Termination Mechanism in the Radical Polymerization of Methyl

Article pubs.acs.org/Macromolecules

Termination Mechanism in the Radical Polymerization of Methyl Methacrylate and Styrene Determined by the Reaction of Structurally Well-Defined Polymer End Radicals Yasuyuki Nakamura†,‡ and Shigeru Yamago*,†,‡ †

Institute for Chemical Research, Kyoto University, Gokasyo, Uji, Kyoto 611-0011, Japan Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency, Gokasyo, Uji, Kyoto 611-0011, Japan

‡

Downloaded by UNIV OF CAMBRIDGE on September 8, 2015 | http://pubs.acs.org Publication Date (Web): September 4, 2015 | doi: 10.1021/acs.macromol.5b01532

S Supporting Information *

ABSTRACT: A novel method to determine the termination mechanism of radical polymerization, i.e., the selectivity between disproportionation (Disp) and combination (Comb), is developed. The method relies on product analyses of the reaction of polymer-end radicals, which are generated from structurally well-controlled living polymers, and the analyses of molecular weight and end-group structure of the product polymers by GPC, mass spectroscopy, and 1H NMR unambiguously determined the contribution of two competing pathways. The termination mechanism in the polymerization of methyl methacrylate (MMA) and styrene was investigated as a proof of principle of the method by using the corresponding polymers prepared by organotellurium-mediated radical polymerization. The ratios of Disp and Comb (D/C) of poly(methyl methacrylate) (PMMA) or polystyrene (PSt) end radicals at 25 °C were 73/27 or 15/85, respectively, and the results agreed well with the previous reports. The contribution of the Comb increased at higher temperature in both cases, though the temperature dependence was less pronounced in PSt radicals (D/C = 67/37 and 13/87 at 100 °C for PMMA and PSt, respectively). Thermodynamic parameters were determined as ΔΔG‡d/c = (−6.9 ± 0.3) − T × (−14.4 ± 1.0) × 10−3 (kJ mol−1) for PMMA and ΔΔG‡d/c = (−2.0 ± 0.5) − T × (−20.8 ± 1.5) × 10−3 (kJ mol−1) for PSt, in which ΔΔG‡d/c and T are difference in Gibbs energy undergoing Disp and Comb, and temperature in Kelvin, respectively, by carrying out the same experiments between −20 to +100 °C. The parameters reveal that Comb is enthalpically less favored but entropically more favored than Disp in both cases. The effects of molecular weight (chain length) were also investigated, and the D/C ratio became constant when the molecular weight of polymers was more than about 3000.

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reaction using small molecules,9,10 and so on. However, no definitive conclusions including quantitative thermodynamic parameters were obtained. Methyl methacrylate (MMA) and styrene (St) were the most extensively studied monomers, and almost entire reports concluded the major pathways of termination of MMA and St polymerizations are Disp and Comb, respectively. However, the selectivity of Disp and Comb (D/C) differs significantly in each report. For example, the D/C ratio in MMA polymerization varies from 85/15 to 57/43 even under nearly identical conditions.6a,c For St polymerization, the contribution of Comb varied from nearly 60% to ∼100% depending on the reports. Furthermore, though several factors as exemplified in temperature and chain length (molecular weight) are discussed to affect the selectivity, no definitive conclusion has been made so far. For example, the increase of Comb at higher temperature is reported using the reactions of a polystyrene (PSt) and poly(methyl methacrylate) (PMMA) model radi-

INTRODUCTION Radical polymerization is the most important polymerization technique industrially and produces about 40−50% of all synthetic polymers, which corresponds to over 105 metric tons annually. It proceeds by three elementary steps: initiation, propagation, and termination (Figure 1).1 The mechanisms of the initiation and propagation steps have been unambiguously clarified, mainly by kinetic studies and product analysis, for many initiators and monomers. However, the termination mechanism, i.e., the selectivity between combination (Comb) and disproportionation (Disp), is still unclear and controversial. The termination affects the chain length and the end-group structure of the resulting polymers and leads to different polymer properties such as toughness and thermal stability, which ultimately affect the final polymer materials and their products.2 Therefore, an accurate understanding of the reaction mechanism is crucial to the rational design of polymer materials. There were numerous reports for studying the selectivity of the termination by employing various methods, such as viscosity and gelation,3 thermal degradation,4 molecular weight and its distribution,5 end-group structure,6,7 mass spectroscopy,8 model © XXXX American Chemical Society

Received: July 10, 2015 Revised: August 21, 2015

A

DOI: 10.1021/acs.macromol.5b01532 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

structurally controlled bromine-terminated PSt, which was prepared by atom transfer radical polymerization and was activated by employing excess amount of Cu(0) and Cu(I) as reductants.14 However, no detail analyses to clarify the temperature and chain length were reported. Furthermore, since the work was limited to PSt, the generality of the method was not fully elucidated. Here we report on the details of the study.


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EXPERIMENTAL SECTION

General Experimental Procedures. All reaction dealing with oxygen sensitive compounds were carried out in a dry Pyrex reaction vessel under nitrogen atmosphere. A 500 W high pressure mercury lamp (Ushio) with the combination of a cutoff filter (Asahi Techno Glass), and 6 W white LED (Panasonic) with combination of neutral density filter (Sigma Koki) were used as light sources. In the photoirradiation experiment, the distance between light source and reaction vessel was set to be 10 cm. Material. Unless otherwise noted, chemicals obtained from commercial suppliers were used as received. Benzene, trifluoromethylbenzene were distilled over CaH2. Methyl methacrylate (MMA) and St were washed with 5% aqueous NaOH solution and were distilled over CaH2. Ethyl 2-(phenylltellanyl) isobutyrate was prepared as reported.15 Characterization. 1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were measured for a CDCl3 or C6D6 solution of a sample and are reported in ppm (δ) from internal tetramethylsilane or from solvent peak. Gel permeation chromatography (GPC) was performed on a machine equipped with PSt mixed gel columns (two linearly connected Shodex LF-804) at 40 °C using RI and UV detectors. THF or CHCl3 were used as an eluent for GPC. The GPC was calibrated with PMMA standards for PMMA and PSt standards for PSt samples. High resolution mass spectra (HRMS) were obtained under fast atom-bombardment (FAB) conditions. Matrix-assisted laser desorption/ionization time-offlight mass spectrometry (MALDI−TOF-MS) spectra were recorded using positive reflector mode with 20 kV acceleration voltages. Samples were prepared by mixing a THF solution of a polymer (10 mg/mL), dithranol (10 mg/mL), and sodium trifluoroacetate (5 mg/mL) in a ratio of 1:1:1. Peak Resolution Analysis of GPC Data. The GPC trace after the reaction was divided into two components by peak resolution method reported by Fukuda et al.14 The Mn of low- or high-molecular weight components was usually identical to or almost double to that of the starting polymer, respectively. The low- and high-molecular weight components were attributed to the Disp and Comb products, respectively. The D/C ratio was determined from the integration of the peaks after subtracting the contribution of dead polymer formed during preparation and purification of the starting living polymer. PMMA, 1. A solution of MMA (1.86 mL, 17 mmol) and ethyl 2(phenylltellanyl)isobutyrate (150 μL, 0.66 mmol), diphenyl ditelluride (270 mg, 0.66 mmol) was irradiated using 6 W white LED lamp equipped with neutral density filter (8% transmittance) at 65 °C for 3 h under a nitrogen atmosphere. A small portion of the reaction mixture was removed, and the conversion of the monomer (92%) was determined by 1H NMR spectroscopy. The reaction mixture was dissolved in degassed chloroform and then poured into vigorously stirred degassed hexane under nitrogen atmosphere. The product was collected by suction filtration and dried under reduced pressure to give 1 (1.51 g, 87% yield). Mn (3300) and Mw/Mn (1.14) were determined by GPC. PSt, 7. A solution of St (1.5 mL, 13.3 mmol) and ethyl 2(phenylltellanyl)isobutyrate (100 μL, 0.44 mmol) was heated at 100 °C for 16 h. A small portion of the reaction mixture was removed, and the conversion of the monomer (78%) was determined by 1H NMR spectroscopy. The reaction mixture was dissolved in degassed chloroform and then poured into vigorously stirred degassed methanol under nitrogen atmosphere. The product was collected by suction filtration and dried under reduced pressure to give 7 (1.01 g, 74% yield). Mn (2800) and Mw/Mn (1.10) were determined by GPC.

Figure 1. Reaction mechanism of radical polymerization. Three elementary steps occur in radical polymerization. I, R•, M, and Mn denote a radical initiator, initiating radical, monomer, and polymer chain with n repeating monomer unit, respectively.

cals,9b,10c but the opposite temperature effect is reported based on the end-group analysis in the polymerization of MMA using a radioactive initiator3b or the gelation study.6a,b These results clearly call for the necessity of a new method which would unambiguously determine the contribution of Comb and Disp. We propose here a new method to clarify the termination mechanism based on simple product analysis, i.e., the analysis of the molecular weights and end-group structures of the product polymers. Since the molecular weight of the Comb product is twice as that of the Disp products, analysis of the molecular weight is, in principle, the ideal method to determine the contribution of the two termination pathways. In addition, when the Disp takes place, the products should consist of a 1:1 mixture of end-hydrogenated and end-dehydrogenated products, Mn(+H) and Mn(−H), respectively, as shown in Figure 1c. However, this simple analysis could not be applied thus far due to the difficulty in controlling the molecular weight in radical polymerization. We envisioned, however, that a simple product analysis would become possible by starting from structurally well-defined polymers in terms of molecular weight and molecular weight distribution as precursors of the polymer-end radical species. Such polymers are now available by using controlled/living radical polymerization. We utilized organotellurium-mediated radical polymerization (TERP) in this study for the preparation of the precursors of the polymer-end radicals,11 although several methods for the living radical polymerization have also been developed.12 The most important advantage of TERP over other methods in clarifying the termination mechanism is the efficient activation of the organotellurium dormant species by photoirradiation to generate the corresponding polymer-end radicals.13 Since the photochemical generation of radicals does not require chemical reagents, side reaction(s) derived from the radicals and chemical reagents, such as multiple redox reactions, can be minimized. Furthermore, as radicals can be generated without heating, the reaction temperature can be arbitrarily selected. Herein, we apply this method to clarify the termination mechanism in the polymerization of the representative vinyl monomers, MMA and St. We also examine factors affecting the termination reaction, such as temperature and chain length (molecular weight). Fukuda and co-workers already reported the clarification of termination mechanism of St starting from B


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Figure 2. Termination reaction of PMMA end radicals and product analysis. (a) Reaction scheme for termination reaction of PMMA end radicals. (b) GPC profile of the reaction mixture, and the result of peak separation into two components. (c) Full and (d, e) partial MALDI−TOF MS spectra of the termination product of PMMA end radicals. Molecular ion peaks were observed as Na+ adducts. Simulated spectra of a 1:1 mixture of the disproportionation products (3 and 4, n or n′ = 28) and combination product (2, n + n′ = 43) are shown in parts d and e, respectively. Photoinduced Termination Reaction of Polymer End Radicals. A solution of 1 or 7 (typically, Mn = 1900−31600, Mw/Mn ≤ 1.22) in a solvent in a Pyrex glass tube under nitrogen atmosphere was photoirradiated with a 500 W high pressure Hg lamp through a 390 nm cutoff filter for 1 to 6 h. After the completion of the reaction (as monitored by 1H NMR for the formation of diphenyl ditelluride), the reaction mixture was analyzed by 1H NMR, GPC and MALDI−TOF MS. Dimethyl 2,2,4-Trimethyl-4-(phenyltellanyl)pentanedioate (6a). To a solution of methyl trimethylsilyl dimethylketene acetal (1.0 g, 5.8 mmol)16 and ZnBr2 (128 mg, 0.58 mmol) in CH2Cl2 (4 mL) was added methyl methacrylate (0.59 mL, 5.4 mmol) at room temperature under nitrogen atmosphere, and the resulting solution was stirred for 4.5 h. To this solution was added a solution of phenyltellanlyl bromide, which was prepared by mixing (TePh)2 (1.4 g, 3.4 mmol) and bromine (0.16 mL, 3.2 mmol) in THF (10 mL) at 0 °C for 1 h, and the resulting solution was stirred at room temperature for 7 h. The solvent was removed under reduced pressure. The residue was purified by preparative GPC under nitrogen atmosphere to give 6a (0.80 g, 35% yield) as a yellow orange greasy solid. 1H NMR (C6D6): δ 1.03 (s, 3H, CH3), 1.13 (s, 3H, CH3), 1.83 (s, 3H, CH3), 2.75 (d, J = 14.3 Hz, 2H, CH2), 2.96 (d, J = 14.3 Hz, 2H, CH2), 3.23 (s, 3H, OCH3), 3.28 (s, 3H, OCH3), 6.95 (t, J = 7.4 Hz, 2H, Ph), 7.16 (t, J = 7.4 Hz, Ph), 7.87 (d, J = 7.2 Hz, Ph). 13C NMR (C6D6): δ 22.1. 23.7, 29.3, 35.0, 43.8, 50.3, 51.3, 51.5, 114.6, 129.2, 129.3, 142.6, 176.3, 177.7. HRMS (FAB) m/z: found, 408.0586; calcd for C16H22O4Te [M]+, 408.0580. Methyl 2-(Phenyltellanyl)isobutyrate (6b). Phenyl lithium (20.0 mL, 1.74 M in Bu2O, 34.6 mmol) was added slowly to a suspension of tellurium powder (4.4 g, 34.6 mmol) in THF (35 mL) at 0 °C under nitrogen atmosphere, and the resulting solution was stirred at room temperature for 30 min. To this solution was added methyl 2bromoisobutyrate (4.5 mL, 36.3 mmol) at 0 °C, and the mixture was

stirred at room temperature for 2.5 h. Deaerated water was added, and the aqueous phase was separated under a nitrogen atmosphere. The remaining organic phase was washed with deaerated saturated aqueous NH4Cl solution, dried over MgSO4 and filtered under nitrogen atmosphere. The solvent was removed under reduced pressure followed by distillation (75−77 °C/0.08 mmHg) to give 6b (5.2 g, 49%) as orange oil. 1H NMR (CDCl3): δ 1.73 (s, 6H, CH3), 3.61 (s, 3H, OCH3), 7.29 (t, J = 7.7 Hz, 2H, Ph), 7.41 (t, J = 7.4 Hz, 1H, Ph), 7.88 (d, J = 6.9 Hz, 2H, Ph). 13C NMR (CDCl3): δ 28.5, 29.8, 52.1, 113.5, 129.0, 129.2, 142.0, 177.1. HRMS (FAB) m/z: found, 308.0058; calcd for C11H14O2Te [M]+, 308.0056. Phenyl 1-Phenylethyltelluride (12).11c Phenyl lithium (25 mL, 1.15 M in cyclohexane and Et2O, 28.7 mmol) was added slowly to a suspension of tellurium powder (3.8 g, 29.7 mmol) in THF (25 mL) at 0 °C under nitrogen atmosphere, and the resulting solution was stirred at room temperature for 30 min. To this solution was added (1bromoethyl)benzene (3.9 mL, 28.7 mmol) at 0 °C, and the mixture was stirred at room temperature for 2.5 h. Deaerated water was added, and the aqueous phase was separated under a nitrogen atmosphere. The remaining organic phase was washed with deaerated saturated aqueous NH4Cl solution, dried over MgSO4 and filtered under nitrogen atmosphere. The solvent was removed under reduced pressure followed by distillation (118−120 °C/0.06 mmHg) to give 12 (3.0 g, 35%, purity ∼95%) as orange oil. 1H NMR (CDCl3): δ 1.95 (d, J = 7.4 Hz, 3H), 4.80 (q, J = 7.1 Hz, 1H), 7.12 (m, 1H), 7.15−7.23 (m, 6H), 7.31 (t, J = 7.4 Hz, 1H), 7.70 (d, J = 8.0 Hz, 2H). 13C NMR (CDCl3): δ 23.8, 24.7, 113.8, 126.2, 126.7, 128.1, 128.2, 129.9, 140.5, 145.2. HRMS (FAB) m/z: found, 312.0163; calcd for C14H14Te [M]+, 312.0158. C


Article

Macromolecules Table 1. Radical−Radical Reaction Starting from PMMA 1 under Photoirradiationa precursor 1

product

entry

Mnb

Mw/Mnb

solvent

temp (°C)

Mnb

Mw/Mnb

D/Cc

kd/kcc

1 2 3 4 5 6 7 8 9 10

3300 3300 3300 3300 3300 3300 31600 1900 6ad 6bd

1.14 1.14 1.14 1.14 1.14 1.14 1.19 1.10 1.00 1.00

C6H6 PhCF3 PhCF3 PhCF3 PhCF3 PhCF3 C6H6 C6H6 C6D6 C6D6

25 −20 0 25 60 100 25 25 25 25

3800 3500 3700 3800 3800 4000 38300 2200

1.20 1.22 1.22 1.22 1.23 1.22 1.24 1.19 e e

73/27 83/17 78/22 73/27 68/32 63/37 73/27 65/35 56/44 55/45f

2.7 4.9 3.6 2.7 2.1 1.7 2.7 1.9 1.3 1.2

e e

A solution of 1 (0.12 g/mL) in a solvent was irradiated with a 500 W high pressure Hg lamp through a 390 nm cutoff filter for 1−4 h. bNumberaverage molecular weight (Mn) and molecular weight distribution (Mw/Mn, where Mw is the weight-average molecular weight) of the polymers were determined by GPC calibrated with PMMA standards. cThe ratio of disproportionation and combination products (D/C) was determined by the GPC peak resolution method. The ratio of the rates of the disproportionation and combination reactions (kd/kc) was calculated from the D/C ratio. d 6a or 6b was used as the precursor instead of 1. eResults were analyzed exclusively by 1H NMR. fMethyl 2-methylpropioate and MMA were obtained as Disp products.


a

Figure 3. Effect of reaction conditions in the termination reaction of PMMA-end radicals. (a) GPC profiles of the termination reaction at various temperatures. (b) Eyring plot for the D/C selectivity of the termination reaction.

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RESULTS AND DISCUSSION Termination Mechanism of MMA Polymerization. The termination mechanism in MMA polymerization, i.e., the reaction of the PMMA-end radical was examined at first to clarify the validity of the current method. Structurally welldefined PMMA 1 (Mn = 3300, Mw/Mn = 1.14), bearing a phenyltellanyl group at the ω-end, was dissolved in benzene (0.12 g/mL) and photolyzed at 25 °C with a 500 W Hg lamp through a 390 nm short-wavelength cutoff filter (Figure 1a). 1 was completely consumed after 2 h, as suggested by the quantitative formation of diphenyl ditelluride by 1H NMR analysis (Figure S1 in Supporting Information). The GPC analysis of the product(s) revealed a bimodal trace which was divided into two components by peak resolution (Figure 2b).14 The Mn of the major component was identical to that of precursor 1 and that of the minor component was almost double that of 1; the former and the latter components were assigned as the Disp and Comb products, respectively. The selectivity (D/C) between Disp and Comb was determined to be 73/27 from the molar ratio of the major and minor components after correction for the amount of dead polymer (4%) terminated during the preparation of 1 (Table 1, entry 1). The selectivity is nearly identical to several previous reports.6b,3b Since the termination proceeds irreversibly, the D/C selectivity is identical to the relative reactivity of the two pathways, as given by kd/kc (=2.7), in which kd and kc are the rate constants for the Disp and Comb reactions, respectively.

The occurrence of the two termination pathways was unambiguously proven by the structural analysis of the products by matrix-assisted laser desorption/ionization−time-of-flight mass spectroscopy (MALDI−TOF MS) (Figure 2c) and 1H NMR (Figure S1). The mass spectrum showed two series of monoisotopic molecular ion signals separated by a molecular mass of MMA (m/z = 100.1), as indicated by red and blue lines in Figure 2c. For the major series (red), the isotopic distribution of each peak was superimposable on that of the theoretical distribution of a 1:1 mixture of Disp products 3 and 4 (Figure 2d). For the minor series (blue), the isotopic distribution was identical to the simulated distribution of Comb product 2 (Figure 2e). The 1H NMR spectrum of the reaction mixture showed characteristic olefinic peaks at 6.20 and 5.47 ppm corresponding to the vinylidene proton of 4. The D/C ratio was estimated to be 71:29 from the intensity of the vinylidene signals relative to the methylene signals derived from the ethyl ester moiety at the α-polymer end. This value agreed well with that determined from GPC analysis. A control experiment with the addition of MMA (5 equiv) under otherwise identical conditions revealed that the D/C ratio was essentially the same as that without MMA, although an accurate ratio could not be determined due to the competitive occurrence of polymerization. The effects of reaction temperature and molecular weight were next examined, and these factors considerably influenced the D/ C selectivity (Figure 3). When the reaction was carried out at −20 °C, the contribution of the Disp increases and the D/C ratio changed to 83/17. In contrast, the contribution of the Comb D


Article

Macromolecules Table 2. Radical−Radical Reaction Starting from PSt 7 under Photoirradiationa precursor 7

product

entry

Mnb

Mw/Mnb

temp. (°C)

Mnb

Mw/Mnb

D/Cc

kd/kcc

1 2 3 4 5 6

2800 26200 12d 2800 2800 2800

1.10 1.22 1.0 1.10 1.10 1.10

25 25 25 60 80 100

4300 41300

1.20 1.26 e 1.22 1.22 1.23

15/85 14/86 15/85f 14/86 14/86 13/87

0.18 0.17 0.18 0.17 0.17 0.15

e 4300 4300 4400

A solution of 7 (0.12 g/mL) in a solvent was irradiated with a 500 W high pressure Hg lamp through a 390 nm cutoff filter for 2 h. bNumberaverage molecular weight (Mn) and molecular weight distribution (Mw/Mn, where Mw is the weight-average molecular weight) of the polymers were determined by GPC calibrated with PSt standards. cThe ratio of disproportionation and combination products (D/C) was determined by the GPC peak resolution method. The ratio of the rates of the disproportionation and combination reactions (kd/kc) was calculated from the D/C ratio. d12 was used as the precursor instead of 7. eResults were analyzed exclusively by 1H NMR. fEthylbenzene and styrene were obtained as Disp products.


a

Figure 4. Termination reaction of PSt end radicals and product analysis. (a) Reaction scheme for termination reaction of PSt end radicals. (b) GPC profile of the reaction mixture, and the result of peak separation into two components. (c) Full and (d, e) partial MALDI−TOF MS spectra of the termination product of PSt end radicals. Molecular ion peaks were observed as Ag+ adducts. The partial MS spectrum and the corresponding simulated spectra of a 1:1 mixture of the disproportionation products (9 and 10, n = 22), or combination product (8, n + n′ = 37) are shown in parts d and e, respectively.

increased at higher temperature, affording a D/C ratio of 63/37 at 100 °C (Table 1, entries 2−6). Since all experiments were carried out below the ceiling temperature of MMA (155.5 °C),17 the effect of depolymerization is negligible. The Eyring plot of ln(kd/kc) versus the inverse of the temperature in Kelvin showed an excellent linear correlation (Figure 3b). The difference in the Gibbs energies, ΔΔG‡d/c undergoing Disp and Comb was determined as

from the slope and intercept of the plot. The result suggests that the Comb is enthalpically less favored but entropically more favored than the Disp. While the observed temperature effect is opposite to those of previous reports,6b,a,d the excellent linear correlation in the Eyring plot strongly supports the validity of the current results. Furthermore, the same tendency, i.e., increase of Comb over Disp at higher temperature was also reported in the radical−radical reaction of simple alkyl radicals.18 This is the first example to clearly and quantitatively show an apparent temperature effect in the termination mechanism. Tanner reported that the effect of viscosity on the Comb/Disp selectivity in the reaction of t-butyl radical; increase of Comb over Disp at

ΔΔG‡ d/c=( −6.9 ± 0.3) − T × ( − 14.4 ± 1.0) × 10−3 (kJ mol−1) E


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Figure 5. Effect of reaction conditions in the termination reaction of PSt-end radicals. (a) GPC profiles of the termination reaction at various temperatures. (b) Eyring plot for the disproportionation/combination selectivity of the termination reaction. The selectivity ln(kd/kc), in which kd/kc is the ratio of the rates of disproportionation and combination, is plotted against the inverse absolute temperature (1/T).

low viscous media.19 Since viscosity of reaction media decreases at high temperature, it is possible that viscosity is the real reason for the observed temperature effect. Further studies should be required to clarify this point. The effect of the molecular weight (chain length) was next examined. When high-molecular-weight PMMA 1 (Mn = 31600, Mw/Mn = 1.19) was photolyzed under the standard conditions (irradiation of 0.12 g/mL sample in benzene under a 500 W Hg lamp through a 390 nm short-wavelength cutoff filter at 25 °C), an essentially identical D/C ratio (73/27) was observed (entry 7). The contribution of the Comb increased (D/C = 65/35) when low-molecular-weight PMMA 1 (Mn = 1900, Mw/Mn = 1.10) was used (entry 8). This effect became more significant when dimer and monomer models of PMMA 6a and 6b, respectively, were employed under the standard conditions (entries 9 and 10, Figures S2 and S3). These results clearly show the existence of a molecular weight effect. However, the D/C ratio becomes constant when the Mn of the precursor exceeds ≈3000, and the Mn of a conventional radical polymerization usually reaches to above 104. Therefore, the results presented in this work with Mn ≈ 3000 represent the selectivity of the termination reaction of MMA polymerization. Termination Mechanism of St Polymerization. The termination in St polymerization was next examined using the structurally well-defined PSt 7 (Mn = 2800, Mw/Mn = 1.10). After photoirradiation of 7 by using the standard conditions used above, the polymer products were analyzed. The GPC trace of the product was largely shifted to the high molecular weight side, indicating the major termination pathway was Comb (Figure 2b). The D/C ratio was determined to be 15/85 (Table 2, entry 1). The results were further supported by MS analysis which exhibited two series of distributions of molecular ion peaks (Figure 2c). The minor peaks at the lower molecular weight region corresponded to a 1:1 mixture of Disp products 9 and 10 (Figure 2d), and the major and higher molecular weight peak corresponded to the Comb product 8 (Figure 2e). The effect of the chain length was next examined by using high-molecular-weight PSt 7 (Mn = 26200, Mw/Mn = 1.22) and low-molecular-weight model compound 12 under the standard conditions. The D/C ratios were obtained as 14/86 and 15/85, respectively (Table 2, entries 2 and 3, and Figure S4), which were almost the same with that obtained from 7 of Mn = 2800. The results suggest that the reactivity of PSt end radicals is less sensitive to the chain length than that of PMMA end radicals. The effect of temperature was also examined at 25−100 °C (Figure 5). The contribution of Comb also increased as the higher temperature from D/C = 15/85 at 25 °C to D/C = 13/87 at 100 °C, though the D/C ratio was less sensitive to the

temperature than that of PMMA end radical (Table 2, entries 4− 6). The Eyring plot of ln(kd/kc) versus the inverse of the temperature in Kelvin also showed a good linear correlation, giving the difference in the Gibbs energy, ΔΔG‡d/c undergoing Disp and Comb as ΔΔG‡ d/c=( −2.0 ± 0.5) − T × ( −20.8 ± 1.5) × 10−3 (kJ mol−1)

The result also confirmed the Comb is enthalpically less favored but entropically more favored than the Disp is as is the case of PMMA termination. The results would suggest that Comb becomes favored at high temperature regardless of the monomer species.

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CONCLUSION A definitive method to determine the mechanisms of the termination reactions in radical polymerization was developed, and the method was applied for MMA and St polymerizations. The results clearly and quantitatively clarified the two pathways involving the corresponding polymer-end radical species. The major pathway for MMA or St polymerizations is Disp or Comb, respectively, and the conclusions agree well with the previous reports. Furthermore, this work clearly quantified the effects of temperature and chain length for the first time. The contribution of Comb increased at higher temperature in both PMMA and PSt terminations, and the results are also in good agreement to the previous report using small molecule radicals. The method developed in this work is operationally simple, and a wide range of radical precursors for the polymer-end radicals can be prepared by TERP. Therefore, this method would be applicable for clarifying termination mechanisms of a wide range of monomers.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01532. NMR spectra of the reactions of model compound and of the new compounds (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(S.Y.) E-mail: [email protected]. Notes

The authors declare no competing financial interest. F


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(16) Kita, Y.; Tamura, O.; Itoh, F.; Yasuda, H.; Kishino, H.; Ke, Y. Y.; Tamura, Y. J. Org. Chem. 1988, 53, 554. (17) Bywater, S. Trans. Faraday Soc. 1955, 51, 1267. (18) (a) Gibian, M. J.; Corley, R. C. Chem. Rev. 1973, 73, 441. (b) Schuh, H.-H.; Fischer, H. Helv. Chim. Acta 1978, 61, 2463. (19) Tanner, D. D.; Rahimi, P. M. J. Am. Chem. Soc. 1982, 104, 225.

ACKNOWLEDGMENTS This work was partly supported by the Core Research for Evolution Science and Technology (CREST) of the Japan Science and Technology Agency (S.Y.) and by Grants-in-Aid for Scientific Research from the Japan Society for the Promotion of Science and Ministry of Education, Culture, Sports, Science, and Technology, Japan (Grants 24350057 and 24109005 for S.Y. and Grant 25810021 for Y.N.).


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REFERENCES

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