WO1995023131A1

WO1995023131A1 - Process for the preparation of 2-cyanoacrylic acid

Info

Publication number: WO1995023131A1
Application number: PCT/IE1995/000003
Authority: WO
Inventors: Valery Alexandrovich Dyatlov; Viktor Maleev
Original assignee: Saldane Limited
Priority date: 1994-02-23
Filing date: 1995-01-09
Publication date: 1995-08-31
Also published as: AU1424395A

Abstract

A process for the preparation of 2-cyanoacrylic acid from poly(alkyl 2-cyanoacrylates), comprises the steps of: a) subjecting oligomeric or polymeric cyanoacrylate material to catalysed pyrolysis in a reactor under polymerisation inhibiting conditions at a temperature greater than 200 °C and under conditions sufficient to vaporise the 2-cyanoacrylic acid formed; b) cooling the vapours which emerge from the reactor on completion of the pyrolysis; and c) collecting the 2-cyanoacrylic acid formed. The oligomeric cyanoacrylate or polymeric material can be selected from crude oligomer, purified oligomer, oligomer produced as a by-product in the conversion of crude oligomer to purified oligomer, oligomer produced as a by-product during the depolymerisation of purified oligomer, oligomer produced during the conversion of crude monomer to purified monomer and polymer produced during storage of purified monomer.

Description

Process for the preparation of 2-cyanoacrylic acid

Technical Field

This invention relates to a process for the preparation of 2- cyanoacrylic acid, more particularly, a process for the preparation of 2-cyanoacrylic acid from alkyl 2-cyanoacrylate oligomers produced as intermediate products of alkyl 2-cyanoacrylate production and poly(alkyl 2-cyanoacrylate)-containing products thereof which are normally produced as an unrecoverable by-product in the production of alkyl 2-cyanoacrylate monomers.

Background Art

2-Cyanoacrylic acid having the formula

CN

/ H₂C= C

\ COOH

can potentially be used for the preparation of a wide range of cyanoacrylate monomers. The ability of 2-cyanoacrylates to polymerise rapidly under the influence of moisture or nucleophilic substances has led to their exploitation as instantaneous adhesives. However, the inherent ability of 2-cyanoacrylates to undergo rapid anionic polymerisation gives rise to complications as regards the synthesis of free 2-cyanoacrylic acid. Accordingly, whereas esters of 2-cyanoacrylic acid are known and well characterised since 1940, the first reported synthesis of free 2-cyanoacrylic acid was described in 1984 by Henkel KGaA (DE 34 15 181 Al). The only known method of obtaining 2-cyanoacrylic acid is described in DE 34 15 181 Al . The method described in DE 34 15 181 Al for the preparation of 2-cyanoacrylic acid involves pyrolysis of alkyl esters of 2- cyanoacrylic acid or Diels-Alder adducts of such esters. The highest reported yield was 13.4% of a crude product having a melting point of 80-81°C. Purified 2-cyanoacrylic acid has a melting point of 92-93°C.

Apart from the low yield of 2-cyanoacrylic acid resulting from the method of DE 34 15 181 Al, a further limitation is that the starting material consists of purified alkyl esters of 2-cyanoacrylic acid which are expensive. Accordingly, the method of DE 34 15 181 Al is not a practical means of obtaining new alkyl esters of 2-cyanoacrylic acid, starting from the free acid.

At present the main commercial route for the preparation of cyanoacrylate esters is the Knoevenagel Condensation Method (H. Lee. (Ed.) (1981) Cyanoacrylic Resins - The Instant Adhesives, Pasadena Technology Press, Pasadena, U.S.A.). This method is used commercially for the synthesis of low molecular weight esters. The Knoevenagel Method is a two stage method. In the first stage a cyanoacetate ester and formaldehyde are reacted together in the presence of an amine to give relatively crude oligomers of polyalkylcyanoacrylates. After appropriate purification, the oligomer, in a second stage is thermally depolymerised to yield monomeric ester in accordance with the following reaction scheme:

CN CN

/ catalyst /

CH₂0 + CH OH - CH - C OH + H₂0

\ \

COOR COOR

In the above scheme R is an alkyl, cycloalkyl or alkenyl moiety of up to ten carbon atoms. The catalyst is normally a secondary amine, a basic salt or a combination thereof.

When the Knoevenagel Condensation Method is applied on an industrial scale four main types of oligo(alkyl 2-cyanoacrylate)- containing products result as follows:

1. Oligomer formed at the condensation stage;

2. Purified oligomer formed following purification of crude oligomer which is used as a starting material for the depolymerisation reaction;

3. Oligomer containing by-product from the purification step; and

4. Oligomer containing residue formed during the depolymerisation step, which residue is made up of oligomer which cannot be depolymerised under the conditions used to form esters of 2-cyanoacrylic acid.

Industrial processes for the production of cyanoacrylate adhesives or glues, hereinafter referred to as adhesives, involve the purification of crude cyanoacrylates by vacuum distillation, followed by depolymerisation. Following distillation a residue is obtained as a by-product which contains oligomer of relatively high molecular weight which is normally discarded as a waste product, which must be disposed of.

Furthermore, the shelf life of cyanoacrylate adhesives is dependent on storage conditions. For example, it is usual following the distillation step to add a free radical stabiliser such as hydroquinone to inhibit free radical polymerisation during storage. Free radical polymerisation can be initiated, for example, by exposure to light. Accordingly, under certain conditions, such as when insufficient stabiliser is used, these adhesives may undergo spontaneous polymerisation with the formation of highly viscous liquid or solid products which cannot be used as an adhesive or as a component of an adhesive. The polymer products formed on storage are solutions of high molecular weight poly(alkyl 2-cyanoacrylates) in the monomeric alkyl 2-cyanoacrylates or, sometimes they are completely polymerised solid poly(alkyl 2-cyanoacrylates). Some monomer can be recovered by distillation of viscous liquid products of the type hereinbefore referred to. However, residual high molecular weight solid polymers cannot be converted into monomer even by thermal depolymerisation and must be disposed of. The oligomers which arise as by-product at various stages during the production of adhesives cannot generally be converted into monomer by depolymerisation.

T e following scheme indicates various stages at which oligomers are formed which cannot be further processed according to conventional methods to form adhesives or be converted into adhesives.

(1) (2)

Condensation Crude Purified oligomer oligomer

Purified monomer by-product depolymerisaton

by-product (4)

Crude monomer

Products (3), (4), (5) and (7) cannot presently be processed to commercially useful materials, more specifically material which can be used directly as an adhesive or as a component of an adhesive. The cost of disposing of such oligomers is high and also there are environmental problems with the disposal of such oligomers. In the case of products (1), (2) and (6) a number of steps are required to obtain a purified monomer which can be used as an adhesive or as a component of an adhesive.

The present invention provides a process for the preparation of high purity 2-cyanoacrylic acid in high yield from polymeric cyanoacrylate materials of the type hereinabove defined.

Disclosure of Invention

Accordingly, the invention provides a process for the preparation of 2-cyanoacrylic acid from poly(alkyl 2-cyanoacrylates), which process comprises the steps of:

a) subjecting oligomeric or polymeric cyanoacrylate material to catalysed pyrolysis in a reactor under polymerisation inhibiting conditions at a temperature greater than 200°C and under conditions sufficient to vaporise the 2-cyanoacrylic acid formed;

b) cooling the vapours which emerge from the reactor on completion of the pyrolysis; and

c) collecting the 2-cyanoacrylic acid formed.

The oligomeric or polymeric cyanoacrylate material used as a starting material in the process according to the invention is any type of poly(alkyl 2-cyanoacrylate)-containing material.

The term oligomer as used herein is any poly(alkyl 2- cyanoacrylate) synthesised by any method, including the Knoevenagel Condensation Method using different catalysts and formaldehyde to alkyl cyanoacetate ratios. Typically the oligomers will be those which are formed by the Knoevenagel Condensation Method. However, by¬ products of commercial 2-cyanoacrylate monomer production and purification and also oligomers formed by the polymerisation of esters of 2-cyanoacrylic acid can be used as starting material in the process according to the invention.

Thus, the process according to the invention can be used to obtain 2-cyanoacrylic acid from any of the oligomeric or polymeric materials hereinbefore defined, including the products identified as (1)- (7) in the scheme hereinabove set out. Thus, high molecular weight oligomers which cannot be used to prepare esters of 2-cyanoacrylic acid by depolymerisation can be used as starting material in the process according to the invention. Starting material in accordance with the invention includes also polymer-monomer solutions of the type hereinabove defined and residual polymers obtained in known commercial processes.

Preferably, the oligomeric or polymeric cyanoacrylate material is fed to the reactor in a carrier gas.

However, the process according to the invention can be carried out without a carrier gas as hereinafter described.

Pyrolysis in accordance with the invention can be carried out over a relatively broad temperature range, typically between 250° and 700°C. However, preferably the pyrolysis is carried out at a temperature in the range 300-600°C.

Polymerisation inhibiting conditions in the process according to the invention are conditions wherein one or both of an anionic polymerisation inhibitor and a free radical polymerisation inhibitor is added to one or both of the oligomeric or polymeric cyanoacrylate material and the carrier gas, when a carrier gas is used.

Suitable anionic polymerisation inhibitors are those known in the art and include sulfur dioxide, carbon dioxide and nitrous oxide. Suitable free radical polymerisation inhibitors are those known in the art and include hydroquinone and methylhydroquinone.

The type of reactor used to carry out the process according to the invention is typically a tube-type reactor or a standard pyrolysis reactor in which the pyrolysis is carried out in the environment of a pseudoliquid layer of a catalyst.

When a tube-type reactor is used, the reactor is preferably a quartz reactor. A tube-type reactor for use in accordance with the invention can be a spiral reactor. Such a reactor can be filled with a catalyst as hereinafter defined adsorbed or fixed to a stationary support.

However, the surface of the reactor can also serve as an integral catalyst. For example, quartz can serve as a catalyst in that the surface of the quartz tube reactor serves as a catalyst for pyrolysis. Likewise quartz rings and quartz sand act as catalysts for pyrolysis.

Furthermore, by providing the inner surface of the quartz tube with indentations or other surface configurations which increase the surface area of the quartz tube also increases the catalysing surface. Furthermore, such configured surfaces improve heat transfer and heat exchange.

A reduced pressure can be used during pyrolysis according to the invention. Under these conditions the pressure and temperature must be sufficient to vaporise the 2-cyanoacrylic acid formed and maintain it in the vapour phase. Preferably, the reduced pressure is in the range 0.05-100 mm Hg (6.64 Pa - 13.3 kPa).

The pressure, temperature and gas flow rate through the reactor are selected so as to provide a sufficient contact time in the reactor of the material being subjected to pyrolysis to achieve complete pyrolysis of vapours formed. The catalyst, when such is used, is suitably a metal oxide, more particularly a transition metal oxide. Preferably, the catalyst is bonded to a solid support. For example, the solid support can be in the form of rings in the case of a tube-type reactor and solid quartz sand where pyrolysis is carried out in the environment or conditions of a pseudoliquid layer of a catalyst.

The carrier gas, when such is used, is suitably an inert gas or an acidic gas.

An example of a suitable tube-type reactor is described herein with reference to Fig. 1.

An example of a suitable pseudoliquid layer reactor is described herein with reference to Fig. 2.

A carrier gas must be used when the pyrolysis reactor is a pseudoliquid layer reactor. However, in the case of a tube-type pyrolysis reactor the use of a carrier gas is optional.

Suitable inert gases include argon and nitrogen.

Suitable acidic gases include sulfur dioxide, carbon dioxide, carbon monoxide and nitrous oxide. Acidic gases are especially useful as carrier gases in accordance with the invention because they also serve as stabilisers which prevent anionic polymerisation of cyanoacrylic acid. The most preferred acidic carrier gas is sulfur dioxide.

The flow rate of carrier gas depends on the pressure and the inner diameter of the reactor and is suitably at a rate of 0-0.1 1/min for a pyrolysis reactor of the tube-type.

For a reactor wherein pyrolysis is carried out in conditions using a pseudoliquid layer, the gas flow rate required to provide a suitable pseudoliquid layer is suitably in the range 0.1-2 1/min. The carrier gas, when such is used, is preferably heated to the pyrolysis temperature before being injected into the pyrolysis zone.

The oligomeric cyanoacrylate material is preferably injected into the pyrolysis reactor in the form of a melt, solid or solution, including a solution of oligomer in an ester of 2-cyanoacrylic acid as a solvent.

The rate of flow of the carrier gas, if employed, is adjusted so as to lead to an acceptable residence time for oligomer vapour within the heated zone of the pyrolysis tube or vessel so that efficient reaction will take place. This time τ may be calculated using the expression

τ = V_r/V_tp+F*_p

where

V = T_pG-22.4-2T_o/T₀MP_p

V_r = free volume of the reactor in litres

M = molecular weight of the sub-unit of the oligomer

G = rate of addition of oligomer in g/sec

T_p = temperature of pyrolysis in K

F₀ = flow rate of carrier gas in litres/sec at pressure P₀ (760 mm Hg (101 kPa)) and temperature T₀ (273K) F_tp = flow rate of carrier gas under pyrolysis conditions T_p and P_p in litres/sec

V_tp = flow rate of pyrolysis gas at T_p and P_p in litres/sec Thus, for example, an unfilled tube-type reactor of the type shown in Fig. 1 used for the pyrolysis of oligo(ethyl 2-cyanoacrylate) and having a heated area of dimensions 30 cm x 1.8 cm diameter would have a residence time τ of 0.085 seconds when operated without carrier gas under the conditions P_G = 750 mm Hg (99.7 kPa), P_p = 1 mm Hg (133 Pa), T_p = 600°C (873K) and G = 10^"3g/sec.

The emergent vapours can be cooled using a two step cooling process. In a first step, the emergent vapours are cooled by passing through a condenser cooled by cold water and the bulk of the 2- cyanoacrylic acid is collected, in a second step the cooled vapours are passed through a condenser cooled by dry ice or by liquid nitrogen and an olefin as a by-product containing the remainder of the 2- cyanoacrylic acid is collected.

Oligomeric material for use in the process according to the invention can also be specially prepared from a cyanoacrylate monomer for the purposes of demonstrating the process according to the invention under experimental conditions as set forth in Example 3.

Brief Description of the Drawings

Fig. 1 is a schematic representation of a tube-type reactor which can used for carrying out the process according to the invention; and

Fig. 2 is a schematic representation of a pseudoliquid layer reactor which can be used for carrying out the process according to the invention.

Referring to Fig. 1 , there is indicated generally at 10, a tube -type reactor suitable for carrying out the process according to the invention. The reactor 10 consists of a thermocouple-controlled electrically- heated dosing funnel 11 which is pressurised by admission of an inert gas such as argon through inlet 12, and which is equipped with a valve 13 to control the rate of addition of reactant to a heated quartz tube 14. Carrier gas (if required) is supplied from a cylinder 15, dried and freed from oxygen by gas purifier 16, passed through a regulator 17 capable of measuring the rate of flow, and admitted to the reactor 10 close to the point of entry of reactant from the dosing funnel 11. The quartz tube 14 is surrounded by a thermocouple-controlled, temperature-programmable electric heater 18, and is optionally supplied with indentations to extend its surface area and to improve heat-transfer. Alternatively, the quartz tube 14 may be provided with a porous internal wall which supports a catalyst, or it may be filled with a catalyst such as sand or silica. The emergent 2-cyanoacrylic acid product is largely collected in an air- or water-cooled vessel or trap 19, and the by-product alkene together with any 2-cyanoacrylic acid not condensed in vessel 19 is collected in a cooled trap 20. The dosing funnel 11, the quartz tube 14 and traps 19 and 20 are connected together by suitable vacuum-tight joints and the outlet from trap 20 is connected to a vacuum pump (not shown) via a capillary-type vacuum regulator 21. During operation, the pressure within the reactor is measured using a Pirani or other suitable vacuum gauge 22 connected at a point between traps 19 and 20.

Typically, the heated areas of the quartz tube 14 are 30 cm in length with an internal diameter of 1.8 cm, and with an internal free volume of 76 cm³ for an unpacked tube or 50 cm³ for a tube containing sand, silica or other solid support.

Referring to Fig. 2, there is indicated generally at 30, a pseudoliquid reactor suitable for carrying out the process according to the invention. The reactor 30 is fitted with dosing, collection and control systems identical to those forming part of the tube-type reactor 10 depicted in Fig. 1. The reactor 30 is not connected to a vacuum pump, is operated at atmospheric pressure, and has provision for pre- heating the carrier gas. The reactor 30 includes a reaction vessel 31 which is made of quartz and has a porous bottom 32 suitable for supporting a solid, granular catalyst 33 such as sand or silica. The reaction vessel 31 is heated by a programmable electric heater 34 and the internal temperature is monitored by means of a thermocouple 35. Fluidisation of the solid, granular catalyst 33 is achieved by vibrating the entire reaction vessel 31 by means of a rod 36 connected eccentrically to a revolving wheel 37. A vibrational amplitude of about 5 mm is employed, and the frequency is regulated by varying the speed of rotation of wheel 37 so that the granular catalyst 33 becomes pseudoliquid and is dispersed throughout the entire volume of the reaction vessel 31.

The reaction vessel 31 typically has a heated area which is 6 cm in length and an internal diameter of 3 cm, and its internal free volume is typically 30 cm³ after addition of the granular catalyst 33.

The invention will be further illustrated by the following Examples.

Example 1

Preparation of iron-containing catalysts

(a) Preparation of a catalyst containing 5% iron

Quartz sand (500 g) was added to a stirred solution of ferrous nitrate (80 g) in water (2 1). The water was evaporated, and the residue was dried at 300°C to constant weight. An appropriate portion of the dry residue was placed into a pyrolysis reactor and further dried at 350°C while being purged with dry inert gas. The adsorbed iron salt was then reduced by heating at 200-300°C whilst purging with dry hydrogen, and the resulting catalyst was finally dried at 600°C during further purging with dry inert gas.

In the above described preparation quartz sand can be replaced by silica gel (500 g) and ferrous nitrate by ferric chloride (75 g). (b) Preparation of a catalyst containing 3% iron

Quartz sand (500 g) was added to a stirred solution of ferrous nitrate (48 g) in water (2 1). Further operations were carried out exactly as described under (a) supra.

In the above described preparation quartz sand can be replaced by silica gel (500 g) and ferrous nitrate by ferric chloride (45 g).

(c) Preparation of a catalyst containing 1% iron

Quartz sand (500 g) was added to a stirred solution of ferrous nitrate (16 g) in water (1 1). Further processing was carried out exactly as described under (a) supra.

In the above preparation quartz sand can be replaced by silica gel (500 g) and ferrous nitrate by ferric chloride (15 g).

Example 2

Activation of reactors and/or catalysts before first use and after each subsequent use

(a) An empty quartz tube-type reactor as shown in Fig. 1 is heated at 800°C during 40 min with sparging by oxygen or air. It is then cooled and washed sequentially with aqueous sulfuric acid (10%), acetone and ether, and then dried at 200°C. The reactor is further dried for 1 hr in vacuo at 600°C with sparging by inert gas prior to use.

(b) A quartz tube-type reactor of the type shown in Fig. 1 and containing sand or silica is heated at 800°C during 2 hr with sparging by oxygen. It is then cooled and washed sequentially with aqueous sulfuric acid (10%), acetone and ether, and then dried at 200°C. The reactor is further dried for 1 hr in vacuo at 600°C with sparging by inert gas prior to use. (c) A quartz tube-type reactor of the type shown in Fig. 1 and containing a metal catalyst on a silica-gel support is heated at 600°C during 2 hr with sparging of oxygen and is then sparged at the same temperature with hydrogen during 2 hr. The reactor is finally dried for 1 hr at 600°C in vacuo with sparging by inert gas.

(d) A quartz reactor of the type shown in Fig. 2 employing a pseudoliquid layer of quartz sand is heated at 600°C during 2 hr with sparging of oxygen. It is then cooled and washed sequentially with aqueous sulfuric acid (10%), acetone and ether, and dried at 200°C. The reactor is further dried prior to use by sparging inert gas at atmospheric pressure and 600°C for 1 hr.

Best Modes for Carrying out the Invention

Example 3

Preparation of 2-cyanoacrylic acid from po ethyl 2-cyanoacrylate obtained by anionic polymerisation of ethyl 2-cyanoacrylate

Preparation of polv(ethyl 2-cvanoacrylate

A 500 ml flask arranged for distillation fitted with a mechanical stirrer was charged with a solution containing 100 ml of ethyl 2- cyanoacrylate in 95 ml of acetone. 5 ml of water was added with stirring and the mixture was heated with reflux and stirring for 3 hours. The solvent was distilled off to give a yellow transparent solid residue. The residue was dried in vacuo at 100°C under 0.5 mm Hg (66.4 Pa) to drive off monomeric ethyl 2-cyanoacrylate and acetone to give a transparent solid poly (ethyl 2-cyanoacrylate) NMR !H ppm (CD3hCO: 1.311 CH₃; 1.39t CH₃; 2.71 m CH₂; 2.82 m CH₂; 4.28 m CH₂0; 4.31 m CH₂0; 4.36 m CH₂0; Calculated for C₆H₇N0₂: C 57.6, H 5.6, N 1 1.2; found: C 57.10 H 5.92 N 10.97. Preparation of starting composition

A 150 ml flask fitted with a mechanical stirrer was filled with lOg of poly (ethyl 2-cyanoacrylate) and 0.2 g of phosphorus pentoxide was added to the melt of polymer with stirring at 80°C. The mixture was stirred at 80°C for 30 min. 1.8 ml of ethyl 2-cyanoacrylate containing 5 mg of hydroquinone was added with stirring and the mixture was cooled to 20°C to give a viscous liquid starting composition.

Pyrolysis of polvfethyl 2-cvanoacrylate

The dosing funnel of a tube-type quartz pyrolysis reactor fitted with a capillary tube for sparging SO₂ and heated to 100°C was filled with 4.63 g (5 ml) of poly(ethyl 2-cyanoacrylate) starting composition. The composition was added dropwise at a rate of 3-4 droplets per min. into the pyrolysis tube heated to 590°- 600°C, with constant sparging of SO2, at a flow rate of 0.5 ml/min., and at a pressure of 0.5-1 mm Hg

(66-133 Pa). Emerging vapours were cooled to give 2.95 g (77.3%) of solid crude 2-cyanoacrylic acid. The solid was recrystallised from toluene to give 1.55 g (43%) of 2-cyanoacrylic acid m.p. = 93-94°C.

Example 4

Preparation of 2-cvanoacrylic acid from high molecular weight polyCeth l 2-cyanoacrylate) formed during long-term shelf storage of industrial glue (by-product 7 supra)

Isolation of polvmer and preparation of starting composition

100 g of Cyacrin (Cyacrin is a Trade Mark) industrial glue based on ethyl 2-cyanoacrylate monomer stabilized with hydroquinone which had a high viscosity after two years of shelf storage due to partial polymerisation was distilled in vacuo in the presence of phosphorus pentoxide with sparging of SO2 to give 25 g of a solid transparent glass-like residue. The residue was melted and oligomer was separated from residual phosphorus pentoxide.

Pyrolysis of high molecular weight polvCethyl 2-cvanoacrylate

The dosing funnel of a tube-type quartz pyrolysis reactor heated to 100^PC was filled with 10 g of solid polymer. The polymer was melted and 100 mg of phosphorus pentoxide was added to the melt. The reactor, which was provided with indentations, was heated in vacuo under 1 mm Hg (133 Pa) to 600°C and sparged with SO2 for 30 min. The sparging was stopped and the melt of polymer was added dropwise into the heated zone of the reactor at a speed of ten droplets per min. 6.2 g of solid was collected in the cooled zone of the reactor. The solid was recrystallised from toluene to give 3.82 g (47%) of pure 2-cyanoacrylic acid m.p. 93-94°C.

Example 5

Preparation of 2-cvanoacrylic acid from a solution of oligo(ethyl 2- cyanoacrylate^') in ethyl 2-cyanoacrylate formed during long-term storage of industrial glue (by-product 6 supra)

A high-viscosity solution of oligo(ethyl 2-cyanoacrylate) in ethyl 2-cyanoacrylate formed after two years storage of the industrial adhesive Cyacrin (Cyacrin is a Trade Mark) was analysed by *H NMR spectroscopy and shown to contain 20% oligomer. This mixture cannot be purified by distillation due to the occurrence of spontaneous polymerisation even under normal polymerisation-inhibiting conditions, and the high molecular weight of the contained oligomer prevents its thermal depolymerisation to ethyl 2-cyanoacrylate.

A tube-type quartz pyrolysis reactor of the type shown in Fig. 1 30 cm in length and having a free volume of 50 cm³ was filled with quartz sand. The reactor was evacuated to a pressure P_p of 10 mm Hg (1.33 kPa), heated to T_p = 623K, and purged for 30 min with sulfur dioxide. After this time, the sulfur dioxide was replaced by argon and the flow rate was adjusted to 3 cm³/min giving F₀ = 5 x 10^"5l/sec. The dosing funnel was filled with 3.5 ml of the oligomer-monomer solution mentioned supra and this was added dropwise into the heated zone of the reactor at a rate G = 10^"3 g/sec so that τ = 0.77 sec. 2-Cyanoacrylic acid (1.5 g) was collected at the exit from the reactor and was recrystallised from toluene to give 1.1 g of pure material.

Example 6

Preparation of 2-cyanoacrylic acid from a solution in ethyl 2- cyanoacrylate of oligo(ethyl 2-cyanoacrylate formed during purification of crude oligomer (by-product 3 supra)

A 500 ml four-necked flask provided with a mechanical stirrer, a dosing funnel, a thermometer and a condenser arranged for reflux was charged with a 40% solution of formaldehyde in isopropyl alcohol (75 g: 1 mole) and diethylamine (0.49 ml). The mixture was stirred and heated at 60°C, and ethyl cyanoacetate (113 g: 1 mole) was added dropwise. After refluxing during a further 10 min, residual free formaldehyde was determined by titration of an aliquot with sulfite. When this had reached a level < 0.1% the condenser was arranged for distillation and an azeotropic mixture of isopropanol and water was distilled off and collected in a cooled receiver. Dry benzene (300 ml) was added to the distillation flask and a further ternary azeotrope of isopropanol, water and benzene was distilled and collected in the cooled receiver.

Residual solvent was then likewise distilled and collected, combined with the earlier distillates, and evaporated. The residue was dissolved in acetone and, after filtration, solid oligomer was precipitated using hexane and dried in vacuo (1.2 g). This oligomer was dissolved in ethyl cyanoacrylate (1 g) to give a viscous solution to which was added phosphorus pentoxide (100 mg).

A tube-type quartz pyrolysis reactor of the same type and dimensions as that described in Example 5 was charged with a quartz catalyst containing 5% iron prepared as described in Example 1, was heated to 623K, and was purged with hydrogen during 2 hr. The hydrogen was replaced by inert gas and the system was evacuated to Pp = 10 mm Hg (1.33 kPa). The dosing funnel was charged with 2.2 g of the oligomer-monomer mixture obtained as described above, and this was introduced into the heated zone of the reactor at a rate of G = 5 x 10^"4 g/sec with a carrier gas flow rate of F₀ = lO^l/sec giving a pyrolysis time τ = 1.12 sec. Crude 2-cyanoacrylic acid (1.3 g) was collected in the cooled receiver and was recrystallised from toluene to give 0.96 g of pure material.

Example 7

Preparation of 2-cyanoacrylic acid from oligofethyl 2-cvanoacrylate) obtained by amine-catalvsed Knoevenagel condensation of ethyl cyanoacetate with formaldehyde (purified oligomer) (by-product 2 supra)

The still-pot residue remaining after removal of solvents from the reaction mixture prepared as described in Example 6 consisted of solid oligo(ethyl 2-cyanoacrylate) (125 g). This was heated until it melted, and phosphorus pentoxide (1 g) was then added with stirring. The molten mixture was filtered and then cooled to give purified solid oligomer (120 g).

An empty, indented tube-type quartz reactor 60 cm long and having V_r = 140 cm³ was arranged as described in relation to Fig. 1, heated to T_p = 873K, evacuated to P_p = 1 mm Hg (133 Pa), and purged with sulfur dioxide during 30 min. The flow of sulfur dioxide was shut off, and the dosing funnel was heated to 100°C and charged with molten, purified oligomer (7.5 g). Phosphorus pentoxide (100 mg) was added, and the melt was added dropwise into the heated zone of the reactor at a rate G = 5 x 10^"4 g/sec so that the residence time was τ = 0.35 sec. Crude 2-cyanoacrylic acid (5.2 g) was collected in the cooled receiver and was recrystallised from toluene to give 3.2 g of pure material. Example 8

Preparation of 2-cyanoacrylic acid by pyrolysis of oligo(ethyl 2- cyanoacrylate obtained by the Knoevenagel condensation of ethyl cyanoacetate with formaldehyde in the presence of a heterogeneous amphoteric catalyst

A 500 ml four-necked flask fitted with a mechanical stirrer, a dosing funnel, a thermometer and a condenser arranged for reflux was charged with a 40% solution of formaldehyde in isopropyl alcohol (75 g: 1 mole) and aluminium oxide (40 g). The mixture was stirred and heated to 60°C, and ethyl cyanoacetate (113 g: 1 mole) was added dropwise. After refluxing during a further 20 min, residual free formaldehyde was determined by titration of an aliquot with sulfite When this was less than 0.1%, the mixture was filtered and an isopropyl alcohol - water azeotrope was distilled off. Dry benzene (300 ml) was added and a ternary azeotrope of benzene - water - isopropanol was distilled off. Residual solvent was then evaporated in vacuo to yield liquid oligo(ethyl 2-cyanoacrylate) (115 g). To this was added phosphorus pentoxide (1 g) with stirring at 60°C, and the mixture was filtered and cooled to give purified liquid oligomer (100 g).

An indented tube-type quartz pyrolysis reactor of the type shown in Fig. 1 60 cm in length and having a free volume of 140 cm³ was evacuated to P_p = 1 mm Hg (133 Pa) and heated to T_p = 873K and purged with sulfur dioxide for 30 min. The dosing funnel was heated to 60°C and charged with liquid oligomer (9.5 g) obtained as described supra to which was added phosphorus pentoxide (100 mg). The flow of sulfur dioxide was stopped and molten oligomer was added to the heated zone of the reactor at a rate G = 5 x 10^"4 g/sec giving a residence time τ = 0.35 sec. Crude 2-cyanoacrylic acid (6.9 g) was collected in the cooled receiver and recrystallised from toluene to give pure material (3.9 g). Example 9

Preparation of 2-cyanoacrylic acid from a still-pot residue obtained after thermal depolymerisation of oligo(ethyl 2-cyanoacrylate) (by¬ product 4 supra)

A 500 ml flask provided with a mechanical stirrer, a sulfur dioxide inlet adapter and a condenser arranged for distillation in vacuo was charged with phosphorus pentoxide (6 g), p-toluenesulfonic acid (1 g), γ-propanesultone (1 g), hydroquinone (2 g) and liquid oligo(ethyl 2-cyanoacrylate) (125 g) obtained exactly as described in Example 8. The oligomer was depolymerised by heating in vacuo at 170-180°C with continuous sparging of sulfur dioxide to give ethyl 2- cyanoacrylate (100 g) which was collected in a cooled receiver, and a viscous residue which consisted of a mixture of dicyanoglutarate- terminated oligo(ethyl 2-cyanoacrylate) and phosphorus pentoxide. The hot residue was decanted from solid phosphorus pentoxide to give liquid oligomer (20 g).

A quartz pyrolysis apparatus of the type shown in Fig. 2 was charged with quartz sand after which its free volume was 30 cm³. This was fluidised and was heated to 700°C and sparged with oxygen for 30 min. The flow of oxygen was stopped and replaced by inert gas preheated to 350°C and with a flow rate F₀ = 1.07 x 10^"3 1/sec. The temperature of the reactor, which was operated at P_p = 760 mm Hg (101 kPa), was reduced to T_p = 623K. The dosing funnel was charged with oligomer obtained as described supra (2.4 g) which was heated to 80°C in order to decrease its viscosity. The oligomer was added to the heated zone of the reactor at a rate G = 5 x 10^"3 g/sec giving a residence time τ = 5 sec. Crude 2-cyanoacrylic acid (0.72 g) was collected in the cooled receiver and recrystallised from toluene to give pure material (0.56 g). Example 10

Preparation of 2-cyanoacrylic acid from the residue remaining after distillation of crude ethyl 2-cyanoacrylate (bv-product 5 supra)

A 500 ml flask arranged for distillation in vacuo was charged with phosphorus pentoxide (2 g), γ-propanesultone (0.5 g), hydroquinone (0.6 g) and crude liquid ethyl 2-cyanoacrylate obtained exactly as describe in Example 8. The monomer was distilled in vacuo to give pure ethyl 2-cyanoacrylate (85 g) and a viscous liquid residue (18 g). This, while still hot, was decanted from phosphorus pentoxide to give dicyanoglutarate-terminated oligo(ethyl 2-cyanoacrylate) (10 g).

This oligomer (2 g) was pyrolysed in the same apparatus and under the same conditions as described in Example 9 to give crude 2- cyanoacrylic acid (0.52 g) which was recrystallised from toluene to yield pure material (0.44 g).

Example 11

Preparation of 2-cvanoacrylic acid from oligo(ethyl 2-cyanoacrylate) obtained bv the amine-catalysed Knoevenagel condensation of ethyl cyanoacetate (1 mole) with formaldehyde (1.1 mole)

A 500 ml four-necked flask provided with a mechanical stirrer, a dosing funnel, a thermometer and a condenser arranged for reflux was charged with a 40% solution of formaldehyde in isopropyl alcohol (83 g: 1.1 mole) and diethylamine (0.49 ml). The mixture was heated at 60°C with stirring, and ethyl cyanoacetate (113 g: 1 mole) was added dropwise. After refluxing during a further 10 min the residual formaldehyde level was determined by titration with sulfite. When this was less than 0.1 % an azeotrope of isopropyl alcohol and water was distilled off. Dry benzene (300 ml) was added and a ternary azeotrope of benzene - water - isopropanol was distilled off. Residual solvent was removed in vacuo to leave solid oligo(ethyl 2-cyanoacrylate) (125 g). This was heated until molten and phosphorus pentoxide (1 g) was added with stirring. The mixture was then filtered and cooled to give purified oligomer (120 g).

A quartz pyrolysis apparatus of the type shown in Fig. 2 was charged with quartz sand having an iron content of 5% which had been prepared according to Example 1. The free volume V_r of the reactor was then 30 cm³. The catalyst was fluidised, and the reactor was heated to 350°C and sparged with hydrogen during 120 min. The flow of hydrogen was stopped and replaced by a flow of inert gas preheated to 350°C (T_p = 623K) at a rate of F₀ = 1.07 x 10^"3 1/sec and at a pressure Pp = 760 mm Hg (101 kPa). The dosing funnel was charged with oligomer (3.2 g) prepared as described supra and heated to 80°C in order to decrease its viscosity. Oligomer was added to the heated zone of the reactor at a rate G = 5 x 10^*3 g/sec, giving a residence time τ = 5 sec. Crude 2-cyanoacrylic acid (1.22 g) was collected in the cooled receiver and recrystallised from toluene to give pure material (1.05 g).

Example 12

Preparation of 2-cvanoacrylic acid by pyrolysis of oligo(ethyl 2- cvanoacrylate under conditions of long residence time τ

The same oligomer as was used in Example 11 (3.4 g) was pyrolysed in exactly the same reactor and under the same conditions except that the carrier gas flow rate F₀ was 1.2 x 10⁴ 1/sec and the rate of addition of oligomer G was 10 ³ g/sec, giving a residence time τ = 30 sec. Crude 2-cyanoacrylic acid (0.4 g) was collected in the cooled receiver and recrystallised from toluene to give pure material (0.35 g). Example 13

Alternative preparation of 2-cvanoacrylic acid from oligo(ethyl 2- cyanoacrylate obtained by the Knoevenagel condensation of equimolar ethyl cyanoacetate and formaldehyde using an amphoteric catalyst

The oligo(ethyl 2-cyanoacrylate) was prepared exactly as described in Example 8.

A quartz pyrolysis apparatus of the type shown in Fig. 2 was charged with quartz sand when the free volume V_r was 30 cm³. The sand was fluidised, and the reactor was heated to 700°C and sparged with oxygen during 30 min. The temperature was reduced to 350°C

(T_p = 623K) and the oxygen was replaced by a flow of inert gas at F₀ = 1.09 x 10^"3 1/sec, preheated to 350°C and at a pressure P_p = 760 mm Hg (101 kPa). The dosing funnel was charged with oligomer (3 g) heated to 80°C in order to reduce its viscosity. The oligomer was added to the heated zone of the reactor at a rate G = 8 x 10^"3 g/sec, giving a residence time τ = 3 sec. Crude 2-cyanoacrylic acid (1.52 g) was collected in the cooled receiver and recrystallised from toluene to give pure material (1.44 g).

The same oligomer (2.9 g) was pyrolysed with a longer residence time τ in the same apparatus under conditions of Tp = 823K, F₀ = 1.8 x 10-5 i/sec, P_P = 760 mm Hg (101 kPa) and G = 10-3 g/_sec giving a value for the residence time τ = 30 sec. Crude 2-cyanoacrylic acid (0.2 g) was collected in the cooled receiver.

Claims

Claims: -

1. A process for the preparation of 2-cyanoacrylic acid from poly (alkyl 2-cyanoacrylates), which process comprises the steps of:

c) collecting the 2-cyanoacrylic acid formed.

2. A process according to Claim 1, wherein the oligomeric or polymeric cyanoacrylate material is fed to the reactor in a carrier gas.

3. A process according to Claim 1 or 2, wherein the pyrolysis is carried out at a temperature in the range 250-700°C.

4. A process according to any preceding claim, wherein the pyrolysis is carried out at a temperature in the range 300-600°C.

5. A process according to any one of Claims 2-4, wherein the polymerisation inhibiting conditions are conditions wherein one or both of an anionic polymerisation inhibitor and a free radical polymerisation inhibitor is added to one or both of the oligomeric or polymeric cyanoacrylate material and the carrier gas.

6. A process according to any preceding claim, which is carried out at a pressure in the range 0.05 to 100 mm Hg (6.64 Pa - 13.3 kPa).

7. A process according to any preceding claim, wherein the reactor is a tube-type reactor.

8. A process according to Claim 7, wherein the surface of the reactor serves as an integral catalyst.

9. A process according to Claim 7 or 8, wherein the tube of the reactor is made of quartz.

10. A process according to any one of Claims 1-6, wherein the reactor is a reactor wherein the pyrolysis is carried out in the environment of a pseudoliquid layer of a catalyst.

11. A process according to any one of Claims 1-7, 9 when dependent on Claim 7 and Claim 10, wherein the catalyst is a metal oxide.

12. A process according to Claim 11, wherein the catalyst is a transition metal oxide.

13. A process according to Claim 11 or 12, wherein the catalyst is bonded to a solid support.

14. A process according to any one of Claims 2-13, wherein the carrier gas is an inert gas.

15. A process according to any one of Claims 2-13, wherein the carrier gas is an acidic gas.

16. A process according to Claim 15, wherein the acidic gas is selected from sulfur dioxide, carbon dioxide, carbon monoxide and nitrous oxide.

17. A process according to Claim 16, wherein the acidic gas is sulfur dioxide.

18. A process according to any preceding claim, wherein the oligomeric or polymeric cyanoacrylate material is in the form of a melt, solid or solution.

19. A process according to Claim 18, wherein the oligomeric or polymeric cyanoacrylate material is in the form of a solution with a cyanoacrylate ester as solvent.

20. A process according to any preceding claim, wherein the oligomeric or polymeric cyanoacrylate material is a product of a Knoevenagel Condensation.

21. A process according to any one of Claims 1-20, wherein the oligomeric or polymeric cyanoacrylate material is selected from crude oligomer, purified oligomer, oligomer produced as a by-product in the conversion of crude oligomer to purified oligomer, oligomer produced as a by-product during the depolymerisation of purified oligomer, oligomer produced during the conversion of crude monomer to purified monomer and polymer produced during storage of purified monomer.

22. A process according to any preceding claim, wherein the emergent vapours are cooled by passing through a condenser cooled by air or cold water.

23. A process according to Claim 22, wherein the cooled vapours are passed through a condenser cooled by dry ice or by liquid nitrogen and the 2-cyanoacrylic acid collected.