The Transacetalization Reaction During the Etherification of Poly( oxymethy1ene)diol [ a-Hydro- o-Hydroxypoly(oxymethylene)] with Orthoesters in

the Presence of Lewis Acids

PAOLO COLOMBO, SERGIO CUSTRO, and PIERINO RADICI, SOC. Italiana Resine S.I.R., Technopolymers Research Laboratory, Via Grazioli

33, Milano, Italy

Synopsis

A transacetalization reaction occurs during the etherification of poly(oxymethy1ene)diol [a- hydro-a-hydroxypoly(oxymethylene)] with orthoesters and an important modification of molecular structure takes place. The intermediates formed during the transacetalization reaction are em- phasized. The connection between this reaction and the other reactions during the etherification of poly(oxymethy1ene)diols is discussed.

INTRODUCTION

It is known that during the etherification of poly(oxymethy1ene)diols with orthoesters in the presence of Lewis acids the molecular weight (M,) of polymers decreases.' Because there is complete recovery of the product, this phenomenon must be related to the acid attack of electrophilic agents inside the macromo- lecular chains.2

In addition to the decrease in molecular weight, there is an important molecular _ _ weight distribution rearrangement, which can be observed in the changed MJM, ratio and in the form of the molecular weight distribution curves.

The purpose of this article is to demonstrate that the change of the molecular weight distribution, which takes place during the etherification of poly(oxy- methy1ene)diols with orthoesters, is due to a chemical reaction (transacetaliza- tion). This reaction occurs in the chain cleavage operated by the poly(oxy- methylene) oxonium ions:

. . .-OCHf + . . .-OCH2-OCH2-OCH2_0CHz-. . . - . . .-OCH2-OCH2-OCHf' + . . .-OCH2-OCH2-OCH2-. . . (1)

EXPERIMENTAL

Materials

a-Hydro-w-hydroxypoly(oxymethy1ene) [poly(oxymethylene)diols] with [q] = 0.30 dl g-l, [q] = 1.50 dl g-l, [q] = 1.56 dl g-l, [q] = 1.98 dl g-l, and [q] = 2.30 dl 8-l were obtained by polymerization of pure gaseous formaldehyde in cyclo- hexane suspension with tributylamine as initiator.

N,N-Dimethylacetamide (DMA) was purified by fractional distillation in the

Journal of Polymer Science: 0 1980 John Wiley & Sons, Inc.

Polymer Chemistry Edition, Vol. 18,681-689 (1980) 0360-6376/80/0018-0681$01.00

682 COLOMBO, CUSTRO, AND RADICI

TABLE I Comparison of the Molecular Characteristics of POM-Diacetates and POM-Diethylethers

Z W - (GPC) Mn

Sample 1 POM-diol POM-diacetate POM-diethylether

1.98 2.02 1.60

- 4.0 2.8

Sample 2 POM-diol 1.50 - POM-diacetate 1.51 3.55 POM-diethylether 1.36 2.40

presence of benzene to remove water azeotropically. It was then evaporated in an inert atmosphere.

The orthoester (TEOF) was purified by standing at room temperature for 4 days over metallic sodium; it was distilled twice and collected on anhydrous so- dium carbonate.

Acetic anhydride (ANAC) was refluxed over calcium hydride and later distilled in uacuo.

Natrium acetate was a commercial pure-grade product.

Etherification Reaction of Poly(oxymethy1ene)diol The reaction was carried out with stirring in a metallic vessel equipped with

a thermometer, a reflux condenser, and pressure-regulating system under ni- trogen. The vessel was jacketed with a circulating thermostated liquid. The reactions, listed in Tables I, 11, 111, and IV, were carried out under different conditions: quantity of reactants, temperature, duration of reaction, yields, and values of [q] . The operating pressure was maintained constant at 1.5 abs atm with nitrogen. The system was never boiling. The reaction mixtures were fil- tered and the polymers were washed twice by stirring in methanol and again filtered. The cake was washed with acetone and dried to constant weight in an oven in uacuo at 60OC. The amounts of methanol and acetone were five times that of the dry polymer.

TABLE I1 Results of the Etherification of a-Hydro-wHydroxypoly(oxymethy1ene) (100 g; [7] = 1.65 dl g-l)

with Triethvlorthoformate (500 a) in the Presence of EtzSOa a t Different Concentrationsa

Test Catalyst hl - =W

No. (g) (dl g-l) M , x 10-3 b M , x 10-3 c Mn

1 1.5 1.31 126.0 42.0 2.95 2 2.5 1.18 102.8 43.0 2.45

~ ~ ~~ ~~~~~

Reaction times: 30 min; reaction temperature: 140OC. Weight-average molecular weight from GPC data. Number-average molecular weight from GPC data.

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684 COLOMBO, CUSTRO, AND RADICI

TABLE IV Results of Etherification of a-Hydro-w-Hydroxypoly(oxymethy1ene) (50 g; [q] = 1.56 dl g-l) with

Triethylorthoformate (500 g) in the Presence of HzS04 (0.2 g) at Various Temperaturesa

Temperature ("C)

60 80

110

1.55 1.40 1.30

4.32 3.01 2.75

* Reaction time: 40 min. From GPC data.

Esterification Reaction of Poly(oxymethy1ene)diol

The reaction was carried out in the same apparatus utilized for the etherifi- cation reaction. The reactions corresponding to Table I and the acetylation of poly(oxymethy1ene)diol of Table IV were carried out under the following con- ditions: quantity of polymer = 100 g; quantity of anhydride = 1500 g; quantity of catalyst (sodium acetate) = 0.75 g; temperature = 150OC; duration of reaction = 30 min; yields = 95%. Values of [v] are indicated in the table and text.

The operating pressure was kept constant a t 2.0 abs atm with nitrogen. The reaction mixture was filtered and the polymer was washed by stirring in acetone (10 parts per part of polymer) and again filtered. After two washes by stirring in water (each with 10 parts per part of polymer) and filtration the cake was washed with acetone and dried to a constant weight in an oven in uacuo at 6OOC.

Measurements

Intrinsic Viscosity

Intrinsic viscosity data were obtained in the usual way from the reduced and inherent viscosities a t C - 0. The determinations were carried out with a so- lution of the polymer in 4-chlorophenol a t 6OoC in the presence of 2% a-pi- nene.

G PC Analysis

A Waters Associates GPC model C-200 was used for the determination of the molecular weight distribution. The polymer was dissolved in N,N-dimethyl- acetamide (DMA) at 15OOC under nitrogen in the presence of antioxidants. The dissolution was complete in 10 min and the solution was filtered before injection. Concentration of polymer in the GPC analysis was 0.3% and the running tem- perature was 140OC. A set of four columns with 105-A Styragel was used.

RESULTS AND DISCUSSION

The hydroxyl end groups of a-hydro-whydroxypoly(oxymethy1ene) (POM) can be chemically and thermally stabilized by acetylation with acetic anhydride.

ETHERIFICATION OF POLY (0XYMETHYLENE)DIOL 685

- €/ution counh

Fig. 1. Molecular weight distribution curve (differential) of poly(oxymethy1ene)diaceht.e (POM). Sample prepared from poly(oxymethy1ene)diol (50 g; [q] = 1.50 dl g-l) and acetic anhydride (500 g) in the presence of sodium acetate (0.5 9). Reaction time: 30 min; reaction temperature: 150OC.

During the acetylation reaction there is a certain loss of product due to the de- polymerization reaction that starts from the end groups of poly(oxymethy- 1ene)diols. The depolymerization does not take place after main chain scission caused by electrophilic attack inside the chains. Effectively the molecular weight does not ~ h a n g e . ~

Therefore the molecular weight and the molecular weight distribution of poly(oxymethy1ene)diacetates are close to those of the initial samples of the poly(oxymethy1ene)diols.

Table I compares the molecular characteristics of poly(oxymethy1ene)diace- tates and poly(oxymethy1ene)diethylethers prepared from the same poly(oxy- methy1ene)diols.

Figures 1 and 2 report the molecular weight distribution curves (differential) of the POM-diacetate and POM-diethylether prepared from POM-diol sample 2.

a ‘0, e c

* 8 # 3 2 3 0 a ? l e W - &/&on ewnk

Fig. 2. Molecular weight distribution curve (differential) of poly(oxymethy1ene)diethylether (POM). Sample prepared from poly(oxymethy1ene)diol (50 g; [s] = 1.50 dl g-l) and triethylor- thoformate (TEOF) (50 g) in the presence of BF3Et.20 (1 8). Solvent, N,N-dimethylacetamide (450 9); reaction time: 30 min; reaction temperature: 150°C.

686 COLOMBO, CUSTRO, AND RADICI

38 36 34 32 30 26 z8 a - E/uhon counh

Fig. 3. Molecular weight distribution curve (differential) of sample 1 in Table 11.

The results listed in Table I and plotted in Figures 1 and 2 show that after the etherification reaction with orthoesters a change in the molecular weight and molecular weight distribution of POM takes place.

These results may be explained by reference to the statistical scission of poly(oxymethy1ene) chains operated by electrophilic agents of the reaction media with a decrease in Mw and a variation of polydispersity ratio MwIM,.

Nevertheless this hypothesis is not probable, as shown by the following con- siderations. In statistical scission long chains are preferred; therefore, an increase in the number of chains would occur, particularly in those with low molecular weight.

Figures 1 and 2 indicate that the statistical scission of the chain<is not the sole reaction. The following data will clarify this statement.

The very small change in M,, which corresponds to the large decrease in Mw (Table I1 and Figs. 3 and 4) indicates the presence of an important reaction.

In the reaction media there is only one electrophilic agent that is allowed to change the molecular structure of the POM. This agent is the oxymethylene cation . . .-OCHf which is formed by acid decomposition of orthoformyl end

_ _

b

3# 36 w 32 JO 28 26 24 - rFlu/)bn counh

Fig. 4. Molecular weight distribution curve (differential) of sample 2 in Table 11.

ETHERIFICATION OF POLY (0XYMETHYLENE)DIOL 687

groups in the following reactions

H I I

8 ...-m H,-s-C-OR - ...- WH? + H-COOR (3)

Therefore the reaction responsible for the molecular rearrangement, previously described, is the “transacetalization reaction.”

The cation. . .-OCH% attacks the macromolecular chains on the oxymeth- ylene bridge according to the mechanism in eq. (1). This type of reaction has already been described for the cationic copolymerization of trioxane with cyclic ethers5; however, in that case the transacetalization takes place during poly- merization; in the present case it occurs during the stabilization phase.

To point out the transacetalization reaction, two samples of the same weight but with different molecular weight of poly(oxymethy1ene)diol were mixed and etherified. The results are reported in Table 111.

The molecular distribution curves of the etherified mixture and the curves of the single-etherified polymers are shown in Figure 5. It can be seen clearly that the molecular structure of the initial polymers is changed by the transace- talization reaction.

In the scheme proposed in eq. (1) the oxymethylene cation. . .-OCHF is the electrophilic agent of the transacetalization reaction which is formed by acid decomposition of orthoformyl end groups [eqs. (2) and (3)]. This fact suggests a close connection between the transacetalization reaction and the number of orthoformyl end groups formed during the etherification of POM with or- thoesters. In the third test the polymers were mixed after separate etherification. The orthoformyl end group content was much lower.

Fig. 5. Comparison of (differential) molecular weight distribution curves of POM-diethylethers. (A) POM-diol (Y etherified; (B) POM-diol fl etherified; (C) mixture of (A) and (B) reetherified; (D) mixture POM-diol (Y and POM-diol fl etherified.

688 COLOMBO, CUSTRO, AND RADICI

In the fourth test the polymers were mixed in the diol form; therefore they were together during the transformation of the orthoformyl end groups to oxymeth- ylene ions and during the next transacetalization reaction due to them. In the third test the transacetalization reaction was reduced because of the low amount of oxymethylene ions [Fig. 5(C)].

In both tests a reaction took place among the chains of the two different polymers:

. .kOCHf' + . . .-'OCH2-0CH2--. . .b 4.. .a-OCH2-OCH2--. . .b + "CH2-. . .'

. . .b-OCH? + . . .a-OCHyOCH2--. . .a + . . .b-OCHpOCH2-. . .' + "CHz-. . .' (4)

(5)

This reaction was much more evident in test 4 than in test 3. In an earlier article4 the complete mechanism of competitive reactions during

the POM etherification with orthoesters in the presence of protonic or Lewis acids was demonstrated. This was made possible by using selective analytical tests to determine the number of end groups. The behavior of the reactions corre- lation to the temperature was also studied.

The proposed scheme of reactions and particularly the influence of temper- ature was again confirmed by comparing the molecular weight distribution curves obtained with the GPC technique.

The molecular weight distribution data of POM etherified at various tem- peratures are given in Table IV.

The poly(oxymethy1ene)diol used for the tests in Table IV was also acetylated. The polydispersity ratio of the latter is 4.20 from GPC data. The change in the form of the molecular weight distribution curve of POM after etherification with triethylorthoformate (TEOF) in the presence of acid catalysts is therefore due to the transacetalization reaction operated by the oxymethylene cation . . .-OCHf'. The data in Table IV and the polydispersity value of the acetylated polymer indicate that at temperatures lower than 60°C the macrocation is not present and the transacetalization reaction cannot take place.

A t temperatures below 60°C, in the absence of oxymethylene ions, ether end groups are formed only in the direct way [eq. (6))

OR I

...- W H @ H + H-k-OR A@ - ...- -OCH20R + HCOOR + HA (6) Q

A t temperatures of more than 60°C the ether end groups are formed according to eqs. (7), (€9, and (9).

H Q A8

I I I

...- -OCH,O-C-OR + HA - ...- WH,-O-C-OR + ROH (7)

H OR

ETHERIFICATION OF POLY (0XYMETHYLENE)DIOL 689

@ --HCOOR ...- -OCH,-O--C-OR ... - -OCHp Ae I H

OR I I

... - -oCHp Ae + H 4 - O R ...- CH,OR + H-C-OR + RA (9) II 0 OR

This mechanism anticipates the intermediate formation of the ma~rocation.~

CONCLUSION

During the etherification reaction of POM with orthoesters there is the for- mation of poly(oxymethy1ene) cations which can attack an oxygen atom'in a poly(oxymethy1ene) chain that involves a process of cleavagqand the formation of new bonds.

The experimental data discussed here indicate an intermolecular mechanism. The intramolecular reaction with the formation of macrocycles cannot be

excluded, however. Until now their formation has not been enhanced. The transacetalization explains the decrease in weight-average molecular weight and the best structural uniformity of the final polymers.

The authors are grateful to Mr. Olcese and Mr. Delvino of LAR-SESTO S.G. for their stimulating discussions of GPC chromatographs. Thanks are due to S.I.R. Consorzio Industriale SPA for per- mission to publish this work.

References

1. T. Okaya, T. Imada, and R. Matsubayshi, Makromol. Chem., 133,227 (1970). 2. P. Colombo, P. Radici, S. Custro, and M. Ermoni, Makromol. Chern., 178,l (1977). 3. J. Schweitzer, R. N. MacDonald, and J. 0. Punderson, J. Appl. Polym. Sci., 1,158 (1959). 4. P. Colombo, S. Custro, M. Ermoni, and P. Radici, Makrornol. Chern., in press. 5. K. Weissermel, E. Fischer, K. Gutweiler, H. D. Hermann, and H. Cherdron, Angew. Chem. Znt.

Ed. Eng., 6(6), 526 (1967).

Received September 12,1978 Accepted November 7,1978