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Anionic Copolymerization of A-Methylstyrene and 2, 3-Dimethylbutadiene

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Anionic Copolymerization of A-Methylstyrene and 2, 3-Dimethylbutadiene Polymer Journal, Vol. 3, No. 4, pp 442-447 (1972) Anionic Copolymerization of a-Methylstyrene and 2, 3-Dimethylbutadiene Heimei YUKI, Yoshio OKAMOTO, Hidekazu TAKANO, and Koji OHTA Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka, Japan. (Received July 22, 1971) ABSTRACT: a-Methylstyrene(a-MeSt) and 2, 3-dimethylbutadiene (DMB) were copoly­ merized in THF and in hydrocarbon solvents with n-BuLi. The copolymer produced at 60°C in THF was composed of almost equimolar amounts of a-MeSt and DMB, if a-MeSt was charged in excess. In the copolymer a-MeSt and DMB were considered to be arranged alternately, and DMB was incorporated in the form of 1, 4-structure. In the copolymerization below 60°C the copolymer composition was affected by the ratio of initial monomer concentrations. The content of a-MeSt in the copolymer increased with decreasing polymerization temperature and reached 91% at -78°C. The copoly­ merization at 30°C gave the copolymer composed of (2: 1) a-MeSt and DMB under the existence of large excess of a-MeSt. It was presumed that the major part of the copolymer was composed of repeated sequences of [a-MeSt-a-MeSt-DMB]. In benzene a copolymer rich in DMB (71%) was obtained at 60°C. The addition of THF to the polymerization system in benzene decreased the content of DMB. KEY WORDS Anionic Copolymerization / a-Methylstyrene / 2, 3- Dimethylbutadiene / Alternating Copolymer / NMR / The authors have reported the anionic alter­ anionic copolymerization of a-MeSt and styrene nating copolymerizations1 including 1, l-di­ in THF and determined the monomer reactivity 3 phenylethylene2 or trans-stilbene (M2), which ratios around room temperature. The polymer can not alone be polymerized. The comono­ produced in the copolymerization was considered mers (M1) combined with M2 were styrene, to have such a tapered structure that a sequence some substituted styrenes and conjugated dienes composed of styrene units at one end changes such as butadiene, isoprene and 2, 3-dimethyl­ continuously into that of alternating units of butadiene. a-Methylstyrene (a-MeSt) can not styrene and a-MeSt at the other end in the be homopolymerized above 61 °C because of the molecule, because the reaction mixture forms a low ceiling temperature. 4 Therefore, a-MeSt living system and the reactivity of styrene will be a candidate of the monomer M 2 in such toward anion is higher than that of a-MeSt. an alternating copolymerization as those describ­ In the previous paper8 we reported the anionic ed above, if a suitable comonomer M 1 is chosen polymerization of 2, 3-dimethylbutadiene (DMB) as the propagation rate constant k 12 being larger and proposed that the polymer produced above than k 11 and the polymerization reaction is -30°C in THF was composed of alternating conducted above 60°C. units of 1, 2- and 1, 4-structure. The 1, 2- There seems to be very few reports on the structure of DMB is sterically very similar to anionic copolymerization including a-MeSt as the structure of a-MeSt unit. In this paper the one of the comonomers. Szwarc and his co­ results of the anonic copolymerization of a-MeSt workers have studied and determined the rate and DMB are reported. constants of the reactions between living poly­ EXPERIMENTAL styrene and a-MeSt monomer5 and also between living poly-a-MeSt and a number of styrene deriv­ Materials atives6 in THF. Tolle, et al.7, have studied the 2, 3-Dimethylbutadiene was synthesized by the 442 Anionic Copolymerization of a-MeSt and 2, 3-Me2Bd. dehydration of pinacol as described by Allen.9 hypodermic syringes to the ampoule, which was The monomer boiling at 69.5°C (760 mm) was held in a thermostat, under dry nitrogen. The dried over lithium aluminum hydride and dis­ ampoule was sealed and the reaction was tilled in vacuo before use. allowed to proceed. After the reaction was a-Methylstyrene was distilled under reduced terminated by adding a small amount of pressure. The purified monomer was dried with methanol, the reaction mixture was poured into lithium aluminum hydride and distilled in vacuo a large amount of methanol. The precipitated before use. polymer was filtered, washed with methanol and n-Butyllithium (n-BuLi) was prepared in n­ dried. The polymer was stored at -20°C. heptane according to the method of Ziegler10 Measurements from n-butyl chloride and metallic lithium. The NMR spectrum of the polymer was Tetrahydrofuran (THF) was first refluxed over measured at 60°C with a JNM-4H-100 spectro­ metallic sodium and then over lithium alumi­ meter at 100 MHz by using carbon tetrachloride num hydride. This was distilled onto Na-K as a solvent. The concentration of the sample alloy and naphthalene. From the green solu­ was ca. 10% (w/v). The chemical shifts are tion the solvent was transferred to a reaction refered to the signal of tetramethylsilane used vessel on a vacuum line just before use. as an internal standard. A small amount of Benzene and n-heptane were purified by the methylene dichloride was added to the sample useal methods and were stored over sodium. solution and its signal was used as a lock signal. Before use, each of the solvents was transferred The solution viscosity of the polymer was to a flask containing a small amount of n-BuLi determined in toluene at 30±0.03°C, the con­ in n-heptane and then distilled under high centration of the polymer being 0.Sg/d/. vacuum. Polymerization RESULTS Using a vacuum system, a solvent was first transferred to a glass ampoule which had been The anionic copolymerization of DMB (M1) evacuated and baked with a gas burner. Then and a-MeSt (M2) was carried out with n-BuLi the monomers and an initiator were added with in THF at [M2 ] 0 > [Milo where [M2] 0 and [Milo Table I. Anionic copolymerization of a-methylstyrene (M2) and 2, 3-dimethylbutadiene (M1)a Polymer Time, No. [M2]0/[M1]0, Temp, M1 content, mol % mol/mol oc hr Yield, '}sp/c,d sp, e % Jb nc d//g oc 3.62 60 0.5 46.1 44.2 46.9 0.22 2 3.09 60 0.5 55.5 44.1 0.17 98-108 3 3.05 60 0.5 52.1 45.4 47.4 0.23 100-111 4 2.96 60 0.5 56.0 45.1 0.13 5 2.39 60 0.5 61.6 47.8 0.22 100-110 6 1.50 50 2.5 81.8 49.0 49.0 0.30 7 3.06 30 24.0 68.8 36.5 35.9 0.39 125-135 8 1.48 30 18.0 92.2 43.7 0.32 9 3.02 0 24.0 93.3 26.1 26.6 0.43 108-120 10 1.54 0 18.0 98.8 40.3 39.4 0.26 11 1.08 -78 20.0 37.5 8.5 • [M1] 0, 4.0-4.3 mmol; solvent, THF 15 ml; initiator, n-BuLi 0.064 mmol. b Determined from NMR spectrum. 0 Determined from polymer yield. d Toluene solution at 3O°C, c=0.50 g/dl. • sp, softening point. Polymer J., Vol. 3, No. 4, 1972 443 H. YuKr, Y. OKAMOTO, H. TAKANO, and K. OHTA are the initial monomer concentrations. The under the reaction conditions employed. Thus results are shown in Table I. The copolymeriza­ the composition of the copolymer can be cal­ tion at 60°C was rather fast. In order to check culated from the initial monomer concentrations the consumption of the monomers in this poly­ and the polymer yield. merization a control experiment was performed. The NMR spectrum of the copolymer prepared After a reaction period of 30 min, the volatile in THF with n-BuLi at 50°C is shown in Figure fraction was collected from the reaction mixture 1, as an example. The copolymer composition by vacuum distillation, and the fraction was was also calculated from the intensity ratio of examined by gas chromatography. The result the peaks of phenyl protons (7.0-7.4 ppm) to indicated that DMB was completely consumed the others. Both the values obtained from in the polymerization. It was also found that polymer yield and from NMR spectrum were 18 hours suffice for the complete consumption in good agreement and within experimental of DMB in the copolymerization at 30 and 0°C error, as shown in Table I. The copolymers produced at 60°C were com­ posed of almost equimolar amounts of M1 and M2 , when the ratio [M 2 ] 0/[Mil0 was 2.4-3.6. (Al On the other hand, in the copolymerization below 60°C, not only the content of a-MeSt in the copolymer increased, but also the copoly­ mer composition became strongly affected by the ratio of initial monomer concentrations, with decreasing polymerization temperature. At - 78 °C the DMB content was only 8 .5 % in the polymer formed from equimolar concentrations (Bl of M 1 and M 2 • The copolymerization in THF was carried out at 30°C under various ratios of initial monomer concentrations, where [Milo was kept constant. The results are shown in Table II. At [M2 ] 0/ (Cl [Milo= 1.5 the M 1 content in the polymer was 44%. With increasing [M2 ] 0/[Milo, the M1 content decreased gradually and reached about 33%, i.e., the polymer was composed of 2: 1 molar ratio of a-MeSt and DMB, at [Mil0/[Mil0 (D) =3. When the copolymerization was carried wt 'A1 out at high, but constant ratio of initial monomer concentrations ([M2] 0/[Milo= 10) and the total monomer concentration varied, the M 1 content in the polymer increased with a de­ (ppm l crease in the monomer concentration as shown Figure 1.
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