Introduction

Various poly(meth)acrylates bearing bulky ester groups, such as bornyl,1 isobornyl,1, 2, 3 adamantyl4, 5, 6, 7 and its derivatives,4, 5, 6, 8, 9, 10, 11 have been synthesized to improve the thermal properties, such as the glass transition temperature and the thermal degradation temperature, of poly(methyl methacrylate) (PMMA). For example, the incorporation of adamantyl groups into polymer structures generally produces not only a high thermal stability but also an improvement in other physical and chemical properties, such as transparency to ultraviolet light, low dielectric constant, hydrophobicity, high oxidation resistance, low surface energy and high density. Although these characteristics are interesting, there are only few studies on the synthesis of poly(1-adamantyl methacrylate) (PAdMA); Otsu and colleagues5 reported the conventional radical polymerization of 1-adamantyl methacrylate (AdMA) and its kinetic properties, and Nakahama and colleagues4 reported the living anionic polymerization of AdMA leading to well-defined PAdMA.

Controlled/living radical polymerizations, such as nitroxide-mediated radical polymerization, atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain transfer polymerization, have been applied to various methacrylate monomers.12, 13, 14 The controlled/living radical polymerization of methacrylate monomers bearing bulky ester groups, such as AdMA, which guarantees the generation of well-defined macromolecular architectures such as the block copolymer, graft copolymer and star-shaped polymer on a preparative scale, is still of great interest. In addition, we have previously reported that the simultaneous control of the molecular weight and stereochemistry15 of PMMA was accomplished using the designed initiating system based on ATRP.16, 17 Therefore, it is interesting to expand our method to methacrylates bearing bulky group, such as AdMA.

We now report the polymerization of AdMA using the ATRP-initiating system, methyl α-bromoisobutyrate (MBiB) and copper(I) bromide (CuBr) with 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA), 2,2′-bipyridyl (bpy) and tris[2-(dimethylamino)ethyl]amine (Me6TREN), as shown in Scheme 1. The livingness of the ATRP of AdMA is confirmed through a kinetic investigation using the CuBr/CuBr2/HMTETA-initiating system, and the stereocontrol of the ATRP of AdMA is carried out using the CuBr/CuBr2/Me6TREN-initiating system in a fluorinated solvent. Poly(methyl methacrylate)-block-poly(1-adamantyl methacrylate) (PMMA-b-PAdMA) was prepared by the ATRP of AdMA using the PMMA macroinitiator, PMMA-Br, as shown in Scheme 2. The effects of the molecular weight and tacticity of the well-defined PAdMAs on glass transition temperature are discussed.

Scheme 1
figure 1

ATRP of 1-Adamantyl methacrylate (AdMA).

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Scheme 2
figure 2

Synthesis of poly(methyl methacrylate) macroinitiator (PMMA-Br) and poly(methyl methacrylate)-block-poly(1-adamantyl methacrylate) (PMMA-b-PAdMA).

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Materials and methods

Materials

Copper(I) bromide (CuBr, 99.999%), copper(II) bromide (CuBr2, 99.999%) and HMTETA (>97%) were purchased from Sigma-Aldrich Chemicals Co. (St Louis, MO, USA) Me6TREN was supplied by Mitsubishi Chemical Co. Ltd. (Tokyo, Japan) HMTETA and Me6TREN were purified by distillation over CaH2 under reduced pressure. AdMA was donated by Idemitsu Kosan Co. Ltd. (Tokyo, Japan) and distilled over CaH2 and CuBr2 under argon. bpy was purchased from Merck KGaA (Darmstadt, Germany) and purified by recrystallization from hexane. MBiB was purchased from Fluka (Buchs, Switzerland) and distilled over CaH2 under argon before use. Dry toluene (>99.5%; water content, <0.001%) was purchased from Kanto Chemicals Co. Inc. (Tokyo, Japan) and was used as received. Tin(II) 2-ethylhexanoate (Sn(EH)2) was purchased from Wako Pure Chemical Industries Ltd. (Osaka, Japan) and was used as received. 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) was supplied by Central Glass Co. Ltd. (Tokyo, Japan) and was distilled from CaH2 under argon.

Measurements

The polymerization solution was prepared in an MBRAUN stainless steel glovebox (MBRAUN, Garching, Germany) equipped with a gas purification system (molecular sieves and copper catalyst) under a dry argon atmosphere (H2O, O2 <1 p.p.m.). The moisture and oxygen contents in the glovebox were monitored by MB-MO-SE 1 and MB-OX-SE 1, respectively. 1H (400 MHz) and 13C (100 MHz) NMR spectra were recorded using a JEOL JNM-A400II (JEOL, Akishima, Japan). Size exclusion chromatography (SEC) was performed at 40 °C in chloroform (0.8 ml min–1) by a Jasco GPC-900 system (Jasco, Hachioji, Japan) equipped with a Tosoh TSKgel GMHHR-M column (Tosoh, Tokyo, Japan) (linear; 7.8 mm × 300 mm; pore size=16 nm; bead size=5 μm; exclusion limit=4 × 106), Shodex KF-804L columns (linear; 8 mm × 300 mm; pore size=20 nm; bead size=7 μm; exclusion limit=4 × 105) and a refractive index detector. The number-average molecular weight (Mn) and polydispersity index (Mw/Mn) of the polymers were calculated on the basis of PMMA calibration. The monomer conversion in the polymerization was calculated from the concentration of the residual monomer in the reaction mixture, as determined by a Shimadzu GC-17A gas chromatography (GC) system (Shimadzu, Kyoto, Japan) equipped with an SGE BPX70 capillary column (SGE, Ringwood, Australia) (30 m × 0.25 mm; film thickness, 0.25 μm; temperature, 120 °C) and an flame ionization detector (temperature, 120 °C) using toluene or HFIP internal standards. The inverse-gated decoupling 13C NMR spectra for the tacticity analysis of PAdMA were measured using a 20% (w/v) sample in CDCl3 at 55 °C with a 45° pulse angle, 7.0 s delay of the inverse-gated decoupling and 10 000 scans. The triad tacticity of a PAdMA molecule was determined from the integral values of the quaternary carbon on an adamantyl group of the polymer in the 13C NMR spectrum. The glass transition temperature (Tg) of the polymers was measured by Differential scanning calorimetry (DSC) using Seiko Instruments SSC 5200 DSC 220 (Seiko, Chiba, Japan). Samples in 3–10 mg portions were first heated above their Tg at a heating rate of 10 °C min−1, then cooled to −20 °C at a cooling rate of 40 °C min−1. The samples were reheated to 280 °C at a heating rate of 10 °C min−1. All Tg values were obtained from the second scan after removing the thermal history.

Polymerization of AdMA

A typical procedure for the polymerization is as follows: MBiB (5.88 μl, 45.4 μmol) was added to a solution of AdMA (1.00 g, 4.54 mmol), CuBr (6.5 mg, 45 μmol) and HMTETA (10.6 mg, 45.4 μmol) in toluene (3 ml) under argon. Polymerization was initiated by heating at 60 °C. After 9 h, the reaction mixture was exposed to air to quench the polymerization. The product was passed through an alumina column using toluene as the eluent to remove the copper catalyst, followed by two reprecipitations in a mixed solvent of methanol and water (1/1, v/v). The purified polymer was obtained as a white powder. The reaction yield was 0.475 g (47.1%), conversion was 57.0%, Mn was 8.3 kg mol−1 and Mw/Mn was 1.13. Polymerizations using the other ligands, solvent and additional CuBr2 were conducted using a similar procedure.

Block copolymerization of AdMA with poly(methyl methacrylate) macroinitiator

The procedure for the preparation of the poly(methyl methacrylate) macroinitiator (PMMA-Br) in toluene at 60 °C is the same procedure used for the polymerization of AdMA. The amounts of the necessary reagents are shown as follows: MMA (22.7 g, 227 mmol), MBiB (0.294 μl, 2.27 mmol), CuBr (325 mg, 2.27 mmol), HMTETA (525 mg, 2.28 mmol) and toluene (50 ml). The PMMA macroinitiator (PMMA-Br) was obtained as a white powder. The reaction yield was 8.96 g (38.8%), conversion was 45.1%, Mn was 9.9 kg mol−1 and Mw/Mn was 1.15.

Block copolymerization of AdMA and MMA using the CuBr/CuBr2/HMTETA catalytic system was carried out by the following procedure: a stock solution of HMTETA (5.1 mg, 22 μmol), CuBr (2.6 mg, 18 μmol) and CuBr2 (0.8 mg, 4 μmol) in toluene (0.16 ml) was added to a solution of PMMA-Br (242 mg, 24.4 μmol) and AdMA (0.407 g, 1.85 mmol) in toluene (1.46 ml) under argon. Polymerization was initiated by heating at 60 °C. After 9 h, the reaction mixture was exposed to air to quench the polymerization. The subsequent purification procedure was the same as that for PAdMA. PMMA-b-PAdMA was obtained as a white powder. The reaction yield was 170.2 mg, conversion was 24.7%, Mn was 15.5 kg mol−1 and Mw/Mn was 1.18.

The block copolymerization of AdMA and MMA using the CuBr2/HMTETA/Sn(EH)2 catalytic system was carried out using the following procedures: Sn(EH)2 (8.3 mg, 20.5 μmol) was added to a solution of PMMA-Br (629 mg, 62.6 μmol), AdMA (1.00 g, 4.54 mmol), CuBr2 (10.2 mg, 45.7 μmol) and HMTETA (10.5 mg, 45.6 μmol) in toluene (3.00 ml) under argon to initiate polymerization by heating at 60 °C. After 2 h, the reaction mixture was exposed to air to quench the polymerization. The subsequent purification procedure was the same as that for PAdMA. PMMA-b-PAdMA was obtained as a white powder. The yield was 1.17 g, conversion was 42.4%, Mn was 18.7 kg mol–1 and Mw/Mn was 1.13.

Results and discussion

Atom transfer radical polymerization of 1-adamantyl methacrylate

We first carried out the polymerization of AdMA in toluene at 60 °C for 9 h using MBiB, copper(I) bromide (CuBr) and an amine ligand ([AdMA]0/[MBiB]0=100/1). The proper choice of ligand for copper is necessary to control the ATRP. Thus, three ligands, Me6TREN, bpy and HMTETA, were used to optimize the polymerization conditions (Scheme 1). Table 1 summarizes the results of the polymerizations, and Figure 1 shows the SEC traces of the obtained polymers. The CuBr/Me6TREN catalytic system (Table 1, run 1) produced an uncontrolled polymerization, which resulted in PAdMA having an Mn of 16.4 kg mol−1, a bimodal molecular weight distribution and Mw/Mn of 2.34. Although the polymerization using the CuBr/bpy catalytic system (Table 1, run 2) produced PAdMA with a unimodal molecular weight distribution, the Mw/Mn of the obtained PAdMA was still as high as 1.35. However, polymerization using the CuBr/HMTETA catalytic system (Table 1, run 3) produced the narrow-dispersed PAdMA with an Mn of 8.3 kg mol−1 and an Mw/Mn of 1.13. However, the Mn value of the obtained PAdMA was lower than the theoretical molecular weight (Mn,theo.) of 12.7 kg mol−1 predicted from the monomer conversion and the initial ratio of AdMA and MBiB. Therefore, we carried out an additional AdMA polymerization using the CuBr/copper(II) bromide (CuBr2)/HMTETA catalytic system in toluene at 60 °C for 8.5 h (Table 1, run 4). It is well known that the addition of copper(II) species to an ATRP system using a copper(I) catalyst provides good control of the molecular weight.18, 19 After polymerization, the Mn value of the obtained PAdMA (10.9 kg mol−1) was the same as the Mn,theo value, and the Mw/Mn value of 1.08 was less than that of PAdMA prepared by the CuBr/HMTETA system. Thus, it was revealed that the CuBr/CuBr2/HMTETA catalytic system is the most suitable for the ATRP of AdMA.

Table 1 Atom transfer radical polymerization of AdMA using the MBiB/CuBr-initiating system with Me6TREN, bpy and HMTETA
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Figure 1
figure 3

SEC traces of PAdMA prepared by ATRP at 60 °C in toluene using CuBr/Me6TREN (run 1), CuBr/bpy (run 2), CuBr/HMTETA (run 3) and CuBr/CuBr2/HMTETA (run 4) as the catalytic system.

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A kinetic investigation was subsequently carried out to confirm the controlled nature of the ATRP of AdMA using the CuBr/CuBr2/HMTETA catalytic system in toluene at 60 °C. Figure 2a shows the relationship between the monomer conversion of the polymerization and the Mn value of the obtained PAdMA. The Mn values of the product linearly increased with the increasing conversion of AdMA, and the Mw/Mn value of 1.08–1.11 remained constant for up to at least 79% monomer conversion. Figure 2b shows the first-order kinetic plot of the polymerization. The consumption rate of AdMA with increasing polymerization time was linear with the concentration of the remaining monomer in the reaction mixture. In conclusion, the ATRP system for AdMA was successfully established.

Figure 2
figure 4

(a) Dependence of the number-average molecular weight (Mn) and polydispersity (Mw/Mn) on monomer conversion and (b) kinetic plots for the polymerization of AdMA catalyzed by CuBr/CuBr2/HMTETA in toluene at 60 °C ([AdMA]0=1.14 mol l−1; [AdMA]0/[MBiB]0/[CuBr]0/CuBr2]0/[HMTETA]0=100/1/1/0.2/1.2).

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Moreover, we carried out the ATRP of AdMA in HFIP, a representative fluoroalcohol, to simultaneously control the molecular weight and tacticity15 of PAdMA (Scheme 1). The use of fluoroalcohols as the solvent for radical polymerization induces a tacticity change in the resultant polymer.20, 21 We also previously succeeded in the simultaneous control of the molecular weight and tacticity of PMMA by ATRP using HFIP.16, 17 The ATRP of AdMA in HFIP was carried out using the CuBr/CuBr2/Me6TREN catalytic system at 20 °C for 15 h (Supplementary Table S1, run 7) and at −20 °C for 40.5 h (Supplementary Table S1, run 11). Although the polymerization at 20 °C yielded PAdMA with a broad Mw/Mn value of 1.85, the polymerization at −20 °C produced narrow-dispersed PAdMA with a Mw/Mn ratio of 1.14 and Mn of 14.1 kg mol−1. The tacticity of PAdMA prepared in toluene at 60 °C (Table 1, run 3) and in HFIP at −20 °C was compared. The triad tacticity of PAdMA can be analyzed from the resonance of the quaternary carbon in the adamantyl group appearing around 80–81 p.p.m. in the 13C NMR spectrum.5 Inverse-gated decoupling was applied to obtain the quantitative 13C NMR spectra, which are shown in Supplementary Information (Supplementary Figure S1). The triad tacticity contents (mm/mr/rr) of PAdMA prepared in toluene and HFIP were 4/27/69 and 1/21/78, respectively. The rr triad contents of the PAdMA prepared in HFIP were 9% higher than that of the PAdMA prepared in toluene. Thus, the simultaneous control of the molecular weight and the tacticity of AdMA was also successfully achieved by polymerization in fluoroalcohol.

Block copolymerization of AdMA and methyl methacrylate

To use the controlled nature of the ATRP of AdMA, we first tried to prepare poly(1-adamantyl methacrylate)-block-poly(methyl methacrylate) (PAdMA-b-PMMA) from PAdMA with bromine end functionality (Mn=9.6 kg mol−1, Mw/Mn=1.08), which was synthesized by the ATRP of AdMA using the MBiB/CuBr/CuBr2/HMTETA-initiating system in toluene at 60 °C for 6 h (Supplementary Scheme S1). However, PAdMA could not sufficiently initiate the block copolymerization of MMA using the CuBr/CuBr2/HMTETA catalytic system in toluene at 60 °C for 9 h, resulting in the mixture of PAdMA-b-PMMA and unreacted PAdMA, as shown in Supplementary Information (Supplementary Figure S2). This result could be caused by the low reactivity of the PAdMA radical toward MMA, which was reported by Otsu et al.6

Thus, we changed the synthesis strategy, as shown in Scheme 2. The PMMA macroinitiator (PMMA-Br) was prepared by the ATRP of MMA using the MBiB/CuBr/HMTETA-initiating system. The resulting PMMA-Br had an Mn value of 9.9 kg mol−1 and an Mw/Mn value of 1.15 (Figure 3a). The block polymerization of AdMA with PMMA-Br was then carried out using the CuBr/CuBr2/HMTETA catalytic system in toluene at 60 °C under the condition of [AdMA]0/[PMMA-Br]0/[CuBr]0/[CuBr2]0/[HMTETA]0=76/1.0/0.74/0.16/0.90 (Table 2, run 5). Although the obtained block copolymer, PMMA-b-PAdMA, showed a relatively narrow and unimodal SEC trace for the Mw/Mn value of 1.18 and the observed Mn value agreed with the theoretical value, the monomer conversion of 24.7% was very low for the polymerization time of 9 h.

Figure 3
figure 5

SEC traces of the (a) PMMA macroinitiator (PMMA-Br) and PMMA-b-PAdMA obtained by the AGET ATRP of AdMA using PMMA-Br in toluene at 60 °C after (b) 1 h, (c) 2 h, (d) 3 h and (e) 4 h ([AdMA]0/[PMMA-Br]0/[CuBr2]0/[HMTETA]0/[Sn(EH)2]0=73/1.0/0.73/0.73/0.33).

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Table 2 Synthesis of PMMA-b-PAdMA by block polymerization of AdMA using the PMMA macroinitiator (PMMA-Br) in toluene at 60 °C
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Given those results, we applied the AGET ATRP method22 for the block copolymerization of AdMA with PMMA-Br (Scheme 2). Polymerization was carried out under the condition of [AdMA]0/[PMMA-Br]0/[CuBr2]0/[HMTETA]0/[Sn(EH)2]0=73/1.0/0.73/0.73/0.33 in toluene at 60 °C (Table 2, run 6). The molecular weight of PMMA-b-PAdMA increased with polymerization time, as shown in Figure 3. The obtained block polymers however showed bimodal SEC traces for polymerization times greater than 3 h (Figures 3d and e). However, the SEC trace of the block polymer for 2 h is unimodal and the Mw/Mn value was 1.13 (Figure 3c). The Mn value of 18.7 kg mol−1 agreed with the predicted value. These results indicate that the AGET ATRP method was effective for the block copolymerization of AdMA using the PMMA macroinitiator.

Molecular weight and tacticity effect on the thermal properties of PAdMA

The thermal properties of polymers containing the adamantyl group were intensively investigated from the viewpoint of high thermal stability. We first investigated the relationship between the glass transition temperature (Tg) of PAdMA and its molecular weight. The Tg of each polymer was measured by DSC. Figure 4 shows the change in Tg of PAdMA with increasing Mn, and the representative DSC trace is shown in Supplementary Information (Supplementary Figure S3). The Tg value varied within the range of 200–244 °C, with an increase in molecular weight from 4.6 to 32.1 kg mol−1. Although the previously reported PAdMA showed no glass transition in almost all cases,4, 5, 6 all the PAdMAs prepared by ATRP in this study showed glass transitions, presumably because of the low molecular weights obtained. The effect of tacticity on the Tg of PAdMA was investigated next. PAdMA with a 69% rr triad content (Mn=14.5 kg mol−1, Mw/Mn=1.12) showed a Tg of 230 °C. However, PAdMA with a 78% rr triad content (Mn=14.1 kg mol−1, Mw/Mn=1.14) showed a Tg of 243 °C. Although these two polymers had narrow molecular weight distributions and almost the same molecular weight, the difference in their glass transition temperatures was large. This result indicates that the Tg of PAdMA increases with the increasing rr triad contents, corresponding to the results reported by Otsu et al.5 The relationship between tacticity and the Tg of PAdMA was the same as the relationship for PMMA.23

Figure 4
figure 6

Dependence of the glass transition temperature (Tg) on the Mn of PAdMA (mm/mr/rr=4/27/69).

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Conclusions

The controlled/living polymerization system for AdMA was successfully established by the ATRP method using CuBr/CuBr2/HMTETA. The simultaneous control of the tacticity and molecular weight of PAdMA was achieved by ATRP conducted in HFIP at low temperature using the CuBr/CuBr2/Me6TREN catalytic system. In addition, the block copolymerization of AdMA with MMA proceeded reasonably using the PMMA macroinitiator/CuBr2/HMTETA/Sn(EH)2-initiating system based on the AGET ATRP method. The dependence of the glass transition temperature of PAdMA on molecular weight and tacticity was clearly revealed by DSC analyses.