Living Anionic Polymerization of α-Methyleneindane: An Exo

Sep 24, 2015 - α-Methyleneindane (MI), possessing an exo-methylene group, was synthesized by the Wittig reaction of 1-indanone and methyltriphenylpho...
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Living Anionic Polymerization of α‑Methyleneindane: An ExoMethylene Hydrocarbon Monomer Haruka Ohishi, Yuki Kosaka, Keita Kitazawa, Raita Goseki, Susumu Kawauchi, and Takashi Ishizone* Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1-S1-13 Ohokayama, Meguro-ku, Tokyo 152-8552, Japan S Supporting Information *

ABSTRACT: α-Methyleneindane (MI), possessing an exo-methylene group, was synthesized by the Wittig reaction of 1indanone and methyltriphenylphosphonium bromide in the presence of potassium tert-butoxide in THF. Since the synthesized MI was contaminated with 0.2−1.5 mol % of the isomeric byproduct, 3-methylindene (3MI), showing a high acidity (pKa ∼ 22), the basic anionic initiators, such as sec-BuLi, were immediately destroyed by the acidic 3MI prior to the polymerization of MI. After the partial deactivation of the initiator, the polymerization of MI quantitatively proceeded with the residual anionic initiator including sec-BuLi, lithium naphthalenide, and potassium naphthalenide in THF at −78 °C to form the polymers having predicted molecular weights based on the molar ratios between MI and the residual initiators and narrow molar mass dispersities (ĐM = Mw/Mn < 1.1). Since the polymerization of MI, a cyclic analogue of α-methylstyrene (αMeSt), quantitatively occurred at 0 °C in THF, the polymerizability of MI significantly differed from that of αMeSt having the low ceiling temperature around 0 °C. A tailored block copolymer of MI and styrene, poly(MI)-b-polystyrene, was obtained with a quantitative efficiency in THF at −78 °C by the sequential anionic copolymerization of MI and styrene, indicating the living character of the propagating carbanion of poly(MI). The relative anionic polymerizability of MI was estimated by the density functional theory (DFT) calculations. MI also underwent the free-radical polymerization with α,α′-azobis(isobutyronitrile) (AIBN) and the cationic polymerization with boron trifluoride diethyl etherate (BF3OEt2). The resulting poly(MI) possessed indane ring structures vertical to the main chain via the addition polymerization of the exo-methylene group and showed the glass transition temperature at 137 °C.

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INTRODUCTION

Very recently, we succeeded in the living anionic polymerization of benzofulvene (BF, α-methyleneindene) with either sec-BuLi or diphenylmethylpotassium (Ph2CHK) in THF at −78 °C (Scheme 1).6−8 BF possessing an exo-methylene group can be considered as a cyclic analogue of 2-phenyl-1,3butadiene and actually reacts as a 1,3-butadiene derivative during the anionic polymerization, since the resulting polymers always possess 1,4- and 1,2-structures in the repeating units.

Living anionic polymerization is well-known as a powerful tool for the synthesis of well-defined polymers, such as endfunctionalized polymers, block copolymers, star-shaped polymers, graft polymers, and dendritic polymers.1−5 It has been established that hydrocarbon vinyl monomers, such as styrene, α-methylstyrene (αMeSt), 1,3-butadiene, and isoprene, undergo anionic polymerization to provide the highly reactive but stable living polymers with predicted molecular weights and narrow molar mass dispersities (ĐM = Mw/Mn < 1.1). It is noteworthy that these hydrocarbon monomers show a significantly low anionic polymerizability compared to the highly reactive polar monomers, such as vinylpyridines, alkyl methacrylates, and N,N-dialkylacrylamides, similarly capable of the living anionic polymerizations. This means that the highly reactive anionic initiators, such as sec-butyllithium (sec-BuLi) and alkali metal naphthalenides, are required to initiate the anionic polymerization of the hydrocarbon monomers. © XXXX American Chemical Society

Scheme 1. Living Anionic Polymerization of BF

Received: July 17, 2015 Revised: September 14, 2015

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treatment of the reaction mixture with p-toluenesulfonic acid. The resulting 1-hydroxy-1-methylindane was readily converted to 3MI and not to MI via the selective dehydration according to the well-known Zaitsev’s rule in organic chemistry. We then synthesized MI by the Wittig reaction of 1indanone and methyltriphenylphosphonium bromide in the presence of potassium tert-butoxide (tBuOK) in THF (Scheme 4). In this case, MI, the target monomer, could be isolated in

Very interestingly, BF could be sequentially polymerized with even a propagating enolate anion of methyl methacrylate (MMA) with a quantitative efficiency to form a tailored poly(MMA)-b-poly(BF). The high anionic polymerizability of BF is thus demonstrated, since the low nucleophilic living poly(MMA) anion cannot initiate the polymerization of hydrocarbon monomers, such as styrene and isoprene. In this study, we focused on the anionic polymerization behavior of α-methyleneindane (MI)9,10 having an exomethylene group and a five-membered ring fused with a benzene ring similar to BF. It should be emphasized that the electronic environment of MI should be remarkably different from that of BF, since the former possesses 8 π-electrons including the exo-methylene group, but the latter has extended 10 π-electrons in the molecule. Based on its chemical structure, MI corresponds to a cyclic analogue of αMeSt showing an equilibrium anionic polymerizability.11,12 Ueda and co-workers reported that MI could be polymerized with radical, cationic, and anionic initiators.9 In contrast to the mixed microstructures of poly(BF),6−8 the repeating units of the resulting poly(MI)s exclusively contained ring structures perpendicular to the main chain via the addition polymerization of the exo-methylene group.13−28 In the case of the anionic polymerization, a polymer was obtained with n-BuLi in THF at 20 °C for 20 h in 96% yield. However, the molecular weight of the polymer was much higher than the calculated value (Mn,calcd = 6190, Mn,obsd = 18 200), and the molecular weight distribution was very broad (ĐM = 3.8). Under similar conditions, it has been confirmed that no apparent polymerization of αMeSt anionically occurred at all due to its low ceiling temperature (∼0 °C).11,12 This result suggests that the polymerizability of MI significantly differs from that of αMeSt. We then attempted to polymerize MI under the various conditions to clarify the reactivity in detail, as shown in Scheme 2. Although the anionic polymer-

Scheme 4. Synthesis of MI via Wittig Reaction of 1-Indanone

79% yield. However, the synthesized MI was always contaminated with 0.2−1.5 mol % of byproduct, i.e., 3MI (Figure S1).29 Although we changed the reaction conditions, such as the reaction time, concentration of the reaction mixture, and stoichiometry of the starting materials, the formation of 3MI could not be completely avoided. A short time reaction, high concentration, and small excess amount of base (tBuOK) are practically recommended in order to obtain high purity MI.30 In fact, the byproduct, 3MI, acted as a serious impurity during the anionic polymerization of MI because of the high acidity (pKa ∼ 22) of the CH2 protons in the indene framework.31 As expected, the polymerization results of MI were strongly affected by the content of 3MI. When the molar amount of the basic anionic initiator, such as sec-BuLi, was lower than the molar amount of 3MI in MI, no polymerization of MI occurred at all. Instantaneous proton abstraction from 3MI certainly took place with the basic anionic initiator to form the indenyl anion of 3MI, as shown in Scheme 5. The resulting

Scheme 2. Anionic Polymerization of MI

Scheme 5. Reaction of 3-Methylindene and Basic Anionic Initiators

ization of MI occasionally suffers from an inherent acidic byproduct in the monomer, the formation of a stable anionic living polymer of MI is clearly demonstrated by the molecular weight control and the sequential copolymerization.

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yellow indenyl anion is highly stabilized and cannot initiate the polymerization of MI due to its low nucleophilicity. Therefore, we attempted to thoroughly remove 3MI, an isomer, from MI. However, various attempts including fractional vacuum distillations in the presence of low nucleophilic bases, such as ethylmagnesium bromide, phenylmagnesium chloride, and triphenylmethyllithium, did not have a significant effect on improving the purity of MI. Consequently, the synthetic pathway and work-up process are more important to realize the high quality of the MI monomer, as already described. In fact, the anionic polymerization of MI smoothly proceeded, if the molar amount of the initiator was higher than that of 3MI. This means that the polymerization of MI can be initiated with the residual anionic initiator after partial quenching with the acidic impurity. Therefore, in this study, we discuss the polymer-

RESULTS AND DISCUSSION Synthesis of MI. We first attempted the reaction of 1indanone and methylmagnesium iodide and the following dehydration to synthesize MI (Scheme 3). However, 3methylindene (3MI) was nearly quantitatively obtained after Scheme 3. Reaction of 1-Indanone and Methylmagnesium Iodide and Following Dehydration

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formation of the propagating carbanion. The coloration was maintained during the polymerization but immediately disappeared by the addition of methanol to afford a colorless solution. A polymer was quantitatively obtained by precipitation in a large amount of methanol and was characterized by 1 H and 13C NMR spectroscopies (Figure S2). The polymerization of MI with sec-BuLi was completed within 1 h at −78 °C. As shown in Table 1, the resulting polymer possessed a molecular weight predicted from the molar ratio between MI and the residual initiator, partially quenched with the acidic 3MI in MI. The size exclusion chromatography (SEC) curve of the poly(MI) obtained with sec-BuLi showed a unimodal shape. The ĐM value was within 1.1, indicating the narrow molecular weight distribution of the poly(MI). For example, the SEC curve of the poly(MI) maintains a unimodal and narrow shape even after 24 h, as can be seen in Figure 1A (run 3). In the case of a high molecular weight sample (run 5), the conversion of MI was 80% at −78 °C after 1 h probably due to the very high viscosity of the polymerization system. In this case, the polymerization system seemed to be a reddish gel, while a soluble polymer was obtained after quenching with methanol. The resulting polymer still possessed a narrow ĐM and the predicted Mn value based on the monomer conversion. It is considered from the unimodal narrow shapes of the SEC curves of poly(MI) that the deactivation of sec-BuLi occurs with 3MI during the initial stage of polymerization. The residual sec-BuLi immediately initiates the polymerization of MI as a strong nucleophile. On the other hand, the anionic polymerization of MI similarly proceeded with sec-BuLi at 0 °C and produced a polymer with a very broad molecular weight distribution (ĐM = 2.15, Figure 1B) in 100% yield (run 6). This is consistent with the polymerization result reported by Ueda and co-workers.9 The observed anionic polymerizability of MI is quite different from that of αMeSt, the counterpart of MI, since αMeSt forms no polymeric product at 0 °C due to the low ceiling temperature (∼0 °C).11,12 We next polymerized MI with various initiators, such as oligo(α-methylstyryl)lithium (αMSLi), lithium naphthalenide (Li-Naph), and potassium naphthalenide (K-Naph) in THF at −78 °C. The polymerization with the organolithium initiators including αMSLi and Li-Naph (runs 7−9) quantitatively

ization results of MI by reducing the initial molar amount of the initiator using the estimated concentration of 3MI in the feed MI monomer, as shown in Table 1. Table 1. Anionic Polymerization of MI in THF at −78 °C Mn (kg/mol) run

MI (mmol)

1

7.85

2e

9.92

3 4 5

12.8 8.32 15.4

6f

7.68

7

5.55

8

7.79

9

9.81

10

6.12

11

6.93

12

7.12

initiator (mmol) sec-BuLi, 0.129 sec-BuLi, 0.118 sec-BuLi, 0.156, sec-BuLi, 0.0445 sec-BuLi, 0.0145 sec-BuLi, 0.0863 αMSLi,g 0.0854 αMSLi, 0.0417 Li-Naph, 0.120 K-Naph, 0.141 K-Naph, 0.135 Ph2CHLi, 0.0282

a

time (h)

conv (%)

calcdb

obsdc

ĐM d

1

100

8.0

8.0

1.08

1.5

100

11

10

1.08

24

100

11

12

1.06

2

100

24

26

1.10

1

80

138

129

1.04

24

100

12

1

100

18

100

26

30

1.03

1

100

22

21

1.12

6

89

12

14

1.12

48

100

13

13

1.05

24

0

8.7

9.8

2.15

8.7

1.06

Estimated by 1H NMR. bMn(calcd) = (MW of monomer) × [M]/[I] × conversion + MW of initiator residue. cMn(obsd) was determined by SEC-RALLS equipped with triple detectors, such as refractive index (RI), light scattering (LS), and viscometer detectors. dĐM was determined by SEC calibration using polystyrene standards in THF. e After 1.5 h, TMS2DPE was reacted with the propagating carbanion of poly(MI). fPolymerized at 0 °C. gOligo(α-methylstyryl)lithium prepared by the oligomerization of αMeSt with sec-BuLi. a

Anionic Polymerization of MI. The anionic polymerization of MI was first examined with sec-BuLi in THF at −78 °C. Upon the addition of MI to sec-BuLi, the reaction mixture immediately turned a dark red color, indicating the rapid

Figure 1. SEC curves of poly(MI)s obtained in THF at −78 °C after 24 h (A, run 3) and at 0 °C after 24 h (B, run 6); peak (A), Mn,calcd = 11 000, Mn,RALLS = 12 000, ĐM = 1.06; peak (B), Mn,calcd = 12 000, Mn,RALLS = 9800, ĐM = 2.15. C

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Macromolecules Table 2. Anionic Polymerization of MI with sec-BuLi in Benzene Mn (kg/mol) run

MI (mmol)

sec-BuLi (mmol)

temp (°C)

time

conva (%)

calcdb

obsdc

ĐM d

13 14 15

6.30 6.34 6.59

0.0981 0.0999 0.115

0 35 35

3 days 20 h 4 days

28 42 35

2.0 3.0 2.3

3.1 2.5 2.6

1.56 1.42 1.45

Estimated by 1H NMR. bMn(calcd) = (MW of monomer) × [M]/[I] × conversion + MW of initiator residue. cMn(obsd) was determined by SECRALLS equipped with triple detectors, such as refractive index (RI), light scattering (LS), and viscometer detectors. dĐM was determined by SEC calibration using polystyrene standards in THF. a

Scheme 6. Reaction of Poly(MI) Anion and TMS2DPE

Table 3. Block Copolymerization of MI with Styrene, αMeSt, and Isoprene in THF at −78 °Ca block copolymer (homopolymerb) Mn (kg/mol) run

initiator

first monomer

second monomer

16 17 18 19 20

sec-BuLi sec-BuLi K-Naph sec-BuLi K-Naph

MI MI MI styrene isoprene

styrene αMeSt isoprene MI MI

calcd 19 18 24 23 10

(9.5) (5.3) (4.4) (6.1) (3.6)

obsdc

ĐM d

22 (9.7) 19 (5.3e) 25 (5.4e) bimodalf 10 (3.0)

1.08 (1.10) 1.06 (1.10) 1.08 (1.08) bimodalf 1.26 (1.18)

a Yield of copolymer was quantitative in each case. bHomopolymer was obtained at the first-stage polymerization. cMn(obsd) was determined by SEC-RALLS equipped with triple detectors, such as refractive index (RI), light scattering (LS), and viscometer detectors. dĐM was determined by SEC calibration using polystyrene standards in THF. eMn(obsd) was determined by SEC calibration using polystyrene standards in THF. fBimodal SEC curve due to the partial deactivation of living polystyrene with 3MI in the second monomer of MI.

Next, the polymerization of MI was examined with sec-BuLi in the hydrocarbon solvent of benzene at 0 °C, as shown in Table 2. In benzene at 0 °C, the polymerization system showed a yellow coloration during the polymerization, and MI was very slowly consumed to form a poly(MI) with a fairly broad molecular weight ditribution (ĐM = 1.4) (run 13). Even after 3 days, the conversion of MI was only 28%. We then increased the polymerization temperature from 0 to 35 °C. At 35 °C after 20 h, the conversion of MI reached 42%, but the longer reaction time of 4 days did not produce a higher conversion. Thus, the anionic polymerization of MI in benzene was far from a controlled fashion. Nevertheless, the polymerization of MI in the hydrocarbon solvent certainly proceeds to form the polymeric product at 0−35 °C. Again, this observed polymerizability of MI is quite different from that of αMeSt showing the ceiling temperature around 0 °C.11,12 Furthermore, it should be noted that BF possessing 10π electrons quantitatively gave the stable living polymer in benzene at 0 °C.7 Stability of Propagating Carbanion of Poly(MI). The stability of the propagating carbanion of poly(MI) was examined by the end-functionalization with a 1,1-diphenylethylene derivative, 1,1-bis(4-trimethylsilylphenyl)ethylene (TMS2DPE),33,34 showing no homopolymerizability. MI was

proceeded within 1 h to afford the polymers with tailored molecular weights. On the other hand, K-Naph produced a poly(MI) in 89% yield after 6 h (run 10). The longer polymerization time of 48 h was necessary to complete the polymerization with an organopotassium initiator (run 11), indicating the significant effect of the countercation of the initiator.32 The resulting poly(MI) still maintained well-defined chain structures even after a 48 h reaction at −78 °C. Finally, we examined the polymerization of MI with diphenylmethyllithium (Ph2CHLi) showing a low nucleophilicity in THF at −78 °C (run 12). No polymerization of MI virtually occurred with Ph2CHLi even after 24 h, and MI was quantitatively recovered from the polymerization system. The observed relative anionic polymerizability of MI is significantly lower than the other hydrocarbon monomers, such as αMeSt, styrene, and BF. In fact, a highly reactive BF, an indene counterpart of MI, underwent the polymerization with Ph2CHLi with a quantitative initiation efficiency to form a polymer with a tailored Mn value.8 On the other hand, αMeSt could be initiated with Ph2CHLi at a 2.5% initiation efficiency to form a polymer having a very high molecular weight under identical conditions, as shown in Table S1. D

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Macromolecules polymerized with sec-BuLi in THF for 1.5 h at −78 °C to complete the propagation, and TMS2DPE in THF was added to the polymerization system (Scheme 6 and Table 1, run 2). The coloration of the polymerization system instantaneously changed from dark red to reddish violet, indicating the formation of the 1,1-diphenylalkyl anion derived from TMS2DPE.33,34 The reaction mixture was allowed to react for a further 1.5 h and then quenched with degassed methanol. The SiMe3 groups in the TMS2DPE residue at the polymer terminal (∼0.25 ppm) are excellent probes similar to two CH3 groups in the sec-C4H9 initiator residue (∼0.6 ppm) in order to estimate the end-functionality using 1H NMR spectroscopy. In fact, the TMS2DPE end-functionality in the resulting poly(MI) was almost quantitative by comparing the integral ratio between the terminal SiMe3 group and the CH3 groups of the initiator residue. This means that the propagating carbanion of the poly(MI) is sufficiently stable in THF at −78 °C at least for 1.5 h, although the initiation step significantly suffered from the inherent acidic byproduct, 3MI. Block Copolymerization of MI. We next examined the sequential anionic copolymerization of MI with comonomers, such as styrene, αMeSt, and isoprene, for the synthesis of block copolymers (Table 3). MI was first polymerized with sec-BuLi in THF at −78 °C for 2 h to achieve the quantitative conversion (run 16). Styrene was then added to the poly(MI) anion at −78 °C to reinitiate the copolymerization. The color of the polymerization system rapidly changed from dark red to orange upon the addition of styrene, indicating the instantaneous crossover initiation of the poly(MI) anion with styrene. A copolymer was obtained in a quantitative yield after quenching with methanol after 30 min. Figure 2 shows the SEC

propagating chain end of the poly(MI) is stable and maintains the reactivity to initiate the second stage polymerization even in the presence of the indenyl anion formed from 3MI (Scheme 5). Similarly, tailored block copolymers, poly(MI)-b-poly(αMeSt) and polyisoprene-b-poly(MI)-b-polyisoprene (runs 17 and 18) were obtained by the sequential copolymerizations of MI with αMeSt or isoprene with sec-BuLi or K-Naph, respectively, in THF at −78 °C. We then changed the addition order of the comonomers and employed MI as a second monomer to synthesize block copolymers in the reversed sequences. The living polystyrene as a macroinitiator was prepared by the polymerization of styrene with sec-BuLi in THF at −78 °C for 30 min. The second stage polymerization of MI certainly occurred with the living polystyrene, and MI was completely consumed at −78 °C within 2 h (run 19). However, the SEC curve of the copolymer in Figure 3 shows a bimodal distribution, indicating the

Figure 3. SEC curve of a mixture of polystyrene and polystyrene-bpoly(MI) (Table 3, run 19).

formation of a mixture of a homopolymer of styrene and a block copolymer. The SEC traces in the lower and higher molecular weight regions corresponded to the polystyrene prepared during the first stage polymerization and the block copolymer of styrene and MI, respectively. A possible explanation about forming the mixture is that the partial initiation of MI has occurred with the living polystyrene due to the low reactivity of MI. A more plausible explanation is that a partial deactivation of the living polystyrene (initial concentration = 0.107 mmol) immediately occurred with the acidic impurity, 3MI (0.040 mmol), in the added MI. In this case, the residual living polystyrene (0.067 mmol) initiated the polymerization of MI to give the block copolymer, polystyrene-bpoly(MI). In fact, the observed deactivation content of polystyrene (33%) was consistent with the estimated value (37%) using the molar ratio of the living polystyrene and 3MI in the second feed MI. In the case of isoprene, a block copolymer, poly(MI)-b-polyisoprene-b-poly(MI), with a rather broad molecular weight distribution (ĐM = 1.26) was produced by the copolymerization of isoprene and MI with K-Naph (run 20). Thus, the living polymer of styrene or isoprene could initiate the sequential copolymerization of MI similar to the living poly(αMeSt), αMSLi (runs 7 and 8). It has thus been demonstrated that the reversible copolymerization of MI with styrene, αMeSt, or isoprene is possible to form the corresponding block copolymers. These

Figure 2. SEC curves of poly(MI) (A, dotted line) and poly(MI)-bpolystyrene (B, solid line); peak (A), Mn,calcd = 9500, Mn,RALLS = 9700, ĐM = 1.10; peak (B), Mn,calcd = 21 000, Mn,RALLS = 22 000, ĐM = 1.08 (Table 3, run 16).

curves of the starting poly(MI) and the copolymer. The SEC curve of the copolymer shifts toward the higher molecular weight side from the SEC curve of poly(MI) by maintaining a unimodal narrow molecular weight distribution (ĐM = 1.08). The composition of the block copolymer analyzed by the 1H NMR measurement agreed with the calculated value. Thus, the formation of the well-defined AB diblock copolymer of MI and styrene, poly(MI)-b-polystyrene, was confirmed. Furthermore, the stability of the propagating carbanion of poly(MI) is clearly demonstrated in THF at −78 °C at least for 2 h. The E

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Macromolecules Table 4. Structural Data, Atomic Charge, and LUMO Value of BF, MI, and αMeSta dihedral angleb (deg)

a

bond angle (deg)

monomer

∠C1−C2−C3−C4

∠C1−C2−C3′−C4′

∠C3−C2−C3′

atomic charge on C1c

LUMO (eV)

BF MI αMeSt

0.0 11.7 36.9

0.0 19.0

105.4 106.5 117.3

−0.301 −0.388 −0.380

−0.162 +0.673 +0.872

All structures were optimized by ωB97X-D/6-311G(d,p). bDefined that trans conformation is 0°. cNatural charges were obtained by NBO Analysis.

aromatic ring of MI (11.7°) suggests that MI still maintains the effective π-conjugation compared to the larger value of αMeSt (36.9°). The other dihedral angles of MI (19.0°) and BF (0.0°) indicated that the conjugated indene ring of BF has a planarity higher than the partially saturated indane ring of MI. The bond angle (∠C3−C2−C3′) might reflect the strains of the sp2conjugated C2 carbons of the monomers. Each value of MI and BF (106.5° and 105.4°) is much smaller than the values for the typical sp2 carbon (120°) and αMeSt (117.3°), indicating that the inherent large strains at the C2 carbons are present in the five-membered rings of MI and BF. On the other hand, the calculated atomic charge on the CH2= groups of BF (−0.301) is significantly lower than those of MI (−0.388) and αMeSt (−0.380), indicating that the exo-methylene carbon of BF readily accepts the nucleophilic attack of the anionic species. Similarly, the calculated LUMO value of BF (−0.162) is significantly different from those of MI (+0.673) and αMeSt (+0.872) and strongly supports the exceptionally high anionic polymerizability of BF among the three monomers. All these DFT calculation results supported the lower polymerizability of MI compared to BF, but it is difficult to explain the relationship between MI and αMeSt. In fact, only the atomic charge on the CH2= group supported the observed anionic polymerizability of MI being slightly lower than αMeSt, while other calculation results, including the planarity of the molecule estimated by the dihedral angle, the strain at the C2 carbon, and the energy level of LUMO, predicted the higher polymerizability of MI compared to αMeSt. The difference in the reactivity between MI and αMeSt might be very small, since the crossover reaction of the two monomers is experimentally possible in the sequential copolymerization (runs 7, 8, and 17). Cationic Polymerization and Radical Polymerization of MI. The cationic polymerization of MI was carried out with boron trifluoride diethyl etherate (BF3OEt2) in CH2Cl2 at −78 °C for 24 h (Table 5). Although an attempt using 6.3 mol % of BF3OEt2 failed and gave no polymeric product, a higher concentration of the initiator (43 mol %) provided the polymer in a quantitative yield. The poly(MI) obtained with BF3OEt2 had a low molecular weight (Mn = 6300) and broad molecular weight distribution (ĐM = 2.65). Thus, although the cationic polymerization of MI proceeded as previously reported,9 the cationic polymerizability of MI seemed rather low. The cationically obtained poly(MI) was readily soluble in various solvents, while the limited solubility of the polymer was suggested in a previous report.9 Next, the radical polymerization of MI was investigated using α,α′-azobis(isobutyronitrile) (AIBN, 1.2 mmol) in bulk at 70 °C for 96 h. Although the polymerization proceeded to give a polymer (Mn = 3000, ĐM = 1.56), the yield of the polymer was only 14%. This is also consistent with a previous report, in which a poly(MI) was obtained with AIBN in bulk at 60 °C for 240 h in 16.7% yield.9 The ceiling temperature of MI did not hinder the radical polymerization between 60 and 70 °C and might be fairly high compared to that of αMeSt (∼0 °C). The

results of sequential copolymerizations substantiate the fact that the anionic polymerizability of MI is comparable to typical hydrocarbon monomers and significantly differs from the highly reactive BF.6−8 Evaluation of Polymerizability of MI by DFT Calculation. As already described, it was suggested that the anionic polymerizability of MI is comparable to αMeSt, since the reversible sequential copolymerization was possible. In fact, the crossover reaction between the poly(MI) anion and αMeSt quantitatively proceeded to form the tailored poly(MI)-bpoly(αMeSt) (run 17), and vice versa (runs 7 and 8). On the other hand, based on the results of the homopolymerization of MI, BF, and αMeSt using Ph2CHLi as shown in Table S1, the relative anionic polymerizability of these three monomers was estimated as follows: BF > αMeSt > MI. In fact, the initiation efficiencies of BF, αMeSt, and MI with Ph2CHLi in THF at −78 °C were 100, 2.5, and 0%, respectively. The anionic polymerizability of BF was thus much higher than those of αMeSt and MI. We now discuss the relative polymerizabilities of these hydrocarbon monomers from the viewpoint of the molecular structures estimated by the DFT calculations. We calculated the following four factors: the two dihedral angles related to the CH2C linkage, the bond angle of the sp2 carbon adjacent to the exo-methylene CH2= group, the atomic charge on the βcarbon of the CH2C linkage, and the LUMO value of the monomer, as shown in Table 4. Figure 4 shows the molecular frameworks and the calculated equilibrium structures of the monomers. The planarity of the monomers was estimated by the two kinds of dihedral angles: a twist angle between the vinyl and aromatic ring (∠C1−C2−C3−C4) and a twist angle in the other direction (∠C1−C2−C3′−C4′). The planarity of BF is significant, since both calculated values are 0.0°. On the other hand, the twist angle between the CH2C linkage and

Figure 4. Molecular frameworks and equilibrium structures of BF, MI, and αMeSt determined by the DFT calculation. F

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Macromolecules Table 5. Polymerization of MI with BF3OEt2 and AIBN run

MI (mmol)

initiator (mmol)

solvent

temp (°C)

time (h)

yielda (%)

Mn (kg/mol), obsdb

ĐM c

21 22 23

8.62 4.15 8.00

BF3OEt2, 0.578 BF3OEt2, 3.17 AIBN, 0.092

CH2Cl2 CH2Cl2 bulk

−78 −78 70

24 24 96

0 100 14

6.3 3.0d

2.65 1.56

a

Methanol-insoluble part. bMn(obsd) was determined by SEC-RALLS equipped with triple detectors, such as refractive index (RI), light scattering (LS), and viscometer detectors. cĐM was determined by SEC calibration using polystyrene standards in THF. dMn(obsd) was estimated from SEC calibration using polystyrene standards in THF.

dominated by the main chain structures and is close to that of poly(αMeSt). The glass transition behavior of poly(MI) was analyzed by differential scanning calorimetry (DSC) (Figure S3). In each measurement, only one glass transition behavior was observed in the DSC profile, and neither the crystallization nor melting transition was detected before the thermal degradation. Figure 6 shows the dependence of the Tg values on the molecular

normal vinyl polymerization (1,2-addition) of the exomethylene group of MI exclusively proceeded under the cationic and radical conditions, since the resulting poly(MI)s obtained with BF3OEt2 and AIBN showed 1H NMR spectra similar to those obtained using the anionic initiators. In the 13C NMR spectra, it is also difficult to find a significant difference in the stereoregularity of the polymers obtained by the anionic, cationic, and radical initiators, although the stereoregularity of the poly(MI) has been previously reported.9 In fact, the splitting of the carbon signals of the repeating units was not obvious for the reliable assignment of the stereoregularity. Solubility and Thermal Property of Poly(MI). Poly(MI) obtained by the anionic polymerization was soluble in benzene, chloroform, and THF but insoluble in n-hexane, diethyl ether, ethyl acetate, acetone, DMF, DMSO, methanol, and water. These observed solubilities are similar to poly(αMeSt) but lower than polystyrene, since the latter is soluble in ethyl acetate, acetone, and DMF. The poly(MI)s produced with BF3OEt2 and AIBN showed solubilities similar to the anionically obtained poly(MI). The thermal stability of the resulting poly(MI) was investigated by a thermogravimetric analysis (TGA) under nitrogen. Figure 5 shows the TGA thermograms of poly(MI),

Figure 6. Relationship between Mn and Tg of poly(MI) and poly(αMeSt).

weight along with that of poly(αMeSt). The Tg of poly(MI) increased to 137 °C, and the poly(αMeSt)s presented higher Tgs around 170 °C. This suggests that the more restricted conformation of poly(MI) compared to poly(αMeSt) did not play an important role in the glass transition behaviors, even if the poly(MI) possessed a stiff ring structure vertical to the main chain.

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CONCLUSIONS The synthesized MI monomer, a typical exo-methylene hydrocarbon monomer, was contaminated with 0.2−1.5 mol % of an inherent acidic isomeric impurity, 3MI, formed during the synthetic procedure. Although the partial deactivation of the anionic initiator occurred with 3MI during the early stage of the polymerization, the residual initiator quantitatively induced the polymerization of MI. Once the polymerization proceeded, the resulting propagating carbanion of poly(MI) was sufficiently stable to initiate the sequential copolymerization in a living fashion. Therefore, the resulting poly(MI) possessed the narrow molecular weight distribution (ĐM < 1.1) and the predicted molecular weight based on the molar ratio between the monomer and the residual initiator in addition to the ring structures vertical to the main chain via the 1,2-addition polymerization of the exo-methylene group. MI, a cyclic analogue of αMeSt, showed a much higher ceiling temperature compared to αMeSt. The polymerization behavior of MI was

Figure 5. TGA thermograms of polymers under nitrogen flow with a heating rate of 10 °C min−1: (A) poly(MI), T10 = 315 °C; (B) poly(αMeSt), T10 = 322 °C; (C) polystyrene, T10 = 398 °C.

poly(αMeSt), and polystyrene. Poly(MI) and poly(αMeSt) showed similar decomposition behaviors, and the degradations started around 280 °C upon heating. The 10% weight loss temperatures (T10) of poly(MI) and poly(αMeSt) were 315 and 322 °C, respectively. The complete weight losses of poly(MI) and poly(αMeSt) were observed at 370 °C. On the other hand, polystyrene started to degrade around 350 °C, and the T10 of polystyrene was observed at 398 °C. These results indicate that the thermal stability of the poly(MI) is mainly G

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Macromolecules close to that of styrene, but not αMeSt, indicating the effect of the exo-methylene framework. The DFT calculation method was an effective tool to predict the relative polymerizability of a series of hydrocarbon monomers, including MI, BF, and αMeSt.

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Purification of MI. MI was purified by the vacuum distillation over CaH2. The purified MI and 3−5 mol % of BzMgCl in THF were sealed off in an all-glass apparatus equipped with a break-seal under the vacuum conditions. The sealed MI was stirred over BzMgCl in THF at room temperature overnight and distilled into an ampule fitted with a break-seal on the vacuum line. The monomer was further diluted with dry THF, and the resulting solution (ca. 1.0 M) was stored for the polymerization in the ampule equipped with a break-seal at −30 °C. Anionic Polymerization. All anionic polymerizations of MI were carried out in THF or in benzene in a sealed all-glass apparatus equipped with break-seals under high-vacuum conditions (10−6 mmHg).35 A typical polymerization procedure (Table 1, run 4) was carried out as follows: a THF solution (10 mL) of MI (1.08 g, 8.32 mmol including 0.0166 mmol of 3MI), precooled at −78 °C, was added to sec-BuLi (0.0611 mmol) in n-heptane (2 mL) with vigorous stirring. The color of the reaction system immediately changed from colorless to dark red by the initiation with the residual sec-BuLi (0.0445 mmol). The polymerization was continued for 2 h at −78 °C and terminated with degassed methanol. The polymer was precipitated by pouring the mixture into a large amount of methanol and isolated by the filtration. The resulting polymer (1.08 g, 100%) was further purified by freeze-drying from benzene and characterized by NMR and IR spectroscopies, SEC, TGA, and DSC measurements. The 1H and 13 C NMR and IR data of poly(MI) obtained with sec-BuLi are as follows. 1H NMR (300 MHz, CDCl3): δ = 0.9−2.1 (4H, H2 and main chain CH2), 2.2−2.8 (2H, H3), 6.4−7.2 (4H, aromatic). 13C NMR (75 MHz, CDCl3): δ = 31.0 (C2), 32.5−3.6 (C3), 51.2−53.6 (main chain CH2 and C1), 123.6−126.0 (C5, C6, C7, C8), 142.2 (C4), 152.8 (C9). IR (KBr): 2846, 1477, 1456, 863, 770, 754 cm−1. Measurement. Both 1H and 13C NMR spectra were measured on a Bruker DPX300 in CDCl3. Chemical shifts were recorded in ppm downfield relative to CHCl3 (δ 7.26) and CDCl3 (δ 77.1) for 1H and 13 C NMR as standard, respectively. IR spectra were recorded on a JASCO FT/IR-4100 instrument using either an attenuated total reflectance (ATR) attachment or a KBr disk. SEC curves for determination of ĐM were obtained in THF at 40 °C at flow rate of 1.0 mL min−1 with a Viscotek TDA302 equipped with three polystyrene gel columns (TSKgelG5000HHR + G4000HHR + G3000HHR). The combination of viscometer, right angle laser light scattering detection (RALLS), and refractive index (RI) detection was applied for the online SEC system in order to determine the absolute molecular weights of polymer. The Tgs of the polymers were measured by DSC using a Seiko instrument DSC6220 apparatus under nitrogen flow. The polymer sample was first heated to 130 °C, cooled to 30 °C, and then scanned at a rate of 20 °C min−1 under nitrogen. A Seiko Instrument TG/DTA6200 was used for TGA analysis between 30 and 600 °C with heating rate of 10 °C min−1 under nitrogen. The ground state geometries were fully optimized by using DFT calculations as well as long-range and dispersion corrected ωB97X-D36 functional with the polarized split-valence triple-ζ 6-311G(d,p) basis set,37,38 denoted as ωB97X-D/6-311G(d,p). Then, the vibrational frequencies were computed at the same level to check each optimized structure as an equilibrium structure. All calculations were carried out using the Gaussian 09 program.39 Natural charges were obtained by using the NBO 6.0 program.40

EXPERIMENTAL SECTION

Materials. All reagents were purchased from Tokyo Chemical Industry Co., Ltd., unless otherwise stated. 1-Indanone, potassium tertbutoxide, and methyltriphenylphosphonium bromide (Wako Pure Chemical Industries, Ltd.) were used without purification. Benzylmagnesium chloride (BzMgCl) was prepared by the reaction of benzyl chloride and magnesium in THF. THF used as a polymerization solvent was refluxed over sodium wire for 3 h, distilled from LiAlH4 under nitrogen, and finally distilled from sodium naphthalenide solution on a vacuum line. Styrene and αMeSt were distilled over CaH2 under reduced pressure and finally distilled over dibutylmagnesium on the vacuum line. Isoprene was distilled over CaH2 and finally distilled over BzMgCl on the vacuum line. TMS2DPE as an endcapping reagent was synthesized as previously reported.33,34 n-Heptane or benzene was washed with concentrated H2SO4 and dried over MgSO4 and then dried over P2O5 for 1 day under reflux. It was then distilled in the presence of n-BuLi under nitrogen. CH2Cl2 for the cationic polymerization was distilled over CaH2 on the vacuum line. Initiators. Commercially available sec-BuLi (1.0 M in cyclohexane, Kanto Chemical Co., Inc.) was used without purification and diluted with dry n-heptane. Li-Naph and K-Naph were prepared from the corresponding alkali metal and 1.5-fold naphthalene in dry THF under argon at room temperature for 24 h. Ph2CHLi was synthesized by the reaction of Li-Naph and 1.5-fold diphenylmethane in dry THF under argon at room temperature for 48 h. These initiators were sealed off under high-vacuum conditions in ampules equipped with break-seals and stored at −30 °C. On the other hand, αMSLi was prepared prior to the polymerization from sec-BuLi and 3-fold αMeSt in THF at −78 °C for 20 min. The concentrations of initiators were determined by colorimetric titration using standardized 1-octanol in THF in a sealed reactor under vacuum, as previously reported.35 AIBN for radical polymerization was purified by recrystallization from methanol. BF3OEt2 was distilled in the presence of CaH2 on a vacuum line and then diluted with CH2Cl2. Synthesis of MI. A three-necked flask was charged with methyltriphenylphosphonium bromide (37.2 g, 104 mmol) suspended in dry THF (200 mL) under nitrogen. Potassium tert-butoxide (16.1 g, 144 mmol) was then added, and the resulting yellow mixture was stirred at room temperature for 30 min. The mixture was cooled to 0 °C, and 1-indanone (10.6 g, 80.2 mmol) in dry THF (70 mL) was dropwise added; the mixture was stirred for 3 h at room temperature. The mixture was filtered through silica gel to remove the precipitated salts. The silica gel was rinsed with n-hexane, and the solvent was evaporated under reduced pressure. The residue was poured into nhexane (ca. 300 mL) to precipitate triphenylphosphine oxide as the byproduct. The mixture was filtered, and the solvent was evaporated under reduced pressure. The residual crude yellow oil was purified by flash column chromatography (silica gel, pretreated with 3 vol % of triethylamine in n-hexane)30 with n-hexane as the eluent. The resulting oil was purified by the repeating vacuum distillations over CaH2 to obtain a colorless liquid of MI (7.70 g, 59.2 mmol, 74%, bp 66−68 °C/ 5 mmHg). The resulting MI contained 0.2−1.5 mol % of 3MI as a byproduct. 1H NMR (300 MHz, CDCl3): δ = 2.82 (m, 2H, H2), 2.99 (m, 2H, H3), 5.03 (s, 1H, Hb), 5.45 (s, 1H, Ha), 7.15−7.22 (m, 3H, H5−H7), 7.50 (m, 1H, H8). 13C NMR (75 MHz, CDCl3): δ = 30.2 (C3), 31.3 (C2), 102.6 (CH2=), 120.7 (C8), 125.5, 126.5, and 128.4 (C7, C6, C5), 141.2 (C9), 146.8 (C4), 150.7 (C1). IR (ATR): 2924, 2847, 1638 (CC), 1472, 1459, 863, 770, 724 cm−1.

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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.5b01601. Table S1; Figures S1−S4 (PDF) H

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(23) Cappelli, A.; Pericot Mohr, G.; Anzini, M.; Vomero, S.; Donati, A.; Casolaro, M.; Mendichi, R.; Giorgi, G.; Makovec, F. J. Org. Chem. 2003, 68, 9473−9476. (24) Cappelli, A.; Anzini, M.; Vomero, S.; Donati, A.; Zetta, L.; Mendichi, R.; Casolaro, M.; Lupetti, P.; Salvatici, P.; Giorgi, G. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 3289−3304. (25) Cappelli, A.; Galeazzi, S.; Giuliani, G.; Anzini, M.; Donati, A.; Zetta, L.; Mendichi, R.; Aggravi, M.; Giorgi, G.; Paccagnini, E.; Vomero, S. Macromolecules 2007, 40, 3005−3014. (26) Cappelli, A.; Galeazzi, S.; Giuliani, G.; Anzini, M.; Aggravi, M.; Donati, A.; Zetta, L.; Boccia, A. C.; Mendichi, R.; Giorgi, G.; Paccagnini, E.; Vomero, S. Macromolecules 2008, 41, 2324−2334. (27) Cappelli, A.; Galeazzi, S.; Zanardi, I.; Travagli, V.; Anzini, M.; Mendichi, R.; Petralito, S.; Memoli, A.; Paccagnini, E.; Peris, W.; Giordani, A.; Makovec, F.; Fresta, M.; Vomero, S. J. Nanopart. Res. 2010, 12, 895−903. (28) Cappelli, A.; Paolino, M.; Grisci, G.; Giuliani, G.; Anzini, M.; Donati, A.; Mendichi, R.; Boccia, A. C.; Botta, C.; Mroz, W.; Samperi, F.; Scamporrino, A.; Giorgi, G.; Vomero, S. J. Mater. Chem. 2012, 22, 9611−9623. (29) Buchanan, G. W.; Selwyn, J.; Dawson, B. A. Can. J. Chem. 1979, 57, 3028−3033. (30) It should be noted that MI was completely isomerized to 3methylidene in CDCl3 in a sealed tube at room temperature after 10 days. This isomerization was presumably induced by the catalytic amount of acid and water in CDCl3. In addition, MI occasionally isomerized to 3MI during the course of flash column chromatography using silica gel. Silica gel should be treated with bases, such as triethylamine, to quench the acidic sites of the silica gel prior to the flash column chromatography. (31) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456−463. (32) Ishizone, T.; Hirao, A.; Nakahama, S. Macromolecules 1989, 22, 2895−2901. In this work, a significant effect of the countercation of the initiator was observed for the anionic polymerization of tert-butyl 4-vinylbenzoate. Larger countercations of the initiators, such as potassium and cesium, were preferable for the anionic polymerization of styrene derivative possessing electrophilic carbonyl substituent. (33) Ishizone, T.; Hirao, A.; Nakahama, S. Macromolecules 1991, 24, 625−626. (34) Ishizone, T.; Tsuchiya, J.; Hirao, A.; Nakahama, S. Macromolecules 1992, 25, 4840−4847. (35) Hirao, A.; Takenaka, K.; Packrisamy, S.; Yamaguchi, K.; Nakahama, S. Makromol. Chem. 1985, 186, 1157−1166. (36) Chai, J.-D.; Head-Gordon, M. Phys. Chem. Chem. Phys. 2008, 10, 6615−6620. (37) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650−654. (38) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639− 5648. (39) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision D.01; Gaussian, Inc.: Wallingford, CT, 2009. (40) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.; Bohmann, J. A.; Morales, C. M.; Landis, C. R.; Weinhold, F. NBO 6.0; Theoretical Chemistry Institute, University of Wisconsin, Madison, 2013.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.I.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grant-in Aid (No. 18550105) from the Ministry of Education, Science, Sports, and Culture, Japan. T.I. appreciates the financial support from the Yazaki Foundation and JX Corporation. Y.K. appreciates the support by Grant-in Aid for JSPS fellows (No. 26·11883) from Japan Society for the Promotion of Science (JSPS). The numerical calculations were carried out on the TSUBAME2.5 supercomputer at the Tokyo Institute of Technology, Tokyo, Japan, and on the supercomputer at the Research Center for Computational Science, Okazaki, Japan.

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DOI: 10.1021/acs.macromol.5b01601 Macromolecules XXXX, XXX, XXX−XXX