Copolymerization of chloral with trioxane

European Polymer Journal, Vol. 13, pp. 815 to 818. Pergamon Press 1977. Printed in Great Britain.

COPOLYMERIZATION OF CHLORAL WITH TRIOXANE ARNE HOLMSTROMand ERLING M. SORVIK The Polymer Group, Department of Organic Chemistry, Chalmers Universit); of Technology and University of Gothenburg, FacL S-402 20 G6teborg, Sweden

(Received 15 February 1977) Abstract--Copolymers of trichloroacetaldehyde (chloral) and 1,3,5-trloxane have been prepared in solvent-free systems with aluminium bromide as an initiator at 4 and - 1 5 °. CH2C12 inhibited polymerization. From i.r.-spectra and chlorine determinations, the copolymers were found to have degrees of polymerization of about 40 and to contain 4-t7 mole ~ oxymethylene units. X-ray diffraction studies indicated a completely amorphous material and no melting was observed with differential scanning calorimetry. The latter method indicated.a decomposition temperature of 300 °, compared with 180 and 220 ° for polyoxymethylene and polychloral respectively. The copolymers were stable towards chemical treatments deleterious to the corresponding homopolymers viz. 10~ aq. KOH at 25 ° and concentrated H2SO4 at 130°. The stability towards alkaline solutions shows that the haloform reaction with polychloral proceeds via depolymerization and not via direct attack on the polymer chain.

INTRODUCI'ION Trichloroacetaldehyde (chloral) has been considered a technically interesting monomer for a rather long time since its high chlorine content should result in materials with high fire resistance. However, the very rigid chains in polychloral make it intractable and impossible to process. Furthermore, although it is extremely stable against strong acids, it is easily degraded by bases. One possible way to overcome these drawbacks would be the incorporation of oxymethylene units in the polychloral chain. Apart from improving the flexibility, the chemical stability would be improved as polyoxymethylene ( P O M ) is fairly stable towards bases (but deteriorates in acid environments). Copolymerization between ultra-pure formaldehyde and chloral has been claimed in patents [1, 2]; the polymerizations were carried out at low temperature in hydrocarbon diluents using phosphines or amines as initiators. Thinius et al. [3] made a thorough investigation on similar polymerization systems. No products containing chloral were Obtained with the above-mentioned initiators. Metal alkyis on the other hand gave polymers containing minor amounts of chloral units. The thermal stability was not improved and Thinius was doubtful whether indeed copolymers, and not a mixture of homopolymers, were formed. 1,3,5-trioxane (trioxane) is a convenient starting material for P O M . Little has been reported on copolymerization of trioxane and chloral. Rosen et al. [4] tried cationic copolymerization of trioxane with various cyclic compounds including parachloral and 2,6-bis(trichloromethyl)- 1,3,5,7-tetra oxacyclooctane. Whenever polymer was formed, it was P O M . On the other hand, claims in patents indicate that minor or moderate amounts of chloral might have been introduced in trioxane polymers using B F 3 " O E t 2 [5] and ethylpolyphosphate [6] as catalysts. To the best of our knowledge, there are no publications about copolymers which can be regarded as

polychioral modified by minor amounts of oxymethylene units. In connection with an investigation on haloaldehydes, we found that such copolymers could be prepared in solvent-free systems with aluminium bromide as initiator. EXPERIMENTAL

Materials Chloral and chloral hydrate (puriss) were obtained from Fluka AG, Switzerland. Before use chloral was dried over CaCI2 and distilled under nitrogen using a CaC12 packed column. Trioxane of polymerization grade was kindly supplied by Perstorp AB, Sweden and used without further purification. 1,1 dichloromethane CH2C12(puriss; Kebo AB, Sweden) was dried and distilled over phosphorus pentoxide and then kept over molecular sieves in the dark. AIBr3 (Merck AG, Germany), boron trifluorideetherate BF3"OEt2, (Fluka AG, Switzerland), sodium peroxide, (Kebo AB, Sweden), ethyleneglycol (Kebo AB, Sweden) and nitric acid (Kebo AB, Sweden) were of pro analysi grade and were used without further purification.

Polymerization procedure All polymerizations were carried out in 100ml Edenmeyer flasks equipped with screw caps with silicon rubber membranes. The flasks were kept at the desired temperature by immersion in a thermostated bath regulated to better than +0.05 °. All glassware was washed, in the following order, with chromic acid, distilled water, sodium bicarbonate solution and distilled water and dried for at least 3hr at 150° in air. During cooling it was flushed with dry nitrogen. When possible, hypodermic syringes were used for the addition of monomers, initiators and solvent. The homo- and copolymerizations are summarized in Tables 1 and 2 respectively. The following procedure represent a typical run. 3 g (0.033 mole) trioxane dissolved in 14.75 g (0.10 mole) chloral was placed in an Erlenmeyer flask. After attainment of thermal equilibrium, 0.2g (0.00075 mole) powdered solid AIBr 3 (alternatively dissolved in 2 ml (0.32 mole) CHIC12) was added under a stream of dry nitrogen. The flask was shaken by hand and kept in the thermostated bath for the predetermined time. 815

ARNE HOLMSTROMand ERLING M. SORVIK

816

Table 1. Homopolymerization of chloral and trioxane in solution Run No.

Initiator*

Solventt

Temp., °C

Time, hr

Conversion

Monomer

1. 2. 3. 4.

Trioxane Chloral Chloral Trioxane

BFa. OEt 2 BF 3 - OEt 2 AIBr 3 AIBr 3

CH2CI 2 CH2CI 2 CH2C12 CH2C12

21 5 5 5

21 24 1 72

32 0 8 0

%

* AIBr 3 added dissolved in 0.032 mole CH2C12. t 0.5 mole. The reaction mixture was poured into a large excess of ethanol. The white polymer product was filtered off on a glass filter, and crushed and washed five times with hot ethanol. The product was first dried for 72 hr over phosphorus pentoxide in a vacuum dessicator at ambient temperature and then under nitrogen at 120° to constant weight. In addition, homopolymers for reference purposes were synthesized. Chloral was polymerized at 0 ° by treating the hydrate with concentrated sulphuric acid according to Chattaway and Kellet [7] and trioxane in bulk at 100° with BF3.OEt z according to Kern et al. [8].

Determination of chlorine content The chlorine content was determined by the use of a Parr peroxide bomb [9]. The organic bonded chlorine was converted to chloride by mixing the organic material with ethylene glycol and igniting it in a large excess of sodium peroxide. The reaction mixture was dissolved in deionized water and r~eutralized with nitric acid. Chloride was determined by potentiometric titration with silver nitrate solution according to Dyrssen and Jagner [10]. All runs were duplicated and gave, in all reported cases, identical results. This procedure gave 71.8~o chlorine for the polychloral mentioned above compared with 72.l% for monomeric chloral, (72.16% theor.). The difference of 0.3% is probably due to the hydroxylic endgroups of the polymer and corresponds to a degree of polymerization (DP) of about 40, in good agreement with previously published data [11] for similarly prepared polymers. Other analyses A Perkin-Elmer Differential Scanning Calorimeter DSC-1 was used for the thermal analysis. The decomposition temperature was taken as the temperature at which an endotherm began to develop when the samples were heated at 8°/min. Infra-red spectra were recorded on a Beckman IR-9 infra-red spectrophotometer. The X-ray dif-

fractogrammes were obtained with a Philips Powder Diffractometer PW 1050/25 using Cu K~-radiation with a Ni filter kindly put at our disposal by Dr. R. Carlsson of the Dept. of Inorganic Chemistry, Chalmers University of Technology, G6teborg. RESULTS AND DISCUSSION

Polymerization The o p t i m u m conditions for polymerization are quite different for chloral a n d trioxane. Polychloral may be obtained by both cationic and anionic initiators, the latter giving considerably higher D P ' s [4]. Polymerization temperatures of about - 3 0 ° are reported to be most favourable. Trioxane cannot be polymerized by anionic initiators [-12]. Cationic initiators are usually employed and the optimum polymerization rate is obtained above 65 ° [8]. For copolymerization of chloral and trioxane, we decided to use cationic initiators and an intermediate polymerization temperature. At first, homopolymerizations in CH2Cl2-solutions with BF3"OEt2 and A1Br3 as initiators were tried. As seen from Table 1, trioxane polymerized reluctantly with BF3"OEt2/ CH2C12 at 21 ° while chloral as expected [4] polymerized rapidly with AIBra/CH2C12 at 5 °. N o n e of the initiators tested were successful for both monomers. In spite of this, copolymerization was attempted with AIBr 3 as initiator and CH2C12 as a solvent at - 1 5 , + 4 and + 21° (Table 2, runs 1~). N o products were obtained, n o t even polychloral and apparently trioxane acts as an inhibitor. In the next series of experiments (Table 2, runs 7-10), CH2C12 was omitted and trioxane was dis-

Table 2. Copolymerization of chloral (M 0 and trioxane (M2) using AIBr 3 as initiator -O-CH~

I

Run* no. 1,2,3 4, 5, 6 7 8 9 10 11 12, 13

M1 :M2 mole ratio in feed 1:1 1:2 3:1 3:1 3:2 3:2 2:1 2:1

Solventj

Temp, °C

Time, hr

Conversion:~ %

Chlorine content wt %

CH2C! 2 CH2CI 2 -------

21,4,-15 21,4, - 1 5 4 - 15 4 -- 15 --15 4, 21

96 96 60 60 60 60 24 24

0 0 5 13 1 5 3 0

71.2 69.8 69.0 70.6 70.6 --

* In runs 1~5, 12 and 13, A1Br3 was added dissolved in 0.032 mole CH2CI 2. "j"0.5 mole. Based on total weight of monomer used. §Calculation is based on an assumed DP = 40, see p. 818.

CCI 3 units mole %

- O - C H 2-~, units mole %

96. 88 83 92 92 --

4 12 17 8 8 --

Copolymerization of chloral with trioxane solved in chloral. As these experiments resulted in polymer products, a third series was carried out (Table 2, runs 11-13) where a small amount of CH2C12 was introduced in runs 12 and 13 by supplying the initiator as 2 ml CH2C12-solution. No polymer whatsoever was formed and it is clear that it is the simultaneous presence of trioxane and CH2C12 which makes A1Br3 inactive. This should be due to the formation of inactive and stable complexes with aluminium as a central atom. In our systems the presence of water in trace amounts cannot be excluded. In spite of this no active initiating species is apparently formed. The complex formation is in accordance with Chmelir's findings [13] that AIBr 3 can form complexes with monomers resulting in inhibition or retardation of the polymerization. Chmelir 1-14] also showed that A1Br3, contrary to most Friedel-Crafts initiators, does not need a cocatalyst such as water. This is believed to be due to the ability of A1Br3 to form ionized dimeric complexes. Brown [15] also noticed a difference between AIBr3 and most other Lewis acids, e.g. SnBr4, in the polymerization of formaldehyde. For AIBr3, he suggested a coordinative cationic insertion mechanism.

Polyoxymethylene

i

Copolymer

The chlorine content (Table 2) indicates that the polymerization products contain a major amount of chloral units. It also indicates that other groups are present. In Fig. 1, i.r.-spectra typical for runs 7-11 Polyoxymethylene Polyc hlorol Copolyrner cm -I

cm -I

700

600

500

700

600

50O

tl i J

ill

[ 50--

i

i

cm -I

3000

c r n -1

2800

3000

2800

90

I

J

stole I: 2 5

- Y L_.__

Characterization o f the polymerizates

......

817

r

Fig. 1. Infra-red spectra in the 500-700cm-' and 2800-3000 cm-l ranges for POM, polychlo[al and a typical copolymerization product (run 8 in Table 2).

i I0 °

i 20 °

30 °

40 °

z8 Fig. 2. X-ray diffractogrammes for POM, polychloral and a typical copolymerization product (run 8 in Table 2) obtained with Cu K~ radiation using Ni filter. (Table 2) are shown together with the reference homopolymers. The X-ray diffractogrammes (Fig. 2) reveal that the narrow crystalline peaks from the homopolymers are completely absent while the broad amorphous bands of polychloral are preserved. This is strong evidence that the products consist of copolymers and not of a mixture of homopolymers. Being.poor in POM, such a mixture of presumably incompatible polymers should contain domains of pure polychloral large enough to be visible as crystalline peaks in X-ray. The lack of homopolymers is further strongly supported by the thermal and chemical stability tests. In accordance with the X-ray results, no crystalline melting was observed in the DSC-analysis. The decomposition temperature was 180° for the POM reference, 220 ° for the polychloral reference, and for the polymer products about 300 ° at a heating rate of 8°/min. When kept at 200 ° for l hr the polymer products showed no weight loss. At 250 :' the weight loss was 2%/min. These results should be compared with those obtained by Thinius et al. [3]. For most products they reported weight losses at 222 ° in the range 10-15%/min. The chemical stability tests give further strong evidence of copolymer formation. None of the treatments indicated in Table 3 had any visible effect on the copolymers. It should be noticed that treatments Nos. 1 and 2 rapidly depolymerize polychloral [11]. Treatment No. 3 causes a rapid deterioration and haloform reaction of this polymer. Treatment No. 4

ARNE HOLMSTROMand ERLING M. SORVIK

818

Table 3. Tests of chemical stability

I. 2. 3. 4. 5. 6.

Pyridin Dimethylformamide, DMF 10% Aq KOH Conc. H2SO 4 p-chlorphenol Hexafluoroacetone sesquihydrate

on the other hand immediately depolymerizes P O M and treatment Nos. 5 and 6 dissolve it. Thus, all the evidence show conclusively that the polymer products consist of copolymers containing chloral and oxymethylene units. As is well known and confirmed by our i.r.-spectra, the polymerization system results in polymers with hydroxylic end-groups [11]. Provided that the D P is known, the chemical composition of the copolymers can be calculated from the chlorine content. Comparison of the i.r.-spectra for the copolymers with the spectrum for the reference polychloral show similar relative hydroxyl absorption. Thus, we have concluded that the copolymers have about the same D P 40 as the reference polychloral giving the copolymer compositions indicated in Table 2. The result of Treatment No. 3 in Table 3 elucidates the course of the haloform reaction, when treating polychloral with an alkaline solution. As the copolymer is stable, the only possible way of forming chloroform is via depolymerization and not via direct attack on the polymer chain. O

\

60 30 30 30 30 30

100 130 25 130 130 65

REFERENCES

1. E. F. T. White, Brit. Pat. 902, 602 (1962). 2. C. E. Lorenz, Brit. Pat 995, 770 (1965). 3. W. Thiimmler, G. Lorenz and K. Thinius, Plaste Kautsch. 11, 386 (1964). 4. I. Rosen, C. L. Sturm, C. H. McCain, R. M. Wilhjelm and D. E. Hudgin, J. Polym. Sci. A3, 1545 (1965). 5. Belg. Pat., 609, 208; C. A. 58, 2516b (1963). 6. French Pat. 1,391,539; C.A. 63, 11,809b (1965). 7. F. Chattaway and F. Keller, J. chem. Soc. 2, 2709 (1928). 8. D. Braun, H. Cherdron and W. Kern, Praktikum der Makromolekularen Organischen Chemie. p. 154. Dr. Alfred Hiitig Verlag, Heidelberg (1966). 9. I. M. Kolthoff and P. J. Elving (Editors), Treatise on Analytical Chemistry, Part II, Vol. 14. Wiley-Interscience, New York (1971) p. 8 and references therein. 10. D. Dyrssen and D. Jagner, Anal. Chim. Acta 35, 407 (1966). 11. A. Novak and W. Whalley, Trans. Faraday Soc. 55, 1490 (1959). 12. O. Vogl, J. Macromol. Sci. Cl2, 109 (1975).

O

+ oCCI3-~ H__C /

\

H

Temp., °C

O

+ o O H - ~ H--C / /

CI3C--C~

Time, min

+ CHCI 3

\

OH

Acknowledgement--Financial support from the Swedish Board for Technical Development is gratefully acknowledged. The authors also thank Prof. E. Adler for valuable discussions and Dr. R. Carlsson for performing the X-ray analysis.

Oe 13. M. Chmelir, Makromolek. Chem. 176, 2099 (1975). 14. M. Chmelir, M. Marek and O. Wichterle, J. Polym. Sci. C16, 833 (1967). 15. N. Brown, J. Macromol. Sci. A1. 209 (1967).