DE1906252B2 - PROCESS FOR THE PREPARATION OF POLYMERIZES OF TRIOXANE - Google Patents

Description

It is known that polyoxymethylenes can be prepared from trioxane either by solution polymerization or bulk polymerization at high temperatures in the presence of ionic catalysts or by irradiating solid trioxane with ionizing radiation (German Auslegeschrift 1,157,783 and 1,247,020). However, since the polyoxymethylenes produced by this process have only a low thermal stability, they have to be stabilized by esterification or etherification of the end groups in order to obtain a thermoplastic material that can be used for technical purposes (H ο ube η - W ey 1, methods of organic chemistry , Vol. 14/2, p. 407, 4th edition, 1963). The esterification of the end groups is z. B. carried out by reacting polyoxymethylene with 2 to 20 parts by weight of an anhydride of a saturated monocarboxylic acid per part of polyoxymethylene at at least 50 ^° C. (see US Pat. No. 2,964,500). To etherify the end groups, the polyoxymethylene can be treated with an alkyl acetal (cf. British patent specification 868 365). Furthermore, a method for preventing discoloration of the polyoxymethylene is known, in which a urea derivative is added during the etherification of the polyoxymethylene (cf. French Patent 1,364,410). Finally, the noticeable reduction in the molecular weight of polyoxymethylene during the esterification is prevented by adding a carbodiimide to the reaction mixture (cf. British patent specification 864 403).

Of the two stabilization options mentioned, the etherification of the end groups is considered to be the one cheaper. A polyoxymethylene stabilized by etherification of the end groups is in any case one superior to polyoxymethylene stabilized by esterification of the end groups in terms of alkali resistance, and it has the further advantage that when it decomposes, no acid is liberated which would cause the further degradation accelerated. However, an acidic catalyst is usually used for the etherification, which is known to lead to a decomposition of the polymer during the etherification (see M. S i 11 i g, "Hydrocarbon Processing", Vol. 41, No. 114, p. 151).

The etherification or esterification means an additional process step and is therefore undesirable.

Furthermore, from the German Auslegeschrift 1 194145 and from the French patent specification 1,314,286 Process for the polymerization of molten trioxane or trioxane dissolved in a solvent known in the presence of an alkyl acetal with cationic catalysts which lead to polyoxymethylenes with etherified, stabilized end groups lead. A disadvantage of the polymerization with catalysts is however, the fact that the polyoxymethylenes produced in this way often contain catalyst residues, which is the case in turn has a detrimental effect on the polymer properties, in particular on the thermal stability. In addition, working with acidic catalysts is obviously associated with disadvantages (German Auslegeschrift 1 157 783, column 1, lines 30 to 34).

The object of the invention is to provide an improved process for the production of polyoxymethylenes with to create etherified end groups, which is free from the disadvantages of the aforementioned methods and thermally delivers highly stable, non-discolored polymers. This object is achieved by the inventive Procedure solved.

The invention thus provides a process for the preparation of polymers of trioxane by Polymerizing trioxane in the solid state at temperatures between room temperature and 64 ° C in the presence of 0.1 to 5 percent by weight of an alkyl acetal of the general formula

Ri-OCOR ¹

R ²

in which R ¹ denotes an alkyl radical having 1 to 6 carbon atoms and R ² denotes a hydrogen atom or an alkyl radical having 1 to 5 carbon atoms, which is characterized in that the reaction mixture is added before or during the polymerization, or the trioxane by itself before mixing treated with the alkylacetal with ionizing rays.

The process of the invention has the advantage that the polymerization and the etherification the end groups with the alkyl acetal takes place in the absence of catalysts and that one In addition, a product with advantageous properties is obtained, namely a snow-white polyoxymethylene with excellent thermal stability.

The prior art has believed the use of extremely stringent reaction conditions necessary if, in the polymerization of solid trioxane, a monomer copolymerizable therewith added to increase the thermal stability of the polyoxymethylene formed. This assumption was based on the theory that the chain growth reaction in radiation-induced polymerization of solid trioxane is controlled by the crystal lattice structure. Surprisingly, it has now been found that when the alkyl acetal of the general formula I is added to the trioxane, the polymerization takes place and the etherification take place side by side, thermally stable macromolecular polyoxymethylenes can be produced in one process step. It was unlikely that the next to the fixed Trioxane present in the liquid or gaseous phase alkylacetal in such a way at the radiation-induced Polymerization of the trioxane would result in an excellently stabilized polyoxymethylene receives. Rather, the person skilled in the art had to assume that he was receiving extremely uneven products.

Any trioxane can be used as the starting material for the process of the invention, but a trioxane having a melting point of at least 59 ° C., the lowest possible amount of impurities and a moisture content of at most 0.1 ° / ₀ is preferably used.

Examples of suitable alkyl acetals of the general formula I are formaldehyde dimethylacetal, formaldehyde diethylacetal, acetaldehyde dimethylaceta ^ acetaldehyde diethylacetal, Propionaldehyde dimethylacetal or propionaldehyde diethylacetal. Particularly preferred is formaldehyde dimethyl acetal (bp 43 ° C), which at the preferred polymerization temperature of 45 to 58 ° C is gaseous and can be brought into contact with solid tnoxane without this a noticeable destruction of the crystal lattice takes place, and thus the polymerization is not significant Extent adversely affected.

The alkyl acetals used according to the invention

must not contain excessive amounts of impurities, e.g. B. alcohols contain.

In general, 0.01 to 10 percent by weight, preferably 0.1 to 5 percent by weight, of the alkyl acetal, based on the weight of the trioxane used. The alkylacetal can be liquid or in the form add a spray to achieve a more even distribution. When using formaldehyde dimethyl acetal the best effect is achieved when this is added in gaseous form. Excellent Results are obtained if, after the addition of the alkyl acetal, optionally under Cooling, evacuated. As a result, the even distribution of the alkyl acetal is achieved more quickly. On the on the other hand, the polymer yield is somewhat increased in the presence of air.

Gamma, X-ray and electron beams can be used as ionizing radiation in the method of the invention, the latter being particularly preferred. Fast neutron and deuteron beams, alpha particles or nuclear fission pieces can also be used as ionizing radiation. There is no limit to the radiation dose to be used, but radiation doses of about 10 * to 10 ⁷ rad are preferred. The polymerization process according to the invention can be carried out by irradiating the reaction mixture during the polymerization or by first irradiating the reaction mixture and then polymerizing it. If the polymerization is carried out during the irradiation, better results are obtained if a reaction mixture is used which is for some time after the addition of the alkyl acetal, e.g. B. than when irradiated for 24 hours or more, allowed to stand under air at 4O ⁰ C, the reaction mixture immediately after the Alkylacetalzugabe. If, on the other hand, the process is carried out by first irradiating the polymerization mixture and then heating it to start the polymerization, the alkyl acetal can be added at any time before or after the irradiation, provided that the addition is only made before the start of the polymerization reaction he follows. However, if the time interval between the irradiation and the polymerization is too long, the number of active centers decreases, as a result of which the polymer yield drops. It is therefore advantageous to add the alkyl acetal shortly after irradiation, for example within 7 days at room temperature. If the temperature is lower, this period can be extended a little.

In the case of the embodiment in which the alkyl acetal is added before the polymerization of the trioxane the following three process steps should preferably be carried out in the order given be performed:

1. After the addition of the alkyl acetal, the reaction mixture is left to stand at or below room temperature, in particular at at least ⁰ ° C., for at least 24 hours.

2. The reaction mixture is irradiated with ionizing radiation.

3. The temperature of the reaction mixture is increased to temperatures below 64 ° C, z. B. 45 to 58 ⁰ C, so that the polymerization begins.

Those made by the process of the invention Polyoxymethylenes have a slightly lower molecular weight than polymers that can be obtained without Addition of an alkyl acetal is obtained. However, the decrease in molecular weight is only so small that it brings no practical disadvantage. Furthermore, leads in the previously described embodiment the addition of a larger amount of alkyl acetal leads to a certain reduction in the polymer yield. However, this reduction in yield can be simple

ίο can be avoided by doing that Irradiate and repeat the polymerization one or more times. In particular, can be repeated by Irradiation and polymerization, the polymer yield and the stability of the polyoxymethylene obtained can be increased to a greater extent in the procedure according to the invention than in one Working without the addition of alkylacetal. Unreacted alkyl acetal can be removed from the reaction product in a simple manner separate and recover, since the alkyl acetals by washing with various organic solvents or with plenty of water, by circulating air under reduced pressure or remove it from the polyoxymethylene by heating.

Both the German Auslegeschrift 1,194,145 and the French patent specification 1,314,286 work with cationic catalysts in solution. The addition of an acetal has a strong chain-transferring effect, which is shown in a significant decrease in the molecular weight. This can be seen from the following table in which the results of French Patent 1,314,286 (page 3, table) with those of the invention Procedure to be compared.

35 example Addition in moles
per 1000 moles
Trioxane Enamel
index Solution
viscosity
(dl / g) 40 French
Patent specification without 2.5 1.7 *) French
Patent specification 0.81 methylal 21.0 1.0 *) 45 French
Patent specification 0.54 acetal 19.5 1.0 *) 1 (comparison
example)
2 without
1.8 methylal 2.4
1.4 50 10 (compar
example)
10 without
3.8 acetal 1.9
1.2

*) Calculated according to H. L. Wa g η e r and K. F. Wissbrum, Die makromolekulare Chemie, 81, p. 14 (1965).

Although in Example 10 of the invention the acetal concentration is about five times that in the Examples of the French patent, in the first case is the relative lowering of viscosity Polyoxymethylene lower than in the French patent.

Furthermore, in the method of the invention, the irradiation can be repeated several times and the polymerization process even achieve a higher polymer yield than when working in Absence of an acetal, although the presence of the alkyl acetal generally with a decrease the polymer yield is connected.

The examples illustrate the invention. irradiated. This polymerizes during the irradiation

The term »average thermal cerium trioxane. The ampoule contents are worked up according to Example 1, the settlement rate K ₂₂₂ * gives the average. This gives 0.79 g of white polymer borrowed weight loss of the polyoxymethylenes in pro (40 ° / ₀ ) with a K ₂₂₂ value of 0.10 and a viscosity number of 0.5 when heated in a nitrogen viscosity number of 0.5.
current to 222 ° C. The loss is referred to as the weight loss 1 4
difference 10 minutes after the start of the treatment and 60 minutes after the start of the treatment, the 1.5 g of trioxane are determined with 0.03 g of formaldehyde di-polyoxymethylene. methyl acetal added and polymeric according to Example 1

The viscosity numbers (η) of the polyoxymethylene ίο siert. The reaction mixture is then at -78 ^° C

are at 60 ⁰ C in p-chlorophenol with a content of 1 hour with gamma rays with a radiation

2% a-pinene determined, output of 2 · 10 ⁵ r / h. irradiated what you get it

. -I can weiterpolymerisieren ₁ for 24 hours at 45 ⁰ C. then

Example 1 _w j _{r (j} ^ ₈ R _ea ktionsprodukt according to Example 1 listed

10 g of trioxane in a 25 ml glass ampoule 15 works. 1.2 g of white polymer (80%) are obtained

are made using an injection syringe with 0.2 g with a K ₂₂₂ value of 0.16 and a viscosity

Formaldehyde dimethyl acetal added. The ampoule number of 0.90.

is melted with the admission of air and 7 days for comparative example 4a
Left to stand at room temperature. Then, the vial at -78 ⁰ C with gamma radiation from a ⁶⁰ Co 20 Repeating this experiment without addition of radiation source of 100 Kilo Curie with a radiation of formaldehyde, one obtains only 1.13 g lung dose of 1 × 10 ⁶ R ( Radiation output 5 · 10 ⁵ R / polymer (75%). The polymer is colored yellow (hours) irradiated. Immediately thereafter, the increases and has a viscosity of 1.3 and a temperature of the reaction mixture to 55 ⁰ C. K ₂₂₂ value of 0.6.
After 4 hours of polymerization, the ampoule 25 comparative example 4b
content poured into methanol. The precipitated poly-

oxymethylene is carefully washed and then Example 1 is repeated using

ßend filtered off. The polymer is 15 hours at 1.5 g of trioxane without the addition of formaldehyde dimethyl

Room temperature under reduced pressure acetal with a polymerization time of 28 hours,

dries. This gives 2.8 g of white polymer 30, so 1.05 g of polymer (70%) is obtained with a

(28%) with a viscosity number of 0.85 and a viscosity number of 2.5 and a K ₂₂₂ value of 1.0.

average thermal decomposition rate. .

speed (K ₂₂₂ ) of 0.22. The determination of the methoxyl example 5

groups according to time 1 (cf. T. A. K o c h and P. E. 2.0 g of trioxane are melted into an ampoule.

Lindvig, J. Appl. Polymer. Sei., Vol. 1, p. 166) 35 zen. After the ampoule according to Example 1

gives a value of 0.2 percent by weight. has irradiated, the contents of the ampoule are left open

... ... warm up to room temperature. Then the

Comparative example 1 ampoule opened and 0.02 g formaldehyde di-

A mixture according to Example 1 without the addition of formaldehyde methyl acetal is added. Then the ampoule is

Polyoxymethylene produced in dimethylacetal has 40 melted shut again and stand for 3 days at 0 ° C

leave a viscosity number of 2.40 and a K ₂₂₂ value. Polymerization is then carried out at 55 ^° C. for 4 hours

from 1.1. and worked up according to Example 1. You get

For example, the 1 2 ^> ^ ^ white polymer (48%) with a viscosity number of 1.8 and a K ₂₂₂ value of 0.44.

2.0 g of trioxane in a 10 ml glass ampoule 45.
are treated with 0.02 g of formaldehyde dimethyl acetal. B e 1 s ρ 1 e 1 6
mixes. The ampoule is drawn in with the admission of air. With admission of air, an aluminum vessel is melted and left to stand for 3 days. Then 500 g of trioxane are charged, so that the bottom is 4 mm, the ampoule is cooled to -78 ° C. and is evenly covered with. Then 5 g of shaped gamma rays from a ⁶⁰ Co radiation source of 50 aldehyde dimethylacetal are sprayed in. The 100 kilocurie container is not closed with a radiation dose of 1 · 10 ⁶ R and irradiated at room temperature for 3 days (radiation ^{output 5 · 10 5} R / hour). Then left standing. The reaction mixture is then the whole reaction mixture at -78 ° C. for 7 days at room temperature with an electric stand. Then the temperature is increased the overall nenstrahl of 2 MeV of a Elektronenbeschleunimisches at 55 ° C. After 4 hours the polymerization ger 55 with a radiation dose of 1-10 ⁶ R, the reaction product is irradiated in accordance with Example 1 listed and allowed to stand 48 hours. Then they work for 3 hours. This gives 0.92 g of white polymer polymerized at 55.degree. After washing and (46%) with a K ₂₂₂ value of 0.30 and drying, 220 g of white polymer (44%) viscosity number of 1.4 are obtained. with a viscosity number of 1.3 and a K ₂₂₂ value

_π . . . - 6o of 0.39.

Example 3

2 g of trioxane and 0.02 g of formaldehyde dimethyl acetal are placed in a 10 ml glass ampoule. 2.0 g of trioxane and 0.02 g of liquid formaldehyde diethyl acetal are melted into an ampoule with the access of air. After giving the reaction mixture 24 hours 65. The ampoule is sealed and left to stand at 40 ° C, it is left to stand for 2 hours at room temperature for 5 days. Then 55 ° C with gamma rays from a ⁶⁰ Co radiation one cools them down to -78 ° C and irradiated according to Example 1. source with a radiation output of 2 · 10 ⁵ R / hour. The irradiated ampoule is at 0 ^° C. for 24 hours

left to stand and then heated to 55 ° C. After 4 hours of polymerization, the contents of the ampoule are worked up in the usual way. 0.85 g of polymer (42.5%) with a viscosity number of 1.6 and a K ₂₂₂ value of 0.55 are obtained.

Example 8

2.0 g of trioxane are mixed with 0.02 g of acetaldehyde dimethyl acetal in an ampoule with the admission of air. The procedure is then continued as in Example 7, but polymerizing ^{at 55 ° C. immediately after the irradiation.} 0.92 g of polymer (46 ° / ₀ ) with a viscosity number of 1.6 and a K ₂₂₂ value of 0.46 are obtained.

Example 9

2.0 g of trioxane are mixed with 0.03 g of liquid formaldehyde dimethyl acetal in a glass ampoule. The ampoule is then evacuated at -78 ° C and sealed. After standing for 16 hours at room temperature, the reaction mixture is ^{irradiated at 0 °} C. with gamma rays from a ¹³⁷ Cs radiation source with a radiation dose of 9 ^{· 10 5} R (radiation output 5 · 10 ⁴ R / hour). Immediately thereafter, the reaction mixture is heated to 55 ° C. and allowed to polymerize at this temperature for 5 hours. After working up as in Example 1, 0.76 g of white polymer (38%) with a viscosity number of 1.1 and a K ₂₂₂ value of 0.19 are obtained.

B e i s ρ i e 1 10

2.0 g of trioxane and 0.01 g of liquid acetaldehyde diethylacetal are placed in a glass ampoule. The ampoule is then evacuated and sealed. After standing at 45 ° C. for 24 hours, the reaction mixture is cooled to -78 ° C. and irradiated with gamma rays from a ⁶⁰ Co radiation source with a radiation dose of 1.3 · 10 ⁶ R (radiation output 5 · 10 ⁵ R / hour). Immediately after the irradiation, the reaction mixture is heated to 55 ° C. Polymerization is allowed to take place at this temperature for 5 hours. When working up the reaction product according to Example 1, 0.86 g of white polymer (43 ° / ₀ ) with a viscosity number of 1.2 and a K ₂₂₂ value of 0.72 is obtained. Comparative example 10

When this experiment is repeated without the use of acetaldehyde diethyl acetal, a polymer is obtained with a viscosity number of 1.9 and a K ₂₂₂ value of 1.2.

Example 11

Example 10 is repeated, but propionaldehyde dimethyl acetal is used instead of acetaldehyde diethyl acetal, and the mixture of trioxane and propionaldehyde dimethyl acetal is left to stand for 7 days at room temperature before irradiation. 0.80 g of white polymer (40 ° / ₀ ) with a viscosity number of 1.8 and a K ₂₂₂ value of 0.72 are obtained.

Example 12

Example 11 is repeated, but propionaldehyde diethyl acetal is used instead of propionaldehyde dimethyl acetal. 0.6 g of white polymer (38%) with a viscosity number of 1.5 and a K ₂₂₂ value of 0.80 is obtained.

Claims (1)

Patent claim: Process for the preparation of polymers of trioxane by polymerizing trioxane in solid state at temperatures between room temperature and 64 ° C in the presence of 0.1 to 5 percent by weight of an alkyl acetal of the general formula R ¹ OCOR ¹ in which R ^{1 is} an alkyl radical having 1 to 6 carbon atoms and R ^{2 is} a hydrogen atom or an alkyl radical having 1 to 5 carbon atoms, characterized in that the reaction mixture before or during the polymerization or the trioxane by itself before mixing with the Alkylacetal treated with ionizing rays. 109 531/367