BACKGROUND OF THE INVENTION

The present invention relates to the preparation of alkylthiopropionic pentaerythritol esters. Such pentaerythritol esters derived from alkylthioalkanoic acids are useful as stabilizers of materials otherwise sensitive to thermal or oxidative degradation, such as polymer resins. Several different methods are known in the art for preparing the alkylthiopropionic acid starting materials, as well as the pentaerythritol ester derivatives thereof. For example, U.S. Patent No. 4,349,468 discloses the preparation of pentaerythritol tetrakis (3-laurylthiopropionate) by heating an alpha-olefin such as l-dodecene with a beta- mercaptopropionic acid or ester in the presence of an azonitrile or peroxide catalyst. The resulting alkylthiopropionic acid is then esterified with pentaerythritol to produce the polyolefin stabilizer.

Similarly, EP 0413 562 discloses the preparation of a 3- alkylthiopropionic acid by reacting an alkyl mercaptan with an alkali metal acrylate in the presence of a strong base catalyst. The reaction is then acidified to recover the acid, which can then be esterified with pentaerythritol. EP 0 413 563 discloses a purification process for esters so prepared, or for esters prepared by other methods. More specifically, unreacted acid and tris ester components of the reaction are removed from the desired tetraester product by solvent refinement with a blend of at least two organic solvents.

JP 63-077854 discloses the manufacture of pentaerythritol tetrakis (3-alkylthiopropionate) by the Michael addition of C ₈ -

C ₃₀ alkyl mercaptans to acrylic acid esters or amides, followed by hydrolysis of the resulting adducts to 3-alkylthiopropionic acid, and subsequent esterification with pentaerythritol.

As the foregoing demonstrates, conventional processes for preparing the pentaerythritol esters result in impure product, and generally require further purification procedures which add to the cost of the final product. It therefore would be desirable to provide a process for the preparation of pentaerythritol esters of alkylthiopropionic acids that is simple and more direct, thereby resulting in a relatively pure product without the necessity for additional purification steps.

It is therefore an object of the present invention to provide a novel process for the preparation of pentaerythritol esters of alkylthiopropionic acids.

It is a further object of the present invention to provide a direct process for preparing alkylthiopropionic pentaerythritol esters of acceptable purity without a recrystallization step.

These and other objects of the present invention will become apparent upon consideration of the following detailed description of the invention.

SUMMARY OF THE INVENTION

The problems of the prior art have been overcome by the present invention, which provides a process of the preparation

of alkylthiopropionic pentaerythritol esters from pentaerythritol tetra-3-mercaptopropionate. In a first embodiment of the invention, pentaerythritol tetra-3- mercaptopropionate and an alpha-olefin are caused to react in the presence of a radical initiator. In a second embodiment of the invention, pentaerythritol tetra-3-mercaptopropionate and an alpha-olefin are caused to react photochemically. The process utilized is an inexpensive, simple method to prepare the esters from a commercially available starting material. Recrystallization can be used to further purify the product.

DETAILED DESCRIPTION OF THE INVENTION

The starting material for the instant process is pentaerythritol tetra-3-mercaptopropionate (PTM) , which is a commercially available product. The PTM can be prepared by any suitable means, such as by the vacuum esterification of 3- mercaptopropionic acid and pentaerythritol withmethanesulfonic acid as a catalyst. In general, commercially available PTM is typically washed with water to remove residual 3- mercaptopropionic acid and esterification catalyst prior to packaging.

Suitable radical initiators include azonitriles, such as azo-tert-butane, azobisisobutyronitrile, 2,2' -azobis (2- methylbutyro-nitrile) , 2,2' -azobis- (2-methylpropionitrile) and 2,2' -azobis(2,4-dimethylvaleronitrile) , and organic peroxides, such as peroxides having 1 to 2 peroxide groups and 4 to 40 carbon atoms. Examples of suitable organic peroxides include t-alkyl and aralkyl peroxides such as t-butylhydroperoxide,

cumyl-t-butyl peroxide, 2,5-dimethyl-2, 5-di(t-butylperoxy hexane) , di-tubutylperoxide and dicumyl peroxide, monoperesters such as t-butyl peracetate, t-butyl peroxylisobutyrate, t- butylperbenzoate, t-butylperpivalate, t-butylper-2- ethylhexoate, t-butylperoxyneodecanoate, t-butylperlaurate, and mono-tubutylperoxymaleic acid, diperesters such as 2,5-bis(2- ethylhexanoylperoxy) - 2 , 5 -dimethylhexane , di-t- butylperoxyphthalate, and 2, 5-bis (benzoylperoxy) -2, 5- dimethylhexane, aromatic diacyl peroxides such as 2,4- dichlorobenzoylperoxide, benzoyl peroxide and o- toluoylperoxide, ketone peroxides such as methyl ethyl ketone peroxide, cyclohexanone peroxide, and 1, 1-bis (t-butylperoxy) - 3, 3, 5-trimethylcyclohexane, and peroxycarbonate esters such as di - s e c - butylperoxydi carbona t e , di - t - butylcyclohexylperoxydicarbonate and t - butylperoxyisopropylcarbonate, and aliphatic diacyl peroxides such as acetyl peroxide, acetylpropionic peroxide, acetyl 2- ethylhexanoyl peroxide, 3,5,5-trimethylhexanoylperoxide, lauroyl peroxide, octanoyl peroxide, stearoyl peroxide, propionyl hexacosanoyl peroxide and succininc acid peroxide.

Preferably the radical initiator is used in an amount ranging from about 0.1 to about 10 mole % based on PTM, most preferably 0.6 to 1 mole %.

Suitable alpha-olefins include alpha-olefins of the formula CH ₂ =R, wherein R represents an alkyl group having from 5 to 27 carbon atoms. These alpha-olefins are commercially available, and can be prepared by conventional means well known to those skilled in the art. Examples of alpha-olefins include

hexene-l, 2-methylpentene-l, 4-methylpentene-l, heptene-1-, octene-1, 2-ethylhexene-l, nonene-1, decene-1, 2,4,4- trimethylpentene-1, dodecene-1, octadecene-1, hexadecene-l-, eicosene-1-, tetracosene-1- and octacosene-1. Preferred alpha- olefins are 1-dodecene and 1-octadecene.

The amount of alpha-olefin used is a function of the thiol or "SH" content of the starting pentaerythritol tetra-3- mercaptopropionate. In case the starting PTM is not the fully substituted ester, an analysis should be carried out to determine the thiol content of the PTM to be used. This can be accomplished by titrating the PTM acidified with concentrated HC1, for example, with iodine to the first permanent yellow endpoint. The alpha-olefin should be used in an amount sufficient to react with all of the thiol groups present. The amount ranges from 3.5-4.5 equivalents based on PTM, preferably 3.6 to 3.9 equivalents.

The PTM, alpha-olefin, and a first charge of initiator are combined and heated to about 40-90°C. Preferably the batch is heated to 80°C and allowed to exotherm. The batch is then held at a temperature from about 40 to about 190°C, preferably about 130°C, for 1-10 hours, preferably 1-4 hours. The remaining initiator charges can be added slowly during the reaction (such as small portions every hour) , or can be added at the end of the reaction. The batch is then held for about 4 hours at 40 - 190°C, preferably 130°C, and the resulting product is analyzed for thiol content (such as by titration as discussed above) . If the thiol content is not 0.22 or lower, more initiator can be added to complete the reaction. Yields greater than 95%

(based on starting material ester distribution and thio groups present) can be easily obtained. The resulting material contains lower levels of 3-mercaptopropionic acid, alkyl thioalkanoic acid, and esterification catalyst than product made by conventional methods . The material can be recrystallized using conventional methods well known to those skilled in the art if even purer product is desired.

In an alternative embodiment of the present invention, the alkylthiopropionic pentaerythritol esters are formed using photochemistry instead of a radical initiator. Specifically, PTM and the alpha-olefin are combined in a reaction vessel and subjected to photochemical reaction, such as by exposure to UV light at a temperature above the melting range of the specific derivative, preferably from about 60-100°C. The UV light is maintained for about four hours, or until the thiol content is reduced to an acceptable level, typically less than 0.22 milliequivalents per gram. A suitable source of UV light is a mercury vapor lamp. Thiyl radicals are known to be generated at 253.7 nm.

EXAMPLE 1

DETERMINATION OF THIOL CONTENT IN PTM AND ALKYLTHIOPROPIONIC PENTAERYTHRITOL ESTERS

About 0.300g of PTM or alkylthiopropionic pentaerythritol ester was accurately weighted into a 250 ml. standard taper

Erlenmeyer flask. 100 ml of 2-propanol (reagent grade) and 10 ml of concentrated hydrochloric acid were added. The mixture was heated to 55-60°C with stirring, and was then titrated with

0.IN iodine to the first permanent yellow color. The amount of thiol in the sample was calculated as follows.-

Millequiv. SH/gram = V(N) /grams of sample

where: V = Volume of iodine to titrate sample N = Normality of the iodine

EXAMPLE 2

PREPARATION OF PENTAERYTHRITOL TETRA KIS(3- STEARYLTHIOPROPIONATE)

The following materials and equipment were used:

Material M.W. Moles Grams

PTM 488.39 0.0522 25.5

1-octadecene 252.49 0.2089 52.75

Butanenitrile, 2-methyl,

2,2' -azobis (Initiator) 192.26 0.0003 0.0608 (0.0152x4)

Equipment

3 neck 100 ml flask, condenser, thermometer, heating mantle, magnetic stirrer and stirring plate.

The PTM, 1-octadecene and a first initiator charge were combined in the 100 ml flask. The batch was heated to 80°C and allowed to exotherm for approximately 15 minutes. The batch was then heated to 130°C and held for 1 hour. Three remaining equal portions of initiator charges were added at one hour intervals at 130°C. The reaction medium was held at 130°C for an additional four hours, and was then titrated for thiol content in accordance with the procedure of Example 1. The product was a hard solid which melted at 54-58°C.

EXAMPLE 3

PREPARATION OF PENTAERYTHRITOL TETRA KIS(3- LAURYLTHIOPROPIONATE) 162

The following materials and equipment were used:

Material M.W. Moles Grams

PTM 488.39 0.2096 102.4

Dodecene 168.32 0.7761 130.64

Butanenitrile, 2-methyl,

2,2'-azobis (Initiator) 192.26 0.0013 0.2432 (0.0608x4)

Equipment

3 neck 500 ml flask, condenser, thermometer, heating mantle, magnetic stirrer and stirring plate.

The PTM was charged to the 500 ml flask and titrated for milliequivalents SH/gram in accordance with the procedure set forth in Example 1. The dodecene charge was adjusted by the following calculation:

A = ;rcc∑:cr-.

Y = grams ofFTM

( X Meq. 5H \ / 1 eq \ ₍ Yg PTM \ / 168.32g \ _P

^V e FTM ^J ^ 100G- Meq ^; ^ 1 M i Eq dcd-tcene ^ ^{" srams Gt} dodsceae charge

The dodecene and the first initiator charge were added to the reaction vessel. The batch was heated to 80°C and allowed to exotherm for approximately 15 minutes. The batch was then heated to 130°C and the three remaining equal portions of initiator charges were added at one hour intervals at 130°C.

The reaction medium was held at 130°C for an additional four hours, and was then titrated for thiol content in accordance with the procedure of Example 1. The product was a semi-soft solid which became fluid at 40°C.

EXAMPLE 4

PHOTOCHEMICAL PREPARATION OF PENTAERYTHRITOL TETRA KIS(3- LAURYLTHIOPROPIONATE

The following materials and equipment were used:

Material M.W. Moles Grams

PTM 488.39 0.1573 76.8

Dodecene 168.32 0.5821 97.98

Equipment

250 ml photochemical jacketed reactor with quartz cell and UV light, thermocouple, heating and cooling unit, magnetic stirring and nitrogen sparge for mixing.

The PTM and dodecene were combined in the photochemical reactor. The UV light was turned on and the reactor was heated with hot water to 60°C. The reaction was held at 60°C for four hours or until the thio content was less than 0.22 milliequivalents per gram. The product had a melting point range of 47-49°C.

EXAMPLE 5

PHOTOCHEMICAL PREPARATION OF PENTAERYTHRITOL TETRA KIS(3- STEARYLTHIOPROPIONATE

The following materials and equipment were used:

Material M.W. Moles Grams

PTM 488.39 0.4176 204.0

1-octadecene 252.49 1.6712 422.0

Equipment

1000 ml photochemical jacketed reactor with quartz cell and UV light, thermocouple, heating and cooling unit, magnetic stirring and nitrogen sparge for mixing.

The PTM and 1-octadecene were combined in the photochemical reactor. The UV light was turned on and the reactor was heated with hot water to 60°C. The reaction was held at 60°C for four hours or until the thiol content was less than 0.22 milliequivalents per gram. The product had a melting point range of 59-62°C.