Living Metathesis and Metallotropy Polymerization Gives Conjugated

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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Living Metathesis and Metallotropy Polymerization Gives Conjugated Polyenynes from Multialkynes: How to Design Sequence-Specific Cascades for Polymers Cheol Kang,†,∥ Seongyeon Kwon,‡,§,∥ Jong-Chan Sung,† Jinwoo Kim,‡,§ Mu-Hyun Baik,*,‡,§ and Tae-Lim Choi*,† †

Department of Chemistry, Seoul National University, Seoul 08826, Republic of Korea Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea § Center for Catalytic Hydrocarbon Functionalizations, Institute for Basic Science (IBS), Daejeon 34141, Republic of Korea

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‡

S Supporting Information *

ABSTRACT: On the basis of a combined experimental and computational study, a novel method for preparing fully conjugated polyenynes via cascade metathesis and metallotropy (M&M) polymerization of various multialkynes is developed. DFT calculations elucidate the detailed mechanism of the metallotropic 1,3-shift, which is a key process of M&M polymerization. An α,β-(C,C,C)-agostic interaction stabilizing the metallacyclobutadiene transition state is found to be critically important for the successful polymerization with excellent specificity. The polymerization efficiency displayed by the tetrayne monomer is controlled by the steric demands of its substituents, and more complex hexayne monomers can be successfully polymerized to give access to highly conjugated polyenynes via a series of intramolecular metathesis and metallotropic shift cascade reactions. Furthermore, living polymerization led to the synthesis of block copolymers consisting of fully conjugated polyenyne backbones. The implementation of pentayne monomers provides polyenynes with successive C− C triple bonds via consecutive metallotropic 1,3-shift. In short, the design of multialkynes enables the preparation of diverse conjugated polyenyne motifs via selective M&M cascade reactions.

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INTRODUCTION

conjugated polyenyne materials. Furthermore, most synthetic routes to polyenynes rely on topochemical reactions in the solid state, which also limits their versatility in various applications.8 The synthesis of soluble polyenynes was realized more recently employing a step-growth mechanism based on Glaser−Hay,9 Sonogashira coupling,7 or alkyne metathesis reactions,10 while short oligoenynes could be prepared by multistep iterative syntheses.3−6 The preparation of conjugated polyenynes with high molecular weights in a controlled manner remained impossible, however. Very recently, we reported the first example of chain-growth polymerization to access conjugated polyenynes by combining olefin metathesis and metallotropic 1,3-shift reactions based on the organic reaction developed by the Lee group.11,12 With the fast-initiating third-generation Grubbs catalyst (G3),13 tetradeca-1,6,8,13-tetrayne monomers underwent a cascade transformation of ring-closing/metallotropic 1,3-shift/ring-closing reactions (RCM−MS−RCM) in a sequence-specific manner to efficiently generate a conjugated polyenyne backbone with a Z−

Among various conjugated polymers, polyenynes received much attention due to their unique and intriguing optoelectronic properties that make them an excellent platform for developing molecular sensors,1 for instance. Although there have been extensive studies for the synthesis of various polyenynes since their discovery in 1969,2 only a handful of polyenyne motifs including polydiacetylene (PDA),1,3 polytriacetylene (PTA),4 their cross-conjugated isomers (iso-PDA5 and iso-PTA6), and poly(cyclopentadienylene ethynylene) (PCE)7 have been reported (Figure 1). This narrow scope of accessible polymers and the difficulty of the methods by which they are prepared limit our understanding of how different sequences of C−C double bonds and triple bonds affect the properties of the π-

Received: September 22, 2018

Figure 1. Previous examples of conjugated polyenynes. © XXXX American Chemical Society

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DOI: 10.1021/jacs.8b10269 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Journal of the American Chemical Society Scheme 1. Mechanism of Cascade Olefin Metathesis/Metallotropic 1,3-Shift (M&M) Polymerization

Scheme 2. (a) Structure of the Original and Modeled Catalyst. (b) Two Different Mechanisms of Metallotropic 1,3-Shift

Figure 2. Metallotropic 1,3-shift of ruthenium carbene along (a) the bottom-bound pathway and (b) the side-bound pathway. The numbers in the parentheses indicate the relative solution phase Gibbs free energies (units in kcal/mol).

E−Z alkene sequence and one triple bond (Scheme 1). This new polymerization, which we call cascade metathesis and metallotropy (M&M) polymerization, allowed for preparing block copolymers when combined with other living polymerization methods such as ring-opening metathesis polymerization or cyclopolymerization.14,15 Based on the initial success, we envisioned that a careful monomer design may not only improve the efficiency of the M&M polymerization but also enable more complex sequences of cascade polymerization. In order to understand the mechanism in detail and obtain a precise design plan for controlling the M&M polymerization with broader monomer scope, we carried out density functional theory (DFT) calculations. Based on the computational analysis, we developed novel strategies for designing new oligo-alkynes that

will carry out multiple selective and sequence-specific cascade reactions within the general framework of M&M polymerization. Furthermore, living polymerization was possible using these newly designed monomers to produce fully conjugated polyenynes, thereby providing new, convenient, and effective routes to conjugated polyenyne motifs.

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RESULTS AND DISCUSSION To elucidate the mechanism of metallotropic 1,3-shift of Grubbs catalyst in detail, we first examined whether the shift takes place via a concerted or stepwise process. For a model study, we made minor simplifications on third-generation Grubbs catalyst (Scheme 2a). Scheme 2b illustrates two different mechanisms of metallotropic 1,3-shift using the imaginary model interB


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Journal of the American Chemical Society

Figure 3. Molecular orbital correlation diagrams describing (a) σ-interactions and (b) π-interactions between the metal fragment (Ru-catalyst) and substrate fragment (bent diene) in the bottom-bound transition state.

hindrance is expected.17 Therefore, we examined and compared both reaction pathways to determine which of them is preferred. Parts a and b of Figure 2 illustrate the bottom- and side-bound approaches of the metallotropic shift. Both bottom- and sidebound processes require an isomerization from the resting state 1 to the four-membered transition state with surprisingly low barriers of 12.9 and 10.9 kcal/mol, respectively. Along intrinsic reaction coordinate calculations on 2-TS and 2′-TS, we have shown that the reactions follow the structures illustrated on Figure 2 (Figure S2−S5). The side-bound pathway requires a thermodynamically unfavorable isomerization from 1 to 2′, but its overall barrier is 2.0 kcal/mol lower than in the bottombound case, leading to an overall preference toward the sidebound pathway. The alkyne-coordinated intermediate 2′ and 3′ are metastable and the rotation of carbene moiety decreases the orbital interaction between the metal center and carbene. Finally, intermediate 3′ isomerizes into the thermodynamically stable intermediate 3. These results suggest that the side-bound pathway is kinetically preferred. Figure S6 depicts the ORTEP structures of each transition state obtained by DFT calculations, while two transition states have similar structural parameters. The low barriers found for the metallotropic 1,3-shifts are remarkable because significant destabilization of the allene moiety is anticipated in both transition states. To understand the underlying foundation of the unique stabilization, molecular orbitals of the transition state were analyzed. Figure 3 shows

mediate a. If the reaction follows the stepwise pathway, ringclosing forms thermodynamically unstable and labile ruthenacyclobutadiene intermediate b, followed by ring-opening to afford carbene species c, which has a different position of M−C double bond and C−C triple bond relative to the species a. On the other hand, metallotropic 1,3-shift via a concerted carbene exchange will result in the formation of species c from a directly. To clarify the nature of species b, we explored an electronic energy surface with respect to the distances between metal center and bond forming/breaking carbons (Figure S1). Interestingly, we found that the geometry of species b corresponds to a saddle point along the reaction coordinate, where both Ru−C bond lengths are about 2.0 Å, which suggests that the reaction proceeds in a concerted manner traversing a four-membered metallacycle transition state. In general, the Grubbs catalyst can adopt two distinct geometries for the alkyne substrate insertion step, which have been labeled the bottom-bound and side-bound pathways.16 Likewise, the metallotropic shift may occur following two different trajectories, where ruthenacyclobutadiene is trans or cis to the N-heterocyclic carbene (NHC) ligand throughout the reaction. The bottom-bound pathway refers to the trans configuration where the NHC ligand may impose a trans effect toward the ruthenacyclobutadiene, whereas the side-bound pathway invokes the cis geometry in which severe steric C


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Table 1. Improved Controlled M&M Polymerization of a Tetrayne Monomer Containing Triisopropylsilyl Ether Groups

entry

monomer

M/I/add

time (h)

conva (%)

yieldb (%)

Mnc (kDa)

Đc

1 2 3 4 5

M2 M2 M2 M2 M2

15/1/2 30/1/4 45/1/6 60/1/8 75/1/10

1 1.7 2.5 4 5

98 >99 96 97 91

94 97 95 97 83

9.8 17.9 28.9 34.9 42.8

1.19 1.18 1.23 1.32 1.42

a

Calculated from crude 1H NMR. bIsolated yield. cDetermined by chloroform size-exclusion chromatography (SEC) calibrated using polystyrene standards.

Figure 4. Plots of Mn vs M/I and corresponding Đ values for (a) P2 and (b) P5.

prepared. Interestingly, the controlled polymerization proceeded well by using G3 and 3,5-dichloropyridine (3,5-Cl2Py) additive in chloroform to give a linear increase in Mn with increasing monomer to initiator ratio (M/I) from 15 (9.8 kDa) to 75 (42.8 kDa, 137 cyclopentene units in one chain) with excellent conversions and yields as well as narrow Đ values with M/I = 15 (1.19), 30 (1.18), 45 (1.23), and 60 (1.32) (Table 1, Figure 4a, and Figure S9a). Presumably, better living polymerization of M2 was due to the bulky TIPS group suppressing the decomposition of the propagating carbene, which was supported by in situ NMR analysis (Figure S10) that showed a higher amount of living propagating carbene (M1: 68−82% vs M2: 78−90%) during polymerization. In addition, slower propagation (M1: kobs = 0.11 min−1 vs M2: kobs = 0.054 min−1) would also improve the ratio of initiation rate over propagation rate thereby the control of the polymerization. Furthermore, the polymerization followed first-order kinetics with respect to the monomer concentration indicating that all the intramolecular reaction steps, especially the metallotropic 1,3-shift, are faster than the intermolecular propagation reaction. Figure 5 illustrates the formation of monomer product from A, while A is the product of ring-closing metathesis of dialkyne substrate (box in Scheme 1 and Figure S8).19 At this stage, A isomerizes into B or C by rotating the alkyne moiety adjacent to the ruthenium carbene to enable a facile metallotropic 1,3-shift. The transition states B-TS and C-TS are associated with the aforementioned bottom- and side-bound pathways, respectively. Although higher levels of steric clashes are expected in the sidebound geometry, the equatorial orientation of the substituents

quantitative molecular orbital correlation diagrams of the model catalyst and diene substrate that have been simplified significantly to only show the most salient features of the MO diagram, and σ- and π-orbitals are shown separately for clarity. There are two distinct interactions involving σ-orbitals in the diene substrate, responsible for the stability of the transition state: strong σ-interactions in the four-membered ring (MO-50 and MO-51) and α,β-(C,C,C)-agostic interaction between Rudyz and out-phase combination of σ-bonds between three carbons (MO-40), as showcased previously in a related study.18 The π-orbitals of the diene substrate also engage in strong πinteractions with the Ru-dxy and Ru-dzx orbitals and support the planar four-membered ring structure. Finally, the π-orbitals 8 and 11 of the diene substrate interact with the metal-based orbitals leading to a notable reorganization of MOs to give the MO-45 and MO-61. These MO interactions compensate for the destabilization of the transition state originating from the substrate distortion and give a barrier that is lower than one may have expected, showing much higher Mayer bond order between metal center and the β-carbon of the diene substrate than previously reported (Table S1).18 Previously, we achieved controlled polymerization of a tetradeca-1,6,8,13-tetrayne monomer having a sterically bulky di-tert-butyl malonate moiety (M1).11 However, the control was limited to DP = 50 with dispersity (Đ) of 1.39 because of competing chain termination by the decomposition of propagating carbene.14 In order to further enhance polymerization efficiency, a new monomer (M2) having even bulkier substituents, triisopropylsilyl (TIPS) ether groups, was D


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envisioned to undergo three RCM and two MS reactions sequentially to generate conjugated polymers with four double bonds, two triple bonds, and three cyclopentene moieties in each repeat unit (Scheme 3). Accordingly, four new monomers containing various bulky substituents were prepared, and their M&M polymerizations were tested using G3 catalyst with 3,5-Cl2Py additive (Table 2, M3−M6). First, a monomer M3 having a di-tert-butylmalonate moiety was polymerized in DCM, reaching only 68% conversion with M/I = 25 (entry 1). A slightly higher temperature (35 °C) enhanced polymerization of M3, giving 96% and 88% conversions with better control for M/I = 10 (Mn = 9.6 kDa, Đ = 1.20) and M/I = 25 (Mn = 19.9 kDa, Đ = 1.35), respectively (entries 2 and 3). Another hexayne monomer M4 containing pivaloyl groups was also polymerized successfully, giving >99% and 90% conversions with M/I = 10 (Mn = 9.9 kDa, Đ = 1.23) and M/I = 25 (Mn = 24.1 kDa, Đ = 1.38) at room temperature (entries 4 and 5). Similar to the case for the analogous tetrayne monomer, M2, a hexayne monomer M5 having a bulky TIPS ether group, showed the best result in chloroform with excellent conversions, yields, and control to give linear increase in Mn up to 30 kDa from M/I = 10 to 35 (up to 105 cyclopentene units in one chain corresponding to 105 RCM and 70 MS reactions) with narrow Đ values below 1.3 (entries 6−9, Figure 4b and Figure S9d). Furthermore, the good living character of M5 was indicated by in situ NMR analysis, which showed a high ratio of the propagating carbene (ca. 70%) during polymerization. Also, a clear first-order kinetic relationship with the monomer concentration suggested that even with the long series of cascade reactions involving five independent steps the intermolecular propagation reaction remains rate-limiting (Figure S11). Finally, from a monomer M6 containing two different substituents, di-tert-butylmalonate (X) and TIPS ether groups (Y), P6 having substituents with the specific XYX sequence in a repeat unit was successfully obtained with excellent conversion and high Mn of 30.8 kDa (entry 10). 1H and 13 C NMR analyses confirmed that only one well-defined microstructure was produced, suggesting that the cascade sequence of RCM−MS−RCM−MS−RCM proceeded selectively and exclusively. Using tetrayne and hexayne monomers that showed good controlled M&M polymerizations, we synthesized block copolymers having fully conjugated enyne chains in both blocks. First, M1 was polymerized under the optimized condition11 to prepare the first block (M/I = 15, Mn = 10.6 kDa, Đ = 1.15), followed by addition of 15 equiv of M2. This resulted in the clear shift of SEC trace corresponding to P1-b-P2 (Mn = 19.5 kDa, Đ = 1.31), confirming the successful block copolymerization from two tetraynes (Figure 6a, c). Furthermore, another block copolymer was constructed by polymerizing a tetrayne M2 as

Figure 5. Second ring-closing metathesis of dialkyne using thirdgeneration Grubbs catalyst (black: side-bound shift, favored; red: bottom-bound shift, disfavored) (R = COOEt).

significantly reduces the steric hindrance and lowers the energy. More importantly, the trans influence between NHC ligand and ruthenacyclobutadiene moiety in the bottom-bound intermediates weakens metal−allene interactions, thus reversing the trend from steric hindrance to result in a more favorable side-bound shift. The stronger α,β-(C,C,C)-agostic interaction in C-TS compared to B-TS allows for the barrier to be much lower in side-bound shift case and this agrees with the fact that this shift is not involved in the rate-determining step. After the carbene shifts through the internal alkyne, the intermediate D isomerizes to give the intermediate E by coordinating the remaining alkyne moiety through the metal center. Finally, ring-closing metathesis on E produces the final product F through E-TS, which is thermodynamically stable and thus makes the reaction irreversible. This mechanism is in good agreement with the experimental observation that the propagation is the ratedetermining step, specifically, the insertion of the monomer be most difficult with an energy barrier of 22.6 kcal/mol (Figure S8). To broaden the scope of cascade M&M polymerization, we attempted increasing the number of the cascade sequences. Based on the computational and experimental conclusion that RCM and MS steps are relatively fast, new complex monomers were designed to contain a total of six alkynes including two internal diynes, which are crucial for the stability.11 They were

Scheme 3. Mechanism of Cascade M&M Polymerization of Henicosa-1,6,8,13,15,20-hexayne Derivatives undergoing Three RCMs and Two Metallotropic 1,3-Shifts

E


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Journal of the American Chemical Society Table 2. Cascade M&M Polymerization of Various Hexayne Monomers

entry

monomer

M/I/Add

solvent

conc (M)

temp (°C)

time (h)

conv (%)a

yield (%)b

Mnc (kDa)

Đc

1 2 3 4 5 6 7 8 9 10

M3 M3 M3 M4 M4 M5 M5 M5 M5 M6

25/1/5 10/1/2 25/1/5 10/1/4 25/1/10 10/1/2 15/1/3 25/1/5 35/1/7 25/1/5

DCM DCM DCM DCM DCM CHCl3 CHCl3 CHCl3 CHCl3 DCM

0.1 0.3 0.3 0.1 0.1 0.1 0.1 0.1 0.1 0.15

rt 35 35 rt rt rt rt rt rt rt

1 0.33 0.67 1 2 1.5 2 4 6 1

68 96 88 >99 90 92 96 94 >99 >99

66 95 82 91 90 88 91 86 87 92

16.7 9.6 19.9 9.9 24.1 8.8 12.7 21.8 30.3 30.8

1.48 1.20 1.35 1.23 1.38 1.12 1.22 1.19 1.29 1.59

a


Figure 6. Syntheses of block copolymers via sequential M&M polymerization of (a) M1 and M2, (b) M2 and M5, and (c, d) their corresponding SEC traces.

F


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Scheme 4. Mechanism of Cascade M&M Polymerization of Hexadeca-1,6,8,10,15-pentayne Moiety undergoing Two RCMs and Two MS Sequences

Table 3. Cascade M&M Polymerization of Various Pentayne Monomers

entry

monomer

time (h)

conva (%)

yieldb (%)

Mnc (kDa)

Đc

1 2 3 4 5

M7 M8 M9 M10 M11

1 1 1 2 1

59 59 43 43 53

50 53 43 43 49

10.5 17.4 11.9 14.4 12.0

2.32 1.57 1.39 1.61 2.05

a


Figure 7. Various conjugated polyene and polyenynes synthesized by G3 and their UV−vis spectra in chloroform solution.

the first block (M/I = 15, Mn = 9.8 kDa, Đ = 1.19) followed by the polymerization of a hexayne monomer M5 (15 equiv) to cause complete shift of SEC trace, thereby making P2-b-P5 with Mn of 22.6 kDa and Đ of 1.26 (Figure 6b,d). This is the first example of a block copolymers synthesis consisting of fully conjugated polyenynes only, highlighting the uniqueness and versatility of this chain-growth M&M polymerization. A second class of new monomers was designed by introducing an internally conjugated 1,3,5-triyne functionality so that two consecutive metallotropic 1,3-shift may occur and form conjugated diyne structures.15,20 We expected that hexadeca1,6,8,10,15-pentayne derivatives would undergo the cascade

M&M polymerization via the RCM−MS−MS−RCM sequence, as illustrated in Scheme 4, to give an unprecedented polyendiyne structure containing three conjugated alkenes (Z−E−Z) and one conjugated diyne in each repeat unit. Using the similar optimized polymerization condition, M7 containing a dimethylmalonate moiety was polymerized to 59% conversion by using G3 (M/I = 25) and 3,5-Cl2Py in DCM (Table 3, entry 1, Mn = 10.5 kDa, Đ = 2.32). We switched to monomers having bulkier ester groups such as di-tert-butylmalonate (M8) and pivaloyl group (M9), hoping that the conversion would improve. But under the same conditions, the conversions were still 59% and 43% to give P8 and P9 with Mn of 17.4 kDa and G


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1,6,8,13,15,20-hexayne monomers underwent successful cascade living polymerization via five independent sequences of the intramolecular transformations to give new conjugated polyenynes consisting of four double bonds and two triple bonds in the repeat unit. Mechanistic studies using in situ NMR analyses revealed that tetrayne and hexayne monomers followed firstorder kinetics, indicating that metallotropic 1,3-shift is a fast transformation, and it was further supported by mechanistic DFT calculations. From tetrayne and hexayne monomers, block copolymers having fully conjugated polyenyne motifs in both blocks were successfully synthesized for the first time, highlighting the utility of M&M polymerization. Furthermore, other unique conjugated polyenynes containing conjugated diynes were prepared from hexadeca-1,6,8,10,15-pentayne monomers, undergoing two consecutive migratory shifts in a row. In brief, genuine designs of multiyne monomers allowed easy and efficient syntheses of conjugated polyenynes with unique backbone motifs.

11.9 kDa, respectively (entries 2 and 3). Furthermore, other monomers containing an ether such as TIPS (M10) and acetal (M11) were polymerized to give similar conversions (43% and 53%) with Mn of 14.4 and 12.0 kDa, respectively (entries 4 and 5). Unlike the previous M&M polymerizations, sterically bulky substituents did not enhance the polymerization efficiency of pentayne monomers, likely because the distance between Rucarbene in dialkynyl carbene intermediate and the side chain (Scheme 4) is too far to stabilize the propagating carbene effectively (Figure S12). Despite relatively low turnover numbers and loss of control, Mns of P7−P11 were fairly high from 11 to 17 kDa performing up to the average of 60 selective and specific transformations without any side reactions or defects, which was confirmed by various characterizations such as 1H and 13C NMR, MALDI (Figure S13), and IR (Figure S14) analyses. In short, from various tetrayne, pentayne, and hexayne monomers, 10 different conjugated polyenynes having unique sequences of double and triple bonds were successfully synthesized with perfect cascade sequences. After preparing a library of diverse conjugated polyenynes, we measured their optical and electronic properties (Table S4). As shown in Figure 7, compared to the analogous conjugated polyene (P12) prepared by cyclopolymerization14a,21,22 which showed absorption maxima (λmax) at 552 and 594 nm, polyenynes presented here having different portions of triple bond (P2: 25%, P5: 33%, P10: 40%) showed much more blueshifted UV−vis spectra without any 0−0 vibronic peak. In particular, P2 containing fewer triple bonds (25%) exhibited a higher λmax value (470 nm) compared to those of P5 (33%, λmax = 462 nm), P10 (40%, λmax = 457 nm) and conventional soluble polydiacetylenes (50%, λabs = 450−460 nm).23 This implies that higher ratio of triple bond in the backbone tends to lower the conjugation length due to higher rotational freedom of single bonds adjacent to triple bonds.23−25 Accordingly, the optical band gaps of P2, P5, and P10 (2.2−2.3 eV) were significantly higher than that of P12 (2.0 eV).21b In addition, most polymers showed similar UV−vis spectra in both solution and film, but interestingly, TIPS ether containing polymers P2 and P5 exhibited strong 0−0 vibronic peaks in the film state (Figure S16) presumably because the sterically bulky side chains would extend the backbone more rigidly. Furthermore, all the polymers P2−P11 showed weak emission at 522−546 nm (Figure S17, φPL < 0.2%), analogous to soluble yellow polydiacetylenes.26 Finally, the highest occupied molecular orbital levels of P2−P11 measured by cyclic voltammetry (Figure S18, −5.66 to −5.42 eV) were deeper than that of P12 (−5.12 eV), implying that conjugated polyenynes would be more robust to aerobic oxidation.

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ASSOCIATED CONTENT

S Supporting Information *

Computed Cartesian coordinates, vibrational frequencies, and energy components of all of the DFT-optimized structures (XYZ). Experimental procedures, characterizations, NMR spectra for new compounds and polymers, kinetic data, and other supporting experiments. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b10269. Experimental procedures, characterizations, NMR spectra for new compounds and polymers, kinetic data, and other supporting experiments (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Mu-Hyun Baik: 0000-0002-8832-8187 Tae-Lim Choi: 0000-0001-9521-6450 Author Contributions ∥

C.K.and S.K. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This paper is dedicated to Professor Mitsuo Sawamoto on his retirement from Kyoto University and for his tremendous achievements in polymer science and education. T.-L.C. acknowledges financial support from the Creative Research Initiative Grant and the Creative Material Discovery Program through NRF, Korea. M.-H.B. acknowledges support from the Institute for Basic Science (IBS-R10-D1) in Korea. C.K. and J.C.S. are supported by the NRF-Global PhD Fellowship program (Grant Nos. NRF2015H1A2A1030158 and NRF2018H1A2A1062399).

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CONCLUSION In conclusion, novel designs of various new monomers allowed for preparing complex and well-defined conjugated polyenynes via highly sequential cascade M&M polymerization. DFT calculations suggested a detailed mechanism and showed that the metallotropic 1,3-shift is energetically viable, constituting a key process of the M&M polymerization methodology. Molecular orbital analysis suggests that the α,β-(C,C,C)-agostic interaction lowers the energy of the transition state containing the ruthenacyclobutadiene motif. Compared to previous reports, the living polymerization efficiency of the tetradeca1,6,8,13-tetrayne monomer could be enhanced by increasing the steric bulk of the side chains to give controlled Mn up to DP = 75 and narrow Đ. More exotic substrates such as henicosa-

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REFERENCES

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Living Metathesis and Metallotropy Polymerization Gives Conjugated

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