End-Capping Reaction of Living Anionic Poly(benzyl methacrylate

Article Cite This: Macromolecules 2019, 52, 4828−4838

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End-Capping Reaction of Living Anionic Poly(benzyl methacrylate) with a Pentafluorophenyl Ester for a Norbornenyl-ω-End Macromonomer with a Long Flexible Spacer: Advantage in the WellControlled Synthesis of Ultrahigh-Molecular-Weight Bottlebrush Polymers Downloaded via IDAHO STATE UNIV on July 19, 2019 at 00:33:55 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Chang-Geun Chae, Yong-Guen Yu, Ho-Bin Seo, Myung-Jin Kim, Zuwang Wen, and Jae-Suk Lee* School of Materials Science and Engineering and Grubbs Center for Polymers and Catalysis, Gwangju Institute of Science and Technology (GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea S Supporting Information *

ABSTRACT: 12-(cis-5-Norbornene-exo-2,3-dicarboximido)dodecanoate pentafluorophenyl ester (exo-NBC12-PFP) was used as a norbornene-substituted end-capping terminator of living anionic polymers. Polystyrene with a terminal 1,1diphenylethyllithium (PSt(DPE)−Li+), poly(2-vinylpyridine) with a terminal 2-pyridinylethyllithium (P2VP−Li+), and poly(benzyl methacrylate) with a terminal lithium ester enolate (PBzMA−Li+) reacted with exo-NBC12-PFP under appropriate reaction conditions to generate norbornenyl-ω-end macromonomers, NBC12-PSt, NBC12-P2VP, and NBC12PBzMA, respectively, each with a 12-carbon spacer. To estimate the efficiency of end-capping, these macromonomers were polymerized by grafting-through ring-opening metathesis polymerization (ROMP). The end-capping reaction of PSt(DPE)−Li+ and P2VP−Li+ suffered from side reactions resulting in low end-capping efficiencies and the generation of by-products. On the other hand, the side reactions were minimal in the end-capping reaction of PBzMA−Li+, resulting in a high end-capping efficiency of 95%. The ROMP of NBC12-PBzMA allowed the synthesis of poly[12-(5-norbornene-exo-2,3-dicarboximido)dodecanoylate]-graft-poly(benzyl methacrylate)s (PNBC12-g-PBzMAs) with a wide range of controlled molecular weights (Mn = 436−4048 kDa, Đ = 1.07−1.23).

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INTRODUCTION Starting from the pioneering contribution of Michael Szwarc,1−4 a number of controlled/living polymerizations have been developed to achieve model polymers with precisely controlled molecular properties.5−9 A conspicuous merit of these living systems is the efficiency with which the living ends link with a myriad of functionality to construct polymers with complex architectures and unique physical characteristics.10−17 A large chunk of such synthetic approaches have relied on an efficient chain-end functionalization because the end-functionalized polymers can be versatile building blocks for new architectures.17 Utilizing an initiator and/or a terminator containing an appropriate functional group has allowed the convenient one-pot synthesis of the desired mono-end or diend-functionalized polymer.18−35 The quality of the end-functionalized polymer would rely on the degree of livingness of the polymerization reaction. Until © 2019 American Chemical Society

now, many end-functionalization methods for vinyl polymers have been effective with reversible deactivation radical polymerizations, such as atom transfer radical polymerization,23−28 reversible addition−fragmentation chain-transfer polymerization,28−32 and nitroxide-mediated radical polymerization.33−35 Those synthetically expedient protocols have mostly used functional initiators, leading to polymers of predictable molecular weights (MWs), low dispersities, and high-end functionalities. However, getting products with nearly 100% purity is compromised by various side reactions (recombination, disproportionation, chain transfer, etc.).28,36,37 Living anionic polymerization is suitable for the synthesis of model end-functionalized polymers because of its excellent Received: March 19, 2019 Revised: May 31, 2019 Published: June 20, 2019 4828

DOI: 10.1021/acs.macromol.9b00559 Macromolecules 2019, 52, 4828−4838

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Scheme 1. (a) End-Capping Reaction of Three Different Types of Living Anionic Polymers with exo-NBC12-PFP and (b) Grafting-through ROMP of Resulting Norbornenyl-ω-End Macromonomers Initiated by Ru in THF at 25 °C To Afford PNBC12-g-PSt, PNBC12-g-P2VP, and PNBC12-g-PBzMA

livingness.38 Functional anionic initiators and electrophilic terminators have shown their efficacy in the incorporation of designed end groups to polymers.39−53 Especially, various mechanisms of the end-capping reactions have been developed by functional terminators.39−53 One of the useful end-capping reactions is the nucleophilic acyl substitution reaction with acyl chlorides.46−49,52 This reaction is effective especially for weak anionic nucleophiles, as the carbonyl chloride is very reactive because of its powerful leaving group (chloride anion). Unfortunately, the functional acyl chlorides with high purity have been limited to only volatile liquids that are quickly isolated through distillation after synthesis. Other long-term purification procedures including chromatography for viscous liquids or solids cannot prevent the rapid hydrolysis of the carbonyl chloride group in air. For this reason, isolation of pure acyl chlorides with appropriate molecular designs is very difficult to achieve. “Activated esters” are alternative acyl compounds with high reactivity.54−59 Because of their hydrolytic resistance, such compounds can be easily isolated by chromatography after synthesis.60 In recent times, two activated esters, Nhydroxysuccinimide ester and pentafluorophenyl (PFP) ester, have been used for side-chain or chain-end functionalization of polymers.61−73 We have successfully synthesized nearly 100% norbornenyl-ω-end-functionalized polystyrene (PSt) derivatives through nucleophilic substitution reaction between primary amino-ω-end PSt derivatives and a norbornenesubstituted PFP ester.51 This protocol is based on the postpolymerization modification, and thus the overall synthetic route is complex with many reaction steps.

The norbornenyl-end polymers are macromonomers suitable for ring-opening metathesis polymerization (ROMP)74,75 to furnish bottlebrush polymers with unique properties.27,28,32,40,41,49,51−53,76−78 As the current interest is growing in the various applications of ultrahigh-MW bottlebrush polymers with large degrees of polymerization (DPs),79−82 the one-pot synthesis of well-defined macromonomer with a desirable linker structure would be highly useful. In this paper, we report the potential of a norbornenesubstituted PFP ester containing a long flexible spacer as a functional terminator to end-cap the living anionic polymer of a methacrylate monomer for the convenient one-pot synthesis of norbornenyl-ω-end macromonomers. Two other vinyl monomers are also similarly functionalized with the norbornenyl group to compare the end-capping efficiency. The resulting norbornenyl-ω-end macromonomers are used in the grafting-through ROMP to identify the accessibility to bottlebrush polymers with ultrahigh MWs.

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RESULTS AND DISCUSSION End-Capping Reaction of Living Anionic Polymers with a Norbornene-Substituted PFP Ester Containing a Long Flexible Spacer. 12-(cis-5-Norbornene-exo-2,3dicarboximido)dodecanoate pentafluorophenyl ester (exoNBC12-PFP) was synthesized as a norbornene-substituted PFP ester with a 12-carbon spacer by an esterification reaction between 12-(cis-5-norbornene-exo-2,3-dicarboximido)dodecanoic acid and pentafluorophenol. The exo-NBC12PFP is nonvolatile but hydrolytically resistant liquid, which allowed its isolation from the reaction mixture through column chromatography. The nucleophilic substitution of this 4829


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Table 1. Results of the End-Capping Reaction of Living Anionic Species of PSt, P2VP, and PBzMA with exo-NBC12-PFPa polymer code NBC12-PSt-1 NBC12-PSt-2 NBC12-P2VP-1 NBC12-P2VP-2 NBC12-PBzMA-1 NBC12-PBzMA-2 NBC12-PBzMA-3

[M]0/[I]0 [St]0/[s-BuLi]0 = 23.0 [St]0/[s-BuLi]0 = 24.4 [2VP]0/[MDPPLi]0 = 20.0 [2VP]0/[MDPPLi]0 = 22.6 [BzMA0/[MDPPLi]0 = 16.0 [BzMA]0/[MDPPLi]0 = 16.0 [BzMA]0/[MDPPLi]0 = 15.3

anionic species −

+

PSt(DPE) Li PSt(DPE)−Li+ P2VP−Li+ P2VP−Li+ PBzMA−Li+ PBzMA−Li+ PBzMA−Li+

end-capping temperatureb (°C) −78 −981st −781st −981st 0 −481st −781st 1st

+ + + +

2nd

0 −782nd + 03rd 02nd −782nd + 03rd

+ 02nd + 02nd

Mn,theoc (kDa)

Mnd (kDa)

Đd

Fe

2.98 3.12 2.71 2.96 3.40 3.40 3.49

2.77 3.44 5.67 4.16 3.73 3.77 3.89

1.04 1.03 1.14 1.09 1.02 1.03 1.02

0.70 0.87 f f

0.76 0.87 0.95

a

The procedures for the living anionic polymerization are described in the Experimental Section of the Supporting Information. 100% monomer conversion was expected. bReaction time: 1 h for −98 °C, 16 h for −78/−48 °C, and 24 h for 0 °C. cMn,theo = MW of terminal residue + [M]0/[I]0 × conv/100% × MW of monomer. dDetermined from SEC-MALLS. eF: End-capping efficiency determined as the fraction of bottlebrush polymers in the resulting SEC−dRI trace of the mixture resulting from the grafting-through ROMP of the macromonomer with [MM]0/[Ru]0 = 250 at [MM]0 = 0.05 mol L−1. fNot possible to determine due to its uncontrolled MW and incomplete grafting-through ROMP.

ing theoretical value (Mn,theo). The chemical structures of the polymers were identified through proton nuclear magnetic resonance (1H NMR) spectroscopy in chloroform-d (CDCl3) containing 0.03 vol % tetramethylsilane (TMS). It is necessary to understand possible side reactions during the end-capping reaction of living anionic polymer with a functional acyl compound.48,83,84 One way is the nucleophilic addition of living anionic polymer to the carbonyl carbon of an end-capped polymer.48 This reaction generates a coupled polymer with a nearly double MW. The other way is the proton abstraction of living anionic polymer from the acidic methylene moiety next to the carbonyl carbon.48 This reaction leads to the production of the non-end-functionalized polymer. In some cases, the proton abstraction catalyzes the reaction between the deprotonated acyl compound and a normal one,83,84 which generates a polymer with a bifunctional end. To avoid these side reactions, living anionic polymers with strong reactivities such as polystyryllithium must react with an epoxide compound to generate less reactive oxyanion before the end-capping reaction.46,49 Lithium ester enolates of polymethacrylates have appropriate reactivities for the direct end-capping reaction with an acyl compound.47,52 The results of the end-capping reaction are listed in Table 1. The anionic polymerization of St, 2VP, and PBzMA based on the typical procedures was performed to afford PSt(DPE)−Li+, P2VP−Li+, and PBzMA−Li+, respectively, with the target Mn values of 2−4 kDa. The living anionic polymers reacted with exo-NBC12-PFP at controlled temperatures. After preparation of the macromonomers, the grafting-through ROMP with [MM]0/[Ru]0 = 250 was performed at [MM]0 = 0.05 mol L−1 for the determination of their end-capping efficiencies. The complete disappearance of macromonomers needed to be confirmed from the 1H NMR spectra of the isolated mixtures. The presence of bottlebrush polymers with low-MW impurities was observed in the SEC−differential refractive index (dRI) traces of the mixtures. The low-MW fractions were considered as non-end-functionalized polymers. The maximum yield of each bottlebrush polymer was determined as the fraction of the bottlebrush polymers in the SEC−dRI trace. The end-capping efficiency (F) for each macromonomer was then derived from the maximum yield of the bottlebrush polymer. It was confirmed from the 1H NMR spectroscopy that most of the NBC12-PSts and NBC12-PBzMAs were completely converted to the bottlebrush polymers by the grafting-through ROMP. Thus, their end-capping efficiencies could be determined directly. Only NBC12-P2VPs failed to determine their end-capping efficiencies because the grafting-through

compound was tested in the end-capping reaction of living anionic polymers. Styrene (St), 2-vinylpyridine (2VP), and benzyl methacrylate (BzMA) were selected as vinyl monomers in the living anionic polymerization to prepare living anionic species of polystyrene (PSt), poly(2-vinylpyridine) (P2VP), and poly(benzyl methacrylate) (PBzMA). The anionic polymerization of St, 2VP, and BzMA is performed in tetrahydrofuran (THF) at −78 °C under 10−6 Torr using appropriate initiating agents, sec-butyllithium (s-BuLi) for St, 3-methyl-1,1-diphenylpentyl lithium (MDPPLi) for 2VP, and 1:3 mixture of MDPPLi:lithium chloride (LiCl) for BzMA to generate PSt with a terminal styryllithium (PSt−Li+), P2VP with a terminal 2pyridinylethyllithium (P2VP−Li+), and PBzMA with a terminal lithium ester enolate (PBzMA−Li+), respectively. The highly reactive PSt−Li+ is not suited for the end-capping reaction because of possible side reactions. Therefore, PSt−Li+ should react with 1,1-diphenylethylene (DPE) to form more stabilized PSt with a terminal 1,1-diphenylethyllithium (PSt(DPE)−Li+) prior to the end-capping reaction. The nucleophilicity of the three living anionic polymers follows the order: PSt(DPE)−Li+ > P2VP−Li+ > PBzMA−Li+. The one-pot synthesis of norbornenyl-ω-end macromonomers proceeded by the living anionic polymerization of St, 2VP, and BzMA and the subsequent end-capping reaction of the corresponding living anionic species [PSt(DPE)−Li+, P2VP−Li+, and PBzMA−Li+, respectively] with exo-NBC12PFP in THF under 10−6 Torr. The use of exo-NBC12-PFP generates three macromonomers with a 12-(cis-5-norborneneexo-2,3-dicarboximido)dodecanoyl ω-end, NBC12-PSt, NBC12-P2VP, and NBC12-PBzMA (Scheme 1a). Optimal temperature condition was studied to improve the end-capping efficiency. These macromonomers (MMs) were used in the grafting-through ROMP initiated by RuCl2(pyridine)2(H2IMes)(CHPh) (Ru) in THF at 25 °C. After final conversion, the ROMP was terminated by adding ethyl vinyl ether to produce three kinds of bottlebrush polymers, poly[12-(5-norbornene-exo-2,3-dicarboximido)dodecanoylate]-graf t-polystyrene (PNBC12-g-PSt), poly[12(5-norbornene-exo-2,3-dicarboximido)dodecanoylate]-graf tpoly(2-vinylpyridine) (PNBC12-g-P2VP), and poly[12-(5norbornene-exo-2,3-dicarboximido)dodecanoylate]-graf t-poly(benzyl methacrylate) (PNBC12-g-PBzMA) (Scheme 1b). The number-average MW (Mn) and the dispersity (Đ) of every polymer were determined through size exclusion chromatography-multiangle laser light scattering (SECMALLS). Every Mn value was compared with the correspond4830


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Figure 1. 1H NMR of (a) NBC12-PSt-2 (Mn = 3.44 kDa, Đ = 1.03), (b) NBC12-P2VP-2 (Mn = 4.16 kDa, Đ = 1.09), and (c) NBC12-PBzMA-3 (Mn = 3.89 kDa, Đ = 1.02) in CDCl3.

the most desired for the appropriate reactivity of PBzMA−Li+ to minimize the proton abstraction. The molecular characterization of NBC12-PSt-2, NBC12P2VP-2, and NBC12-PBzMA-3 through 1H NMR spectroscopy showed the presence of the ω-end group (Figure 1). Specific proton resonances corresponding to this group clearly appeared at 6.27, 3.43, 3.26, and 2.66 ppm. These resonances are identical to those of exo-NBC12-PFP. However, an additional resonance was observed at 3.6−3.5 ppm in the 1H NMR spectrum of the NBC12-P2VP-2. This resonance was considered to be associated with the presence of by-products with higher MWs. The MW distribution shapes of NBC12-PSt-2, NBC12P2VP-2, and NBC12-PBzMA-3 were identified from their SEC−dRI traces (Figure 2). Both NBC12-PSt-2 and NBC12-

ROMP incompletely stopped probably due to the presence of an excess of coordinating pyridine pendants. This made it hard to determine their end-capping efficiencies. The end-capping reaction of PSt(DPE)−Li+ was performed initially at −78 °C for 16 h and continued at 0 °C for 24 h. The reaction at −78 °C caused an unusual color change of the reaction solution from deep red to light green. The latter color was assumed as a result of the creation of enolate ion pairs by the considerable proton abstraction of the PSt(DPE)−Li+ from an acidic methylene in exo-NBC12-PFP. Although the Mn value of the product was in accordance with the Mn,theo value, a low end-capping efficiency of 0.70 was achieved (Table 1, NBC12-PSt-1). To stabilize the PSt(DPE)−Li+, the endcapping reaction was performed at −98 °C for 1 h, −78 °C for 16 h, and 0 °C for 24 h in sequence. As a result, the color change to light green was almost prevented. However, the endcapping efficiency was just 0.87 despite the predictable Mn value of the final product (Table 1, NBC12-PSt-2). The end-capping reaction of P2VP−Li+ with exo-NBC12PFP at −78 °C for 16 h and 0 °C for 24 h in sequence resulted in a Mn value higher than the Mn,theo and high Đ value of the product (Table 1, NBC12-P2VP-1). When the reaction was performed at −98 °C for 1 h, −78 °C for 16 h, and 0 °C for 24 h in sequence to stabilize the P2VP−Li+, the discrepancy between the Mn and Mn,theo values was not dramatically reduced (Table 1, NBC12-P2VP-2). The uncontrolled MWs for the two NBC12-P2VPs were attributed to the presence of by-products with higher MW. The end-capping reaction of PBzMA−Li+ with exo-NBC12PFP was performed at broad temperature conditions. The reaction at 0 °C for 24 h resulted in a low end-capping efficiency of 0.76 (Table 1, NBC12-PBzMA-1). The reaction at −48 °C for 16 h and 0 °C for 24 h slightly increased the end-capping efficiency to 0.87 (Table 1, NBC12-PBzMA-2). When the initial temperature was set at −78 °C, the endcapping efficiency increased up to 0.95 (Table 1, NBC12PBzMA-3). The end-capping reaction did not proceed at −98 °C because of the insufficient reactivity of the lithium ester enolate. The Mn values of the products were well in accordance with the Mn,theo values with the low dispersities. Thus, the main type of side reaction was revealed to be the proton abstraction that produces non-end-functionalized polymers. −78 °C was

Figure 2. SEC−dRI traces of (a) NBC12-PSt-2 (Mn = 3.44 kDa, Đ = 1.03), (b) NBC12-P2VP-2 (Mn = 4.16 kDa, Đ = 1.09), and (c) NBC12-PBzMA-3 (Mn = 3.89 kDa, Đ = 1.02).

PBzMA-3 having predictable MWs (Mn ≈ Mn,theo) showed narrow and unimodal traces (Figure 2a,c). Thus, the endcapping efficiencies lower than unity were attributed to the proton abstraction. However, the trace of NBC12-P2VP-2 showed a bimodal MW distribution with the presence of an 4831


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Table 2. MW Results of the Grafting-through ROMP of NBC12-PSt-2, NBC12-P2VP-2, and NBC12-PBzMA-3 (MMs) Initiated by Ru in THF at 25 °Ca polymer code

MM

[MM]0/[Ru]0

[MM]0 (mol L−1)

yieldb (%)

Mn,theoc (kDa)

Mnb (kDa)

Đb

PNBC12-g-PSt-0.05 PNBC12-g-PSt-0.1 PNBC12-g-P2VP-0.05 PNBC12-g-P2VP-0.1 PNBC12-g-PBzMA-0.05 PNBC12-g-PBzMA-0.1

NBC12-PSt-2 NBC12-PSt-2 NBC12-P2VP-2 NBC12-P2VP-2 NBC12-PBzMA-3 NBC12-PBzMA-3

250 250 250 250 250 250

0.05 0.1 0.05 0.1 0.05 0.1

87 87 80 83 95 95

748 748 832 863 924 924

835 1045 2041 4389 1021 1016

1.13 1.45 1.19 1.33 1.04 1.09

Polymerization time: 24 h. bDetermined from SEC-MALLS. cMn,theo = [MM]0/[Ru]0 × conv/100% × MW of MM.

a

Figure 3. 1H NMR of (a) PNBC12-g-PSt-0.05 (Mn = 835 kDa, Đ = 1.13), (b) PNBC12-g-P2VP-0.05 (Mn = 2041 kDa, Đ = 1.19), and (c) PNBC12-g-PBzMA-0.05 (Mn = 1021 kDa, Đ = 1.04) in CDCl3.

of Mn and Đ values with increasing [NBC12-PBzMA-3]0. Their Mn values were very similar to the Mn,theo values and the Đ values maintained less than 1.1. The 1H NMR spectra of PNBC12-g-PSt-0.05, PNBC12-gP2VP-0.05, and PNBC12-g-PBzMA-0.05 showed the conversion of the norbornenyl-ω-end macromonomers to bottlebrush polymers (Figure 3). The characteristic proton resonances of the norbonene-dicarboximido group disappeared or became broader. However, the sharp resonances corresponding to the macromonomer still appeared in the spectra of PNBC12-g-P2VP-0.05, indicating that the complete conversion failed in the ROMP of NBC12-P2VP-2. We assumed that the pyridine pendants would act as extra coordinating ligands for the ruthenium-based active center thereby hindering the metathesis reaction. The SEC−dRI traces of the bottlebrush polymers coexisted with low-MW traces corresponding to linear polymers that were not ω-end-functionalized. PNBC12-g-PSt-0.05 obtained from [NBC12-PSt-2]0 = 0.05 mol L−1 exhibited a bimodal trace. The minor trace corresponding to higher-MW products grew bigger drastically at PNBC12-g-PSt-0.1 obtained from [NBC12-PSt-2]0 = 0.1 mol L−1 (Figure 4a). The main traces of PNBC12-g-P2VP-0.05/0.1 obtained from [NBC12-P2VP2]0 = 0.05/0.1 mol L−1 appeared broad, and the trace shape became broader with increasing [NBC12-P2VP-2]0. In addition, the fraction of coupled polymers contained in the NBC12-P2VP-2 did not decrease (Figure 4b). In contrast, PNBC12-g-PBzMA-0.05/0.1 obtained from [NBC12PBzMA-3]0 = 0.05/0.1 mol L−1 showed two very small traces corresponding to non-end-functionalized polymers and higherMW products. The main traces corresponding to the desired

higher-MW trace corresponding to coupled polymers (Figure 2b). Grafting-through ROMP of Norbornenyl-ω-End Macromonomers Obtained from End-Capping Reaction with a Norbornene-Substituted PFP Ester Containing a Long Flexible Spacer. To evaluate the purity of the macromonomers, NBC12-PSt-2, NBC12-P2VP-2, and NBC12-PBzMA-3 were employed in the grafting-through ROMP initiated by Ru in THF at 25 °C. While the feed ratio of macromonomer to initiator was fixed at [MM]0/[Ru]0 = 250, the initial molar concentration of macromonomer was set at [MM]0 = 0.05 or 0.1 mol L−1. This protocol produces PNBC12-g-PSts, PNBC12-g-P2VPs, and PNBC12-gPBzMAs. A long polymerization time of 24 h was allowed for the three macromonomers to reach the final conversions. The MW results of these bottlebrush polymers are listed in Table 2. When the grafting-through ROMP reached a final conversion, the yield of each bottlebrush polymer relied on the fraction of non-end-functionalized polymers. Two PNBC12-gPSt products, PNBC12-g-PSt-0.05/0.1, obtained from [NBC12-PSt-2]0 = 0.05/0.1 mol L−1 showed the Mn and Đ values increasing with the [NBC12-PSt-2]0 value. This result implies the presence of by-products bifunctionalized with norbornenyl group. Such trend was also observed in the two PNBC12-g-P2VP products, PNBC12-g-P2VP-0.05/0.1, obtained from [NBC12-P2VP-2]0 = 0.05/0.1 mol L−1. The Mn values of these two products were much higher than the Mn,theo values, and the gap between Mn and Mn,theo increased with [NBC12-P2VP-2]0. In contrast, two PNBC12-g-PBzMA products, PNBC12-g-PBzMA-0.05/0.1, obtained from [NBC12-PBzMA-3]0 = 0.05/0.1 mol L−1 showed little change 4832


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of the propagating active center. This effect consequently expands the MW range of a bottlebrush polymer. Our previous route for the one-pot synthesis of norbornenylω-end PBzMA is the sequential process of the living anionic polymerization and end-capping reaction with exo-5-norbornene-2-carbonyl chloride (exo-NBCOCl) as a terminator. This route produces norbornenyl-ω-end PBzMA without the spacer, exo-5-norbornene-2-carbonyl-ω-end poly(benzyl methacrylate) (NBC0-PBzMA).52 To verify the significance of spacer group in the bottlebrush polymer synthesis, we prepared a NBC0PBzMA as a control of NBC12-PBzMA-3. From the synthesis and molecular analysis, the resulting NBC0-PBzMA was found to have a predictable MW and a low dispersity (Mn,theo = 3.14 kDa, Mn = 3.84 kDa, Đ = 1.02). The determination of endcapping efficiency for the NBC0-PBzMA was implemented according to the same method for the NBC12-PXs (X: St, 2VP, and BzMA). As a result, the end-capping efficiency was found to be 98%. This macromonomer was used to produce poly(exo-5-norbornene-2-carbonylate)-graf t-poly(benzyl methacrylate)s (PNBC0-g-PBzMAs) for comparison with PNBC12-g-PBzMAs (Scheme 2).

Figure 4. SEC−dRI traces of (a) PNBC12-g-PSt-0.05/0.1 ([NBC12-PSt-2]0 = 0.05/0.1 mol L−1), (b) PNBC12-g-P2VP0.05/0.1 ([NBC12-P2VP-2]0 = 0.05/0.1 mol L−1), and (c) PNBC12-g-PBzMA-0.05/0.1 ([NBC12-PBzMA-3]0 = 0.05/0.1 mol L−1). BBs: bottlebrush polymers. MM: macromonomer. Non-MMs: non-macromonomers.

Scheme 2. End-Capping Reaction of Living Anionic Species of PBzMA (PBzMA−Li+) with exo-NBCOCl and Graftingthrough ROMP of Resulting Norbornenyl-ω-End PBzMA (NBC0-PBzMA) Initiated by Ru in THF at 25 °C To Afford PNBC0-g-PBzMA

products appeared narrow. In addition, the minor trace corresponding to the higher-MW products little grew by increasing [NBC12-PBzMA-3]0 (Figure 4c). We assumed the possible side reactions during the endcappring reaction from the MW results of the macromonomers and bottlebrush polymers and the previous reports.48,83,84 The end-capping reaction of PSt(DPE)−Li+ would simultaneously cause the proton abstraction of PSt(DPE)−Li+ from the acidic methylene moiety next to the carbonyl group in exo-NBC12PFP. This side reaction would generate a non-end-functionalized polymer with a similar MW of NBC12-PSt. Then, the subsequent reaction between the deprotonated exo-NBC12PFP and normal one would generate a dinorbornenesubstituted PFP ester. This compound would lead to the production of a macromonomer undesirably ω-end-bifunctionalized with norbornenyl group with a similar MW of NBC12PSt. The higher-MW products contained in the desired bottlebrush polymers would be the result of the partial intermolecular metathesis reaction caused by the incorporation of the dinorbornenyl-ω-end macromonomers. An additional by-product with the double MW would be generated in the end-capping reaction of P2VP−Li+ with exo-NBC12-PFP, probably because the sequential addition of two P2VP−Li+ species to the carbonyl carbon of an exo-NBC12-PFP would be catalyzed by pyridine pendants.48 Comparison of a Norbornene-Substituted Carbonyl Chloride with No Spacer and a Norbornene-Substituted PFP Ester with a Long Flexible Spacer in the Bottlebrush Polymer Synthesis. Several previous studies on the ROMP of norbornenyl monomers containing sterically demanding pendant groups have shown the effect of the spacer group on the polymerization kinetics.53,85,86 The main emphasis is that the spacer group should be designed to relieve the propagating active center from the steric crowding of polymeric grafts for well-controlled synthesis of bottlebrush polymers with ultrahigh MWs. The successful approach is to introduce a long flexible spacer to the monomer to separate the propagating active center from the sterically demanding pendants,53,85,86 which helps to overcome a static shielding

Prior to bottlebrush polymer synthesis, the 1H NMR spectra of exo-NBCOCl, NBC0-PBzMA, exo-NBC12-PFP, and NBC12-PBzMA-3 were compared to observe the resonance transition of the vinylene protons by the conversion from the terminator to macromonomer (Figure 5). The exo-NBCOCl exhibited a bimodal resonance distribution of the asymmetric vinylene group (Figure 5a). When the electron-withdrawing chlorine atom of the exo-NBCOCl was replaced with the PBzMA chain, the overall chemical shifts of the vinylene resonances decreased as the two protons were more shielded (Figure 5b). One unusual observation was the asymmetric multimodal resonance distribution of the vinylene group after substitution of PBzMA. The bonds around the vinylene group consist of tertiary and quaternary carbons. As these bonds are rigid, the electron density of the vinylene protons in the NBC0-PBzMA would likely be affected by several possible chain configurations at the ω-end position. This would imply that the vinylene group is sterically shielded by the PBzMA chain. 4833


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[NBC12-PBzMA-3]0 = 389 mg mL−1) as the maximum value, even though this concentration causes the generation of higher-MW products. The ROMP was allowed for a long time of 24 h to reach the final conversion. The MW results of the grafting-through ROMP are listed in Table 3. As the end-capping efficiency for the NBC0-PBzMA was 0.98, the 98% yield of PNBC0-g-PBzMA should be expected for the quantitative macromonomer conversion. This yield was well achieved from the ROMP at low [NBC0PBzMA]0/[Ru]0 values of 125 and 250. However, the yield began to decrease to 94 and 81% as the [NBC0-PBzMA]0/ [Ru]0 increased to 500 and 1000, respectively. PNBC0-gPBzMA-125/250/500/1000, the four products from [NBC0PBzMA]0/[Ru]0 = 125/250/500/1000, showed the Mn values of 457/1005/2218/3605 kDa and the Đ values of 1.08/1.08/ 1.14/1.35. The Đ value increased as the Mn value increased from 1005 to 3605 kDa. In contrast, the ROMP of NBC12-PBzMA-3 maintained a constant yield of 95% at the [NBC12-PBzMA-3]0/[Ru]0 values up to 1000. This yield was well in accordance with the end-capping efficiency of 0.95 for the NBC12-PBzMA-3, which indicates that the macromonomer conversion was quantitative at the [NBC12-PBzMA-3]0/[Ru]0 = 125−1000. The PNBC12-g-PBzMA-125/250/500/1000, the four products from [NBC12-PBzMA-3]0/[Ru]0 = 125/250/500/1000, showed the Mn values of 436/891/1670/4048 kDa and the Đ values of 1.08/1.08/1.15/1.23. The Đ value increased as the Mn value increased from 891 to 4048 kDa similar to the case for the ROMP of NBC0-PBzMA. However, the range of the Đ values was relatively narrow. The SEC−dRI traces of PNBC0-g-PBzMA-125/250/500/ 1000 were compared to those of PNBC12-g-PBzMA-125/ 250/500/1000 (Figure 6). The PNBC0-g-PBzMA-125/250 showed narrow MW distributions and uniform main trace shift to the higher-MW region. The PNBC0-g-PBzMA-500 contained higher-MW products as well as a small fraction of unreacted macromonomers. The PNBC0-g-PBzMA-1000 showed the drastic increase of their fractions. In addition, this polymer showed a less uniform main trace shift with a lower-MW tail (Figure 6a). These results provide the following information. First, the end-capping reaction of PBzMA−Li+ with exo-NBCOCl also competes with the proton abstraction generating minor impurities that are non-end-functionalized and dinorbornenyl-ω-end PBzMAs. Second, the grafting-through ROMP of NBC0-PBzMA is kinetically depressed significantly with increasing steric hindrance at the propagating chain end because of the rigid bonds around the active center.

Figure 5. 1H NMR spectra of vinylenes of (a) exo-NBCOCl, (b) NBC0-PBzMA (Mn = 3.84 kDa, Đ = 1.02), (c) exo-NBC12-PFP, and (d) NBC12-PBzMA-3 (Mn = 3.89 kDa, Đ = 1.02) in CDCl3.

The vinylene group of the exo-NBC12-PFP is symmetric, and thus a single proton resonance appeared (Figure 5c). The same chemical shift was retained after substitution of PBzMA (Figure 5d). As the norbornene-dicarboximido group was separated from the ω-end of the PBzMA by its long flexible 12carbon spacer, the electron density of the vinylene protons would not be affected by substituent change. In addition, the vinylene group in the NBC12-PBzMA-3 would not be sterically shielded because of the conformational change of the long flexible 12-carbon spacer. Consequently, the electron density of these protons would overcome the influence of chain configurations at the ω-end position. This would be the main reason why the NBC12-PBzMA-3 maintained the single resonance of its vinylene protons that is nearly identical with that of exo-NBC12-PFP. Both NBC0-PBzMA and NBC12-PBzMA-3 were used as macromonomers (MMs) in the grafting-through ROMP initiated by Ru in THF at 25 °C. The [MM]0/[Ru]0 value was varied to 125, 250, 500, and 1000. High concentration of propagating active center is required for the synthesis of an ultrahigh-MW bottlebrush polymer. Accordingly, the [MM]0 was fixed at 0.1 mol L−1 ([NBC0-PBzMA]0 = 385 mg mL−1;

Table 3. MW Results of the Grafting-through ROMP of NBC0-PBzMA and NBC12-PBzMA-3 (MMs) initiated by Ru in THF at 25 °Ca polymer code

MM

[MM]0/[Ru]0

[MM]0 (mol L−1)

yieldb (%)

Mn,theoc (kDa)

Mnb (kDa)

Đb

PNBC0-g-PBzMA-125 PNBC0-g-PBzMA-250 PNBC0-g-PBzMA-500 PNBC0-g-PBzMA-1000 PNBC12-g-PBzMA-125 PNBC12-g-PBzMA-250 PNBC12-g-PBzMA-500 PNBC12-g-PBzMA-1000

NBC0-PBzMA NBC0-PBzMA NBC0-PBzMA NBC0-PBzMA NBC12-PBzMA-3 NBC12-PBzMA-3 NBC12-PBzMA-3 NBC12-PBzMA-3

125 250 500 1000 125 250 500 1000

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

98 98 94 81 95 95 95 95

470 941 1805 3110 462 924 1848 3696

457 1005 2218 3605 436 891 1670 4048

1.08 1.08 1.14 1.35 1.08 1.08 1.15 1.23

Polymerization time: 24 h. bDetermined from SEC-MALLS. cMn,theo = [MM]0/[Ru]0 × conv/100% × MW of MM.

a

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Figure 7. Plots of Mn and Đ versus [NBC12-PBzMA-3]0/[Ru]0 for PNBC12-g-PBzMA-125/250/500/1000 (Mn = 436/891/1670/4048 kDa, Đ = 1.08/1.08/1.15/1.23) produced from the grafting-through ROMP of NBC12-PBzMA-3 initiated by Ru in THF at 25 °C. Figure 6. SEC−dRI traces of (a) PNBC0-g-PBzMA-125/250/500/ 1000 (Mn = 457/1005/2218/3605 kDa, Đ = 1.08/1.08/1.14/1.35) and (b) PNBC12-g-PBzMA-125/250/500/1000 (Mn = 436/891/ 1670/4048 kDa, Đ = 1.08/1.08/1.15/1.23).

even though this macromonomer contained the dicarboximide as the anchor group. This result implies that the relief of steric hindrance at the propagating chain end by a long flexible spacer efficiently overcame the reactivity constraints. Thus, we suggest that the long flexible spacer needs to be considered as an essential requirement to afford bottlebrush polymers with a wide range of controlled DPs.

In contrast, all of PNBC12-g-PBzMA-125/250/500/1000 contained only small traces of non-end-functionalized impurities because of the quantitative macromonomer conversion. Each main trace maintained the narrowness and uniform shift to higher-MW region without leaving lower-MW tail. Although the fraction of higher-MW products increased as the MW of the desired product increased, the growth appeared not to be significant (Figure 6b). From the results, two possible conclusions are reached. The probability of the dinorbornenyl-ω-end PBzMA is lower in the end-capping reaction with exo-NBC12-PFP rather than exoNBCOCl because of milder reactivity of the PFP ester group. During grafting-through ROMP, the propagating active center can avoid the steric barricade of the crowding polymeric grafts because of the flexible conformational change of 12-carbon spacers. This effect helps most of the propagating active centers to preserve their active state maintaining the uniform chain growth throughout the polymerization. When the Mn and Đ values of the PNBC12-g-PBzMA-125/ 250/500/1000 were plotted against the [NBC12-PBzMA3]0/[Ru]0 values, the Mn values of 436/891/1670/4048 kDa almost overlapped the line corresponding to the theoretical Mn value at 100% yield (Mn,theo(100%)) (Figure 7). Although the Mn scaling appeared pseudolinear and the Đ value gradually increased with the feed ratio, the Mn value of PNBC12-gPBzMA was proven to be well controlled up to around 4000 kDa by the grafting-through ROMP of NBC12-PBzMA-3 because of the effect of the long flexible 12-carbon spacer. The chemical structure of the norbornenyl anchor group is another consideration for the preparation of effective macromonomers. Radzinski et al. have demonstrated that the type of anchor group affects the metathesis activity between the propagating ruthenium-carbene center and norbornenyl group.32 In their study, a PSt macromonomer containing a dicarboximide anchor and a short spacer showed insufficient reactivity and thus its grafting-through ROMP with [MM]0/ [Ru]0 = 1000 resulted in a low DP of ∼100 with a low macromonomer conversion of ∼10%. In our study, the ROMP of NBC12-PBzMA with the same feed ratio achieved a predictable DP with a complete macromonomer conversion,

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CONCLUSIONS We successfully demonstrated the end-capping reaction of the living anionic polymers of PSt, P2VP, and PBzMA with a norbornene-substituted PFP ester as a facile route to the synthesis of norbornenyl-ω-end macromonomers for graftingthrough ROMP. The living anionic species of PSt and P2VP were too reactive and hence underwent considerable side reactions based on the proton abstraction. On the other hand, the living anion species of PBzMA allowed the one-pot synthesis of a norbornenyl-ω-end PBzMA macromonomer with high end-capping efficiency and monofunctionality because of its mild reactivity tolerant to the side reactions. The long flexible 12-carbon spacer introduced to the PBzMA macromonomer relieved the steric hindrance at the norbornenyl-ω-end. This spacer effect facilitated the well-controlled synthesis of ultrahigh-MW methacrylate-based bottlebrush polymers with main chain DPs up to ∼1000 through the grafting-through ROMP. We expect that the end-capping protocol using well-designed functional PFP esters, living anionic polymers, and appropriate additives will be useful in creating polymers with sterically demanding macromolecular architectures.

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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/acs.macromol.9b00559. Description on the experimental details, and NMR spectra of exo-NBC12-PFP, NBC0-PBzMA, and PNBC0-g-BzMA-125 (MM: NBC0-PBzMA) (PDF)

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Corresponding Author

*E-mail: [email protected]. Phone: +82-62-715-2306. 4835


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Chang-Geun Chae: 0000-0001-8805-6743 Jae-Suk Lee: 0000-0002-6611-2801 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2015R1A2A1A01002493 and NRF2018R1A2B6003616). This work was also supported by “Nobel Research Project” grant for Grubbs Center for Polymers and Catalysis funded by the GIST in 2019.

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End-Capping Reaction of Living Anionic Poly(benzyl methacrylate

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