Synthesis and Stereospecific Polymerization of a Novel Bulky Styrene

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Synthesis and Stereospecific Polymerization of a Novel Bulky Styrene Derivative Rong Wang,† Dongtao Liu,‡ Xiaohong Li,§ Jie Zhang,† Dongmei Cui,*,‡ and Xinhua Wan*,† †

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China ‡ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China § Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China S Supporting Information *

ABSTRACT: A novel vinylbiphenyl monomer, 2-methoxy-5phenylstyrene (MOPS), was designed and efficiently synthesized to investigate the stereospecific polymerization of bulky and polar styrenic derivative. Regardless of its large side group and electron-donating o-methoxy substituent, this compound showed a high polymerizability and was readily converted to the corresponding polymers with moderate to high molecular mass through radical, anionic, and coordination polymerizations. The resultant polymers were characterized by a combination of 1H/13C NMR spectrometry, thermal analysis, and wide-angle X-ray diffraction. Radical polymerization initiated by AIBN in toluene at 60 °C produced a syndiotactic-rich (rr = 0.37) polymer as most bulky vinyl monomers, whereas anionic polymerizations induced by n-BuLi yielded only isotactic-rich polymers no matter if polar tetrahydrofuran (−78 °C, mm = 0.54) or apolar toluene (−40 °C, mm = 0.78) was employed as the solvent. The isotactic-rich microstructure obtained by anionic polymerization in polar solvent at low temperature, the condition that usually leads to syndiotactic-rich polymer, manifested the strong interactions between the o-methoxy groups of the growing chain end and the penultimate unit with the lithium counterion. Highly isotactic (mm = 0.95) and perfect syndiotactic (rr > 0.99) polymers were obtained via coordination polymerizations in toluene at ambient temperature with the β-diketiminatoyttrium precursor (I) and the heterocyclic-fused cyclopentadienylscandium complex (III) as the catalytic precursor, respectively. All the polymers were thermally stable with 5% weight loss temperatures above 360 °C. They underwent glass transitions in the temperature range of 124−140 °C depending on the tacticity, much higher than polystyrene, implying the dominant role of congestion effect of large side groups on the segment movement restriction of polymer chain. Both isotactic and syndiotactic polymers were crystalline and had melting points higher than 300 °C, although the atactic and less stereoregular polymers were amorphous. The facile synthesis in conjunction with stereostructure tailorability, high thermal stability, glass transition temperature, and melting point makes the polymer a promising candidate for not only helical functional material but also engineering plastics.

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INTRODUCTION Helix is a widely observed asymmetric architecture in many biomacromolecules and closely related to the abundant and delicate biological functions of living systems. 1−3 The identification of double helices of DNA, where the bases sitting inside are exactly paired, reveals the mechanism of selfduplication and greatly promotes our understanding about life process.4 Inspired by Mother Nature, many synthetic helical polymers have been prepared5−12 and found wide practical and potential applications, such as chiral stationary phase of high performance liquid chromatography (HPLC),13,14 redoxtriggered chiroptical switches,15 and asymmetric catalysts.16,17 Biopolymers such as DNA, collagen, and myosin display precise control over their macromolecular helicity due to the © 2016 American Chemical Society

homochirality of the stereocenters in nucleotide or peptide backbones.18 A small structure modification could induce the collapse of helical conformation and invite a severe biological disaster. In some α-helical antimicrobial peptides, the substitution of D-amino acid by L-enantiomer would cause a remarkable depression in hemolytic action.19 For artificial polymers, the main chain stereoregularity plays an important role in the generation and stabilization of helical conformation as well as the concomitant functions. For example, the optical activity of poly[(S)-3-methyl-1-pentene] would be maximum Received: February 12, 2016 Revised: March 10, 2016 Published: March 23, 2016 2502

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Macromolecules Chart 1

for the 100% isotactic polymer.20 Only is the optical activity induced in isotactic-rich poly(2-vinylpyridine) by chiral acids but not in atactic one.21 Poly(phenylacetylene)s bearing cinchona alkaloid pendants catalyze the Henry reaction of benzaldehyde and nitromethane with a high enantioselectivity (94% ee) when the cis-transoidal helical conformation is taken. In a sharp contrast, the corresponding trans-rich, nonhelical ones just show a poor enantioselectivity (18% ee).22 As for vinyl polymers, the backbone rigidity is an additional prerequisite to take a stable helical conformation in solution. This is usually achieved by utilizing steric congestion substituents, which restrict the free rotations around the σ-bonds linking main chain carbon atoms.5,23,24 The attachment of large pendant groups to polymer main chain increases the internal rotation barrier and has led to numerous helical bulky poly(meth)acrylates and poly(meth)acrylamides.25−29 Despite these remarkable successes, most of the efforts to obtain optically active helical polystyrene derivatives end with undesired results. Isotactic-rich poly(3-methyl-4-vinylpyridine), poly(2-methoxystyrene), and some polystyrenes bearing ortho-amino groups are capable of taking dominant helical conformations with an excess screw sense at low temperature but lose their optical activities quickly even at ambient temperature.30−33 The helical conformations of vinylterphenyl polymers are stable because of high population of rod-like, laterally attached p-terphenyl pendants.34−38 However, they can only be prepared by radical polymerization and are therefore endowed with disordered configurations. In addition, the p-terphenyl-based vinyl monomers demand multistep tedious synthetic procedures. It is highly desired to develop a facial synthon to approach novel bulky vinyl aromatic monomers and a catalytic system to polymerize these monomers efficiently and stereospecifically. Since the possibility to synthesize iso- and syndiotactic polystyrenes was fully appreciated in the 1950s and 1980s, respectively, owing to the pioneering works of Ziegler−Natta and Ishihara,39,40 the investigations into the polymerization of styrene (St) in a more active and stereospecific manner have led to the discovery of various homogeneous metallocene, halfmetallocene, and non-metallocene group 3 and 4 transition metal based catalysts.41−45 In spite of these marvelous achievements, stereospecific polymerizations of styrene derivatives bearing polar groups have been far from successful. The efficient catalysts for St polymerization are based mainly on hard Lewis-acidic transition metals that are swiftly killed by the Lewis-basic polar groups. Therefore, masking the polar atoms with bulky groups such as trialkylsilyl and separating them from the vinyl group using long spacers are commonly adopted strategies,46−48 which, however, cause dramatic drops in the activity and selectivity. Recently, we achieved the unprecedented highly isoselective coordination polymerization of omethoxystyrene (oMOS) with high activity by using the cationic β-diketiminatoyttrium complex ([(BDI)Y-

(CH2SiMe3)2(THF)] (I), BDI = bis(2,6-dimethylanilido)ketimine, Chart 1).49 The strong Lewis-basic polar methoxy group, which is a typical poison to the transition-metal-based catalysts, behaves as an activator to the hard Lewis-acidic rareearth metal catalyst since it remains to coordinate to Y3+ throughout the polymerization. Additionally, using a constrained geometry catalyst (CGC, [(PyCH 2 Flu)Y((CH2SiMe3)2(THF))] (II), Chart 1), we achieved the polymerization of unmasked oMOS with moderate activity and excellent syndioselectivity.50 This is partly attributed to the presence of a pyridine side arm in the CGC yttrium catalyst that reduces the oxophilicity of the metal center. Therefore, the coordination of methoxyl to the metal center is weak, and the competitive coordination of the vinyl group to the metal center results in rapid insertion of oMOS molecules. In the present work, we extend the scope of these catalysts to bulky styrene derivatives bearing polar groups. A novel vinylbiphenyl monomer, 2-methoxy-5-phenylstyrene (MOPS), was synthesized and converted to the corresponding polymers via radical, anionic, and coordination polymerizations. The stereostructures of the resultant polymers were characterized by 1 H and 13C NMR spectroscopies. Radical and anionic polymerizations yielded syndiotactic- and isotactic-rich polymers, separately, whereas the coordination polymerizations catalyzed by I and (2,3,4,5,6-Me5-4H-cyclopenta[b]thiophenyl)Sc(CH2SiMe3)2THF (III)51 produced highly isotactic polymer and perfect syndiotactic polymer. All the polymers were thermally stable and underwent glass transitions in the temperature range from 124 to 140 °C. The polymers resultant from radical polymerization or anionic polymerization in THF were amorphous. However, the polymers obtained via anionic polymerization in apolar toluene and coordination polymerization were crystalline.

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EXPERIMENTAL SECTION

Solvents and Reagents. 4-Hydroxybiphenyl (99%, J&K), dimethyl sulfate (AR, Beijing Chemical Co.), acetic acid (AR, Beijing Chemical Co.), hydrogen bromide/acetic acid solution (33 wt %, Beijing Chemical Co.), triphenylphosphine (99%, Acros), aqueous formaldehyde (40%, AR, Beijing Chemical Co.), n-butyllithium (nBuLi, 1.6 M hexane solution, TCI), chloroform-d (CDCl3, 99.8 atom % D, J&K), 1,2-dichlorobenzene-d4 (99.5 atom % D, J&K), and dichloromethane-d2 (CD2Cl2, 99.5 atom % D, J&K) were used as purchased. Azodiisobutyronitrile (AIBN, AR, Beijing Chemical Co.) was recrystallized from ethanol and dried under vacuum at room temperature. Tetrahydrofuran (THF) and toluene used as the polymerization solvents were refluxed over sodium and distilled out just before use. The catalysts’ structures are shown in Chart 1.49−51 Measurements. Room temperature 1H and 13C NMR spectra were recorded on a Bruker ARX 400 MHz spectrometer with CDCl3 or CD2Cl2 as the solvent and tetramethylsilane (TMS) as the internal reference. High temperature 13C NMR spectra were obtained with a Bruker ARX 400 MHz using o-dichlorobenzene-d4 as the solvent at 120 °C. Heteronuclear single quantum correlation (HSQC) and 2503

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Macromolecules Scheme 1. Synthesis of 2-Methoxy-5-phenylstyrene (MOPS)

Figure 1. 1H NMR (left) and 13C NMR (right) spectra of MOPS measured in CDCl3 at 25 °C. H, 4.73. Found: C, 60.72; H, 4.80. MS (ESI, m/z): 299.00453 (M + 23+). 2-Methoxy-5-phenylstyrene (MOPS). Into a 1000 mL flask, 3bromomethyl-4-methoxybiphenyl (0.05 mol, 14.1 g), triphenylphosphine (0.1 mol, 26.7 g), and acetone (250 mL) were introduced. The reaction mixture was refluxed for 3 h. The corresponding phosphonium salts were obtained by filtration and dried under infrared lamp. Yield: 78%. To the mixture of above obtained phosphonium salts (0.03 mol, 17.65 g) and formalin (123 mL, 40 wt %) cooled by a ice−water bath, 45.2 mL of 40% potassium carbonate aqueous solution was added dropwise. After addition, the mixture was stirred further for 3 h at room temperature and then extracted with dichloromethane (150 mL × 3). The organic layers were combined and washed by water and brine, respectively. After drying over anhydrous Na2SO4, the solvent was taken away under reduced pressure. The residue was purified by silica gel column chromatography (dichloromethane/petroleum ether: 1/5 (v/v) as eluent) to afford the product as white solids. Yield: 90%. 1 H NMR (400 MHZ, CDCl3, δ ppm): 3.89 (s, 3H, OCH3), 5.28−3.35 (dd, 1H, vinyl), 5.66−5.85 (dd, 1H, vinyl), 6.91−6.97 (d, 1H, Ar), 7.03−7.15(q, 1H, vinyl), 7.27−7.35 (t, 1H, Ar), 7.38−7.50 (m, 3H, Ar), 7.53−7.60 (d, 2H, Ar), 7.67−7.72 (d, 1H, Ar). 13C NMR (100 MHZ, CDCl3, δ ppm): 55.67, 111.16, 114.89, 125.40, 126.77, 126.83, 126.98, 127.50, 128.74, 131.71, 133.74, 140.89, 156.35. Anal. Calcd for C15H14O: C, 85.68; H, 6.71. Found: C, 85.68; H, 6.54; MS (ESI, m/z): 211.11169 (M + 1+). Radical Polymerization. MOPS (0.20 g, 0.95 mmol), BPO (3.12 mg, 0.002 mmol), and toluene (1.2 mL) were added into a polymerization tube. After three freeze−pump−thaw cycles, the tube was sealed under vacuum and heated at 60 °C for 24 h. After the reaction was completed, the solution was diluted with THF and then poured into methanol. The polymer (P1) was collected by filtration and dried under vacuum at 40 °C for 24 h as white powders (0.14 g). Yield: 70%. Anionic Polymerization. As a general procedure, the Schlenk tube containing MOPS (0.50 g, 2.4 mmol) was first attached to a vacuum line. Then the dry and clean solvent (2.5 mL) was injected into the tube under argon and cooled to a desired temperature. After that, a predetermined amount of n-BuLi (1.6 M hexane solution) was added into the reaction vessel via a syringe to initiate polymerization. After a

heteronuclear multiple bond correlation (HMBC) were measured with a Bruker ARX 600 MHz spectrometer with CDCl3 as the solvent. Mass spectra were recorded with a Finnigan-MAT ZAB-HS spectrometer. Elemental analyses were run on an Elemental Vario EL instrument. Differential scanning calorimetry (DSC) traces were obtained with a TA DSC Q100 instrument at a heating rate of 20 °C min−1 and cooling rate of 2 °C min−1. Thermogravimetric analyses were performed on a TA SDT 2960 instrument under a nitrogen atmosphere. The number-average molecular mass (Mn), weightaverage molecular mass (Mw), and polydispersity (Mw/Mn) of polymer were determined by a gel permeation chromatography (GPC) apparatus equipped with a Waters 2410 refractive index detector and a Waters 515 pump. THF was employed as the eluent at a flow rate of 1.0 mL/min at 35 °C. All the data were calibrated against a series of monodispersed polystyrenes. One-dimensional powder wide-angle Xray diffraction (1D-WAXD) experiments were carried out on a Philips X’Pert Pro diffractometer equipped with a 3 kW ceramic tube as the Xray source (Cu KR) and an X’celerator detector. Synthesis. 4-Methoxybiphenyl. Into a 1000 mL flask, 4hydroxybiphenyl (0.29 mol, 50.0 g), dimethyl sulfate (0.35 mol, 44.1 g), K2CO3 (0.35 mol, 48.7 g), and acetone (750 mL) were sequentially added. The reaction mixture was refluxed for 18 h and then poured into 2500 mL of hot water. The precipitates were collected by filtration and dried under vacuum at 50 °C for 24 h. Yield: 99%. 1H NMR (400 MHZ, CDCl3, δ ppm): 3.81−3.88 (t, 3H; OCH3), 6.94−7.00 (d, 1H; Ar), 7.38−7.45 (t, 2H; Ar), 7.50−7.58 (t, 4H; Ar). 13C NMR (100 MHZ, CDCl3, δ ppm): 55.36, 114.22, 126.67, 126.76, 128.17, 128.74, 133.80, 140.85, 159.16. Anal. Calcd for C13H12O: C, 84.75; H, 6.57. Found: C, 84.93; H, 6.53. MS (ESI, m/z): 185.09594 (M + 1+). 3-Bromomethyl-4-methoxybiphenyl. To a mixture of 4-methoxybiphenyl (0.1 mol, 18.4 g), paraformaldehyde (0.15 mol, 4.5 g), and glacial acetic acid (50 mL), 60 mL of 33 wt % hydrogen bromide/ acetic acid solution was added rapidly. The mixture was kept at 50 °C for 4 h and then poured into 100 mL of water. The precipitates were collected by filtration and dried under vacuum at 50 °C for 24 h. Yield: 85%.1H NMR (400 MHz, CDCl3, δ ppm): 3.93 (s, 3H, OCH3), 4.62 (s, 2H, CH2Br), 6.93−6.7 (d, 1H, Ar), 7.28−7.35 (t, 1H, Ar), 7.35− 7.45 (t, 2H, Ar), 7.50−7.59 (m, 4H, Ar). 13C NMR (100 MHz, CDCl3, δ ppm): 29.03, 55.80, 111.33, 126.41, 126.75, 126.93, 128.74, 128.79, 129.68, 133.87, 140.26, 156.99. Anal. Calcd for C14H13OBr: C, 60.67; 2504

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Macromolecules

Figure 2. HMBC (left) and HSQC (right) spectra of MOPS measured in CDCl3 at 25 °C.

Table 1. Polymerization Results of MOPS triadc a

run

initiator

solvent

T (°C)

time (h)

yield (%)

1 2 3 4 5

AIBN n-BuLi n-BuLi I III

toluene THF toluene toluene toluene

60 −78 −40 25 25

24 2 36 1 1

70 94 86 90 99

−5 b

Mn × 10

b

PDI

mm

mr

rr

md

re

2.06 2.94 1.14 0.48 0.52

2.57 1.65 2.40 1.15 1.10

0.16 0.54 0.78 0.95 0

0.47 0.24 0.13 0.04 0

0.37 0.22 0.09 0.01 >0.99

0.40 0.66 0.85 0.97 0

0.60 0.34 0.15 0.03 >0.99

a Menthol-insoluble part. bDetermined by GPC in THF on the basis of standard polystyrene calibration. cCalculated by the integral of the peaks. dm = mm + 1/2mr. er = 1 − m.

Scheme 2. Iso- and Syndioselective Coordination Polymerizations of MOPS

certain time, a few drops of methanol were added to stop the reaction. The reaction mixture was diluted with THF and then poured into methanol. The resultant polymer was collected by filtration and dried under vacuum at 40 °C for 24 h. Coordination Polymerization. The following example describes the syndioselective polymerization of MOPS in toluene at 25 °C with the initiation system of [III]0/[AliBu3]0/[[Ph3C][B(C6F5)4]]0/[MOPS]0 = 1/10/1/300. In a glovebox, a toluene solution (3.0 mL) of III (0.01 mmol, 4.8 mg) was added into a 25 mL flask. Then, 10 equiv of AliBu3 (0.2 mL, 0.5 mol/L), 1 equiv of [Ph3C][B(C6F5)4] (0.01 mmol, 9.2 mg) dissolved in toluene (1.0 mL), and MOPS (0.62 g, 3 mmol) were introduced into the flask. After stirring for 2 h, methanol (0.5 mL) was injected to terminate the polymerization. The viscous mixture was poured into a large quantity of methanol. The obtained white solids were collected by filtration, washing with methanol, and then dried under vacuum at 50 °C to a constant weight.

sulfate, followed by bromomethylation with paraformaldehyde and hydrogen bromide. After reflux with triphenylphosphine in acetone, (2-methoxy-5-phenylbenzyl)triphenylphosphonium bromide was produced. The Wittig reaction of phosphonium salt with formaldehyde aqueous solution under alkaline condition yielded the target molecule. The intermediates and monomer were fully characterized by 1H/13C NMR, mass spectrometry, and elementary analysis. All the date agreed well with the expected structure. Figure 1 exhibits the 1H and 13C NMR spectra of MOPS. The assignments of 1H NMR spectrum were made in terms of the integral and split of resonance peaks, while the 13C NMR spectrum was assigned by means of HSQC and HMBC spectra (Figure 2). The nonquaternary carbon atoms (C1−9) were identified by analyzing 1JCH correlation (HSQC). The quaternary carbon atoms (C10−13) were further analyzed based on HMBC, taking advantages of 2 J CH and 3 J CH correlations. The carbon atom C10 was correlated with He, Ha, and Ha′; C11 was correlated with Hb, Hc, Hd, He, and Hf;

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RESULTS AND DISCUSSION Synthesis. The monomer MOPS was prepared via four steps reactions as illustrated in Scheme 1. The synthesis began with the etherification of 4-hydroxybiphenyl and dimethyl 2505

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Macromolecules C12 was correlated with Hg and He; C13 was correlated with Hd, Hf, and Hh. Therefore, the 13C NMR spectrum of MOPS was fully assigned. Polymerization. The monomer MOPS was polymerized under various reaction conditions. It was readily converted to the corresponding polymers via radical, anionic, and coordination polymerizations, regardless of its large substituent and the ortho electron-donating methoxy group. The results are summarized in Table 1. The radical polymerization in toluene at 60 °C induced by AIBN gave a polymer yield of 70% in 24 h. The anionic polymerization in THF with n-BuLi as the initiator proceeded quickly even at a temperature as low as −78 °C and consumed 94% monomer in 2 h. Because of the limited solubility of MOPS, the anionic polymerization in toluene could not be conducted at below −40 °C. At −40 °C, both initiation by n-BuLi and propagation run slowly, and thus, 36 h was required to reach 86% monomer conversion. Three complexes (Chart 1) were employed to catalyze the coordination polymerization of MOPS. The cationic complexes [(BDI)Y(CH2SiMe3) (THF)][B(C6F5)4], which was obtained in situ by the treatment of I with the organoborate [Ph3C][B(C6F5)4] in toluene, promoted the isoselective polymerization efficiently at room temperature (Scheme 2). Within 1 h, 90% MOPS was converted to the polymer. The constrained geometry catalyst II was known to catalyze, combined with AliBu3 and [Ph3C][B(C6F5)4], the coordination polymerization of oMOS with high activity and perfect syndioselectivity.50 However, it was ineffective to the polymerization of MOPS, probably due to the crowded ligands that hinder the insertion of the monomer with large pendant group. Therefore, we tried another rare earth metal complex, (2,3,4,5,6-Me5-4H-cyclopenta[b]thiophenyl)Sc(CH2SiMe3)2THF (III), which was previously reported to show high syndioselectivity for homopolymerization of styrene and regioselectivity for copolymerization of ethylene and dicyclopentadiene.51 Fortunately, the catalytic activity of III was not blocked by the polar methoxy group and bulky substituent of MOPS and afforded the polymer with high molecular mass and narrow molecular mass distribution. The resultant polymers are white powders and soluble in many common organic solvents like anisole, THF, chloroform, chlorobenzene, and toluene. For the sake of clarity, the polymer prepared by radical polymerization, anionic polymerizations in THF and toluene, and coordination polymerizations under the catalyzes of I and III were named as P1, P2, P3, P4, and P5, respectively. Figure 3 presents the 1H NMR spectra of the resultant polymers prepared under various conditions. After polymerization, the typical resonance peaks of vinyl groups (Ha, Ha′, Hb) disappeared completely, indicating the conversion of monomer to polymer. Broad absorptions were observed for methine and methylene protons of the polymer yielded by radical polymerization. The signals of the polymers resulted from anionic and coordination polymerization were rather sharp, an indicative of higher regularity.52 The difference in chemical shift between methine and methylene protons was greater in the polymers obtained by anionic initiator and isoselective catalyst than in that obtained by syndioselective catalyst, and the methine proton signal of P5 was observed at the highest magnetic field. This is consistent with the polystyrenes with various tacticities.53 Because of the shielding effect of large pendant groups, proton NMR was not able to give the tacticity quantitatively.

Figure 3. 1H NMR spectra of PMOPS obtained under various conditions measured in CDCl3 at 25 °C.

Tacticity. The tacticities of polystyrene and its derivatives are usually estimated by the resonance absorptions of ipso carbon atoms in 13C NMR spectra, which are sensitive to the configurational regularity of polymer main chain.54,55 The difference in 13C NMR spectra between the monomer and the polymer is mainly caused by the saturation of double bond and the steric hindrance of pendant groups. To identify the resonance peak of the ipso carbon atom (C10) of PMOPS, HSQC and HMBC measurements of the model compound, 3ethyl-4-methoxybiphenyl, were first made in CDCl3 at room temperature to provide a reference (Figure S1). Owing to the electronic effect variation caused by the hydrogenation reaction of the vinyl group, the resonance absorption of ipso carbon C10 (132.7 ppm) of model compound shifted to lower field compared to that of the monomer (126.7 ppm), whereas all other aromatic carbons were only slightly influenced (Table S1). Figure 4 exhibits the 13C NMR spectra of P4 and P5. Like

Figure 4. 13C NMR spectra of P4 and P5 measured in CD2Cl2 at 25 °C.

the model compound, only did the absorption of C10 (135.5 ppm) shift obviously, and those of the other three quaternary carbon atoms kept almost unchanged. In this way, the absorption of ipso carbon was identified. This speculation was further supported by the HMBC spectrum of P4 (Figure S2). Although not much correlations were observed because of the 2506

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Macromolecules limited mobility of polymer chains, C12 interacted strongly with He, excluding the uncertainty of two near absorptions (i.e., C10 and C12). The quantitative analyses of the polymer tactic structures were made by 13C NMR spectra measured in o-dichlorobenzene at 120 °C (Figure 5). By analogous to PoMOS and

Figure 6. Schematic diagram of the interactions of the lithium counterion with the oxygen atoms of penultimate unit and growing chain end.

Table 2. Thermal Behaviors of the Polymers Prepared under Different Conditions run

m

Td(5%)a (°C)

Tgb (°C)

P1 P2 P3 P4 P5

0.40 0.66 0.85 0.97 0

374.4 373.7 372.0 369.4 367.0

126.3 123.9 126.0 127.9 140.3

Tmc (°C)

crystallined

319.1 302.5/314.5 317.4

no no yes yes yes

a

Temperature at which 5% weight loss was observed under N2. bGlass transition temperature of the polymers after quenching at molten state. c Melting temperature determined by DSC. dDetermined by DSC and temperature variable wide-angle X-ray diffraction.

Figure 5. Expanded 13C NMR signals of ispo carbon atoms (C10) of PMOPS measured in o-dichlorobenzene at 120 °C.

other polystyrene derivatives,49,50,54,55 the absorptions in the ranges of 134.10−134.4, 134.4−134.6, and 134.6−135.0 ppm were assigned as syndiotactic (rr), heterotactic (mr), and isotactic (mm) triads, respectively. The tacticities of the polymers prepared under various conditions are listed in Table 1. Complex I was known to catalyze isoselective polymerization of oMOS. It was still efficient for MOPS and yielded highly isotactic polymer (P4, mm = 0.95) regardless of the enlarged pendant groups. The polymer P5 exhibited only a narrow singlet at 133.3 ppm, indicating almost no stereo error was made in the polymerization catalyzed by the complex III. The polymer obtained by radical polymerization initiated by AIBN in toluene at 60 °C was syndiotactic-rich (rr = 0.37). This was held for by Bernoullian statistics and suggested a growing chain end controlled stereochemistry.52,55 The anionic polymerization induced by n-BuLi in apolar toluene at −40 °C yielded a isotactic-rich polymer with mm = 0.78. The substitution of toluene with THF reduced the stereoselectivity but still produced isotactic-rich polymer, which conflicted with the general believing that the anionic polymerization in polar solvent produce syndiotactic-rich polymers.52 This interesting phenomenon was probably due to the interactions of lithium counterions with the oxygen atoms of penultimate unit and growing chain end (Figure 6), which favored the formation of meso diads as the case of poly(2-vinylpyridine) and poly(3methyl-2-vinylpyridine).56,57 Thermal Behaviors. The thermal properties of the polymers are listed in Table 2. All the polymers are quite thermal stable. The 5% weight loss temperature under inert atmosphere is above 360 °C. The glass transition temperatures (Tg) of the polymers as prepared are obvious except for P4 and P5, suggesting that the polymers are predominantly crystalline (Figure S4). After quenching from melt, all the samples showed obvious glass transitions in the temperature range from 124 to 140 °C (Figure 7). The perfect syndiotactic polymer displayed the highest Tg. The dependence of glass transition temperatures on tacticity was probably derived from the different local

Figure 7. DSC curves of PMOPS after quenching at molten state.

conformations of the polymers with various stereoregularity. Moreover, the glass transition temperatures of the polymers are higher than that of polystyrene, implying the prominent roles of congestion effect of large side groups on the segment movement restriction of polymer chain. For polystyrene, the tacticity of the polymers affect the melting point greatly. Syndiotactic polystyrene has the highest melting point of 270 °C, which is higher than that of isotactic polystyrene by 40 °C. In contrast to polystyrene, the tacticity of the polymers nearly has no effect on the melting temperature. P3, P4, and P5 display melting temperature (Tm) in the range of 317−320 °C, which is also higher than that of polystyrene. The much better thermal stability of the polymer than that of polystyrene likely associates with the large steric hindrance and rigidity, which makes the polymer a promising candidate for engineering plastic. Crystal Structure. The crystal structures of P3 and P5 are analyzed by temperature variable wide-angle X-ray diffraction. 2507

DOI: 10.1021/acs.macromol.6b00325 Macromolecules 2016, 49, 2502−2510

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Macromolecules

Figure 8. One-dimensional WAXD patterns of P4 and P5.

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With increasing temperature, the crystal structure developed, and entered into isotropic state above melting temperature. The WAXD patterns of P3 and P5 are different from each other, suggesting the crystal structure is different because of their various stereoregularities. This is in accord with the results of polystyrene, for which the X-ray diffraction spectra of isotactic and syndiotactic polystyrene are quite different. The identity period of crystallized isotactic polystyrene is 6.65 Å, indicating a 3-fold helical structure, while that of syndiotactic polystyrene is much smaller (5.06 Å), and the polymer takes a planar-zigzag conformation in the crystalline state. For our polymers, the different crystal structures also suggesting different conformations of the polymers, and relevant research is underway.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00325. 1 H/13C NMR spectra of model compound and polymers and DSC curves of the polymers (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.M.C.). *E-mail: [email protected] (X.H.W.). Author Contributions

R.W. and D.L. contributed equally to this paper. Notes

The authors declare no competing financial interest.

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CONCLUSIONS

ACKNOWLEDGMENTS The financial support of the National Natural Science Foundation of China (No. 21274003) and the Research Fund for Doctoral Program of Higher Education of MOE (No. 20110001110084) is greatly appreciated.

The synthesis and stereospecific polymerization of a novel vinylbiphenyl monomer, MOPS, were described. In spite of its large side group and ortho polar methoxy substituent, the monomer showed high polymerization activity toward radical, anionic, and coordination polymerizations. Isotactic-rich polymers were formed with anionic polymerization in either apolar toluene or polar THF at low temperature due to the complexion of oxygen atom with the countercation. Perfect syndiotatic and highly isotactic polymers were prepared via coordination polymerization under the catalysis of III and I as the precursors, respectively. All of the polymers had good thermostability. They possess glass transition temperatures ranging from 124 to 140 °C depending on the tacticity, much higher than polystyrene (∼100 °C), suggesting a larger backbone rigidity caused by bulky pendant groups. The polymers with high stereoregularity, i.e., P3, P4, and P5, were crystalline and melted at above 300 °C. Given the facile monomer synthesis, stereostructure tailorability, in conjunction with high thermal stability, glass transition temperature, and melting point, the polymer could be a promising candidate for not only helical functional materials but also engineering plastics. Further works, in continuity of this one, will investigate the possibility to synthesize helical vinylbiphenyl polymers with a dominant helical screw sense induced by chiral environment or pendant auxiliary.

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