Synthesis of Polyketones Containing Substituted Six-Membered Rings

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Synthesis of Polyketones Containing Substituted Six-Membered Rings via Pd-Catalyzed Copolymerization of Methylenecyclohexanes with Carbon Monoxide Daisuke Takeuchi,* Keisuke Watanabe, and Kohtaro Osakada Chemical Resources Laboratory R1-04, Tokyo Institute of Technology, 4259 Nagatsuta, Yokohama 226-8503, Japan

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S Supporting Information *

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INTRODUCTION The alternating copolymerization of olefins such as ethylene, propylene, and styrene with CO is catalyzed by the Pd complexes with chelating N,N- and P,P-ligands and produces the polyketones formulated as −(CHR−CH2−CO)n−.1−4 Stereoselective and/or living copolymerization was reported to form the polymers with regulated structure and narrow molecular weight distribution. Polyketones with five- and sixmembered rings are expected to show high thermal stability. Such copolymers were prepared by copolymerization of ringstrained cycloolefins, such as norbornene5 and 7-methylenebicyclo[4.1.0]heptane,6 as well as from α,ω-dienes.7 The copolymer of 7-methylenebicyclo[4.1.0]heptane shows high glass transition temperature (152 °C). The polyketones with lower Tg value would be suited for casting or molding at the temperature close to room temperature. Recently, we investigated isomerization polymerization of methylenecyclohexane and methyl- or propyl-substituted methylenecyclohexanes catalyzed by the diimine−Pd catalyst and obtained the hydrocarbon polymers with trans- or cis-1,4and trans-1,3-cyclohexylene groups.8 Herein we report that the diimine−Pd catalyst promotes copolymerization of the substituted and unsubstituted methylenecyclohexanes with CO. The produced polyketones contain both cyclohexylene and aliphatic ketonyl groups, and their thermal properties could be controlled by varying the length and structure of the spacer between the cyclic units.

The copolymerization of I with CO catalyzed by 1/NaBARF ([I]/[1] = 300) for 48 h produces the polymer with Mn = 12 000 (Mw/Mn = 1.30) in 69% isolated yield (75% monomer conversion) (Table 1, run 1). (bpy)PdMeCl, which is a common catalyst for the copolymerization of olefins with CO,1 does not promote the copolymerization of I with CO under similar conditions. 13 C{1H} NMR measurement of the copolymer showed CH carbon signals at δ 33−34 (b1) and 55−56 (g1) (Figure 1(i)), in addition to the signals due to carbonyl (δ 212−213, l1) and CH2 carbons (δ 25−32, a1 and c1−f1). The peak positions are much closer to the corresponding signals of trans-1-acyl-2methylcyclohexane than with the cis isomer.9 Thus, the copolymer contains a 1,2-trans-cyclohexenylene group in the monomer unit with high trans/cis ratio. The signals of carbonyl carbon and neighboring CH carbon (g1) are split in a pair peaks, which correspond to diisotactic and disyndiotactic microstructures of neighboring trans-cyclohexylene groups. Catalyst 1/NaBARF also promotes the copolymerization of 3- or 4-methyl-1-methylenecyclohexanes (II and III) with CO ([monomer]/[Pd] = 100), although conversion of the monomer is lower (27 and 42% after 24 h) (Table 1, runs 2 and 3). 13C{1H} NMR spectrum of poly(II-co-CO) contains a carbonyl carbon signal at δ 213 and eight major signals at δ 20− 60 (Figure 1(ii)). DEPT analysis clearly showed that the signal at δ 20.1 is assigned to methyl carbon (h2), and those at δ 53−54 (g2), 28.8 (b2), and 27.0 (d2) are to CH carbons. Thus, the cyclohexane group of the repeating unit is bonded to a methyl group. Calculation (GIAO method, mPW1PW/631G(d) level) of 13C{1H} NMR chemical shift of possible triads indicates that the above peak positions are consistent with the repeating structure shown in eq 1. Thus, the copolymer contains a 1,2-trans-cyclohexylene group in the monomer unit. Chemical shift of the carbonyl carbon signal and those of CH2 and CH attached to carbonyl group in poly(IIIco-CO) are close to those of poly(II-co-CO) (Figure 1(iii)) and also to the calculated chemical shift of model compound with triad structure. The two polyketones do not contain repeating units composed of a bis(methylene)cyclohexylene group, which is confirmed by the absence of the carbonyl carbon signals near 210 ppm.

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RESULTS AND DISCUSSION Pd complex 1 promotes the copolymerization of methylenecyclohexane (I) as well as of 3- and 4-methyl-substituted methylenecyclohexanes (II, III) with CO in the presence of NaBARF (BARF = B{C6H3(CF3)2-3,5}4), as shown in eq 1.

Received: July 2, 2015 Revised: August 19, 2015

© XXXX American Chemical Society

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DOI: 10.1021/acs.macromol.5b01458 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Copolymerization of Methylenecyclohexanes with CO by 1/NaBARFa

a


b

run

monomer

[monomer]/[1]

time (h)

convb (%)

yield (%)

Mnc

Mw/Mnc

Tg (°C)

1 2 3 4 5 6 7

I II III IV IV IV V

100 100 100 100 50 200 100

48 24 24 4 4 4 24

75 27 42 quant quant quant 31

69

12000 4030 4150 16400 8070 27700 5390

1.30 1.81 2.03 1.14 1.12 1.11 1.05

98

90

38

Reaction conditions: 1 = 0.010 mmol, NaBARF = 0.012 mmol, [monomer]/[Pd] = 100, solvent = CH2Cl2 (0.5 mL), at room temperature. Determined by 1H NMR. cDetermined by GPC based on polystyrene standard in THF as eluent.

into two peaks, assigned to diisotactic and disyndiotactic dyads. Thus, poly(IV-co-CO) is composed of the repeating unit −(CH2−CH(C4H8)CH−CH2−CO−)− with a cis-1,2-cyclohexylene group. The copolymerization of IV with CO catalyzed by 1/NaBARF obeys first-order kinetics with respect to the concentration of IV. The observed rate constant is 2.2 × 10−4 s−1 with [I]0 = 20 mM at 20 °C (kobsd/[1] = 1.1 × 10−5 s−1 cat. mmol−1). As the copolymerization obeys first-order kinetics with the concentration of IV, the rate-determining step of the polymer growth is in the intermolecular coordination and is insertion of IV into the Pd−polymer bond rather than the insertion of CO and isomerization of the polymer end. Figure 2(i) shows a linear relationship between conversion of the monomer and molecular weight of the produced polymer as well as the low polydispersity.

Figure 1. 13C{1H} NMR spectra (CDCl3, rt) of (i) poly(I-co-CO), (ii) poly(II-co-CO), (iii) poly(III-co-CO), (iv) poly(IV-co-CO), and (v) poly(V-co-CO).

The copolymerization of 2-methyl-1-methylenecyclohexane (IV) with CO ([IV]/[1] = 100) proceeds much more smoothly compared to I, II, and III, and yields the polyketone in 90% isolated yield after 4 h. The produced polymer has narrow molecular weight distribution (Mn = 16 400 and Mw/Mn = 1.14). 13 C{1H} NMR of poly(IV-co-CO) exhibited one CH carbon signal at δ 34.3 (b4) and three CH2 carbon signals at δ 44.5 (a4), 29.1 (c4), and 23.2 (d4) (Figure 1(ii)). The appearance of the four signals due to the CH and CH2 carbons of the polyketone and the absence of a CH3 carbon signal indicate the symmetrical structure of the repeating unit, shown in eq 2. The

Figure 2. (i) Relationship between monomer conversion and Mn (Mw/Mn) of the produced copolymer ([IV]/[1] = 100, the dashed line shows the Mn calculated from monomer conversion) and (ii) GPC profiles of the produced polymer ((a) first stage ([IV]/[1] = 50) and (b) second stage ([IV]/[1] = 100)) in the copolymerization of IV and CO by 1/NaBARF.

Change in the ratio of [IV] to [1] from 50 to 200 caused linear increase of the molecular weight of the product up to Mn = 27 700 (Mw/Mn = 1.11) (Table 1, runs 4−6). The molecular weight of the produced polymer is in good agreement with the calculated value based on the initial [IV]/[1] molar ratio. Another evidence for living character of the polymerization is obtained by addition of a second batch of the monomer after conversion of the monomer, which resumes the polymer growth. Figure 2(ii) depicts GPC profiles of the polymer obtained after first-stage copolymerization ([IV]/[1] = 50, quantitative conversion in 2 h, Mn = 8070, Mw/Mn = 1.12) and second-stage copolymerization ([IV]/[1] = 100, quantitative conversion in 4 h, Mn = 23 700, Mw/Mn = 1.17). The elution was shifted to the higher molecular weight region keeping narrow molecular weight distribution. Thus, the copolymerization of IV proceeds smoothly without chain transfer.

chemical shifts of the CH2 carbon signals of cyclohexane group (δ 29.1 and 23.2) are close to those of cis-1,2-dimethylcyclohexane (δ 31.5 and 23.7) rather than its trans isomer (δ 36.0 and 26.9).10 The carbonyl carbon signal at δ 209.8 is split B


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Macromolecules Methylenecyclohexanes can be easily derived from cyclohexanones, which are frequently found in naturally occurring terpenes. We synthesized 2-isopropyl-5-methyl-1-methylenecyclohexane (V) by Wittig reaction of (−)-menthone and used as the monomer for the Pd-catalyzed copolymerization with CO. The copolymerization of V with CO catalyzed by 1/NaBARF ([V]/[1] = 100) attained 31% conversion of V after 24 h (Table 1, run 7). The produced polymer has narrow molecular weight distribution, with Mn = 5390 and Mw/Mn = 1.05. 13 C{1H} NMR measurement of the copolymer showed two CH3 carbon signals at δ 22.5 (h5) and 18.5−18.0 (j5) (Figure 1v). The chemical shift of carbonyl carbon of poly(V-co-CO) (δ 210.7) is close to that of poly(IV-co-CO) (δ 209.8), which is consistent with CH2−CO−CH2 linkage. The signals of CH2 carbon attached to carbonyl group were observed at δ 49.6−48.2 (k5) and 41.0−39.0 (a5). Both signals are split in 3−4 peaks due to the relative stereochemistry between methyl branch and cyclohexylene group and between diads (Scheme 1). All the spectroscopic data indicate the structure, shown in eq 3, and the presence of the diads shown in Scheme 1.

Scheme 2. Plausible Mechanism of Copolymerization of (a) I, II, and III, (b) IV, and (c) V with CO

Scheme 1. Four Diad Structures of Poly(V-co-CO)

chain walking of the Pd center to the neighboring cyclohexane carbon and insertion of CO into the Pd−C bond leads to the 1,2-trans-cyclohexylene ring. Relative stereochemistry between the methyl substituent and the acyl group attached to the cyclohexane ring is controlled to cis in poly(II-co-CO) and to trans in poly(III-co-CO). It is consistent with the above reaction pathway that involves stereoselective β-hydrogen elimination (Scheme 2a). CO insertion occurs into the bond between Pd and the carbon at the 2-position, although polymerization of I catalyzed by the same catalyst involves exclusive insertion of the monomer into the Pd−C bond at the 4-position.8 Coordination and insertion of CO into the Pd−C bond is an easier process than that of methylenecyclohexane. The CO insertion occurs even into the bond of Pd with sterically more crowded 2-carbon, prior to further chain walking to form the bond between Pd and the carbon at the 4-position. The polymerization of IV involves insertion of CO into the bond between Pd and the cyclohexylmethyl ligand, as shown in Scheme 2b. After the 2,1-insertion of IV to the Pd−polymer bond, the chain-walking reaction moves the Pd center to the methyl group at the cyclohexane ring (Scheme 2b, B). The subsequent insertion of CO leads to the cis-1,2-cyclohexylene unit. Copolymerization of V with CO also proceeds via a similar pathway, involving chain walking, and the insertion of CO occurs into the Pd−C bond at the end of the isopropyl group (Scheme 2c). Preference of the chain walking of the Pd center to the 2-position with a substituent rather than 6position results in selective formation of the polymers

Differential scanning calorimetric (DSC) measurements of poly(I-co-CO) and poly(IV-co-CO) showed glass transition of these polymers at 98 and 38 °C, respectively. Those values are lower than that of the polyketone having a 1,2-cis-cyclohexylene group, formed by Pd-catalyzed copolymerization of 7methylenebicyclo[4.1.0]heptane with CO (Tg = 152 °C).6 Scheme 2 summarizes mechanism of the alternating copolymerization of the methylenecyclohexanes with CO. Coordination and 2,1-insertion of the methylidene group of I to the Pd−polymer bond is followed by chain walking of the Pd center to the 2 (or 6)-position of the cyclohexane ring (Scheme 2a, A). CO insertion occurs into the bond between the Pd center and the cyclohexyl carbon next to the polymer chain. Copolymerization of II and III proceeds via a similar pathway. The methyl-substituted methylenecyclohexanes adopt a chair conformation with an equatorial methyl group and undergo addition of the Pd−polymer bond from the equatorial direction so as to avoid severe 1,3-diaxial interaction. The subsequent C



Macromolecules

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containing cis-1,2-bis(methylene)cyclohexylene groups. Selective chain walking of the Pd center to 2-position rather than 6position may be attributed to the preferred formation of stable tetrasubstituted olefin coordinated to the Pd center via abstraction of a β-hydrogen of the cyclohexyl−Pd intermediates. In summary, we have presented that isomerization copolymerization of methylenecyclohexanes with CO proceeds smoothly, in spite of low reactivity of olefins with 1,1disubstituted CC double bond. Both the coordination/ insertion of methylenecyclohexanes and CO and isomerization of the Pd center take place in controlled manner to afford the polyketones composed of substituted methylenecyclohexylene unit. The Pd catalysis is applied to the monomer derived from (−)-menthone, leading to the polymer with high selectivity. Many new monomers, which are easily accessible, will be converted to the polyketones via the copolymerization with CO.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS Grant-in-Aid for Young Scientist (22685012). We thank Dr. Tomohito Ide for the calculation of the 13C{1H} NMR chemical shift.

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REFERENCES

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General. All manipulations of the reactions were carried out under nitrogen or argon using standard Schlenk techniques. Anhydrous CH2Cl2 was purchased and used as received. Ether was distilled from Na under nitrogen or argon. Diimine ligands,11 PdCl(Me) (diimine),12 NaBARF,13 and methylenecyclohexanes8,14 were prepared according to the reported procedure. NMR (1H and 13C) spectra were recorded on Varian Mercury 300 or JEOL JNM-500 spectrometer. The peaks were referenced to CHCl3 (δ 7.26) in the CDCl3 solvent for 1H and CDCl3 (δ 77.0) for 13C. Gel permeation chromatography (GPC) measurement was performed at 40 °C on a TOSOH HLC-8020GPC equipped with a differential refractometer detector and a variable wavelength UV−vis detector, using THF as eluent at a flow rate of 0.6 mL min−1 with TSKgel Super HM-L and Super HM-M column. DSC was recorded on Seiko DSC6200R instruments. Copolymerization of Methylenecyclohexanes with CO. Typically, to a 25 mL Schlenk flask containing a CH2Cl2 solution (0.5 mL) of Pd complex 1 (0.010 mmol, 6.6 mg) was added NaBARF (0.012 mmol, 10.6 mg) under Ar. After stirring for 3−5 min, the reaction mixture was degassed, flushed with CO (1 atm), and stirred for 3−5 min at room temperature. Methylenecyclohexanes (1.00 mmol) and naphthalene (inner standard) were added, and the reaction mixture was stirred at room temperature. A small portion of the reaction mixture was taken out from the flask and was subjected to 1H NMR and GPC to check conversion of the monomer and molecular weight of the polymer. The mixture was poured to methanol to isolate the produced polymer. 13C{1H} NMR (125 MHz, CDCl3, rt) data are summarized: Poly(I-co-CO): δ = 213.1, 212.6 (l1), 56.0, 55.5 (g1), 46.8 (a1), 34.0, 33.5, 33.2 (b1), 31.5, 29.7 (c1, f1), 25.9, 25.5 (d1, e1). Poly(II-co-CO): δ = 212.6 (l2), 53.9, 53.4 (g2), 45.2 (a2), 36.8, 36.6 (c2), 31.1 (e2), 28.8, 28.6 (b2), 27.0 (d2), 23.5, 23.3 (f2), 20.1, 19.6 (h2). Poly(III-co-CO): δ = 212.8 (l3), 50.6 (g3), 44.9 (a3), 34.7 (f3), 31.4, 31.8 (b3), 20.3 (d3), 27.5 (e3), 26.4, 26.0 (c3), 19.5 (h3). Poly(IVco-CO): δ = 209.8 (l4), 44.5 (a4), 34.3 (b4), 29.1 (c4), 23.2 (d4). Poly(V-co-CO): δ = 210.7 (l5), 49.6, 48.4, 48.2 (k5), 45.2 (i5), 41.0, 40.3, 39.5, 39.0 (a5), 35.5 (c5), 30.9, 30.6 (b5, g5), 26.5 (d5), 25.8 (f5), 25.0 (e5), 22.5 (h5), 18.5, 18.1, 18.0 (j5).

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01458. Calculated 13C{1H} NMR chemical shifts of possible triads for poly(II-co-CO) and poly(III-co-CO) (GIAO method) (PDF) D


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Synthesis of Polyketones Containing Substituted Six-Membered Rings

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