FI57432B - FOERFARANDE FOER VEHICLE STABILIZATION AV EN NORMALT FAST OXIMETHYLENEAMPOLYMER - Google Patents

Description

RSF ^ I M (11) ADVERTISEMENT 57470 jksTq ibj vn; utlAggn I NGSSKRIFT D 'H 0 £ C (45): lb:;: i' '. "Ό (51) Kv.ik.3 / int.ci.3 C 08 J 3/00, C 08 G 2/36 FINLAND I - FI N LAN D (21) P »t * nttlh» k «Nuf - PitanauwMtnIng 1323/73 (22) HtfcimltpUvf - Arubknlnpdtg 25.01 + .73 (23) Alkuplivi—> Giltlgh« t> dag 25.01 + .73 ( 41) Tullut julklselul - Bllvlt offamllg 27.10.73

Patent and Registration Office NihUviksipuon j. Heard.Outdoor Date—

Patent · ech registerstyreiaen AntökM utiagd oeh uti.ikrift «n pubifcend 30.0U. 80 (32) (33) (31) «uolkwit — Begird a priori * · * 26.0l +. 72 USA (US) 21 + 7625 (71) Celanese Corporation, 522 Fifth Avenue, New York, NY, USA (US) (72) Michael David Golder, Bronx, New York, USA (US) (7 ^) Oy Kolster Ab (5! +) The present invention relates to a process for the thermal stabilization of a normally solid oxymethylene copolymer having a melting point of more than about 60 ° C. # relatively unstable monomeric oxymethylene units mixed with relatively stable monomeric -OR units in which the chain R contains at least two carbon atoms directly bonded to each other, in the polymer chain between two valencies, all substituents in this chain R are inert, at least some of these , and this copolymer and about 2 to about 25 parts by weight stepped from the copolymer, water, alcohol or trioxane or mixtures of these substances, is placed in the reaction zone.

Certain polymers contain both relatively stable and relatively unstable monomer units, and often the stability of such polymers depends on the relative position of the above-mentioned stable and unstable monomer units. For example, if the polymer is readily degraded according to a reaction that begins at the ends of the polymer molecules, the degradation is rapid when the monomer units at the terminal positions readily react.

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The process according to the invention is characterized in that a) a heterogeneous system is formed, consisting essentially of a copolymer in the molten state and water, alcohol or trioxane vapor, or a vapor mixture of these substances, by keeping the reaction zone at a higher temperature than the copolymer melting sufficient to keep the copolymer in the molten state and in the vapor state of said reactant or mixture thereof; and b) reacting the molten copolymer with said substance or mixture of substances under the aforementioned temperature and pressure conditions for about 0.1 to about 15 minutes to remove unstable monomeric oxymethylene units from the end portions of the copolymer molecules so that at least 90% of the resulting molecular polymer chains remain.

Oxymethylene polymers with repeating -OCH 2 units directly attached to each other have been known for many years. They can be prepared by polymerizing anhydrous formaldehyde, or by polymerizing trioxane, which is a cyclic trimer of formaldehyde. High molecular weight oxymethylene polymers have varying strengths, and according to a preferred embodiment of this invention, the polymer to be stabilized is an oxymethylene polymer containing simple carbon-carbon bonds in the main polymer chain.

The application relates to a new improved process for the normalization of a solid oxymethylene copolymer by hydrolysis of said copolymer in the presence of water, alcohol or trioxane, or a mixture of these substances at elevated temperature and pressure, the copolymer being in the molten state, and water, alcohol or trioxane. , i.e., the hydrolysis side zone has two phases.

U.S. Patent 3,301,821 relates to a hydrolysis process in which the polymer is in solid form and the reactants are in the form of vapors; instead, according to the invention, the polymer is in the molten state. In addition, the known method achieves a worse result than the method according to the invention (cf. the working examples).

U.S. Patents 3,318,8U8 and 3 1 * H 280 relate to Applicant's prior hydrolysis process. According to this known method, a very high pressure is maintained in the hydrolysis zone, whereby the polymer-reactant mixture is in liquid form (see U.S. Pat. No. 3,318,8U8, column 7 »lines 6-10 and U.S. patent 3 U18 280, column 5, lines 9 -1 * 0 (cf. the experiments explained in Comparative Examples 1 to 1).

The object of the method according to the invention is to provide a thermally stable product; the aim is that at least 90% of the polymer molecules are relatively stable (thermally resistant), i.e. that the polymer chains do not contain more than 10% of unstable chains (at least 90% of permanent end groups).

3,57432

Copolymers which can be used according to this aspect of the invention, are those having a structure comprising recurring units of the formula "" ν> ^ sn where n is an integer of 0? To 5 and wherein n is zero # 60 to 99.6 R and R 1 are inert substituents, i.e. they do not contain interfering functional groups and do not cause undesired reactions.

A preferred group of copolymers are those whose structure comprises oxymethylene and oxymethylene repeating units, with 60-99.6% of the repeating units being oxymethylene units.

Particularly preferred oxymethylene polymers are those combined with oxyalkylene units having adjacent carbon atoms derived from cyclic ethers having adjacent carbon atoms. These copolymers can be prepared by copolymerizing trioxane a with a cyclic ether having the structure CH 2 - (° CH 2> n where n is an integer 0, 1 or 2).

Examples of preferred polymers are copolymers of trioxane and cyclic ethers containing at least two adjacent carbon atoms, such as the copolymers disclosed in U.S. Patent No. 3,027,352.

Specific cyclic ethers that can be used include ethylene oxide, 1,3-dioxolane, 1,3,5-trioxepane, 1,3-dioxane, trimethylene oxide, pentamethylene oxide, 1,2-propylene oxide, 1,2-butylene oxide, neopentyl glycol - formal, pentaerythritol diformal, paraldehyde, tetrahydrofuran and butadiene monoxide.

The term "copolymer" refers to polymers having two or more monomer groups, including tripolymers and higher polymers, suitable tripolymers are disclosed in U.S. Patent 3,686-1 ^ 2.

Preferred polymers to be treated in accordance with the invention are thermoplastic materials having a melting point of at least 150 ° C and which are normally in a moldable state at a temperature of 200 ° C. These polymers have a good heat resistance of 11,57432 before the treatment according to the invention, but this resistance is significantly improved by such treatment. For example, if a polymer sample treated in accordance with this invention and also chemically stabilized as described below is placed in an open container in a convection oven at 230 ° C and its weight loss is measured without taking a sample from the oven, its thermal shrinkage is less than 1 , 0 weight - # / minute during the first minute of i * 5, preferably less than 0.1 weight - # / ml in about the same time.

Preferred polymers treated in this invention have a melt index (M.I) of less than 50, preferably less than 30, as measured by ASTM D 1238-62T. After treatment, the preferred copolymers have significant alkali resistance. For example, if the treated copolymers are heated to reflux at about 1U2-11 at + 5 ° C in 50% aqueous sodium hydroxide for i + 5 minutes, the weight of the copolymer is reduced to less than 1 #.

Preferred catalysts used in the preparation of the desired copolymers are boron fluoride and boron fluoride coordination complexes with organic compounds, especially those in which oxygen or sulfur is a donor atom.

The boron fluoride coordination complex may be complexed with, for example, phenol, ether, ester or dialkyl sulfide. Boron fluoride dibutyl etherate, a boron fluoride coordination complex with dibutyl ether, is the preferred coordination complex. The boron fluoride complex with diethyl ether is also very effective. Other boron fluoride complexes that can be used include complexes with methyl acetate, ethyl acetate, phenyl acetate, dimethyl ether, methyl phenyl ether, and dimethyl sulfide. Suitable catalysts are disclosed in U.S. Patents 2,989,505, 2,989,506, 2,989,507, 2,989,509, 2,989,510 and 2,989,511.

The boron fluoride coordination complex should be present in the polymerization zone in such amounts that the boron fluoride concentration ranges from about 0.001 to about 1.0% by weight, based on the weight of the monomers in the polymerization zone. Preferably, amounts between about 0.003 and about 0.1% by weight should be used.

The monomers are preferably anhydrous or substantially anhydrous.

Small amounts of moisture, such as may be present in commercial grade reactants or upon contact with atmospheric air, do not prevent polymerization, but should be removed to obtain the best possible yield.

In preparing the preferred copolymers, the trioxane, cyclic ether and catalyst are dissolved in a common anhydrous solvent such as cyclohexane and allowed to react in a closed reaction zone. The temperature in the reaction zone is from about 0 ° C to about 120 ° C. The reaction time is about 5 minutes to about 72 hours. Up to about 100 atmospheric overpressures can be used; generally atmospheric pressure is preferred.

5,57432

The chemical structure of the cyclic ether must be taken into account. Thus, 1,3-dioxolane contains both an oxymethylene group and an oxyethylene group. Its incorporation into the copolymer molecule increases both the oxymethylene and oxyethylene content of the polymer molecule.

In general, the cyclic ether is present in the reaction mixture in amounts of about 0.2 to about 30 mole percent, based on the total moles of monomer. The optimum ratio depends on the particular copolymer desired, the expected degree of conversion and the chemical structure of the cyclic ether used.

The copolymers prepared from the preferred cyclic ethers have a structure consisting primarily of oxymethylene and oxyethylene groups in a ratio of from about 250: 1 to about 1.5: 1.

Once the polymerization reaction is complete, it is desirable to neutralize the effect of the polymer catalyst, as prolonged contact with the catalyst will degrade the polymer. The polymerization product can be treated with an aliphatic amine such as tri-n-butyl amine n or tri. with ethyl, amine, n in an excess of the free catalyst relative to the amount of free catalyst in the reaction product, and preferably in an organic washing liquid which is a solvent for the unreacted trioxane solution, or, if desired, the reaction product can be washed with water to neutralize the catalyst. A detailed description of suitable methods for neutralizing the effect of a catalyst can be found in U.S. Patent No. 2,990,509. · According to this invention, relatively unstable monomer moieties or polymer units can be removed by a process comprising treating the polymer with a reactant under elevated temperature and pressure. , so that the polymer / reactant system is heterogeneous, i.e., a two-phase system; the polymer being in the molten state and the reactant in the vapor state; and the reaction is continued for a time sufficient for relatively relatively unstable parts or. to remove the units from the ends of the polymer molecule so that the molecules terminate in relatively durable units.

The polymer / reactant system can be converted to this two-phase form in a variety of ways, such as 1) melting the polymer and adding the reactant to it under conditions such that the reactant remains in the vapor space, or 2) mixing the polymer and reactant and then heating under pressure. until the polymer / reactant system is in the molten liquid / vapor state.

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When the copolymer is a heterogeneous copolymer having relatively unstable units of oxymethylene units, the "hydrolysis" treatment under alkaline conditions is preferred. In a preferred embodiment, the polymer is reacted with from about 2 to about 25% by weight of the hydrolyzing agent. The reaction must take place at elevated temperature and pressure so that the polymer is in the molten liquid state and the reactant is in the vapor, i.e., gaseous state. Thus, the treatment can be said to be "heterogeneous melt hydrolysis" as opposed to the homogeneous melt hydrolysis disclosed in U.S. Patents 3,318,8i + 8 and 3,118,280.

The hydrolyzing agent may be water, or a primary, secondary or tertiary aliphatic or aromatic alcohol, or trioxane. First of all, it must be taken into account when selecting an alcohol that its thermodynamic properties are such that under the conditions of the process according to the invention the alcohol and / or alcohol / water and / or alcohol / water / trioxane mixtures are in the vapor state. Suitable alcohols include aliphatic alcohols, and are preferably methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, and the like.

The term "hydrolysis" as used herein includes the reaction of a polymer with water or the aforementioned hydroxy-containing substances or trioxane or a mixture thereof.

The end units of the oxymethylene polymer are often hydroxy-substituted oxymethylene (C-C (CH 2 -OH)) units, and the separation of the oxymethylene group using a polymer molecular hydrolysis reaction has the effect of transferring the hydrogen atoms of the hydroxyl group to the proximal oxygen atom group.

For example, when oxyethylene units are combined in a polymer chain by mixed polymerization, as described above, sequential separation of oxymethylene units occurs until the oxyethylene unit becomes a chain end group. Oxyethylene groups with carbon-carbon bonds are relatively resistant to such separation and remain attached to the polymer chain at the end position and protect the internal oxymethylene units from further hydrolytic exits. Since the oxyethylene units are also resistant to separation under the influence of heat, a molecule shortened in its chain has better initial heat resistance than the original copolymer from which it is derived.

It has been found that the products of the selective treatment of this invention, after a substantially constant weight has been reached, are also, in a preferred embodiment, the polymer maintained under the reaction conditions until it reaches a substantially constant weight. Thus, an oxymethylene copolymer which has been hydrolytically treated is not only heat resistant but also excellently resistant to such hydrolytic treatment under even more stringent conditions than in the treatment, despite the fact that the polymer still contains a substantial number of internal oxymethylene units which are normally prone to unless they are protected by stable end groups.

Preferably, the hydrolysis reaction takes place under alkaline vapor conditions, generally such that the pH of the added hydrolyzing agent is above 9 ° 0. To achieve such a pH, an alkaline substance must be present. The alkaline substance is preferably water-soluble or soluble in the hydroxy-containing substance; it may be a strongly basic hydroxide such as an alkali metal or alkaline earth metal hydroxide, or it may be a salt of a strong base and a weak acid, or it may be ammonia or an organic base such as an amine or amidine.

Suitable basic substances include sodium hydroxide, potassium hydroxide, sodium carbonate, sodium acetate, ammonium hydroxide, triethyleneamine, tripropylamine, tetramethylguanidine, trimethylamine, triethylamine, tri-butylamine, melamine and calcium hydroxide. The amount of the basic substance present in the chemical reaction is from about 0.001 to about 10.0% by weight, preferably from about 0.001 to about 1.0% by weight.

One advantage of alkaline hydrolysis over neutral hydrolysis is that the alkaline hydrolysis is faster and the alkaline neutralizes the excess polymerization catalyst present or any acidic substances formed during the reaction that could otherwise tend to degrade the polymer during the hydrolysis step.

In certain cases, it is preferable to use as the basic substance, e.g. triethylamine, which is used in such an excess that it maintains the basic conditions throughout the course of the hydrolysis reaction and reacts with any acidic substance formed. In suitable embodiments, an amount of about 0.25% by weight of triethylamine, based on the weight of the polymer, is sufficient. Therefore, if 5? hydrolysing reagent was added to the polymer, the hydrolyzed solution could contain 5 x 0% by weight? triethylamine α and the pH of the hydrolysis solution could be adjusted before adding the polymer. In some cases, it may be that the polymer contains some water in addition to that added to the hydrolysis solution.

After the polymerization reaction, the polymer is washed and dried to neutralize the active catalyst and to remove unreacted monomers, solvent and catalyst residues. In particular, water or a mixture of an alcohol such as methanol and water may be used, which may contain small amounts of ammonia or an amine such as triethyl amine.

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In some cases, it is preferable to mix the polymer with a larger amount of reactant, and after the catalyst is neutralized, remove some of the reactant by filtration, evaporation, etc., leaving only 2 to 25% by weight of reactant with the polymer during the hydrolysis reaction.

In another embodiment, the catalyst is neutralized and then the polymer is filtered, washed and dried. The polymer can then be easily stored until it is subsequently subjected to a hydrolysis treatment.

According to another preferred embodiment, one or more chemical stabilizers are combined with the copolymer to further reduce the amount of thermal damage. The amount of combined stabilizer depends on the particular stabilizer used. An amount between about 0.05 and 10% by weight (based on the weight of the polymer) has been found to be suitable for most stabilizers.

A suitable stabilizing system is a combination of 1) an antioxidant, such as a phenolic antioxidant, and most preferably a substituted bisphenol, and 2) an anti-chain component, generally a polymeric compound containing moth nitrogen atoms.

A suitable group of substituted bisphenols are alkylene bisphenols comprising compounds having 1-1 + carbon atoms in the alkylene group and having zero to 2 alkyl substituents on each benzene ring and each alkyl substituent having 1-1 + carbon atoms. Preferred alkylene bis phenols are 2,2'-methylene bis (1 + -methyl-6-tert-butylphenol) and 1+, 1 +'-butylidene bis (6-tert-butylphenol) 3-methylphenol). Suitable phenolic stabilizers, other than alkylene bisphenols, include 2,6-di-tert-butyl-1 + -methylphenol, octylphenol and p-phenylphenol.

Suitable anti-cleavage agents include carboxylic polyamides, polyurethanes, substituted polyacrylamides, polyvinylpyrrolidone, hydrazides, compounds having 1 to 6 amide groups, proteins, compounds having tertiary amine and terminal amide groups, compounds having amidine groups. cycloaliphatic amine compounds and aliphatic acyl ureas. Stabilizers may be present in the heterogeneous melt hydrolysis step or may be added to the hydrolyzed polymer after this step.

When the heterogeneous melt hydrolysis reaction is complete and a satisfactory number of unstable monomer units have been removed from the polymer molecules, the remaining reactant is removed from the treated polymer. Separated parts or reaction products from the chain and any unreacted substances such as trioxane should also be removed. Formaldehyde is 9,57432 the major reaction product resulting from chain shortening from oxymethylene polymers in hydrolysis and is believed to consist of the sequential detachment of oxymethylene terminal units at the end of the polymer chain. In some cases, especially when the polymerization reaction product is rapidly hydrolyzed, the hydrolyzed material may contain some unreacted trioxane. According to a preferred embodiment of the present invention, the chemical reactant, formaldehyde, trioxane and other volatile substances can be removed by suddenly reducing the pressure under which these substances have been kept, which causes the volatiles to evaporate with respect to temperature. The lower pressure should be in the range of about 0.01-3.5 atm, and is preferably achieved by subjecting the agents to atmospheric pressure or low vacuum (about 0.01 + atm). If desired, the stabilized polymer can then be extruded and further processed. In some cases, the extruded filaments are pelletized after compression and stored until the polymer is ready for use.

In a preferred embodiment of the present invention, the time that the molten polymer is maintained at elevated temperatures and pressures in the presence of a reactant in the vapor state (this time is known as the residence time) is from about 0.1 minutes to about 15 minutes. The temperature range is preferably about 16-20 ° C.

The pressure range is that required to maintain the 2-phase reaction, i.e., the polymer in the molten state and the reactants in the vapor state; preferably it is less than 11 atm.

The residence time, temperature and pressure are interdependent and are preferably maintained so that the polymer / reactant system remains in the state described above and the reaction proceeds sufficiently to remove the desired number of unstable units from the ends of the polymer molecules so that they terminate in relatively durable units.

In the following examples, a trioxane-ethylene oxide copolymer containing about 2% by weight of oxyethylene groups divided into oxymethylene chains was used. The reaction product obtained from the polymerization reaction vessel was also washed with water and dried to neutralize and remove the catalyst and to remove unreacted trioxane.

In Comparative Examples 1-1U, a two-stage extrusion according to a known method, i.e. a known homogeneous (liquid-liquid) hydrolysis method, has been used. The K i value of the product means the weight loss of the polymer per minute, as a percentage, determined by heating the polymer stabilized with an antioxidant and a formaldehyde detergent in an open vessel in a convection oven at 230 ° C.

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Comparative Examples 1-lU

The polymer is pressure fed to a one inch single screw extruder with a length to diameter ratio of 20/1. The feed section of the extruder had 5 turns and the groove depth was 11.7 mm. Thus, the polymer is fed under pressure to a melt hydrolysis measuring section with 6 turns and a groove depth of 1.5 mm. The reactant is pumped to the extruder at the beginning of the melt hydrolysis section. Next is a reduced section with one and a half turns with a groove depth of 0.56 mm.

This reduced section maintains the pressure in the melt hydrolysis section. The polymer / reactant system then passes through a portion with air holes (which can be called a low pressure portion) with four turns at a groove depth of 5.6 mm. When the heated, pressurized polymer / reactant system enters the air-sloped portion, the pressure drops abruptly and formaldehyde, reactant, and other volatile components escape through the perforation. The treated polymer then passes through a pumping (i.e. pre-compression) section with 3 1/2 turns and a groove depth of 1.5 mm where the polymer condenses. The polymer is then extruded through a die. The chemical stabilizers are then blended into the polymer and re-extruded.

The operating parameters and the obtained Kjj23Q_arv ° t are summarized in Table I.

Table I

Comparative Solvent TEA Hydrolysis Hydrolysis Product

example%% temperature pressure, atm ^ D230 ° C

n ° o c 1 5.3 b 177 More than 53 0.029 2 5.3 k 205 "53 0.0l17 3 5.0 U 20b" 53 0.023 li 8.6 b 202 "53 0.025 5 10.0 b 178" 53 0.0211 6 10.0 U 178 "53 0.02b 7 9.7 11 20b" 53 0.027 8 10.0 11 178 "53 0.020 9 10.0 U 177" 53 0.018 10 10.0 b 20b "53 0.031 11 7.6 2.5 198 "53 0.021 12 7.6 2.5 192" 53 0.027 13 7.5 2.5 190 "53 0.022 111 7.8 2.5 191" 53 0.023

Triethylamine (TEA) and water 57432

The following Examples 15--29, which illustrate the unexpectedly low KD23o values obtained by adhering to the process principles taught by this invention, are implemented using an extruder with two co-rotating toothed screw coils. The overall length / diameter ratio of the machine is U3 and it has feed and melting sections with a length to diameter ratio of 12.5; a hydrolysis section and a solvent injection port having a length / diameter ratio of 20.5 and volatile matter removal and extraction pumping compartments having a length to diameter ratio of 10. The melt lock is formed at the beginning and end of the liquid polymer vapor reactor hydrolysis zone. Otherwise, the entire screw comprises right-handed screw sleeves; i.e., barrier barriers have not been used. As in Comparative Examples 1-1, the chemical stabilizers are then mixed with the hydrolyzed polymer and the mixture is re-extruded. The operating parameters and the obtained K ^^ Q values are summarized in Table II.

Table II

Example Solvent TEA Hydrolysis Hydrolysis Product No.%% Temperature Pressure, atm ^ 230 ^

° C

15 5.0 li, 5 207 7 0.015 16 5.0 li, 5 193 7 0.016 17 5.0 li, 5 210 7 0.016

18 5.0 li, 5 193 7 0.01U

19 8.1 lt, 0 196 7 0.015 20 9, U 1 +, 0 2U1 7 0.013 21 9, li U, 0 2ll1 7 0.015 23 10.0 li, 5 202 7 0.017 2k 10.0 li, 5 202 7 0,015

25 13.0 l, 5 193 7 0.01 U.

26 13.0 li, 5 210 7 0.013 27 15.0 li, 5 193 7 0.0llt 28 15.0 li, 5 210 7 0.0llt 29 12.0 li, 0 202 7 0.012 1

Triethylamine (TEA) and water.

To eliminate the possibility that unexpectedly low K i values would be caused by higher shear ratios in the twin-screw extruder, a series of experiments were performed, as in Examples 15-29, with the screw speed being more than 12,574,32 times double. The negative results obtained in Examples 30-39, Table III, support the conclusion that the significantly improved K i values are the result of two-phase, i.e. molten polymer / vapor reactive hydrolysis.

Table III

Example TEA amount Screw batch speed ^ D230 ° C

No. r / min 30 Excess 70 0.013 31 "90 0.013

32 "110 0.01H

33 "130 0.015 3 * t" 150 0.017 35 Stoichiometric 70 0.015 36 "90 0.013 37" 110 0.015 38 "130 0.013 39" 150 0.012

In all of the above examples, the following stabilizers were combined with the polymer prior to K 2 analysis (weights based on polymer weight): 0.03 wt% melamine, 0.1 wt cyaniguanidine, and 0.5 wt% 2,2'-methylene bis (U-methyl-6-tert-butylphenol).

The results show that at a hydrolysis pressure above 53 atm Kp230 ° C above 0.018, generally about 0.02-0.0 U; with a low hydrolysis pressure of 7 atm,

Kj) 230 ° C is less than 0.018, average 0.01U. Because two different devices were used (different devices in the comparative experiments than in the experiments according to the invention), a further series of experiments was performed to determine the significance of the device type; as shown in Examples 30-39, the screw coil speed is not significantly affected. The copolymers of this invention are useful for conversions into films, fibers, castings, etc. Using melt extrusion, compression molding, injection molding, and the like. known manufacturing methods.

Claims (6)

57432 13 A process for the thermal stabilization of a normally solid oxymethylene copolymer having a melting point above about 150 ° C, wherein the molecules of this copolymer contain 60-99 * 6 moles of relatively unstable monomeric oxy-methylene units mixed with relatively stable monomeric -OR monomers. at least two carbon atoms directly bonded to each other, in the polymer chain between two valencies, all substituents in this chain R being inert, wherein at least a portion of the molecular end moieties contain these unstable monomer units, and this copolymer and about 2 to about 25 weight percent water, alcohol, or trioxane or mixtures of these substances is introduced into the reaction zone, characterized in that a) a heterogeneous system consisting mainly of a copolymer in the molten state and water, alcohol or trioxane vapor, or a vapor mixture of these substances, is formed, keeping the reaction a ion zone at a temperature higher than the melting point of the copolymer and at a pressure sufficient to keep the copolymer in the molten state and in the vapor state of said reactant or mixture thereof; and b) reacting the molten copolymer with said substance or mixture of substances under the aforementioned temperature and pressure conditions for about 0.1 to about 15 minutes to remove unstable monomeric oxymethylene units from the end portions of the copolymer molecules so that at least 90% of the resulting molecular polymer chains remain. Process according to Claim 1, characterized in that the temperature is 160 to 240 ° C. Method according to Claim 1, characterized in that the pressure is less than 10.2 atm. Process according to Claim 1, characterized in that the permanent units are oxyethylene units. Process according to Claim 1, characterized in that the alcohol is methyl alcohol, ethyl alcohol, n-propyl alcohol or isopropyl alcohol. A method according to claim 1, characterized in that the copolymer is reacted with said substance or mixture of substances under basic conditions.