Effect of Ring Functionalization on the Reaction Temperature of

May 5, 2016 - ... benzocyclobutene-terminated imides with high organosolubility. Xianfeng Que , Yurong Yan , Zhiming Qiu. European Polymer Journal 201...
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Effect of Ring Functionalization on the Reaction Temperature of Benzocyclobutene Thermoset Polymers Colin O. Hayes,† Peng-hao Chen,† R. Paxton Thedford,‡ Christopher J. Ellison,‡ Guangbin Dong,† and C. Grant Willson*,† †

Department of Chemistry and ‡McKetta Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712-1224, United States S Supporting Information *

ABSTRACT: The temperature required to induce cross-linking in typical benzocyclobutene-based thermosets is near 250 °C, which exceeds the use temperature of many chemical components. A new and versatile synthesis of BCB-functionalized monomers has allowed access to monomers that can be incorporated into a variety of macromolecular platforms to enable significantly reduced cure temperatures. Incorporation of BCB-functionalized comonomers in polystyrene and polynorbornene enabled insolublization of thin films by curing at only 120 °C for 1 h.

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INTRODUCTION Benzocyclobutene (BCB) chemistry has been exploited in the design of thermally cross-linkable thin films1 and well-defined nanoparticles.1,2 The BCB chemistry is attractive in part because it takes place with very little densification and shrinkage.3,4 BCB chemistry has also found a niche in total synthesis as a latent Diels−Alder diene5 as well as a substrate for C−C activation,6 and its use in industrial thermosets is widespread. For example, Dow Chemical Company’s thermoset product Cyclotene is based on BCB chemistry, and it is used in a variety of microelectronics applications, such as wafer-level packaging, liquid-crystal displays, and GaAs interlayer dielectrics. Unfortunately, classical BCB chemistry requires cure temperatures near 250 °C,7 a temperature that is incompatible with sensitive processes. It has been reported that functionalization of the alkyl ring on BCB can significantly lower the temperature at which the electrocyclic ring opening reaction responsible for cross-linking takes place.8 Pugh et al.9 described the synthesis of 1-ethoxy-4vinylbenzocyclobutene, which cures at a significantly reduced temperature. The key step in their synthesis of substituted BCB involves use of an ortho-diazonium carboxylate salt as a benzyne precursor. This route facilitated the synthesis of a BCB monomer that was polymerizable and cross-linked at a reduced temperature, but the BCB forming step was low yielding and potentially dangerous on large scale. Wilson et al. report that 1,2 BCBs substituted with long polymer chains can undergo an ultrasound-induced electrocyclic ring opening at 6−9 °C, but synthesizing long tethers to the BCB ring is often impractical, and sonication is not a viable means of curing a thermoset in an industrial application.10 Harth et al. designed a 1-substituted © XXXX American Chemical Society

BCB that underwent reduced cross-linking, but the material had to be “grafted-to” an acrylic-acid-containing polymer, and only the synthesis of chain collapsed nanoparticles was demonstrated.11 An efficient route has been devised for the formation of the phenolic benzocyclobutenone 5, which that can be conveniently derivatized to incorporate polymerizable functionality, and the ketone can be modified to provide derivatives with a range of ring opening temperatures. The ring-opening reaction of several such derivatives was studied by differential scanning calorimetry (DSC). Applying Mitsunobu conditions on 5 using a perfluoro-tertbutanol in tetrahydrofuran (THF) produced an unusual result: a THF inserted major product. The THF inserted compound underwent opening at 148 °C (maximum of the exotherm). Polystyrene and polynorbornene copolymers containing this alkoxy BCB were studied in thin films, and both were rendered insoluble upon heating to 120 °C for 1 h.

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RESULTS AND DISCUSSION Synthesis. Most routes to 1-functionalized BCBs utilize benzyne as a reactive intermediate. While there are a variety of ways to generate benzyne,10 the benzocyclobutenone phenol 5 was synthesized in four steps efficiently, as previously reported by Chen et al.,6 and this route was reproduced in this study. The meta-bromophenol 1 was alkylated quantitatively to give the corresponding benzyl ether 2 (Scheme 1). Next, the key Received: February 10, 2016 Revised: April 14, 2016

A

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Macromolecules Scheme 1. Synthesis of Benzocyclobutenone Phenol

Allyl BCB 6a was derivatized for DSC studies. The ketone was reduced with sodium borohydride in methanol to produce the benzocyclobutenol 6b. From 6b, two silyl-protected derivatives (tertiary butyl dimethyl silane (TBS) and tertiary butyl diphenyl silane (TBDPS)) were synthesized by the Corey protocol,15 as shown in Scheme 3. The alcohol 6b could not be successfully alkylated in a typical Williamson ether synthesis protocol.16 It was observed that basic conditions even at room temperature caused the ring to undergo isomerization to the ortho-methyl benzaldehyde. The Mitsunobu reaction was attempted using perfluoro-tert-butanol as a coupling partner, intending to synthesize compound 11. When the reaction was run in THF, the unexpected THF inserted product 6c was produced. This reaction is unusual as it is uncommon for the Mitsunobu reaction to make alkyl ethers efficiently. Other ethereal solvents such as dioxanes and diethyl ether were tested; however, no other ether insertion product was observed. In the case of ethyl ether, only starting material was recovered and no other products, suggesting diethyl ether decreases the overall rate of the reaction. Furthermore, when the reaction was run at −78 °C then cooled to room temperature, the yield of 6c increased from 71 to 87% and 11 could no longer be observed. This suggests that the THF insertion compound is the kinetic product of this transformation. These data are summarized in Table 1. A control reaction was carried out using the same reaction conditions except p-nitrobenzoic acid (which has comparable acidity) was employed as the coupling partner. In this case, no THF insertion product was observed. This observation undermines the hypothesis that the reaction proceeds via acid-catalyzed ring opening of THF and subsequent coupling. Rather, because the anion of perfluoro-tert-butanol is such a bulky and therefore poor nucleophile, it is possible that there is a competition between THF and the perfluoro-anion as a nucleophile. The oxonium ion resulting from a nucleophilic addition of THF might then be opened by the perfluoro anion, resulting in 6c. A pathway to 6c that is consistent with the experimental results is provided in Scheme 4. A model system of allyl benzocyclobutenols was prepared and studied to determine their exothermic reactions compared with that of unfunctionalized benzocyclobutene. The allyl functionality was chosen as a model for its similarity to other polymerizeable groups (both alkyl in nature and containing a double bond), yet it is much less likely to undergo autopolymerization compared with a styrene. Samples were

benzocyclobutenol ring-forming step was performed by treating the aryl bromide with lithium tetramethylpyridine to form benzyne, followed by coupling to the enolate of acetaldehyde, which was generated by stirring n-butyl lithium in THF at room temperature for 16 h. The coupling afforded benzocyclobutenol 3. Swern oxidation produced ketone 4, and deprotection of the benzyl ether via hydrogenation gave the benzocyclobutenone phenol 5 in 75% yield overall in four steps. (It was also found that compound 5 could be isolated by treating 4 with BCl3 in DCM.) Various monomers could then be accessed by derivatization of benzocyclobutenone 5. Mitsunobu reaction on 5 with corresponding alcohols produced allyl BCB 6a, styrene BCB 7a, and norbornene BCB 8a. 4-Vinylphenyl methanol and 5-norbornene methanol were prepared via literature methods from commercially available starting materials,12−14 as depicted in Scheme 2. Scheme 2. Synthesis of Allyl, Styrene, and Norborene BCBs

Scheme 3. Synthesis of Silyl-Protected Benzocyclobutenols

B

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Macromolecules Table 1. Mitsunobu Reaction Results Using Perfluoro-tert-butanol

Conditions: Solvent (1.0 M) PPh3 (1.8 eq) DIAD (2.0 eq) Alcohol (2−5 eq) 0° to RT

17% (0%−78 °C to RT)

THF

71% (87%−78 °C to RT)

DCM

62%

0%

Dioxanes

94%

0%

Ethyl Ether

34%

0%

Scheme 4. Proposed Pathway to the THF Insertion Product

benzocyclobutenones. Most significant are the results of compounds 6b and 6c, which have exothermic transitions at significantly reduced temperatures (176 and 148 °C, respectively). It should be noted that compound 6c performs similarly to the 1-ethoxy styrene synthesized by Pugh et al. (135 °C as reported),9 and thus lends credence to the hypothesis that the 1-alkoxy group is a key contributor to cross-linking temperature because the delta-trifluoromethyl groups appear to have no effect. The work by Harth et al. did not utilize DSC data; therefore, a direct comparison of cross-linking temperature cannot be made. The alcohol 6b has the possibility of keto−enol tautomerization competing with Diels−Adler addition;17 therefore, alkoxy-BCB 6c was chosen as a better candidate for a cross-linking application. The styrene and norbornene analogues were both prepared analogously to the allyl model compounds (Table 2). The Mitsunobu reaction was used to couple the requisite styrene

prepared in hermetically sealed DSC pans and scanned from 25 to 300 °C twice. The results are highlighted in Figure 1. Compounds 6a and 12 (5-(allyloxy)-8-methylbicyclo[4.2.0]octa-1,3,5-trien-7-one (Scheme 5), see Supporting Information) were prepared from a known method,6 while cyclotene, donated from Dow Chemical, served as a source of unfunctionalized BCB 13. The maximum of the exotherm in BCB 13 was consistent with literature accounts of BCB curing at 250 °C. Interestingly, both silyl-protected BCBs 9 and 10 had exactly the same exothermic maximum, 260 °C (see Supporting Information). This result suggests that the steric bulk of the different silyl groups had no affect the electrocyclic ring opening. Both benzocyclobutenone derivatives 6a and 12 had exothermic transitions at 231 and 238 °C, slightly below that of unfunctionalized BCB, indicating that the 2-methyl functionalization has little effect on changing the ring opening in C

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Figure 1. Normalized differential scanning calorimetry exotherms of functionalized benzocyclobutene derivatives.

products 7b and 8b were produced cleanly with NaBH4. The THF-inserted Mitsunobu products 7c and 8c were also isolated in reasonable yield. Statistical copolymers were synthesized from monomers 7c and 8c (Scheme 6). The ring-opening metathesis copolymerization (ROMP) of 8c and norbornene proceeded cleanly at room temperature to provide polymer 15. Classical freeradical copolymerization (AIBN, 60 °C) with styrene and monomer 7c resulted in an insoluble solid that was likely crosslinked. Using a bulk, light-initiated polymerization held at room

Scheme 5. Synthesis of BCB Methyl Ketone 12:

and norbornene functionalities via the available phenol. (The coupling partners were synthesized according to literature procedures; see the Supporting Information.) The reduction

Table 2. Synthesis of Target Monomers from Benzocylobutenone 5

D

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Macromolecules Scheme 6. Synthesis of Low-Temperature Cross-Linkable Polymers

Figure 2. Relative film thickness of BCB containing polymers under different cure conditions.

50 to 200 nm, which were measured by spectroscopic ellipsometry. Film thicknesses were measured before curing, after curing, and after a toluene wash (2 × 10 s rinse of toluene), which completely stripped control homopolymers polystyrene and polynorbornene subject to the same curing conditions. Curing was carried out in a vacuum oven, which was purged with argon gas, then heated to the requisite temperatures for 1 h. The results are outlined in Figure 2. Both polymers 14 and 15 showed strong retention of thickness at 120 °C (for 1 h) after toluene washing. Polynorbornene 15 had a sharper onset and reached complete film retention at 150 versus 180 °C for the polystyrene derived copolymer 14. This is likely due to a high concentration of double bonds in the thin film that could serve as readily available sites for Diels−Alder addition. BCB cross-linking readily occurs through a Diels−

temperature resulted in a THF soluble polymer that was precipitated in methanol. The ROMP and light-initiated polymerizations both proceeded cleanly at room temperature, which could imply that the temperature required for AIBN initiation caused ring opening of the benzocyclobutene monomer, which, in turn, generated an insoluble cross-linked network. This result suggests that the alkoxy-substituted BCB is not thermally stable in solution for extended time. The lightinitiated radical polymerization produced soluble polymer (not cross-linked), which suggests that the substituted BCB does not open in the presence of radicals and rather responds only to temperature. Polymers 14 and 15 were prepared to evaluate their thermal cross-linking behavior in thin films. Solutions were spun from toluene on bare silicon wafers yielding films of thicknesses from E

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Figure 3. Proton NMR of compound 6c in DMSO over time at 120 °C. Protons Ha and Hb have been highlighted for clarity.

BCB. After 24 h, the signal from proton Ha disappears completely, suggesting a complete conversion of the thermal reaction. Not only do these results demonstrate the disappearance of the BCB ring as a function of temperature but also they are also consistent with the thin film data that show no cross-linking until 120 °C. As a control experiment, compound 11, which has no exothermic peak on the DSC trace up to 300 °C, was subjected to the same thermal treatment. The compound was shown to be completely inert to any reaction over the course of 48 h at 120 °C, which is consistent with DSC results (see Supporting Information).

Alder reaction, but for a cross-link to form, each BCB unit must encounter some type of dienophile. In the case of polymer 15, the polynorbornene backbones contain one alkene per repeat unit, meaning that the concentration of dienophiles is very high in the thin film. By contrast, polymer 14 contains no dienophile except each BCB unit itself (when ring opened). Thus, polymer 15 may have reached a greater cross-linking density at a reduced temperature due to many available dienophiles, which, in turn, resulted in greater film retention than polymer 14. The results demonstrate that monomers 7c and 8c have the effect of insolubilizing thin films upon mild thermal treatment, while ordinary unfunctionalized BCB monomers require temperatures up to 250 °C to cure and polymers 14 and 15 have been rendered insoluble at 120 °C, a significant reduction. In conjunction with DSC data, which show a single irreversible exothermal transition (Figure 1), the results suggest that copolymers have cross-linked as a result of a reduced temperature electrocyclic ring opening of the BCB monomers. Compounds 6c and 11 were subjected to a time and temperature study using H1 NMR to further probe the mechanism of cross-linking. The spectrum of compound 6c showed no noticeable changes at 80 and 100 °C over 24 h; however, at 120 °C the protons assigned to the 1-alkoxy BCB ring began to decrease in intensity relative to other protons. Figure 3 shows that proton Ha, the 1-alkoxy proton on the ring, disappears, while proton Hb, the perfluoro-alkoxy protons, is constant in signal over the course of a ring-opening isomerization event. After 4 h, the signal of Ha decreased to 63% of its original intensity relative to Hb. Furthermore, Hb began as a triplet but after 4 h became two overlapping triplets, consistent with a mixture of starting material and ring-opened

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CONCLUSIONS AND FUTURE DIRECTIONS The synthesis of several 1-functionalized BCBs, the synthesis of BCB monomers, and their respective polymers are described, and thickness retention and cross-linking behavior of those polymers in the thin film is reported as a function of temperature. This work emphasizes the fact that there is likely an electronic relationship between the structure of the benzocyclobutene ring and the temperature at which electrocyclic ring opening occurs. The most important implication for the macromolecular chemist is that judicious use of chemistry can afford a thermal cross-linking group that operates at greatly reduced temperatures (150 °C or less) from that of ordinary, saturated benzocyclobutenes. The synthesis of these compounds has also been designed in such a way that a useful functionality (the free phenol) has been built into the ring so that custom monomers may be appended. This approach is applied to norbornene and styrene. It should be cautioned, however, that polymer synthetic methods that F

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using hexanes and ethyl acetate (1:1) as an elutent. The product was collected and concentrated in vacuo to yield a white solid in 87% yield (175 mg). 1H NMR (400 MHz, CDCl3) δ 7.43−7.22 (m, 4H), 7.19− 7.09 (m, 1H), 6.72 (d, J = 8.4 Hz, 1H), 6.69−6.58 (m, 2H), 5.67 (dd, J = 17.6, 0.7 Hz, 1H), 5.29−4.92 (m, 4H), 3.46 (dd, J = 14.5, 4.6 Hz, 1H), 2.89 (dd, J = 14.5, 0.9 Hz, 1H), 2.33 (s, 1H). 13C NMR (101 MHz, CDCl3) δ 153.49, 144.01, 137.19, 137.01, 136.44, 131.37, 130.99, 127.41, 126.37, 115.91, 114.91, 114.02, 70.85, 70.77, 42.37. HRMS: Calculated for C17H16O2 (M+Na)+ = 275.104, found 275.104.

require heat may cause premature gelation, as was the case in the attempt to synthesize polymer 14 thermally. The thin-film data show that the alkoxy/THF-inserted BCB polymers in this paper are useful for rendering desired films insoluble.

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EXPERIMENTAL SECTION

General Procedures. Size-exclusion chromatography (SEC) data were collected with an Agilent 1100 Series isopump and autosampler with a Viscotek model 302 TETRA detector platform and THF as an eluent at 23 °C. Three I-series mixed bed high-MW columns were calibrated relative to PS standards. A Brewer CEE 100CB spin-coater was used to coat all thin films. Ellipsometry was performed with a J.A. Woollam VB 400 VASE ellipsometer withwavelengths from 382 to 984 nm and a 65° angle of incidence. Thermogravimetry and differential scanning calorimetry were performed using a TA Instrument Q500 TGA and TA Q100 DSC, respectively, each with a 50 mL flow rate of nitrogen. DSC method used was a constant heating/cooling rate of 10 °C/min from 25 to 300 °C and back to 25 °C for two cycles. TGA samples were ramped to 1000 °C at 10 °C/min heating rate under a nitrogen atmosphere. All thin-film samples were placed in a vacuum oven, purged with nitrogen three times, and cured at the specified temperature for 1 h. Reagents. Triphenyl phosphine and di-(4-chlorobenzyl)azodicarboxylate were purchased from Combi Blocks. Perfluoro-tertbutanol was purchased from Oakwood Chemical. Diisopropyl azodicarboxylate, tert-butyldimethylsilyl chloride, tert-butyl(chloro)diphenylsilane, imidazole, styrene, norbornene, THF, DCM, hexanes, and ethyl acetate were purchased from Sigma-Aldrich. 5-Hydroxybicyclo[4.2.0]octa-1,3,5-trien-7-one (compound 5),6 (4vinylphenyl)methanol [18], and (bicyclo[2.2.1]hept-5-en-2-yl)methanol [19] were synthesized according to literature procedures. Synthetic Data. 5-((4-Vinylbenzyl)oxy)bicyclo[4.2.0]octa-1,3,5trien-7-one. 5-Hydroxybicyclo[4.2.0]octa-1,3,5-trien-7-one (150 mg, 1 equiv), (4-vinylphenyl)methanol (150 mg, 1 equiv), and triphenyl phosphine (526 mg, 1.8 equiv) were dissolved in tetrahydrofuran (6.5 mL) in a 100 mL round-bottomed flask equipped with a magnetic stir bar. The mixture was stirred at room temperature, at which point diisopropyl azodicarboxylate (DIAD, 439 μL, 2.0 equiv) was added dropwise. The reaction was stirred at room temperature overnight, at which point reaction was loaded onto silica gel and purified using hexanes and ethyl acetate (9:1) as an elutant. The product was collected and concentrated in vacuo as a colorless oil in 68% yield (190 mg). 1H NMR (400 MHz, CDCl3) δ 7.47−7.40 (m, 5H), 7.04 (dd, J = 7.1, 0.5 Hz, 1H), 6.88 (dd, J = 8.4, 0.5 Hz, 1H), 6.72 (dd, J = 17.6, 10.9 Hz, 1H), 5.76 (dd, J = 17.6, 0.9 Hz, 1H), 5.45 (s, 2H), 5.26 (dd, J = 10.9, 0.9 Hz, 1H), 3.94 (t, J = 0.8 Hz, 2H). 13C NMR (101 MHz, CDCl3) δ 184.99, 152.20, 150.55, 137.77, 137.47, 136.41, 135.93, 132.53, 128.07, 126.32, 116.48, 115.31, 114.16, 73.69, 51.22. HRMS: calculated for C17H14O2 (M+H)+ 251.106, found 251.106.

5-(Bicyclo[2.2.1]hept-5-en-2-ylmethoxy)bicyclo[4.2.0]octa-1,3,5trien-7-one. 5-Hydroxybicyclo[4.2.0]octa-1,3,5-trien-7-one (313 mg, 1 equiv), (bicyclo[2.2.1]hept-5-en-2-yl)methanol (289 mg, 1 equiv), and triphenyl phosphine (611 mg, 1 equiv) were dissolved in dichloromethane (40 mL) in a 100 mL round-bottomed flask equipped with a magnetic stir bar. The mixture was stirred at room temperature at which point a solution of di(4-chlorobenzyl)azodicarboxylate (DCAD, 856 mg, 1 equiv) in DCM (10 mL) was added dropwise. The reaction was stirred at room temperature overnight, at which point the white precipitate was filtered and then the filtrate was columned on silica gel using hexanes and ethyl acetate (9:1) as an elutant. The product was collected and concentrated in vacuo as a colorless oil in 52% yield (291 mg). 1H NMR (400 MHz, CDCl3) δ 7.37−7.28 (m, 1H), 6.90 (dd J = 7.0, 5.5, 1H), 6.72 (td, J = 8.7, 0.6 Hz, 1H), 6.09 (dd, J = 5.7, 3.0 Hz, 1H, endo), 6.02 (ddd, J = 13.2, 5.4, 3.0 Hz, 2H exo), 5.89 (dd, J = 5.7, 2.9 Hz, 1H endo), 4.38 (dd, J = 10.3, 6.2 Hz, 1H exo), 4.18 (dd, J = 10.3, 9.2 Hz, 1H exo), 4.06 (dd, J = 10.2, 6.8 Hz, 1H endo), 3.87 (dd, J = 10.2, 9.2 Hz, 1H endo), 3.81−3.77 (m, 2H), 2.92 (s, 1H), 2.81−2.71 (m, 1H), 2.49−2.37 (m, 1H), 1.87−1.72 (m, 1H), 1.42−1.14 (m, 2H), 0.62 (ddd, J = 11.7, 4.4, 2.6 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ 184.74, 152.76, 150.53, 137.56, 136.90, 136.32, 132.31, 116.16 (exo), 116.10 (endo), 114.76 (exo), 114.66 (endo), 76.60 (exo), 75.93 (endo), 51.12, 49.33 (endo), 45.02 (exo), 43.81 (endo), 43.55 (exo), 42.21 (endo), 41.62 (exo), 38.60 (exo), 38.36 (endo), 29.39 (exo), 28.82 (endo). HRMS calculated for C16H16O2 (M+Na)+ 263.104, found 263.104.

5-(Bicyclo[2.2.1]hept-5-en-2-ylmethoxy)bicyclo[4.2.0]octa-1(6),2,4-trien-7-ol. 5-((Bicyclo[2.2.1]hept-5-en-2-yl)methoxy)bicyclo[4.2.0]octa-1(6),2,4-trien-7-one (120 mg, 1 equiv) was dissolved in methanol (15 mL) in a 25 mL round-bottomed flask equipped with a magnetic stir bar at 0 °C. Sodium borohydride (23 mg, 1.2 equiv) was added, and the reaction was capped with a nitrogen balloon and warmed to room temperature over 2 h. Dilute HCl (0.5M, 10 mL) was added, and the reaction was stirred until a white solid formed (∼30 min). The reaction was transferred to a separatory funnel and extracted with ethyl acetate (5 × 15 mL). The combined organic extract was washed with brine and dried with magnesium sulfate, filtered, and columned on silica gel using hexanes and ethyl acetate (1:1) as an elutent. The product was collected and concentrated in vacuo to yield a colorless oil in 93% yield (112 mg). 1H NMR (400 MHz, CDCl3) δ 7.15−7.05 (m, 1H), 6.70−6.56 (m, 2H), 5.98 (m, 2H), 5.24−5.10 (m, 1H), 4.29−3.59 (m, 2H), 3.54−3.36 (m, 1H),

5-((4-Vinylbenzyl)oxy)bicyclo[4.2.0]octa-1(6),2,4-trien-7-ol. 5-((4Vinylbenzyl)oxy)bicyclo[4.2.0]octa-1(6),2,4-trien-7-one (200 mg, 1 equiv) was dissolved in methanol (15 mL) in a 25 mL roundbottomed flask equipped with a magnetic stir bar at 0 °C. Sodium borohydride (36 mg, 1.2 equiv) was added, and the reaction was capped with a nitrogen balloon and then warmed to room temperature over 2 h. Dilute HCl (0.5M, 10 mL) was added, and the reaction was stirred until a white solid formed (∼30 min). The reaction was transferred to a separatory funnel and extracted with ethyl acetate (5 × 15 mL). The combined organic extract was washed with brine and dried with magnesium sulfate, filtered, and columned on silica gel G

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equiv). The mixture was stirred at 0 °C, and diisopropyl azodicarboxylate (DIAD) was added dropwise (248 μL, 2 equiv). The reaction was warmed to room temperature overnight, at which point the reaction was loaded onto a column of silica gel and purified using a gradient of hexanes to hexanes and ethyl acetate (8:2). As the second compound to elute, the product was concentrated in vacuo and isolated as a colorless oil in 61% yield (210 mg). 1H NMR (400 MHz, CDCl3) δ 7.44−7.19 (m, 5H), 6.83−6.66 (m, 3H), 5.75 (ddd, J = 17.6, 4.7, 0.9 Hz, 1H), 5.30−5.07 (m, 3H), 4.93 (dd, J = 4.2, 1.8 Hz, 1H), 4.08−3.96 (m, 2H), 3.68−3.53 (m, 2H), 3.41−3.29 (m, 1H), 3.11− 2.98 (m, 1H), 1.88−1.66 (m, 4H). 13C NMR (101 MHz, CDCl3) δ 153.63, 144.14, 137.17, 136.43, 132.68, 131.28, 129.38, 127.23, 126.32, 121.88 (q, J = 292 Hz), 118.96, 115.90, 114.21, 113.94, 76.15, 70.45, 69.62, 67.55, 38.08, 26.78, 25.91.19F NMR (376 MHz, CDCl3) δ −70.40 (s). HRMS: calculated for C25H23F9O3 (M+Na)+ 565.140, found 565.139.

2.93 (s, 1H), 2.87 (dd, J = 14.0, 5.5 Hz, 1H), 2.76 (d, J = 7.3 Hz, 1H), 2.46 (s, 1H), 1.90−1.64 (m, 1H), 1.44−1.09 (m, 2H), 0.55 (ddd, J = 11.8, 7.2, 4.7 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ 153.82, 144.08, 137.52, 136.85, 132.39, 131.29, 115.41, 114.46, 73.70 (exo), 72.96 (endo), 70.86, 49.39, 43.85, 42.31, 38.82, 38.63, 29.55 (exo), 28.98 (endo). HRMS = calculated for C16H18O2 (M+Na)+ 265.120, found 265.120.

2-(Allyloxy)-8-((1,1,1,3,3,3-hexafluoro-2-(trifluoromethyl)propan2-yl)oxy)bicyclo[4.2.0]octa-1,3,5-triene. 5-(Allyloxy)bicyclo[4.2.0]octa-1,3,5-trien-7-ol (152 mg, 1 equiv) was dissolved in THF (5.0 mL, 1.0 M) along with triphenyl phosphine (407 mg, 1.8 equiv) and perfluoro-tert-butanol (407 mg, 2 equiv). The mixture was stirred at 0 °C, and diisopropyl azodicarboxylate (DIAD) was added dropwise (339 μL, 2 equiv). The reaction warmed to room temperature overnight, at which point the reaction was loaded onto a column of silica gel and purified using a gradient of hexanes to hexanes and ethyl acetate (8:2). As the first compound to elute, the product was concentrated in vacuo and isolated as a colorless oil in 17% yield (58 mg). 1H NMR (400 MHz, CDCl3) δ 7.24−7.17 (m, 1H), 6.71−6.62 (m, 2H), 5.99−5.86 (m, 1H), 5.62 (d, J = 2.5 Hz, 1H), 5.31 (dq, J = 17.3, 1.7 Hz, 1H), 5.17 (dq, J = 10.6, 1.5 Hz, 1H), 4.59−4.44 (m, 2H), 3.47 (dd, J = 14.4, 4.0 Hz, 1H), 3.22 (d, J = 14.4 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ 153.76, 143.19, 132.82, 132.61, 127.28, 121.80 (q, J = 290 Hz), 118.89, 117.14, 115.41, 113.56, 75.44, 69.53, 40.83. 19 F NMR (376 MHz, CDCl3) δ −70.08 (s). HRMS: calculated C15H11O2F9 (M+) 394.061, found 394.061.

2-(Bicyclo[2.2.1]hept-5-en-2-ylmethoxy)-8-(4-((1,1,1,3,3,3-hexafluoro-2-(trifluoromethyl)propan-2-yl)oxy)butoxy)bicyclo[4.2.0]octa-1(6),2,4-triene. 5-(Bicyclo[2.2.1]hept-5-en-2-ylmethoxy)bicyclo[4.2.0]octa-1(6),2,4-trien-7-ol (66 mg, 1 equiv) was dissolved in THF (2.0 mL, 1.0 M) along with triphenyl phosphine (128 mg, 1.8 equiv) and perfluoro-tert-butanol (75 μL, 2 equiv). The mixture was stirred at 0 °C, and diisopropyl azodicarboxylate (DIAD) was added dropwise (128 μL, 2 equiv). The reaction was warmed to room temperature overnight, at which point the reaction was loaded onto a column of silica gel and purified using a gradient of hexanes to hexanes and ethyl acetate (8:2). As the second compound to elute, the product was concentrated in vacuo and isolated as a colorless oil in 69% yield (104 mg). 1H NMR (400 MHz, CDCl3) δ 7.25−7.08 (m, 1H), 6.69−6.56 (m, 2H), 6.13−5.80 (m, 2H), 4.98 (dtd, J = 13.5, 4.1, 1.7 Hz, 1H), 4.23−3.43 (m, 8H), 3.30 (dt, J = 14.0, 4.1 Hz, 1H), 3.03−2.90 (m, 2H), 2.76 (s, 1H), 2.51−2.36 (m, 1H), 1.89−1.58 (m, 4H), 1.43−1.15 (m, 3H), 0.60−0.44 (m, 1H). 13C NMR (101 MHz, CDCl3) δ 153.84, 144.13, 137.33, 132.42, 131.18, 129.68, 121.87 (q, J = 292 Hz), 118.94, 115.35, 113.61, 76.30, 72.30, 69.66, 67.60, 49.37, 43.85, 42.19, 38.56, 38.09, 28.91, 26.84, 25.88. 19F NMR (376 MHz, CDCl3) δ −70.43 (s). HRMS: Calculated for C24H25F9O3 (M+Na)+ 555.154, found 555.155.

2-(Allyloxy)-8-(4-((1,1,1,3,3,3-hexafluoro-2-(trifluoromethyl)propan-2-yl)oxy)butoxy)bicyclo[4.2.0]octa-1,3,5-triene. 5(Allyloxy)bicyclo[4.2.0]octa-1,3,5-trien-7-ol (152 mg, 1 equiv) was dissolved in THF (5.0 mL, 1.0 M) along with triphenyl phosphine (407 mg, 1.8 equiv) and perfluoro-tert-butanol (407 mg, 2 equiv). The mixture was stirred at 0 °C, and diisopropyl azodicarboxylate (DIAD) was added dropwise (339 μL, 2 equiv). The reaction was warmed to room temperature overnight, at which point the reaction was loaded onto a column of silica gel and purified using a gradient of hexanes to hexanes and ethyl acetate (8:2). As the second compound to elute, the product was concentrated in vacuo and isolated as a colorless oil in 71% yield (286 mg). 1H NMR (400 MHz, CDCl3) δ 7.15 (dd, J = 8.4, 7.2 Hz, 1H), 6.66 (dd, J = 7.7, 5.7 Hz, 2H), 5.97 (ddt, J = 17.3, 10.5, 5.2 Hz, 1H), 5.30 (ddd, J = 17.3, 2.9, 1.5 Hz, 1H), 5.17 (ddd, J = 10.5, 2.9, 1.1 Hz, 1H), 4.99 (dd, J = 4.2, 1.8 Hz, 1H), 4.71−4.55 (m, 2H), 3.97 (t, J = 5.8 Hz, 2H), 3.60−3.49 (m, 2H), 3.31 (dd, J = 14.0, 4.3 Hz, 1H), 3.00 (dd, J = 14.0, 1.1 Hz, 1H), 1.79−1.61 (m, 4H). 13C NMR (101 MHz, CDCl3) δ 153.44, 144.09, 133.69, 131.22, 129.32, 121.86 (q, J = 291 Hz), 118.99, 117.02, 115.75, 113.93, 76.14, 69.56, 67.41, 38.05, 26.73, 25.83. 19F NMR (376 MHz, CDCl3) δ −70.43 (s). HRMS: Calculated for C25H23F9NaO3 (M+Na)+ = 565.140, found 565.139.

((5-(Alylloxy)bicyclo[4.2.0]octa-1,3,5-trien-7-yl)oxy) (tert-butyl)dimethylsilane. 5-(Allyloxy)bicyclo[4.2.0]octa-1,3,5-trien-7-ol (89 mg, 1 equiv) was dissolved in dimethylformamide (2 mL) in a 5 mL round-bottomed flask equipped with a magnetic stir bar. Imidazole (85 mg, 2.5 equiv) was added; then, tertbutyldimethyl silyl chloride (92 mg, 1.2 equiv) was added. The solution was capped with a nitrogen balloon, then stirred for 1 h. The reaction mixture was then directly loaded onto a column of silica and collected using hexanes and ethyl acetate (9:1) as an elutent. The product was concentrated in vacuo and isolated as a colorless oil in 95% yield (138 mg). 1H NMR (400 MHz, CDCl3) δ 7.05 (ddd, J = 8.4, 7.1, 0.5 Hz, 1H), 6.64−6.48 (m, 2H), 5.91 (ddt, J = 17.3, 10.5, 5.3 Hz, 1H), 5.23 (dq, J = 17.3, 1.4 Hz, 1H), 5.20−5.17 (m, 1H), 5.10 (dq, J = 10.5, 1.4 Hz, 1H), 4.71− 4.48 (m, 2H), 3.32 (ddt, J = 13.8, 4.4, 0.8 Hz, 1H), 2.88 (ddt, J = 13.8, 1.8, 0.8 Hz, 1H), 0.79 (s, 9H), 0.00 (s, 6H). 13C NMR (101 MHz, CDCl3) δ 158.47, 148.59, 138.71, 136.17, 135.88, 122.12, 120.54,

8-(4-((1,1,1,3,3,3-Hexafluoro-2-(trifluoromethyl)propan-2-yl)oxy)butoxy)-2-((4-vinylbenzyl)oxy)bicyclo[4.2.0]octa-1(6),2,4-triene. 5((4-vinylbenzyl)oxy)bicyclo[4.2.0]octa-1(6),2,4-trien-7-ol (160 mg, 1 equiv) was dissolved in THF (5.0 mL, 1.0 M) along with triphenyl phosphine (297 mg, 1.8 equiv) and perfluoro-tert-butanol (175 μL, 2 H

DOI: 10.1021/acs.macromol.6b00316 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules 119.67, 75.46, 75.18, 47.11, 30.74, 22.99, 0.45. HRMS: calculated for C17H26O2Si (M+) 290.169, found 290.170.

((5-(Allyloxy)bicyclo[4.2.0]octa-1,3,5-trien-7-yl)oxy) (tert-butyl)diphenylsilane. 5-(Allyloxy)bicyclo[4.2.0]octa-1,3,5-trien-7-ol (61 mg, 1 equiv) was dissolved in dimethylformamide (2 mL) in a 5 mL round-bottomed flask equipped with a magnetic stir bar. Imidazole (59 mg, 2.5 equiv) was added, then tert-butyldiphenyl silyl chloride (112 mg, 1.2 equiv) was added. The solution was capped with a nitrogen balloon then stirred for 1 h. The reaction mixture was then directly loaded onto a column of silica and collected using hexanes and ethyl acetate (9:1) as an elutent. The product was concentrated in vacuo and isolated as a colorless oil in 95% yield (134 mg). 1H NMR (400 MHz, CDCl3) δ 7.80−7.66 (m, 4H), 7.48−7.32 (m, 6H), 7.17 (dd, J = 8.4, 7.1 Hz, 1H), 6.72 (d, J = 8.4 Hz, 1H), 6.61 (d, J = 7.1 Hz, 1H), 5.97 (ddt, J = 17.2, 10.5, 5.4 Hz, 1H), 5.39 (dd, J = 4.3, 2.0 Hz, 1H), 5.31 (dq, J = 17.2, 1.4 Hz, 1H), 5.21 (ddd, J = 10.5, 2.9, 1.4 Hz, 1H), 4.71 (m, 2H), 3.08 (dd, J = 14.0, 4.3 Hz, 1H), 3.01−2.93 (m, 1H), 1.09 (s, 9H). 13C NMR (101 MHz, CDCl3) δ 153.40, 143.73, 135.87, 134.00, 133.61, 131.64, 130.88, 129.80, 127.64, 117.35, 115.55, 113.96, 71.03, 70.24, 41.87, 26.93, 19.26. HRMS: calculated for C27H30O2Si (M+) 414.201 found 414.201.

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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.6b00316. NMR sepectra of polymers, DSC, and cross-linking procedures. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Blomberg, S.; Ostberg, S.; Harth, E.; Bosman, A. W.; Van Horn, B.; Hawker, C. J. Production of crosslinked, hollow nanoparticles by surface-initiated living free-radical polymerization. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 1309−1320. (2) Harth, E.; Van Horn, B.; Lee, V. Y.; Germack, D. S.; Gonzales, C. P.; Miller, R. D.; Hawker, C. J. A Facile Approach to Architecturally Defined Nanoparticles via Intramolecular Chain Collapse. J. Am. Chem. Soc. 2002, 124, 8653−8660. (3) Ohba, K. Overview of Photo-definable Benzocyclobutene Polymer. J. Photopolym. Sci. Technol. 2002, 15, 177−182. (4) Patel, K. S.; Kohl, P. A.; Bidstrup-Allen, S. Novel technique for measuring through-plane modulus in thin polymer films. IEEE Trans. Compon., Packag., Manuf. Technol., Part B 1998, 21, 199−202. (5) Sadana, A.K.; Saini, R. K.; Billups, W. E. Cyclobutarenes and Related Compounds. Chem. Rev. 2003, 103, 1539−1602. (6) Chen, P.; Savage, N. A.; Dong, G. Concise Synthesis of Functionalized Benzocyclobutenes. Tetrahedron 2014, 70, 4135−4146. (7) Zuo, X.; Yu, R.; Shi, S.; Feng, Z.; Li, Z.; Yang, S.; Fan, L. Synthesis and characterization of photosensitive benzocyclobutenefunctionalized siloxane thermosets. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 6246−6258. (8) Kirchhoff, R. A.; Bruza, K. J. Benzocyclobutenes in polymer synthesis. Prog. Polym. Sci. 1993, 18, 85−185. (9) Pugh, C.; Baker, James S.; Storms, William K. Synthesis of a Polymerizable Benzocyclobutene that Undergoes Ring-Opening Isomerization at Reduced Temperature. Synlett 2013, 25, 148−152. (10) Hickenboth, C. R.; Moore, J. S.; White, S. R.; Sottos, N. R.; Baudry, J.; Wilson, S. R. Biasing Reaction Pathways with Mechanical Force. Nature 2007, 446, 423−427. (11) Dobish, J. N.; Hamilton, S. K.; Harth, E. Synthesis of lowtemperature benzocyclobutene cross-linker and utilization. Polym. Chem. 2012, 3, 857−860. (12) Tadross, P. M.; Stoltz, B. M. A Comprehensive History of Arynes in Natural Product Total Synthesis. Chem. Rev. 2012, 112, 3550. (13) Bloome, K. S.; McMahen, R. L.; Alexanian, E. J. PalladiumCatalyzed Heck-Type Reactions of Alkyl Iodides. J. Am. Chem. Soc. 2011, 133, 20146−20148. (14) Ishihara, J.; Fukuzaki, T.; Murai, A. The total synthesis of (±)-naphthyridinomycin. Tetrahedron Lett. 1999, 40, 1907−1910.

Polymer 15. Norbornene (75 mg, 80 equiv) and 2-(bicyclo[2.2.1]hept-5-en-2-ylmethoxy)-8-(4-((1,1,1,3,3,3-hexafluoro-2(trifluoromethyl)propan-2-yl)oxy)butoxy)bicyclo[4.2.0]octa-1(6),2,4triene (104 mg, 20 equiv) were combined in dichloromethane (20 mL) in a 50 mL round-bottomed flask with a magnetic stir bar and stirred at room temperature. Second-generation Grubbs’ catalyst (8.5 mg, 1 equiv) was added, and the reaction was stirred for 1 h at room temperature. Ethyl vinyl ether (2 mL) was added; then, the reaction was concentrated in vacuo and redissolved in THF (2 mL). The reaction was precipitated in methanol, filtered, and dried in vacuo to give the corresponding polymer (fibrous solid) in 81% yield (145 mg). Tg = 52 °C. Td5 = 407 °C. GPC Mn = 65.8 kDa, Mw = 115.3 kDa, Đ = 1.75.

Polymer 14. Author: Neat styrene (123 mg, 80 equiv) and 8-(4((1,1,1,3,3,3-hexafluoro-2-(trifluoromethyl)propan-2-yl)oxy)butoxy)2-((4-vinylbenzyl)oxy)bicyclo[4.2.0]octa-1(6),2,4-triene (160 mg, 20 equiv) were combined in a 5 mL round-bottomed flask with Igracure 819 (6 mg, 1 equiv). The mixture was capped and purged with nitrogen gas for 30 min, then exposed in a Rayonet photochemical reactor with eight 40 W broadband UV bulbs for 16 h. The reaction was then dissolved in THF (2 mL), precipitated in methanol, filtered, and dried to give the corresponding polymer (white powder) in 63% yield (173 mg). Tg = 74 °C. Td5 = 269 °C. GPC Mn = 48.6 kDa, Mw = 100.3 kDa, Đ = 2.06. I

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Macromolecules (15) Corey, E. J.; Venkateswarlu, A. Protection of hydroxyl groups as tert-butyldimethylsilyl derivatives. J. Am. Chem. Soc. 1972, 94 (17), 6190−6191. (16) Burgstahler, A. W.; Worden, L. R. Hexaphenylbenzne. Org. Synth. 1966, 46, 28. (17) Baker, J. S. Synthesis of Functional Vinylbenzocyclobutenes for Use as Crosslinkers in the Preparation of Amphiphilic Nanoparticles. Ph.D. Dissertation. University of Akron, 2011.

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DOI: 10.1021/acs.macromol.6b00316 Macromolecules XXXX, XXX, XXX−XXX