Acyclic Diene Metathesis ( ADMET ) Polymerization for Precise Synthesis of Defect-Free Conjugated Polymers with Well-Defined Chain Ends

This accounts introduces unique characteristics by adopting the acyclic diene metathesis (ADMET) polymerization for synthesis of conjugated polymers, poly(arylene vinylene)s, known as promising molecular electronics. The method is more suitable than the other methods in terms of atom efficiency affording defect-free, stereo-regular (exclusive trans) polymers with well-defined chain ends; the resultant polymers possess better property than those prepared by the conventional methods. The chain ends (vinyl group) in the resultant polymer prepared by ruthenium-carbene catalyst(s) can be modified by treating with molybdenum-alkylidene complex (olefin metathesis) followed by addition of various aldehyde (Wittig type cleavage), affording the end-functionalized polymers exclusively. An introduction of initiating fragment, the other conjugated segment, and one-pot synthesis of end-functionalized block copolymers, star shape polymers can be achieved by adopting this methodology.

In this accounts, unique characteristics by adopting the ADMET polymerization methods in terms of properties in the resultant polymers as well as synthesis of various advanced materials especially by the end modifications, including the points that have to be taken into consideration for the efficient synthesis, have been introduced.

Synthesis of Conjugated Polymers by ADMET Polymerization
Table 1 summarizes the results for ADMET polymerization of 2,7-divinyl-9,9-dialkylfluorenes by ruthenium-carbene catalysts (Scheme 4).Synthesis of high molecular weight PFVs with uniform molecular weight distributions could be achieved by adopting this method under optimization of the reaction conditions (catalyst, monomer/catalyst molar ratios, initial monomer concentration, etc.), although the perfect control of the repeating units cannot be obtained in this condensation (step growth) polymerization (Mw/Mn = ca.2).Continuous removal of ethylene by-product from the reaction medium should be prerequisite for obtainment of the high molecular weight polymers in the present polymerization due to an equilibrium shown in Scheme 3 [21][22][23].The resultant polymers possess all-trans internal olefinic double bonds with defect-free nature (without termination of conjugation).The polymerizations of 2,7-divinyl-9,9-di-n-octyl-fluorene using Ru(CHPh)(Cl)2(IMesH2)(PCy3) (Ru(B)) in toluene proceeded under the optimized conditions, affording rather high molecular weight polymers with unimodal molecular weight distributions.Dichloromethane could also be used as the solvent, especially in terms of improvement of the solubility in the resultant PFV.The similar results were observed in the ADMET polymerization of the 2'-ethylhexyl analogue.The polymerization did not complete if the n-hexyl analogue was used as the monomer under the similar conditions, probably due to precipitation of the resultant polymer after a certain period; the polymerization under low initial monomer concentration conditions or in CH2Cl2 was effective.
The ADMET polymerization of the n-octyl analogue using RuCl2(CHPh)(PCy3)2 [Ru(A)] did not proceed, whereas the polymerization by Ru(B) afforded high molecular weight polymers under the same conditions.High molecular weight polymers could also be obtained by RuCl2(CH-2-O i Pr-C6H4)(IMesH2) (Ru(C)).However, the polymerization using RuCl2(CHPh)(IMesH2)(3-BrC5H4N)2 (Ru(D)) afforded low molecular weight oligomers even if the reaction was conducted at high temperature (90 °C).This would be explained as due to a formation of the pyridine-coordinated dormant species (probably after formation of the methylidene species, Ru=CH2).Taking into account of these results, it is clear that Ru(B) and Ru(C) are suited to this polymerization [23].
Figure 1 shows typical 1 H NMR spectrum in the resultant polymer prepared by the ADMET polymerization using ruthenium-carbene catalyst [25].Olefinic double bonds in the resultant PFVs possessed exclusive trans regularity confirmed by 1 H NMR spectra [21,23], because the reaction proceeds via metallacyclobutane intermediate (Scheme 3).Moreover, the resultant PFVs prepared by Ru catalysts possessed vinyl groups at the both polymer chain ends [22,23,25].These are remarkable contrasts with that prepared by the previous (precursor) method, especially two broad resonances ascribed to protons at 4.5-4.8ppm in addition to rather broad resonances ascribed to aromatic protons in the polymer prepared by the precursor method [23].
Figure 2a shows UV-vis spectra (in THF, 1.0 × 10 −5 M at 25 °C) for PFVs (n-hexyl analogue) prepared by the ordinary method [available from Aldrich, Mn = 24,100, Mw/Mn = 1.70 and Mn = 1720, Mw/Mn = 1.11, prepared from the sulfonium salt precursor monomer under high-temperature (180-300 °C) and vacuum conditions (10 −6 mbar)], and by the ADMET polymerization using Ru(B) (Mn = 32,000, Mw/Mn = 2.0).The spectrum prepared by the ADMET polymerization displays two absorption bands at 427 and 455 nm, which can also be attributed to π-π* transitions of the conjugated backbone [31].Three absorption peaks at 455, 427, and 400 nm are attributed to 0-0, 0-1, and 0-2 transitions [32], respectively, with corresponding emission peaks at 465, 496, and 530 nm (described below, Figure 2b).The sample prepared by the ADMET polymerization showed a rather sharp contrast to that prepared by the Hormer-Emmons (HWE) route [reaction of 2,7-bis(methylenediethyl phosphate)-9,9-di-n-octylfluoene with 9,9-di-n-octylfluorene-2,7-dicarbaldehyde in the presence of KO t Bu] [31], probably because that the resultant polymer possessed higher stereo-regularity (all trans in the internal olefinic double bond) than that prepared by the HWE route (mixture of cis/trans).Note that the spectrum prepared by the ordinary method (the precursor route) shows a broad absorption band withλmax = 414 nm; this band can be attributed to π-π* transitions of the conjugated backbones with a shoulder at ca. 380 nm; the bands for the samples prepared by the ADMET polymerization are intensified and red-shifted when compared to that for the sample prepared by the precursor method.The observed difference is due to presence of defect, leading to an enhanced conjugation length [31].Figure 2b shows the fluorescence spectra (in THF, 1.0 × 10 −6 M at 25 °C, excitation wavelength at 390 nm or 426 nm).The spectrum of PFV prepared by the ADMET method showed a strong emission band at 465 nm with a shoulder at 496 nm along with a slight shoulder at 530 nm.It is explained that the shoulder arises from coupling between the fluorene and vinylene units to form a new electronic state with a lower energy.No significant differences were observed in the spectra for the other alkyl analogues [23].The spectrum of PFV prepared by the precursor route also showed a rather strong emission band at 464 nm with a shoulder at 494 nm, but remarkable differences in their intensities were seen.Moreover, a shoulder observed at 450 nm in the sample prepared by the HWE route was not observed in the samples prepared by the ADMET route and the emission band at 465 nm possessed narrow half-width.The observed difference would be probably due to that the resultant polymer possessed high stereo-regularity (all trans olefinic double bond) without any defects.Therefore, it is clear that PFVs prepared by the ADMET polymerization are defect-free polymers with high stereo-regularity as well as with better optical property compared with samples prepared by the other methods.
A facile, exclusive end-functionalization of PFV prepared by ADMET polymerization using Ru(B) has been achieved by treating the vinyl groups in PFV chain ends with Mo cat.followed by Wittigtype cleavage with 4-Me3SiOC6H4CHO; precise synthesis of ABA type amphiphilic triblock copolymers could be accomplished by grafting PEG into both the PFV chain ends (Scheme 5) [25].
The SiMe3 group in the resultant polymers (PFV-OTMS) was easily cleaved by treating with HCl aq. to afford PFV-OH in high yields.No significant changes in the Mn values were seen before/after the procedure.The OH groups in the PFV chain ends were then treated with KH in THF, and the subsequent reaction with mesylated poly(ethylene glycol) (PEGMs 2 ) gave ABA-type amphiphilic triblock copolymers in rather high yields (69%-90%).The resultant copolymers possessed uniform molecular weight distributions; the copolymers were identified by 1 H (Figure 3) and 13 C NMR spectra and confirmed that no residual PEG was seen in GPC traces for the isolated polymer(s).The Mn values in the resultant triblock copolymers estimated based on the integration ratios with methylene protons of the PEG segment (Figure 3b) were very close to those estimated by both GPC [Mn(calc.)= Mn(GPC)/1.6]and the value in the starting PEG [25].Scheme 5. Synthesis of amphiphilic triblock copolymers [25].Various block (graft) copolymers have been prepared by combination of the ADMET polymerization of 9,9-dialkyl-2,7-divinyl-fluorene with Cu-catalyzed atom transfer radical polymerization (ATRP) of styrene using macroinitiators prepared by an introduction of initiating functionalities into PFVs chain ends (Scheme 6) [27].The Mn value in the macroinitoator, PFV(C6H4OCOCMe2Br)2, estimated by 1 H NMR spectra, was relatively close to that estimated based on the exact Mn value of PFV (corrected from GPC data), strongly suggesting that both polymer chain ends could be exclusively modified by adopting the present approach.The resultant macroinitiator was added in styrene in the presence of CuBr, dNbipy (4,4-dinonyl-2,2-dipyridyl) at 90 °C for conducting subsequent ATRP.A precise synthesis of the amphiphilic ABCBA-type block copolymers could be then attained by subsequent combination with click reaction after modification of the chain end with NaN3. 1 H NMR spectra for the resultant polymers possessed protons ascribed to PEG units, suggesting incorporation of PEG segment.Note that the Mn values estimated by 1 H NMR spectra (on the basis of methylene protons in the PEG segment) were very close to those calculated (based on Mn value of PFV and integration ratio of PFV and styrene).The formation of regular one-dimensional conjugated structures on the nanoscale should be thus expected by exploiting the specific assembling properties of rod-coil block copolymers, and the precise control of the amphiphilic nature as well as of the block lengths via synthesis shall open the way to fine-tuning the lateral dimensions of these nanostructures.

Precise One-Pot Synthesis of End-Functionalized Conjugated Multi-Block Copolymers via Combined Olefin Metathesis and Wittig-Type Coupling
Emerging applications of conjugated polymers require the patterning of materials on the nm length scale, and block copolymers made of covalently linked polymers represent an ideal route to control the self-assembly of these nano-sized morphologies.Formation of regular one-dimensional conjugated structures on the nano-scale should be thus expected by exploiting the specific assembling properties of rod-coil block copolymers, and a precise control of the block lengths via synthesis shall open the way to fine-tuning the lateral dimensions of these nano-structures.However, the vast majority of a significant number of studies for the generation of nano-scale morphologies from block copolymers have involved one or more of the blocks on the basis of random copolymers or random copolymers with conjugated units attached as side chains.Therefore, more relevant systems would be envisaged based on fully conjugated block copolymers from an application and scientific viewpoints.Scheme 6. Syntheses of macroinitiator, block copolymers by combination of ADMET polymerization with Cu catalyzed atom transfer radical polymerization (ATRP).Synthesis of amphiphilic ABCBA block copolymers, PFV-(PS-bl-PEG) 2 , by combination of ATRP with click coupling [27].
As a more facile "one-pot" procedure for the synthesis (expressed as method B), the vinyl groups in the PFVs' chain ends were treated with Mo cat.(1.8 equiv to PFV) to afford the "bis-alkylidene" species containing "mono-alkylidene" species partially in situ.The mixture was then added a dichloromethane solution containing 0.5 equiv of OPVCHO (3PVCHO, 7PVCHO) or 3T(CHO) 2 .The resultant reaction mixture was added into a toluene solution containing Mo cat (2.5 equiv) to complete the olefin metathesis (with the vinyl group in the PFV chain ends), and the subsequent addition of aldehyde (ArCHO) in excess amount afforded end-functionalized triblock copolymers, expressed as [(PFV) 2 -7PV]Ar 2 , [(PFV) 2 -3PV]Ar 2 or [(PFV) 2 -3T]Ar 2 (Ar = C6H5, C6F5, terthiophene (3T), ferrocene (Fc)), in high yields (70%-88% yields on the basis of PFVs, Table 2), which were isolated simply by pouring the reaction mixture into methanol.The Mn values in [(PFV) 2 -7PV]Ar 2 , [(PFV) 2 -3T]Ar 2 were analogous to the estimated values with uniform molecular weight distributions (Mw/Mn = 1.33-1.63)without significant increases after these modification procedures, and were close to those adopted in the above approach (method A).Moreover, resonances ascribed to protons of the vinyl groups in the starting PFVs were not observed in the 1 H NMR spectra, whereas resonances ascribed to protons in the middle segment, especially 7PV or 3PV, were clearly observed (Figure 4).Moreover, the estimated Mn values on the basis of integration [of the middle segments, expressed as Mn(NMR)] were highly analogous to the calculated values [expressed as Mn(calcd)] on the basis of molar ratios.These results also strongly suggest that exclusive formations of [(PFV) 2 -7PV]Ar 2 , [(PFV) 2 -3PV]Ar 2 , [(PFV) 2 -3T]Ar 2 have been achieved by adopting this methodology.The latter method should be more promising and useful, because the target end-functionalized fully conjugated block copolymers can be easily prepared in one-pot in a precise manner.groups in the PFV chain ends were then treated with KH in THF and the subsequent reaction with mesylated poly(ethylene glycol) [PEGMs2, MsO(CH2CH2O)nMs, Ms = MeSO2: Mn = 4600] gave ABCBA type amphiphilic block copolymers in moderate yields (60, 75%, Scheme 8).The resultant copolymers possessed uniform molecular weight distributions; the copolymers were identified by 1 H NMR spectra and confirmed that no residual PEG was seen in GPC traces for the isolated polymer(s).The Mn values in the resultant block copolymers estimated based on the integration ratios with methylene protons of the PEG segment were very close to the calculated values (Table 3).The facts clearly indicate that facile, efficient attachments of a pseudo phenol terminus on the PFV to PEGMs2, could be achieved in a precise manner by adopting the present "grafting to" approach.Moreover, 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.

Polymers
We have shown that a facile, efficient synthesis of ABCBA type amphiphilic block copolymers has been established in a precise manner (as a rare example) by attachment of PEG into the both chain ends of the all-trans, defect-free, high molecular weight PFVs.Formation of regular one-dimensional conjugated structures on the nanoscale should be thus expected by exploiting the specific assembling properties of rod-coil block copolymers, and the control of the block lengths via synthesis opens the way to fine tuning the lateral dimensions of these nanostructures.We thus believe that the present approach should offer unique, important methodology for precise synthesis of end functionalized block conjugated polymers for targeted device materials as well as synthesis of various block copolymers containing conjugated polymer fragments.Scheme 8. Synthesis of amphiphilic block copolymers (by grafting PEG) [29].

Precise One-Pot Synthesis of Fully Conjugated End-Functionalized Star Polymers
On the basis of the above methodology, a facile, precise one-pot synthesis of end-functionalized star (triarm) polymers consisting of PFVs, the triblock copolymers (by incorporation of tri(2,5-dialkoxy-1,4-phenylene vinylene) or terthiophene units as the middle segment), have been prepared by olefin metathesis followed by Wittig-type coupling.The key step of the synthesis is (i) olefin metathesis of the vinyl group in the PFV chain ends, (ii) treatment with core molecule [tris(4-formylphenyl)amine etc.], and (iii) reaction with various aldehyde (Scheme 9) [30].Scheme 9. Synthesis of end-functionalized triblock conjugated copolymers [30].
As shown in Scheme 9, the vinyl groups in the PFVs' chain ends were treated with Mo cat.(1.8 equiv to PFV), and the mixture was then added a CH2Cl2 solution containing TPA(CHO) 3 .The reaction solution was added a toluene solution containing Mo cat (3 equiv) for completion of the olefin metathesis (with the vinyl group in the PFV chain ends), and the subsequent addition of aldehyde (ArCHO) in excess amount afforded the end-functionalized star (triarm) copolymers, TPA[PFV-Ar] 3 (Ar = C6F5, ferrocene (Fc)).The Mn values in TPA[PFV-Ar] 3 , estimated by GPC, were analogous to the estimated values with unimodal molecular weight distributions.Resonances ascribed to protons of the vinyl groups in the starting PFVs were not observed in the 1 H NMR spectra, whereas resonances ascribed to protons in ferrocene moiety were clearly observed.Moreover, the estimated Mn values on the basis of integration in TPA[PFV-Fc] 3 were highly analogous to the calculated values on the basis of molar ratios, suggesting an exclusive formation of end-functionalized star conjugated polymers, TPA[PFV-Ar] 3 .The method can also be applied for synthesis of the star polymers containing triblock copolymers as the arm segment [30].

Polymersa
M n(GPC) b M n(calcd) c M n(NMR) d M w /M n b Yield e Synthetic conditions, see Scheme 7, PEG(Ms) 2 : M n = 4600; b GPC data in THF vs polystyrene standards; c Calculated on the basis of molar ratio; d Estimated by the integration ratio; e Isolated yield.