Macromolecules 1998, 31, 9141-9145
9141
Anionic Living Polymerization of 2,3-Diphenyl-1,3-butadiene Akira Hirao,* Yashunori Sakano, Katsuhiko Takenaka,† and Seiichi Nakahama Department of Polymer Chemistry, Tokyo Institute of Technology, 2-12-1, Meguro-ku, Tokyo 152-8552, Japan Received July 13, 1998; Revised Manuscript Received October 2, 1998 ABSTRACT: The anionic polymerization of 2,3-diphenyl-1,3-butadiene (1) was investigated in THF at -78 °C as well as in benzene at 40 °C. Compound 1 was found to undergo anionic living polymerization in THF with cumylpotassium or s-BuLi to afford quantitatively the polymers of well-controlled molecular weights and narrow molecular weight distributions (Mw/Mn < 1.1). Each of the microstructures of the polymers was observed to be only the 1,4-addition product of poly(2,3-diphenyl-1,3-butadiene). Furthermore, the polymer consisted of 90% cis and 10% trans 1,4-structures, although the assignment of these configurations was tentative. It was observed by DSC that the polymer showed a glass transition temperature of 98 °C and a melting point of 170 °C. Surprisingly, on standing for a few hours the polymer precipitated completely from the polymer solution in THF. The polymer thus precipitated became insoluble in most organic solvents except for hot toluene and o-dichlorobenzene. This may therefore be due to the crystallization of the polymer, but not cross-linking as previously reported.
Introduction Anionic polymerizations of 1,3-diene monomers have so far been extensively investigated.1,2 Under the appropriate conditions of polymerization, 1,3-butadiene, isoprene, and many 2-alkyl- and 2-aryl-substituted 1,3butadiene derivatives are known to undergo anionic living polymerization to afford the poly(1,3-diene)s with controlled molecular weights and narrow molecular weight distributions.2 Furthermore, block copolymers of styrene with 1,3-butadiene or isoprene prepared by anionic living polymerization are commercially available as thermoplastic elastomers.3 The stereochemistry of anionic polymerization of dienes depends on the polymerization variables such as initiator, solvent, and temperature and can be controlled to a certain extent. For example, a high cis-1,4 polyisoprene analogous natural rubber can be obtained by the polymerization of isoprene with organolithium compounds in hydrocarbon solvents.4 Little attention has been, however, paid to the polymerization of 2,3-diphenyl-1,3-butadiene (1). To the
and the 1,2-isomeric structure was not observed by IR or 1H and 13C NMR analyses. Later, similar results on the radical polymerization of 1 were reported by Asami and co-workers.6 Vogl and co-workers also attempted to polymerize 1 anionically with either n-butyllithium (n-BuLi) or sodium naphthalenide.5 Unfortunately, n-BuLi was not effective and the polymer yields were less than 10% yields. On the other hand, a nearly quantitative conversion of the monomer in the polymer was observed with sodium naphthalenide in THF at 30 °C. It turned out, however, that the polymer was completely insoluble in organic solvents and totally cross-linked. Unfortunately, no further information was available in the literature. From these results, it is considered that 1 possess anionic polymerizability to a certain extent. However, we believe that the polymerization temperature of 30 °C in THF might be unusual as a general condition of the anionic polymerization. It therefore occurred to us that it should be possible to control the polymerization of 1, affording a soluble polymer, even with molecular weight control by carefully setting the condition of anionic polymerization. We present in this paper our successful results of the anionic living polymerization of 1 in THF at -78 °C. The microstructures of the resulting polymers will be discussed. Experimental Section
best of our knowledge, only two studies on the polymerization were previously reported.5,6 Vogl and coworkers first demonstrated in 1977 that 1 underwent AIBN-initiated free-radical polymerization to afford the polymers in 60-96% yields, while (t-Bu)3Al-TiCl4 and (i-Bu)3Al-TiCl4 Ziegler type initiators were not effective for the polymerization.5 The resulting polymer was shown to be 57% trans-1,4-poly(2,3-diphenyl-1,3-butadiene) and 43% cis-1,4-poly(2,3-diphenyl-1,3-butadiene), †
Nagaoka University of Technology.
Materials. Styrene, R-methylstyrene, isoprene, methyl methacrylate (MMA), THF, heptane, and benzene were purified by the procedures reported previously.7 2,3-Diphenyl-1,3-butadiene (1). The monomer 1 was previously synthesized by Vogl and co-workers.5,8 In this study, we synthesized 1 by the following new method using a nickelmediated coupling reaction between R-bromostyrene and 1-phenylvinylmagnesium bromide, prepared from R-bromostyrene and Mg in THF. To 30 mL of a dry ether solution of R-bromostyrene (4.91 g, 26.8 mmol) and NiCl2-bis(diphenylphosphino)propane (40 mg, 0.10 mmol) was added dropwise a THF solution of 1-phenylvinylmagnesium bromide (40 mL of a 0.69 M THF solution, 27.6 mmol) at 0 °C under an atmosphere of nitrogen. The resulting mixture was stirred for 12 h at room temperature. It was then hydrolyzed with 2 N HCl and extracted with ether (30 mL × 3). The ether layer
10.1021/ma981094o CCC: $15.00 © 1998 American Chemical Society Published on Web 12/08/1998
9142 Hirao et al. was washed with 10% NaHCO3 and H2O and dried over MgSO4. After evaporation, the residual oil was fractionally distilled twice at 98-99 °C (1 Torr) to give 2.53 g (12.3 mmol, 46% yield) of 1 as a colorless liquid which crystallized on standing. It was further purified by repeating recrystallization from methanol, mp 48.5-49.0 °C (lit.5 mp 49-50 °C). The purity of 1 thus obtained was analyzed by GC to be more than 99.3%: 1H NMR (CDCl3) δ 7.54-7.18 (10H, m, Ar), 5.53 (2H, d, J ) 1.65 Hz, CH2d), 5.31 (2H, d, J ) 1.65 Hz, CH2d); 13C NMR (CDCl3) δ 149.9 (CH2dC), 140.1 (C3 of Ar), 128.2 (C5 and C6 of Ar), 127.5 (C4 of Ar), 116.3 (CH2d); IR (KBr) 3030, 3023, 1608, 1573, 1495, 1443, 1096, 1071, 1028, 904, 776, 704 cm-1. Compound 1 thus synthesized was dried under high-vacuum condition (10-6 Torr) in the presence of P2O5 for 48 h. It was diluted with THF (0.05 M) or benzene (0.15 M) on a vacuum line and divided into ampules with breakseals that were prewashed with potassium naphthalenide in THF or 1,1diphenylhexyllithium in benzene. Polymerization Procedure. Polymerizations were carried out under high-vacuum condition in all-glass apparatus with breakseals in the usual manner. The resulting polymers all were first soluble in THF after terminating with a few drops of degassed methanol, but they precipitated completely on standing after a few hours. The polymers that precipitated were insoluble in not only THF but also most organic solvents. They were soluble only in hot o-dichlorobenzene. The polymers with Mns of less than 7000 were also soluble in hot toluene. The polymerization mixtures soon after terminating with a few drops of degassed methanol were directly injected into a SEC instrument to characterize the molecular weights and molecular weight distributions of the polymers using polystyrene calibration. Block Copolymerization. Block copolymerizations were conducted by the sequential addition of two monomers in THF at -78 °C. The block copolymer of poly(styrene-b-1) was prepared in THF at -78 °C with s-BuLi by polymerizing styrene first for 30 min and then 1 for 48 h. When 1 was used as a first monomer, the polymerization of 1 was carried out with s-BuLi in THF at -78 °C for 48 h, and then either styrene or MMA was added as a second monomer to polymerize in THF at -78 °C for 1 h. In the polymerization of MMA, 5 equiv of LiCl was added prior to the polymerization. With use of isoprene as a second monomer, cumylpotassium instead of s-BuLi was used as an initiator in the first polymerization of 1, and the polymerization mixture was allowed to stand for 5 h in THF at -78 °C after the addition of isoprene. The block copolymers poly(styrene-b-1) and poly(1-b-MMA) thus obtained were soluble and did not precipitate on standing. They could be purified by precipitation from THF to methanol. The block copolymers were characterized by NMR and SEC. Measurements. IR spectra were recorded on a JASCO IR-G spectrophotometer. 1H and 13C NMR spectra were recorded on a JEOL GSX-270 (270 MHz for 1H and 67.80 MHz for 13C) in CDCl3 and toluene-d8. Size exclusion chromatography (SEC) was conducted on a TOYO SODA HLC-802 instrument at 40 °C with THF as the carrier solvent. Differential scanning calorimetric (DSC) measurements were carried out with a Perkin-Elmer DSC Model II at a scanning rate of 2 °C/min.
Results and Discussion Anionic Polymerization of 1. The anionic polymerization of 1 was first carried out in THF at -78 °C with cumylpotassium. On mixing 1 in THF with cumylpotassium, an immediate color change from the bright red characteristic of cumylpotassium to dark red was observed, indicating that the polymerization occurred very rapidly to generate a new carbanion from 1. This color remained stable as long as the temperature was kept at -78 °C. Only a trace amount of oligomer was obtained after 1 h. A polymer was obtained in 66%
Macromolecules, Vol. 31, No. 26, 1998
Figure 1. SEC curves of poly(2,3-diphenyl-1,3-butadiene)s (peak A, Mn ) 2600, Mw/Mn ) 1.07; peak B, Mn ) 13000, Mw/ Mn ) 1.05). Table 1. Anionic Polymerization of 1 in THF at -78 °C 1, mmol
initiator
1.99 2.68 2.67 2.89 2.71 2.16 2.20 2.27e
cumyl-Kb cumyl-K cumyl-K K-naphc s-BuLi/R-MeStd s-BuLi s-BuLi s-BuLi
mmol
Mn time, yield, h % calcd obsda Mw/Mna
0.128 1 0.156 20 0.0430 168 0.0402 200 0.102 48 0.0509 20 0.0330 25 0.856 4
