Insight into the Intermolecular Interaction and Free Radical Polymerizability of Methacrylates in Supercritical Carbon Dioxide

High pressure in situ Fourier transfer infrared/near infrared technology (HP FTIR/NIR) along with theoretical calculation of density functional theory (DFT) method was employed. The solvation behaviors and the free radical homopolymerization of methyl methacrylate (MMA), methacrylate acid (MAA), trifluoromethyl methacrylate (MTFMA) and trifluoromethyl methacrylate acid (TFMAA) in scCO2 were systematically investigated. Interestingly, the previously proposed mechanism of intermolecular-interaction dynamically-induced solvation effect (IDISE) of monomer in scCO2 is expected to be well verified/corroborated in view that the predicted solubility order of the monomers in scCO2 via DFT calculation is ideally consistent with that observed via HP FTIR/NIR. It is shown that MMA and MAA can be easily polymerized, while the free radical polymerizability of MTFMA is considerably poor and TFMAA cannot be polymerized via the free radical initiators. The α trifluoromethyl group (–CF3) may effectively enhance the intermolecular hydrogen bonding and restrain the diffusion of the monomer in scCO2. More importantly, the strong electron-withdrawing inductive effect of –CF3 to C=C may distinctly decrease the atomic charge of the carbon atom in the methylene (=CH2). These two factors are believed to be predominantly responsible for the significant decline of the free radical polymerizability of MTFMA and the other alkyl 2-trifluoromethacrylates in scCO2.


Introduction
Owing to the unique structure and superior performances, methacrylate-based monomers and polymers have attracted much attentions [1][2][3], especially for the fluorinated ones. For example, it was reported that fluorinated methacrylate polymers have excellent surface properties, impressive optical performances [4,5] and thus are extensively applied in the fields of ice-phobic coatings, functional thin film, medical materials, optical devices/fiber and nanocomposites [6][7][8][9][10]. Among the Scheme 1. Structure of model monomers used.

Apparatus
High pressure in situ FTIR/NIR experiments were performed using a PerkinElmer Spectrum 400 FTIR/NIR spectrometer supported by Spectrum Software (V 6.3.5, PerkinElmer, Bucks, UK) for data acquisition and processing. The spectrometer was intentionally modified either by replacing the standard sample accessories with a set of specially designed fiber sensor in attenuated total reflection (ATR) mode as was described in our previous work [21,29] or by integrating with a 50.0 mL stainless steel view cell to construct a high-pressure trans FTNIR in situ monitoring system [22]. The view cell was equipped with two sapphire windows, facilitating the in-line FTNIR spectroscopic monitoring of the phase behaviors during the polymerization process. ATR-FTIR spectra were recorded over a wavenumber range of 4000-600 cm −1 for the solvation behaviors of the methacrylate monomers in scCO2. The transmission FTNIR spectra were recorded over a wavenumber range of 7000-5500 cm −1 for the polymerization of every methacrylate in scCO2. Twenty scans were taken in every FTIR/FTNIR spectrum with a resolution of 2 cm −1 .

Solvation Process and Phase Behaviors of the Monomers in CO2
The solvation process of monomers in gaseous and supercritical CO2 was monitored by using the high-pressure in situ ATR-FTIR monitoring system, following the similar procedure in our previous work [21,29]. The view cell was heated to the target temperature (60 °C) and then purged with N2 and degassed by a vacuum pump alternately for at least three times to eliminate the possible residuals in the system, then charged with a certain amount of the monomer (10 mL) via a special syringe and stirred at a speed of around 300 rpm. CO2 was introduced into the view cell in a stepwise fashion to solvate the previously added monomer. The FTIR spectra of the monomer were collected throughout the whole solvation process from 0 to 38.00 MPa at specified pressure intervals while stirring continued. At every selected pressure, the system was stirred for about five minutes until two identical FTIR spectra were obtained. The phase behaviors of the monomer + CO2 system were directly observed through the sapphire windows during the solvation process.

Apparatus
High pressure in situ FTIR/NIR experiments were performed using a PerkinElmer Spectrum 400 FTIR/NIR spectrometer supported by Spectrum Software (V 6.3.5, PerkinElmer, Bucks, UK) for data acquisition and processing. The spectrometer was intentionally modified either by replacing the standard sample accessories with a set of specially designed fiber sensor in attenuated total reflection (ATR) mode as was described in our previous work [21,29] or by integrating with a 50.0 mL stainless steel view cell to construct a high-pressure trans FTNIR in situ monitoring system [22]. The view cell was equipped with two sapphire windows, facilitating the in-line FTNIR spectroscopic monitoring of the phase behaviors during the polymerization process. ATR-FTIR spectra were recorded over a wavenumber range of 4000-600 cm −1 for the solvation behaviors of the methacrylate monomers in scCO 2 . The transmission FTNIR spectra were recorded over a wavenumber range of 7000-5500 cm −1 for the polymerization of every methacrylate in scCO 2 . Twenty scans were taken in every FTIR/FTNIR spectrum with a resolution of 2 cm −1 .

Solvation Process and Phase Behaviors of the Monomers in CO 2
The solvation process of monomers in gaseous and supercritical CO 2 was monitored by using the high-pressure in situ ATR-FTIR monitoring system, following the similar procedure in our previous work [21,29]. The view cell was heated to the target temperature (60 • C) and then purged with N 2 and degassed by a vacuum pump alternately for at least three times to eliminate the possible residuals in the system, then charged with a certain amount of the monomer (10 mL) via a special syringe and stirred at a speed of around 300 rpm. CO 2 was introduced into the view cell in a stepwise fashion to solvate the previously added monomer. The FTIR spectra of the monomer were collected throughout the whole solvation process from 0 to 38.00 MPa at specified pressure intervals while stirring continued. At every selected pressure, the system was stirred for about five minutes until two identical FTIR spectra were obtained. The phase behaviors of the monomer + CO 2 system were directly observed through the sapphire windows during the solvation process.

Free Radical Homopolymerization of the Monomers in scCO 2
The free radical homopolymerization of the monomers in scCO 2 was studied via the high-pressure FT-NIR monitoring system. The view cell was preheated to the target temperature (60 • C) and purged with N 2 and degassed by a vacuum pump alternately for at least three times to eliminate the impurities in the system. Then the view cell was charged with a certain amount of the monomer (approximately 0.05 mol) by using a special syringe, stirred at a speed of around 300 rpm and filled with pressurized CO 2 to approximately 12 MPa (which is slightly higher than the corresponding P T of the monomer + CO 2 system) so as to dissolve the added monomer. Then BPO (or AIBN) was added into the view cell via a set of high-pressure sample-in tube by the aid of the pressurized CO 2 to initiate the polymerization. After the pressure was rapidly increased to 25 MPa (within 3 min), FT-NIR spectra were started to be in situ collected periodically during the polymerization processes, via which the monomer conversion was determined using the identical method recently reported [22]. When the system continuously polymerized for 8 hours or became cloudy [22,30,31] (where the baseline absorbance exceeded 3.0 [21]), the view cell was cooled and depressurized. After CO 2 in the view cell was slowly released, the raw product was collected and characterized.

Characterization
Hydrogen nuclear magnetic resonance ( 1 H NMR) and carbon-13 nuclear magnetic resonance ( 13 C NMR) spectra were detected via an Avance 400 superconducting Fourier digital NMR instrument (400 MHz for proton, Bruker, Karlsruhe, Germany) in deuterated chloroform (CDCl 3 ) at 25 • C, where tetramethylsilane (TMS) and the residual chloroform in CDCl 3 were used as references of chemical shift. The Fourier transform infrared (FT-IR) spectra of the products were measured using a Tensor 27 FT-IR spectrometer (Bruker, Karlsruhe, Germany) in potassium bromide (KBr) disks in the wavenumber range of 4000 to 400 cm −1 . The number-average molecular weight (M n ) of the obtained non fluorinated polymers was analyzed using a Waters-Breeze gel permeation chromatography (GPC, Waters, Milford, CT, USA) where tetrahydrofuran (THF) was used as the eluent at a flow rate of 1.0 mL·min −1 and monodisperse polystyrene standards were applied to calibrate the relative molecular weight. The molecular weight of the poly 2,2,2-trifluoromethyl methacrylate (PMTFMA) was analyzed using a Microflex matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF mass, Bruker-Dalton, Bremen, Germany) followed a similar process to that in our recent work [22] except that dithranol was used as the matrix.

Computational Methods
The theoretical calculations have been performed using the Gaussian 2009 software package [32]. The geometries of monomer-(CO 2 ) n complexes as well as the monomer clusters are optimized using DFT-M062X/6-311G (2d, p) method which has been used in the similar system [33]. The absorption enthalpy(∆H abs ) between the monomer and CO 2 or two monomer molecules was calculated and used to evaluate the solubility of the monomers in scCO 2 . In such calculation the solvent treatment is not considered similarly to that reported in monomer + scCO 2 system [23]. Moreover, the atomic charge of every monomer and the corresponding monomer radical and the binding energy during the addition of the monomer with the initial free radical are also calculated in the present work.

Characterization of the Monomers and Obtained Polymers in scCO 2
The FT-IR (A), 1 H NMR (B)and 13 C NMR (C) spectra of MTFMA and the corresponding polymers (PMTFMA) are shown in Figures 1 and 2, in which the attribution of every spectrum is also presented. The MALDI-TOF mass spectrum of PMTFMA is shown in Figure 3. While since the spectra of PMMA and PMAA have been reported elsewhere, such results are not listed here.
due to the spin coupling of the directly connected three F atoms, as shown in the 13 C NMR spectra of PMTFMA in Figure 2C. More importantly, as shown in the MALDI-TOF mass spectrum of PMTFMA in Figure 3A, the molecular weight is measured as 1413 g·mol −1 . Additionally, it is seen that the intervals of marked peaks are all close to the molecular weight of MTFMA (154.1, shown in the expansion in Figure 3B). Since the spectral results mentioned above can be well attributed and confirmed with each other, it is indicated that MTFMA has been successfully polymerized in scCO2.  PMTFMA in Figure 2C. More importantly, as shown in the MALDI-TOF mass spectrum of PMTFMA in Figure 3A, the molecular weight is measured as 1413 g·mol −1 . Additionally, it is seen that the intervals of marked peaks are all close to the molecular weight of MTFMA (154.1, shown in the expansion in Figure 3B). Since the spectral results mentioned above can be well attributed and confirmed with each other, it is indicated that MTFMA has been successfully polymerized in scCO2.

Solvation Bahaviors of the Monomers in Gaseous and Supercritical CO2
We believe that it is the intermolecular interactions in monomer/polymer + CO2 system that dominate the solvation process as well as the phase behaviors of monomer/polymer in scCO2 and may also play an important role to the polymerizability of the monomer in scCO2. Previously, we As shown in the FTIR spectrum of PMTFMA in Figure 2A, the peaks at 1114 and 1182 cm −1 are attributed to the stretching vibration of C-F bonds (v(C-F)). Moreover, as compared with FT-IR spectrum of MTFMA (shown in Figure 1A), it is seen that the peak of v(C=C) around 1630-1650 cm −1 disappeared. Additionally, v(C=O) is found to blue-shift from 1741 cm −1 ( Figure 1A) to 1755 cm −1 due to the destruction of the π-π conjugation effect in MTFMA. As shown in the 1 H NMR spectra of PMTFMA ( Figure 2B) and MTFMA ( Figure 1B), the olefinic =CH 2 peaks in MTFMA (located at 6.44 and 6.23 ppm) completely disappeared. Furthermore, the chemical shift of -OCH 3 slightly declined from 3.86 ppm to 3.79 ppm because of the previous p-π conjugation effect in MTFMA weakened. While the signal of the carbon atom in -CF 3 (centered near 125 ppm) splits into multiple peaks mainly due to the spin coupling of the directly connected three F atoms, as shown in the 13 C NMR spectra of PMTFMA in Figure 2C. More importantly, as shown in the MALDI-TOF mass spectrum of PMTFMA in Figure 3A, the molecular weight is measured as 1413 g·mol −1 . Additionally, it is seen that the intervals of marked peaks are all close to the molecular weight of MTFMA (154.1, shown in the expansion in Figure 3B). Since the spectral results mentioned above can be well attributed and confirmed with each other, it is indicated that MTFMA has been successfully polymerized in scCO 2 .

Solvation Bahaviors of the Monomers in Gaseous and Supercritical CO 2
We believe that it is the intermolecular interactions in monomer/polymer + CO 2 system that dominate the solvation process as well as the phase behaviors of monomer/polymer in scCO 2 and may also play an important role to the polymerizability of the monomer in scCO 2 . Previously, we presented the concept of transition pressure (P T ) to demonstrate the unexpected vibrational absorption evolution of the functional groups during the solvation process of liquid monomers and polymers in gaseous and supercritical carbon dioxide [21,29]. P T is defined as the lowest pressure at which the liquid solute could be completely dissolved or miscible with scCO 2 under isothermal conditions. A lower P T that the solute + CO 2 presents, a better solubility that the solute has in scCO 2 . In the present work, the absorption band centers of C=O and C-F are extracted from the initial in situ FTIR spectra monitored via the high-pressure ATR-FTIR system and plotted versus the CO 2 pressure. In this way, the absorption evolution of these functional groups in the MTFMA (MMA, MAA, TFMAA) + CO 2 binary system of the probe functional groups (v(C-F) and v(C=O)) was sketched and from which the corresponding P T of every binary system was measured, as shown in Figures 4-6.

Solvation Bahaviors of the Monomers in Gaseous and Supercritical CO2
We believe that it is the intermolecular interactions in monomer/polymer + CO2 system that dominate the solvation process as well as the phase behaviors of monomer/polymer in scCO2 and may also play an important role to the polymerizability of the monomer in scCO2. Previously, we presented the concept of transition pressure (PT) to demonstrate the unexpected vibrational absorption evolution of the functional groups during the solvation process of liquid monomers and polymers in gaseous and supercritical carbon dioxide [21,29]. PT is defined as the lowest pressure at which the liquid solute could be completely dissolved or miscible with scCO2 under isothermal conditions. A lower PT that the solute + CO2 presents, a better solubility that the solute has in scCO2. In the present work, the absorption band centers of C=O and C-F are extracted from the initial in situ FTIR spectra monitored via the high-pressure ATR-FTIR system and plotted versus the CO2 pressure. In this way, the absorption evolution of these functional groups in the MTFMA (MMA, MAA, TFMAA) + CO2 binary system of the probe functional groups (v(C-F) and v(C=O)) was sketched and from which the corresponding PT of every binary system was measured, as shown in  As shown in Figures 4-6, it is found that the PT of the MTFMA + CO2 system is measured as 10.4 MPa at 60.0 °C , either v(C-F) or v(C-O) was used as the probe. While at the identical temperature, the PT of MMA + CO2, MAA + CO2 and TFMAA + CO2 system was obtained as 9.5 MPa, 11.0 MPa and 11.5 MPa, respectively. Such differences in the PT values are attributed to the differences in the monomer structures and the resulted interactions in the corresponding monomer + CO2 system and are demonstrated as follows.
First, the carboxyl group and the resulted strong hydrogen bonding among the monomers are believed to negatively impact on the miscibility of the monomer with CO2 and thus contribute to a higher PT of the binary system. While after the carboxyl group is esterified, the PT is found to effectively decrease since the negative impact of hydrogen bonding is eliminated. For example, from TFMAA to MTFMA, the PT of the corresponding binary system is found to decrease from 11.5 MPa to 10.4 MPa, from MAA to MMA, the PT of the system is found to decrease from 11.0 MPa to 9.5 MPa.
Second, the dispersion interaction among the monomers along with the attraction between -CF3 of the monomer and CO2 plays a contrary role to the miscibility of the monomer with scCO2. It is inferred that the dispersion interaction may play a predominant role in solvation behaviors of most monomers/polymers in scCO2 [21]. In view that the dispersion interaction among the monomers increases notably with the increase of the molecular weight (Mn), the corresponding PT should also  As shown in Figures 4-6, it is found that the PT of the MTFMA + CO2 system is measured as 10.4 MPa at 60.0 °C , either v(C-F) or v(C-O) was used as the probe. While at the identical temperature, the PT of MMA + CO2, MAA + CO2 and TFMAA + CO2 system was obtained as 9.5 MPa, 11.0 MPa and 11.5 MPa, respectively. Such differences in the PT values are attributed to the differences in the monomer structures and the resulted interactions in the corresponding monomer + CO2 system and are demonstrated as follows.
First, the carboxyl group and the resulted strong hydrogen bonding among the monomers are believed to negatively impact on the miscibility of the monomer with CO2 and thus contribute to a higher PT of the binary system. While after the carboxyl group is esterified, the PT is found to effectively decrease since the negative impact of hydrogen bonding is eliminated. For example, from TFMAA to MTFMA, the PT of the corresponding binary system is found to decrease from 11.5 MPa to 10.4 MPa, from MAA to MMA, the PT of the system is found to decrease from 11.0 MPa to 9.5 MPa.
Second, the dispersion interaction among the monomers along with the attraction between -CF3 of the monomer and CO2 plays a contrary role to the miscibility of the monomer with scCO2. It is inferred that the dispersion interaction may play a predominant role in solvation behaviors of most monomers/polymers in scCO2 [21]. In view that the dispersion interaction among the monomers increases notably with the increase of the molecular weight (Mn), the corresponding PT should also  Figures 4-6, it is found that the P T of the MTFMA + CO 2 system is measured as 10.4 MPa at 60.0 • C, either v(C-F) or v(C-O) was used as the probe. While at the identical temperature, the P T of MMA + CO 2 , MAA + CO 2 and TFMAA + CO 2 system was obtained as 9.5 MPa, 11.0 MPa and 11.5 MPa, respectively. Such differences in the P T values are attributed to the differences in the monomer structures and the resulted interactions in the corresponding monomer + CO 2 system and are demonstrated as follows.

As shown in
First, the carboxyl group and the resulted strong hydrogen bonding among the monomers are believed to negatively impact on the miscibility of the monomer with CO 2 and thus contribute to a higher P T of the binary system. While after the carboxyl group is esterified, the P T is found to effectively decrease since the negative impact of hydrogen bonding is eliminated. For example, from TFMAA to MTFMA, the P T of the corresponding binary system is found to decrease from 11.5 MPa to 10.4 MPa, from MAA to MMA, the P T of the system is found to decrease from 11.0 MPa to 9.5 MPa.
Second, the dispersion interaction among the monomers along with the attraction between -CF 3 of the monomer and CO 2 plays a contrary role to the miscibility of the monomer with scCO 2 . It is inferred that the dispersion interaction may play a predominant role in solvation behaviors of most monomers/polymers in scCO 2 [21]. In view that the dispersion interaction among the monomers increases notably with the increase of the molecular weight (M n ), the corresponding P T should also distinctly increase in the present work. While from MMA to MTFMA, the M n increases by 54, the P T is found to increase only by 0.9 MPa. Similarly, the identical increase of M n from MAA to TFMAA merely results in an increment of 0.5 MPa in the P T of the binary system. We believe that it is the attraction between CO 2 and -CF 3 in the fluorinated monomers, as well as the unique fluorine repulsion among the fluorinated molecules, that effectively weakens/counteracts the negative effects of the dispersion interaction among the monomers on the P T of the binary system and thus contribute potently to the enhanced solubility of fluorinated monomers in scCO 2 .
So as to further discern the intermolecular interactions during the solvation process of the methacrylate monomers in scCO 2 , the theoretical calculations of DFT method is intentionally employed as the complement to the high pressure in situ FTIR technique.

Calculation of the Intermolecular Interactions in Methacrylate + CO 2 System
We believe that the evolution of σ(A-B) (the average resultant force/interaction of the intermolecular interactions between the CO 2 and the monomer) and σ(B-B) (the average resultant force/interaction of the self-interactions between the monomers) dominate the solvation behaviors of monomer in scCO 2 [21,29]. So far, several function groups, such as C=O [34], C-O [35], C-F [15], are reported to definitely contribute to σ(A-B) due to the special attraction between the specific group(s) and CO 2 . It is also reported that the interaction between the carbonyl group and CO 2 may contribute more than that between C-F and CO 2 to the attraction of FOA with CO 2 in FOA + scCO 2 system [22]. In the present work, M . . . CO 2 is used to represent the cluster of monomer(s) with CO 2 under lower CO 2 pressure and M . . .  Table 1. It can be seen that the monomer molecule may be better solvated by CO 2 when the pressure increases since more stable cluster of M . . . nCO 2 may form (shown in Figures 7 and 8). Moreover, as shown in Table 1 (Entry 1), the calculated H abs (M . . . M) is found to be more than twice of H abs (M . . . CO 2 ), indicating that the average resultant interaction between MMA is stronger than that between CO 2 and MMA under lower CO 2 pressure, namely, σ(B-B) > σ(A-B). More importantly, it is clearly seen that the obtained H abs (M . . . 3CO 2 )(−9.2 kJ/mol) is distinctly higher than H abs (M . . . CO 2 ) (−3.4 kJ/mol), certifying that σ(A-B) increases with the increase of CO 2 pressure. Contrarily, the initially strong σ(B-B) may be gradually weakened/undermined because of the slight electron redistribution along with the expansion of MMA by CO 2 during pressuring [20]. With the increase of CO 2 pressure, σ(A-B) may increase further, versus that σ(B-B) is increasingly weakened. There must be a pressure value where σ(A-B) may equal to σ(B-B), the macroscopic force on the MMA molecule may equilibrate and MMA may completely disperse and fully miscible with CO 2 under isothermal condition. Such pressure is exactly the P T of MMA + CO 2 system, which has been accurately determined via our high-pressure FTIR monitoring system (9.5 MPa). It is inferred that, as the CO 2 pressure further increased, σ(A-B) may probably become greater than σ(B-B), which has been partly proved and for the obtained H abs (M . . . 3CO 2 ) the value is found to be higher than that of the initial H abs (M . . . M) (−7.8 kJ/mol).
Polymers 2020, 12, x FOR PEER REVIEW 9 of 14 pressure further increased, σ(A-B) may probably become greater than σ(B-B), which has been partly proved and for the obtained △Habs(M…3CO2) the value is found to be higher than that of the initial △Habs(M…M) (−7.8 kJ/mol). Similarly, the solvation behaviors of MTFMA in gaseous and supercritical CO2 can also be well understood via the calculation as well as the IDISE hypothesize. While the calculated △Habs(M…M) in MTFMA is found to be obviously higher than that in MMA (partly due to the increase in Mn), resulting in a higher PT of MTFMA + CO2 system.  Unexpectedly, the obtained △Habs(M…M) in MAA (−70.1 kJ/mol) or TFMAA (−103.6 kJ/mol) is found to be dramatically higher than that in MMA or MTFMA, suggesting that the σ(B-B) in these systems may be severely greater than that in MMA or MTFMA. Such strong self-interactions are believed to be mainly derived from the existence of -COOH and the resulted effective hydrogen bonding in the system. Moreover, it is clearly that the initial value of △Habs(M…M) is considerably Similarly, the solvation behaviors of MTFMA in gaseous and supercritical CO 2 can also be well understood via the calculation as well as the IDISE hypothesize. While the calculated H abs (M . . . M) in MTFMA is found to be obviously higher than that in MMA (partly due to the increase in M n ), resulting in a higher P T of MTFMA + CO 2 system. Polymers 2020, 12, x FOR PEER REVIEW 9 of 14 pressure further increased, σ(A-B) may probably become greater than σ(B-B), which has been partly proved and for the obtained △Habs(M…3CO2) the value is found to be higher than that of the initial △Habs(M…M) (−7.8 kJ/mol). Similarly, the solvation behaviors of MTFMA in gaseous and supercritical CO2 can also be well understood via the calculation as well as the IDISE hypothesize. While the calculated △Habs(M…M) in MTFMA is found to be obviously higher than that in MMA (partly due to the increase in Mn), resulting in a higher PT of MTFMA + CO2 system.  Unexpectedly, the obtained △Habs(M…M) in MAA (−70.1 kJ/mol) or TFMAA (−103.6 kJ/mol) is found to be dramatically higher than that in MMA or MTFMA, suggesting that the σ(B-B) in these systems may be severely greater than that in MMA or MTFMA. Such strong self-interactions are believed to be mainly derived from the existence of -COOH and the resulted effective hydrogen bonding in the system. Moreover, it is clearly that the initial value of △Habs(M…M) is considerably pressure further increased, σ(A-B) may probably become greater than σ(B-B), which has been partly proved and for the obtained △Habs(M…3CO2) the value is found to be higher than that of the initial △Habs(M…M) (−7.8 kJ/mol). Similarly, the solvation behaviors of MTFMA in gaseous and supercritical CO2 can also be well understood via the calculation as well as the IDISE hypothesize. While the calculated △Habs(M…M) in MTFMA is found to be obviously higher than that in MMA (partly due to the increase in Mn), resulting in a higher PT of MTFMA + CO2 system.  Unexpectedly, the obtained △Habs(M…M) in MAA (−70.1 kJ/mol) or TFMAA (−103.6 kJ/mol) is found to be dramatically higher than that in MMA or MTFMA, suggesting that the σ(B-B) in these systems may be severely greater than that in MMA or MTFMA. Such strong self-interactions are believed to be mainly derived from the existence of -COOH and the resulted effective hydrogen bonding in the system. Moreover, it is clearly that the initial value of △Habs(M…M) is considerably Unexpectedly, the obtained H abs (M . . . M) in MAA (−70.1 kJ/mol) or TFMAA (−103.6 kJ/mol) is found to be dramatically higher than that in MMA or MTFMA, suggesting that the σ(B-B) in these systems may be severely greater than that in MMA or MTFMA. Such strong self-interactions are believed to be mainly derived from the existence of -COOH and the resulted effective hydrogen bonding in the system. Moreover, it is clearly that the initial value of H abs (M . . . M) is considerably greater than that of H abs (M . . . 3CO 2 ) in MAA + CO 2 system, indicating that the self-interaction of σ(B-B) may be too strong to be surpassed by σ (A-B), even CO 2 pressure is considerably high. In this case, MAA must exist in form of clusters instead of single molecule in scCO 2 , especially in the dimer form, as shown in Figure 9. Additionally, probably owing to the strong electron-withdrawing effect of -CF 3 in TFMAA, the intermolecular hydrogen bonding among TFMAA is predicted to be further enhanced, leading to a greater σ(B-B) in TFMAA. Such prediction has been largely proved since the calculated H abs (M . . . M) in TFMAA is much greater than that of the other three methacrylate monomers, along with that the experimentally measured P T of TFMAA + CO 2 system is the highest among that of the four methacrylate + CO 2 systems in the present work. It is inferred that the TFMAA . . . TFMAA clusters may restrain the diffusion of TFMAA in scCO 2 and decrease the polymerizability of TFMAA in scCO 2 to some extent.
Based on the calculation mentioned above, it is clear that the ∆H abs may be a rational and promising function to evaluate the evolution of σ(A-B) and σ(B-B) during the solvation process of the monomers in CO 2 . In view that the predicted order of the solubility of the four monomers inscCO 2 (MMA > MTFMA > MAA > TFMAA) is ideally consistent with that of the experimental P T (MMA < MTFMA < MAA < TFMAA), it is believed that the previously proposed mechanism of intermolecular-interaction dynamically-induced solvation effect (IDISE) of monomer in scCO 2 system is well certified/corroborated.

Free Radical Polymerization of the Monomers in scCO 2
The free radical polymerization of the four monomers in scCO 2 was investigated so as to understand the impacts of the monomer structure, especially -CF 3 in the alkyl 2-trifluoromethacrylate(s), on the polymerizability of the monomer. The results are listed in Table 2.  Figure 2). e No polymeric product is obtained whatever the free radical polymerization of TFMAA were performed in scCO 2 , trifluorotoluene or toluene and initiated by AIBN or BPO.
As shown in Table 2, it is clear that MMA and MAA can be easily polymerized under the experimental conditions mentioned above. For example, after polymerized for 8 hours, the conversion of MAA reaches to 70.5% and the M n comes up to 23,800. While for MTFMA, it is found that at the identical polymerization time to that in MAA, the conversion of MTFMA is 3.1%. Even if the polymerization lasts for 40 h, the conversion is merely 10.8%. Simultaneously, the molecular weight of the polymeric product is found to be severely low as 1634. Namely, the polymerization degree is around 10. It is shown that the free radical polymerizability of MTFMA is considerably poor, though such monomer has been polymerized via the initiation of BPO in the present work. What's more, it is found that TFMAA cannot be polymerized by the initiation of free radical initiator in either scCO 2 or organic solvent. Clearly, -CF 3 may significantly decrease the free radical polymerization activity of the corresponding methacrylate(s) in scCO 2 .
We believe that the existence of -CF 3 and the resulted strong electron-withdrawing effect may distinctly change the electron distribution in C=C. May this be the reason for the reduction of polymerizability of the MTFMA and TFMAA? Theoretical calculation of DFT-M062X/6-311G (2d, p) method was employed to understand the homopolymerization activity of the monomers in scCO 2 , especially the contribution of -CF 3 to the free radical polymerizability of MTFMA in scCO 2 . The atomic charge of every monomer (M) and the corresponding monomer radical (IM·, produced by the addition of the monomer with the initial free radical I) and binding energy ( E, the energy change in the formation of IM, I + M→IM) of every monomer with I· are calculated and listed in Table 3. Table 3. Geometric parameters and the binding energy of every monomer with I. regard to the chain propagation, the steric hindrance of C6 in IM is obviously bigger than that of C2 in the corresponding monomer, especially when MTFMA or TFMAA is polymerized. These two factors are both unfavorable for the chain growth during the free radical polymerization. In this case, the monomer should be active enough if the monomer/polymer chain is expected to continuously propagate, namely, the polymerization successfully occurs. It is inferred that the electron distribution in C=C, especially the atomic charge in C1 of the monomer, may play a key role to the free radical polymerizability of the methacrylate monomers. The bigger the absolute value of the atomic charge of C1 in C=C is, the more easily the chain propagates. As shown in Table 3 (Entry 1), it is found that after -CH 3 is replaced by -CF 3 , the absolute value of the atomic charge of C1 in C=C distinctly decreases. For example, it declines from 0.300 e (in MMA) to 0.235 e (in MTFMA) or from 0.293 e (in MAA) to 0.223 e (in TFMAA). It is believed that such a decrease in the atomic charge of C1 in MTFMA or TFMAA is predominantly derived from the strong electron-with drawing inductive effect of -CF 3 to C=C and the resulted electron redistribution in C=C (namely, the electron between C1 and C2 may shift to C2 to some extent) and may be mainly responsible for the decline of the free radical polymerizability of MTFMA and TFMAA. Possible reasons are presented as follows.
First, generally, C1 in C=C is much easier to be attacked by the initial free radicals since the steric hindrance of C1 is much smaller than that of C2 in methacrylate monomers along with that more favorable/stable intermediate of monomer radical IM may be produced if C1 of the monomer is bonded with the initial free radical. In this case, it is suggested that the decrease in the atomic charge of C1 may be severely unfavorable for the chain initiation. Such suggestion is believed to be verified by the calculation of the binding energy. For example, it is seen in Table (Entry 7) that the absolute value of the binding energy of MTFMA with I· (77.239 kJ·mol −1 ) is clearly found to be much lower than that of MMA with the identical I·(95.513 kJ·mol −1 ), indicating that the initiation activity of I· may decline significantly when MTFMA is used instead of MMA. Similar results can also be obtained when TFMAA is applied instead of MAA.
Second, the chain propagation activity may dramatically decline in step with the decrease of atomic charge of C1 in C=C. As shown in Table 3 (Entry 6), the absolute value of the atomic charge of C6 is found to be considerably small when MMA or MAA is in involved in the monomer radical of IM· and which is even smaller in the monomer radical of I· with MTFMA or TFMAA. Moreover, with regard to the chain propagation, the steric hindrance of C6 in IM is obviously bigger than that of C2 in the corresponding monomer, especially when MTFMA or TFMAA is polymerized. These two factors are both unfavorable for the chain growth during the free radical polymerization. In this case, the monomer should be active enough if the monomer/polymer chain is expected to continuously propagate, namely, the polymerization successfully occurs. It is inferred that the electron distribution in C=C, especially the atomic charge in C1 of the monomer, may play a key role to the free radical polymerizability of the methacrylate monomers. The bigger the absolute value of the atomic charge of C1 in C=C is, the more easily the chain propagates.
Pulsed laser polymerization (PLP) is reported to be an effective technique to determine the propagation rate coefficients (kp) of a series of monomers (MMA, MAA, styrene and vinylidene fluoride, etc.) in the free radical polymerization in both scCO 2 and organic solvent systems [36][37][38]. While the report on such determination of the alkyl 2-trifluoromethacrylate(s) monomer (such as MTFMA or TFMAA) has not been seen/available so far. Moreover, ab initio calculations are also used to investigate the polymerization kinetics [39,40]. It is expected that these methods may be successfully applied in the next work so as to achieve further understanding on the polymerizability of alkyl 2-trifluoromethacrylate(s) as well as other fluorinated monomers in scCO 2 .
In brief, since the absolute value of the atomic charge of C1 in MTFMA or TFMAA is distinctly small as compared with that in MMA or MAA, the chain initiation as well as the chain propagation of MTFMA or TFMAA is predicted to be severely difficult to occur, which is well in accordance with what has been observed experimentally in the free radical polymerization in the present work.

Conclusions
The solvation behaviors and the free radical homopolymerization of four methacrylate monomers, including MMA, MAA, MTFMA and TFMAA in scCO 2 were systematically investigated via HP FTIR/NIR technology along with the theoretical calculation of DFT-M062X/6-311G (2d, p) method in the present work. The intermolecular interactions of every monomer + CO 2 system during the solvation is demonstrated and discerned via the concepts of P T , σ(A-B), σ(B-B) and ∆H abs , via which the solubility of the monomer in scCO 2 is successfully evaluated and the previously proposed mechanism of intermolecular-interaction dynamically-induced solvation effect (IDISE) of monomer in scCO 2 may be well verified/corroborated. More importantly, the homopolymerization activity of the four methacrylates in scCO 2 , especially the special contribution of -CF 3 to the free radical polymerizability of MTFMA in scCO 2 is revealed. The free radical polymerization of MMA and MAA in scCO 2 is easy to occur, while that of MTFMA is much difficult though it does have been successfully polymerized via the initiation of BPO. TFMAA cannot be polymerized via the free radical initiators. It is believed that there are mainly two factors that are predominantly answer for the significant decline of the free radical polymerizability of MTFMA, TFMAA and other alkyl 2-trifluoromethacrylates in scCO 2 . First, the diffusion of the monomer in scCO 2 may be restrained since the intermolecular hydrogen bonding is enhanced by -CF 3 . Second, the atomic charge of C1 in C=C may be the distinctly decreased by the strong electron-withdrawing inductive effect of -CF 3 to C=C. Whereas there are still many issues to be further solved, the present work may be the first example that successfully combine the HP FTIR/NIR technology with the DFT method to explore and disclose the intermolecular interaction mechanism as well as the free radical polymerizability of methacrylate in scCO 2 and is expected to improve the research and application in many related fields.