Regio- and Stereoselective Ring-Opening Metathesis Polymerization

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Regio- and Stereoselective Ring-Opening Metathesis Polymerization of Enantiomerically Pure Vince Lactam Mathis J. Benedikter,† Georg Frater,‡ and Michael R. Buchmeiser*,†,§ †

Institute of Polymer Chemistry, University of Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany XiMo AG, Altsagenstr. 3, CH-6048 Horw/Lucerne, Switzerland § German Institutes of Textile and Fiber Research (DITF) Denkendorf, Körschtalstr. 26, D-73770 Denkendorf, Germany ‡

S Supporting Information *

ABSTRACT: The ring-opening metathesis polymerization (ROMP) of (+)-Vince lactam [(S)-azabicyclo[2.2.1]hept-5-en-3-one] (1) and its Nbenzyl, N-trimethylsilyl (TMS), and N-tert-butoxycarbonyl (Boc) derivatives (2a−c) is reported. Highly cis-syndiotactic (st) poly(Vince lactam) was readily accessible by using the cyclometalated ruthenium complex Ru[CH(2-OiPr-Ph)](Piv)(1-mesityl-3-C4H8-imidazol-2-ylidene) (Piv =2,2-dimethylpropanoate) (4); however, small amounts of trans double bonds (ca. 5%) formed. Highly cis-st (>98%) polymers were accessible by the action of the monoaryloxide pyrrolide (MAP) type complexes W(N-2,6-iPr2C6H3)(CHCMe2Ph)(Pyr)(HMTO) (Pyr = pyrrolide, HMTO = 2,6-(2,4,6-Me3C6H2)2C6H3O) (7) and W(O)(CHCMe2Ph)(PMe2Ph)(Me2Pyr)(TPPO) (TPPO = 2,3,5,6-tetraphenylphenolate) (8). Complementary, cis-isotactic (>98% cis-it) polymers were prepared by the action of Mo(N-2,6-Me2C6H3)(CHCMe2Ph)(OBiphen) (OBiphen = 3,3′-di-tertbutyl-5,5′,6,6′-tetramethyl-1,1′-biphenyl-2,2′-diolate) (5) and its tungsten analogue W(N-2,6-Me2C6H3)(CHCMe2Ph)(OBiphen) (6). Notably, none of these Mo- and W-based initiators polymerize unprotected Vince lactam. Deprotection of poly(N-TMS Vince lactam) and poly(N-Boc Vince lactam) with neat trifluoroacetic acid allowed for the isolation of all-cis highly tactic poly(Vince lactam).

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INTRODUCTION In recent years, ring-opening metathesis polymerization (ROMP) has become an important tool for the preparation of specialty polymers, on both a laboratory and industrial scale.1,2 Generally, ROMP-derived polymers usually contain both cis and trans olefinic bonds. In recent years, however, both Z- and E-selective catalysts for ROMP have been developed.3−13 Some of these catalysts also offer access to either isotactic (it) or syndiotactic (st) structures.3−13 Apart from molecular weight and polydispersity (Đ), tacticity is one of the most important factors that influence the physical properties of a polymer such as glass transition temperature (Tg) and melting point (Tm).3 Within the context of highly tactic, high-cis, or high-trans polymers, so far, molybdenum and tungsten imido and oxo alkylidene Schrock complexes provide tailored polymer structures probably best. Thus, molybdenum imido alkylidenes containing chiral biphenolate ligands allow for the preparation of cis-it poly(norbornene)s through enantiomorphic site control.3,4,8−10 Complementary, the chiral-at-metal mono aryloxide pyrrolide (MAP) complexes, described by the same group, form cis-st poly(norbornene)s through stereogenic metal control.3−5,7,8,10,11 Grubbs et al. recently also reported on cyclometalated ruthenium complexes capable of forming cis-st polymers.12,14 With molybdenum imido alkylidenes, high-transst structures can form via chain end control15,16 while hightrans-it polymers, though only reported for one single monomer, are accessible via the action of MAP catalysts via © XXXX American Chemical Society

turnstile rearrangement of the intermediary metalacyclobutanes.5 Generally, tacticities of ROMP-derived polymers can readily be determined from the olefinic coupling patterns in the 1H NMR spectra, provided chiral monomers are used. However, the formation of these tacticites has only been proven for a comparably low number of monomers.5,6,8,9 We therefore set out to prepare highly tactic polymers from enantiomerically pure (S)-azabicyclo[2.2.1]hept-5-en-3-one (1), a bicyclic γlactam, which is often referred to as Vince lactam (Scheme 1). Scheme 1. (+)-Vince Lactam (1) and the Preparation of NBenzyl (2a), N-TMS (2b), and N-Boc Vince Lactam (2c)a

Reagents and conditions: (a) NaH, THF, 0 °C, 30 min, BnBr, rt, 3 days; (b) NEt3, TMSCl, CH2Cl2, rt, 16 h; (c) Boc2O, 4-(N,Ndimethylamino)pyridine (DMAP), NEt3, CH2Cl2, rt, 16 h.

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Received: February 9, 2018 Revised: March 1, 2018

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DOI: 10.1021/acs.macromol.8b00318 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 1. Ruthenium, molybdenum, and tungsten complexes for the polymerization of (+)-Vince lactam and its derivatives.

correspond to the various possible triads (such as mmm, rrr, mmr, etc.). Furthermore, no distinct coupling pattern was observable for the olefinic protons, thus indicating an atactic (at) polymer structure. Polymerization of (+)-1 by the Rubased initiator 4, which has already been used for the preparation of cis-st polymers from norborn-2-ene-derived monomers,12 resulted in a polymer that consisted ca. 95% of cis-double bonds as revealed by 1H NMR (Figure 2B) and IR spectroscopy (Figure S10). However, the polymer also contained around 5% trans double bonds as evidenced by the presence of signals at δ = 5.71, 4.33, and 3.48 ppm. The cis fraction of the double bonds at δ = 5.58 and 5.54 ppm showed the characteristic coupling pattern consisting of two doublets with coupling constants of 3JHH = 6.2 and 3JHH = 6.4 Hz, respectively. The latter stem from the coupling of the olefinic protons with the methine protons. No coupling between the olefinic protons was observed, confirming the assumed cis-st structure (Figure 3B). Cis-st polymers prepared from an enantiomerically pure monomer will consist of alternating head-to-head and tail-to-tail-linkages between the individual repeat units and therefore possess C2 symmetry.10 Consequently, there are two distinct types of olefinic bonds in the resulting polymer, and the two protons on each type of olefinic bond are equivalent to each other. Because of the poor solubility of the polymer, size exclusion chromatography (SEC) could only be performed at elevated temperature (70 °C) using m-cresol as solvent. Results of all SEC measurements are summarized in Table 1. Polymerization of (+)-1 and Its N-Protected Versions by the Action of Mo- and W-Based Initiators. Reaction of (+)-(1) with the MAP initiators 7 and 8 did not result in any noticeable conversion of 1. This is at a first glance surprising since Schrock-type catalysts are known to tolerate amides.25 However, because of the bicyclic nature of 1, the nitrogen lone pair overlaps very well with the π-orbital of the carbonyl group as evidenced by IR spectroscopy (νCO = 1662 cm−1) and outlined in the literature.17 The resulting conjugation facilitates enolization, and the enol may deactivate the group 6 imido and oxo alkylidenes. This theory was substantiated by attempting to polymerize norborn-2-ene using initiator 7, which had previously been exposed to 1. To prevent enolization, a benzyl group was introduced at the lactam’s nitrogen (Scheme 1). And

This compound is available in large quantities since it is used in the preparation of several drugs or drug candidates, among them the anti-retroviral blockbuster medication Abacavir.17 Apart from synthesizing highly tactic polymers, we were also interested in determining the physical properties of hydrogenated, highly tactic poly(Vince lactam). Since the monomer contains a lactam group, the formation of hydrogen bonds might result in interesting polymer properties similar to polyamides. Furthermore, the lactam nitrogen offers a magnitude of possibilities for N-derivatization18−23 including benzylation and protection with the trimethylsilyl (TMS) or tert-butoxycarbonyl (Boc) group (Scheme 1) and should thus provide facile access to versatile functional polymers. Cho and Choi already published on the ROMP of rac-Vince lactam using ill-defined tungsten hexachloride-based initiators; however, their approach suffered from low conversion and the formation of atactic polymers.24

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RESULTS AND DISCUSSION Complexes 4−8 (Figure 1) were chosen since they exhibit excellent selectivity in the polymerization of norborn-2-ene and some of its derivatives.3,4,8,12 Initiator 3 was chosen in order to determine the chemical shifts of both the cis and trans double bonds in poly(Vince lactam) (Scheme 1). Polymerization of (+)-Vince Lactam ((+)-1) by the Action of Ruthenium-Based Initiators. Addition of the third-generation Grubbs catalyst 3 (Figure 1) to a solution of 200 equiv of (+)-1 in CH2Cl2 led to the precipitation of polymer from the reaction mixture. The resulting off-white solid was insoluble in THF or CHCl3 but readily dissolved in acidic solvents such as formic acid, trifluoroacetic acid, or mcresol. 1H NMR (Figure 2A) and 13C NMR (Figure S5) spectra in CF3COOD revealed a mixture of cis and trans double bonds in the polymer backbone. Similar to other ROMP-derived polymers, the chemical shifts of the methine protons showed the largest difference between cis and trans configured polymers.4 Thus, in the 1H NMR (Figure 2A) the signals of the methine protons next to the cis double bonds were found at δ = 4.69 and 3.84 ppm, while those next to a trans double bond showed two signals at δ = 4.37 and 3.53 ppm. In the 13C NMR spectrum (Figure S5), multiple different signals can be observed in the range from 133.4 to 136.6 ppm; these B


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Figure 2. 1H NMR spectra of poly((+)-Vince lactam) prepared by (A) initiator 3 (at), (B) initiator 4 (95% cis-st), (C) deprotected poly(2b) prepared by the action of 7 (cis-st), and (D) deprotected poly(2b) prepared by the action of 5 (cis-it) (all in CF3COOD, 400 MHz).

indeed, N-benzyl Vince lactam (2a) could be polymerized by complexes 5−8. Complex 5 led to complete monomer conversion within seconds, forming an off-white polymer, which precipitated from solution. Similar to the unprotected polymers, the polymer was only soluble in acidic solvents such as trifluoroacetic acid. The IR spectrum (Figure S25) showed a strong absorption band at 697 cm−1, characteristic for cis double bonds, while no trans band at ν ≈ 970 cm−1 could be observed. In the 1H NMR spectrum (Figure 4A) the olefinic signals, visible at δ = 5.87 and 5.28 ppm, showed a pseudotriplet coupling pattern. Furthermore, 1H−1H COSYNMR spectroscopy revealed coupling of the olefinic protons (Figure S24). The observed coupling pattern has already been reported for several other cis-it ROMP-derived polymers8,9 and can be explained by the presence of head-to-tail linkages (Figure 3A). Because of the lack of C2 symmetry, the olefinic protons in poly-2a prepared by the action of 5 are inequivalent and couple to each other and to the methine protons. Since the olefinic coupling is fairly small for cis double bonds, overlapping of the two central signals of the resulting doublet of doublet

leads to the formation of a pseudotriplet. No other tacticities were visible, neither by 1H NMR nor by 13C NMR (Figure S23). Polymerization of 2a by initiator 6 resulted in poly-2a with virtually identical NMR spectra (Figures S26−S28). The MAP complex 7 was found to be significantly less reactive; i.e., full conversion of 2a required 3 h. The resulting polymer was easily soluble in chlorinated solvents; therefore, NMR spectra were recorded in CDCl3. In the 1H NMR (Figure 4B) the olefinic protons formed doublets at δ = 5.58 and 5.42 ppm with coupling constants of 3JHH = 3.9 and 3JHH = 4.7 Hz, respectively, with no coupling between the olefinic protons. 1 H−1H COSY-NMR (Figure S13) further confirmed the absence of any coupling between the olefinic protons. Again, no trans-olefin bands were observable in the IR spectrum (Figure S14), and the 13C NMR (Figure S12) showed only one set of signals. We therefore conclude that the polymer must consist of a single tactic, i.e., cis-st structure. Finally, polymerization of monomer 2a using initiator 8 gave a cis-st polymer as evidenced by the identical NMR spectra (Figures S15 and S16). C


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precipitated from n-pentane. Unfortunately, recording of the NMR spectra in CDCl3 failed due to gelation of the samples, which resulted in low quality spectra. We suspect that gelation occurs due to a partial deprotection of the polymer by traces of HCl present in CHCl3. In order to determine tacticity and proof full deprotection at the same time, we decided to determine tacticity after deprotection. Complete deprotection of the polymer was easily accomplished by dissolving the polymer in neat trifluoroacetic acid followed by precipitation from methanol. After deprotection, both poly(2b) and poly(2c) prepared by the action of 7 showed virtually identical 1 H NMR spectra (for poly(2b) see Figure 2C). They displayed two sets of doublets at δ = 5.61 and 5.57 ppm, indicating a cis-st structure. Since the difference in chemical shift between the olefinic signals in NMR is marginal, 1H−1H COSY-NMR also does not resolve any potential coupling of the olefinic signals. However, 13C NMR (Figure S31) does only show one single set of signals; therefore, a single polymer structure must have formed. Furthermore, no trans signal around ≈970 cm−1 was observed by IR spectroscopy (Figure S32). SEC measurements were carried out with the protected polymer in CHCl3, since gelation did not occur at low concentrations. Complete deprotection of the polymers was clearly proven by the absence of any signals resulting from the TMS or Boc group. Disappointingly, initiator 8 did not yield the expected all-cis-st polymer structures from monomers 2b and 2c. Thus, poly(2b) prepared by the action of 8 was found to have a cis-st content of ca. 75% while only minor amounts of polymer were formed with 2c. Polymerization of 2b and 2c with initiator 5 followed by deprotection yielded a polymer, which showed only one olefinic resonance with an integral of two protons in the 1H NMR (Figure 2D). Therefore, the pseudotriplet coupling pattern that is characteristic for cis-it polymers was not observed. However, both the methine signals and the IR spectrum (Figure S35) suggest that only cis-double bonds had formed. Since only one set of signals was observed in 13C NMR (Figure S36), it is again reasonable to assume that only one single polymer structure had formed. Since initiator 5 is known to yield cis-it polymers3,4,8 and also polymerized 2a in that fashion, it is very likely that poly(2b) and poly(2c) prepared by the action of 5 are in fact cis-it. Polymerizing 2b and 2c with the tungsten analogue 6 was very low yielding and was thus not further investigated.

Figure 3. Structure of (A) cis-it, (B) cis-st, (C) cis-it, alt, and (D) cis-st, alt poly(Vince lactam) (R = H) and its benzyl, TMS, and Boc derivatives. Since enantiomerically pure (+)-1 was used, structures C and D can be excluded.

Polymerization of N-TMS- and N-Boc-(+)-1 Using Moand W-Based Initiators. Since the benzyl group could not be removed from poly-2a by conventional means, more labile protecting groups were explored. Because of the ease of removal, both the trimethylsilyl (TMS) and tert-butyloxycarbonyl (Boc) groups were chosen. The synthesis of N-Boc-Vince lactam 2c has been reported earlier,22 but to the best of our knowledge no synthesis of TMS-Vince lactam 2b has been reported so far. We found that this compound can easily be prepared by reacting Vince lactam with chlorotrimethylsilane in CH2Cl2 in the presence of triethylamine. However, purification of the compound turned out to be difficult because 2b is easily deprotected upon heating or upon contact with silica or aluminum oxide. Nonetheless, the crude product that was obtained after removing all volatiles, dissolving the product in n-pentane and filtering off the triethylamine hydrochloride was found to be sufficiently pure for polymerizations using complexes 5−8. In fact, this easily scalable protocol allows for the protection of large amounts of lactam (up to 100 g) at a time. Both monomers were found to polymerize upon addition of complexes 5−8; however, yields varied (Table 1). All resulting polymers were easily soluble in organic solvents such as CH2Cl2, THF, and methanol and were simply

Table 1. Cis Content, Tacticities, Molecular Weights, and Polydispersities (Đ) of Polymers Prepared by the Action of Initiators 3−8

a

initiator

monomer

R

cis [%]a

tacticity [%]a

Mn [g mol−1]b

Đb

yield [%]d

3 4 5 5 5 6 7 7 7 8 8

1 1 2a 2b 2c 2a 2a 2b 2c 2a 2b

H H Bn TMS Boc Bn Bn TMS Boc Bn TMS

≈65 95 >98 >98 >98 >98 >98 >98 >98 >98 75

at st it it it it st st st st st

8000c 39000c 65000c 19000 34000 n.d. 29000 50000 72000 65000 31000

2.1c 4.6c 2.1c 1.23 1.15 n.d. 1.65 1.25 1.04 1.27 1.31

88 60 65 74 91 98 94 76 88 87 92

Determined via 1H NMR. bUnless otherwise noted, Mn and Đ were determined by SEC in CHCl3. cDetermined by SEC in m-cresol at 70 °C. Isolated yields. n.d. = not determined.

d

D


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Figure 4. 1H NMR spectra of (A) cis-it poly(2a) prepared by the action of 5 (CF3COOD, 400 MHz), (B) cis-st poly(2a) prepared by the action of 7, and (C) partially hydrogenated cis-st poly(H-2a) prepared by the action of 7 (both in CDCl3, 400 MHz).

Hydrogenation of Poly(N-benzyl Vince lactam). Hydrogenation of both cis-it and cis-st poly((+)-2a) was attempted using tosyl hydrazide in CHCl3 at 130 °C. Under these conditions, many ROMP-derived polymers can be fully hydrogenated.3−5,26 However, cis-it poly(2a) could not be hydrogenated due to its insolubility in nonacidic solvents. In the case of cis-st poly(2a), hydrogenation of 50% of the double bonds was observed. The complete disappearance of one olefinic signal in 1H NMR indicates that one type of double bond was selectively hydrogenated. Two new signals for the resulting aliphatic protons were found at δ = 1.93 and 1.43 ppm. Assuming that the methine proton next to the nitrogen is shifted further downfield than the one neighboring the carbonyl, it can be deduced from 1H−1H COSY-NMR (Figure S20) that the HACCHA double bond is hydrogenated while the HBCCHB is retained. Similar observations were reported by Schrock et al. for the hydrogenation of cis-st poly[(S)PhEtNNBE] (PhEtNNBE = methyl-N-(1-phenylethyl)-2azabicyclo[2.2.1]hept-5-ene-3-carboxylate) and were attributed to the steric shielding of the HBCCHB double bonds.8 Properties of Poly(Vince lactam). One dominant and somewhat inconvenient property of all poly(Vince lactam) samples is their insolubilities in essentially all solvents short of organic acids. This was found for all polymers independent of tacticity. The hydrogenation of deprotected poly(Vince lactam) was also investigated; however, due to the insolubility of the polymers, no successful hydrogenation procedure could be

developed. Gratifyingly, TGA measurements (Figure S42) revealed high temperature stability of the polymers. Thus, cis-it and cis-st samples of poly(2a), whether unsaturated or partially hydrogenated, were temperature-stable up to ca. 400 °C. However, no melting point could be observed by DSC up to 350 °C. Wide-angle X-ray scattering (WAXS, Figures S43− S45) showed that the cis-st poly(2a) and its partially hydrogenated form were amorphous; consequently, a melting point cannot be expected. Cis-it poly(2a), however, showed partial crystallinity in WAXS. In view of the finding that no melting was observed, it is not unlikely that the polymer decomposes before melting.

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CONCLUSION We further widened the scope of monomers that can be polymerized in a cis-selective and stereospecific manner using well-defined Mo and W imido and oxo alkylidene Schrock catalysts. The use of biphenolate complexes 5 and 6 resulted in the formation of cis-it polymers, while MAP complexes 7 and 8 allowed for the synthesis of cis-st polymers from N-protected Vince lactam. Furthermore, ruthenium-based initiator 4 was found to polymerize unprotected Vince lactam (1), yielding an almost entirely cis-st polymer. From an analytical point of view, enantiomerically pure N-benzyl Vince lactam appears to be a promising compound for screening selectivities of metathesis initiators since the coupling in 1H NMR is very well resolved and thus allows for the facile determination of tacticity in the E


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mL of CH2Cl2 was added, the suspension was centrifuged, and the supernatant liquid was decanted. The polymer was washed with CH2Cl2 and dried in vacuo overnight to give an off-white solid (96 mg, 0.88 mmol, 88%). 1H NMR (CF3COOD): δ 5.82−5.51 (m, 2H), 4.69 (s, 0.7H, cis), 4.37 (s, 0.3H, trans), 3.84 (s, 0.6H, cis), 3.53 (s, 0.4H, trans), 2.67 (s, 1H), 1.82 (s, 1H) ppm. 13C NMR (CF3COOD): δ 184.9, 184.3, 183.6, 136.6, 136.1, 135.4, 134.4, 134.0, 133.4, 59.7, 59.4, 55.5, 49.3, 44.8, 38.2, 37.8 ppm. SEC (m-cresol): Mn = 8000 g/mol, PDI = 2.1. IR: 3244 (m), 1675 (s), 1266 (m), 1080 (w), 968 (w), 729 (m) cm−1. Polymerization of 2-Azabicyclo[2.2.1]hept-5-en-3-one Using Complex 4. Polymerization was performed analogously to the polymerization using the third-generation Grubbs catalyst, but CH2Cl2 was replaced by THF. The polymerization yielded an off-white solid (65 mg, 0.60 mmol, 60%). 1H NMR (CF3COOD): δ 5.58 (d, J = 6.2 Hz, 1H), 5.54 (d, J = 6.4 Hz, 1H), 4.69 (d, J = 6.8 Hz, 1H), 3.72 (d, J = 7.2 Hz, 1H), 2.74 (s, 1H), 1.78 (d, J = 12.0 Hz, 1H) ppm. 13C NMR (CF3COOD): δ 184.8, 135.4, 133.2, 55.0, 44.9, 38.7 ppm. SEC (mcresol): Mn = 39 000 g/mol, PDI = 4.6 IR: 3227 (w), 1676 (s), 1238 (m), 1075 (w), 731 (m) cm−1. General Polymerization Procedure with Schrock-Type Initiators. The monomer (1 mmol, 200 equiv) was dissolved in 2 mL of CH2Cl2, and a solution of the initiator (0.005 mmol, 1 equiv) in 1 mL of CH2Cl2 was added under vigorous stirring. After stirring for 3 h at room temperature, ferrocene aldehyde (5.4 mg, 0.025 mmol, 5 equiv) was added, and the resulting solution was stirred for a further 15 min. The polymer was then precipitated by dropwise addition of the solution to a suitable solvent. Methanol was used in the case of (S)-N-benzyl-2-azabicyclo[2.2.1]hept-5-en-3-one while the trimethylsilyl- and tert-butyloxycarbonyl-protected polymers were precipitated from n-pentane. General Deprotection Procedure. 2 mL of trifluoroacetic acid was added to the trimethylsilyl- or tert-butyloxycarbonyl-protected polymer prepared from 1 mmol of monomer, and the mixture was placed in an ultrasound bath until the polymer had completely dissolved. The polymer was precipitated from methanol, centrifuged, washed with methanol, and dried in vacuo at 50 °C. Partial Hydrogenation of cis-St Poly((S)-N-benzyl Vince lactam). Tosyl hydrazide (120.5 mg, 0.650 mmol, 3 equiv), 2,6-ditert-butyl-4-methylphenol (5.0 mg, 0.23 mmol, 0.1 equiv), and tributylamine (120 mg, 150 μL, 0.650 mmol, 3 equiv) were added to a pressure tube, and a solution of cis-st poly((S)-benzyl Vince lactam) (43.2 mg, 0.216 mmol) in 3 mL of CHCl3 was added. The mixture was heated to 130 °C for 16 h. After cooling to room temperature, the polymer was precipitated by dropping the solution into 50 mL of methanol. The suspension was centrifuged, and the supernatant solution was decanted. The polymer was washed with methanol and dried in vacuo to give a colorless solid (37 mg, 0.186 mmol, 86%). 1H NMR (CDCl3): δ 7.42−6.95 (m, 5H), 5.39 (s, 1H), 4.99 (d, J = 15.1 Hz, 1H), 3.83 (d, J = 14.6 Hz, 2H), 2.31 (s, 1H), 1.93 (s, 1H), 1.82 (s, 1H), 1.43 (s, 1H), 1.23 (s, 1H) ppm. 13C NMR (CDCl3): δ 176.5, 136.3, 134.0, 128.9, 127.8, 127.8, 52.6, 44.4, 41.2, 32.3, 28.0 ppm. IR: 2922 (w), 1681 (s), 1405 (m), 732 (w), 699 (m) cm−1.

resulting polymer. The Vince lactam derivatives used in this study were shown to be polymerizable with benzyl, trimethylsilyl, and tert-butyloxycarbonyl substituents at the lactam nitrogen.

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EXPERIMENTAL SECTION

General. All manipulations were carried out in a N2-filled glovebox (Lab Master 130, MBraun, Garching, Germany) or by standard Schlenk techniques. CH2Cl2, diethyl ether, toluene, pentane, and THF were dried by a solvent purification system (SPS, MBraun). Starting materials and all reagents were purchased from Sigma-Aldrich (Munich, Germany), Alfa Aesar (Karlsruhe, Germany), and ABCR (Karlsruhe, Germany), dried, and, where appropriate, distilled prior to use. (S)-2-Azabicyclo[2.2.1]hept-5-en-3-one (1) was received from Soneas Chemicals Ltd., Hungary (Budapest, Hungary), and sublimed prior to use. For column chromatography, silica gel (Fluka, 60M, 0.040−0.063 mm grain size, 230−400 mesh ASTM) was used as stationary phase. NMR data were recorded on a Bruker Avance III 400 at 400.13 MHz for 1H, 100.61 MHz for 13C, and 376.50 MHz for 19F. Chemical shifts are reported in ppm relative to the solvent signal. One SEC system was operated in CHCl3 and consisted of a 1260 Infinity System (Agilent Technologies Inc.) equipped with an Agilent 1200 Series G1362A RI detector. The apparatus was equipped with a precolumn (8 × 50 mm) and three consecutive separation columns (8 × 300 mm, PSS, Mainz, Germany) with a porosity of 100 000 Å and a particle size of 5 μm. A flow rate of 1.0 mL/min and a column oven temperature of 35 °C were used. The injection volume was set to 100 μL. The setup was calibrated using polystyrene (800 ≤ Mn ≤ 5 800 000 g/mol). For SEC run in m-cresol, a Polymer Laboratories PL-GPC 220 system with an RI detector was used. One precolumn (7.5 × 50 mm) and three consecutive separation columns (7.5 × 300 mm, Agilent PLgel Olexis) were used. The system was operated at 70 °C applying a flow rate of 1.0 mL/min. The injection volume was set to 100 μL. For calibration, narrow polystyrene standards (680 g/mol ≤ Mn ≤ 7 270 000 g/mol) were used. IR spectra were measured in the range of 4000 to 400 cm−1 on a Bruker IFS 128 ATR/FT-IR spectrometer and analyzed using the OPUS software (Version 7.2). Wavenumbers are reported in cm−1. DSC measurements were performed under nitrogen on a PerkinElmer DSC 4000; data were analyzed using the Pyris software. All measurements were carried out applying a heating rate of 50 K/min and a cooling rate of 10 K/min. TGA measurements were performed under nitrogen using a PerkinElmer TGA 7 with a heating rate of 20 K/min. GC-MS data were recorded on an Agilent Technologies device consisting of a 7693 autosampler, a 7890 A GC, and a 5975C quadrupole MS. Dodecane was used as internal standard. An SPB-5 fused silica column (34.13 m × 0.25 mm × 0.25 mm film thickness) was used. The injection temperature was set to 150 °C. The column temperature ramped from 45 to 250 °C within 8 min and was then held for a further 5 min. The column flow was 1.05 mL/min. WAXS experiments were carried out on a Rigaku D/Max Rapid II (0.8 collimator, Ω = 0° oscillation) in the 2Θ range of 5°−100°. Mo(N-2,6-Me2C6H3)(CHCMe2Ph)[OBiphen] (OBiphen = 3,3′-di-tert-butyl-5,5′,6,6′-tetramethyl-1,1′-biphenyl-2,2′diolate) (5),27 W(N-2,6-Me2C6H3)(CHCMe2Ph)[OBiphen] (6),27 W(N-2,6-iPr2C6H3)(CHCMe2Ph)(pyr)(HMTO) (HMTO = 2,6(2,4,6-Me3C6H2)2C6H3O, 7),28 the third-generation Grubbs catalyst (3),29 ruthenium complex (4),30 and (S)-N-tert-butyloxycarbonyl-2azabicyclo[2.2.1]hept-5-en-3-one (2c)19 were prepared according to literature procedures. WO(CHCMe2Ph)(PMe2Ph)(Me2Pyr)(TPPO) (TPPO = 2,3,5,6-tetraphenyl phenolate) (8) was supplied by XiMo AG, Switzerland. Polymerization of (S)-2-Azabicyclo[2.2.1]hept-5-en-3-one Using the Third-Generation Grubbs Catalyst. (S)-2-Azabicyclo[2.2.1]hept-5-en-3-one (1, 109 mg, 1.00 mmol, 200 equiv) was dissolved in 2 mL of CH2Cl2, and a solution of the third-generation Grubbs catalyst (3, 4.4 mg, 0.005 mmol, 1 equiv) in 1 mL of CH2Cl2 was added. After stirring for a few seconds, an off-white solid started to precipitate. The suspension was stirred for 3 h at room temperature, and ethyl vinyl ether (0.1 mL) was added. After stirring for 15 min, 20

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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.8b00318.

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Additional experimental details, 1H NMR, 13C NMR, and IR data of all polymers (PDF)

AUTHOR INFORMATION

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*E-mail [email protected] (M.R.B.). F


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Michael R. Buchmeiser: 0000-0001-6472-5156 Funding

XiMo AG, Switzerland, is gratefully acknowledged for financial support. Special thanks to Dr. J. Spörl, DITF Denkendorf, for all WAXS measurements. Notes

The authors declare no competing financial interest.

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REFERENCES

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Regio- and Stereoselective Ring-Opening Metathesis Polymerization

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