Iso- and Syndio-Selective ROMP of Norbornene and

Nov 7, 2014 - Iso- and Syndio-Selective ROMP of Norbornene and Tetracyclododecene: Effects of Tacticity Control on the Hydrogenated Ring-Opened ...
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Iso- and Syndio-Selective ROMP of Norbornene and Tetracyclododecene: Effects of Tacticity Control on the Hydrogenated Ring-Opened Poly(cycloolefin)s Shigetaka Hayano* and Yuki Nakama Zeon Corporation R&D Center, 1-2-1 Yako, Kawasaki-ward, Kawasaki City, Kanagawa Prefecture 210-9507, Japan S Supporting Information *

ABSTRACT: Isospecific and syndioselective ring-opening metathesis polymerizations (ROMP) of norbornene (NB) and tetracyclododecene (TCD), as well as their tactic (and atactic) hydrogenated ring-opened polymer products have been thoroughly investigated for the first time. In the case of NB polymerization, cis-isospecific ROMPs of NB were achieved with Mo(O)(rac-5,5′,6,6′-Me4-3,3′-t-Bu2-biphenolate)2-based catalyst (cis = 100%, isotacticity =100%). Cis-syndioselective ROMPs of NB proceeded in the presence of W(NPh)Cl4· Et2O- and W(N-2,6Me2Ph)Cl4·Et2O-based catalysts (cis = 90%, syndiotacticity =90%). Hydrogenated poly(NB)s exhibited high crystallinity irrespective of their stereostructures. The crystalline nature of syndiotactic hydrogenated poly(NB) (syndiotacticity = 90%: Tm = 130−136 °C, ΔH = 60−75 J/g, wc = 0.55−0.70) was close to that of conventional atactic one (meso/racemo = 50/50: Tm = 145−150 °C, ΔH = 65−70 J/g, wc = 0.65). On the contrary, isotactic hydrogenated poly(NB) was characterized by a remarkably unique crystalline nature such as high melting points (isotacticity > 90%: Tm = 160−180 °C, ΔH = 50−90 J/g, wc = 0.50−0.65). In the case of TCD polymerization, W(NPh)Cl4·Et2O-based catalyst introduced cissyndioselective ROMP of TCD (cis = 90%, syndiotacticity = 86%). Syndiotactic hydrogenated poly(TCD) obtaining from W( NPh)Cl4·Et2O-based catalyst was entirely amorphous and soluble in ordinary organic solvents. On the other hand, atactic hydrogenated poly(TCD) showed poor solubility and semicrystallinity (Tm = 290 °C, ΔH < 10 J/g). In addition, Mo(O)(rac5,5′,6,6′-Me4-3,3′-t-Bu2-biphenolate)2-based catalyst provided all-cis poly(TCD), of which hydrogenated product was completely insoluble and highly crystalline (Tm > 350 °C, wc = 0.65). These tendencies are in significant contrast to those of the hydrogenated poly(endo-dicyclopentadiene)s, wherein atactic polymer is amorphous and both tactic polymers are crystalline (syndiotacticity = 90%; Tm = 270 °C and isotacticity > 95%; Tm = 295 °C, respectively).

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INTRODUCTION Polymerizations of cycloolefins have attracted intensive research from both academia and industry. To date, various cycloolefin monomers have been investigated to provide polymers through ring-opening metathesis polymerization (ROMP).1 Among them, norbornenes are recognized as versatile monomers, which easily polymerize in the presence of olefin metathesis catalysts. Norbornene (NB), endodicyclopentadiene (DCP), and tetracyclododecene (TCD) are the most simple, versatile and important cycloolefin monomers. These are made by Diels−Alder addition of cyclopentadiene and ethylene, which are one of the main fractions of C5 stream and the most versatile olefin monomer, respectively (Scheme 1).2 Diels−Alder reaction of equimolar amounts of cyclopentadiene and ethylene gives norbornene, and further addition of cyclopentadiene affords tetracyclododecene. Cyclopentadiene dimerizes via the Diels−Alder mechanism to yield dicyclopentadiene. NB and TCD, a bicyclic olefin and a tetracyclic olefin respectively, are able to polymerize through ROMP to form ring-opened polymers to high molecular weight without undesirable side reactions. DCP, a tricyclic olefin, can also be © XXXX American Chemical Society

polymerized by various metathesis catalysts through opening of the strained norbornene ring selectively (Scheme 2).3 The properties of the ring-opened poly(cycloolefin)s are affected by the steric structural factors (head−head/head−tail/tail−tail configurations, cis/trans configuration of the double bond and meso/racemo of the optically active carbon) in the polymer main chain. However, the unsaturated nature makes poly(cycloolefin)s susceptible to oxidative degradation. Hydrogenation of the double bonds eliminates this problem. In addition, the saturation of the double bonds in the polymer chain drastically changes the physical/thermal properties of poly(cycloolefin)s. Therefore, hydrogenated ring-opened NB, DCP and TCD polymers appear to be promising materials for practical applications. Syntheses and properties of hydrogenated ring-opened poly(cycloolefin)s (H-poly(cycloolefin)) have been under intensive research for these decades.4−8 In an early study of this field, Hamilton and co-workers reported the microReceived: September 30, 2014 Revised: October 15, 2014

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were reported two decades before.6 Recently, we have fully studied isotactic and syndiotactic hydrogenated poly(DCP)s produced by Mo- and W-based stereospecific ROMP catalysts.7 Both the isotactic and the syndiotactic hydrogenated poly(DCP)s are highly crystalline (isotacticity > 95%: Tm = 295 °C, syndiotacticity = 90%: Tm = 270 °C), while the atactic hydrogenated poly(DCP) is amorphous (meso/racemo = 50/ 50: Tg = 102 °C). In the case of TCD polymerization, poly(TCD) as well as its hydrogenated product were also studied.8 1H and 13C NMR spectra of poly(TCD) and of its hydrogenated polymer are now assigned. Despite of these achievements, microstructures of these polymers such as cis/ trans-structure and stereostructure (tacticity) are not still understood.8a,c Moreover, thermal properties of atactic hydrogenated poly(TCD) were not characterized well. Therefore, in order to investigate the properties of tactic and atactic hydrogenated poly(TCD)s, full characterization and precise control of its microstructures appear to be a matter of priority. To recap the above-stated background, iso- and syndio-specific ROMPs of NB and TCD need to be investigated preferentially for the further study of tactic hydrogenated polymers. The stereospecific ROMP of cycloolefins has been studied energetically in recent years. It is recognized that the precise design of catalysts and ligands enable control of the stereochemistry of ROMP process, and cis/trans-selective ROMPs of various monomers were successfully achieved by using of various well-defined catalysts.1 On the other hand, there little has been reported about the stereocontrol (tacticity control) of ROMP in the past decades, as the simultaneous regulation of both cis/trans-ratio and tacticity is assumed to be fairly complicated. Schrock and his co-workers shed light on a strategy of catalyst design for stereospecific ROMP: cisisospecific ROMPs of various norbornene derivatives were realized by the development of molybdenum imido alkylidenes that contains a sterically demanding biphenolate.9 Recently, cissyndiospecific ROMPs of norbornene- and norbornadienederivatives were investigated very well by use of newly developed molybdenum and tungsten monoalkoxide pyrrolide catalysts.10 In addition, suitable choice of monomer and catalyst attained trans-isospecific ROMP of 5,6-dicarbomethoxynorbornene.10d More recently, new Ru-based catalysts attained cissyndiospecific ROMP of 2,3-dimethoxynorbornadiene and cis 2,3-dimenthoxynorbornadiene.11 These studies focus on the stereoselectivity of the ROMPs of norbornene derivatives having functional/steric groups for the ease of characterization of the stereostructures of the yielding polymers. Monomers that do not contain functionality have not been studied much. We have reported that the Mo- and W-based binary catalysts promoted cis-isoselective and cis-syndioselective ROMPs of nonpolar and steric DCP. However, stereoselective ROMPs of other monomers were out of our scope.7 Study of the stereoselective ROMP of simple aliphatic cycloolefins seems to be insufficient. For these reasons, this work focuses on the stereoselective ROMPs of nonpolar polycyclic olefins such as steric small NB and steric bulky TCD, as well as their tactic (and atactic) hydrogenated ring-opened polymers using Mo- and W-based catalyst precursors illustrated in Scheme 3. Especially, we have succeeded in preparing and characterizing isotactic and syndiotactic hydrogenated poly(NB) and poly(TCD) using NMR, DSC, and XRD analyses. The fundamental factors such as catalyst structure and monomer bulkiness that would affect

Scheme 1. Diels−Alder Reaction to Produce Norbornene Monomers in the Present Study

Scheme 2. Possible Microstructures of Ring-Opened Poly(norbornene) and Its Hydrogenated Product

structures of poly(NB), poly(DCP), hydrogenated poly(NB), and hydrogenated poly(DCP) in detail.4 Hydrogenated poly(NB)s were synthesized by various classic catalysts such as OsCl3−phenylacetylene for the purpose of characterization of their microstructures. However, the properties of hydrogenated poly(NB)s were not studied.4b Recently, thermal properties and crystallization behavior of crystalline atactic hydrogenated poly(NB) were investigated in minute detail by Register et al. (meso/racemo = 46/54: Tm = 143 °C, ΔH = 65 J/g).5 However, the nature of isotactic and syndiotactic hydrogenated poly(NB)s is still unknown due to the lack of information on the stereospecific ROMP of NB. In the case of DCP, thermal and mechanical properties of atactic hydrogenated poly(DCP) B

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for 1H, 100.53 MHz for 13C) equipped with high-temperature probe CH8HT at 25−210 °C or on a Bruker Avance III 500 MHz spectrometer (500.13 MHz for 1H, 125.77 MHz for 13C) with Cryoprobe DCH 500/3 at 25−60 °C or on a Bruker Avance III HD 600 MHz spectrometer (150.91 MHz for 13C) equipped with CRYO probe DCH 600S3 at 125 °C. Chemical shifts were determined with reference to the tetramethylsilane (δ 0.00 ppm), o-dichlorobenzene (δ 127.5 ppm for 13C), tetrachloroethane (δ 5.97 ppm for 1H, δ 73.8 ppm for 13C) or chloroform (δ 7.24 ppm for 1H, δ 77.2 ppm for 13C). Cis/ trans ratios of the poly(cycloolefin)s were calculated by the integration ratios of each peaks of 1H NMR spectra, and meso/racemo ratios were estimated by peak separation of each peaks of 13C NMR spectra using Lorentzian function on JEOL Delta v5.0.2 NMR software. The molecular weight distributions (MWD) of the polymers were estimated on a gel-permeation chromatograph (GPC) (Tosoh HLC8220 GPC; eluent tetrahydrofuran). The relative number- and weightaverage molecular weights (Mn and Mw, respectively) were acquired by the use of a calibration curve obtained using polystyrene standards. The absolute degree of polymerization (DP) of the polymers were evaluated using the integration ratio of the proton signals of all the main chain peaks/the proton signals of the chain transfer end peaks of 1 H NMR (i.e., poly(NB): methyl group of the terminal n-C6 (1H NMR (CDCl3) δ 0.86−0.88 (t, 3H, CH3)) and the terminal vinyl group (1H NMR (CDCl3) δ 5.78 (m, 1H, CHCH2), 4.84−4.98 (dd, 2H, CHCH2); poly(TCD): the terminal vinyl group (1H NMR (CDCl3) δ 5.92 (m, 1H, CHCH2), 4.85−4.98 (dd, 2H, CH CH2))). The absolute Mn was estimated by the following equation: Mn(absolute) = formula weight of the monomer x DP. Differential scanning calorimeter (DSC) measurements were performed on SII NanoTechnology X-DSC7000 in a dry nitrogen stream. Wide-angle Xray diffraction (WAXD) of the polymers was recorded on RIGAKU RINT 2500 and Bruker D8 DISCOVER. Catalyst Preparation. Polymerization catalyst solutions were prepared as follows unless otherwise stated: (i; for W imidos 1−3) A toluene solution of W(NPh)Cl4·Et2O (1) (0.0278g, 56.7 μmol) (green solution) was mixed with three equivalents of Et2Al(OEt) at room temperature, and the mixture was stirred. The catalyst mixture was aged at room temperature for an additional 15 min resulting in a light brown solution. (ii; for Mo and W phenolates 4−7) The toluene solution of W(NPh)((R)-(+)-Biphenolate)2 (7) (0.0556g, 56.7 μmol) (dark-red solution) was mixed with two equivalents of n-BuLi at −78 °C and allowed to warm up to room temperature, and the mixture was stirred for 15 min at room temperature. The reaction mixture turned into a light brown solution. Polymerization. Polymerization was carried out in a prebaked ampule tube equipped with a rubber septum at 50 or 80 °C. A cyclohexane solution of NB (7.50g, 79.7 mmol) and 1-octene (0.450g, 3.98 mmol) was added to the catalyst mixture at the prescribed temperature. After stirring the reaction for a fixed time, the polymerization was quenched with a small amount of ethanol. The obtained polymer was reprecipitated from ethanol and dried in vacuo at 40 °C for 24 h. The polymer yield was determined by gravimetric measurement. Hydrogenation. A p-xylene solution of poly(cycloolefin) and a pxylene solution of 4-folds of p-Tos-NHNH2 were mixed in a glass flask equipped with a three-way stopcock. The mixture was allowed to heat up to the reaction temperature while stirring. Chemical transfer hydrogenation was carried out at 125 °C for 5 h. After the hydrogenation reaction, the reaction mixture was cooled slowly to room temperature. In the course of the hydrogenation reaction, the recrystallization of crystalline H-poly(cycloolefin)s might have proceeded from the dilute solution to form a fine powder of crystalline H-poly(cycloolefin)s. While amorphous H-poly(cycloolefin)s were basically soluble in p-xylene and no recrystallization proceeded from the solution.

Scheme 3. Tungsten- and Molybdenum-Based Catalyst Precursors Employed in the Present Study

the stereocontrol of the ROMPs of NB and TCD will also be discussed.

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EXPERIMENTAL SECTION

General Remarks. All operations were carried out in a glovebox (nitrogen atmosphere, O2 < 1 ppm and H2O < 1 ppm) or under an argon atmosphere by using standard Schlenk techniques. WCl6 (Soegawa Chemical), MoOCl4 (Strem), Et2Al(OEt) (Kanto Chemical), and n-BuLi (Kanto Chemical) were used without further purification. WOCl4 was synthesized from WCl6 and hexamethyldisiloxane in methylene chloride.7d W(NPh)Cl4·Et2O (1), W(N-2,6Me2-Ph)Cl4·Et2O (2), and W(N-2,6-i-Pr2−Ph)Cl4·Et2O (3) were synthesized from WOCl4 and isocyanates in n-octane as previously reported.7d MoO(2,6-Me2-phenolate)4 (4), MoO(3,3′,5,5′-Me4-biphenolate)2 (MoO(Me4-biphenolate)2) (5) and MoO(racemic-5,5′,6,6′Me4-3,3′-t-Bu2-biphenolate)2 (MoO(rac-Biphenolate)2) (6) were prepared from MoOCl4 and phenolates in diethyl ether as previously reported.7a W(NPh)((R)-(+)-5,5′,6,6′-Me4-3,3′-t-Bu2-biphenolate)2 (W(NPh)((R)-(+)-Biphenolate)2) (7) were prepared from 1 and (R)-(+)-Biphenolate-Li2 in diethyl ether as previously reported.7f nPentane, n-hexane, n-octane, and diethyl ether were distilled over sodium metal under an argon atmosphere before use. Toluene and methylene chloride were distilled over calcium hydride. Cyclohexane and p-xylene were degassed and stored over molecular sieves. Hexamethyldisiloxane was distilled over molecular sieves under reduced pressure. Phenyl isocyanate (Tokyo Kasei) was distilled from calcium hydride just prior to use. 2,6-Me2-phenyl isocyanate (Tokyo Kasei) and 2,6-i-Pr2-phenyl isocyanate (Tokyo Kasei) were used as received. Commercially available racemic-5,5′,6,6′-tertamethyl3,3′-di-tert-butyl-1,1′-biphenyl-2,2′-diol (rac-Biphenol) (Strem), (R)(+)-5,5′,6,6′-tertamethyl-3,3′-di-tert-butyl-1,1′-biphenyl-2,2′-diol ((R)(+)-Biphenol) (Strem), 3,3′,5,5′-tertamethyl-1,1′-biphenyl-2,2′-diol (Me4-biphenol) (Strem) and 2,6-dimethylphenol (Wako Chemicals) were used as received. Norbornene (Aldrich), endo-dicyclopentadiene (Zeon Corporation) and tetracyclododecene were stored as cyclohexane solution over molecular sieves 3A. Tetracyclododecene was synthesized from NB and DCP as according to the literature (vide inf ra). 1-Octene (Wako Chemicals) was distilled over calcium hydride. p-Tos-NHNH2 (Aldrich) was used as received. Analyses of Complex and Polymer. 1H and 13C NMR spectra were recorded on a JEOL JNM-EX400WB spectrometer (399.78 MHz

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RESULTS AND DISCUSSION Monomer Structure and Catalyst Design for the Present Stereospecific ROMPs. In general, the stereoC

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focused our attention on the effects of steric bulkiness of norbornene monomers on the stereoselectivity of ROMP. Norbornene, the simplest bicyclic olefin, and tetracyclododecene, a versatile and simple tetracyclic olefin are employed for the present study. endo-Dicyclopentadiene, an industrially highly important tricyclic olefin, is also used as a monomer for comparison. In most Diels−Alder reactions, when the product distribution is under kinetic control, the endo adduct is preferentially, sometimes exclusively, formed. Dimerization of cyclopentadiene can provide two possible products, the endo and the exo adducts. In the case of DCP, the exo-form is kinetically unfavorable; therefore, endo-dicyclopentadiene is exclusively produced above 99.9% purity by Diels−Alder reaction. TCD can potentially exist in four stereoisomers, (endo, anti)-, (exo, syn)-, (endo, syn)- and (exo, anti)-forms (Scheme 5).13,14

selectivity of the enchainment reaction of metal-catalyzed polymerization would be influenced by a combination of catalyst structure and monomer structure. The propagation species derived from metal catalysts would possess their chirality centers both in their ligands and in their polymeryl group attached to the metal. Stereoregularity of the polymer chain can therefore be derived from the catalyst asymmetry (enantiomorphic site control/catalyst control) or the polymer asymmetry (chain end control/penultimate effect) or, most likely, a combination of the two. In the case of ROMP, monomer coordination step may determine the stereoselectivity, and the formed stereostructure might be retained during the following steps such as cycloaddition and ringopening (Scheme 4). The absolute configurations of Scheme 4. Elementary Reaction of ROMP

Scheme 5. Possible Stereoisomers of DCP and TCD

norbornene monomer’s two bridgehead carbons, of which chiralities are opposite, would also be retained during the ROMP process. Therefore, the next enchainment of ROMP holds not only the asymmetry of the ligand but also the asymmetry of the one of the bridgehead carbons of the last inserted norbornene monomer. Namely, the alignment of the two bridgehead carbons of the penultimate repeating units directly correlates to the polymer asymmetry of the propagation species. In addition, when the substituent on the penultimate repeating units has functionality, it may coordinate to the metal and may affect the stereochemistry of the elementary reaction. Consequently, stereocontrol effect by polymeryl group would be influenced by steric/functional structures of substituent on the repeating units. Steric structure of the approaching monomer has a predominantly large effect on the insertion behavior of it to the propagation species. As reported, steric demanding norbornenes might generally need higher energy to coordinate to the catalyst.12 This means that the existence of the bulky substituent may result in the increase of the energy of coordination of the olefin moiety to the metal center. On the other hand, the functionality of the monomer may also affect the enchainment process. When functionalized norbornene is employed as a monomer, metal center would be coordinated by the olefin moiety of the norbornene-ring or by the polar group of substituent or by both of them in a chelating fashion during polymerization. However, the effect from substituent would be much smaller when the functional group does not contain polar heteroatom. To summarize the above, the stereoselectivity of the ROMP process would possibly be affected by both the steric bulkiness and the functionality of the incoming monomer. Thus far, we have reported that the Mo- and W-based binary catalysts promoted cis-isospecific and cis-syndiospecific ROMPs of DCP.7 However, the effect of the monomer structure on the stereocontrol has not been investigated much. In this study, we

Simulation data suggest that the (endo, syn)- and the (exo, anti)forms are kinetically and thermodynamically unfavorable. The estimated formation enthalpies of the (endo, anti)- and the (exo, syn)-forms are similar. On the other hand, the activation enthalpy of the (exo, syn)-form was calculated to be much higher than that of the (endo, anti)-form. This means the (endo, anti)-form might be kinetically more preferred than the (exo, syn)-form. These assumptions are in good agreement with experimental results. For instance, the Diels−Alder reaction of equimolar of norbornene and cyclopentadiene at 230 °C yielded isomers of TCD, which comprised 95% of the (endo, anti)-TCD and 5% of the (exo, syn)-TCD. It was determined by 1 H NMR that there was no detectable amount of the (endo, anti)- and the (exo, syn)-form in the yielded TCD. In the present study, the mixture of stereoisomers (the (endo, anti)TCD; 95%, the (exo, syn)-TCD; 5%) was prepared and employed as TCD monomer. Isospecific and Syndioselective ROMPs of Norbornene with Various W- and Mo-Based Catalysts. Isospecific ROMPs has been realized by several well-defined catalysts. Molybdenum alkylidene biphenolate complexes are well-known as effective initiators for cis-isospecific ROMPs of several functional cycloolefins.9 Previously, we have reported that cisisoselective ROMP of DCP was introduced by oxomolybdenum (and phenylimidotungsten) bisbiphenolate-based binary catalysts, such as MoO((R)-(+)-5,5′,6,6′-Me4-3,3′-t-Bu2-biphenolate)2−n-BuLi.7a,b,f Isoselectivity of the above-stated two ROMP systems appears to have come from the enantiomorphic site control caused by the biphenolate ligand having axial D

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Table 1. Polymerization of NB by Various Catalysts and Sequential Hydrogenation of the Obtained Ring-Opened Poly(NB)sa

Key: (a) Polymerized in cyclohexane at 50 °C for 3h; [NB] = 20 wt % (2.12 M), [1-octene] = 106 mM, [W complex] = 2.12 mM, [W complex]: [Et2Al(OEt)] = 1:3, all polymer yields were 100%. Hydrogenated in p-xylene at 125 °C for 5h; [p-Tos-NHNH2]/[NB unit] = 4/1 in mol ratio, [poly(NB)] = 5 wt %, all polymer’s hydrogenation ratios were 100%. (b) Polymerized in cyclohexane at 80 °C for 3h; [NB] = 20 wt % (2.12 M), [1octene] = 53 mM, [Mo or W complex] = 2.12 mM, [W complex]:[n-BuLi] = 1:2, all polymer yields were 100%. Hydrogenated in p-xylene at 125 °C for 5 h; [p-Tos-NHNH2]/[NB unit] = 4/1 in mol ratio, [poly(NB)] = 2.5 wt %, all polymer’s hydrogenation ratios were 100%. (c) Tacticities were estimated based on 13C NMR measurements in C2D2Cl4 at 125 °C using Bruker Avance III HD 600 MHz spectrometer equipped with CRYO probe DCH 600S3. a

the stereospecific polymerization of NB by binary catalyst systems. Table 1 summarizes the results of the optimized NB polymerizations by various molybdenum- and tungsten-based catalysts and the following hydrogenations of the obtained poly(NB)s. 1−3 were activated by 3-fold of Et2Al(OEt), while 2-fold of n-BuLi was used as cocatalyst for 4−7. At first, we performed all the NB polymerizations without chain transfer agent. Similar to the case of the ROMP of DCP by them, the polymerization mixtures became very viscous gels, and the following hydrogenation of the obtained poly(NB)s leveled off. Therefore, we decided to add 1-octene as a chain transfer agent

chirality. Thus far, syndiospecific ROMP has not been studied much. Our recent study has verified that tungsten(VI) “small”imido tetrachloride complexes such as W(NPh)Cl4·Et2O (1) are useful for cis-syndioselective ROMP of DCP.7c,d,f More recently, molybdenum alkylidene monoalkoxide pyrrolide catalysts were developed and revealed to be effective for cissyndiospecific ROMPs of various functional norbornenes.10 However, there has been little reported about stereospecific ROMP of norbornene itself which has no polar substituent. Moreover, the nature of tactic hydrogenated poly(norbornene)s has not been clarified yet. These reasons led us to the study of E

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in all the polymerization systems, for the purpose of keeping the molecular weights in the moderate range to hydrogenate them with ease. The 1−7 induced the ROMP of NB smoothly to provide ring-opened poly(NB)s in quantitative yields under the presence of 1-octene. All the obtained poly(NB)s were highly soluble in ordinary organic solvents at room temperature. Therefore, the molecular weights of the poly(NB)s were characterized by the combination of 1H NMR and GPC at 25 and 40 °C, respectively. GPC traces of all the poly(NB)s in Table 1 are described in Supporting Information, Figures S26−S-32. It was found that the molecular weights of all the poly(NB)s were kept in the moderate range. Relative Mn of the poly(NB)s acquired by GPC using polystyrene calibration were somewhat higher than absolute Mn measured by 1H NMR. It is likely that not only the cis/trans-ratio but the tacticity would affect the hydrodynamic volume of the poly(NB)s. Unfortunately, we could not discuss it in detail by the present results because we failed to vary cis/trans while keeping stereoregularity intact: both cis/trans-selectivity and stereoselectivity were influenced by catalyst structure in a different manner. Assuming that the high-trans poly(NB) such as obtaining from 4 would have a relatively rigid main chain as compared to highcis polymer, the hydrodynamic volume of high-trans polymer becomes larger and consequently, the relative Mn of it by GPC becomes much higher than the absolute Mn. Next, microstructures of the obtained polymers were investigated. Ratios of cis junction of the double bonds of the main chain could be determined based on the splitting of vinyl proton by 1H NMR measurements of poly(NB) (in CDCl3 at 25 °C; 1H NMR charts of poly(NB)s from 1 and 6 are described in Supporting Information, Figures S-1 and S-2). On the other hand, it has already reported by Hamilton et al. that the tacticity of the poly(NB) could be determined only by 13C NMR measurements of hydrogenated poly(NB)s (H-poly(NB)).4b Therefore, the hydrogenation of the poly(NB)s was conducted in order to investigate both tacticities and properties of their hydrogenated products. The saturation of the polymer backbone proceeded fully by chemical transfer hydrogenation using p-Tos-NHNH2. 13C NMR spectra of the syndiotacticand the atactic-H-poly(NB)s were recorded in CDCl3 at 60 °C, and those of all the H-poly(NB)s were recorded in C2D2Cl4 at 125 °C in order to estimate their tacticities. The selected 13C NMR charts of methylene carbons C5,6 measured in CDCl3 at 60 °C were described in Figure 1(a), and those of methine carbons C1,4 and methylene carbon C7 in C2D2Cl4 at 125 °C were described in Figure 1(b). 13C NMR charts of all the Hpoly(NB)s in Table 1 are shown in Supporting Information, Figures S-11−S-21. As seen in Table 1, it is noteworthy that W(NPh)Cl4·Et2O (1) promoted cis-syndioselective ROMP of NB. The splitting of methylene carbons in Figure 1a clearly suggested the formation of syndiotactic polymer. It is in contradiction to our expectation that Me-substituted version, W(N-2,6-Me2-Ph)Cl4·Et2O (2), was also verified to be a useful precatalyst for cis-syndioselective ROMP of NB; syndioselectivity of 2 is 92% and almost identical to that of 1. Even W imido catalyst having relatively bulky isopropyl substituent, W(N-2,6-i-Pr2−Ph)Cl4·Et2O (3), yielded cis-syndio-biased polymer. In the case of DCP polymerization, syndioselectivity largely decreased with increasing the size of imido substitute.7c Whereas stereoselectivity of NB polymerization by 1−3 seems to be less affected by steric demands of the substituent on the imido group. Molybdenum oxo phenolate complex, MoO(2,6-Me 2-phenolate)4 (4),

Figure 1. Selected 13C NMR spectra of (a, top) methylene carbons C5,6 (recorded in CDCl3 at 60 °C, [H-poly(NB)] = 0.5 wt %.) and of (b, bottom) methylene carbon C7 and methine carbons C1,4 (recorded in C2D2Cl4 at 125 °C, [H-poly(NB)] = 0.5 wt %) of H-poly(NB)s shown in Table 1

afforded trans-rich and entirely atactic polymer (Figure 1a). This result suggested that the 2,6-Me2-phenolate ligand is not effective for the stereocontrol of the ROMP process of NB. We have found that the syndiotactic and the atactic Hpoly(NB)s were soluble in CDCl3 at 60 °C to be relatively easily determined their meso/racemo ratios. In contrast, the Hpoly(NB)s obtained from 5−7 were entirely insoluble at the same condition. In the early stage of this study, the tacticities of isotactic polymers could not be clearly observed by changing deuterated solvent, measuring temperature or polymer concentration. Finally we have found that the methylene carbon C7 and the methine carbons C1,4 were resolved into triplet and doublet, respectively, in CD2Cl4 at 125 °C when cryoprobe was employed and measurement conditions were carefully optimized for gaining high resolution (Figure 1b). Unfortunately, C5,6 carbons were resolved not into doublet but F

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the thermally annealed samples were heated again at the same rate up to 200 °C. In the DSC measurement, each Hpoly(NB)s described in Table 1 exhibited first order transitions at elevated temperatures (Figure 2 and Figures S-37−S-50). Namely, it is clarified that all the H-poly(NB)s are crystalline polymers irrespective of their stereoregularity. During the cooling segment, melt-crystallization of all the NB polymers has proceeded fully, reflecting high crystallization rates of them. However, their crystalline nature is largely influenced by their tacticities. As summarized in Table 1, melting points and melting enthalpies of the syndio- and the syndio-biased-Hpoly(NB)s obtained from 1−3 are considered to be essentially identical except for the effects by their molecular weights (Tm = 130−136 °C; ΔH = 60−75 J/g). Their thermal behaviors are comparable to that of the atactic polymer from 4 (meso/ racemo = 50/50: Tm = 145−150 °C, ΔH = 70−75 J/g, wc = 0.65). Before the present investigation, we speculated that syndio-H-poly(NB) might be characterized by high melting and high crystallinity as compared to those of conventional ata-Hpoly(NB), because tactic polymers tend to form highly stable crystal structures. Beyond our expectation, the melting points of the syndio-H-poly(NB)s are 10 °C lower than that of the ata-H-poly(NB), and their crystal structures after annealing would be very close (vide inf ra). In contrast, it can be emphasized that the iso-H-poly(NB)s were characterized by their unique crystalline nature. The isoH-poly(NB)s provided by 6 and 7 possessed first order transitions above 170 °C, that are more than 30 °C higher than those of atactic and syndiotactic polymers (Table 1). This result suggests that crystal of isotactic polymer is thermally more stable than those of atactic and syndiotactic polymers. DSC thermogram of the annealed iso-H-poly(NB) by 7 exhibited two different endothermic peaks at 165 and 175 °C, while melting point of highly tactic polymer from 6 was broad and only at 178 °C after annealing (Figures S-48 and S50). Melting enthalpy of the iso-H-poly(NB) by 6 is lower than that of less tactic polymer by 7, probably due to the effect of its high molecular weight. The iso-biased polymer obtained from 5 also has high melting at 164 °C after annealing. As a conclusion, it is interesting that the iso-H-poly(NB)s showed multi endothermic peaks irrespective of their thermal history and their ratio of heat quantities might be affected by its isotacticity and molecular weight. In order to gain the information on their crystal structures, WAXD investigation of fully annealed samples was conducted next. Figure 3 shows diffraction patterns for the syndio-Hpoly(NB) from the 2-based catalyst, the ata-H-poly(NB) from the 4-based catalyst and the iso-H-poly(NB) from the 7-based catalyst. WAXD patterns of all the annealed H-poly(NB)s in Table 1 are described in Supporting Information, Figures S61−S-67. The diffraction patterns of the each polymers exhibited sharp crystalline peaks, supporting their crystalline nature. The WAXD patterns of the syndio-H-poly(NB)s were similar to those of the already reported ata-H-poly(NB). On the other hand, the diffraction patterns of the iso-H-poly(NB)s were different from those of the atactic and the syndiotactic polymers. This result suggests that the crystal structures of the H-poly(NB)s depend on tacticities of their polymer backbone. The WAXD patterns of syndio-H-poly(NB)s showed each diffraction peaks clearly and sharply comparing other Hpoly(NB)s, suggesting its highly regulated crystal structure. It is interesting that the crystallinities of the present H-poly(NB)s were relatively high: The weight fraction crystallinities (wc) of

multiplet at the optimized condition. Therefore, at present we did not choose C5,6 carbons to determine tacticity. As seen in Figure 1b and Figures S-19−S-21 in Supporting Information, H-poly(NB)s obtained from 5-7 were isotactic. It is interesting to note that MoO(rac-Biphenolate)2 (6) exhibited remarkable ability to regulate the polymerization process into cisisospecific: the obtained poly(NB) had almost all-cis geometry, while isotacticity of the hydrogenated product was also almost 100%. 13C NMR spectrum of the poly(NB) from 6 also suggests its highly regulated structure: all-cis geometry was verified and only 4 carbon peaks were observed (see Supporting Information, Figures S-5−S-7). In contrast, 13C NMR spectra of another poly(NB)s were complicated and not fully assignable. It is clarified that MoO(Me4-biphenolate)2 (5) promoted cis-isoselective ROMP of NB, however, the isotacticity of the obtained H-poly(NB) was moderate and around 80%. W(NPh)((R)-(+)-Biphenolate)2 (7) was verified to be a useful catalyst for cis-isoselective ROMP of NB. Its isoselectivity appeared to be around 90%. Therefore, biphenolate ligands were thought to be effective ligands for the isoselective ROMPs of NB. To recap this section, the cis-syndioselective ROMP and the cis-isospecific ROMP of NB were accomplished for the first time. Properties of the Isotactic and the Syndiotactic Hydrogenated poly(NB). Properties of the tactic hydrogenated ring-opened poly(NB)s and that of the atactic polymers were investigated in detail by the combination of DSC and WAXD. It is worth emphasizing that the isotacticand the syndiotactic H-poly(NB)s were verified to be crystalline polymers with different crystalline natures. Figure 2 illustrates the selected DSC charts of first heating, cooling and second heating profiles of the syndiotactic H-

Figure 2. DSC thermograms of (a) the syndio-H-poly(NB) from the 1-based catalyst, (b) the syndio-biased H-poly(NB) from the 3-based catalyst, (c) the iso-biased H-poly(NB) from the 5-based catalyst, and (d) the iso-H-poly(NB) from the 7-based catalyst described in Table 1 (determined under N2 for 10 °C/min for each step).

poly(NB) (syndio-H-poly(NB)) by 1-based catalyst, the syndio-biased H-poly(NB) by 3-based catalyst, the iso-biased H-poly(NB) by 5-based catalyst and the isotactic H-poly(NB) (iso-H-poly(NB)) by 7-based catalyst. The detailed DSC thermograms of all the as synthesized polymers and the annealed polymers written in Table 1 are depicted in Supporting Information, Figures S-37−S-50. In Figure 2, each of the as synthesized samples without thermal hysteresis were heated up to 200 °C at the rate of 10 °C/min at first, then cooled at the same rate until ambient temperature, and finally G

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NMR measurement of poly(TCD). Fortunately, the cis/trans of the double bonds of the polymer chain could be determined based on the resolution of signals attributable to allyl proton (in CDCl3 at 60 °C; 1H NMR charts of poly(TCD)s from 1 and 6 are described in Supporting Information, Figures S-3 and S4).15 On the other hand, tacticity of the poly(TCD) was successfully determined by 13C NMR measurement of hydrogenated poly(TCD) (H-poly(TCD)). Figure 4 illustrates the DEPT measurement chart of H-poly(TCD) obtained from 1-based catalyst represented in Table 2, in which repeating units derived from the (endo, anti)-TCD and the (exo, syn)TCD were separately observed. This indicates the successful saturation of poly(TCD)s by using p-Tos-NHNH2 to form corresponding hydrogenated polymers without any isomerization. Composition of repeating units of the yielded polymers was identical to that of monomer’s stereoisomers: repeating units of the (endo, anti)-form was 95%, while that of the (exo, syn)-form was 5%. As described in Figure 5, methine carbons C5,10 of the repeating units of H-poly(TCD) derived from (endo, anti)-adduct was observable as doublet which is likely to be due to diad splitting (13C NMR charts of soluble Hpoly(TCD)s from 1−4 in Table 2 are described in Supporting Information, Figures S-22−S-25). It is worth noting that W( NPh)Cl4·Et2O (1)−Et2Al(OEt) catalyst introduced cis-syndioselective ROMP of steric tetracycle monomer, TCD. However, syndiotacticity of the obtained polymer was not very high and around 86%. The results in Figure 5 suggest that syndioselectivity of TCD polymerization decreased by increasing the size of imido substitute, and more specific, that substituent on 2,6-position of phenylimido ligand affected the stereochemistry of TCD polymerization largely as compared to the case of NB and DCP polymerizations: cis-rich and syndiobiased poly(TCD) was yielded in the presence of W(N-2,6Me2-Ph)Cl4·Et2O (2)-based catalyst, while the catalyst precursor W(N-2,6-i-Pr2−Ph)Cl4·Et2O (3) which has relatively bulky isopropyl group on phenylimido ligand promoted slightly trans- and iso-biased polymerization of TCD. As expected, WOCl4 yielded poly(TCD) with no regularity in its microstructures. According to our previous study, the 2,6Me2-phenolate ligand poorly regulated the stereochemistry of the ROMPs of less steric NB and DCP. Beyond our expectation, the MoO(2,6-Me2-phenolate)4 (4)-based catalyst promoted isoselective ROMP of more steric TCD. This result implies that the combination of the steric bulkiness of both catalyst ligand and monomer substituent governs the nature of the elementary metathesis reaction. It is remarkable that catalysts derived from 5−7 introduced cis-selective ROMPs of TCD. Cis-selectivity of MoO(Me4-biphenolate)2−n-BuLi (5) was about 80%. Similar to the case of NB polymerization, almost all-cis poly(TCD) is preparable by a MoO(racBiphenolate)2 (6)-based catalyst. 13C NMR spectrum of the all-cis poly(TCD) from the 6-based catalyst was very simple and only 7 signals were observed (see Supporting Information; Figure S-10). It suggested all-cis geometry and highly regulated stereostructure. In contrast, 13C NMR spectra of the poly(TCD)s from 1 and 4 were complicated (see Supporting Information; Figures S-8 and S-9). Unfortunately, we do not have enough information to assign them, therefore it is only assumable that 6-based catalyst induced cis-isospecific ROMP for (endo, anti)-form. W(NPh)((R)-(+)-Biphenolate)2 (7) was also an effective catalyst precursor for cis-selective ROMP of TCD. Unfortunately, H-poly(TCD)s from 5−7 were entirely insoluble in any deuterated solvents even at 210 °C probably

Figure 3. WAXD patterns of the thermally annealed (a) syndio-Hpoly(NB) from the 2-based catalyst, (b) ata-H-poly(NB) from the 4based catalyst and (c) iso-H-poly(NB) from the 7-based catalyst represented in Table 1

the fully annealed syndio-H-poly(NB)s were evaluated by peak separation of crystalline peaks and amorphous halo, yielding wc = 0.5−0.7. Similarly, the wc of the fully annealed iso-Hpoly(NB)s was 0.5−0.7. To summarize this section on the results by DSC and WAXD measurements, it can be concluded that both newly developed tactic H-poly(NB)s can be regarded as highly crystalline polymers. Though it is not clarified yet about the reason why the iso-H-poly(NB) showed such a unique crystalline nature, we presume at this moment that the iso-Hpoly(NB) has plural crystalline structure systems in spite of its very simple WAXD diagram. Deeper insight of the influence of stereoregularity on the crystal nature of H-poly(NB)s is under intensive research and will be reported elsewhere. Iso- and Syndio-Selective ROMPs of Tetracyclododecene with Various W- and Mo-based Catalysts. So far, synthesis of poly(TCD) and hydrogenated poly(TCD) (Hpoly(TCD)) was already reported by several groups. However, the microstructure of H-poly(TCD) formed from WCl6 was not assigned, and the thermal property of it was not clear in the previous report, possibly because of isomerization of the polymer chain caused by Ni-based hydrogenation catalyst.8a Moreover, synthesis, characterization and properties of tactic H-poly(TCD)s have not been achieved yet. Therefore, stereoselective polymerization of TCD was investigated at first. Table 2 is a summary of the results of the optimized TCD polymerization by various molybdenum- and tungsten-based ROMP catalysts and subsequent hydrogenations of the provided poly(TCD)s. Polymerization conditions were same as those of NB polymerizations. Catalyst precursors 1−7 introduced smooth polymerizations of TCD to produce ringopened poly(TCD)s in quantitative yields under the presence of suitable amount of chain-transfer agent. The poly(TCD)s obtained from 1−4 were highly soluble in ordinary organic solvents at room temperature, while those from 5−7 were less soluble, probably due to their isotacticities. Therefore, the molecular weights of the poly(TCD)s were characterized by 1H NMR in CDCl3 at 60 °C for those from 5−7 and by GPC at 40 °C for those from 1−4. GPC traces of the poly(TCD)s from 1−4 are described in in Supporting Information, Figures S-33− S-36. The molecular weights of all the poly(TCD)s were controlled in the suitable range for further analyses. Microstructures of the obtained polymers were investigated next. The splitting of the vinyl proton was not detectable by 1H H

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Table 2. Polymerization of TCD by Various Catalysts and Sequential Hydrogenation of the Obtained Ring-Opened Poly(TCD)sa

Key: (a) Polymerized in cyclohexane at 50 °C for 3 h; [TCD] = 20 wt % (1.25 M), [1-octene] = 62 mM, [W complex] = 1.25 mM, [W complex]: [Et2Al(OEt)] = 1:3, all polymer yields were 100%. (b) Polymerized in cyclohexane at 80 °C for 3 h; [TCD] = 20 wt % (1.25 M), [1-octene] = 62 mM, [Mo or W complex] = 1.25 mM, [Mo or W complex]:[n-BuLi] = 1:2, all polymer yields were 100%. (c) Relative Mn determined by GPC. (d) Absolute Mn determined by 1H NMR. (e) Hydrogenated in p-xylene at 125 °C for 5 h; [p-Tos-NHNH2]/[TCD unit] = 4/1 in mol ratio, [poly(TCD)] = 2.5 wt %, all polymer’s hydrogenation ratios were 100%. (f) Not detectable because it is entirely insoluble even at 210 °C. (g) No glass transition or crystal melting were observable below 350 °C. (h) Evaluated for the H-poly(TCD) samples without heat hysteresis. a

“isotactic” hydrogenated ring-opened poly(TCD)s were investigated by means of DSC and WAXD. Figure 6 displays the selected DSC charts of the second heating of syndiotactic H-poly(TCD) (syndio-H-poly(TCD)) obtained from W(NPh)Cl4·Et2O (1), of iso-biased Hpoly(TCD) by MoO(2,6-Me2-phenolate)4 (4) and of “isotactic” H-poly(TCD) (“iso”-H-poly(TCD)) by MoO(racBiphenolate)2 (6). The detailed DSC thermograms of all the annealed polymers in Table 2 are depicted in Supporting

owing to their extremely high melting points (vide inf ra). Therefore, we could not determine their tacticities, but their high crystallinity implies that they are tactic polymers. Regarding the results of the ROMP of NB and DCP, it is probable that 5−7 induced isoselective ROMPs of TCD, of which isoselectivities would be much higher than 70%. Properties of the Tactic and the Atactic Hydrogenated Poly(TCD)s. Properties of syndiotactic, atactic and I

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Figure 6. DSC thermograms of the (a) the syndio-H-poly(TCD) from the 1-based catalyst, (b) the iso-biased-H-poly(TCD) from the 4based catalyst, and (c) the “iso”-H-poly(TCD) from the 6-based catalyst described in Table 2 (determined on the second scan under N2 for 10 °C/min after full annealing).

such as toluene, cyclohexane and chloroform at room temperature. As described in Table 2, syndio-biased Hpoly(TCD) with 2-based catalyst is also amorphous. However, it is less soluble in organic solvents at ambient temperature: a solution needed to be heated above 60 °C to dissolve it. On the other hand, atactic H-poly(TCD)s obtained from WOCl4 and 3 displayed endothermic peaks at elevated temperatures on the DSC measurements. These are likely to be crystal melting points of them, however they would be categorized as semicrystalline polymers: the thermograms of the second heating of ata-H-poly(TCD)s showed exothermal coldcrystallization peaks just below melting points (see Supporting Information Figure S-53), reflecting a low crystallization rate. The polymers were less soluble, supporting their semicrystalline nature. The iso-biased H-poly(TCD) obtained from 4 was proven to be a crystalline polymer. DSC thermogram of it showed large crystal melting at elevated temperature above 300 °C. The iso-biased polymer was insoluble at ambient temperature, and needed to be heated up to 210 °C to dissolve in NMR solvent. The melting points of H-poly(TCD)s appeared to increase with increasing their isotacticities of the main chain. It is regrettable that the H-poly(TCD)s produced by 5−7 showed no phase transition below 350 °C. As stated above, those polymers were entirely insoluble at 210 °C and infusible below 350 °C. Therefore, X-ray measurements of the as synthesized polymers were conducted to gain further information about their ordered structures. WAXD patterns of the as synthesized H-poly(TCD)s in Table 2 are described in Supporting Information, Figures S-68−S-74. Figure 7 shows the WAXD patterns of the syndio-H-poly(TCD) produced by the 1-based catalyst, of the iso-biased H-poly(TCD) produced by the 4-based catalyst, and of the “iso”-H-poly(TCD) produced by the 6-based catalyst described in Table 2. It is worth emphasizing that the diffraction patterns of the syndio-Hpoly(TCD) only possessed an amorphous halo, which supports the DSC results. Whereas the WAXD patterns of the atactic and the “isotactic” polymers exhibited relatively broad crystalline peaks. The weight fraction crystallinities (wc) of the atactic H-poly(TCD) produced by the 3-based catalyst, of the iso-biased H-poly(TCD) produced by the 4-based catalyst and of the “iso”-H-poly(TCD) produced by the 6-based catalyst were evaluated by peak separation, yielding wc = 0.26,

Figure 4. DEPT measurement of the H-poly(TCD) produced by 1based catalyst shown in Table 2 (in o-dichlorobenzene-d4 at 120 °C; carbons of the repeating units derived from (endo, anti)-TCD are fully assigned and, C*, are attributable to carbons of the repeating units derived from (exo, syn)-form).

Figure 5. Selected 13C NMR spectra of methine carbons C5,10 of the repeating units of H-poly(TCD) derived from (endo, anti)-form yielded by 1, 2, 3 and 4-based catalysts shown in Table 2 (recorded in o-dichlorobenzene-d4 at 140 °C or in o-dichlorobenzene-d4/trichlorobenzene at 210 °C; chemical shift were slightly tuned for clarity).

Information, Figures S-51−S-57. As seen in Figure 6, the DSC thermogram of the syndio-H-poly(TCD) showed only a glass transition at 160 °C and no melting point, implying its amorphous nature. In addition, it is noteworthy that this syndiotactic polymer is soluble in ordinary organic solvents J

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0.36, and 0.62, respectively, when those has no heat hysteresis. These data clearly suggest that the ata- and the “iso”-Hpoly(TCD)s are crystalline polymers. It is also interesting that the cis-“isotactic”-poly(TCD) produced by the 6-based catalyst is also a very unique semicrystalline polymer, of which DSC thermograms with different heat history are described in Supporting Information (see Figures S-58−S-60). While the annealed polymer only possessed glass transition at 213 °C, as synthesized one had a broad melting at 207 °C. WAXD study also revealed that the as synthesized polymer was a crystalline polymer (see Figure S75). Another poly(TCD)s such as cis-syndiotactic one were entirely amorphous. To our best knowledge, melting temperature of the polymer is always higher than glass transition temperature. Unusually low melting indicates instability of polymer crystals of cis-“isotactic”-poly(TCD). As reported, cisisotactic-poly(DCP) is a crystalline polymer having high melting (Tg = 150 °C, Tm = 265 °C, ΔH = 30 J/g). It is

Figure 7. WAXD patterns of (a) the syndio-H-poly(TCD) from the 1based catalyst, (b) the iso-biased-H-poly(TCD) from the 4-based catalyst and (c) the “iso”-H-poly(TCD) from the 6-based catalyst represented in Table 1.

Table 3. Effects of Cocatalysts on the Polymerization of NB and TCD by 1- and 6-based catalysts and Sequential Hydrogenation of the Obtained Polymersa

Key: (a) Polymerized in cyclohexane at 50 °C for 3 h; [NB] = 20 wt % (2.12 M), [1-octene] = 106 mM, [1] = 2.12 mM. (b) Polymerized in cyclohexane at 80 °C for 3 h; [NB] = 20 wt % (2.12 M), [1-octene] = 53 mM, [6] = 2.12 mM, [W complex]:[n-BuLi] = 1:2. (c) Polymerized in cyclohexane at 50 °C for 3 h; [TCD] = 20 wt % (1.25 M), [1-octene] = 62 mM, [1] = 1.25 mM. (d) Polymerized in cyclohexane at 80 °C for 3 h; [TCD] = 20 wt % (1.25 M), [1-octene] = 62 mM, [6] = 1.25 mM. (e) Relative Mn and Mw/Mn determined by GPC. (f) Absolute Mn determined by 1 H NMR. (g) Hydrogenated in p-xylene at 125 °C for 5 h; [p-Tos-NHNH2]/[CC] = 4/1 in mol ratio, [polymer] = 2.5 wt %, all polymer’s hydrogenation ratios were 100%. (h) Not detectable by 13C NMR because it is entirely insoluble in any solvents even at 210 °C. (i) No glass transition or crystal melting were observable below 350 °C. a

K

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yields were 31% and 28% for NB and TCD after 3 h of polymerization. At 60 °C, polymerizations of NB and TCD ended instantaneously to form polymers in quantitative yields. The stereostructures of these obtained polymers were essentially identical and within analytical errors irrespective of the polymerization temperature: cis = 85−90%, racemo = 87− 90% for poly(NB), cis = 88−90%, racemo = 83−86% for poly(TCD). The thermal properties of the each hydrogenated polymers were almost same. As a summary, the reaction temperature did not largely affect the stereochemistry of the enchainment reaction by 1−Et 2 Al(OEt) (1:3) catalyst irrespective of the reaction temperature in the range 20−60 °C. Effect of polymerization solvent was studied briefly. nHexane and n-octane were not preferable solvents for the present polymerization, because they do not dissolve poly(NB)s and poly(TCD)s. When toluene was employed as solvent, polymerizations proceeded smoothly to yield cissyndiotactic polymers from 1-based catalyst and cis-isotactic polymers from 6-based catalyst quantitatively (at 50 °C for 3 h for 1; at 80 °C for 3 h for 6). It was verified that the microstructures of them were identical to those polymerized in cyclohexane. THF as typical polar solvent was not suitable for the present stereoselective ROMPs. It just decelerated the polymerizations by 6-based catalyst. In contrast, not only polymerization rate but stereocontrol ability of 1-based catalyst received unfavorable impact by using THF as a polymerization solvent. Comparison of Stereospecific ROMPs of NB, DCP, and TCD. Scheme 6 summarizes the stereocontrol of ROMPs of NB and TCD by the present catalyst systems. Results obtaining from DCP polymerizations were also added for comparison. The data of cis/trans of the poly(DCP)s and tacticity of the Hpoly(DCP)s were precisely recorded again by newly developed

assumable that the difference of crystal stability came from both the structure of repeating unit and the purity of isomers. To conclude this section, we have found that the syndio-Hpoly(TCD) is amorphous and soluble in organic solvents at room temperature. On the other hand, the atactic and the isobiased polymers were found to be semicrystalline and insoluble at room temperature. It is evident by WAXD measurement that the “iso”-H-poly(TCD)s were highly crystalline polymers. Effects of Reaction Conditions on the Stereoselective ROMPs of NB and TCD. Effects of reaction conditions on the polymerization of NB and TCD by 1- and 6-based catalysts were investigated in more detail. Effect of cocatalysts was examined (Table 3). The complexes 1 and 6 had no polymerization activity for NB and TCD in the absence of cocatalysts. Neither poly(NB) nor poly(TCD) was obtained when equimolar amount of n-Bu4Sn was employed as cocatalyst for both complexes. The color of catalyst mixtures did not change after mixing cocatalyst, indicating the transalkylation between 1 (or 6) and n-Bu4Sn would be extremely slow due to low basicity of n-Bu4Sn and low acidity of both complexes. Et3Al activated 1 and 6 to polymerize NB and TCD to yield analyzable polymers, however the stereocontrol by them was partially unsuccessful: 1−Et3Al (1:1) was not a stereoselective ROMP catalyst for NB and TCD, while 6−Et3Al (1:1) promoted cis-isospecific ROMP of NB and cisselective ROMP of TCD. Cis-selectivity and isoselectivity of 6− Et3Al (1:1) for NB and TCD polymerizations were almost identical to those of 6−n-BuLi (1:2). However, the activity of 6−Et3Al is lower than that of 6−n-BuLi, indicating that n-BuLi is more favored for 6. As stated above, Et2Al(OEt) was a useful cocatalyst for 1 to induce syndioselective ROMP of NB and TCD, but ROMP by 6−Et2Al(OEt) (1:3) yielded gel polymers which were unanalyzable by NMR or GPC. When n-BuLi was added to 6, polymerizations of NB and TCD were induced to result in the formation of highly tactic polymers. It was found that 1−n-BuLi was not an efficient catalyst for NB and TCD, which afforded viscous gel. In addition, no polymer was obtained when n-BuLi was employed in 3-fold excess for 6. Unfortunately, no information has been obtained about propagating species of the present binary ROMP systems. At least one can say that the suitable choice of cocatalysts might be a key for catalyst activation. When the catalyst precursors 1 and 6 were insufficiently or excessively alkylated by cocatalyst, catalyst systems would not work well as single-site stereoselective ROMP catalysts. Effect of polymerization temperature was examined for 6−nBuLi (1:2) at first. It is interesting that rate of polymerization of NB and TCD was relatively higher than that of DCP: It took 50 min to consume 1000 fold of DCP by 6−n-BuLi (1:2) at 80 °C, while polymerizations of NB and TCD finished within 20 min. At 20 °C, polymer yields were 70% and 65% for NB and TCD respectively after 2 h reaction. At 50 °C, polymer yields were 100% after 3 h. It is worth noting that the obtained polymers from 6−n-BuLi (1:2) were all-cis and “isotactic”: cis > 99%, meso > 99% for poly(NB), cis > 99% for poly(TCD). To conclude, the stereoselectivity of 6−n-BuLi (1:2) might be constant for NB and TCD irrespective of the reaction temperature in the range of 20−80 °C. Next 1−Et2Al(OEt) (1:3) was investigated about the effect of polymerization temperature. Again NB and TCD polymerized rapidly as compared to DCP: at 50 °C 1-based catalyst needed 3 h to polymerize 1000 fold of DCP, on the other hand 1000 folds of NB and TCD were consumed within 1 h. At 20 °C, polymer

Scheme 6. Summary of the Stereocontrol of ROMPs of NB, DCP and TCD by the Tungsten- and the Molybdenumbased Catalysts

L

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method using high temperature NMR system.16 It can be emphasized that the stereoselectivity of the present polymerization systems was highly dependent on the catalyst design and the monomer structure. We have found that the catalysts derived from tungsten chloride with “small” imido ligand could regulate the enchainment reaction of NB, DCP and TCD into syndioselective in the present study. However, its ability for stereocontrol is likely to be limiting. Less steric NB yielded cissyndiotactic poly(NB)s from 1 and 2, while more steric tricyclic DCP also yielded cis-syndiotactic polymer only from 1. In the case of sterically bulky TCD monomer, 1-based catalyst introduced syndioselective ROMP, but its syndioselectivity was not very high. Syndioselectivity of NB polymerization with W-imidos seems to be less affected by steric demands of the imido group. Stereoselectivity of TCD polymerization with Wimidos appears to be largely influenced by steric bulkiness of the substituent on the imido group. From these results, one can say that syndioselectivity of the ROMPs by W imido-based catalysts decreases with increasing the steric demands of the imido group as well as of the monomer substituents. Since the propagation species generated from W imido complexes 1−3 would only have polymer asymmetry and no catalyst asymmetry at the metal or on the ligand, it is speculative that the syndioregularity might be derived mainly from chain end control/penultimate effect. In contrast to the ROMP results by tungsten imido-based catalysts, molybdenum oxyphenolates 4−6 tend to induce the cis-isoselective ROMPs of these three monomers. Concerning cis-selectivity, catalyst 6 introduced all-cis polymerization of bicyclic NB and tetracyclic TCD, while cis-selectivity of 6 for tricyclic DCP polymerization was rather low. This result suggests a possibility of the interaction between metal center and unsaturated bond of cyclopentene ring of approaching DCP or that of polymeryl group. In regard to the stereostructure, isoselectivity of the ROMP by 6 is 100% for NB and above 95% for DCP. 5 yielded iso-biased polymers of which isotacticities were rather low. 4-based catalyst was ineffective for isoselective ROMP. Considering the excellent study on the isospecific ROMPs of various monomers by Schrock catalysts,9 it is assumable that the phenolate ligands of 4−6 such as racemic-5,5′,6,6′-tertamethyl-3,3′-di-tert-butyl-1,1′-biphenolate would remain on the metal after catalyst aging, and in turn, they would influence the polymerization process. The origin of the stereocontrol effect of Mo oxyphenolates can be explained by the idea that the effects from the phenolate ligands such as ligand asymmetry of bulky biphenolate might overwhelm the effect from the polymer asymmetry. As a conclusion for this paragraph, the isoselectivity of the ROMPs by molybdenum oxyphenolate catalysts might come from catalyst control. Catalyst precursor 7 is also a preferable instance to compare the effects of the ligand for the stereoregulation. Effects of terminal oxo and substituted imido for cis-selectivity and stereoselectivity are vague at this moment; terminal oxo type 6 is effective to attain all-cis polymerization of NB and TCD, while imido type 7 is evident to useful for highly cis-selective ROMP of DCP. With respect to stereoregularity, terminal oxo type 6 induced isospecific ROMP of NB (and TCD), while imido type 7 yielded less tactic polymers. However, both catalyst precursors proved to have almost identical ability for stereocontrol of ROMP of DCP. To compare the results by 1 and 7, it can be said that the influences of the biphenolate

ligand on the polymerization process could be compared favorably with that of the phenylimido ligand. Scheme 7 is a summary of the properties of the tactic and the atactic H-poly(NB), H-poly(DCP), and H-poly(TCD). It is Scheme 7. Summary of the Effects of the Stereostructure on the Thermal Properties of H-poly(NB), H-poly(DCP) and H-poly(TCD)

interesting that the H-poly(NB)s were revealed to be crystalline polymers irrespective of their tacticities. Crystal structures of the annealed syndio- and the annealed ata-H-poly(NB)s were supposed to be very close, but the melting points of the syndioH-poly(NB)s are 10 °C lower than that of the ata-H-poly(NB). In contrast, crystal structure of the annealed isotactic polymers would be quite different. Melting points of the iso-Hpoly(NB)s are much higher than those of the syndio- and the ata-H-poly(NB)s. The iso- and iso-biased-H-poly(NB)s showed plural endothermic peaks in DSC thermograms. Further, temperatures of endothermic peaks of them increased with increasing their isotacticities. It is presumable at this moment that the iso-H-poly(NB) has plural crystalline structures and/or plural crystal rearrangements. Register et al. reported that the small endothermic peak was detectable below the melting point in DSC measurement of ata-H-poly(NB), which was suggested as an endothermic crystal rearrangement.5 The authors believe that crystal rearrangement might play a key role for determining thermal properties of tactic H-poly(NB)s. Presently the crystal nature of each H-poly(NB)s is under intensive research and will be published in the near future. On the contrary, tacticity control of the main chain gave drastic influences on the nature of H-poly(DCP)s and of Hpoly(TCD)s. Previously we reported that both the iso- and the syndio-H-poly(DCP)s were crystalline and less soluble, which might have their own different crystal structures. On the other hand, ata-H-poly(DCP) is amorphous and soluble above 50 °C.7 It is an unexpected character that the ata-H-poly(TCD) was found to be a semicrystalline polymer. So far, the ata-Hpoly(TCD) has been reported and recognized as an amorphous and soluble polymer, of which hydrogenation was conducted mainly by Ni-based catalyst.8a,c Catalytic hydrogenation of poly(cycloolefin)s by Ni-based catalysts is known to be often accompanied by an isomerization of tertiary carbons of the polymer chain. We have presently employed chemical transfer M

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ACKNOWLEDGMENTS We would like to express appreciation to Professor Koji Tashiro (Toyota Technological Institute) for helpful discussion on the property of the polymers. We are thankful to Mr. Tomohiko Ushijima (Zeon Corporation) for his support on the DFT calculation of tetracyclododecene. 13C NMR measurements of the iso-H-poly(NB)s were conducted with assistance of RIKEN using Bruker Avance III HD 600 MHz spectrometer equipped with CRYO probe DCH 600S3, and we thank Dr. Muneki Oouchi (NMR facility, RIKEN) for his kind guidance for the 13 C NMR measurement.

hydrogenation using p-Tos-NHNH2 to saturate poly(TCD)s in order to avoid the unfavorable isomerization reaction, and the high-temperature NMR study of the obtained H-poly(TCD)s confirmed that no isomerization proceeded in parallel with hydrogenation. To summarize the above, it is presumed that the isomerized ata-H-poly(TCD) is amorphous. The syndio-Hpoly(TCD) is amorphous and soluble at ambient temperature, this result is in sharp contrast with that of the highly crystalline syndio-H-poly(DCP). In general, tactic polymers tend to be crystalline and atactic polymers are inclined to be amorphous. From this point of view, it is worthwhile to emphasize that the propensity of the syndio-H-poly(TCD) seems to be an extraordinary instance of the crystalline polyolefin. It is also noteworthy that the iso-H-poly(DCP) and the “iso”-Hpoly(TCD) were characterized by unique crystalline nature: melting points of isotactic polymers are relatively higher than those of syndiotactic and atactic polymers.

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SUMMARY The isospecific and the syndioselective ROMPs of NB and TCD were precisely investigated using various W- and Mobased catalysts. While W(NPh)Cl4·Et2O (1) was a useful catalyst precursor for the syndioselective ROMPs of these monomers, MoO(rac-Biphenolate)2 (6)-based catalysts were effective for the isospecific ROMPs of them. The hydrogenated stereoregular NB and TCD polymers were well characterized by DSC and WAXD analyses. In fact, the H-poly(NB)s exhibited high crystallinities irrespective of their tacticities. Crystal structures of the syndio-H-poly(NB) and the ata-Hpoly(NB) were presumably close, but the melting points of the syndiotactic polymers are 10 °C lower than that of the atactic polymers. Melting point and WAXD patterns of the iso-Hpoly(NB) were quite different from those of the syndio- and the ata-H-poly(NB)s, suggesting different crystal structure. On the other hand, the syndio-H-poly(TCD) was amorphous and soluble at room temperature. However, the ata-H-poly(TCD) and the “iso”-H-poly(TCD) were found to be crystalline polymers. Although the stereoselective polymerizations of NB and TCD with the W- and Mo-based catalysts were not well clarified at the moment, we have considered that the steric structures of both catalysts and monomers would strongly affect the stereoregularity of the polymer chain. The polymerization process of less steric NB appears to be easily controlled by suitably designed catalyst, whereas larger effect from the catalyst may be necessary to govern the stereoselectivity of ROMP of more steric TCD. ASSOCIATED CONTENT

S Supporting Information *

Analytical details (1H NMR, 13C NMR, DSC, WAXD) of the poly(NB)s, poly(TCD)s, H-poly(NB)s, and H-poly(TCD)s. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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Corresponding Author

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

The authors declare no competing financial interest. N

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Soc. 2011, 133, 1784. (g) Flook, M. M.; Gerber, L. C. H.; Debelouchina, G. T.; Schrock, R. R. Macromolecules 2010, 43, 7515. (11) Rosebrugh, L. E.; Marx, V. M.; Keitz, B. K.; Grubbs, R. H. J. Am. Chem. Soc. 2013, 135, 10032. (12) Rules, J. D.; Moore, J. S. Macromolecules 2002, 35, 7878. (13) Sagane, T.; Yamaguchi, H.; Minami, S.; Mizuno, A.; Wamura, H. US Patent 5,106,931, 1992. (14) DFT calculation of enthalpies of formation and activation of each stereoisomers of tetracyclododecene was performed at the B3LYP/6-31G* level using Gaussaian03: (exo, syn)-form: activation enthalpy H‡ = 26.0 kcal/mol, formation enthalpy ΔH = −19.7 kcal/ mol; (exo, anti)-form: H‡ = 34.7 kcal/mol, ΔH = −17.4 kcal/mol; (endo, syn)-form: H‡ = 31.6 kcal/mol, ΔH = −15.9 kcal/mol; (endo, anti)-form: H‡ = 22.6 kcal/mol, ΔH = −19.5 kcal/mol. (15) The cis/trans-ratio of poly(TCD) was estimated based on 1H NMR measurement. Proton signals attributable to allyl proton on C1,4 in the main chain was observed as doublet, which is likely to be owing to the cis/trans splitting. No resolution of the repeating units derived from stereoisomers was observable. 1H NMR (CDCl3) δ 2.93 (brs, Hallyl‑cis), 2.67 (brs, Hallyl‑trans). (16) The cis/trans-ratio of poly(DCP) was estimated by resolution of allyl proton of the main chain using following equation based on 1H NMR data recorded CDCl3 in at 60 °C: cis % = 100(2Hallyl‑A − Hallyl‑B)/(2Hallyl‑A + 2Hallyl‑B). 1H NMR (CDCl3) δ 2.84 (brs, Hallyl‑A), 2.59 (brs, Hallyl‑B). 13C NMR of H-poly(DCP) was recorded at 200 °C in o-dichlorobenzene-d4/trichlorobenzene (1/2, wt/wt) mixture to determine its tacticity.

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dx.doi.org/10.1021/ma502020u | Macromolecules XXXX, XXX, XXX−XXX