Synthesis of Functional Polyacetylenes via Cyclopolymerization of

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Synthesis of Functional Polyacetylenes via Cyclopolymerization of Diyne Monomers with Grubbs-type Catalysts Gregory I. Peterson, Sanghee Yang, and Tae-Lim Choi*

Acc. Chem. Res. Downloaded from pubs.acs.org by UNIV OF EDINBURGH on 01/28/19. For personal use only.

Department of Chemistry, Seoul National University, Seoul 08826, Republic of Korea

CONSPECTUS: Metathesis cyclopolymerization (CP) of α,ω-diynes is a powerful method to prepare functional polyacetylenes (PAs). PAs have long been studied due to their interesting electrical, optical, photonic, and magnetic properties which make them candidates for use in various advanced applications. Grubbs catalysts are widely used throughout synthetic chemistry, largely due to their accessibility, high reactivity, and tolerance to air, moisture, and many functional groups. Prior to our entrance into this field, only a few examples of CP using modified Grubbs catalysts existed. Inspired by these works, we saw an opportunity to expand the accessibility and utility of Grubbs-catalyzed CPs. We began by exploring CP with popular and commercially available Grubbs catalysts. We found Grubbs third-generation catalyst (G3) to be an excellent catalyst when we used strategies to stabilize the propagating Ru carbene, such as decreasing the polymerization temperature or using weakly coordinating solvent or ligands. Controlled living polymerizations were demonstrated using various 1,6-heptadiyne monomers and yielded polymers with exclusively 5-membered rings (via α-addition) in the polymer backbone. The strategy of stabilizing the Ru carbene was also critical to successful CP with Hoveyda-Grubbs second-generation (HG2) and Grubbs first-generation (G1) catalysts. We found that decomposed Ru species were catalyzing side reactions which could be completely shut down by decreasing the reaction temperature or using weakly coordinating ligands. While HG2 generally led to uncontrolled polymerizations, we found it to be an effective catalyst for monomers with very large side chains. G1 displayed broader functional group tolerance and thus broader monomer scope than G3. We next looked at our ability to change the regioselectivity of the polymerization by using Z-selective catalysts which favor β-addition and the formation of 6-membered rings in the polymer backbone. While modest β-selectivity could be obtained using Grubbs Z-selective catalyst at low temperatures, we found that by using one of Hoveyda and co-workers’ catalysts with decreased carbene electrophilicity, we could achieve exclusive formation of 6-membered rings. We also pursued alternative routes to achieve 6+-membered rings in the polymer backbone by using diyne monomers with increased distance between alkynes. We found that optimizing the monomer structure for CP was an effective strategy to achieve controlled polymerizations. By using bulky substituents (maximizing the Thorpe-Ingold effect) and/or using heteroatoms (shorter bonds) to bring the alkynes closer together, controlled living CP could be achieved with various 1,7-octadiyne and 1,8-nonadiyne monomers. Finally, we took advantage of several inherent properties of controlled CP techniques to prepare polymers with advanced architectures and nanostructures. For instance, the living nature of the polymerization enabled production of block copolymers, the tolerance of very large substituents enabled production of dendronized and brush polymers, and the insolubility or crystallinity of some monomers was utilized for the spontaneous self-assembly of polymers into various one- and two-dimensional nanostructures. Overall, the strategies of stabilizing the propagating Ru carbene, modulating the selectivity and reactivity of the Ru carbene, and enhancing the inherent reactivity of monomers were key to improving the utility and performance of CP with Grubbs-type catalysts. The insight provided by these studies will be important for future developments of CP and other metathesis polymerizations utilizing ringclosing steps.

1. INTRODUCTION

having increased solubility, processability, and enhanced

Polyacetylenes (PAs) have long been studied for their interesting electrical, optical, photonic, and magnetic properties.1 Metathesis cyclopolymerization (CP) of α,ω-diynes is an important method for preparing functional PAs. In addition to

properties (compared to nonfunctionalized PAs), 2 PAs

© XXXX American Chemical Society

Received: November 23, 2018

A

DOI: 10.1021/acs.accounts.8b00594 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Figure 1. Mechanism of α- and β-addition in the CP of α,ω-diynes.

Figure 2. Grubbs-type catalysts used for CP of α,ω-diynes.

prepared by CP can be obtained with controlled regiochemistry, stereochemistry, molecular weight, and molecular weight dispersity (ĐM). This, however, was not always the case. Initial polymerizations using ill-defined catalysts gave uncontrolled results in terms of both molecular weight and regioselectivity. The first controlled living CPs were demonstrated by Schrock and co-workers using Mo and W catalysts with 1,6-heptadiyne monomers (HDYs).3,4 They found that polymerizations yielded polymers containing both 5- and 6-membered cycloalkenes in the backbone via α- or β-addition, respectively (Figure 1). They later modified their catalysts to obtain β-addition selectively.5,6 Buchmeiser and co-workers further expanded this chemistry and developed living polymerizations with Mo catalysts that could give α-addition selectively.7−9 Despite the beautiful chemistry in these examples, the use of air- and moisture-sensitive Mo and W catalysts remained a limitation. Grubbs catalysts are widely used throughout synthetic chemistry, largely due to their accessibility, high reactivity, and tolerance to air, moisture, and many functional groups. However, even decades after their invention, these user-friendly catalysts had not given successful CP. Thus, a major breakthrough in CP chemistry came when Buchmeiser and coworkers modified the Grubbs-type Ru alkylidenes, by replacing the chloride ligands with electron-withdrawing groups such as

trifluoroacetate (Figure 2), and achieved controlled CP of HDYs.10−13 Polymerizations yielded 5-membered ring repeat units (α-addition) selectively, and control of polymer molecular weight was demonstrated, but broad ĐM values were observed, with a few exceptions, presumably due to slow initiation by the catalysts. Inspired by these works, in 2011 our research group entered into this field, and since then, we have made significant contributions to expanding the accessibility and utility of Grubbs-catalyzed CPs. In this Account, we describe our efforts to expand the scope of catalysts and monomers, further elucidate the mechanism of CP, identify key criteria for achieving controlled living polymerizations, and prepare topologically complex polymers and nanostructures using CP. For a broader overview of CP,2,14,15 or PAs,1,16 we direct the reader to these cited review articles.

2. REACTION DEVELOPMENT One of our initial objectives was to achieve CP with popular and commercially available Grubbs catalysts. In addition to making the polymerization more accessible, we envisioned that we would be able to take advantage of the high reactivity and good stability of these catalysts to achieve better control of the polymerization. We expected that this would enable us to also substantially expand the monomer scope and increase the B


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polymers, we tuned the substitution at the 4-position of HDYs (e.g., M2 and M3). In addition, we decreased the temperature (0 to −10 °C), which led to significant improvement in the control of the polymerization (e.g., for M2 at the same monomer/ initiator ratio, the ĐM decreased from 1.81 at RT to 1.16 at 0 °C). With these conditions, we observed linear increases in number-average molecular weight (Mn) with increasing monomer/initiator ratio, up to a degree of polymerization (DP) of ∼150, and typically narrow ĐM values (1.06−1.44) across the studied monomers. Importantly, polymerizations maintained the α-selectivity observed by Buchmeiser and coworkers and also showed living behavior, which was demonstrated through the successful preparation of block copolymers (see section 3.1). In order to expand our monomer scope and obtain more controlled polymerizations, we wanted to understand why THF was the key to successful polymerization. Our hypothesis was that the THF might weakly coordinate and stabilize the propagating carbene intermediate. Therefore, we expected that we could switch solvents to DCM (which is typically a better solvent for conjugated polymers) and add small amounts of a coordinating additive to maintain control of the polymerization.18 This turned out to be an effective strategy, and our best results were obtained with 3,5-dichloropyridine (3,5-Cl2Py) at decreased temperature. With these conditions, we could now polymerize M1 in a controlled manner (with 20 mol % 3,5-Cl2Py at 10 °C: Mn = 49.9 kDa, ĐM = 1.16; without additive at RT: Mn = 12.6 kDa, ĐM = 2.56). Across all monomers, we observed ĐM values less than 1.38, with DPs as high as ∼300. To determine the role of the additive, we conducted 1H NMR experiments and found that 3,5-Cl2Py suppressed the decomposition of the propagating carbene and increased turnover numbers (the same was true for using THF as solvent), enabling polymers with higher DP to be prepared. On a related note, M6 was also shown to have longer carbene lifetimes, suggesting that steric congestion can also shield the carbene from degradation. Overall, our results suggested that the key to efficient CP was suppressing catalyst decomposition and maximizing the carbene lifetime. We next wondered why previous attempts at CP with Grubbs second-generation (G2) or Hoveyda-Grubbs second-generation (HG2) catalysts were unsuccessful.10 Previous studies with modified Grubbs catalysts suggested side reactions via an olefin metathesis mechanism might be possible.19 Through careful mechanistic investigations we determined that dimers and trimers of diynes (Figure 4a) were formed via cycloaddition reactions that were catalyzed by decomposed Ru species (Figure 4b).20 These side reactions led to very low yields of polymer (with the side products accounting for the majority of the remaining yield). We turned to our previously established methods for stabilizing the propagating carbene (Figure 4c) to try and address this problem. With HG2, decreasing the polymerization temperature led to an improvement in yields (e.g., for M7, the polymer yield increased from 37% to >95% by decreasing the temperature from RT to 0 °C). Using additives also improved the yield (e.g., for M2, the polymer yield increased from 5% to 94% by adding 3,5-Cl2Py). In each of these cases the production of dimers and trimers was completely shut down, further demonstrating the importance of stabilizing the propagating carbene. However, CP was still uncontrolled and yielded polymers with broad dispersity (the best performing monomers M4 and M8 had ĐM values of 2.86 and 2.74, respectively). The lack of control was attributed to slow

structural diversity of PAs obtainable with CP. Furthermore, we hoped that these catalysts might be amenable to the synthesis of polymers with advanced architectures (e.g., block, dendronized, and graft polymers) and nanostructures. To achieve these goals, we had to develop a strong mechanistic understanding of CP, identify the primary modes of failure (which we later found out to be decomposition of the Ru carbene species), and identify how the monomer structure influences its polymerizability. With this knowledge, we have utilized three overarching strategies to ensure successful CP, which are stabilizing the propagating Ru carbene, modulating the selectivity and reactivity of the Ru carbene, and enhancing the inherent reactivity of monomers. In this section, we will discuss how we have used these techniques to expand the catalyst scope and monomer scope, switch the regioselectivity of CP, and obtain controlled polymerizations. 2.1. Expanding the Catalyst Scope

In our first foray into this field, we explored the CP of HDYs using Grubbs third-generation catalyst (G3).17 We envisioned that the fast initiating G3 would give larger initiation to propagation rate ratios, which would lead to narrower ĐM values than those obtained with the catalysts developed by Buchmeiser and co-workers. We began by attempting to polymerize M1 (Figure 3) in dichloromethane (DCM) at room temperature (RT). Unfortunately, only poor conversion (18−68%) was observed. By screening solvents, we found that tetrahydrofuran (THF) led to high conversions (92%), but the resulting polymers were insoluble in this solvent. To obtain soluble

Figure 3. Representative monomer scope and polymerization results for the α-selective CP of HDYs. C


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Figure 4. Side products formed during CP of HDYs (A), mechanism of side product formation after Ru catalyst decomposition (B), and methods for suppressing carbene decomposition (C).

Grubbs-type catalysts studied to that point) gave >99:1 α- to βselectivity.22 We were interested in determining if we could tune the selectivity toward β-addition and the formation of 6membered rings in the polymer backbone. Our first report looking at β-selective CP utilized a Z-selective Grubbs catalyst (GZ).23 With this catalyst, polymerizations proceeded with moderate control, reaching Mn values up to 16 kDa with ĐM values ranging 1.49−1.96. The degree of β-selectivity increased as the number or size of substituents on the monomer increased (e.g., M10 < M1 < M11, in the order of least to most selective; see Figure 5). Decreasing the temperature (to −40 °C from RT) also improved the regioselectivity, enabling the polymer from M11 to reach 93% incorporation of 6-membered rings. We hypothesized that the regioselectivity of GZ was driven by steric interactions between the monomer and catalyst ligands. Due to steric and electronic effects of GZ,24 monomer addition occurs via a side-bound pathway (Figure 6a), which makes α-addition less favored as the monomer substituent size increases, due to steric interaction with the mesityl moieties of the NHC ligand. Unfortunately, the monomer scope with GZ appeared to be limited to bulky malonate type monomers (for instance, the βselectivity for M10 was only 71%), which led us to more deeply pursue the origin of selectivity. We, in collaboration with Baik and co-workers, studied the mechanism of CP using a density functional theory (DFT) computational model.22 Computed reaction energy diagrams for alkyne metathesis, using both G3 and GZ, were in good agreement with the experimental results. For instance, αinsertion was favored over β-insertion by ∼9 kcal/mol for G3, and β-insertion was favored over α-insertion by ∼4 kcal/mol (likely an overestimate given the experimentally observed

initiation rates and chain transfer reactions. Fortunately, we did observe narrower ĐM values (as low as ∼1.5) with HG2 using macromonomers with slower propagating rates and increased steric bulk (see section 3.2). Achieving CP with HG2 led us to envision that CP with Grubbs first-generation catalyst (G1) should also be possible if an appropriate stabilizing additive could be identified. Polymerization of M2 with G1 in DCM failed, giving only 8% yield of polymer, similar to the results obtained when using HG2 without additives.21 The addition of pyridine ligands generally led to only modest improvement of the polymer yield (up to 26%). Fortunately, we found two excellent additives, benzoic acid and sodium benzoate, with which we could achieve between 82% and 98% yield of polymer for M2 and reasonable control of the polymerization (e.g., Mn = 23.3, ĐM = 1.53). We explored other monomers (see Figure 3) and obtained nearly equivalent results (Mn up to 12 kDa and ĐM values ranging 1.32−2.13). Importantly, G1 displayed increased monomer scope (e.g., M9 does not undergo polymerization with G3). From 1H NMR experiments we found that both additives stabilized the propagating carbene via coordination and additionally enhanced catalyst activity by phosphine protonation or partial exchanging with the chloride ligands. Despite the slightly decreased performance in CP compared to G3, G1 still represents a viable option given its functional group tolerance and that it is one of the cheapest and the most readily available olefin metathesis catalysts. 2.2. Controlling Regioselectivity

One of the intriguing aspects of Grubbs-type catalysts was their α-selectivity. Typical polymerizations with G3 (in fact all D


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electronic demand of CP for α-addition (Figure 6b) were limiting the selectivity for β-addition. We envisioned that tuning the electrophilicity of the carbene, by using a catalyst with stronger π-backdonation, would reduce the catalyst reactivity as well as make α-addition less favored electronically (enabling the sterics to play a bigger role). This led us to pursue using one of Hoveyda and co-workers’ Z-selective catalysts (HZ) which has enhanced π-backdonation due to its trigonal-bipyramidal geometry.22 DFT calculations supported the viability of this catalyst, as β-insertion was associated with a barrier of only 13.5 kcal/mol and was favored over α-insertion by 7 kcal/mol. When we employed this catalyst, as predicted, we found very high degrees of β-selectivity (92% to >99%), and importantly all polymerizations could be conducted at RT. Furthermore, we were able to obtain polymers with Mn values up to 25.8 kDa with ĐM values ranging 1.51−1.92. As might be expected from previous studies, addition of 3,5-Cl2Py, in most cases, enabled obtaining polymers with narrower ĐM values (1.30−1.81).25 Overall, we found that the strategy of exploiting the electronic properties of the Fischer carbenes was quite powerful as we could completely switch the regioselectivity of CP for various monomers. 2.3. Expanding the Monomer Scope via Monomer Design

In addition to tuning the stability and reactivity of the catalyst, we found that the monomer design was often a critical element to the success of CP. This was particularly true in the case of monomers with increased distance between alkynes, such as 1,7octadiynes (ODYs). Buchmeiser and co-workers demonstrated that modified Grubbs catalysts were able to polymerize ODYs, albeit with poor performance.26 We envisioned that the CP of ODYs might be improved by using G3 and modifying the monomer structure.27 We began by changing the number and size of the substituents at the 4-position. As the number or size was increased, the conversion and yield of polymer increased (Figure 7). Of note: the α-selectivity of G3 was maintained despite the change of monomer structure. In addition to achieving higher DP (up to ∼80), the dispersity also narrowed as substituent size was increased (from as high as 1.95 down to 1.18). We hypothesized that CP was enhanced by the Thorpe-

Figure 5. Representative monomer scope and polymerization conditions for the β-selective CP of HDYs.

selectivities) for GZ. The insertion barrier for GZ was also calculated to be >10 kcal/mol lower than that of G3, which was in good agreement with the need to use low temperature to achieve modest selectivity. We expected that the high reactivity associated with the side-bound GZ pathway and the intrinsic

Figure 6. Stereochemical model that describes the regioselectivity of G3 and GZ (A), and the intrinsic electronic demand of CP (B). E


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with Mn and ĐM of 28 kDa and 1.35 in 24 h and 18 kDa and 1.28 in 50 min, respectively).29 Our final strategy was to introduce heteroatoms into the monomer, either at the 4-position or at both the 4- and 5-positions.30 We expected that the shorter C− N bonds would bring the alkynes in closer proximity, facilitating ring-closing. For monosubstituted monomers, the polymerization times improved (e.g., 2 h for M25, compared to 24 h for M17). For disubstituted monomers, significant improvement in ĐM values (1.12−1.44) and yields (≥88%) were observed with polymerization times as short as 5 min. Overall, we developed several effective methods to modify the monomer structure and achieve more controlled CP. Our success with ODYs motivated us to pursue a more challenging monomer. We wondered if we could introduce 7membered rings into the backbone via the CP of 1,8-nonadiynes (NDYs).31 We first attempted polymerization of M28 (allcarbon monomer backbone, Figure 8) with various Grubbs

Figure 7. Representative monomer scope and polymerization conditions for the CP of ODYs.

Figure 8. Representative monomer scope and polymerization conditions for the CP of NDYs.

Ingold effect and that the larger substituents forced the monomer into a conformation in which the alkynes were brought closer together, facilitating cyclization. Furthermore, we expected that the increased sterics suppressed carbene decomposition and chain transfer reactions. To better understand the reactivity of these monomers, we conducted kinetics experiments and observed higher conversion rates with increased substituent size.27 Regardless, the conversion rates for ODYs were still ∼100 times slower than HDY monomers. We pursued three different strategies, pertaining to modifying the monomer structure, to make cyclization faster. Our first strategy was to introduce additional dimethyl substitution onto the α-position of the side chain (e.g., M20 and M21) to maximize the Thorpe-Ingold effect.28 This cut the polymerization times from 24 h down to as little as ∼1 h. Our second strategy was to move one of the substituents from the 4-position to the 5-position (e.g., M22−M24). We observed similar reactivity to the dimethyl-substituted monomers (polymerizations in ∼1 h) and comparable ĐM values to the 4,4-substituted monomers (e.g., M19 and M24 gave polymers

catalysts but did not obtain any polymer. Fortunately, utilizing our previously developed monomer design strategies, in particular, the incorporation of heteroatoms, led to successful CP. After optimizing conditions, we found that HG2-mediated CP of acetal monomers (e.g., M29−M31) yielded polymers with Mn values up to 8.2 kDa with ĐM values ranging 1.69−4.49 in 4−8 h. Better results were obtained with aminal monomers (e.g., M32 and M33), which gave polymers with Mn values up to 28 kDa with ĐM values ranging 1.49−2.18. In an attempt to enhance the control of the reaction, we switched catalysts to G3. For M33, we obtained polymers with ĐM values between 1.16 and 1.29. Polymerizations also proceeded in a living manner, enabling the production of block copolymers containing 7membered rings (see section 3.1). This work further demonstrates how rational design of monomers can lead to successful polymerizations. F


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Figure 9. BCPs via: two different metathesis polymerizations (A), CP of both blocks while maintaining consistent ring size (B), and CP of all blocks while changing ring size (C).

3. ADVANCED POLYMER STRUCTURES AND NANOPARTICLES

preparation of advanced polymers and nanoparticles. For instance, we have utilized the living nature of the polymerization in the synthesis of BCPs and the tolerance/requirement of sterically bulky substituents to our advantage in the synthesis of dendronized and brush polymers. Furthermore, synthesis of these polymers and polymers with limited solubility has

One of the ways that the versatility of CP has been demonstrated is by preparing polymers with advanced architectures, such as block copolymers (BCPs) and star polymers.32−34 We found that features of our new controlled CPs also enabled the G


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Figure 10. Scheme for the synthesis of denpols from HDYs (A), AFM images of denpol P35 showing its elongated conformation (B), scheme for the synthesis of denpols from ODYs (C), scheme for the synthesis of brush polymer P37 (D), and AFM image of P37 showing its elongated conformation (E).

3.2. Dendronized and Brush Polymers

contributed to our ability to prepare nanostructures during CP. In this section, we will discuss how we have used these techniques to increase the functional/structural diversity of PAs.

An interesting feature of CP is that bulky substituents often aid the polymerization, presumably by forcing the alkynes closer together (Thorpe-Ingold effect) or by shielding the propagating carbene and suppressing decomposition. We used this to our advantage when we prepared dendronized polymers (denpols) in a graft-through approach. The graft-through approach is an important route to making well-defined polymers as it enables the preparation of polymers whose dendrons do not have defects, as well as ensuring a dendron is located on every repeat unit. Initially, we prepared denpols from HDYs functionalized with second- or third-generation ester-type dendrons (Figure 10). We obtained polymers with DP up to ∼200 with ĐM values between 1.08 and 1.36.17 One of the interesting features of denpols is that they have elongated structures due to steric repulsion between dendrons. We confirmed the elongated conformation of these denpols by visualization of individual polymers by atomic force spectroscopy (AFM) (Figure 10b). Using ODYs, denpols with two dendrons per repeat unit were also prepared (Figure 10c), although these monomers required heating and use of the more thermally stable HG2 for sufficient conversion during polymerization.29 Analogous to denpols, we also prepared brush polymers, where each repeat unit was functionalized with a pendant polymer chain.38 Using living ring-opening polymerization (ROP), poly(L-lactide) and poly(ε-caprolactone) macromonomers were prepared from HDY initiators (Figure 10d). Using HG2 we could produce high molecular weight (>0.5 MDa) brush polymers with moderate

3.1. Block Copolymers

One of the ways that we demonstrated the structural diversity of functional PAs obtainable with CP was by preparing BCPs. We have successfully utilized the chain end from ring-opening metathesis polymerization (ROMP) and tandem ring-closing/ ring-opening metathesis polymerization (RCROMP, which uses monomers containing alkyne and cycloalkene moieties) to make BCPs (Figure 9a).28,35−37 We have also used the living chain end from CP to prepare fully conjugated polymers with consistent ring size (Figure 9b) and varying functionality either outside of the ring (e.g., BCP4)17,18,30 or inside of the ring (e.g., heterocyclic and carbocyclic rings in BCP530). BCPs with varying ring size have also been prepared (Figure 9c).27,29,31 The best results were obtained when the relatively fast cyclizing HDYs were polymerized before ODYs or NDYs. Failure to do so resulted in loss of blockiness or homopolymer impurities. One exception was with heterocyclic ODYs (e.g., M26), which could be polymerized first (e.g., BCP8) as they had similar reactivity to HDYs.30 Triblock copolymers containing blocks of 5-, 6-, and 7membered rings were also prepared by sequential polymerization of a HDY, ODY, and NDY (BCP9).31 Overall, living CP is a powerful method to make conjugated BCPs with diverse functionality. H


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Figure 11. Graphical scheme of INCP and light-mediated isomerization to trigger formation of various nanostructures (A), AFM images showing morphological changes for BCP10 (B), AFM images showing morphological changes for BCP11 (C), TEM images of various 2D-nanosheets from P38 (D), and TEM image and controlling the length of 1D-nanoribbons from P39 (E).

branched, or 2D-island-like nanostructures (Figure 11a−c).36 We found the combination of CP and INCP methods to also be applicable to homopolymers that have marginal solubility. For instance, the CP of HDY monomers containing a fluorene moiety with neohexyl side chains at the 4-position led to formation of 2D-nanosheets whose shapes varied in different solvents (Figure 11d).48 By adding an additional phenyl group between the fluorene moiety and neohexyl side chains, 1Dnanoribbons with tunable width were obtained via the INCP mechanism (Figure 11e).49 Further control of their length was successfully achieved via crystallization-driven self-assembly (CDSA) from smaller nanoribbon seeds. Overall, these examples show the versatility of CP for preparing various semiconducting nanomaterials.

dispersity. We again confirmed, with AFM imaging, that the polymeric side chains led to extended polymer conformations (Figure 10e). We envision that both of these types of polymers may be used as insulated molecular wires as their conjugated backbones are insulated by nonconjugated macromolecules. 3.3. Nanoparticles via Self-Assembly

Self-assembly of polymers into nanoparticles has been a hot topic in recent years because it provides access to various intriguing nanostructures.39−41 In particular, semiconducting materials produced from conjugated polymers have been investigated because of their optoelectronic potential.42−44 We envisioned using CP as a new tool to prepare polymers that could undergo self-assembly to form semiconducting nanoparticles. In particular, we were interested in seeing if we could achieve in situ nanoparticlization of conjugated polymers (INCP),45,46 in which nanoparticles would be spontaneously formed during CP. We began by preparing a BCP in which the second block was derived from a HDY based on Meldrum’s acid (e.g., BCP10 and BCP11), with which we observed spontaneous micellization during CP.35 The rigid and insoluble PA block formed the micelle core due to strong π−π interactions, and the polynorbornene block formed the solubilizing corona. Interestingly, we could use the photoinduced cis- to trans-olefin isomerization (related polymers initially have up to 16% cis-alkenes)47 to trigger morphological changes from micelles to higher-order nanocaterpillar,

4. OUTLOOK AND CONCLUSIONS CP is a powerful tool to prepare functional PAs. By utilizing conditions that stabilize the propagating Ru carbene (decreased temperature, bulky monomer design, and/or weakly coordinating additives) we could achieve living controlled CP of various diyne monomers. By tuning the steric and electronic nature of the carbene (making the carbene less electrophilic), we could control the regiochemistry of CP and obtain α- or β-addition selectively. Furthermore, by carefully designing the monomer structure (primarily using the Thorpe-Ingold effect, or using heteroatoms to decrease bond lengths), we could achieve more I


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Accounts of Chemical Research Author Contributions

controlled and efficient CP of monomers which produce polymers with increased ring sizes in their backbone. These techniques were key to the preparation of PAs with diverse structures, including BCPs, denpols, graft polymers, and nanoparticles. Outside of our lab, these techniques have also been utilized to prepare a wide variety of functional PAs. For instance, Xie and co-workers have further shown the significance of decreased reaction temperature, bulky monomers, and coordinating solvents or ligands in the G3-mediated CP of HDYs. Using these methods they have prepared ladder/bridgetype PAs,50,51 triphenylamine-functionalized PAs (which form interesting cylindrical nanostructures),52 dendronized PAs,53 and PA ionomers.54 We envision that these methods will continue to play an important role in the future development of functional PAs. The strategies to enhance CP have also been utilized in the development of other polymerization methodologies which rely on ring-closing metathesis (RCM) steps. For instance, by decreasing the temperature, using a weakly coordinating solvent or additives, and tuning monomer structures to maximize the Thorpe-Ingold effect, we achieved controlled RCROMP using G3.37,55 Gutekunst and Hawker showed that the “polymerization trigger” in the macrocyclic ROMP (which relies on the same G3 mediated RCM and ring-opening steps as RCROMP) enabled controlled polymerizations by using 3,5-Cl2Py as a weakly coordinating additive.56 These strategies were also critical to the success of cascade metathesis and metallotropy polymerizations (M&M, which combines RCM with a metallotropic shift).57 We expect these techniques to continue to lead to advancements in polymerization methodologies involving Ru alkylidenes. While we have achieved significant improvement in Grubbscatalyzed CP, there are still areas that will benefit from improvement. In terms of monomer scope, new methods are needed to polymerize monomers with increased distance between alkynes, which are expected to be significantly harder to polymerize due to the slow cyclization step. Also, despite the functional group tolerance of Grubbs catalysts, there are only a few examples of exploring that tolerance in CP.58 Further exploration and optimization of polymerization conditions of monomers containing more diverse functional groups would greatly expand the utility and demonstrate the strength of Grubbs-catalyzed CP. In terms of polymerization performance, the biggest limitation is achieving higher DP polymers (typically limited to DPs < 200). At higher DPs, we generally observe much broader dispersity and loss of living character due to catalyst decomposition. New methods (including new catalysts, additives, optimized monomer structures, etc.) are needed which can achieve high DP. Finally, more investigations on semiconducting nanostructures and their optoelectronic properties would further broaden the utility of CP. These future developments will help further cement CP as a go-to method to prepare functional PAs.

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All authors have contributed to the manuscript and have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. Biographies Gregory I. Peterson is a Research Professor in the Department of Chemistry at Seoul National University. He obtained his Ph.D. in Chemistry at the University of Washington. His research interests include functional and stimuli-responsive polymeric materials. Sanghee Yang is currently a Ph.D. candidate in the Department of Chemistry at Seoul National University. Her current research involves studying polymer self-assembly. Tae-Lim Choi is a Professor in the Department of Chemistry at Seoul National University. He is also the editor for J. Polym. Sci., Part A: Polym. Chem. His research interests include new polymer synthetic methods and polymer self-assembly.

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ACKNOWLEDGMENTS

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REFERENCES

The authors thank the Korean NRF for the following funding: Creative Research Initiative Grant, Young Investigator Grant (NRF-2018R1C1B6003054), and Global Ph.D. Fellowship Program (NRF-2015H1A2A1033703).

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Sanghee Yang: 0000-0001-7944-6635 Tae-Lim Choi: 0000-0001-9521-6450 J


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Synthesis of Functional Polyacetylenes via Cyclopolymerization of

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