Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Group-Transfer Polymerization of Various Crotonates Using Organic Acid Catalysts Yasumasa Takenaka* and Hideki Abe Bioplastic Research Team, RIKEN Center for Sustainable Resource Science, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
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S Supporting Information *
ABSTRACT: Various poly(alkyl crotonate)s (PRCrs) were synthesized by group-transfer polymerization (GTP) using organic superacid catalysts such as 1-[bis(trifluoromethanesulfonyl)methyl]-2,3,4,5,6pentafluorobenzene (C6H5CHTf2) and 1-trimethylsiloxyl-1-methoxy2-methyl-1-propene (MTS) as an initiator. Under the optimized reaction conditions, the corresponding poly(ethyl crotonate) (PEtCr) was obtained with a narrow molecular weight distribution (MWD = 1.14) in quantitative yield (>99%). The thermal stabilities of the PRCrs obtained by organic acid-catalyzed GTP were generally superior to those of poly(alkyl methacrylate)s. The glass-transition temperature (Tg) of poly(methyl crotonate) (PMeCr) (122 °C) was higher than that of poly(methyl methacrylate) (PMMA) prepared by GTP (104 °C). The 5% weight loss temperature (Td5) of PMeCr (359 °C) was also higher than that of PMMA (304 °C). The values of the total light transmittance (Tt) (reaching 91.9%) and haze (99 93 10
41 69 96 86 h
8.3 9.2 14.4 21.3 0
7.5 12.0 27.2 42.2
7.7 13.5 29.8 43.2 1.6
1.15 1.20 1.14 1.35 1.28
1.0 0.77 0.52 0.50
a
Monomer (M) is ethyl crotonate (EtCr), initiator (I) is 1-methoxy-1-trimethylsilyloxy-2-methyl-1-propene (MTS), and catalyst (Cat.) is C6F5CHTf2, CH2Cl2 = 25 mL and [M]0/[I]0/[Cat.]0 = 100/1/0.1. bCalculated by the weight of the crude sample after removing the monomer (NR: no reaction). cThe obtained polymer was purified by reprecipitation with hexane. dCalculated from a conversion of the monomer and a feed ratio of monomer to initiator. eDetermined by 1H NMR spectra. fDetermined by SEC in CHCl3 using polystyrene standards with an RI detector. g Initiator efficiency: f = [molecular weight of polymer calculated by a feed ratio and conversion of monomer]/[number-average molecular weight of polymers determined by 1H NMR]. hNot isolated.
catalysts at −40 °C. Using C6H5CHTf2, HNTf2, and TMSNTf2 as catalysts, EtCr is successfully polymerized, and the obtained PEtCr shows a good yield (>66%) with a narrow MWD (Mw/Mn = 1.14−1.20) (entries 1−3). However, other or g a n i c ac i d an d b a s e c a t a ly s t s , s u c h a s tr i s(trifluoromethanesulfonyl)methane (HTf3), TiBP, o-carborane, and m-carborane do not promote the polymerization of EtCr (entries 4−7). These results suggest that organic superacids known to catalyze the Diels−Alder reaction32 or Mukaiyama−Michael reaction33,34 can also catalyze the GTP of EtCr. In the GTP of EtCr using these organic acid catalysts, the reaction temperature is critical for achieving PEtCr with a
narrow MWD and excellent yield (Table 2). The maximum monomer conversion is 91% for polymerization performed for 24 h at −20 °C (entry 10), but the MWD is broader than those obtained at lower temperatures such as −60 °C. The minimum MWD is obtained in the polymerization at −60 °C (entry 11), but the polymerization rate is very low. However, for polymerization at −40 °C, the obtained polymer shows a good monomer conversion (72%) and polymer yield (69%) with a narrow MWD (Mw/Mn = 1.20) (entry 3). Figure 1a depicts the time-dependent change in Mn values of the obtained PEtCr against the reaction time at −40 °C. The molecular weight of the polymer linearly increases while maintaining a narrow MWD (Figure 1a,b), suggesting a C
DOI: 10.1021/acs.macromol.9b00272 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Scheme 2. Proposed Mechanism for the Organic Acid-Catalyzed GTP of Crotonates15
Figure 2. MALDI-TOF MS spectra of poly(ethyl crotonate)s (a) prepared by GTP at −40 °C for 12 h and (b) prepared by GTP at 20 °C for 24 h. Series A and A′ are the protonated linear polymer [Mn = 101.12 + (114.14)n + 1.01 + 22.99 (Na+)] and [Mn = 101.12 + (114.14)n + 1.01 (not ionized)]. Series B is the cyclization polymer [Mn = 101.12 + (114.14)n−3 + 297.37 + 22.99 (Na+)]. Series C is the double cyclization polymer [Mn = 101.12 + (114.14)n−6 + (297.37)2 + 22.99 (Na+)].
smooth polymer growth without chain-transfer reactions. Additionally, the analysis of the polymerization kinetics in Figure 1c reveals a pseudo-first-order relationship between the monomer conversion and reaction time. Furthermore, we investigated the effect of the amount of the C6H5CHTf2 catalyst for the GTP of EtCr at −40 °C (Table 3). The polymer yield increases as the amount of the catalyst increases; a quantitative yield of the obtained polymer is achieved within 24 h for 0.10 mmol of the catalyst (entries 3, 15, and 16). However, further increases in the amount of catalyst decrease the polymer yield (entries 17 and 18). On the other hand, the initiator efficiency (f) decreases as the polymerization temperature and the amount of catalyst increase. These results suggest that side reactions, such as
isomerization and cyclization of propagating chain-end groups (Scheme 2),15,28 occur under polymerization at high temperatures and high catalyst concentrations. MTS is then changed to an inactive compound, 1-trimethylsiloxy-1-methoxy-2methyl-1-propane, as an initiator by the generation of a silicon Lewis acid. When quantitative amounts of an initiator and a catalyst are used, almost no polymerization occurs (entry 18). This result suggests that most of the MTS is consumed by the generation of the silicon Lewis acid. To provide detailed insight into the polymerization reaction, MALDI-TOF MS measurements were performed. In the MALDI-TOF MS analysis of the PEtCr obtained by GTP at −40 °C for 12 h (see Supporting Information), two series of peaks are observed (Figure 2a). The major peaks (series A) D
DOI: 10.1021/acs.macromol.9b00272 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
as the steric hindrance of the alkyl ester groups increases, the yields of the obtained PRCrs decrease (entries 22−26). tBuCr is not polymerized in this system (entry 25). All monomers except tert-butyl crotonate and p-tolyl crotonate polymerize to give the obtained polymers with quantitative yields for the optimal conditions (see Supporting Information, Table S1). To obtain higher-molecular-weight PRCrs with suitable physical properties, polymerization was conducted at a monomer/initiator/catalyst (M/I/C) feed ratio of 400/1/0.2 under otherwise identical conditions. Alkyl crotonates (MeCr, EtCr, nPrCr, iPrCr, nBuCr, and sBuCr) were successfully polymerized to obtain polymers with high number-average molecular weights (Mn) from 4.1 × 104 to 7.2 × 104 g/mol (Table 5, entries 27−31 and 33). However, the polymer yields are low, reaching only 40%, and the MWD values are somewhat broader (Mw/Mn = 1.17−1.32) because of the increased viscosity of the reaction solutions.
agree with the molecular weight of linear PEtCr with the MTS initiator residue and the desilylated chain end. The minor peaks (series B) agree with the molecular weight of PEtCr with cyclization of the propagating chain-end groups (cyclization polymer) (see Supporting Information, Scheme S1). In contrast, the major peaks (series B) in the MALDI-TOF MS analysis of PEtCr obtained by GTP at 20 °C for 24 h are cyclization polymers (Figure 2b, entry 8). The isomerization polymer (Scheme 2) is not found under our polymerization conditions. These results suggest that this organic strong acidmediated GTP at lower temperatures under −40 °C proceeds in a living manner with the inhibition of side reactions, such as backbiting cyclization reactions. Thus, organic strong acid catalysts, such as C6H5CHTf2, without using co-catalysts such as iodotrimethylsilane, overcome the low reactivity of the conventional metal-based Lewis acid catalysts used in the GTP.27 Various alkyl crotonates were successfully polymerized as the corresponding polymers by using the organic acidcatalyzed GTP system at −40 °C for 24 h. The results are shown in Table 4. Linear alkyl crotonates such as EtCr, nPrCr, Table 4. GTP of Alkyl Crotonate (RCr) in CH2Cl2 at −40 °C for 24 ha entry
R
yieldb(%)
Mnc(kg/mol)
Mw/Mnc
19 3 20 21 22 23 24 25 26
methyl ethyl n-propyl n-butyl isopropyl isobutyl sec-butyl tert-butyl p-tolyl
46 72 78 64 35 47 58 trace 5
7.3 13.5 10.9 9.4 6.7 6.2 4.2
1.12 1.20 1.18 1.18 1.17 1.14 1.18
1.3
1.08
a
Monomer (M) is alkyl crotonate (RCr), initiator (I) is 1-methoxy-1trimethylsilyloxy-2-methyl-1-propene (MTS), and catalyst (Cat.) is C6F5CHTf2, M = 50.0 mmol, I = 0.50 mmol, Cat. = 0.050 mmol, CH2Cl2 = 25 mL, [M]0/[I]0/[Cat.]0 = 100/1/0.1, and [M]0 = ca. 1.6 M. bThe obtained polymer was purified by reprecipitation. c Determined by SEC in CHCl3 using polystyrene standards with an RI detector.
and n BuCr are smoothly polymerized to obtain the corresponding polymers with good yields of 72, 78, and 64%, respectively, over 24 h (entries 3, 20, and 21). However,
Figure 3. (a) 1H NMR spectrum (CDCl3, room temperature (rt), 500 MHz) and (b) 13C NMR spectrum (CDCl3, rt, 125 MHz) of PMeCr.
Table 5. GTP of Alkyl Crotonate (RCr) in CH2Cl2 at −40 °C for 14 Days and the Optical Properties of the Obtained HighMolecular-Weight Polymersa entry
R
yieldb(%)
Mnc (kg/mol)
Mw/Mnc
Td5d (°C)
Tge (°C)
Ttf (%)
hazef (%)
thicknessg (mm)
27 28 29 30 31 32 33
methyl ethyl n-propyl n-butyl isopropyl isobutyl sec-butyl
68 86 92 65 60 19 40
40.8 59.3 71.7 50.8 41.2 30.4 48.7
1.24 1.32 1.27 1.22 1.23 1.13 1.17
359 330 338h 347 293 349 315
121.7 81.6 48.5h 32.7 120.9 73.2 96.7
92.0 92.4 91.9h 92.6 92.6 94.3 93.9
1.88 1.51 3.20h 3.71 4.95 1.80 2.39
0.170 0.160 0.175h 0.200 0.140 0.170 0.175
a
Monomer (M) is alkyl crotonate (RCr), initiator (I) is 1-methoxy-1-trimethylsilyloxy-2-methyl-1-propene (MTS), and catalyst (Cat.) is C6F5CHTf2, M = 200.0 mmol, I = 0.50 mmol, Cat. = 0.10 mmol, CH2Cl2 = 25 mL, [M]0/[I]0/[Cat.]0 = 400/1/0.2, and [M]0 = ca. 4.02 M. bThe obtained polymer was purified by reprecipitation with hexane. cDetermined by SEC in CHCl3 using polystyrene standards with an RI detector. d Determined by TGA. eMeasured by DMA. fThe values of transmissivity (Tt) and haze were measured by a haze meter. gMeasured by a vernier caliper. hMn is 122.1 kg/mol, and the MWD value is 2.02. E
DOI: 10.1021/acs.macromol.9b00272 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
show high Tt values, reaching 91.9%, equal to those of both PMMA films and glass plates (Table 5 and see Supporting Information). In addition, the haze values of the pressed PRCr films are very low at
