Synthesis and Isomeric Characterization of Well-Defined 8-Shaped

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Article pubs.acs.org/Macromolecules

Synthesis and Isomeric Characterization of Well-Defined 8‑Shaped Polystyrene Using Anionic Polymerization, Silicon Chloride Linking Chemistry, and Metathesis Ring Closure Qiming He,† Jialin Mao,‡ Chrys Wesdemiotis,*,†,‡ Roderic P. Quirk,† and Mark D. Foster*,† †

Department of Polymer Science and ‡Department of Chemistry, The University of Akron, Akron, Ohio 44325, United States S Supporting Information *

ABSTRACT: A methodology to efficiently synthesize well-defined, 8-shaped polystyrene using anionic polymerization, silicon chloride linking chemistry, and metathesis ring closure has been developed, and the 8-shaped architecture was ascertained using the fragmentation pattern of the corresponding Ag+ adduct, acquired with tandem mass spectrometry. The 4-arm star precursor, 4-star-α-4-pentenylpolystyrene, was formed by linking α-4-pentenylpoly(styryl)lithium (PSLi) with 1,2-bis(methyldichlorosilyl)ethane and reacting the excess PSLi with 1,2-epoxybutane to facilitate purification. Ring closure of 4-star-α-4-pentenylpolystyrene was carried out in dichloromethane under mild conditions using a Grubbs metathesis catalyst, bis(tricyclohexylphosphine)benzylidine ruthenium(IV) chloride. Both the 4-arm star precursor and resulting 8-shaped polystyrene were characterized using SEC, NMR, and MALDI-ToF mass spectrometry (MS). Tandem mass spectrometry (MS2) was used for the first time to study the fragmentation pattern of 8shaped polystyrene. The results confirmed the formation of the intra-silicon-linked, 8-shaped polystyrene isomer, but the observed spectra left open the possibility that the inter-silicon-linked, 8-shaped polystyrene isomer was also produced.

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functional groups.33−37 Hayashi et al.24 synthesized 4-starpoly(methyl acrylate) (PMA) telechelics having four allyl end groups through atom transfer radical polymerization (ATRP) of methyl acrylate using a tetrafunctional initiator followed by an end-capping, radical reaction with allyltributyltin. They subsequently condensed the telechelics using double ringclosure metathesis (RCM) to form a 4.6 kDa, 8-shaped PMA in 38% yield and characterized the cyclization product with MALDI-ToF mass spectrometry. Huang and co-workers19 prepared a 4-star-poly(ethylene glycol) (PEG) precursor having four alkyne chain-end groups through ring-opening polymerization of ethylene oxide using a tetrafunctional initiator followed by alkyne chain-end functionalization. The purity of the cyclization reaction product was determined with MALDIToF mass spectrometry utilizing the mass shift (4.04 Da for 2 × H2 formation) of the Glaser alkyne−alkyne coupling reaction, by the disappearance of the alkyne FTIR stretch absorption after cyclization, and by the increase in SEC elution time after cyclization. They showed that only 8-shaped PEO formed when the precursor concentration was 0.625 g/L, while a shoulder peak corresponding to a side product appeared at a lower elution time when the concentration was increased to 1.000 g/ L. In the first publication about the synthesis of an 8-shaped PS,

INTRODUCTION Elimination of chain ends provides cyclic and multicyclic polymers with unique properties.1−11 For example, low molecular weight, bicyclic eight-shaped polystyrene (8-shaped PS) has a glass transition temperature (Tg) higher than that of linear polystyrene.4,12 In addition, diffusion dynamics of 8shaped polytetrahydrofuran [poly(THF)] chains in linear matrices have two distinguishable modes, while just one mode has been observed for diffusion of 4-arm star poly(THF) in linear matrices.5 Association properties of amphiphilic 8shaped polystyrene-b-poly(acrylic acid) in solutions differs from those of linear polystyrene-b-poly(acrylic acid). The average aggregation number for an 8-shaped block copolymer analogue was lower than that for the linear counterpart, while the size of the aggregate formed by the 8-shaped molecule was slightly larger.6 Advances in structure−property investigations for 8-shaped polymers have been limited by the challenges faced in the synthesis and characterization of such polymers with welldefined structures. Syntheses of cyclic polymers have been reviewed by Laurent and Grayson,13 and strategies for both cyclics and 8-shaped polymers have been reviewed by Tezuka,14,15 Jia and Monteiro,16 and Hadjichristidis and coworkers.17 In general, synthetic routes to 8-shaped polymers can be divided into three main categories: (a) cyclization of 4arm stars,18−28 (b) cyclization of linear precursors,4,6,12,29−32 and (c) postpolymerization linking of single cyclic chains with © XXXX American Chemical Society

Received: May 29, 2017 Revised: July 13, 2017

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DOI: 10.1021/acs.macromol.7b01121 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Antonietti and Folsch12 reported the polymerization of a linear precursor, α,ω-disodium polystyrene, initiated by sodium naphthalene in THF followed by cyclization using 1,2bis(methyldichlorosilyl)ethane. Characterization of the product was limited to SEC and 1H NMR analyses. After cyclization, the SEC curve gave evidence of a mixture of linear precursor, 8shaped PS, and dimer; the dimer could not be removed even after three fractionations. Tezuka and co-workers29,32 developed a synthetic route for 8-shaped poly(THF) using an electrostatic self-assembly and covalent fixation (ESA-CF) process based on terminal cyclic ammonium salt groups and dicarboxylates as counteranions. MALDI-ToF mass spectrometry, together with IR and SEC, was used to confirm the structure of the 8-shaped poly(THF), isolated in 65% yield after preparative thin layer chromatography (TLC). Using postpolymerization linking reactions between single cyclic chains, Monteiro and co-workers33,35 demonstrated synthesis of 8-shaped PS by linking two cyclics with azide and alkyne pendent groups, respectively, using an improved, rapid Cucatalyzed azide/alkyne cycloaddition (CuAAC) “click” reaction. After purification by preparative SEC, 8-shaped PS (82% yield) was characterized using SEC, FT-IR, and 1H NMR spectroscopy. Tezuka et al.36 prepared 8-shaped poly(THF) by intermolecular metathesis condensation of cyclic poly(THF) functionalized with an allyl group. After purification by SEC fractionation, the 8-shaped poly(THF) structure of the product (28% yield) was characterized using 1H NMR spectroscopy. Although the methods mentioned above were successful in preparing 8-shaped polymers, the concomitant formation of higher molecular weight impurities in these methods required extra steps, like fractionation, to obtain pure products. Here we present an efficient synthesis of well-defined 8-shaped polystyrenes using a recently developed, efficient synthetic method for macrocycles based on living alkyllithium-initiated anionic polymerization, silicon chloride linking chemistry, and metathesis ring closure.38 An 8-shaped polymer product should be a mixture of isomers if the core structure has an asymmetric nature, and since the precise nature of the core may affect details of the dynamics of the 8-shaped polymer, it is desirable to differentiate the isomers using advanced characterization methods. Ishikawa et al.23 first analyzed isomeric 8-shaped poly(THF) with a trans-3-hexenyl core synthesized using the ESA-CF process. Subsequent metathesis cleavage of the olefinic group in the 8-shaped poly(THF) was conducted using a second-generation Grubbs catalyst in the presence of ethyl vinyl ether, forming two simple loops of distinct sizes consisting of one and two prepolymer units, respectively. MALDI-ToF mass analysis was performed for the SEC-fractionated metathesis cleavage products to determine the structure of the 8-shaped poly(THF). Chen and co-workers18 determined the ratio of inter isomer (with disulfide group in the loop) and intra isomer (without disulfide group in the loop) of an 8-shaped PS with a disulfide linkage core. The two isomers were confirmed by cleavage of the disulfide bond and analysis with SEC, 1H NMR, and MALDIToF mass spectrometry. The disadvantage of this approach is that it requires the cleavage reaction to distinguish the isomers formed. In the present work, electrospray ionization ion mobility mass spectrometry (ESI-IM-MS)39−43 and matrixassisted laser desorption/ionization tandem mass spectrometry (MALDI-MS2) were used for the first time to investigate isomers of 8-shaped polystyrene; these methods do not require any postcyclization chemical reaction to alter the polymer for

analysis, which could potentially cause misleading structural assignments.

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SAMPLES AND EXPERIMENTAL METHODS

Materials. Styrene (99%, Sigma-Aldrich), benzene (99%, EMD), tetrahydrofuran (THF) (>95%, Fisher Scientific), 5-bromo-1-pentene (99%, Sigma-Aldrich), heptane (99%, EMD), diethyl ether (99%, EMD), and dichloromethane (99%, EMD) were purified as described previously.44 1,2-Bis(methyldichlorosilyl)ethane (Gelest) was stirred over calcium hydride (95%, Sigma-Aldrich) on the vacuum line at 40 °C, transferred into an ampule by short-path distillation, and transferred into a drybox. After dilution with benzene in the drybox, the ampule was flame-sealed under high vacuum. 1,2-Epoxybutane (99%, Sigma-Aldrich) was stirred sequentially over calcium hydride and dibutylmagnesium on the vacuum line, distilled into an ampule, and flame-sealed. Bis(tricyclohexylphosphine)benzylidine ruthenium(IV) chloride (97%, Sigma-Aldrich), lithium (stabilized, 1% Na, FMC), toluene (certified ACS, Fisher Scientific), acetic acid (>99%, Sigma-Aldrich), and methanol (GR ACS, EMD) were used as received. Synthesis of 4-star-α-4-Pentenylpolystyrene. 5-Lithio-1-pentene was prepared according to the procedures of Takano et al.,45 and the concentration of this initiator (0.49 M) was determined using double titration.46 Ampules of styrene (5.3 mL, 42.3 mmol), THF (2.2 mL, 28.2 mmol), 1,2-epoxybutane (ca. 2.5 mL, 28.7 mmol), and acidic methanol (ca. 5 mL with ca. 0.05 mL of acetic acid) were prepared by distillation on the high vacuum line. After degassing, these ampules were flame-sealed from the vacuum line. 1,2-Bis(methyldichlorosilyl)ethane (0.1086 g, 0.42 mmol) and benzene (2 mL) were transferred into an ampule in the drybox, and then the ampule was flame-sealed on a high vacuum line. After flame-sealing all ampules to a 250 mL, Morton-creased flask, the reactor was attached to the high vacuum line. After injection of 5-lithio-1-pentene (5.8 mL, 2.8 mmol) through a glass side arm on the reactor, the side arm was flame-sealed, and benzene (ca. 100 mL) was distilled into the reactor. After the reactor was sealed off from the line, THF and styrene were added to the reactor by smashing the respective break-seals of their ampules sequentially. The reactor was kept at 0 °C for 1 h before a base polymer sample was transferred into a side arm and flame-sealed. After that, the break-seal of the 1,2-bis(methyldichlorosily)ethane solution ampule was smashed, and the reactor was kept at 0 °C for 24 h. Finally, 1,2-epoxybutane and acidic methanol (added 5 min after 1,2epoxybutane) were added into the reactor by smashing their respective break-seals. After characterizing the product distribution using TLC, the α-vinyl-ω-hydroxypolystyrene fraction was separated from the star polymer fraction by silica gel chromatography using toluene as eluent. The 4-star-α-4-pentenylpolystyrene precursor for the macrocycle was isolated after column chromatography in 93% yield and was characterized using SEC, 1H NMR, 13C NMR, and MALDI-ToF MS. Synthesis of 8-Shaped Polystyrene. Bis(tricyclohexylphosphine)benzylidine ruthenium(IV) chloride (0.2761 g, 0.34 mmol) and 4-star-α-4-pentenylpolystyrene (0.5005 g, 0.082 mmol) were each dissolved in separate ampules with dichloromethane (25 mL) in the drybox. The ampules were flame-sealed on the vacuum line before connecting them to a 1 L, Morton-creased reactor. After attaching the reactor to the high vacuum line, dichloromethane (550 mL) was vacuum-distilled into the reactor, and the reactor was removed from the vacuum line by flame-sealing. The ruthenium catalyst and 4-star-α-4-pentenylpolystyrene were added into the reactor by smashing the respective break-seals of their ampules. After heating under reflux at 40 °C for 40 h, the reactor was reconnected to the vacuum line and the solvent removed by flash distillation. The residual catalyst was removed by silica gel column chromatography using toluene as eluent. The polymer product was isolated by precipitation into methanol. The yield after drying the sample in a high-vacuum oven was 0.4138 g (83% yield). The polymer was characterized using SEC, 1H NMR, 13C NMR, and MALDI-ToFMS; the isomers were characterized by ESI-IM-MS and MALDI-MS2. B

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Scheme 1. Reaction Pathway for the Synthesis of 4-star-Polystyrene Using 1,2-Bis(methyldichlorosilyl)ethane as Linking Agent

cell N2 gas flow rate 22.7 mL/min, and sample flow rate through ESI capillary 5 μL/min. The IM-MS experiments were performed by applying a traveling-wave height of 15 V and a traveling-wave velocity of 350 m/s to the IM cell.

Characterization. SEC analyses were performed using a Waters 150-C Plus instrument equipped with three HR-Styragel columns [100 Å, mixed bed (50/500/103/104 Å), mixed bed (103/104/106 Å)], a differential refractometer (Waters 410) detector, and a laser light scattering detector (Wyatt Technology, DAWN EOS, λ = 670 nm). THF was used as eluent with a flow rate of 1.0 mL/min at 30 °C. Samples were dissolved in THF (4.5 mg/mL) and passed through a 0.45 μm Teflon filter before analysis. Results were analyzed using Wyatt ASTRA software (version 4.73.04). 1 H and 13C solution NMR spectra were obtained on a Varian 500 spectrometer (500 MHz) using approximately 50 mg of sample dissolved in 1 mL of CDCl3 (D, 99.8%, Cambridge Isotopes). MALDIMS and MS2 data were collected using a Bruker Ultraflex-III MALDI tandem time-of-flight (ToF/ToF) mass spectrometer (Bruker Daltonics) equipped with a Nd:YAG laser (355 nm). The following solutions were made in THF (Aldrich, 99.9%): 20 mg/mL of the matrix molecule 2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2enylidene]malononitrile (DCTB, Alfa Aesar, 99+%), 10 mg/mL of the cationizing salt, silver trifluoroacetate (Aldrich, 98%), and 10 mg/ mL of the polymer. Matrix and salt were mixed in a ratio of 10/1 (v/ v). About 0.5 μL of the matrix/salt mixture was then deposited onto the 384-well ground steel target plate and allowed to dry. Sample solution was deposited onto the matrix/salt spot, followed by another layer of matrix/salt mixture on the dried sample spot. Analyses were run in positive reflectron mode, and the mass-to-charge ratio (m/z) scale was calibrated using a polystyrene standard with a molecular weight close to that expected for the sample. Monoisotopic peaks were compared to masses calculated for the proposed structure. ESI-IM-MS experiments were performed on a Waters Synapt HDMS Quadrupole/ ToF (Q/ToF) mass spectrometer (Waters Corp.) containing a trap cell, IM cell, and transfer cell located between the quadrupole and ToF mass analyzers and equipped with the traveling-wave version of IMMS.43 A 0.3 mg/mL polymer solution was prepared in a chloroform/ methanol mixture (v/v = 8/2). A 0.06 mg/mL solution of the cationizing salt, sodium trifluoroacetate (Aldrich, 98%), was then added to the polymer solution (salt/polymer = 3/97, v/v). The ESIIM-MS experiments were carried out with a capillary voltage of 3.5 kV, sample cone voltage of 35 V, and extraction cone voltage of 3.2 V. The source temperature was 120 °C, and the desolvation temperature was 150 °C. Nitrogen was used as desolvation gas, with a flow rate of 500 L/h. Other instrument parameters were trap collision energy (CE) 6 eV, transfer CE 4 eV, trap gas flow rate 1.5 mL/min (Ar), ion-mobility

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RESULTS AND DISCUSSION It was envisioned that a new, efficient synthesis of 8-shaped polymers could be developed based on our previous work for the synthesis of macrocyclic polystyrenes utilizing living anionic polymerization, silicon chloride linking chemistry, and metathesis ring closure.38 Living alkyllithium-initiated polymerization is one of the most reliable methods for the synthesis of polystyrenes with controlled, predictable molecular weights, narrow molecular weight distributions, and quantitative chainend functionalizations.47−49 The use of 4-pentenyllithium as initiator ensures that every α chain end has a vinyl group.45 It is known from previous work50 that the reaction of polymeric organolithium compounds with silyl halides is a very efficient reaction that is not complicated by the competing side reactions that are generally observed for linking reactions with organic halides. For the study of polymer chain behavior, particularly dynamics, it is desirable to produce a single isomer, and therefore linking was first attempted with silicon tetrachloride as a linking agent.50 While it is known that for sufficiently large arms steric hindrance prevents achieving quantitative functionality with silicon tetrachloride, it was thought that for the linking reaction of α-(4-pentenyl)poly(styryl)lithium with small arms (M n = 1500 g/mol) functionality of 4.0 could be achieved. However, a mixture of 2-arm star, 3-arm star, and 4-arm star was obtained; even after several fractionations it was not possible to obtain a pure 4-arm star. Therefore, a more efficient linking agent, 1,2-bis(methyldichlorosilyl)ethane, was chosen for further study because the ethylene spacer between the two silicon groups reduces steric crowding in the linking reaction with poly(styryl)lithium arms;48,51−53 in addition, the presence of protons in the ethylene bridge was found to facilitate the characterization of C

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demonstrates the high efficiency of the chlorosilane linking reaction with 1,2-bis(methyldichlorosily)ethane to form 4-starpolystyrene. The polydispersity index based on the differential refractometer trace is Mw/Mn = 1.07. The presence of the characteristic vinyl resonances (−CH=, Ha) and (=CH2, Hb, Hb′) at δ 5.7 and 4.9 ppm in the 1H NMR spectrum (Figure 2a) with an integration ratio of 1.00:2.14

both the 4-star-polystyrene precursor and 8-shaped polystyrene using 1H NMR. 4-star-α-4-Pentenylpolystyrene Macrocycle Precursor. Polymerization of styrene was carried out using 5-lithio1-pentene45 as the initiator and using benzene as the solvent. 10 equiv of THF was added to the initiator to promote dissociation of the aggregates of this primary organolithium initiator;54,55 lower average degrees of aggregation favor initiation over propagation, promoting formation of narrow molecular weight distribution polymers.47−49,54,56 The resulting α-4-pentenyl PSLi chains were linked with 1,2-bis(methyldichlorosily)ethane. A 20% excess of PSLi chain ends relative to the Si−Cl groups in 1,2-bis(methyldichlorosily)ethane was added to drive the linking reaction to completion.17,38,50,57 The addition of THF not only increased the rate of initiation to propagation, it also dramatically accelerated the rate of the chlorosilane linking reaction as reported by Burns and Register.55 In our previous work with silicon chloride linking chemistry, the excess living chains were terminated using ethylene oxide to form ω-hydroxyethylpolystyrene to facilitate their separation from the star polymer.38 However, due to the hazards of working with ethylene oxide, an alternative, liquid epoxy functionalizing agent, 1,2-epoxybutane, was utilized. The functionalization reaction with 1,2-epoxybutane results in efficient functionalization with regiospecific attack of poly(styryl)lithium at the least substituted carbon of the epoxide ring to form the corresponding secondary alcohol.58,59 The difference of polarity between the desired 4-star-polystyrene and the hydroxylfunctionalized polystyrene enabled their facile separation using silica gel column chromatography. This method does not require the usual procedure of fractionation to remove unlinked, excess polymer chains after alcohol termination.50 The SEC chromatogram of 4-star-polystyrene after silica gel chromatography (Figure 1, blue dashed curve) shows a symmetric, monomodal distribution, and the number-average molecular weight, Mn, determined by SEC coupled with light scattering is 6200 ± 620 g/mol [Mn(calcd) = 6000 g/mol], which is about 4 times the Mn of the base polystyrene, Mn = 1200 ± 120 g/mol (Figure 1, black dotted curve). This

Figure 2. (a) 1H NMR spectrum of 4-star-α-4-pentenylpolystyrene and (b) 1H NMR spectrum of 8-shaped polystyrene.

(Ha:Hb, Hb′) demonstrates the incorporation of vinyl groups at the ends of the 4-star-polystyrene.38,44 A broad peak between δ −0.1 and −0.9 ppm corresponds to the protons of methyl groups [−SiCH3, Hc] and methylene groups (−SiCH2CH2Si−, Hd) which are bonded to silicon. The integration of this peak (10 H) relative to that for the vinyl protons (12 H) is 2.57:3.14, while a ratio of 2.50:3.00 should be observed if the α-4pentenylpoly(styryl)lithium linking reaction with 1,2-bis(methyldichlorosily)ethane is quantitative. Thus, the 1H NMR spectrum taken after silica gel chromatography is consistent with the formation of 4-star-α-4-pentenylpolystyrene from the linking reaction.38,44 The 13C NMR spectrum of 4star-polystyrene, as shown in Figure 3a, corroborates the 1H NMR results.38,44 The MALDI-ToF-MS spectrum of purified 4-star-α-4pentenylpolystyrene (Figure 4a) exhibits a clean, monomodal distribution, which indicates the exclusive formation of 4-arm star polymer and that the excess linear chains with hydroxyl end groups have been efficiently removed by column chromatography. The calculated monoisotopic mass based on the structure of the 32-mer 4-star-α-4-pentenylpolystyrene is 3827.22 Da [4 × 69.07 (C5H9) + 32 × 104.06 (C8H8) + 2 × 43.00 (Si-CH3) + 28.03 (CH2CH2) + 106.91 (Ag+)], while the corresponding experimental peak is observed at m/z 3827.30. The distance between closest monoisotopic peaks is 104.1 m/z units (104.1 Da for singly charged ions), corresponding to the mass of a styrene unit. The MALDI-ToF mass spectral

Figure 1. SEC chromatograms of α-(4-pentenyl)polystyrene (black, dotted curve), purified 4-star-α-4-pentenylpolystyrene (blue, dashed curve), and the corresponding 8-shaped polystyrene (red, solid curve). D

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Figure 3. 13C NMR spectra of (a) 4-star-α-4-pentenylpolystyrene and (b) 8-shaped polystyrene.

Mn = 1.06. The elution volume for the 8-shaped polystyrene is larger than that of the starting 4-arm star precursor; this is consistent with the SEC results from previous studies for the synthesis of 8-shaped polymers from 4-arm star precursors.18−27 When the 1H NMR spectrum of the 8-shaped polystyrene (Figure 2b) is compared to that of the 4-starpolystyrene precursor (Figure 2a), it is obvious that the terminal vinyl resonances at δ 4.9 ppm (=CH2, Hb, Hb′) and δ 5.7 ppm (−CH=, Ha) of the 4-star-polystyrene precursor have disappeared, and new internal vinyl proton resonances at δ 5.1 ppm (−CHCH−, Ha) are observed. This is consistent with previous results.38,44 The integration ratio of the broad peak between δ −0.1 and −0.9 ppm corresponding to the protons of the two silicon-bonded methyl groups [−SiCH3, Hc] and silicon-bonded methylene groups (−SiCH2CH2Si−, Hd) compared to that of the new vinyl protons is 5.12/2; this is in good agreement with the expected number of silicon-bonded C−H protons relative to vinyl protons (5/2) for the 8-shaped polystyrene structure (see Scheme 2). By comparing the 13C NMR spectrum of the star precursor (Figure 3a) to that of the 8-shaped polystyrene (Figure 3b), it is apparent that the resonances characteristic of the terminal vinyl carbons at δ

characterization, the SEC chromatogram, and the NMR spectra corroborate the efficient synthesis of 4-star-α-4-pentenylpolystyrene with controlled molecular weight, narrow molecular weight distribution, and quantitative vinyl chain-end functionality. 8-Shaped Polystyrene. The 4-star-α-4-pentenylpolystyrene was effectively cyclized at a concentration of 1.42 × 10−4 M using bis(tricyclohexylphosphine)benzylidine ruthenium(IV) chloride (Grubb’s first-generation catalyst; 5.9 × 10−4 M) in dichloromethane (see Scheme 2) as reported for cyclic metathesis ring closure.44 These dilute solution conditions favor intramolecular ring closure over the corresponding intermolecular reactions. When the cyclization reaction was analyzed after 24 h, the vinyl resonances at δ 5.7 and 4.9 ppm for the star polymer precursor were still present in the 1H NMR spectrum; therefore, the reaction time was extended to 40 h at 40 °C to achieve quantitative cyclization. After 40 h, the residual catalyst was removed using gel column chromatography. The SEC chromatogram of the cyclization product (Figure 1; red, solid curve) exhibited a symmetric monomodal distribution, with Mw = 6100 ± 610 g/mol by light scattering, and Mw/ E

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Figure 4. MALDI-ToF-MS spectra of (a) 4-star-α-4-pentenylpolystyrene and (b) 8-shaped polystyrene.

114.1 ppm and at δ 139.3 ppm for the 4-star-polystyrene precursor have disappeared, and a new internal vinyl carbon resonance has appeared at δ 130.1 ppm. These peak shifts are consistent with previous work for metathesis ring-closure reactions.37,39 MALDI-ToF mass spectrometry is the most sensitive and distinctive characterization method to confirm metathesis ringclosure products because formation of byproduct ethylene molecules during metathesis coupling leads to an unambiguous mass change characteristic of the closure; ring closure to form the 8-shaped polystyrene from the 4-star precursor results in a molecular weight reduction of 56.06 Da (formation of two ethylene molecules). The MALDI-ToF mass spectrum of the 8shaped polystyrene (Figure 4b) shows a monomodal major distribution, with a tiny minor distribution for which the peaks are 29.0 Da less than the corresponding major peaks. This minor distribution has been assigned to products from chain cleavage during ionization, which has been reported previously for small cyclic polystyrenes containing a silicon atom.38 The calculated monoisotopic mass of the 32-mer of the 8-shaped polystyrene is 3771.16 Da [2 × 110.11 (C8H14) + 32 × 104.06 (C8H8) + 2 × 43.00 (Si−CH3) + 28.03 (CH2CH2) + 106.91 (Ag+)], while the corresponding experimental peak is observed at m/z 3771.20. Comparison of the monoisotopic peaks of the 32-mer for 4-star-polystyrene precursor (Figure 4a) and with those for 8-shaped polystyrene (Figure 4b) shows a reduction of m/z 56.10 after metathesis ring closure; these results confirm the loss of two ethylene molecules (56.06 Da) in forming the product of the cyclization reaction. More strikingly, no peaks

corresponding to the precursor 4-star polystyrene are observed in the MALDI-ToF mass spectrum of the 8-shaped polystyrene product, further supporting the conclusion that this is an efficient cyclization reaction. The SEC chromatograms, NMR spectra, and most importantly the MALDI-ToF mass spectral results provide direct evidence for the structural characterization and efficient synthesis of 8-shaped polystyrene from the metathesis ring closure reaction of 4-star-α-4-pentenylpolystyrene. The cumulative characterization data show that the combination of alkyllithium-initiated polymerization, silicon chloride linking chemistry, and metathesis ring closure provides an efficient methodology for the synthesis of well-defined 8shaped polystyrene with controlled molecular weight and narrow molecular weight distribution. It should be possible to extend the approach to higher molecular weights. Because of the high efficiency of the Grubbs catalyst, the only limitation of the double metathesis cyclization reaction is the intermolecular metathesis reaction. As previously reported, we have already achieved synthesis of a 37K cyclic polystyrene using the metathesis cyclization,44 so synthesis of a 37K 8-shaped polystyrene should be possible. The size of the 4-arm star precursor for the 8-shaped molecule is smaller than the linear precursor for the cyclic at the same molecular weight, which makes the intermolecular condensation reaction less probable for the double metathesis reaction. We note that an alternative route to an 8-shaped polymer would use CuAAC or other “click” chemistry33,35 for the cyclization. While the cyclization is efficient both with the click approach and methathesis ring closure, there are advantages F

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Macromolecules Scheme 2. Reaction Pathway for the Metathesis Ring Closure To Form 8-Shaped Polystyrene

product using SEC. These methods require postcyclization chemical reaction to form loops with two distinctive sizes. Analysis with mass spectrometry, however, can circumvent the need for postcyclization reactions. Electrospray ionization ion mobility mass spectrometry (ESIIM-MS) experiments have demonstrated the ability to distinguish isomeric oligomers.39−42,60−64 However, the present study represents the first application of this method to polystyrene macrocycles. Ion mobility spectrometry (IMS) separates gas-phase ions according to their charge, size, and shape. Ion mobility mass spectrometry (IM-MS) couples IMS to mass analysis, thus providing two-dimensional separation; the ion mobility dimension probes the time it takes for an ion to drift through a chamber against a buffer gas (nitrogen) under an electric field, while the MS dimension measures the mass-tocharge ratio (m/z) of the ion. IM-MS is useful for distinguishing isomers and conformers, and ions with the same m/z value but different charges, as long as their drift times through the IM chamber are different. The rotationally averaged forward-moving area of an ion, which is sensed during the collisions with the buffer gas, is called collision cross section (CCS);65−67 this parameter, which can be deduced from the ion’s drift time through the IM cell, reveals useful information about the ion’s size and shape (viz. architecture). Larger CCSs cause longer drift times and vice versa. Further, a larger CCS indicates a more extended ion architecture, whereas a smaller CCS is indicative of a smaller, more compact architecture. The ability of IM-MS to unveil such molecular structure features has led to steadily increasing applications of

with the metathesis approach. The cyclization using metathesis condensation generates a macrocycle with a mass 28 mass units lower than that of its precursor, while “click” chemistry generates a macrocycle with a mass identical to that of its precursor. Thus, MALDI-ToF mass analysis can be used unambiguously for characterization of the product of the metathesis ring-closure reaction. In addition, the “click” reaction leads to a relatively polar entity in the macrocycle, while our method incorporates a nonpolar entity less likely to modify the PS chains’ interactions with the air and substrate interfaces of supported films.8−11 For fundamental physical studies attempting to isolate the effect of chain architecture on film behavior the well-defined 8-shaped polystyrene synthesized using anionic polymerization, silicon chloride linking chemistry and metathesis ring closure presents a model compound. Advanced Isomer Characterization Using Mass Spectrometry. After a thorough characterization to confirm the formation of 8-shaped polystyrene, it remains to distinguish the isomers resulting from the cyclization reaction. It is a challenge to characterize the isomers of this 8-shaped polystyrene because both the size of the whole molecule and the chemical environment at the core are so similar for the two isomers that it is impossible to differentiate them using SEC, MALDIToF-MS, or NMR. Tezuka and co-workers23 analyzed isomeric 8-shaped poly(THF) with a trans-3-hexenyl core using a metathesis cleavage reaction of the olefinic group. Then MALDI-ToF mass analysis could elucidate isomer formation due to the isomers having two distinct loop sizes. Wang et al.18 quantitatively determined the ratio of isomers by cleaving a disulfide linkage core in an 8-shaped PS and characterizing the G

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Macromolecules

Figure 5. MALDI-ToF/ToF-MS2 spectrum of the silverated 32-mer from 8-shaped polystyrene.

Measured and calculated CCS values agree reasonably well (within