Polymer Journal, Vol. 24, No. 7, pp 669-677 (1992) Selection of Catalyst for Group Transfer Polymerization of 1-Butadienyloxytrimethylsilane Hiroshi SuMI, Tadamichi HIRABAYASHI, Yoshihito INAI, and Kenji YoKOTA Department of Materials Science & Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466, Japan (Received November 19, 1991) ABSTRACT: Catalytic activity of various Lewis acids in the group transfer polymerization (GTP) of 1-butadienyloxytrimethylsilane (BdTMS) initiated with benzaldehyde was investigated. Among the Lewis acids studied, three zinc halides (ZnBr2 , Znl2 , ZnC1 2) and HgBr2 gave soluble polymers with narrow molecular weight distributions in quantitative yields. The polymerizations proceeded apparently on the insoluble solid catalyst. The other soluble Lewis acid catalysts, including (C2H 5),A1Cl, A1Cl 3 , SnC14 , SnBr4 , and TiBr4 , gave polymers in moderate to poor yields and often gelled polymers. Through 1 H NMR monitoring of the polymerization, time vs. monomer and initiator consumption curves were obtained, and then initiation and propagation rates were estimated from slopes of the curves. Both rates were enhanced in an order of ZnBr2 >Znl2 >ZnC12 . This order is consistent with that of chemical shifts of formyl carbon of equimolar mixture of ZnX2 with benzaldehyde, as well as crotonaldehyde which is a model for a polymer end, in chloroform-d. This order was also compatible with the decreasing order in MwiM. values of the resulting polymers. Thus, use of ZnBr2 as the catalyst resulted in P(BdTMS) with the smallest MwiM. (1.26) which was determined by a GPC method. KEY WORDS Group Transfer Polymerization I Aldol Condensation I 1-Butadienyloxytrimethylsilane I Lewis Acid I Zinc Bromide I Kinetics/ Since the anionic living polymerization was that BdTMS in the presence of benzaldehyde 1 established, synthesis of polydienes with well­ (BAld) as an initiator with ZnC1 2 could give controlled molecular weight has been explored its polymer (P(BdTMS)) according to the extensively owing to both academic interests manner of the aldol-type group transfer and industrial applications. However, anionic polymerization (aldol-GTP). An outline of initiators did not work in the case of mono­ the above polymerization can be proposed as mers with substituents like hydroxyl group etc. Scheme 1. Improved anionic living polymerization for such polar monomers were reported2 through + CHz=CH-CH=CH. masking the sensitive functional groups by I 0 BdTMS OSi(CHa)3 organosilyl groups. A diene compound and also silyl enol ether, 1-butadienyloxytri­ (Q)-cH-CHz-CH=CH-CH ZnCiz methylsilane (BdTMS), is applicable to an 6si(CHa)3 g n BdTMS alternative living polymerization process, namely group transfer polymerization clas­ I I Tn II sified into an aldol-condensation type. 3 OSi(CHa)a OSi(CHa)3 0 In our previous paper,4 it was ascertained Scheme l. 669 H. SUM! et al. Though the number-averaged molecular distilled on calcium hydride under reduced weight (Mn) at full consumption of the pressure to give a fraction of bp 65-66oC/ monomer was controlled by the ratio of 65 mmHg. BdTMS was confirmed as a mix­ [BdTMS]0 /[BAld]0 and increased by succes­ ture of two geometric isomers (92% E and sive addition of the monomer, the molecular 8% Z based on gas chromatographic data), weight distribution was about 1.5 in terms of however, used as a BdTMS monomer without Mwl M"" Our attention has been directed to further separation from each isomer. (In this improving the Mw/Mn value of P(BdTMS) connection, we must correct a description in smaller. At the first attempt, 5 this value could our previous papers,4 •5 where E-isomer was be decreased by employing the initiator having misunderstood for Z-isomer because of its a strongly electron-donating substituent (e.g., proton coupling constant of 11.8 Hz at = at para position of formyl moiety.) group, because the initiation reaction was ZnBr2 , Zni2 , A1Cl 3 , and HgBr2 were accelerated by rapid formation of a carbo­ sublimed in vacuo at each appropriate tem­ cation from such aldehyde with assistance of perature. ZnC1 2 was sublimed under dry ZnC1 2 . nitrogen atmosphere. SnC1 4 , SnBr4 , and In this paper, we describe catalytic effects (C 2 H 5LA1Cl were distilled under reduced of various Lewis acids in the aldol-GTP of pressure of nitrogen atmosphere. Guaranteed BdTMS. At first, screening of Lewis acids grade of TiBr4 was commercially available, was carried out to obtain the smaller M wl M" and used without further purification. All value and quantitative yield of the soluble Lewis acid catalysts were stored and handled polymer. For the heterogeneous aldol-GTP under the dry nitrogen atmosphere in a dry catalyzed by zinc halides in a NMR tube, box. In addition, Znl2 and HgBr2 were stored kinetic analysis was also put into practice. As and handled in the dark to avoid liberation a result, ZnBr 2 could be recommended as the of halogen. excellent catalyst in a viewpoint of not only Benzaldehyde (BAld) and crotonaldehyde achievement of the smallest M wl M" value and were fractionally distilled immediately before easy handling but also kinetic data. Electron use. Benzene was purified by the conventional density on formyl carbon in equimolar mix­ method and dried over sodium, distilled just tures of zinc halide with benzaldehyde prior to each polymerization. Benzene-d6 was (initiator) as well as crotonaldehyde (=a commercially available in 99.9% purity. model of a polymer end) was estimated by means of 13C NMR, and the estimation was Procedures useful to elucidate polymerization behavior A mixture of Lewis acid catalyst, solvent, observed experimentally. Finally, we demon­ and BAld in a dry Schlenk-tube was stirred strate that soluble zinc halides never played a for 30 min at 30oC under the dry nitrogen primary role in this heterogeneous aldol-GTP atmosphere. Then BdTMS was quickly in­ system. jected through the septum by a syringe. After an adequate period, the mixture was filtered EXPERIMENTAL through glass filter with fine meshes (No. 4-grade), and the filtrate was subjected to Materials evaporate in vacuo. P(BdTMS) was obtained 1-Butadienyloxytrimethylsilane (BdTMS) as a highly viscous liquid. was prepared according to Danishefsky's pro­ Polymerization was also carried out in a cedure, 6 of which details was given also in our NMR-tube at the probe temperature (35°C), previous papers.4 •5 The crude product was and directly observed at regular intervals. In 670 Polym. J., Vol. 24, No. 7, 1992 GTP of 1-Butadienyloxytrimethylsilane this case, exact amounts of certain catalyst for the aldol-GTP of BdTMS. The results are were placed in the tube, then a mixture of summarized in Table I. Zinc halides (ZnX2 : benzene-d6 , BAld, and BdTMS was added. X=Cl, Br, I) and HgBr2 were practically The consumption of BdTMS was evaluated insoluble in reaction mixtures (No. 1-4 in from the NMR peak area of the monomer Table I). On the other hand, (C2 H 5)zAICl, and polymer. A1Cl 3 , SnC1 4 , SnBr4 , and TiBr4 were soluble (No. 5-9 in Table 1). Polymerizations with Analyses the insoluble catalysts proceeded smoothly 1 H and 13C NMR spectra were recorded rather than that with the soluble ones. The with a Varian XL-200 spectrometer in deu­ soluble catalysts were often accompanied by terochloroform. For kinetic studies, 1 H NMR serious gelation, resulted in the low or no spectra were recorded with a Hitachi R-90 yield of the soluble polymer. Such strong NMR spectrometer. Molecular weight and its Lewis acids must promote not only aldol­ distribution were determined by a gel perme­ GTP process also side reactions remarkably, ation chromatography (GPC), calibrating the even though concentrations of the catalyst molecular weight with standard polystyrenes. lowered to merely 4 mol% to the initiator. GPC was measured by differential refrac­ Among zinc halides, ZnBr 2 was most tometry in tetrahydrofuran as eluent on a effective to achieve the narrow molecular Tosoh HLC-8030 instrument equipped with weight distribution. The molecular weight four gel packed columns (TSKgel, G5000-, distribution of the polymer obtained with G4000-, G3000-, G2000-HXL). ZnBr2 , Znl2 , and ZnC1 2 came to broaden in this order. Figure 1 shows GPC curves of the polymers obtained with these zinc halides. RESULTS AND DISCUSSION ZnBr2 catalyzed the aldol-GTP of BdTMS Screening of Lewis Acid more effectively than Zni2 or ZnC1 2 • The Various Lewis acid catalysts were examined polymerization with ZnBr 2 was initiated Table I. GTP of BdTMS with benzaldehyde and various Lewis acids• Time Yield No. Lewis acid M. (calcd)c h % Heterogeneous 1 ZnBr2 3 89 3640 2960 1.26 2 Znl2 3 89 2630 2790 1.58 3 ZnCI2 5 94 3190 2950 2.16 4 HgBr2 12 90 2690 2890 1.82 -----··--- Homogeneous (C 2 H 5) 2 A1Cl 24 61 2220 2600 1.51 TiBr4 72 52 1110 3060 1.94 SnBr4 2 (2780) 2880 (1.74) SnC1 4 Gelation AlCI 3 Gelation • BdTMS, 10.0 mmol; benzaldehyde, 0.5 mmol; Lewis acid, 0.5 mmol; benzene, 8.25 ml; temp. 30oC. h Determined by GPC, calibrated by standard polystyrene samples. c Calcurated from [BdTMS] 0 /[aldehyde] 0 . rlSoluble polymer was not obtained. To determine M. aliquot of reaction mixture was analyzed by GPC. Polym. J., Vol. 24, No.7, 1992 671 H. SUM! et al. a) ZnBr2 c) ZnCI2 _:__jl Elution volume/ mJI. Elution volume/ mJI. Elution volume/ mJI. Figure I. GPC curves of polymerization mixture with ZnX2 in benzene at 30oC. BdTMS, lOmmol; benzaldehyde, 0.5 mmol; ZnX2 , 0.1 mmol; benzene, 8.25 ml. rapidly and proceeded smoothly to give each zinc halide before rapid consumption of P(BdTMS) in quantitative yield. On the other monomer.