Makromol. Chem. 179, 1905-1911 (1978)

Advances in Etherification Reaction Mechanism of Poly(oxymethylene)diol[oc-Hydro~-hydroxypoly(oxymethylene)] with Triethyl Orthoformate

Paolo Colombo*, Sergio Custro, Mario Ermoni, and Pierino Radici

SOC. Italiana Resine S. I. R., Technopolymers Research Laboratory, Via Grazioli 33, Milano, Italy

(Date of receipt: November 21, 1977)

SUMMARY: The etherification of poly(oxymethylene)diol[a-hydrow-hydroxypoly(oxymethylene)] with orthoesters

in the presence of Lewis acids leads to the simultaneous formation of various types of end groups: orthoformyl [bis(alkoxy)methoxy] groups (A), formyl (oxyformyl) (B), and ether (alkoxy) groups (C). It was elucidated theoretically and experimentally that the orthoformyl end groups are intermediates of formyl end groups, and partially of ether end groups. The kinetics of the reactions was also evaluated at various temperatures.

Introduction

As reported in a previous paper'), during the etherification of poly(oxymethylene)diols[cr-hydro- o-hydroxypoly(oxymethylene)] with orthoesters in the presence of Lewis acids, the simultaneous formation of three types of end groups is observed : the bis(a1koxy)methoxy group (orthoformyl group) (A), the oxyformyl group (formyl group) (B), and the alkoxy group (ether group) (C)

H I

-0-C-OR I

OR A

- -0- C-H

0 II

B

-OR

C

The aim of this work is to study experimentally the formation of ether end groups either by direct transformation of the poly(oxymethy1ene)diol end groups to ether groups or by a complex mechanism with intermediate formation of orthoformyl end groups.

Discussion and Results

Some authors have examined the reactions of hydroxyl groups of compounds with or high molecular") weight with orthoesters in the presence of Lewis or protonic acids.

Nevertheless the complete mechanism and the competitive reactions have not been cleared neither theoretically nor experimentally. Also the influence of operating conditions on the various reactions has not been proved completely. The following proposed mechanism is close to the behavior of the reactive species and defines the intermediates.

The proposed scheme of the reaction between cc-hydrow-hydroxypoly(oxymethy1ene) and triethyl orthoformate is composed of various reactions. Carrying out the etherification at different temperatures, we have obtained interesting informations concerning the various steps.

1906 P. Colombo, S. Custro, M. Ermoni, and P. Radici

CH,OH + 1 -+ ...- CH,O-&OR + HA I

OR

B CH20CH,- ... + 1 --+ CH,O-$-OR + AQ + H280- ...

I

OR

r CH,O-C-OR + HA + ...-CH20:,C<H AQ + ROH

I .... OR OR

PR 8Hz A' + H-C-OR -+ CH,OR + 1

I OR

EH-CH20- ... + CH20CH,OCH,-..

R = CH2CH3 A' = conjugated base ofprotonic or Lewis acid

( 0

(ii)

(iii)

(iv)

B + CHZO- ... (vii)

AQ

At the beginning reactions at temperatures up to 60°C were studied. The results are reported in Tab. 1.

Tab. 1. with triethyl orthoformate (400g) catalyzed by sulfuric acid (1,O 9). Reaction time: 60 rnin

Results of the etherification of ac-hydrow-hydroxypoly(oxymethylene) (100g; [ q ] = 1,40dl. g-l)

~

Temp. Yield [TI/(dl' g- L, VK"' VDb' 100 - (VD + VKY' in "C in % in % in % in %

30 99,85 1,41 49 30 21 45 99,9 1,39 38,5 36,5 25 60 99,9 1,40 34,2 41,7 24,l

a) VK: Alkali-stable fraction determined in KOH/water/methanol at 80°C (hydroxyl and formyl end

b, VD: Fraction stable under basic conditions determined in N,N-dimethylacetamide at 160°C (ether

') 100 - (VD + VK): Fraction with orthoformyl end groups.

groups) (See Exptl. Part).

end groups) (See Exptl. Part).

One can observe that already at low temperatures ( Z 6 0 ° C ) , a great amount of hydroxyl end groups has reacted; ether and formyl groups are prevailing, yet orthoformyl end groups are formed. Their formation corresponds to Eq. (ii) rather than Eq. (iii), as resulting from the complete recovery and the unchanged molecular weight of the product.

Advances in Etherification Reaction Mechanism of Poly(oxymethylene)diol . . . 1907

According to the general reaction scheme previously described, the ether end groups are formed either by the direct way (Eq. (i)) or through the orthoformyl intermediate (Eqs. (ii), (iv), (v), and (vi)).

The cation ...- OCH; is an intermediate in the formation of ether end groups (eq. (vi)) and in chain scission of a-hydro-whydroxypoly(oxymethy1ene) (Eq. (vii)).

Therefore, the unchanged molecular weight of the product indicates that the preferred reaction at low temperatures is the direct formation of ether end groups according to Eq. (i). Etherification reactions at temperatures > 60°C were also examined. The data are shown in Tab. 2 and in Figs. 1 and 2.

Fig. 1 shows that during the etherification of a-hydroo-hydroxypoly(oxymethy1ene) the content of hydroxyl end groups decreases and that of ether end groups increases as a function

Tab. 2. with triethyl orthoformate (400 g) catalyzed by BF3 .(CZH5)20 (0,6 g). Reaction time: 60 min

Results of the etherification of 1-hydro-whydroxypoly(oxymethy1ene) (100g; [q] =2,0dl. g- ')

VK"' in %

VDb) in %

100-(VD +VK)" in %

30 1,99 60 1,98 80 1,87

110 1,60

150 1 3

a,b,c) Cf. Tab. 1

130 1,4

37,s 30,8 17,6

6,O 4.0

9,7

52,7 46,6 59,7 71,s 80 88.8

9,8 22,6 22,7 183 14 732

Fig. 1. Effect of temperature in the ether- ification of poly(oxymethylene)diol (POM) (100g; [q]=2,0dl.g-') with triethyl orthoformate (400g). Catalyst: BF3 . (C~H5)20 (0,8 8); reaction time: 60min. (0 ) : POM containing two ether end groups; (0 ) : POM containing one orthoformyl end group; (A): POM con- taining one hydroxyl or formyl end group

I f I I I

30 60 90 110 1.50 Temperature in 'C

P. Colombo, S. Custro, M. Ermoni, and P. Radici

Fig. 2. Decrease of intrinsic viscosi- ty [ q ] , of poly(oxymethy1ene)diol with increasing reaction temperature of the etherification with triethyl ortho-. formate

I I I I I

30 60 9D 120 1550 Temporatwe in 'C

of temperature. The content of orthoformyl end groups increases up to about 80°C, however, they decrease at temperatures >80°C. A similar behavior of the polymer molecular weight can be seen in Fig. 2.

By the chemical degradation method (VK) (See Exptl. Part) the presence of formyl end groups formed during the reaction cannot be demonstrated. This was done by IR spectroscopy. Etherification of a low molecular weight sample of a-hydro-o-hydroxypoly(oxymethy1ene) [(CHZ0)20] confirms the observations shown in Fig. 1. After reaction at T> 130°C hydroxyl end groups are absent (Tab. 3).

An absorption band at 5,78 microns (C+O in ester group) is present in the IR spectrum of the etherified samples. This confirms the formation of formyl end groups.

Tab. 3. Etherification of a-hydrow-hydroxypoly(oxymethy1ene) (paraformaldehyde ( C H Z O ) ~ ~ ) (1OOg) with triethyl orthoformate (900 g) catalyzed by sulfuric acid (9,O g). Reaction time: 60 min

Temp. in "C

Hydroxymethyl end groups") in %

Formyl end groupsb' in %

30 60 90

120 135 150

0,02 t 0,l 0,02 t 0,1 0,02 t 0,l 0,02 t 0.1 0,02 + 0.1 0,02 t 0,l

a) Hydroxymethyl end groups determined by IR spectroscopy (see Exptl. Part). Content in the initial

b' Formyl end groups determined by IR spectroscopy (see Exptl. Part). sample: 10,5%.

Advances in Etherification Reaction Mechanism of Poly(oxymethy1ene)diol.. . 1909

One can observe that formyl groups are formed also at low temperatures and they do not increase with temperature. During the etherification of polyoxymethylene with triethyl orthoformate in the presence of protonic or Lewis acids, the molecular weight of the sample decreases at temperatures >80°C, as shown in Fig. 2. Moreover the product is completely recovered after the reaction. This degradation can be explained by an acid attack inside the macromolecular chain (Eq. (viii)).

.- CH~O-CHZO-CH~O- ... + He - OCH: + HO-CH20-CH20-..

(viii)

However, the decrease of molecular weight occuring during the etherification of poly(oxymethy- 1ene)diol is mainly caused by the attack of the OCHY cation on oxymethylene links ((Eqs. (v) and (vii)).

In order to verify this statement, a sample of poly(oxymethy1ene) containing two ether end groups was again treated under the conditions of etherification at reaction temperatures =c 12O"C, whereby the ether end groups are not changed (Tab. 4).

Tab. 4. triethyl orthoformate (400 g) catalyzed by sulfuric acid (0,8 g). Reaction time: 30 min

Results of the reaction of a-ethyl-o-ethoxypoly(oxymethy1ene) (100g; [q] = 1,45 dl. g- ') with

Temp. in "C

Yield in %

80 100 120

99,6 99,6 99,8

1,45 1 9

0,020 0,021 0,019

a' Decomposition rate constant (at 220°C in nitrogen) of the reaction product in percent weight loss per min.

Carrying out this reaction under conditions which cause a decrease of molecular weight no change of molecular weight was observed. This shows that the chains break due to an electrophilic attack of the cationic intermediate ...- OCHY and that the latter is formed through an acid attack at the orthoformyl end groups (Eqs. (iv) and (v)).

During the etherification reaction, at temperatures > 8O"C, the increase of ether end groups is observed at the same time as the decrease of orthoformyl end groups, which may only be explained by the decomposition of the orthoformyl end groups with the formation of the cation . . .-OCHY (Eqs. (iv) and (v)).

The reactions of the cation ...- OCH? lead to an increase of the amount of polymer with ether end groups (Eqs. (vi) and (vii)). The effect of temperature in these reactions was examined; a-hydrow-hydroxypoly(oxymethy1ene) was reacted under the same conditions but at different temperatures.

The influence of temperature on the stability of ions is known. Tab. 5 shows that the rates of the two reactions (Eqs. (vi) and (vii)) are influenced by temperature in a different way. Since at low temperatures the preferred reaction is that corresponding to Eq. (vii), it is derived that the activation energy of the reaction of Eq. (vi) is higher.

1910 P. Colombo, S. Custro, M. Ermoni, and P. Radici

Tab. 5. The etherification of 2-hydro-w-hydroxypoly(oxymethylene) (1OOg; [q] = 230 dl . g- l ) with triethyl orthoformate (100g) in the presence of BF3. EtzO (1,Og) at different temperatures. Solvent: N,N-dimethylacetamide (900g); reaction time: 60min

Temp. in "C

Yield in %

VD"' in %

105 135

99,2 99,s

75 85

1,30 1,50

a) Cf. Tab. 1.

Conclusion

The formation of ether end groups during the reaction of poly(oxymethy1ene)diol with ortho- esters can occur either by a direct way or by formation of an intermediate containing orthoformyl [bis(alkoxy)methoxy] groups.

The transformation of the orthoformyl groups into ether end groups takes place via the forma- tion of a cationic intermediate, which is responsible of the decrease of molecular weight.

ExperimentaI Part

Materials: cr-Hydro-w-hydroxypoly(oxymethylene)s[poly(oxymethylene)diols] with [q] = 1,4 dl. g- ', [q]=1,45dl.g-', [q]=2,0dl.g-', and [q]=2,5dl.g-' were obtained by polymerization of pure gaseous formaldehyde in cyclohexane suspension using tributylamine as initiator.

Paraformaldehyde [(CH,O),,], a commercial grade product, was dried i. vac. at 80°C for 24 h. After standing at room temperature for 4 days over metallic sodium, triethyl orthoformate was

distilled twice and collected on anhydrous sodium carbonate. N,N-Dimethylacetamide (DMA) was purified by fractional distillation in the presence of benzene

to remove water azeotropically. Successively it was evaporated in an inert atmosphere. Boron trifluoride etherate was distilled and stored in an inert atmosphere in glass tubes.

Etherification reaction of poly(oxymethylene)diol: The reaction was carried out with stirring in a metallic vessel, provided with a thermometer, a reflux condenser, and a pressure regulating system under nitrogen. The vessel was jacketed with a circulating thermostated liquid.

The reactions, corresponding to Tabs. 1, 2, 4, and 5, were carried out under different conditions: quantity of reactants, temperature, duration of reaction, yields and values of [q] are indicated in the Tabs. The operating pressure was maintained constant at 1,5 abs. atm. (1,52 bar) with nitrogen. The system was never boiling.

The reaction mixture (500g) was filtered and the polymer was washed twice by stirring in S00cm3 of methanol and again filtered. The cake was washed with 500cm' of acetone and dried to constant weight in an oven i. vac. at 60°C.

Etherification reaction of paraformaldehyde: The reaction was carried out as described for poly(oxymethy- 1ene)diol. When the reaction was completed, solvent and reactant were removed by distillation i. vac. The solid was removed by filtration, washed with methanol and water, and dried.

Alkali treatment (VK). Determination of the part containing hydroxyl or formyl end groups: TO remove the alkali instable fraction, crude polyoxymethylene (1 0 g) containing hydroxyl or formyl end groups, was suspended at 80°C in water (40g)/methanol (60g) and KOH (7g) for 4h.

After reaction the mixture was cooled down to 30°C and filtered. The insoluble polymer containing orthoformyl and ether groups was washed with water (to pH 7) and dried i. vac. for 4 h.

Basic treatment (VD). Determination of the part containing ether end groups: To remove the thermally instable fraction, crude poly(oxymethy1ene) (10 g) containing hydroxyl, formyl and orthoformyl end

Advances in Etherification Reaction Mechanism of Poly(oxymethylene)diol . . . 1911

groups, was dissolved at 160°C in N,N-dimethylacetamide (lo0 g) and 2,2’,2”-nitrilotriethanol (triethanol- amine) (5 g) for 60 min. The solution was cooled to 90°C and filtered. The precipitated polymer containing ether end groups, was washed with toluene at 80°C and dried.

Measurements

Therrnalstability: k z z o , the percent weight loss per min during the first 30min at 220°C, was measured in a nitrogen atmosphere on a thermobalance.

Intrinsic viscosity: Intrinsic viscosity data were obtained in the usual way from the reduced and inherent viscosities at c-0. The determinations were carried out with a solution of poly(oxymethylene)diol in 4-chlorophenol at 60°C in the presence of 2% a-pinene.

IR analysis: IR spectra were obtained in KBr sintered disks with a Perkin-Elmer 577 spectrometer. The errors due to the preparation of the samples were eliminated calculating the values of absorbance of hydroxymethyl and oxyformyl groups referring to that of the same sample at the wave length of 4,32 pm (standard absorbance).

The authors are very grateful to Dr. G. Cancellieri, of the EUTECO SPA, for stimulating discussion of IR spectra. Thanks are due to S. I . R. Consorzio Industriale SpA for permission to publish this work.

I ) P. Colombo, P. Radici, S. Custro, M. Ermoni, Makromol. Chem. 178, 1 (1977)

’) B. Smith, Acta Chem. Scand. 10, 1006 (1956) 4, T. Okaya, T. Imada, K. Matsubayashi, Makromol. Chem. 133, 227 (1970)

J. P. Dusza, J. P. Joseph, S. Bernestein, Steroids 8, 495 (1966)