Crosslinked poly(methyl methacrylate) with perfluorocyclobutyl aryl ether moiety as crosslinking unit: thermally stable polymer with high glass transition temperature

Crosslinked poly(methyl methacrylate) (PMMA) with high glass transition temperature (Tg) and thermal decomposition temperature was prepared by simple thermal crosslinking of PMMA-containing random copolymers bearing aryl trifluorovinyl ether (TFVE) moieties. A methacrylate monomer consisting of aryl TFVE moiety, 4-((1,2,2-trifluorovinyl)oxy)phenyl methacrylate (TFVOPMA), was first synthesized followed by radical copolymerization with methyl methacrylate (MMA) initiated by AIBN, providing the random copolymer containing aryl TFVE moieties, poly(4-((1,2,2-trifluorovinyl)oxy)phenyl methacrylate)-co-poly(methyl methacrylate) (PTFVOPMA-co-PMMA). Finally, crosslinked PMMA polymer with perfluorocyclobutyl (PFCB) aryl ether moieties as crosslinking units was obtained by [2π + 2π] cycloaddition reaction of aryl TFVE moieties in PTFVOPMA-co-PMMA copolymer. Thermal properties of both PTFVOPMA-co-PMMA and crosslinked PTFVOPMA-co-PMMA were examined by TGA and DSC. Compared to pure PMMA, Tg of PTFVOPMA-co-PMMA increased by 15.1 °C and no Tg was found in the DCS test of the crosslinked PTFVOPMA-co-PMMA. Thermal decomposition temperature (Td,5%) of crosslinked PMMA was 47 °C higher than that of pure PMMA. Furthermore, the water absorption of crosslinked PMMA film greatly reduced in comparison with that of pure PMMA.


Introduction
Poly(methyl methacrylate) (PMMA) is a kind of transparent polymeric material possessing diverse excellent properties such as superior light transmittance, light weight, chemical stability, weathering corrosion resistance, electrical insulation and good processability, which make PMMA widely applied in many elds including aerospace, building construction and optical instruments. [1][2][3][4] However, the glass transition temperature (T g , 100 C) and heat-deformation temperature of PMMA are relatively low so as to limit its applications. 5,6 In order to improve T g and the thermal stability of PMMA, investigators have explored various methods, 3,7-15 including the copolymerization of methyl methacrylate (MMA) with comonomers bearing rigid or bulky groups to overcome the miscibility puzzle, or the formation of a threedimensional network structure by the addition of crosslinking agent. Wilkie et al. prepared MMA-DVB crosslinked polymers using divinylbenzene (DVB) as crosslinking agents and the degradation temperature (T d,5% ) of resulting polymers increased from 162 C to 294 C. 12 Kuo et al. suggested an approach to raise the T g of PMMA through copolymerization with methacrylamide (MAAM) since hydrogen-bonding interactions exist between these two monomer segments. [16][17][18][19] Fluoropolymers have attracted much curiosity in material science due to their unique properties including thermal stability, chemical resistance, ame retardancy, superior electrical insulating ability, low dielectric constant and refractive index, and unique surface property. [20][21][22] To avoid low processability of peruoropolymers, partially-uorinated polymers like poly(vinylidene uoride) (PVDF), 4,5 alternating ethylene-tetrauoroethylene (ETFE) 23 and alternating ethylene-chlorotriuoroethylene (ECTFE) copolymers, 24,25 have been prepared and exhibited improved processability. Among partially-uorinated polymers, peruorocyclobutyl (PFCB) aryl ether polymers [26][27][28][29][30][31] are an intriguing class of uoropolymers, which not only retain the general outstanding properties of uoropolymers originating from the low polarity, strong electronegativity and small van der Waals radius of uorine atom and strong C-F bond, but also possess many other advantages such as improved processability and optical transparency, structural versatility and tunable properties. These PFCB aryl ether-based polymers have been studied on the potential application in photonics, polymer light-emitting diodes, proton exchange membrane in fuel cell and atomic oxide resistance materials on space-cra etc. [32][33][34][35][36][37][38][39][40] Inspired by the fact that PFCB aryl ether-based polymers possess high heat resistance and optical transparency, Prof. Huang of Shanghai Institute of Organic Chemistry et al. prepared a series of PFCB-containing copolymers with high heat resistance. [41][42][43] The resulting polymethacrylate bearing sulfonyl functionality exhibits excellent thermal stability (T d > 300 C) and polyimide (PFCBBPPI) containing PFCB biphenyl ether moieties shows excellent thermal stability and high transparency. PFCB aryl ether polymers are most commonly prepared through a thermal [2p + 2p] cycloaddition of aryl tri-uorovinyl ethers (TFVE) at temperatures of between 150 C and 200 C without additional agent and no condensation byproducts are produced. Thus, TFVE group may act as a crosslinkable group and the crosslinked region, peruorocyclobutyl (PFCB) moieties, may give rise to better chemical resistance. Lee et al. synthesized a novel uorinated aromatic polyether monomer containing TFVE group and the resulting crosslinked polymers aer thermal crosslinking showed higher T g (239-271 C). 44 All aforementioned discussions intrigued us to synthesize methacrylate monomer containing TFVE group, which can be copolymerized with MMA to provide polymethacrylate copolymers containing TFVE groups so that the subsequent thermal cycloaddition of TFVE groups can produce crosslinked polymethacrylate copolymers with PFCB aryl ether moieties as the crosslinked region. The resulting crosslinked polymethacrylate copolymers may have higher thermal stability and good optical transparency because of the below points: (1) PFCB aryl ether-based polymers exhibit higher heat resistance and good transparency, (2) PFCB aryl ether moieties with large volume may decrease the activity of PMMA main chain, which might increase the heat resistance of resultant polymers, (3) three-dimensional network structure may limit the mobility and rotation ability of polymeric chain, which benets heat resistance of resultant polymers. Herein, we report the preparation of transparent crosslinked poly(methyl methacrylate) (PMMA) copolymers with high T g and high T d via copolymerization of MMA with a methacrylate monomer containing TFVE moiety, 4-((1,2,2-triuorovinyl)oxy)phenyl methacrylate (TFVOPMA), followed by thermal crosslinking process as shown in Scheme 1. These resultant PMMA-based copolymers containing TFVE or PFCB aryl ether groups were investigated in detail by NMR, FT-IR, differential scanning calorimetry (DSC) and thermogravimetry analysis (TGA).
Measurements 1 H, 13 C and 19 F NMR spectra of intermediates were recorded on a JEOL resonance ECZ 400S spectrometer (400 MHz) in CDCl 3 . Tetramethylsilane (TMS) and CDCl 3 were used as internal standards for 1 H and 13 C NMR, respectively; CF 3 CO 2 H was used as an external standard for 19 F NMR. FT-IR spectra were recorded on a Nicolet AVATAR-360 FT-IR spectrophotometer with a resolution of 4 cm À1 . Number-average molecular weights (M n ) and molecular weight distributions (M w /M n ) were obtained on a conventional gel permeation chromatography (GPC) system equipped with a Waters 515 Isocratic HPLC pump, a Waters 2414 refractive index detector, and a set of Waters Styragel columns (HR3 (500-30 000), HR4 (5000-600 000) and HR5 (50 000-4 000 000), 7.8 Â 300 mm, particle size: 5 mm). GPC measurement was carried out at 35 C using tetrahydrofuran (THF) as eluent with a ow rate of 1.0 mL min À1 . The system was calibrated with linear polystyrene standards. Differential scanning calorimetry (DSC) was performed on a TA Q200 DSC instrument in N 2 with a heating rate of 10 C min À1 . Thermogravimetry analysis (TGA) was conducted on a TA Discovery TGA 55 thermal analysis system in N 2 with a heating rate of 10 C min À1 . UV/vis spectra were acquired on a Hitachi U-2910 spectrophotometer. (Scheme 2). 1-Methoxy-4-(2-bromo-1,1,2,2-tetrauoroethyl) benzene was rstly prepared by uoroalkylation reaction of 4methoxyphenol. To a 500 mL dried three-neck round-bottom ask tted with a condenser and a constant pressure dropping funnel, 4-methoxyphenol (20.00 g, 0.16 mol) and caesium carbonate (33.60 g, 0.19 mol) were added followed by evacuating and backlling with N 2 three times. Next, DMSO (200 mL) was added via a gastight syringe and the mixture was stirred for 30 min. 1,2-Dibromotetrauoroethane (22.60 mL, 0.19 mol) was then dropped slowly, keeping the temperature below 35 C. The resulting mixture was heated at 50 C for 5 h. Finally, 500 mL of deionized water was added and the organic layer was separated and extracted by dichloromethane, washed by water and brine and then dried over magnesium sulfate (MgSO 4 ). 1-Methoxy-4-(2-bromo-1,1,2,2-tetrauoroethyl)benzene (40.69 g, 84%) was obtained by silica column chromatography using hexane as eluent. 1  In a pre-dried ask, 1-methoxy-4-(2-bromo-1,1,2,2-tetrauoroethyl)benzene (3.88 g, 13 mmol) was dissolved in 300 mL of anhydrous dichloromethane. The solution was cooled to 0 C followed by adding dichloromethane solution of BBr 3 (25 mL, 25 mmol) dropwise within 30 min. The mixture was slowly warmed up to room temperature and stirred at room temperature for another 12 h. The resulting mixture was ltrated and the residual salt was removed by washing with dichloromethane and ltrating three times. The solvents were removed by rotary evaporation, and the residue was subjected to column chromatography (eluent: hexane) to afford 4-(2-bromo-1,1,2,2-tetrauoroethoxy)phenol as a colorless liquid (3.15 g, 84% yield). 1  In a pre-dried ask, 4-(2-bromo-1,1,2,2-tetrauoroethoxy) phenol (3.70 g, 13 mmol) and triethylamine (1.49 mL, 0.015 mol) were dissolved in 300 mL of 2-butanone. The solution was cooled to 0 C followed by adding the solution of methacryl chloride (1.48 mL, 15 mmol) in 10 mL of 2-butanone dropwise. The mixture was warmed up to room temperature and stirred for another 1 h. The salt was removed by ltration and the ltrate was washed with water and dried over MgSO 4 . The organic layer was ltrated and evaporated under reduced pressure, and then the residue was puried by ash column chromatography (eluent: hexane) on silica gel to give 3.79 g (82%) of 4-(2-bromo-1,1,2,2-tetrauoroethoxy)phenyl methacrylate as a colorless oil. Newly activated zinc (1.98 g, 0.035 mol) was added to a dried three-neck round-bottom ask followed by evacuating and backlling with N 2 three times. Next, dry acetonitrile (40 mL) was added and the oil temperature was lied to 90 C. 4-(2-Bromo-1,1,2,2-tetrauoroethoxy)phenyl methacrylate (15.86 g, 29 mmol) was dropped slowly into the above mixture, keeping the mixture boiling slightly. The mixture was reuxed for 10 h and aer cooled to room temperature, the mixture was ltered. The salt was washed by dichloromethane to extract the product. The organic phase was collected and dried over MgSO 4 . Aer CH 2 Cl 2 was evaporated, the crude product was puried by ash column chromatography (eluent: hexane) to afford 4-((1,2,2-triuorovinyl) oxy)phenyl methacrylate (TFVOPMA) as a colorless oil (3.98 g, 53%). 1  Copolymerization of TFVOPMA and MMA AIBN (17.8 mg, 0.108 mmol) was rstly added to a 25 mL Schlenk ask (ame-dried under vacuum prior to use) sealed with a rubber septum for degassing. Next, TFVOPMA (1.28 g, 5 mmol), MMA (0.50 g, 5 mmol) and 2-butanone (3.4 mL) were introduced via a gastight syringe. The solution was degassed by three cycles of freezing-pumping-thawing followed by immersing the ask into an oil bath preset at 60 C to start the polymerization. The polymerization was terminated by putting the ask into liquid nitrogen aer 3 h. Aer repeated purication by dissolving in 2butanone and precipitating in ethanol three times, 1.39 g (78%) of white powder, poly(4-((1,2,2-triuorovinyl)oxy)phenyl methacrylate)-co-poly(methyl methacrylate) (PTFVOPMA-co-PMMA), was obtained aer drying in vacuo at 30 C. GPC: M n ¼ 50 900 g mol À1 , M w /M n ¼ 1.84. 1  Crosslinking of PTFVOPMA-co-PMMA PTFVOPMA-co-PMMA copolymer was dissolved in ethyl acetate and the solution was then spin-cast onto the freshly cleaned This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 1981-1988 | 1983 glass substrate followed by heat treatment at 30 C for 30 min, 40 C for 30 min and 65 C for 1 h. The obtained thin lm was placed into a beaker and then the beaker was placed into a tube furnace, followed by evacuating and backlling with N 2 three times. The temperature of tube furnace was raised to 100 C with a rate of 5 C min À1 . Aer keeping at 100 C for 30 min, the temperature was raised to 150 C with a rate of 5 C min À1 . Aer keeping at 150 C for 30 min, the temperature was raised to 180 C with a rate of 5 C min À1 . Aer reacting at 180 C for 10 h, the furnace was cooled and crosslinked PTFVOPMA-co-PMMA copolymer lm was obtained. FT-IR (KBr): n (cm À1 ) : 2998, 2950,  1732, 1501, 1488, 1452, 1276, 1199, 1150, 969, 842, 807, 752.

Water absorption of crosslinked PMMA-based copolymer
The water uptake experiments were carried out as follows: sample lms (PMMA and crosslinked PTFVOPMA-co-PMMA) were dried in vacuo at 55 C for 24 h and a constant weight (AE0.0001 g) can be obtained for the lm (about 0.5 g). The lms were immersed in ultra-pure water at 25 C. At certain interval, the lms were then taken out of water and the water absorbed on the surface was wiped off by a cleaning cloth. The weight of the lms was weighed immediately.
The chemical structure of TFVOPMA monomer was characterized by FT-IR, 1 H NMR and 19 F NMR. The characteristic peaks of double bond are found to be located at 5.75 ('b') and 6.31 ('c') ppm in 1 H NMR spectrum of TFVOPMA (Fig. 1A). The proton resonance signal of methyl linked to the double bond appears at 2.03 ppm ('a'), while the resonance signals at 7.17 ('d') and 7.23 ppm ('e') belong to phenyl protons. In 19 F NMR spectrum of TFVOPMA (Fig. 1B), characteristic peaks of -OCF] CF 2 group appear at À120.3, À127.8 and À135.3 ppm, respectively. And the appearance of the peak at 1832 cm À1 in FT-IR spectrum of TFVOPMA ( Fig. 2A) further conrms the presence of -OCF]CF 2 group. The sharp peak located at 1740 cm À1 was attributed to stretching vibration of carbonyl, while the peak at 1640 cm À1 belongs to the stretching vibration of double bond. The absorption at 812 cm À1 indicates para-substitution of benzene ring. All these results conrm the successful synthesis of TFVOPMA, the methacrylate monomer with TFVE moiety.
Preparation and crosslinking of PTFVOPMA-co-PMMA copolymer Compared to living/controlled polymerization, conventional free radical polymerization allows achieving higher molecular weights and the related industrial technology is mature. Therefore, free radical copolymerization of MMA and TFVOPMA was conducted in 2-butanone at 60 C using AIBN as initiator to provide the corresponding random copolymer, poly(4-((1,2,2-triuorovinyl)oxy)phenyl methacrylate)-co-poly(methyl methacrylate) (PTFVOPMA-co-PMMA).
The resulting copolymer was characterized by GPC, FT-IR, 1 H NMR and 19 F NMR. GPC retention curve of PTFVOPMA-co-PMMA shows a unimodal elution peak with a relatively broad molecular weight distribution of 1.84 and the molecular weight of PTFVOPMA-co-PMMA is about 50 900 g mol À1 (Fig. 3). In the FT-IR spectrum of PTFVOPMA-co-PMMA (Fig. 2B), the typical signal of double bond at 1640 cm À1 disappeared aer polymerization and the signal of TFVE was located at 1834 cm À1 , which indicated that TFVE group was not affected during the copolymerization of TFVOPMA and MMA. Other typical signals such as 1731 (C]O, symmetric stretch), 1499 (phenyl ring, stretch), 812 and 838 (Ar-H deformation) are visible and the stretching vibration of -CH 3 (2998 cm À1 ) and -CH 2 (2947 and 2845 cm À1 ) become stronger, which also illustrate the preparation of PTFVOPMA-co-PMMA copolymer.  Fig. 3A). Moreover, the existence of TFVE group appeared at À119.2, À126.1 and À134.0 ppm in 19 F NMR spectrum (Fig. 4B) further conrmed the preservation of TFVE groups in PTFVOPMA-co-PMMA copolymer. All the above evidence veried that TFVE group did not affect the radical copolymerization of TFVOPMA and MMA and the PMMA-based copolymer containing TFVE aryl ether moieties has been successfully prepared.

Preparation and properties of crosslinked PMMA-based copolymer
Triuorovinyl ether groups (-OCF]CF 2 ) can form per-uorocyclobutane units via [2p + 2p] cycloaddition reaction at high temperature, thus we rstly monitored this cross-linking reaction by DSC. As we can see from Fig. 5, T g of PTFVOPMAco-PMMA was about 137.6 C in round 1, higher than that of PMMA (122.5 C). We supposed that the introduction of rigid benzene ring with large volume makes rotational hindrance of polymeric chain difficult, which leads to higher T g of PTFVOPMA-co-PMMA. Interestingly, T g of PTFVOPMA-co-PMMA increased with the scan times and the values of T g reached 169.7 C in the sixth scan. The phenomenon demonstrated that [2p + 2p] cycloaddition reaction of triuorovinyl ether groups in PTFVOPMA-co-PMMA copolymer occured at higher temperature during the DSC measurement and the formed PFCB crosslinking unit further limited the mobility of polymer chains, endowing the polymer with higher T g .
Next, we conducted thermal crosslinking process of PTFVOPMA-co-PMMA copolymer. Firstly, thin lm of PTFVOPMA-co-PMMA was obtained by dissolving PTFVOPMAco-PMMA into ethyl acetate and spin-casting the solution onto the freshly cleaned glass substrate followed by heat treatment at 30 C for 30 min, 40 C for 30 min and 65 C for 1 h. Then, this lm was performed in a tube furnace under N 2 at 180 C for 10 h, affording a transparent lm with a pale yellow color. From FT-IR spectrum of PTFVOPMA-co-PMMA aer thermal crosslinking (Fig. 6C), the peak at 1836 cm À1 corresponding to TFVE group disappeared and a new peak at 969 cm À1 corresponding to PFCB ring was found, which indicated the formation of PFCB aryl ether moieties in the copolymer by thermal cycloaddition of aryl TFVE groups. Other typical characteristic peaks can also be seen in FT-IR spectrum aer crosslinking (Fig. 6C).
As it can be seen from Fig. 5, no T g was found on the DSC curve of crosslinked PTFVOPMA-co-PMMA below 250 C, that is,   This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 1981-1988 | 1985 the resultant crosslinked polymer is in glass state below 250 C, which demonstrated that the formed network with PFCB aryl ether moieties as crosslinking segment largely improved the heat resistance of the obtained PMMA-based copolymer. Thermal degradation of PTFVOPMA-co-PMMA before and aer crosslinking was investigated by TGA as shown in Fig. 7. The 5% weight loss temperatures (T d,5% ) for PMMA, PTFVOPMA-co-PMMA and crosslinked PTFVOPMA-co-PMMA are 254 C, 250 C and 301 C, respectively; while the 10% weight loss temperatures (T d,10% ) are 269 C, 276 C and 326 C, respectively. PTFVOPMA-co-PMMA containing TFVE groups shows lower thermal degradation temperature at rst stage before T d,6% (257.6 C) as compared to that of PMMA, which may be attributed to the polarity and soness of TFVE group. However, PTFVOPMA-co-PMMA containing TFVE groups shows higher T d at late stage (aer T d,6% ¼ 257.6 C), which may be attributed to the formation of enough PFCB rings at higher temperature. As we can see, crosslinked PTFVOPMA-co-PMMA with PFCB aryl ether moieties as crosslinked units shows higher thermal stability compared to PMMA and PTFVOPMA-co-PMMA. This can be explained that the formation of PFCB ring aer thermal treatment decreases the activities of PMMA main chain because of its large volume, and three-dimensional network structure limits the mobility and rotation ability of polymer chain, which may improve the thermal stability of the resultant PMMA-based copolymer.
The light transmittance of PTFVOPMA-co-PMMA and crosslinked PTFVOPMA-co-PMMA lms were investigated by UV/vis spectroscopy. As shown in Fig. 8, the transmittance of PTFVOPMA-co-PMMA lm are 74% at 500 nm and 80% at 600 nm, and the transmittance of the crosslinked lm decreased to 62.5% at 600 nm.
Among the unique properties of uorinated polymers, low water absorption is a typical one. The moisture resistance was evaluated by measuring the weight change of PMMA and crosslinked PTFVOPMA-co-PMMA lms aer immersing into pure water for a certain time (Fig. 9). It is noted that the water    absorption of crosslinked PTFVOPMA-co-PMMA lm greatly decreased in comparison with pure PMMA (0.03% vs. 1.73% aer 8 h). Furthermore, the water absorption of crosslinked PTFVOPMA-co-PMMA lm retained below 0.06%, while that of PMMA increased with the immersing time. The decreasing water absorption could be due to the highly hydrophobic nature of aryl PFCB moieties.

Conclusions
In summary, a methacrylate monomer containing aryl TFVE group, TFVOPMA, was synthesized and copolymerized with MMA to provide the copolymer containing aryl TFVE groups, PTFVOPMA-co-PMMA, which was treated at 180 C to give crosslinked network PTFVOPMA-co-PMMA copolymer via the formation of PFCB moieties. The glass transition temperature of crosslinked network PTFVOPMA-co-PMMA copolymer is high and they are thermally stable with less water absorption.

Conflicts of interest
There are no conicts to declare.