Preparation of crystalline polyaldehydes

From the Department of Synthetic Chemistry, Kyoto University, Japan

Preparation of Crystalline Polyaldehydes * By JUNJI FURUKAWA, TAKEO SAEGUSA, and HIROYASU FUJII

(Eingegangen am 14. November 1960)

SUMMARY: We found that some organometallic compounds and metal alkoxides polymerized

aldehydes including acetaldehyde, propionaldehyde and trichloroacetaldehyde (anhydrous chloral) to give crystalline polyaldehydes. Crystalline polyacetaldehyde was much less soluble in organic solvents than the amorphous one.

Active species of this polymerization was supposed to be metal alkoxided and the mechanism of polymerization was considered in connection with several synthetic organic reactions which involve metal alkoxides and carbonyl compounds. It was assumed that the coordination of aldehyde to the metal alkoxide was essential in propagation reaction which determined the stereospecificity of the polymer produced.

ZUSAMMENFASSUNG: Aldehyde, niimlich Acetaldehyd, Propionaldehyd und Trichloracetaldehyd (wasser-

freies Chloral), polymerisieren unter dem EinfluR einiger metallorganischer Verbindungen und Metallalkoxyde und geben kristalline Polyaldehyde. Kristalline Polyacetaldehyde sind in organischen Losungsmitteln weniger loslich als amorphe.

Es wird angenommen, daB Metalloxyde die aktiven Anteile bei dieser Polymerisation sind. Der Polymerisationsmechanismus wurde im Hinblick auf einige organische, synthe- tische Reaktionen betrachtet, die mit Metallalkoxyden und Carbonylverbindungen ver- kniipft sind. Es wird angenommen, daR ein Koordinationskompfex des Aldehyds mit dem Metallakoxyd bei der Wachstumsreaktion entscheidend ist und die Stereospezifitat des Polymerisats bestimmt.

Introduction

High polymerization of acetaldehyde was reported for the first time in 1936 by LETORT~) and by TRAVERS~), in which freezing of the monomer was an indispensable process. We have already found another new meth- od3), in which y-alumina was used as the catalyst and the polymerization was executed without freezing of the monomer. Polyacetaldehyde prepared by these two methods was an elastic material having a polyacetal structure (methyl polyoxymethylene). The configuration of the carbon

*) This paper presented before the Division of Rubber Chemistry at the New York Meet-

l) M. LETORT, C. R. hebd. SBances Acad. Sci. 202 (1936) 767. 2 , M. S. TRAVERS, Trans, Faraday SOC. 32 (1936) 246. a) J. FURUKAWA, T. SAEGUSA, T. TSURUTA, H. FUJII, A. KAWASAKI, and T. TATANO;

ing of the h e r . Chem. SOC., September 14-16, 1960.

J. Polymer Sci. 36 (1959) 546; Makromolekulare Chem. 33 (1959) 32.

398

Preparation of Crystalline Polyaldehydes

of the main chain was assumed t o be irregular. Therefore, it had been expected, that there could exist stereoregular (crystalline) polyacetaldehyde besides irregular (amorphous) ones.

Recently NATTA et al.*) and we5) succeeded, independently, in the preparation of crystalline polyacetaldehyde by using some organometallic compounds, i.e., diethylzinc or triethylaluminum.

Organometallic compounds have also polymerized other aldehydes such as propionaldehyde and trichloroacetaldehyde (anhydrous chloral) to give crystalline polymers.

The present paper is the detailed description of the stereoregular polymerization of aldehyde.

In general, the polymerization must be carried out at lower temperatures, i.e., the temperature of Dry Ice, otherwise, a side reaction prevails over the polymerization. The polyacetaldehyde, thus obtained, was shown to be crystalline by X-ray diffraction5). From infrared spectrum the crystalline polyacetaldehyde was found to have the same chemical structure as that of thk amorphous one, but its absorption was much sharper and intenser5). The crystalline polyacetaldehyde differs maIkedly from the amorphous one in solubility. The amorphous polymer is easily soluble in common organic solvents including alcohol, ester, ketone, aromatic hydrocarbons, and chlorinated hydrocarbons, while the crystalline one is only partly soluble in chloroform. Because the molecular weights of both polymers do not differ very much, the difference in solubility may be attributed to the regularity in the arrangement of the polymer chain.

As is illustrated by the GRIGNARD reaction, an organometallic compound is known to add carbonyl unsaturation to give the corresponding metal alkoxide. Accordingly, it may be suggested, that the active species in the polymerization by metal alkyl is the corresponding metal alkoxide. In fact, we found that some metal alkoxides were active catalysts for the stereospecific polymerization of aldehydes.

Experimental Materials a) Acetaldehyde

Acetaldehyde was prepared by the decomposition of paraldehyde which had been treated with sodium carbonate and rectified. Under nitrogen, it was fractionated through a column cooled by water, b.p. 20.6-20.8 "C.

G. NATTA, G. MAZZANTI, P. CORRADINI, and I. W. BASSI, Makromolekulare Chem. 37 (1960) 156.

5, J. FURUKAWA, T. SAEGUSA, H. FUJII, A. KAWASAKI, H. IMAI, and Y. FUJII, Makro- molekulare Chem. 37 (1960) 149.

399

J. FURUKAWA, T. SAEGUSA, and H. FUJII

b) Propionaldehyde

distilled, b.p. 48.549.0”C.

c ) Trichloroacetaldehyde

sulfate and distilled, b.p. 97.0-98.0 “C.

d) Solvents

e) Organometallic compounds All the organometallic compounds were prepared and used under dry nitrogen. Butyl-

lithium was prepared by grinding the mixture of the calculated amounts of metallic lithium and butyl chloride in hexane with use of several glass beads in a beverage bottle a t room temperature. The upper clear solution was used As the catalyst solution. Diethylzinc was prepared from ethyl halogenide and zinc powder according to “Organic Syntheses”6) ; b.p. 112-115 “C. Diethylcadmium was synthesized by the reaction of ethyImagnesium bromide and cadmium halide7); b.p. 61-62 “C./20 mm. Hg. Ethylmagnesium bromide was prepared in diethyl ether by usual GRIGNARD method under nitrogen, whose ether solution was used in polymerization. Tributylboron was prepared according to the method given by BROWN^); b.p. 90 “C./9 mm. Hg.

Tetraethyltin was prepared by a GRIGNARD methods), b.p. 96”C./30 mm. Hg. Commer- cial triethylaluminum and lithium aluminum hydride were used without purification.

f) Metal alkoxides

Commercial sample (extra pure grade) was dried over anhydrous sodium sulfate and

Commercial anhydrous chloral (extra pure grade) was dried over anhydrous sodium

Solvents were dried and purified by ordinary methods.

Aluminum isopropoxide, ethyl orthosilicate and ethyl orthotitanate were commercial samples, which were purified by distillation. Other metal alkoxides were prepared by various methods. Alkoxides of sodium, magnesium and calcium were prepared by the reaction of the metal and excess alcohol, followed by distillation of the free alcohol in vacuo. Lithium ethoxide and zinc ethoxide were prepared by the reaction of the corresponding metal alkyl and ethanol. Silver ethoxide was prepared by the reaction of silver nitrate and sodium ethoxidelO). Alkoxides of tinll), antimonyl’), chromium (111)12), manganese(II)lS), and iron (III)l*) were prepared from ‘the corresponding metal halide and sodium ethoxide. Vanadyl triethoxide, VO(OC,H,), was made from vanadium pentoxide and ethanol 15).

Method The procedure is illustrated by the following example, in which acetaldehyde was poly-

merized by diethylzinc. The same apparatus as that used in the polymerization by alu-

6, C. R. NOLLER, Org. Syntheses, Coll. Vol. 2, 184. ’) E. KRAUSE, Ber. dtsch. chem. Ges. 50 (1917) 1813. 8, H. C. BROWN, J. h e r . chem. SOC. 67 (1945) 374. 9, G. E. COATES, “Organometallic Compounds”, Methuen, London 1956, p. 119.

lo) W. L. GERMAN and T. W. BRANDON, J. chem. SOC. [London] 1942, 526. 11) H. MEERWEIN and R. SCHMIDT, Liebigs Ann. Chem. 444 (1925) 221. 12) P. A. THIESSEN and B. KANDELAKY, Z. anorg. Chem. 181 (1929) 285. lS) B. KANDELAKY, I. SETASCHFYILI, and I. TAWBERIDZE, Kolloid-Z. 73,(1935) 47. 14) P. A. THIESSEN and 0. KOERNER, Z. anorg. Chem. 191 (1930) 74. 15) W. PRANDTL and L. HESS, Z. anorg. Chem. 82 (1913) 103.

400


mina3) was employed, which is shown in Fig. 1. In the tube A, 15 ml. of acetaldehyde (0.25 mole) was placed, and 1.3 ml. of diethylzinc (0.0125 mole) was dissolved in 30 ml. of n-hexane in the tube B. The two tubes A and B were cooled in a Dry Ice/acetone bath and the system was evacuated to 3-5 mm. Hg. Then the Dry Ice/acetone bath of A was removed in order to allow the monomer to be distilled into B through the capillary C. The distillation rate was about a sixth mole of monomer per hour. After the monomer had been

1

I'

I 2 3

4

i- 1: Fig. 1. Polymerization apparatus

added to the catalyst solution, the mixture was kept at -78°C. Resinous polymer was deposited on the wall of the tube B as polymerization proceeded. After 20 hrs., 50 ml. of methanol were added to the reaction mixture. From the methanol extract, a small quanti- t y of elastomeric polymer (Fraction-I) was obtained. The methanol insoluble residue was then extracted with chloroform. On evaporation of chloroform, 3.62 g. of white resinous polymer (Fraction-11) was obtained, whose intrinsic viscosity in chloroform at 25.0 "C. was 0.92 (dl./g.). The insoluble residue in chloroform was 2.33 g., which was insoluble in common organic solvents. If zinc hydroxide derived from the catalyst was included in the insoluble residue, the amount of organic substance (Fraction-111) was 2.33 g. - 1.25 g. = 1.08 g. The total yield of the polymer (Fraction-I1 and Fraction-111) was 43%. The procedure of polymerization by metal alkoxide was almost the same as above. In the case of an insoluble metal alkoxide, it was dispersed in hexane, to which the monomer was added gradually. Polymer so produced was treated similarly.

Polymerizations of propionaldehyde and of chloral were carried out similarly. Poly- chloral, insoluble in the most organic solvents, was purified by washing with water, alcohol and acetone succeedingly.

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Results and Discussion

The results of the polymerizations of aldehydes by metall-organic compounds are shown in Table I.

Table I. Yield of polymerization of aldehydes by metal-organic compounds (Monomer, 0.25 mole; metal-org. comp., 0.0125 mole; hexane, 30 ml.; at -78°C.

for 20 hrs.)

45 0

34. - 25 0

26 32 0

2 1 0

5 - - - 4 27

0 1 1 5

0 43 o - - -

0 45 0 12

- -

0 0 0

0

I = Methanol soluble. I1 = Chloroform soluble.

25 25 45 45 23 23

0 0 3 3

0

14 14 0 9 9

- -

- -

LiBu ...... ZnEt, ..... CdEt, BBu, ...... AlEt, ......

.....

SnEt, ...... EtMgBr .... ZnEt,+H,O LiAlH, ....

5 O 1 0 0 33 ~ 12

38 11 14

3 0 1 23 5 6 i 21 0 0 1 2

- - _

43

45 12

I11 - Insoluble.

0 - 0

0 -

0 0

‘By X-ray diffraction the methanol soluble fraction (Fraction-I) was found to be amorphous, while the chloroform soluble fraction (Fraction- 11) and the insoluble fraction (Fraction-111) were crystalline5). As is seen in Table I, diethylzinc, triethylaluminum, ethylmagnesium bromide (GRIGNARD Reagent) and the binary mixture of diethylzinc and water are excellent catalysts. The mixture of diethylzinc and water was prepared by the addition of equimolar amount of water to diethylzinc in hexane, whose chemical structure had been shown by us 16) to be CzH,ZnO(ZnO)nZnC,H,, CzH,OZnO(ZnO)nZnCzH, or CzH,OZnO(ZnO)nZnOCzH,.

It is noteworthy, that propionaldehyde and chloral were also polymerized by these catalysts, although the freezing and adsorption methods (alumina catalyst) are effective only for the polymerization of acetaldehyde.

Polychloral prepared by diethylzinc was insoluble in common organic solvents, whose infrared spectrum was found t o be almost the same as that of “metachloral” *) given by NOVAK et al. 17). The polychloral of this

la) R. SAHATA, T. TSURUTA, T. SAEGUSA, and J. FURUKAWA, Makromolekulare Chem. 40 (1960) 64.

17) A. NOVAK and E. WHALLEY, Trans. Faraday SOC. 55 (1959) 1490. *) NOVAK et al.I7) established that “metachloral“ had the structure

They proposed to call it “polychloral’’.

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study had great resistance to hot pyridine although "metachloral" was reported to decompose easily in even cold pyridine. This difference may be ascribed to the difference of the degree of polymerization.

Table I1 shows the effect of polymerization solvent. It is to be noted that a nonpolar solvent such as hexane gave the best result and polar solvents such as ether and acetone afforded poor results.

Table I11 illustrates the effect of a metal halide on the polymerization by diethylzinc. It may be deduced that a metal halide generally reduces the degree of polymerization, the stereoregularity and the yield.

Table IV shows the polymerization by metal alkoxides. Alkoxides of alkali metals were inactive in this polymerization, while alkoxides of group-I1 and group-111 elements were active catalysts.

2 17 8

Table 11. Polymerization of acetaldehyde"). Effect of solvent (Acetaldehyde, 0.25 mole; diethylzinc, 0.0125 mole; solvent, 200 ml.; at-78 "C. for 20 hrs.)

6 4 0

Solvent Yield of polymer (yo)

~ I 1 I1 I I11 1 Total

Toluene ................ 6 Hexane ................. 29

. . . . . . . . . . . . 6 7 ................ 0

Diethyl ether Acetone

a) Cold monomer was added dropwise to the catalyst solution a t -78 "C. I, 11, 111: see Table 1.

Table 111. Polymerization of acetaldehyde. Effect of metal halide") (Acetaldehyde, 0.25 mole; diethylzinc, 0.0125 mole; metal halide, 0.005 equivalent;

solvent 30 ml.; a t -78°C. for 20 hrs.)

1 I

I 1 Metal halide Solvent

Yield of polymer (yo) I I1 I I11

None ............. FeCl, ............ AlCl, ............. SnCl, .............. TiCl, ............. FeCl, .............

BF,(C,H,),O ...... AICl, .............

~~ ~~

Hexane Hexane Hexane Hexane Hexane

Diethyl ether Diethyl ether Diethyl ether

0 10

7 4 6 3 4

11

Total

45 19 17

6 10 11 25 19

a) Monomer was added as vapor to cold catalyst solution and was condensed; I, 11,111: see Table 1.

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0

1.7 1.7 4.8

14 0

Table IV. Yield of polymerization af acetaldehyde-bymtd al- (Monomer, 0.25 mole: metal alkoxide, 0.0125 mole: hexane, 20 ml.; at-78 “C. for 20 hrs.)

0

3.0 0 5.0

25 0

Metal alkoxide

0 0 0 0 0 0 0

LiOEt . . NaOEt . . AgOEt .

Ca(OEt), Zn(0Et ), Al(OPr), Si(OEt),

MdOEt), O 1 ; 0

0 22 2.3 17.5

I = Methanol soluble.

Metal

Ti(OEt), . . . . Sn(OEt), ... VO(OEt), . . .

Fe(OEt), .. .

I1 = Chloroformsoluble. I11 = Insoluble.

b) rota1

10.9 15.6 0 0 0 0 .

22 19.8

-

It is interesting to note that tin tetraethoxide gives a polymer while tetraalkyltin gives no polymer. This may be due to the fact that tetraalkyltin is reluctant to react with an aldehyde to give tin alkoxide although tin alkoxide itself can induce polymerization.

Another peculiarity of using metal alkoxide can be seen in the cases of alkoxides of titanium and iron. Organometallic compounds of these metals are usually unstable and only special organometallic compounds can be prepared. Alkoxides of these metals, however, can be prepared easily, which were found to induce the polymerization successfully.

The mechanism of the polymerization may be considered as follows. AS mentioned before, it is well known that a metal alkyl (R-M) reacts with an aldehyde to give a metal alkoxide of the corresponding secondary alcohol.

RM + R’CHO -+ RR’CHOM (M = Metal)

Therefore, it may be assumed that the metal alkoxide is also the active species in the polymerization by metal alkyl. There are many synthetic reactions which involve metal alkoxide and carbonyl compound. The relation between the polymerization reaction and these synthetic reactions may be written as shown in Fig. 2.

The complex (I) is well known as “MEERWEIN Complex”, in which the oxygen of the carbonyl group coordinates t o the metal alkoxide 18, l’).

The “MEERWEIN Complex” resembles alumina 3, adsorbed by acetaldehyde in which the carbonyl group of acetaldehyde coordinates t o aluminum

18) H. MEERWEIN, Liebigs Ann. Chem. 4.55 (1927) 227. Is) E. PFEIL, Chem. Ber. 84 (1951) 229.

404


CH,\ /CH, R M + ,."=O + M-OCH

1 \R

P /R MEERWEIN-PONNDORF O=C;_CH,

/ M' Reduction "0 = C k H , /

\ OPPENAUER - Oxidation H

(1)

1 /R

OCH M \ I \CH3

0-C-CH, I

k (111)

1 /R \

,OCH

O-C-CH, CH3 TISCHTSCHENKO / \

'0= CLCH3 \

, Reaction ,.H

H

/ M...

(IV)

-1 /R

OCH \ I CH3

M, 0-CH-CH, \

0-CH-CH,

(VI) 1 /R

OC H

, I \"H< 0-- c H-CH,

1 'P Chain Transfer

/R

I \CH,

'o-cH,-CH,

OCH

,O = C-CH, M'

(V)

0 C d R

, I \CH< 0-CH-CH3

I /P O=C-CH,

(VIII)

'O=C'lCH, 4 \

H M-OCH2CH, (VII)

Fig. 2. The relation of the polymerization and some synthetic organic reactions

405


of the surface of alumina. In the case of alumina, the coordination was confirmed by infrared spectrum, e.g., the absorption of the carbonyl band was found to shift when the aldehyde was adsorbed on alumina3). I n the complex (I) hydride ion transfer from alkoxy group to the carbon atom of carbonyl group produces the complex (11), and this reaction is the “MEERWEIN-PONNDORF Reduction”. The reverse reaction is the “OPPENAUER Oxidation”. In the complex (I), the alkoxy anion can also transfer to the carbonyl group to produce a new metal alkoxide (111), to which another molecule of the aldehyde coordinates to produce another MEERWEIN complex (IV). The hydride ion transfer reaction in the complex (IV) corresponds to so-called “TISCHTSCHENKO Reaction”, which gives the ester (V) and the metal alkoxidel9). On the other hand, the transfer of the alcoholate anion in complex (IV) produces another metal alkoxide of higher order (VI). By reiteration of two consecutive processes, e.g., the coordination of aldehyde and the transfer of alcoholate anion, the polymerization reaction continues t o propagate until the hydride ion transfer happens in a “MEERWEIN Complex” of much higher order (VII). The hydride ion transfer reaction gives a metal alkoxide and a polymer molecule, whose end is an ester group. As the metal alkoxide thus produced will initiate another polymerization, this reaction may be regarded as chain transfer.

The reaction temperature of MEERWEIN-PONNDORF reduction is the highest, that of TISCHTSCHENKO reaction is the middle and that of polymerization is the lowest. Therefore, it may be reasonable to assume that the activation energy of the hydride transfer is higher than that of the alcoholate transfer.

The mechanism of stereoregularity may be imaginated as follows. Metal alkoxides, such as aluminum alkoxide, are known to aggregate to form tetramerzo’ W, in which the central aluminum is six-coordinated and the remaining three aluminums are four-coordinated zz). At lower temperatures the degree of aggregation of aluminum alkoxide is supposed to be larger, where the aluminum of surrounding alkoxides are four-coordinated. Regular coordination of monomer on aluminums of the surrounding alk-

20) R. C. MEEROTRA, J. Indian chem. SOC. 31 (1954) 85. 21) R. A. ROBINSON and D. A. PEAK, J. physic. Chem. 39 (1935) 1125. 221 D. C. BRADLY, 131st Nat. Meeting of h e r . Chem. SOC., Miami, Florida, April 1957.

(Symposium on Metal-Organic Compounds).

406


oxides which provide general steric hindrance seems to play an influencial role in stereoregular polymerization. Stereospecific MEERWEIN-PONNDORF reduction~=-~6) may be taken to illustrate the regular coordination of carbonyl compound to aluminum alkoxide.

23) W. YON E. DOERING and R. W. YOUNG, J. Amer. chem. SOC. 72 (1950) 631. 24) L. M. JACKMAN, J. A. MILLS, and J. S. SHANNON, J. Amer. chem. SOC. 72 (1950) 4814. 25) L. M. JACKMAN, J. A. MILLS, and A. K. MACBETH, J. chem. SOC. [London] 1949, 2641.

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Preparation of crystalline polyaldehydes