CA1105187A - Process for the polymerization of formaldehyde - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Polyoxymethylene is prepared by polymerizing formaldehyde in a reaction medium comprising an organic diluent non-solvent for the polyoxymethylene, a chain transfer agent and a catalyst consisting of a polylactone car-rying a terminal ionic couple on its macromolecular chain, said catalyst being used in an amount of from 0.001 to 10%
by weight with respect to the feed in formaldehyde.

Description

~ he present inv~ntion relates to the prepar~lt10n of formaldehyde polyn~ers in the presence of a new type o~
catalytic compound.
Formaldehyde polymers (or polyoxymethylenes) having a molecular weight of at least 10,000, endowed with mechanical characteristics such as toughness, hardness, resilience, exceptional dimensional stability and dielectric properties, such as to make them useful as plastic materials for engineering purposes, are known in the art.
These polyoxymethylenes are generally produced by introducing anhydrous formaldehyde monomer in a liquid organic reaction medium, operating in a large range of temperatures and in the presence of catalysts for the polymerization reaction.
In particular, the known catalysts suitable for this purpose are of various natures such as: amino com-pounds, e.g. aliphatic, cycloaliphatic and aromatic amines;
or arsines, stibines and phosphines in which the hydrogen atoms respectively bonded to arsenic, antimony and phos-~0 phorus are substituted by hydrocarbon organic radicals.
Other catalysts used in the art are hydrazines,organic acid salts, such as alkaline earth metal stearates, and compounds of the boron halides type (boron trichloride and trifluoride) and aluminum trichloride.
These known catalysts do not produce completely satisfactory results, mainly because they often do not allow an effecl:ive control of the molecular weight of the formaldehyde polymer. As a result, the polyoxymethylenes produced may present a great dispersion of the molecular waight with the related drawbacks~

Besides, said catalysts must be accurately re~oved from tlle polymer at the end of the polymerization ; ~ ~
.~ . ç, ç
i ~ .
pc/r~

~7 and there~orc the~ lnvolve costly purification treatments.
In fact, the residues of s~id catalysts which are present in the polyoxymethylene, bring,,about undesired depolymerization phenomenaO
These drawbacks are avoided, or at least greatly reduced, by the process of the present inventiorl, which essentially consists of polymerizing formaldehyde in the presence of a new catalytic compound.
Thus, the invention provides a process for the preparation of polyoxymethylene, characterized by feeding anhydrous formaldehyde monomer into a reaction medium comprising an organic diluent, which is liquid under the reaction conditions, non-solvent for the polyoxymethylene and inert (non-reactive) towards the other constituents of the reaction medium, a chain transfer agent, and a catalyst dissolved or dispersed in the reaction medium, .said catalyst being used in an amount of from 0.001 to 10 by weight with respect to the feed in formaldehyde and consisting of a polymer A carrying a terminal ionic couple on its macromolecular chain, said polymer A being a polylactone block cons.isting of recurring units:
r ~~
C ---(PMl)--O _ L .
obtained from one or more lactone monomers of the general formula:

C - ~PM ) Il 1 O
wherein PMl is a linear polymethylene chain having from 2 to 13 carbon atoms, non substituted or having at least one :
hydrogen atom replaced by a substituent chosen from alkyl, alkylenyl, cycloalkyl, aryl and aralkyl radicals and halo~en atoms.

pC/~LJ . - 2 - ' The preerred lac-tones are: ~-proplolactone, ~butyrolactone, ~-valerolactolle, E~caprolactone, t~-enan-tholact~ne, -the methyl derivatives of E-caprolactone in the position beta, gamma and delta~ p:ivolactone and alpha, alpha-dimethyl-~-isopropylidene-~-propiolactone.
The catalyst, or polymer ~, has preEerably a molecular weight of from 1,000 to 120,000 and is preferably used in an amount of from 0.01 to 1.0~ by weight with respect to the feed in formaldehyde.
As is known in the art, the polymerization of lactone monomers in the presence of an anionic catalyst gives rise to a polymer A having a ionic couple on the terminal group of the chainO
Reference is made in this connection to Makromoleculare Chemie 56, 179 (1962) and 97, 139 (1966) and to Journ. Polym. Sci., Chem. Ed.ll, 425 (1973).
It has been found that this ionic couple originates the formaldehyde polymerization with production of high molecular weight polyoxymethylene, according to a trend typical of the living polymers.
Whatever -the reaction mechanism may be, the polymer A acts as a homogeneous or heterogeneous catalyst, according to the preselected polymerization medium, and affords almost quantitative polymerization yields with respect to the formaldehyde feed.
The high catalytic activity is a particular characteristic of said polymers A.
Preparation of the catalyst (polymer A) The catalyst of the present invention is prepared by anionic polymerizarion of one or more lactones chosen ~rom those previously defined, the best results being obtained with -caprolactone, ~-propiolactone, ~-valerolactone -;~
and ~butyrolactone.

'~- PC/~uu ~ 3 ~
- ~', ., : :' The lactone polymerization i3 preferabl~
carried out in the presence o~ an anionic catalyst chosen from compounds defined by -the general fo~nulas:

(i) MeZ (R)z or (ii) Me (OR) wherein Me is a metal of Group IA, IIA, IIB or IIIA of the Periodic System of the Elements, R is an alkyl, aryl, alkylaryl, cycloalkyl or naphthyl radical, or else, only in the case of formula (i), a hydrogen atom, and z is 1 or 2.
Among these substances, those preferred are the metal-alkyl and metal-alkoxy derivatives of alkal.i or alkaline earth metals, and the metal-alkyl derivatives of metals of Group IIB of the Periodic System of the Elements, such as, e.g., zinc-dibutyl and cadmium-dibutylO
Other anionic catalysts useful for the lactone polymerization are those defined by the general formula:

X ~ CN - (PM2) - C,1 L 8~
wherein X is a metal of Group IA, IIA or IIB of the Periodic System of the Elements, or a substituted quaternary ammonium group, PM2 is a polymethylene chain having from 3 to 13 carbon atoms, having possibly one or more hydrogen atoms replaced by hydrocarbon radicals, such :.
as alkyl or aryl radicals, and y is 1 or 2.
: Other anionic catalysts suitable for t~Le purpose : are alkali metcLls, such as lithium, sodium and potassium and the magnesi.um alkyl derivatives (Grignard's compounds).
The prepcLration of the polymer A is preferably carried out in an anhydrous medium and in the absence of . . ~..: -oxygen, at a temperature which may vary in a large range and generally from -100 to 2~0C. The most suitable ' ' ; ~ pC/~UJ ~ 4 ~

5~
temperatur~ is chosen as a unction of thc type of preselected l~ctone monomer and the other predetermined objects.
Accordingly, the pol.ymerization times may vary from 1 minute to 120 hours.
In any case, the catalyst is used in an amount of from 0.01 to 10 moles per each 100 moles of lactone monomer subjected to polymerlzat.ion~
Polymerization is preferably effected with pure monomers, above all free from moisture and from o-ther compounds with active hydrogen ions.
The lactone polymerization may be carried out by the suspension or solution technique, or else in the absence of solvents and diluentsO
The polymer A thus obtained is separated from the reaction medium and may be used directly for the prepaxation of the polyoxymethyleneO . ~-. .
In the preferred embodiment, the raw polymer A
is purified to recover the polymer A containing the active catalytic centres on the macromolecular chain, in a parti-cularly pure form.
Preferably, the polymer A is brought to a particulate form during or possibly before said purifica-tion treatment, inasmuch as said polymer A is then in a physical form suitable for the subsequent reaction with formaldehyde. . ~.
In particular, the polymer A may be purified by dissolvin~ :it in a solvent, this being followed by precipitation and washing of the polymer A.
Solvents suitable for these treatments are aromatic hyd~o~-arbons, such as benzene and toluene, ~ .

pC~tLJ - 5 ~

q~ 7 chlorinated aliphatic ancl aromatic hydrocarbo7ls, ~nd ethers, especially aliphatic ethers.
It i5 ~lso convenient to use aprotic polar solvents, such as substituted amides, e.g. dimethyl-formamide and dimethylacetamide, sulfoxides, e.g.
dimethyl sulfoxide, and substituted phosphoramides such as hexamethylphosphoric triamide.
The non-solvents, suitable for the precipita-tion of the dissolved polymer A, are aliphatic hydro-carbons, such as hexane and heptane, and cycloaliphatic hydrocarbons, such as cyclohexane.
The dissolution and precipitation operations may be carried out at ambient temperature or at temperatures higher than ambient. The polymer A may be used in dissolved or suspended form, according to the reaction medium selected for the formaldehyde polymeriza-tion.
When the polymer A is used in the form of a suspension in a diluent it is advantageous to bring said polymer to a very divided particulate form, in order to promote contact of its active centres with formaldehyde.
In this case, the precipitation conditions are preferably adjusted so as to obtain -the polymer A in the form of particles having a size of from 1 to 300 microns and preferably from 10 to 100 micronsO
When the polymer A is obtained by the suspension technique, operation is conveniently carried out so as to directly obtain said polymer A in a particulate form suitable for the subsequent polymerization of formaldehyde.
In this case~ it may be sufficient to filter the suspension - and thoroughly wash the polymer A to remove the undesired impurlties <, . . .
~ pc/~uU ~ 6 -The polymer A obtained by the solution technique is usually precipita-ted by adding a non~so].vent for said polymer A, and the precipitate is submitted to the washing operations already described.
Preparation oE the polyoxymethylene .
The polyoxymethylene is prepared by feeding gaseous and anhydrous formaldehyde monomer into a reaction medium containing a diluent, which is liquid under the operative conditions, non-solvent for the polyoxymethylene and inert (non-reactive) towards the other cons-tituents of the reaction medium.
The diluents useful for the purpose are of various natures, such as, e.g., ethers (diethyl ether), hydro-carbons (pentane, hexane, heptane, octane, cyclohexane, benzene, toluene, xylene) and chlorinated hydrocarbons ; (methylene chloride). Preferred diluents are hydrocarbons, especially those having 5-10 carbon atoms per molecule.
In any case, said diluents should not interfere with the active centres of the polymer A and should be inert (non-reactive) towards the other constituents of the reaction medium.
The formaldehyde used should be endowed with high purity and dryness, in order to obtain a polyoxymethylene with hi~h molecular weight (at least 10,000). Such a formaldehyde may be obtained, e.g., by the purification processes described in U.SO patents 3,118,747 and 3,184,900.
The process of the invention is carried out in the presence of a chain transfer agent. The amount of chain transfer agent mainly depends on the amount of catalyst used, and generally ranges from 0.05 to 5 parts by weight for each part by weight of catalyst. Best results are obt:ained with an amount of chain transi-er agent X

~ pC/~U~J _ 7 _ a~: $~ ~

of Erom 0.5 to 1.5 par-ts by weigh~ for each part by weiyht of catalyst. The chain transfer agent may be chosen from a wide class of compounds~ such as carboxylic acids and esters having at least 2 carbon atoms, aliphatic, aromatic and cycloaliphcltic alcohols, anhydrides of carboxylic acids, amides, imines and other polar compounds, such as water.
It should be noted that the presence of the chain transfer agent affords the production of a reaction produc-t consisting mainly of polyoxymethylene with only small amounts of copolymer consisting of a polyoxymethylene with only small amounts of copolymer consistin~ of a polyoxy-methylene block chemically bonded to a polyme.r A block, the said copolymer being disclosed in our Canadian Patent 1,054,741, issued May 15, 1979.
Operating according to the process of the invention, one obtains a polyoxymethylene which, upon suitable stabilization, is suitable for spinning or for processing by injection moulding, by extrusion or by the processes known as "roto-molding" and "blow-molding", to produce articles and molded parts useful in the practice.
The polymerization temperature may vary wi-thin large limits, i.eO from about -100C up to the boiling - point of the or~anic diluent used, but, however, not higher than 110C, while the pressure is usually maintained at atmospheric value, even if it is possible to operate at a pressure above or below atmospheri!c. Operation is preferably carried out at a temperat~re from -30 ~o 70C.
The i.-ormaldehyde polymerization may be carried out in a discontinuous, semi-continuous or continuous way. :
When operating in a continuous way,:t~he catalyst is conveniently introduced into the react:ion medium in the form of a solul:ion or suspension in an inert organic medium.
. ~ ' ' .,,,-, PC/~UJ - 8 ~

- ' ~ , , ' ' - ' ''' ' ' , , A continuous technique is di~,closed ~or example by U.S.
pakent 3,458,~79.
In any case, one obtains a suspension o~ the polyoxymethylene, wherefrom the polymer is recovered in the form of granules having a size of from 20 to 700 microns and generally from about 20 to about 100 microns, when polymerization is carried out with the catalyst in solution, and from about 60 to about 700 microns, when the catalyst is in suspension.
In the second case, the particle size of the polyoxymethylene depends on that of the catalyst used.
The apparent density of the polyoxyme-thylene, upon separation from the reaction medium and drying, also depends on the way of using the catalyst. More particularly, said apparent density is of the order of 0.15-0.25 g/ml, when the catalyst is in solution, and 0.4-0.6 g/ml when the catalyst is in suspension.
The polyoxymethylene thus obtained contains at least one terminal hydroxyl group for each macromolecular 2Q chain, which renders the said polymer thermically unstable.
For the purpose of avoiding depolymerization from the chain extremity, the hydroxyl groups are conveniently substituted by ester or ether groups, e.g. by treatment with acetic anhydride or, respectively, with methyl or ethyl orthoformate, or by urethane groups by treatment with isocyanates. Processes known in the art may be used for such treatments.
Antioxidants (substituted phenols or bîs-phenols) as well as weakly basic substances capable of blocking formaldehyde or its oxidation products, such as formic acid, are addecl to the polyoxymethylenes thus treated.
These basic substances are preferably of a e~i pC/ft-~J
::

polymeric nature, such as, e.g. polyamicles and poly-es-~eramides.
In the following examples, the parts and percentages are intended by weight unless otherwise speci~ied.
Exampl 1.51 parts of sodium-naphthyl are added to 100 parts of pure ~-valerolactone, dissolved in 200 parts of anhydrous benzene maintained under a~itation in an inert atmosphere.
Polymerization, which shows itself by an increase in viscosity of the solution, starts after a few minutes.
Polymerization is carxied out for 60 minutes, the mass is cooled and the polymer A is precipitated by adding heptane.
The polymer A is thoroughly washed with heptane, until the reaction solvent is removed.
Analysis of a sample of the polymer A gives the following results:
- Conversion: 68.3%
- Reduced viscoslty: 0.31 (as measured at 30C in benzene with a polymer A -concentration of 0.2 g/dl) - particle size: 3125 microns : 1~8%
125-88 microns : 50O4%
~8-44 microns : 28.6%
; <44 microns : 19.2%~
The suspension of polymer A in heptane is stored at OC and used as such for the subsequent polymerizakion o formaldehyde.
Gaseous and pure formaldehyde monomer is mtx~uced at a rate of 2.5 parts per minute into a polymerization reactor contain;ng 1000 parts of anhydrous heptane and 2.25 ' PC/'t~ ~ ~ ~

~5~7 parts of the polymcr A described above.
The reactor is supplied with a vigorous stirrer, a suit~ble system to ensure strictly anhydrous conditions, and a thermostating jacket.
0.26 parts of anhydrous ethanol are also introduced into the reactor.
Formaldehyde is fed in for 190 minutes, polymerization being carried out at 25-30C. At the end of this period of time, the polyoxymethylene slurry is maintained under agitation Eor 10 minutes and then filtered.
The residual solid is dried in a vacuum oven at 50C, thus recovering 448 parts of polyoxymethylene with a yield of 94.3% with respect to the feed in formaldehyde.
This polyoxymethylene has an intrinsic viscosity of 1.40 (as measured in para-chlorophenol with 2% of alpha-pinene and at a polymer concentration of 0.5 g/dl).
One part of the polyoxymethylene is esterified under strictly inert conditions, by a reactive system : consisting of 1.5 parts of pure acetic anhydride and 3.0 parts of a mix-~ure of C~2 and C13 linear paraffins (in a ; 30/70 ratio), at a temperature of 153-155C and for a period of 30 minutes.
: The system is malntained at boiling point and, upon completion of the said period, the suspension is cooled, the acetylated polyoxymethylene is filtered and thoroughly washed with acetone, with water and a~ain with ~: acetone.
The polyoxymethylene diacetate is finally dried : : ~ . in a vacuum oven at 50C and recovered with a yield of ~0 94.5%~ This polyoxymethylene diacetate is submitted to the following tests: :
. - intrinsic viClcosity 1.41:(as measured in parachlorophenol) . .

~ pc/~4~ : - }1 -.: ~ ~ : : . , : , . .. , , . , . . .. ~ . . , . : . . . . . .. .

-~ apparen-t density : 0.55 g/m]
- the~mal degrada-tion tes-t: K220 = 0.06 (thermal degradation at 220C in a nitrogen atmosphere, expressed as the decomposition rate in weight percent per minute during the first 30 minutes and measured by a thermoscale. The degradation products are continuously discharged by flushing with a nitroqen flow).
- The polyoxy~ethylene diacetate is fractionated in a steel column filled with Celite (R.T.M.), using dimethylforma-mide as a solvent and operating at a programmed temperatureO
Analysis of the single Eractions shows that the polymer has a polydispersivity ratio (Mw / Mn) equal to

2.3~.
Example 2 . _ 0.7 parts of lithium-caprolactam are added to 114 parts of pure ~-caprolactone dissolved in 200 parts of anhydrous toluene maintained under agitation at 15 C
under inert conditions. Polymerization, which shows 20 itself by an increase in viscosity o~ the solution, starts after a few minutes. Polymerization is carried out for 60 minutes with a conversion of 90.1~.
The polymer A thus obtained is purified in hep-tane, filtered and thoroughly washed still with heptane until the reaction solvent is removed.
Analysis of a sample of the polymer A gives the following results~
- reduced viscosity: 0.38 (as measured in benzene) - melting point: 60-61C
- particle size: ~88 microns o 5.9%
88-44 microns o 62.3~

<~l4 microns O 310 8%.

~ '' pc~UlJ ~ 12 -.

. . . . . ... . .

5~ ~',,) Formaldehyde polymerizatioll is carried out in the same manner as in the first Example, introducing in the polymerization reactor which contains 1000 parts of heptane and 0,92 parts of -the above described polymer A, a stream of pure and ~aseous formaldehycle monomer, at the rate of 2.5 parts per minute, for 124 minutes. 0.15 parts of pure ethanol are also introduced into the reactor. The temperature is maintained at 20-25C and, upon completion of the said period, the polyoxymethylene is maintained under agitation for 10 minutes, then filtered and dried in a vacuum oven at 50C.
300.7 parts of polyoxymethylene are recovered with a yield of 96O8% with respect to the formaldehyde feed.
The polyoxymethylene is submitted to the following tests:
; - intrinsic viscosity = 1O79 (as measured in parachlorophenol) - apparent density : 0.49 g/ml - particles size: ~350 microns : 20.2~
350-250 microns 41.0%
250-125 microns : 35.8%
<125 microns : 3.0%
One part of the polyoxymethylene is esterified with acetic anhydride in a reactive system similar to ; that of Example 1. The esterification yield is equal to 96.7% and the esterified polyoxymethylene has an intrinsic j viscosity equal to that of the non-esterified polyo~y~ethylene.
The following tests are effected on the esterified polyoxymethylene:
- Thermal degrcldation test: K220 = 0.07 - Other physical-mechanical properties are determined upon addltion of 0.5~ of a block polyesteramide (consisting of 93% of polycaprolactam and 7% of polycaprolactone) and .
' ;
' ' ' .
~ PC/~UJ - 13 -. . , . : , .
-: ., , . . . . , : ~,. . .

of 0.4~ of n-oc~adecyl-beta(~'-hydroxy-3',5'-d.i-tert-butylphenyl)propionate alld setting into melt. The results are reported in Table 1 under (POM-2).
Example 3 Operating as in Example 1, the polymerization reactor is charged with 1000 parts of toluene and 0.12 parts of polymer A prepared as described in the first -.~ part of Example 2. Gaseous and pure formaldehyde monomer is then introduced at a rate of 2.5 parts per minute, for 48 minutes. 0.09 parts of n-propanol are also introduce.^l into the .reactor. At the end of the said period of time, the polyoxymethylene suspension is maintained under agitation for 10 minutes and the polymer is then filtered and dried in a vacuum oven at 50C.
118 parts of polyoxymethylene are recovered with a yield of 98.3% with respect to the formaldehyde feed.
The polyoxymethylene thus obtained has the fo1lowing characteristics:
- intrinsic viscosity: 1.52 (as measured in p-chlorophenol) - apparent density: 0~22 g/ml - particle size D >63 microns : 2.5 63~44 microns : 55.2'~
~: <44 microns : 42.3~
A portion of the polyoxymethylene is stabili~ed ; by esterification wlth acetic anhydride, operating exactly in the same manner as in Example 1, with a reaction yield of 93.8%~ The acetylated polyoxymethylene is ~; subjected to the following tests:
- intrinsic viscosity: 1.51 (as measured in p-chlorophenol) - thermal degr.-.. dation test . K220 = 0~09 - Other physical-mechanical properties are determined after addition of t:he stabilizers and setting into melt in the ~ pC/~tuJ ~ 4~-: . :

same manner as in Ex~mple 2. The results of the tests ar~
summarized in Table 1 under (~OM-3).
Tahle 1 Tensile strength Kg/cm ~ASTM D 638) 704 700 Elongation % (ASTM D638) 79 30 Impact strength Izod with notch Kg.cm/cm (~STM D 256) 15.3 9.4 Example 4 0,012 parts o~ lithium-sec-bu-tyl are added to 114 parts of pure ~-caprolactone dissolved in 200 parts of anhydrous heptane maintained under agitation at 10 C
in an inert atmosphere.
A suspension of the polymer gradually forms and is maintained under the said conditions for 60 minutes.
; The polymer A thus obtained is filtered and thoroughly washed with anhydrous heptane under inert conditions.
Analysis o~ a sample of the polymer A gives the following results:
- Conversion : 96.2~
- Reduced viscosity : 0.70 (as measured in benzene) - Melting point : 60-62C
- Particle size: >88 microns : 7.8%
88-44 microns :69.3%
<44 microns :22.9%.
Operating in the same manner as in the Example 1, the polymeriæation reactor containing lQOQ parts of cyclohexane and 0.64 parts of polymer A prepared as described above, is fed with a flow of pure and gaseous formaldehyde monomer, at a rate of 2.5 parts per minute~
for a period of 108 minutesO
The polymerization temperature is maintained at pc~ -' . ~ : , .. ..

S' ~

10-15C and ti~ polyoxyrnethylene is fincllly filter~cl, dried at 50C under v~cuum, recovering 263 parts oP
polymer (yield o~ 97.5~ with respect to formaldehyde).
The polyoxyme-thylene has the ~ollowing charactexistics:
~ Apparent density: 0.45 g/ml - Particle size: >350 microns = 7.3~
350-250 microns = 46.2%
250-lZ5 microns = 34.5%
<125 microns = 12.0%
One part of the polyoxymethylene is stabilized by etherification in a reactive system consisting of 0.4 parts of triethyl orthoformate, 0.8 parts of anhydrous dymethyl acetamide, 2 parts of n-heptane and 0.05 parts of ethyl sulphate. The reaction mixture is maintained at 150-152C for 15 minutes. The suspension is then cooled, the etheri~ied polyoxymethylene is filtered and washed with toluene containing 1~ oE triethanolamine, and then with methanol~
Upon drying, 95% of the polymer is recovered.
The etherified polyoxymethylene, which has undergone no variations in particle size and apparent density, is subjected to the following tests:
- Intrinsic viscosity : 1.70 (as measured in p-chlorophenol) - thermal degradation : K220 = 0.04 - alkaline attack stability : FAS.
A sample of etherified polyoxymethylene is brought into solution in benzyl alcohol containin~ 1% o triethanolamine at the temperature of 150-152C for a period of 30 minutesO The polymer/benzyl alcohol ratio is equal to 1:10. At the end of the operation the polymer is precipitated by cooling~ filtered, washed and dried. -.
', , ~ pc/'~
. . ~.

The resi.dual percentaye of etherified polyo~ymethy].ene is the alkali-stahle frclction (Table 2 under POM-4).
- Othex physical-mechani.cal tests are determined upon addition of 005% of polyvinylpyrrolidone (molecular wei~ht 30,000), 0.3% of 2,2'~methylene~bis-(4~meth~1-6~
tert-butylphenol) and setting into melt (Table 2 under POM~4).
~ The etherified polyoxymethylene is fractionated in the same manner as in Example 1. The polydispersivity ratio is 1.65.
Example 5 Operation is carried out as in Example 1 in a polymerization reactor containing 1000 parts of toluene and 0.10 parts of polymer A prepared in the first part of Example 4. 126 parts of polyoxymethylene are obtained in 51 minutes with a yield of 98.5% with respect to the formaldehyde feed.
The polyoxymethylene has the following characteristics:
- apparent density : 0.23 g/ml - particle size : >63 microns : 2.1% .
63-44 microns o 73.8%
<44 microns : 24.1%
The polyoxymethylene is etherified as ln Example ~ .
4 with a reacti.on yield of 94.8~, and is then subjected to the following tests:
- intrinsic viscosity : 1.38 - thermal degradation: K220=0.05 : - alkaline attack stability : FAS
~Table 2 under POM-5).
- The physical-mechanical characteristics are determined upon addition of the stabilizers as in Examplé 4 (Table P

~: . . . . .

2 under ~OM-5). 5~
- The etherifled polyoxyme-thylene is fractionated in a manner similar to that indicated in Example 1. The polydispersivity ra-tio is equal to 1.70O
Example 6 1DO7 parts of zinc-n-butyl in paraffin solution are added to 140 parts oE ~ dimethyl-~-isopropylidene-~--propiolactone, operating under strictly inert conditions in a nitrogen atmosphere. The system is maintained at 50C for 20 hours. At the end of the operation the polymer A thus obtained is finely ground under inert conditions and thoroughly washed with heptane at 30 C.
Analysis of the polymer A gives the following results:
- Conversion : 83.4~
- Reduced viscosity O 0.50 (as measured at 20C in chloro-form with a polymer A concentration of 0.5 g/dl) - melting point : 195-198C
- Particle sizeO ~125 microns : 0.9~
125-88 microns O 73.1%
20<88 microns : 26.0~ `
Operating in the same manner as in Example 1, in a polymerization reactor containing 1000 parts of toluene and 0.40 parts of polymer A described above, ~,79.2 parts of polyoxymethylene are produced in a period of 32 minutes with a yield of 99O0% with respect to the formaldehyde feed.
The polyoxymethylene has the following characteristics:
- Apparent density : 0.18 g~ml ~30- Particle sizle : >63 microns : lo 2~ -: 63-44 microns :28,4%

~44 microns 70O4%
.: : , .

~L ~ pc/~

The polyoxymeth~l.ene is etherif:ied in the m~nner indicat~d in ~xample ~, wi-th a reaction yield of 95.0~.
The etherified polyoxyme-thylene, which has undergone no variations in physi.cal characteristics with respect to the non-etherified polyoxymethylene, has the following characteristics:
- Intrinsic viscosity : 1O17 (ad measured in p-chlorophenol) - thermal deyradation : K220 = 0.05 - alkaline attack stability : FAS = 98.1~.
Table 2 POM-4 POM-5 Melt-Index (g/10 minutes at 195 C) 2.3 9.5 Tensile strength kg/cm (ASTM D638) 715 700 Elongation ~ (ASTM D638) 78 29. 5 Impact strength Izod with noth Kg.cm/
cm (ASTM D256) 16 10 FAS (%) 98.0 98.5 Example 7 0.016 parts of lithium-n-butyl in a 15% solution in hexane axe added to 114 parts of pure -caprolactone maintained at 65C under agitation in a nitrogen atmosphere.
Polymerization starts immediately and after 10 minutes 250 parts of anhydrous benzene are rapidly introduced, while the mass is cooledO
The polymer A is precipitated by adding 800 parts of anhydrous heptane.
The suspension is filtered~ thoroughly washed with heptane, operating so as to avoid any contact with moisture and air, until the organic solvent is completely .
removed.

Analysis gives the following results:
- Conversion : 99.1%

-- . 19 ~
P~

, ! 5 ~

- Reduced viscosity: 0.52 (as measured at 30C i.n benzene with a polymer A
concentra-tion of 0.2 g/dl) - Melting point : 60 C
- Particle size: ~88 microns : 4.2 88-44 microns : 39.2 <4~ microns : 56.6~
One uses the reactor o~ Example 1 modified so as to continuously feed in heptane and the above polymer A and continuously discharge the suspension of polyoxymethylene formed. A stream of gaseous and pure formaldehyde monomer is first introduced for 144 minutes, at the rate of 2.5 parts per minute, into the reactor ; containing 1000 parts of heptane and 2.30 parts of polymer A.
The slurry is then continuously discharged, while introducing heptane so as to main~ain the level constant in the reactor. Besides, 160 parts of polymer A and 24 . parts o n-butanol are introduced during the continous :20 run of 200 hours. The average production per hour is of 142.5 parts of polyoxymethylene having an intrinsic viscosity of 1.81 and an apparent density of 0.51 g/ml.
At the end of the said period of time, the mass . is maintained under agitation in the reactor for 10 minutes . and then dischargedO
; The polymerization apparatus, i.e. both the static part (walls, pipe for conveying the suspension, thermometric sheath, etc.) and the moving part (such as ~: the stirrer) are perfectly clean~at the end ofpolymerization.
One part o~ the polyoxymethylene is stabilized :. ~
by esterification with acetic anhydride in the same manner ~: as in Example 1, with a react1on yield of 96.1%.

pC~
~..

The following -tests are effected on -the esterified po]yoxymethylene:
- Intrinsic viscosity : 1.80 (as measured in p-chlorophenol) - apparent density : 0.51 g/ml - Particle size : >350 microns : 2.3~
350-250 microns : 27.2%
250-125 microns : 68.4~
<125 microns : 2.1%
- thermal degradation : K220 - 0.06 - The polymer is fractionated as in Example 1 and a poly~
dispersivity ratio of 2.50 is noted.
Example 8 Operating in the same manner as in Example 7 and with the same polymer A, formaldehyde is polymerized in a continuous manner, using toluene as diluent medium.
The formaldehyde monomer is first introduced for 144 minutes, at the rate o~ 2.25 parts per minute r into the reactor containing 1000 parts of toluene and 0.4 parts of polymer A. During the subsequent run of 200 hours, there are introduced 150 parts per hour of formaldehyde and 0.17 parts per hour of polymer A. Besides, n-butanol is also introduced at the rate o 0.17 parts/hour.
147.5 parts of polyoxymethylene are produced per hour, with a yield of 98.7~ with respect to the formal-dehyde feed.
The polyoxymethylene is stabilized by means of the same etherification system as in Example 4, with a reaction yield of 94~1%o The i--ollowing tests are effected on the etherified polyoxymethylene:
- Intrinsic viscosity 1.20 Apparent density ~ 0O20 g/ml ~, c/~u ~5~
- Particle size: >63 m.icrons : 5.B~
63-~ microns : 58.2~
<4~ mi.crons : 36.0%
- Thermal stability : K220 = 0.04 - Alkaline attack stability : FAS = 98DO~
- The etherified polyoxymethylene is fractionated as described in Example 1, the polydispersivity ratio being of 1.61.

pc/~

- , '

Claims (8)

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

1. A process for the preparation of polyoxymethylene, which comprises feeding anhydrous formaldehyde monomer into a reaction medium comprising an organic diluent, which is liquid under the reaction conditions, non-solvent for the polyoxymethylene and inert (non-reactive) towards the other constituents of the reaction medium, a chain transfer agent, and a catalyst dissolved or dispersed in the reaction medium, said catalyst being used in an amount of from 0.001 to 10% by weight with respect to the feed in formaldehyde and consisting of a polymer A carrying a ter-minal ionic couple on its macromolecular chain, said polymer A being a polylactone consisting of recurring units:

obtained from one or more lactone monomers of the general formula:
wherein PM1 is a linear polymethylene chain having from 2 to 13 carbon atoms, non substituted or having at least one hydrogen atom replaced by a substituent chosen from alkyl, alkylenyl, cycloalkyl, aryl and aralkyl radicals and halogen atoms.

2. The process of claim 1, wherein said catalyst is used in an amount of from 0.01 to 1.0% by weight with respect to the feed in formaldehyde.

3. The process of claim 1, wherein said polymer A has a molecular weight of from 1,000 to 120,000.

4. The process of claim 1, wherein said lactone monomers are chosen from .beta. propiolactone, .beta.-butyrolactone, .delta.-valerolactone, .epsilon.-caprolactone, .omega.-enantholactone, .beta.-.gamma.-and .delta.-methyl .epsilon.-caprolactones, pivololactone and .alpha.,.alpha.,dimethyl-.beta.-isopropylidene-.beta.-propiolactone.

5. The process of claim 1, wherein the reaction temperature is from -100°C up to the boiling point of the organic diluent, and anyway not above 110°C.

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

7. The process of claim 1, wherein the polymer A is obtained by polymerization of one or more of said lactone monomers either in the presence of a solvent for the polymer A or a diluent non-solvent for the latter, or in the absence of said solvent and diluent, at a temperature of from.-100°C to 200°C and for a period of from 1 minute to 120 hours, and in the presence of from 0.01 to 10 moles, for each 100 moles of lactone monomer, of an ionic catalyst chosen from among:
(a) compounds corresponding to the general formulas:
(i) Mez+ (R)? and (ii) Mez+ (OR)?
where Me is a metal of Group IA, IIA, IIB or IIIA of the Periodic System of the Elements, R is an alkyl, aryl, aral-kyl, cycloalkyl or naphthyl radical, or else, only in the case of formula (i); a hydrogen atom, and z is 1 or 2;
(b) compounds corresponding to the general formula:
where X is a metal of Group IA, IIA or IIB of the Periodic System of the Elements, or else a quaternary ammonium group, PM2 is a polymethylene chain having from 3 to 13 carbon atoms, non substituted or having at least one hydrogen atom replaced by a hydrocarbon radical, and y is 1 or 2:
and (c) alkali metals and magensium-alkyl derivatives.

8. The process of claim 1, wherein said polymer A is used in the form of particles of from 1 to 300 microns in size.