High-Glass-Transition-Temperature Hydrocarbon Polymers Produced

Sep 21, 2017 - A series of prepolymers were treated with CF3SO3H to cationically cyclize adjacent diene–styrene/diene–diene units into main-chain ...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/Macromolecules

High-Glass-Transition-Temperature Hydrocarbon Polymers Produced through Cationic Cyclization of Diene Polymers with Various Microstructures Yang Cai, Jianmin Lu,* Gaifeng Jing, Wantai Yang, and Bingyong Han* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China S Supporting Information *

ABSTRACT: A series of prepolymers were treated with CF3SO3H to cationically cyclize adjacent diene−styrene/ diene−diene units into main-chain cyclic units. Diene− diphenylethylene alternating copolymers synthesized using anionic copolymerization were first cyclized completely without intermolecular linking reactions. The cyclization produced much higher Tg values (approximately 200 °C) than those of cyclized random diene−styrene copolymers (approximately 130 °C). To eliminate the number of spacer carbons between cyclic repeating units, poly(1-phenyl-1,3butadiene) (PPB) was used as a prepolymer in cationic cyclization. The Tg increased substantially from 200 to 271 °C as the 3,4structure content increased from 8% to 80%. The cyclized PPB with high 3,4-structure content is the highest Tg hydrocarbon polymer yet reported. All results indicated that the backbone rigidity of the cyclic hydrocarbon polymer can be drastically increased by reducing the number of spacer carbons and increasing carbon sharing between cyclic repeating units. adamantylstyrene) (Tg = 234 °C),5 which can effectively increase the Tg of the parent polymer; and (3) the introduction of cyclic structures in the main chain to improve the rigidity of the backbone. To date, methods that have been developed for the introduction of cyclic structures in the main chain can be classified as follows: (i) the polymerization of cyclic hydrocarbon monomers often followed by hydrogenation reactions, such as hydrogenated poly(β-pinene) (Tg = 130 °C),6 polybenzofulvene (PBF, Tg = 199 °C),7 and hydrogenated poly(1,3-cyclohexadiene) (Tg = 231 °C);8 and (ii) the polymerization of commercially available noncyclic hydrocarbon monomers followed by simple postpolymerization modifications. Most cyclic monomers are rare and expensive, in contrast to commercially available noncyclic hydrocarbon monomers. Thus, another promising route for the fabrication of hightemperature hydrocarbon polymers would be the postpolymerization modification of general polymers, which could benefit both academic research and industrial application. One of the suitable methods would be the cationic cyclization of unsaturated hydrocarbon polymers under acidic conditions. This method is more simple and practical than the hydrogenation of cycloolefin polymers. Recently, Kamigaito et al. obtained thermoplastics with high optical and thermal performance and a high Tg (approximately 130 °C) through the anionic random copolymerization of styrene and diene derivatives followed by well-controlled cyclization.9−11 Additionally, Lal

1. INTRODUCTION In the plastics market, high-temperature thermoplastics are the most specialized and significantly growing area. The major areas in which high-temperature thermoplastics are applied are the medical market and the automotive, aerospace, and electronics industries.1 High glass transition temperature (Tg) or high melting temperature (Tm) and high decomposition temperature are the most crucial thermal properties of high-temperature thermoplastics. High-temperature thermoplastics can be divided into two groups. One group contains heteroatoms in the polymer chain; for example, polysulfones such as poly(phenyl sulfone) (Tg = 288 °C),1 poly(aryl ether ketone)s such as poly(ether ether ketone) (Tm = 370 °C),1 polyimides such as “Kapton” (Tg = 410 °C), and polyamides such as “Kevlar” (Tm > 500 °C).2 Most high-temperature thermoplastics belong to this group. The other group contains simpler non-heteroatom-containing hydrocarbon polymers. High-temperature hydrocarbon polymers have attracted increasing attention because they have high service temperatures. In addition, hydrocarbon polymers lacking polar heteroatoms can have a low dielectric constant and nonhygroscopicity and frequently exhibit favorable transparency, which make them attractive materials, especially in optoelectronic fields. A number of studies in recent years have attempted to prepare high-temperature hydrocarbon polymers. The strategies used involved the following: (1) the enhancement of the tacticity of polymer chains such as the highly isotactic polypropylene (PP, Tm = 160 °C);3 (2) the incorporation of bulky pendant groups in the side chain such as PS derivatives with bulky pendant groups, for example, poly(α-methylstyrene) (Tg = 173 °C)4 or poly(p© XXXX American Chemical Society

Received: May 22, 2017 Revised: August 25, 2017

A

DOI: 10.1021/acs.macromol.7b01075 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Table 1. Copolymers of Butadiene/Isoprene and Diphenylethylene Prepared Using Anionic Polymerizationa microstructure in dieneb (%) entry

copolymers

FDPEb (mol %)

1 2 3 4 5 6 7e 8e

a-PBD-1 a-PBD-2 a-PID a-PBTs-1 a-PBTs-2 a-PITs r-SBR r-SIR

48.2 47.4 45.7

FTsb (mol %)

FIpb (mol %)

51.8 52.6 54.3 47.9 48.5 48.2

48.0 48.0

FBdb (mol %)

52.1 51.5 51.8 52.0 52.0

1,4

1,2

76 90 91 90 69 93 76 67

24 10 ∼0 10 31 ∼0 24 1

3,4

9

7 32

Mnc (g/mol)

Mw/Mnc

Tg (°C)

Td5d (°C)

19400 27200 23000 24800 28900 18000 113400 112400

1.74 1.85 1.11 1.51 1.76 1.70 1.04 1.04

105 109 92 104 103 117

300 317 310 334 340 321

a

Anionic copolymerization was performed using n-butyllithium in a dry nitrogen atmosphere for 24 h; the conditions of these copolymerizations are listed in Supporting Information (Table S1). bMolar ratio of monomers in the copolymer calculated from the 1H NMR spectrum. The microstructure in diene was also determined using the 1H NMR spectrum. cDetermined using conventional GPC against PSt standards. dThe 5% weight-loss temperature (Td5) was determined using TGA under N2 gas flow. eThe data for entries 7 and 8 were reported by Masami Kamigaito (rSBR/r-SIR: random St−Bd/Ip copolymer, St content = 48.0 mol %).10

2. EXPERIMENTAL SECTION

synthesized a new class of amorphous thermoplastic elastomers by selectively cyclizing polyisoprene blocks in polyisoprene− polybutadiene−polyisoprene; the cyclized polyisoprene block had a Tg that was approximately 15 °C higher than that of the PS block.12 This postpolymerization modification has been used for diene polymers such as natural rubber,13,14 synthetic polyisoprene,15,16 cis-1,4-polybutadiene,17,18 and their copolymers with styrene.19,20 The cyclized products have been utilized in adhesives, inks, paints, photoresists, and tires.21 The key factors in the described method for obtaining high-Tg amorphous hydrocarbon polymers are the degree of cyclization (CC consumption can reach 100%) and the structural integrity of the cyclized product (no spacer carbons22 between cyclic repeating units), which both depend on the microstructure of the precursor polymer used for cationic cyclization. The high Tg of the polymers is due to rigid cyclic repeating units in the backbone that are formed through intramolecular cyclization between the remaining double bonds in the main chain and/or the pendants. However, the Ip−Ip or Bd−Bd units and St−St units in random St-Ip (r-SIR) or random St-Bd (r-SBR) copolymer largely worsen the thermal properties of their cyclized products, which may also result in incomplete cyclization and some intermolecular reaction. Therefore, the control of the monomer sequence distribution in the copolymer for cationic cyclization is critical if intramolecular cyclization reactions along the chain are to be favored to generate linear polymers with rigid cyclic repeating units. Inspired by previous studies, we used a series of diene− diphenylethylene alternating copolymers as precursor polymers for cationic cyclization. The Tg obtained was approximately 200 °C, substantially higher than that obtained for cyclized random styrene−diene copolymers. Furthermore, to eliminate the spacer carbons between the cyclic repeating units in the backbone of the cyclized products, we employed poly(1-phenyl-1,3-butadiene) (PPB) as a prepolymer, and this drastically increased the Tg of the cyclized products to 271 °C. To our best knowledge, this is the highest Tg of a hydrocarbon polymer yet reported. In this study, the cyclization of the aforementioned prepolymers with various microstructures was investigated in detail, and the thermal behavior was characterized. Additionally, the relationship between Tg and cyclized polymer structure was discussed.

2.1. Materials. The following chemicals were used as received: transcinnamaldehyde (99%, Acros), methyltriphenylphosphonium iodide (98%, Alfa Aesar), potassium tert-butoxide (t-BuOK, >98%, Acros), nbutyllithium (1.6 M solution in hexanes, InnoCHEM), trifluoromethanesulfonic acid (CF3SO3H, >98%, Alfa Aesar), tin(IV) chloride (SnCl4, 99%, anhydrous, Acros), nickel(II) 2,4-pentanedionate (Ni(acac)2, 95%, Alfa Aesar), methylaluminoxane (MAO, with 10% in toluene, Aldrich), sodium carbonate (Na2CO3, Beijing Chem. Works), calcium hydride (CaH2, Beijing Chem. Works), n-hexane (>95%, Beijing Chem. Works), ethanol (>99.7%, Beijing Chem. Works), and 1,3-butadiene (Bd, polymer grade; Yanshan Petrochemical, Beijing, China). 1,1-Diphenylethylene (DPE, 98%, Acros) was purified by the dropwise addition of sec-butyllithium until a red color persisted; it was then freshly distilled prior to use. trans-Stilbene (Ts, 96%, Acros) was recrystallized three times from methanol and twice from n-hexane. Isoprene (Ip, >99%, Aladdin) was dried over CaH2 for 48 h and distilled under dry nitrogen. The following chemicals were all purchased from Beijing Chem. Works and purified after they were received: toluene, benzene, dichloromethane, ethylene dichloride (EDC), and cyclohexane, which were all dried over CaH2 for 48 h and distilled under dry nitrogen. Tetrahydrofuran (THF) was distilled from sodium naphthalene after it had been dried over CaH2 for 48 h. 1-Phenyl-1,3-butadiene (1-PB) was prepared according to a known procedure through a Wittig reaction between trans-cinnamaldehyde and methyltriphenylphosphonium iodide in the presence of t-BuOK; purification was performed using column chromatography to obtain the monomer as a transparent oil in an 89% yield.23 The monomer was characterized using 1H and 13C NMR; please refer to the Supporting Information. 2.2. Synthesis of a-PBD, a-PID, a-PBTs, and a-PITs. Anionic copolymerization was performed under dry nitrogen in baked threenecked flasks. A typical example procedure for the anionic copolymerization of poly(butadiene-alt-1,1-diphenylethylene) (aPBD) is explained. A solution of Bd (0.545 g, 10.09 mmol) in THF was added to a solution of DPE (2.0 g, 11.10 mmol) in THF. The mixture was stirred, and then n-butyllithium (0.2414 mmol) was added to initiate the copolymerization at 0 °C, which was allowed to occur for 24 h. The polymerization was then quenched with degassed ethanol, and the polymerization mixture was poured into a large amount of ethanol. The precipitated solid was filtered and dried in vacuum overnight. We obtained a-PBD-1 (entry 1 in Table 1) as a white solid. DPE/Ip, Ts/Bd, and Ts/Ip alternating copolymerization was performed similarly. The yields of DPE-based copolymers were all higher than 50%, and those of Ts-based copolymers were higher than 70%. No 1,2-structure was discovered in poly(isoprene-alt-1,1-diphenylethylene) (a-PID) or poly(isoprene-alt-trans-stilbene) (a-PITs) copolymers, as evidenced B

DOI: 10.1021/acs.macromol.7b01075 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Scheme 1. Anionic Copolymerization of Butadiene/Isoprene and Diphenylethylene and the Subsequent Cationic Cyclization of the Copolymer

by the absence of transmittances at 990 and 910 cm−1 in the corresponding infrared (IR) spectrum (Figure S1). 2.3. Synthesis of PPB. Anionic polymerization was performed under dry nitrogen in baked three-necked flasks. A typical example, 1-PB anionic polymerization, is explained. The polymerization was initiated by adding n-butyllithium (0.2 mmol) to a solution of 1-PB (2.0 mL, 15.2 mmol) in benzene (20 mL). The mixture was stirred for 5 h at 50 °C. The polymerization was then quenched with degassed ethanol, and the polymerized mixture was poured into a large amount of ethanol. The precipitated solid was filtered and dried under vacuum overnight. PPB8 (entry 1 in Table 3) was obtained as a white solid. Coordination polymerizations were performed under dry nitrogen in baked three-necked flasks by sequentially introducing toluene, MAO, and 1-PB. Polymerizations were started by introducing Ni(acac)2 dissolved in toluene; they were quenched with degassed ethanol, and the polymerization mixture was then poured into excess acidified ethanol, washed several times with ethanol, and dried under vacuum overnight. Cationic polymerizations were performed under dry nitrogen in baked three-necked flasks by sequentially introducing 1-PB, EDC, and SnCl4. Polymerizations were quenched by adding ethanol that contained a small amount of aqueous ammonia solution. The precipitated solid was filtered and dried under vacuum overnight. 2.4. Cyclization Procedures of Alternating Copolymers and PPB. All cyclizations were performed under dry nitrogen in baked threenecked flasks. As an example, CF3SO3H (0.68 mmol, 12 mL) was added to PPB solution containing PPB8 (0.6 g) and cyclohexane (12 mL) at 50 °C. The mixture was stirred for 4 h, and the reaction was then terminated with 1% aqueous solution of Na2CO3 (5 mL). The mixture was washed with water three times, dropped into ethanol, centrifuged, and then dried under vacuum overnight. The cyclized product of PPB8 was obtained as an amber-colored powder (entry 1 in Table 3). The cyclizations of a-PBD, a-PID, poly(butadiene-alt-trans-stilbene) (aPBTs), and a-PITs copolymers were performed similarly. 2.5. Measurements. 1H and 13C NMR spectra were obtained at 25 °C using a Bruker ARX400 (400 MHz) spectrometer with CDCl3 and tetramethylsilane as the solvent and internal reference, respectively. Polymer molar masses were determined through gel permeation chromatography (GPC) at 30 °C by using THF as the eluant on a Waters apparatus equipped with a 515 HPLC pump, a 2410 refractive index detector, and three μ-Styragel columns (HT3 + HT4 + HT5) at an elution rate of 1.0 mL/min. Linear polystyrene standards were used for calibration. The absolute weight-average molecular weight (Mw) of the polymers was determined by a chromatographic system (Wyatt Technology) equipped with a refractive index (RI, Optilab rEX), four-

bridge capillary viscometer (Wyatt ViscoStar) and DAWN HELEOS II multiangle laser light scattering (MALLS, laser wavelength was 658 nm) triple detectors at 35 °C. THF was used as the solvent. The Tg of each polymer was determined using differential scanning calorimetry (DSC) (Pyris 1, PerkinElmer). For the PPB polymers, the samples were heated to 200 °C at 10 °C/min and subsequently cooled to 25 °C at 10 °C/min, followed by a heating cycle from 25 to 200 °C at the same rate. For the C-PPB (cyclized PPB) polymers, the samples were heated to 300 °C at 10 °C/min, then cooled from 300 to 25 °C at 10 °C/min, and subsequently heated from 25 to 300 °C at the same rate. All Tg values were obtained from the second scan and thus after the removal of thermal history. The thermal stability of the polymers was determined using thermogravimetric analysis (TGA; Mettler Toledo TGA Thermal Analysis). Samples were heated from 30 to 800 °C at a heating rate of 10 °C/min in a nitrogen atmosphere. The IR spectra of the polymers were obtained using a Thermo Fisher Nicolet 6700 Fourier transform infrared spectrometer in the spectral range 400−4000 cm−1. All samples were dispersed in potassium bromide by grinding.

3. RESULTS AND DISCUSSION 3.1. Cationic Cyclization of the Alternating Copolymer of Butadiene/Isoprene and 1,1-Dipenylethylene/transStilbene. During the cyclization of styrene−diene copolymers, the formed cationic carbon preferentially attacks the ortho position of the adjacent benzene ring of the styrene unit through intramolecular Friedel−Crafts alkylation, which results in a tetrahydronaphthyl bicyclic unit in the backbone chain.9−11 The reactivity ratios for St and Ip or Bd were not close to 0, indicating that the formation of an alternating copolymer was difficult. Ip− Ip or Bd−Bd units and St−St units exist in random SIR or SBR copolymers, which may result in incomplete cyclization and some intermolecular reaction. The ideal precursor polymers for cationic cyclization are alternating copolymers. Therefore, we selected diphenylethylene instead of styrene to copolymerize with diene, which cannot homopolymerize because of steric hindrance. The anionic copolymerization of diphenylethylene and Ip or Bd has previously been demonstrated to result in alternating copolymers.24−27 The Ip−Ip or Bd−Bd unit content in these alternating copolymers is much lower than that in styrene−diene copolymers through the control of the monomer C

DOI: 10.1021/acs.macromol.7b01075 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

copolymers, which resulted in a more twisted main chain (Scheme 1). In addition, no higher Mn products were formed, which indicates that no intermolecular linking occurred, unlike for r-SIR and r-SBR.9−11 This difference was predominately engendered by the well-controlled alternating sequence and the high ratio of phenyl to alkene units in diene−diphenylethylene copolymers, which can efficiently suppress intermolecular reaction and favor intramolecular cyclization between the C C bond and the adjacent phenyl group. Furthermore, additional pendant phenyl groups can suppress intermolecular reaction because of steric hindrance. Figures 2 and 3 show the 1H NMR spectra of the alternating copolymers and their cyclized products. The signals (d and i) at 3.8−5.7 ppm, which were assigned to the olefinic protons, disappeared completely after cyclization, implying 100% cyclization. By comparison, the cyclization degrees of r-SIR and r-SBR rarely reach 100% because of their random comonomer distribution.9−11 In addition, the peaks corresponding to aromatic protons (b′) at 6.0−7.5 ppm became broader due to the formation of tetrahydronaphthyl bicyclic units. These results indicate that cyclization most probably proceeded between the CC double bond of the diene unit and the benzene ring of the adjacent diphenylethylene unit via the Friedel−Crafts reaction. The thermal properties of the alternating copolymers and their completely cyclized products were evaluated using DSC (DSC curves, see Figure S6) and TGA. The Tg and 5% weight-loss temperature (Td5) are listed in Tables 1 and 2. The Tg values of the alternating copolymers were drastically increased after 100% cyclization (entries 1−6). Furthermore, they were approximately 70 °C higher than those of the cyclized r-SIR and r-SBR (entries 7 and 8). These differences resulted from the alternating comonomer sequence distribution, which favored intramolecular cyclization reactions and the introduction of more pendant phenyl groups along the main chains. The content of Bd−Bd (Ip−Ip) units in these alternating copolymers is much lower than that in r-SBR/r-SIR. The Bd−Bd (Ip−Ip) units in these copolymers could decrease the Tg values of their cyclized products due to more spacer carbons generated in the backbone. Therefore, the Tg values of these cyclized alternating copolymers were higher than that of cyclized r-SIR/r-SBR. The introduction of pendant phenyl groups was also helpful to increase the Tg vuales. In addition, the Tg values of completely cyclized a-PITs increased linearly with Ts content increased. It indicated the effect of alternating comonomer sequence and pendant phenyl groups on the Tg vaules of the cyclized products (Figures S7 and S8). The thermal stability of these alternating copolymers was also improved after complete cyclization. The Td5 values of the DPE-based alternating copolymers increased from 300 to 400 °C, whereas those of the Ts-based alternating copolymers increased from 320 to 360 °C. This difference was probably due to the thermoreversible fracture C−C bonds in the backbone of the completely cyclized Ts-based alternating copolymers. This C−C bond between the tetrahydronaphthyl bicyclic unit and the spacer carbon connected to the phenyl group was easily fractured when heated because of its low bond energy.28 3.2. Synthesis and Cationic Cyclization of PPB. The comonomer sequences of the synthesized alternating prepolymers were highly but not perfectly alternating. More importantly, spacer carbons were still present between the cyclic repeating units after cyclization, even for perfectly alternating prepolymers, which can weaken the rigidity of the backbone of the cyclized polymers. Thus, some ideal prepolymers, which have

feed ratio. Additionally, no DPE−DPE or Ts−Ts units are present in these alternating copolymers. A series of diene− diphenylethylene alternating copolymers were prepared: a-PBD, a-PID, a-PBTs, and a-PITs (Table 1). The copolymer composition and the microstructure of the diene component of the a-PBD were determined from 1H NMR spectroscopy according to the previous study.27 The copolymer composition of other alternating copolymers (a-PID, a-PBTs, and a-PITs) was calculated similar to that of a-PBD. The 13C NMR spectra of these alternating copolymers have been shown in the Supporting Information (Figures S2−S5). The diphenylethylene content in these alternating copolymers (entries 1−6) was approximately equal to the styrene content in r-SIR or r-SBR (entries 7 and 8). Different than most living anionic copolymerization of DPE and Ip/Bd reported,24,25,27 the anionic copolymerization we performed was not with standard high-vacuum techniques. In addition, side reactions that occurred during anionic copolymerization of Ts and Ip/Bd have been demonstrated previously.26 Therefore, the dispersity values were high in our work. The cationic cyclization of the obtained alternating copolymers was then examined using CF3SO3H in cyclohexane at 25 °C to produce cyclized copolymers (Scheme 1). All cyclized products were amber-colored powders and easily soluble in THF (entries 1−6 in Table 2). Typical examples of the GPC Table 2. 100% Cationic Cyclization of Copolymers of Butadiene/Isoprene and Diphenylethylenea entry

copolymers (prepolymers)

Mnb (g/mol)

Mw/Mnb

Tg (°C)

Td5c (°C)

1 2 3 4 5 6 7d 8d

a-PBD-1 a-PBD-2 a-PID a-PBTs-1 a-PBTs-2 a-PITs r-SBR r-SIR

15700 22900 14500 6100 7000 11400 71800 76500

1.82 1.89 1.45 1.94 2.11 1.91 2.22 1.55

204 200 196 188 183 202 134 129

396 400 404 352 356 346

a Cyclization was performed using CF3SO3H in cyclohexane for 4 h at room temperature; [prepolymer]0 = 6.0 wt %; [CF3SO3H]0 = 19 mM; 100% cyclization was determined from the olefinic proton ratio in the 1 H NMR spectrum. bDetermined using conventional GPC against PSt standards. cThe 5% weight-loss temperature (Td5) was determined using TGA under N2 gas flow. dThe data for entries 7 and 8 were reported by Masami Kamigaito (entry 7: the cyclization of r-SBR was 100% with 41% intermolecular linking; entry 8: the cyclization of r-SIR was 92% with 19% intermolecular linking).10

curves obtained for a-PBD, a-PID, a-PBTs, and a-PITs before and after cyclization are presented in Figure 1. For all these copolymers, the curve peak shifted to a lower molecular weight, and the molecular weight distributions were broader after the cyclization because of a reduction in hydrodynamic volume; the absolute molecular weight did not decrease during the cyclization, as has been demonstrated by previous studies.9−11 The molecular weight determined by conventional GPC was based on the hydrodynamic volume of polymer. The reduced hydrodynamic volume after the cyclization resulted in inaccurate values for the molecular weight by this measure method. The reduction in the relative molecular weight of the cyclized Tsbased alternating copolymers was higher than that of the DPEbased alternating copolymers, and this was because of the phenyl group in the spacer carbon in the Ts-based alternating D

DOI: 10.1021/acs.macromol.7b01075 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 1. GPC traces for intramolecular Friedel−Crafts cationic cyclization of (A) a-PBD (entry 1 in Table 1), (B) a-PID (entry 3), (C) a-PBTs (entry 4), and (D) a-PITs (entry 6) with CF3SO3H in cyclohexane at 25 °C: [copolymer]0 = 6.0 wt %; [CF3SO3H]0 = 19 mM.

and cationic polymerization to prepare PPB prepolymers with high 3,4-structure content (1H NMR spectra in Figure S10), which cannot be obtained using anionic polymerization. A typical 1 H NMR spectrum of PPB is presented in Figure 4A. No 1,2 units were formed, as evidenced by the absence of absorption at 990 and 910 cm−1.31 The CC consumption kinetics of PPB at different CF3SO3H concentrations was studied to determine how 100% cyclization could be achieved. The CC consumption of PPB rapidly increased with the reaction time and then reached a steady-state value. The cyclization rate strongly increased as the CF3SO3H concentration increased. The CC consumption did not reach 100% at a low CF3SO3H concentration (