Introduction

The radical polymerization of dialkyl fumarates yields poly(dialkyl fumarate)s (PDRFs) [1,2,3] with rigid chain conformations [4,5,6,7,8]. PDRFs are high-temperature and transparent polymer materials that are useful for information and electronics applications, such as in coatings, disks, fibers, displays, electronic devices, solar cells, etc [9,10,11,12]. PDRFs do not contain a methylene spacer in their main chain in contrast to conventional vinyl polymers that contain a flexible methylene spacer in every repeating monomer unit. The rigidity of the PDRF chains was evaluated using an analysis of the viscosity of PDRFs and light scattering data of a dilute PDRF solution using worm-like chain models [13,14,15,16,17]. The alignment and anisotropic features of these PDRF chains and their resulting unique solid-state properties have also been reported in the literature [9, 10]. Recently, we reported the reversible addition-fragmentation chain transfer (RAFT) polymerization of diisopropyl fumarate (DiPF) and the synthesis of block copolymers consisting of a less-flexible poly(diisopropyl fumarate) (PDiPF) segment and a flexible polyacrylate segment [18,19,20].

Adamantane (tricyclo[3.3.1.13,7]decane) is a thermodynamically stable and highly symmetrical tricyclic hydrocarbon and consists of fused chair-form cyclohexane rings with the same structure as a diamond lattice [21]. The syntheses of many polymers containing an adamantyl moiety in their main or side chains have been reported in the literature [22,23,24,25,26,27,28,29,30,31,32,33] and exploit the unusual physical and chemical properties of adamantane, for example, its thermodynamic stability, high melting point, thermal and oxidation stabilities, low surface energy, volatility, high density, and hydrophobicity. Thirty years ago, Otsu et al. reported the superior thermal properties of various acrylic polymers containing adamantyl moieties in their side chains, for example, acrylates [34], methacrylates [34,35,36], itaconates [37], fumarates [38], crotonates [6], and sorbates [39]. Later, Ishizone and coworkers reported the living anionic polymerization of adamantane-containing styrene and butadiene derivatives to synthesize functional polymers and block copolymers [40,41,42,43,44,45,46,47]. Several years ago, Nakano et al. reported the synthesis of sequence-controlled block copolymers comprising poly(1-adamantyl acrylate) (PAdA) and poly(n-butyl acrylate) sequences, which were used as the hard and soft segments, respectively, by organotellurium-mediated living radical polymerization (TERP) [48]. The thermal, optical, and mechanical properties of the adamantane-containing block copolymers were investigated in detail. More recently, the dynamic mechanical properties of random copolymers of DiPF containing 1-adamantyl acrylate (AdA) or bornyl acrylate repeating units were studied to determine the chain dynamics specific to a rigid poly(substituted methylene) structure [49, 50].

In this study, four types of adamantane-containing DRFs, i.e., di(1-adamantyl) fumarate (1), bis(3,5-dimethyl-1-adamantyl) fumarate (2), 1-adamantyl isopropyl fumarate (3), and 3,5-dimethyl-1-adamantyl isopropyl fumarate (4) were prepared (Fig. 1). A preliminary study by Matsumoto and Otsu previously reported the synthesis of these monomers using conventional radical polymerization [38], but a detailed study has not been carried out in the past thirty years to synthesize and characterize these adamantane-containing fumarate polymers. In this study, we polymerized these monomers in the presence of a radical initiator either in solution or in bulk to obtain thermally stable PDRFs. Furthermore, reversible addition-fragmentation chain transfer (RAFT) polymerization using 1,1′-(1,2-ethanediyl) bis[2-[[(dodecylthio)thioxymethyl]thio]-2-methylpropionate] (T2) and 2,2,2-trifluoroethyl 2-[[(dodecylthio)thioxymethyl]thio]-2-methylpropionate [51] (T1F) as the chain transfer agents (Fig. 1) was performed to produce block copolymers consisting of rigid PDRF and flexible poly(2-ethylhexyl acrylate) (P2EHA) segments. The alignment of the PDRF segments in the solid-state was investigated by X-ray diffraction.

Fig. 1
figure 1

Chemical structure of the DRFs and RAFT agents used in this study

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Experimental methods

Materials

DiPF and 2-ethylhexyl acrylate (2EHA) were purchased from Tokyo Chemical Industry, Co., Ltd. (Tokyo, Japan) and distilled under a reduced pressure before use. Dimethyl 2,2′-azobisisobutyrate (MAIB) was purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan) and recrystallized from n-hexane. 2,2′-Azobisisobutyronitrile (AIBN) was purchased from Nacalai Tesque, Ltd. (Kyoto, Japan) and used after recrystallization from methanol. Benzoyl peroxide (BPO, containing 25% water) was purchased from Nacalai Tesque, Ltd. (Kyoto, Japan) and used without further purification. 1-Adamantanol, N,N-dimethylaminopyridine (DMAP) and N,N’-dicyclohexylcarbodiimide (DCC) were purchased from Tokyo Chemical Industry, Co., Ltd. (Tokyo, Japan) and used without further purification. Maleic anhydride and sulfuric acid were purchased from Kishida Chemical Industry, Co., Ltd. (Tokyo, Japan) and used as received. 3,5-Dimethyl-1-adamantanol, piperidine, and 2-propanol were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan) and used without further purification. The solvents were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan) and used after distillation. T2 was synthesized according to a previously reported method [20].

Synthesis of 1

Three drops of concentrated sulfuric acid were added to maleic anhydride (0.983 g, 10 mmol) and 1-adamantanol (3.35 g, 22 mmol) in 50 mL of cyclohexane, and then, the mixture was refluxed for 4 h. After the reaction, the solution was washed twice with saturated aq. NaHCO3 and then once with saturated aq. NaCl. After drying over sodium sulfate, the filtrate was concentrated under a reduced pressure, resulting in the precipitation of a white solid. The crude yield was 2.48 g (65%). The product was recrystallized from a mixture of ethanol and toluene (5/1 by volume) and then dried in vacuo at room temperature. The product included a small amount of the corresponding monoester but was used for the subsequent isomerization without further purification.

Di-1-adamantyl maleate

Yield 1.18 g, white solid, m.p. 103‒110 °C (lit. 146 °C) [38]. 1H NMR (400 MHz, CDCl3) δ (ppm) 6.05 (s, CH = , 2 H), 2.17 (m, CH2 and CH, 18 H), 1.75–1.63 (m, CH2, 12 H).

Di-1-adamantyl maleate (1.00 g, 2.6 mmol) in 40 mL of cyclohexane was refluxed for 3.5 h in the presence of 0.25 mL of piperidine. After removing the solvent, a white solid was isolated. Silica gel chromatography was performed using a mobile phase containing hexane/ethyl acetate/chloroform = 85/5/10, by volume, and subsequent drying provided the corresponding fumarate 1.

1: Yield 0.31 g (31%), m.p. 191.8 °C (lit. 192 °C) [38]. 1H NMR (400 MHz, CDCl3) δ (ppm) 6.65 (s, CH = CH, 2 H), 2.18 (s, CH, 6 H), 2.14 (m, CH2, 12 H), 1.71‒1.64 (m, CH2, 12 H); 13C NMR (100 MHz, CDCl3) δ (ppm) 164.3 (C = O), 134.8 (C = C), 81.9 (OC), 41.4 and 36.3 (CH2), 31.0 (CH).

Synthesis of 2

Bis(3,5-dimethyl-1-adamantyl) maleate was synthesized from maleic anhydride and 3,5-dimethyl-1-adamantanol according to a procedure similar to that used to synthesize 1.

Bis(3,5-dimethyl-1-adamantyl) Maleate

Yield 2.9 g (66%), m.p. 64.5‒65.5 °C (Lit. 86.0 °C) [38]. 1H NMR (400 MHz, CDCl3) δ (ppm): 6.03 (s, CH = CH, 2 H), 2.31–1.74 and 1.43–1.09 (m, CH and CH2, 26 H), 0.88 (s, CH3, 12 H).

2 was obtained by the isomerization of bis(3,5-dimethyl-1-adamantyl) maleate in the presence of piperidine and purified by silica gel chromatography using a mobile phase containing hexane/ethyl acetate/chloroform = 80/10/10, by volume.

2: Yield 0.99 g (37%), m.p. 70.2 °C (Lit. 70.0 °C) [38]. 1H NMR (400 MHz, CDCl3) δ (ppm) 6.62 (s, CH = CH, 2 H), 2.27‒1.65 and 1.47‒1.07 (m, CH and CH2, 26 H), 0.88 (s, CH3, 12 H). 13C NMR (100 MHz, CDCl3) δ (ppm) 164.3 (C = O), 134.8 (C = C), 83.2 (OC), 50.5, 47.2, 42.3, 39.8, 34.1, 31.2, and 30.0 (CH, CH2, and CH3).

Synthesis of 3 and 4

3 was synthesized according to the following procedures. Maleic anhydride (4.9 g, 0.05 mol) and 2-propanol (3.0 g, 0.05 mol) were refluxed in 130 mL of cyclohexane for 13 h in the absence of an acidic catalyst to synthesize isopropyl maleate monoester. The resulting solution was used for the subsequent reactions without isolating the product.

Isopropyl maleate monoester

1H NMR (400 MHz, CDCl3) δ (ppm) 6.465 (d, J = 12.8 Hz, CH = CHCOOCH(CH3)2, 1 H), 6.335 (d, J = 12.8 Hz, CH = CHCOOH, 1 H), 5.24‒5.14 (m, CH, 1 H), 1.355 (d, J = 6.2 Hz, CH3, 6 H).

To the abovementioned solution, 1-adamantanol (7.6 g, 0.05 mol) and ca. 0.5 mL of concentrated sulfuric acid were added, and then, the mixture was refluxed for 22 h. After the reaction, the solution was washed twice with saturated aq. NaHCO3 and then once with saturated aq. NaCl. After drying over sodium sulfate, the filtrate was concentrated under a reduced pressure to produce a white paste. The product was a mixture of the diesters and monoesters of maleic acid. Repeated silica gel chromatography using a mobile phase containing hexane/diethyl ether/acetone = 70/10/20, by volume, yielded 1-adamantyl isopropyl maleate as the main product, which was a colorless liquid.

1-Adamantyl isopropyl maleate

Yield 1.8 g (12%). 1H NMR (400 MHz, CDCl3) δ (ppm) 6.145 (d, J = 10.7 Hz, CH = CHCOOCH(CH3)2, 1 H), 6.095 (d, J = 10.7 Hz, CH = CHCOOC10H15, 1 H), 5.18‒5.08 (m, CH, 1 H), 2.17 and 1.71‒1.63 (m, C10H15, 15 H), 1.29 (d, J = 6.4 Hz, CH3, 6 H).

1-Adamantyl isopropyl maleate (1.8 g) in 40 mL of toluene was refluxed for 2 h in the presence of 0.25 mL of piperidine. After removing the solvent, a yellow viscous liquid was isolated. After the product was diluted with chloroform, it was washed twice with saturated aq. NaHCO3 and once with saturated aq. NaCl and then dried over sodium sulfate. After the filtrate was concentrated under a reduced pressure, silica gel chromatography using a mobile phase containing hexane/diethyl ether/acetone = 85/5/10, by volume, yielded 3, which was a colorless liquid. The yield was 1.51 g (84%). 4 was synthesized according to a similar procedure.

3: 1H NMR (400 MHz, CDCl3) δ (ppm) 6.755 (d, J = 12.0 Hz, CH = CHCOOCH(CH3)2, 1 H), 6.705 (d, J = 12.0 Hz, CH = CHCOOC10H15, 1 H), 5.15‒5.06 (m, CH, 1 H), 2.19 and 1.71‒1.64 (m, C10H15, 16 H), 1.285 (d, J = 6.4 Hz, CH3, 6 H). 13C NMR (100 MHz, CDCl3) δ (ppm) 164.9 and 164.0 (C = O), 135.6 and 133.2 (C = C), 82.0 (OC), 68.8 (OCH), 41.3 and 36.2 (CH2), 30.9 (CH), 21.8 (CH3).

4: 1H NMR (400 MHz, CDCl3) δ (ppm) 6.750 (d, J = 12.0 Hz, CH = CHCOOCH(CH3)2, 1 H), 6.703 (d, J = 12.0 Hz, CH = CHCOOC12H19, 1 H), 5.14‒5.05 (m, CH for COOCH(CH3), 1 H), 2.18‒1.60 and 1.31‒1.23 (m, CH and CH2 for COOC10H13(CH3)2, 13 H), 0.88 (s, CH3 for COOC10H13(CH3)2, 6 H). 13C NMR (100 MHz, CDCl3) δ (ppm) 164.9 and 164.1 (C = O), 135.6 and 133.2 (C = C), 82.0 (OC), 68.9 (OCH), 50.5, 47.2, 42.6, 41.3, 39.8, 36.2, 34.1, 31.2, 31.0, 30.0, and 21.9 (CH, CH2, and CH3).

Synthesis of T1F

T1F was synthesized as follows. To a mixture of 2- [[(dodecylthio)thioxymethyl]thio]-2-methylpropanoic acid (1.10 g, 3.0 mmol), 2,2,2-trifluoroethanol (0.908 g, 9.1 mmol), and DMAP (36.7 mg, 0.30 mmol) in 20 mL of chloroform (super dehydrated grade), DCC (0.65 g, 3.2 mmol) in 5 mL of chloroform was added dropwise with stirring at 0 °C, and the mixture was further stirred at 0 °C for 1 h and then stirred at room temperature for 72 h. The filtrate was concentrated and purified by column chromatography (silica gel, hexane/chloroform = 1/1, by volume).

Yellow liquid. Yield 0.50 g (38%). 1H NMR (400 MHz, CDCl3) δ (ppm) 4.46 (q, J = 8.5 Hz, CF3CH2, 2 H), 3.27 (t, J = 7.4 Hz, SCH2, 2 H), 1.72 (s, CH3, 6 H), 1.68‒1.26 (m, (CH2)10, 20 H), 0.88 (t, J = 6.6 Hz, CH2CH3, 3 H). 13C NMR (100 MHz, CDCl3) δ (ppm) 221.5, 172.0, 123.0 (q, J = 276 Hz), 61.7 (q, J = 36 Hz), 55.6, 37.2, 32.1, 29.77, 29.76, 29.68, 29.56, 29.48, 29.0, 27.9, 25.3, 22.8, and 14.3.

Conventional radical polymerization

A monomer and a radical initiator were placed in a glass tube in the presence or absence of a solvent. After freeze–thaw cycles (three times) were performed to remove any oxygen, the solution was heated at a specific temperature. After polymerization, the solution was diluted with a small amount of chloroform or toluene and then poured into a large amount of methanol to precipitate the polymers. Hot ethanol was used as the precipitant for the polymerization of 1. The polymer yield was determined based on the weight of the isolated polymers. The isolated polymers were purified by reprecipitation and used for the SEC and NMR measurements.

RAFT polymerization

A monomer, radical initiator, and CTA were placed in a glass tube. After freeze–thaw cycles (three times) were performed to remove any oxygen, the solution was heated at 80 or 60 °C. An aliquot of the polymerization solution was analyzed by 1H NMR spectroscopy to determine the conversion of the monomers. After polymerization, the solution was diluted with a small amount of chloroform and then poured into a large amount of methanol to precipitate the polymers. The polymer yield was determined based on the weight of the isolated polymers. The isolated polymers were purified by reprecipitation and used for the SEC and NMR measurements. The theoretical number-average molecular weight (Mn,cacld) was calculated according to Eq. 1.

$$ M_{{\mathrm{n}},{\mathrm{calcd}}} = \ \left( {\mathrm{MW}}_{{\mathrm{monomer}}} \times \left[ {{\mathrm{monomer}}} \right]_0 \right. \\ \left. \times \;\% \;{\mathrm{conversion}} \times 0.01 \right)/\left[ {{\mathrm{CTA}}} \right]_0 + {\mathrm{MW}}_{{\mathrm{CTA}}}$$
(1)

where MWmonomer and MWCTA are the molecular weights of the monomers and CTAs, respectively, and [monomer]0 and [CTA]0 are the initial concentrations of the monomers and CTAs, respectively.

Synthesis of block copolymers

Triblock copolymers of PDRF and P2EHA [or poly(methyl acrylate) (PMA)] were synthesized according to a procedure similar to that reported in a previous study [20]. The PDRFs prepared by RAFT polymerization using T2 as the CTA were used as the macro-CTA. An acrylate monomer, a radical initiator, and the macro-CTA were placed in a glass tube. After freeze–thaw cycles (three times) were performed to remove any oxygen, the solution was heated at 60 °C. An aliquot of the polymerization solution was analyzed by 1H NMR spectroscopy to determine the conversion of the acrylate monomers. After polymerization, the solution was diluted with a small amount of chloroform and then poured into a large amount of methanol to precipitate the polymers. The polymer yield was determined based on the weight of the isolated polymers. The isolated polymers were purified by reprecipitation and used for the SEC and NMR measurements. The DRF content in the copolymers were estimated by the results of the NMR spectroscopy and SEC analysis.

Measurements

The 1H NMR spectra were recorded using JEOL ECS-400 and ECX-400 spectrometers with chloroform-d at room temperature. The number average and weight average molecular weights (Mn and Mw) were determined by size exclusion chromatography (SEC) with tetrahydrofuran as the eluent at 40 °C and using a system consisting of a JASCO PU-2080-Plus pump, a JASCO DG-2080-53 degasser, a Chromato Science CS-300C thermostat chamber, Tosoh TSK gels GMHHR-N and GMHHR-H columns, a JASCO RI-2031-Plus RI detector, and a JASCO UV-2031-Plus UV detector (300 nm). The molecular weights were calibrated using polystyrene standards (Tosoh Corporation, Ltd., Japan) [19]. The thermogravimetric (TG) analysis was carried out using a Shimadzu DTG-60 analyzer, and measurements were obtained under a nitrogen stream at a heating rate of 10 °C/min. The onset temperature of decomposition (Td5) was determined to be the temperature at which 5% weight loss was observed. The maximum decomposition temperature (Tmax) was determined by taking the derivative of a peak in the TG curves. The synchrotron radiation experiments were performed at the BL40B2 of the SPring-8 beamline (Hyogo, Japan). X-rays with a wavelength of 1.0 Å were exposed for 1 s at room temperature. The scattering patterns were recorded using a 1032 × 1032 pixel flat panel detector (Hamamatsu Photonics, C9278DK, pixel size = 50 μm × 50 μm) with a camera distance of 85.37 mm. The UV-vis spectra were recorded using a UV-2450 spectrophotometer (Shimadzu, Kyoto, Japan) to evaluate the transmittance of the polymer films, which were prepared by casting and drying a 5 wt% polymer solution in chloroform on a quartz plate. The thickness of the polymer films was 55–65 μm. The refractive indices (nD, nD, nF, and nC) were measured at wavelengths of 486, 546, 589, and 656 nm, respectively, using an Abbe-type refractometer (Atago DRM2, 1-bromonaphthalene, a halogen lamp, Tokyo, Japan), and the polymer films (thickness 100‒120 μm) were prepared by casting 10 wt% chloroform solutions. The Abbe number (νD) was calculated using the following equation.

$$\nu _{\mathrm{D}} = (n_{\mathrm{D}} - 1)/(n_{\mathrm{F}} - n_{\mathrm{C}})$$
(2)

The calculated refractive index values were fitted with the experimental results using the Cauchy equation (Eq. 3) as the two-parameter (n∞ and D) analysis method [52].

$$n_\lambda = n_\infty + D/\lambda ^2$$
(3)

where λ is the wavelength, n∞ is the refractive index at an infinite wavelength, nλ is the refractive index at wavelength λ, and D is a constant.

Results and discussion

Synthesis of Adamantane-containing DRF

1 and 2 were synthesized from maleic anhydride and 1-adamantanol or 3,5-dimethyl-1-adamantanol, respectively, in the presence of an acidic catalyst in cyclohexane after refluxing for 4 h, and then, the resulting maleates were isomerized to the corresponding fumarates in the presence of piperidine in the same solvent while refluxing for 3.5 h (Scheme 1). Silica gel column chromatography yielded 1 and 2, which presented as white crystals, with 31–37% yields. The melting points were 191.8 and 70.2 °C for 1 and 2, respectively. The 1H NMR spectra of 1 and 2 are shown in Fig. 2.

Scheme 1
scheme 1

Synthesis of the symmetric DRFs containing adamantyl esters 1 and 2

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Fig. 2
figure 2

1H NMR spectra of a 1 and b 2 in CDCl3. Asterisk indicates the peak attributed to H2O

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The asymmetric fumarates were synthesized according to the reactions shown in Scheme 2. The equimolar amounts of maleic anhydride and 2-propanol were reacted in cyclohexane while refluxing in the absence of an acid for 13 h. Then, 1-adamantanol and a small amount of sulfuric acid were added to the solution, and the reaction mixture was refluxed for 22 h. After the evaporation of the solvent, the 1H NMR spectrum of the crude product indicated the presence of three types of the maleic diesters in a product mixture; i.e., 1-adamantyl isopropyl maleate was the main product and diisopropyl maleate and di-1-adamantyl maleate were the side products, as shown in Fig. 3a. The symmetric diesters were produced by transesterification during reflux in the presence of an acid. In addition to the diesters, the maleic acid monoesters as well as an unreacted maleic anhydride were also detected. The peaks characteristic of the isopropyl monoester and the 1-adamantyl monoester of maleic acid [see the spectra in Fig. 3b, c] are included in the 1H NMR spectrum. The unreacted maleic anhydride and the monoesters of maleic acid were readily removed from the reaction mixture by washing with saturated aq. NaHCO3. In contrast, the isolation of 1-adamantyl isopropyl maleate, which is the asymmetric fumaric diester, from the mixture of the three diesters was achieved by repeated silica gel column chromatography (n-hexane/ethyl acetate/chloroform = 7/1/2). We isolated 1-adamantyl isopropyl maleate as a colorless liquid with a purity of 92%. The isolated yield was 12%.

Scheme 2
scheme 2

Synthesis of the asymmetric DRFs containing adamantyl esters 3 and 4

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Fig. 3
figure 3

1H NMR spectra of a a reaction mixture containing isopropyl maleate and 1-adamantanol in the presence of an acid, refluxed in cyclohexane for 22 h, b a reaction mixture containing maleic anhydride and isopropanol in the absence of an acid, refluxed in cyclohexane for 19 7h, c a reaction mixture containing maleic anhydride and isopropanol in the absence of an acid, refluxed in cyclohexane for 10 h, and d 1-adamantyl isopropyl maleate isolated after purification by column chromatography and containing a small amount of diisopropyl maleate and di-1-adamantyl maleate. Measured in CDCl3

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The isomerization of 1-adamantyl isopropyl maleate to the corresponding fumarate, i.e., 3, was carried out in cyclohexane while refluxing in the presence of piperidine (Scheme 2). Silica gel column chromatography yielded 3, which presented as a colorless liquid, with a high purity (>96%) and 84% yield, which was calculated based on the use of 1-adamantyl isopropyl maleate (Fig. 4). Similarly, 4 was synthesized and isolated as a colorless liquid. The NMR spectral data of the DRFs used in this study are summarized in Table 1.

Fig. 4
figure 4

1H NMR spectrum of 3 in CDCl3. Asterisk indicates the peak attributed to H2O

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Table 1 NMR spectral data in ppm for the carbon-carbon double bonds in the DRFsa
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Conventional radical polymerization

The adamantane-containing DRFs were polymerized in the presence of a radical initiator (Scheme 3) under the polymerization conditions shown in Table 2. 1 had a high melting point and exhibited a low solubility in organic solvents. The solution polymerization of 1 yielded an oligomer with an Mn less than 104. The separation of the resulting P1 from the remaining monomer 1 was achieved after repeated precipitation using chloroform as the solvent and hot ethanol as the precipitant.

Scheme 3
scheme 3

Conventional radical polymerization of the symmetric DRFs 1 and 2

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Table 2 Radical polymerization of the adamantane-containing DRFs in the presence of a radical initiator either in bulk or in toluene
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The introduction of methyl substituents to the 3- and 5-positions of the adamantyl group led to a drastic decrease in the melting point and the improved solubility of the monomer. The bulk polymerization of 2 with BPO at 80 °C yielded a polymer with a 26% yield. The obtained polymer P2 was supposed to have a high molecular weight, because the viscosity increased during polymerization. In fact, the chloroform solution of P2 had a high viscosity, but P2 was insoluble in THF. Therefore, the molecular weight was not determined by the SEC measurement in this study. The solution polymerization provided a soluble polymer with an Mn less than 104.

The asymmetric DRFs, i.e., 3 and 4, yielded high molecular weight polymers in high yields during bulk polymerization in the presence of MAIB at 60 °C (Scheme 4). The solution polymerization again yielded polymers with a low Mn. Previous studied reported that polymerization at a high monomer concentration was required to produce PDRFs with a high molecular weight and a high yield [3, 4]. The monomers 3 and 4 were liquids at room temperature, so they are suitable for high polymerization under bulk conditions. The produced polymers P3 and P4 were soluble in toluene, chloroform, and THF. The 1H NMR spectra of the isolated polymers are shown in Fig. 5. The peaks characteristic to the repeating units, i.e., the methyl, methylene, and methine hydrogens of the 1-adamantyl and 3,5-dimethyl-1-adamantyl ester groups, were observed as the broad peaks in the range of 0.7–2.5 ppm. For P3 and P4, the methyl and methine hydrogens of the isopropyl group were also detected ca. 1.2 and 4.9 ppm, respectively. In contrast to the detection of the peaks characteristic of the hydrogen in the side chain, the methine hydrogen of the main chain was hardly observed because of peak broadening. The difficult observation of the peaks attributed to the main chain hydrogen of the PDRFs has often been reported in the literature [2,3,4]. This suggests the highly restricted motion of the main chain of these adamantane-containing PDRFs.

Scheme 4
scheme 4

Conventional radical polymerization of the asymmetric DRFs 3 and 4

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Fig. 5
figure 5

1H NMR spectra of a P1, b P2, c P3, and d P4 in CDCl3

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RAFT Polymerization of Adamantane-Containing DRFs

In our previous study [20], it was reported that the polymer chain structures of PDiPF was controlled by the RAFT polymerization using T2 as the bifunctional chain transfer agent (CTA). In this study, therefore, we carried out the RAFT polymerization of the adamantane-containing fumarates using T2. We first investigated the RAFT polymerization behavior of 3 (Scheme 5) in order to synthesize the polymers with functional trithiocarbonate groups at the both chain ends.

Scheme 5
scheme 5

Synthesis of P3 and the block copolymer of P3 with P2EHA (P2EHA-block-P3-block-P2EHA) by RAFT polymerization

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When polymerization was carried out with a [3]/[T2] molar ratio = 100/1 and at 60 °C for 4–6 h with different MAIB concentrations ([3]/[MAIB] molar ratio = 100/2.0–0.35), the corresponding polymer P3 with a trithiocarbonate moiety located at both chain ends was produced with a 43‒50% yield (47‒58% conversion). The observed Mn and Mw/Mn values were in the ranges of 1.12‒1.33 × 104 and 2.01, respectively. The Mn and Mw/Mn values decreased to 0.56 × 104 and 1.75, respectively, when the [DRF]/[T2] ratio was changed from 100 to 50. The higher CTA concentration resulted in better control of the reactions, but not enough control was obtained. For the RAFT polymerization of the DRFs, it has been pointed out that the steric hindrance of the leaving group R of the CTAs suppressed the initiation of the DRF polymerization [19], which differs from the RAFT polymerization of conventional monomers, such as methyl methacrylate and styrene. It has been speculated that the larger Mw/Mn values occur due to the slow initiation of the polymerization. We studied the polymerization behavior at a higher temperature (80 °C), but no temperature dependence was observed, as shown in Table 3.

Table 3 RAFT polymerization of 3 and 4 in bulk using either T2 or T1F as the CTAs
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The 1H NMR analysis of the produced polymer chains (Fig. 6(a)) revealed the introduction of the T2 fragment at the polymer chain ends and the R group at the center of the polymer chains with an 83‒93% functionality (see the footnotes in Table 3). This indicates that the RAFT polymerization of 3 was capable of controlling the chain-end structure, but control of the molecular weight was still limited. This result was different from the well-controlled polymerization of DiPF using the same CTA and similar polymerization conditions [20]. The isopropyl and adamantyl groups have Taft σ* parameters, which are the polar substituent constants that describe the inductive effects of the substituents, that are similar to one another, i.e., −0.19 and −0.26, respectively [53]. As shown in Table 3, the polymerization of 4 resulted in smaller Mw/Mn values (1.44–1.64). Unfortunately, the functionality for introducing the CTA fragments into P4 could not be determined because of the overlap of the characteristic peaks in the 1H NMR spectrum (Fig. 6b). The electron-donating effect of the adamantyl groups was weak but detected by NMR spectroscopy, as shown in Table 1. The 1H NMR chemical shifts of the CH=CH groups changed in the order of 6.810 (for DiPF) > 6.755/6.705 (for 3) ≥ 6.750/6.703 (for 4) > 6.655 (for 1) > 6.625 (for 2) ppm. In the 13C NMR spectra, the peaks attributed to the C=C carbons were observed at 133.2‒135.6 ppm for the adamantane-containing DRFs and 130.9 ppm for DiPF. Introducing an electron-withdrawing group into the ester alkyl group of the primary radical produced from CTA may promote the initiation of the polymerization of the DRFs with electron-donating ester alkyl groups. Actually, we examined this assumption by studying the RAFT polymerization process using the CTA including the electron-withdrawing ester group T1F (Fig. 1). As a result, the degree of control over the living radical polymerization was not improved, even by the introduction of a trifluoromethyl group. These results suggested the importance of the steric hindrance of the adamantyl groups, rather than the polar effect, to the initiation process. We concluded that the steric hindrance of the adamantyl groups obstructed the precise control of the polymerization of the adamantane-containing DRFs. In this study, we decided to further use the produced PDRFs as the macro-CTAs, because the relatively high functionality for the introduction of the trithiocarbonate groups at the chain ends was enough.

Fig. 6
figure 6

1H NMR spectra of a P3 and b P4 prepared by RAFT polymerization with T2 at 60 °C

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Block copolymer synthesis

Block copolymerization was carried out in toluene at 60 °C using 2EHA as the second monomer and P3 as the macro-CTA (Mn = 1.25–1.33 × 104, Mw/Mn = 2.01‒2.89), which was prepared by the RAFT polymerization of 3 with T2. The copolymer yields were 68–71%, and the Mn increased according to the conversion of 2EHA, as expected. The Mw/Mn values decreased during the polymerization of 2EHA, and finally, copolymers with a narrower molecular weight distribution were isolated (Mw/Mn = 1.61‒1.68). The block copolymers containing the P4 segment and the random copolymer segment of 3 and DiPF were synthesized by a similar method. Figure 7 shows the SEC curves for P4 and P2EHA-block-P4-block-P2EHA, which was synthesized by RAFT polymerization using T2. The peak in the SEC curve shifted to the high molecular weight region after the block copolymerization. In Fig. 8, the 1H NMR spectra of the resulting block copolymers are shown. Based on the peak intensity ratio of the characteristic peaks in the spectra, we calculated the DRF contents in the copolymers, which were compared with those determined from the change in the Mn values for the macro-CTA and the resulting block copolymer. As shown in Table 4, both the DRF content values agreed with each other.

Fig. 7
figure 7

SEC curves for P4 (---) and P2EHA-block-P4-block-P2EHA (―) synthesized by RAFT polymerization using T2

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Fig. 8
figure 8

1H NMR spectra of a P2EHA-block-P3-block-P2EHA and b P2EHA-block-P4-block-P2EHA in CDCl3

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Table 4 Block polymerization of 2EHA using the PDRFs as the macro-CTA in toluene at 60 °Ca
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Next, the thermal properties of the PDRFs and their block copolymers combined with polyacrylates were investigated by TG analysis. The TG curves of the selected PDRFs are shown in Fig. 9. The determined Td5 and Tmax values are summarized in Table 5. The onset temperature for the decomposition of the DRFs increased by the introduction of adamantyl ester groups, while the decomposition of DiPF started below 250 °C. The Td5 and Tmax values for P1 and P2 were much higher than those of PDiPF. The P3 and P4 compounds containing both the adamantyl and isopropyl ester groups had moderate thermal stabilities (Td5 = 286–288 °C). The triblock copolymers of the adamantane-containing PDRF with P2EHA also exhibited high decomposition temperatures. An excellent thermal stability was already reported for the polymers containing AdA repeating units [48, 49].

Fig. 9
figure 9

TG curves of P1, P4, P2EHA-block-P3-block-P2EHA, and PDiPF measured under a nitrogen stream at a heating rate of 10 °C/min

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Table 5 Thermal decomposition temperatures of adamantane-containing PDRFs and the triblock copolymers with P2EHAa
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The triblock copolymers containing the rigid poly(substituted methylene)s and the soft P2EHA segments were soluble or swelled in many types of organic solvents, including hexane, cyclohexane, ethyl acetate, and methyl ethyl ketone (refer to Table S1 in the Supporting Information). Some of the corresponding PDRF homopolymers were insoluble in hexane, ethyl acetate, methyl ethyl ketone, and acetone. The introduction of asymmetric structures to the adamantane-containing DRF monomers improved the solubility of not only the DRF monomers but also the corresponding PDRFs. The triblock copolymer of P4 with P2EHA exhibited the best solubility.

The optical properties of the polymers were also investigated. The homopolymers P1‒P4 were too brittle to prepare a cast film to evaluate their optical properties. In contrast, a highly transparent thin film was successfully prepared from the block copolymer of P3 with P2EHA by casting and was similar to the PDiPF and PAdA films previously reported in the literature [20, 48]. The results are shown in Fig. 10 and Table 6. The transmittance of visible light at 400 nm was higher than 90% for P2EHA-block-P3-block-P2EHA. The microphase separated structure of the copolymers may form in the block copolymers [48], which had no significant effect on the light scattering, because the domain size was smaller than the wavelength of the light. The nD value of P2EHA-block-P3-block-P2EHA (1.494) was similar to the nD values of the other adamantane-containing polyacrylates, such as PAdA and its block copolymer [48], and conventional polymethacrylates, including poly(methyl methacrylate) (nD = 1.49‒1.51) [54], and slightly higher than the nD values of the polymers including the PDiPF segment (nD = 1.463‒1.473), as shown in Table 6. A previous study [20] demonstrated that the transmittance of the block copolymers synthesized by the RAFT polymerization increased after the reduction of the RAFT moiety at the polymer chain end, and simultaneously, the nD values slightly decreased because of the presence of trithiocarbonate groups with C=S bonds that are highly polarizable. These results indicated that the trithiocarbonate groups surely influenced the optical properties, such as the refractive indices at a short wavelength and the Abbe number, but this effect was much smaller compared with the effect of introducing the adamantyl moiety in the side chain.

Fig. 10
figure 10

Wavelength dependence of the refractive indices of P2EHA-block-P3-block-P2EHA (red) and P2EHA-block-PDiPF-block-P2EHA before (blue, solid curve) and after (blue, dotted curve) reduction to remove the RAFT groups at the polymer chain ends

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Table 6 Optical properties of the PDRFs and polyacrylates and their block copolymers
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PDRFs are one of least flexible polymers. For example, the Kuhn’s statistical segment length, which corresponds to twice the persistence length, was determined to be 22 nm for PDiPF based on the dilute solution properties [14,15,16]. It has also been verified that the rigidity of the PDRF chains is sensitive to the bulkiness of the ester alkyl groups in the side chain [17]. Figure 11 shows the X-ray diffraction profiles of various PDRFs and triblock copolymers including PDRF segments. In Table 7, the d values calculated from the position of the diffraction peak are summarized together with the diameter of the polymer chains, which were estimated based on the molecular models. All the PDRFs and their block copolymers had strong peaks located in the low angle region of q = 4–6 nm‒1 and amorphous halos in the wide angle range of q = 10–15 nm‒1. The d values increased with an increase in the bulkiness of the ester alkyl groups of the DRF repeating unit in the following order: isopropyl < adamantyl <3,5-dimethyl-1-adamantyl. The P3 with an asymmetric repeating structure exhibited an intermediate d value between those of PDiPF and P2. These characteristic peaks at low angles are due to the rigid PDRF segments. Diffraction in the low angle region was not observed for the vinyl polymers containing the adamantyl groups, such as PAdA. It should be noted that these d values agree with the diameters of the PDRF chains. The rigid PDRF chains possibly assembled to form any aggregates present in the solid-state.

Fig. 11
figure 11

X-ray diffraction profiles obtained for P2, P3, and PDiPF

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Table 7 X-ray diffraction data for the PDRFs and their block copolymers
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Conclusion

In this study, we synthesized four kinds of DRFs containing adamantyl groups and used them to carry out conventional radical polymerization and RAFT polymerization. It has been demonstrated that the introduction of methyl substituents into the adamantyl moiety and an asymmetric ester structure into the fumarates lowered the melting points and changed the solubility of the adamantane-containing fumarate monomers. Block copolymers consisting of a rigid PDRF segment and a flexible polyacrylate segment were prepared by RAFT polymerization. It was confirmed that the adamantane-containing PDRFs and their block copolymers with a polyacrylate were thermally stable. The X-ray diffraction experiments revealed that the adamantane-containing PDRF chains aggregated in the solid-state. We are now continuing to study the aggregation and chain dynamics of the DRFs with different chain rigidities.