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. The induction periods were ob­ hand, Znl2 did not so, possibly due to some served longer, for ZnBr2 , Znl2 and ZnC1 2 in irregular side reaction occurrence. An induc­ that order. Induction periods are often ob­ tion period appeared on the initial stage of served in such polymerization as a slower the polymerization with ZnC1 2 , resulted in initiation step precedes a faster propagation broadening of the molecular weight distibu­ step. Therefore, judging from the length tion. According to our proposed mechanism the above induction period, the efficiency of of the aldol-GTP of BdTMS, 5 the acidity of these halides to the initiation step should be

Lewis acid catalysts should play an important in the order of ZnBr 2 > Znl2 > ZnC1 2 , as is role for the rapid initiation and consequently coincident with the narrowing order of the achievement of the narrower molecular weight distribution of the molecular weight of the distribution. However, it dose not seem that polymer obtained by each catalyst. After the the order of the efficiency observed in this induction periods, every plots are straight polymerization is the same as in Friedel-Crafts lines (solid lines in Figure 3) with different acylation and so on. 7 Therefore, this order slopes depended on the catalyst used. Since have to be ensured by other experimental these slopes should reflect rates of propaga­ data, e.g., kinetic studies and spectroscopic tion, the efficiency of the catalysts to propaga­ measurements. tion was in the order of ZnBr 2 > Znl2 > ZnC1 2 , too. Kinetic Investigation Although the present aldol-GTP system is The aldol-GTP of BdTMS with zinc halides heterogeneous, typical kinetics of homogene­ was carried out in a NMR tube for kinetic ous living polymerization model may be investigation. Figure 2 shows 1 H NMR adopted for the purpose of qualitative com­ spectra of the polymerization mixture. The parison of catalytic activity among zinc consumption of BdTMS was evaluated from halides. Assuming that (1) the polymerization the NMR peak area of the monomer (b, c, d) rates, RP, is proportional to monomer and polymer (b' c', d'). Figure 3 shows time­ concentration ([M]) and (2) the concentra­ consumption curves for the aldol-GTP of tion of active propagating ends ([P*]) is BdTMS with zinc halides. There was an nearly constant and probably equal to the induction period on the polymerization with initial concentration of an initiator ([1]0 ),

672 Polym. J., Vol. 24, No.7, 1992 GTP of 1-Butadieny1oxytrimethy1si1ane lOmin

e d c b a CH2=CH-CH=CH I a 0Si(CH3b b,c,d e r-----1 ,----,

ds-benzene f I l_ .111

2h -- e' d' c' b' f'

I Tn II 0Si(CH3)3 0Si(CH3b 0 g' a' a'

c',d' e'

b'

f' _l

10 9 8 7 6 5 4 3 2 1 0 8/ppm

1 Figure 2. H NMR spectra of the GTP mixture of BdTMS with ZnBr 2 in benzene-d6 at 35oC: BdTMS, 0.351 mmol; benzaldehyde, 0.0176mmol; benzene-d6 , 0.44ml; ZnBr2 , 0.0176mmol. kinetic scheme would be simplified as follows. Consequently, the apparent rate constants (kp') of polymerization were calculated from P*+M--+P* (1) the plots of ln([M]0 /[M]1) vs. time, which were shown in Figure 4a. The kP' values Hence, obtained from Figure 4a for usig ZnBr2 , Znl2 , and ZnC1 2 were summarized in Table II. RP= -d[M]/dt=kp[P*][M] (2) Dashed lines in Figure 3 show the con­ sumption of the initiator during the po­ ln([M]0 /[M]1) = kP [P*] t = kP' t (3) lymerization with zinc halides. On the 1 H

Polym. J., Vol. 24, No. 7, 1992 673 H. SuMI et a!.

100 ()'/ 1.0 I / " 60 I • _-C) 80 " / 0.8 0 _.(} '#. •I " /(j c '#. 0.6 0 "·.;::; '2 60 ' 0 40 0. 0 E ·;; ::J 0. :: 0.4 ZnCI2 II) E c ::J .E 0 40 u 0 20 u 0.2

20 0 6 8

8 2.0 Time/h b)

Figure 3. Monomer and initiator consumption vs. time 1.6 80 cruves in GTP of BdTMS with ZnX2 in benzene-d6 at '#. 35°C: BdTMS, 0.351 mmol; benzaldehyde, 0.0176mmol; " / " c , , ; 1.2 / "0 benzene-d6 0.44ml; ZnX2 0.0176mmol: e. ZnBr2 0. / 0 :;; Znl2 ; (), ZnC1 2 : -,monomer consumption;---, initiator 'ZnCI2 0. 60 E consumption. ::J II)c .E 0 40 u

NMR spectrum of the polymerization mixture 20 (Figure 2), a sharp singlet peak at .:5 = 9.92 0 (ppm) due to a formyl proton of benzaldehyde 2 4 6 8 decreased with time, and a doublet-doublet Time/h peak at .:5=9.50 (ppm) appeared instead. The Figure 4. First order plots for GTP of BdTMS: a) latter should be assigned to a formyl proton monomer; b) initiator. in an adduct of the monomer and initiator (or also polyer ends). Therefore, the initiator Table II. Kinectic results for GTP of BdTMS consumption was also calculated by com­ paring the peak intensity of the formyl group kp' X 104 k;' X 104 Catalyst of benzaldehyde to that of propagating chain s-1 s-1 ends. An approximately linear relationship can be accepted in the first stage, and the slope ZnBr2 1.8 4.5 depends upon zinc halide used. The rate of Znl2 0.9 1.1 ZnC1 0.4 0.6 the initiator consumption estimated in this 2 manner agreed well with the length of the induction period observed in Figue 3. The tion ([M]) is approximated to be constant initiation rate constants (kJ were determined ( [M]0 ). Thus, in first-order kinetics,

ln([l]0 /[l]) = ki [M]0 t = k/ t ( 6) Ri= -d[I]/dt=kJI][MJ (4) The k/ values obtained by using of ZnBr2 , ln([I] 0 /[I]) = kJ [M]dt (5) Znl2 , and ZnC1 2 are summarized in Table II, At the initial stage, the momomer concentra- too.

674 Polym. 1., Vol. 24, No. 7, 1992 GTP of 1-Butadienyloxytrimethylsilane

In comparison with the kP' and k/ values observed chemical shift of formyl carbon of for the different zinc halides, ZnBr 2 is most aldehyde with zinc halide does not always effective to both processes. Its influence comes reflect the absolute chemical shift of co­ out strongly on initiation rather than prop­ ordinated complex itself. In other words, the agation. However, Zni2 and ZnC1 2 resulted present difference (L1<5) in chemical shift of in relatively small and comparable kP' and k/ formyl carbon between with and without zinc values. Initiation must be still forced to halide does not reflect the absolute acidity of compete with propagation, and it must be each zinc halide. To embarrass the matter, the difficult to achieve the narrower molecular net quantities of each zinc halide responsible weight distribution of the resulting polymer. for complexation is not same in spite of feeding equivalent zinc halide to the mixture. Activity of Zinc Halide Catalysts Irrespective of the problems mentioned Electron density on the formyl carbon above, changes of the chemical shift of formyl should be varied by coordination with zinc carbon in the equimolar mixture of aldehyde halides, depending on their acidity. 13C chem­ and zinc halide could be correlated with the ical shifts of the formyl carbon of crotonal­ estimated kP' or k/ value. Figure 5 would dehyde and benzaldehyde were measured in help to elucidate the "practical" catalytic the presence of the equimolar amounts of zinc ability of each zinc halide in this aldol-GTP halide. Figure 5 shows relationships between process. Since these chemical shifts indicate the rate constants, kP' or k;' , and the 13C the net electron density on formyl carbon of chemical shifts of aldehydes. Crotonaldehyde the initiator or the polymer end under given would be accepted as a model compound of a polymerization conditions, it is reasonable to propagating polymer end. In all cases, 'formyl recognize that the L1<5 values must be a scale of carbon of aldehyde with zinc halide gave only the "practical" catalytic ability of zinc halide one resonance peak at a lower magnetic field under the same polymerization conditions. than that of corresponding aldehyde alone. Based on 13C NMR data, the net electron Therefore, complexation of aldehyde with density on formyl carbon in both the initiator zinc halide must be in rapid equilibrium and polymer end was obviously decreased within a NMR time scale. However, the by mixing with zinc halide in the order of ZnBr2 >Zni2 >ZnC12 . In other words, cat­ ionic character of those formyl carbons was -3.0 ©-nH enhanced by mixing with zinc halide in the CH3CH==CHUH 0\ same order. Thus the order of the "practical" 0 ZnBr2e ZnXz \ ZnXz bO ZnBr2 Initiation: .Q -3.8 @-lfH +LA '- \ = 0 ZnlzO\ 0 0-LA Znl2\ (LA: Lewis acid)

bO ZnCiz(), .Q -4.6 ' I ' ,none 'none Propagation : I I EB

1 e 200 198 196 194 192 190 0-LA 8 ppm w-yH-CH2-CH==CH-IfH + LA Figure 5. Relationships between rate constants (k;', kp') 0Si(CH3)3 0 and 13C chemical shifts of aldehydes complexed with

ZnX2 : aldehyde, 5 wtjvol% in chloroform-d. Scheme 2.

Polym. J., Vol. 24, No. 7, 1992 675 H. SUM! eta/. catalytic ability observed as ZnBr 2 > Zni2 > 100 ZnBr2 ZnCI2 ZnC1 2 is not inscrutable, even though the electronegativity of halogen atoms lines up '? .Q 60 such as Cl > Br >I. Such the order of the net ....a 540 cationic character of formyl carbon is again (/) 20 coincident with the order of degree of con­ u trolling molecular weight. 0 0 2 4 6 14 16 18 20 22 Time/h General Mechanism of Aldol-GTP of BdTMS Figure 6. GTP of BdTMS with benzaldehyde and ZnX2 Consequently, it could be confirmed that in benzene-d6 at 35°C: -,no filtration; ---,filtrated at the the well-designed molecular weight of the point of the arrow. BdTMS, 0.571 mmol; benzaldehyde, polymer in the aldol-GTP of BdTMS resulted 0.029 mmol; benzene-d6 , 0. 90 ml; ZnX2 , 0.029 mmol. from formation of a moderate cationic center on formyl carbon of the initiator as well as the of their solubility. However it was denied polymer end with a suitable Lewis acid. The by the experiment to clarify how much zinc general mechanism for the GTP of BdTMS halide dissolved in benzene and how the could be again proposed as follows. In both dissolved catalyst influenced the aldol-GTP initiation and propagation, a carbocation process. should be formed through complexation of The solid part of the catalyst was filtered the formyl group with Lewis acid, followed out of a mixture through a glass filter with by nucleophilic addition of the monomer and fine meshes (No. 4 grade) in the course of migration of the trimethylsilyl group from the polymerization in a NMR tube. The NMR coming monomer to carbonyl oxygen. For acquisition was allowed to continue for the these processes, ZnBr 2 can be recommended filtrate. The monomer consumption behavior as the most excellent catalyst in the present before and after filtration of each catalyst is work. Popular Lewis acids, other than zinc shown in Figure 6, where the catalyst was halides, were too strong because of occurrence filtered away at the time pointed by an arrow. of undesired side reactions so long as hydro­ Reproducibility of data was very good. carbon solvent was employed. Among zinc Apparently the monomer consumption ceased halides, Zni2 and ZnC1 2 showed rather week after removing the solid part of the catalyst acidity than ZnBr 2 as a whole and were inferior from the mixture. Therefore, the solid zinc to control the molecular weight ofP(BdTMS). halide must work primarily. By using other solvent beside hydrocarbon, However it does not mean that none of zinc for example diethyl ether in which zinc halide halides dissolved in benzene at all. In fact, a can dissolve to make homogeneous solution, mixture of benzaldehyde and zinc halide but the circumstances will become quite different. without the monomer in benzene was stirred A kinetic study for the aldol-GTP of BdTMS under the same conditions as of the practical in such homogeneous solution is now in polymerization, and then insoluble parts were progress. removed by filtration through the identical galss filter. Amounts of zinc in each filtrate Active Site of Zinc Halide were determined by atomic absorption spec­ Although zinc halides were maintained in a troscopy. As a result, it was found that 2.8, solid state during this GTP process, they 6.0, and 0.4 (±0.05)mol% of fed ZnBr2 , might be partly soluble in benzene. If dissolved Znl2 , and ZnC1 2 , respectively. These values catalyst might play an important role, the were independent of the "practical" activity observed activity would be under the control discussed but might be rather depend on

676 Polym. J., Vol. 24, No. 7, 1992 GTP of 1-Butadienyloxytrimethylsilane covalent nature of halogen atoms. Anyway, REFERENCES these data insist that soluble zinc halide m hydrocarbon is negligible in consideration of I. M. Morton, "Anionic Polymerization: Principles and Practice," Academic, New York, N.Y., 1983. the catalytic activity. 2. A. Hirao, K. Takenaka, S. Packirisamy, K. Yamagauchi, and S. Nakahama, Makrornol. Chern., Acknowledgments. The authors thank Dr. 186, 1157 (1985). I. Kojima and Dr. T. Uchida of the division 3. D. Y. Sogah and 0. W. Webster, Macromolecules, of analytical chemistry in Department of 19, 1775 (1986); F. P. Boettcher, Makrornol. Chern., Makrornol. Syrnp., 13/14, 193 (1988). Applied Cheistry, Nagoya Institute of Tech­ 4. T. Hirabayashi, T. Ito, and K. Yokota, Polyrn. J., nology for their help on quantitative analysis 20, 1041 (1988). of zinc. This study is partly supported by a 5. T. Hirabayashi, T. Kawasaki, and K. Yokota, Polyrn. J., 22, 287 (1990). Grant-in-Aid for Scientific Research (No. 6. S. Danishefsky and T. Kitahara, J. Org. Chern., 40, 01550716) from the Ministry of Education, 538 (1975). Sceience, and Culture of Japan. 7. G. A. Olah, "Friedel-Crafts and Related Reactions," Vol. I, Interscience, New York, N.Y., 1963, p 860.

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