Significantly Enhanced Thermotropic Liquid Crystalline

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Significantly Enhanced Thermotropic Liquid Crystalline Columnar Mesophases in Stereoregular Polymethylenes with Discotic Triphenylene Side Groups Xiao Li,†,§ Bin Mu,‡,§ Changlong Chen,∥ Jian Chen,† Jiang Liu,† Feng Liu,*,∥ and Dongzhong Chen*,†

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†

Key Laboratory of High Performance Polymer Materials and Technology of Ministry of Education, Collaborative Innovation Center of Chemistry for Life Sciences, Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China ‡ School of Natural and Applied Sciences, Northwestern Polytechnical University, Xi’an 710072, China ∥ State Key Laboratory for Mechanical Behavior of Materials, Shaanxi International Research Center for Soft Matter, School of Materials Science & Engineering, Xi’an Jiaotong University, Xi’an 710049, China S Supporting Information *

ABSTRACT: Side-chain discotic liquid crystalline polymers (SDLCPs) with discotic (disclike) mesogens (discogens) attached as side groups through flexible spacers constitute a class of fascinating organic polymer semiconducting materials. While so far almost all reported SDLCPs belong to conventional C2 polymers based on polymerization of vinyl monomers, limiting their structure diversities, substitution density, and efficiency promotion. In this article we present the synthesis of a series of syndiotactic polymethylene SDLCPs of Pm(TPn) with discotic triphenylene (TP) side groups of variant peripheral alkoxy substituents (n = 6, 4, 10) and different length alkyl spacers (m = 3, 4, 6, 8, 10, 12) through an indirect two-step rhodium-complex-catalyzed C1 carbene polymerization pathway. The thermal properties and ordered organization structures of the precursor polymers and polymethylene SDLCPs have been systematically investigated with differential scanning calorimetry (DSC) and polarized optical microscopy (POM), especially through synchrotron radiation variable-temperature small/wide-angle X-ray scattering (SAXS/WAXS) analyses. All of the series C1-type syndiotactic polymethylenes with high densely substituted TP side groups exhibit various hierarchical ordered columnar mesophases comparable to that of the typical side-chain C2 polymers of well-defined polyacrylates with TP discogens. Moreover, they possess a remarkably broadened temperature range of columnar structures persistent to very high temperatures in virtue of the stiff helical polymethylene backbone. This work provides a feasible route to prepare the C1-type SDLCPs with high densely substituted functional side groups and may offer an in-depth understanding for the hierarchical organization of ordered columnar structures with significantly increased thermal stability.

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alkynylbenzene,12,13 perylene,14−17 porphyrin, and phthalocyanine (Pc),18−20 triphenylene (TP) derivatives are among the most extensively investigated and particularly promising discogens attracting persistent attention due to their relative ease in synthesis, chemical stability, and rich mesophases.21−23 Many TP-based SDLCPs have been explored through different polymerization methods with various polymer backbones such as polysiloxane,24−26 poly(methyl)acrylate,27−32 polystyrene, 31,33−36 polynorbornene, 31,37 conjugated polyacetylene,38,39 and polythiophene.40,41 Among them polyacrylate

INTRODUCTION Discotic liquid crystals (DLCs) are promising and useful optoelectronic materials for their advantageous properties such as capability of long-range self-assembly, self-healing, and high charge carrier mobilities along the stacking axis in variant columnar mesophases.1−5 Combining the virtues of DLCs with good processability as well as the film-forming and mechanical properties of polymers, side-chain discotic liquid crystalline polymers (SDLCPs) composed of discotic (disc-like) mesogens attached as side groups through flexible spacers constitute a class of fascinating organic semiconducting materials.6−8 Although some impressive examples of SDLCPs have been reported adopting various discotic mesogens (discogens) such as the derivatives of hexabenzocoronene (HBC),9−11 multi© XXXX American Chemical Society

Received: July 9, 2019 Revised: August 19, 2019

A

DOI: 10.1021/acs.macromol.9b01433 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules with moderate flexibility is one of the most preferred backbones for the development of SDLCPs. In recent years, our group has focused on the rational design and controlled synthesis of TP-based polyacrylate SDLCPs via reversible addition−fragmentation chain transfer (RAFT) polymerization for systematically clarifying and revealing some fundamental key issues such as the molecular weight (MW) effect and the spacer length influence of the well-defined SDLCPs.42−49 On the basis of a huge homologous family of polyacrylate SDLCPs with different length spacers and a series of narrow distributed homopolymers with gradually increased molecular weights, a remarkable MW effect or polymer effect at a critical degree of polymerization (DP) around 20 has been revealed.42 Moreover, a discrete columnar stack (DCS)-based hierarchical selforganization model was first proposed, that is, a certain number of neighboring TP discs of the same polymer chain self-assembled into DCS subunits; then the DCS subunits further surrounded the polymer backbone to form a multicolumn-bundle cylindrical superstructure, which further selforganized into variant columnar mesophases with different two-dimensional (2D) intercolumnar orders. Such a DCSbased multicolumn-bundle hierarchical self-organization model accounted well for various well-organized columnar superlattices and their transformation, which was strongly supported by a combination of various modern instrument analyses, thermotropic phase transitions, and their kinetic investigations together with a coarse-grained dissipative particle dynamics (DPD) theoretical simulation.42−45 Furthermore, it was demonstrated that for SDLCPs proper coupling between side-chain discogens and the polymer backbone through shorter spacers was desirable for achieving well-organized ordered columnar superstructures; thus, a positive coupling effect (PCE) was proposed for SDLCPs,46−49 in sharp contrast to the renowned classical spacer decoupling principle, which has been widely demonstrated to be suitable and highly effective for their counterpart side-chain calamitic liquid crystalline polymers with longer spacers preferred. On the other hand, rhodium (Rh) complex-mediated polymerization of carbene monomers or certain carbene precursors provides unprecedented attractive opportunities to synthesize stereoregular carbon-chain polymers with high densely substituted side groups.50−52 This kind of carbene polymerization can prepare unique polymers with functional side groups at each carbon atom of the generally stereospecific saturated polymer backbone.53 Such kind of carbon-chain polymers with high densely substituted side groups at each backbone carbon atom are called Cl polymers to distinguish them from the conventional C2 polymers of often atactic nature and with side groups at every two main-chain carbons usually originated from vinyl monomers. In particular, the RhI(diene)-mediated carbene polymerization of diazoester monomers can obtain syndiotactic C1 polymers of stiff helical backbone with densely functionalized side groups, and syndiotactic polymethylenes with side-chain alkyloxycarbonyl or calamitic mesogens showing thermotropic liquid crystalline phases have been reported;54−56 such unique functional C1 polymers are not available via the conventional vinyl monomer-based polymerization pathway. Thus far, almost all reported SDLCPs belong to conventional C2 polymers based on polymerization of vinyl monomers, with the side-chain groups attached every other carbon of the polymer backbone, limiting their structure diversities, substitution density, and efficiency promotion.

Although there were some impressive reports on C1 polymers based on Pd-catalyzed polymerization of isocyanide,57−60 an interesting example of a C1 polymer composed of a helical rigid polyisocyanide backbone with side-chain-tethered porphyrin discogen was reported by Scolaro and Rowan in 2003, whereas the hydrogen-bonding interactions between the side-chain amide group and the CN double-bond linkage with the stiffened polymer backbone prevented the π-stacking between the side-chain porphyrin discs.61 Very recently we demonstrated the successful synthesis of a group of highly stereoregular C1 polymers with tetraphenylethene (TPE)based aggregation-induced emission (AIE) fluorescent side groups via Rh-catalyzed carbene polymerization as compared to the usual C2 polyacrylate counterparts with the same TPE side groups;62 they exhibited thermodynamically stable and significantly enhanced high fluorescence quantum yields due to enhanced restriction of intramolecular rotation (RIR) of TPE luminogens and blocking the nonradiative channels.63 Herein, we present the synthesis of a series of syndiotactic polymethylenes with densely substituted side-chain TP discogens through different length spacers via an indirect two-step carbene polymerization route. The thus obtained C1type SDLCPs with high densely substituted TP discogens are capable of self-assembling into various well-defined columnar mesophases and discotic columnar superlattices mainly regulated by the spacer length and significantly promoted thanks to the rigid helical polymethylene backbones as revealed by a combination of synchrotron X-ray scattering and other modern characterization techniques, which may offer an in-depth understanding for the hierarchical organization of complex structures and improve the optoelectronic properties of DLC polymer semiconducting materials for various applications.

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EXPERIMENTAL SECTION

Materials. The monohydroxyl triphenylene derivatives TPn-OH with peripheral substituents pentakis-hexyloxy, -butoxy, or -decyloxy (n = 6, 4, or 10), respectively, around the TP core were synthesized according to our previously reported procedures.30,42,48 Rhodium (Rh) complex catalyst and the intermediate N,N’-ditosylhydrazine (DTHZ) were prepared according to the same procedures as described in our previous report.63 Other chemicals and solvents were commercially available and used directly as received. Measurements. 1H NMR and 13C NMR analyses were carried out on a Bruker AVANCE III 400 NMR spectrometer at 25 °C with tetramethylsilane (TMS) as internal standard. Fourier transform infrared (FTIR) spectra were recorded on an iS50 (Nicolet) infrared spectrometer with a scanning range of 500−4500 cm−1. Gel permeation chromatography (GPC) measurements were performed on (1) Viscotek GPCmax VE-2001 with tetrahydrofuran (THF) used as the eluent at a flow rate 1.0 mL min−1, column temperature 35 °C, or (2) Agilent Technologies Inc. PL-GPC 50 gel permeation chromatography with dimethylformamide (DMF) used as the eluent at a flow rate 1.0 mL min−1, column temperature 35 °C. In both cases polystyrene (PSt) standard polymer samples were adopted for relative molecular weight calculations. Thermal gravimetric analysis (TGA) curves were recorded on a synchronized thermal analyzer TGA/DSC 1 from Mettler-Toledo under a nitrogen atmosphere with solid samples of around 6−8 mg at a heating rate of 20 °C min−1. The differential scanning calorimetry (DSC) thermograms were recorded on a Mettler-Toledo DSC 1 calorimeter under a nitrogen atmosphere with a heating and cooling rate of 10 °C min−1, and the amounts of investigated samples were typically 6−10 mg. The phase transitions and liquid crystalline textures were investigated via polarized optical microscopy (POM) with a PM6000 microscope equipped with a Leitz-350 heating stage and a Nikon D3100 digital camera. For the B


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Scheme 1. Synthesis of Precursor Polymers PmBr through C1 Carbene Polymerization and Preparation of the Corresponding C1 Syndiotactic Polymethylene SDLCPs of Pm(TPn) with Side-Chain TP Discogens via Williamson Etherification

(120 mL). The mixture solution was cooled to 0 °C, and then 1,8diazabicyclo[5.4.0]undec-7-ene (DBU, 11 mL, 75 mmol) was slowly added dropwise under a nitrogen atmosphere. The resulting reaction system was stirred at room temperature for 15 min and then quenched with a saturated sodium bicarbonate aqueous solution (100 mL) followed by extraction with dichloromethane. The solvents of the organic phase were removed by rotary evaporation to give a crude product, and further purification of the crude product was carried out by silica-gel column chromatography with dichloromethane as the eluent. After drying at 46 °C under vacuum for 7 h, 3.0 g of yellowgreen clear liquid 6-bromohexyl diazoacetate was obtained (yield 82%). 1H NMR (400 MHz, CDCl3) δ (ppm): 4.74 (s, 1H, OOCCHN2), 4.16 (t, 2H, CH2OOC), 3.41 (t, 2H, BrCH2), 1.87 (m, 2H, CH2CH2OOC), 1.67 (m, 2H, BrCH2CH2), 1.48 (m, 2H, CH2(CH2)2OOC), 1.38 (m, 2H, Br(CH2)2CH2). 13C NMR (100 MHz, CDCl3, 298 K) δ (ppm): 166.74, 64.53, 46.00, 33.58, 32.42, 28.45, 27.59, 24.88. FTIR (cm−1): 3118, 2936, 2859, 2107, 1687, 1459, 1394, 1354, 1238, 1184, 1041, 739, 644. Synthesis of Precursor C1 Polymer P6Br through Rh(I)Catalyzed Carbene Polymerization. Monomer 6-bromohexyl diazoacetate (2.49 g, 10 mmol) was added to a clear yellow solution of Rh(I) catalyst (0.065 g, 0.02 equiv to the monomer) in anhydrous chloroform (8 mL) under a nitrogen atmosphere, and then the mixture was stirred at room temperature for 24 h. The reaction solution was slowly added dropwise to a mixed solvent of methanol and dichloromethane (MeOH:CH2Cl2 = 2:3, v/v) (400 mL), and then the mixture solution was strongly stirred overnight. The solid product was separated and collected by centrifugation. After drying at 75 °C under vacuum for 6 h, 1.0 g of yellow-green solid was obtained (75%). Preparation of the Target Polymer P6(TP6) via Williamson Etherification. Potassium carbonate (0.55 g, 4 mmol), potassium iodide (small catalytic quantity), and 2-hydroxyl-3,6,7,10,11-pentakis(hexyloxy)triphenylene (TP6-OH, 1.49 g, 2 mmol) were mixed in DMF (66 mL) and stirred at 85 °C for 1.5 h under a nitrogen atmosphere to get a dark green mixture solution. Then the precursor polymer P6Br (0.221 g, 1.0 mmol) in THF (45 mL) was added dropwise into the solution; after finishing the addition, the obtained orange reactive solution was refluxed for 3 days. Then the resulted solution was concentrated by rotary evaporation to remove all of the solvents. After washing with water to eliminate inorganic salts and extracting with dichloromethane, the obtained organic solid was purified from a mixed solvent of isopropanol and dichloromethane for more than three times and finally collected by centrifugation. After

precursor polymers PmBr encapsulated in aluminum foil, small/wideangle X-ray scattering (SAXS/WAXS) diffractograms were obtained using a high-flux small-angle X-ray equipment (Anton Paar SAXSess mc2) with λ = 0.1542 nm of Cu Kα radiation. For the series polymethylenes Pm(TPn) with TP side groups, the SAXS/WAXS measurements were performed at the beamline BL16B1 of the Shanghai Synchrotron Radiation Facility (SSRF).64 Synthesis of Syndiotactic Precursor C1 Polymers and Polymethylenes with Side-Chain TP Discotic Mesogens. As provided in the Supporting Information, the Rh-catalyzed carbene polymerization directly from the TP-based diazo monomers was proved infeasible with a very low yield or even no polymer harvested. Therefore, an indirect two-step route of Rh-catalyzed C1 carbene polymerization of halogenated precursors followed with a highly efficient postfunctionalization to introduce TP side groups was employed here for preparation of a series of TP-based polymethylene SDLCPs. Scheme 1 shows the synthesis route of halogenated precursor C1 polymers PmBr with various length alkyl spacers of m = 3, 4, 6, 8, 10, 12 through Rh-catalyzed carbene C1 polymerization, and the preparation of corresponding TP-based SDLCPs of Pm(TPn) with alkyl spacers of m = 3, 4, 6, 8, 10, 12 and peripheral substituent hexyloxy (n = 6), as well as two comparative polymer samples with shorter butoxy or longer decyloxy substituents (n = 4 or 10) for comparing the effects of substituent length with the same short spacer of m = 4. Typical synthesis procedures for a representative polymer P6(TP6) with a six-methylene spacer and hexyloxy substituents are exemplifed below. The starting compound 6-bromo-1-hexanol was synthesized referring to a literature method of preparing ω-bromoalkanols in high yields from α,ω-diols reaction with HBr in toluene.65 6-Bromohexyl Bromoacetate. 6-Bromo-1-hexanol (4.6 g, 25 mmol) and potassium carbonate (17.3 g, 125 mmol) were mixed in dichloromethane (180 mL), and then bromoacetyl bromide (15 g, 75 mmol) was slowly added dropwise at 0 °C under a nitrogen atmosphere. After stirring at room temperature for 2.0 h, the reaction mixture was quenched with saturated sodium bicarbonate aqueous solution (100 mL). The solvents in the collected organic phase were removed by rotary evaporation to give a crude product; after further purification by silica-gel column chromatography with a mixed solvent eluent of dichloromethane and petroleum ether (1:1, v/v) and drying at 50 °C under vacuum for 10 h, 6.8 g of a colorless clear liquid 6bromohexyl bromoacetate was obtained (yield 90%). 6-Bromohexyl Diazoacetate. A solution of 6-bromohexyl bromoacetate (4.5 g, 15 mmol) in THF (30 mL) was added to a solution of N,N’-ditosylhydrazine (DTHZ, 10 g, 30 mmol) in THF C


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Figure 1. Typical NMR spectra comparison of a representative precursor C1 polymer P6Br and the corresponding target polymer P6(TP6) attached with side-chain TP discogens: (a) 1H NMR spectra (400 MHz) and (b) 13C NMR spectra (100 MHz) of P6Br and the corresponding P6(TP6) in CDCl3.

Table 1. Yields and Molecular Characterizations of the Precursor C1 Polymers PmBr and the Target Polymethylene SDLCPs of Pm(TPn) with Side-Chain TP Discogens samples

yield (%)

Mn (g/mol)a

PDIa

samples

yield (%)

Mn (g/mol)a

PDIa

P3Br P4Br P6Br P8Br P10Br P12Br P4(TP4)

74 68 75 67 73 72 77

74 800 51 300 55 700 28 400 36 100 28 900 21 600

1.54 2.21 2.28 2.15 2.50 2.12 1.47

P3(TP6) P4(TP6) P6(TP6) P8(TP6) P10(TP6) P12(TP6) P4(TP10)

27 75 79 78 72 81 71

9300 23 900 25 800 23 600 19 000 24 500 27 500

1.13 1.42 1.42 1.38 1.35 1.39 1.48

a

Determined from GPC with THF (or DMF for P3Br, P4Br, and P6Br) as the eluent and polystyrene (PSt) standard samples for the relative molecular weights calculation.

drying at 70 °C under vacuum for 10 h, 0.498 g of a pale yellow solid was obtained (yield 79%). The other precursor C1 polymers and target stereoregular polymethylenes with TP side groups were prepared according to the above-described similar procedures.

analysis. Figure 1 shows the typical 1H NMR and 13C NMR spectra of a representative precursor polymer P6Br with a sixmethylene spacer and the corresponding target C1 polymer P6(TP6) attached with TP6 discogens of peripheral hexyloxy substituents. The highly efficient preparation of C1 syndiotactic SDLCPs of P6(TP6) is verified by the indicative change in the 1H NMR chemical shift with the complete disappearance of proton resonance of the bromomethyl group (−CH2Br) at δ = 3.42 ppm (c′) in the precursor polymer P6Br (Figure 1a) and also the absence of carbon resonance of the bromomethyl (−CH2Br) group at about δ = 33.88 ppm (c8′) in the 13C NMR spectrum (Figure 1b), as compared with the corresponding precursor polymer P6Br. Moreover, though the proton signal of the backbone hydrogen in the 1H NMR spectra was clearly identified both for the precursor polymer P6Br (d′ in Figure 1a) and for C1 polymer with nonplanar TPE side-chain groups,63 no backbone proton signal was observed in the 1H NMR spectrum of P6(TP6) (Figure 1a), which was reasonably ascribed to the shielding effect of the closely surrounded discotic columns of the side-chain TP discogens as discussed in the following sections. Furthermore, the well-resolved featured singlet peaks at δ = 45.52 (c7, c7′) and 171.24 ppm (c1, c1′) (Figure 1b) well attributed to the backbone carbon and the proximate side-chain carbonyl group,

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RESULTS AND DISCUSSION Synthesis of C1 Syndiotactic Polymethylene SDLCPs with Side-Chain TP Discogens. Although the target C1 polymer cannot be synthesized directly via Rh-catalyzed carbene polymerization with the side-chain TP-attached diazoester monomers, we adopted an indirect two-step synthesis pathway. First, a group of precursor C1 polymers from brominated alkyl diazoacetate monomers was synthesized via Rh-catalyzed carbene polymerization, and then TP discogens were introduced as the side-chain groups of the well-synthesized precursor polymers upon grafting with monohydroxyl triphenylene derivatives TPn-OH through the highly efficient Williamson etherification reaction (Scheme 1), referring to the literature method reported by Attias and coworkers for attaching similar TP discogens onto a rigid conjugated polythiophene backbone.40,41 The composition and structure of the synthesized C1 polymers were first characterized and confirmed by NMR D


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Figure 2. DSC thermograms for the TP side-chain C1 polymers of Pm(TPn) during different running cycles with a scanning rate of 10 °C min−1.

polymers only with the exception for those of very short spacers.63 In sharp contrast, the diazoester monomers with side-chain aromatic conjugated TP discogens could not polymerize directly, whereas the TP side-chain C1 polymers with medium and long spacers were well prepared through etherification reaction of the precursor C1 polymers with bromoalkyl side groups. The distinctly different polymerization performance between side-chain TP and TPE groups, despite both being bulky groups, is reasonably attributed to the strong π-stacking among coplanar TP discs, while no π-stacking was detected in the case of twisted TPE side-chain groups.63 Thermal Properties and Phase Behaviors of C1 Syndiotactic Polymethylene SDLCPs with Side-Chain TP Discogens. The thermal properties and liquid crystalline behaviors of the precursor polymers PmBr and the corresponding TP side-chain C1 polymers Pm(TPn) were investigated by TGA, DSC, and POM. The detailed analysis of the precursor polymers PmBr is provided in the Supporting Information; all of the polymers PmBr with bromoalkyl side groups exhibited cylindrical hexagonal columnar mesophases (Ch) similar to that reported for C1 syndiotactic side-chain liquid crystalline polymers with various length alkoxy side groups.55 The TGA analysis results of a representative precursor polymer P6Br and the TP side-chain C1 polymers Pm(TPn) are provided in the Supporting Information (Figure S6). The overall trend is that the thermal decomposition temperatures of TP side-chain C1 polymers are significantly higher than those of the precursor polymers as a result of the encircling of the lateral TP aromatic macrocyclic units; the temperatures of 5% weight loss for P6Br and P6(TP6) were determined to be 312 and 378 °C and their char yields to be 6% and 24% at 700 °C, respectively. The thermal stability of the somewhat special P3(TP6) with only about 56% side-chain TP grafting ratio was between the precursor polymer and other fully grafted TP side-chain C1 polymers. The DSC thermograms of the polymethylene SDLCPs of Pm(TPn) during the first heating, the immediate first cooling, and subsequent second heating runs are presented in Figure 2. For the series C1 polymers Pm(TP6) with the same TP6 discogen of pentakishexyloxy substituents and various length alkyl spacer m

respectively, manifested unambiguously the syndiotactic structure character of thus obtained C1 polymers.50−56 It is worth noting that the indirect two-step route worked well in an almost complete quantitative Williamson etherification efficiency with higher practical separation yields after purification more than 70% for the synthesis of all TP-based SDLCPs with spacer length m ≥ 4 (Table 1), while it turned out to be extremely challenging for the preparation of C1 homologous SDLCPs with very short alkyl spacers. The etherification grafting ratio of TP discogens onto the C1 precursor polymer P3Br with a short three-methylene spacer was determined to be only around 56% from 1H NMR integral area ratio estimation (Figure S4, Supporting Information) in a quite lower yield of 27% (Table 1). Moreover, our several experimental attempts on the etherification for the precursor polymer P2Br of an even shorter two-methylene spacer failed with no TP groups grafted on. The huge difficulties encountered in the side-chain functionalization of the precursor C1 polymers with very short spacers obviously correlate closely with the steric hindrance of the bulky TP groups. Furthermore, it is quite unexpected to notice that the calculated apparent relative molecular weights of the series SDLCPs after introducing the large side-chain TP discogens were even significantly smaller than the corresponding precursor polymers as determined via GPC against PSt standards (Table 1). In addition to the significant differences in hydrodynamic characteristics between these TP side-chain C1 polymers and the standard polymers PSt samples, such underestimation by GPC measurements may be mainly caused by the remarkable contraction behavior resulted from the strong π-stacking aggregation of large conjugated aromatic TP moieties, which was already reflected significantly in their monomer solutions (Figure S5, Supporting Information). Such deviation with distinctly lower relative molecular weights due to TP aggregation shrinkage has also been observed in previously reported TP side-chain C2 polymers.29,30,42,46 Here it is also worth noting that the direct C1 carbene polymerization of the monomers with nonplanar twisted TPE side groups proceeded smoothly to produce highly efficient AIE E


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indicating the formation of an ordered organization similar to that of mesogen-jacketed liquid crystalline polymers (MJLCPs).46,66 The preparations from C1 polymers of P6(TP6) and P8(TP6) exhibited slightly altered textures upon heating or cooling, as exemplified with the texture comparison of P6(TP6) at 25, 80, and 170 °C (Figure S10), reflecting various columnar mesophases developed as defined by the two transition temperatures at around 42 and 139 °C, which will be exploited in detail and confirmed with SAXS/ WAXS analyses below. For P10(TP6) and P12(TP6) of longer spacers a distinct contrast change between strong birefringence textures and a fluidic state of full dark field was observed at around 110 °C/95 °C for P10(TP6) and 85 °C/75 °C for P12(TP6), respectively, during the heating/cooling processes, indicating the phase transition between a columnar mesophase and the isotropic state. The transition temperatures were essentially consistent with those obtained from DSC measurements (Figure 2, Table 2).

= 3, 4, 6, 8, 10, and 12, no DSC thermal transitions were detected for P3(TP6) and P4(TP6) with very short spacers due to the amorphous character for P3(TP6) with a special composition and some kind of mesogen-jacketed structure for P4(TP6) as discussed later on. For P6(TP6) and P8(TP6) with medium length spacers, two endothermic peaks during the first heating, no transition peak during the immediately following cooling process, and one endothermic peak during the second heating run were exhibited in the DSC thermograms. Then for P10(TP6) and P12(TP6) with longer spacers, besides the similar thermal behaviors as that observed for C1 polymers with medium length spacers, one exothermic peak was also displayed in the first cooling run (Figure 2); such thermal behaviors and difference are the macroscopic reflection of their various organized structures as will be discussed next. Figure 3 shows the typical POM images of some representative TP side-chain C1 polymers Pm(TPn). Under

Table 2. Thermal Properties and Phase Transition Assignments for All of the Investigated TP Side-Chain C1 Polymers of Pm(TPn) SDLCPs

transition temperature (°C) and enthalpy change (J g−1) during heating processa

P3(TP6) P4(TP6) P6(TP6) P8(TP6) P10(TP6) P12(TP6) P4(TP4) P4(TP10)

∼125 TSoftening Dh‑s ∼170 Ch > 250 decomp Dob‑s 42(4.42) Dr‑s 139(0.46) Ch > 250 decomp Dh1‑s 42(9.42) Dh2‑s 121(0.71) Ch > 250 decomp Dh1‑s 47(10.41) Dh2‑s 110(0.86) Iso Dh 40(13.74) Dhd 84(2.08) Iso Dob‑s ∼80 Dr‑s ∼200 Ch > 250 decomp Dh1‑s 42(5.08) Dh2‑s 108(0.73) Ch 230 Iso

a

Phase transition temperature and corresponding enthalpy changes obtained from DSC, POM and SAXS/WAXS. Abbreviations: Dh = discotic columnar hexagonal mesophase; Dh‑s = discotic columnar hexagonal superlattice; Dr‑s = discotic columnar rectangular superlattice; Dob‑s = discotic columnar oblique superlattice; Dhd = disordered discotic columnar hexagonal mesophase; Ch = cylindrical hexagonal columnar mesophase; Iso = isotropic state.

In comparison to the C1 polymers P4(TPn) with the same four-methylene short spacer and different length alkoxy peripheral substituents of TP discogens, P4(TP4) with shorter butoxy substituents exhibited no detectable thermal transitions in the DSC thermograms (Figure 2), while obvious birefringence was displayed in the whole temperature range from 25 °C to decomposition temperature over 250 °C by POM observation (Figure 3), similar to that of P4(TP6). As shown in Figure 2, the polymer P4(TP10) with longer decyloxy substituents presented one endothermic peak at around 42 °C, revealing intracolumnar organization change upon first heating, one exothermic peak at about 104 °C upon first cooling, and one endothermic peak at around 108 °C in the second heating run in the DSC thermograms, implying columnar order changes as demonstrated by SAXS/WAXS analyses discussed later on. The birefringent POM textures of P4(TP10) persisted from ambient temperature 25 °C to the decomposition temperature at around 230 °C, which was slightly lower than that of other homologous polymers with shorter alkoxy substituents. The detailed thermal properties and phase transition assignments based on a combination of DSC, POM, and SAXS/WAXS analyses are summarized in Table 2.

Figure 3. Representative POM images of the C1 polymers of Pm(TPn) after cooling to room temperature.

crossed polarizers, except P3(TP6), all other polymers displayed obvious birefringence, indicating the formation of organized ordered structures. For P3(TP6) with only about a 56% grafting rate of TP6 side group, no significant birefringence appeared in the whole investigated temperature range; only a softening temperature at about 125 °C was observed, indicating its amorphous nature as also revealed by X-ray scattering analysis discussed next. The melt-pressed P4(TP6) polymer films showed significant birefringence in the whole temperature range investigated from room temperature 25 °C to the near decomposition temperature beyond 250 °C, F


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Figure 4. Variable-temperature synchrotron radiation SAXS/WAXS diffractograms during the first heating process of (a) P4(TP6) and (b) P6(TP6).

Figure 5. Schematic illustration of various columnar mesophases and supercylindrical structure with variant number TP columns surrounding the backbone evolving versus the spacer length of the C1-type polymethylene SDLCPs of Pm(TPn) with multicolumn-bundle-based stacking mode: (a) P4(TP6), P4(TP10) at 25 °C, Dh‑s (p3m1); (b) P6(TP6), P4(TP4) at 25 °C, Dob‑s (p2); (c) P6(TP6), P4(TP4) at 80 °C, Dr‑s (p2mm); (d) P8(TP6), P10(TP6) at 25 °C, Dh‑s (p6mm); (e) P12(TP6) at 25 °C, Dh (p6mm); (f) Pm(TPn) (m = 4, 6, 8; n = 4, 6, 10) at 200 °C, Ch (p6mm).

Columnar Structural Evolution and Sub-10 nm Liquid Crystalline Orders Regulated by Varied Alkyl Spacer

Lengths for the C1 Syndiotactic Polymethylene SDLCPs. For probing into the structural evolution and subG


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as shown in Figure 5a with a reasonable calculated density of 1.06 g cm−3 (Table S5). With the increase of temperature the π-stacking peak in the WAXS region became weaker until disappearing at about 170 °C, indicating complete disassociation of TP columnar stacking; the clearing point temperature was estimated to be above 250 °C combining with DSC analysis and POM observation. It is interesting to see that the peaks at q = 3.49 and 6.04 nm−1 assigned as (110) and (300) for the Dh‑s hexagonal superlattice got sharper and stronger as the temperature increased from room temperature to around 150 °C (Figure 4a), which was ascribed to the superimposed [10] and [11] signals (with associated d = 18 and 10.4 Å) of TP column stacking in a subhexagonal lattice. It is worth noting that here within the three-column-bundle structure instead of being randomly located the three TP columns around the main chain are in an inverted triangle orientation arrangement as similarly observed in TP side-chain polyacrylates with shorter spacers,46,48 so the discotic columnar hexagonal superlattice Dh‑s belongs to the p3m1 space group rather than the usual p6mm (Figure 5a). Upon heating to 170 °C, with the side-chain TP stacking column disassociated, the compact polymer backbone is surrounded by the attached freely distributed TP discs in a whole to act as a supramolecular cylinder, which further constructed into a cylindrical hexagonal structure Ch (p6mm) with a remarkably sharpened (100) peak (Figures 4a and 5f). P6(TP6). As shown in Figure 4b, with two-methylene lengthened spacers, the SAXS/WAXS diffractograms of the polymer P6(TP6) revealed quite different structural organizations below 200 °C as compared to the homologue P4(TP6). At around ambient temperature the P6(TP6) selforganized into a discotic columnar oblique superlattice Dob‑s (p2) with lattice parameters of a = 44.8 Å, b = 38.7 Å, and β = 83.0°, adopting a four-TP-column bundle superstructure around the polymer backbone with a calculated density of 0.96 g/cm3 (Figure 5b). With temperature increasing to 80 °C, the scattering peaks significantly changed in the SAXS region and the π-stacking peak became weaker in the WAXS region (Figure 4b); a discotic columnar rectangular superlattice Dr‑s (p2mm) was well assigned with lattice parameters of a = 41.3 Å, b = 34.4 Å, and β = 90° in a modified four-TP-column bundle superstructure through a thermal-induced local rearrangement (Figure 5c). Meanwhile, the two peaks at q = 3.38 and 5.87 nm−1 with associated d = 18.6 and 10.7 Å, respectively, for (210) and (320) were exactly in the ratio of

10 nm liquid crystalline columnar orders regulated by varied length alkyl spacers, both variable-temperature SAXS and WAXS analyses were performed for the LC mesophases and in other interested temperature region. For the precursor polymers PmBr encapsuled in aluminum foil, SAXS/WAXS diffractograms were simultaneously carried out with a high-flux small-angle X-ray equipment (Anton Paar SAXSess mc2); all of the polymers PmBr constructed into simple cylindrical hexagonal columnar mesophases (Ch) with gradually increasing lattice parameters a = 13.2, 14.5, 17.0, 18.4, 21.1, 21.7 Å, respectively, for those with various length alkyl spacers of m = 3, 4, 6, 8, 10, and 12 (Supporting Information, Figure S9, Table S4), consistent with that reported for C1 polymers with various length alkoxy side groups.55 For the series TP sidechain C1 syndiotactic polymethylenes of Pm(TPn), the SAXS/ WAXS measurements were conducted at beamline BL16B1 of SSRF;64 the interesting and inspiring ordered structures evolved with the variation of the spacer length are presented and discussed as follows. P3(TP6). The variable-temperature SAXS/WAXS diffractograms of P3(TP6) are provided in the Supporting Information (Figure S11). One can clearly see that before entering the totally isotropic state beyond the softening temperature of 125 °C, the C1 polymer of P3(TP6) with the shortest threemethylene spacer possessed only two quite broad peaks at around q = 3.5 and 2.0 nm−1, reflecting the average sizes of TP discs and the contour profile of the polymer chain as a whole, respectively. Such results manifested the amorphous nature of the C1 polymer of P3(TP6), which was not unexpected when considering the shortest spacer and especially the random copolymer character due to the low-efficiency grafting of only about 56%. P4(TP6). Figure 4a shows the variable-temperature synchrotron radiation SAXS/WAXS diffractograms of P4(TP6) with the associated structure assignments provided in the Supporting Information (Table S5). At room temperature, a broad halo was centered at around q = 14.25 nm−1 (d = 4.41 Å) featuring the average distance of alkyl chains and an obvious shoulder peak at q = 17.60 nm−1 (d = 3.57 Å) reflecting the π-stacking of side-chain TP discs exhibited in the WAXS region. Combined with the characteristic peaks in the SAXS region, a discotic columnar hexagonal superlattice Dh‑s (p3m1) induced by the TP discotic columns surrounding the polymer backbone adopting a three-TP-column bundle structure with a lattice parameter of a = 36 Å was proposed H


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mesophase Ch as displayed by other homologous SDLCPs with shorter spacers. P12(TP6). With a two more methylene elongated spacer after P10(TP6), P12(TP6) showed a quite different and simplified SAXS/WAXS diffractogram (Supporting Information, Figure S12 and Table S9). Through a longer and more flexible spacer binding to the main chain the SAXS signals at around 1.4−2.0 nm−1 observed in those C1 polymethylene SDLCPs with shorter spacers, reflecting a certain degree of ordered organization of the bound polymer backbone, disappeared in the SAXS diffractograms of P12(TP6). At lower temperatures, P12(TP6) only showed one main peak of 3.65 nm−1 and one small peak of 7.30 nm−1 in the small-angle region together with a π-stacking peak of 18.16 nm−1 in the wide-angle region; in combination with the POM observation and DSC analysis, it was assigned as a discotic columnar hexagonal mesophase Dh (p6mm) packed only by the sidechain TP discogens with a lattice parameter a = 19.8 Å (Figure 5e), which was reduced by more than one-half in size, as compared with the lattice parameters of the homologues with shorter spacers in their various discotic columnar superlattices with the whole polymer chain closely surrounded by the TP columns as a supercylindrical building block. When heating above the first thermal transition temperature of 40 °C as revealed by DSC analysis, it changed into a disordered discotic columnar hexagonal phase Dhd, with almost disappearing πstacking signal, while an even significantly sharpened SAXS peak at 3.65 nm−1 was observed (Figure S12), which was ascribed to the softening of the peripheral alkyl chains, the diminishing or dissociation of π-stacking, and the significant weakening of the backbone constraint due to introduction of long spacers. Then upon the temperature exceeding the second DSC transition at around 84 °C, the polymer P12(TP6) entered the isotropic phase showing only broad halos, confirming its lower clearing point temperature. P4(TP4). For the comparative C1 polymer P4(TP4) with the same four-methylene short spacer but shorter butoxy peripheral substituents as compared to that of P4(TP6), it exhibited quite different and fascinating columnar mesophases with varied temperatures. Figure 7a shows the variabletemperature SAXS/WAXS diffractograms of P4(TP4) with the diffraction data and detailed phase assignments provided in the Supporting Information (Table S10). It is interesting to note that though the P4(TP4) behaved more like a MJLCP without detectable transition signals in the DSC thermal

1:√3, manifesting the superposition of [10] and [11] of an ordered hexagonal columnar lattice by the side-chain TP discogen stacking columns on the discotic columnar rectangular mesophase Dr‑s (p2mm). Upon heating to a higher temperature of 200 °C a cylindrical hexagonal mesophase Ch (p6mm) was formed similar to that of P4(TP6) (Figure 5f); the structure assignments at different temperatures are given in the Supporting Information (Table S6). P8(TP6) and P10(TP6). Figure 6 shows the variabletemperature synchrotron radiation SAXS/WAXS diffractograms of P8(TP6) and P10(TP6), with the diffraction data and detailed assignments are provided in the Supporting Information (Tables S7 and S8). For P8(TP6) at a lower temperature a discotic columnar hexagonal superlattice Dh1‑s (p6mm) was formed, adopting a six-TP-column bundle superstructure with lattice parameters of a = 48 Å (Figure 5d). Upon heating to 80 °C above the temperature corresponding to the lower transition temperature in the DSC first heating scan, the π-stacking peak became weaker in the wide-angle region and the peaks in the small-angle region were almost unchanged but slightly broadened; thus, another slightly changed discotic columnar hexagonal superlattice Dh2‑s (p6mm) was assigned. Upon heating to 200 °C beyond the second transition temperature revealed by the DSC analysis, the π-stacking peak disappeared completely while showing a very sharp (100) peak, indicating the disassociated individual TP discs surrounded the compact polymer backbone to construct into a cylindrical hexagonal lattice Ch (p6mm) as for the homologous polymers P4(TP6) and P6(TP6) with shorter spacers at high temperatures (Figure 5f). The structure changes with temperature of P10(TP6) are comparable to that of P8(TP6) (Figure 6b, Table 2); a discotic columnar hexagonal superlattice Dh1‑s(p6mm) was also established with the lattice parameter of a = 49.3 Å at lower temperature in a six-TP-column bundle superstructure organization (Figure 5d). With the elongation of the twomethylene spacer P10(TP6) exhibited a more obvious πstacking peak and a significant increase in the (120) peak at around 3.90 nm−1 (Figure 6b), reflecting an enhanced sidechain TP-column organization less restrained by the polymer backbone. Moreover, with the temperature increasing, P10(TP6) underwent a phase transition from Dh1‑s to Dh2‑s at around the lower transition temperature determined by DSC and then changed directly into the isotropic fluidic phase at about 110 °C with the absence of the cylindrical hexagonal I


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Figure 8. (a) WAXS diffractograms for Pm(TP6) at 25 °C with elongated spacer length. (b) Plot of the discotic columnar structure disassociation temperatures for C1-type polymethylene and C2-type polyacrylate SDLCPs versus the spacer length, wherein the data in empty diamonds and connected via a dashed line for the C2-type polyacrylates are adapted from the previous report for comparison.46

same TP6 discogens, except P3(TP6) which special in composition with a lower grafting degree exhibiting amorphous state, P4(TP6) presented a discotic columnar hexagonal superlattice Dh‑s (p3m1), adopting a three-columnbundle structure. With the spacer length increasing, P6(TP6) self-organized into a discotic columnar oblique superlattice Dob‑s (p2) in a four-column-bundle structure and converted into a discotic columnar rectangular superlattice Dr‑s (p2mm) through a thermal-induced local rearrangement. Then both P8(TP6) and P10(TP6) self-organized into a discotic columnar hexagonal superlattice Dh‑s (p6mm) in a sixcolumn-bundle organization. With further lengthened spacer, P12(TP6) was constructed into a simple discotic columnar hexagonal mesophase Dh (p6mm) dominated by the stacking of the side-chain TP discogens. In addition, for the comparative C1 polymers of P4(TP4) and P4(TP10) with the same four-methylene short spacer and variant length alkoxy substituents, P4(TP4) formed superlattices of Dob‑s (p2) and Dr‑s (p2mm) in a four-column-bundle structure comparable to the ordered superstructures of P6(TP6), both of which share the common character with equal length in the alkyl spacer and peripheral substituents. P4(TP10) self-organized into a discotic columnar hexagonal superlattice Dh‑s (p3m1) with a three-column-bundle structure, comparable to that of P4(TP6) though with longer decyloxy substituents. Moreover, for all of the C1-type polymethylene SDLCPs of Pm(TPn) with shorter to medium spacers of m = 4, 6, 8, a cylindrical hexagonal mesophase Ch (p6mm) persistent to higher temperatures developed by the helical polymethylene backbone closely surrounded by the tethered TP discs in a whole to act as a supramolecular cylinder (Figure 5f). Varied π-Stacking Spacings and Discotic Columnar Structure Disassociation Temperatures with the Lengthened Alkyl Spacers. Figure 8a shows the WAXS diffractograms for the series C1 polymers of Pm(TP6) at 25 °C and their change trend with the elongated spacer length. It is noteworthy that the π-stacking shoulder peaks of TP discs appeared at about 17.5−18.2 nm−1 in the WAXS regions, which slowly moved to higher q values, revealing that the spacings between TP discogens decreased gradually from 3.57 Å for P4(TP6) to 3.46 Å for P12(TP6) with the increase of the spacer length (Supporting Information, Tables S5−S9). Also, an obvious shoulder peak at around 15.4 nm−1 developed and gradually increased when the spacer length is larger than eight

analyses, as revealed by the synchrotron radiation SAXS/ WAXS investigations, actually they displayed similar rich columnar mesophases as that of P6(TP6) with accordingly higher transition temperatures, where there is a common feature with the same length of the spacer and the peripheral alkoxy substituents. At low temperatures, a discotic columnar oblique superlattice Dob‑s (p2) was formed with lattice parameters of a = 43.9 Å, b = 35.1 Å, and β = 84°, adopting a four-TP-column bundle superstructure. With the temperature increasing to above about 80 °C, another discotic columnar rectangular superlattice Dr‑s (p2mm) was constructed with lattice parameters of a = 35.5 Å, b = 30.6 Å, and β = 90° in a slightly rearranged four-TP-column bundle superstructure (Figure 5c). Then it changed into a cylindrical hexagonal columnar mesophase Ch (p6mm) of a = 36.0 Å at a higher temperature of about 200 °C until decomposition far beyond 250 °C (Table 2, Table S10). P4(TP10). As shown in Figure 7b, with longer decyloxy peripheral substituents, the variable-temperature SAXS/WAXS diffractograms of P4(TP10) displayed columnar mesophases more comparable to that of P4(TP6). At low temperatures, a discotic columnar hexgonal superlattice Dh1‑s (p3m1) was constructed with a lattice parameter of a = 40.5 Å, adopting a three-TP-column bundle superstructure (Figure 5a). With the temperature increasing, a slightly altered discotic columnar hexgonal superlattice Dh2‑s developed. When the temperature was raised above 108 °C, for instance, at 200 °C, with complete disassociation of TP π-stacking, the P4(TP10) polymer chain as a whole was constructed into a cylindrical hexagonal columnar mesophase Ch (p6mm) of a = 40.1 Å (Figure 5f, Table 2, Table S11) until the isotropic temperature of 230 °C as determined by POM observation. Multicolumn-Bundle Stacking Mode Evolution with Increased Spacer Length for the Series C1-Type TP Side-Group Polymethylene SDLCPs. On the basis of the foregoing analyses and comparative discussions it can be concluded that the TP-based C1 syndiotactic polymethylene SDLCPs adopted the similar multi-TP-column bundles-based stacking mode and supercylindrical structure evolution with the elongated spacers as that demonstrated in the TP sidechain polyacrylate C2 polymers;46 however, the columnar mesophase specifics and the evolution pace with the increased spacer length were significantly different as illustrated in Figure 5. Among the investigated series C1 polymethylenes with the J


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without any alkyl spacers the MJLCPs with directly attached TP discogens exhibited only a hexagonal columnar phase due to the strong coupling effect between the TP moieties and the MJLCP main chain.35 Also, a hierarchical nanostructure of a hexagonal columnar phase of the whole polymer chain together with a ND phase associated with the side-chain TP moieties formed in a hydrogen-bonded supramolecular complex system; even a smectic A phase developed with a more rigid side-chain terphenyl core of the MJLCP.36 Therefore, on one hand, the TP-based C1 syndiotactic polymethylene SDLCPs exhibited enhanced hierarchical ordered columnar mesophases with significantly expanded temperature ranges especially for those possessing a hightemperature Ch mesophase with shorter to medium length spacers, as compared with the side-chain TP polyacrylate C2 polymers.46 On the other hand, they shared the characteristic of broadened columnar mesophases persistent to much higher temperatures with the MJLCPs attached with side-chain TP discogens,33−36 and supplemented with an additional advantage of ordered columnar mesophases with the side-chain TP groups organized into ordered columns through π-stacking rather than the ND phase of lowest symmetry as observed for the MJLCPs with side-chain TP discogens.33,34 In summary, the C1-type polymethylene SDLCPs integrated the advantages of both well-defined TP side-chain polyacrylates and MJLCPs, exhibiting ordered columnar mesophases with remarkably widened temperature ranges persistent to very high temperatures. All of the hierarchical structure character and enhanced ordered columnar phases of the C1-type polymethylene SDLCPs may be well attributed to the fact that the backbone in a helical conformation is oriented in a favorable direction consistent with the side-chain columns, and the significantly rigid main chain closely surrounded by high densely substituted TP side groups helps to keep the ordered columnar mesophases with remarkably enhanced thermal stability.

methylenes, indicating partial crystallization of the longer alkyl spacer, which manifested an enhanced decoupling between the side-chain TP discogens and the stiff helical backbone. Moreover, as shown in Figure 8b, it is very interesting to note that the discotic columnar structure disassociation temperatures decreased with elongation of the spacer lengths, and cylindrical hexagonal columnar mesophases were formed at higher temperatures after disassociation of the π-stacking of TP discs from lower temperature discotic columnar superlattices for the C1-type SDLCPs with shorter to medium spacers, in contrast to their turning directly into the isotropic state for those with longer 10- or 12-methylene spacers, which corroborated the weakened coupling interactions between side-chain TP discogens and the polymer backbone for the series C1 polymers of Pm(TP6) with increased spacer length. More importantly, when compared with those TP side-chain polyacrylate C2 polymers of the same spacer length,42,46,48 the TP-based C1-type polymethylene SDLCPs exhibited at least 40 °C higher of the TP π-stacking discotic columnar structure disassociation temperature (Tdisa.), and possessed much broader temperature ranges of columnar mesophases especially for those with shorter to medium spacers additionally showing a Ch mesophase persistent to higher temperatures until the isotropic transition or decomposition beyond the various ordered columnar superlattices (Figure 8b, Table 2), which was mainly attributed to the rather rigid backbone in helical conformation. The helical backbone conformations are common for various high densely substituted polymers;57−61,66,67 in particular, the helical structure characteristics of the syndiotactic C1 polymethylenes substituted with functionalized side groups have been well corroborated by Tokita et al.54−56 Although there were no reports of C1 polymers with sidechain discogens in the literature for comparison, the phase behaviors of mesogen-jacketed liquid-crystalline polymers (MJLCPs) with TP side groups provided some helpful references to the C1 polymer systems discussed here. A group of MJLCPs with side-chain TP discogens were synthesized via the conventional free radical polymerization, wherein every two TP discogens were attached to one repeating unit of the poly(vinyl terephthalate) main chain through various length alkyl spacers.33−36 It is worth noting that the MJLCPs with one side-chain TP discogen indirectly connected to each backbone carbon atom; thus, at this point it is comparable to our C1 polymers, while they are essentially a kind of polystyrene-derivatized C2 polymer with an extra benzene ring or other rigid aromatic core inserted between the backbone and TP discs. Both side-chain TP-based MJLCPs with three- or six-methylene spacers formed rectangular columnar phases at relatively high temperatures, while at low temperatures, TP side groups formed a discotic nematic phase (ND) in conjunction with the rectangular columnar phase generated by the rod-like supramolecular mesogenthe MJLCP chain as a whole, and also a hexagonal columnar mesophase developed at high temperatures for the one with six methylene spacers.33 The MJLCPs with TP discogens attached through a longer 12-methylene spacer produced a simple hexagonal columnar phase at low temperatures self-organized by side-chain TP discogens with a smaller lattice parameter a = 2.06 nm. After a re-entrant isotropic phase at medium temperatures, a high-temperature columnar nematic phase with a larger dimension of a′ = 4.07 nm developed by the MJLCP chain as a whole supramolecular mesogen.34 Then

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CONCLUSIONS In summary, a series of high densely substituted C1 syndiotactic TP-based polymethylene SDLCPs of Pm(TPn) with different length alkyl spacers of m = 3, 4, 6, 8, 10, 12 and peripheral alkoxy substituents (n = 6, 4, 10) have been synthesized through an indirect two-step Rh-catalyzed C1 carbene polymerization route. The chemical structures of the precursor C1 polymers and polymethylene SDLCPs were fully characterized and confirmed by 1H and 13C NMR, FTIR, and other spectroscopic techniques. Then the thermal properties and ordered organization structures were systematically investigated by DSC and POM, especially with variabletemperature synchrotron radiation SAXS/WAXS analyses. For the series C1 polymers with the same TP6 discogens, P4(TP6) exhibited a discotic columnar hexagonal superlattice Dh‑s (p3m1) in a three-column-bundle structure with a lattice parameter a = 3.60 nm. P6(TP6) self-organized into a discotic columnar oblique superlattice Dob‑s (p2) or a rectangular superlattice Dr‑s (p2mm) in a four-column-bundle structure, and both P8(TP6) and P10(TP6) constructed into a discotic columnar hexagonal superlattice Dh‑s (p6mm) in a six-columnbundle structure. With the longest 12-methylene spacer P12(TP6) produced a simple discotic columnar hexagonal mesophase Dh (p6mm) based on the stacking of the side-chain TP discogens almost unrestricted by the main chain with an obviously diminished lattice parameter a = 1.98 nm. Then for the comparative C1 polymers of P4(TP4) and P4(TP10) with K


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the same four-methylene spacer, P4(TP4) with the same length of alkyl spacer and peripheral substituents formed a discotic columnar oblique superlattice Dob‑s (p2) in a fourcolumn-bundle structure comparable to the superstructure of P6(TP6). Also, P4(TP10) constructed into a discotic columnar hexagonal superlattice Dh‑s (p3m1) in a threecolumn-bundle structure similar to that of P4(TP6) but with a slightly increased lattice parameter a = 4.05 nm. It is very interesting to note that the TP-based C1 syndiotactic polymethylene SDLCPs combined the advantages of both the well-defined polyacrylates and MJLCPs with side-chain TP discogens. Therefore, this work provides a feasible route to prepare the C1-type SDLCPs with high densely substituted functional side groups and may offer an in-depth understanding for the hierarchical organization of ordered columnar structures with remarkably increased thermal stability and hopefully improved optoelectronic properties of DLC polymer semiconducting materials for various applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b01433.

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Attempts on direct carbene polymerization route, variable-temperature SAXS/WAXS diffractograms and assignments, thermograms of DSC and TGA, POM images, GPC traces, synthesis details, FTIR, and 1H NMR and 13C NMR spectra (PDF).

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Bin Mu: 0000-0002-6604-276X Feng Liu: 0000-0001-6224-5167 Dongzhong Chen: 0000-0001-7303-4533 Author Contributions §

X.L. and B.M. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21574062, 21875098, 21805228, and 21761132033) and also partially by the Program for Changjiang Scholars and Innovative Research Team in University (IRT_16R38) and the Fundamental Research Funds for the Central Universities and the Scientific Research Foundation of Graduate School of Nanjing University (Grant 2017ZDL06). We thank Beamline BL16B1 at SSRF (Shanghai Synchrotron Radiation Facility, China) for providing the beamtime.

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REFERENCES

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Significantly Enhanced Thermotropic Liquid Crystalline

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