]0 values of 0.10 mol L−1 (entry 2-5) and 0.010 mol L−1 (entry 2-6). The respective polymerizations quantitatively afforded the products labeled P(MOI-61)/0.1 M and P(MOI-61)/0.01 M. Only the P(MOI-61)/0.01 M system was obtained as a solution state even after the polymerization as shown in Table 2. It should be worth noting that the conventional homogeneous radical polymerization of MMA at such low monomer concentrations yields only the oligomeric PMMA with a very low conversion. Hence, the polymerizations of MOI-n would proceed via an intramolecular template polymerization mechanism, eventually producing P(MOI-n)/ 60 °C with a ladder-like structure. The form of P(MOI-30)/60 °C was, however, an insoluble gel, suggesting the occurrence of a side reaction such as an intermolecular cross-linking reaction. The results for other representative polymerizations are described. The low-temperature radical polymerization at −5 °C was performed using the redox initiating system consisting of benzoyl peroxide (BPO) and N,N-dimethylaniline (DMA). The polymerization produced P(MOI-30)/−5 °C in the yield of 92.6% (entry 2-2). Furthermore, the polymerization at −50 °C was initiated by the B(n-Bu)3/O2 system, producing P(MOI30)/−50 °C in 83.7% yield (entry 2-3). Both products were gels. The radical polymerizations of MOI-EO-38 were performed at 60, −5, and −50 °C to produce P(MOI-EO-38)/60 °C, P(MOIEO-38)/−5 °C, and P(MOI-EO-38)/−50 °C as gel-like forms (entries 2-7, 2-8, and 2-9, respectively). AOI-20 was polymerized with AIBN using the [vinyl group]0 of 1.0 mol L−1 at 60 °C, producing P(AOI-20)/60 °C (entry 2-10). The occurrences of the polymerizations for these systems were also confirmed by the disappearance of the peaks due to the CC bonds (Figures S3).

(5)

where ML is the molecular weight per unit contour length and related to the local conformation, that is, the pitch of the helical structure. The ML value for the PHIC chain with the 83 helical conformation was reported to be 730 ± 50 g mol−1nm−1.26 Assuming the same helical structure of MOI-26 and MOI-EO-38 as the PHIC chain, the ML value was calculated to be 890 ± 60 g mol−1 nm−1 for MOI-26 and 1140 ± 80 g mol−1 nm−1 for MOIEO-38. The observed values of 800 ± 50 g mol−1 nm−1 for MOI26 and 1020 ± 50 g mol−1 nm−1 for MOI-EO-38 were obtained by eq 5, which are just slightly lower than the calculated one. This implies that the local conformation, i.e., the pitch of the helix of MOI-26 in HFIP and MOI-EO-38 in THF, is imperceptibly different from that of PHIC in hexane and THF. The bulkiness of the 2-(methacryloxy)ethyl and 2-[2-(methacryloxy)ethoxy]ethyl groups has little effects on the helical structure of the originally rigid nylon-1 main structure. A conclusion, which was obtained from the solution properties, together with 1H NMR spectra is that the polyisocyanates are considered to be rod molecules composed of the pendant (meth)acryloxy groups regulated onedimensionally around a rigid helical backbone. Radical Polymerizations of (Meth)acryloxy Group. Table 2 lists the recipes and the results of the radical polymerization of MOI-n, MOI-EO-n, and AOI-n. The polymerizations of MOI-30 were performed using the molar concentration for the vinyl group ([vinyl group]0) of 1.0 mol L−1 (entries 2-1, 2-2, and 2-3). AIBN was used as the initiator for the polymerization at 60 °C, which produced a gel-like product in quantitative yield (entry 2-1). The occurrences of the polymerizations were confirmed by the IR spectra, in which the adsorptions due to the CC bonds at 1635 and 1070 cm−1 were significantly decreased (Figure S3); hence, the product was 3400


Article

Macromolecules Table 3. Results for Methyl Esterification and Characterizations of the Products diad tacticityb

a

a

−1

entry

methyl esterification product

yield (%)

Mn,SEC (kg mol )

Mw/Mn

3-1 3-2 3-3 3-4 3-5 3-6 3-7 3-8 3-9 3-10

PMMAMOI‑30/60 °C PMMAMOI‑30/−5 °C PMMAMOI‑30/−50 °C PMMAMOI‑61/1.0 M PMMAMOI‑61/0.1 M PMMAMOI‑61/0.01 M PMMAMOI‑EO‑38/60 °C PMMAMOI‑EO‑38/−5 °C PMMAMOI‑EO‑38/−50 °C PMAAOI‑20/60 °C

32 49 13 40 21 70 80 53 53 100

22.3 7.71 3.96 11.8 15.2 4.85 52.4 25.3 20.5 6.32

1.96 2.18 2.45 2.11 1.58 2.02 1.81 1.76 1.63 4.40

a

triad tacticityb

m

r

mm

mr

rr

0.44 0.42 0.44 0.44 0.43 0.47 0.28 0.28 0.34 0.52

0.56 0.58 0.56 0.56 0.57 0.53 0.72 0.72 0.66 0.48

0.21 0.21 0.22 0.21 0.20 0.23 0.08 0.08 0.13

0.47 0.43 0.44 0.46 0.46 0.49 0.39 0.39 0.42

0.32 0.36 0.34 0.33 0.34 0.28 0.53 0.53 0.45

Determined by SEC in THF based on the calibrations using PMMA standards. bDetermined by 1H NMR spectra.

Removal of Polyisocyanate Template and Methyl Esterification To Produce PMMA. A total of 10 samples obtained for the radical polymerization stages were treated with NaOH in order to remove the polyisocyanate template moieties. Figure S4a shows a typical 1H NMR spectrum of the products, which were obtained by the hydrolysis of P(MOI-30)/60 °C and subsequent purifications by dialysis, passage through mixed beds of ionic exchange resins, and freeze-drying. Although a very small signal that might be due to the cleaved polyisocyanate template appeared at 3.5 ppm (below 0.1%), the large main signals were assignable to the target poly(methacrylic acid), called PMAAMOI‑30/60 °C. Figure S4b shows the 1H NMR spectrum of the hydrolyzed product obtained from P(AOI-20/60 °C), which was also assigned to poly(acrylic acid), called PAAAOI‑20/60 °C. Table 2 summarizes the codes for a total of 10 samples of the hydrolyzed products and their yields from the corresponding polymerization products, which ranged from 15% to 68%. These values were calculated on the assumption that all of the vinyl groups were polymerized for the radical polymerization stage. However, the vinyl groups were not quantitatively consumed as judged from the IR spectra of the radical polymerization products; hence, the exact yields for the hydrolysis would be greater than the above values. It should be emphasized that the degrees of hydrolysis were >99.9% for all samples, which was supported by the 1H NMR spectra. The respective hydrolyzed products were reacted with diazomethane to afford products in yields ranging from 13% to 100% as listed in Table 3. Figures 4a and 4b show the typical 1H NMR spectra of the products, which exhibited the characteristic signals due to the methyl ester groups of poly(methyl methacrylate) (PMMA) and poly(methyl acrylate) (PMA) in the regions from 3.4 to 3.7 ppm. Hence, the products were assignable to the final 10 samples, called PMMAMOI−30/60 °C, PMMAMOI−30/−5 °C, PMMAMOI−30/−50 °C, PMMAMOI−61/1.0 M, PMMAMOI−61/0.1 M, PMMAMOI−61/0.01 M, PMMAMOI−EO−38/60 °C, PMMA M O I − E O − 3 8 / − 5 ° C , PMMA M O I − E O − 3 8 / − 5 0 ° C , and PMAAOI−20/60 °C. The degrees of methyl esterification were quite high values between 98.8% and 99.6% as calculated from the 1H NMR spectra. Table 3 lists the Mn,SEC and Mw/Mn values for the final samples, which were based on the calibrations using linear PMMA standards. The Mn values ranged from 3.96 to 52.4 kg mol−1. The Mw/Mn values were between 1.58 and 4.40. A notable difference in the Mn values was observed; i.e., the Mn values were 22.3 kg mol−1 for PMMAMOI‑30/60 °C (entry 3-1), 7.71 kg mol−1 for PMMAMOI‑30/−5 °C (entry 3-2), and 3.96 kg mol−1 for PMMAMOI‑30/−50 °C (entry 3-3). A typical SEC trace of PMMAMOI‑30/60 °C is shown in Figure S5. The Mn values

Figure 4. 1H NMR spectra of (a) PMMAMOI‑30/60 °C, (b) PMAAOI-20/60 °C, and (c) PMMAMMA/60 °C in CDCl3.

decreased with the decreasing temperature of the radical polymerization stage. Furthermore, the number-averaged degree of polymerizations of PMMAMOI‑30/60 °C and PMMAMOI‑30/−5 °C were greater than the n value, 30 for the starting polymer of MOI30. This would imply the occurrence of the intermolecular crosslinking reactions for the radical polymerization of MOI-30. These tendencies were also observed for the PMMAMOI‑EO series (entries 3-7, 3-8, and 3-9). The Mn values were 11.8 kg mol−1 for PMMA M O I ‑ 6 1 / 1 . 0 M (entry 3-4), 15.2 kg mol − 1 for PMMA MOI‑61/0.1 M (entry 3-5), and 4.85 kg mol −1 for PMMAMOI‑61/0.01 M (entry 3-6). The Mn of PMMAMOI‑61/0.01 M was a low value but close to the calculated value of 6.1 kg mol−1, suggesting that the intermolecular reactions were suppressed by using such dilute conditions. In order to prepare controlled samples, the radical polymerizations of MMA were also performed at 60, −5, and −50 °C, producing products, called PMMAMMA/60 °C, PMMAMMA/−5 °C, and PMMAMMA/−50 °C, re3401


Article

Macromolecules Table 4. Recipes and Results for Radical Polymerizations of MMAa and Characterizations of the Products diad tacticityd −1

b

entry

[vinyl group]0 (mol L )

initiator

4-1 4-2 4-3

1.0 1.0 1.0

AIBN BPO/DMA B(n-Bu)3/O2

c

−1

product

yield (%)

Mn,SEC (kg mol )

Mw/Mnc

PMMAMMA/60 °C PMMAMMA/−5 °C PMMAMMA/−50 °C

53 28 3.3

15.8 8.39 23.0

2.29 1.80 1.78

triad tacticityd

m

r

mm

mr

rr

0.22 0.15 0.14

0.78 0.85 0.86

0.06 0.02 0.02

0.32 0.25 0.24

0.62 0.73 0.74

Solvent, chloroform; polymerization time, 48 h. b[AIBN]0, 0.01 mol L−1; [BPO]0 = [DMA]0, 0.04 mol L−1; [B(n-Bu)3]0, 0.02 mol L−1. Determined by SEC in THF based on the calibrations using PMMA. dDetermined by 1H NMR spectra.

a c

Figure 5. 13C NMR spectra of (a) PMMAMOI‑30/60 °C and (b) PMMAMMA/60 °C in CDCl3.

spectively (Table 4). The Mn,SEC values ranged from 8.39 to 23.0 kg mol−1 and the Mw/Mn ranged from 1.78 to 2.29 for the PMMAMMA series. Tacticity Determined by 1H NMR Spectra. Figure 4 shows the typical 1H NMR spectra for the PMMAMOI, PMAAOI, and PMMAMMA series. Tables 3 and 4 list the diad tacticity (m: isotactic; r: syndiotactic) and triad tacticities (mm: isotactic; mr: heterotactic; rr: syndiotactic), which were determined from the peak areas due to the α-CH3 groups for PMMAs and CH2 groups for PMA. The tacticity of “conventional” PMMA is first described. PMMAMMA/60 °C was a syndiotactic enriched polymer with the m/r of 0.22/0.78 and mm/mr/rr of 0.06/0.32/0.62 (entry 4-1). This syndiotactic-specific trend was further enhanced with the decreasing polymerization temperature, PMMAMMA/−5 °C (entry 4-2) and PMMAMMA/−50 °C (entry 4-3). Thus, the polymerizations of MMA showed a tendency to proceed in an enthalpy-dominating manner. The tacticities of the PMMAMOI series, the most featured results in this study, are described. The m/r ratio was 0.44/0.56 for PMMAMOI‑30/60 °C (entry 3-1); therefore, the polyisocyanate template effectively reduced the syndiotacticity. The mm/mr/rr was 0.21/0.47/0.32, and this ratio was close to 1/2/1. We will focus on this result later because the preparation of PMMA with such a 1/2/1 main chain configuration has not been achieved to date by the radical polymerization. There has been no significant difference in the tacticities among the following five samples: PMMAMOI‑30/60 °C (entry 3-1), PMMAMOI‑30/−5 °C (entry 3-2), PMMAMOI‑30/−50 °C (entry 3-3), PMMAMOI‑61/1.0 M (entry 3-4),

and PMMAMOI‑61/0.1 M (entry 3-5). Thus, the temperatures and concentrations for the radical polymerization stage and the n value of the precursor polymer hardly influenced the main chain configurations for the PMMAMOI series. However, a noticeable difference was observed for the triad tacticity of PMMAMOI‑61/0.01 M (entry 3-6) in which the isotactic selectivity was enhanced to m = 0.47. We now recalled that only P(MOI61)/0.01 M was a species that was soluble in some solvents in which intermolecular side reactions were effectively suppressed due to extremely high dilution condition (Table 2). These results implied that the mm/mr/rr = 0.23/0.49/0.28 of PMMAMOI‑61/0.01 M would reflect the tacticity constructed entirely by intramolecular template polymerizations. The tacticities of the PMMAMOI‑EO series are also described. The m/r was 0.28/0.72, and mm/mr/rr was 0.08/0.39/0.53 for PMMAMOI‑EO‑38/60 °C (entry 3-7). Hence, the syndiotacticity was reduced in the order of PMMAMMA/60 °C > PMMAMOI‑EO‑38/60 °C > PMMAMOI‑30/60 °C. The tacticity of PMMAMOI‑EO‑38/5 °C (entry 3-8) was same as that of PMMAMOI‑EO‑38/60 °C. On the other hand, the m/r and mm/mr/rr were 0.34/0.66 and 0.13/0.42/0.45 for PMMAMOI‑EO/−50 °C (entry 3-9); thus, the isotactic selectivity was increased at the low temperature of −50 °C. This tacticity− temperature relationship should be characteristic of the PMMAMOI‑EO series, which was contrary to the PMMAMMA series in which the syndiotactic specificity was enhanced with the decreasing polymerization temperature. A plausible explanation is as follows. The methacryloxy groups would be somewhat flexible for the MOI-EO series due to the presence of the 3402


Article

Macromolecules

chain stereoregularity for all the other samples were also determined from the 13C NMR spectra, which satisfactorily agreed with the Bernoulli statistics as shown in Tables S2-2, S2-3, and S2-4. Figure 6 shows the plots of the number fractions of the triad tacticities (mm: isotactic; mr: heterotactic; and rr: syndiotactic)

oxyethylene spacer. However, their molecular mobility should be significantly reduced at the low temperature of −50 °C so that the restriction from the polyisocyanate template should effectively occur. The m/r was 0.52/0.48 for PMAAOI; hence, the meso-diad was enriched by the polyisocyanate template for the acrylate-type polymerizations (entry 3-10). Tacticity Determined by 13C NMR Spectra. The above section indicated that the isotactic selectivity was enhanced for the PMMAMOI and PMAAOI series as compared to that for the PMMAMMA series. It should be especially noted that the mm/mr/ rr ratio was almost equal to 1/2/1 for the PMMAMOI series, which is categorized as PMMA with a nonstereospecific main chain. There have been few reports on the synthesis of PMMA with such an “ideally” atactic main chain configuration with the mm/mr/rr of 1/2/1 by the radical polymerization; however, one has to take into account that such an mm/mr/rr ratio may be observed for the 1:1 mixture of the isotactic-enriched and syndiotactic-enriched PMMAs. In order to exclude such a possibility, the triad tacticity (mm/mr/rr) was determined from the peaks due to the α-CH3 group (15−25 ppm) and/or those due to quaternary carbons (43−57 ppm) for the 13C NMR spectra (Figure 5a). Furthermore, the pentad tacticities (mmmm/mmmr/rmmr/mmrr + rmrr/mmrm + rmrm/mrrm/ rrrm/rrrr) were also determined from the peaks due to the carbonyl carbons (175−180 ppm). The tacticities were compared to those calculated values based on the Bernoulli statistics. Table S2-1 lists the triad tacticities for PMMAMOI‑30/60 °C to be 0.24/0.48/0.28. The pentad tacticities were 0.06/0.08/0.06/0.28/0.23/0.06/0.14/0.09. The calculated values for the number fractions of the triad and pentad tacticities were determined using the following equations on the basis of the Bernoulli statistics.

Figure 6. Plots of the number fractions of the triad tacticities (mm: isotactic; mr: heterotactic; rr: syndiotactic) against the number fraction of the meso-diad (Pm) for the PMMAMOI (○), PMMAMOI‑EO (□), and PMMAMMA (◇) series.

versus the number fraction of the meso-diad (Pm) for the PMMAMOI, PMMAMOI‑EO, and PMMAMMA series. Figure 6 also exhibits theoretical curves of the Bernoulli statistics together with the literature data.30,31 The result again indicated that the experimental data obtained in this study were in good agreement with the Bernoulli statistics. Furthermore, the synthesis of PMMA possessing a nearly “ideally” atactic tacticity with the 1/ 2/1 mm/mr/rr ratio was first achieved by the radical polymerization. We describe the temperature dependence of the tacticity. Figure 7 shows the plots of ln(Pm/Pr) as a function of 1/T (K−1),

Pmm = Pm 2 Pmr = 2Pm(1 − Pm) Prr = (1 − Pm)2 Pmmmm = Pm 4 Pmmmr = 2Pm 3(1 − Pm) Prmmr = Pm 2(1 − Pm)2 Pmmrr + Prmrr = 2Pm 2(1 − Pm)2 + 2Pm(1 − Pm)3 Pmmrm + Prmrm = 2Pm 3(1 − Pm) + 2Pm 2(1 − Pm)2 Pmrrm = Pm 2(1 − Pm)2 Prrrm = 2Pm(1 − Pm)3 (7)

Figure 7. Fordham plots for the polymerization of MOI-30 (○), MOIEO-38 (□), and MMA (◇).

where Pm means the number fraction of the meso-diad, which was determined from the peak intensities due to the α-CH3 groups in the 1H NMR spectra (Figure 4a). The Pm value was 0.44 for PMMAMOI‑30/60 °C, providing the calculated triad tacticities of 0.20/0.49/0.31. These values were very close to the experimental values. Similarly, the pentad tacticity was calculated to be 0.04/ 0.10/0.06/0.27/0.22/0.06/0.15/0.10, which also fairly agreed with the experiment values. Thus, the tacticities determined by the NMR spectrum were reliable. The obtained sample was proved to be not a mixture of it-specific PMMA and st-specific PMMA, but PMMA with an “ideally” atactic tacticity. The main

namely the Fordham plots,32 where Pm/Pr denotes the number fraction of the meso-diad divided by that of the racemo-diad and T is the temperatures for the radical polymerization stage such as 333, 268, and 223 K. The activation parameters, such as the difference in the activation enthalpy (ΔH‡) and that in the activation entropy (ΔS‡) between the meso- and racemo-specific propagations, can be determined by the plot according to the equation

Prrrr = (1 − Pm)4

3403


Article

Macromolecules ln

ΔSm‡ − Sr‡ ΔHm‡ − Hr‡ Pm = − Pr R RT −1

Finally, we describe the glass transition temperature (Tg) of the “ideally” atactic PMMA. As shown in Figure S6, the Tg value was determined to be 95.1 °C for PMMAMOI30/60 °C (mm/mr/rr = 0.21/0.47/0.32, entry 3-1). This value was much lower than the Tg of 121 °C for the conventional syndiotactic-enriched PMMA, PMMAMMA/60 °C (mm/mr/rr = 0.06/0.32/0.62, entry 4-1). We now refer to the previously reported PMMA samples, which were prepared by the anionic polymerization1,33 and the coordination polymerization with a zirconocene catalyst,34 called PMMAAnion and PMMACoordination, respectively. The Tg value of 95.1 °C for PMMAMOI30/60 °C (mm/mr/rr = 0.21/0.47/0.32) was very close to that of 99.2 °C for PMMAAnion (mm/mr/rr = 0.28/0.36/ 0.36)33 and also to that of 93 °C for PMMACoordination (mm/mr/rr = 0.24/0.42/0.34).34 This comparison also suggested that there has been not so crucial correlation between the Tg values and the triad tacticities. Figure S7 shows the plots of the Tg value vs the number fraction of meso-diad (Pm) for PMMAMOI30/60 °C, PMMAMMA/60 °C, PMMAAnion,33 and PMMACoordination.34 The plot of PMMA prepared by the controlled radical polymerization such as ATRP,35 called PMMAATRP, was also added. It can be seen from Figure S7 that the Tg value was almost constant at 120−130 °C for the samples with low meso-contents (Pm < 0.2). On the other hand, the Tg values linearly decreased from ca. 125 to 50 °C with the increasing Pm values from ca. 0.2 to 1.0. These results implied that the diad tacticity plays a dominant role in the Tg values of PMMA, rather than the triad, pentad, and higher tacticities.

(8) −1

. The ΔS‡m − ΔS‡r

where R is the gas constant of 8.314 J K mol values are negative for all three samples as listed in Table 5; Table 5. Activation Parameters for the Radical Polymerization Stagea

a

initiator

ΔH‡m − ΔH‡r (kJ mol−1)

ΔS‡m − ΔS‡r (J mol−1 K−1)

MOI-30 MOI-EO-38 MMA

0.15 −1.70 3.01

−1.61 −13.5 −2.10

Calculated with eq 8.

hence, the difference in the tacticity observed for the present study might not be well explained by the activation entropy. A notable difference was observed for their ΔH‡m − ΔH‡r values, which are 0.15, −1.70, and 3.01 kJ mol−1 for the radical polymerizations of MOI-30, MOI-EO38, and MMA, respectively (Table 5). The ΔH‡m − ΔH‡r value was negative for the polymerization of MOI-EO-38; thus, a meso-propagation is favored by the enthalpy, eventually producing PMMAMOIEO38 with an isotactic enriched main chain configuration with decreasing the polymerization temperature. On the other hand, the ΔH‡m − ΔH‡r value was positive for the polymerization of MMA producing the PMMAMMA with a syndiotactic enriched main chain configuration. In remarkable contrast, the ΔH‡m − ΔH‡r was nearly zero for the MOI-30 system; thus, there has been no significant specificity between the meso- and racemopropagations, eventually generating PMMA possessing an “ideally” atactic tacticity with the 1/2/1 mm/mr/rr ratio. Figure 8 shows the illustration to support the proposed mechanism for

■

CONCLUSIONS The radical polymerizations of the (meth)acryloxyl groups covalently bonded to a polyisocyanate were demonstrated with the desire to provide insights into the stereoselectivity for the “one-dimensionally regulated polymerization” using the rigid helical polymer backbone as the molecular template. A series of polyisocyanates with pendant (meth)acryloxy groups were prepared by the living coordination polymerization, and their strong molecular perturbation together with the main chain stiffness was characterized by the 1H NMR and SAXS measurements. The radical polymerizations of the pendant (meth)acryloxy groups were performed using diverse conditions in terms of the polymerization temperatures and concentrations to produce products with gel-like forms, whereas that obtained under highly diluted conditions afforded soluble materials. Thus, the present radical polymerizations would mainly proceed via intramolecular template polymerization mechanism accompanied by intermolecular cross-linking as a minor side reaction. The subsequent extensive hydrolysis reaction and methyl esterification process provided the target poly[methyl (meth)acrylate]s. A typical poly(methyl methacrylate) (PMMA) showed the triad tacticity (mm/mr/rr) of 0.23/0.49/0.28. These values suggest that there has been no significant specificity between the mesoand racemo-propagations for the radical polymerizations of pendant methacryloxy groups perturbed strongly on the rigid polyisocyanate template, eventually generating PMMA possessing an “ideally” atactic tacticity with the approximate 1/2/1 mm/ mr/rr ratio, which was supported by the observation for the activation enthalpy (ΔH‡) between the meso- and racemo-specific propagations.

Figure 8. Illustration for the radical polymerizations of pendant methacryloxy groups on the rigid helical polyisocyanate template producing polymers with the “ideally” atactic main chain configurations.

the MOI-30 system. A propagating radical attacks the adjacent methacryloxy group to construct a new C−C bond with a stereocenter. In this study, the adjacent methacryloxy group would be so strongly perturbed by the rigid template on the position that the meso- and racemo-propagations would occur with equal probability. This is a feasible explanation why the stereoreospecificity is independent of the temperature for the polymerization of MOI-n.

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR spectra, SEC traces, and solubilitues of MOI-n, MOIEO-n, and AOI-n, IR spectra of P(MOI-n), P(MOI-EO-n), and 3404


Article

Macromolecules P(AOI-n), 1H NMR spectra of PMAAMOI and PAAAOI, and SEC trace of PMMA M O I ‑ 3 0 / 6 0 ° C , DSC thermograms of PMMAMMA/60 °C, and PMMAMOI‑30/60 °C, and summaries for number fractions of diad, triad, and pentad for the PMMA series. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00771.

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(21) Połowiński, S. Prog. Polym. Sci. 2002, 27, 537. (22) Saito, R.; Kobayashi, H. Macromolecules 2002, 35, 7207. (23) Kikuchi, M.; Mihara, T.; Jinbo, Y.; Izumi, Y.; Nagai, K.; Kawaguchi, S. Polym. J. 2007, 39, 330. (24) Kawaguchi, S.; Mihara, T.; Kikuchi, M.; Lien, L. T. N.; Nagai, K. Macromolecules 2007, 40, 950. (25) Kikuchi, M.; Lien, L. T. N.; Narumi, A.; Jinbo, Y.; Izumi, Y.; Nagai, K.; Kawaguchi, S. Macromolecules 2008, 41, 6564. (26) Norisuye, T.; Tsuboi, A.; Teramoto, A. Polym. J. 1996, 28, 357. (27) Patten, T. E.; Novak, B. M. Macromolecules 1993, 26, 436. (28) Org. Synth. 1963, Collect. Vol. 4, 250. (29) Saito, N.; Ikeda, Y. J. Phys. Soc. Jpn. 1951, 6, 305. (30) Bovey, F. A.; Tiers, G. V. D. J. Polym. Sci. 1960, 44, 173. (31) Miller, R. L. J. Polym. Sci. 1962, 56, 375. (32) Fordham, J. W. L. J. Polym. Sci. 1959, 39, 321. (33) Biroš, J.; Larina, T.; Trekoval, J.; Pouchlý, J. Colloid Polym. Sci. 1982, 260, 27. (34) Ning, Y.; Cooney, M. J.; Chan, E. Y.-X. J. Organomet. Chem. 2005, 690, 6263. (35) Percec, V.; Guliashvili, T.; Popov, A. V.; Ramirez-Castillo, E. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 1935.

AUTHOR INFORMATION

Corresponding Author

*Tel and Fax +81-238-26-3182; e-mail [email protected]. ac.jp (S.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Yuji Jinbo at Yamagata University for the many helpful discussions about the SAXS measurements. The authors also thank Showa Denko K. K., Japan, for kind supplies of MOI, MOI-EO, and AOI. 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 is gratefully acknowledged. The synchrotron radiation experiments were performed at the BL10C in the Photon Factory of the High Energy Accelerator Organization at Tsukuba, Ibaraki, Japan. We thank Prof. Koichi Ito, Emeritus Professor, Toyohashi University of Technology, for critically reading our manuscript.

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Radical Polymerization of Pendant (Meth)acrylates on a Rigid

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