Successive Synthesis of Miktoarm Star Polymers Having up to Seven

Jan 16, 2013 - This reaction sequence was repeated three times to successively synthesize 3-arm ABC, 4-arm ABCD, 5-arm ABCDE, 6-arm ABCDEF, and ...
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Successive Synthesis of Miktoarm Star Polymers Having up to Seven Arms by a New Iterative Methodology Based on Living Anionic Polymerization Using a Trifunctional Lithium Reagent Shotaro Ito, Raita Goseki, Takashi Ishizone, Saeko Senda, and Akira Hirao* Polymeric and Organic Materials Department, Graduate School of Science and Engineering, Tokyo Institute of Technology, S1-6, 2-12-1, Ohokayama, Meguro-ku, Tokyo 152-8552, Japan S Supporting Information *

ABSTRACT: A new stepwise iterative methodology based on living anionic polymerization using a trifunctional lithium reagent substituted with trimethylsilyl (TMS), tert-butyldimethylsilyl (TBDMS), and tetrahydropyranyl (THP) ethers of protected hydroxyl functionalities has been developed in order to obtain synthetically challenging many-armed μ-star polymers. In each reaction sequence of the new methodology, these three ether functions were selectively deprotected in turn under carefully selected conditions as designed, followed by conversion to three α-phenyl acrylate (PA) reaction sites step by step at different reaction stages. They were used for the introduction of two different arm segments and the reintroduction of the above same three ethers. This reaction sequence was repeated three times to successively synthesize 3arm ABC, 4-arm ABCD, 5-arm ABCDE, 6-arm ABCDEF, and 7-arm ABCDEFG μ-star polymers with well-defined structures. Herein, the A, B, C, D, E, F, and G arms were poly(cyclohexyl methacrylate), polystyrene, poly(4-methoxystyrene), poly(4methylstyrene), poly(methyl methacrylate), poly(ethyl methacrylate), and poly(2-methoxyethyl methacrylate) segments, respectively. The trifunctional lithium reagent was also demonstrated to satisfactorily function as a convenient and useful core agent access to the general synthesis of 4-arm ABCD and 6-arm A2B2C2 μ-star polymers.

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INTRODUCTION Asymmetric star-branched polymers composed of chemically different arm segments, the so-called mixed arm or miktoarm star-branched polymers, abbreviated as μ-star polymers,1−8 have recently received much attention, since their morphological suprastructures and supramolecular assemblies formed in bulk as well as selected solvents are new, characteristic, and, more importantly, quite different from those produced by linear block polymers.9−22 Such nanoscale ordered materials may possibly play an important role in the fields of nanoscience and nanotechnology with many potential applications. Accordingly, μ-star polymers are expected as promising next-generation multiphase polymeric materials. Although the well-defined structure of a μ-star polymer is essential to investigate detailed morphological behavior, a major problem is currently the lack of a general and established methodology to synthesize well-defined μ-star polymers composed of three or more different arm segments.10,23−37 In order to overcome this problem, we have, since 2001, developed a general and versatile methodology, which is based on a new conceptual “iterative approach”.5−7,38−40 In this methodology, the reaction system is designed in such a way that the same reaction site is always regenerated after the introduction of an arm segment in each reaction sequence, and this “arm introduction and regeneration of the same reaction © 2013 American Chemical Society

site” sequence is repeatable. Therefore, arm segments can be successively and, in principle, limitlessly introduced by repeating the reaction sequence to afford a series of μ-star polymers with many arms. The first successful demonstration was a stepwise iterative methodology based on living anionic polymerization using a 1,1-diphenylethylene (DPE) function as a reaction site. By developing this methodology and further modified ones, we successfully synthesized a variety of welldefined μ-star polymers with many arms, such as 3-arm ABC, 4arm ABCD, 5-arm ABCDE, 6-arm ABCDEF, 7-arm ABCDEFG, 6-arm A2B2C2, 9-arm A3B3C3, 4-arm ABC2, 7-arm AB2C4, 15-arm AB2C4D8, and 31-arm AB2C4D8E16 μ-star polymers.41−52 However, since the living anionic polymers usable in these methodologies were always required to react with the DPE reaction site, they were limited to only highly reactive living polymers of styrene, 1,3-butadiene, isoprene, and their derivatives. Very recently, we have developed a “second-generation” stepwise iterative methodology using an α-phenyl acrylate (PA) function as a new reaction site, in which less reactive living anionic polymers of 2-vinylpyridine and alkyl methacrylate Received: December 5, 2012 Revised: January 9, 2013 Published: January 16, 2013 819

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Scheme 1. “Second-Generation” Stepwise Iterative Methodology Based on Living Anionic Polymerization Using 1

reacted with living polymer (C). A 3-arm ABC μ-star polymer core-functionalized with TBDMS ether was thus synthesized. The PA reaction site was prepared from the TBDMS ether, followed by reacting it with the functionalized DPE anion, to reintroduce both the TMS and TBDMS ethers. By repeating the same reaction sequence two more times, a 4-arm ABCD, followed by a 5-arm ABCDE μ-star polymer, could be successively synthesized. Typically, the A, B, C, D, and E segments are poly(methyl methacrylate), poly(ethyl methacrylate), poly(tert-butyl methacrylate), poly(benzyl methacrylate), and poly(2-methoxyethyl methacrylate), respectively. Herein, the 3-arm ABC, 4-arm ABCD, and 5-arm ABCDE μ-star polymers thus synthesized were the first successful well-defined μ-star polymers composed of all different poly(methacrylate)based arm segments. Furthermore, living anionic polymers of styrene, isoprene, and 2-vinylpyridine could also be used in the synthesis of μ-star polymers by this methodology because the PA reaction site was readily and quantitatively reacted with these more reactive living polymers, as previously reported by us.53−58 Thus, a wide variety of living anionic polymers with different reactivities were advantageously usable in the secondgeneration stepwise iterative methodology, which makes it more general and versatile. As can be seen in Scheme 1, the arm segment is introduced one by one in each process by repeating the reaction sequence. Therefore, four reaction sequences are needed to synthesize the 5-arm ABCDE μ-star polymer. Herein, we report on the development of a new iterative methodology using a trifunctional lithium reagent, with which two different polymer segments can be sequentially introduced in each process. Accordingly, with the use of this methodology, the same 5-arm ABCDE μ-star polymer may be synthesized by repeating the reaction sequence only twice. Thus, the new methodology is more efficient in the process of μ-star polymer synthesis than that previously reported.53

monomers are usable in addition to the above highly reactive living polymers.53 Scheme 1 shows the synthetic outline of this iterative methodology. In order to repeat “arm introduction and regeneration of the same reaction site” sequence in the methodology, a specially designed DPE derivative substituted with trimethylsilyl (TMS) and tert-butyldimethylsilyl (TBDMS) ethers, 1, is newly synthesized and used as a dualfunctionalized key agent. As can be seen in Scheme 1, TMS and TBDMS ethers of two silyl-protected hydroxyl functionalities convertible to two PA reaction sites were introduced at the chain end of polymer (A). This was accomplished by the living anionic polymerization of the corresponding monomer (a) with the functionalized DPE anion derived from 1 and RLi. The TMS ether was then selectively deprotected to regenerate the hydroxyl group by treatment with methanol containing a very small amount of acetic acid, while the TBDMS ether remained unchanged under such conditions. The first PA reaction site was prepared by reacting α-phenylacrylic acid with the regenerated hydroxyl group under the Mitsunobu esterification reaction conditions and subsequently reacted with living polymer (B). As a result, an in-chain-(TBDMS ether)-functionalized A-block-B was obtained. Next, the second PA reaction site was prepared from the TBDMS ether present between the A and B segments via deprotection by treatment with (C4H9)4NF, followed by conversion to the PA function. Then, the functionalized DPE anion derived from 1 and RLi was reacted with the resulting inchain-PA-functionalized A-block-B to reintroduce both the TMS and TBDMS ethers. Since the two ethers are the same as those introduced in the starting polymer, A, the abovementioned reaction sequence can be repeated. Accordingly, this reaction is a key step to continue the next reaction sequence in the iterative methodology. In practice, the PA reaction site was again prepared from the TMS ether exactly in the same manner and subsequently 820

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derivative thus prepared (0.697 g, 2.15 mmol) and hexamethyldisilazane (0.807 g, 5.01 mmol) were dissolved in nitromethane (10 mL), and the mixture was allowed to react at 25 °C for 20 min under nitrogen. Removal of all volatile compounds by vacuum pump (10−1 Torr), followed by high-vacuum conditions (10−6 Torr), yielded 1-((3(2-Tetrahydro-2H-pyranyloxy)methyl)phenyl)-1-(3trimethylsilyloxymethylphenyl)ethylene (2) (0.832 g, 2.10 mmol, 98%). The resulting 2 was used without further purification. It was diluted with dry THF and then divided into ampules with break-seals under high-vacuum conditions. 300 MHz 1H NMR (CDCl3) δ = 7.38−7.16 (m, 8H, Ar), 5.46 (s, 2H, =CH2), 4.80−4.68 (m, 4H, Ar− CH2−), 4.46, 4.50 (m, 1H, −O−CH−O−), 3.95−3.87, 3.57−3.50 (m, 2H, −O−CH2−CH2−), 1.95−1.48 (m, 6H, −CH2−CH2−CH2−), 0.14 (s, 9H, −Si−(CH3)3). General Procedure of μ-Star Polymer Synthesis. Except for deprotection and the Mitsunobu esterification reaction, all of the polymerizations and linking reactions were carried out under highvacuum conditions (10−6 Torr) in sealed handmade glass reactors equipped with break-seals. The reactor was sealed off from the vacuum line and prewashed with red 1,1-diphenylhexyllithium (ca. 0.05 M) in heptane solution prior to the polymerization and linking reaction. All operations were performed according to the usual high-vacuum technique with break-seals.62 Synthesis of 4-Arm ABCD μ-Star Polymers. The synthesis was carried out in similar manners reported by our previous paper.53 The synthetic details are reported in the Supporting Information. Successive Synthesis of μ-Star Polymers by a New Stepwise Iterative Methodology. The synthesis was started from the chainend-(TMS, TBDMS, and THP ethers)-functionalized PCHMA. A core-PA-functionalized 3-arm ABC μ-star polymer composed of PCHMA, PS, and P4MOS segments was synthesized by the same procedures as those used in the synthesis of 3-arm ABC μ-star polymers previously reported.53 By reacting a trifunctional lithium reagent, 3, prepared from TBDMS-PLi and 2, with the resulting corePA-functionalized polymer, TMS, TBDMS, and THP ethers were reintroduced at the core. Since the resulting core-(TMS, TBDMS, and THP ethers)-functionalized ABC μ-star polymer possesses the same three ethers as those of the starting PCHMA, a 5-arm ABCDE followed by a 7-arm ABCDEFG μ-star polymer was synthesized by repeating the same reaction sequence twice. Herein, the A, B, C, D, E, F, and G arms were PCHMA, PS, P4MOS, P4MS, PMMA, PEMA, and PMOEMA segments, respectively. Mn(RALLS) = 74.2 kg/mol and Mw/Mn = 1.04 (SEC). 1H NMR (CDCl3) (300 MHz): δ = 7.24− 6.18 (m, Ar), 5.80 (s, −CHCH2), 4.66 (s, −O−CH−), 4.18−3.98 (m, −O−CH2−CH3, −O−CH2−CH2−OCH3) 3.74 (s, Ar−OCHH3), 3.60 (s, COOCH3, −CH2−OCH3), 3.38 (s, −CH2−OCH3), 2.33− 0.49 (broad, backbone), 2.27 (s, −C6H4−CH3). The synthetic details are reported in the Supporting Information. Synthesis of a 6-Arm A2B2C2 μ-Star Polymer. The target polymer was synthesized by the reaction of 1,4-dilithio-1,1,4,4tetraphenylbutane with a core-PA-functionalized 3-arm ABC μ-star polymer composed of PMMA, PαMS, and PMOEMA segments. The synthetic details are reported in the Supporting Information.

EXPERIMENTAL SECTION

Materials. The reagents (>98% purities) were purchased from Aldrich, Japan, and used as received unless otherwise stated. Tetrahydrofuran (THF) was refluxed over sodium wire, distilled over LiAlH4 under nitrogen, and then distilled from its sodium naphthalenide solution under high-vacuum conditions (10−6 Torr). Heptane and tert-butylbenzene were washed with concentrated H2SO4, water, and aqueous NaHCO3, dried over P2O5, and finally distilled from its 1,1-diphenylhexyllithium solution under high-vacuum conditions. Styrene, α-methylstyrene (αMS), 4-methylstyrene (4MS), 4-methoxystyrene (4MOS), and 1,1-diphenylethylene (DPE) were washed with 5% NaOH solution, water, and then dried over MgSO4. After filtration of MgSO4, they were distilled over CaH2 under reduced pressures. Finally, styrene, αMS, 4MS, and 4MOS were distilled from their Bu2Mg solutions (ca. 3 mol %) under high-vacuum conditions. DPE was finally distilled from its 1,1-diphenylhexyllithium solution (ca. 3 mol %) under high-vacuum conditions. 2-Vinylpyridine (2VP) was stirred over KOH overnight. After filtration of KOH, 2VP was distilled over fine powder CaH2 twice under reduced pressures. Finally, 2VP was distilled over fine powder CaH2 under high-vacuum conditions. Methyl methacrylate (MMA), ethyl methacrylate (EMA), cyclohexyl methacrylate (CHMA), benzyl methacrylate (BnMA), allyl methacrylate (AMA), and 2-methoxyethyl methacrylate (MOEMA) were washed with 5% NaOH, water, dried over MgSO4, and finally distilled from their (C8H17)3Al solutions (ca. 3 mol %) under highvacuum conditions. (2,2-Dimethyl-1,3-dioxolan-4-yl)methyl methacrylate (acetal-DIMA) was synthesized and purified according to our procedure previously reported.59 LiCl was dried with stirring at 120 °C for 72 h under high-vacuum conditions. All monomers and LiCl were diluted with dry THF and then divided into ampules equipped with break-seals under high-vacuum conditions. α-Phenylacrylic acid and 1,1-bis(3-hydroxymethylphenyl)ethylene were synthesized according to the procedures previously reported.60,61 3-tert-Butyldimethylsilyloxy-1-propyllithium (TBDMS-PLi, in cyclohexane, FMC Corporation Lithium Division) was diluted with dry heptane and divided into ampules with break-seals under high-vacuum conditions. Measurements. Both 1H and 13C NMR spectra were measured on a Bruker DPX300 in CDCl3. Chemical shifts were recorded in ppm downfield relative to CHCl3 (δ = 7.26) and CDCl3 (δ = 77.1) for 1H and 13C NMR as standard, respectively. Molecular weight and polydispersity indices were measured on an Asahi Techneion AT2002 equipped with a Viscotek TDA model 302 triple detector array using THF as a carrier solvent at a flow rate of 1.0 mL/min at 40 °C. Three polystyrene (PS) gel columns (pore size (bead size)) were used: 650 Å (9 μm), 200 Å (5 μm), and 75 Å (5 μm). The relative molecular weights were determined by SEC with RI detection using standard PS calibration curve. The combination of viscometer, right angle laser light scattering detection (RALLS), and RI detection was applied for the online SEC system in order to determine the absolute molecular weights of homopolymers, diblock copolymers, and star-branched polymers. Polymer mixtures were separated by using a preparative SEC (KNAUER, Smartline) equipped with RI detector using THF as a carrier solvent at a flow rate of 5.0 mL/min at room temperature. The diameter of the column is 21.5 mm. The polymer mixture of ca. 0.5 g can be separated. Synthesis of 1-((3-(2-Tetrahydro-2H-pyranyloxy)methyl)phenyl)-1-(3-trimethylsilyloxymethylphenyl)ethylene (2). To the solution of 1,1-bis(3-hydroxymethylphenyl)ethylene (2.35 g, 9.79 mmol) and 3,4-dihydro-2H-pyran (0.493 g, 5.87 mmol) dissolved in dry CH2Cl2 (10 mL) was added p-toluenesulfonic acid monohydrate (0.186 g, 0.979 mmol), and the mixture was stirred at 25 °C for 12 h under nitrogen. The reaction was quenched with saturated aqueous NaHCO3 (10 mL). The reaction mixture was extracted with diethyl ether, and the ether layer was washed with water and dried over anhydrous MgSO4. After the removal of solvents under reduced pressure, the silica gel column chromatography treated with triethylamine (hexane/ethyl acetate = 2/1 (v/v)) afforded 1-(3hydroxymethylphenyl)-1-((3-(2-tetrahydro-2H-pyranyl)oxymethyl)phenyl)ethylene in 22% yield (0.704 g, 2.17 mmol). The DPE

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RESULTS AND DISCUSSION General Synthesis of 4-Arm ABCD μ-Star Polymers by Using a Trifunctional Lithium Reagent. Throughout the methodologies herein developed, a new trifunctional lithium reagent, 3, substituted with TMS, TBDMS, and THP ethers of protected hydroxyl functionalities is first prepared by reacting 2 with 3-(tert-butyldimethylsilyloxy)-1-propyllithium (TBDMSPLi). These three ethers are designed to be selectively deprotected in turn to regenerate hydroxyl groups under carefully selected conditions. Then, the hydroxyl groups are converted to three PA reaction sites step by step by different reaction stages. The resulting three PA reaction sites are used for the introduction of three-arm segments.

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Scheme 2. Synthesis of 4-Arm ABCD μ-Star Polymer

Table 1. Synthesis of 4-Arm ABCD μ-Star Polymers Mn (kg/mol) polymer d

A ABd ABCd ABCDd Ae ABe ABCe ABCDe

calcd

RALLSb

8.95 21.9 33.9 43.2 22.9 50.4 67.6 80.9

11.2 22.2 34.8 43.2 30.6 50.3 67.3 80.0

composition (wt %)a

Mw/Mn H NMR

SECc

10.6 21.7 34.1 45.8 28.5 50.6 65.9 81.0

1.05 1.05 1.03 1.03 1.02 1.03 1.03 1.04

1

calcd

1

H NMR

49/51 31/33/36 24/25/28/23

49/51 31/33/36 23/23/28/26

60/40 44/30/26 37/24/22/17

60/40 45/31/24 38/22/23/17

a

A/B/C/D polymer segments. bDetermined by SEC with triple detectors. cDetermined by SEC using PS standards. dA/B/C/D = PCHMA/ PMMA/PBnMA/PMOEMA. eA/B/C/D = PAMA/PS/PMOEMA/P2VP.

methacrylate (CHMA) in THF at −78 °C for 2.5 h to afford a PCHMA chain end functionalized with TMS, TBDMS, and THP ethers. Since TBDMS−PLi was observed to react to a certain extent with the TMS ether of 2 under the above conditions, it was end-capped with styrene, followed by reacting it with 2. Prior to the polymerization, a 3-fold excess of LiCl was added to narrow the molecular weight distribution. The polymerization efficiently proceeded to result in the chain-endfunctionalized PCHMA. The resulting polymer was observed to have a narrow molecular weight distribution (Mw/Mn = 1.05) and the quantitative introduction of the TMS, TBDMS, and THP ethers was confirmed by 1H NMR analysis. A small deviation between the Mn value determined by RALLS and that calculated may be attributed to the reaction loss of 3 with a trace amount of impurities in 2.63 Among these three ethers, the TMS ether was first selectively deprotected by treatment of the polymer with methanol containing a catalytic amount of K2CO3, while both the TBS and THP ethers remained completely intact under such conditions. The regenerated hydroxyl group was converted to the PA reaction site by the Mitsunobu esterification reaction with α-phenylacrylic acid. The conversion was observed to be quantitative by 1H NMR analysis, in which vinylidene protons of the PA function were clearly observed at 6.31 and 5.87 ppm in expected intensities. The resulting chain-end-(PA, TBDMS and THP ethers)-functionalized PCHMA was then reacted with a 3-fold excess of living PMMA in THF at −40 °C for 20 h. The SEC profile of the reaction mixture exhibited only two sharp peaks corresponding to the linked product of PCHMAblock-PMMA and the PMMA used in excess in the reaction. The linking efficiency was estimated to be nearly quantitative by comparing the two peak areas. The target diblock copolymer was isolated in 93% yield by fractional precipitation. It was

The key point for the synthesis of μ-star polymers is the selective deprotection of TMS, TBDMS, and THP ethers at each reaction stage. Unfortunately, conventional deprotection procedures were not always useful for such purpose. For example, the TMS ether was not completely deprotected only by the treatment of methanol even after 48 h. Deprotection of the TBDMS ether with (C4H9)4NF was problematic in the presence of the benzyl ester function present around the linking point. The benzyl ester linkage was partly cleaved under such conditions. Accordingly, we failed the selective deprotection of these three ethers several times under usual reported conditions. In the former case, the addition of a catalytic amount of acetic acid or K2CO3 was necessary for the complete deprotection of the TMS ether. Both TBDMS and THP ethers were observed to be stable under such conditions. In the latter case, the TBDMS ether was selectively deprotected by the addition of phenol equal to (C4H9)4NF in amount, while the benzyl ester function remained intact. Thus, careful treatments are essential to realize the selective deprotection of TMS, TBDMS, and THP ethers. In order to examine the selective deprotection of the three ethers and the subsequent conversion of regenerated hydroxyl groups to PA reaction sites, the synthesis of a 4-arm ABCD μstar polymer is carried out, as illustrated in Scheme 2. It should be noted that the synthesis of well-defined 4-arm ABCD μ-star polymers are still very difficult and not well established in methodology even at the present time.40 The first example is a 4-arm ABCD μ-star polymer composed of four different poly(alkyl methacrylate) arm segments. The trifunctional lithium reagent, 3, was prepared by the reaction of 2 with TBDMS-PLi end-capped with a few units of styrene in THF at −78 °C for 15 min and in situ used as an initiator in the living anionic polymerization of cyclohexyl 822

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observed that the isolated polymer possessed a narrow molecular weight distribution (Mw/Mn = 1.05), and the molecular weight (Mn = 22.2 kg/mol) determined by RALLS was in good agreement with that calculated (Mn = 21.9 kg/ mol), as summarized in Table 1. The TBS ether present between two blocks was then deprotected by treatment with (C4H9)4NF in THF at 25 °C for 16 h in the presence of phenol. The THP ether was completely stable under such conditions. The regenerated hydroxyl group was converted to the PA reaction site by the same manner as that mentioned above. The in-chain-(PA and THP ether)functionalized PCHMA-block-PMMA thus prepared was reacted with a 3-fold excess of living poly(benzyl methacrylate) (PBnMA) at −40 °C for 20 h. The linking reaction cleanly and quantitatively proceeded under the employed conditions. The linked polymer was isolated in 85% yield by SEC fractionation. The isolated polymer possessed a narrow molecular weight distribution (Mw/Mn = 1.03), and the molecular weight observed by RALLS (Mn = 34.8 kg/mol) agreed well with that calculated (Mn = 33.9 kg/mol). Moreover, the composition observed by 1H NMR was consistent with that calculated from the feed ratio. Thus, a 3-arm ABC μ-star polymer, composed of PCHMA, PMMA, and PBnMA segments, was successfully synthesized. Finally, the THP ether present at the core was deprotected by treatment with HCl in THF at 25 °C for 16 h, and the regenerated hydroxyl group was converted to the PA reaction site in the same manner as that mentioned above. The resulting core-PA-functionalized 3-arm ABC μ-star polymer was reacted with a 3-fold excess of living poly((2,2-dimethyl-1,3-dioxolan-4yl)methyl methacrylate) (P(acetal-DIMA)) at −40 °C for 20 h. The linking reaction efficiently proceeded under such conditions. The target 4-arm ABCD μ-star polymer was isolated in 83% yield by fractional precipitation. The expected and well-defined structure of the isolated polymer was confirmed by the results listed in Table 1. Thus, obviously, the TMS, TBDMS, and THP ethers were satisfactorily deprotected in turn under carefully selected conditions as designed, followed by conversion to the PA reaction sites used step by step for linking reactions, resulting in the successful synthesis of a 4-arm ABCD μ-star polymer, composed of PCHMA, PMMA, PBnMA, and P(acetal-DIMA) leading to a water-soluble poly(2,3-dihydroxypropyl methacrylate). As reported in our previous papers,53−58 the PA reaction site is capable of reacting with living poly(alkyl methacrylate)s as well as living PS and P2VP with higher reactivities. Therefore, one more 4-arm ABCD μ-star polymer having PS and P2VP segments was synthesized without changing the entire reaction system. The synthesis was started from the chain-end-(TMS, TBDMS, and THP ethers)-functionalized poly(allyl methacrylate) (PAMA), prepared by the living anionic polymerization of AMA with 3. A 2-fold excess of living PS end-capped with DPE, a 3-fold excess of living poly(2-methoxyethyl methacrylate) (PMOEMA), and a 2-fold excess of living P2VP were sequentially reacted in each linking reaction step. These linking reactions were carried out at −78 °C for 12 h, at −40 °C for 20 h, and at −78 °C for 12 h. Each linking reaction efficiently proceeded under such conditions to afford targeted PAMAblock-PS, a 3-arm μ-star polymer composed of PAMA, PS, and PMOEMA segments, and a 4-arm μ-star polymer composed of PAMA, PS, PMOEMA, and P2VP segments. As shown in Figure 1, the SEC profile of each polymer exhibits a sharp monomodal distribution and the peak moves to a higher

Figure 1. SEC profiles of A, AB, 3-arm ABC, and 4-arm ABCD μ-star polymers.

molecular weight by increasing the number of arm segments. Their expected and well-defined structures were confirmed by the characterization results listed in Table 1. Thus, a 4-arm ABCD μ-star polymer having PS and P2VP segments was also successfully synthesized. This clearly indicates that a variety of living anionic polymers with different reactivities is applicable to the PA reaction site and any reaction order of living polymers is possible. Interestingly, the trifunctional lithium reagent, 3, actually behaves as a tetrafunctional core agent in the synthesis of 4-arm ABCD μ-star polymers, as shown in Scheme 2. Accordingly, it is recognized that 3 is a convenient and useful core agent access to the general synthesis of various well-defined 4-arm μ-star polymers difficult to synthesize up to now, as mentioned above. As reported in our previous paper,53 the amount of living polymer to be linked is very important. A 2-fold excess (or even a 1.5-fold excess) of living polymer of styrene or 2-VP to the PA reaction site is enough to complete the linking reaction at −78 °C. Living polymer of 2VP is not stable at −40 °C for a long time, since the side reaction between the chain-end anion and the pendant pyridine ring gradually occurs. Therefore, the linking reaction with living P2VP must be carried out at −78 °C. More importantly, the use of a 3-fold or more excess of living poly(alkyl methacrylate) is required in each linking reaction. The reaction temperature of −40 °C is also critical. All attempts under different conditions were not successful to complete the linking reaction. In practice, linking efficiencies of 50−80% were obtained by the use of a 2-fold excess of living polymer or the reaction temperature of −78 or −20 °C. Successive Synthesis of μ-Star Polymers by a New Stepwise Iterative Methodology. In the preceding section, we successfully demonstrated the facile synthesis of 4-arm ABCD μ-star polymers by using 3. However, μ-star polymers with more arms cannot be synthesized because no more reaction sites remained at all after the final reaction step. For the further synthesis of μ-star polymers with more arms, we previously developed a methodology based on the “iterative” approach. As mentioned in the Introduction, the reaction system of the iterative methodology is designed so that the same reaction site is always regenerated after the introduction of an arm segment, and this “arm introduction and regeneration of the same reaction site” sequence is repeatable. With the iterative methodology as shown in Scheme 1, the arm segment was successively introduced one by one by repeating the reaction sequence to afford 3-arm ABC, 4-arm ABCD, and 5arm ABCDE μ-star polymers.53 823

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Scheme 3. Successive Synthesis of μ-Star Polymers by a New Iterative Methodology Based on Living Anionic Polymerization Using a Trifunctional Lithium Reagent, 3

In this section, we have developed a new iterative methodology, which is more efficient in synthetic process than the above-mentioned methodology previously reported. The outline of the new iterative methodology is illustrated in Scheme 3. The superiority of the new methodology to the previous one is that the arm segment to be introduced in each process increases from one to two in number and, thereby, many-armed μ-star polymers may be synthesized by fewer processes. As can be seen in Scheme 3, the starting polymer is a chainend-(TMS, TBDMS, and THP ethers)-functionalized polymer (A), prepared by the living anionic polymerization of monomer (a) with 3. A 3-arm ABC μ-star polymer was synthesized by the selective deprotection of TMS ether, followed by the TBDMS ether, the subsequent conversion to the PA reaction sites, and the introduction of two arm segments with the use of living polymer (B), followed by living polymer (C), similar to those shown in Scheme 2. The THP ether at the core was then converted to the PA reaction site in the same manner as that mentioned before. The resulting core-PA-functionalized 3-arm ABC μ-star polymer was reacted with the trifunctional lithium reagent, 3, to reintroduce the TMS, TBDMS, and THP ethers of protected hydroxyl functionalities convertible to three PA reaction sites. Accordingly, the above-mentioned reaction sequence may possibly be repeated from the resulting core(TMS, TBDMS, and THP ethers)-functionalized 3-arm ABC μ-star polymer. Thus, the reaction between the PA reaction site and 3 is the key step to continue the next reaction sequence in the new methodology. The repetition of the same reaction sequence two more times may give a 5-arm ABCDE, followed by a 7-arm ABCDEFG μ-star polymer. The practical synthesis was carried out as follows: A chainend-(TMS, TBDMS, and THP ethers)-functionalized PCHMA, used as the starting polymer, was prepared by the living anionic polymerization of CHMA with 3. Living PS and poly(4methoxystyrene) (P4MOS) were sequentially linked with PA

reaction sites via the TMS and TBDMS ethers. A core-PAfunctionalized 3-arm ABC μ-star polymer composed of PCHMA, PS, and P4MOS segments was obtained by converting the THP ether to the PA reaction site. The conversion to the PA reaction site and the linking reaction were carried out in similar manners mentioned above. The first and second linking reactions were performed at −78 °C for 12 h. The core-PA-functionalized ABC μ-polymer was then reacted with a 2-fold excess of 3 in THF at −78 °C for 12 h to reintroduce the TMS, TBDMS, and THP ethers at the core. The reaction was observed by 1H NMR analysis to be almost quantitative. Thus, the resulting 3-arm ABC μ-star polymer was successful in possessing the same three ethers of protected hydroxyl functionalities as those in the starting PCHMA. The second reaction sequence, involving conversion to the PA reaction site twice and linking reactions to introduce two arm segments, was performed in order to synthesize a core(TMS, TBDMS, and THP ethers)-functionalized 5-arm ABCDE μ-star polymer. The fourth (D) and fifth (E) arms were sequentially introduced by linking reactions of living poly(4-methylstyrene) (P4MS) and PMMA under the conditions of −78 °C for 12 h and −40 °C for 20 h, respectively. After conversion of the THP ether to the PA reaction site, the same TMS, TBDMS, and THP ethers were reintroduced by the reaction of the core-PA-functionalized 5arm ABCDE μ-star polymer with 3 at −78 °C for 12 h. The third reaction sequence was also carried out to introduce poly(ethyl methacrylate) (PEMA) and PMOEMA segments as the sixth (F) and seventh (G) arms by linking the PA reaction site with living PEMA, followed by living PMOEMA at −40 °C for 24 h. As a result, a core-(THP ether)-functionalized 7-arm ABCDEFG μ-star polymer composed of PCHMA, PS, P4MOS, P4MS, PMMA, PEMA, and PMOEMA segments was synthesized. The linking efficiency was estimated to be always more than 95% yield in each of all the reaction steps and even 824

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Table 2. Successive Synthesis of μ-Star Polymers by a New Iterative Methodology Mn (kg/mol) polymer d

ABC ABCDd ABCDEd ABCDEFd ABCDEFGd

calcd

RALLS

35.0 45.9 56.6 62.8 73.8

34.9 47.1 53.5 64.9 74.2

b

composition (wt %)a

Mw/Mn 1

c

H NMR

SEC

calcd

34.2 45.9 56.0 62.2 74.6

1.03 1.03 1.04 1.05 1.04

30/35/35 23/26/27/25 20/21/22/20/17 17/18/19/17/15/14 14/16/17/15/13/13/12

1

H NMR

31/35/34 24/25/27/24 20/23/21/20/16 18/20/18/17/13/14 16/17/16/14/12/12/13

a A/B/C/D/E/F/G polymer segments. bDetermined by SEC with triple detectors. cDetermined by SEC using PS standards. dA/B/C/D/E/F/G = PCHMA/PS/P4MOS/P4MS/PMMA/PEMA/PMOEMA.

in the final reaction step of a core-(PA and THP ether)functionalized 6-arm ABCDEF star with living PMOEMA. The characterization results are summarized in Table 2. Their molecular weights measured by RALLS and compositions observed by 1H NMR were in good agreement with those calculated within experimental limits. Narrow molecular weight distributions were attained in all of the polymers, as shown in Figure 2. Thus, not only the final 7-arm ABCDEFG μ-star but

previous methodology, where an arm segment was introduced one by one in each reaction sequence. Accordingly, six reaction sequences will be needed to synthesize the same 7-arm μ-star polymer. Although the 7-arm ABCDEFG μ-star polymer herein synthesized is composed of three polystyrene and four poly(alkyl methacrylate) derivatives, other 7-arm μ-stars composed of all poly(methacrylate)-based arm segments and arm segments including polyisoprene and P2VP can be synthesized. Synthesis of a 6-Arm A2B2C2 μ-Star Polymer. As is seen in Scheme 2, the intermediate 3-arm ABC μ-star polymer is core-functionalized with the PA reaction site. It may be possible to synthesize a new 6-arm A2B2C2 μ-star polymer by linking this core-PA-functionalized ABC star polymer with an appropriate dianionic species, as illustrated in Scheme 4. For Scheme 4. Synthesis of 6-Arm A2B2C2 μ-Star Polymer

Figure 2. SEC profiles of 3-arm ABC, 4-arm ABCD, 5-arm ABCDE, 6arm ABCDEF, and 7-arm ABCDEFG μ-star polymers synthesized by a new iterative methodology.

this purpose, a core-PA-functionalized 3-arm ABC μ-star polymer composed of PMMA, PαMS, and PMOEMA segments was synthesized from a chain-end-(TMS, TBDMS, and THP ethers)-functionalized PMMA by manners similar to those mentioned above. As a dianionic species, 1,4-dilithio1,1,4,4-tetraphenylbutane (4) was prepared by the reaction of lithium naphthalenide with a 1.8-fold excess of DPE in THF at −78 °C for 30 min and in situ reacted with the above core-PAfunctionalized 3-arm ABC star polymer. To the THF solution of the ABC μ-star polymer, the anionic species, 4, was added slowly in a titration manner in THF at −78 °C. The deep red color characteristic of 4 disappeared immediately when 4 was mixed with the star at an early stage of the reaction but faded gradually with time by time. The addition was continued until the red color remained, followed by allowing the reaction mixture to stand at −78 °C for 12 h. As shown in Figure 3a, the SEC profile of the reaction mixture exhibits two peaks, corresponding to the linked product and the ABC star polymer, respectively. No higher molecular weight polymers were formed under such conditions. The linking efficiency was estimated to be 68% on the basis of the SEC peak area ratio. The target linked polymer was isolated in 50% yield by SEC fractionation. The isolated polymer exhibits a sharp monomodal distribution (Mw/Mn = 1.03) (also see Figure 3b). The molecular weight and composition determined

also 3-arm ABC, 4-arm ABCD, 5-arm ABCDE, and 6-arm ABCDEF μ-star polymers with expected and well-defined structures were successfully synthesized. This successful synthesis demonstrated that the new iterative methodology proposed herein works very satisfactorily, and the reaction sequence is efficiently repeated in each process at least three times. Most importantly, the TMS, TBDMS, and THP ethers of the three protected hydroxyl functionalities were selectively and in turn deprotected under carefully selected conditions as designed, followed by conversion to PA reaction sites at different reaction stages. Accordingly, seven arm segments could be introduced step by step on demand. Since the final 7arm star possesses the THP ether at the core, the same reaction sequence can be repeated to further introduce arm segments into the star. As mentioned above, two polymer segments were introduced in each reaction sequence, and accordingly, the 7-arm ABCDEFG μ-star polymer could be synthesized by repeating the same reaction sequence only three times in the present new methodology. On the other hand, four reaction sequences were needed even for the synthesis of a 5-arm μ-star polymer by the 825

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reagent behaved as a tetrafunctional core compound was also recognized as a convenient and useful core agent access to the general synthesis of 4-arm ABCD and 6-arm A2B2C2 μ-star polymers in a quite simple manner.

■

ASSOCIATED CONTENT

■

AUTHOR INFORMATION

S Supporting Information *

Text giving the synthetic procedure of 4-arm ABCD μ-star polymers, a 6-arm A2B2C2 μ-star polymer and the successive synthesis of μ-star polymers having up to 7-arms by a new iterative methodology using a trifunctional lithium reagent. This material is available free of charge via the Internet at http://pubs.acs.org. Figure 3. SEC profiles of the polymer mixture (a) and 6-arm A2B2C2 μ-star polymer isolated by SEC fractionation (b).

Corresponding Author

*E-mail [email protected].

by RALLS and 1H NMR are in good agreement with those calculated, as listed in Table 3. All of these results clearly show

Notes

The authors declare no competing financial interest.

■

Table 3. Synthesis of 6-Arm A2B2C2 μ-Star Polymer Mn (kg/mol) polymer d

ABC A2B2C2d

calcd

RALLSb

29.1 57.4

28.7 58.0

Mw/Mn 1 H NMR

27.8

ACKNOWLEDGMENTS A.H. gratefully acknowledges the financial support by a grant (B:21350060) from a Grant-in-aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture of Japan.

composition (wt %)a

SECc

calcd

1.03 1.03

35/31/34 35/31/34

1

H NMR

35/33/32 35/33/32

■

a

A/B/C polymer segments. bDetermined by SEC with triple detectors. cDetermined by SEC using PS standards. dA/B/C = PMMA/PαMS/PMOEMA.

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CONCLUSIONS We have successfully developed a new stepwise iterative methodology based on living anionic polymerization using a trifunctional lithium reagent substituted with the TMS, TBS, and THP ethers of three protected hydroxyl functionalities convertible to three PA reaction sites. The key step is the reaction between the PA reaction site and the trifunctional lithium reagent to reintroduce these three ethers, with which the next reaction sequence is continued. One more important and advantageous point is that such three ethers are designed to be selectively and in turn deprotected under carefully selected conditions to convert them to three PA reaction sites step by step at different reaction stages. With these PA reaction sites, two arm segments, followed by the three ethereal functions, are introduced. A series of well-defined μ-star polymers with up to 7 arms and 7 components could be successively and efficiently synthesized. The success of the proposed new iterative methodology makes it possible to synthesize many-armed μstar polymers by fewer processes. The trifunctional lithium 826

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