Article pubs.acs.org/Macromolecules
Free Radical Copolymerization of Ethylene with Vinyl Acetate under Mild Conditions A. Zarrouki, E. Espinosa, C. Boisson,* and V. Monteil* CPE Lyon, CNRS, UMR 5265, Laboratoire de Chimie, Catalyse, Polymère et Procédés (C2P2), LCPP Group, Université de Lyon, Université Lyon 1, 43 Bd. du 11 Novembre 1918, F-69616 Villeurbanne, France S Supporting Information *
ABSTRACT: This work highlights a medium pressure and temperature radical polymerization process in organic solvents for the synthesis of ethylene−vinyl acetate copolymers (EVA). Organic solvents play a crucial role in copolymerization yields, copolymer molar masses, chemical composition, and branching degree. THF and dimethylcarbonate (DMC) were selected, yielding EVA copolymers exhibiting molar masses from 1000 up to 35000 g mol−1 and vinyl acetate contents between 4 and 32 mol %. In the case of DMC, an additional transfer agent (propanal) was required to reach the molar-mass range targeted for the specific application of EVA copolymers as cold flow improver (CFI): an essential diesel fuel additive.
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INTRODUCTION Polyolefins account for more than half of thermoplastics’ production. They feature excellent chemical and physical properties and are based on the least expensive monomers. Among them, polyethylene (PE) is the most produced polymer. Three major grades of PE are synthesized, discriminated by their densities: HDPE (high density polyethylene) and LLDPE (linear low density polyethylene), both produced by catalytic polymerization (Ziegler−Natta, Phillips, and metallocene catalysis), and LDPE (low density polyethylene) obtained by free-radical polymerization. The lack of polarity often prevents polyethylene from exhibiting important end-use properties such as chemical compatibility with other materials (fillers, other polymers), adhesion, or dyeability. Direct incorporation of polarity into polyethylene via copolymerization of ethylene with polar vinyl comonomers allows for a drastic improvement of these properties, but the catalytic path, despite numerous studies, is still not industrially relevant due to the poisoning by polar monomers of almost all olefin polymerization catalysts.1−5 Even though efficient systems based on palladium have been developed for copolymerization of ethylene with various polar monomers,6 in particular acrylates, catalysts’ prices and low activities hamper industrial development. In the specific case of vinyl acetate, coordination copolymerization (with α-olefins, especially ethylene) is much more tedious, and only a few percent of vinyl acetate has been successfully incorporated.6−8 The current commercial processes for the synthesis of copolymers of ethylene with polar vinyl monomers are thus exclusively based on free radical polymerization. Free radical homopolymerization of ethylene has been accidentally discovered by Imperial Chemical Industries (ICI) in 1933. The polymerization process, either in a tubular or stirred autoclave reactor, requires very harsh reaction conditions: high temperatures (usually above 200 °C) and © XXXX American Chemical Society
very high pressures (Pethylene > 800 bar, usually above 2000 bar), due to low reactivity of ethylene in radical polymerization.1,2,4,9 As aforementioned, copolymers of ethylene with polar vinyl comonomers, especially vinyl esters, acrylates or acrylic acids, are of great industrial importance. Generally, these copolymers display low amounts of polar units ( 5) and butyl branches. LV is the average block length of vinyl acetate. a
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Our group has recently shown that the production of PE via a free radical pathway can still be efficient under much milder temperature and pressure conditions in organic solvent media: from 10 to 250 bar of ethylene and below 90 °C. Polymerization has been carried out in organic solvents using 2,2′-azobis(isobutyronitrile) (AIBN) as organosoluble initiator.16,17 An unprecedented solvent activation effect has been highlighted. Highest activities have been obtained in polar solvents such as dialkyl carbonates or THF. This unusual major solvent effect on polymerization yield has been linked to the permittivity (ε) and the dipole momentum (μ) of the polymerization medium.18 Following these studies, several other significant breakthroughs on ethylene radical polymerization have been achieved. In aqueous media, free radical polymerization of ethylene led to stable aqueous dispersions of latex of high molar-mass LDPE.19,20 Importantly, the first development of a controlled radical polymerization of ethylene using reversible addition−fragmentation chain transfer (RAFT) technique21 has been reported recently. Detrembleur et col. have also succeeded in controlling the copolymerization of ethylene with vinyl acetate under mild conditions harnessing cobalt mediated radical polymerization (CMRP).22,23 The present work focuses on the copolymerization of ethylene and vinyl acetate via free radical chemistry under the less energy demanding mild conditions used for PE synthesis (50−125 bar, 70 °C) in various organic solvent media. Among all industrial applications of EVA copolymers, the focus is on cold flow additives for diesel fuel in automotive industries. As a consequence these EVA have to fulfill several requirements: low molar masses, relatively low vinyl acetate content and crystallinity. For this purpose the control of molar masses has been particularly studied through the use of propanal as chain transfer agent. The impact of chemical composition, molar mass and microstructure of EVA copolymers on their efficiency as cold flow improver (CFI) diesel fuel additives has been evaluated.
EXPERIMENTAL SECTION
Materials. Ethylene (99.95%, Air Liquide) and 2,2′-azobis(isobutyronitrile) (AIBN, 98%, Fluka) were used as received. Propanal (97%, Sigma-Aldrich), dimethyl carbonate (99%, Sigma-Aldrich), and diethyl carbonate (99%, Sigma-Aldrich) were dried over molecular sieves 3 Å and purged from oxygen with argon. Vinyl acetate (98% stabilized with hydroquinone, Sigma-Aldrich) was dried over CaH2 and then distilled under vacuum. Toluene and tetrahydrofuran (Biosolve) were purified through M.Braun solvent purification system MB-SPS. EVA Characterizations. Size Exclusion Chromatography in THF (SEC-THF). SEC measurements were performed with a Viscotek TDAmax system from Malvern Instruments that consists of an integrated solvent and sample delivery module (GPCmax) including a four-capillary differential viscometer and a differential refractive index detector (RI). Tetrahydrofuran was used as the mobile phase at a flow rate of 1 mL min−1. All polymers were injected at a concentration of [3−5] mg mL−1 after filtration through a 0.45 μm pore-size membrane. The separation was carried out on three Polymer Standard Service columns [SDVB, 5 μm, 300 × 7.5 mm] and a guard column. Columns and detectors were maintained at 40 °C. The OmniSEC 5.02 software was used for data acquisition and data analysis. Because of the low molar-mass range of most of the samples, analyses were done using a conventional calibration based on narrow polystyrene standards using only the signal from the refractometer detector. Nuclear Magnetic Resonance (NMR). All NMR experiments were performed at 90 °C in 2:1 (v:v) tetrachloroethylene/deuterated benzene solutions in 10 mm tubes using a Bruker Avance II 400 Ascend spectrometer with a 10 mm SEX-13C/1H probe at the NMR Polymer Center of Institut de Chimie de Lyon (FR5223) (Villeurbanne, France). Chemical shifts are given in parts per million (ppm) and referenced to tetramethylsilane as internal standard. Differential Scanning Calorimetry (DSC). Conventional measurements and TOPEM measurements were performed on a Mettler Toledo DSC 1, and on a Mettler Toledo DSC 3. Conventional measurement was carried out at a heating, and cooling, rate of 10 °C min−1. Two successive heating and cooling of the samples were performed under nitrogen atmosphere. Crystallinity (Xc) and crystallization temperatures were obtained during the second cooling and heating cycle. Xc was calculated with the following equation Xc = (ΔHcrystallization + ΔHmelting)/2ΔHc∞) where ΔHc∞ is 291 J g−1. TOPEM is a method of stochastic temperature-modulated DSC which allows a separation of thermal phenomena: nonreversing heat flow B
DOI: 10.1021/acs.macromol.6b02756 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules phenomena (e.g., crystallization) and reversing heat flow phenomena (e.g., glass transition).24 Cooling was done at 1 °C min−1, with randomly generated temperature pulses of ±0.025 °C, with a period of 15−30 s. EVA Synthesis. Caution! All polymerizations involve high pressures and explosive gas. The reaction was initiated by AIBN (6.1 × 10−3 mol L−1). In a Schlenk tube, AIBN was dissolved, in a mixture of the polymerization solvent and vinyl acetate (and possibly propanal as chain transfer agent). The volume of the reaction medium was kept constant and equal to 50 mL. The mixture was subsequently introduced through a cannula into a 160 mL stainless steel reactor (from Parr Instrument Co.) equipped with safety valves and a mechanical stirrer. Ethylene was then introduced until the desired pressure was reached, and at the same time the mixture was heated at 70 °C under stirring (400 rpm). An intermediate 1.5 L tank filled with ethylene (240 bar of ethylene at 20 °C) was used to charge the reactor and to maintain a constant pressure of ethylene in the reactor by successive manual ethylene additions. After the desired reaction time, the reactor was slowly cooled down and degassed. The polymer was then recovered by evaporation of the solvent at room temperature. EVA Evaluation as Cold Flow Additives. The evaluation of polymers efficiencies as cold flow additives was performed according to the normalized method defined by the norm NF EN 116 (1998 version). It consists of the suction of 20 mL (normalized depression) through a filter (porosity of 45 μm) at each °C, with a cooling rate of 0.5 °C min−1. The CFPP (cold filter plugging point) was achieved if the suction time exceeded 60 s.
Scheme 1. Transfer Reactions to Solvent
DMC which can thus be considered as a “non-transferring” solvent. The vinyl acetate content of EVA copolymers determined by 1 H NMR varied from 7 to 21 mol % depending on the ethylene pressure and was slightly influenced by the nature of the solvent. Further investigations of EVA microstructures by 13C NMR (Figure S5) allowed for calculation of the average block length of VAc (LV). LV decreases with the higher incorporation of ethylene, and is not really impacted by the nature of the solvent. On the other hand the solvent slightly influenced the branching content and the nature of branches (from 13C NMR, Figures S6 and S7, Table S1, and eq S1), particularly at lower pressures. Higher branch content was observed in THF and toluene in comparison to dialkylcarbonate solvents. This could originate from the higher solubility of low molar-mass EVA produced in THF (and toluene) in comparison to DMC and DEC. Short-chain branching (especially butyl branches) was favored over longer-chain branching (over 5 carbons) in carbonates. On the basis of results reported in Table 1 and in view of the use of EVA as cold flow improver (CFI) diesel fuel additives, ethylene pressure was fixed at 75 bar in the following to reach a good compromise between yield and VAc content. EVA additives must be soluble in diesel fuels and display low molar masses for maximal efficiency. In addition THF and DMC were selected, both promoting high yields and having very different transfer abilities. Molar masses of EVA synthesized in the “transferring” solvent THF were actually directly compatible with the requirements of their use as cold flow improver (CFI) diesel fuel additives. On the contrary, molar masses of EVA obtained in “non-transferring DMC” were too high to guarantee their solubility in diesel fuel at low temperature. The use of an additional transfer agent (propanal) was mandatory to reach the suitable molar-mass range. Control of Molar Masses in Nontransferring Solvent (DMC). The use of a chain transfer agent is common in free radical polymerization. Thiol compounds are widely used, but Sulfur incorporation in diesel fuel is prohibited. Thus, aldehyde compounds exhibiting high transfer abilities are preferred and are used in the high pressure and high temperature process for EVA synthesis to control molar masses. The mechanism of transfer from growing macroradical to propanal is given in Scheme 2. EVA syntheses were performed in DMC at 75 bar of ethylene pressure in the presence of various amounts of propanal. Results are summarized in Table 2. As expected, an increase in propanal concentration in the copolymerization medium permitted to decrease the EVA molar masses from 35000 to 1000 g mol−1. The suitable molarmass range for cold flow improver (CFI) diesel fuel application was reached. Corresponding chain-end structures from transfer
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RESULTS AND DISCUSSION Solvent Effects in EVA Synthesis. Four different solvents were selected based on previous studies of ethylene polymerization: toluene, diethyl carbonate (DEC), dimethyl carbonate (DMC), and tetrahydrofuran (THF).17,18 The pressure of ethylene was varied from 50 to 125 bar. The results of polymerization tests are given in Table 1. As observed in ethylene homopolymerization, yields were also strongly impacted by the solvent choice in copolymerization of ethylene with vinyl acetate. Extremely low yields were obtained in the less polar solvent (toluene), whereas higher yields were achieved in more polar solvents: toluene ≪ DEC ∼ DMC < THF. It is rather unsurprising as ethylene and vinyl acetate homopolymerization productivities are sensitive to solvent effects as reported in the literature.16−18,25−27 An increase of the yield while increasing the ethylene concentration (directly related to the ethylene pressure) was also observed in THF, and more surprisingly, an increase followed by a decrease was noticed in DMC and DEC. Further studies based on theoretical modeling of ethylene/solvent/vinyl acetate mixtures are in progress to explain these phenomena, which could originate from a phase change from biphasic to supercritical medium. Solvents also had a strong influence on molar masses of EVA copolymers. As illustrated in Scheme 1, transfer of the growing macroradical to the solvent molecules likely took place in the polymerization medium. This transfer was much higher in the case of THF (and toluene) than for DEC and DMC as evidenced by EVA molar masses (Table 1). EVA exhibiting molar masses (Mn) in the range 1200−2100 g mol−1 were obtained in THF while much higher molar masses (14000− 30000 g mol−1) were observed in dialkylcarbonates. Chain-end functionalities formed by transfer reactions to solvent were clearly evidenced by 13C NMR (Figure S1 for the full spectrum of EVA copolymer) in the case of THF and toluene (Figures S2 and S3, respectively). They were also observed in the case of DEC (weak signals, Figure S4) while they were not observed in C
DOI: 10.1021/acs.macromol.6b02756 Macromolecules XXXX, XXX, XXX−XXX
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A closer investigation of EVA microstructures by 13C NMR evidenced an impact of propanal concentration on branching. By increasing the concentration in propanal, global branch content increased from 6 to 12 br/1000C while the ratio Bn/B4, also increased from 0.6 to 3.7. This phenomenon might be driven by a higher solubility of lower molar mass polymer chains, hence more plausible intermolecular transfer reactions. Variation of Copolymer Composition in THF and DMC/Propanal. Two polymerization conditions were selected for the synthesis of low molar mass EVA: (1) copolymerization in “transferring” solvent, THF and (2) copolymerization in “non-transferring” solvent DMC in the presence of propanal as chain transfer agent. To further vary the copolymer composition (VAc content) the pressure of ethylene was fixed to 75 bar and the amount of vinyl acetate was changed keeping the total volume (solvent + VAc) constant (Table 3 and Table 4). In the case of DMC, the volume of propanal was fixed to 2.5 mL in order to obtain low molar masses, about 2000 g mol−1. Note that variations of VAc/solvent ratio certainly impacted ethylene solubility and physical properties of the copolymerization medium (permittivity (ε) and dipole momentum (μ)) and consequently could impact yields, copolymer composition and molar masses. In both solvents, similar trends were observed. An increase of yields with increasing amount of vinyl acetate in the polymerization medium was observed in both cases. At the same time a global increase of molar masses with the incorporation of polar units was noticed. It was expected in THF since an increase of the initial amount of vinyl acetate was accompanied by a decrease of the concentration of THF, which behaved as a transfer agent. It is more surprising in DMC since the quantity of propanal was kept constant; however this observation could be explained, at least partially, by the increase of VAc content in the copolymers. The VAc incorporation in EVA copolymers determined by 1 H NMR was directly proportional to the initial concentration of VAc in both solvents (Figure S10). A globally higher incorporation of VAc was noticed in DMC as compared to THF. Reactivity ratios were calculated using the Mayo−Lewis equation (eq S4), from copolymerization results presented in Tables 2−4. To this purpose several approximations were made: (1) a constant solubility of ethylene whatever the
Scheme 2. Mechanism of Radical Transfer Reaction to Propanal
reactions to propanal were clearly evidenced by 13C NMR (Figure S8). Propanal consumption during copolymerization was found to be limited (below 5%) which guaranteed no drift in molar masses. A determination of the transfer constant of propanal was done using conventional Mayo eq (Figure S9 and eq S2). To this purpose several approximations were made: (1) a constant solubility of ethylene whatever the medium composition was considered, (2) the composition drift in VAc was neglected which was also realistic as VAc consumption during copolymerization remained below 16%, (3) the composition drift in propanal was also neglected (below 5%), and (4) polymer chains initiated by AIBN were neglected. The ethylene solubility determination was reported in a previous work.16 A highly linear correlation coefficient was obtained (>0.99) and Ctr,propanal = 0.3581. This value is relatively close to the reported value for benzaldehyde (Ctr = 0.1970), and approximately 10 times higher than the transfer constant to THF (Ctr = 0.0288).28 This difference is consistent with molar masses of polymers reported in Table 3 and Table 4. THF concentration was around ten times higher than propanal concentration, and similar molar masses were obtained in THF and in DMC+propanal. The decrease of EVA molar masses with increasing amounts of propanal was also accompanied by a decrease in copolymerization yield, probably due to the particular reactivity of radical stemming from transfer reaction to propanal. However, propanal concentration did not influence copolymer composition (13.9−15.5 mol % VAc). Average block length LV was found to be also constant (LV ∼ 1.2). 13C NMR was also used to calculate the average block length LE of ethylene sequences in EVA copolymers (eq S3 and Table S2) which was found to also be constant (LE ∼ 7.5) whatever the amount of propanal introduced.
Table 2. EVA Synthesis in DMC in the Presence of Additional Transfer Agent (Propanal)a run DMC
17 18DMC* 19DMC 20DMC 21DMC 22DMC 23DMC 24DMC 25DMC 26DMC 27DMC* 28DMC*
propanal (mL)
η (g)
VAc (mol %)b
Mn (g mol−1)b
Mn (Đ) (g mol−1)c
Brd
Bn/B4d
LVd
LEd
Xc (%)e
0 0.125 0.25 0.375 0.5 1 1.5 2 2.5 3 3.5 4
4.5 3.95 3.64 3.8 4.29 3.37 3 2.8 2.73 2.5 1.9 1.79
14.9 14.7 14.3 14.5 15.1 15.5 15.4 13.9 13.9 14.8 14 14.2
− 21000 13000 9400 7700 4200 3100 2300 1800 1500 1400 1200
34800 (2) 20700 (1.8) 13300 (2) 9300 (1.9) 7500 (2.1) 3900 (2.4) 2700 (2.5) 2100 (2.5) 1700 (2.5) 1200 (2.7) 1100 (2.7) 900 (2.9)
6 4 6 6 6 9 10 9 12 9 10 11
0.6 0.8 1.5 1.2 1.1 2.5 3.0 2.2 3.7 2.5 2.1 2.7
1.21 1.19 1.20 1.19 1.20 1.22 1.21 1.18 1.21 1.22 1.22 1.24
7.3 7.2 7.5 7.3 7.1 7.1 7.2 7.6 7.1 7.3 7.6 7.9
15 15 16 16 16 15 16 16 17 15 13 11
a All experiments were carried out for 4 h (*3 h) at T = 70 °C with 40 mL of DMC and 10 mL of VAc at 75 bar ethylene pressure under standard conditions described in the Experimental Section. bDetermined by 1H NMR. cDetermined by SEC−THF (equivalent PS). dDetermined by 13C NMR. Br is the number of branching per 1000 CH2. Bn/B4 is the ratio between chain branches over 5 carbons (n > 5) and butyl branches. LV is the average block length of vinyl acetate. LE is the average block length of ethylene. eDetermined by DSC.
D
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Macromolecules Table 3. Variation of VAc Concentration in Copolymerization with Ethylene in THFa run
VAc (mL)
η (g)
VAc (mol %)b
Mn (g mol−1)b
29 * 30THF 31THF 32THF 33THF 34THF* 35THF 36THF
5 7.5 10 12.5 15 17.5 20 22
1.84 2.62 4.67 5.44 6.23 6.12 4.12 5.41
5.7 9.3 10.8 13.2 16.4 19.6 22.1 24.6
1400 1500 2000 2000 2200 2500 3300 3000
THF
Mn (Đ) (g mol−1)c 1900 2100 2100 2300 2700 2900 3300 3200
(2.2) (2.4) (2.7) (2.6) (2.6) (2.6) (2.7) (2.8)
Brd
Bn/B4d
LVd
LEd
Xc (%)e
6 6 7 8 5 6 4 4
1.3 1.6 1.8 2.0 1.6 2.8 0.7 1.2
1.11 1.18 1.18 1.24 1.25 1.31 1.32 1.35
18.8 11.5 9.9 7.5 6.2 5.6 5 4.7
32 27 24 18 14 11 5.5 4.2
All experiments were carried out for 3 h (*4 h) at T = 70 °C in THF under standard conditions (described in the Experimental Section). Determined by 1H NMR. cDetermined by SEC−THF (equivalent PS). dDetermined by 13C NMR. Br is the number of branching per 1000 CH2. Bn/B4 is the ratio between chain branches over 5 carbons (n > 5) and butyl branches. LV is the average block length of vinyl acetate. LE is the average block length of ethylene. eDetermined by DSC.
a b
Table 4. Variation of VAc Concentration in Copolymerization with Ethylene in DMC/Propanala run
VAc (mL)
η (g)
VAc (mol %)b
Mn (g mol−1)b
37DMC 38DMC* 39DMC 25DMC 40DMC 41DMC 42DMC 43DMC 44DMC 45DMC
2.5 5 7.5 10 12.5 15 17.5 22 24 26
1.4 1.14 1.01 2.73 2.77 3.5 4.25 4.84 5.58 5.31
4.1 8.8 10.8 13.9 17 19.8 22.4 28.7 30.5 32.4
1100 1500 1700 1800 2000 2000 2400 2800 2600 3200
Mn (Đ) (g mol−1)c 800 1200 1500 1700 1700 1800 2000 2500 2400 2800
(2.5) (2.7) (2.3) (2.5) (2.6) (2.6) (2.5) (2.3) (2.6) (2.5)
Brd
Bn/B4d
LVd
LEd
Xc (%)e
13 11 11 12 9 9 6 5 5 4
3.9 3.2 1.7 3.7 2.4 2.0 2.7 0.9 1.2 1.2
1.05 1.13 1.18 1.21 1.25 1.30 1.37 1.49 1.58 1.56
24.4 11.2 8.6 7.1 6.1 5.4 4.8 4 3.8 3.5
33 29 21 17 11 9.5 6.6 2.7 1.8 1.2
All experiments were carried out for 4 h (*3 h) at T = 70 °C in DMC with 2.5 mL of propanal under standard conditions (described in the Experimental Section). bDetermined from 1H NMR. cDetermined by SEC−THF (equivalent PS). dDetermined by 13C NMR. Br is the number of branching per 1000 CH2. Bn/B4 is the ratio between chain branches over five carbons (n > 5) and butyl branches. LV is the average block length of vinyl acetate. LE is the average block length of ethylene. eDetermined by DSC. a
agreement with syntheses of random copolymers with a good correlation between the composition of the feed and that of the copolymers. 13C NMR analysis of copolymers showed a slightly higher branching content in polymers produced in DMC than those produced in THF (Figure S11). In addition the average block length of vinyl acetate LV increases from 1.0 to 1.6 with increasing VAc incorporation while the average block length of ethylene LE strongly decreases (Table 3 and Table 4). Actually EVA copolymers exhibiting ethylene sequences of variable lengths (from 3 to 24 ethylene units) separated by mainly isolated polar units were synthesized. Investigation of Thermal Properties of EVA Copolymers in View of Their Use as Cold Flow Improver (CFI) Diesel Fuel Additives. The cooling of a diesel fuel causes crystallization of linear paraffins from the solution, which can lead to the plugging of filters in diesel motors. Additives, in particular EVA copolymers, are classically used to decrease the temperature of filter plugging. This phenomenon is evaluated by a standardized test (described in the Experimental Section) which gives a temperature, the cold filter plugging point (CFPP), for a couple diesel fuel/additive. The newly available EVA copolymers from less-energy demanding polymerization process were evaluated as CFPP additives and compared to three commercial EVA obtained from the industrial LDPE-like process. Regarding patent literature, EVA used as CFPP improver have VAc contents and molar masses in the same range as EVA synthesized in this work, the major difference being a higher branching content for commercial polymers.29−31 Indeed their syntheses were done at higher temperature, which is known to favor transfer reactions.
medium composition was assumed, which was realistic as solubilities of ethylene in THF, DMC, or VAc were close, and (2) a composition drift in VAc was neglected, which was also realistic as VAc consumption during copolymerization remained below 15% except for run 34THF. High linear correlation coefficients were obtained in both solvents. Likewise, a good fit between computed composition curves and experimental data was observed (Figure 1). Reactivity ratios were determined in both solvents: rE = 1.72, rVAc = 1.09, rErVAc = 1.87 in THF while rE = 1.16, rVAc = 0.78, rErVAc = 0.91 in DMC/propanal. They are similar and close to 1, in
Figure 1. Composition diagrams of ethylene−vinyl acetate copolymerization in THF and DMC. Key: (−) y = x; (red line) THF, computed curve; (red ▼)THF, experimental data; (green line) DMC, computed curve; (green ▲) DMC, experimental data. E
DOI: 10.1021/acs.macromol.6b02756 Macromolecules XXXX, XXX, XXX−XXX
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could also result in the crystallization of a broader range of ethylene sequences than for low molar masses. This assumption was confirmed by the study of the influence of cooling rate. For high molar-mass polymers, an increase of the crystallization temperature and a decrease of the initial exothermy were noticed with the decrease of the cooling rate (10, 5, and 1 °C), whereas no effect of the cooling rate was noticed for low molarmass polymers. On the other hand, the decrease of the crystallization temperature as well as of crystallinity values for low molar-mass EVA samples might be explained by a delay in crystallization due to a solubilization effect of the smallest chains on the rest of polymer chains. Polar-unit content is a key factor for controlling the crystallinity of EVA since the polar units hamper the crystallization of ethylene sequences. As shown in Figure 3,
Crystallization phenomena being involved in the control of CFPP of diesel fuel by additivation, thermal properties of EVA copolymers, and in particular their crystallinity and crystallization profiles, are critical parameters. These properties were investigated using DSC. The first observation on EVA copolymers was their extremely broad crystallization transitions (from +80 down to −30 °C). It originated from the microstructure of copolymer chains exhibiting ethylene sequences of various lengths, disturbed by branches and polar units. Moreover, the overlapping of crystallization and glass transition of EVA could render uneasy an accurate determination of the overall crystallinity value. Modulated DSC studies were thus done to separate glass transition and crystallization phenomena. The conclusion is a slight overestimation of crystallinity values determined by conventional DSC, regardless of the EVA composition. Only conventional DSC were performed thereafter. Crystallinities of EVA samples 17−45 were measured by DSC (Tables 2−4). EVA synthesized with various amounts of propanal in DMC (Table 2) exhibited similar crystallinity values (15−17%), except for the lowest molar masses ( rE in DMC), which was confirmed by calculations of average block length of ethylene LE from 13C NMR (Table 3 and Table 4). It explains the difference, as well as the sooner crystallization onset in THF (Figure 4) for similar polymer composition. Moreover, crystallization starts more intensely in THF, especially for high content in ethylene units. The lower
Figure 2. DSC thermogramms of EVA crystallization. Influence of molar masses on thermal properties.
On the one hand, a decrease of the temperature of the crystallization onset was noticed with the increase of molar masses. A higher chemical heterogeneity for low molar-mass polymers could be an explanation. Indeed, it is statistically more likely to form polymer chains with different chemical compositions for low molar masses, whereas a higher number of units in a polymer chain average this composition. However, this assumption was not supported by the simulation of the distribution of ethylene sequences based on the determined reactivity ratios. A modification of crystallization profile was also noticed since a more exothermic crystallization onset was observed for polymer of higher molar masses. We propose that the decrease in chain mobility due to higher molar masses delayed the crystallization. In addition, a lower branching content, therefore a higher number of longer ethylene sequences without default, which crystallized first, could possibly explain the higher exothermy at the crystallization onset. Moreover, the delay effect due to high molar masses
Figure 4. Differences of crystallization profiles (DSC thermogramms) of EVA obtained in THF and DMC. F
DOI: 10.1021/acs.macromol.6b02756 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules branching content together with the longer ethylene sequences for copolymers obtained in THF could explain these phenomena. Evaluation of Copolymers as CFPP Additives. This section aims at evaluating EVA produced under mild conditions as CFPP additives of diesel fuel and at establishing relationships between the structure of EVA and their cold flow improver (CFI) properties. Commercial EVA efficiencies as CFPP additives are presented in Table 5. The untreated diesel fuel selected for Table 5. Efficiencies of Commercial EVA as CFPP Additives commercial EVAa
ϕ
A
B
C
CFPP (°C)b
−11
−28
−27
−27
Figure 5. Influence of copolymers compositions on efficiency as CFPP additives.
a
Used at a concentration of 210 ppm. bNormalized method for a diesel fuel.
< 7%) and ethylene sequences are probably too short to be able to cocrystallize with n-paraffins. In summary, the new process under mild conditions for EVA synthesis allows one to easily and precisely control the VAc content and the molar masses of EVA copolymers, leading to very efficient cold flow improver diesel fuel additives.
this study exhibited a CFPP of −11 °C while values between −27 and −28 °C were measured in the presence of 210 ppm of commercial EVA. CFPP measurements were performed according to a normalized method (exposed in the Experimental Section). A range of EVA copolymers with various molar masses while containing similar percentage of VAc were produced in DMC under mild conditions using increasing concentration of propanal (Table 2). The influence of EVA molar masses on CFPP values (Table 6) was thus first investigated. CFFP between −25 and −26 °C (comparable with values obtained with commercial additives) were obtained in a large range of molar masses (900−9300 g mol−1) but the use of higher molar masses EVA (13300 g mol−1) result in an increase of CFPP (−15 °C), corresponding to a decrease in additive efficiency. The composition of the copolymer (polar-unit content) is another key characteristic of EVA, and its influence on CFPP efficiency was thus also investigated. CFPP values obtained with EVA copolymers featuring VAc contents between 4 and 25 mol % synthesized in DMC and in THF are reported in Figure 5. Independently of the solvent used for their synthesis, EVA copolymers containing from 8 to 20 mol % of VAc contribute to an improvement of CFPP of diesel fuel (between −24 and −30 °C). The two EVA exhibiting a lower vinyl acetate content are less efficient as CFPP additives (CFPP > −17 °C) because of their high crystallinity (Xc = 32−33%). It can be related to both a diminution of the fraction of soluble EVA chains and to the decrease of the proportion of “useful” ethylene sequences for cocrystallization with n-paraffins. Longest ethylene sequences could probably not stymie enough the crystal growth due to a lack of polar units, and perhaps not being able to cocrystallize at all with n-parafins. On the other hand, high vinyl acetate contents also inhibit the improvement of CFPP. The three EVA containing more than 22 mol % of VAc were giving CFPP higher than −15 °C. In this case, EVA copolymers are soluble in fuel diesel but their crystallinities are too low (Xc
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CONCLUSION Free radical copolymerization of ethylene with vinyl acetate was successfully performed in organic solvent medium at relatively low pressure (75 bar) and low temperature (70 °C). The selection of the organic solvent was crucial to control both EVA yields and molar masses. Two solvents were selected: tetrahydrofuran (THF) and dimethyl carbonate (DMC). A wide range of EVA was synthesized in both solvents allowing for determinations of reactivity ratios. EVA exhibiting higher molar masses (Mn > 30000 g mol−1) and slightly higher incorporations of VAc were obtained in DMC as compared to THF (Mn = 1500−3500 g mol−1). The use of increasing amounts of an additional transfer agent (propanal) permitted to control molar masses of EVA copolymers in DMC in the range of 1000 to 35000 g mol−1. In view of the targeted application as cold flow improver diesel fuel additives low molar masses EVA (Mn < 3500 g mol−1) containing from 4 to 32 mol % were synthesized using two different polymerization conditions: (1) THF and (2) DMC + propanal. The impact of molar masses and VAc content of copolymers (related to their crystallinity) on their efficiency for decreasing the CFPP temperature was investigated. Low molar masses EVA with VAc contents between 8 and 20 mol % of VAc corresponding to crystallinities between 9 and 29% exhibited similar efficiency than commercial EVA additives obtained at very high temperature and pressure. Beyond the specific use of low molar-mass EVA as cold flow improver diesel fuel additives highlighted in this paper, the mild conditions process allowed for the synthesis of higher molarmass EVA copolymers in DMC in the absence of additional
Table 6. Influence of Molar Masses on Additive Efficiency as CFPP Additive copolymera −1
Mn (g mol ) (Đ) CFPP (°C)c a
b
19
20
21
22
24
25
26
28
13300 (2.0) −15
9300 (1.9) −26
7500 (2.1) −26
3900 (2.4) −26
2100 (2.5) −26
1700 (2.5) −26
1200 (2.7) −25
900 (2.9) −26
Used at a concentration of 210 ppm. bSEC-THF (eq PS). cNormalized method on a diesel fuel (crude −11 °C). G
DOI: 10.1021/acs.macromol.6b02756 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
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transfer agents, which could be interesting materials for other EVA applications, in the packaging industry for instance.
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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.6b02756. 13 C NMR spectra of EVA copolymers, calculation of branching contents and of average block lengths of ethylene LE and vinyl acetate LV of EVA copolymers, and determination of transfer constant of propanal in ethylene−vinyl acetate copolymerization (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*(C.B.) E-mail:
[email protected]. *(V.M.) E-mail:
[email protected]. ORCID
V. Monteil: 0000-0003-3530-1789 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Total M&S is acknowledged for financial support. The authors thank Olivier Boyron and Manel Taam for SEC analyses and Dr. Sébastien Norsic for technical support. We are grateful to Dr. Fernande Boisson and the NMR Polymer Center of Institut de Chimie de Lyon (FR5223) for assistance and access to the NMR facilities. We are also grateful to Dr. Franck Colas and Mettler Toledo for assistance to the DSC facilities. We also thank Dr. Roger Spitz, Dr. Jean Raynaud, Dr. Julie Prévost and Dr Ana Maria Cenacchi Pereira for fruitful discussions.
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REFERENCES
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DOI: 10.1021/acs.macromol.6b02756 Macromolecules XXXX, XXX, XXX−XXX
