Synthesis of Well-Defined Novel Reactive Block Polymers Containing

Mar 19, 2014 - Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1-S1-13,...
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Synthesis of Well-Defined Novel Reactive Block Polymers Containing a Poly(1,4-divinylbenzene) Segment by Living Anionic Polymerization Shunsuke Tanaka,† Raita Goseki,† Takashi Ishizone,† and Akira Hirao*,†,‡,§ †

Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1-S1-13, Ohokayama, Meguro-ku, Tokyo 152-8552, Japan ‡ Institute of Polymer Science and Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan § College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 199 Ren Ai Road, Suzhou Industrial Park, Suzhou 215123, China S Supporting Information *

ABSTRACT: In order to synthesize a variety of block polymers having poly(1,4-divinylbenzene) (PDVB) segments, the living anionic block polymerizations of DVB with styrene, 2-vinylpyridine (2VP), tert-butyl methacrylate (tBMA), methyl methacrylate (MMA), N-(4-vinylbenzylidene)cyclohexylamine (1), 2-(4′-vinylphenyl)-4,4-dimethyl-2-oxazoline (2), or 2,6-ditert-butyl-4-methylphenyl 4-vinylbenzoate (3) were conducted in THF at −78 °C with the anionic initiator bearing K+ in the presence of a 10-fold excess of potassium tert-butoxide. With the sequential addition of DVB and each of these monomers, the following block polymers having PDVB segments were successfully synthesized: PS-b-PDVB, P2VP-b-PDVB, PDVBb-P2VP, PDVB-b-PtBMA, PDVB-b-P(1), PDVB-b-P(2), PDVB-b-P(3), PS-b-PDVB-b-PtBMA, PS-b-P2VP-b-PDVB-b-PtBMA, and PS-b-PDVB-b-P2VP-b-PtBMA. The resulting polymers are all novel block polymers with well-defined structures (predictable molecular weights and compositions and narrow molecular weight distributions) and possess reactive PDVB segments capable of undergoing several postreactions. Based on the results of such sequential block polymerizations, the anionic random copolymerization of DVB and 2VP, the polymerizability with (C4H9)2Mg, and some other addition reactions, it was found that the comparable reactivity of the chain-end anions follows the sequence of PS− > PDVB− > P2VP− > PtBMA−. Accordingly, the reactivity of the corresponding monomers increases as follows: styrene < DVB < 2VP < tBMA.

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INTRODUCTION 1,4-Divinylbenzene (DVB) and its derivatives are well-known as cross-linking agents in the various polymerization systems. It has been believed that their polymerizations readily lead them to cross-linked insoluble gels even at the early stage of the polymerization. Recently, we successfully achieved the living anionic polymerization of DVB, in which one of the two vinyl groups of DVB exclusively and selectively polymerized to afford soluble and well-controlled linear PDVBs in chain length.1,2 The polymerization was carried out in THF using a specially designed anionic initiator system prepared from oligo(αmethylstyryl)lithium and a 10-fold or more excess of potassium tert-butoxide (KOBut). The resulting living anionic polymers were stable only for a few minutes at −78 °C, but stable even after 30 min at −95 °C. Under such conditions, the unwanted addition reaction of the chain-end anion to the pendant vinyl group was almost suppressed, and soluble polymers with controllable Mn values of up to 60 500 g/mol and narrow molecular weight distributions (Mw/Mn ≤ 1.05) were quantitatively obtained. The living nature of the polymerization © 2014 American Chemical Society

of DVB was further supported by the sequential addition of DVB and tert-butyl methacrylate (tBMA) to successfully synthesize the well-defined AB diblock copolymer, PDVB-bPtBMA. The suppression of the unwanted addition reaction to the pendant vinyl group, which essentially occurred during the polymerization of DVB, may be explained as follows: the Li+ countercation of the chain-end anion may be completely replaced by K+ due to the presence of excess K+, and the resulting PDVB− K+ significantly shifted to the ion-pair form via equilibrium in a high concentration of KOBut. Some KOBut molecules may possibly be coordinated to the chain-end anion by the ionic interaction, providing the steric bulkiness (or hindrance) around the chain-end anion. Thus, the chain-end anion may become less reactive and more sterically bulkier than that in the absence of KOBut. Received: December 31, 2013 Revised: March 7, 2014 Published: March 19, 2014 2333

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Figure 1. Monomers used in this study. three styrene derivatives of 1,6 2,7 and 3,8 and 1,1-bis(3-(tertbutyldimethylsilyloxymethyl)phenyl)ethylene (4)9 were synthesized according to our previous papers. 4-Vinylbenzoic acid, used as the precursor of 2 and 3, was generously donated by Hokko Chemical Industry Co., Ltd. (Tokyo, Japan). Styrene, α-methylstyrene (αMS), 1,1-diphenylethylene (DPE), MMA, and tBMA were washed with 5% NaOH(aq) and water and then dried over MgSO4. After filtration of MgSO4, they were distilled over CaH2 twice under reduced pressures. Finally, both styrene and αMS were distilled from their (C4H9)2Mg solutions (ca. 3 mol %) on the vacuum line (∼10−6 Torr). DPE was finally distilled from its 1,1-diphenylhexyllithium (ca. 3 mol %) solution on the vacuum line. Both MMA and tBMA were finally distilled from their (C6H13)3Al solutions (ca. 3 mol %) on the vacuum line. 2VP was distilled over CaH2 twice under reduced pressures and then distilled from its (C6H13)3Al (ca. 3 mol %) solution on the vacuum line. Heptane was stirred with concentrated H2SO4 overnight. After removal of H2SO4 by decantation, heptane was dried with P2O5 overnight. Decanted heptane was further purified by refluxing with 1,1diphenylhexyllithium, followed by distillation under a nitrogen atmosphere. It was finally distilled from its 1,1-diphenylhexyllithium solution on the vacuum line. Tetrahydrofuran (THF) was refluxed over sodium wire for 10 h and distilled over LiAlH4 under a nitrogen atmosphere. It was finally distilled from its sodium naphthalenide solution (ca. 3 mol %) on the vacuum line. tert-Butanol was distilled over CaH2 twice under nitrogen and finally distilled over CaH2 on the vacuum line. KOBut was prepared according to our preceding papers.1,2 Potassium naphthalenide (K-Naph) was prepared by the reaction of potassium metal with a small excess molar amount of naphthalene in THF under high vacuum conditions. Oligo(αmethylstyryl)lithium was prepared from the reaction of sec-BuLi and 3−5 equiv of αMS in THF at room temperature for 10 s and at −78 °C for 30 min under high vacuum conditions.1,2 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. Size exclusion chromatography (SEC) was performed on an Asahi Techneion AT-2002 equipped with a Viscotek TDA model 305 triple detector array using THF as a carrier solvent at a flow rate of 1.0 mL/min at 40 °C. Three polystyrene 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 UV or RI detection using a 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 polymers. General Procedure of Block Copolymerization. All the polymerizations and reactions for preparing KOBut were carried out under high vacuum conditions (∼10−6 Torr) in sealed handmade glass reactors equipped with break-seals. The reactor was sealed off from the vacuum line and washed with a red heptane solution of 1,1diphenylhexyllithium (ca. 0.05 M) prior to the polymerizations and reactions. All operations were performed according to the usual high vacuum technique with break-seals.10−12 The block copolymers were synthesized by the sequential addition of the corresponding monomers to appropriate initiators in THF at −78 °C. The polymerization time for each monomer is shown in

A further important point is that the pendant vinyl group attached to the main chain obtained after the polymerization dramatically decreases in reactivity in comparison to the two vinyl groups of DVB. Prior to the polymerization, the β-carbon chemical shifts by 13C NMR are observed to be 113.9 ppm for the two vinyl groups, strongly indicating that they are exactly equivalent in reactivity. The vinyl groups are activated to each other by the long conjugation system from the vinyl group to the other one at the para position via the benzene ring. After the polymerization, the pendant vinyl group attached to the main chain becomes less reactive both due to the disappearance of the long conjugated system by converting one vinyl group to the corresponding −CH2−CH− chain and the electrondonating effect of the generated −CH2−CH− chain. This is also indicated by the 13C NMR analysis that the β-carbon chemical shift of the pendant vinyl group is significantly shifted from 113.9 to 113.1 ppm after the polymerization.3,4 Thus, the above-indicated less reactive and sterically bulkier chain-end anion may possibly add much more slowly to the less reactive pendant vinyl group, while one of the activated vinyl groups of DVB preferentially undergoes the polymerization. One of the most important synthetic applications of the living anionic polymerization is the possible synthesis of block polymers by the sequential polymerization in which different monomers are sequentially added to an appropriate anionic initiator.5 Accordingly, the success of the living anionic polymerization of DVB is also expected to allow access to novel block polymers having reactive PDVB block(s), whose vinyl groups are capable of undergoing a variety of postreactions, such as cross-linking, epoxidation, and hydrosilylation. In addition to the block polymer synthesis, it is possible to evaluate the reactivities of DVB and its chain-end anion during the anionic polymerization by the results of block polymerization. We now report the successful synthesis of a variety of novel block polymers containing PDVB block(s) with well-defined structures by a sequential polymerization. Furthermore, the reactivities of DVB and its chain-end anion will be discussed based on the results of the sequential block polymerizations, the anionic random copolymerization of DVB and 2VP, and some other addition reactions. Monomers used for this purpose are four conventional monomers including styrene, 2-vinylpyridine (2VP), tert-butyl methacrylate (tBMA), and methyl methacrylate (MMA), and three para-substituted styrene derivatives, N-(4-vinylbenzylidene)cyclohexylamine (1), 2-(4′vinylphenyl)-4,4-dimethyl-2-oxazoline (2), and 2,6-di-tert-butyl4-methylphenyl 4-vinylbenzoate (3), which are anionically more reactive than styrene, as shown in Figure 1.

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EXPERIMENTAL SECTION

Materials. All the reagents (>98% purities) were purchased from Aldrich Japan and used as received unless otherwise stated. DVB,2 2334

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the reactivities of the monomer and the chain-end anion in exactly the reverse direction. For instance, a monomer substituted with a strong electron-withdrawing group has a higher anionic reactivity due to the reduction of the electron density on the vinyl group, while the chain-end anion derived from the monomer becomes lower in reactivity by the same electron-withdrawing effect that reduces the electron density on the anion. During block polymerization using different monomers, this often causes a serious problem that the less reactive chain-end anion cannot initiate the polymerization of other less reactive monomers. In other words, the nucleophilicity of the chain-end anion does not match the electrophilicity of the other monomer. Therefore, it is essential to consider the sequential addition order of monomers that the less reactive monomer must be the first to polymerize, followed by the addition of the more reactive monomer to polymerize. Accordingly, block polymers can be synthesized by the sequential addition of monomers in the order of decreasing chain-end anion reactivity. Thus, there is a fatal restriction in the synthesis of block polymers by the sequential polymerization using monomers having different reactivities. The series of sequential polymerizations of DVB with each of the three conventional monomers, styrene, 2VP, and tBMA, were carried out for the synthesis of various block copolymers. Needless to say, these three monomers undergo the living anionic polymerization under the same conditions in THF at −78 °C. Before the sequential polymerizations using DVB, the reactivities of such monomers and their chain-end anions are discussed based on previous results. It is known that the monomer reactivity follows the sequence of styrene < 2VP < t BMA, while the reactivities of their chain-end anions are in the opposite direction as follows: living polystyrene (PS−) > living poly(2-vinylpyridine) (P2VP−) > living poly(tert-butyl methacrylate) (PtBMA−).5,13−15 In addition, it has been reported that the relative anionic polymerizability of vinyl monomers can be estimated by the β-carbon chemical shifts of vinyl groups in the 13C NMR spectra.3,4 The monomers showing downfield shifts suggest their lowered π-electron densities on the vinyl βcarbons and usually possess higher anionic polymerizability than the monomers showing upfield shifts. In fact, the β-carbon shifts of styrene, 2VP, and tBMA are observed at 113.8, 118.1, and 124.1 ppm, respectively, supported the above-mentioned relative anionic polymerizability of these monomers and the reactivity of the corresponding anionic living polymers. Since PS− is the most reactive living polymer among them, it readily polymerizes both 2VP and tBMA, which are more reactive than styrene, to give the corresponding block

Table 1. Summary of Crossover Polymerizations among Styrene, 2VP, and tBMAa monomer living polymer

styrene

−

PS P2VP− PtBMA−

++ + −

t

2VP

b

++ ++b −

BMA ++ ++ ++b

a ++: Living polymerization; +: low initiation efficiency; and −: no polymerization. bHomopolymerization.

Tables 1 and 2. As a typical example, the synthesis of PDVB-b-PtBMA was shown as follows: A precooled THF solution (8.73 mL) of DVB (0.470 M, 4.10 mmol) was mixed at once with the initiator solution (15.6 mL) of oligo(α-methylstyryl)lithium (3.43 mM, 0.0534 mmol) in the presence of KOBut (0.649 mmol) at −78 °C with stirring vigorously. After 1 min, tBMA in THF (0.960 M, 3.49 mmol, 3.64 mL) precooled to −78 °C was added to polymerize at −78 °C for 3 h. The polymerization mixture was quenched with degassed methanol at −78 °C. The polymer was precipitated by pouring the mixture into a large amount of methanol, purified by reprecipitation twice from THF to methanol, and freeze-drying from its absolute benzene solution overnight in the dark to avoid cross-linking of the PDVB segment. A polymer yield was quantitative. The resulting copolymer was characterized by SEC, 1H NMR, and 13C NMR analyses. 1H NMR (CDCl3, 300 MHz, ppm): δ 7.3−6.1 (br, m, Ph and CH2CH−Ph of PDVB segment), 5.8−5.5 (br, cis-CHHCH− of PDVB segment), 5.2−5.0 (br, trans-CHHCH− of PDVB segment), 2.2−1.3 (br, m, main chain and alkyl substituents).

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RESULTS AND DISCUSSION Sequential Block Copolymerization of DVB with Styrene, 2VP, or tBMA. Well-defined block polymers are usually synthesized by the sequential addition of monomers in which two or more monomers are sequentially added to an anionic initiator, known as the “sequential polymerization procedure”.5 With the use of monomers having similar reactivities, the resulting living chain-end anions also exhibit similar reactivities, and thereby crossover polymerization is possible among such monomers to afford almost all block polymers with any sequence without difficulty. On the other hand, the synthesis of block polymers is problematic and considerably limited in block type with the use of monomers having different reactivities. In such a case, one consideration for the successful synthesis of block polymers is the order of monomer addition. The reason is that a more reactive monomer usually produces a less reactive living polymer chain-end anion and vice versa because the inductive and/or resonance effects of the substituent strongly influence

Table 2. Block Copolymerizations of DVB with Styrene, 2VP, tBMA, and MMAa Mn × 10−3 (g/mol)

monomer

composition 1st/2nd (wt %)

first

second

time (min)

yield (%)

calcd

SEC

styrene 2VP t BMA DVB DVB DVB DVB

DVB DVB DVB styrene 2VP t BMA MMA

10 + 1 30 + 1 3 h + 30 1+2 1+4 1+3h 1+1h

∼100 ∼100

26.9 23.4

∼100 ∼100 ∼100

20.7 20.1 23.1

19.7 26.9 1.03 17.7 24.6 1.18 no block copolymer was obtained no block copolymer was obtained 14.4 19.4 1.09 12.9 20.2 1.05 17.0 21.8 1.28

b

c

RALLS

d

Mw/Mnc

calcd

1

H NMR

39/61 41/59

43/57 42/58

36/64 51/49 39/61

38/62 48/52 39/61

a All the polymerizations were carried out at −78 °C in THF with oligo(α-methylstyryl)lithium in the presence of KOBut. bPolymerization times were shown as [reaction time for first monomer] + [reaction time for second monomer]. cEstimated by SEC calibrating with standard polystyrenes. d Estimated by SEC equipped with triple detectors.

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copolymers of PS-b-P2VP and PS-b-PtBMA.16 On the other hand, the least reactive styrene and even 2VP cannot be polymerized with the least reactive PtBMA−, and the tBMA homopolymer was always recovered in either case. P2VP−, having a reactivity between PS− and PtBMA−, polymerizes the most reactive tBMA to afford P2VP-b-PtBMA. The polymerization of the least reactive styrene with P2VP− can occur only with an insufficient initiation efficiency, and most of the starting P2VP and unexpected high-molecular-weight P2VP-b-PS are obtained. The undesired addition reaction of PS− to the pendant pyridine moiety occurs to a certain extent. Thus, the well-defined P2VP-b-PS cannot be obtained by this sequential addition order. Such reactivity relationships among the three monomers and their chain-end anions are summarized in Table 1. Based on the above discussion summarized in Table 1 and the successful living anionic polymerization of DVB, the first series sequential polymerizations in which styrene, 2VP, or t BMA was first polymerized to prepare the corresponding living anionic polymer, followed by addition of DVB, was conducted. With these polymerizations, the reactivity of DVB was directly determined for the three living polymers of PS−, P2VP−, and PtBMA− with different reactivities. During the sequential polymerization of styrene with DVB, styrene was first polymerized with oligo(α-methylstyryl)lithium in THF at −78 °C for 10 min in the presence of a 10-fold excess of KOBut and DVB was then added at −78 °C. The polymerization of DVB immediately occurred and the polymer was quantitatively produced after 1 min. As shown in Figure 2a,

composition, although the SEC profile exhibited a somewhat broad molecular weight distribution (Mw/Mn = 1.18) (see Figure 2). In the results of the first series sequential polymerizations, both PS− and P2VP− initiate the polymerization of DVB, while DVB cannot be polymerized with PtBMA−. Accordingly, the result clearly shows the reactivity of DVB as follows: styrene < DVB, 2VP < tBMA. Because PDVB− is capable of polymerizing DVB, the reactivity of PDVB− may possibly be positioned between PS− and P2VP− and is higher than that of PtBMA−. In order to evaluate the reactivity of PDVB−, the second series sequential polymerization was carried out by the addition of DVB, followed by adding either styrene, 2VP, or tBMA, to the initiator system prepared from oligo(α-methylstyryl)lithium and a 10-fold excess of KOBut in THF at −78 °C. The polymerization times were 1 min for the first polymerization of DVB and 2, 4, or 180 min for the second step polymerization. Thus, the reactivity of the first formed PDVB− is directly determined by the sequential polymerization with each of the three monomers. During the sequential polymerization of DVB with styrene, only a soluble DVB homopolymer having a multimodal broad molecular weight distribution was obtained, while styrene monomer was quantitatively recovered. The obtained PDVB after 3 min (1 min + 2 min) was almost consistent in the Mn value and the SEC shape with the polymer obtained by the polymerization of DVB for 3 min at −78 °C in a separate experiment. This result showed that PDVB− cannot initiate the polymerization of styrene and the chain-end anion gradually undergoes the addition reaction to the pendant vinyl group in the PDVB− after 1 min. This clearly indicates that PDVB− is less reactive than PS− capable of polymerizing styrene. During the sequential polymerization of DVB with 2VP, the PDVB− readily and quantitatively polymerized 2VP. The resulting polymer exhibits a sharp monomodal SEC distribution, and the peak corresponding to the DVB homopolymer is not detectable. As also listed in Table 2, the Mn value measured by RALLS was in good agreement with that calculated. Moreover, the composition of both segments observed by 1H NMR completely agreed with the calculated value. Accordingly, PDVB− effectively polymerized 2VP in a living manner to afford the expected block copolymer of PDVB-b-P2VP. Considering the result obtained by the first sequential polymerization of 2VP with DVB, a crossover polymerization is possible between DVB and 2VP, indicating that the reactivities of both monomers and their chain-end anions are similar to each other. As previously reported,1 the successful synthesis of the welldefined block copolymer of PDVB-b-PtBMA was already achieved by the sequential polymerization of DVB and tBMA. The same polymerization was performed in order to ascertain the reproducibility. As expected, a well-defined block copolymer similar to the previous sample was again obtained without difficulty. The results are also listed in Table 2. Thus, both 2VP and tBMA, which are more reactive than styrene, were readily polymerized with PDVB−, while PDVB− could not initiate the polymerization of the least reactive styrene among the three monomers. As already discussed, block copolymers can be synthesized by the sequential addition of monomers in the order of decreasing chain-end anion in reactivity. The results herein obtained show that PDVB− is less reactive than PS− capable of polymerizing styrene, while it is more reactive than or even comparable to

Figure 2. SEC profiles of diblock copolymers: (a) PS-b-PDVB and (b) P2VP-b-PDVB.

a sharp SEC peak (Mw/Mn = 1.03) is observed, while the PS formed during the first stage is negligible. The observed small peak at higher molecular weight side might be a coupling product (∼4%), as previously suggested.1,2 Good agreements of the Mn values and compositions are attained between the calculated values and those observed by RALLS and 1H NMR, also as listed in Table 2. Thus, obviously, PS− initiates the polymerization of DVB with quantitative efficiency to result in the formation of the requisite PS-b-PDVB. On the other hand, PtBMA− could not polymerize DVB at all even after 30 min, and only the tBMA homopolymer was recovered. The sequential polymerization of 2VP, followed by addition of DVB, efficiently proceeded to quantitatively afford the corresponding block copolymer of P2VP-b-PDVB. The resulting block copolymer possessed predictable Mn value and 2336

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P2VP− and PtBMA−. The higher reactivity of P2VP− than PtBMA− is already established by the results summarized in Table 1. Accordingly, the comparative reactivity of the chainend anions follows the sequence of PS− > PDVB−, P2VP− > PtBMA−. This sequence is similar to that estimated from the results of the first series sequential polymerization mentioned above. Accordingly, the reactivity of the corresponding monomers increases in exactly the opposite order as follows: styrene < DVB, 2VP < tBMA. Since methyl methacrylate (MMA) as the representative methacrylate monomer is very similar in reactivity to tBMA, the sequential polymerization of DVB, followed by the addition of MMA, was carried out under the identical conditions employed for the synthesis of PDVB-b-PtBMA, except for the polymerization time of MMA (1 h). Under these conditions, a THFsoluble polymer was obtained in 100% yield. Although the observed Mn value and composition of the resulting polymer were close to the predictable values, as listed in Table 2, the SEC profile showed a main sharp monomodal peak with a small broad shoulder corresponding to dimeric and/or highermolecular-weight products, which caused broadening of the SEC distribution (Figure 3). Obviously, the occurrence of some side reactions, probably the addition reaction of PDVB− to the less sterically hindered methyl ester group, was indicated.

The direct proof of the living polymerization of DVB is also supported by the successful synthesis of the last two block copolymers. Since oligo(α-methylstyryl)lithium/KOBut is always used as the initiator for the polymerization of DVB, a diblock copolymer of poly(αMS)-b-PDVB can be synthesized without difficulty. Sequential Block Copolymerizations Using DVB and Three Styrene Derivatives, 1, 2, and 3. As described in the preceding section, it was found that the reactivities of DVB and its chain-end anion were similar to those of 2VP and the chainend anion. The sequential polymerization was further performed between DVB and each of the three title functional styrene derivatives, 1−3, in order to synthesize new block copolymers with additional functional sequences. As shown in Figure 1, 1, 2, and 3 are styrene derivatives para-substituted with N-cyclohexylimine, 2-oxazoline, and 2,6-di-tert-butyl-4methylphenyl ester functions of strong electron-withdrawing character. Because of their characteristics, it was already established that these styrene derivatives were all more reactive than styrene and, therefore, their chain-end anions were less reactive than PS−.3,6−8 From the crossover polymerization results previously reported, it was found that both 1 and 2 were similar in reactivity to 2VP, while the reactivity of 3 corresponded to that of tBMA. Considering such reactivities of 1, 2, and 3 in mind, we carried out the sequential polymerizations in which DVB was first polymerized, followed by addition of 1, 2, or 3. In addition to the synthetic possibility of new block copolymers, the reactivity of PDVB− can also be examined by these polymerizations. As expected, 1 and 2, whose reactivities are similar to that of 2VP, were readily polymerized with PDVB−, resulting in the quantitative formation of well-defined block copolymers, PDVB-b-P(1) and PDVB-b-P(2). As listed in Table 3, agreements between the calculated Mn values and compositions and those observed are satisfactory. As a matter of course, PDVB− initiated the polymerization of the much more reactive 3 having a reactivity close to tBMA to give a requisite block copolymer of PDVB-b-P(3). The successful synthesis of these three block copolymers provides further good evidence that the anionic reactivity of PDVB− is similar to that of P2VP−. Thus, the approach using functional styrene derivatives successfully allows access to the three new block copolymers with additional functional sequences. One more sequential polymerization, in which 3 was first polymerized, followed by the addition of DVB, was conducted to examine the reactivity of DVB. The first-formed living polymer of P(3)− having the reactivity similar to PtBMA− could not polymerize DVB, and the homopolymer of 3 was recovered. This result again demonstrates that P(3)− as well as PtBMA− is less reactive than PDVB− capable of polymerizing DVB. This is also consistent with the result shown in the

Figure 3. SEC profile of PDVB-b-PMMA.

At the present time, the reactivity between DVB and 2VP cannot yet be differentiated by both series of sequential polymerization results. Although there are some limitations in the sequential polymerizations using DVB, four novel welldefined block copolymers of PS-b-PDVB, P2VP-b-PDVB, PDVB-b-P2VP, and PDVB-b-PtBMA having reactive PDVB segments were successfully synthesized. The second block copolymer of PDVB-b-P2VP has the same structure as the third P2VP-b-PDVB, if their end-group structures are not considered. Table 3. Block Copolymerizations of DVB with 1−3a

Mn × 10−3 (g/mol)

monomer b

c

composition 1st/2nd (wt %)

1st

2nd

time (min)

yield (%)

calcd

SEC

RALLS

DVB DVB DVB

1 2 3

1+2h 1.5 + 2 h 1 + 20 h

90 100 100

18.6 16.8 26.3

13.3 13.5 16.9

19.0 16.2 25.3

d

Mw/Mnc

calcd

1.17 1.14 1.05

63/37 56/44 59/41

1

H NMR 63/37 55/45 58/42

a All polymerizations were carried out in THF at −78 °C with oligo(α-methylstyryl)lithium in the presence of KOBut. bPolymerization times were shown as [reaction time for first monomer] + [reaction time for second monomer]. cEstimated by SEC calibrating with standard polystyrenes. d Estimated by SEC equipped with triple detectors.

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the segments. Agreement between the calculated compositions and those observed by 1H NMR was satisfactory in each of the three block polymers. These analytical outcomes clearly prove that all the polymerization stages proceed as designed to successfully synthesize the requisite multiblock polymers containing the PDVB segment. Further Estimation of Reactivities of DVB and PDVB−. Throughout the sequential block copolymerization results with other monomers, DVB appears similar in reactivity to 2VP. The results also show no reactivity difference between PDVB− and P2VP−. In order to further evaluate the reactivities of both monomers and their chain-end anions, the anionic random copolymerization of DVB with 2VP was carried out at a 1:1 molar ratio in THF at −78 °C with oligo(α-methylstyryl) lithium in the presence of a 10-fold excess of KOBut. If their reactivities are almost comparable, they should be equally consumed to afford a random copolymer with 50/50 composition. The polymerization was quenched with degassed methanol before the monomers were completely consumed. At the very early stage of the polymerization, the reaction mixture showed a vivid reddish color, which is specific to P2VP−, but gradually turned a dark red color indicative of the generation of PDVB−. Under the employed conditions, 2VP was completely polymerized, but 67% of DVB remained unreacted in the polymerization mixture. In fact, the observed composition of the resulting polymer by 1H NMR was close to that estimated from their consumptions by the polymerization (see Table 5).

preceding section. Accordingly, the reactivity of the monomers is shown as follows: styrene < DVB, 2VP, 1, 2 < tBMA, 3. On the other hand, the overall comparative reactivity of all the chain-end anions herein used follows the sequence of PS− > DVB−, P2VP−, P(1)−, P(2)− > PtBMA−, P(3)−. Synthesis of Triblock and Tetrablock Polymers Containing PDVB Segments. Three types of novel triblock and tetrablock polymers containing PDVB segments were synthesized by the sequential polymerizations in THF at −78 °C. The synthesized block polymers were an ABC triblock terpolymer of PS-b-PDVB-b-PtBMA and two ABCD tetrablock quarterpolymers of PS-b-P2VP-b-PDVB-b-PtBMA and PS-bPDVB-b-P2VP-b-PtBMA. In each polymerization, styrene was first polymerized with sec-BuLi in the presence of a 10-fold excess of KOBut, and then the monomers were sequentially added according to the previously established monomer reactivity. Each polymerization step was the same as that employed in the block copolymerization, and the detailed polymerization conditions are described in the Supporting Information. In all steps, the monomers were completely consumed, and the polymer yields were always quantitative. As shown in Figure 4, the resulting three block polymers exhibit sharp monomodal

Table 5. Random Copolymerization of 2VP and DVB in THF at −78 °C for 1 mina composition 2VP/DVB (mol %)

Mn × 10−3 (g/mol) calcdb

SECc

RALLSd

Mw/Mnc

calcdb

obsde

11.0

7.07

10.3

1.12

76/24

75/25

a

Polymerization was carried out with oligo(α-methylstyryl)lithium in the presence of KOBut. bConsidering the monomer conversion estimated by 1H NMR of the crude polymerization mixture. c Estimated by SEC using standard polystyrene calibration curve. d Estimated by SEC equipped with triple detectors. eEstimated by 1H NMR.

Figure 4. Synthesis of tri- and tetrablock polymers containing PDVB segment: (a) PS-b-PDVB-b-PtBMA, (b) PS-b-P2VP-b-PDVB-bPtBMA, and (c) PS-b-PDVB-b-P2VP-b-PtBMA.

This result clearly indicates that the crossover reaction from P2VP− to DVB is much slower than that from PDVB− to 2VP. This means that DVB possesses a reactivity lower than 2VP, although the crossover reaction is possible between the two monomers. Another proof for the superiority of 2VP to DVB in reactivity is provided by the polymerizability difference between DVB and 2VP with (C4H9)2Mg. It was found that DVB remained completely intact with (C4H9)2Mg in THF at room temperature even after 1 h and no polymer was formed, while 2VP was immediately polymerized with (C4H9)2Mg under the same conditions. As reported in our previous paper,2 PDVB− quantitatively reacted with 1,1-diphenylethylene (DPE) in a 1:1 addition manner under the conditions in THF at −78 °C within 30 min. In order to compare the reactivity of PDVB− with that of P2VP−, the reaction of each polymer anion with 1,1-bis(3-(tertbutyldimethylsilyloxymethyl)phenyl)ethylene (4) of a DPE derivative was examined under the same conditions. The tertbutyldimethylsilyl group is suitable proof for the 1H NMR analysis. Under the conditions in THF at −78 °C, PDVB−

SEC distributions. In the tetrablock quarterpolymer of PS-bPDVB-b-P2VP-b-PtBMA, a small amount of dimerized product (∼5%) was formed, possibly due to the accidental coupling reaction. As summarized in Table 4, their Mn values observed by RALLS agreed well with those calculated. The 1H NMR spectra revealed the presence of protons corresponding to all Table 4. Synthesis of Tri- and Tetrablock Copolymers Containing PDVB Segmenta Mn × 10−3 (g/mol) polymer t

PS-b-PDVB-b-P BMA PS-b-P2VP-b-PDVB-b-PtBMA PS-b-PDVB-b-P2VP-b-PtBMA

calcd

SECb

RALLSc

Mw/Mnb

31.8 45.3 45.0

31.5 45.4 35.4

31.2 47.4 44.3

1.06 1.08 1.10

All the polymerizations were carried out in THF at −78 °C with oligo(α-methylstyryl)lithium in the presence of KOBut. The polymers were always quantitatively obtained. bEstimated by SEC calibrating with standard polystyrenes. cEstimated by SEC equipped with triple detectors. a

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dx.doi.org/10.1021/ma402657t | Macromolecules 2014, 47, 2333−2339

Macromolecules

Article

addition of the second monomer is required to synthesize welldefined block polymers. However, we previously demonstrated that the PDVB− quantitatively reacted with DPE in a 1:1 addition manner to convert to the 1,1-diphenylalkylanion at the chain end,2 which was compatible with the pendant vinyl group at −78 °C even after 5 h. Therefore, it is worth noting that the end-capping of PDVB− with DPE, followed by the addition of the other monomer, is more convenient instead of the direct use of PDVB− in the block polymerization.

quantitatively underwent the 1:1 addition reaction with 4 within 30 min, while the reaction of P2VP− with 4 proceeded only in 25% for 30 min. Thus, PDVB− was more reactive than P2VP− in this addition reaction. The reactivity difference between PDVB− and P2VP− can also be estimated from the stability of the chain-end anion toward the pendant DVB group. As mentioned in the sequential polymerization, the molecular weight distribution of the living block copolymer of PDVB-b-P2VP− remained narrow after standing it for 4 min. This means that the P2VP− cannot react with the pendant vinyl groups in the PDVB block within, at least, 4 min. For the polymerization of DVB for 4 min under the same conditions, on the other hand, the DVB− gradually reacted with the pendant vinyl group, leading to the multimodal distribution of the resulting PDVB. Accordingly, PDVB− appears more reactive than P2VP− for the addition reaction to the pendant vinyl group. Thus, the results of the above two addition reactions strongly indicate that PDVB− is more reactive than P2VP−. Furthermore, the superiority of 2VP to DVB in anionic reactivity is supported by the above result of the random copolymerization and the observation that 2VP was polymerized with (C4H9)2Mg, which was not active toward DVB. Eventually, the overall comparative reactivity of the chain-end anions follows the sequence of PS− > PDVB− > P2VP− > PtBMA−. The reactivity of the monomers is therefore as follows: styrene < DVB < 2VP < tBMA. This conclusion may be reasonably explained. Although DVB is a styrene derivative para-substituted with a vinyl group, DVB is considered to be a more reactive monomer than styrene due to the longer conjugation system from the vinyl group to the other one via the benzene ring. Although DVB possesses the longer conjugation system, 2VP possesses a strong electron-withdrawing moiety based on the CN bond, allowing 2VP to be more reactive than DVB having no electron-withdrawing group.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (A.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.T. greatly appreciates the support by Grant-in-Aid for JSPS fellows from Japan Society for the Promotion of Science (JSPS). A.H. is thankful for the financial support from DIC Corporation in Japan.

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REFERENCES

(1) Hirao, A.; Tanaka, S.; Goseki, R.; Ishizone, T. Macromolecules 2011, 44, 4579−4582. (2) Tanaka, S.; Matsumoto, M.; Goseki, R.; Ishizone, T.; Hirao, A. Macromolecules 2013, 46, 146−154. (3) Ishizone, T.; Hirao, A.; Nakahama, S. Macromolecules 1993, 26, 6964−6975. (4) Hamer, G. K.; Peat, I. R.; Reynolds, W. F. Can. J. Chem. 1973, 51, 897−914. (5) Hsieh, H. L.; Quirk, R. P. In Anionic Polymerization: Principles and Applications; Marcel Dekker: New York, 1996; pp307−331. (6) Hirao, A.; Nakahama, S. Macromolecules 1987, 20, 2968−2972. (7) Hirao, A.; Ishino, Y.; Nakahama, S. Macromolecules 1988, 21, 561−565. (8) Ishizone, T.; Kato, H.; Yamazaki, D.; Hirao, A.; Nakahama, S. Macromol. Chem. Phys. 2000, 201, 1077−1087. (9) Hirao, A.; Hayashi, M. Macromolecules 1999, 32, 6450−6460. (10) Hirao, A.; Takenaka, K.; Packrisamy, S.; Yamaguchi, K.; Nakahama, S. Makromol. Chem. 1985, 186, 1157−1166. (11) Hadjichristidis, N.; Iatrou, H.; Pispas, S.; Pitsikalis, M. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 3211−3234. (12) Uhrig, D.; Mays, J. W. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 6179−6222. (13) Giebeler, E.; Stadler, R. Macromol. Chem. Phys. 1997, 198, 3815−3825. (14) Goldacker, T.; Abetz, V.; Stadler, R.; Erukhimovich, I.; Leibler, L. Nature 1999, 398, 137−139. (15) Ludwig, S.; Böker, A.; Abetz, V.; Müller, A. H. E. Polymer 2003, 44, 6815−6823. (16) For the synthesis of PS-b-P2VP and PS-b-PtBMA, the first formed PS− is usually end-capped with 1,1-diphenylethylene prior to the second monomer addition to avoid the undesired attack of the PS− on the −CN− bond of the pyridine ring or the ester carbonyl.

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CONCLUSIONS In order to synthesize block polymers having PDVB segments, we have carried out the living anionic block polymerizations by the sequential addition of DVB and other monomers including styrene, 2VP, tBMA, MMA, and three styrene derivatives, 1, 2, and 3, which are para-substituted with N-cyclohexylimine, 2oxazoline, and 2,6-di-tert-butyl-4-methylphenyl ester groups of strong electron-withdrawing character. With developing two series of sequential polymerizations, in which DVB was added either first or second to the anionic initiator, the following welldefined block polymers were successfully synthesized: PS-bPDVB, P2VP-b-PDVB, PDVB-b-P2VP, PDVB-b-P t BMA, PDVB-b-P(1), PDVB-b-P(2), PDVB-b-P(3), PS-b-PDVB-bPtBMA, PS-b-P2VP-b-PDVB-b-PtBMA, and PS-b-PDVB-bP2VP-b-PtBMA. The resulting block polymers were all novel functional block polymers having a reactive PDVB segment, which can be subject to facile and efficient postfunctionalization by various reactions. Throughout two series of sequential block polymerizations, the anionic random copolymerization, the polymerizability with (C4H9)2Mg, and two addition reactions of chain-end anions to 4 and the pendant vinyl group, the reactivities of DVB and its chain-end anion have been compared to those of other monomers and their chain-end anions. As a result, the following comparative reactivities of the chain-end anions and monomers were found: PS− > PDVB− > P2VP− > PtBMA− and styrene < DVB < 2VP < tBMA. Since the chain-end anion of PDVB− is stable only within a few minutes at −78 °C, as previously reported,1,2 the quick 2339

dx.doi.org/10.1021/ma402657t | Macromolecules 2014, 47, 2333−2339