CA1054159A - Monoacetals of aromatic 1,2-diketones - Google Patents

Abstract

Abstract of the Disclosure Monoacetals of aromatic diketones in which the acetal group is derived from primary mono- or dialkohols are useful as sensitizers in the photochemical polymerisation or photochemical crosslinking of polymers. They are prepared from the diketones (benzils) by reaction with sulfites.
The sulfites and the benzils may be formed immediately before the reaction in the same vessel without isolating these compounds. Most of the benzil monoacetals are new compounds.

Description

` ~()5415~
The invention relates to new benzil-monoacetals, to a new process for tne production of benzil-monoacetals and to their use as sensitisers for photopol~nerisation of polymerisable systems containing unsaturated S compounds, as well as to their use for the photo-chemical cr~ss-linking of polymerisates.
It is known that unsaturated monomers or mixtures thereof with unsaturated polymers can be photochemically polymerised in the presence of suitable sensitisers such as carbonyl compounds containing a halogen in the a-position with respect to the carbonyl group, mercaptans, aromatic disulphides, nitroso compounds, azo compounds, benzoins and benzoln e~hers. In industry there is now a demand for sensitisers which, with good storage-stability, initiate photopolymerisation more rapidly,and at the same time produce a polymer yield per unit of time higher than that possible with the hitherto known sensitisers. By virtue of the employment of such improved sensitisers, the expensive industrial UV-irradiation equipment could be utilised more economically.
Compounds as defined below have now been found thatare suitable, in a surprisingly advantageous manner, as sensitisers for photopolymerisation of polymerisable systems containing unsaturated compounds. The advantages they offer are, in particular, a more rapid start of .

2 --~, ~ `
``:
`~ `
;
: 1054159 photopolymerisation and the higher time-yield attainable in consequence, with, at the same time, excellent dark-storage stability. Moreover the compounds are suitable as sensitisers for the photochemical cross-linking of -~ -linear polymerisates, such as, e.g. polyethylene.
The compounds usable according to the invention correspond to formula I
~C=~ ~

C~OCH2R )2 wherein Rl represents hydrogen, alkyl having l to 5 carbon atoms, alkenyl having 2 or 3 carbon atoms, aralkyl having 7 to 9 carbon atoms or a group -(CH2)n-X, wherein X is halogen, alkoxy having 1 to 4 carbon atoms or phenoxy and n is an integer from l to 3.
Particularly preferred is the use of compounds of formula I
wherein R represents hydrogen, alkyl having l to 3 carbon atoms or a group -CH2-X, and X represents chlorine, bromine or -OCH3. -With the exception of the dimethylacetal and monoethylene-acetal of benzil, these monoacetals of aromatic 1,2-diketones of formula I

are new compounds. The invention concerns therefore also new rompounds of formula I

wherein Rl represents alkyl having 1 to 5 carbon atoms, alkenyl having 2 or 3 carbon atoms, aralkyl having 7 to 9 carbon atoms or a group -~CH2)n-X, wherein X is halogen, alkoxy having l to 4 carbon atoms or phenoxy and n is an integer from l to 3.
Preferred compoundsof formula I are compounds wherein R1 represents alkyl having l to 3 carbon atoms, vinyl, benzyl, ~-styrene or a group -(CH2)n-X, X represents chlorine, bromine, or alkoxy having l to 4 carbon

- 3 -. .
, atoms, and , n represents 1 or 2.
Examples of compounds of formula I are: benzil-diethylacetal, benzil-dipropylacetal, benzil-dibutylacetal, benzil-di~-phenylethyl)acetal, benzil-diallylacetal, benzil-di(y-phenylallyl)acetal, benzil-di(2-methoxy-ethyl)acetal, benzil-di(2-ethoxyethyl)acetal, benzil-di(2-chloroethyl)acetal, benzil-di(2-bromoethyl)acetal, benzil-di(2-chloropropyl)acetal, benzil-di(2-methylthioethyl)acetal, benzil-di(2-tert.butylthioethyl)acetal, benzil-di(2-phenylthioethyl)acetal, benzil-di(2-phenoxyethyl)acetal, benzil-di(2-acetoxy-ethyl)acetal, benzil-di(2-butyroxyethyl)acetal, benzil-di(2-ethoxycarbonyl-ethyl)acetal, benzil-di(3-methoxycarbonylpropyl)acetal, 4,4'-dimethylbenzil-dimethylacetal, 4,4'-diphenylbenzil-diethylacetal, 2,2'-dimethoxybenzil-dipropylacetal, 4,4'-dichlorobenzil-dimethylacetal, 4,4'-dibromobenzil-dimethylacetal, 4,4'-diisopropylbenzil-di(2-chloroethyl)acetal, 2,4,2',4'-tetramethylbenzil-diethylacetal, 4-chlorobenzil-dimethylacetal, 4-ethoxy-benzil-diethylacetal, 4-phenylbenzil-dimethylacetal, 2,4,6-trimethylbenzil-di(2-methoxyethyl)acetal.
No generally applicable processes for the production of non-cyclic monoacetals of aromatic 1,2-diketones of formula I have hitherto been known. The dimethylacetal of benzil was produced by reaction of benzil with a large excess of barium oxide and methyliodide in dimethylformamide (Chem. Berichte 94, 2258 (1961)). After separation of the benzilic acid methyl ester, occurring as by-product, in several purifying operations, benzil-dimethylacetal is obtained in 40% yield. This process is too unecon-omical for commercial production, since it requires expensive reagents and produces low yields. Nor is it commercially applicable for the production, not described, of the higher homologues.
A process has now been found which enables monoacetals of aromatic 1,2-diketones of formula I to be produced, in a generally applicable manner, simply and in high yield and degree of purity. This process com-prises the reaction of an aromatic 1,2-diketone either with a sulphurous acid ester in the presence of an anhydrous acid and of an alcohol, or with thionyl - E
i .
.

., chloride and an alcohol.
There is preferably used for this purpose an aromatic 1,2- -diketone of formula II
O O
~ 2 (II) Ar - C - C - Ar wherein Arl and Ar2 each represent a phenyl radical and this reacted with a sulphurous acid ester of the formula (RlCH20)2SO in the presence of an anhydrous acid and of an alcohol of the formula RlCH20H, or with thionyl chloride and an alcohol of the formula RlCH20H, whereby Rl represents hydrogen, alkyl having 1 to 5 carbon atoms, alkenyl having 2 or 3 carbon atoms, aralkyl having 7 to 9 carbon atoms or a group -(CH2)n-X, wherein X is halogen, alkoxy having 1 to 4 carbon atoms or phenoxy and n is an integer from 1 to 3.
Examples of sulphurous acid esters which can be used for this -~
process are dimethylsulphite, diethylsulphite, dipropylsulphite, dibutyl-sulphite, dihexylsulphite, diallylsulphite, di(~-phenylethyl)sulphite, di(y-phenylallyl)sulphite, di(2-chloroethyl)sulphite, di~2-methoxyethyl)sulphite, di(2-phenoxyethyl)sulphite, di(2-methylthioethyl)sulphite, di(2-phenylthio-ethyl)sulphite, di(2-acetoxyethyl)sulphite or di(3-carbomethoxypropyl)sulphite.
These sulphites are p:roduced by processes known from the literature, e.g. by reaction of the corresponding alcohols with thionyl chloride.
The anhydrous acid employed for the purpose of accelerating the formation of acetal can be an anhydrous mineral acid such as, e.g.
; sulphuric acid or hydrochloric acid, or a Lewis acid such as, e.g. boro-` fluoride or its complexes. It is preferable to use concentrated sulphuric acid, and in an amount of at least one mole per mole of diketone.
The alcohol employed is advantageously the alcohol correspond-ing to the sulphurous acid ester, that is, for example, methanol with application of dimethylsulphite, or isopropanol with application of diiso-propylsulphite. If the diketones are reacted with thionyl chloride and a j primary monoalcohol, then as such there can be used, e.g. methanol, ethanol, ;.~ .
propanol, butanol, hexanol, allyl alcohol, ~-phenylethyl alcohol, y-phenyl-allyl alcohol, 2-chloroethanol, Z-methoxyethanol, 2-phenoxyethanol, 2-methyl-_ 5 _ ~ ' .. . ....... ..
~, . . , . . - :

thioethanol, 2-phenylthioethanol, 2-acetoxyethanol, hydracrylic acid methyl ester or glycolic acid butyl ester.
The reaction of the diketones with sulphurous acid esters in the presence of an acid and of an alcohol is generally performed at a temp-erature of 20 to 120C, preferably at 40 to 100C.
; The reaction of the diketones with a mixture of thionyl chlor-ide and al~ohol is advantageously performed with cooling to ca. O to 20C, whereby the sulphite forms. The subsequent reaction is, as described above, carried out at elevated temperature.
A modification of the process consists in reacting a benzoin firstly with sulphuryl chloride and subsequently with thionyl chloride and an alcohol. Preferably used in this case is a benzoin of formula III
Ar C = O
2 1 ~III) Ar CH - OH
which is reacted with sulphuryl chloride and then with thionyl chloride and an alcohol, RlCH20H, whereby Ar1, Ar2 and Rl have the above defined meanings. t In the case of this process variant, there is formed from the benzoin employed, by an oxidation reaction with the sulphuryl chloride, the corresponding benzil which, ' "J

. : :
:
. .
- :

1054~59 without being isolated, is converted by reaction with thionyl chloride and an alcohol into benzil-monoacetal.
The procedure used is to suspend the benzoin in the sulphuryl chloride and to heat the suspension slowly to about 25 - 50 C. After completion of oxidation, the --excess sulphuryl chloride is distilled off and~ after ~-cooling to room temperature, the reaction with thionyl chloride and the alcohol performed as described above. -~
The isolation of benzil-monoacetals from the reaction mixtures is effected by known processes; e.g. by concentration by evaporation of the solution, by the addition of water or by cooJing. Yields of over 80% of crude monoacetal are obtained; as required, the resulting product can be purified by recrystallisation or distillation or by other known methods.
The physical properties of the benzil-monoacetals are dependent to a great extent on the nature and position of the substituents; in general~ the products are low-melting or oily compounds~ which at room temperature are stable to an unlimited extent.
The benzil-monoacetals producible according to the invention can be used as sensitisers for photopolymerisation of polymerisable systems containing unsaturated compounds.
Such systems are~ for example~ unsaturated monomers such as acrylic acid methyl ester, -ethyl ester, -n- or tert.

., :

. .

.. . . . . . . . ..

~054~59 butyl ester, methacrylic acid alkyl esters such as methylmethacrylate or ethylmethacrylate, di-~meth)-acrylates of aliphatic diols, acrylonitrile, methacrylo-nitrile~ acrylamide~ methacrylamide~ N~N~disubstituted acrylamides and methacrylamides, vinylacetate, vinyl-acrylate, vinylpropionate, succinic acid divinyl ester, isobutylvinyl ether, butanediol-1,4_divinyl ether, -styrene~ alkylstyrenes, halostyrenes, divinylbenzenes, vinylnaphthalene, N-vinylpyrrolidone, vinyl chloride, ~0 vinylidene chloride, diallylph~halate, diallylmaleate~
triallylisocyanurate, triallylphosphate, ethylene glycol diallyl ether, pentaerythritol-tetraallyl ether, and mixtures of such monomers.
Photopolymerisable systems are, in addition, un-saturated polymers and mixtures thereof with unsaturated monomers. These inc]ude~ in particular~ themiEturesof unsaturated polyesters with unsaturated monomers. By unsaturated polyesters are meant, for example, polycondensation products from ~ -unsaturated dicarboxylic acids or their derivatives with polyols. Examples of ~ ~-unsaturated dicarbox-ylic acids or their derivatives are maleic acid~ maleic acid anhydride~ fumaric acid~ mesaconic acid, and citraconic acid. ~esides the unsaturated dicarboxylic acids, it is also possible to incorporate, for the purpose of adjustment of the degree of unsaturation, saturated dicarboxylic acids or dicarboxylic acids inert to polymerisation~ Examples ~-in this case are succinic acid, sebacic acid, isophthalic - -.

acid, phthalic acid, halogenated phthalic acids or 3,6-endomethylene- ~ 4~
tetrahydrophthalic acid, as well as the anhydrides of these dicarboxylic acids.
The polyols employed for the produc~ion of poly-condensation products are mainly glycols such as ethylene glycol, propanediol-1,2, diethylene glycol, 1,3-propylene glycol, 1,4-tetramethylene glycol as well as triethylene glycol.
Further modifications of unsaturated polyester resins are possible by incorporation of monocarboxylic acids or monoalcohols.
These unsaturated polyesters are usually employed in admixture with unsaturated monomers containing allyl or vinyl groups, preferably with styrene. After the addition of benzil-monoacetals of formula I, such mix-tures can be photopolymerised in an advantageous manner to thus obtain moulding and coating compounds.
Moulding compounds which can be photopolymerised with compounds of formula I are, for example, so-called air-drying molding compounds.
These are unsaturated polyesters containing, besides ~,~-unsaturated di-carboxylic acid esters, also ~,y-unsaturated ether radicals.
Coating compounds that can be photopolymerised with compounds 2~ of formula I are, for example, lacquer coatings from unsaturated monomers and unsaturated polymers.

, . . .

-- :

105~159 These lacquers may also be photopolymerised by the so-called active-base process (Aktivgrundverfahren).
The coating compound is in this case applied with the photo-initiator to a peroxide-containing layer previously applied to the substrate, and subsequently photopolymerised.
The photopolymerisable compounds or mixtures can be stabilised by addition of the usual thermal inhibitors which are employed in the production of light-sensitive compounds. As examples of these, mention may be made of hydroquinone, p-quinone, p-methoxyphenol~ B-naphthylamine~
~-naphthol and phenols. For enhancement of the dark-storage stability, additions can moreover be made of copper compounds such as copper naphthenate, -stearate ; or ~octoate, phosphorus compounds such as triphenylphosphine, tributylphosphine, triethylphosphite, triphenylphosphite or tribenzylphosphate, quaternary = onium compounds such as tetramethylammonium chloride or trimethylbenzyl-ammonium chloride or hydroxylamine derivatives such as N,N-diethylhydroxylamine. Furthermore, the photopolymerisable compounds or mixtures can contain chain-transfer agents ~
such as triethanolamine or cyclohexene. `
In order to exclude the inhibiting action of atmospheric oxygen, it is advantageous to make additions of paraffins, waxes or wax~like substances to the coating compounds with the photosensitisers. These added substances float out at the commencement of photopolymerisation and thus prevent - 1~

''' : `~ . ' ` 1054~59 the inhibiting action of the atmospheric oxygen.
A further possibility of preventing the inhibiting action of the atmospheric oxygen is for the process to be performed under inert gas, or for fillers permeable to W -light, such as~ e.g. certain silicates~ to be added to the polyester resin. The formulations filled in this way cure~ even in air, rapidly under W -irradiation, because the content of bonding agent is reduced at the surface.
Also the introduction of autoxidisable groups into ;
the resin to be cured can eliminate the inhibi~ing action -~
of the atmospheric oxygen. For example~ this can be effected by copolymerisation with certain allyl compounds.
Furthermore, small amounts of conventional W -absorbers ~ ;
can be added to the moulding and coating compounds, without the reactivity of the photosensitisers being appreciably impaired. The coating and moulding compounds may also contain slight amounts of the usual carriers and fillers~
; as well as so-called thixotropic agents, such as glass fibres~ synthetic fibres~ silicic acid and `
talcum.
A further application of compounds of formula I is the photochemical cross-linking of polymerisates, specially of olefin polymerisates. By olefin polymerisates are meant in this connection polyolefins such as high-and low-pressure polyethylene, polypropylene, polybutylene, polyisobutylene and ethylene-vinylacetate-copolymerisates.
In addition, copolymerisates from olefins such as ethylene, propylene~ butylene or isobutylene can be cross-linked by W -irradiation with the aid of the benzil-monoacetals.
Ben~ monoacetals are moreover applicable for the production of photopolymerised elements from which~
after irradiation and by washing-out, reliCf profiles for printing purposes can be produced. Suitable as unsaturated polymers in photopolymerisable layers for the production ;`of relief profiles for printing purposes are, in particular, linear synthetic polyamides. Photo-polymerisable unsaturated monomers, which are used in the mentioned polymers in li~ht-sensitive layers for the production of relief profiles~ are preferably those which contain at least two polymerisable olefinic double bonds and, besides the double bonds, also amide groups such as, e.g. methyPene-bis-acrylamide~ methylene-bis-methacryl-amide as well as bis-acryl- or bis-methacrylamides of diamines.
A further application of the benzil-monoacetals as photosensitisers is in the drying, by W -irradiation~ of printing pastes containing as bonding agents unsaturated monomers and unsaturated polymers. Based on bonding agents having, for example, conjugated double bonds, printing , :

pastes which dry in a short time under the action of W -rays can be produced.
Examples of such bonding agents are natural or synthetic conjuen oils~ unsaturated polyester resins or polyfunctional acrylates or methacrylates. Such r'' printing-paste bonding agents frequently contain as additives chain-transfer agents such as triethanolamine or cyclohexene, anti-inhibiting agents such as diallyl-phthalate-prepolymers, or stabilisers such as diethyl-hydroxylamine. For such printing-paste bonding agents~
the benzil-monoacetals according to the invention are p~rticularly well suited catalysts for photochemical curing.
,. .
The benzil-monoacetals of formula I or Ia are used for the mentioned fields of application advantageously in amounts of 0.1 to 20 per cent by weight~ preferabl~ in amounts of ca. 005 to about 5 per cent by weight, and either singly or in admixture with each other.
The addition of the sensitisers to the photopolymer-isable systems is generally effected by a simple stirring-in, since most of these systems are liquid. Usually there is obtained a solution of the sensitisers according to the invention~ in consequence of which their homogeneous -~ distribution and the transparency of the polymerisates are ensured.
~.

' - - . . . i 105~159 - Polymerisation of the systems sensitised in such a manner is performed, using known methods of photopolymerisation, by irradiation with light rich in short-wave radiation. Suitable light sources for the irradi-ation of the substrates containing the photo-sensitisers of formula I are i medium-pressure, high-pressure and low-pressure mercury vapour lamps, as - well as superactinic fluorescent lamps, of which the emission maxima is in the range of between 300 and 400 m~.
The production and use according to the invention of the said benzil-monoacetals are further illustrated in the following examples.
The term 'parts' denotes parts by weight and temperatures are expressed in degrees Centigrade.
, _ ~_ , ., , ~ .

.: . , - .
~- :: . - -E.xflm~l Pro~uct~on of ~)enzi~dl ~t~yl cetaL
hod ~
:
210.0 g of benzil and 440 g of dimethylsulphite are dissolved at reflux temperature in 1000 ml of anhydrous methallol. An addition is then made dropwise to this solution at 60 - 65 in the course of 4 hours, w;th stirring, of 200.0 g of concentrated sulphuric acid.
This reaction mixture is subsequently refluxed for a further

4 hours. After this length of time, a sample examined ~ ~
by thin-layer chromatography (neutral silica gel; ~- ;
solvent: toluene/ligroin 9:1) shows only traces of benzil. -~
The slightly yellow solution is cooled and then neutralised with potassium carbonate. The precipitated potassium sulphate is removed and an amount of 2 ml of trimethyl-phosphite then added to the filtrate to effect separation of the residual traces of benzil; the temperature is maintained for 2 hours at room temperature, the filtrate then concentrated by evaporation to dryness and the residue distilled in vacuo. Benzildimethylacetal distills at 140 - 141tO.5 mm in the form of a colourless oil, which crystallises in the receiver; M.P. 62 - 63. Benæil-dimethylacetal is obtained by this process in a yield of 85 - 90% of theory, calculated on the amount of benzil used. The absorption of the carbonyl bands of the benzil-dimethylacetal obtained lies in the IR-spectrum at

5-91 ~u; J~max. 250.5 nm-E ~

:
~ 054159 b) ~et~ocl B
An amount of 210.2 g of benæil is suspended in 238.0 g oE ~hionyl chloride. This suspension is cooled to 3`
4 and, with slow stirring, 128~0 g of methanol added dropwise in the course of 2 hours, whereby the temperature is maintained between 2 and 7. After completion of the addition, the reaction mixture is allowed to slowly warm up to room temperature to obtain a clear yellow solution. This is thereupon heated within 30 minutes to 50 and stirred for 1 hour at this temperature. The excess dimethylsulphite is distilled off in vacuo, -~
the oily residue then taken up in 400 ml of isopropanol, . .
and 25 g of potassium carbonate and 7 ml of trimethyl-phosphite are added to the clear solution. A crystalline -precipi.tate forms as the solut10n cools. An addition ,; . ... .
is made dropwise at between 7 and 10 of 350 ml of water, and the reaction mixture stirred. The crystalline precipitate is filtered off under suction, washed with isopropanol/
water 1:1 and dried at 40 in vacuo.
! Benzildimethylacetal is obtained by this process in the form of white crystals in a yield of 84% of theory.

c) Method C -An amount of 106.0 g of benzoin is suspended in 135 g of sulphuryl chloride and this suspension stirred for 12 hours at room temperature. Excess sulphuryl chloride is then distilled off and the residue suspended in 120 g _ /6 . . .
.

- `
~ 054159 of thiorlyl chloride, An addition is made dropwise to this suspension at be~ween 2 and 7, in the course of 2 hours, of 65 g of me~hanol. After the addition, the reaction mixture is allowed to slowly come up to room temperature, whereby a clear ~Jellow-brown solution is obtained, This is heated within 30 minutes to 50 and stirred for 1 hour at this temperature, The excess dimethyl-sulphite is distilled off in vacuo and the oily residue taken up in 200 ml of isopropanol, A clear solution is obtained, to which are added 15 g of potassium ca~bonate and 5 ml of trimethylphosphite, A crystalline precipitate is formed on cooling, A further addition is made of 200 ml of water and the crystal mass then filtered off under suction; it is washed with isopropanol/water 1:1 and dried at 40 in vacuo.
By this method, benzildimethylacetal is obtained in the form of wh~te crystals in a yield of 79% of theory.

Examples 2 - 4 Production of ben~ dialkylacetals An amount of 21 g of benzil is suspended in 23,8 g of thionyl chloride, The suspension is cooled to ca. 5 and alkanol slowly added dropwise in the amount given in column 2 of the following table, Cooling is removed and stirring carried out for 4 hours at room temperature and for a further 1 hour at 50. The yellow solution is concentrated in vacuo, the oily residue dissolved in 40 ml ; 17 . ~

-of isopropanol, ~nd 3 g of potassium carbonate and 0.6 ml of trimethylphosphite -then added to the soLution.
Af~er the addition o~ 30 ml of water, a crystalline precipitate of the product listed in column 3 of the table i5 formed, which is filtered off and recrystallised from isopropanol. ~

Example Alcohol employed Product Melt. point - -_ _ Boil. point 2 19 g of ethanol benzene-diethylacetal M.P. 61-62 ~ -: 3 24 g of n-propanol benzene-di n-propyl M.P. 46-47 acetal ; 10_ _ 30 g oE n-butanol benzene-di-n-butyl- B- 0.5 175 Example 5 Production of benzil-di-~ methoxyethyl-acetal a) Method B
An amount of 21.0 g of benzil is suspended in 23.8 g of `-; thionyl chloride and the suspension cooled to 0. An addition ` ~-is made dropwise to this suspension, within 30 minutes, of 30.4 g of ~-methoxyethanol, whereby the temperature is maintained at between 2 rlnd 7 by cooling. The reaction mixture is raised within 6 hours to room temperature to obtain a clear solution.
This is heated for 1 hour at 60. The clear solution is concentrated in a rotary evaporator; the residue is then taken up in 40 ml of isopropanol, and 3 g of potassium E

lQSa~159 .:
carbonate and 0.6 ml of trimethylphosphite are added.
Cooling is applied as 35 ml of water is added dropwise, and white crystals co~mence to precipitate. These are filtered ~` off under suc.ion and recryskallised Erom ethanol.
Benzene-di-~-methoxyethyl-acetal is o~tained by this means in the form of white crystals having a melting point of 67-69.
b) Method C `
.:
An amount of 21.2 g of benzoin is placed into 27.0 g of sulphuryl chloride. This mixture is stirred for 6 hours at room temperature, whereby the reaction mixture gradually turns yellow as an intense evolution of gas occurs. When all the benzoin has oxidlsed to benzil, the -excess sulphuryl chloride is distilled off in vacuo, and the residue taken up in 23.8 g of thionyl chloride. An addition is then made dropwise at 5 of 30.4 g of ethylene glycol monomethyl ether, and the process continued as described in Example 1. Benzil-di-p-methoxyethyl-acetal is obtained in the form of white crystals having a melting point of 67-69.

Example 6 Produc _on of benzil-di-~-chloroethyl-acetal A suspension of 21.0 g of benzil in 23.8 g of thionyl-chloride is cooled to 0, and 35.3 g of 2-chloroethanol added dropwise to the suspension at between 0 and 5 in - the course of 30 minutes. The reaction mixture is stirred ~ /9 ` ~054159 for 6 hours a~ room temperature and subscquently for 1 hour at S0 - 60; i-~ is then concentrated in a rotary evapora-tor~ and 3 g of potassium carbona~e and 0.6 ml of trimethylphosphite are added to the residue; the whole is afterwards taken up in 40 ml of isopropanol. Water is added and a crystalline precipitate formed. This is filtered off under suction and recrystallised from isopropanol to obtain benzil-di-~-chloroethyl-acetal in the form of white crystals, M.P. 58 - 59. --E~ample 7 Production of benzil-di-~-bromoethyl-acetal 50.0 g of 2-bromoethanol is added dropwise, with cooling and in the course of 30 minutes, to a suspension, cooled to 0, of 21.0 g of benzil in 23.8 g of thionyl chloride. After completion of the addition, the reaction mixture is stirred for 8 hours at room temperature and then for 1 hour at 50 - 60. It is afterwards concentrated in vacuo, the residue dissolved in 40 ml of isopropanol, and 3.0 g of potassium carbonate and 0.6 ml of trimethyl- -phosphite added to the solution. Water is added to obtain a crystalline precipitate, which is filtered off under suction and recrystallised from isopropanol. The resulting product is benzil-di-~-bromoethyl-acetal in the form of -white crystals having a melting point of 79 - 80.

~ 0 :

` 1054159 E~a~ e 8 ~ Pho~o~o~ eL^isiltinn of rletlly~acry~late : An amount of 0.1 g in each case of various kno~
sensitisers and of sen.sitisers according to the invention is dissolved in 10.0 g of freshly distilled acrylic acid methyl ester. This solution contained in a quartz glass tube of 1.5 cm diame~er is irradiated, in a thermostatically control~ed water bath a~ 25, with a high-pressure mercury-vapour lamp. The lamp is situated at a distance of 10 cm from the quartz tube. Nitrogen is passed for 1 minute before irradiation through the solution of the respective sensitisers, and the flow of nitrogen is maintained during irradiation. The polymerisation of the monomers occurring during exposure to the light is indicated by a rise in temperature of the irradiated solution. The exposure time is 20 seconds. Immediately after irradiation, the irradiated solution is cooled in order to prevent a thermal polymerisation. The solution of the formed ; polymerisate in the monomer is rinsed with small amounts of ethylacetate in a round flask, and the solvent and the unpolymerised monomeric fraction are subsequently distilled off in a rotary evaporator. The polymeric residue is dried in a vacuum drying chamber at 50 - 60 and afterwards weighed.
The amounts of polyacrylic acid methyl ester obtained with various sensitisers by the above test procedure are shown in the following table.
:.
E ~ ~
.

. ~ . . . .

Table 1 ~ ~_~ . . .. --~
Per cent SensLtiser Start time Amount of fornled poly-~i by weight in seconds acrylic acid methyl ester in per cent by weight ~ _ . _ _ ___~__ _ . 1 benzoin 11 6.2 . 1 benzoinisopro- 8 7.7 --pyl ether _ _ acetal . 16.1 -Without the photo-initiator, the amount of polymerisation ~ -is below 0.1%.
From the values shown in Table 1, it is clear that, compared with.known photo-initiators, the photosensitisers '. according to the invention initiate polymerisation more -rapidly and give higher polymer yields. ~ ;~
.

: Photopolymerisation of methylacrylate ~.
.
.; With the same test procedure as described in Example 8, solutions of known photosensitisers and photosensitisers . according to the invention are irradiated in acrylic acid lS methyl ester. After various exposure times, an aliquotic specimen of the irradiated solution is concentrated by evaporation, and the polymeric residue dried and weighed.
The results of this test series are given in the followlng ~ .
t Table 2.
,.. .
,...................................................................... .

~;
.

- ~OS4~59 Table 2 . ~er C~llt Sensitiser Anount of formed polyacrylic acid methyl : by welght . es~er in per cent by weight after an.irradiation time of:
_ . _ ........................ ..
. 10 seconds 20 seconds 30 seconds .. . . ~ ~ _ . . ._ ~__ 1 benzoin _ 6.2 11.8 1 benzoiniso- 4.2 7.7 13.4 ~ropyl ether .. _. . benzLldimethyl- 8.0 16.1 26.7 ~:
From the values.listed in Table 2, it is clearly shown that, compared with known photo-initiators, the photo-sensitisers according to the invention initiate polymerisation more rapidly and give higher polymer yields.
.~
. Example 10 Guring of polyester resin lS An amount of 0.2 parts by weight of known photo-sensitisers and of photosensitisers according to the invention are i.ncorporated into unsaturated polyester . resin according to the following formulation:
10.0 parts of unsaturated polyester resin (polyester based 0 on maleic acid having a styrene content of 35%) 0.2 part of photosensitiser 0.1 part of a 10% solution of paraffin in toluene.
This mi~ture is stirred until the constituents are completely dissolved, and the solution subsequently applied, .
with a film-applier (500 m~), to glass plates. The films ~:

~3 .

-, ~

-lOS~lS9 are irradla~ed wi~h a fluorescent lamp giving oEf a high propor~ion of W -light, at a dLstanc~ of 5 cm. ~fter an exposure time of 20 minutes, the hardness of the films is determined by means of a pendulum apparatus (pendulum hardness according to Konig). The results of this series of tests are given in Table 3:
Table 3 _ . ~ . ~
Sensitiser Pendulum hardness according to -K~nig, after 20 min. irradiation _ benzoin 55 benzoinisopropyl ethe r 82 benzildimethylacetal 95 ; benzildiethylacetal 93 benzildipropylacetal 86 benzildibutylacetal 90 benzil-di-~-methoxy- 83 ethylacetal :
Example 11 Determination of dark-storage stabilitY
A 2% solution of known photosensitisers and of photo-sensitlsers according to the invention in unsaturated ' 20 polyester resin (polyPster based on maleic acid having a i styrene content of 35%) is prepared at 25, and in each case the flow-through-time of a specific amount of the particular ~;
'3 solution through a graduated buret determined. The solution is then stored in the dark at room temperature and, after ; 25 2, 4 and 8 weeks' storage time, the flow-through-time E ~

.

.

. ~ , . `.
. .
.

- ~0~4159 determined in the same mallner. Polyester resin containing no additive is talcen as a comparison. The results of these tests are given in Table 4:
l'able 4 . Flow-through-time in seconds Sensitlser _ _ _ _ _ at star-tafter 2 weeks 4 weeks 8 weeks ~_ _ __ polyester resin 275 265 283 278 without additive benzoin 268 267 284 296 benzoinisopropyl- 272 278 281 283 The same mixtures were stored in the dark at 60 and the time before gelling occurred determined; the times lS are given in Table S:

Table 5 --. _ _. .
Storage time at 60 .
~ Sensitiser before gelling occurs benæoin 70 h -~-20 benæoinisopropyl- 190 h : benzildimethyl- ~ 230 h ~:
acetal _ : :
~:

:~:

.

: "

ample 12 PL~otocro~sllnlc;n~_~o~ ylene Benzildimethylacetal in a concentration of 0.5% is worked, on mixing rolls, into polyethylene of density 0.92. From the rolled sheet obtained are pressed films ; 5 0.1 mm in thickness, which are irradiated for 40 minutes with a high-pressure mercury vapour lamp at a distance of 10 cm. The irradiated film specimens are then extracted in boiling toluene for 5 hours. The extraction residue, which corresponds to the cross-linked proportion of the polyethylene, amounts to 24~/o of the weight of the film used. Without photosensitiser, but otherwise under the same conditions as those described above, no extraction residue remains.
xample l3 " . :.
Curing of printin~ pastes lS ~ mixture of 70 parts o trimethylolpropane-trisacrylate, , 10 parts of diallylphthalate-prepolymer and ; 20 parts of benzil-di-~-methoxyethyl-acetal, as bonding agent for inorganic and organic pigments, gels on being irradiated with a high-pressure mercury vapour lamp in less than 1 second.
The same result is obtained if, instead of methoxy-ethylacetal, the identical amount of benzil-dimethylacetal, benzil-dibutylacetal or benzil-di-~-bromoethylacetal is used.

6 _ , .

.
SUPPLEMENTARY:DISC~OSURE

Examples 14 to 18 21.0 g of benzil (0.1 mole) are suspended in 23.8 g of thionyl chloride (Q.2 mole) under cooling to 0C. 48 g of ~-butoxyethanol (0.4 mole) are drop-ped into the stirred suspension at a temperature of 2 to 7C. The reaction mixture is sti`rred for further 6 hours during which period the temperature is gradually raised to room temperature. A clear solution resul~s which is now stirred 10 hours at room temperature and 3 hours at 60C. The resulting re-action mixture is evaporated in a rotation still and the residue is distilled under high vacuum. There is obtained benzil di-butoxyethyl-acetal as a yell-owish oil boiling at 180C/0.02 mm Hg.
If the above procedure is carried out with using an equivalent amount of allyl alcohol, ~-allyloxyethanol, ~-isopropoxyethanol and 3-methoxy-butanol respectively instead of the butoxyethanol, there are obtained benzil-diallylacetal, benzil di-2-allyloxyethyl-acetal, benzil di-2-isopropoxyethyl-acetal and ~, benzil di-3-methoxy*utyl-acetal as yellowish, viscous oils. ~
Examples 19 and 20 ~ -21 g of benzil (0.1 mole) are suspended in 23.8 g of thionyl chloride (0.2 mole) under cooling to 0C. 56 g of 2-phenoxyethanol ~0.4 mole) are dropped to this suspension within 30 minutes at a temperature of 2 to 7C. The re-action mixture is stirred to room temperature. A clear solution results which is stirred further 10 hours at room temperature and 3 hours at 80C.
The solution is evaporated in a rotation still and the residue is heated to 180C under vacuum in order to distill a little amount of non-reacted benzil.
The residue crystallizes at room temperature and is recrystallized from iso-propanol. The ohtained benzil di-2-phenoxyethyl-acetal melts at 85C.
By substituting the phenoxyethanol by the equivalent amount of 2-phenylethanol in the a~ove procedure, there is ohtained benzil di-2-phenylethyl-~l acetal, melting at 72C.

- 3Za -. ., " ' - - ' Example 21 The acetals from Examples 14 to 20 are used as sensitizers for the photopoly-merization of methyl acry~late under exactly the same conditions as described in Example 8. The results are summerized in Table 6.
Table 6 Per cent Sensitizer Start time Amount of formed polymethyl by weight in seconds acrylate in percent by weight . .
without 60 0,1 1 benzoin 11 6,2 1 benzil-di- 3 13,5 2-butoxy-ethyl-acetal 1 benzil-di-2- 4,5 17 phenox~-ethyl-acetal 1 benzil-di- 4 14,5 2-phenyl-ethyl-acetal . 1 benzil-di- 3 7 .1 2-isopropoxy-ethyl-acetal 1 benzil-di- 3 18 2-allyloxy-ethyl-acetal 1 benzil-di- 4 8,4 ~
3-meJthoxy~ . ~ :
etfiyl-acetal 1 benzil-di- 5 5,3 ally~l-acetal Example 22 The acetals from Examples 14 to 20 are used as sensitizers for the photopoly-merization of a polyester resin exactly as described in Example 10~ The re-sults are listed in Table 7.

. ~ ~ ~ - ~h -~05415 ,' Table 7 r Sensitizer Pendulum hardness according to K~nig, after 20 min. irradiation ' benzoin 55 . benzil-di-2-phen~lethyl-acetal 85 benzil-di-2-phenoxyethyl-acetal 83 benzil-di-2-Butoxyethyl-acetal 90 . benzil-di-2-allyloxyethyl-acetal 70 benzil-diallyl-acetal 74 ': ',' ~`.
'', .:
. ~

~ - ~c - : ,

Claims (17)

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

1. Compounds of formula I

I

wherein R1 represents alkyl having 1 to 5 carbon atoms, alkenyl having 2 or 3 carbon atoms, aralkyl having 7 to 9 carbon atoms or a group -(CH2)n-X, wherein X is halogen, alkoxy having 1 to 4 carbon atoms or phenoxy and n is an integer from 1 to 3.

2. Compounds of formula I according to claim 1, wherein R1 rep-resents alkyl having 1 to 3 carbon atoms, alkenyl having 2 or 3 carbon atoms or aralkyl having 7 or 8 carbon atoms.

3. Process for the sensitized photopolymerisation of photopoly-merisable systems containing unsaturated compounds wherein the photopolymer-isation is performed in the presence of at,least one compound of formula I as defined in claim 1.

4. Process according to Claim 3 wherein the photopoly-merisation is performed in the presence of at least one compound of formula I as defined in Claim 2.

5. Photopolymerisable system consisting of at least one unsaturated, polymerisable compound and 0.1 to 20 per cent by weight of a compound of formula I as defined in Claim 1 or 2 and optionally other conventional additives.

6. Photopolymerisable system according to Claim 5, where-in the unsaturated, polymerisable compound is an acrylic or methacrylic ester.

7. Photopolymerisable system according to Claim 5, where-in the unsaturated, polymerisable compound is an un-saturated polyester resin in admixture with styrene.

8. Photopolymerisable system according to Claim 5, where-in the conventional additive is a stabiliser, a chain-transfer agent, a wax, a pigment or a filler.

9. Process for the production of compounds of formula I

I

wherein R1 represents hydrogen, alkyl having 1 to 5 carbon atoms, alkenyl having 2 or 3 carbon atoms, aralkyl having 7 to 9 carbon atoms or a group -(CH2)n-X, wherein X is halogen, alkoxy having 1 to 4 carbon atoms or phenoxy and n is an integer from 1 to 3, which process comprises (a) the reaction of benzil with a sulfite of the formula (R1CH2O)2SO in the presence of the alcohol R1CH2OH and in the presence of at least the molar amount of sulfuric acid, or (b) the reaction of benzil with thionyl chloride and an alco-hol of the formula R1CH2OH.

10. Process according to claim 9(a) wherein 1 mol of benzil is reacted with 3 to 4 moles of the sulfite and with at least 4 moles of the alcohol and wherein the reaction is performed at 20 to 120°C.

11. Process according to claim 10, wherein the reaction is per-formed at 40 to 100°C.

12. Process according to claim 9(a) wherein benzil is reacted with dimethyl sulfite in methanol in the presence of at least molar amounts of sulfuric acid.

13. Process according to claim 9(b) wherein 1 mol of benzil is reacted with at least 2 moles of SOC12 and at least 4 moles of the alcohol R1CH2OH.

14. Process according to claim 9(b) wherein the reaction is per-formed firstly at 0 to 20°C and subsequently at 20 to 120°C.

15. Process according to claim 14, wherein the second reaction step is performed at 40 to 100°C.

16. Process according to claim 9(b) wherein benzil is reacted with thionyl chloride and methanol.

17. Process according to claim 13, wherein the benzil is formed in situ from benzoin by reacting it with sulfuryl chloride.