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Mechanistic Investigations on the Competition between the Cyclopolymerization and [2 + 2 + 2] Cycloaddition of 1,6-Heptadiyne Derivatives Using Second-Generation Grubbs Catalysts Eun-Hye Kang, Cheol Kang, Sanghee Yang, Elina Oks, and Tae-Lim Choi* Department of Chemistry, Seoul National University, Seoul 08826, Korea S Supporting Information *

ABSTRACT: Until recently, the cyclopolymerization (CP) of 1,6heptadiyne derivatives using Grubbs catalysts had been unsuccessful, leading to the misbelief that these catalysts were inactive in these circumstances. However, a recent breakthrough has changed this previous perspective of CP, where a successful living CP was reported using a third-generation Grubbs catalyst with the aid of weakly coordinating ligands. Although it became clear that weakly coordinating ligands greatly enhanced the efficiency of CP by suppressing the decomposition of the propagating carbene, it was still unclear as to what was actually occurring during the previous attempts at CP using ligand-free conditions, especially in the case of the Hoveyda−Grubbs catalyst. Here, we have found that second-generation Grubbs or Hoveyda−Grubbs catalysts in dichloromethane (DCM) formed predominantly side products, i.e., dimers and trimers of 1,6-heptadiyne derivatives, instead of producing the desired conjugated polymers. Further mechanistic studies disclosed that [2 + 2 + 2] cycloaddition reactions by the decomposed Grubbs catalyst were responsible for these side products, not the commonly presumed olefin metathesis pathway. Furthermore, a control experiment revealed that pyridine not only stabilized the propagating carbene but also suppressed the dimer formation by poisoning the newly generated catalytic species that would have promoted [2 + 2 + 2] cycloaddition. This observation explained why the third-generation Grubbs catalyst successfully and selectively cyclopolymerized 1,6-heptadiyne monomers. Another significant observation was that depending on the nature of the substituents of the 1,6-heptadiynes, different ratios of polymers and side-products were obtained as a result of competition between CP and cycloaddition. Monomers containing more coordinating substituents favored the undesired cycloaddition products owing to slower polymerization and faster decomposition of the carbene, while weakly chelating monomers strongly favored CP. Finally, with this new mechanistic understanding of the factors that contribute to CP propagation and decomposition of the Grubbs catalysts, we could maximize the efficiency of CP by modifying the monomer structure, lowering the reaction temperature, or adding stabilizing ligands. This report demonstrates a successful result of how mechanistic investigation has turned a previously unattainable polymerization into an efficient one.

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INTRODUCTION Olefin metathesis polymerizations have revolutionized the field of synthetic polymer chemistry not only because efficient living polymerizations become possible but also because these polymerizations produce various functional materials.1 The polymerization of 1-alkynes2 is one of the unique olefin metathesis polymerization, producing soluble polyacetylene derivatives.3 Similar to olefin metathesis reaction between olefins, the metal alkylidene catalyzes the transformation of alkynes to dienes via metallocyclobutene intermediate and produces conjugated polyenes.2b In particular, the cyclopolymerization (CP) of 1,6-heptadiyne derivatives is a very attractive method of converting alkynes into conjugated polyenes.4 However, compared to ring-opening metathesis polymerization (ROMP) and acyclic diene metathesis polymerization (ADMET), CP has been much less investigated because of the lack of general and user-friendly catalysts that are able to promote this living polymerization efficiently. Decades ago, early investigations into CP were carried out using the Ziegler− Natta catalyst,5 MoCl5, and WCl6,6 but with little under© 2016 American Chemical Society

standings of the reaction mechanism. Using Schrock alkylidenes, the first well-defined olefin metathesis catalysts, the Schrock group reported the first breakthrough in the understanding of the mechanism by introducing the concept of α- and β-addition to explain the issues of selectivity of the ring structures (Scheme 1) and also achieved the first living CP.7 Later, to solve the regioselectivity issue, the Schrock and Buchmeiser groups modified Mo−alkylidenes to achieve β-8 and α-selective9 CP, respectively. The second breakthrough came when the Buchmeiser group developed functional groupand air-tolerant Ru-based alkylidenes by modifying Grubbs catalysts with electron-withdrawing groups such as trifluoroacetate or isocyanate, and they showed that they could achieve efficient CP via selective α-addition with a broadened monomer scope.10 However, initial attempts to use commercially available user-friendly Ru−alkylidenes, such as second-generation Received: May 25, 2016 Revised: July 21, 2016 Published: August 18, 2016 6240

DOI: 10.1021/acs.macromol.6b01110 Macromolecules 2016, 49, 6240−6250

Article

Macromolecules Scheme 1. Mechanism of Cyclopolymerization of 1,6Heptadiyne and Structures of the Resulting Repeat Units

Scheme 2. Cyclopolymerization Using Ru−Alkylidene and the Formation of Dimer and Trimer

nistic study revealed which factors influenced the competition between CP and cycloaddition and how to understand the structure−reactivity relationship of this reaction in order to fully maximize the efficiency of CP. With these conclusions, we were able to successfully cyclopolymerize 1,6-heptadiynes using HG2 and suggest a general guideline for successful CP using Grubbs catalysts.

Grubbs (G2) or Hoveyda−Grubbs catalysts (HG2) (Figure 1), did not facilitate a CP, and this led to the misunderstanding

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RESULTS AND DISCUSSION Mechanistic Investigation on Dimer Generation. We initially screened various CP conditions to examine the events that occur during a typical polymerization of the most commonly employed monomers, dipropargyl malonate esters, 1a and 1b, using various Grubbs catalysts and compared their efficiencies (Table 1). Tetrahydrofuran (THF) and dichloromethane (DCM) were selected as coordinating and noncoordinating solvents, respectively, and three different Grubbs catalysts (Figure 1) were tested. Reactions using G2 or HG2 in DCM afforded only dimer (3) or trimer (4) at room temperature (Table 1, entries 1, 2, and 4), respectively, while reactions using the pyridine-containing G3 in DCM (Table 1, entry 3) or HG2 in a coordinating solvent, THF (Table 1, entry 5), instead underwent predominantly CP to form polymer 2 and did not produce 3 or 4. However, repeating the same reaction using HG2 in THF at an elevated temperature (50 °C) resulted in formation of 3 and decrease in CP (Table 1, entry 6). Alternatively, instead of using G3 (entry 3), adding pyridine ligands such as 3-chloropyridine (3ClPy) and 3,5-dichloropyridine (3,5-Cl2Py) to ligand-free HG2 also selectively promoted CP with higher conversion to form 2 (Table 1, entries 7 and 8). Adding a benzoquinone derivative as a weaker ligand, however, increased the CP conversion only slightly while maintaining the high yields of 3 and 4 (Table 1, entry 4 vs 9). These di- and trimerizations of 1,6-heptadiynes were previously reported by the Buchmeiser group when modified Grubbs catalysts containing trifluoroacetate or isocyanate were reacted with 1,6-heptadiyne derivatives containing nitrogens and ethers (Scheme 2).10c,14e With regards to the mechanism of dimer formation, they proposed an intramolecular backbiting reaction via an olefin metathesis mechanism (Cycle I in Scheme 3). Because of this side reaction, lower formation of the polymer was inevitable. Initially, we assumed that the mechanism of dimerization by Grubbs catalysts (Table 1, entries 1 and 2) would be very similar to that proposed by the Buchmeiser group, even though the catalysts employed were different. The Buchmeiser group reported that the coordination of heteroatoms on the monomers and the presence of 2-PrOstyrene ligand promoted the occurrence of the undesirable backbiting side reaction.10c,14e The group further explained that the pyridine impeded this backbiting reaction because it competed with the alkenes of the polymer backbone for

Figure 1. Chemical structures of common Grubbs catalysts.

that Grubbs catalysts were not active enough for CP.10a,11 The third breakthrough came when we recently reported a general living CP via selective α-addition using the rapidly initiating and readily available third-generation Grubbs catalyst (G3, Figure 1) in weakly coordinating solvents.12 After thorough mechanistic studies, we reported that weakly coordinating ligands such as THF and pyridine significantly suppressed the decomposition of the active propagating metal−carbene species during the polymerization, thereby promoting efficient living CP.13 Therefore, we concluded that the previously assumed reasons for unsuccessful CP by Grubbs catalysts was due to the low stability of the active species in the absence of ligands, not their intrinsically low activity. With these recent findings and breakthroughs, several applications of CP such as well-defined polymerizations of various 1,6-heptadiyne14 and 1,7-octadiyne derivatives15 and the preparation of optoelectronic materials5b,10e,16 have been conducted. After learning a valuable lesson regarding the crucial ligand effect in catalysis, we hypothesized as to why the previous attempts at CP using G2 and HG2 were unsuccessful. Interestingly, there were recent reports of successful CPs to produce conjugated brush polymers via a macromonomer approach using HG2 in THF14d and ionomers in the mixed solvent system of THF and ionic liquid using G2.16g These results brought to question, however, as to why CP had been impossible using highly active and stable G2 or HG2, the most widely used catalysts. In fact, our previous studies suggested that the intrinsic activities of these catalysts should also be very high for CP.12,13 Herein, from numerous mechanistic investigations, we rationalize the previous failures of CP by addressing the possible competing reaction pathways of 1,6heptadiyne derivatives that produce not only the desired conjugated polymers but also their dimers and even trimers as major side products when a ligand-free catalyst (HG2) is used under various reaction conditions (Scheme 2). Also, we conclude that this major side reaction, whose mechanism had been in dispute, is the [2 + 2 + 2] cycloaddition of alkynes catalyzed by decomposed Ru−alkylidene. Lastly, this mecha6241


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Macromolecules Table 1. Cyclopolymerizations of 1a and 1b under Various Reaction Conditions

a

entry

cat.

monomer

solvent

1 2 3 4 5 6c 7 8 9

HG2 G2 G3 HG2 HG2 HG2 HG2 HG2 HG2

1a 1a 1a 1b 1b 1b 1b 1b 1b

DCM DCM DCM DCM THF THF DCM DCM DCM

additivea

2b (%)

3b (%)

4b (%)

conv (%)

3-ClPy 3,5-Cl2Py 2,6-Cl2BQ

2 3 55 5 93 49 73 94 13

47 24 trace 47 0 27 0 0 52

6 trace 0 9 0 0 0 0 17

60 33 65 81 99 93 75 97 88

10 mol % of monomer was added. bCalculated from 1H NMR spectroscopy. cThe reaction was performed at 50 °C.

Scheme 3. Two Plausible Aromatization Mechanisms Based on the Olefin Metathesis Reaction (Cycle I) and Ru-Catalyzed [2 + 2 + 2] Cycloaddition Mechanisms (Cycle II)

olefin (blue) of the backbone to generate the aromatic dimer (F). However, the issue that arises with this proposed mechanism is that E requires cis-dienes for successful cyclizations to produce the aromatic compound F, and therefore Buchmeiser proposed a trans-to-cis isomerization to explain the production of F.10c However, our previous study, as well as reports from the Buchmeiser group, showed that CP mediated by Ru catalysts predominantly forms trans-vinylene, and even a small amount of cis-vinylene spontaneously isomerizes into thermodynamically more stable trans-vinylene.19 Hence, it is highly unlikely that the proposed nonspontaneous trans-to-cis isomerization would occur rapidly, if at all, and efficiently promote the dimer formation. Instead, moving forward with this work, we turned to an alternative pathway: a well-investigated transition metalcatalyzed [2 + 2 + 2] cycloaddition reaction that produces 3 and 4.20−22 The mechanism of this Ru-catalyzed cycloaddition reaction involves a ruthenacyclopentatriene intermediate (G)

coordination to the Ru metal. As a result, they proposed a scheme, Cycle I, to describe this backbiting mechanism and coordination models via an olefin metathesis pathway (Scheme 3).10c It was a similar mechanism to the cascade olefin metathesis reaction, which the Blechert group reported as the first intramolecular trimerization or cyclization of triynes using first-generation Grubbs catalyst (G1).17 Furthermore, Witulski and co-workers adopted this olefin metathesis pathway to explain their observations with regioselectivity during the cycloaddition of diynes and terminal alkynes.18 According to the model proposed by Buchmeiser, after the Ru−alkylidene (A) reacted with a 1,6-heptadiyne derivative (B) by α-addition to produce C, it underwent rapid cyclization to form the initial propagating carbene (D). Then, by reacting with another molecule of B, D formed the second intermediate E, which, in theory, should have, upon cyclization with the alkyne, undergone polymerization to form poly(B). Instead, E underwent a backbiting side reaction by cyclization with the 6242


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Figure 2. Crossover experiment between poly(1d) and 1a.

Figure 3. Cyclopolymerization and cycloaddition reactions of 1a′, a 3,3-dimethyl-substituted analogue of 1a.

(Scheme 3, Cycle II).21a,23 In fact, the Pérez-Castells group proposed this mechanism as a result of the decomposition of Grubbs catalysts under harsh reaction conditions (>60 °C)20 for the cycloaddition reaction using G2 and HG2. However, there remains some ambiguities in the [2 + 2 + 2] cycloaddition mechanism because they also observed another olefin metathesis reaction that still occurs at room temperature.20d Despite these observations and hypothesis, the mechanism for dimer production of 1,6-heptadiyne derivatives during CP is still unclear. Although Cycle II seemed to be a more plausible mechanism than Cycle I that required unfavorable trans-to-cis isomer-

ization, more systematic investigations and evidence into the formation of dimers and trimers over polymers were necessary. Therefore, we designed the following control experiments to support the correct mechanism (Figure 2). First, we conducted a crossover experiment to check if Cycle I was plausible. To a purified poly(1d), HG2 was added to generate the propagating carbene (poly(1d)-D) by the reaction between the catalyst and the conjugated olefin of the polymer. When 1a was added to this reaction mixture, we obtained not only poly(1d)x-copoly(1a)y (x:y ∼ 0.84:0.16), via the formation of poly(1d)-E, but also the dimer and trimer of 1a (3a and 4a). According to Cycle I, the intermediate poly(1d)-E should produce 6243


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Macromolecules heterocycloaddition product (3da) via backbiting mechanism ((i), Figure 2). However, we could not detect 3da at all, but only obtained 3a and 4a, which were the product between the decomposed Ru−carbene and 1a. This suggested that Cycle II was the more probable mechanism. As a counter experiment, the reaction between poly(1a) and 1d was repeated, and it also produced only the homodimer of 1d and no crossover products at all, suggesting that the proposed Cycle I is not the mechanism at play in this reaction (Figure S3). To provide support for the occurrence of Cycle II over Cycle I, we attempted the same dimerization experiments using an asymmetric monomer, 3,3-dimethyl-substituted diyne (1a′, Figure 3). If the steric effects were negligible, 1a′ would produce all four possible isomers (3a′, i−iv, Figure 3). However, the dimethyl group on 1a′ is so bulky that the products of this catalysis would be biased by steric hindrance. For example, if Cycle I occurred, the dimerization by olefin metathesis would produce 3a′-i as the sole product because the propagating carbene would only react with the sterically less hindered alkyne (H in Figure 3), thereby producing a highly regular head-to-tail microstructure, 2a′, via CP and 3a′-i via dimerization.13 In contrast, Cycle II would produce two isomers, 3a′-i and 3a′-ii, because a steric effect on the alkyne containing dimethyl groups would suppress its binding to the metallacyclopentatriene intermediates (I in Figure 3, not forming 3a′-iii and 3a′-iv). Interestingly, the products observed from reaction of 1a′ using HG2 in DCM produced not only the expected regioregular polymer 2a′ (31%) but also a mixture of the isomers of 3a′. From two-dimensional NMR spectroscopic (COSY and NOESY) and GC/MS analyses on the mixture of isomers (Figures S4 and S5), we concluded that these isomers of 3a′-i and 3a′-ii were present in an almost 1:1 ratio (9% and 8%, respectively). Furthermore, the much higher CP efficiency of 1a′ (33%) over 1a (2%, Table 1, entry 1) supported the more favorable Cycle II mechanism. The CP of 1a′ should have been suppressed because the gem-dimethyl at the 3-position of the diyne would have slowed down the intramolecular cyclization, thereby, according to Cycle I, favoring the backbiting process ((i), Figure 3) over the polymerization ((ii), Figure 3).13 The reaction of 1a′, however, produced more polymer than 1a. Again, this higher yield of the polymer from 1a′ also made sense because we reported that the dimethyl group significantly suppressed carbene decomposition. Therefore, when compared to the analogous reaction with 1a (Table 1), the carbene survived longer to continue CP and give a higher yield of 2a′, while dimerization was significantly retarded owing to slower decomposition of the propagating carbene. Previously, we reported that CP of 1,7-octadiyne derivatives was much slower than that of 1,6-heptadiyne derivatives due to greater distances between the two alkyne components, thereby leading to slower cyclization.24 Therefore, if Cycle I was operative, the backbiting reaction should dominate over CP, as shown in Figure 3. However, from the reactions of 5a and 5b, we observed good conversion to the corresponding polymer (>50%) but found almost no dimer (Figure 4). Furthermore, it has been reported that [2 + 2 + 2] cycloadditions of 1,7octadiynes are less efficient than that of 1,6-heptadiynes because the longer tether disfavors the formation of the metallacyclopentatriene intermediate (Figure 4),23b,25 a fact that further supported the occurrence of Cycle II. To support the hypothesis that the decomposed product of Ru−alkylidene catalyzed the cycloaddition to produce the dimers observed, we designed a reaction whereby we took a

Figure 4. Reaction of 1,7-octadiyne derivatives using HG2.

residue from the reaction mixture of HG2 and 1,6-heptadiyne and reused it for the catalysis of the cycloaddition reaction. First, the reaction of 1,6-heptadiyne26 and HG2 produced the oligomer, which could be easily removed after precipitation and filtration (Figure 5). Then, the filtrate solution containing the

Figure 5. Cycloaddition of 1a using the decomposed Grubbs catalyst.

decomposed Ru species, and the dimer product of 1,6heptadiyne was recovered and analyzed by 1H NMR spectroscopy to confirm that the residue contained no remaining carbene. However, we observed that the reaction of this residue and 1a produced the dimer 3a in 16% conversion, hence confirming that the decomposed Ru species promoted the cycloaddition via Cycle II to produce the observed sideproducts. Furthermore, to confirm that the Ru−carbenes completely decomposed, we added norborene to the reaction mixture and found no ROMP product at all (Figure S6). After confirming the correct pathway to be Cycle II, we could explain the observations reported in Table 1. First, using coordinating solvents such as THF or pyridine-type additives in noncoordinating solvents stabilized the carbene, and the efficient polymerization occurred (Table 1, entries 3, 5, 7, and 8)13 while Grubbs catalysts without any stabilizing ligands readily underwent decomposition and produced almost no polymers (Table 1, entries 1, 2, and 4). Furthermore, the addition of 2,6-Cl2BQ, very weakly coordinating or less stabilizing ligands, improved the CP only slightly (Table 1, entry 9). Lastly, the reaction at a higher temperature even in coordinating THF induced more dimerization instead of improving the metathesis activity because of the more facile decomposition of the catalyst (Table 1, entry 6). Role of Pyridine Ligand. After coming to these conclusions, we became curious as to why the dimerization 6244


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intermediate for CP but also for suppressing the major side reaction that occurs, i.e., cycloaddition. Substituent Influence on CP. The Buchmeiser group reported that when using modified Ru catalysts, monomers that contained heteroatoms (nitrogen or oxygen), which could coordinate to the metal, favored the dimerization reaction.10c,14e This observation led us to hypothesize that the structure of the monomer and its coordination to a metal would also affect both the decomposition of the Ru species and the cycloaddition reaction. Therefore, we screened several 1,6heptadiyne derivatives containing different substituents to monitor the structure−reactivity relationship upon altering the electronic and steric natures of the substituents (Figure 7, 1a−1h). Similar to entry 1 in Table 1, reactions of HG2 and these monomers in DCM were investigated with an extended reaction time of 18 h (Table 2).

was not observed for CP catalyzed by G3. The CP of 1a using G3 in DCM was more efficient than when G2 or HG2 was used because G3 already contains stabilizing pyridine ligands. In the absence of additional ligands or coordinating solvents, however, we did observe significant decomposition of the propagating carbene when G3 was employed, which resulted in very low conversion to the products (17% for 1 mol % catalyst loading, Figure 6a, or 55% for 2 mol %, Table 1, entry 3).13

Figure 7. Various 1,6-heptadiyne derivatives screened by reaction with HG2.

Table 2. Screening Monomers for CP Using HG2

Figure 6. (a) Inefficient cycloaddition in the reaction between G3 and 1a. Conversion and the composition of products were monitored by 1 H NMR spectroscopy. (b) Monitoring reaction profiles before and after the addition of pyridine in the middle of the reaction.

Interestingly, even with longer reaction times over 7 h, the decomposed Ru species, which should promote the cycloaddition reaction, produced only negligible amounts (only 2%) of the dimer from the remaining 1a. In contrast, the same reactions using HG2 in THF at high temperatures (Table 1, entry 6) or with the addition of 2,6-Cl2BQ in DCM at room temperature (Table 1, entry 9) produced large amounts of the cycloaddition products, suggesting that the pyridine played a major role in the cycloaddition reaction. To confirm this, we monitored the reaction by 1H NMR spectroscopy of 1a and HG2 in CD2Cl2 before and after adding 3-chloropyridine (3ClPy) (Figure 6b). As the reaction progressed, 1a was converted to 3a (and a small amount of 4a), showing that cycloaddition was dominant. After 35 min when 89% of HG2 was initiated, 5 equiv of 3-ClPy to the catalyst was added to the NMR tube. At this point, the cycloaddition reaction stopped immediately (Figure 6b), implying that pyridine poisoned or strongly coordinated to the decomposed Ru catalyst that was responsible for the cycloaddition, whereas weakly coordinating THF or 2,6-Cl2BQ did not. In brief, the pyridine ligand was essential not only for stabilizing the propagating carbene

a

entry

monomer

2a (%)

3a (%)

4a (%)

conva (%)

1 2 3 4 5 6 7

1a 1c 1d 1e 1f 1g 1h

99

51 19 44 42 32 0 0

39 0 0 44 22 0 0

>99 33 66 >99 >99 >99 >99

Calculated from 1H NMR spectroscopy.

In general, bis-substituted heptadiynes (1a, 1c, 1d) showed little preference toward CP; instead, they favored cycloaddition reactions (Table 2, entries 1−3). Surprisingly, monosubstituted monomers (1e−1h) resulted in much more efficient CP than bis-substituted monomers (Table 2, entries 4−7). Among them, the strongest coordinating amide produced the smallest amount of polymer 2e (14%), while large amounts of 3e (42%) and 4e (44%) formed (Table 2, entry 4). Alternatively, the CP reaction of 1f, containing a less coordinating ester group, improved to a yield of 37%, and its proportion of cycloaddition 6245


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monomers, the CP of 1f,28 which originally preferred cycloaddition products at room temperature, now produced the high-molecular-weight polymer exclusively with a TON of at least 48, or even up to 95 (five times), at 0 °C (Table 3, entries 3−5). These results suggest that the reaction conditions employed can be optimized to maximize the CP, even with the previously considered as unsuccessful condition, HG2 in DCM. Discussion of Substituent Effects. These results made us curious as to how the electronic and steric properties of the substituents on the monomers influenced the carbene decomposition. Our first hypothesis was the possibility of intermolecular coordination of the monomers to the metal carbene, and this was readily checked by adding 10−250 equiv of diethyl malonate (DEM) to the reaction between HG2 and 1h, the most reactive monomer (Table S1). However, the ratio of the products and the molecular weights of 2h did not decrease, even with increasing amounts of DEM; therefore, we could rule out the poisoning of the catalyst by intermolecular coordination of the carbonyl group. Alternatively, an intramolecular coordination or chelation in the intermediate states (H or H′, Scheme 4) could facilitate the decomposition of the carbenes. For instance, in the case of propagation, a ring-closing reaction of the 1,1-disubstituted Ru−carbene intermediate should occur upon binding to the other alkyne (I or I′). This step would be retarded, however, for the monomers containing carbonyl groups due to the formation of a stable six-membered ring (H or H′). Therefore, the longer the intermediates were trapped as the chelated H or H′, the slower the propagation and the faster catalyst decomposition would occur. The chelation state (H) of the bis-substituted monomers (1a−1d) would be more dominant, retarding the ring closing of I to J (k2), while the monosubstituted monomers would have relatively less chelation, thereby promoting faster propagation (k2′). In addition, the stronger coordination of electron-rich carbonyl groups (1e and 1f) would favor chelation, while an eight-membered ring formed for 1g and sterically bulky ether in 1h would disfavor the chelation, thereby enhancing the CP pathway (k2′). This hypothesis also explains why we could previously achieve successful graft CP by HG2 in THF for the first time, since the macromonomer used resembled that of 1g in Figure 7.14d In brief, the chelation of the carbonyl group versus alkyne coordination determines the competition between the propagation of CP (k2 or k2′) and the carbene decomposition (k3 or k3′) that leads to the cycloaddition sidereactions. This model gives an insight into the correlation between the monomer structure and the carbene decomposition and explains why the cycloaddition pathway was favored over CP in cases of strong chelation such as that

products decreased (32% for 3f; 22% for 4f) (Table 2, entry 5). Remarkably, 1g, containing a distant carbonyl group, and 1h, containing a sterically hindered triisopropyl silyl (TIPS) protecting group, were exclusively converted to high molecular weight polymers with no side products (Table 2; Mn = 14.8K/ PDI = 2.86 for 2g and Mn = 11.0K/PDI = 2.74 for 2h, respectively, for entries 6 and 7). It is notable that these are the first examples of a successful CP catalyzed by HG2 in DCM without any externally stabilizing ligands. Through in-depth investigations of coordinating substituents, we conclude that the cycloaddition reaction is favored with an increase in the strength and number of the coordinating groups present on the monomer. To understand how the coordination affects the efficiency of the CP, we quantified the remaining percentages of the propagating carbenes for each monomer (Figure 8, 1e−1h, and

Figure 8. Comparison of remaining carbene% after the CP of 1e−1h.

Figure S7). The fast-initiating G3 was used instead of HG2 because signals from the propagating 14-electron Ru− alkylidenes from HG2 are undetectable using 1H NMR spectroscopy.27 The remaining carbene in CD2Cl2 after full monomer conversion was the lowest (16%) for 1e and increased up to 52% for 1h according to the efficiency of the CP as shown in Table 2. This observation showed that the functional groups on the monomers indeed affected the decomposition of the propagating carbene, thereby decreasing the efficiency of CP and increasing the preference for cycloaddition. Temperature Effect on CP Efficiency Using HG2. From these results, it became clear that suppressing the carbene decomposition was the most important factor for enhancing the CP and reducing the side-reaction, cycloaddition. To improve the CP efficiency in the ligand-free system, the reaction temperature was lowered to 0 °C to prevent the decomposition of the propagating carbene.13 The turnover number (TON) of 1c for CP increased 9 times from 2.5 at room temperature to 22.5 at the lower temperature to give a polymer with a Mn of 8.6K (Table 3, entries 1 and 2). Among the monosubstituted Table 3. Enhancement of CP under Low Temperature

entry

monomer

M/I

temp

time (h)

2a (%)

3a (%)

4a (%)

TON for CP (2)

c

1c 1c 1f 1f 1f

50 50 50 50 100

RT 0 °C RT 0 °C 0 °C

18 8 18 5 5

99 >99 >99

Calculated from 1H NMR spectroscopy. bDetermined by CHCl3 SEC calibrated using polystyrene (PS) standards. cA result of entry 2 of Table 2. A result of entry 5 of Table 2.

d

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Macromolecules Scheme 4. Proposed Model Showing How Chelation Influenced the Competing Reaction Pathways

reaction and maximize the efficiency of the CP. We expect that this report will provide general guidelines for successful CP using Grubbs catalysts by revealing a deeper and more insightful understanding into the mechanisms of olefin metathesis reactions between diynes and Ru−alkylidenes. Based on this work, the study of cyclopolymerization using the first-generation Grubbs catalyst is ongoing.

possible with 1a. Now, we can explain as to why the previous CP of 1,6-heptadiynes were unsuccessful using HG2 and understand the reactions that actually occurred, especially for the most common monomer employed in these reactions, 1a.

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CONCLUSION In summary, we report detailed investigations for the reactions, both the desired CP and undesired side-products, which occur with 1,6-heptadiyne derivatives catalyzed by Grubbs catalysts. The results of these studies shed a light on why G2 or HG2 could not cyclopolymerize 1,6-heptadiyne derivatives and that the actual products of these attempted reactions were, in fact, dimers and trimers of diynes formed via cycloaddition reactions instead of polymerization. We examined how the structures of monomers, the catalysts used, solvent, temperature, and additives influenced the two competing pathways of CP and cycloaddition to give the different ratios of conjugated polymers and side products. Based on these observations, detailed mechanistic studies disclosed that, in fact, the decomposed Ru− carbene catalyzed the unwanted dimerization of alkynes by [2 + 2 + 2] cycloaddition, rather than the previously proposed olefin metathesis pathway. Therefore, minimizing the carbene decomposition of HG2 using weakly coordinating agents, such as pyridine ligands in DCM, could suppress the cycloaddition, thereby leading to more successful CPs of the various 1,6-heptadiynes. In addition, pyridine is an additive of choice because it suppressed not only the carbene decomposition but also the cycloaddition pathway. Therefore, we could rationalize why CP using G3 containing pyridine ligands did not produce dimers, even in nonstabilizing solvents like DCM. Furthermore, the efficiency of the CP reaction was highly dependent on the substituents of the 1,6-heptadiyne monomers because those with stronger chelation abilities retarded polymerization, which then led to the decomposition of the active carbenes. Although these studies clarified why HG2, in general, was a poor catalyst for the CP of 1,6heptadiynes in noncoordinating solvents, by understanding the detailed mechanism, one could modify the experimental conditions or monomer structures to minimize the side-

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EXPERIMENTAL SECTION

Materials and Characterization. All reactions were carried out under dry argon atmospheres using standard Schlenk-line techniques. All reagents which are commercially available from Sigma-Aldrich, Tokyo Chemical Industry Co. Ltd., Acros Organics, and Alfa Aesar, without additional notes, were used without further purification. 1a,29 1a′,13 1b,12 1c,14e 1d,30 1g,13 5a,15d 5b,15c and G312 were prepared by literature methods. Dichloromethane (DCM) for the polymerization was purified by Glass Contour Organic Solvent Purification System, and tetrahydrofuran (THF) for the polymerization was distilled from sodium and benzophenone. Both were degassed further by Ar bubbling for 10 min before performing reactions. Thin-layer chromatography (TLC) was carried out on MERCK TLC silica gel 60 F254 and flash column chromatography was performed using MERCK silica gel 60 (0.040−0.063 mm). For SEC analysis, HPLC grade chloroform was purchased from J. T. Baker. CDCl3 (99.50% D) and CD2Cl2 (99.90% D, 0.75 mL) were purchased from Euriso-top and used without further purification. 1H NMR and 13C NMR was recorded by Varian/Oxford As-500 (500 MHz for 1H and 125 MHz for 13C) spectrometer and Agilent 400-MR (400 MHz for 1H). Chloroform size exclusion chromatography (SEC) analyses were carried out with a Waters system (515 pump, 2410 refractive index detector), Viscotek 270 dual detector, and Shodex GPC LF-804 column on samples diluted in chloroform (0.001−0.003 wt %; HPLC grade, J. T. Baker) and filtered with a 0.2 μm PTFE filter (Whatman). The flow rate was 1.0 mL/min, and the temperature of column was maintained at 35 °C. The SEC data were analyzed using OmniSEC 4.2 (Viscotek). Monomer Synthesis. Preparation of 4-(1,6-Heptadiyne)-N,Ndiethylformamide (1e). 4-Carboxy-1,6-heptadiyne31 (441.8 mg, 3.25 mmol) was added to a 50 mL round-bottom flask containing a magnetic stirring bar, and the flask was purged with argon. DCM (10 mL) was added, and the mixture was cooled down to 0 °C. A solution of oxalyl chloride (2.0 M in DCM, 2.43 mL, 4.87 mmol) was added, 6247


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and 2 drops of DMF was added under the control of atmospheric pressure. Generated CO2 gas was trapped by a balloon. The reaction mixture was stirred for 2 h at room temperature and concentrated to give a yellow-colored liquid. After this flask was filled with argon, DCM (10 mL), diethylamine (0.41 mL, 3.99 mmol), and triethylamine (0.56 mL, 3.99 mmol) were added. After stirring 2 h at room temperature, the reaction was quenched by saturated NaHCO3 aqueous solution. The organic layer was washed with water and extracted by ethyl acetate, dried with MgSO4, and concentrated. The product was purified by flash column chromatography on silica gel (gradient elution: ethyl acetate:hexane = 1:10 to 1:5) to afford the compound as colorless liquid (569.6 mg, 2.98 mmol, 91.8%). 1H NMR (500 MHz, CDCl3) δ: 3.43 (dq, J = 9.4, 7.2 Hz, 4H), 3.10−3.02 (m, 1H), 2.55−2.44 (m, 4H), 1.99 (t, J = 2.7 Hz, 2H), 1.24 (t, J = 7.2 Hz, 3H), 1.13 (t, J = 7.1 Hz, 3H). 13C NMR (125 MHz, CDCl3) δ: 171.90, 81.53, 70.17, 42.40, 40.93, 39.81, 21.97, 15.10, 13.17. HRMS (ESI): m/z for C12H17NNaO [M + Na]+, calcd 214.1202; found 214.1201. Preparation of 4-(Decylcarboxy)-1,6-heptadiyne (1f). 4-Carboxy1,6-heptadiyne31 (305.0 mg, 2.24 mmol) was added to a 50 mL roundbottom flask containing a magnetic stirring bar, and the flask was purged with argon. DCM (8 mL) was added, and the mixture was cooled down to 0 °C. A solution of oxalyl chloride (2.0 M in DCM, 1.46 mL, 2.91 mmol) was added, and 2 drops of DMF was added under the control of atmospheric pressure. Generated CO2 gas was trapped by a balloon. The reaction mixture was stirred for 2 h at room temperature and concentrated to give yellow-colored liquid. After this flask was filled with argon, DCM (8 mL), n-decanol (0.56 mL, 2.91 mmol), and triethylamine (0.81 mL, 5.82 mmol) were added. After stirring overnight at room temperature, the reaction was quenched by saturated NaHCO3 aqueous solution. The organic layer was washed with water and extracted by ethyl acetate, dried with MgSO4, and concentrated. The product was purified by flash column chromatography on silica gel (ethyl acetate:hexane = 1:50) to afford the compound as colorless liquid (536.7 mg, 1.94 mmol, 86.7%). 1H NMR (400 MHz, CDCl3) δ: 4.13 (t, J = 6.7 Hz, 2H), 2.76 (m, 1H), 2.70− 2.58 (m, 4H), 2.01 (t, J = 2.6 Hz, 2H), 1.68−1.60 (m, 2H), 1.40−1.18 (m, 14H), 0.88 (t, J = 6.9 Hz, 3H). 13C NMR (125 MHz, CDCl3) δ: 172.51, 80.65, 70.60, 65.39, 43.20, 32.03, 29.67, 29.45, 29.35, 28.73, 26.01, 22.82, 20.05, 14.26. HRMS (ESI): m/z for C18H28NaO2 [M + Na]+, calcd 299.1982; found 299.1983. Preparation of 4-(Triisopropylsilyloxy)-methyl-1,6-heptadiyne (1h). Triisopropylsilyl trifluoromethanesulfonate (2.43 mL, 9.02 mmol) was added to the mixture of 4-hydroxymethyl-1,6-heptadiyne32 (919 mg, 7.52 mmol), TEA (3.14 mL, 22.6 mmol), and DMAP (45.9 mg, 0.376 mmol) dissolved by DCM (24 mL) in 100 mL roundbottom flask at 0 °C. The mixture was stirred for 5 h at room temperature, and then saturated NaHCO3 aqueous solution was added. The mixture was washed with NH4Cl aqueous solution and extracted by ethyl acetate. The organic layer was dried with MgSO4 and concentrated. It was purified by flash column chromatography on silica gel (hexane only) to afford compound as a colorless liquid (1.94 mg, 6.96 mmol, 92.5%). 1H NMR (400 MHz, CDCl3) δ: 3.76 (d, J = 5.5 Hz, 2H), 2.43−2.31 (m, 4H), 1.97 (t, J = 2.7 Hz, 2H), 1.95 (m, 1H), 1.13−1.03 (m, 21H). 13C NMR (125 MHz, CDCl3) δ: 82.47, 69.73, 64.11, 40.06, 19.70, 18.15, 12.12. Anal. Calcd for C17H30OSi: C, 73.31; H, 10.86. Found: C, 73.09; H, 10.86. General Procedure for the Reaction of 1,6-Heptadiyne Derivatives and Grubbs Catalysts. Monomer (0.100 mmol) and a magnetic bar were added to a 4 mL vial with a cap containing PTFE−silicon septum. Dry solvent (0.10 mL) was added after the vial was purged with argon three times, and the solution of catalyst (mixed with additive in cases) (0.1 mL) prepared from an inert atmosphere was rapidly injected at given temperature. The reaction was quenched by excess ethyl vinyl ether (0.2 mL) after desired reaction time and dried under vacuum. The ratio of products was calculated from the crude 1H NMR spectrum; then, the mixture was precipitated in methanol (10 mL). The polymer was filtered, and the dimer and trimer were purified from the filtrate by flash column chromatography on silica gel.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01110. Characterizations for new compounds and polymers, NMR spectra, supporting experiments, and results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (T.-L.C.). Present Address

E.O.: Technische and Makromolekulare Chemie, Bundesstrasse 45, 20146 Hamburg, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from Basic Science Research Program, and the Nano-Material Technology Development Program through NRF is acknowledged. K.E.H. is supported by NRF2011-Fostering Core Leaders of the Future Basic Science Program (Grant NRF-2011-0012779). We thank National Center for Inter-University Research Facilities at SNU for supporting GC/MS experiments.

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REFERENCES

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