Note: Descriptions are shown in the official language in which they were submitted.
WO 92/13904 PCT/US91l09476
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FREE RADICAL POLYMERIZATION OF VINYL AROMATIC MONOMERS
The present '_nvention relates to a process for
polymerization of vinyl aromatic monomers. More
particularly the present invention relates to an
improved process for the free radical polymerization of
vinyl aromatic monomers to make high molecular weight
polymers.
Currently, production of high molecular weight
vinyl aromatic polymers, particularly polymers having
weight average molecular weights (Mw) of greater than
300,000, is best performed by the use of anionic
polymerization techniques. This is due to the extremely
slow polymerization rates required to make high
molecular weight vinyl aromatic polymers using free
radical chemistry. Disadvantageously however, anionic
polymerization processes require expensive anionic
initiators and tend to produce discolored products due
to the presence of residual lithium-containing salts.
In addition, anionic processes utilize different
equipment than free radical processes. Consequently
commercial producers of. vinyl aromatic polymers by means
of free radical chemistry must invest in anionic
polymerization equipment in order to prepare very nigh
molecular weight aolymers. Finally, anionic
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polymerization cannot be employed to prepare many
copoiymeric products. In many eases the monomer is not
amenable to anionic polymerization. In other cases
block copolymers are formed due to unequal reactivities
of the comonomers.
It would be desirable if it were possible to
produce high molecular weight polyvinyl aromatic resins
utilizing free radical polymerization equipment while
obtaining rates that. are commercially practical. Thus
~0 it is to the attainment of the preparation of such high
molecular Weight polymers via free radical polymeri-
zation techniques that the present invention is
directed.
According to the present invention there is
provided a process for free radical polymerization of a
vinyl aromatic monomer to prepare a high molecular
weight polymer characterized in that the polymerization
is conducted in the presence of from 5 to 5000 parts per
million (ppm) of a soluble organic acid having a pKa
from 0.5 to 2.5 at 25°C. It has been surprisingly
discovered that in the presence of such an amount of
these acids, the free radical polymerization rate is
substantially increased, thereby allowing the attainment
of high molecular weight polymers in reasonable reaction
times.
The vinyl aromatic monomers usefully employed
according to the present process include styrene, ring
alkyl substituted styrene, particularly C~-~ alkyl and
-- especially methyl; ring substituted styrenes and
a-methylstyrene. A preferred monomer is styrene. The
polymerization can also include a comonomer to prepare
vinyl aromatic copolymers. The comonomer must be
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noninterfering with the acid. Examples include
(meth)acrylonitrile, (meth)acrylic acid and C1_4 alkyl
esters thereof, N-C~_~ alkyl maleimide, N-phenyl
maleimide, etc. In addition the polymerization may be
conducted in the presence of predissolved elastomer to '
prepare impact modified, grafted rubber containing
products.
By the term "soluble" is meant that the acid is
sufficiently soluble in the reaction mixture to achieve
the indicated.coneentration of organic acid. Preferred
organic acids are miscible with neat styrene monomer.
Suitable organic acids include the C1-20 alkyl and aryl
substituted sulfonic and phosphoric acids. Examples
include methane sulfonic acid, toluene sulfonic acid,
camphorsulfonic acid, napthalene sulfonic acid, methyl
phosphoric acid, phenyl phosphoric acid, etc. Strong
acids, that is, organic acids having a pKa less than
0.5, are not desired due to increased incidence of
cationio polymerization as opposed to the desired free
radical initiation. Preferred acids have pKa from 1.0
to 2Ø A preferred organic acid is camphorsulfonic
acid.
It has been discovered that at increased
concentrations of organic acid, cationic polymerization
becomes prevalent. Generally, acids with higher pK,
that is, weaker acids, may be employed in higher
concentration without detrimental effect. Stronger
acids are employed in relatively lower concentration. I
;,ationic polymerization is undesirable because~it
results in extremely low~molecular weight oligomer~
formation. Even small quantities of such low
molecular weight product would significantly reduce the
l
molecular weight average of the resulting product. Most
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preferred are amounts of organic acid from 50 to 5000
ppm. The amount of acid is measured with respect to the
molar quantity of vinyl aromatic monomer.
A free radical initiator may be employed to
further improve the rate of free radical initiation.
Suitable initiators include organic peroxides, and
hydroperoxides having up to 10 carbons and other well
known free radical initiators. Preferably such
initiator is used in an amount from 50 to 2000 ppm based
on total monomer, preferably from 100 to 1000 ppm.
The monomer may be polymerized in bulk, that
is, in the absence of a diluent, or in the presence of a
diluent, that is, in solution. Suitable diluents
include toluene, ethylbenzene, and other noninterfering
organic liquids. Preferably the reaction is conducted
under bulk polymerization conditions. Suitable
polymerization temperatures are from 25 to 200°C,
preferably from 85 to 180°C.
The polymerization rate according to the
present process is substantially increased and the
resulting product has substantially increased molecular
weight compared to products prepared by free radical
polymerization in the absence of an organic acid. --
However, because the product has increased molecular
weight, .the conversion rate is less at higher acid
concentrations compared to lower acid concentrations.
That is, the higher molecular weight polymers require
longer reaction times despite incrementally faster
polymerization,rates. Preferred. polymer_produet:has a
molecular weight (Mw) from 300,000 to 1,000.000, more
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preferably 500,000 to 800,000, based on a polystyrene
standard as measured by size exclusion chromatography.
The products are employed in applications where
high molecular weight vinylaromatic polymers have
previously found suitable uses. Particularly preferred
are molding polymers comprising the presently prepared
polymeric products. The product may be blended with
other ingredients such as mold release additives,
lubricants, colorants, ignition resistant additives,
impact modifiers, glass fibers, as well as other resins
such as polyvinylaromatic resins having different
molecular weights, polyphenylene oxides, polycarbonates,
elastomeric copolymers such as styrene-butadiene block
copolymers, polybutadiene, etc.
Having described the invention the following
examples are provided as further illustrative and are
not to be construed as limiting.
Examples 1-3
Aliquots of styrene monomer which was purified
by degassing and contacting with alumina were placed in
glass tubes. To each tube was added an amount of
methane sulfonic acid further identified in Table 1.
The tubes were sealed under vacuum and placed in an oil
bath at 150°C for 1 hour. The tubes were withdrawn and
the weight average molecular weight of the polystyrene
in each tube measured using size exclusion
chromatography. Results are contained in Table 1.
It may be seen~that significant increase in
molecular weight is observed upon addition of small
quantities of the acid.
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Tabls 1
_Run Methane Sulfonie Acid (ppm) _Mw
A'~ 0 270 , 000
1 100 460,000
2 300 580,000
3 500 830>000
'comparative
Examples 4-8
The reaction conditions of Examples 1-3 were
substantially repeated employing various concentrations
of camphorsulfonic acid (CSA) at a reaction temperature
of 140°C. Weight average molecular weight of the
resulting polystyrene and conversion rates are provided
in Table II.
Table II
Conversion Rate
Run Amt. CSA ppm ~percent/hr) Mw
B~ 0 43 28000000
4 100 25 400000
5 250 18 510000
6 500 14 650000
7 750 12 700000
8 1000 10 710000
comparative
Examples 9-10
The reaction conditions of Examples 1-3 are
substantially repeated utilizing phenylphosphonie acid
(PPA) at a reaction temperature of 150°C. Weight
average molecular weight of the resulting polystyrene
and conversion rates are~provided in Table III.
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Table III
Amt. PPa Conversion Rate
_Run (wt. percent)(pereent/hr) _Mw
C'~ 0 50 240000
9 0.1 40 280000
0.5 25 430000
5
*'comparat i ve
Examples 11 and 12
10 Aliquots of styrene monomer which was purified
by degassing and contacting with alumina were placed in
glass tubes. In examples 11 and 12, 500 ppm of camphor
sulfonic acid was added to the tube. In Example 12 and
comparative run D, 500 ppm t-butyl hydroperoxide was
added. Thus, no camphor sulfonic acid is present in run
D and Example 12 contained both camphor sulfonic acid
and t-butyl hydroperoxide. The tubes were dried over
anhydrous calcium chloride, sealed under reduced
pressure and placed in an oil bath at 110°C for 2 hours.
The tubes were withdrawn and the polymer was recovered.
The weight average molecular weight of the polystyrene
in each tube was measured using size exclusion
chromatography. Conversions were determined by weight
loss after drying a portion of the polymer at 240°C.
Results are contained in Table IV.
Table IV
Run Percent Solids Mw Mn
D~ 19.9 336,000 11,000
11 3.3 1,257,000 754,000
12 24.5 411,000 221,000
snot an example of the invention
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It may be seen that the use of both a
hydroperoxide initiator and an acid gave both high
molecular weight polymer and high polymer conversion.
Examples 12-16
The reaction conditions of Examples 1-3 were
substantially repeated employing oamphorsulfonic acid
(500 ppm based on styrene) and 500 ppm of the following
organic peroxides, ditertiary butyl peroxide (Ex. 12),
2,5-dimethy-2,5-dihydroperoxyhexane (Ex. 13), 2,5w
dimethyl-2,5-t-butylperoxyhexane (Ex. 14), 9,9,12,12-
tetramethyl-7,8, 13, 1~4-tetraoxaspiro [5.8] tetradecane
(Ex. 15), and 1,1-di-t-butylperoxycyelohexane (Ex. .6).
Examples 12-15 were conducted at 150°C for two hours.
Example 16 was conducted at 110°C for two hours.
Results are contained in Table V.
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Table
V
Example ppm, csal Pereent MW X 103 Mn x
'03
Solids
12 500 98.5 211 89
E# 0 98.8 171 76
13 500 83.4 223 110
F~ 0 86.0 186 93
1u 500 98.9 25u 100
G~' 0 98.8 20o 86
500 71.2 325 158
H~' 0 79.6 236 118
16 500 41.7 348 145
I'~ 0 42.8 266 135
15 ~ Comparative
1 Camphorsulfonie ac id
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