The mechanism of anionic polymerization of dimethylketene

Die Makrornolekulare Chernie 100 (1967) 175-185 (ATr. 2282)

From the Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Nagoya, Japan

The Mechanism of Anionic Polymerization of Dimethylketene

By KENJI YOSHIDA and YUYA YAMASAITA

(Eingegangen am 28. Februar 1966)

SUMMARY: In the reaction of dirnethylketene with GRIGNARD reagent of ether polyketonic type

oligomers were formed, but the non-distillable fraction was of polyester type. Therefore it is suggested that the propagation reaction to polyketone proceeds slowly, but those to polyester is a fast one.

From the results of fractionations of polymers prepared under various conditions, the polymerization mechanism is explained by three kinls of growing chain ends which are in dynamic equilibrium.

ZUSAMMENFASSUNG: Bei der Reaktion van Dimethylketen mit GRIGNARD-Reagenz in dther werden Oligo-

mere mit Polyketonstruktur gebildet. Der nicht destillierbare Anteil besitzt Polyester- struktur. Daraus wird geschlossen, da13 die Polyketonbildung langsam ablauft, wahrend die Polyesterbildung eine schnelle Reaktion ist.

Aus Fraktionierungsergebnissen an Polymeren, die unter verschiedenen Bedingungen hergestellt wurden, wird abgeleitet, daB drei Arten von wachsenden Ketten irn dynamischen Gleichgewicht fiir den Polymerisationsrnechanismus bestimrnend sind.

Introduction

It has been reportedl-7) that dimethylketene can polymerize anion- ically and yields three types of polymers, i.e., polyester, polyacetal and polyketone, and the composition of these structures depends on the polymerization condition. The variation of composition was explained by the ambident nature of polydimethylketene anion, and tentative polymerization mechanism was proposed. But there was little direct experimental evidence. In this report, we will describe studies on the elementary reaction of dimethylketene and on the block character of polydimethylketene.

Results Elementary reaction

Generally, if polymerization reaction is successive, its mechanism may be estimated by the reaction of monomer with catalyst. Hznce, we at-

175

K. YOSHIDA and Y. YAMASHITA

tempted to analyze the reaction product of dimethylketene with GRIGNARD reagent.

In to the solution of isopropyl magnesium chloride in ether, the solution of dimethylketene in ether was added between -30 and -20°C. under nitrogen atmosphere. After decomposition of the reaction mixture cold aq. HC1, products were analyzed by gas chromatography. One by analytical example of these reactions is shown in Fig. 1. From this figure a t least nine components were clearly observed, named A, B, . . . a , I.

I I I I I I I I I I I I I I 4 I I I I I I I I I O I I

0 1 2 3 5 I0 min 15 20 25

Fig. 1. Analysis of reaction products of dimethylketene with isopropy 1 magnesium chloride by gas chromatography: analytical conditions; column, Silicone D.C H.V.G (20 wt.-%) 2m; carrier gas, helium 43 ml./min.; temperature, 185OC. (at arrowed time, chart speed

and sensitivity were changed)

I n this figure, chart speed and sensitivity were changed halfway, such tha t area ratios were directly comparable. I n this case, mole ratio of dimethylketene to GRIGNARD reagent was ca. 2.6, and decreasing this ratio, the content of components with higher boiling points (with long retention time) and distillation residue were decreased. Each product was isolated by gas chromatography, and identified mainly by IR and NMR spectra.

Compounds A, B and C were identified to be diisopropyl ketone, diisopropyl carbinol and l-hydro~y-2,2,4-trimethylpentanone-3~~~), respectively, comparing I R spectra with those of authentic samples synthesized through other routes.

Compounds D and I were identified to be 2,4,4,6-tetramethylheptadione-3,5 and 2,4,4,6,6,8-hexamethylnonatrione-3,5,7, respectively, from

176

Anionic Polymerization of Dimethylketene

IR and NMR spectra. It is noted that NMR spectra of these compounds are identical except the area intensity, and IR spectra are analogous in whole region.

Compound E may be (CH3)2C=CHOCOC(CH3)2COCH(CH3)2.

Compounds F, G and H could not be identified, but it may be presumed from IR and NMR spectra that these are reduced (cyclic) compounds of dimer or trimer of dimethylketene having carbonyl group in respective molecule. Other minor products were not examined. But the compound

CH3 C(CH3)2 CH3 \ / I /

CH-C-0-C-CH II \ 0 CH,

/ CH3

which may be formed by the 0-acylation of the anion

C(CH3)2 II

(CH3),CH-C-O'

with dimethylketenes) was not produced. Principal data of these spectra are summarized in Table 1.

The distillahle fraction of reaction products (< 100°C./l mm. Hg) could be assigned as before, but sizable fractions having higher boiling points ( >lOO°C./l mm. Hg) were also formed at the same time. This oily residue was fractionated with acetone-water system. Early separated fraction by the addition of water to the acetone solution of the residue gave almost the same IR spectrum of polydimethylketene with polyester structure. The IR spectrum of later separated one was also analogous to those of polyester except peak broadening and appearance of absorption in the 3500 cm-l region. This absorption attributed to OH stretching vibration may be caused hy the addition of GRIGNARD reagent to the carbonyl group in the chain with certain length.

In some cases, small quantities of solid polymer were produced as by- product, being rich in polyester units and containing a small quantity of polyketone units.

Employing isopropyl magnesium bromide instead of chloride the reaction products were almost identical in both, distillable fraction and residue. So there is negligible effect by the kind of halogen in GRIGNARD reagent on the reaction.

Analogous reaction between dimethylketene and diisopropyl magnesium (mole ratio 1.8:l) was performed, comparing with the reaction of

177

- -

A

R

C

D

E

F

G

H

I


Table 1. Identification of products A-I with IR and NMR spectra ~ ~ _ _ _ _ ~ _ _ _ _

Compound

CH3 c_H3 a\ / a

CH-CH-CH / I \

CH3 OH C_H3 a a

c_H3 CH3 a\ I b

a\ I b / a c~-c-c-c-cgc / c l l I I1 \ c_H3 0 C H 3 O c_H3

a b a

c e b a

a b b a

IR (cm-’)&)

VC,~; 1715

polymeric OH -3440

polymeric OH -3440

VC,~; 1700

1702 vc-0; 1722

1721 vc-0; 1760

vc-0; 1718 1749

vc-0; 1717 1758

vc-0; 1715 1742

V C , ~ ; 1689 1715

NMR T(ppm)b)

a 8.91 doublet b 7.24

a 9.14 doublet.

a 8.97 doublet b 8.86, d 6.56 c 6.97

a 8.92 doublet b 8.61 c 7.15

a 8.94 doublet b 8.63 d 7.17 c, c‘ 8.32, 8.33 e 3.10

a 8.86 doublet b 8.64 c 6.95

8) neat ;I,”) CCI, solution, tetramethylsilane as internal standard.

178


dimethylketene with GRIGNARD reagent. In this reaction a solid polymer was formed rapidly. The polymer consisted of polyester 92 % and of polyketone 8 %, only trace of a liquid component was produced. Hence, the presence of halide (in the case of GRIGNARD reagent) may retard the propagation to polymer.

As was reportedg-ll), dimethylketene can copolymerize with carbonyl compounds and yields alternating copolymer. So i t is interesting to study the reaction between dimethylketene, carbonyl compound and GRIGNARD reagent. Using benzaldehyde and isopropyl magnesium chloride as car- bony1 compound and GRIGNARD reagent, respectively, this reaction was performed, mole ratio of dimethylketene t o benzaldehyde t o GRIGNARD reagent being 1:2:1 or 2 : l : l . As soon as the solution of dimethylketene- benzaldehyde mixture in ether was added into the GRIGNARD solution in ether, solid polymer was formed immediately in both cases. And this polymer was confirmed to be alternating copolymer of dimethylketene and benzaldehyde with polyester structure (examined by I R spectrum). Liquid component was analyzed by gas chromatography. I n both cases the main product was phenyl isopropyl carbinol, and as minor products, diisopropyl ketone, diisopropyl carbinol, and 2,4,4,6-tetramethylheptadione-3,5 were formed. Unreacted benzaldehyde was also detected. A product with longer retention time than phenyl isopropyl carbinol was not found. So in this case the alternating propagation t o polymer was very rapid like in the reaction of dimethylketene with diisopropyl magnesium.

Fractionation of polydimethylketene

The polymerization conditions of samples used are listed in table 1. I n many cases, polymers were fractionated continuously by the same solvent series, i.e., ether, acetone, benzene, and toluene, employing SOXHLET’S extractor for 20 hrs. The composition of each fraction was measured by the KBr disk method, using the bands at 1746 cm-I for polyester, at 1710 cm-1 for polyacetal, and at 1670 em-I for polyketone as key.

As dimethylketene can be polymerized in dimethylformamide in the absence of other catalytic agent, the polymer obtained under these conditions (P-8) was referenced for the P-7 sample prepared with sodium naphthalene in dimethylformamide. As shown in Table 2, conversion of P-8 was very low, therefore polymerization with dimethylformamide alone had negligible effect on P-8. In Table 3, the compositions of the whole polymers and fractionated polymers are listed.

179


i-PrMgC1 n- BuLi d)

NaNaphb) NaNaphb) NaNaphb)

n-BuLie) -

Table 2. Samples used")

0.28 Ether 0.5 Ether 0.2 THF 0.2 Toluene 0.9 DMF - DMF

0.5 Toluene

Sample No.

P-3 P-4 P-5 P-6 P-7 P-8 2-34

18.5 13.0 7.5 7.5 8.0 5.0 -

20 20 20 20 20 10 30

Time (hrs.)

20 20 17 17

20 182

0.5

Conv. (%)

3.6 45 22 57 22

trace 34

8) polymerization temperature: -78OC. b) NaNaph: sodium naphthalene; C) THF: tetrahydrofuran; DMF: dimethylformamide; d, prepared in ether; e) prepared in toluene.

From the I R measurements o i fractionated polymers, the following is noticed:

a ) Insoluble fraction in boiling benzene or toluene (P-3-TR or 2-34-BR) of the polymer having relatively high polyketonic content (P-3 or 2-34) contains more polyketonic structure.

b) In some cases, ether extracts contain more polyacetalic structure than original polymer. Especially, in the case of P-4, the fraction containing 53 yo of polyacetalic structure was obtained from polymer containing 8 % of polyacetalic structure. This fraction did not show the absorption of isolated polyester groups8p4).

c) Polymers being rich in polyacetalic structure always showed the isolated polyester band a t 1773 cm-l except P-4-E. So the analytical data contain more ambiguity than the experimental error for these polymers. As the ratio between the optical densities a t 1710 cm-l and 1773 cm-l was assumed as a criterion to evaluate the purity of the polyacetalic structure in high-molecular-weight products4), the ratio of D,,,, to D,,,, was foot-noted for these polymers. Though polymer P-7 showed both absorptions of polyester block and isolated polyester bands a t 1746 and 1773 cm-l, respectively, the absorption a t I746 cm-l was absent in ether-extractable fraction (P-7-E). The insoluble fraction (P-7-EA an3 P-7-AB) did not show the absorptions of the isolated polyester band.

d ) Though compositions of P-4, P-5 and P-6 are almost identical, polyacetal content in ether extract of P-4 (P-4-E) is very much larger than those of P-5-E or P-6-E, and in the last two cases each fraction has almost the same composition.

Discussion

In the reaction of dimethylketene with GRIGNARD reagent, it was found that the polyketonic type oligomers, i.e., diisopropyl ketone, 2,4,4,6- tetramethylheptadione-3,5, and 2,4,4,6,6,8-hexamethylnonatrione-3,5,7 (peak A, D, and I in Fig. 1, respectively) formed by successive C-acylation of growing anion had been produced. The formation of these compounds suggests that the propagation to polyketone is successive, and may be

180


Table 3. Composition of fractionated polymers

Sample No.

Insoluble fract. in boiling solv.

P-3 P-3-A P-3-AB P-3-BT P-3-TR

P-4 P-4-E P-4-EA P-4-AB P-4-BR


P-6 P-6-E P-6-EA P-6-AB Pd-BR


p-a 2-34

2-34-A 2-34-AB 2-34-BR

7 25 11 7

15

a 53 6 2 -

5

1 5

10 -

11 17 16

2 -

61 . C)

- Acetone Benzene Toluene

- Ether Acetone Benzene




- Acetone Benzene

28 16 14 52 73

12 7

13 11 -

a

a 10

14 -

4 7

10 2 -

3 0

Soluble fract. in boiling solv.

Acetone Benzene Toluene

-

Ether Acetone Benzene

-


-


-

Ether Acetme Benzene

-

Acetone Benzene

-

88 9 68 28 - -

100b,d) 39 13 58 6 19 8 5 11

:wt.-%)

3 4 -

0 48 36 73 a4

100 16 61 6

17

100 9

17 74 0

100 59 17 24 0

100 25 5

70 0

100 88

2 10

0

100 100 69 11 20

Polymer compos. (%)a)

P E

65 59 75 41 12

a0

a1 a7

40

-

87 85 91 76 -

a5 76 74 96 -

36 b, 10

a) PE: polyester, PA: polyacetal, PK: polyketone; b) isolated polyester band is observed a t 1773 cm-l; C) PE, PA, D1,,,/Dl,,, 2.28, absorption a t 1740 cm-l is not present; d) PE, PA, Dl,l,/Dl,,, 2.36.

explained by the proposed mechanism'), in which ion pair chelate complex is formed between growing end and monomer in the transition state, because acylation of the enolate anion derived from isobutyrophenone

181

K. YOSIIIDA and Y. YAMASHITA

or diisopropyl ketone with dimethylketene yields exclusively 0-acylated product in the case of potassium as a counter ion in dimethoxyethaneQ.

The fraction which was not distillable or not detectable by gas chromatography, and polymer which was formed occasionally as by-product were oligomer or polymer with prevailing polyester structure (50-70 % of total product when dimethylketene/GRIGNARD reagent = 2.6). Oligomers which would be formed by 0-, successive 0-, C- and C-, 0-acylation of enolate anion derived from dimethylketene and GRIGNARD reagent, such as:

CH, CH3 C(CH3)2 CH, \ I II /

and CH-C-C-C-0-C-CH / I l l II \

CH, 0 CH, 0 CH,

were not polyester

formed. Hence, i t may be concluded that the propagations to and polyketone occur almost independently, and the formation

of polyester proceeds very'fast in this system. This was supported by the observation that a fraction rich in polyketone was separated from polymer prepared with isopropyl magnesium chloride in ether as boiling toluene insoluble part (P-3-TR).

may be deduced from the same tendency of fractionation results. In n-BuLiltoluene system, the same conclusion as in the above system

Boiling ether extractable fraction of the polymer obtained with n-BuLi in ether (P-4-E), although being very little part in the whole polymer, consists of polyester (40 %) and polyacetal (53 %). This fraction did not show the isolated polyester band, so it seemed to have a polyacetalic chain of considerable length. The propagation t o polyacetal in this system may be selective 0-acylation of growing end (perhaps solvated free ion) with dimethylketene, and the propagation rate is more rapid than the exchange rate of polyacetal growing end t o those of polyester or polyketone.

182


On the other hand, boiling ether extractable fraction of the polymer obtained with sodium naphthalene (P-7-E, 88 % of whole polymer) showed isolated polyester band, and no absorption at 1746 cm-l attributed to polyester block was observed. Thus, in this system propagation proceeds through random C- and 0-acylation of the free ion-like growing ends with dimethylketene, but the ratio 0-/C-acylation is very large as a result of high polarizability of carbonyl groups in polar medium and differ- ence of electronegativity between carbon and oxygen atoms.

Boiling ether insoluble fraction of this polymer (P-7-EA and P-?-AB), being minor part, was rich in polyester and did not show the isolated polyester band. Therefore, it is assumed that propagation to polyester is very rapid when applying such a system which does not allow to form a chelate structure easily.

From the above consideration of results, following scheme is proposed as a tentative polymerization mechanism, considering the two state mechanism of COLEMAN and

EEn, EAn and EKn are growing ends of polyester, polyacetal and polyketone, respectively, and there are equilibria between those ends, adding monomer becoming E E ~ + ~ , AAn+l and E K n + i with rates of KE, kA and kK. Positions of equilibria and kinetic constants are dependent on both solvent and catalyst employed.

In i-PrMgC1 or i-PrMgBr/Et20 or n-BuLi/toluene system, EA is practi- cally absent, and exchange reaction between EE and EK is very slow, then polydimethylketene having high block character is produced. As kE > kK (from the results of elementary reaction) and contents of polyester and polyketone were not so different, the ratio [EK]/[EE] may be considerably larger than one.

In n-BuLi/Et20 or NaNaph/THF or toluene system propagation proceeds exclusively through EE with some exchange reactions between EE

183


and EK, since insoluble fraction in boiling benzene must be present if long polyketonic sequence was formed. But in the first system above mentioned long polyacetalic sequence was formed, so exchange reaction between EA and EE is slower than the propagation to polyacetal, and in the last two systems propagation to polyacetal and exchange reaction between EA and EE occur with comparable rate.

In NaNaph/DMF system propagation proceeds exclusively through EA to produce mainly polyacetal, but a small quantity of polyester is formed with rate faster than exchange reaction through traces of EE.

Experimental Elementary reaction

Materials : Ether predried by sodium metal was completely dried over LiAIH,, and distilled immeliately before use. GRIGNARD reagents and diisopropyl magnesium were prepared by usual methods in ether. Dirnethylketene was prepared by pyrolysis of the dimer tetramethyl-l,3-~yclobutanedione~~), and distilled over CaH, in a vacuum system. As very pure dimethylketene prepared a t lower pyrolysis temperature are so reactive as to polymerize in a dry ice trap, it was necessary to prepare rather impure monomer a t higher pyrolysis temperature4).

Reac t ion : The solution of dimethylketene in ether cooled to -78OC. was added into the GRIGNARD solution with stirring a t -20 to -3OOC. under nitrogen atmosphere. Exo- thermic reaction occurred violently. Mter stirring the mixture was recooled a t -20 to -3OOC. for 30 min., then raised to OOC., and decomposed by the addition of cold aq. HCI. Pro- ducts were extracted with ether. Ether layer was washed, dried with anhydrous sodium sulfate, and solvent evaporated. An aliquote of this condensed extract was analysed by gas chromatography. The products were distilled and separated to several fractions.

The products were analyzed and isolated by gas chromatography, employing a Hitachi model KGL-2 B operated with a 2 m. column packed with Silicone D.C.H.V.G. (20 wt.-yo) using helium as carrier gas a t 15O-19O0C.

Measurements of I R a n d NMR spec t ra : The I R and NMR spectra were measured using JASCO model IR-S an.l Japan Electron Optics JNM C-60 type (60 Mc.) spectro- meters, respectively. The I R measurements were performed for neat state, and for the NMR measurement ca. 5 yo solution in carbon tetrachloride and tetramethylsilane as internal standard were used.

Analys is a n d i so la t ion of p r o d u c t s b y g a s c h r o m a t o g r a p h y :

Polymerization and fractionation Catalysts and sohents were prepared or purified by the usual methods. Into a graduated

polymerization tube, monomer was placed by distillation under vacuum, then solvent and catalyst were added which after shaking stood overnight a t dry ice temperature. The polymerization was stopped by the addition of methanol. The precipitated polymer was further washed with N/20 HC1-methanol solution and a little amount of water and dried under vacuum. SOXFILET extraction was performed to fractionate the polymer, employing about 1 g of polymer and solvent series demonstrated in Table 3.

184


Determination of polymer structure The IR spectra were used to measure the polymer structure employing the KBr disk

method. The absorptions a t 1746 cm-l for polyester, a t 1710 cm-I for polyacetal and a t 1670 cm-1 for polyketone respectively were used as key hands. As there is some band overlap between these three bands, true optical densities of respective absorptions were calculated according to Eq. (2). Eq. (2) was derived from Eq. ( l ) , which was obtained graphically:

or

DPE* 1 0.10 0 [ DPA* ] = [ 0.16 1 0.11 DPK* 0 0.05 1

1.02 -0.10 0.01 -0.16 1.02 -0.11

DPK 0.01 -0.05 1.00

[#,I [ ;;

Here, DYE*, D ~ A * and DPK* are observed optical densities a t 1746, 1710 and 1670 cm-I respectively, and DPE, DPA and DPK are true optical densities arising from polyester, polyacetal and polyketone a t respective wave numbers.

1) G. NATTA, G. MAZZANTI, G. F. PREGAGLIA, M. BINAGHI, and M. PERALDO, J. Amer.

2, G. NATTA, G. MAZZANTI, G. F. PREGAGLIA, and M. BINAGHI, Makromolekulare Chem.

3) G. NATTA, G. MAZZANTI, G. F. PREGAGLIA, M. BINAGHI, and M. GRANSBINI, Makro-

4, G. F. PREGAGLIA, M. BINAGHI, and M. CAMBINI, Makromolekulare Chem. 67 (1963) 10. 5, Y. YAMASHITA and S. NUNOMOTO, J. chem. SOC. Japan, ind. Chem. Sect. [K8gy6 Kagaku

a) Y. YAMASHITA and S. N~NOMOTO, Makromolekulare Chem. 58 (1962) 244. ') Y. YAMASHITA, S. MIURA, and M. NAKAMURA, Makromolekulare Chem. 68 (1963) 31. *) K. YOSHIDA and Y. YAMASHITA, Tetrahedron Letters [London] 1966, 693. 9, G. NATTA, G. MAZZANTI, and G. F. PREGAGLIA, J. Amer. ckem. SOC. 82 (1960) 5511.

lo) G. NATTA, G. MAZZANTI, G. F. PREGAGLIA, and G. POZZI, J. Polymer Sci. 58 (1962) 1201. 11) R. G. J. MILLER, E. NIELD, and A. TURNER-JONES, Chem. and Ind. 1962,181. 12) B. D. COLEMAN and T. G. FOX, J. Polymer Sci. C 4 (1963) 345. 13) W. E. HANFORD and J. C. SAUER, Organic Reactions, Vol. 3, p. 108.

chem. SOC. 82 (1960) 4742.

44-46 (1961) 537.

molekulare Chem. 51 (1962) 148.

Zasshi] 66 (1963) 467.

185

Documents

The mechanism of anionic polymerization of dimethylketene