Note: Descriptions are shown in the official language in which they were submitted.
11249~7
Case 3-12341/CGM 229/1+2
Process for the manufacture of brominated products of
polystyrene
The present invention relates to a process for the
manufacture, by direct bromination, of brominated products
of polystyrene obtained by radically initiated polymerisation,
said products having a high bromine content.
The halogenation of polystyrenes is desirable
whenever it is intended that the final products should
have flame-resistant properties. A relatively low bromine
content usually suffices for this purpose. However, if it
is desired to incorporate halogenated polystyrene as
flame retardant in other plastics, a high bromine content
is advantageous in order that only small amounts of flame
retardant may be used. For this utility, the halogenated
polystyrene should have at most an insignificant natural
colour and sufficient thermal stability.
The direct bromination at low temperature,and using
relatively short reaction times,of polystyrenes obtain~d
by radically initiated polymerisation to produce products
having the desired properties, has proved to be an
insoluble problem. It is, however, most desirable, as the
use of these commercially available polystyrenes would be
more economic. The known difficulties which occur in the
direct bromination of these polystyrenes in halogen-
containing solvents at low temperature and in the presence
of a Lewis acid catalyst, are described e.g. in German
Auslegeschrift 2 537 385 and German Offenlegungsschrift
, 9~
llZ49~7
2 651 435: premature precipitation by gel formation, low
bromine content of the products, pronounced natural colour,
and thermal instability. These disadvantages are overcome
in the prior art by reacting selectively hydrogenated
oligomers of styrene,or polystyrene freshly prepared by
cationic polymerisation, with bromine.
It is known from Chemical Abstracts77,75 880 k (1972)
and 85,6331 k (1976) to carry out the direct bromination
of polystyrene in CC14 at elevated temperature in the
presence of iron as catalyst. Although a premature gel
formation is prevented in this physiologically hazardous
solvent, a noticeable and undesirable reduction in the
molecular weight of the brominated polystyrene is observed.
It has also been found that, at low temperature, only an
insufficient bromine content is obtained.
Accordingly, it is the object of the present
invention to provide a process for the direct bromination
of commercially available polystyrene, i.e. polystyrene
obtained by radically initiated polymerisation, which
process overcomes the disadvantages referred to and yields
thermally stable brominated products having at most an
insignificant natural colour, a high bromine content, and
good solubility in the reaction medium.
The present invention therefore relates to a process
for the manufacture of brominated products of polystyrene
which contain at least 50% by weight of bromine, by direct
bromination of polystyrene obtained by radically initiated
polymerisation, in solution and in the presence of a Lewis
acid catalyst at low temperature, which process comprises
carrying out the bromination in a partially halogenated
112494~
hydrocarbon as solvent, in the presence of a divalent and/or
trivalent iron salt as catalyst, in the temperature range
from -30 to +30C.
It is preferred to use bromine or bromine chloride
as brominating agent in the direct bromination. The bromine
chloride can be obtained in known manner, e.g. by
continuously adding bromine and passing a flow of chlorine
gas through the reaction solution. This procedure may also
result in the formation to an insignificant degree of
chlorinated final products.
The process of the invention affords the particular
advantage that it is possible to use essentially polystyrene
which is ready prepared and commercially available. The
brominated polystyrenes obtained in the process of this
invention have a bromine content of preferably at least
55% by weight, most preferably of at least 60% by weight.
The degree of polymerisation of the polystyrene employed
is preferably at least 100 and is especially between 500
and 2500, most preferably between 1000 and 2500. Partially
halogenated hydrocarbons are used as solvents. These can
have aromatic and especially aliphatic or cycloaliphatic
character and additionally contain at least one hydrogen
atom in the molecule. Preferred halogens are bromine,
fluorine and especially chlorine. The use of readily
volatile halogenated hydrocarbons is also advantageous, as
they are easiest to remove from the products obtained.
Examples of such solvents are: chloroform, methylene
chloride, dibromomethane, chloroethane, bromoethane,
dibromoethane, chlorobenzene, hexafluorobenzene, as well
as correspondingly mixed halogenated hydrocarbons or
mixtures of partially halogenated hydrocarbons. Preferred
solvents are methylene chloride and 1,2-dichloroethane.
llZ4947
The amount of polystyrene is not crucial for the
process. It is advantageous to employ an amount of 5 to
60% by weight, based on the reaction solution.
Divalent and/or trivalent iron salts or oxides are
used as catalysts. These can be both salts of inorganic
and organic acids and complex salts. Examples are: iron
fluoride, iron chloride, iron bromide, iron sulfate, iron
nitrate, iron phosphate, iron oxalate, iron acetate or
iron benzoate, as well as hexacyanoferrate and iron oxides
such as Fe304. When using an iron bromide, it is especially
advantageous to form this in situ by reacting iron with
bromine. Iron is therefore used in this case, and the
catalytically active iron bromide is formed during the
bromination. The catalyst is normally used in an amount
of 0.05 to 20V/o by weight, with the preferred amount being
from 0.5 to 5% by weight, based on the polystyrene
employed.
Preferred catalysts are iron halides, especially
FeC13, and iron. However, very good results are also
obtained by substituting other iron salts for these
catalysts in the process of the invention.
The bromination is preferably carried out in the
temperature range from 0 to 25C in apparatus ordinarily
used for the purpose. To avoid secondary reactions, it is
advantageous to perform the reaction while excluding light.
The reaction can be performed e.g. by dissolving
the polystyrene in the partially halogenated hydrocarbon,
then adding the catalyst, e.g. iron powder. Then, while
excluding light and with stirring and cooling, the required
amount of bromine, if desired dissolved in the same solvent,
11~4947
-- 5 --
is slowly added dropwise. The reaction mixture can be
stirred for a time in order to bring the reaction to
completion.
The reaction mixture is worked up by conventional
methods. Excess bromine can be removed by a reducing
agent, e.g. an aqueous hydrogen sulfite solution. The
reaction solution is then washed free of bromide with
water. The brominated polystyrene can then be isolated by
removing the solvent e.g. by distillation. However, it is
also possible to precipitate the brominated polystyrene
from the solution by addition of a polar solvent e.g. an
alcohol. The isolated brominated polystyrene can then be
dried at elevated temperature and in vacuo in order to
remove the solvent more completely.
The brominated polystyrenes obtained by the process
of this invention are soluble, uncrosslinked thermoplastic
products which are distinguished by a high bromine content,
an insignificant natural colour, and very good thermal
stability. Surprisingly, these products are obtained by
the process o~f the invention using radically polymerised
polystyrene as simple starting material. These brominated
polystyrenes are useful flame retardants for plastics,
especially for thermoplastics.
The invention is illustrated in more detail by the
following Examples.
11~4947
-- 6 --
Example 1: 26 g of commercially available polystyrene
(Hostyren N 5000) are dissolved in 600 ml of methylene
chloride. After addition of 0.5 g of iron powder, 99.8 g
of bromine are slowly added dropwise at 0C while
excluding light and with stirring and cooling. After
stirring for 6 hours at 0C, the methylene chloride solution
is stirred at room temperature with 100 ml of 3% sodium
hydrogen sulfite solution and then washed free of bromide
with water (S x 500 ml). The methylene chloride solution
is then poured into lS00 ml of methanol with stirring.
The precipitated product is filtered with suction and dried
for 12 hours at 60C/1 mbar, affording 59 g of brominated
polystyrene in the form of a colourless powder with a
bromine content of 64.9%. The product starts to decompose
at 315C (determination by thermogravimetric analysis).
Example 2: 26 g of commercially available polystyrene
are dissolved in 600 ml of methylene chloride. After
addition of 1 g of iron powder, 99.8 g of bromine are
added slowly dropwise at 15-20C while excluding light
and with stirring and cooling. After stirring for 6 hours
at 15-20C, the reaction mixture is washed free of
bromide as in Example 1. The methylene chloride solution
is concentrated and the residue is kept for a further hour
at 250C/10 mbar, affording 61 g of slightly yellowish
brominated polystyrene with a bromine content of 64.4 % by
weight. The product starts to decompose at 325C
(determination by thermogravimetric analysis).
Example 3: 26 g of commercially available polystyrene
are dissolved in 600 ml of dichloroethane. After addition
of 1 g of iron powder, 99.8 g of bromine are slowly added
11~4947
-- 7 --
dropwise at 15-20C while excluding light and with
stirring and cooling. After atirring for 6 hours at 15-20C,
the reaction mixture is washed free of bromide as in
Example 1. Precipitation of the polymer solution in
methanol yields 58 g of almost colourless polystyrene with
a bromine content of 64.3% by weight. The product starts
to decompose at 305C (determination by thermogravimetric
analysis).
Example 4: 26 g of commercially available polystyrene
are dissolved in 600 ml of methylene chloride. After
addition of 0.5 g of iron powder, 79.9 g of bromine are
slowly added dropwise at -10C while excluding light and
with stirring and cooling. After stirring for 24 hours at
-10C, the reaction mixture is washed free of bromide
as in Example 1. Precipitation of the polymer solution in
methanol yields 55 g of almost colourless brominated
polystyrene with a bromine content of 61.6% by weight.
The product starts to decompose at 325C (determination by
thermogravimetric analysis).
Example 5: 26 g of commercially available polystyrene
are dissolved in 300 ml of methylene chloride. After
addition of 4 g of FeP04, 79.9 g of bromine are slowly
added dropwise at 0C while excluding light and with
stirring and cooling. After stirring for 24 hours at 0C,
the reaction mixture is washed free of bromide as in
Example 1. Precipitation yields 62 g of brominated
polystyrene in the form of a yellowish powder with a
bromine content of 62.4% by weight. The product starts to
decompose at 318C(determination by thermogravimetric analysis).
11~4947
Example 6: 26 g of commercially available polystyrene are
dissolved in 300 ml of methylene chloride. After addition
of 2 g of dried Fe(II) oxalate (10 hours/140C/l mbar),
51.3 g of bromine and 21.3 g of chlorine are added at 0C
in the course of 4 hours while excluding light and with
stirring and cooling. The addition of the halogens is made
as follows: first only bromine (1 hour), later bromine
and chlorine simultaneously (2 hours) and finally only
chlorine (1 hour). The reaction mixture is stirred for
18 hours, then washed free of bromide as in Example 1 and
worked up, affording 69.6 g of brominated polystyrene in
the form of a yellowish powder with a bromine content of
63.3% by weight and a chlorine content of 0.9% by weight.
The product starts to decompose at 349C (determination
by thermogravimetric analysis).
Example 7: 26 g of commercially available polystyrene
are dissolved in 300 ml of methylene chloride. After
addition of 2 g of finely powdered Fe304, 51.3 g of bromine
and 21.3 g of chlorine are added in the manner described
in Example 6. The reaction mixture is stirred for 18 hours
and worked up as in Example 1, affording 72.6 g of bromin-
ated polystyrene in the form of a yellowish powder with a
bromine content of 64.0% by weight and a chlorine content
of 1.2% by weight. The product starts to decompose at
334C (determination by thermogravimetric analysis).
Comparison Example: 26 g of commercially available
polystyrene are brominated as described in Example 1 using
CCl4 as solvent. The brominated polystyrene has a bromine
content of only 45% by weight.
