A new method of high polymerization of acetaldehyde

From the Department of Industrial Chemistry, Faculty of Engineering, Kyoto University, Kyoto, Japan

A New Method of High Polymerization of Acetaldehyde

By JUNJI FURUKAWA, TAKEO SAEGUSA, TEIJI TSURUTA, HIROYASU FUJII, AKIHIRO KAWASAKI, and TOSHIO TATANO

(Eingegangen am 26. Juni 1959)

SUMMARY : It has long been known that freezing of acetaldehyde was an indispensable process for

high polymerization of it. We have, however, found that acetaldehyde was converted into high polymer above its melting point by aluminium oxide as catalyst. Polyacetaldehyde thus obtained is white, nontacky and bighly elastic material, whose structure was shown to be methyl polyoxymethylene by infra-red spectrum. G O , and MOO, gave high polymers, but in much less yield than alumina. Other metal oxides such as B,O,, P,O,, MgO, Tho,, SiO,, CuO, TiO,, ZrO,, V,O,, ZnO, MnO,, Fe,O,, BaO, PtO,, PdO, Ni,O,, CaO and silica- alumina (SiO, 87 yo, Al,O, 13 %) were inactive. It was suggested that the adsorption of Metaldehyde on alumina played an influential role in this polymerization.

ZUSAMMENFASSUNG: Es war lange bekannt, da13 das Gefrieren des Acetaldehyds notwendig ist, wenn man ihn

zu hochmolekularen Produkten polymerisieren will. Wir haben nun gefunden, da13 Acet- aldehyd mittels Katalysatoren auch oberhalb seiner Gefriertemperatur in Polymere iiberfiihrt werden kann. Der hergestellte Polyacetaldehyd ist eine weiBe, nicht klebrige und hochelastische Masse, deren Struktur nach Infrarot-Untersuchungen die eines Methylpolyoxymethylens ist. CrO, und MOO, ergeben auch Hochpolymere, jedoch in kleinen Ausbeuten. Andere Metalloxyde wie B,O,, P,C),, MgO, Tho,, SiO,, CuO, TiO,, V,O,, ZnO, MnO,, Fe,O,, BaO, PtO,, PdO, Ni,O,, CaO und Kieselerde-Aluminium waren unwii-ksam. Es wird angenommen, da13 die Adsorption des Acetaldehyds bei der Poly- merisation eine bedeutende Rolle spielt.

I n 1936 i t was observed by TRAVERS~) and by LETORT~) that when the vapour of acetaldehyde was condensed on a surface a t a temperature below its freezing point (-123.3"C.) or when it was frozen, part of the aldehyde was polymerized. Since the first description of high polymerization, many s t ~ d i e s ~ - ~ ) have been performed on it, and i t has heen con-

l) M. S. TRAVERS, Trans. Faraday SOC. 32 (1936) 246. ,) M. LETORT, C. R. hebd. SBances Acad. Sci. 203 (1936) 767. 9 M. LETORT et al., C. R. hebd. SBances Acad. Sci. 216 (1943) 58, 608; 224 (1947) 50;

231 (1950) 519; 240 (1955) 86; 241 (1955) 651,1765; 242 (1956) 371; J. Chim. physique (France) 48 (1951) 594.

4, H. A. RIGBY, C. J. DANBY, and C. N. HINSHELWOOD, J. chem. SOC. [London] 1948,234. 5, J. C. BEVINGTON and R. G. W. NORRISH, Proc. Roy. SOC. [London] A 196 (1949) 363.

32


firmed tha t freezing of monomer was an indispensable process. This poli- merization was suggested to be an entirely new type of reaction, i.e., the site of polymerization was supposed to be the boundary hetween the solid crystalline and liquid phase of acetaldehyde.

It was found by us that acetaldehyde was converted into high polymer even above its melting point by aluminum oxide catalyst and this method of preparation of polyacetaldehyde was advanced.

Polyacetaldehyde thus obtained is white, nontacky and highly elastic material, whose structure was shown to be similar to tha t of the polymer obtained by i'reezing method6), i. e., it is methyl polyoxymethylene.

-CH-0-CH-0-CH-0- 1 I I

CH, CH, CH,

So far as we have studied, no polymerization was observed with other aldehydes such as formaldehyde, Fropionaldehyde, acrolein and croton- aldehyde.

Experimentcil Materials

Acetaldehyde. Acetaldehyde was prepared by the decomposition of paraldehyde by sulfuric acid. Under nitrogen atmosphere it was fractionated through a reflux condenser cooled with water, b.p. 20.6-20.8"C.

Alumina. Pure alumina was prepared by hydrolysis of aluminum isopropoxide followed by calcination a t higher temperature. Alumina thus obtained was pulverized in an agate-mortar prior to use. In several experiments commercial alumina granules (8-14 mesh) was used without purification, which was calcinated at 500--600°C. for 15 hours.

Other metal oxides. The purest available materials were chosen, which were pulverized and dried over phosphorous pentaoxide.

Petroleum ether. It was washed with concentrated sulfuric acid, with water, dried by sodium dispersion and distilled.

Triethyl amine. Commercial sample was dried with pottasium hydroxide and distilled.

Apparatus and Method A typical procedure is as follows. Acetaldehyde was placed in the tube A of the appa-

ratus shown in Figure 1, which was cooled by a dry ice-acetone bath, while the catalyst (alumina) was added to the tube B. After the system had been evacuated to 3-5 mm. Hg, the dry ice-acetone bath of A was removed and B was cooled with the bath. The monomer in A was allowed to warm to room temperature so that it was distilled into B through the capillary C a t the rate of about 1 mole of monomer per 6 hours. Sometimes monomer was distilled on to cold alumina at atmospheric pressure of nitrogen (SF-20-1, Table 1). After

@) G. B. B. M. SUTHERLAND, A. R. PHILPOTTS, and G. H. TWIGG, Nature [London] 157 (1946) 267.

33

J. FURUKAWA, T. SAEGUSA, T. TSURUTA, H. FUJII, A. KAWASAKI, and T. TATANO

100

% 60-

f ;:- 0 -

monomer had been distilled, the tube B was kept at low temperature in the dry ice-acetone bath for a definite time of polymerization. To obtain the polymer it was extracted with acetone containing 1 yo of P-naphthylamine (stabilizer) followed by precipitation by water from acetone solution, and by drying in vacuo at room temperature. The polymer was purified, if necessary, by resolution in acetone and by precipitation by carbon tetrachloride.

Fig. 1. Apparatus.

8 0 - 9

Viscosity measurement and molecular weight determination

The viscosity of the solution of polymer in methyl ethyl ketone was measured at 27.6 O C . using an OSTWALD viscometer. The intrinsic viscosity was determined by extrapolation method, from which the molecular weight was calculated using the following equation given by BOVEY and WANDS').

[ql = 3.36.10-4 ~ 0 . 6 6

') F. A. BOVEY, R. C. WANDS, J. Polymer Sci. 14 (1954) 113.

34


20.3 3.6 1.4 0.5 0.2 0.0

13.5 1.6 0.2 1.6 1.6

1.6

from the solution in carbon tetrachloride was examined over the range between 2 and 15 p, as given in Figure 2.

Results and Discussion

Polymerization by Alumina

The results of the polymerization of acetaldehyde by alumina are given in Table 1. Alumina was calcined a t a definite temperature for 15 hours. One fourth mole of acetaldehyde (15 ml.) was distilled onto 5 g. of alu-

-70

-70 -70 -70 -70 -70 -70

-70 -70 -70 -70

-15 to -10

-184 -184

Table 1. Polymerization of acetaldehyde by alumina (acetaldehyde 15 ml., alumina 5 9.)

Expt. No.

SF-20-4

SF-105-1 SF- 106-1 SF-26-1 SF-29-1 SF-31-1 SF-31-2 SF-20-2

SF-17-1 SF-22-1 SF-20-1') SF-19-2

SF-2 1-59 SF-21-3')

Origin&)

Al-iso- mropoxide

A A A A A A B

B B B B

B -

Alumina

:alcinat. temp. ("C.)

200 400 600 800

1000 1150

not cal- cinedb)

600 1000 600 600

600

Polymerization Vatere)

Time @.5.) _-

43

43 43 43 43 43 43

65 65 43 65 20

-1.59) 1.5h)

- Con- ver- sion (%) -

0

0 49 55 91 76 8

23 66 57 63f)

0 34 9

a) A: Prepared by hydrolysis of aluminum isopropoxide.

b) Commercial alumina granules were used without calcination. C) Weight loss when calcined a t 1150°C. d) Time elasped since monomer had been distilled onto alumina. 0 ) In methyl ethyl ketone a t 27.6OC.

B: Commercial alumina granules (8-14 mesh).

Polymer

7.07 7.17 5.40 6.65 0.03

3.88 2.40 4.30 4.07

4.35 5.60

DP

50,000 52,000 33,000 40,000 -

17,500 9,400

23,000 21,000

23,500 35,000

f ) Monomer was distilled to alumina under nitrogen a t atmospheric pressure. 9 ) Alumina was cooled by liquid nitrogen, to which acetaldehyde was distilled during

1.5 hours. All the monomer had been distilled, the reaction mixture was warmed to room temperature, and the polymer was isolated by the usual procedure.

h) Without alumina the vapour of aldehyde was condensed in a tube immersed in liquid nitrogen during 1.5 hours.

35

J-. FURUKAWA, T. SAEGUSA, T. TSURUTA, H. FUJII, A. KAWASAKI, and T. TATANO

mina during about 1.5 hours a t 3-5 mm. Hg. After all monomer had been distilled onto alumina, the reaction mixture was kept a t a definiet temperature for a given period.

I n Table 1 it will be seen tha t the degree of dehydration of alumina has a striking effect upon the catalytic activity, i.e., the conversion was small when the degree of dehydration was low, and aluminum isopropoxide (SF-20-4) and alumina calcined at 200 "C. (SF-105-1) were inactive. It is also to be noted that alumina calcined a t 1150°C. gave only low polymer in low yield.

Furthermore a t -15 to -10°C. no polymerization was observed (SF-19-2), while at -184°C. polymer having higher degree of polymerization was obtained in lower conversion than a t -70 "C. (SF-21-5). The last two experiments in Table 1 show that alumina is obviously active even a t a temperature far below the freezing point of acetaldehyde where so-called freezing polymerization takes place without any special catalyst. I n addition to Table 1 another point worthy to mention is that the polymerization was much influenced by the mode of adding the monomer t o the catalyst. Thus, when the liquid monomer instead of its vapour was added to alumina (case - 1) or when alumina granules were added gradu- ally to cold liquid monomer (case - 2), the conversion was very low; i.e., 5.3 yo (case - 1) or 9.6% (case - 2), respectively, whereas distillation method 60-70 yo.

Pure alumina prepared from aluminum isopropoxide seems more active than commercial granules. This difi'erence in activity may be ascribed to either the purity or the physical nature of the catalyst surface.

Polyacetaldehyde thus obtained is nontacky and highly elastic material. It is readily soluble in a number of solvents including acetone, diethyl ether, butyl acetate, carbon tetrachloride, ethyl alcohol and ben- zene. It is insoluble in water, petroleum ether and carbon disulfide.

The infra-red spectrum of one of these polymers (SF-17-1) is given in Figure 2. Although the complete spectrum cannot be interpreted, the main features provide strong evidence for the structure of methyl polyoxymethylene. Thus the methyl group is presumably responsible for strong band a t 7.25p, the C-0-C linkage for strong band at 9.61p, while the isolated methylene group may be connected with the band a t 7.46 p. On infra-red evidence, the alternative possible structure for the polymer of acetaldehyde, polyvinyl alcohol, can be ruled out at once, as there is no sign of the characteristic hydroxyl absorption near 3 p. Fur- ther the disappearance of the carbonyl group on polymerization is es- tablished by the absence of characteristic carbonyl absorption near 5.8 p.

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8 l 5

These observations also show that polyacetaldehyde obtained by alum.ina catalyst consists of the same structural unit as those synthesized by freezing method6).

Tacky semisolid

Table 2. Effects of basic substances on the polymerization of acetaldehyde by alumina (Distillation procedure, commercial alumina granules 5 g, acetaldehyde 15 ml. (0.25 mole)

at -7O"C., 43 hours)

Expt. No.

SF-100-2 SF-100-3 1 SF-101-1 SF-101-2 SF-101-3 SF-101-4

Basic substances added MoIe * 103

127.5

1.0 1 5.0

Conversion Remarks %

a) Containing considerable amount of metaldehyde.

The effects of basic substances are shown in Table 2, where alumina was treated beforehand in the following manner. I n the first series of experiments (SF-100-1 to -3) commercial alumina granules were immersed in methanol containing a given amount of potassium hydroxide, and then i t was dried in vacuo and calcined at 500-600°C. for 15 hours. I n the second series (SF-101-1 to -4) calcined alumina granules were immersed in petroleum ether solution of triethylamine, and volatile matter was removed by distillation in vacuo (150 "C. under 10 mm. Hg for 1.5 hours). Polymerization was carried out according t o the usual distillation procedure. It is evident tha t basic substance has an inhibiting effect upon the polymerization.

Polymerization by Other Metal Oxides

I n addition to alumina the following metal oxides were examined in catalytic activity; CrO,, MOO,, B,O,, P2OS, MgO, Tho,, SiO, (siIica gel), silica-alumina (fluid catalyst for cracking of petroleum, SiO, 87 yo, Al,O, 13%), CuO, TiO,, ZrO,, V,O,, ZnO, NnO,, Fe,O,, BaO, PtO,, PdO, Ni,O,, CaO.

Pulverized metal oxide dried over phosphorous pentaoxide was used instead of alumina. Of these oxides, CrO, and MOO, gave high polymers, but in much less conversion than alumina; B,O,, F,O,, MgO, Tho,.

37

J. FURUKAWA, T. SAEGUSA, T. TSURUTA, H. FUJII, A. KAWASAKI, and T. TATANO

Silica-alumina gave trace of tacky soft polymers having lower molecular weight; while others gave no polymer. When CrO, was used as catalyst, the reaction mixture happened to explode during after-treatment. This may be due t o peroxide formed by the oxidation of acetaldehyde. I n addition, active carbon and diethyl zinc were found t o be inactive. Thus it may be concluded that alumina is unique in its catalytic activity. in the polymerization of acetaldehyde.

Mechanism of Polymerization of Alumina

As is shown in Table 1, alumina samples calcined a t 200 "C. and 1150 "C. were both found t o be inactive in the polymerization of acetaldehyde. This result has an interesting bearing on the role of alumina. It was re- porteda) that alumina heated below 200°C. and above 1100°C. showed no measurable adsorption of nitrogen or of carbon tetrachloride, although samples prepared at the temperature between 300 "C. and 1000 "C. showed well defined adsorption isotherms.

%

t I I

5 6 7 a - 4

Fig. 3. Infra-red spectra of acetaldehyde. curve-1 : adsorbed on alumina

- - - - curve-2: liquid

8 , S. J. GREGG and K. S. W. SING, J. physic. Chem. 56 (1952) 388.

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These findings support the view that the adsorption of acetaldehyde on alumina plays an influencial role in this polymerization. Figure 3 shows the infra-red spectrum of acetaldehyde absorbed on alumina (curve - 1) and that of liquid acetaldehyde (curve - 2). Acetaldehyde was added to cold alumina powder, and the mixture was kept a t room temperature for about 30minutes, then the excess acetaldehyde was removed by evaporation. Curve - 1 is the differential absorption spectrum between the alumina thus treated with acetaldehyde and tha t untreated. It is clear tha t the carbonyl band of acetaldehyde which can be observed a t 5.8 p in liquid shifted to about 6.4 p when it was adsorbed on alumina. This phenomenon may be attributed to reduction of the double-bond character of carbonyl bond by polarization such tha t

0

l 0

CH, )C+-0- + Al-

H .O-

alumina

Such a shift of carbonyl bond was also observed in certain p-diketones by RASMUSSEN et al. 9). Both acetylacetone and dibenzoylmethane were known to exist largely in the mono-en01 form, but neither shows any carbonyl band corresponding t o a normal conjugated ketone. Instead a very strong band was observed in the range 6.1-6.5 p. These workers believed this absorption t o arise from a carhonyl group whose double- bond character was reduced by resonance between the following forms.

+ OH 0 OH.. . . O - I /I II I

RCOCH2COR G+ R-C=CH-C-R ++ R-C-CH=C-R

(R = CH, or C,H,)

The above view seems to be favourable for the interpretation that the shift of the carbonyl band of acetaldehyde absorbed on alumina is due to the polarisation of the carbonyl group.

The polymerization mechanism can be speculated as follows. When polarized monomer is attacked by ionic initiator, polymerization is in- duced and the chain of polymerization propagates by successive attack

O ) R. S. RASMUSSEN, D. D. TUNNICLIFF, and R. R. BRATTAIN, J. Amer. chem. SOC. 71 (1949) 1068.

39

J. FURUICAWA, T. SAEGUSA, T. TSURUTA, H. FUJII, A. KAWASAKI, and T. TATANO

‘7% ~

HO- C - O - - A + I

H

of the active center on the monomer adsorbed on a neighbouring po- sition.

The polymerization is supposed to be cationic by the following findings. 1. Basic substances inhibited this polymerization (Table 2). 2. Freezing polymerization was shown to be cationic typelo). 3. Alumina was shown to be active in the cationic polymerization of

ethylenell), and its acidity was much reduced when alumina was calcined a t 1030°C.

The polymerization reaction can be formulated as follows,

(termination) CH,

+ A I

PH I

CH3 CH3 I I I I

H H H+ + -O-C+ + HO-C+ (initation)

CH, y H3 CH, I 7%- HO-k+ + - 0 - C + + HO-C-0-C+ :- HO-

I I t H H H H

(propagation)

where A inplies an anion present in alumina. Generally acetaldehyde is known to cyclize t o trimer (paraldehyde) by

acidic catalyst. I n bigh polymerization catalyzed by alumina premature termination by ring closure may be prevented by masking of growing center with alumina. Solidification in freezing polymerization may play the similar role t o that effected by adsorption on alumina.

lo) M. LETORT and P. MATHIS, C. R. hebd. SBances Acad. Sci. 241 (1955) 1765. 11) T. SHIBA and E. ECHIGAYA, Bull. chem. SOC. Japan 76 (1955) 1046, 1144.

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A new method of high polymerization of acetaldehyde