Polymerization of Alkyl Diazoacetates Initiated by Pd

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Polymerization of Alkyl Diazoacetates Initiated by Pd(Naphthoquinone)/Borate Systems: Dual Role of Naphthoquinones as Oxidant and Anionic Ligand for Generating Active Pd(II) Species Hiroaki Shimomoto, Shohei Ichihara, Hinano Hayashi, Tomomichi Itoh, and Eiji Ihara*

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Department of Materials Science and Biotechnology, Graduate School of Science and Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama 790-8577, Japan S Supporting Information *

ABSTRACT: For polymerization of alkyl diazoacetates, the combination of a Pd complex bearing 1,4-naphthoquinone (NQ) or its derivatives as a ligand and borate, NaBPh4, was found to be an efficient initiating system. The polymerization of ethyl diazoacetate by a Pd(0) complex having two NQ molecules [Pd(nq)2] with NaBPh4 proceeded to give poly(ethoxycarbonylmethylene)s with a relatively high molecular weight (up to Mn = 36 kDa) in good yield (∼70%). This initiating system was also effective for polymerizing other diazoacetates, benzyl and cyclohexyl diazoacetates. In addition, in the presence of NaBPh4, a novel Pd(II) complex bearing an anionic naphthoquinonyl ligand derived from 2,3-dichloro-1,4-naphthoquinone (dichlone), [Pd(cod)(Clnq)Cl] (cod = 1,5-cyclooctadiene), which was newly prepared by treatment of a Pd(0) precursor, Pd2(dba)3·CHCl3 [dba = (E,E)-dibenzylideneacetone], with COD and dichlone, was capable of affording poly(alkoxycarbonylmethylene)s with much higher stereoregularity compared to the previously reported Pd-based initiating systems, despite a rather low polymer yield (∼20%). For both Pd(nq)-based initiating systems, Pd−Ph species generated by transmetalation with NaBPh4 were responsible for the initiation of diazoacetates based on MALDI-TOF-MS analyses of the resulting polymers, and naphthoquinones played unique important dual roles as both an oxidant and an anionic ligand for generating the active Pd(II) species.

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INTRODUCTION Carbon−carbon (C−C) main chain polymers are obviously one of the most important synthetic polymers in terms of both fundamental research and industrial application. Almost exclusively, C−C main chain polymers have been prepared by vinyl polymerization, which employs vinyl compounds (vinyl monomers) as a monomer and utilizes repetitive additions to their CC bonds as propagation (Scheme 1A). Owing to prominent achievements, such as controlled/living polymerization and stereospecific polymerization as a result of intensive fundamental research on vinyl polymerization, precise syntheses of polymers with well-defined structures with respect to chain length, stereostructure (tacticity), and molecular architecture have been realized and have contributed to development of a variety of functional polymeric materials.1−11 Even so, there still remains a prospect of developing a totally new strategy for the synthesis of C−C main chain polymers, different from vinyl polymerization constructing the C−C main chain from two carbon units, because such new polymerization will provide us with a powerful tool for preparing a variety of new functional C− C main chain polymers. One strategy for such a synthetic method is the so-called C1 polymerization, which constructs C−C main chains from one carbon unit (Scheme 1B). As a representative example for the C1 polymerization along with organoboron-initiated polymer© XXXX American Chemical Society

ization of dimethylsulfoxonium methylide developed by Shea and co-workers,12 polymerization of diazoacetates has attracted much attention because the method can yield C−C main chain polymers with an alkoxycarbonyl group (ester) as a functional group on each main-chain carbon atom, which can be considered as C1 polymer counterparts of poly(alkyl acrylate)s.13−17 Although the polymer structure can be obtained by radical polymerization of dialkyl fumarate or maleate,18 the C1 polymerization has been shown to have important advantages over the radical vinyl polymerization with respect to monomer variation, molecular weight control, and stereoregulation. In addition, it has been demonstrated that the dense packing of the ester substituents along the polymer main chain, which is the most striking characteristic of the polymers obtained from the C1 polymerization of diazoacetates (poly[alkoxycarbonylmethylene]), brings about unique properties,19−34 which cannot be realized by the vinyl polymers bearing the same substituents on every other carbon atom in the main chain. Thus, C1 polymerization has been considered to be a promising method for developing new polymeric materials based on C−C main chain polymers. Received: April 26, 2019 Revised: August 25, 2019

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DOI: 10.1021/acs.macromol.9b00857 Macromolecules XXXX, XXX, XXX−XXX

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stereoregulation.38,39 Meanwhile, we have developed two types of Pd-based initiating systems for the polymerization of diazoacetates, namely, Pd(NHC)(nq)/borate (NHC: N-heterocyclic carbene, nq: 1,4-naphthoquinone)40,41 and π-allylPdClbased systems;42 the former is effective for high Mn polymer synthesis, and the latter is effective for affording polymers in high yield (Figure 1). Importantly, we have found that polymerization of diazoacetates with bulky ester substituents by the πallylPdCl/NaBPh4 system proceeds in a controlled manner,21,26 and recently, Wu and co-workers reported that π-allylPdCl with a bidentate phosphine ligand34 enabled controlled polymerization of a variety of diazoacetates. Furthermore, Toste and coworkers have just reported that (π-allyl)palladium carboxylate dimers are capable of conducting polymerization of diazoacetate in a controlled manner where the dinuclear Pd framework plays a critical role in the efficient initiating ability.43 Even with this recent significant progress in the diazoacetate polymerization, further improvements are desirable for the initiating systems, particularly in the comparison with a variety of sophisticated initiating systems for the precisely controlled vinyl polymerization. In this paper, we will present our important findings on the new initiating systems based on Pd complexes with NQ ligands, which are indeed developed from the Pd(NHC)(nq)/borate system40,41 (Figure 1). Initially, we considered that NHC plays a crucial role in the initiating ability of the Pd(NHC)(nq)/borate system, with NQ acting just as a stabilizing auxiliary ligand. On the contrary and somewhat surprisingly, as revealed in this paper in detail for the Pd(nq)2/NaBPh4 system, NQ is indeed the key reagent for (1) oxidizing Pd(0) to Pd(II) to generate an active species and (2) stabilizing the propagating Pd center as an anionic ligand after the oxidation. Along with the efficient

Scheme 1. (A) Vinyl Polymerization and (B) C1 Polymerization

Since the first report on C1 polymerization of diazoacetate using Cu powder as a catalyst,35 some Rh- and Pd-based initiating systems have been reported to be effective for affording high-molecular-weight polymers from diazoacetates. In particular, the Rh(diene)-based initiating system reported by de Bruin and co-workers is quite effective for producing high−molecularweight polymers [Mn (number-average molecular weight) > 200 kDa] stereospecifically to give syndiotactic polymers.36,37 Furthermore, a combination of experimental and theoretical investigations revealed the most plausible active species, cationic allylRh(III)OH, and a detailed proposed mechanism of the high

Figure 1. Pd-based initiating systems effective for polymerization of diazoacetate: (A) π-allylPd-based and (B) Pd(naphthoquinone)/borate initiating systems. B

DOI: 10.1021/acs.macromol.9b00857 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Polymerization of Diazoacetates Initiated by Pd(nq)2/NaBPh4 Systema run

Pd complex

cocatalystb

monomer (M)

[M]/[Pd]

temp

time

yield (%)c

1f 2f 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

[Pd(IMes)(nq)]2 [Pd(IPr)(nq)]2 Pd(cod)(nq) Pd(cod)(nq) Pd(nq)2 Pd(nq)2 Pd(nq)2 Pd(nq)2 Pd(nq)2 Pd(nq)2 Pd(nq)2 Pd(nq)2 Pd(nq)2 Pd(nq)2 Pd(nq)2 Pd(nq)2 Pd(nq)2 Pd(nq)2

NaBPh4 NaBPh4 NaBPh4 none NaBPh4 none NaBPh4 NaBPh4 NaBPh4 NaBPh4 NaBPh4 NaBPh4 NaBPh4 NaBPh4 NaBPh4 NaBPh4 NaBPh4 NaBPh4

EDA EDA EDA EDA EDA EDA EDA EDA EDA EDA EDA EDA EDA EDA BDA BDA cHDA cHDA

100 100 100 100 100 100 100 100 100 200 300 400 50 20 100 200 100 200

50 °C 50 °C 50 °C 50 °C 50 °C 50 °C RT 0 °C 50 °C 50 °C 50 °C 50 °C 50 °C 50 °C 50 °C 50 °C 50 °C 50 °C

13 h 13 h 13 h 13 h 13 h 13 h 13 h 13 h 5 min 1h 3h 5h 30 min 1h 1h 2h 1h 2h

52 57 36 trace 72 trace 63 trace 65 73 75 70 47 59 73 77 65 76

Mn,RI × 10−3d

Mw,RI × 10−3d (Mw,MALS × 10−3e)

Mw/Mn,RId (Mw/Mn,MALSe)

16.5 19.8 5.2

1.40 1.70 1.71

17.2

1.42

17.2

1.27

16.9 26.9 33.0 36.2 12.2 5.9 20.9 24.9 20.4 32.0

1.37 1.80 1.99 1.73 (1.51) 1.20 1.20 1.57 1.76 (1.56) 1.73 1.83 (1.61)

62.6 (78.4)

43.8 (73.4) 58.6 (72.7)

In THF; [monomer]0 = ∼0.3 M (runs 3−14), ∼0.2 M (runs 15 and 17), ∼0.4 M (runs 16 and 18); EDA was used as a CH2Cl2 solution with a concentration of 1.9−2.8 M. b[NaBPh4]/[Pd] = 1.1−1.2. cAfter purification with preparative SEC. dDetermined by SEC using PMMA standards. e Determined by SEC-MALS. fPolymerization data were quoted from our previous work;40 [Pd(IMes)(nq)]2: 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (1,4-naphthoquinone)palladium(0) dimer; [Pd(IPr)(nq)]2: 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene(1,4naphthoquinone)palladium(0) dimer. a

initiating ability for diazoacetate polymerization, these findings on the mechanistic aspect of the Pd(nq)2/NaBPh4 system is fundamentally quite significant for further development of the diazoacetate polymerization. In addition, we found that a Clsubstituted derivative of NQ [2,3-dichloro-1,4-naphthoquinone (dichlone)] provided a well-defined Pd(II) complex, Pd(cod)(Cl-nq)Cl, which possessed an initiating ability with the assistance of NaBPh4 for stereoselective polymerization of diazoacetates although the polymer yield was rather low (∼20%). The results of the detailed investigation using the Pd(cod)(Cl-nq)Cl/NaBPh4 system are also quite significant for understanding the essential part of the Pd-initiated polymerization and further development of more effective initiating systems.

Then, we have subsequently found that replacing COD in Pd(cod)(nq)/NaBPh4 with another NQ to afford Pd(nq)2/ NaBPh4 further improved the initiating ability in terms of molecular weight and yield of the product (run 5): a polymer with a similar Mn (17.2 kDa) to that obtained with the Pd(NHC)(nq)/NaBPh4 system was produced in higher yield (72%). The presence of NaBPh4 was also essential to produce polymers for the initiating system (run 6). The effect of polymerization temperature was examined to reveal that the reactivity was maintained at room temperature but diminished at 0 °C (runs 7 and 8). The polymerization rate was relatively fast: the polymer yield already reached around 70% in 5 min (run 9). Noteworthy is that the molecular weight of the products can be roughly controlled by changing the monomer feed ratio in the polymerization initiated by the Pd(nq)2/NaBPh4 system although the polymerization did not proceed in an ideal living/controlled manner, and thus polymers with broad molecular weight distribution (Mw/Mn = 1.2−2.0) were obtained. The Mn values of the products increased with the increase in the feed ratio to [EDA]/[Pd] = 200 (Mn = 26.9 kDa), 300 (Mn = 33.0 kDa), and 400 (Mn = 36.2 kDa), while the polymer yield remained relatively high (> 70%) (runs 10−12). As expected, the polymerization with lower monomer feed ratios gave polymers with lower Mn values (runs 13 and 14). Even though the quality of control is not so high, the high yield synthesis of poly(ethoxycarbonylmethylene) [poly(EDA′)] with Mn up to 36 kDa44 is a remarkable result for Pd-based initiators for diazoacetate polymerization. The polymerization results of monomers other than EDA by the Pd(nq)2/NaBPh4 system are summarized in runs 15−18. The polymerization of benzyl diazoacetate (BDA) and cyclohexyl diazoacetate (cHDA) was carried out with a feed ratio of [monomer]/[Pd] = 100, yielding polymers with Mn values of ∼20 kDa in ∼70% yield (runs 15 and 17), which were comparable with those of the product obtained from EDA under

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RESULTS AND DISCUSSION (I) Pd(nq)2/NaBPh4 Initiating System. Polymerization Behavior. In the course of our investigation for improving the performance of the Pd(NHC)(nq)/borate system40,41 for diazoacetate polymerization, we happened to find that a Pdbased system, Pd(cod)(nq)/NaBPh4, where a Pd complex bearing COD in place of NHC in Pd(NHC)(nq)/NaBPh4 was employed, possessed the initiating ability although the activity was significantly lower than the Pd(NHC)(nq)/NaBPh4 system. As shown in Table 1, while the polymerization of ethyl diazoacetate (EDA) conducted with the Pd(NHC)(nq)/ NaBPh4 system {[Pd(IMes)(nq)]2/NaBPh4 or [Pd(IPr)(nq)]2/NaBPh4} at 50 °C with a feed ratio of [EDA]/[Pd] = 100 afforded polymers with Mn = 15−20 kDa in 50−60% yield (runs 1 and 2), both the yield and Mn became much lower with the Pd(cod)(nq)/NaBPh4 system under a similar condition (run 3). Pd(cod)(nq) alone did not produce polymers as was the case with the NHC-ligated Pd(0)-based complexes (run 4), indicating that the borate played an essential role in the initiating ability. C

DOI: 10.1021/acs.macromol.9b00857 Macromolecules XXXX, XXX, XXX−XXX

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Figure 2. (A) Part of the MALDI-TOF-MS spectrum of a polymer sample (Mn = 6.3 kDa, Mw/Mn = 1.18) obtained with Pd(nq)2/NaBPh4-initiated polymerization of EDA with a feed ratio of [EDA]/[Pd] = 50 in 1,4-dioxane at 50 °C for 1 h and (B) theoretical isotopic distribution of a Na adduct of a Ph-initiated polymer (degree of polymerization = 29) terminated by protonolysis.

Scheme 2. (A) Oxidation of Pd(0) in the Presence of p-Benzoquinone and a Strong Proton Donor (HX), (B) Proposed Mechanism Involving Oxidative Transmetalation, and (C) Plausible Mechanism for the Polymerization Initiated by the Pd(nq)2/ NaBPh4 System

a similar condition (run 5). Again, the Mn values of the products became higher with the increase in the monomer feed ratio to

[monomer]/[Pd] = 200 for both monomers (runs 16 and 18). As a whole, the Pd(nq)2/NaBPh4 system is quite effective for the D

DOI: 10.1021/acs.macromol.9b00857 Macromolecules XXXX, XXX, XXX−XXX

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by protonation with an acidic quencher, resulting in the formation of H at the ω chain end. As a whole, in this proposed mechanism, one of the NQ ligands initially attached to Pd(0) plays a unique dual role for generating the active species: (1) as an oxidant, it oxidizes Pd(0) to Pd(II), and (2) as an anionic ligand, it is coordinated to the Pd(II) in an η3-fashion as the πallyl ligand in other active Pd-based initiating systems. (II) Pd(cod)(Cl-nq)Cl/NaBPh4 Initiating System. Preparation and Characterization of Pd(cod)(Cl-nq)Cl. Inspired by the unique role of NQ in the Pd(nq)2/NaBPh4 system, we attempted to prepare analogous Pd(0) complexes with NQ derivatives with a series of substituents such as Cl and MeO on the 2- and 3-positions following the procedures for the syntheses of Pd(cod)(nq) and Pd(nq)2. While most of the attempts failed to give a well-defined complex, 2,3-dichloro-1,4-naphthoquinone (dichlone) gave an air-stable crystalline product in good yield (54%) in a synthetic procedure for the preparation of Pd(cod)(nq) where a precursor Pd(0) complex, Pd2(dba)3· CHCl3 [dba = (E,E)-dibenzylideneacetone], was treated with COD and dichlone in acetone at room temperature and the resulting solution was concentrated in vacuo followed by recrystallization from acetone/ether (Scheme 3). While the

polymerization of diazoacetates in terms of molecular weight, yield, and available monomer range. MALDI-TOF-MS Analysis and Plausible Polymerization Mechanism. In order to clarify the chain-end structures and obtain information on the initiation and termination mechanism, MALDI-TOF-MS analysis was carried out. Figure 2A shows a part of the MS spectrum (high resolution mode) of a low-molecular-weight poly(EDA′) (Mn = 6.3 kDa, Mw/Mn = 1.18) obtained with the Pd(nq)2/NaBPh4 initiating system; we can observe a major set of signals whose peak intervals correspond to the molecular weight of the repeating unit derived from EDA (m/z = 86.1, CHCO2C2H5), while there exist several sets of minor signals with the same peak intervals. On the basis of comparison of one of the major peak clusters with simulated signal appearance (Figure 2B), the major set of signals was assigned to a Na+ adduct of the polymer with Ph and H at the α and ω chain ends, respectively.45 From the results, we can propose a plausible initiation and termination mechanism of polymerization of diazoacetates with the Pd(nq)2/NaBPh4 system (Scheme 2C) although the presence of some minor sets of signals in the spectrum indicates that there existed other modes of initiation and/or termination in the polymerization, resulting in the moderate control of the polymerization. According to the MALDI-TOF-MS analyses described above, the polymerization should be initiated by a Pd−Ph species. In our previous publication on the polymerization of diazoacetates initiated by π-allylPdCl/NaBPh4, we have proposed that a Pd− Ph species generated by transmetalation from the borate BPh4− to the Pd(II) center initiated the polymerization.42 In contrast to the initiation with the divalent π-allylPdCl precursor, in the Pd(nq)2/NaBPh4 system using zero-valent Pd(nq)2, oxidation of the Pd should occur during the formation of a Pd−Ph species. Although we do not have any experimental evidence at present, a reasonable mechanism is proposed below on the basis of literature on the related oxidation processes.46,47 It was reported that Pd(0) was oxidized to Pd(II) by quinone as an oxidant in the presence of strong proton donors such as acetic acid and trifluoroacetic acid (Scheme 2A).46 In addition, even in the absence of a strong acid, similar Pd(0)-to-Pd(II) oxidation by quinone was reported in the presence of a methanol/arylboron complex where arylboron-activated methanol behaved as a strong proton donor, promoting the oxidation of Pd(0) (Scheme 2B).47 Quite interestingly, transmetalation of a Ph group from the organoboron to Pd simultaneously occurred with the oxidation to afford a Pd−Ph species via an “oxidative transmetalation” mechanism, resulting in the formation of a Pd(II) complex η3-ligated with an anionic hydroquinone. The unique mechanism and the formation of the η3-ligated Pd(II) complex were also supported by theoretical calculation.47 Then, as described in Scheme 2C, the oxidative transmetalation mechanism can be reasonably applied to the generation of an initiating species in our Pd(nq)2/NaBPh4 system, resulting in the formation of a Pd(II)-Ph complex η3ligated with an anionic hydroquinone. Particularly noteworthy with the proposed initiating species is the η3-π-allyl-type ligation because its effectiveness for the Pdbased initiating system has been well-established so far.34,42,43 After the formation of the active Pd−Ph species, a diazo-bearing carbon atom of a monomer engaged in the initiation should be inserted into the Pd−Ph through possible formation of a Pd− carbene species43 accompanied by N2 elimination, affording the propagating species with a new Pd−C bond. After propagation with excess monomers, the polymerization should be terminated

Scheme 3. Synthesis of Pd(cod)(Cl-nq)Cl

result of elemental analysis supported the expected composition of Pd(cod)(dichlone), 1H and 13C NMR spectra indicated that both ligands of COD and dichlone were ligated asymmetrically (Figures S1 and S2 in the Supporting Information). Then, the structure of the complex was unambiguously determined by Xray crystallographic analysis, as illustrated in Figure 3. Surprisingly to us, one of the Cl−C bonds in dichlone oxidatively added to the Pd(0) center, resulting in the formation of divalent Pd(cod)(Cl-nq)Cl, which was a very rare example of a Pd(II) complex bearing σ-bonded naphthoquinonyl group as an anionic ligand. To our knowledge, there are only a few reported examples where a p-quinone derivative is σ-bonded to Pd(II) so far,48,49 and this is the first example of a Pd(II) complex with a σ-bonded naphthoquinonyl anion. As revealed in Figure 3, each center of two CC bonds in COD, the chloride anion, and the naphthoquinonyl carbon atom constitute a square planar geometry on the Pd center, with the NQ plane located perpendicularly to the coordination plane of the Pd(II). Polymerization Behavior. The newly obtained Pd(II) complex, Pd(cod)(Cl-nq)Cl, was employed as an initiator for the polymerization of EDA, revealing that the combination of Pd(cod)(Cl-nq)Cl and NaBPh4 was effective to produce poly(EDA′) (Mn = 20.5 kDa) despite a low polymer yield (20%), while Pd(cod)(Cl-nq)Cl alone did not produce polymers (runs 1 and 2 in Table 2). The polymer yield became slightly higher with the increase of polymerization temperature: the polymerization of EDA was conducted at 70 °C where 1,4dioxane was used as a solvent with a higher boiling point than THF (run 3), giving a polymer in 28% yield (run 3). On the other hand, the Pd(cod)(Cl-nq)Cl/NaBPh4 system was not E

DOI: 10.1021/acs.macromol.9b00857 Macromolecules XXXX, XXX, XXX−XXX

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sensitivity of the Pd(cod)(Cl-nq)Cl/NaBPh4 system to the steric congestion derived from the monomer ester substituents. Polymer Tacticity. The most striking characteristic of the Pd(cod)(Cl-nq)Cl/NaBPh4 system is that it can produce polymers with high stereoregularity, while the Pd(nq)2/ NaBPh4 system gave atactic polymers. The polymer tacticity was evaluated by NMR analyses; in Figure 4, 1H and 13C NMR spectra of poly(EDA′) obtained with the Pd(nq)2/NaBPh4 and Pd(cod)(Cl-nq)Cl/NaBPh4 systems are shown and compared to that prepared with a diene-ligated Rh complex, [Rh(cod)(Lprolinate)], 36,37 which can afford polymers with high syndiotacticity. The appearance of some signals of the polymers is dependent on the tacticity, particularly main chain methine (CH) signals in 1H NMR and side chain carbonyl carbon (C O) signals in 13C NMR. While a highly stereoselective polymer obtained with the Rh(diene)-based initiator exhibits only one signal in each region of the CH and CO (Figure 4C), split and broad signals are observed for the product obtained with the Pd(nq)2/NaBPh4 system (Figure 4A), suggesting the product is an atactic polymer. On the other hand, the polymer prepared with the Pd(cod)(Cl-nq)Cl/NaBPh4 system exhibits a dominant peak at the same position as for syndiotactic polymers obtained with the Rh(diene)-based initiator in each 1H and 13C NMR spectrum (Figure 4B). Although the existence of some minor peaks on both sides of the dominant one suggests that the stereoregularity is not so high as that in Rh(diene)-initiated polymerization, the appearance of signals in Figure 4B indicates that much higher stereoregulation is achieved with the Pd(cod)(Cl-nq)Cl/NaBPh4 system compared to the Pd(nq)2/ NaBPh4 system as well as previously reported Pd-based initiating systems.34,40−43,50,51 Likewise, high stereoregulation is also observed in the spectra of the polymers obtained from BDA (Figure S3 in the Supporting Information). As described below, the high stereoregularity could be ascribed to the steric congestion around the propagating Pd center in the Pd(cod)(Cl-nq)Cl/NaBPh4 system. MALDI-TOF-MS Analysis and Plausible Polymerization Mechanism. In the MS spectrum for the polymer obtained with the Pd(cod)(Cl-nq)Cl/NaBPh4 system (Figure 5), the same set of signals as that for the product obtained with the Pd(cod)2/ NaBPh4 system is observed as a major component. Thus, we can assume that the polymerization was initiated by a Pd−Ph species, which should be generated by replacement of Cl with transmetalation of a Ph group from NaBPh4 (Scheme 4). The Pd−Ph species should initiate the polymerization with insertion of the monomer into Pd−Ph accompanied by N2 elimination,

Figure 3. X-ray structure of Pd(cod)(Cl-nq)Cl (upper: side-view, lower: top-view) with 50% thermal ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Pd1−C1, 1.988(2); Pd1−C11, 2.178(2); Pd1−C12, 2.1996(19); Pd1−C15, 2.308(2); Pd1−C16, 2.344(2); Pd1−Cl2, 2.3163(5); C11−Pd1−C12 36.56(7); C15−Pd1−C16, 33.73(7); Cl2−Pd1−C1, 86.44(6).

active at a lower temperature of 0 °C, as was the case with the Pd(nq)2/NaBPh4 system (run 4). While the increase of the monomer feed ratio to [EDA]/[Pd] = 200 decreased both yield and Mn (run 5), the polymerization with the low feed ratio of [EDA]/[Pd] = 10 for 1 h (run 6) yielded a polymer with a lower Mn (7.0 kDa), whose MALDI-TOF-MS analysis afforded information about the polymer chain-end structures as described below. As a cocatalyst, the addition of phenyllithium (PhLi) instead of NaBPh4 was found to be effective for the polymerization (run 7), indicating that the transmetalation from organolithium reagents was also effective for generating the active species. As for monomers other than EDA, BDA was able to be transformed into polymers (run 8), while the polymerization of cHDA did not afford polymers (run 9), suggesting the

Table 2. Polymerization of Diazoacetates Initiated by the Pd(cod)(Cl-nq)Cl/NaBPh4 systema run

Pd complex

cocatalystb

monomer (M)

[M]/[Pd]

temp

time

yield (%)c

Mn × 10 −3d

Mw/Mnd

1 2 3e 4 5 6 7 8 9

Pd(cod)(Cl-nq)Cl Pd(cod)(Cl-nq)Cl Pd(cod)(Cl-nq)Cl Pd(cod)(Cl-nq)Cl Pd(cod)(Cl-nq)Cl Pd(cod)(Cl-nq)Cl Pd(cod)(Cl-nq)Cl Pd(cod)(Cl-nq)Cl Pd(cod)(Cl-nq)Cl

NaBPh4 none NaBPh4 NaBPh4 NaBPh4 NaBPh4 PhLif NaBPh4 NaBPh4

EDA EDA EDA EDA EDA EDA EDA BDA cHDA

100 100 100 100 200 10 100 100 100

50 °C 50 °C 70 °C 0 °C 50 °C 50 °C 50 °C 50 °C 50 °C

13 h 13 h 13 h 13 h 13 h 1h 13 h 13 h 1h

20 trace 28 trace 16 5.5 19 26 trace

20.5

1.46

20.0

1.98

14.1 7.0 15.5 11.9

2.19 1.42 2.30 2.31

In THF; [monomer]0 = ∼0.3 M; EDA was used as a CH2Cl2 solution with a concentration of 1.9−2.4 M. b[NaBPh4]/[Pd] = 1.1−1.5. cAfter purification with preparative SEC. dDetermined by SEC using PMMA standards. e1,4-dioxane was used as solvent. fPhLi was used as a butyl ether solution with a concentration of 1.6 M ([PhLi]/[Pd] = 1.1). a

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DOI: 10.1021/acs.macromol.9b00857 Macromolecules XXXX, XXX, XXX−XXX

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Figure 4. 1H and 13C NMR spectra of EDA polymers obtained with (A) Pd(nq)2/NaBPh4 (Mn = 24.0 kDa, Mw/Mn = 1.54), (B) Pd(cod)(Cl-nq)Cl/ NaBPh4 (Mn = 15.6 kDa, Mw/Mn = 2.16), and (C) Rh(cod)(L-prolinate) (Mn = 69.3 kDa, Mw/Mn = 2.62).

about the high activity for the diazoacetate polymerization as observed in π-allylPd-based initiating systems.34,42,43 On the other hand, in the Pd(cod)(Cl-nq)Cl/NaBPh4 system, the coordination of Cl−NQ or both Cl−NQ and COD results in the rather sterically constricted environment around the Pd center, leading to the high stereoregulation in the polymerization. However, the steric congestion would bring about the lower rate of the polymerization, and a large part of the monomer would be consumed to yield low-molecular-weight oligomeric products, which were removed in purification with preparative SEC. Nonetheless, we believe that the finding of the Pd(cod)(Clnq)Cl/NaBPh4 system is meaningful for understanding the fundamental aspect of the diazoacetate polymerization, and further development based on the system will lead to new initiating systems with improved initiating ability. Polymerization with in Situ Generated Pd Complexes with an NQ Derivative. As described above, attempts to prepare welldefined NQ-ligated Pd complexes with other NQ-derivatives were unsuccessful. Then, we tried to establish a method for conducting the polymerization with in situ generated NQcontaining Pd-based systems by treatment of Pd2(dba)3·CHCl3 with COD and an NQ derivative followed by the addition of NaBPh4 (Scheme 5A and Table 3). While the polymerization of EDA without the addition of NQ derivatives [Pd2(dba)3· CHCl3/NaBPh4] did not afford polymers (run 1), the reaction with the addition of Cl-containing NQ derivatives, dichlone, 2chloro-3-methoxy-NQ, and 2-chloro-NQ, yielded polymeric products (runs 2−4). When dichlone was added as a ligand precursor for the in situ procedure, a similar polymerization result was observed to that for the polymerization with the combination of the isolated Pd(cod)(Cl-nq)Cl and NaBPh4 in terms of yield, Mn, and Mw/Mn (run 2 in Table 3 vs run 1 in

followed by propagation to afford a Pd(II) species bearing a Phinitiated propagating polymer chain as a ligand on the metal center. The transmetalation can be carried out with PhLi as the polymerization result in run 7 in Table 2 indicated. The other set of signals with the second highest intensity agrees well with the structure having the Ph group at the α chain end but a different ω chain end group: a cyclic ketone framework resulting from back-biting of the propagating chain end to an ester carbonyl of its own side chain. The occurrence of the back-biting was observed in diazoacetate polymerization initiated by πallylPdCl-based systems and reported in our previous publication.42 Although a Pd−OEt species generated from the back-biting could initiate the polymerization of EDA,42 the lack of signals derived from EtO-initiated polymer chains in the MALDI-TOF-MS spectrum indicated that the back-biting occurred only in the very late stage of the polymerization where the monomer was almost completely consumed. Based on the proposed mechanism, it is apparent again that dichlone plays a dual role in generating the active species for the polymerization with the Pd(cod)(Cl-nq)Cl/NaBPh4 system: (1) oxidizing the initial Pd(0) to Pd(II) via its oxidative addition across the C−Cl bond and (2) stabilizing the active Pd(II) center through coordination as an anionic ligand. While an η3coordinating anionic ligand derived from an unsubstituted NQ and another neutral NQ are attached to the active Pd center in the Pd(nq)2/NaBPh4 system (Scheme 2), a σ-bonded Cl−NQ anion and COD stabilize the propagating Pd center in the Pd(cod)(Cl-nq)Cl/NaBPh4 system. The contrastive initiating abilities of the two initiating systems could be ascribed to the difference in the coordinating environment around the Pd center. In the Pd(nq)2/NaBPh4 system, the η3-coodination of the NQ-derived anion would bring G

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Figure 5. (A) Part of the MALDI-TOF-MS spectrum of a polymer sample obtained with Pd(cod)(Cl-nq)Cl/NaBPh4-inititated polymerization of EDA (run 6 in Table 2), (B) theoretical isotopic distribution of a Na-adduct of the Ph-initiated polymer (degree of polymerization = 38) terminated by protonolysis, and (C) theoretical isotopic distribution of a Na-adduct of the Ph-initiated polymer (degree of polymerization = 39) terminated by backbiting.

Scheme 4. Plausible Mechanism for Polymerization Initiated by Pd(cod)(Cl-nq)Cl/NaBPh4

Table 2). Also, a similar dominant signal is observed in the CH region in the 1H NMR spectrum of the product (Figure 6A).

The comparison of the signal appearance in the NMR spectra suggests that the stereoregularity of the product obtained with H

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Macromolecules Scheme 5. (A) Polymerization of EDA Initiated by in Situ Generated Pd Complexes and (B) Plausible Propagating Species

Figure 6. 1H NMR spectra of polymers obtained by polymerization of EDA initiated by in situ-generated Pd complexes [(A) run 2, (B) run 3, and (C) run 4 in Table 3].

Table 3. Polymerization of EDA Initiated by in Situ Generated Pd complexesa run

NQ derivative

yield (%)c

Mn × 10 −3d

Mw/Mnd

1 2b 3b 4b

none dichlone 2-chloro-3-methoxy-NQ 2-chloro-NQ

trace 20 24 41

19.0 16.9 18.0

1.64 1.84 1.48

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CONCLUSIONS We have demonstrated that Pd(naphthoquinone)/borate initiating systems are highly effective for the polymerization of diazoacetates. Compared to our previously reported Pd-based initiating systems such as the Pd(NHC)(nq)/borate and πallylPdCl-based systems, the Pd(nq)2/NaBPh4 system is capable of affording polymers with higher Mn values and in higher yield. Furthermore, we have newly synthesized a Pd(II) complex with an NQ derivative as the ligand, [Pd(cod)(Cl-nq)Cl], and found that the novel Pd complex activated with NaBPh4 can produce polymers with high stereoregularity. In both Pd(nq)-based systems, it was revealed that NQs play unique and important dual roles as an oxidant of Pd(0) and an anionic ligand. Although ideal stereospecific polymerization of diazoacetate using Pd complexes has not yet been achieved so far, Pd-based systems in general have the important characteristic of affording polymers with different tacticities depending on their ligand structures, in contrast to the Rh-based systems, which always give highly syndiotactic polymers irrespective of their ligand structures. Based on the new findings described here, we are now trying to further improve the performance of the Pd-based initiating systems.

In THF; [EDA]0 = ∼0.3 M; [NaBPh4]/[Pd] = 1.1−1.5; EDA was used as a CH2Cl2 solution with a concentration of 2.5 M. b[COD]/ [NQ derivative]/[Pd] = 4/2/1. cAfter purification with preparative SEC. dDetermined by SEC using PMMA standards. a

the in situ technique was somewhat lower than that of the product obtained with the isolated complex (Figure 4B). These results suggest that even with the in situ method, the oxidative addition of dichlone occurred to afford Pd(cod)(Cl-nq)Cl followed by the generation of the initiating Pd−Ph species by transmetalation with NaBPh4; however, the presence of excess COD and dichlone would inflict a negative effect on the stereoregulation. Interestingly, almost the same level of stereoregularity is observed in the NMR spectrum for the product obtained with 2-chloro-3-methoxy-NQ (Figure 6B). On the other hand, such a dominant signal is not observed in the NMR spectrum obtained with 2-chloro-NQ (Figure 6C), indicating that an atactic polymer was obtained in this case. These results suggest that tacticity of the product depended on the steric effect of the NQ derivative attached to the Pd center and, in particular, the substituent on the 3-position of the NQ framework (Cl for dichlone and OMe for 2-chloro-3-methoxyNQ) was crucial probably because it would exert an essential effect on the stereo control of the propagation (Scheme 5B). In any case, this in situ initiator preparation technique has an advantage that polymers can be easily obtained from commercially available reagents alone without isolation of Pd complexes and polymer tacticity can be controlled to a certain extent by the steric effect from NQ derivatives employed.

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EXPERIMENTAL SECTION

Materials. Sodium tetraphenylborate (NaBPh4, Tokyo Chemical Industry, >99.5%), 1,5-cyclooctadiene (COD, Tokyo Chemical Industry, 99%), 1,4-naphthoquinone (NQ, Tokyo Chemical Industry, >98.0%), 2,3-dichloro-1,4-naphthoquinone (dichlone, Tokyo Chemical Industry, >99.5%), 2-chloro-1,4-naphthoquinone (Tokyo Chemical Industry, >98.0%), phenyllithium (PhLi, Tokyo Chemical Industry, 1.6 M in butyl ether), 1,4-dioxane (FUJIFILM Wako Pure Chemical, >99.5%, super dehydrated), chloroform (Junsei Chemical, 99%), dichloromethane (Junsei Chemical, 99%), methanol (Yoneyama Yakuhin Kogyo, 99%), hydrochloric acid (Nacalai Tesque, 35−37%), I

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with CHCl3 three times, and the combined organic layer was washed with NaCl aqueous solution and water. The organic layer was dried over Na2SO4, and concentrated under reduced pressure. Purification with preparative recycling SEC gave a polymer (21.5 mg, 20% yield). Measurements. The number-average molecular weight (Mn) and polydispersity ratio [weight-average molecular weight/number-average molecular weight (Mw/Mn)] were measured by means of SEC on a Jasco-ChromNAV system equipped with a differential refractometer detector using THF as the eluent at a flow rate of 1.0 mL/min at 40 °C, calibrated with six poly(methyl methacrylate) (PMMA) standards (Shodex M-75; Mp = 2400−212000, Mw/Mn < 1.1) and dibutyl sebacate (molecular weight = 314.5). The columns used for the SEC analyses were a combination of Styragel HR4 and HR2 (Waters; exclusion limit molecular weight = 600 and 20 kDa for polystyrene, respectively; column size = 300 mm × 7.8 mm i.d.; average particle size = 5 μm). The absolute molecular weight of the polymers was determined by SEC coupled with multiangle light scattering (SECMALS) in THF at 40 °C on a Dawn HELEOS II 8+ (Wyatt Technology; λ = 661.5 nm). The refractive index increment (dn/dc) values were measured assuming 100% mass recovery. The columns used for the SEC-MALS analyses were a combination of a KF-805 L and a KF-804 L (Shodex; exclusion limit molecular weight = 4000 and 400 kDa for polystyrene, respectively; column size = 300 mm × 8.0 mm i.d.; average particle size = 10 and 7 μm, respectively). Purification by preparative recycling SEC was performed on a JAI LC-918R equipped with a combination of JAIGEL-3H and JAIGEL-2H columns (Japan Analytical Industry; exclusion limit molecular weight = 70 and 5 kDa for polystyrene, respectively; column size = 600 mm × 20 mm i.d.) using CHCl3 as the eluent at a flow rate of 3.8 mL/min at room temperature. By using a recycling procedure when it was required, polymeric products were completely isolated from concomitant oligomeric products with this purification. 1H (500 MHz) and 13C (126 MHz) NMR spectra were recorded on a Bruker Avance III HD 500 spectrometer in CDCl3 at room temperature (Pd complexes and monomers) or at 50 °C (polymers). Matrix-assisted laser desorption/ ionization time-of-flight mass spectrometry (MALDI-TOF-MS) analyses were performed on a JMS-S3000 (JEOL, spiral mode) using dithranol as a matrix and sodium trifluoroacetate as an ion source. The calibration was carried out using poly(ethylene glycol) (Mn = 2700− 3500). Elemental analyses were performed on a YANAKO CHN Corder MT-5.

CaH2 (Nacalai Tesque, >90.0%), and Na2SO4 (Nacalai Tesque, > 98.5%) were used as received. Tetrahydrofuran (THF, Kanto Chemical, >99.5%, dehydrated Super Plus grade) was further purified using Glass Contour MINI (Nikko Hansen & Co.). Acetone (FUJIFILM Wako Pure Chemical, 99%) was dried over CaH2 and used without further purification. Diethyl ether (Junsei Chemical, 99%) was dried over CaH2 and distilled before use. Synthesis. Pd(cod)(nq),52 Pd(nq)2,53 Pd2(dba)3·CHCl3 [dba = (E,E)-dibenzylideneacetone],54 Rh(cod)(L-prolinate),36,37 and 2chloro-3-methoxy-1,4-naphthoquinone55 were prepared according to the literature studies. Ethyl diazoacetate (EDA) was prepared according to the literature56 and stored as a dichloromethane solution. The solution was dried over CaH2, and the concentration was determined with trichloroethylene (Katayama Chemical) as an internal standard. Benzyl diazoacetate (BDA) and cyclohexyl diazoacetate (cHDA) were prepared according to the general procedure reported by Fukuyama and co-workers.57 The characterization data for BDA57 and cHDA58 were reported in the literature. Caution! Extra care must be taken for preparation and handling of the diazoacetates because of their potential explosiveness. Synthesis of Pd(cod)(Cl-nq)Cl. Under a nitrogen atmosphere, Pd2(dba)3·CHCl3 (0.16 g, 0.15 mmol), dichlone (0.28 g, 1.3 mmol), and COD (0.15 mL, 1.3 mmol) were dissolved in acetone (3.0 mL). The resulting suspension was stirred at room temperature for 1 h and then filtered to give a red solution. Diethyl ether was added to the solution, and the mixture was left to stand at 4 °C for overnight. The crystal was collected by vacuum filtration and washed by rinsing with a small amount of ice-cold acetone and diethyl ether, yielding a yellowbrown crystal (75 mg, 54%). 1H NMR (500 MHz, CDCl3, δ): 8.09− 8.07 (m, 1H, Ar), 8.04−8.02 (m, 1H, Ar), 7.70−7.64 (m, 2H, Ar), 6.22−6.14 (m, 2H, CHCH), 5.64−5.54 (m, 2H, CH CH), 3.01−2.57 (m, 8H, CH2CH2). 13C NMR (126 MHz, CDCl3, δ): 185.7, 174.7, 164.6, 148.9, 133.6, 133.5, 132.7, 131.6, 127.5, 126.9, 124.3, 124.1, 107.0, 106.9, 31.6, 31.3, 28.5, 27.9. Anal calcd for C18H16O2Cl2Pd: C, 48.95; H, 3.65. Found: C, 49.07; H, 4.04. Polymerization Procedure. As a representative example, the procedure for the polymerization of EDA with Pd(nq)2/NaBPh4 (run 5 in Table 1) is described as follows. Under a nitrogen atmosphere, Pd(nq)2 (4.23 mg, 1.0 × 10−2 mmol) was placed in a Schlenk tube and was cooled to −78 °C. A THF (3.0 mL) solution of NaBPh4 (3.76 mg, 1.1 × 10−2 mmol) was gradually added to the Schlenk tube. The resulting yellow suspension was stirred at −78 °C for 10 min and then 0 °C for 10 min, giving a deep red solution. The polymerization was started by the addition of a dichloromethane solution of EDA (2.47 M, 0.40 mL). After 10 min, the reaction mixture was warmed to 50 °C and stirred for 13 h. After the volatiles were removed under reduced pressure, 10 mL of 1 N HCl/methanol, 10 mL of 1 N HCl aqueous solution, and 20 mL of CHCl3 were added to the residue. The resulting mixture was then extracted with CHCl3 three times, and the combined organic layer was washed with NaCl aqueous solution and water. The organic layer was dried over Na2SO4 and concentrated under reduced pressure. Purification with preparative recycling size-exclusion chromatography (SEC) gave a polymer (62.0 mg, 72% yield). As a representative example for the polymerization of EDA with the in situ-generated Pd complex with an NQ derivative, the procedure with dichlone as a NQ derivative (run 2 in Table 3) is described as follows. Under a nitrogen atmosphere, Pd2(dba)3·CHCl3 (5.18 mg, 5.0 × 10−3 mmol) and dichlone (4.54 mg, 2.0 × 10−2 mmol) were soluble in THF (2.5 mL) in a Schlenk tube. To the solution, a diluted solution of COD [0.50 mL out of a mixed solution of COD (0.10 mL) and THF (10.1 mL), ∼4.0 × 10−2 mmol] was added at room temperature. The resulting solution was stirred at the temperature for 10 min and then cooled to −78 °C. NaBPh4 (4.60 mg, 1.2 × 10−2 mmol) was added to the Schlenk tube, and the resulting solution was stirred at the temperature for 10 min. The polymerization was started by the addition of a dichloromethane solution of EDA (2.50 M, 0.40 mL). After 10 min, the reaction mixture was warmed to 50 °C and stirred for 13 h. After the volatiles were removed under reduced pressure, 10 mL of 1 N HCl/ methanol, 10 mL of 1 N HCl aqueous solution, and 20 mL of CHCl3 were added to the residue. The resulting mixture was then extracted

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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.9b00857.

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X-ray analysis data, 1H and 13C NMR spectra of Pd(cod)(Cl-nq)Cl, and 1H NMR spectra of poly(EDA′), poly(BDA′), and poly(cHDA′) (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone & Fax: +81-89-927-8547. ORCID

Hiroaki Shimomoto: 0000-0002-9515-8681 Tomomichi Itoh: 0000-0001-8579-939X Eiji Ihara: 0000-0002-0279-5105 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Young Scientists (B) (JSPS KAKENHI grant no. 16K17916), a Grant-in-Aid for Scientific Research (B) (JSPS KAKENHI grant no. 18H02021), J

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(19) Ihara, E.; Okada, R.; Sogai, T.; Asano, T.; Kida, M.; Inoue, K.; Itoh, T.; Shimomoto, H.; Ishibashi, Y.; Asahi, T. Pd-Mediated Polymerization of Diazoacetates with Aromatic Ester Group: Synthesis and Photophysical Property of Poly(1-pyrenylmethoxycarbonylmethylene). J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 1020−1023. (20) Shimomoto, H.; Itoh, E.; Itoh, T.; Ihara, E.; Hoshikawa, N.; Hasegawa, N. Polymerization of Hydroxy-Containing Diazoacetates: Synthesis of Hydroxy-Containing “Poly(substituted methylene)s” by Palladium-Mediated Polymerization and Poly(ester−ether)s by Polycondensation through O−H Insertion Reaction. Macromolecules 2014, 47, 4169−4177. (21) Shimomoto, H.; Asano, H.; Itoh, T.; Ihara, E. Pd-initiated controlled polymerization of diazoacetates with a bulky substituent: synthesis of well-defined homopolymers and block copolymers with narrow molecular weight distribution from cyclophosphazene-containing diazoacetates. Polym. Chem. 2015, 6, 4709−4714. (22) Shimomoto, H.; Shimizu, K.; Takeda, C.; Kikuchi, M.; Kudo, T.; Mukai, H.; Itoh, T.; Ihara, E.; Hoshikawa, N.; Koiwai, A.; Hasegawa, N. Synthesis of polymers with densely-grafted oligo(ethylene glycol)s by Pd-initiated polymerization of oxyethylene-containing diazoacetates. Polym. Chem. 2015, 6, 8124−8131. (23) Shimomoto, H.; Oda, A.; Kanayama, M.; Sako, T.; Itoh, T.; Ihara, E.; Hoshikawa, N.; Koiwai, A.; Hasegawa, N. Pd-Initiated Polymerization of Diazo Compounds Bearing Dialkoxyphosphinyl Group and Hydrolysis of the Resulting Polymers and Oligomers to Afford Phosphonic Acid-Containing Products. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 1742−1751. (24) Shimomoto, H.; Uegaito, T.; Yabuki, S.; Teratani, S.; Itoh, T.; Ihara, E.; Hoshikawa, N.; Koiwai, A.; Hasegawa, N. Lithium ion conductivity of polymers containing N-phenyl-2,6-dimethoxybenzamide framework in their side chains: Possible role of bond rotation in polymer side chain substituents for efficient ion transport. Solid State Ionics 2016, 292, 1−7. (25) Shimomoto, H.; Kikuchi, M.; Aoyama, J.; Sakayoshi, D.; Itoh, T.; Ihara, E. Cyclopolymerization of Bis(diazocarbonyl) Compounds Leading to Well-Defined Polymers Essentially Consisting of Cyclic Constitutional Units. Macromolecules 2016, 49, 8459−8465. (26) Kato, F.; Chandra, A.; Tokita, M.; Asano, H.; Shimomoto, H.; Ihara, E.; Hayakawa, T. Self-Assembly of Hierarchical Structures Using Cyclotriphosphazene-Containing Poly(substituted methylene) Block Copolymers. ACS Macro Lett. 2018, 7, 37−41. (27) Shimomoto, H.; Kudo, T.; Tsunematsu, S.; Itoh, T.; Ihara, E. Fluorinated Poly(substituted methylene)s Prepared by Pd-Initiated Polymerization of Fluorine-Containing Alkyl and Phenyl Diazoacetates: Their Unique Solubility and Postpolymerization Modification. Macromolecules 2018, 51, 328−335. (28) Shikinaka, K.; Suzuki, K.; Masunaga, H.; Ihara, E.; Shigehara, K. Stiff and hierarchical chain nature of atactic and stereoregular poly(substituted methylene)s. Polym. Int. 2018, 67, 495−499. (29) Takaya, T.; Oda, T.; Shibazaki, Y.; Hayashi, Y.; Shimomoto, H.; Ihara, E.; Ishibashi, Y.; Asahi, T.; Iwata, K. Excited-State Dynamics of Pyrene Incorporated into Poly(substituted methylene)s: Effects of Dense Packing of Pyrenes on Excimer Formation. Macromolecules 2018, 51, 5430−5439. (30) Tokita, M.; Shikinaka, K.; Hoshino, T.; Fujii, K.; Mikami, J.; Koshimizu, N.; Sakajiri, K.; Kang, S.; Watanabe, J.; Shigehara, K. Thermotropic behavior of syndiotactic polymethylenes with alkyloxycarbonyl side chains. Polymer 2013, 54, 995−998. (31) Koshimizu, N.; Aizawa, Y.; Sakajiri, K.; Shikinaka, K.; Shigehara, K.; Kang, S.; Tokita, M. Thermotropic Behavior of Syndiotactic Polymethylenes with ω-[4-(trans-4-Pentylcyclohexyl)phenoxy]alkyloxycarbonyl Side Chains. Macromolecules 2015, 48, 3653−3661. (32) Franssen, N. M. G.; Ensing, B.; Hegde, M.; Dingemans, T. J.; Norder, B.; Picken, S. J.; Alberda van Ekenstein, G. O. R.; van Eck, E. R. H.; Elemans, J. A. A. W.; Vis, M.; Reek, J. N. H.; de Bruin, B. On the “Tertiary Structure” of Poly-Carbenes; Self-Assembly of sp3-CarbonBased Polymers into Liquid-Crystalline Aggregates. Chem. − Eur. J. 2013, 19, 11577−11589.

a Grant-in-Aid for Scientific Research (C) (JSPS KAKENHI grant no. 19K05586), and a Grant-in-Aid for Challenging Exploratory Research (JSPS KAKENHI grant no. 19K22219). The authors thank Applied Protein Research Laboratory in Ehime University for its assistance in NMR, Ehime Institute of Industrial Technology for its assistance in MALDI-TOF-MS measurements, and the Advanced Research Support Center in Ehime University for its assistance in elemental analysis. The authors are grateful to Prof. Shigeki Mori (Ehime University) for the X-ray single crystal structural analyses. The authors are also grateful to Profs. Sadahito Aoshima and Arihiro Kanazawa (Osaka University) for their assistance in SEC-MALS measurements.

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DOI: 10.1021/acs.macromol.9b00857 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.9b00857 Macromolecules XXXX, XXX, XXX−XXX