FORM 2
THE PATENT ACT 1970
(39 of 1970)
&
The Patents Rules, 2003
PROVISIONAL SPECIFICATION
(See section 10 and rule 13)
1. TITLE OF THE INVENTION:
"Improved process for the preparation of polythiols"
2. APPLICANT
(a) NAME: CALYX CHEMICALS AND PHARMACEUTICALS LTD.
(b) NATIONALITY: Indian Company incorporated under the Indian
Companies ACT, 1956
(c) ADDRESS: 2, Marwah's Complex, Saki-Vihar Road, Sakinaka,
Andheri (East), Mumbai - 400072, Maharashtra, India 3.PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention.

FILED OF THE INVENTION:
The present invention relates to an improved process for the preparation of mercaptocarboxylates of trimethylolpropane and pentaerythritol (herein after referred to as polythiols). In particular, the present invention relates to an improved process for the preparation of trimethylolpropane trimercaptoacetate compound of formula 1 (herein
after referred to as TMPMA)

o
6
formula 1

and pentaerythritol tetramercaptoacetate compound of formula 2 (herein after referred to as PETMA)


o
SH

The present invention more particularly relates to an improved process for the preparation of compound of formula I and compound of formula II in high purity and yield.
BACKGROUND OF THE INVENTION:
Polythiols are well known for their use in the preparation of poly-thiourethanes for optical lenses having a high refractive index. The use of polythiols as crosslinking/curing agents includes special compositions for coatings, adhesives or sealants, e. g. flame-retardants, corrosion-resistant coatings or electrically inductive transparent films. Some of them are useful for floor coatings on concrete or for construction sealants.


Acting as chain-transfer agents, polythiols are used in the production of different polymers showing interesting properties, e.g. improved moldability, viscosity, storage stability or heat resistance. The applications of these polymers include compositions for special coatings, inks and optical devices.
Further applications of polythiols are for example their use as heat-stabilizers and antioxidants in different polymers. Polythiol with lower cross-linking capability like TMPMA lead to materials with lower hardness. TMPMA and PETMA are used as polymer cross-linking agents.
There are some processes described in the prior art for the preparation of polythiols.
US5932681 describes the preparation of polythiol by esterification or transesterification reaction between maercaptoacid and polyol in the presence of strong acid catalyst e.g. methane sulfonic acid
US 3144422 describes the preparation of novel esters of pentaerythritol and alkane 1,1,1-triols such as 1,1,1-trimethylolpropane by reacting pentaerythritol or alkane 1,1,1-triols with mercaptoalkanoic acid in toluene in the presence of para-toluenesulfonic acid as an acid catalyst.
JP 2001031646 describes the reaction of thioglycolic acid with pentaerythritol in dichloromethane in the presence of para-toluenesulfonic acid as a catalyst to obtain pentaerythritol tetrakis(thioglycolates).
Processes reported in the prior art suffer from drawbacks. At the end of the reaction, acid catalyst has to be removed which requires an additional step of aqueous work up. Thus, the process becomes tedious and costly.
Hence there is a need to develop a cost effective, easier to operate and technically viable process for the preparation of polythiols which also provides high purity and yield.


The present inventors have surprisingly found out that mercaptocarboxylates of trimethylolpropane and pentaerythritol can be prepared in high yield and purity by reacting mercaptoalkanoic acids with trimethylolpropane and pentaerythritol respectively in the presence of a cation exchange resin as a catalyst in a suitable organic solvent.
OBJECT OF THE INVENTION:
An object of the present invention is to provide an improved process for the preparation of mercaptocarboxylates of trimethylolpropane and pentaerythritol (polythiols).
Another object of the present invention is to provide an improved process for the preparation of trimethylolpropane trimercaptoacetate compound of formula 1 and pentaerythritol tetramercaptoacetate compound of formula 2.
Another object of the present invention is to provide an improved process for the preparation of compound of formula I and compound of formula II in high purity and
yield.
Yet another object of the present invention is to provide a cost effective process for the preparation of polythiols.
Yet another object of the present invention is to provide an environmentally friendly process for the preparation of polythiols.
SUMMARY OF THE INVENTION:
According to an aspect of the present invention there is provided an improved process for the preparation of mercaptocarboxylates of trimethylolpropane and pentaerythritol (polythiols) comprising reacting mercaptoalkanoic acid with trimethylolpropane and pentaerythritol respectively in the presence of a cation exchange resin as a catalyst in a suitable organic solvent.


In particular, the present invention relates to an improved process for the preparation of trimethylolpropane trimercaptoacetate compound of formula 1 (TMPMA)

o
formula 1
and pentaerythritol tetramercaptoacetate compound of formula 2 (PETMA)

SH
formula 2
in high purity and yield comprising,
reacting mercaptoacetic acid with trimethylolpropane and pentaerythritol respectively in
the presence of a cation exchange resin as a catalyst in a suitable organic solvent.
DETAILED DESCRIPTION:
The invention will now be described in detail in connection with certain preferred and optional embodiments, so that various aspects thereof may be more fully understood and appreciated.
Polythiols are used for the preparation of poly-thiourethanes for optical lenses having a high refractive index. The use of polythiols as crosslinking/curing agents includes special compositions for coatings, adhesives or sealants, e. g. flame-retardants, corrosion-resistant coatings or electrically inductive transparent films. Some of them are useful for floor coatings on concrete or for construction sealants. Acting as chain-transfer agents, polythiols are used in the production of different polymers showing interesting properties, e.g. improved moldability, viscosity, storage stability or heat resistance.


The present invention circumvents drawbacks of the prior art and provides a cost effective and environmentally friendly process.
The present invention relates to an improved process for the preparation of mercaptocarboxylates of trimethylolpropane and pentaerythritol (polythiols) comprising reacting mercaptoalkanoic acid with trimethylolpropane and pentaerythritol respectively in the presence of a cation exchange resin as a catalyst in a suitable organic solvent
In an embodiment of the present invention mercaptoalkanoic acid is selected from mercaptoacetic acid and mercaptopropionic acid, preferably mercaptoacetic acid.
The present invention particularly relates to an improved process for the preparation of trimethylolpropane trimercaptoacetate compound of formula 1 (TMPMA) in high purity
and yield comprising,
8 o
0
formula 1
reacting mercaptoacetic acid with trimethylolpropane in the presence of a cation exchange resin as a catalyst in a suitable organic solvent.
In particular, the present invention further relates to an improved process for the preparation of pentaerythritol tetramercaptoacetate compound of formula 2 (PETMA) in high purity and yield comprising,



SH
formula 2
reacting mercaptoacetic acid with pentaerythritol in the presence of a cation exchange resin as a catalyst in a suitable organic solvent.
The present inventors have found out that with an improved process of the present invention the time required for the reaction is less. The process does not require any aqueous work up. Products are obtained by removing solvent and volatile raw materials under vacuum. Also cation exchange resin the catalyst, used in the reaction can be recovered and recycled. Thus, it makes process of the present invention a cost effective and technically viable.
Cation exchange resin used in the reaction is commercially available strong cation exchange resin; preferably selected from Dowex-50, Amberlite IR 120, Amberlyst A-15, INDION 130, INDION 140 and INDION 190.
Preferably, cation exchange resin used in the reaction is 0.01 to 0.1 part by weight per part of the trimethylolpropane and pentaerythritol.
Preferably, the mole ratio of trimethylolpropane and pentaerythritol:mercaptoalkanonic acid used in the reaction is from 1:3.5 to 1:5
The suitable organic solvent is selected from hydrocarbons and halohydrocarbons, preferably aromatic hydrocarbons, more preferably toluene.
Preferably, the ratio of trimethylolpropane and pentaerythritol : solvent (w:v) used is from 1:0.5 to 1:3,


The reaction is carried out at a temperature ranges from 80°C-160°C, preferably from 100°C-150°C, more preferably from 120°C-150°C.
The water formed during the reaction is removed azeotropically. After the reaction is completed, the cation exchange resin used as a catalyst is recovered by filtration and recycled. The product obtained in 95-97% yield. Purity of the product measured by standard method of titration was 98-99%.
All the raw materials used in the experiments are commercially available. Mercaptoacetic acid used is 80% aqueous solution. Cation exchange resin is used as such. Purity of the product was found out by standard method of titration.
The following examples, which include preferred embodiments, will serve to illustrate the practice of this invention, it being understood that the particulars shown are by way of example and for purpose of illustrative discussion of preferred embodiments of the
invention.
Example 1:
Trimethylolpropane trimercaptoacetate (TMPMA)
1, 1, 1-trimethylolpropane (140g, 1.044M), mercaptoacetic acid (405g, 80%, 3.52 M), cation exchange resin (10.64g) and toluene (420 ml) were mixed and the mixture was heated to 105°C-140°C for 3 hours and 140°C-148°C for 2 hours with stirring and simultaneously removing water azeotropically. After no more water separated, the mixture was cooled to room temperature and the resin was filtered off, and washed with toluene (50 ml). Toluene was stripped off the product at 90°C-95°C. The product was further subjected to 1-2 mm vacuum at 90°C-95°C to get viscous oil.. Yield: 356.6g, 96% IR(neat) 2968.45, 2567.25, 1737.86, 1465.90, 1409.96, 1276.88, 1147.65, 1018.41 cm-1


Example 2:
Pentaerythritol Tetramercaptoacetate (PETMA)
Pentaerythritol (400g, 2.94M), mercaptoacetic acid (1420g, 80%, 12.35M), cation exchange resin (43.12g) and toluene (235 ml) were mixed and the mixture was heated 105°C-140°C for 3 hours and 140°C-148°C for 2 hours with stirring and simultaneously removing water azeotropically. After no more water separated, the mixture was cooled to room temperature and the resin was filtered off, washed with toluene (80 ml). Toluene was stripped off the product at 90°C-95°C. The product was further subjected to 1-2 mm vacuum at 90°C-95°C to get viscous oil Yield: 1220g, 96% IR(neat) 3527.80, 2962.66,2567.25, 1735.93, 1469.76, 1406.11, 1276.88, 1020.34 cm-1
It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative examples and that the present invention may be embodied in other specific forms without departing from the essential attributes thereof, and it is therefore desired that the present embodiments and examples be considered in all respects as illustrative and not restrictive, reference being made to the appended claims, rather than to the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.