Ring-Closing Metathesis

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

Superior Cascade Ring-Opening/Ring-Closing Metathesis Polymerization and Multiple Olefin Metathesis Polymerization: Enhancing the Driving Force for Successful Polymerization of Challenging Monomers Ho-Keun Lee, Jaeho Lee, Johannes Kockelmann, Torben Herrmann, Massih Sarif, and Tae-Lim Choi* Downloaded via STEPHEN F AUSTIN STATE UNIV on July 31, 2018 at 13:01:08 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Department of Chemistry, Seoul National University, Seoul 08826, Korea S Supporting Information *

ABSTRACT: Recently, our group successfully developed two new polymerization methodologies for monomers containing two cycloalkene moieties. These polymerization methods yielded well-defined polymers via a combination of ring-opening and ring-closing metathesis (cascade polymerization) or ring-opening, ring-closing, and cross-metathesis (multiple olefin metathesis polymerization (MOMP)) using a second monomer. However, cascade polymerization had some limitations such as low polymerization efficiency (maximum turnover number (TON) of 250) and narrow monomer scope. Furthermore, one-shot MOMP also showed a very narrow monomer scope because of certain undesired side reactions. To overcome these problems, we designed various new monomers containing cyclopentene and even more challenging ring-strain-free cyclohexene moieties, so that polymerization would produce a thermodynamically favored six-membered-ring backbone repeat unit. With this enhanced driving force for polymerization, these new monomers successfully underwent cascade polymerization with a high polymerization efficiency, leading to a maximum TON of 1940 and maximum number-average molecular weight (Mn) of 343 kDa. Lastly, one-shot MOMP, which uses all three types of metathesis transformations in a single step, was possible with these monomers and gave highly A,B-alternating copolymers with high selectivity as well. This was possible because the newly designed monomers with the appropriate thermodynamic and kinetic preferences suppressed undesired polymerization pathways and reduced defects in the polymer microstructures. In short, we present our strategies for achieving superior cascade polymerization and MOMP using these new monomers.

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INTRODUCTION

reactions. However, most examples of metathesis polymerization methodologies rely on only one of the three types of olefin metathesis reactions to produce simple polymer structures. In order to synthesize well-defined polymers with complex microstructures, tandem polymerization, where more than two types of olefin metathesis transformations are combined in one-shot or one-pot reactions, has been developed successfully, analogous to cascade reactions in organic synthesis.10−15

Olefin metathesis reaction is a powerful method for preparing various molecules that involves the exchange of carbon− carbon double bonds.1 This field has rapidly advanced over the last two decades, with the development of highly active Grubbs2 and Schrock catalysts.3 Organic chemists have developed ring-opening metathesis (ROM),4 ring-closing metathesis (RCM),5 and cross-metathesis (CM)6 for the synthesis of complex organic compounds such as natural products and drugs. Furthermore, polymer chemists have developed polymerizations, such as ring-opening metathesis polymerization (ROMP),7 cyclopolymerization (CP),8 and acyclic diene metathesis polymerization (ADMET)9 using CM © XXXX American Chemical Society

Received: May 29, 2018

A

DOI: 10.1021/jacs.8b05613 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

Scheme 1. Previous Cascade Ring-Opening/Ring-Closing Metathesis Polymerization and MOMP with M1 Derivatives

The first representative example of using more than two types of olefin metathesis processes was ring-opening-insertion metathesis polymerization (ROIMP) using the secondgeneration Grubbs catalyst, which was used to synthesize well-defined A,B-alternating copolymers in an one-shot method.10 The high degree of alternation of this copolymer was due to enthalpically driven selective CM to form α,βunsaturated carbonyl olefins with the in situ generated polymers via fast ROMP of cycloalkene using the secondgeneration Grubbs catalyst. Another example is the two-pot polymerization using two olefin metathesis reactions sequentially (ROMP followed by ADMET) to produce branched polymers in an independent manner.11 Recently, a new concept of cascade polymerization via tandem ring-opening/ ring-closing polymerization of monomers containing cycloalkene and terminal alkyne moieties12 has attracted much attention because this selective cascade polymerization proceeded in a living manner, using the fast-initiating thirdgeneration Grubbs catalyst to produce even sequence-specific polymers.13 The most recently reported living polymerization combining olefin metathesis and metallotropic shift occurred in a perfectly alternating cascade manner to produce unique conjugated polyeneynes.14 Lastly, our group has reported cascade ring-opening/ring-closing polymerization using monomers containing two cyclopentene moieties and multiple olefin metathesis polymerization (MOMP), where all three types of olefin metathesis transformations occurred simultaneously to produce well-defined A,B-alternating copolymers using the same monomers15a (Scheme 1a). Although these two types of polymerization methods were mechanistically unique, the maximum turnover number (TON) of 250 was relatively lower than that of the well-investigated ROMP. Furthermore, the monomer scopes for cascade polymerization and MOMP were very narrow as well.15 To further improve the reactivity and selectivity of the cascade polymerization and MOMP and to expand their polymerization scopes, one needs to investigate the origins of the low reactivity and narrow monomer scope and to develop a new strategy to overcome these limitations. Herein, we report a new design of bis-cycloalkene monomers toward achieving highly efficient cascade polymerization with broad monomer scopes, which resulted in a maximum TON of 1940 and maximum number-average molecular weight (Mn) of

343 kDa. Since these polymerizations using the newly designed monomers produced kinetically and thermodynamically preferred six-membered-ring backbone structures, we were able to broaden the monomer scope, not only for the cascade polymerization but also for the MOMP, by suppressing the generation of side products that caused significant defects in the previous one-shot MOMP method.

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RESULTS AND DISCUSSION In our previous studies, we successfully synthesized welldefined polymers with a five-membered ring in the repeat unit of the backbone from monomers containing two cyclopentene moieties (M1a−c)15 and using the first-generation Grubbs catalyst (G1) (Scheme 1a).16 Notably, intrinsically less active G1 outperformed supposedly more active Ru catalysts containing N-heterocyclic carbenes, because G1 containing a smaller PCy3 ligand showed higher preference to the productive pathway of the cascade polymerization. 15b Furthermore, we noticed that optimizing the polymerization concentration was important because, although polymerization at high concentrations would afford higher conversions, some monomers containing bis-cycloalkene readily underwent crosslinking via intermolecular ROMP when polymerized at above the critical concentration (Figure S70). In addition, when intrinsically more active second-generation Hoveyda−Grubbs catalyst (HG2)17 was used (Scheme 1b (ii)), A,B-alternating copolymers were obtained either through postmodification using selective CM or by one-shot copolymerization using the two monomers M1a and 1,4-butanediol diacrylate. Although this cascade polymerization involving two olefin metatheses (ROM and RCM) in a cascade manner and the MOMP combining all three metathesis reactions (ROM, RCM, and CM) simultaneously were attractive strategies, they had some drawbacks, such as narrow monomer scope and relatively low TON. To address these issues and to understand the factors that influence the polymerizations, we designed and synthesized three new monomers, M2a−c (Scheme 2a), which contain a challenging ring-strain-free cyclohexene (strain energy = 2.5 kcal/mol) moiety instead of the cyclopentene (strain energy = 6.8 kcal/mol)18 moiety present in the M1 derivatives. However, the challenging cascade polymerizations of the M2 monomers failed (Scheme 2b) because the B


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cases, the polymerization efficiency was still low, implying that the driving force for the polymerization of the M3 derivatives was not sufficient because the polymer products still contained less stable five-membered rings in contrast to the monomers that contained cyclohexene. On the basis of the results for M3, we came up with a new strategy for monomer design to increase the thermodynamic gain by improving the stability of the polymer products. As opposed to the formation of a five-membered-ring repeat unit during the polymerization, the formation of a six-membered ring would enhance the driving force for the cascade polymerization. To test this, we synthesized new monomers, M4 (Scheme 3b), by using simple reactions such as SN2 reactions and the Tsuji−Trost allylation to simply shift the position of the olefin from the 3-position to the 4-position on the cyclopentene moiety in M3.19 In addition to the high enthalpic gain due to the six-membered-ring formation, the use of M4, in particular, could lead to much faster and more selective cascade polymerization because the catalyst would approach the less sterically hindered 3-cyclopentene more readily than the more sterically hindered and less reactive 2cyclohexene, and thus the 3-cyclopentene would be opened first. Then, the resulting carbene would undergo a ringclosing/opening reaction with the adjacent 2-cyclohexene (Scheme 3c). The opposite sequence would be impossible not only because 2-cyclohexene is more crowded but also because the resulting carbene from the ring opening would not be able to reach the neighboring 3-cyclopentene (Scheme 3d (i)). Furthermore, even if it could reach the cyclopentene, ninemembered-ring cyclization would not be kinetically or thermodynamically feasible (Scheme 3d (ii)). To our delight, the cascade polymerizations of the M4 derivatives produced well-defined P4 with high efficiency without any cross-linking even at higher concentrations above 0.5 M. It is worth noting that the ring-strain-free cyclohexene moiety in M4, which does not undergo ROMP by itself, prevents cross-linking even at a high concentration above 2.5 M, which is well above the critical concentration of cyclopentene. In particular, cascade polymerization of M4a with M/C = 50−250 was successful, with high efficiency and conversion of over 96%, but the conversion dropped to 76% when M/C = 500. The Mn of P4a increased from 12 kDa to 73 kDa as the TON increased from 49 to 380 (Table 2, entries 1− 4). The structure of P4a was analyzed using 1H NMR spectroscopy, 13C NMR spectroscopy, and MALDI-TOF mass spectrometry (Figure 1, Figure S29, and Figure S63). Although the spectra of P4a were complicated owing to two stereogenic

Scheme 2. Cascade Polymerization of M2 and M3 Derivatives

thermodynamics of cyclohexene favored the ring-closed state of the monomers over cascade polymerization to give the less stable five-membered-ring structures in the polymer products (Table 1, entries 1−3). Hence, we designed other new monomers, M3a−c (Scheme 2a), substituting one of the two cyclohexenes with a cyclopentene moiety, so that the higher ring-strain energy would enhance the enthalpy gain. To our delight, polymerization using M3a at a high concentration (1.5 M) successfully proceeded successfully with 100% conversion at monomer/catalyst ratios (M/C) of 50 and 150, and the Mn of the resulting P3a was up to 20 kDa (Scheme 2c, Table 1, entries 4 and 5). On the other hand, the polymerizations of M3b, which contains an amide linker, and M3c, which contains a carbon linker, at room temperature were less satisfactory, with low conversions of 36% and 30%, respectively (Table 1, entries 6 and 8). To improve the polymerization conversion, we conducted the polymerization at a low temperature, 0 °C, to increase the thermodynamic gain by decreasing the entropic factor (ΔG = ΔH − TΔS). As expected, their conversions increased by approximately 20% to give P3b and P3c with conversions of up to 55% and Mn of 17−19 kDa (Table 1, entries 7 and 9). However, in all three Table 1. Cascade Polymerization of M2 and M3 Monomers entry

monomer

monomer: catalyst

1 2 3 4 5 6 7 8 9

M2a M2b M2c M3a M3a M3b M3b M3c M3c

50:1 50:1 50:1 50:1 150:1 50:1 50:1 50:1 50:1

conc.

time

temp

conva

1.5 1.5 1.5 1.5 1.5 2.5 2.5 2.5 2.5

24 24 24 18 18 48 48 48 48

25 °C 25 °C 25 °C 25 °C 25 °C 25 °C 0 °C 25 °C 0 °C

0% 0% 0% 100% 100% 36% 55% 30% 49%

M M M M M M M M M

h h h h h h h h h

Mnb

ĐMb

yield

20.4 k 19.1 k

1.64 1.55

75% 68%

18.9 k

1.44

40%

17.2 k

1.69

35%

a

Conversion was determined via crude 1H NMR analysis. bMolecular weight and dispersity (ĐM) were determined using THF SEC calibrated using polystyrene standards. C


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Scheme 3. New Strategy for Improving Polymerization Efficiency and Selectivity with M4 Derivatives by Forming a SixMembered Ring in the Polymer

Table 2. Cascade Polymerization of M4 Monomers entry

monomer

M:C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

M4a M4a M4a M4a M4b M4b M4b M4b M4b M4b M4c M4c M4c M4c M4c M4c

50:1 150:1 250:1 500:1 50:1 150:1 250:1 500:1 750:1 1000:1 50:1 150:1 250:1 500:1 750:1 1000:1

conc

time

2.5 2.5 2.5 3.5 1.5 1.5 1.5 1.5 1.5 3.5 1.5 1.5 1.5 1.5 1.5 2.5

16 20 36 36 16 24 30 36 36 36 18 20 24 36 36 60

M M M M M M M M M M M M M M M M

h h h h h h h h h h h h h h h h

Mna

ĐMa

convb

yield

12.2 k 26.4 k 38.8 k 73.4 k 17.9 k 38.5 k 62.0 k 85.2 k 103.6 k 121.0 k 20.3 k 52.7 k 67.0 k 85.0 k 127.5 k 167.0 k

2.00 1.64 1.70 1.77 1.53 1.64 1.62 1.70 1.56 1.69 1.68 1.72 1.75 1.93 1.81 1.86

97% 96% 96% 76% 98% 98% 98% 97% 96% 70% 95% 97% 97% 96% 96% 94%

88% 90% 92% 60% 95% 90% 92% 90% 89% 63% 91% 93% 90% 88% 92% 85%

a

Molecular weight and dispersity (ĐM) were determined using THF SEC calibrated using polystyrene standards. bConversion was determined via crude H NMR analysis.

centers giving four diastereomers and an E/Z mixture of the internal acyclic alkene, the detailed polymer structure was confirmed by 2D COSY 1H NMR (Figure 1b), and MALDITOF mass spectrometry showed a clear mass difference corresponding to the mass of the repeat unit. The cascade polymerization of M4b with a methanesulfonyl amide linker also proceeded well, with excellent efficiency even with M/C = 1000 (a maximum TON of 700), and the Mn increased from

18 kDa to 121 kDa as the TON increased from 48.5 to 700 (Table 2, entries 5−10). Lastly, M4c, which contains a carbon linkage, showed even higher efficiency, with a TON that reached 940, and the resulting Mn increased from 20 kDa to 167 kDa under the optimized conditions (M/C = 50−1000, 2.5 M, for 18−60 h) (Table 2, entries 11−16). These monomers (M4a−c) showed significantly higher polymerization efficiencies than their analogous monomers containing D


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Figure 1. 1H 1D and 2D COSY NMR spectra of P4a.

the same linkers such as the M1 and M3 derivatives, which gave maximum TONs of only 250 and 150, respectively. In short, the formation of the thermodynamically stable sixmembered rings in the polymer backbone provided a sufficient driving force for the successful cascade polymerization, resulting in high molecular weights and high TONs despite involving monomers with low-ring-strained cyclohexene moieties. Encouraged by the results for the M4 derivatives, we designed M5a−d by substituting the cyclohexene moieties in M4 with higher-ring-strained cyclopentene moieties (Scheme 4a). We anticipated that they would show even higher efficiency because ring-strain-free six-membered rings would be formed from the monomers containing two cyclopentenes (Scheme 4b). First, we attempted the polymerization of M5a, which contains an ether linkage, at a high concentration above 1.0 M, but a cross-linked gel was obtained just like with M1a.15a In this case, two cyclopentenes with less steric congestion were likely to undergo a faster ring-opening

Scheme 4. Superior Cascade Polymerization of M5 Derivatives

process, leading to cross-linking, in contrast to the cases for M4 derivatives. In other words, intermolecular ROMP of two cyclopentene moieties was dominant under the highconcentration conditions instead of the desired sequential E


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Figure 2. 1D and 2D COSY 1H spectra of P5a.

highest among the results from all cascade polymerizations attempted, indicating the power of this monomer design. Lastly, we polymerized M5c and M5d, which contain carbon linkages, at high concentrations of above 1.0 M without crosslinking, just like M5b. P5c, containing two methoxyethane moieties, was produced with a maximum TON of 820 in 2−6 h with M/C = 50−1000, and its Mn increased from 14.2 kDa to 117 kDa (Table 3, entries 14−17). A similar monomer, M5d, which contains only one methoxyethane group, showed significantly high reactivity when M/C = 50−2000 with a maximum TON of 1960. However, the Mn was relatively low, between 10.3 and 42.6 kDa, compared to those from other monomers (Table 3, entries 18−22). Also, one should note that regardless of polymerization efficiency, dispersities (ĐM) for all the polymers were rather broad due to extensive chain transfer reactions. Using simple thermodynamic intuition, we successfully demonstrated cascade polymerization and pro-

cascade polymerization. Fortunately, repeating the polymerization at a low concentration of 0.5 M produced well-defined P5a without any cross-linking over a wide M/C range of 50− 1000 with a much shorter reaction time of 30 min. It is remarkable that a maximum TON of 1000 was obtained in only 30 min to give P5a with an Mn of 101 kDa even at this low concentration (Table 3, entries 1−6). Another monomer, M5b, containing a methanesulfonyl amide linker, was successfully polymerized, and, in contrast to M5a, crosslinking did not occur even at a high concentration of 2.0 M. Presumably, this was because of the steric hindrance of the Nmethanesulfonyl protecting group on M5b, which suppressed the undesired independent ROMP of cyclopentene that would have led to cross-linking. At a relatively high concentration (1.0−2.0 M), P5b was produced with Mn ranging from 14.6 to 343 kDa and a TON of up to 1940 (M/C = 50−2000, 2−4 h) (Table 3, entries 7−13). Notably, the Mn of 343 kDa was the F


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Journal of the American Chemical Society Table 3. Cascade Polymerization of M5 Monomers entry

monomer

M:C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 29 20 21 22

M5a M5a M5a M5a M5a M5a M5b M5b M5b M5b M5b M5b M5b M5c M5c M5c M5c M5d M5d M5d M5d M5d

50:1 150:1 250:1 500:1 750:1 1000:1 50:1 150:1 250:1 500:1 750:1 1000:1 2000:1 50:1 250:1 500:1 1000:1 50:1 250:1 500:1 1000:1 2000:1

conc

time

Mna

Đ Ma

convb

yield

0.5 0.5 0.5 0.5 0.5 0.5 2.0 2.0 2.0 2.0 2.0 1.0 1.0 1.0 2.0 2.0 3.0 2.0 2.0 2.0 2.0 2.0

0.5 h 0.5 h 0.5 h 0.5 h 0.5 h 0.5 h 2h 2h 2h 4h 4h 4h 4h 2h 2h 4h 6h 6h 6h 6h 6h 6h

17.2 k 26.4 k 41.4 k 74.4 k 72.8 k 100.7 k 14.6 k 26.8 k 55.0 k 103.9 k 153.0 k 194.2 k 343.4 k 14.2 k 61.8 k 82.7 k 117.0 k 10.3 k 22.6 k 32.5 k 38.1 k 42.6 k

2.01 2.97 2.00 2.49 2.68 2.51 1.61 2.02 1.50 1.69 2.04 1.88 1.50 1.58 1.65 1.79 1.33 1.65 1.83 1.98 2.01 1.96

99% 100% 100% 100% 100% 100% 97% 97% 97% 98% 98% 98% 97% 99% 100% 97% 82% 99% 99% 100% 100% 98%

72% 77% 73% 81% 90% 87% 63% 76% 92% 90% 88% 90% 91% 80% 90% 87% 73% 62% 79% 82% 84% 85%

M M M M M M M M M M M M M M M M M M M M M M

a

Molecular weight and dispersity (ĐM) were determined using THF SEC calibrated using polystyrene standards. bConversion was determined via crude 1H NMR analysis.

Scheme 5. Depolymerization of P1, P3, P4, and P5 Derivatives

vided some insight into what determines its efficiency. Therefore, owing to proper monomer design, the M5 derivatives provided the strongest driving force, and their cascade polymerizations were completed with either a short reaction time of 30 min or the maximum TON of 1960 using only 0.05 mol % of the catalyst.

Since the cascade polymerization process was in thermodynamic equilibrium, even the isolated polymers could be depolymerized to the corresponding monomers via the reverse ROM/RCM process. The degree of such depolymerization would reflect the thermodynamic preference of the monomer state over the polymer state or the inverse of the polymerization efficiency. The depolymerization experiments were G


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Journal of the American Chemical Society Scheme 6. MOMP Results with M3, M4, and M5 Derivatives

Table 4. One-Shot MOMP with M3−M5 Monomers and Sequential MOMP with P3−P5 Polymers entry

monomer

method

M(P):Da:C

time

Mnb

ĐMb

A,B-altc

yield

1 2 3 4 5 6 7 8 9 10

M3a P3a M4a P4a M5a P5a M5b P5b M5c P5c

one-shot sequential one-shot sequential one-shot sequential one-shot sequential one-shot sequential

50:50:1 50:50:1 50:50:1 50:50:1 50:50:1 50:50:1 50:50:1 50:50:1 50:50:1 50:50:1

12 h 12 h 12 h 12 h 4h 4h 8h 12 h 12 h 12 h

7.8 k 5.7 k 7.3 k 4.8 k 7.4 k 6.4 k 11.0 k 9.9 k 7.5 k 7.7 k

1.53 1.36 1.61 1.49 2.24 1.36 1.57 1.23 1.76 1.31

92% 95% 97% 95% 94% 94% 94% 95% 92% 92%

74% 73% 85% 72% 78% 82% 78% 72% 81% 84%

a

1,4-butanediol diacrylate. bMolecular weight and dispersity (ĐM) were determined using THF SEC calibrated using polystyrene standards. Alternation was determined using 1H NMR analysis. Reaction concentration: 0.5 M.

c

cycloalkene followed by consecutive and selective CM with 1,4-butanediol diacrylate20 in an one-shot manner to form A,Balternating copolymers with thermodynamically stable α,βunsaturated carbonyl olefins. However, in our previous report on MOMP, only M1a with the ether linkage underwent successful one-shot MOMP, while M1b and M1c produced A,B-alternating copolymers only by sequential MOMP. The sequential MOMP consisted of two steps: the initial cascade polymerization of M1b and M1c and the insertion of the diacrylate via selective CM into the isolated polymers, P1b and P1c. In other words, one-shot MOMP of M1b and M1c with the diacrylate was less than satisfactory because some side reactions, such as the undesired formation of a sevenmembered ring trapped by the irreversible CM process with the diacrylate, significantly lowered the desired A,B-alternating microstructures (Scheme 3a and Figures S60, S61). Here, we reasoned that these new monomers would undergo MOMP with enhanced selectivity, because the six-membered-ring backbone formation during the polymerization is both thermodynamically and kinetically preferred so that the undesired side pathways such as the slower and more challenging formation of eight- or nine-membered rings would be completely suppressed. Several new monomers, namely, M3a, M4a, and M5a−c, were tested in MOMP with both the one-shot and sequential methods, and the results were compared. First, all of the internal acyclic olefins should undergo selective CM with the diacrylate to generate well-defined A,B-alternating copolymers, and this alternation can be calculated simply from the integration by monitoring the disappearance of the internal olefin peak at 5.5 ppm and the appearance of new α,βunsaturated carbonyl olefin peaks at 7.0 and 5.8 ppm using 1H

conducted by adding fresh G1 (2 mol %) to each dichloromethane (DCM) solution of each polymer at a low concentration of 0.04 M and stirring for 12 h. We previously reported depolymerizations that reverted 13% of M1a, 26% of M1b, and 34% of M1c as measured using 1H NMR spectroscopy (Scheme 5a).15a With these results in mind, the same experiments were repeated with P3a, P4, and P5. In the case of P3a, which showed the lowest polymerization efficiency, the highest amount, 55%, of P3a was depolymerized to M3a (Scheme 5b). In comparison, only 9% for M4a, 16% for M4b, and 21% for M4c were depolymerized, which are less than the analogous P1 derivatives with the same linkers (Scheme 5c vs a). Lastly, the P5 derivatives, which were prepared with the highest polymerization efficiencies, produced the least amounts of depolymerized monomer: 0% for both M5a and M5b and only 6% and 1% for M5c and M5d, respectively (Scheme 5d). These results were consistent with our original design and the polymerization data that suggested that the best cascade polymerizations would be observed with the M5 derivatives, which produced sixmembered rings from monomers containing two fivemembered rings. Next, we envisioned that these new monomers that underwent successful cascade polymerization would undergo MOMP, which involves all three types of olefin metathesis transformations (ROM, RCM, and CM). For MOMP, we used another monomer, 1,4-butanediol diacrylate, as a coupling partner and, instead of G1, intrinsically more active secondgeneration Hoveyda−Grubbs catalyst (HG2),17 which promotes selective CM with electron-deficient olefins such as acrylates (Scheme 6). The MOMP proceeds via an initial ringopening/closing cascade reaction of the monomer having bisH


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Figure 3. Proton spectra of one-shot MOMP with M4a (a) and sequential MOMP with P4a (b).

using these new monomers and understand the design principle of these new monomers to suppress defects promoted by the undesired pathways.

NMR spectroscopy. In the case of M3a, which shows structural similarity to M1a, the one-shot and sequential MOMP methods were carried out using P3a to give the identical polymer, showing over 92% alternation and an Mn of 6−8 kDa, respectively (Table 4, entries 1and 2). In addition, one-shot MOMP using M4a and M5a gave high alternation of over 94% and Mn greater than 7 kDa (Figure 3a, Figure S47a, and Table 4, entries 3 and 5). Moreover, the analogous two-step sequential MOMP using P4a and P5a produced the same copolymer with almost identical A,B-alternation, although their Mn values were slightly lower than those from the one-shot method (Figure 3, Figure S47b, and Table 4, entries 4 and 6). Lastly, we also conducted one-shot MOMP using M5b and M5c, which contain sulfonamide and carbon linkages, as well as sequential MOMP using P5b and P5c, and the two methods gave very similar copolymers, showing nearly identical structural features, according to 1H NMR spectroscopy, with over 92% alternation and Mn ranging between 7 and 11 kDa (Table 4, entries 7−10). Also, other diacrylates can be used as coupling partners, and these resulting polymers are listed in Table S1 in the Supporting Information (Figures S71−S76). In short, we were able to expand the scope of one-shot MOMP

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CONCLUSION In this study, we successfully expanded the monomer scope and greatly improved the efficiency or TON of the cascade polymerization by designing novel monomers. We were able to polymerize challenging monomers containing even lowstrained cyclohexene, by designing a cascade polymerization method that would form a thermodynamically stable sixmembered ring in the polymer backbone, which gave a maximum TON of 940 and maximum Mn of 167 kDa. To further enhance the polymerization efficiency, another series of monomers containing two cyclopentene moieties was prepared, which afforded an enthalpically favored sixmembered ring to give polymers with Mn exceeding 343 kDa and a maximum TON of 1940 in a much shorter time. In addition, these new monomers successfully underwent oneshot MOMP, which combined all three olefin metathesis processes, to give A,B-alternating copolymers with higher selectivity than those in the previous studies. This high I


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selectivity was due to the novel monomer design that promoted thermodynamically and kinetically preferred sixmembered-ring cyclization, thereby suppressing the formation of defects caused by the undesired side reactions. This taught an important lesson that novel monomer design based on an understanding of thermodynamic parameters can not only broaden the polymerization scope but also greatly improve the polymerization efficiency, in this case, by up to 8 times compared to the previous results.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b05613.

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Experimental procedures, NMR data for monomers, polymers, SEC data for polymers, MALDI-TOF data for polymers (PDF)

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Jaeho Lee: 0000-0001-7584-920X Tae-Lim Choi: 0000-0001-9521-6450 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This paper is dedicated to Professor Taihyun Chang at POSTECH on his 65th birthday and his lifelong achievement in research and education. The financial support from Creative Research Initiative Grant, Creative Material Discovery Program, and the Nano-Material Technology Development Program through NRF is acknowledged.

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REFERENCES

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