CA1045741A - Process for the polymerization of formaldehyde - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Polyformaldehyde is prepared and simultaneously stabilized by feeding anhydrous formaldehyde to a reaction medium containing a chain transfer agent and a liquid organic diluent non-solvent for the polyoxymethylene and non-reactive towards the other constituents of the reaction medium and containing in a dispersed form as catalyst-stabilizer a block copolymer A-B carrying ionic couples in which - A is a polylactonic block of recurring units - B is a polyactamic block of recurring units

Description

The invention relates -to a process for the polymerization of formaldehyde and simultaneous stabilization of the forming polyoxymethylene.
The invention further relates to the stabilized polyoxy-methylene obtained by the said process~
Formaldehyde polymers (or polyoxymethylenes) are known in the art, of a molecular weight amounting to 10,000 at least, exhibit-ing mechanical properties such as tenacity, exceptional ~tability in size, hardne5s, resiliency and dielectr.ic properties, such as `
to make them useful as plastics for special uses.
The polyoxymethylene obtained rom formaldehyde is generally prepared by introducing the anhydrous gaseous monomer into an inert (non reactive) organic liquid medium, in the presence of a catalyst.
Cataly~ts useful for the purpose are of various nature, such as amino compoundc, hydrazines, arsines, stibines and pho~phines, alkali metal salts of organic acids, boron and aluminum halides.
The resulting polyoxymethylene contains in each macromolecule at lea~t one terminal hydroxyl which makes the polymer un~table under the action of heat, wherefore the polyoxymethylene i9 txeated in

2~ order to'co~vert the said hydroxyl group~ to further groups of higher stability.
Conversion to ester or ether groups are therefore known, such a~ by reaction with acetic anhydride or orthoformate of ethyl or methyl, respectively, or to urethane group-~ by reaction with i9 ocyanates.
As is well known, notwithstanding the conversion of the tenminal groups, acetal polymers are practically useless as plastics on account of their high sensitivity towards oxygen, heat, ultra-violet rays and traces of impurities always contained in a technical product. - 2 ~ s~

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Consequently, decomposition of the polyoxymethylene occurs, especially in the hot, e.g. during extrusion and moulding in a molten condition.
~ n order to counteract these undesirable phenomena~ the art has resorted to the use of antioxidants which are generally of a phenolic nature (substituted phenols and substituted diphenols) which are incorporated in the formaldehyde polymer.
However, since, notwithstanding this stabilization by means of antioxidants, degra~ation products of the macromolecular chain are still evolved, substance~ adapted to block the fonnaldehyde and the oxidation products thereof (such as formic acid) are incorporated in the polyoxymethylene. These blocking substances are of a funda mentally basic nature, of a rather high molecular weight and are normally selected among various polyamides.
The problem~ arising in the polymerization of formaldehyde and in the ~tabilization of the resulting polyoxymethylene are of various nature.
Thus, e.g., catalysts known heretofora yield a polyoxy-methylene with a certain degree of dispersion of the molecular weight.
Moreover, the said catalysts should be accurately removed from the polymer, for their presence promote~ the polymer decom-position more particularly during the hereinbefore mentioned treat-ment in a molten condition.
Problems moreover arise in the stabilization of the poly-oxymethylene, more particularly during the addition of the polymeric basic sllbstances adapted to block the formaldehyde and the oxidation products theraof. The polyamides normally employed for the purpose exhibit namely a certain incompatibility with the polyoxymethylene and homogenization thereof with the polyoxymethylene meets with at

- 3 -57~least two obstacles of a technical and of a chemical-physical character.
A~ is well known, the addition of a polyamide (or of compounds similar in function) to the polyoxymethylene is effected during ex-trusion of the polymer and a thorough homogenization is usually effected by a machine (extruder) which effects a considerable shear-ing action on the macromolecular chain of the polyoxymethylene.
This leads to splitting phenomana of the polymeric chain and to a "zippering" of the two chains resulting from ~plitting, 10 thereby entailing a sharp decrease of the molecular weight and a loss of useful products.
All the above disadvantages are obviated or at least con-siderably ~educed by the process of the invention, according to which the polymerization of ~ormaldehyde is carried out in the presence of a catalyst which, on completion of polymerization, re-mains permanently linked to the macromolecular chains of the poly-oxymethylene and is sLmilar in function to the stabilizers of the prior art adapted to block the products of degradation of the poly-oxymethylene.
Thus, the invention provide~ a process for preparing and simultaneously stabilizing a polyoxymethylene, characterized by feeding anhydrou~ monomeric formaldehyde to a reaction medium com-prising an organic diluent which i~ liquid and non-solvent for the polyoxymethylene under the reaction conditions and inert (non-reactive) towards the other constitu~nts of the reaction medium, and comprising, dispersed therein, from 0.001 to 5 parts by weight to 100 parts by weight polyoxymethylene of a catalyst-stabilizer con-sisting of a block copolymer carrying ionic couples on its macro-molecular chain, the said block copolymer being defined by the _ L~ _ ~09~5~
general structure A-B, wherein:
-A is a polyactonic block consisting of recurring units: :

t ~-~PMl)-obtainable from at least one monomeric lactone of the general formula:
r (PMl~
~ O

-B is a polylactamic block consisting of recurring units:
_ _ ,.~

_ ~- ( PM2 )~
H

obtainable from at least one monomeric lactam of the ganeral formula:

~(PM2) C - NH

wherein PM~ and PM2 are linear polymethylene chains having from 2 to 13 and 3 to 13 carbon atoms, respectively, non substituted or having at least one hydrogen atom replaced by an alkyl, aryl, ~ alkylaryl or cycloalkyl radical.

The ~aid block copolymer has generally a molecular weight of from at least 1,000 up to 50,000 and contains generally the block B in a proportion of from 50 to 99~ by weight.
As is known in the art, the polymerization of lactamic and lactonic monomer~ in the presence of an anionic catalyst gives rise to a copolymer of the A-B type in which A i~ the polylactonic block, and B i the polylactamic block, On completion of the ': ' . . ' ; ' :
.: .
. ~ . , i7~
polymerization reaction, the copolymer A-B has two ionic couples~
on its lactamic and its lactonic part, respectively.
In this connectionp reference i5 made to Makromolekulare Chemie 115 (1968), pages 33-42, ~ (1969) pages 34-539 89 (1965) pages 27-43 and to Fortschritte der Hochpolymeren-Forschung 2, 1961, pages 578-595.
The ionic couples give rise to the pol~merization of for-maldehyde with the production ~f a polyoxymethylene of a high molecular weight along a typical progress of the living polymers~
Whichever the reaction mechani m, the process according to the invention affords the following advantage~:
- extremely high polymerization rate of formaldehyde with practical-ly quantitative yields with respect to the formaldehyde feed and production of a polyoxymethylene in which the ratio of the average ponderal molecular weight to the numerical one i9 lower than 2;
- possibility of carrying out the polymerization with a high con-centration of the polyoxymethylene, more particularly with a weight ratio of the polyoxymethylene to the diluent up to 1:1, though still obta~nin~ a sufficiently fluid su~pension from which the formaldehyde polymer i9 easily separaked in the form of a powder of a high bulk density, generally of the order of o.6-o.8 g/ml;
- the polymeri~ation i9 carxied out avoiding in a practically com-plete manner ~he polyoxymethylene incrustation~ on the polymeri-zation apparatus;
- the resulting polyoxymethylene contains the stabilizer adapted to block the degradation product evolved from the pol~mer during the subsequent processing at a molten state and, more partieularly, .

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during moulding;
- thorough homogenization at a ~olecular level of the stabilizer and polyoxymethylene, which ultimately makes small quantities of stabiLizer ~ufficient;
- the stabilizer cannot be extracted because it is linked to the macromolecular chains of the polyoxymethylene.
PREPARATION OF THE CATALYST-STABXLIZER
. .~ .
The catalyst-stabilizer consists of the previously defined block copolymer A-B and is prepared by catalytic polymerization of one or more laetonic monomers, with one or more lactamic mono-mer~ selected among those previously defined.
The preferred lactones are ~-caprolactone, ~-valero-lactone, ~-propiolactone, pivalolactone and ~-enanthiolactone.
The preferred lactams a~e: ~-caprolacta~, ~-pyrrolidone, ~V-lauryllactam, a-piperidone, and ~-enanthiolactam.
The said pol~merization is carried out in the presence of an anionic cataly~t consisting of a metal derivative of one of the said lactams.
More particularly, the said metal derivative has the following ~tructure:

C CC__ N ~

wherein Me denotes an alkali or alkaline earth metal and Z is 1 or 2. The preferred al~ali metal are: lithium, ~odium and potassium, calcium being the preferred alkaline earth metal.
The polymerization medium can be admixed with the 14L57~L

pre~ormed catalystg i.e., in the form of a metal derivative of the lactam For practical reasons, the alkali or alkaline ea~th metal or an organo-metallic derivative thereof or its hydride, alkoxide, phenoxide or its hydroxide is preferably introduced into the said medium containing the lactam and the metal derivative of the lactam is formed in the polymerization mediu~.
The preparation of the block copolymer A-B is preferably carried out in an anhydxous atmosphere free from oxygen and at a tPmpera ure which can vary within a wide range, g~nerally from -20 to +300C.
The be~t suited temperature should be selected within the range of values defined above, depending upon the type of monomers and the compo ition of the finished copolymer.
Accordingly, the polymeri2ation period can vary from 2 minutes to 6 hours.
In any ca-qe, the proportion of catalyst (metal-lactam) i~
of from 0.1 to 10 moles to 100 moles of the monomer charge.
The c~polymeri~ation of the lactam with the lactone can be carried out in suspension, in ~olution or in the absence of solvent~
or diluentq.
The re~ulting block copolymer A-B i9 separa~ed from the reaction medium and can be utilized as such for preparing and stabilizing the polyoxymethylene.
However, in the preferred embodiment, the raw copolymer A-B
i9 subjected to treatments adapted to:
- separate in a particularly pure form the copolymer A-B containing the catalytically active nuclei in its macrGmolecular chain, - bring the copolymer A-B to a particulate form suitable for the 1~4~

subsequent formaldehyde polymerization.
More particularly, the A-B copolymer9 is purified in order to separate the unreacted monomers, the lactone homopolymer or copolymer, the fre2 catalytic residue (not chemically bonded to the macromolecular chain) or other possible impurities.
Purification is generally carried out by dissolving the A-B copolymer in a ~olvent, followed by precipitation and washing of the A-B copolymer. Solvents suitable for this treatment are those in which the impurities are highly soluble and which do not destroy the catalytically active nuclei bonded to the macro-molecular chain. Generally, the solvent belongs to the following general classes: chlorinated aliphatic hydrocarbons, aromatic hydrocarbons, chlo~inated aromatic hydrocarbons, nitrogenous aromatic compounds, substituted amides, sulphoxides and others.
Examples of such solvents are: nitrobenzene, chlorobenzene, toluene, xylene, dimethylformamide and dimethyl sulphoxide According to an embodiment, the raw A-B copolymer i9 dis-solved in the solvent at a temperature of from 0 to 200C. The copolymer A-B is then precipitated by cooling the solution and/or addition of a non-solvent for the copolymer. Specially suited non-solvents for thi~ purpose are aliphatic hydrocarbon~ ~uch as hexane, heptane, octane and others~ The A-~ copolymer i~ then filter~d and wa~hed by means o~ the above described solvent, till the impurities are no longer present in the filtrate. The latter steps can be carried out at room temperature or above.
The resulting A-B copolymer is employed for preparing and simultaneou~ly stabilizing the polyoxymethylene by operating with the A-B copolymer disper~ed in a diluent, in order to promote con-tact of its active nuclei with formaldehyde. Accordingly, the A-B

_ g _ 5~4~
copolymer should be advantageously brought to a finely subdivided particulate form.
For this reason, at the stage of the above-discussed puri-fication treatment where tha A-B copolymer forms a precipitate, the conditions are generally so adjusted that the precipitate is from 1 to 300 microns, preferably from 1 to 100 microns in particle size.
The above-discussed treatment may exhibit special aspects depending upon whether the A-B copol~mer derives from polymeriza-tion in solution~ in suspension or in the absence of solvents or diluents.
Thus, eOg., in the case of a bulk polymerization or a poly-merization in a molten s~ate, the A-B copolymer is desirably pul-verised and dissolved in a suitable solvent or it is directly dis-solved in a molten state. The A-B copolymer is then precipitated and washed The dissolution and precipitation steps can be re-peated several times.
The A-B ~opolymer obtained by the ~olution polymeriza~îon is normally precipitated by cooling and/or addition of a non-solvent for the copolymer, followed by filtering and washing.
It will be obvious from the above discussion that the poly-merization is preferably carried out in solution or in suspension, inasmuch as the impurities remain solubilized at least in part in the diluent or solvent. In this ca~e, this dispenses then from dissolving the A-B copolymer in the aforesaid solvent in which the impurities are soluble and even of precipitating it in the case of a suspension polymerization.
In this ca~e also, the suspension technique is generally preferred because the A-B copol~mer is then obtained in the form of finely subdivided particles (1 to 300 micron in size) suitable s~
for the polymerization of the formaldehyde.
PREPARATION OF THE POLYOXYMETHYLENE
As previously stated, the stabilized polyoxymethylene is prepared by supplying anhydrous formaldehyde to a reaction medium containing an organic diluentJ which is liquid under the reaction conditions, non-solvent for the polyoxymethylene and inert (non-reactive) towards the other constituents of the reaction medium, ; the said medium containing the A-B copolymer in a dispersed form.
Diluents useful for the purpose are of various nature such as ethers Idiethyl ether and dimethyl ether), hydrocarbons (pentane, hexane, heptane, decane, cyclohexane, decahydronaphthalene, xylene, benzene and toluene) and chlorinated hydrocarbons (methylene chloride). The preferred diluents are hydrocarbons, more parti-cularly those having 5-10 carbon atoms in the molecule.
The formaldehyde employed shall preferably be highly pure.
In actual practice, the water contqnt can be less than 100 ppm, the content of the other impuritie~ should be less than 100 ppmO
A formaldehyde having these properties can be obtained for instance by ths purifying methods disclosed by U.SO patents 3,118~747 and 20 3,184,g00.
The polymerization temperature can vary within wide limits, iOe. from about -120C to the boiling temperature of the organic thinner employed as reaction medium, in any case not exceeding 110aC, the pressure being generally the atmospheric pressure, ; though a higher or lower pressure can be employed. The best re-; sults are obtained by adopting a temperature range of from -30C
to 70Co ~he polymerization medium can include an adjusting agent for the molecular weight such as water, formic acid or methanol, . - 11 -.' ~

7~
the level of which can be varied during the monomer purification step, or a carboxylic acid having at least three carbon atoms, an aliphatic alcohol having at least two carbon atom~, a cycloaliphatic or aromatic alcohol, an anhydride of a carboxylic acid, an amide, an imide, an imine or others in a proportion of the order 0.000001 up to 0.5% moles to the polymerized formalde~yde.
The formaldehyde polymerization can be carried out dis-continuously, semi~continuously or continuously.
When polymerizing continuouslyg a te~hnique as diRclosed, e.g., by U.S. patent 3,458,479 can be adoptedO The suspension of the polyoxymethylene discharged from the polymerization zone is submitted to filtering and the polymer is separated in the form of granules of from 5 to 700 micron~ depending in any case upon~
the grain size of the A-B copolymer employed.
The dried polyoxymethylene i9 in the form o~ a powder of a bulk density within the previously mentioned range o~ values, The resulting polyoxymethylene contains at least one terminal hydroxyl group in each macromolecule~ which makes the product thermally unstableO In order to avoid a polymer degrada-tion at the end of the chain, the hydroxyl groups are substituted by ester or ether groups ~uch as by treatment with acetic anhydride or orthoformate of methyl or ethyl, raspectively, or by urethane ~OUp9 by treatment with an isocyanateO
The processes known in the art can be employed for such treatment. The thus treated polyoxymethylene is finally admixad with an antioxidant, such as a phenolic or a diphenolic compound, the resulting composition being suitable for the proces~ing to moulded articles or products by injection moulding, extrusion or techniques known as "roto-moulding" or "blow-moulding"O
_ 12 - :

:

.

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It sh~uld be noted that the best results are obtained from a stabilized polyoxymethylene containing the polylactamic block B in a proportion of from Ool to 2 and preferably from 0.2 to oO6~ by weight of the polyoxymethyleneO
In the following Examples, the parts and percentage~ shall be under~tood by weight, unless otherwise specified.
Example 1 Preparation of the A-B coPolYmer 110 parts of ~-caprolactam stirred at 110C in an inert atmosphere are admixed with 0O10 part finely subdivided lithium metal. On completion of the formation of the metal-lactam, 100 parts pure dlmethyl sulphoxide, then 6.0 parts pivalolactone are added, following by heating at 145C and stirring during 4 hours, during which an increase in vi9co~ ity of the ~olution occur~. The pre-cipitation of the polymer in a fine powder is then effected by cooling, followed by thorough washing by decanting with toluene, thereby removing any soluble residue in the polymerization medium, ultimately obtaining the A-B copolymer dispersed in toluene.
~he analysis o~ the A-B copolymer shows:
- ~ conversion of the monomers = 93060 - nitrogen percentage = 11.72 - lactam percentage - 94.60 - reduced vi9c09ity (liters.g 1) = 1.26 In the Examples,the A-B copolymer viscosity is always measured at 35C from a solution of m-cresol containing 0O5 wt% copolymer, and expressed as the ratio ~reduced 4S79~L

in litersOg lo concentration - ~rain size =
\ 125 microns : 0~3%
125-88 microns : 2704~
88-44 microns '' 4302%
c~ 44 microns : 28.1~

Pure monomeric gaseous formaldehyde is charged at a rate of 2.5 parts per minute at the bottom of a polymerization reactor containing 1,000 parts anhydrous toluene and 0.5 part of tha pre-viously described copolymers A-B.
The polymerization is carried out with the stirred mass, in the absence of moisture and air, at a temperature of about 25C, by adding formaldehyde during 100 minutes and simultaneously 0.07 part anhydrous ethanol, followed by stirring during 10 minutes, filtering and drying at 60C in a vacuum oven, 242.5 parts polyoxy-methylene being recovered (yield with respect to the formaldehyde feed 96.8%). The polyoxymethylene has an intrinsic vi~cosity of 1.82 liters.g~l.
In the Ex~mples, the polyoxymethylene viscosity i5 always measured at 60C from a solution o p-chlorophenol with 2~ a-pinene containing 0.5 wt~ polymer, and exprcssed as the ratio7~ intrinsic =

~ relative in liters.g 1.
concentration The powder has a bulk density of 0.70 grams/ml and the following grain æize distribution:
~' 250 microns : 302 250-177 microns : 50.8 30177-125 microns : 45.0~
~125 microns ~ 0.8%.

.

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One part of the polyoxymethylene is esterified at 150-155~C during 20 min with a reac~ive system consisting of 1.5 parts pure acetic anhydride and 3.0 parts of a mixture of Clo-C14 n-paraffines, the pressure being such as to maintain the -~ystem at boiling conditions.
The suspension is cooled and filtered and the polymer is washed with toluene and acetone, dried in a vacuum oven at 60C~
the determined reackion yield being of 9~.3%.
The acetylated polyoxymethylene has an intrinsic viscosity and a bulk density similar to those of the non-stabilized polyoxymethylene.
The resulting polyox~methylene having a content of A-~
copolymer of 0.2~ is melted, e~truded at 190-220C and converted to granules 2x2 mm by a screw extruder with an automatic cutting blade.
The ~ollowing tests are effected on the granules:
- K220 = thenmal dagradation test at 220C in a nitrogen atmosphere, expressed as a rate of decomposition in percentage by weight of the polymer, per minute, during the first 30 minutes.
- D220 - thermal degradation test at 220C in airJ expressed in per-centage of weight lo~s of the polymer a~ter 10 minute-~ and 20 minutes heating, re~pectively.
The measurement~ are effected by a thermo-scale, the gaseous degradation products being continuously drained away by flowing a nitrogen and an air stream, respectivelyO
; The results are summarized in Table 1 sub POM-lo The acetylated polyoxymethylene is admixed with 0.35% pentaerythritol tetra (beta-4'-hydroxy-3',5'-di-tert-butyl phenyl) propionate and converted to granules in the above de9cribed mannex. The degradation , . .

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tests are carried out, the results of which are summarized in Table 1 sub POM-2.
Table 1 colour K220 D220 10' 20' POM-l white -3 5.6 20.8 POM-2 white 0.03 0.4 0.9 Example 2 Preparation of the A-~ copolymer 94 parts pure ~-caprolactam are admi~ed while stirred at 110C in an inert atmosphere with 0.2 part sodium metal in a 50%
paraffin dispersion. The metal-lactam i~ formed and after 20 minute~
6 paxts ~-caprolactone are added and the whole heated at 220C.
After 15 minute~, the melted mass is extruded under inert condition~ into a reactor containing 400 part~ boiling anhydrous pure dLmethyl sulphoxide and easily dis~olved by vigorously ~tirring.
The clear ~olution is cooled while adding anhydrous xylene~

thereby precipitating the A-B copolymer in the form of a fine powder, which i~ washed by decanting with xylene in order to remove the dimethyl sulphoxide and the further impurities present in the ~y3tem at the end of the polymeri~ation.
An A-B copolymer suspension in xylene is obtained and the following checks are made:
- % conversion of the monomers = 96.0 - nitrogen percentage = 11062 - reduced viscosity (liters.g 1) = 1030 ; - melting point (C) = 206 laV4574~

Preparation of the polyoxymethylene The procedure of Example 1 is followed, formaldehyde being supplied to a polymerization reactor containing 1,000 parts n-heptane and 1.4 parts of the above described A-B copolymer.
Formaldehyde is supplied at a rate of 2.5 parts per minute during 140 min and 0.15 part ethanol is also charged, the temperature being about 0C.
Stirring is then carried out during 10 minutes, followed by filtering and drying in a vacuum oven at 60~C, where~y 341.1 parts polyoxymethylen~ are recovered (yield with respect to the formalde-hyde feed of 97.2%). The resulting polyoxymethylene powder has an intrinsic viscosity of 1.50 liters.g , a bulk density of 0.62 grams/
ml and the following grain size distribution:
~ 350 microns : 6.8%
350-250 microns ' 45.3 250-125 micron~ : 44.6~
< 125 microns : 303%o The polyoxymethylene containing 0.4% A-B copolymer i~ acetyl-atad as described in Example 1 with a reaction yield of 9603~o Acetylation i~ not followed by appreciable variations in viscosity and compositlon of the polyoxymethyleneO
This acetylated polymer i9 melted and converted to granules as in Example 1 and subjected to the thermal degradation tests.
The results are summarized in Table 2 sub POM-3 0 The acetylated polyoxymethylene powder is admixed with 0.45~
n-octa~ecyl (beta-4'-hydroxy-3',5'-di-tert-butyl phenyl) propionate, homogenized and converted to granulesO The granules are submitted to the tests the results of which are summarized in Table 2 sub PoM-4.

, ,, .. ': , , . ~ :
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Example 3 (comparative Example) The formaldehyde is polymerized as described in Example 2 by utilizing tri-butylamine as catalyst (0.01%), ethyl alcohol as adjusting agent (0005%~ and n-heptane as polymerization medium, 78 parts polyoxymethylene to 100 parts formaldehyde feed are obtainedO
The polymer has an intrinsic viscosity of 1.53 liters.g 1 a bulk density of 0028 grams/ml and the following grain size distribution:
10~ 125 microns : 0.1%
125- 88 microns : 5.3%
88- 44 microns : 18.4 < 44 microns u 7602~
The polyoxymethylene is acetylated a~ described in Example 1 with a reaction yield of 90.1~, a polymer of an intrinsic vis-cosity of 1.57 liters.g 1 being obtained.
The acetylated polyoxymethylene is melted and converted to granulas in the usual manner. The results of the tests carried out on the granules are summarized in Table 2 sub POM-5.
996 parts acetylated polyoxymethylene powder are admixed with 4 parts polyamide obtained from the copolymerization of hexamethylenediamine adipate, hexamethylenediamine sebacate and ~-caprolactam in a weight ratio of 4:4:3, homogenized and granulat-ed in the usual manner.
The results of the tests carried out on the granulated product are summarized in Table 2 sub PoM-6.
991 parts acetylated polyoxymethylene powder are admixed with 4 parts of the previously described poly~mide and 4.5 parts n-octadecyl(beta-4'-hydroxy-3',5'-di-tert-butylphenyl )propionate .
The resulting acetylated polyoxymethylene which contains 0.4%

.. . .

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polyamide and 0.45% antioxidant is melted and granulabsd in the usual manner.
The granulated product is submitted to the tests the r~sultq of which are summarized~ to Table 2 sub POM-7.
Table 2 Colour K220 D220 10' 20' PoM-3 white 3 3O6 12.0 PoM-4 white 0.03 0.4 0.8 PoM-5 white 0O15 24.1 78.2 PoM-6 wbite oOo6 5~3 l9o9 PoM-7 white 0.05 OO9 1~8.
ExamPle 4 PreParation of the A-B copolymer 70 part~ pure ~-caprolactam maintained at 100~C while stirred in an inert atmosphere (by mean~ of a nitrogen st~ am) are admixed with 0.17 part finaly subdivided sodium metal in a 50~
paraffin disper~ion. After 15 minutes, 30 parts pure ~-caprolactam are admixed and heated to 170Co After 20 minutes, 400 parts an-hydrous dimethyl ~ulphoxide are 810wly added while vigorously stirring till a clear solution i9 obtainedO The temperature is lowered while 600 parts toluene are added to the stirred mass. The precipitation of the A-B copolymer occurs at about 85C with a toluene concentration of about 60%. Cooling to room temperature i9 effected and washing by decantation with anhydrous toluene yields a 16.7~ A-B copolymer suspension in toluene.
The analysis shows: -- ~ conversion of the monomers = 96O8 -nitrogen percentage = 8~76 - lactam percentage = 70.7 ~0 - melting point (C) = 164 ~C~45~4~
Preparation of the polyoxymet~ylene The procedure of Example 1 is followed and the polymerization reactor containing 1,000 parts toluene and 0.90 part A-B copolymer (as above described) is charged with formaldehyde at a rate of 2.5 parts/min during 100 minutes. The reactor maintained at about 20C
is further charged with 2.0 parts cyclohexanol. Stirring during 10 minutes, filtering and drying in a vacuum oven at 60C are effected and 242 parts polyoxymethylene are recovered (yield of 96.5~ with respect to the fonmaldehyde feed).
The polyoxymethylene has an intrinsic viscosity of 1.31 liters.g 1, a buLk density of 0.63 grams/ml and the following grain size distribution:
~ 350 microns : 10.1 350-250 microns : 47.3 250-125 microns : 35.2~
125 microns : 7.4%.
The polyoxymethylene having an A-B content of 0.36% is ;~
;~ acetylated as in Example 1 with a reaction yield of 9607%.
The ViSC05 ity and composition of the acetylated pol~oxy-methylene are similar to those of the non-acetylated polymer.
The acetylated polyoxymethylene in powder form is admixed with the antioxidant 4,4'-butylidenebis(6-tert-butyl-meta-cresol) in a proportion of 0.45~, homogeniæed, melted in a cell (as for thermoplastics) of a Plasti Corder PLV 151 (Brabender)0 The cell is thermostated with heating oil at 220C, the number of revolutions of the rotor being of 60/min.
A test is carried out with a ~welling time of 12 minutes and the percentage of weight loss during plasticizing is determined, the intrinsic viscosity and thermostability being ascertained on the .
:,.

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discharged polymer.
~he results are summarized in Table 3 sub PoM-8. In the ;? table~ ~C denotes the percentage of weight lo~s of the polymex during melting in the cell, in the form of gaseou~ products (formaldehyde and oxidation products thereof)l In the same Table,~ e denotes the intrinsic viscosity4 Example 5 The procedure of Example 4 is followed, formaldehyde being polymerized with the A-B copolymer described in the same Example till a polyoxymethylene of an intrin~ic viscosity of 1030 litersOg 1 is obtained which contain~ 0.45~ A-B copolymer.
~ he polyoxymethylene is acetylated a~ described in Example 1 . and the acetylated polyoxymethylene is admixed with 0.45~ 4,4'-: butylidene-bis(6-tert-butyl-meta-cresol).
. The mixture is homogenized and the tests described in Example

4 are carried out.
The results are given in Table 3 sub POM-9 Example 6 (comParatiVe ExampleL
A sample of finely subdivided polycaprolac~am and 4,4'-butylidenebi~ (6-tert-butyl-meta-cresol), in a proportion of 0.36~
and 0045~, respectively, are added to an acetylated polyoxymethylene in powder form, of an intrinsic vi~cosity of 1~31 liters.g 1, which has been obtainad as described in Example 3. The mixture is homo-genized and submitted to the tests described in Example 4.
The results are summarized in Table 3 sub POM-10 Example 7 (come~tive Example) A sample of finely subdivided polycaprolactam and 4,4'-butylidenebis(6~tert butyl-meta-cresol) in a proportion of 0.45%

and 0.45%, respectively, are added to an acetylated polyoxymethylene in powder form. - 21 -.

S'~4~1 The polycaprolactam and the aceylated polyoxymethylene are the same as those of Example 60 The mixture is homogenized and submitted to the tests de-scribed in Example 40 The results are summarized in Table 3 sub POM-ll.
Example 8 ( comParat ive ExamPle A block copolymer A-B obtained by anionic copolymerizat.ion of ~-caprolactam and ~-caprolactone,in a mutual weight ratio sLnilar to that of the A-B copolymer of Example 4 (~ lactam = 70~7) is ad-10 mixed together with 4,4'-butylidenebi~(6-tert~butyl-meta-cresol) in a proportion of 0.35% and 0.45%J respectively, to an acetylated polyox~methylene in powder form sLmilar to that of Example 6.
The mixture is homo~enized and submitted to the tests de-scribed in Example 4.
The results are summarized in Table 3 sub POM-12.
Example 9 ~comE~ativa Example) A block copolymer A-B, similar to that described in Example 8, i8 added together with 4,4'-butylidenebis (6-tert-butyl-meta-cre~ol) in a proportion of 0.45% and 0.45~, respectively, to an acetylated polyoxymethylene in powder form ~imilar to that de-scribed in Example 6.
The mixture i~ homogenized and submitted to the tests de-scribed in Example 4 .
The results are summarized in Table 3 sub POM-130 ~C K220 ~ e (liters~g PoM-8 0.6 -5 1.27 POM-9 0.5 0.04 1.28 POM-10 1.2 0.10 1.20 POM-ll 1.0 0.07 1.25 POM-12 1.1 0.09 1.20 POM-13 0.9 0.07 1~26 . .

7~1 Example 10 Preparatlon of the A-B_copol~mer 73 parts a-pyrrolidone maintained under ~tirring at 60C in an inert atmosphere are admixed with 0.30 part finely subdivided sodium metal in a 50~ parafin dispersionO
After formation of the metal lactam, 100 parts anhydrous toluene are added7 the temperature is raised to 30C and 15 parts -valerolactone ar~ added. After 3 hours, the su~pension of the A-B copolymer i diluted and thoroughly washed with ~nhydrous toluene, whereby the soluble polymerization residues are removed.
The analysis of the A-B copolymer shows:
- % conversion of the monomers = 94 - nitrogen percentage - 13058 lactam percentage = 82.5 - reduced viscosity (liters.g 1) = 10300 PreParation of the ~olYoxymethylene The procedure of Example 1 i9 followed and formaldehyde is charged to a reactor containing 1,000 parts cyclohexane and 4.0 parts o~ the above described A-B copolymer.
: 20 Formaldehyde i8 fed at a rate of 2.5 part~ per minute during 300 minutes~ and 0.38 part butanol is also introduced, the tempera-ture being maintained at about 35C.
Finally, stirring during 10 further minutes, filtering and drying are effected and 71602 parts polyoxymethylene are recovered (yield with respect tothe formaldehyde feed 95.0%). The polyoxy-methylene has an intrinsic viscosity of 1.90 liters.g 1 a bulk density i: of 0. 78 grams/ml and its A-B copolymer content is of O . 56~.
One part of the polyoxymethylene thus obtained is etherified by a rsactive system consisting of 0.4 part triethyl orthoformate,O.8 ~)4~i74~L
part anhydrous dLmethyl acetamide and 2 parts of a mixture of Clo-C14 n-paraffins, the said system containing ethyl sulphate in a proportion of 0013% of the total of the liquid components.
After et~erification at 150-152cduring 15 minutes, the re-sulting suspension is cooled and filtered. The etherified polyoxy-methylene is washed with toluene containing 1~ triethanolamine, then with methanol, dried in a vacuum oven at 60C, whereupon a reaction yield of 98.8% is ascertained.
The polyoxymethylene is submitted to the following tests:
- intrinsic viscosity (~ e) = 1.85 litera.g~
- bulk density ( ~ a) = 0.79 grams/ml - alkali-stable fraction (FAS) = 97.4.
The latter determination i9 effected by dissolving the etherified polyoxymethylene in benzyl alcohol containing 1~ tri-ethanolamine and maintaining it at 150-152C during 20 minutesO
The polymer/benzyl alcohol ratio is 1:10. Precipitation of the polymer by cooling filtering, wa~hing with methanol~ and drying are then effected. The remaining polymer percentage is referred to as the alkali-stable fraction.
The etherified polyoxymethylene is moreover fractionated in a steel column ~illed with Celite (RoToM~)~ dimethyl acetamide ~eing employed as solvent and the temperature being programmed.
The analysis of the individual fractions shows that the polymer posses~es a polydisperqibility ratio (~w/~n) of 10680 The etherified polyoxymethylene is melted, converted to granules and submitted to the thermal degradation tests similarly as described in Example 1.
The results are swmmarized in Table 4, sub POM-14.
The etherified polyoxymethylene in powder form is admixed . .
:, . . .

57~3iL
with 0.5% of 4,4' thiobis(6-tert-butyl-meta-cresol), homogenized and converted to granules in the usual mannerO ~he granulated product i9 submitted to the tests the results of which are summarized in Table 4 sub POM-15.
Table 4 Colour K220 D220 10' 20' POM-14 white 0~02 4.0 8.5 PoM-l5 white 0002 0.3 0.7

Claims (14)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for preparing a polyoxymethylene, comprising the step of supplying anhydrous monomeric formaldehyde to a reaction medium comprising a chain transfer agent and an organic diluent which is liquid and non-solvent for the polyoxymethylene under the reaction conditions and inert towards the other constituents of the reaction medium, and further comprising, dispersed in said medium, from 0.001 to 5 parts by weight to 100 parts by weight polyoxymethylene of a catalyst-stabilizer consisting of a block copolymer carrying ionic couples on its macromolecular chain, said block copolymer being of the general structure A-B, in which:
A is a polylactonic block consisting of recurring units obtainable from at least one monomeric lactone of the general formula:

B is a polylactamic block consisting of recurring units:

obtainable from at least one monomeric lactam of the general formula:

wherein PM1 and PM2 are linear polymethylene chains having from 2 to 13 and 3 to 13 carbon atoms, respectively, non substituted or having at least one hydrogen atom replaced by a radical selected from the group consisting of the alkyl, aryl, alkylaryl and cycloalkyl radicals.

2. The method of claim 1, wherein the chain transfer agent is selected from the group consisting of water, formic acid, methanol, a carboxylic acid having at least three carbon atoms, an aliphatic alcohol having at least two carbon atoms, a cyclo-aliphatic or aromatic alcohol, an anhydride of a carboxylic acid, an amide, an imide or an imine.

3. The method of claim 2, wherein the chain transfer agent is in the proportion of the order 0.000001 up to 0.5% moles to the polyoxymethylene.

4. The method of claim 1, wherein the block copolymer has a molecular weight of from 1,000 to 50,000 and contains the block B in a proportion of from 50 to 99 wt%.

5. The method of claim 1, wherein the said lactone is selected from the group consisting of .epsilon.-caprolactone, ?-valerolactone, .beta.-propiolactone, pivalolactone and .omega.-enanthiolactone.

6. The method of claim 1, wherein the said lactam is selected from the group consisting of .epsilon.-caprolactam, .alpha.-pyrro-lidone, .omega.-lauryllactam, .alpha.-piperidone and .omega.-enanthiolactam.

7. The method of claim 1, wherein the diluent is selected from the group consisting of ethers, hydrocarbons and chlori-nated hydrocarbons.

8. The method of claim 1, wherein the reaction temperature is from -120° up to the temperature of ebullition of the diluent, said diluent having a temperature of ebullition not exceeding 110°C.

9. The method of claim 1, wherein the reaction temperature is from -30° to 70°C.

10. The method of claim 1, wherein the block copolymer A-B is obtained by catalytic copolymerization of said lactamic and lactonic monomers at a temperature of from -20° to 300°C
and during a period of from 2 min to 6 hours, the catalyst employed being present in a proportion of from 0.1 to 10 moles for 100 moles lactonic and lactamic monomers and being defined by the general formula:

wherein Me is selected from the group consisting of the alkali and alkaline earth metals, Z is 1 or 2 and PM2 is the said polymethylene chain.

11. The method of claim 10, wherein the block copolymer A-B is obtained by copolymerizing said monomers in a solvent for the forming copolymer A-B.

12. The method of claim 10, wherein the block copolymer A-B is obtained by copolymerizing said monomers in a diluent non-solvent for the forming copolymer A-B.

13. The method of claim 1, wherein the said dispersed catalyst-stabilizer is in the form of particles 1 to 300 microns in size.

14. A polyoxymethylene prepared by the method of claim 1, consisting essentially of polyoxymethylene and said catalyst-stabilizer permanently linked to the polyoxymethylene in a proportion of from 0.001 to 5 parts catalyst-stabilizer for 100 parts polyoxymethylene.