Olefin metathesis polymerization: Some recent developments in the precise polymerizations for synthesis of advanced materials (by ROMP, ADMET)

Tetrahedron 74 (2018) 619e643

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Tetrahedron report 1150

Olefin metathesis polymerization: Some recent developments in the precise polymerizations for synthesis of advanced materials (by ROMP, ADMET) Yanjun Chen a, b, 1, Mohamed Mehawed Abdellatif a, c, 1, Kotohiro Nomura a, * a

Department of Chemistry, Faculty of Science and Engineering, Tokyo Metropolitan University, Hachioji, Tokyo, 192-0397, Japan Department of Applied Chemical Engineering, Ningbo Polytechnic, Ningbo, Zhejiang, 315800, China Chemical Industries Division, Chemistry of Tanning, Materials and Leather Technology, National Research Centre, 33 ElBohout St., BP12622, Dokki-Giza, Egypt b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 November 2017 Received in revised form 18 December 2017 Accepted 20 December 2017 Available online 22 December 2017

Recent results for synthesis of end-functionalized polymers (EFP) by using olefin metathesis polymerization have been introduced including basic characteristics in ring-opening metathesis polymerization (ROMP) of cyclic olefins and acyclic diene metathesis (ADMET) polymerization for synthesis of conjugated polymers. Several approaches were demonstrated for synthesis of EFP by living ROMP using molybdenum (exclusive coupling with aldehyde) and ruthenium catalysts (sacrificial ROMP, chain transfer). Cis specific (Z selective) ROMPs were achieved by molybdenum, ruthenium, and vanadium catalysts by the ligand modification. The catalytic synthesis of EFP with high cis selectivity has been achieved by combined ROMP with chain transfer by V(CHSiMe3)(N-2,6-Cl2C6H3)[OC(CF3)3](PMe3)2. The ADMET polymerization using molybdenum and ruthenium catalysts afforded defect-free, high molecular weight poly(arylene vinylene)s containing all trans olefinic double bonds. The methods for precise synthesis of EFPs, exhibiting unique optical properties combined with the end groups, were developed. The catalytic one-pot syntheses for EFPs have also been developed. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Olefin metathesis ROMP ADMET polymerization Endfunctionalization Ring opening metathesis polymerization

Contents 1. 2.

3.

4.

Background: basics in olefin metathesis catalysts and the mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 620 Ring-opening metathesis polymerization (ROMP) for synthesis of end-functionalized polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 621 2.1. Basics and some examples for synthesis of advanced materials by end-modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 621 2.1.1. Synthesis of end-functionalized polymers by living ROMP: some examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 622 2.2. Synthesis of end-functionalized polymers by Z-Selective (cis-specific) chain transfer ring-opening metathesis polymerization (ROMP) . . . . . . 626 2.2.1. Basic catalyst design in Z-selective olefin metathesis, and tacticity control in ROMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 626 2.2.2. (Imido)vanadium-alkylidene catalyst for synthesis of end-functionalized polymers by cis-specific chain transfer ROMP . . . . . . . . . . . 629 Acyclic diene metathesis (ADMET) polymerization for synthesis of conjugated polymers, Poly(Arylene Vinylene)s . . . . . . . . . . . . . . . . . . . . . . . . . . . . 632 3.1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 632 3.2. ADMET polymerization for synthesis of defect-free end-functinalized conjugated polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 634 3.2.1. General characteristics in synthesis of Poly(Arylene Vinylene)s by the ADMET polymerization using ruthenium-carbene catalysts . . 634 3.2.2. Synthesis of end-functionalized conjugated polymers by combined olefin metathesis with Wittig-type coupling . . . . . . . . . . . . . . . . 635 3.2.3. Catalytic one-pot synthesis of end-functionalized conjugated polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 638 Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 639 Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 641 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 641

* Corresponding author. E-mail address: [email protected] (K. Nomura). 1 YC and MMA: Equal contribution as the first authors. https://doi.org/10.1016/j.tet.2017.12.041 0040-4020/© 2017 Elsevier Ltd. All rights reserved.

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1. Background: basics in olefin metathesis catalysts and the mechanism Olefin metathesis has been recognized as the useful method applied for synthesis of various organic compounds (basic, fine chemicals, pharmaceuticals etc.) and polymeric, advanced materials,1e5 and it has been known that metal-carbene (alkylidene) complexes play a key role in this catalysis. These reactions such as ring-opening metathesis (ROM, reaction with cyclic olefins), ring closing metathesis (RCM, intramolecular reaction with acyclic diene), and acyclic diene metathesis (ADMET, intermolecular reaction with acyclic diene), cross metathesis (CM, reaction with acyclic olefins) proceed via metallacycle intermediate, as shown in Scheme 1. These carbon-carbon bond formations have been recognized as most effective and important methods in recent organic synthesis as well as polymer synthesis in terms of better atom efficiency and construction of environmentally benign chemical processes, synthetic methods. RCM and ADMET are the metathesis reactions of acyclic dienes and control of the selectivity (among two reactions) would be possible by the substrate concentration employed; ADMET reactions (intermolecular metathesis) are conducted under high substrate concentrations whereas RCM is generally conducted under the low concentration to avoid the intermolecular reactions. Olefin metathesis catalysts are generally classified as three types shown in Chart 1; (i) transition metal alkylidene (carbene) complexes, metallacycles, (ii) transition metal compounds with metal

alkyl cocatalysts (generating active species in situ), and (iii) transition metal compounds without metal alkyl cocatalysts or preformed alkylidenes (generating active species in situ without alkylating reagent). Both ruthenium-carbene [so called Grubbs type, exemplified as Ru(1)-Ru(4)] and molybdenum-alkylidene [so called Schrock type, Mo-F0, Mo-F6] catalysts (Chart 1) are the known successful examples.1e5 In general, the resultant olefinic double bonds are a mixture of cis- and trans-forms. Recently, several complex catalysts demonstrate high cis-specificity in olefin metathesis reactions including ROMP Scheme 2).1c,2e,6,7 It has been demonstrated that the ligand modifications play essential roles not only to exhibit the high activity, but also to achieve the stereospecific olefin metathesis reactions.1,2 Precise control over macromolecular structure is a central goal in polymer synthesis, and living polymerizations [absence of undesirable side reactions such as chain transfer and termination, accomplished by ring-opening metathesis polymerization (ROMP), group transfer polymerization, controlled radical polymerization, and anionic polymerization] generally provide synthesis of polymers with both controlled molecular weights and narrow molecular weight distributions.8 Introduction of end functionality has also been one of the most important method that enables grafting of the other polymers with different main chain or introduction of functionalities.5d,e,l,9,10 Precise control of every detail of the macromolecular structure and chain architecture has enabled the development of numerous advanced polymeric materials that are needed in fields as diverse as coatings and adhesives, electronics,

Scheme 1. Typical olefin metathesis reactions and their basic mechanisms.

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Chart 1. Classification of olefin metathesis catalysts and typical (traditional) molybdenum and ruthenium complex catalysts.

medicine and cosmetics, environment, and countless others.8e10 Ring-opening metathesis polymerization (ROMP) and acyclic diene metathesis (ADMET) polymerization have been widely used in synthesis of advanced polymeric materials.5 These polymerizations are addition type polymerization induced by ring strain (ROMP) or condensation polymerization with accompanying byproduction of small molecule (such as ethylene or propylene, ADMET), as shown in Scheme 1. In this article, we wish to introduce some recent developments in these two polymerization techniques applied for synthesis of new polymeric materials by adopting exclusive end functionalization. We also wish to introduce our recent results concerning synthesis of end-functionalized polymers by cis-specific catalytic chain transfer ROMP using (imido)vanadium-alkylidene catalyst.11 2. Ring-opening metathesis polymerization (ROMP) for synthesis of end-functionalized polymers 2.1. Basics and some examples for synthesis of advanced materials by end-modification Ring-opening metathesis polymerization (ROMP) is a promising

technique, because the resultant polymers (especially by ROMP of norbornene derivatives) generally possess rather linear structures compared to ordinary vinyl polymers such as poly(acrylamide) s.5e,12 This thus contributes to better maintain nature of the functionality at the side chain and the polymer chain end, because the end functionality would not be (covered and) strongly affected by the polymer main chain. It has been well known that the molybdenum-alkylidene complexes called Schrock type catalysts are useful initiators for the living ROMP of cyclic olefins, especially substituted norbornenes and norbornadienes.2,5a-c,e,g,k,l The absence of chain-transfer (such as intermolecular or intramolecular metathesis with internal olefins in the resultant ROMP polymer) and termination (including catalyst deactivation) reactions in such polymerization systems allows synthesis of the homopolymers and the block copolymers with narrow molecular weight distributions (Scheme 2).2,5b,e,k Importantly, the exclusive end functionalization can be easily achieved in the living ROMP technique by treating the propagating chain end with aldehyde (called Wittig-type coupling, cleavage). Precise control of the functionality in both the initiation and the termination sites can be thus possible.2,5b,e,k Moreover, control of monomer repeating unit can also be possible by varying the monomer/catalyst molar ratios due to the exclusive initiation

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Scheme 2. General scheme for ring-opening metathesis polymerization (ROMP).

efficiency of the molybdenum initiator (especially Mo-F0). The ROMP has thus demonstrated tremendous utility as a powerful synthetic tool in preparing macromolecular materials displaying promising biological, electronic, and mechanical properties. Importance of using the molybdenum catalysts should be thus emphasized for their precise preparations, although the initiators are highly sensitive to moisture and both monomers and solvent have to be thus strictly purified to avoid the catalyst decomposition (deactivation). In contrast, dissociation of ligand (PR3 etc.) should be required to generate the catalytically-active species in the ROMP with the ruthenium-carbene catalysts (Scheme 3)5b,e,k,13; the initiation efficiency is thus not always perfect as seen in the molybdenumalkylidene initiators. An equilibrium between coordination and dissociation of PR3 should be present even in the propagation process, and replacement of halogen with the other anionic ligand

(and/or replacement of PR3 with the other neutral donor ligands/ substrates) can also be considered as the probable side reactions. In general, Ru(1) showed moderate initiation and propagation, whereas Ru(2) showed slow initiation but fast (rapid) propagation. 2.1.1. Synthesis of end-functionalized polymers by living ROMP: some examples As described above, both precise control of molecular weight (monomer repeating unit) and exclusive end functionalization can be achieved in ROMP of cyclic olefins (norbornene derivatives) using the molybdenum-alkylidene catalyst (especially Mo-F0). We already demonstrated a synthetic methodology to prepare various amphiphilic block copolymers (ABCs) by adopting the “grafting to” approach14 whereby poly(ethylene glycol) (PEG) is attached to the ROMP polymers prepared by the living technique using the Schrock-type molybdenum initiator (Scheme 4).14,15 Various block

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Scheme 3. General scheme for generation of the catalytically active species in ring-opening metathesis polymerization (ROMP) by ruthenium-carbene catalysts.

copolymers were prepared by sequential addition of norbornene and its sugar-containing derivative [1,2:3,4-di-O- isopropylidenea-D-galactopyranos-6-O-yl-5-norbornene-2-carboxylate, a mixture endo/exo] in toluene at 25
C with different monomer/

initiator molar ratio. The resultant carbohydrate-containing polymers are expected to exhibit strong specific affinity with cell surface proteins, most probably arising from the clustering and binding of the cells by multivalent arrays, thus leading to a greater

Scheme 4. Precise synthesis of amphiphilic block (star) copolymers by grafting to approach.14

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Scheme 5. Synthesis of ROMP polymers containing hydroxyl group via “sacrificial synthesis” approach.5d,18

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Scheme 6. End-functionalization of ROMP polymer by Ru catalyst.5d,18,20,21

affinity and specificity than their mono-valent counterparts.16 The living ROMPs were terminated by addition of 4-Me3SiOC6H4CHO and 3,5-(Me3SiO)2-C6H3CHO to afford poly(1a,b) in high yields (>94%); this is an established procedure to perform the cleavage of the ROMP polymer-metal bonds (as reaction of the Mo living ends) with aldehyde yielding a carbon-carbon double bond in a Wittig-type reaction. The Mn value of the resultant ring-opened polymers, determined by GPC, increased linearly upon increasing the monomer/Mo molar ratios while the molecular weight distributions remained narrow,14 indicating livingness of the present polymerization. The quantitative removal of SiMe3 group of the block copolymer, poly(1a,b), by treating the polymers with aqueous HCl solution afforded the polymers carrying hydroxy functionality at the chain end (yield 91e99%), whereas the cyclic acetal groups of the carbohydrate residue remained intact under these weakly acidic conditions. The hydroxy group was then treated with KH in THF, and its subsequent coupling with PEGMs2 [MsO(CH2CH2O)nMs, Ms ¼ MeSO2] resulted in the triblock linear ABCs consisting of ROMP polymers and PEG [poly(2)] or triarm ABCs consisting of ROMP polymer and PEG [poly(3)] in high yield. The GPC traces for the resulting polymers were unimodal and displayed appropriate increment in the Mn values with low PDI values (Mw/ Mn ¼ 1.06e1.20, Scheme 4). In addition, the Mn values estimated by the 1H NMR spectra (by the integration ratio of olefinic protons to those of PEG) were in good agreement with those calculated on the basis of monomer/initiator molar ratios. Moreover, the reaction of poly(1) with 0.5 equiv. of PEG in the presence of KH afforded ABA or ABCBA (sandwich) type amphiphilic multiblock copolymers, poly(4), in high yields. Cyclic acetals in the ABCs, poly(2e4), could be selectively removed, without accompanying any ester-cleavages, by using a mixture of CF3CO2H and water (9/1, v/v) at room temperature (for 15 min).17 The deprotected polymers were identified by NMR and FT-IR spectra, and the integration ratios estimated from the 1H NMR

spectra for PEG/sugar/ring-opened NBE protons of the resultant polymers were very close to the calculated values.14 Since precise control of the repeat units of both norbornene (hydrophobic) and the sugar-substituted norbornene derivatives (rather hydrophilic after deprotection) as well as the attached PEG (hydrophilic) could be possible by using this approach, it can thus be concluded as a promising technique for the preparation of new types of ABCs consisting of ROMP and PEG units in a precise fashion. Hydroxy-functionalized ROMP polymers prepared by the ruthenium catalyst were demonstrated by Kilbinger et al. so called “sacrificial synthesis” route.5d,18 The synthetic strategy adopted is shown in Scheme 5, and the route consists of (i) synthesis of diblock copolymers by sequential addition in the living ROMP, and (ii) subsequent treatment of conc. HCl (in MeOH/CH2Cl2) to cleave the olefinic acetal groups in the second block segment. Due to that the polymerization should proceed in a living manner, both Ru(CHPh)(Cl)2(PCy3)2 (Ru1) and exo-N-phenylnorbornene-2,3dicarboximide were chosen according to the previous report.19 Diblock copolymers were obtained by subsequent addition of dioxepine monomer, and the second polymer block was then decomposed under acidic conditions; PPh3 was added in situ to improve the efficiency and substituent in the dioxepine was important for the efficient synthesis. Presence of the hydroxyl group was confirmed by introduction of SiMe3 group etc. (Scheme 5). Although the procedure seems apparently tedious compared to the approach by the molybdenum system (simple termination by addition of functionalized aldehyde) and the application may be limited, the approach enables us to prepare polymers containing polar functionalities (due to improved functional group tolerance in ruthenium catalysis) that seems difficult to achieve by adopting the ROMP using the molybdenum system.5d Synthesis of telechelic ROMP polymers containing two hydroxyl groups at the chain ends was achieved by adopting a route consisting of (i) synthesis of triblock copolymers by sequential addition in the living ROMP, and (ii) subsequent treatment of HCl (in MeOH/

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Scheme 7. Synthesis of end-functionalized glycopolymers as mimetics of chondroitin sulfate proteoglycans.22

CH2Cl2) to cleave the olefinic acetal groups (Scheme 5).20 Use of cleavable monomers (cyclic acetals, R’ ¼ Me, Ph) that can be addressed separately, sequential deprotection was accomplished affording polymeric materials bearing different substituents at their respective chain ends. The resultant polymers possessed relatively narrow molecular weight distributions, but the resultant telechelic polymer contained small amount of mono functionalized ROMP polymers probably due to “incomplete initiation” by Ru(1) under these conditions (as explained in Scheme 3). The resultant polymers after hydrogenation with Raney Ni and subsequent introduction of pyrene moiety, deprotection under acidic conditions consisted of mono functionalized ROMP polymer and small amount of the other ROMP polymers (mono hydroxyl group and phenyl group), probably due to incomplete initiation at the first step.20 The similar approach can be applied for synthesis of various endfunctionalized ROMP polymers, as shown in Scheme 6.21 For example, synthesis of thiol-functionalized ROMP-polymers by employing thioacetal monomers in place of dioxepine monomer, which can be then cleaved by hydrogenation (by Raney Ni) leaving the desired thiol group behind.21b Moreover, facile end-capping technique for ROMP with living ruthenium-carbene chain ends without further chemical transformation steps could be achieved, when vinylene carbonate and 3H-furanone are introduced as the termination agents.21c Efficient synthesis of ROMP polymers containing aldehyde or carboxylic acid end groups could be thus achieved by this new method, which involves the decomposition of acyl carbenes to ruthenium carbides.21a End-functionalized ROMP polymers that mimic the native-like, multivalent architecture carrying on chondroitin sulfate (CS) proteoglycans was also reported by adopting ROMP by using so called 3rd generation Ru catalyst, Ru(4), in the presence of internal olefins containing a biotin moiety as the chain transfer reagents for introduction of the end functionality (Scheme 7).22 More recently, Kilbinger et al. reported chain-end functionalization of ROMP polymers using degenerative chain transfer

metathesis (Scheme 8).23 This strategy is conducted using chain transfer agent (CTA) which regenerates the ruthenium-benzylidene complex, Ru(4), and terminate chain end with a cyclohexenyl group. Substituted cyclohexene rings are worked as good CTA which preserves the living character of ROMP with using less amount of metal complex. That makes the polymerization more cheap and suitable for industrial, biomedical and academic applications which need reduced levels of residual ruthenium catalyst. 2.2. Synthesis of end-functionalized polymers by Z-Selective (cisspecific) chain transfer ring-opening metathesis polymerization (ROMP) 2.2.1. Basic catalyst design in Z-selective olefin metathesis, and tacticity control in ROMP As described above, ring-opening metathesis polymerization (ROMP), addition type polymerization induced by ring strain, has been widely used in synthesis of advanced polymeric materials.5 Although the resultant olefinic double bonds are generally a mixture of cis- and trans-forms, recent catalyst developments have demonstrated high cis-specificity in olefin metathesis reactions including ROMP.1c,2e,6,7,11,24,25 Because olefin metathesis is a reversible process, isomerization of kinetically generated Z olefin products to the thermodynamically favored E alkenes [as exemplified by Ru(2), Ru(3), widely employed catalysts, Scheme 9], particularly at late stages of a process, is frequently a significant complication. The first successful example has been demonstrated by Mo- and W-based monoaryloxide pyrrolide complexes (Scheme 10).6 Subsequent investigations led to Ru catalysts bearing a bidentate P-O ligand (for chemo- and stereoselective ROMP),24 a bidentate N-heterocyclic carbene (NHC) moiety [exemplified as Ru(5)], or a chelate dithiolate ligands [exemplified as Ru(6)] for various olefin metathesis processes7; in many instances, the latter two sets showed moderate to high Z selectivity for reactions of unhindered terminal alkenes (e.g.,

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Scheme 8. Catalytic synthesis of end-functionalized ROMP polymers by degenerative chain transfer metathesis.23

Scheme 9. Key structural features for the design of Z selective Ru catalysts.7i

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Scheme 10. General consideration for syn/anti isomers and coordination of norbornene. Selected “cis-specific (Z selective)” olefin metathesis catalysts and the proposed mechanism for synthesis of cis, syndiotactic polymers.2b,c,e,6b

without a secondary allylic substituent, Scheme 9). Substantial amounts of the E isomer would be generated at higher conversions and/or improved stereoselectivity was achieved at the expense of diminished catalytic activity. It has thus been demonstrated that the ligand modifications play essential roles not only to exhibit the high activity, but also to achieve the stereospecific olefin metathesis reactions.1,2 In order to consider the stereo control of ROMP by Mo-akylidene

catalysts, there are several factors that should be taken into consideration. There is an equilibrium between syn (in which the alkylidene substituent positioned close to imido ligand) and anti (in which the alkylidene substituent positioned away from imido ligand) isomers.2b,c,e Four type of coordination of cyclic olefin (exemplified as norbornene in Scheme 10) should be considered in each isomers as exemplified the case when norbornene coordinate from the front side.

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In the cis-specific ROMP using molybdenum catalyst (MoHIPTO), control of syn/anti isomer ratios (by using small imido and large aryloxide), and of olefin coordination (direction trans to the pyrrolide ligand) can be achieved by the ligand modification (Scheme 10).6b This clearly enables control of tacticity in the resultant polymers. More recently, molybdenum monoaryloxide chloride complexes (Mo3, Mo4, Scheme 11) furnish higher-energy (Z) isomers of trifluoromethyl-substituted alkenes through cross-metathesis reactions with the commercially available, inexpensive and typically inert Z-1,1,1,4,4,4-hexafluoro-2-butene.6k These catalysts also showed highly efficient and stereoselective transformations with Z1,2-dichloroethene and 1,2-dibromoethene.6l The catalyst thus enables the synthesis of representative biologically active molecules and trifluoromethyl analogues of medicinally relevant compounds.

Scheme 11. Z Selective olefin metathesis with dihaloalkenes, Z-1,1,1,4,4,4- hexafluoro2-butene.6l

2.2.2. (Imido)vanadium-alkylidene catalyst for synthesis of endfunctionalized polymers by cis-specific chain transfer ROMP (Imido)vanadium(V)-alkylidene complexes, V(CHSiMe3)(NR)(X)(PMe3)n [R ¼ 1-adamantyl, 2,6-R0 2C6H3 (R’ ¼ H, Me, Cl etc.); X ¼ aryloxo, alkoxo, ketimde, imidazolin-2-iminato, imidazolidin-2-iminato etc.; n ¼ 1 or 2],3f-h,11,25 also exhibited from moderate to high catalytic activities for ROMP of cyclic olefins, and the ligand modification plays an important role for exhibiting the high activity and the selectivity.3g,h,25 In particular, V(CHSiMe3)(N2,6-Cl2C6H3)(OC6F5)(PMe3)2 showed the highest activity for ROMP of norbornene (NBE) and the derivatives.25b The complex polymerizes cis-cyclooctene without any assistances of cocatalyst and the activity increased at high temperature (100
C).25c The resultant polymers in the ROMP of NBE possessed ultrahigh molecular weights with low PDI (Mw/Mn) values. Good linear

Scheme 12. Cis-specific ring-opening metathesis polymerization (ROMP) of norbornene (NBE) by (imido)vanadium(V)-alkylidene catalysts.25a,b

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Scheme 13. Efficient synthesis of end-functionalized ROMP polymers by cis-specific chain transfer ring-opening metathesis polymerization (ROMP) of norbornene (NBE) by (imido) vanadium(V)-alkylidene catalyst.25

relationships between TON (turnover number, polymer yield on the basis of V) and the Mn value consistent with low PDI values were observed. Moreover, linear relationships between ln[NBE]t/[NBE]0 and the polymerization time were also observed {[NBE]0, [NBE]t are the NBE concentration at the initial, the certain time, respectively}, suggesting that the reaction rates were a first order kinetics. The facts strongly suggest a possibility of living polymerization without catalyst deactivation, affording ultrahigh molecular weight polymers with narrow PDI values. Olefinic double bonds in the resultant ring-opened polymers were a mixture of cis- and trans-forms, and control of the selectivity seemed difficult in this catalysis. Therefore, design of the cis-specific ROMP catalysts was the important subject on this project. It turned out that olefinic double bonds in the resultant polymers prepared by the fluorinated alkoxo analogue, V(CHSiMe3)(N2,6-Cl2C6H3)[OC(CH3)(CF3)2](PMe3)2 (V1), at 25
C possess high cis selectivity (92%), whereas the resultant polymers prepared by the aryloxo analogues (exemplified above) possess a mixture of cis/ trans olefinic double bonds, confirmed by 1H and 13C NMR spectra.

Table 1 Ring-opening metathesis polymerization (ROMP) of norbornene (NBE) by V(CHSiMe3)(N-2,6-Cl2C6H3)[OC(CF3)3](PMe3)2 (V3) in the presence of chain transfer agents (CTA) at 80
C.a CTA (mol%)

PMe3/V TONb molar ratio

TOFb/min1 Mnc  104 Mw/Mnc cisd/%

1-hexene (10.0) 1-hexene (10.0) 1-hexene (10.0) 1-hexene (10.0) ATMS (5.0)e ATMS (5.0) ATMS (10.0) ATMS (30.0) ATMS (30.0) ATMS (50.0) 1-octene (10.0) VCH (10.0) VCHE (10.0) DM1B (10.0) cis-4-octene (10.0)

none 3.0 10 100 10 10 10 10 10 10 10 10 10 10 10

596 1070 1450 1520 1630 1830 1860 1910 1390 1890 980 2220 1830 1820 2210

1790 3200 4350 4570 4890 5490 5590 5740 6970f 5660 2940 6660 5490 5450 6620

6.28 4.80 3.90 4.06 42.7 15.2 8.73 3.45 3.35 1.94 5.14 9.85 9.97 104 148

1.51 1.74 2.04 2.03 1.78 2.46 2.43 2.46 2.24 2.43 1.68 2.35 1.95 2.56 2.58

94 97 98 98

98 98 99 97 98 98 98 98 98

a Reaction conditions: complex V3 0.3 mmol, NBE 200 mg (2.12 mmol), benzene 4.8 mL (initial NBE conc. 0.44 mmol/mL), PMe3/V ¼ 10 (molar ratio), 80
C, 3 min. b TON (turnovers) ¼ NBE reacted (mmol)/V(mmol), TOF ¼ TON/time. c GPC data in THF vs polystyrene standards (Mn in g/mol). d Cis percentage (%) estimated by 1H NMR spectra. e Polymerization at 25
C. f Reaction 5 min, yield >98%. ATMS ¼ allyltrimethylsilane, VCH ¼ vinylcyclohexane, VCHE ¼ 4-vinyl-1-cyclohexene, DM1B ¼ 3,3-dimethyl-1-butene.

The cis selectivity in the resultant polymers prepared at 20
C in toluene reached to 96%. It turned out that the activity by the OC(CF3)3 (F9) analogues, V(CHSiMe3)(N-2,6-X2C6H3)[OC(CF3)3](PMe3)2 [X ¼ H (V2), Cl (V3), Scheme 12], were much higher than those by the OC(CH3)(CF3)2 analogue (V1), probably due to an activation of the alkylidene species by introduction of electron-withdrawing alkoxide. The activities by V2,3 increased upon addition of PMe3 but decreased by excess additions. Note that the activities further increased upon addition of PMe3 even at 80
C, suggesting that excess PMe3 would stabilize the active species. Also note that the olefinic double bonds in the resultant polymers prepared at 25
C possess high cis selectivity (98%), and the highly cis percentages (97,98%) did not decrease even though the ROMPs were conducted at 50 and 80
C in the presence of PMe3. Cis percentage in the resultant polymers prepared by V1-3 without addition of PMe3 at 80
C increased in the order: 61% (V1) < 93% (V2) < 98% (V3), clearly indicating that the high cis specificity can be achieved by modification of both the imido and the anionic donor (alkoxo) ligands.15b Although there are successful examples for cis specific olefin metathesis reactions including ROMP using molybdenum and ruthenium complex catalysts,6,7 examples conducted at high temperature are very limited so far (these reactions were conducted mostly at room temperature). High activity at high polymerization temperature should be noteworthy especially from the aspect of practical application, because the solution polymerization process at high temperature improves viscosity in the reaction mixture leading to the better mass transportation and the temperature control; the present catalysts (especially V2 and V3) would be rare examples as thermally robust, highly efficient cis specific ROMP catalysts. On the basis of the facts that an increase in the cis selectivity upon addition of PMe3 or low temperature and that the fast coordination/dissociation exchange was seen in the 31P and 1H NMR spectra, it was thus assumed that coordination of NBE for subsequent metathesis would be controlled in this catalysis; NBE would coordinate to V trans (opposite) to PMe3 and high cis selectivity would be achieved due to a proposed intermediate consisting of a steric bulk of small arylimido and large alkoxo ligands (Scheme 12).3g,h,25a,b More recently, highly efficient synthesis of end-functionalized polymers has been achieved by combined cis-specific (Z selective) ROMP of NBE with terminal olefins [1-hexene, allyltrimethylsilane (ATMS) etc.] used as the chain transfer (cross metathesis, CM) agent at 80
C in the presence of V(CHSiMe3)(N-2,6-Cl2C6H3) [OC(CF3)3](PMe3)2 (V3, Scheme 13).11 The ROMP proceeded with the high activity and the high cis selectivity (97e98%) upon addition of PMe3, and the Mn values could be controlled by varying concentration

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Scheme 14. Conventional methods for synthesis of poly(arylene vinylene)s: (a) condensation polymerization in vacuo, (b) Gilch polymerization, and (c) Heck coupling.38e45

of the terminal olefins, and the activity at 80
C was not affected by the ATMS concentrations. The selected results are summarized in Table 1. The activity in the presence of 1-hexene increased at 80
C [ex. TOF ¼ 1380 min1 (at 25
C) vs 1450 (at 80
C), upon addition of PMe3], whereas the activity decreased if the polymerization was conducted in the absence of PMe3. Both the activity and the Mn value were unchanged by varying the PMe3/V molar ratios (10e100) under these conditions. The Mn values in the polymers prepared at 80
C (in the presence of 1-hexene) were lower than those prepared at 25
C, suggesting that degree of chain transfer (CM) was affected by the polymerization temperature. The resultant polymers showed high cis selectivity (97e98%), although a slight decrease in the selectivity was observed if the ROMP was conducted without addition of PMe3 (94%). Note that the activity in the ROMP in the presence of ATMS at 80
C was not affected by the ATMS concentrations charged (5.0e50.0 mol%), and the Mn values could be controlled by varying the concentration; the reaction reached to completion after 5 min

without change in the Mn value. Resonances corresponding to protons in the SiMe3 and vinyl group (-CH¼CH2) were clearly observed in the 1H NMR spectra in the resultant polymers, and the integration ratios [9(SiMe3):3(-CH¼CH2) in the resultant polymers with various Mn values] strongly suggest that the resultant polymers possessed both SiMe3 and vinyl group at each polymer chain end.11 The ROMPs in the presence of VCH, VCHE and 1-octene afforded the polymers with the Mn values that are relatively similar to those obtained in the presence of ATMS, 1-hexane; protons corresponding to the vinyl group were also observed by their 1H NMR spectra (and the Mn values estimated by the integration ratios showed the similar trends). The results thus suggest that the chain transfer ROMP (tandem ROMP and CM or combined ROMP with CM) proceeded in the presence of these olefins. In contrast, the ROMPs in the presence of DM1B (tert-butyl ethylene) and cis-4octene gave high molecular weight polymer with rather broad molecular weight distributions. These results suggest that the chain transfer (CM) reactions did not occur efficiently under these conditions.

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Scheme 15. Horner-Emmons route to synthesis of conjugated polymer.46,47

3. Acyclic diene metathesis (ADMET) polymerization for synthesis of conjugated polymers, Poly(Arylene Vinylene)s 3.1. Background Conjugated polymers (CPs), such as poly(p-phenylene), poly(fluorene), poly(p-arylene vinylene)s, poly(thiophene)s, are promising organic electronics applied for optoelectronic and electrochemical devices.26e33 It has been widely recognized that precision in the synthesis (defect-free nature such as structurally regular, chemically pure etc.) plays an important role toward their electronic and photophysical properties, and the device efficiency (because these polymers are not defect tolerant). Development of new synthetic method/methodology for synthesis of defect-free and high molecular weight materials has thus been considered as an important subject. Moreover, the fundamental properties of CPs are also generally affected by the supramolecular interactions (such as non-covalent intermolecular p-p, CH/p and van der Waals interactions, hydrogen bonding etc.), and the nano/mesoscale organization34e36; in particular, the communication between distinct polymer chains is essential for light emission, charge transport.37 As described above, precise, efficient synthesis of defect-free end-functionalized conjugated polymers thus attracts considerable attention in terms of the better device performances.32 More recently, important/promising (electronic, optical) properties that are different from the original CPs have been demonstrated by the

end-modification,38,39 as exemplified in poly(fluorene)s containing discrete end-groups (such as triarylamines, perylene monoimide dye, heteroaryl end-groups etc.),40 and in end-functionalized poly(9,9-dialkyl-fluorene-2,7-vinylene)s (PFVs).41 Conventional synthesis of poly(p-phenylene vinylene)s (PPVs) by condensation polymerization in the presence of tetrahydrothiophene generally requires harsh conditions such as high temperature (180e300
C) under vacuum conditions (Scheme 14).42,43 However, a significant decrease in the quantum efficiency of the final PPV film was observed due to formation (byproduction) of oxidation products during the conversion step. Moreover, contamination of impurities (halogen, sulfur etc.) affects severe damages toward the device performance. The other methods such as the Gilch polymerization,44 Suzuki-Heck or Heck coupling,45 and Horner-Wittig-Emmons (HWE) reactions46 have also been employed as the conventional methods. Gilch polymerization provides some advantages for placement of vinylene units along the polymer backbone with high molecular weights and low PDI (Mw/Mn) index. In synthesis of poly(9,9-di-n-octylfluorene-2,7-vinylene) (PFV) and its copolymers with MEH-PPV (Scheme 14),44a,b the key factor for obtainment of high molecular weight polymers was an introduction of a chloromethyl group into the 9,9-di-n-octyl-fluorene unit.44a However, significant defect structure was detected in the PFV (and the copolymer) main chain; degree of the defect structure was roughly estimated to be 10e15%.44a A series of PFV copolymers incorporating bis(phenyl)oxadiazole (OXD), triphenylamine (TPA), or both

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Scheme 16. Synthesis of conjugated polymers by ADMET polymerization.49e59

moieties along the backbone were also synthesized by Heck coupling (Scheme 14).45a These, especially Heck type coupling, are method widely employed for synthesis of poly(arylene vinylene)s exemplified in PPVs (Scheme 14).45b The resultant PFVs (and PPVs) prepared by these methods possessed rather low percentage of structural defects compared to those prepared by the Gilch method, however, the polymers possessed a mixture of cis- and trans-olefinic double bonds and control of molecular weight as well as removal of Pd (and exclusion of a possibility of cross linking even small percentage), which would affect the property (quantum efficiency), seemed difficult. Horner-Emmons method (Scheme 15) would be the simple method.46 The resultant PFVs possessed rather lower molecular weights compared to those prepared by the Gilch route, but possessed fewer defects (and rather high yields).46 Note that the polymers possessed no termination of the conjugation and crosslinking (less structural defect), which showed better optical

properties compared to those especially by the Gilch method.46 A series of oligo(9,9-di-n-octyl-fluorene-2,7-vinylene)s (OFVs) with defined conjugation units were prepared by a step wise approach (Scheme 15).47 The absorption and photoluminescence spectra were red-shifted upon increasing the FV repeat units.47 In summary, several methods have been known for preparing poly(arylene vinylene)s, and some reactions require harsh conditions. The resultant polymers should be strictly purified by removal of impurities (inorganic salts, sulfur, halogen, Pd etc.) that affect damages the device performances (quantum yields, emission etc.). Difficulty of structural control (cis/trans) in addition to control of end groups also affects the property of the resultant polymers. Therefore, development of new synthetic method/ methodology for synthesis of defect-free and high molecular weight materials has thus been considered as an important subject.

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Scheme 17. Basic mechanism on ADMET condensation polymerization.

Table 2 ADMET polymerization of 9,9-dialkyl-2,7-divinylfluorene (DVF) by Ru -carbene catalysts.53,a Alkyl in R

Ru cat.

Solvent

Conc.b

Temp./
C

Time/h

Mnc  104

Mw/Mnc

Yieldd/%

n-octyl n-octyl n-octyl 20 -ethylhexyl 20 -ethylhexyl 20 -ethylhexyl n-hexyl n-hexyl n-hexyl

2 2 4 2 2 2 2 3 2e

toluene toluene C6H5Br toluene toluene toluene toluene toluene CH2Cl2

90 180 180 180 270 180 100 200 103

50 50 90 50 50 50 50 50 40

7.5 8 3.5 3 3 8 3 5 8

1.84 2.75 0.33 2.10 3.30 3.00 2.30 2.10 3.20

1.8 2.0 1.8 2.1 2.2 2.6 2.5 2.1 2.0

75 90 75 75 82 79 93 82 >99

a b c d

Conditions: solvent 1.0 mL, DVF/Ru ¼ 40 (molar ratio). Initial monomer concentration in mmol/mL. GPC data in THF vs polystyrene standards. Isolated yields. eDVF/Ru ¼ 35 (molar ratio), CH2Cl2 2.0 mL.

3.2. ADMET polymerization for synthesis of defect-free endfunctinalized conjugated polymers 3.2.1. General characteristics in synthesis of Poly(Arylene Vinylene)s by the ADMET polymerization using ruthenium-carbene catalysts Acyclic diene metathesis (ADMET) polymerization48 has been considered as an efficient route for the synthesis of p-conjugated polymers (Scheme 16),49e59 because the polymerization does not require harsh conditions and accompany side reactions which might alter the photophysical properties as well as the others required as device performance. This methodology therefore afforded synthesis of defect-free, high molecular weight, all trans poly(9,9-di-n-octyl-fluorene-2,7-vinylene) (PFV),51,53 poly(2,5dialkyl-phenylene-1,4-vinylene)s (PPVs),52,54,57 and poly(N-alkylcarbazole- 2,7-vinylene)s (PCVs),53 poly(thienylene vinylene)s58,59 by using Schrock type molybdenum-alkylidene, and rutheniumcarbene catalysts (Scheme 16). The resultant polymers possess high stereo-regularity (all trans olefinic double bond) and analytically pure (defect-free), and showed unique photophysical properties compared with those prepared by other methods.53 ADMET polymerization of divinyl aromatics is a condensation

polymerization that proceeds with by-production of ethylene (Scheme 17). Note that olefinic double bonds in the resultant polymers/oligomers possess highly trans due to steric bulk in the metallacycle intermediate. The initial trials in the polymerizations (of substituted divinylbenezenes), however, afforded oligomer mixtures,50 but we demonstrated synthesis of high molecular weight PFVs by adopting this approach using Mo(CHCMe2Ph)(N-2,6Me2C6H3)[OCMe(CF3)2]2 (Mo-F6).51 It was revealed that the polymerization should be conducted under high monomer concentration with repetitive removal of ethylene for obtainment of high molecular weight polymers. These are basic contents that should be considered, because the ADMET polymerization is a condensation polymerization as well as intermolecular metathesis reactions of two monomers (containing vinyl or propenyl groups). Use of appropriate catalysts, described below, is important, and the molecular weight distribution (Mw/Mn) should be close to 2 even with unimodal molecular weight distribution due to nature of condensation polymerization accompanied by-production of small molecule (ethylene). Selected results for ADMET polymerization of 9,9-dialkyl-2,7divinylfluorene (DVF) by Ru complex catalysts are summarize in Table 2.53 Attempted polymerization by RuCl2(CHPh)(PCy3)2 [called

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Scheme 18. Synthesis of amphiphilic block copolymers of PFVs by combined olefin metathesis with Wittig-type coupling (by adopting grafting to and grafting from approach, and click reaction).41a,c

first generation Grubbs Ru cat., Ru(1)] recovered DVF, and use of RuCl2(CHPh)(IMesH2)(3-BrC5H4N)2 [Ru(4), called 3rd generation Grubbs cat.] also gave oligomer mixtures, although Ru(4) was effective for synthesis of poly(thienylene vinylene)s.58 Ru(CHPh)(Cl)2(IMesH2)(PCy3) [Ru(2), called 2nd generation Grubbs cat.] and RuCl2(CH-2-OiPr-C6H4)(IMesH2) [Ru(3), called Hoveyda-Grubbs 2nd generation cat.] were effective affording defect-free, all-trans high molecular weight PFVs with unimodal molecular weight distributions.53 It should be noted that olefinic double bonds in the resultant PPVs, PFVs, and PCVs possessed exclusive trans regularity (1H and 13 C NMR spectra), and possessed vinyl groups at the polymer chain ends.41,52,53 Syntheses of poly(thienylene vinylene)s and their derivatives were also reported by the group of Hillmyer and Wagener.58,59 These polymerizations were conducted in 1,2,4trichlorobenzene at 90
C under vacuum (ca. 100 mTorr), and up to five aliquots of 1 mol % of Ru(2) or Ru(4). Solid-state metathesis polycondensation technique was also employed for obtainment of high molecular weight polymers.59 Use of propenyl group instead of vinyl group was the key for the success; the resultant polymers were a mixture of cis/trans olefinic double bonds, although reason for the details was not clear at this moment.

3.2.2. Synthesis of end-functionalized conjugated polymers by combined olefin metathesis with Wittig-type coupling As described above, the resultant PFVs in the ADMET polymerization by Ru catalyst possessed vinyl group exclusively,41,53 the end-modification of the conjugated materials can be thus established especially by combined olefin metathesis of the vinyl chain ends with molybdenum catalyst (reagent) with subsequent Wittig-type cleavage with aldehyde.41 Exclusive synthesis of amphiphilic block copolymers could be thus demonstrated by grafting PEG into both the PFV chain ends (Scheme 18).41a,60 The phenolic OH group protected by SiMe3 group could be introduced by olefin metathesis of the vinyl groups in the PFV chain ends with Mo-F6 cat. (reagent) and subsequent reaction with 4-Me3SiO-C6H4CHO. The SiMe3 group could be then cleaved by HCl aq.; treatment of the resultant OH group with KH followed by reaction with mesylated poly(ethylene glycol) (PEGMs2) to afford ABA-type amphiphilic triblock copolymers.41a The Mn values in the resultant copolymers estimated by the integration ratios with methylene protons of the PEG segment were highly close to the estimated value from both GPC [Mn(calcd) ¼ Mn(GPC)/1.6]61 and the starting PEG. Syntheses of block (graft) copolymers were also demonstrated by combination of the ADMET polymerization with Cu-catalyzed atom transfer radical polymerization (ATRP). Styrene

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Scheme 19. One pot synthesis of end-functionalized star-shape homo polymers and block copolymers by combined olefin metathesis with Wittig-type coupling.41e,f

was polymerized in the presence of macroinitiator, which could be prepared by an introduction of the initiating functionalities into the PFV chain ends (Scheme 18).41c,62 The subsequent end-modification by treatment of the Br with NaN3, and following click reaction with 4-pentanoate terminated PEG methyl ether afforded amphiphilic ABCBA-type block copolymers in a precise manner.41c,62 The incorporation of PEG segment confirmed by the 1H NMR spectra, and the Mn values estimated by the NMR spectra were highly close to those calculated, indicating that each reactions (end-modifications) took place in a precise, quantitative manner.41c Facile, efficient one pot syntheses of the end-functionalized triblock copolymers and the star shape (triarm) (co)polymers containing a PFV main chain were also demonstrated (Scheme 19) by adopting the combined olefin metathesis with Wittig-type coupling. Olefin metathesis of the vinyl end groups in PFV with Mo-F6 cat. (1.8 equiv to PFV) gave a mixture of the “mono- and bisalkylidene” species in situ, and subsequent Wittig-type coupling

with 0.5 equiv of OPVCHO (3PVCHO, 7PVCHO) or 3T(CHO)2 (0.5 equiv) afforded “the triblock intermediate” as a mixture of “monoand bis-alkylidene” species (marked with dashed bracket in Scheme 19). Treatment of the residual vinyl end group with Mo-F6 cat. (2.5 equiv) to complete the olefin metathesis, and subsequent addition of aldehyde (ArCHO) in excess amount afforded the endfunctionalized triblock copolymers, expressed as [(PFV)2-7PV]Ar2, [(PFV)2-3PV]Ar2 or [(PFV)2e3T]Ar2 [Ar ¼ C6H5, C6F5, terthiophene (3T), ferrocenyl (Fc)] in high yields (70e88% on the basis of PFVs).41e The resultant polymers possessed uniform molecular weight distributions without significant increases in their Mn values, and the Mn values, estimated by their integration ratios (on the basis of the middle segments) in the 1H NMR spectra, were highly analogous to those calculated, strongly suggesting their exclusive formations.41e The exclusive synthesis could be further confirmed by grafting PEG,41e analogous to those demonstrated above.41a,c The resultant copolymers, prepared according to Scheme 20, possessed uniform

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Scheme 20. Synthesis of amphiphilic block copolymers (by grafting PEG).41e

Fig. 1. Fluorescence spectra for PFVs containing various oligo(thiophene)s (conc. 1.0  106 M in THF at 25
C, excitation at 460 nm).41b

molecular weight distributions, and the Mn values based on the integration ratios (with methylene protons of the PEG) were very close to the calculated values, clearly indicating that facile, efficient attachments of a pseudo phenol terminus on the PFV to PEGMs2, could be achieved in a precise manner. Importantly, the results strongly demonstrate that the end-functionalization of PFV chain ends, reaction with the middle segments, and subsequent grafting of PEG took place with exclusively in all cases. Instead, treatment of “the triblock intermediate” first with tris(formylphenyl)amine (triarm core segment), and subsequent olefin metathesis with Mo-F6 cat. (2.5 equiv) followed by addition of aldehyde (ArCHO) in excess amount afforded the triarm end-

functionalized triblock copolymers, expressed as TPA [{(PFV)2e3T}-Ar]3 or TPA[{(PFV)2-3PV}-Ar]3 exclusively (Scheme 19).41f The resultant polymers were identified by 1H NMR spectra and GPC traces (unimodal molecular weight distributions). Similarly, treatment of a mixture of the “mono- and bis-alkylidene” species, which were prepared by olefin metathesis of the vinyl end groups in the starting PFV with Mo-F6 cat. (1.8 equiv to PFV)], with tris(formylphenyl)amine [TPA(CHO)3] or 2,4,6-tris(4formylphenyl)-1,3,5-triazine [TPTA(CHO)3], and subsequent olefin metathesis with Mo-F6 cat. (2.5 equiv) followed by addition of aldehyde (ArCHO, in excess) afforded the triarm end-functionalized polymers, expressed as TPA(PFV-Ar)3 or TPTA(PFV-Ar)3 exclusively (Scheme 19).41f As described above, recently, unique optical properties that are different from the original PFVs have been demonstrated by the end-group modification.41 It has been known that the UVevis spectra for PFV displays two absorption bands at 455, 427 (and 400) nm, that can be ascribed to p-p* transitions (0-0, 0e1, and 0e2 transitions, respectively) of the conjugated backbone, and the corresponding emission peaks at 465, 496, and 530 nm are observed in the fluorescence spectra.36,47,53 Note that relative intensities at ca. 497e500, 528e531 nm (compared with that in ca. 465 nm) in the fluorescence spectra for PFVs containing 3 or 4 thiophene repeat units (PFV-3T, PFV-MP3T, PFV-DH4T) were apparently higher than those in the original PFV and the others (PFV-2T, PFV-6T, Fig. 1), and their fluorescence life times (lem ¼ 530e609 nm in THF) (t ¼ 0.654, 0.746, 0.697 ns, respectively) are longer than those in the others (0.521, 0.534 ns, respectively).41b On the basis of recent study concerning effect of terthiophene (3T) as the end-groups in PFVs by means of time-resolved fluorescence and anisotropy technique, it was revealed that the 3T end-modified PFV undergoes structural change in the excited state41i; the longer decay component can be

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Fig. 2. Fluorescence spectra (excitation at 380 nm) for PFV-[Porphyrin(Zn)]2 (left) and PFV-(F-BODIPY)2 (right) in THF (conc. 1.0  106 M) at 25
C.41d

ascribed to a species of which p-system of the excited PFV part is expanded over the end-group after coplanarization of the terthiophene unit upon the excitation.41i Fig. 2 shows fluorescence spectra (in THF at 25
C) for PFVs containing porphyrin, expressed as PFV-[Porphyrin(Zn)]2, or F-BODIPY (boradiaza-indacene or boron-dipyrromethene; 2,6diethyl-1,7,8-trimethyl-4,4-difluoro-4-bora-3a,4a-diaza-sindacene), expressed as PFV-(F-BODIPY)2, with different molecular weights.41d Two absorption bands attributed to Zn(II)-porphyrin (two Q bands) in addition to absorption bands ascribed to PFV were observed at 560, 610 nm, which are shifted from the original (543, 580 nm). It was revealed in Fig. 2 (left) that the relative intensity especially at 637 nm compared to the others (attributed to emissions to PFVs) in the fluorescence spectra increased upon decrease in the Mn value (PFV conjugation units), whereas the relative intensities were also affected by the excitation wavelength. Similarly, a new broad emission band (lmax ¼ 613e614 nm) which is different from that in F-BODIPY itself63,64 was also observed in the fluorescence spectra (Fig. 2, right), and the intensities of a broad emission band at 613e614 nm were affected by the PFV conjugation length, probably due to influence of an energy transfer (from PFV to FBODIPY under these excitation conditions). Note that the THF solutions containing PFV-[Porphyrin(Zn)]2 displayed amethyst-white (white-pale purple, Mn ¼ 6600) or pale white-blue emission (Mn ¼ 16500) under UV irradiation. Moreover, note that the white-orange light emission of the THF solutions containing PFV-(F-BODIPY)2 (Mn ¼ 8400) under UV irradiation can be tuned to “white-light emission” by varying the PFV conjugation units (Mn ¼ 29500). The facts clearly suggest that emission color can be modified from the blue light emission (by PFV) by an introduction of functionality (chromophore such as F-BODIPY or porphyrin) and the emission property can also be tuned by varying the PFV conjugation units.41d Moreover, these polymers especially containing F-BODIPY displayed remarkably high photoluminescence quantum yields (FPL ¼ 89%). The facts should be noteworthy because white light-emitting diodes (WLEDs) have received considerable interest due to their potential applications. 3.2.3. Catalytic one-pot synthesis of end-functionalized conjugated polymers One-pot synthesis of end-functionalized poly(9,90 -di-n-octylfluorene-2,7-vinylene)s (EF-PFVs) could be achieved by combined

ADMET polymerization (of 2,7-divinyl-9,9-di-n-octhyl-fluorene, DVF) with end-functionalization/chain transfer using 1,2-disubstituted olefins [DOs: 1,4-cis-diacetoxy-2-butene (DAB), cis-stilbene, cis-4-octene etc.] in the presence of RuCl2(PCy3)(IMesH2)(CHPh) [Ru(2)].65 The efficient synthesis of high molecular weight PFVs with efficient end-functionalization could be achieved when DOs were added after the initial ADMET polymerization (Scheme 21); 2 step addition seems suited to the purpose. Although certain optimizations are necessary for the exclusive synthesis of EF-PFVs, significant reduction of amount of molybdenum catalyst/ reagent (>2 equiv to the vinyl group), employed above (described in 3-2-2), could attained by adopting this method. Another one-pot synthesis of EF-PFVs was demonstrated by combined ADMET polymerization using Mo(CHCMe2Ph)(N-2,6Me2C6H3)[OCMe(CF3)2]2 (Mo-F6 cat.) with subsequent Wittigcoupling (Scheme 22). Further addition of Mo-F6 after the ADMET polymerization was necessary for the exclusive endfunctionalization, and for completion of olefin metathesis with the vinyl chain ends to allow the subsequent Wittig-type coupling with aldehyde. The present method also enables reduction of molybdenum catalyst (Mo-F6, required rather excess for endfunctionalization of polymers prepared by ADMET polymerization using ruthenium catalyst). Various aldehydes can be used for introduction of the end-functionality as demonstrated in Scheme 22, and the method can be applied for synthesis of the other poly(arylene vinylene) s. More recently, exclusive syntheses of a series of EF-PFVs with different end groups has been achieved by (i) synthesis of EF-PFV containing vinyl chain end by ADMET polymerization using Mo(CHCMe2Ph)(N-2,6-Me2C6H3)[OC(CH3)(CF3)2] (Mo-F6 cat.) terminated with aldehyde, and (ii) subsequent olefin metathesis with the vinyl group using Mo-F6 cat. followed by Wittig-type coupling with another aldehyde.67 The exclusive formation of EF-PFVs containing vinyl end group by the ADMET polymerization could be confirmed especially by grafting PEG, and by synthesis of amphiphilic triblock copolymers by combined atom transfer radical polymerization (ATRP) from the PFV chain end with grafting PEG by click reaction (Scheme 23), as demonstrated previously.41c The bromide in the chain end was treated with NaN3, and the resultant polymer, p-MeC6H4-PFV-(PSN3), was then reacted with 4-pentanoate terminated PEG methyl ether (Mn ¼ 2000) in the presence of CuBr and dNbipy in THF (click reaction). The resultant polymer possessed unimodal and low PDI

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Scheme 21. Catalytic one-pot synthesis of end-functionalized PFVs by ADMET polymerization using ruthenium-carbene catalysts.65

Scheme 22. One-pot synthesis of end-functionalized PFVs by combined ADMET polymerization with Wittig-type coupling.66

value [p-MeC6H4-PFV-PS-PEG, Mn(GPC) ¼ 39100, Mw/Mn ¼ 1.16], and the 1H NMR spectrum showed protons ascribed to PEG units, strongly suggesting incorporation of PEG segment. Note that the Mn value estimated by 1H NMR spectrum [Mn(NMR) ¼ 41200, on the basis of methylene protons in the PEG segment] was very close to the calculated value [Mn(calcd) ¼ 40500, based on Mn value of PFV and integration ratio of PFV and styrene], strongly supports a precise synthesis of the amphiphilic triblock copolymers by adopting this approach. The results also indicate that a preparation of the macroinitiator (ATRP), treatment with NaN3, and subsequent click reaction proceeded exclusively. Various EF-PFVs with different end groups [C6F5, pyridyl, ferrocenyl, terthiophene etc.] have thus been prepared (Scheme 24). The resultant polymers possessed low PDI values and no significant differences in both the Mn and Mw/Mn values from starting PFVC6F5 were observed after the modification. These results also demonstrated that synthesis of EF-PFVs containing two different

end-groups has been achieved by adopting this methodology.67 Synthesis of various PFVs with different end groups has thus been demonstrated by adopting this methodology. It has also been demonstrated that their emission properties were influenced by the end groups and the conjugation repeat units. This would be the rare demonstration of exclusive synthesis of conjugated polymer with different end groups, and the method should be useful for synthesis of new type of conjugated materials that should be promising for basic understanding and exhibit new/unique optical properties by the end-modification, an integration of functionality.

4. Concluding remarks Olefin metathesis has been recognized as the useful method applied for synthesis of various organic compounds and polymeric, advanced materials; ring-opening metathesis polymerization (ROMP) and acyclic diene metathesis (ADMET) polymerization

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Scheme 23. Synthesis of amphiphilic triblock copolymers by combination of ATRP (atom transfer radical polymerization) with subsequent click coupling.67

have been widely used in synthesis of advanced polymeric materials. Precise control over macromolecular structure is a central goal in polymer synthesis, and the introduction of end functionality has also been one of the most important methods that enable grafting of the other polymers with different main chain or introduction (integration) of functionalities. Therefore, some recent developments in these two polymerization techniques applied for synthesis of new polymeric materials by adopting exclusive end functionalization have been summarized. Several approaches have been demonstrated for synthesis of endfunctionalized polymer by living ROMP by using molybdenum and ruthenium catalysts, and the catalytic syntheses have been achieved

by both ruthenium and (imido)vanadium catalysts in the presence of chain transfer agents. In particular, the synthesis with cis selectivity (>97%) has been achieved by combined ROMP with chain transfer (cross metathesis) in the presence of V(CHSiMe3)(N-2,6-Cl2C6H3) [OC(CF3)3](PMe3)2 catalyst. The ADMET polymerization using molybdenum and ruthenium catalysts afforded defect-free, high molecular weight conjugated polymers, [poly(arylene vinylene)s, exemplified as poly(fluorene vinylene)s (PFVs) and poly(phenylene vinylene)s] with all trans olefinic double bonds. The methods also demonstrated precise synthesis of end-functionalized conjugated polymers by combined olefin metathesis with Wittig-type coupling with various aldehydes, and the resultant polymers showed unique

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Scheme 24. Synthesis of end-functionalized PFVs with different end groups by ADMET polymerization, olefin metathesis and Wittig-type coupling.67

optical properties combined with the end groups. The method also demonstrated synthesis of various end-functionalized block copolymers (by grafting to and grafting from approaches) as well as star shaped (tri-arm) copolymers in one pot with high efficiency. Furthermore, catalytic one-pot syntheses of end-functionalized PFVs have been attained by both ruthenium and molybdenum catalysts and the method should provide green route for synthesis of conjugated materials with better device performance. Development of these precise synthetic methods should provide more opportunity for design of advanced functional materials.

The conjugated polymer project is partly supported by Advanced Catalytic Transformation for Carbon utilization (ACT-C JPMJCR12YX), Japan Science and Technology Agency. M.M.A. expresses his thanks to The Follow-up Research Fellowship for former international students (Tokyo Human Resources Fund for City Diplomacy, Tokyo Metropolitan University) for conducting collaboration research, and Y.C. expresses her thanks to Visiting Research Project Foundation of Ningbo Polytechnic for her study with K.N. in 2017. References

Acknowledgment K.N. expresses his heartfelt thanks to former/present group members who contributed this project as coauthors, and the other members for discussion/supports. The vanadium catalyst project by K.N. was partly supported by Grant-in-Aid for Scientific Research on Innovative Areas (No. 26105003, “3D Active-Site Science”) from The Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, and Grant-in-Aid for Scientific Research (B) from the Japan Society for the Promotion of Science (JSPS, No. 15H03812).

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Yanjun Chen received her Ph.D degree at Lanzhou Univ. in 2009 (Profs. K. Gao, W.-H. Sun). She continued her research as postdoctoral associate in Ningbo Institute of Technology, Zhejiang Univ. from 2010 to 2012. She is now working in Ningbo Polytechnic, and was a visiting researcher in Prof. K. Nomura's laboratory in 2017.

Mohamed Mehawed Abdellatif received his Ph.D degree from Tokyo Metropolitan Univ. in 2013 (Prof. K. NOMURA). He was a postdoctoral associate at the Univ. of Lorraine from 2013 to 2016 (with Dr. G. PICKAERT, Dr. AVERLANTPETIT M.-C). He continued his academic career at National Research Center of Egypt in 2013. His research interests include the synthesis of functional polymers and low molecular weight gelators.

Kotohiro Nomura finished his undergraduate and master studies in Saitama Univ. in 1986 (Prof. A. Miyashita) and Univ. of Tokyo in 1988 (Prof. Y. Saito), and joined as a research scientist in the Organic Synthesis Research Laboratory, Sumitomo Chemical Co., Ltd. He received his Ph.D. degree in 1993 from Osaka Univ. (Prof. N. Sonoda), and joined a group of Prof. R. R. Schrock (MIT, USA) as a postdoctoral fellow. He then returned to Sumitomo, and moved to Nara Institute of Science and Technology as an Associate Professor in 1998. He has been a full professor in Department of Chemistry, Tokyo Metropolitan Univ. since April in 2010. He has co-authored ca. 300 publications including reviewing articles and book chapters. His recent research projects focus on design of molecular catalysts for precise olefin polymerization, metathesis reactions, and chemospecific organic transformations.